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Synthesis and functionalization of polymer nanospheres

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Synthesis and functionalization of polymer nanospheres

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Content Synthesis and Functionalization of Polymer Nanospheres Copyright 2000 By Marcia Tuten Greci A Dissertation Presented to THE FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (Chemistry) August 2000 Marcia Tuten Greci Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UNIVERSITY OF SOUTHERN CALIFORNIA THE GRADUATE SCHOOL UNIVERSITY PARK LOS ANGELES, CALIFORNIA 90007 This dissertation, written by M a r c ia T u t e n G r e c i .................. under the direction of hsx.— Dissertation Committee, and approved by all its members, has been presented to and accepted by The Graduate School, in partial fulfillment of re quirements for the degree of DOCTOR OF PHILOSOPHY Dean o f Graduate Studies DISSERTATION COMMITTEE Chairperson Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. To Mama, Daddy, and Mike Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Acknowledgements I would first like to express my appreciation to Dr. G. K. S. Prakash for his support and guidance throughout my graduate career. This work would not have been possible without his advice and suggestions. Dr. George A. Olah’s support and encouragement were also an invaluable part of my studies. I also must acknowledge the many people in the Olah-Prakash group for their Scientific Cooperation as well as help and support. Among these people are Dr. Robert Aniszfeld, Dr. Arwed Burrichter, Dr. Emily Tongco, Dr. Eric Marinez, Tony Atti, Suchi Krishnaraj, Christoph Thiebes, Stefan Salzbrunn, Dr. Sabine Shwaiger, Dr. Markus Etzkom, Dr. Juergen Simon, Karine Mercado, Virginie Pleynet, Neal Devraj, Chulsung Bae, Jin Bo Hu, Dr. Paul Mwashimba, and Dr. Stephen Butala. I would also like to thank Reiko, Jessy, David, and Carole for keeping our group running smoothly. Research support of my work by the Loker Hydrocarbon Research Institute and Air Force Office of Scientific Research (MURI, Program Director Dr. Charles Lee) is gratefully acknowledged. Last, but certainly not least, I would like to thank my friends and family for all of the support they have given me over the years. Dr. Howard Thomas, my undergraduate advisor, was instrumental in fostering my love of Chemistry and helping me to grow as a chemist. I would certainly not be here today if it weren’t for the love and support, not to mention the unwavering belief in my ability to be here, of my parents and primary cheerleaders, Marcia and Alec Tuten. Finally, I would like to thank my husband, Mike Greci, for all of the love, friendship, help, support, and comfort he has provided. iii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Marcia T. Greci G. K. S. Prakash SYNTHESIS AND FUNCTIONALIZATION OF POLYMER NANOSPHERES In this dissertation, we have extended the work previously carried out on the synthesis and functionalization of polymer nanospheres in an attempt to find a general route for the facile surface functionalization of polymer nanospheres. We have focused our attention primarily on the polystyrene-divinylbenzene system. Chapter one summarizes the work carried out in the polymer nanosphere area. In chapter two, we have explored the emulsifier-free emulsion polymerization of polystyrene. Several variables were tested to determine the best way to control the size and spherical integrity of the nanospheres. In chapter three, the synthesis of poly-(vinylpyridine) nanospheres via the emulsifier-free emulsion polymerization technique is discussed. This was the first time poly-(vinylpyridine) nanospheres were synthesized using this method. These nanospheres were then coated with metals and used as catalysts in a variety of organic reactions. In chapter four, a method for the surface functionalization of preformed polystyrene nanospheres is described. This method of surface functionalization was based on a method developed by Frechet in 1979. We were successful in placing several functional groups onto the surface of polystyrene. However, it was determined that the surface coverage was not uniform and another method needed to be developed. iv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In chapter five, a novel in situ grafting of substituted styrene monomers at the end of nanosphere formation is reported. In this method, the substituted monomers were added to the polymerization reaction mixture and were able to react competitively with any remaining styrene monomer to result in surface coverage by the desired functional group. This method was found to be quite versatile and will have many potential applications. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table of Contents Dedication ii Acknowledgements iii Abstract iv List of Figures viii List of Tables xi List of Equations xii Bibliography xiii Chapter 1 Introduction 1.1 Synthesis of Polymer Nanospheres 1 1.1.1 Emulsion Polymerization 1 1.1.2 Emulsifier-Free Emulsion Polymerization 3 1.2 Functionalization of Polymer Nanospheres 6 1.3 Applications of Polymer Nanospheres 8 1.3.1 Catalysis 8 1.3.2 Combinatorial Chemistry 11 1.3.3 Protein Support 14 1.3.4 Magnets 17 1.3.5 Photonic Crystals 19 1.4 References 22 Chapter 2 Synthesis of Polystyrene Nanospheres 2.1 Introduction 2.2 Results and Discussion 2.3 Experimental 2.3.1 General Procedure for Synthesis of Polystyrene Nanospheres 2.4 Conclusions 2.5 References Chapter 3 Synthesis of Poly-(vinylpyridine) Nanospheres 3.1 Introduction 3.2 Results and Discussion 3.3 Experimental 46 49 56 27 28 42 43 44 44 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.4 Conclusions 3.5 References 57 58 Chapter 4 Functionalization of Preformed Polystyrene Nanospheres 4.1 Introduction 60 4.2 Results and Discussion 61 4.3 Experimental 66 4.3.1 General procedure for the lithiation of preformed Polystyrene nanospheres 67 4.3.2 Addition of Electrophiles 67 4.4 Conclusions 72 4.5 References 73 Chapter 5 Polystyrene Functionalization: A New in situ Grafting Technique 5.1 Introduction 74 5.2 Results and Discussion 76 5.3 Experimental 79 5.3.1 General Procedure for Synthesis of Surface- Functionalized Polystyrene Nanospheres 80 5.4 Conclusions 89 5.5 References 90 Chapter 6 Concluding Remarks 6.1 Conclusions 91 6.2 References 94 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. List of Figures Figure 2.1: Polystyrene nanospheres with .5 mole% divinylbenzene 30 Figure 2.2: Polystyrene nanospheres with 5 mole% divinylbenzene 30 Figure 2.3: Polystyrene nanospheres with 4 mole% divinylbenzene 31 Figure 2.4(a): Polystyrene synthesized at 70 °C 31 Figure 2.4(b): Polystyrene synthesized at 90 °C 32 Figure 2.5: Polystyrene nanospheres with immediate addition of initiator 33 Figure 2.6: Polystyrene nanospheres synthesized with too much initiator 34 Figure 2.7: Polystyrene nanospheres with small concentrations of monomer (top) and large concentrations of monomer (bottom) 35 Figure 2.8: Polystyrene nanospheres synthesized in 695 mL water and 5 mL ethanol 36 Figure 2.9: Polystyrene nanospheres synthesized in 690 mL water and 10 mL ethanol 37 Figure 2.10: Polystyrene nanospheres synthesized in 685 mL water and 15 mL ethanol 37 Figure 2.11: Polystyrene nanospheres synthesized in 680 mL water and 20 mL ethanol 38 Figure 2.12: Polystyrene nanospheres synthesized in 675 mL water and 15 mL ethanol 38 Figure 2.13: Polystyrene nanospheres synthesized in 650 mL water and 50 mL ethanol 39 Figure 2.14: Polystyrene nanospheres synthesized in 525 mL water and 175 mL ethanol 39 Figure 2.15: Polystyrene nanospheres synthesized in 700 mL ethanol 40 Figure 2.16: Polystyrene nanospheres synthesized at 250 rpm 41 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.17: Polystyrene nanospheres synthesized at 400 rpm 41 Figure 2.18: Diagram of reaction kettle used for polystyrene synthesis 43 Figure 3.1: Poly-(4-vinylpyridine) nanospheres 50 Figure 3.1: Poly-(2-vinylpyridine) nanospheres 50 Figure 3.3: Poly-(2-co-4-vinylpyridine) nanospheres 51 Figure 3.4: Poly-(4-vinylpyridine) nanospheres synthesized at 90 °C 52 Figure 3.5: Poly-(4-vinylpyridine) nanospheres synthesized at 70 °C 52 Figure 3.6: Auger Spectrum of poly-(2-vinylpyridine) 53 Figure 3.7: Palladium-coated poly-(vinylpyridine) nanospheres before reaction 54 Figure 3.8: Palladium-coated poly-(vinylpyridine) nanospheres after reaction 55 Figure 4.1: General reaction scheme for the surface functionalization of preformed polystyrene nanospheres 61 Figure 4.2: Thiolated polystyrene nanospheres 62 Figure 4.3: Polystyrene nanospheres functionalized with 2-hydroxy propyl groups 63 Figure 4.4: Carboxylated polystyrene nanospheres 63 Figure 4.5: Formylated polystyrene nanospheres 64 Figure 4.6: Aminated polystyrene nanospheres 65 Figure 4.7: EDX of thiolated polystyrene nanospheres 69 Figure 4.8: Surface-Reflectance D R . of hydroxylated polystyrene nanospheres 69 Figure 4.9: Surface-Reflectance IR of carboxylated polystyrene nanospheres 70 Figure 4.10: Surface-Reflectance IR of formylated polystyrene nanospheres 71 Figure 4.11: Surface-Reflectance IR of aminated polystyrene nanospheres 72 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.1: Substituted styrenes successfully grafted onto the surface of polystyrene nanospheres 76 Figure 5.2: Non-functionalized polystyrene nanospheres 77 Figure 5.3: Hydroxylated nanospheres coated with silver colloids (top) and ruthenium colloids (bottom) 78 Figure 5.4: Hydroxylated polystyrene nanospheres formed by the hydrolysis of acetoxy groups 81 Figure 5.5: Auger Spectrum of hydroxylated polystyrene nanospheres after hydrolysis of the acetoxy groups 82 Figure 5.6: Hydroxylated polystyrene nanospheres obtained after hydrolysis of the r-butoxy group 83 Figure 5.7: Auger Spectrum of hydroxylated polystyrene nanospheres obtained after hydrolysis of the r-butoxy groups 83 Figure 5.8: para-Fluorostyrene grafted polystyrene nanospheres 84 Figure 5.9: r ,2 ’,2’-trifluorostyrene grafted polystyrene nanospheres 85 Figure 5.10: 2-vinylpyridine grafted polystyrene nanospheres 85 Figure 5.11: 4-vinylpyridine grafted polystyrene nanospheres 86 Figure 5.12: Auger Spectrum of aminated polystyrene nanospheres 87 Figure 5.13: Polystyrene nanospheres grafted with 4-vinylaniline 87 Figure 5.14: Auger Spectrum of chloromethylated polystyrene nanospheres 88 Figure 5.15: Chloromethylstyrene grafted polystyrene nanospheres 89 x Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 2.1: Table 4.1: List of Tables Time profile of polystyrene nanosphere synthesis Reaction Conditions for Addition of Various Electrophiles to preformed polystyrene nanospheres Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. List of Equations Equation 3.1: Heck Reaction using palladium-coated poly-(vinylpyridine) nanospheres as catalyst Equation 3.2: Suzuki coupling using palladium-coated poly-(vinylpyridine) nanospheres as catalyst Equation 3.3: Stille Coupling using palladium-coated poly-(vinylpyridine) nanospheres as catalyst Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Bibliography 1. D. C. Sherrington, Chem. Comm., 2275 (1998). 2. D. C. Sherrington, in Polymer-Supported Reactions in Organic Synthesis, ed. P. Hodge and D. C. Sherrington, Wiley, Chichester, UK, ch. 3 (1980). 3. E. A. 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Park, K. Gandhi, L. Sun, J. J. Aklonis, and R. Salovey, Polym. Eng. Sci., 30, 1158(1990). 128. M. T. Greci, S. Pathak, K. Mercado, G. K. S. Prakash, M. E. Thompson, and G. A. Olah, Chem. Mat., submitted. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 1: Introduction Polymer nanospheres have attracted increased attention over the past several years. They have been used in a variety of applications, including catalysis, combinatorial chemistry, protein supports, magnets, and photonic crystals. These polymer nanospheres are synthesized primarily by emulsion polymerization and are later surface functionalized by a variety of methods. This chapter will review some reported methods of synthesis and functionalization, followed by a short review of the uses of polymer nanospheres. 1.1 Synthesis of Polymer Nanospheres 1.1.1 Emulsion Polymerization Polymer nanospheres were first synthesized via emulsion polymerization in the 1950s. Since that time, a tremendous amount of effort has been put into investigating emulsion systems and trying to achieve better size and shape control of the nanospheres.1 Early syntheses of polymer nanospheres focused on the styrene-divinylbenzene systems. These were carried out by traditional emulsion polymerization techniques. The reaction mixture used consisted of styrene and divinylbenzene dispersed as spherical liquid droplets in an excess of an immiscible water phase. The styrene -divinylbenzene mixture also contained a source of free radicals, the polymerization initiator, and the aqueous l Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. phase generally contained a low level of some dissolved suspension stabilizer, a surface active species, often a water soluble polymer which helps to maintain the organic monomer droplets separate from each other. The suspension is maintained stable by continuous stirring and the reaction mixture is typically heated to -80 °C for twelve hours. During this period the spherical liquid monomer droplets were converted into hard glassy polymer particles, still retaining the spherical symmetry of the original liquid droplets.2 A paper by Williams and Bobalek in 1966 examined the kinetics of the emulsion polymerization described above.3 In this study, they determined that the kinetics of polymerization did not fit the steady state theory as previously believed. The generation of free radicals from potassium persulfate in the aqueous phase occurs at a constant rate and 100% efficiency. The overall rate of initiation and the rates of initiation per particle are therefore constant and directly derivable from the decomposition kinetics of potassium persulfate. The authors asserted two observations based on the initiation rate. First, with a constant rate of initiation per particle at 100% free-radical efficiency, there is no chain branching, and termination exclusively by combination takes place. Therefore, the rate of termination per particle must decrease during the constant rate period to account for the observed increase in molecular rate. At a constant rate of initiation per particle this can only mean the possibility of non-steady-state polymerization mechanism. Second, since the trend of increasing molecular weight with conversion is generally substantial, it should take more than a minor variation in any of the above-mentioned case conditions to account for this trend. The fact that termination occurs by combination in styrene polymerization was stated to be a well-established experimental fact. Under 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the conditions where the free-radical efficiency is not 100%, it was proposed that the efficiency should increase with increasing conversion, since the surface area is likewise increasing. If such were the case, the rate of initiation per particle should increase, resulting in a decrease of molecular weight. Branching could cause this, but it is believed that branching is a rare phenomenon in the case of polymerization of styrene. Therefore, the authors concluded that steady-state kinetics do not apply to emulsion polymerization of polymer nanospheres.3 In 1996, Margel4 reported another study into the mechanism and kinetics of emulsion polymerization. According to that work, the reaction mechanism and kinetics are complex and poorly understood. The process is composed of two major stages, nuclei formation and then