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Synthesis of Block, Graft and Star Polymers from Inorganic Macroinitiators Krzysztof Matyjaszewski,* Peter J

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Synthesis of Block, Graft and Star Polymers from Inorganic Macroinitiators Krzysztof Matyjaszewski,* Peter J APPLIED ORGANOMETALLIC CHEMISTRY Appl. Organometal. Chem. 12, 667–673 (1998) REVIEW Synthesis of Block, Graft and Star Polymers from Inorganic Macroinitiators Krzysztof Matyjaszewski,* Peter J. Miller, Eric Fossum and Yoshiki Nakagawa Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA Recent advances in the synthesis of block, graft INTRODUCTION and star polymers containing inorganic macro- molecular species are described. Anionic copo- One of the most important elements in the lymerization techniques were used in the for- production of copolymers is control over the mation of diblock copolymers of poly(styrene- polymerization reaction. If the process can be block-methylphenylsilylene) and poly(isoprene- manipulated so that the number of growing chains block-methylphenylsilylene) by the ring-opening is constant and that chain transfer or termination polymerization of 1,2,3,4-tetramethyl-1,2,3,4- reactions are avoided (or eliminated), then the tetraphenylcyclotetrasilane initiated by living functional group at the polymer terminus will be anionic polystyrene and isoprene respectively. conserved, allowing for additional chemistry to Hydrosilation of an attachable initiator onto take place. Such transformation reactions can result telechelic vinyl- or hydrosilyl-terminal or in production of a macroinitiator which can initiate -pendant poly(dimethylsiloxane) (PDMS) polymerization of a different monomer, thereby yielded a PDMS macroinitiator. This macro- producing block copolymers. If a difunctional initiator was used in atom transfer radical initiator is used, the same technique can be applied polymerization (ATRP) of styrene and isobornyl toward triblock copolymers. Furthermore, use of a acrylate to produce ABA triblock copolymers. functional group on a monomer in conjunction with As a model for graft copolymers from a another monomer in a statistical copolymerization polyphosphazene backbone, chemical trans- results in pendant species which can be transformed formation of hexachlorocyclotriphosphazene to initiate polymerization toward graft copolymers. resulted in hexafunctional molecules containing With such techniques the combinations and archi- either benzyl bromide or bromopropionyl moi- tectures of the copolymers can be tailored to yield eties. The initiator 1,1,3,3,5,5-hexakis[4-(2- materials with properties that meet particular needs. bromopropionyloxymethyl)phenoxy]cyclotri- Of particular interest are copolymers that contain phosphazene was used in the ATRP of styrene to an inorganic block. Inorganic polymers generally yield a polymer with a narrow, monomodal possess properties that are different from carbon- molecular weight distribution. Chain extension based polymers.1 The desire to produce block and of this star polymer with isobornyl acrylate is graft copolymers from inorganic and organic blocks also described. # 1998 John Wiley & Sons, Ltd. is generally pursued to exploit the best properties of the individual materials and generate new classes of Keywords: polysilanes; poly(dimethylsiloxane); compounds. For example, polysilylenes (polysi- polyphosphazenes; atom transfer radical poly- lanes) possess unique electronic and optical proper- merization; controlled/‘living’ radical polymer- ties due to s-bond delocalization along the silicon ization backbone, giving them potential applications in Received 2 September 1997; accepted 7 November 1997 electronics and reprographics. Another example is polysiloxanes, which possess high oxygen perme- ability and favorable water and weather resistance. Finally, polyphosphazenes have a broad range of physical properties leading to applications in * Correspondence to: K. Matyjaszewski, Department of Chemistry, biomedicine as well as flame retardance based on Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA. the substituents bonded to phosphorus. Contract/grant sponsor: National Science Foundation. For carbon-based vinyl monomers, controlled Contract/grant sponsor: ATRP Consortium. polymerization has traditionally been achieved by # 1998 John Wiley & Sons, Ltd. CCC 0268–2605/98/100667–07 $17.50 668 K. MATYJASZEWSKI, P. J. MILLER, E. FOSSUM AND Y. NAKAGAWA Scheme 1 Synthesis of block copolymers of PMPS with styrene (top) and isoprene (bottom) by anionic ring-opening polymerization. ionic mechanisms.2 For example, the living anionic styrenes,5–8 acrylates,4 methacrylates3,9,10 and ac- polymerizations of styrene and methyl methacry- rylonitrile.11 late are quite common, resulting in preservation of In this paper we discuss recent advances made in polymer functionality. However, like the inorganic our laboratory to produce novel copolymers and analogues, polymerization by ionic mechanisms is architectures based on the union between inorganic limited to a rather narrow class of monomers under and organic macromolecular species. As the paper conditions of the most stringent purity. Therefore, shows, the two primary synthetic methods are the desire to develop a controlled free radical living anionic polymerization and ATRP. polymerization system has driven research in the area for the last decade. One result of this has been atom transfer radical polymerization (ATRP).3,4 When an activated alkyl halide is stirred with a DISCUSSION vinyl monomer in the presence of a copper catalyst, well-defined polymers are obtained with predeter- Block copolymers of polysilanes mined functionality and molecular weight, making with vinyl-based polymers them ideal for the synthesis of a variety of The most common method of producing poly- copolymers. The key to control is a rapid silanes, the Wurtz reductive coupling of dichlor- equilibrium between active and dormant propagat- osilanes with alkali metals