nuclei growth. At the start of the process, the monomer, surfactant, and initiator are dissolved and form a homogeneous solution in the continuous phase. Upon polymerization, the initiator radicals react with solute monomer molecules to form oligomeric radicals, which at a critical chain length precipitate as small nuclei. These nuclei may then grow to the final size by a variety of mechanisms, such as agglomeration of small nuclei, polymerization of monomer in the swelled nuclei, and seeded polymerization of the monomer on the nuclei surfaces. The stabilizer in the dispersion polymerization is adsorbed on the particle surfaces and thereby stabilizes the particles by a process which is only qualitatively understood. The particles stop growing when all of the monomer is consumed and/or when the stabilizer is adsorbed and forms a relatively packed coating on the particle surfaces.4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1.1.2 Emulsifier-Free Emulsion Polymerization The use of classical emulsion polymerization to prepare polymer nanospheres has certain disadvantages. For example, such lattices are often stabilized by the adsorption of surfactants, as explained above, and removal of these surface active agents can lead to loss of stability.5 Even in cases where the latex particles can be effectively stabilized by groups which are an integral part of the polymer particle, removal of the surface active agent used in the preparation is not always easily accomplished and it is often difficult to establish precisely that the surface active agent has been removed.6 To overcome these difficulties, Goodwin and coworkers developed a method for the emulsifier-free emulsion polymerization of styrene in 1974.7 In emulsifier-free emulsion polymerization, the polymerization is carried out in the same way as in classical emulsion polymerization, except that no emulsifier is used. Accordingly, nucleation takes place by precipitation of macroradicals (and macromolecules) as compared with micelle formation in normal emulsion polymerization. Since there is no emulsifier present, the nuclei thus formed are not stabilized by any emulsifier or stabilizer. As a result, the initially formed polymer nuclei collide and form larger and larger particles as the polymerization proceeds.8 Latex particles produced in the absence of emulsifier are, to some extent, stabilized by the orientation of their own polymer chains, notably the chain ends originating from initiator molecules. Monomer to water ratio in emulsifier-free emulsion polymerization is usually much smaller than that in normal emulsion polymerization, and the size of the resulting particles is in the region of 100-1000 nm.8 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Goodwin and coworkers drew three primary conclusions from their early work7 on emulsifier-free emulsion polymerization. First, they determined that the ionic strength of the aqueous phase has a pronounced influence on the diameter of the particles formed as the final product. Thus, variation of ionic strength provides a convenient means of controlling particle size. The ionic strength is determined by the rate of decomposition of the initiator. Second, monomer concentration, temperature and the initial initiator concentration also significantly influence the ultimate size of the particles obtained in a reaction. These three parameters were determined to be closely interconnected and combinations of these variables could be utilized to obtain a particular particle size. Finally, the authors stated the lattices were colloidally stable within the size range of 100- 1000 nm.7 One technique for synthesizing larger polymer nanospheres is through the use of seeded emulsifier-free emulsion polymerization. Smith first reported polymerization in the presence of seed particles in 1948.9 The method for seeded polymerization is based on the observation that swelling of polymer particles is considerably enhanced in the presence of a small percentage of a second low molecular weight polymer. Thus, monodisperse polystyrene latex particles are treated, first with a suitable oligomer, and then with styrene, divinylbenzene, and an oil-soluble initiator. The mixture is allowed to stand until the oligomer and monomer are completely absorbed by the particles. Subsequent polymerization leads to the formation of the correspondingly larger monodisperse particles. This method is particularly useful for the preparation of monodisperse polymer microspheres in the region of 5-20 |im which are less readily 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. obtained by basic heterogeneous polymerization techniques. Polymerization conditions must be carefully chosen to avoid the formation of new particles.8 Salovey and coworkers studied1 0 the synthesis of crosslinked polymer nanospheres by seeded polymerization in 1992. They found it possible to synthesize beads with diameters greater than 1 |im using a multistage emulsion polymerization, in the absence of emulsifier, by particle nucleation onto pre-existing polymer seeds. In addition to control of bead size, specific crosslinking and compositional gradients could be selected. Monodispersity of final bead size required an adequate seed concentration at the beginning of the polymerization. Otherwise, a polymodal size distribution resulted. In order to obtain monodisperse polystyrene beads in a second stage, using polystyrene seeds approximately 500 nm in diameter, a critical seed concentration of about 16.9 g/L was required. The critical number of seed particles could be reduced by increasing the seed particle size.1 0 1.2 Functionalization of Polymer Nanospheres There has been tremendous effort over the last few decades toward a general route for the surface functionalization of polymer nanospheres. Polymer nanospheres have a large surface area that is rather inert. For the nanospheres to be used for their proposed applications, the surface must be functionalized. Several methods have been developed for the surface functionalization of polymer nanospheres. Nanospheres of polystyrene are the most highly investigated system since these microspheres are rigid and relatively stable.1 1 These include the direct synthesis of monomers containing functional groups, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. copolymerization of polystyrene and other monomers containing functional groups, and surface functionalization on preformed polymer nanospheres. The direct synthesis of monomers containing functional groups is an effective method for the surface functionalization of polymer nanospheres. This method has functional groups incorporated throughout the nanosphere as well as on the surface. In 1986, Margel and coworkers synthesized polymeric nanospheres consisting of chloromethylstyrene, formylstyrene, and styrene sulfonylchloride.1 1 They were successful in synthesizing monodisperse, spherical beads using these monomers in a classical emulsion polymerization, with surfactants. The polymerization was carried out in various organic solvents, with the nanospheres proving to be very stable. They did find that the nanospheres tended to agglomerate in aqueous solutions.1 1 Margel and coworkers extended their work with poly-chloromethylstyrene nanospheres in 1991.1 2 In this work, Margel synthesized the nanospheres in ethanol with polyvinypyrrolidone as surfactant. The initiator used was azobisisobutyronitile (AIBN) and a cosolvent, dimethylsulfoxide (DMSO), was used. These nanospheres were found to be more useful than the previous ones synthesized by Margel because they did not agglomerate in aqueous solvents, making them more useful in a variety of applications. The polychloromethylstyrene nanospheres were then further elaborated to attach amino groups and other ligands.1 2 The major problem with using this method as a way of coating the surface of the polymer nanospheres with functional groups is the cost of using functionalized monomers. Functionalized monomers of styrene are very expensive, and any functional groups within the nanosphere are not accessible for later applications. It is therefore better to only have the functional groups on the surface of the nanosphere. 7 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. One alternative to this problem is the copolymerization of polystyrene and other styrene monomers containing functional groups. While some functional groups will still be located throughout the polymer nanosphere, there will not be as large a cost to synthesizing these nanospheres. One example of this type of polymerization was carried out by Okubo and coworkers in 1992.1 3 Okubo synthesized polymer nanospheres containing styrene, chloromethylstyrene and divinylbenzene in the presence of poly- (acrylic acid) as the surfactant. This method was successful in producing monodisperse, spherical nanospheres.1 3 The third technique used for the surface functionalization of polymer nanospheres is the use of unfunctionalized polymer nanospheres. Frechet and coworkers used this technique to coat the surface of polystyrene nanospheres with thiol and hydroxyl functionalities in 1979.1 4 The surface of the preformed polystyrene nanospheres was first lithiated, followed by an electrophilic substitution reaction. This resulted in a surface coverage of the desired functional group. This technique is discussed in more detail in chapter four of this dissertation. Prakash and coworkers recently developed a fourth technique for the surface functionalization of polystyrene nanospheres.1 5 This technique involves the in situ grafting of a substituted styrene monomer onto the surface of a polystyrene nanosphere. The substituted monomer is added to the polymerization reaction mixture thirty minutes prior to the end of the reaction time. The substituted monomer then reacts competitively with the remaining styrene monomer to coat the surface of the nanosphere with the desired functionalized polystyrene. This technique is discussed in chapter five of this dissertation. 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1.3 Applications of Polymer Nanospheres 1.3.1 Catalysis Research into readily recoverable and recyclable polymer-supported heterogeneous catalysts for oxidative processes has grown steadily in recent years, despite initial skepticism about the adequate stability of the supports under oxidative conditions.1 6 Polymer-supported analogues of the catalysts have utilized anion, cation, and chelating ion-exchange resins as supports. The majority of these have been based upon porous polystyrene resins, and have displayed favorable activity and selectivity, but in terms of recycling, have been disappointingly unstable, often showing excessive metal leaching.1 7 Sherringtion and coworkers reported the development of a polybenzimidazole (PBI) supported MoV I catalyst in 1994, along with the development of other polymer supports carrying imidazole residues as ligands. These catalysts were used in the tert-butyl hydroperoxide (TBHP) epoxidation of propene.1 8 Six different supports for the MoV I catalyst were synthesized. The first, and most active was polybenzimidazole. Three supports were based on polystyrene. These were polystyrene nanospheres functionalized with N-(2-hydroxypropyl) aminomethyl-2- pyridine, polystyrene nanospheres functionalized with 2-pyridyl-2-imidazole, and polystyrene nanospheres functionalized with 5-benzimidazole carboxylic acid. The final two supports were based on poly(glycidylmethacrylate) resins. These were resins functionalized with 2-pyridiyl-2-imidazole, and resins functionalized with 2-aminomethyl pyridine.1 8 9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. These functionalized polymers were then loaded with Mo complexes and used in the catalytic epoxidation of oct-1-ene and propene. After reaction, the catalysts showed no appreciable loss of Mo from the surface of the polymer supports. This catalyst was found to be suitable for use in the laboratory under most routine conditions of 1 f t temperature and pressure. Sherrington then extended his work in this area to include the Wacker Oxidation of Oct-1-ene using a palladium (II) complex supported on cyano-functionalized polyimide beads.1 9 In this work, the polyimide bead carrying cyano groups was reported to be a useful support material for anchoring of Pd(II) complexes, the nitrile ligand being employed to coordinate Pd(II).20 This polymer-supported catalyst was then used to catalyze Wacker-type oxidation reactions. This catalyst was reported to have greater thermo-oxidative stability, offering the ability to operate in more severe oxidative conditions. Sherrington and coworkers later reported the synthesis of functional spherical polyimides and their use as supports for MoV I catalysts in cyclohexene epoxidation.2 1 This catalyst, not unlike those previously reported by Sherrington, showed thermal stability under normal laboratory conditions, and did not leach metal during the reaction. A new, mild heterogeneous catalyst for the tetrahydropyranylation of alcohols and phenols was developed by Marston and coworkers in 1998.2 2 In this set of experiments, the hydrochloride salt of a cross-linked macroreticular poly-(4-vinylpyridine) resin in bead form was found to be an effective, yet mild acid catalyst for the tetrahydropyranylation of alcohols and phenols. The reaction was carried out in the presence of excess dihydropyran without the formation of troublesome oligomeric 10 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. pyrans. High yields of the desired ethers were obtained even in reactions with alcohols having steric restrictions or acid sensitive amine functionalities. Akashi and coworkers developed a method for depositing platinum nanoparticles on polystyrene microspheres in 1998,2 3 and reported further studies on this method in 1 9 9 92 4 xhe microspheres were coated via the reduction of H2PtCl6 by aqueous ethanol in the presence of polystyrene microspheres with surface-grafted poly(N- isopropylacrylamide). The catalyst, when separated from the reaction mixture by centrifugation, retained high activity on recycling in the aqueous hydrogenation of allyl alcohol. Also in 1998, Akashi and coworkers reported25 the in-situ formation of silver nanoparticles on poly (N-isopropylacrylamide)-coated polystyrene microspheres. They noted that the poly (N-isopropylacrylamide) (PNIPAAm) protected silver colloids demonstrated unusual temperature dependence because the PNIPAAm polymer itself is temperature sensitive. As in the above-mentioned example, the aqueous hydrogenation of allyl alcohol was used to demonstrate the catalytic activity of the silver-coated polystyrene nanospheres. Studies showed that there was little to no appreciable loss of metal coverage on the surface of the microspheres after seven recycles.2 5 1.3.2 Combinatorial Chemistry Combinatorial chemistry has developed within a few years from a laboratory curiosity to a method that is taken seriously in drug research. A characteristic of combinatorial synthesis is that a reaction is performed with many synthetic building blocks at once, in parallel or in a mixture, rather than with just one building block. All 11 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. possible combinations are formed in each step, so that a large number of products (called a library) are obtained from only a few reactants.26 All combinatorial library methods involve three main steps, preparation of the library, screening of the library components, and determination of the chemical structures of active compounds.2 7 Combinatorial libraries are often carried out on solid surfaces such as polymer nanospheres. This solid-phase combinatorial synthesis has several advantages. First, reagents can be used in excess without separation problems later, and thus reactions can be driven to completion. Second, simply washing the support results in purified product. Third, simple complete automation of reaction sequences is possible. It is also possible to carry out split synthesis via the one-bead-one-compound method. There are also several disadvantages to solid phase combinatorial synthesis. First, it is not well developed, therefore there is a large expenditure of time for development required. Second, additional reaction steps are needed for the linkage of the starting material and cleavage of the products from the support. Third, the support and linker limit possible chemistry, and fourth, methods for analytical monitoring of the reactions are not well- developed.26 One of the more common uses of solid phase support in combinatorial chemistry is the one-bead-one-compound concept. This was first recognized by Lam and is based on the fact that combinatorial bead libraries, prepared via a “split synthesis” approach, contain single beads displaying only one type of compound although there may be up to 101 3 copies of the same compound on a single 100 |_im diameter bead.28 The split synthesis method was first described by Furka in 1988. A mixture of 27 tetrapeptides 12 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. was prepared, and the peptides were cleaved from the resin and analyzed by paper electrophoresis and Edman degradation.27 The selected polymeric carrier often determines synthetic approaches to one- bead-one-compound libraries. A beaded polymer has to fulfill certain criteria depending on the synthetic and screening strategies. For one-bead-one-compound libraries, the size and substitution homogeneity is of the utmost importance, as well as the resin resistance to the formation of clusters. The ability of the resin to swell in both organic and aqueous media is especially important when on-bead binding methods are used for screening.27 Supports used almost exclusively in combinatorial synthesis are low molecular weight polystyrene crosslinked with 1-2% divinylbenzene. The products are generally cleaved from the resin under acidic conditions and have an acid or amide function at the former linkage site, which is not always desirable. Reactions that take place under strongly acidic conditions are therefore not feasible, and the synthesis products must be stable under the same acidic conditions during cleavage. This requirement for compatibility of the linker with the reaction conditions and for product stability precludes the possibility of a “universal linker” for all syntheses. Highly desirable linkers would not leave any functional groups behind in the target product of the combinatorial synthesis on cleavage. At the same time, the cleavage reagent should preferably be pumped off or easily separated since workup that is complicated or difficult to automate is generally undesirable.26 There are several types of libraries built on solid supports. Some of the most common are peptide libraries; libraries of nonpeptide oligomeric compounds, including peptoids, oligocarbamates, oligoureas, vinylogous sulfonyl peptides, 13 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. peptidosulfonamides, azatides and ketides; and small molecule libraries, including acyclic libraries, libraries on preformed scaffolds, heterocyclic libraries, and structurally heterogeneous libraries.27 There are two types of library screenings carried out on libraries built on solid supports. One is on-bead screening, which includes binding assay studies and functional assay studies. The other screening method is solution-phase screening. This includes the 96-well two-stage releasable assay studies, in situ solution- phase releasable assay studies and the combination of on-bead and solution-phase screening assay studies.27 1.3.3 Protein Support Merrifield first introduced solid-phase peptide synthesis (SPPS) in 1962.2 9 Merrifield’s idea was to employ an insoluble and filterable polymeric support such as cross-linked polystyrene which functions at the same time as the carboxy protecting group for the C-terminal amino acid. Thus, the N-protected C-terminal amino acid is attached to the chloromethylated polystyrene-divinylbenzene nanosphere. After removal of the N-protection, the next N-protected amino acid is coupled and the process is repeated until the entire desired peptide is assembled on the polymeric support. The advantage of this procedure is that a large excess of coupling components and additive reagents can be employed which will be easily separated from the polymer-bound peptide by simple filtration. Consequently, the laborious and cumbersome purification of intermediate peptides, a process typical of the classical solution synthesis, is avoided and •J A results in a tremendous saving of time. 14 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. After completion of the synthesis, the peptide is detached from the support. In the Merrifield strategy, strong acids, such as anhydrous HF, are used for the cleavage of peptides from the polymer. Under these conditions the side-chain protecting groups are simultaneously removed and free peptides are obtained. Owing to standardization of the steps involved, the solid-phase synthesis can easily be automated. Automatic peptide synthesizers have already been developed and are commercially available.3 0,3 1 Polymer nanospheres have also been used as supports for antibodies in immunological applications.32,33 These polymer-supported antibodies are referred to as immunomicrospheres and consist of hydrophilic or hydrophobic cross-linked particles with antibodies covalently bound to their surface. These particles find specific receptors on living or fixed cells and therefore can label cell sub-populations and the labeled cells can be observed by means of a scanning electron or light-microscope. Immunomicrospheres of suitable composition change the electrokinetic behavior of living or aldehyde fixed cells, thus allowing their separation by means of continuous flow electrophoretic instruments. These techniques are used for difficult separations of specific cells occurring only in small numbers, such as fetal cells in peripheral blood, or to isolate specific antigens for biochemical and immuochemical studies. Other application of polymeric microspheres as visual markers for light and scanning electron microscopy include electrophoretic cell separation, magnetic cell separation, and studies of phagocytisis.32 Hidalgo-Alvarez and coworkers have extensively studied the properties of polystyrene microspheres bound to immuno gamma globulin molecules (IgG).34'3 7 The adsorption of IgG onto polymer microspheres as an application for diagnostic test 15 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. systems was first reported by Singer and Plotz.38 One drawback to the use of IgG immunomicrospheres is the relative instability of the complex. Consequently, non specific agglutination of the sensitized latex is often caused by substances which do not operate as antigens, leading to erroneous diagnoses.34 An experimental investigation on the adsorption of F(ab’)2 from rabbit IgG onto polystyrene microspheres was carried out to determine the electrostatic interaction in the adsorption and to determine the stability of the complex. Hidalgo-Alvarez and coworkers found this fragment to be more stable than the Fab fragment, which had earlier been used. It was determined that the F(ab’)2- PS complex was stable under saturated conditions at pH 4 and that the stabilization mechanism under these experimental conditions is not electrostatic. These results suggest improved accuracy in diagnostic testing methods using this immunomicrosphere approach.3 5 Hidalgo-Alvarez next carried out a comparative study on the electrokinetic behavior of serum albumin molecules adsorbed onto different polymer microspheres.36 The degree of hydrophobicity of the polymer surfaces plays an important role in the adsorption of bovine serum albumin (BSA) molecules onto polystyrene microspheres. The aim of this work was to establish the influence of ion participation on the adsorption of monomeric BSA molecules onto polystyrene beads. Towards this goal, four samples of latex particles with different superficial hydrophobicities were used. Adsorption and electrophoretic mobility experiments were performed according to the pH and ionic strength to determine the role played by the electrostatic interactions in the adsorption. It was found that the ionic adsorption effect was much more pronounced with polymers of high hydrophobicity.36 1 6 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Further studies have been undertaken to determine the stability and usefulness of polymer nanospheres as protein supports in biological applications.3 9 '42 Most of these have determined polymer supports to be useful agents for drug delivery and diagnostic test systems.39 This is due to the relative inertness of the surface of the polymer beads. They are also useful due to the well defined and large surface area they possess.40 One of the most recent studies involves the encapsulation and release of rhodium (13) citrate and its association complex with hydroxypropyl-/J-cyclodextrin from biodegradable polymer microspheres 42 A new class of Rh(II) carboxylates has recently been synthesized and shows promise as useful compounds for chemotherapy.4 1 However, due to the high water solubility of Rh(II) citrate, high systemic doses of the drug would be required to achieve efficacious concentrations in tumor sites. Therefore, it was useful to develop controlled release systems wherein the Rh(II)-complex is shielded from the extracellullar milieu to minimize local toxicity and prolong drug action. Shastri and coworkers have examined42 the Host-Guest complexation approach using cyclodextrins to alter solubility and improve the encapsulation and release from polymer microspheres. They found42 that association with cyclodextrins could prolong the release of small molecules from the host polymer microspheres. This promises to be a significant result in the continued fight against cancer. 1.3.4 Magnets Polymer nanospheres have also been magnetized for use as fillers, coatings, pigments, capsule agents, and ultrasonic contrast agents.43 Spherical particles in the nanoscale range have become increasingly important both technologically and for 17 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. fundamental studies due to the tunable anisotropic interaction they exhibit.44 For example, magnetic core-shell properties combine the properties of the individual magnetic particles and non-magnetic sphere dispersions: in the absence of an applied magnetic field the particles have an isotropic sphere dispersion whereas in an external magnetic field the particles form anisotropic structures.45,46 Magnetic polystyrene nanospheres were formed by seeded polymerization of iron salts onto previously formed polystyrene nanospheres. In this example, the core polystyrene nanospheres were synthesized by emulsion polymerization using polyvinylpyrrolidone as surfactant. X-ray diffraction studies demonstrated that the coating on the polystyrene nanospheres was composed of crystallized Fe3C > 4. Unfortunately, the iron oxide coating was not always stable, due to leaching of traces of the coating or iron ion salts from the particle surfaces into the working aqueous solution at high or low pH. This problem was overcome by a further coating of the polystyrene nanospheres with silica nanoparticles.4 7 The researchers extended their work to form hollow magnetic silica nanoparticles. To achieve the results, the magnetic polystyrene nanospheres described above were subjected to high temperature conditions. This burned off the polystyrene core, resulting in hollow magnetic particles. These particles were hydrophilic and could be dispersed in aqueous solutions, with water molecules filling the voids. However, at temperatures greater than 1000 °C, they became hydrophobic due to siloxane bond formation between free hydroxyl groups and would float on the water.48 Caruso and coworkers prepared magnetite multilayers on polymer latex nanospheres in 1999.4 5 Magnetite nanoparticles were alternately deposited with either 18 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. poly(diallyldimethylammonium chloride) or poly(allylamine hydrochloride) on the polystyrene nanospheres. The influence of an external magnetic field to order the composite polystyrene core magnetic shell particles was also investigated. Caruso is currently extending this work to include magnetic spheres coated with silica, which would have the properties of the magnetic core and the surface properties of the silica coating. This would enable exploitation of the individual function of each component incorporated in the multilayer shell.45 1.3.5 Photonic Crystals Photonic crystals are a new class of optical materials which may hold the key to the continued progress towards all-optical integrated circuits.49 The concepts surrounding photonic crystals were developed by Yablonovitch50 and John.5 1 They suggested that structures with periodic variations in their dielectric constant could influence the nature of photonic modes in a material; Yablonovitch’s aim was to control the radiative properties of materials, while John’s was to effect photon localization by introducing a random reffactive-index variation. In a photonic crystal, the periodic potential is due to a lattice of macroscopic dielectric media instead of atoms. If the dielectric constant of the constituent media is different enough, Bragg scattering off the dielectric interfaces can produce many of the same phenomena for photons as the atomic potential does for electrons. Thus a photonic crystal could be designed to possess a complete photonic bandgap- a range of frequencies for which light is forbidden to exist within the interior of the crystal. If a defect is introduced into the crystal, it could lead to the ability of light to exist and travel within the crystal. This ability to manipulate a 19 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. photon provides us with a new dimension in our ability to mold or control the properties of light.49 For example, the inhibition of spontaneous emission or photon localization could be achieved.5 2 Polymer nanospheres have been used to create photonic crystals. One of earliest reports of the use of polystyrene nanospheres to grow porous silica was made by Lenhoff and coworkers.53 In their work, they filtered polystyrene nanospheres carrying either negative surface charges of positive charges in suspension through a smooth narrow-pore membrane. When the particulate layers had been deposited, they could be washed and dried. When the latex particles had accumulated on the membrane surface, they built up closely packed, ordered layers roughly 10 pm thick. Silica polymerization was then carried out between the microspheres. The latex templates inside the polymerized silica were then removed by heating at 450 °C for 4 hours, leaving an ordered silica lattice as the final product.5 3 In 1998, Xia and coworkers reported the synthesis of macroporous membranes with highly ordered and three-dimensionally interconnected spherical pores.54 This work was an outgrowth of earlier, work which had demonstrated a simple and practical procedure for forming crystalline assemblies of mesoscale spherical particles over relatively large areas.5 5 The assemblies could be subsequently used as sacrificial templates to generate macroporous membranes of organic polymers or inorganic ceramics containing spherical pores.56 In this paper, Xia reported the use of polystyrene nanospheres to form highly ordered three-dimensional crystalline assemblies. The void spaces were then filled with a UV-curable liquid prepolymer, such as polyurethanes and polyacrylates, by capillary action. Subsequent solidification of the prepolymer and 20 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. dissolution of the polystyrene nanospheres resulted in a membrane film consisting of a highly organized and three-dimensionally interconnected framework of spherical pores. The pores were uniform in size, with dimensions defined by the diameter of the particles. They were also completely exposed on both top and bottom surfaces of the membrane film. Due to their specified and highly ordered structures, these membranes could be useful as photonic bandgap structures in photonic crystals.54 Three-dimensional arrays of spherical voids were synthesized of titania, zirconia, and alumina by Stein and coworkers in 1998.5 7 As in the previous examples, a cubic array of polystyrene nanospheres was formed. Then, in a one-step process, the metal oxide was added to the polystyrene lattices while under vacuum pressure. After drying in a vacuum dessicator, the polystyrene nanospheres were removed by calcination. The final product was a hard and brittle powder with 320 to 360 nm voids. This method was perceived by the authors to readily lend itself to commercial workup and would be a precursor to photonic crystals. Studies were also carried out using opaline structures in the fabrication of photonic bandgap structures.58 One example was developed by Vos, who employed artificial opals to form photonic crystals made of air pockets in titania.52 Polystyrene nanospheres were used to form a three-dimensional crystal with an opaline structure. In the second step of the process, the voids in the opals were filled with the desired material by precipitation from a liquid-phase chemical reaction. To obtain T iC > 2, a solution of tetra-propoxy-titane in ethanol was used. In the third step, the polystyrene nanospheres were removed by calcination. 21 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Xia and coworkers developed a new method for the formation of artificial opaline structures using polystyrene nanospheres in 1999.5 8 g Their method worked for beads as small as 50 nm. To form the opaline structures, an aqueous dispersion of polystyrene nanospheres was injected into a cell, then a positive pressure was applied through the glass tube to force the water to flow through the channels formed by the polystyrene nanospheres. The particles accumulated at the bottom of the cell and assembled themselves into a cubic-close-packed (c.c.p.) lattice under continuous sonication. It is foreseen that this could be a more efficient method for forming the artificial opals which can be used to synthesize the arrays needed for photonic crystals. 1.4 References 1. D. C. Sherrington, Chem. Comm., 2275 (1998). 2. (a) D. C. Sherrington, in Polymer-Supported Reactions in Organic Synthesis, ed. P. Hodge and D. C. Sherrington, Wiley, Chichester, UK, ch. 3 (1980). (b) E. A. Gulke, in Encyclopedia of polymer Science and Engineering, ed. H. F. Mark, N. M. Bikales, C. G. Overberger, G. Menges, and J. I. Kroschivitz, 2n d ed., Wiley, New York, 16, 443. 3. D. J. Williams, and E. G. Bobalek, J. Polym. Sci., A-l, 4, 3065 (1966). 4. H. Bamnolker and S. Margel, J. Polym. Sci., A, 34, 1857 (1996). 5. (a) J. W. Vanderhoff, H. J. van den Hul, R. H. M. Tausk, and J. Th. G. Overbeck, Clean Surfaces, Their Preparation and Characterization for Interfacial Studies, ed. G. Goldfinger, New York, (1970). (b) R. H. Ottewill and T. Walker, Kolloid Z. u Z. Polymere, 227, 108 (1968). 6. (a) R. Ho. Ottewill and J. N. Shaw, Kolloid Z. u Z. Polymere, 1967,218, 34. (b) J. N. Shaw, J. Polym. Sci., C, 27, 237 (1969). 7. J. W. Goodwin, J. Hearn, C. C. Ho, and R. H. Ottewill, Colloid and Polym Sci., 252, 464 (1974). 22 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8. R. Arshady, Colloid and Polym Sci., 270, 717 (1992). 9. M. W. Smith, J. Am. Chem. Soc., 70,186 (1948). 10. D. Zou, L. Sun, J. J. Aklonis, and R. Salovey, J. Polym. Sci., A, 3 0 ,1463 (1992). 11. M. Shahar, H. Meshulam, and S. Margel, J. Polym. Sci., A, 24, 203 (1986). 12. S. Margel, E. Nov, and I. Fisher, J. Polym. Sci., A, 29, 347 1991. 13. M. Okubo, Y. Iwasaki, and Y. Yamamoto, Colloid and Polym Sci., 270, 733 (1992). 14. J. M. J. Frechet, M. D. de Smet, and M. J. Farrall, Polymer, 20, 675 (1979). 15. Chapter 5 of this dissertation 16. D. C. Sherrington, Pure andAppl. Chem., 60,401 (1988). 17. (a) J. Sobezak, and J. J. Ziolkowski, J. Mol. Cat., 3 ,165 (1978). (b) S. Ivanov, R. Boeva, and S. Tanielyan, J. Cat., 56, 150 (1979). (c) R. Boeva, S. Kotov, and N. I. Jordanov, React. Kinet. Catal. Lett., 24, 239 (1984). (d) S. Bhaduri, and H. Khwaja, J. Chem. Soc. Dalton Trans., 415 (1983). (e) T. Yokoyama, M. Nishizawa, T. Kimura, and T. M. Suzuki, Bull. Chem. Soc. Japan,, 58, 3271 (1985). (f) D. C. Sherrington, and S. Simpson, J. Cat., 131, 115 (1991). (g) D. C. Sherrington, and S. Simpson, React. Polym., 142, 540 (1993). 18. M. M. Miller, D. C. Sherrington, and S. Simpson, J. Chem. Soc., Perkin Trans., 2091 (1994). 19. J. H. Ahn, and D. C. Sherrington, Macromolecules, 29,4164 (1996). 20. (a) M. Kraus, and D. Tamanov, J. Polym. Sci., A, 12, 1781 (1974). (b) W. Keim, P. Masotrilli, C. F. Nobile, N. Ravasio, B. Corain, and M, Zecca, J. Mol. Cat., 8 1 ,167 (1993). (c) H. G. Tang, and D. C. Sherrington, Polymer, 34, 2821 (1993). 21. J. Ahn, and D. C. Sherrington, Chem. Comm., 643 (1996). 22. R. D. Johnston, C. R. Marston, P. E. Krieger, and G. L. Goe, Synthesis, 394 (1988). 23. C. W. Chen, M. Q. Chen, T. Serizawa, and M. Akashi, Chem. Comm., 831 (1998). 24. C. W. Chen, T. Serizawa, and M, Akashi, Chem. Mat., 11, 1381 (1999). 25. C. W. Chen M. Q. Chen, T. Serizawa, and M. Akashi, Adv. Mat., 10, 1122 (1998). 23 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 26. F. Balkenhol, C. von dem Bussch-Hiinnefeld, A. Lansky, and C. Zechel, Angew. Chem., Int. Ed. Eng., 35, 2288 (1996). 27. K. S. Lam, M. Lebl, and V. Krchnak, Chem. Rev., 9 7,411 (1997). 28. K. S. Lam, S. E. Salmon, E. M. Hersh, V. J. Hruby, W. M. Kazmierski, and R. J. Knapp, Nature, 354, 82 (1991). 29. (a) R. B. Merrifield, Fed. Proc. Fed. Am. Soc. Exp. Biol., 21, 412 (1962). (b) R. B. Merrifield, J. Am. Chem. Soc., 85, 2149 (1963). (c) R. B. Merrifield, Biochemistry, 3, 1385(1964). 30. E. Bayer, Angew. Chem. Int. Ed. Eng., 30, 113 (1991). 31. (a) J. M. Stewart, and J. D. Young, Solid Phase Peptides Synthesis. Freeman, San Francisco, (1969). (b) R. B. Merrifield, Adv. Enzymol., 32,221 (1969). (c) A. Dryland, and R. C. Sheppard, J. Chem. Soc., Perkin Trans., 1, 125 (1968). 32. A. Rembaum, Pure andAppl. Chem., 52, 1275 (1980). 33. V. L. Covolan, L. H. Innocentini Mei, and C. L. Rossi, Polym. For Adv. Tech., 8 , 44 (1996). 34. J. Sera, J. Puig, A. Martin, F. Galisteo, MaJ. Galvez, and F. Hidalgo-Alvarez, Colloid and Polym. Sci., 270, 574 (1992). 35. J. L. Ortega-Vinuesa, and R. Hidalgo-Alvarez, Colloids and Surfaces, B, 1, 365 (1993). 36. A. Martin Rodriguez, M. A. Cabrerizo-Vflchez, and R. Hidalgo-Alvarez, Colloids and Surfaces, A, 92, 113 (1994). 37. J. M. Peula, J. Puig, J. Serra, F. J. de las Nieves, and R. Hidalgo-Alvarez, Colloids and Surfaces, A, 92, 111 (1994). 38. J. M. Singer, and C. M. Plotz, Am. J. of Med., 21, 888 (1956). 39. J. M. Puela, and F. J. de las Nieves, Colloids and Surfaces, A, 77,199 (1993). 40. F. Betton, A. Theretz, A. Elaissari, and C. Pichot, Colloids and Surfaces, B, 1,97 (1993). 41. J. M. Puela, and F. J. de las Nieves, Colloids and Surfaces, A, 90, 55 (1994). 42. R. D. Sinisterra, V. P. Shastri, R. Najjar, and R. Langer, J. ofPharm. Sci., 88, 574 (1999). 24 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 43. (a) M. Abe, T. Itoh, and Y. Tamaura, Mat. Res. Soc. Syp. Proc., 232,107 (1991). (b) M. Zhang, Q. Zhang, I. Itoh, and M. Abe, IEEE Trans. Magn., 30, 4692 (1994). (c) N Kawahashi, and E. Matijevic, J. o f Colloid Int. Sci., 143, 103 (1991). 44. J. H. E. Prumislow, and A. P. Gast, Phys. Rev. Lett., 72, 2294 (1994). 45. F. Caruso, A. S. Susha, M. Giersig, and H. Mohwald, Adv. Mat., 11, 950 (1999). 46. A. P. Philipse, M. P. B. van Bruggen, and C. Pathmamanoharan, Langmuir, 10, 92 (1994). 47. H. Bamnolker, B. Nitzan, S. Gura, and S. Margel, J. Mat. Sci. Lett., 16, 1412 (1997). 48. S. Brandriss, and S. Margel, Langmuir, 9 ,1232 (1994). 49. J. D. Juannopoulos, P. R. Villeneuve, and S. Fan, Nature, 386, 143 (1997). 50. E. Yablonovitch, Phys. Rev. Lett., 58, 2059 (1987). 51. S. John, Phys. Rev. Lett., 58, 2486 (1987). 52. J. E. G. J. Wijnhoven, and W. L. Vos, Science, 281, 802 (1998). 53. O. D. Velev, T. A. Jede, R. F. Lobo, and A. M. Lenhoff, Nature, 389,447 (1997). 54. S. H. Park, and Y. Xia, Adv. Mat., 10, 1045 (1998). 55. S. H. Park, D. Qin, and Y. Xia, Adv. Mater., 10, 1028 (1998). 56. (a) D. J. LeMay, R. W. Hopper, L. W. Grubes, and R. W. Pekara, MRS Bull., 15, 19 (1990). (b) W. R. Even, Jr., and D. P. Gregory, MRS Bull., 19, 29 (1994). (c) D. Walsh, and S. Mann, Nature, 377, 320 (1995). (d) A. Imhof, and D. J. Pine, Nature, 389, 948 (1997). (e) S. A. Davis, S. L. Burkett, N. H. Mendelson, and S. Mann, Nature, 385, 420 (1997). (f) G. A. Ozin, Acc. Chem. Res., 30, 17 (1997). (g) O. D. Velev, T. A. Jede, R. F. Lobo, and A. M. Lenhoff, Nature, 389,447 (1997). 57. B. T. Holland, C. F. Blanford, and A. Stein, Science, 281, 538 (1998). 58. (a) 1.1. Tarhan, G. H. Watson, Phs. Ref. Lett., 76, 315 (1996). (b) W. L. Vos, M. Megens, C. M. van Kats, and P. Bosecke, J. Phys: Condis. Matter, 89, 503 (1996). (c) W. L. Vos, R. Sprik, A. van Blaaderen, A. Imhof, A. Lagendijk, and G. H. Wegdam, Phys. Rev. B., 53, 16231 (1996). (d) H. Miguez, C. Lopez, F. Meseguer, A. Blanco, L. Vasquez, R. Mayoral, M. Ocana, V. Fomes, and A. Mifsud, Appl. Phys. Lett., 71, 1148 (1997). (e) R. Mayoral, J. Requena, J. S. Moya, C. Lopez, A. Cintas, H. Miquez, F. Meseguer, L. Vasquez, M. Holgado, and A. Blanco, Adv. Mater., 9, 257 (1997). (f) J. S. Moya, J. Requena, A. Mifsud, V. Fomes, H. Miguez, F. Meseguer, C. Lopez, A. 25 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Blanco, J. S. Moya, J. Requena, A. Mifsud, and V. Fomes, Adv. Mater., 10, 430 (1998). (g) B. Gates, D. Qin, and Y. Xia, Adv. Mater., 11,466 (1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 2: Synthesis of Polystyrene Nanospheres 2.1 Introduction According to Sherrington, 1 polystyrene nanospheres were first synthesized in the 1950s by emulsion polymerization. Using this technique, researchers were able to achieve good size and shape control of the nanospheres. One problem with this method is the presence of stabilizers, or surfactants, in the reaction mixture. These surfactants tend to adsorb onto the surface of the nanospheres during polymerization, limiting the size which can be achieved by this method.2 Another problem with this method is the presence of surfactant after the reaction. Removal of the surface agents can lead to a loss of stability.3 Also, it is difficult to remove the surfactant after reaction and one can never be sure that all of it is removed.4 Goodwin and coworkers developed the method of emulsifier-free emulsion polymerization of styrene in 1974 to alleviate these problems.5 Emulsifier-free emulsion polymerization is very similar to traditional emulsion polymerization. The major difference is the absence of surfactant in the emulsifier-free technique. As a result of the lack of emulsifier, the initially formed nuclei collide and form larger and larger particles as the polymerization proceeds. The polymer nanospheres are stabilized by the orientation of their own polymer chains. Monomer to water ratio is usually much smaller in emulsifier-free emulsion polymerization, and the nanosphere diameter is typically in the range of 100-1000 nm.6 27 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Three primary conclusions were drawn from Goodwin’s early work5 on emulsifier-free emulsion polymerization. First, the ionic strength of the aqueous phase has a pronounced influence on the diameter of the particles formed as the final product. Thus, variation of ionic strength provides a convenient means of controlling particle size. The ionic strength is determined by the rate of decomposition of the initiator. Second, monomer concentration, temperature, and the initial initiator concentration also significantly influence the ultimate size of the particles obtained in a reaction. These three parameters were determined to be closely interconnected and combinations of these variables could be utilized to obtain a particular particle size. Finally, the authors stated that the latices were colloidally stable within the size range of 100-1000 nm.5 Salovey developed a method for the emulsifier-free emulsion polymerization of styrene as a batch reaction.7 We repeated this method in an attempt to optimize the conditions and found it to be the best method to control the size and spherical integrity of the polystyrene nanospheres synthesized (vida infra). This was done by varying each of the reaction conditions and characterizing the resulting polystyrene nanospheres by Scanning Electron Microscopy. 2.2 Results and Discussion Polystyrene nanospheres were successfully synthesized using the emulsifier-free emulsion polymerization procedure originally developed by Salovey in the Chemical Engineering Department at the University of Southern California.7 The nanospheres were synthesized with a uniform diameter of ~500 nm. After this procedure was 28 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. optimized, the experimental conditions were varied to gain a better understanding of the properties of the polystyrene nanospheres. One variable in the synthesis of the polystyrene nanospheres is the concentration of the crosslinking agent. Divinylbenzene (1) was used as the crosslinking agent to prevent swelling of the nanospheres. 1 The amount of crosslinking was varied between .5 mole% (figure 2.1) and 5 mole% (figure 2.2). The spherical integrity and monodisperse character remained the same at low crosslinking concentrations. However, when higher crosslinking concentrations were used the monodisperse character of the nanospheres was lost. The ideal crosslinking concentration for our experiments was 4 mole%. This concentration of crosslinking agent reduced the swelling of the nanospheres while maintaining their monodisperse character (figure 2.3). Another variable in the reaction was temperature. When the temperature of the reaction was varied, the spherical integrity of the nanospheres changed. Temperatures in the range of 75-85 °C had no effect on the spherical integrity, while temperatures higher or lower showed great disparities in nanosphere shape (figure 2.4) The ideal temperature for the reaction was 80 °C. This temperature, when maintained constant throughout the reaction, resulted in uniform, spherical nanospheres. 29 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.1: Polystyrene nanospheres with .5 mole% divinylbenzene Figure 2.2: Polystyrene nanospheres with 5 mole% divinylbenzene 30 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.3: Polystyrene nanospheres with 4 mole% divinylbenzene Figure 2.4 (a): Polystyrene synthesized at 70 °C Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.4 (b): Polystyrene synthesized at 90 °C A third variable in the polymerization was the time of initiator addition. The monomer is added after the water has been heated, stirred, and degassed for one hour. The monomer must reach reaction temperature before the initiator is added. If the initiator is added too soon, the nanospheres lose their monodispersity (figure 2.5). The optimum time for the addition of the initiator is 20 minutes after the addition of the monomer to the reaction kettle. The initiator can be added later than 20 minutes, with no ill effects on the monodispersity of the nanospheres. 32 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.5: Polystyrene nanospheres with immediate addition of initiator A fourth variable which was tested was the concentration of the initiator. The initiator used in this polymerization is potassium persulfate. If too little initiator was added, the reaction was slow and did not go to completion, resulting in spherical nanospheres with unreacted monomer remaining at the end of the reaction. If too much initiator was added the reaction proceeded too quickly, with polydisperse beads with non- spherical shapes (figure 2.6). The ideal initiator concentration is .3 mole% of the monomer concentration. This concentration results in monodisperse, uniform nanospheres as shown in figure 2.3. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. L* 5E1 EHTa 1 0 . 0 KV U[\ 2 .0 0 p m |---------------------- p o l y s t y r e n e 8 /2 4 /3 '9 _ I1hi> K lU .O K . F'HOT0= 5 5 Figure 2.6: Polystyrene nanospheres synthesized with too much initiator A fifth variable in the reaction is the monomer concentration. Larger concentrations of monomer, with a constant initiator concentration (.002 moles), resulted in larger nanospheres. However, these larger nanospheres were polydisperse in size, regardless of reaction time. If smaller concentrations of monomer were used, the beads were small and polydisperse, with loss of spherical integrity (figure 2.7). The concentration of monomer which gave the most control over spherical integrity and monodisperse character was .673 moles per .7 L water. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. L= SEl EHT- 1 0 . 0 \:\j c . 0 0 | j m i---------------- iolMsiMrene ;•;/15/00 riqhl nRi>. X 1 0 . 0 K PHOTO- 770 Figure 2.7: Polystyrene nanospheres with small concentrations of monomer (top) and large concentrations of monomer (bottom) The sixth variable tested was reaction medium. To do this, we used a small amount of ethanol in the reaction mixture. The temperature of these reactions was 75 °C 35 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. to prevent the loss of ethanol during the reaction. Ethanol concentrations ranged from 5 mL to 100% ethanol (figures 2.8-2.15). These reactions gave unreliable results, with varied nanosphere diameter in repeated reactions. These reactions also resulted in polydisperse nanospheres over a wide range of diameters. While larger nanospheres were seen in some of the reactions, we were not able to reproduce these reliably and therefore could not depend on ethanol as a solvent for size control of the nanospheres. Figure 2.8: Polystyrene nanospheres synthesized in 695 mL water and 5 mL ethanol Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.9: Polystyrene nanospheres synthesized in 690 mL water and 10 mL ethanol Figure 2.10: Polystyrene nanospheres synthesized in 685 mL water and 15 mL ethanol Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.11: Polystyrene nanospheres synthesized in 680 mL water and 20 mL ethanol Figure 2.12: Polystyrene nanospheres synthesized in 675 mL water and 25 mL ethanol Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.13: Polystyrene nanospheres synthesized in 650 mL water and 50 mL ethanol Figure 2.14: Polystyrene nanospheres synthesized in 525 mL water and 175 mL ethanol 39 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I1HG- K 1 . 0 4 1 FH0T0- 7 0 4 Figure 2.15: Polystyrene nanospheres synthesized in 700 mL ethanol The next variable tested in the emulsifier-free emulsion polymerization of styrene was the stirring rate. The stirring rate did not significantly affect the nanosphere diameter, except when the rate was very slow, resulting in polymerization of the monomer without nanosphere formation (figure 2.16). Fast stir rates did result in larger nanospheres, although the spherical integrity was affected (figure 2.17). The stir rate which produced the most control over size and spherical integrity was found to be 300 rpm, resulting in beads similar to those shown in figure 2.3. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.16: Polystyrene nanospheres synthesized at 250 rpm Figure 2.17: Polystyrene nanospheres synthesized at 400 rpm The variable which gave the most control over the diameter of the polystyrene nanospheres was reaction time. To test time as the variable, the nanosphere Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. polymerization was carried out with a sample taken every hour and the diameter was measured using Scanning Electron Microscopy. This showed a steady growth rate as seen in Table 2.1. As the table shows, a reaction time of thirty minutes results in beads of 295 nm diameter. A reaction time of five hours results in nanospheres with 550 nm diameter, and a reaction time of twelve hours results in nanospheres with a 700 nm diameter. Time Profile of Polystyrene Growth 800 700 600 E 500 c ® 400 o E « 300 • o 200 100 3 0 0.5 1 2 4 5 6 7 8 12 9 time Table 2.1: Time profile of polystyrene nanosphere synthesis 2.3 Experimental Styrene and divinylbenzene were obtained from Aldrich and vacuum distilled to remove the inhibitor. Potassium persulfate was also obtained from Aldrich and was used as received. All reactions were carried out in a 1-liter reaction kettle (figure 2.18). The kettle was fitted with a condenser, a nitrogen inlet valve, a thermometer, and a mechanical stirring apparatus. 42 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Condenser n Stirrer-300rpm T h e rm o m e te r 8 0 C ■700 m L w a te r Figure 2.18: Diagram of reaction kettle used for polystyrene synthesis The mechanical stirrer used was the IKA Eurostar power control-visc. Characterization was performed using SEM and surface-reflectance infrared spectroscopy. All SEM samples were prepared by placing a drop of sample diluted with 2 mL of water on top of a glass plate. The sample was allowed to dry at room temperature, then sputter-coated with gold. All SEM analyses were performed using a Cambridge 360 Scanning Electron Microscope at 10 kV and 10 K magnification. All Surface-Reflectance Infrared Spectroscopy was carried out on the Bio-Rad FT instrument. IR samples were prepared by drying the samples overnight under vacuum. 2.3.1 General Procedure for Synthesis of Polystyrene Nanospheres: A one-liter reaction kettle, equipped with a condenser, gas inlet, a thermometer, a mechanical stirring apparatus, and containing 700 mL of distilled water was heated to 80 °C, stirred at 300 rpm, and degassed with N2 for 1 hour. After one hour, the gas flow was turned off and 77 mL of styrene (.673 moles) and 4 mL of divinylbenzene (-4 mole percent) were added to the water. The reaction was stirred for twenty minutes to bring the monomer and crosslinking agent to temperature, followed by the addition of .6 g potassium persulfate 43 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (.003 moles) dissolved in 20 mL of water as the initiator. The reaction was stirred at 300 rpm and 80° C for the reaction time needed to give the desired diameter. The nanospheres were removed from solution by vacuum filtering through a coarse filter frit and washing three times with acetone. The resulting polymer was then characterized via scanning electron microscopy to determine the size and integrity of the nanospheres and Surface Reflectance infrared spectroscopy to characterize the polymer as polystyrene. 2.4 Conclusions Polystyrene nanospheres were synthesized using the emulsifier-free emulsion polymerization technique originally developed by Salovey.7 This polymerization process was optimized using a number of variables to gain better size and shape control of the nanospheres. The variables tested were crosslinking concentration, temperature of polymerization, time of initiator addition, concentration of initiator, monomer concentration, reaction media, stir rate, and time of reaction. As discussed above, varying the time of reaction gave the greatest control over the size, monodisperse character, and spherical integrity of the beads. A time profile is also given above to demonstrate the control of nanosphere diameter achievable through time control. 2.5 References 1 D. C. Sherrington, Chem. Comm., 2275 (1998). 2 G. Bamnolker and S. Margel, J. Polym. Sci., A, 34,1857 (1996). 3 (a) J. W. Vanderhoff, H. J. van den Hul, R. J. M. Tausk, and J. Th. G. Overbeck, Clean Surfaces. Their Preparation and Characterization for Interfacial Studies, ed. G. 44 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Goldfmger, New York, (1970). (b) R. H. Ottewill and T. Walker, Kolloid Z. u Z. Polymere, 227, 108 (1968). 4 (a) R. H. Ottelwill and J. N. Shaw, Kolloid Z. u Z. Polymere, 1967, 218, 34. (b) J. N. Shaw, J. Polym. Sci., C, 27, 237 (1969). 5 J. W. Goodwin, J. Hearn, C.C. Ho, and R. H. Ottewill, Colloid and Polym. Sci., 252, 464 (1974). 6 R. Arshady, Colloid and Polym. Sci., 270, 717 (1992). 7 D. Zou, L. Sun, J. J. Aklonis, and R. Salovey, J. Polym. Sci., A, 30, 1463 (1992). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3: Synthesis of Poly-(vinylpyridine) Nanospheres 3.1 Introduction Nanosized metal and semiconductor particles possess unique electronic, optical, and catalytic properties that are remarkably different from bulk microcrystallites.1 These nanoparticles are presently under intense study for potential uses in optoelectronic devices,2 in ultrasensitive chemical and biological sensors,3 and as catalysts in chemical and photochemical reactions,4 etc. They have a characteristic high surface to volume ratio and consequently large fractions of the metal atoms that are exposed at surfaces are accesible to reactant molecules and available for catalysis. They are often coated with an organic shell to prevent agglomeration due to van der waals forces.5 Nord and co-workers as far back as the 1940’s described noble metal colloids stabilized by synthetic polymers and their catlaytic properties.6 Hirai and coworkers first reported the synthesis of catalytically active metal colloids by aqueous alcohol reduction of metal salts in the presence of protective polymer.7 The colloidal particles of 1-3 nm mean diameter with narrow size distributions showed high activity and selectivity for the hydrogenation of olefins and dienes, hydration of acrylonitrile, and the light induced hydrogen generation from water.8 Polymer-stabilized noble metal colloids on supports have great potential for environmental and industrial processes, but many established methods suffer from complex steps for the immobilization and decreased catalytic activity.9 These synthetic routes are based on a two step process involving synthesis of 46 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. metal colloids and immobilization via covalent interaction or ligand coordination between the protective polymers and supports. Recently, palladium dispersions containing small metal particles (20-80 °A) have been of increasing scientific interest as colloidal catalysts for organic C-C bond formation reactions.10,11 Palladium catalyzed carbon-carbon bond reactions are gaining prominence in organic synthesis for their remarkable chemo-, regio-, and stereo selectivities, mild reaction conditions, and high efficiency. Several challenges, however, remain for chemical practitioners to grapple with when performing a palladium catalyzed process at scale. First, the relatively high price of palladium complexes, albeit in catalytic amounts, contributes significantly to the overall cost of production. Second, removal of ligands and byproducts derived from using palladium as homogeneous catalyst often complicates workup and product isolation. To address these concerns, several solid supported palladium catalysts have been developed over last two decades with varying degrees of success. Most of these resin-bound ligand systems, however, are based upon phosphine ligands attached to 1-2 % crosslinked polystyrene, a system which requires several demanding synthetic transformations and expensive reagents to prepare. In most cases additional free ligands such as PPh3 have to be added to stabilize the polymer-phosphine- Pd complexes. Addition of soluble ligands, however increases the chance for metal to leach out of the system through disproportionation between the polymer-bound and free phosphines. An easily prepared, air and moisture stable, and recyclable solid system is highly desirable in all important Pd-catalyzed C-C bond formation reactions. Towards this goal, poly-(vinylpyridine) nanospheres were synthesized. Palladium metal colloids were then adsorbed onto the nanosphere surface. These metal-coated nanospheres were then tested for their catalytic ability. 47 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Poly-(vinylpyridine) has also been used in applications other than catalysis. One such use is as an acid scavenger to remove generated acids from various reactions,1 2 including esterifications using acyl halides,1 3 silylation of a primary alcohol with diphenyl-r-butylchlorosilane,1 4 and mixed acid anhydride preparation from acyl halids and acids.1 5 The polymer is used in these reactions to replace liquid pyridine in both aqueous and non-aqueous media. The polymer is easily recovered at the end of the reaction due to its solid character. Olah and coworkers reported the use of polyvinylpyridinerhydrogen fluoride (PVPHF) as an effective fluorinating agent in 1993.1 6 Liquid pyridine hydrogen fluoride systems had previously been used as fluorinating agents, including the preparation of many selectively fluorinated compounds.1 7 Liquid reagents provide some difficulty in the workup of a reaction, and a solid equivalent to this system was needed. Zupan first reported the preparation and use of a polymer-supported hydrogen fluoride reagent, but had little success.1 8 Olah and coworkers developed1 6 the system of poly-4-vinylpyridium poly(hydrogen fluoride). They found this reagent to show considerable improvement over the liquid form, both in its effectiveness as a fluorinating agent and in its ease of workup. This reagent was found to be useful in the hydrofluorination of alkenes and alkynes, the fluorination of secondary and tertiary alcohols, and the bromofluorination of alkenes.1 6 One problem with this reagent is the polymer shows considerable swelling after reaction. If a higher crosslinking concentration were used, this swelling problem would decrease, resulting in even greater ease of workup. 3.2 Results and Discussion 48 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. For the first time, the emulsifier-free emulsion polymerization technique has been used to successfully synthesize poly-vinylpyridine nanospheres using 4-vinylpyridine (1) and 2-vinylpyridine (2) as monomers. The nanospheres were crosslinked using ~4 mole% divinylbenzene (3). X X X N 1 2 3 This percentage of crosslinking was chosen because it decreases the solubility and swelling of the polymer beads while maintaining the monodisperse character of the nanospheres. Three types of nanospheres have been synthesized using vinylpyridines. These include poly-(4-vinylpyridine) (figure 3.1), poly-(2-vinylpyridine) (figure 3.2), and the copolymer poly-(2-co-4-vinylpyridine) (figure 3.3). All nanospheres were -500 ± 20 nm in diameter and showed uniformity in shape and monodispersity in size. 49 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. HAG- X 1 0 . 0 K PHOTO' Figure 3.1: Poly-(4-vinylpyridine) nanospheres Figure 3.2: Poly-(2-vinylpyridine) nanospheres 50 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.3: Poly-(2-co-4-vinylpyridine) nanospheres Varying the reaction conditions does not appear to affect the size of the beads, although temperatures too high or too low do affect the spherical integrity of the nanospheres (figure 3.4 and 3.5). Reaction times, temperatures, and stirring rates were all varied with the beads still maintaining monodisperse character of -500 nm. Auger spectroscopy showed a nitrogen peak at 379 eV, supporting the presence of nitrogen on the surface of the nanospheres (figure 3.6). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.4: Poly-(4-vinylpyridine) nanospheres synthesized at 90 °C Figure 3.5: Poly-(4-vinylpyridine) nanospheres synthesized at 70 °C. After reaction, the nanospheres quickly settled out of the water rather than staying suspended as in the case of polystyrene. Water is easily decanted off the top, then the 52 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. remaining water is removed by filtration of the nanospheres. Poly-(2-vinylpyridine) settles faster than the copolymer, which settles faster than poly-(4-vinylpyridine). This disparity is a possible result of better hydrogen bonding to the water in the poly-(4- vinylpyridine) nanospheres due to the easy accessibility of the nitrogen in the polymer backbone. The nitrogen in the poly-(4-vinylpyridine) is in the para position, and therefore more exposed than the nitrogen in the poly-(2-vinylpyridine) nanospheres. The copolymer would have hydrogen-bonding capability somewhere between that of the poly-(4-vinylpyridine) and the poly-(2-vinylpyridine) nanospheres. This would explain the settling pattern of the poly-(vinylpyridine) nanospheres. oiE coim m a™ l i H K sagas m m m m Figure 3.6: Auger Spectrum of poly-(2-vmylpyndine) The poly-(vinylpyridine) nanospheres were coated with palladium metal colloids in a separate set of experiments (figure 3.7). These coated nanospheres were used by S. Pahtak1 9 as catalysts in various organic reactions. Some of those include the Heck 53 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. reaction (equation 3.1), Suzuki coupling (equation 3.2), and Stille coupling (equation 3.3). In these reactions, the palladium-coated nanospheres were recovered after the reaction with no appreciable loss of palladium (figure 3.8).1 9 Figure 3.7: Palladium-coated poly-(vinylpyridine) nanospheres before reaction Br—i y —no2 + u V ^ 0 -(n ( Bu) T=120 C, DMA PVP-Pd (0.1mole%) 12 hours o 2n 0-(nBu) Equation 3.1: Heck reaction using palladium-coated poly-(vinylpyridine) nanospheres as catalyst 54 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Br_X ) ^ N0J L J + Na2C° 3 (a,) PVP-PdC('.4 m ^ % )°: 6 hours B(OH2) Equation 3.2: Suzuki Coupling using palladium-coated poly-(vinylpyridine) nanospheres as catalyst T=130 C, DMA / \ . PVP-Pd (0.4mole%l Br- \ / ^ N 0 2+ 4 h o u r s o \ r ~ \ J / SnMe3 Equation 3.3: Stille Coupling using palladium-coated poly-(vinylpyridine) nanospheres as catalyst Figure 3.8: Palladium-coated poly-(vinylpyridine) nanospheres after reaction 55 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.3 Experimental All polymer syntheses were carried out in a 1-liter reaction kettle. The kettle was fitted with a condenser, a nitrogen inlet valve a thermometer, and a mechanical stirring apparatus. The mechanical stirrer used was the IKA Eurostar power control-visc. Analysis was carried out on the Cambridge 360 Scanning Electron Microscope at 10 kV and 10 K magnification, and the Perkin Elmer Phillips 460 Auger Scanning Microscope. SEM samples were prepared by placing a drop of sample diluted with 2 mL of water onto a glass plate and allowing it to air-dry. The sample was then sputter-coated with gold. Auger samples were prepared by placing a drop of sample diluted with 6 mL of water onto a silicon wafer and allowing it to air-dry. The vinylpyridines and divinylbenzene used were purchased from Aldrich. The vinylpyridines were vacuum distilled from calcium hydride to remove the inhibitor and traces of moisture. The divinylbenzene was vacuum distilled to remove the inhibitor. Potassium persulfate was purchased from Aldrich and used without further purification. Synthesis of Poly-vinylpyridine nanospheres A one liter reaction kettle, equipped with a condenser, gas inlet a thermometer, mechanical stirring apparatus, and containing 700 mL of water was heated to 80° C, stirred at 300 rpm, degassed with N, for 1 hour. After one hour , the gas flow was turned off and 73 mL of vinylpyridine (0.673 moles) and 4 mL of divinylbenzene (~4 mole%) were added to the water. The reaction mixture was stirred for twenty minutes to bring the monomer and crosslinking agent to temperature, followed by the addition of 0.6 g (3 mmoles) potassium persulfate dissolved in 20 mL of water as the initiator. The reaction mixture was stirred at 300 rpm and 80 °C for 4 hrs 56 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and then stopped. The resulting polymer was then observed via scanning electron microscopy to determine the size and integrity of the nanospheres, and Auger Spectroscopy to analyze the surface of the nanospheres. SEM showed uniform nanospheres of -500 nm diameter. Auger spectroscopy showed a strong nitrogen peak at 379 eV. The nanospheres had a Tg of 154 °C. 3.4 Conclusions Poly-(vinylpyridine) nanospheres have been successfully synthesized, using 4- vinylpyridine and 2-vinylpyridine as the monomers. The crosslinking agent used was divinylbenzene. The crosslinking agent was used at a concentration of ~4 mole% to control the swelling and solubility of the nanospheres while maintaining the spherical integrity and the monodisperse character of the nanospheres. Three types of poly- (vinylpyridine) nanospheres were synthesized. These are poly-(4-vinylpyridine), poly-(2- vinylpyridine), and poly-(2-co-4-vinylpyridine). All three varieties of poly- (vinylpyridine) nanospheres showed spherical integrity and monodisperse character. The nanospheres synthesized had a diameter of ~500 nm. This size was consistent regardless of the reaction conditions used. Further studies are required to gain better size control of the reaction. One such study is described in chapter 5 of this dissertation. Many uses are envisioned for these poly-(vinylpyridine) nanospheres. Among these are catalysis studies similar to the Heck reaction, Suzuki coupling, and Stille coupling, which were described above. These beads could also be used as acid scavengers. The large surface to volume ratio of the beads will lend itself well to this 57 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. purpose, while the crosslinking of the bead will prevent excessive swelling which would make it difficult to remove the polymer from the reaction mixture. Another envisioned use for these poly-(vinylpyridine) nanospheres coupled with Hf is for Freidel-Crafts alkylations. As in the case of the nanosphere as an acid scavenger, the nanospheres would be easy to remove after reaction, and could easily be used again, with more HF added. 3.5 References 1. (a) G. Schmid, Ed. Clusters and Colloids. VCH Weinhem, (1994). (b) L. N.Lewis, Chem. Rev., 93, 2693 (1993). (c) A. P. Alivisatos, Science, 271, 933 (1996). (d) A. Henglein, Chem. Rev., 89, 1861 (1989). 2. V. L. Colvin, M. C. Schlamp, and A. P. Alivisatos, Nature, 370, 354 (1994). 3. (a) S. R. Emory, and S. Nie, J. Phys. Chem., B, 102, 493 (1998). (b) M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, and A. P. Alivisatos, Science, 281, 2013 (1998). (c) W. C. W. Chan, and S. Nie, Science, 281, 2016 (1998). 4. (a) N. Toshima, K. Nakata, and H. Kitoh, Inorg. Chim. Acta., 265, 149 (1997). (b) T. J. Schmidt, M. Noeske, H. A. Gasteiger, and R. J. Behm, J. Electrochem. Soc., 145, 925 (1998). 5. M. T. Reetz, W. Helbig, and S. A. Quaiser, Chem. Mat., 7, 2227 (1995). 6. L. D. Rampino, and F. F. Nord, J. Am. Chem. Soc., 63, 2745 (1941). 7. H. Hirai, Y. Nakao, and N. Toshima, J. Macromol. Sci., Chem., A12, 1117 (1978). 8. (a) H. Hirai, H. Chawanya, and N. Toshima, React. Polym., 3, 127 (1986). (b) N. Toshima, T. Takahashi, T. Yonezawa, and H. Hirai, J. Macromol. Sci., Chem., A25, 669 (1988). 9. (a) M. Ohtaki, M. Komiyama, H. Hirai, and N. Toshima, Macromolecules, 24, 5567 (1991). (b) Q. Wang, H. Liu, and H. Wang, J. Colloid Int. Sci., 190, 380 (1997). 10. M. Beller, H. Fischer, K. Kuhlein, C. P. Reisinger, and W. A. Hermann, J. Organomet. Chem., 520, 257 (1996). 58 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 11. C. P. Mehnert, and J. Y. Ying, Chem. Comm., 2215 (1997). 12. D. Getman, D. Hagerty, G. Wilson, and W. F. Wood, J. Chem. Educ., 61,550 (1984). 13. W. F. Wood, and M. Fesler, J. Chem. Educ., 63, 92 (1986). 14. G. Cardillo, M. Orena, S. Sandri, and C. Tomasini, Chem. Ind. (London), 643 (1983). 15. W. K. Fife, and Z. D. Zhang, Tet. Lett., 27,4933 (1986). 16. G. A. Olah, X. Y. Li, Q. Wang, and G. K. S. Prakash, Synthesis, 693 (1993). 17. (a) G. A. Olah, J. T. Welch, Y. D. Vankar, M. Nojima, I. Kerekes, and J. A. Olah, J. Org. Chem., 44, 3872 (1979). (b) S. C. Sondej, and J. A. Katzenellenbogen, J. Org. Chem., 51,3508 (1986). (c) G. K. S. Prakash, V. P. Reddy, X. Y. Li, and G. A. Olah, Synlett, 594 (1990). (d) J. Jeko, T. Timar, and J. Jaszberenyi, J. Org. Chem., 56, 6748 (1991). (e) D. P. Matthews, J. P. Whitten, and J. R. McCarthy, Tet. Lett., 27, 4861 (1986). 18. (a) M. Zupan, B. Sket, and Y. Johar, J. Macromol. Sci., Chem., A17, 759 (1982). (b) A. Gregoricic, and M. Zupan, J. Fluorine Chem., 24, 291 (1984). (c) A. Gregorcic, and M. Zupan, Bull. Chem. Soc. Jpn., 60, 3083 (1987). 19. S. Pathak, M. T. Greci, K. Mercado, G. K. S. Prakash, M. E. Thompson, and G. A. Olah, Chem. Mat., submitted. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4: Functionalization of Preformed Polystyrene Nanospheres 4.1 Introduction Polystyrene nanospheres are useful in many applications, such as those summarized in the introduction to this dissertation. Their popularity is due, in part, to their large surface area and surface uniformity.1 Another special property of the polystyrene nanospheres is their rigidity.2 One drawback to their use is the relative inertness of their surface. Therefore, there has been considerable effort directed toward finding new methods for the surface functionalization of polystyrene nanospheres.3 Preformed polystyrene nanospheres were surface-functionalized using a lithiation technique first reported by Frechet in 1979.4 In this method, polystyrene resins were first lithiated by tetramethylethylenediamine (TMEDA) and n-butyllithium. After this, the lithiated resins were treated with sulfur, which replaced the lithium in a substitution-like mechanism. We repeated this work using sulfur, then extended it to include other functional groups (figure 4.1). Functional groups substituted onto the surface of our preformed polystyrene nanospheres include thiol, hydroxy, aldehyde, carboxylic acid, and amine. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. o crosslinked polystyrene nanosphere Figure 4.1 General reaction scheme for the surface functionalization of preformed polystyrene nanospheres 4.2 Results and Discussion These reactions were carried out in a three-neck flask fitted with an attached filter frit. Special care was taken to avoid contact with air during all steps of the reaction. The lithiation step was run under argon atmosphere. The lithiated beads were then vacuum filtered without removing them from the reaction flask. After the filtration, the dry THF was added to the reaction flask followed by the electrophile being used for that reaction. As mentioned above, Frechet first reported this method for the thiolization of polystyrene in 1979. We first repeated this method on our preformed polystyrene nanospheres using sulfur as the electrophile, resulting in surface coverage of thiols. This method also produced disulfide bonds, which were broken by a reduction reaction using lithium aluminum hydride. Characterization was carried out using Energy Dispersive X- Ray Analysis (EDX) and Scanning Electron Microscopy (SEM). EDX showed a strong sulfur peak and SEM showed uniform, monodisperse beads of -500 nm diameter (figure 4.2). 61 T M E D A ^ N -B uLi cyclo hexane Li Li electrophile T H F E E =-SH , -C (C H 3)2O H , -C H O , -c o 2h , -N H 2 ' Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.2: Thiolated polystyrene nanospheres The next functional group added to our preformed polystyrene nanospheres was a 2-hydroxy propyl group. The 2-hydroxy propyl group was added by using acetone as the electrophile and solvent in the step after lithiation. Surface Reflectance IR showed a strong hydroxy peak, after the beads were dried. SEM also showed uniform, monodisperse beads of ~500 nm (figure 4.3). Polystyrene beads were also surface functionalized with carboxylic acid. Bubbling dry CO2 (g) through the reaction mixture of lithiated beads successfully carboxylated the surface of the beads. After 1.5 hours of bubbling, the Surface Reflectance IR showed a strong carbonyl peak at 1720 cm'1 and a strong hydroxy stretch at 3300 cm'1 . SEM showed the integrity of the beads had been maintained under the reaction conditions (figure 4.4). The beads tended to agglomerate as a result of strong hydrogen bonding. 62 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.3: Polystyrene nanospheres surface functionalized with 2-hydroxy propyl groups Figure 4.4: Carboxylated polystyrene nanospheres Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Aldehydes were added to the surface of the polystyrene nanospheres by reacting the lithiated beads with N-formylpiperidine (1). This reaction was carried out overnight, l and resulted in an IR peak associated with the aldehyde frequency. SEM showed uniform nanospheres (figure 4.5) Figure 4.5: Formylated polystyrene nanospheres The final functional group added to the surface of preformed polystyrene nanospheres by this method was amines. The amino group was added by reacting a mixture of cis, trans styryl azides (2) with the lithiated beads overnight. Styryl azides 64 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. were prepared by a literature procedure. This reaction showed a strong amine peak in the Surface Reflectance IR and uniform monodisperse beads in the SEM (figure 4.6). i l l Figure 4.6: Aminated polystyrene nanospheres This method for functionalizing the surface of preformed polystyrene beads had general applications, with several electrophiles successfully functionalizing the surface. SEM showed the beads maintained their integrity throughout the reaction. Surface Reflectance IR and EDX showed peaks in the desired location. Unfortunately, when the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. beads were reacted with various metals, they did not result in even distribution of the metal on the surface of the beads as shown by Transmission Electron Microscopy (TEM).6 Therefore, it can be concluded that while the surface functionalization does take place, it is not uniform across the surface of the beads. These nanospheres were therefore not useful for extended applications such as those listed in the introduction. 4.3 Experimental All reactions were carried out in a three-necked flask with a filter frit attached. The polystyrene beads used in this study were provided by Salovey’s group in the Department of Chemical Engineering at the University of Southern California, where they were prepared by a continuous-feed emulsifier-free emulsion polymerization.7 The polystyrene beads were 2% crosslinked with divinylbenzene and were ~500nm in diameter. All chemicals were obtained from Aldrich. Sulfur, n-butyl lithium, lithium aluminum hydride, and N-formylpiperidine were used as received. THF was dried over sodium metal. Reagent grade acetone was dried over molecular sieves. TMEDA and cyclohexane were distilled over CaH2. Characterization was performed using SEM, EDX, and Surface Reflectance E R Spectroscopy. All SEM and EDX samples were prepared by placing a drop of sample diluted with 2 mL of water on top of a glass plate. The sample was allowed to dry at room temperature, then sputter-coated with gold. All SEM analyses were performed using a Cambridge 360 Scanning Electron Microscope at 10 kV and 10 K magnification. All Surface-Reflectance Infrared Spectroscopy was carried out on the Bio-Rad FT instrument. IR samples were prepared by drying the samples overnight under vacuum. 66 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.3.1 General procedure for the lithiation of preformed polystyrene nanospheres A 50 mL three-necked flask was evacuated and filled with Argon. Polystyrene nanospheres (lg, 500 nm, 2% crosslinked) were placed in flask along with 2.5 mL n-BuLi, 1 mL TMEDA, and 8.3 mL dry cyclohexane. The reaction mixture was stirred at 65 °C for four hours under argon atmosphere. The beads were then vacuum filtered through the attached filter frit and washed two times with dry cyclohexane. 4.3.2 Addition of Electrophiles After lithiation, various electrophiles were added to the lithiated beads according to the following procedures. Table 4.1 gives a summary of the various procedures used for the addition of electrophiles to the lithiated polystyrene nanospheres. Electrophile Reaction Conditions Reaction time Resulting Functional Group Sulfur Dry THF, room temperature Overnight Thiols Acetone Room temperature Overnight Hydroxy groups Carbon Dioxide Dry THF, Room temperature 90 minutes Carboxylic Acid N-formylpiperidine Dry THF, -78 °C to room temperature 2 hours Aldehyde Styryl Azide Dry THF, Room temperature Overnight Amine Table 4.1: Reaction Conditions for Addition of Various Electrophiles to lithiated polystyrene nanospheres 67 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Preparation of thiolated polystyrene nanospheres After the beads were washed with dry cyclohexane, the flask was evacuated three times then filled with argon. Next, 10 mL dry THF and .lg Sulfur were placed in the flask with the lithiated polystyrene beads and stirred at room temperature overnight. The nanospheres were then washed with a 2:1 THF/6N HC1 solution, placed in a Soxhlet extractor, and extracted with dry THF for 72 hours to remove any unreacted sulfur. The thiolated beads were removed from the Soxhlet extractor and dried under vacuum. After drying, the beads were placed in a 50 mL round bottom flask along with 20 mL dry THF and -.17 g LiAlH4 to break any disulfide bonds. This reaction mixture was refluxed for four hours. Following reflux, 25 mL of 3M HC1 were added to the reaction. The beads were then filtered and washed with water, THF, acetone, methylene chloride, and methanol. The solid was collected in a vial and dried under vacuum overnight. IR spectroscopy showed a small peak at -2590 cm'1 . EDX showed a strong sulfur peak (figure 4.7), and SEM showed spherical beads of -500 nm diameter. Preparation of hydroxylated polystyrene nanospheres After washing the lithiated beads two times with dry cyclohexane, the flask was evacuated three times then filled with argon. The beads were returned to the bottom of the reaction flask, then 10 mL of dry acetone were added. The reaction mixture was stirred at room temperature overnight. The nanospheres were then washed with a 2:1 THF:6N HC1 solution and filtered. The beads were washed with water, THF, acetone, methylene chloride, and methanol. Surface Reflectance IR showed a peak in the hydroxy stretching region and no peak in the carbonyl stretching frequency (figure 4.8). SEM showed uniform nanospheres of -500 nm diameter. 68 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. s s X-RRS': 0 - 20 ke<J L ive: 4 3 s P r e se t: 200s Remaining: 15?s Real: 5 4 s 20X Dead_______________________ 4. 1.660 k eU ch 93= M EM I : Figure 4.7: EDX of thiolated polystyrene nanospheres BIQ-RfiD i obo 4000 3000 2000 Wavenumbers (cm -1) Figure 4.8: Surface Reflectance IR of hydroxylated polystyrene nanospheres 69 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Preparation of carboxylated polystyrene nanospheres: After lithiation, the flask was evacuated three times, then filled with CO2 (g > . Next, ~20 mL of dry THF were added to the flask. The reaction mixture was stirred, with constant CO2 bubbling, for ninety minutes. It was then quenched with a 2:1 THF:6N HC1 solution. The beads were then filtered and washed with water, THF, acetone, methylene chloride, and methanol. The solids were collected and dried overnight. Surface Reflectance IR showed a carbonyl stretch at 1726cm'1 and a broad hydroxy stretch centered at 3300cm'1 (figure4.9). SEM showed uniform spheres of -500 nm diameter. 4000 3600 3200 2800 2400 2000 1600 1400 Wavenumbers (cm -1) Figure 4.9: Surface-Reflectance IR of carboxylated polystyrene nanospheres Preparation of formylated polystyrene nanospheres: After lithiation, the beads were washed two times with dry cyclohexane. The flask was then evacuated three times and filled with argon. Next, 15mL of dry THF were added and the reaction flask was cooled Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. to -78 °C. After the flask had reached -78 °C, -.5 mL N-formylpiperidine were added to the reaction. The reaction was stirred at -78 °C for two hours then allowed to slowly warm to room temperature. The reaction was then quenched with water and washed three times with methylene chloride. After drying, the IR showed a carbonyl stretching peak at -1700cm'1 , and only a small hydroxy stretching frequency due to the water from workup (figure 4.10). SEM showed uniform, spherical beads of -500 nm diameter. 4000 lO’ O O 3000 2000 Wavenumbers (cm -1) fiL'JhM'Oii COfiThO F’ i!L ? y rYRkNr. BiifiDS r:-<‘ ■- Figure 4.10: Surface-Reflectance IR of formylated polystyrene nanospheres Preparation of azido-polystyrene nanospheres: After lithiation, the nanospheres were washed three times with dry cyclohexane, then the flask was evacuated three times and filled with argon. Subsequently, 1 mL styryl azide diluted into 15 mL dry THF was added to the reaction flask. The reaction mixture was stirred at room temperature overnight, then washed with dilute HC1 followed by dilute KOH. After drying, the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Surface-Reflectance IR showed an amine stretching frequency at -1600 cm'1 (figure 4.11), and SEM showed uniform, monodisperse nanospheres of -500 nm diameter in size. 1000 2000 3000 4000 Wavenumbers (cm -1) Figure 4.11: Surface-Reflectance IR of aminated polystyrene nanospheres 4.4 Conclusions This method for the surface functionalization of preformed polystyrene nanospheres is effective in placing functional groups onto the surface of the nanospheres. However, further investigation shows uneven distribution of metal colloids onto the surface of the functionalized beads. This indicates uneven distribution of the functional groups on the surface of the preformed polystyrene nanospheres. Therefore, these beads would not be useful in applications such as those summarized in the introduction section 72 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of this dissertation as they require uniform functional group coverage on the bead surface. It was therefore necessary that we find a better way to achieve uniform functionalization of the surface of polystyrene nanospheres. This was achieved via the in situ grafting technique discussed in chapter 5. 4.5 References 1. N. Kawahashi and E. Matijevic, J. Colloid Int. Sci., 138, 534 (1990). 2. M. Shahar, H. Meshulam, and S. Margel, J. Polym. Sci., A, 24, 203 (1986). 3. (a) A. Rembaum, W. Volksen, US Patent 4,123,396, (1978). (b) A. Rembaum, R. C. K. Yen, US Patent 4,534,996, (1985). (c) S. Margel, E. Nov, I. Fisher, J. Polym. Sci., A, 29, 347 (1991). (d) M. Okubo, Y. Iwasaki, and Y. Yamamoto, Colloid and Polym. Sci., 270, 733 (1992). (e) A. Tuncel, R. Kahraman, and E. Piskin, J. ofApp. Polym. Sci., 51, 1485 (1994). (f) M. Chen, A, Kishida, and M. Akashi, J. Polym. Sci., A, 34, 2213 (1996). 4. J. M. J. Frechet, M. D. de Smet, and M. J. Farrall, Polymer, 20, 675 (1979). 5. F. W. Fowler, A. Hassner, and L. A. Levy, J. Am. Chem. Soc., 89, 2077 (1967). 6. In collaboration with S. Pathak, unpublished results. 7. (a) D. Zou, V. Derlich, K. Gandhi, M. Park, L. Sun, D. Kriz, Y. D. Lee, G. Kim, J. J. Aklonis, and R. Salovey, J. Polym. Sci., A, 28, 1909 (1990). (b) M. Park, K. Gandhi, L. Sun, J. J. Aklonis, and R. Salovey, Polym. Eng. Sci., 30, 1158 (1990). (c) D. Zou, L. Sun, J. J. Aklonis, and R. Salovey, J. Polym. Sci., A, 30, 1463 (1992). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5: Polystyrene Functionalization: A New In Situ Grafting Technique 5.1 Introduction Polystyrene nanospheres have been used in a wide variety of applications in the past. Such usage is due in part to the surface uniformity and large surface area of the polystyrene nanospheres. These applications include catalysis, combinatorial chemistry, protein supports, magnets, and photonic crystals. For the polystyrene nanospheres to be used in these applications, the surface must first be functionalized. As a way of maintaining the surface uniformity of the nanospheres, it is important to have uniform functionalization of the surface. The active surface sites are then able to meet the specific demands of the desired function. Several methods of surface functionalization have been employed to provide active surface sites. In 1978, Rembaum and Volksen patented a method for the synthesis of metal-containing polystyrene nanospheres.1 This was done by the copolymerization of polystyrene-vinylpyridine nanospheres. These nanospheres would contain pendant tertiary amine groups that could bind to metals. Rembaum and Yen patented a method for the surface functionalization of polystyrene nanospheres with acrolein or other aldehydes by irradiation with high intensity radiation in 1985.2 This method results in a surface coating of aldehydes which could then be converted to other functional groups. Another method for the surface functionalization of polystyrene nanospheres is the direct 74 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. synthesis of functionalized styrene monomers. Margel, et. al., synthesized nanospheres of chloromethylstyrene, formylstyrene and styrene sulfonylchloride in organic solvents.3,4 One major obstacle to this method is the cost of the monomers. Another method for the surface functionalization was reported by Okubo, Iwasaki, and Yamamoto in 1992.5 In this method, the seeded copolymerization of styrene, divinylbenzene, and chloromethylstyrene resulted in surface-pendant chlorine groups. This method is carried out by dispersion polymerization in ethanol-water medium with poly (acrylic acid) as the stabilizer. Tuncel, Kahraman, and Piskin achieved surface functionalization by first synthesizing polystyrene nanospheres in the first step, then copolymerizing styrene and acrylate comonomers on the polystyrene latex particles.6 Poly (acrylic acid) was also used as the emulsifier in this method. Graft copolymers containing hydrophobic backbones and hydrophilic branches were reported by Chen, Kishida, and Akashi in 1996.7 They synthesized polystyrene nanospheres with poly (A-isopropylacrylamide) branches on the surface. Their method involved free-radical polymerization in an ethanol medium. We have now developed a novel and versatile in-situ grafting technique for the surface functionalization of polystyrene nanospheres. The parent polystyrene nanospheres are synthesized via emulsifier-free emulsion polymerization with the addition of a substituted styrene in the last thirty minutes of reaction. This substituted styrene monomer competitively reacts with the remaining styrene monomer in a grafting fashion, resulting in surface functionalization. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5.2 Results and Discussion The above-mentioned grafting-technique is an efficient way to achieve the uniform surface functionalization of polystyrene nanospheres. Groups which have been successfully grafted onto the surface of the polystyrene nanospheres include para- acetoxy styrene(l),para-f-butoxy styrene (2), para-fluorostyrene (3), r ,2 ’,2’- trifluorostyrene (4), 2-vinylpyridine (5), 4-vinylpyridine (6), 4-vinylaniline (7), and para- chloromethylstyrene (8) (figure 5.1). The monomer to be grafted to the surface of the polystyrene nanosphere is added to the reaction thirty minutes prior to the end of the reaction. In each case, Scanning Electron Microscopy confirmed the maintenance of spherical integrity, and size uniformity throughout the grafting time period (figure 5.2). F / / /■ u. V [ f l If V h3c o J H l H3C CH3 F i 2 3 4 ✓ ( f / A i i f i V nh2 x i 5 6 7 8 Figure 5.1: Substituted styrenes successfully grafted onto the surface of polystyrene nanospheres 76 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.2: Non-functionalized Polystyrene Nanospheres Auger Spectroscopy was used to verify the existence of the desired functional groups on the surface of the polystyrene beads. This method shows a peak at a distinctive electron-volt value for each element. Carbon is located at 272 eV, nitrogen at 379 eV, oxygen at 503 eV, etc. Each sample showed the desired peak, confirming the presence of the desired functional group and the success of the grafting technique. Auger is a qualitative, not quantitative, measurement. Therefore this method can not be used to determine the number of functional units on the surface of the nanosphere. Further experiments on coating the surface with various metals were carried out to prove the uniformity of the surface coating. These results were verified using Transmission Electron Microscopy and showed an even distribution of metal colloids around the nanosphere. Figure 5.3 shows the metal coating of the hydroxylated polystyrene nanospheres.8 77 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.3: Hydroxylated nanospheres coated with silver colloids (top) and ruthenium colloids (bottom) 78 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The various groups grafted to the surface of the polystyrene groups changed the physical characteristics in several ways, para-Acetoxy styrene, para-t-butoxy styrene, para-fluorostyrene, r , 2’,2 ’-trifluorostyrene, and para-chloromethylstyrene coated beads remained dispersed throughout the solution. The nitrogen-containing functional groups caused the beads to precipitate out of solution rather than remaining dispersed. This is consistent with the behavior of the poly-vinylpyridine beads.9 5.3 Experimental Styrene, 2-vinylpyridine, 4-vinylpyridine and divinylbenzene were obtained from Aldrich and vacuum distilled to remove the inhibitor. All other monomers were purchased from Aldrich and used without further purification. Potassium persulfate and potassium hydroxide were also obtained from Aldrich and used without further purification. All reactions were carried out in a 1-liter reaction kettle. The kettle was fitted with a condenser, a nitrogen inlet valve, and a mechanical stirring apparatus. The mechanical stirrer used was the IKA Eurostar power control-visc. Characterization was performed using SEM, TEM, and Auger Spectroscopy. All SEM samples were prepared by placing a drop of sample diluted with 2 mL of water on top of a glass plate. The sample was allowed to dry at room temperature, then sputter-coated with gold. All SEM analyses were performed using a Cambridge 360 Scanning Electron Microscope at 10 kV and 10 K magnification. All Auger samples were prepared by placing a drop of sample diluted with 6 mL of water on a silicon wafer. Auger spectra were run on a Perkin Elmer PHI 440 Auger Scanning Microscope. All Surface-Reflectance Infrared Spectroscopy Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. was carried out on the Bio-Rad FT instrument. IR samples were prepared by drying the samples overnight under vacuum. 5.3.1 General Procedure for Synthesis of Surface-Functionalized Polystyrene Nanospheres: A one-liter reaction kettle, equipped with a condenser, gas inlet, and mechanical stirring apparatus, containing 700 mL of distilled water was heated to 80 °C, stirred at 300 rpm, and degassed with N2 for 1 hour. After one hour, the gas flow was turned off and 77 mL of styrene (.673 moles) and 4 mL of divinylbenzene (~4 mole percent) were added to the water. The reaction mixture was stirred for twenty minutes to bring the monomer and crosslinking agent to temperature, followed by the addition of .6 g potassium persulfate (.003 moles) dissolved in 20 mL of water as the initiator. The reaction mixture was stirred at 300 rpm and 80 °C for 4.5 hours. Subsequently, -.5 mole percent of substituted styrene was added to the reaction. The reaction was allowed to continue for thirty minutes then stopped by removing the heat source and discontinuing the stirring. The nanospheres were removed from solution by vacuum filtering through a coarse filter frit and washing three times with acetone. The resulting polymer was then characterized via scanning electron microscopy to determine the size and integrity of the nanospheres, Auger Spectroscopy to analyze the surface of the nanospheres, and/or Surface Reflectance infrared spectroscopy to verify the existence of the desired functional group on the surface of the nanosphere. Cleavage of acetoxy and t-butoxy groups: The nanospheres substituted with para- acetoxystyrene or para-t-butoxystyrene were stirred overnight in a 5 M KOH solution to cleave the acetoxy or t-butoxy group, leaving a hydroxylated surface. The polymer was 80 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. removed from the aqueous reaction media by filtration through a coarse filter frit for IR analysis. Surface Reflectance IR showed the disappearance of the carbonyl stretch at -1720 cm'1 . para-Acetoxy styrene-grafted nanospheres (1): Beads grafted with p-acetoxystyrene showed a distinct oxygen peak at 503 eV on Auger Spectroscopy. Surface Reflectance IR showed a carbonyl peak at -1720 cm'1 . Scanning Electron Microscopy showed uniform, spherical beads after cleavage (figure 5.4). The oxygen peak was still present at 503 eV on Auger Spectroscopy (figure 5.5), and the carbonyl peak had disappeared in the Surface Reflectance IR. Figure 5.4: Hydroxylated Polystyrene Nanospheres formed by the hydrolysis of acetoxy groups Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. V • ;':'C II [ ■ : : ■ ’ o . C . , *r TTl&m v 'v ;^ :? ^ • ‘ -:::S ’ '}'. :"-.:l''-^'-y : ,'■. I " - 1 -!?:' S'V V i.V '■ ’ . l ‘ . f ^••va':'’ :; '''v'i.->';'.'<>':’ l . ;'rK ••■ •!* i;^!* :'■ • ■ • ' Figure 5.5: Auger spectrum of hydroxylated polystyrene nanospheres after hydrolysis of the acetoxy groups para-f-Butoxy styrene-grafted nanospheres (2): Beads grafted with p-r-butoxy styrene showed a distinct oxygen peak at 503 eV on Auger Spectroscopy. Surface Reflectance IR showed a carbonyl peak at -1720 cm'1 . Scanning Electron Microscopy showed spherical, monodisperse nanospheres (figure 5.6). After cleavage, the oxygen peak was still present at 503 eV on Auger Spectroscopy (figure 5.7) and the carbonyl peak had disappeared in the Surface Reflectance IR. 82 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.6: Hydroxylated Polystyrene Nanospheres obtained after hydrolysis of the t- butoxy groups SCALE FACTOR, GFFSET=311 .9 8 3 , 0 .6 0 6 I COUNTS/SEC 10 t i l l i i_____ i— i-------1-----1— l h 9 U T l 5 m 4 * A 0 111 V 3 BU=3.08kO B I— 6 1 6060uA j i i i I. k ,j\ \ { ‘ » \ A. - f / y v y w - M A / v ^ y .aA/VVVA » r . — I 1 - - - - - - - - 1 - - - - - - - - 1 - - - - - - - - 1 - - - - - - - - I I I I I I I i I I I I I T " 2 0 , 0 8 0 . 0 1 4 0 . 0 2 0 8 .0 2 6 0 . 0 3 2 0 .0 3 8 0 .8 4 4 8 . 8 5 8 0 .0 5 6 8 . 8 6 2 8 .8 K IN E TIC ENERGY, eV Figure 5.7: Auger Spectrum of hydroxylated polystyrene nanospheres obtained after hydrolysis of the t-butoxy groups 83 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. para -Fluorostyrene-grafted nanospheres (3): para-Fluorostyrene-grafted polymer nanospheres showed an Auger peak at 647eV. Scanning Electron Microscopy showed uniform, spherical nanospheres (figure 5.8). Figure 5.8: para-Fluorostyrene grafted polystyrene nanospheres l ’,2’,2’-trifluorostyrene-grafted nanospheres (4): Polystyrene nanospheres grafted with r ,2 ’,2’-trifluorostyrene showed an Auger peak at 647 eV. SEM showed uniform, spherical, monodisperse nanospheres (figure 5.9). 2-vinylpyridine-grafted nanospheres (5): The nanospheres grafted with 2- vinylpyridine did not show an Auger peak at 379 eV as expected. However, these beads showed the same physical characteristics as solid poly-2-vinylpyridine beads. They settled out of solution and showed even coverage of the beads with platinum metal colloids.1 0 The lack of an Auger peak is probably due to the weak nature of the nitrogen Auger electron. SEM showed monodisperse, spherical nanospheres (figure 5.10). 84 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.9: r ,2 ’,2’-trifluorostyrene grafted polystyrene nanospheres Figure 5.10: 2-vinylpyridine grafted polystyrene nanospheres 4-vinylpyridine-grafted nanospheres (6): The nanospheres grafted with 4- vinylpyridine did not show an Auger peak at 379 eV as expected. However, these beads 85 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. showed the same physical characteristics as solid poly-4-vinylpyridine beads. They settled out of solution and showed even coverage of the beads with platinum metal colloids.1 0 The lack of an Auger peak is again probably due to the weak nature of the nitrogen Auger electron. Scanning Electron Microscopy showed spherical, monodisperse nanospheres (figure 5.11). Figure 5.11: 4-vinylpyridine grafted polystyrene nanospheres 4-vinyl aniline-grafted nanospheres (7): Nanospheres grafted with 4-vinyl aniline showed a small Auger peak at 379 eV (figure 5.12). They also were a dull orange color after the grafting time period. Scanning Electron Microscopy showed monodisperse, uniform nanospheres (figure 5.13). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SCALE FACTOR, 0FFS E T =1S 2.9 3 3 , O.OOO k COUHTS/SEC BU=3.00kU B I: 0iOOOOuA 19 3 t ’l 7 t j 5 R u j 4 * A u 0 6 0 . 0 3 2 0 .0 380 K IN E TIC ENERGY, eV 2 0 . 0 8 0 . 0 1 4 0 .0 2 0 0 .0 Figure 5.12: Auger Spectrum of aminated polystyrene nanospheres Figure 5.13: Polystyrene nanospheres grafted with 4-vinylaniline 87 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. para-Chloromethylstyrene-grafted nanospheres (8): para-Chloromethylstyrene grafted polystyrene nanospheres showed an Auger peak at 215 eV (figure 5.14). Scanning Electron Microscopy showed uniform, monodisperse nanospheres (figure 5.15). 16 it v 5 r. u 4 * A 'I U J ^2 SCALE FACTOR, 0 F F S E T = 2 3 4 .193, 0 .6 6 6 K COUNTS/SEC B V = 3 . 8 P B I=6.0006uA i L i i i i i i i i i i l____ i — i — i — i — i — lyl jA /wVVVw ..................................................i i i i i i i i i i i 1 i r 2 6 , 6 8 6 . 6 1 4 6 .0 2 0 6 .6 2 6 6 . 6 3 2 6 .6 3 8 6 .6 4 4 6 .6 5 6 6 .6 5 6 6 .6 6 2 6 .6 KINETIC ENERGY, eV Figure 5.14: Auger Spectrum of chloromethylated polystyrene nanospheres 88 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.15: Chloromethylstyrene grafted polystyrene nanospheres 5.4 Conclusions This new in-situ grafting technique is an efficient method for the surface- functionalization of polystyrene nanospheres. Eight different substituted styrenes have been successfully grafted onto the surface of the polystyrene nanospheres during the emulsifier-free emulsion polymerization of styrene. These different functional groups can then be bound to metals or covalently bonded to other groups to be used in a wide variety of applications.11 There are also several published methods for converting the chlorine group from chloromethylstyrene into other functional groups.4'5 This method can be further expanded to include other substituted monomers, such as 4-vinyl benzoic acid, and nanospheres of different diameters. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5.5 References 1. A. Rembaum, W. Volksen, US Patent 4,123,396, (1978). 2. A. Rembaum, R. C. K. Yen, US Patent 4,534,996, (1985). 3. M. Shahar, H. Meshulam, S. Margel, J. Polym. Sci., A, 24, 203 (1986). 4. S. Margel, E. Nov, I. Fisher, J. Polym. Sci., A, 29, 347 (1991). 5. M. Okubo, Y. Iwasaki, and Y. Yamamoto, Colloid and Polym. Sci., 270, 733 (1992). 6 . A. Tuncel, R. Kahraman, and E. Piskin, J. Appl. Polym. Sci., 51,1485 (1994). 7. M .Chen, A. Kishida, and M. Akashi, J. Polym. Sci., A, 34, 2213 (1996). 8. M. T. Greci, S. Pathak, K. Mercado, G. K. S. Prakash, M. E. Thompson, and G. A. Olah, Chem. Mat., submitted. 9. Chapter 3 of this dissertation. 10. In collaboration with S. Pathak, unpublished results. 11. Chapter 1 of this dissertation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 6: Concluding Remarks 6.1 Conclusions The main purpose of this research project was the synthesis and functionalization of polymer nanospheres. These nanospheres have many applications in the fields of catalysis, combinatorial chemistry, protein supports, magnets, and photonic crystals. Polystyrene nanospheres are often preferred for use in these applications due to their high surface area and the uniformity and rigidity of the surface. The relatively poor reactivity of the surface of polystyrene is one of its biggest obstacles towards use in this field. Therefore, it was necessary to develop various methods for functionalizing the surface of the polystyrene nanospheres. Salovey and coworkers1 developed a method for emulsifier-free emulsion polymerization of monodisperse, crosslinked polystyrene nanospheres. We have repeated Salovey’s polymerization technique to synthesize monodisperse, crosslinked polystyrene nanospheres. We also varied the polymerization conditions to attain size control of the diameter of the beads. We found that varying the time of the reaction gave the best control of size, with monodisperse nanospheres synthesized in the range of 200- 800nm diameter. Varying the stir rate or the concentration of monomer did not have any effect on the size, although addition of small amounts of ethanol did produce larger, polydisperse nanospheres in the range of 200nm-2.3|im. Also, using a crosslinking ratio greater than 4 mole% resulted in polydisperse, nonspherical nanobeads. 91 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Another problem encountered in the synthetic process was if the reaction mixture was not stirred smoothly and rapidly, the beads were often monodisperse, but nonspherical. It was also important to allow the monomer to reach reaction temperature before adding initiator. If the initiator was added too soon, the resulting nanospheres were polydisperse in character. One method to obtain functionalized polymers was direct polymerization using monomers containing functional groups. Therefore, we synthesized poly-vinylpyridine nanospheres. Nanospheres were synthesized using 4-vinyIpyridine, 2-vinylpyridine, and the combination of the two. These nanospheres were monodisperse and uniform. The diameter of these beads was -500 nm. Despite changing the reaction conditions many times, we were unable to find a method for controlling the diameter of the poly- vinylpyridine nanospheres. Conditions such as reaction time, stir rate, and monomer concentration were varied. In all cases, the nanospheres were either uniform and monodisperse at -500 nm or they were nonspherical with little or no uniformity. More work is needed to find a method for controlling the size of poly-vinylpyridine nanospheres while maintaining spherical integrity. While directly synthesizing nanospheres from monomers containing functional groups is a straightforward method towards the production of surface-functionalized polymer nanospheres, the cost of such materials is often high. It was decided that it would be more cost effective to have a solid polystyrene core with a functionalized surface. To that end, we developed a method for the surface functionalization of preformed polystyrene nanospheres based on work earlier done by Frechet.2 In this method, discussed in chapter 4, we were successful in functionalizing the surface of the 92 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. nanospheres with thiols, hydroxy groups, aldehydes, carboxylic acids, and amines through the intermediacy of the lithiated polystyrene beads. These results were verified by Surface-Reflectance Infrared Spectroscopy, Energy Dispersive X-Ray Spectroscopy, and Scanning Electron Microscopy. Unfortunately, when we tried to coat the surface of these functionalized nanospheres with various metals, Transmission Electron Microscopy showed uneven metal coverage of the surface. This result implies uneven coverage of the functional groups on the polystyrene nanosphere surface. Therefore it became necessary to develop another method for the surface functionalization of polystyrene nanospheres. We then investigated a method for an in situ grafting technique to surface- functionalize polystyrene nanospheres as they are being formed. We used the emulsifier- free emulsion polymerization technique discussed in chapters 1 and 2 to carry out these polymerizations. Thirty minutes before the end of reaction time, ~1 mole% of functionalized monomer was added to the reaction. The reaction was allowed to continue for thirty minutes, with constant stir rate and constant temperature. This allowed the functionalized monomer to react competitively with the unreacted styrene monomer in a grafting fashion during the final stages of the polymerization. This resulted in surface coverage by the desired functional group. Substituted monomers added by this method include para-acetoxy styrene, para-r-butoxy styrene, para-fluorostyrene, 1\ 2’,2 ’- trifluorostyrene, 2-vinylpyridine, 4-vinylpyridine, 4-vinylaniline, and para- chloromethylstyrene. The functional groups anchored onto the surface by this method include hydroxy, fluorine, vinylpyridine, amine, and chlorobenzyl. Other monomers, such as vinylanthracene could also be used to make polymer nanospheres. 93 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The poly-(vinylpyridine) nanospheres have been coated with palladium nanoparticles and the resulting metal coated nanospheres were used as catalysts in carbon-carbon bond formation reactions. The Hydroxy functionalized beads were coated with silver and ruthenium nanoparticles. Some of the techniques developed in this work have wide applications in the electronic, electrooptic, catalysis, and fuel cell fields. 6.2 References 1. D. Zou, V. Derlich, K. Gandhi, M. Park, L. Sun, D. Kriz, Y. D. Lee, G. Kin, J. J. Aklonis, and R. Salovey, J. Polym. Sci., A, 28, 1909 (1990). 2. J. M. J. Frechet, M. D. de Smet, and M. J. Farrall, Polymer, 20, 675 (1979). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. U MI MICROFILMED 2002 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. Photographs included in the original manuscript have been reproduced xerographically in this copy. Higher quality 6” x 9" black and white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order. ProQuest Information and Learning 300 North Zeeb Road, Ann Arbor, Ml 48106-1346 USA 800-521-0600 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. 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Creator Greci, Marcia Tuten (author)

Core Title Synthesis and functionalization of polymer nanospheres

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Degree Doctor of Philosophy

Degree Program Chemistry

Degree Conferral Date 2000-08

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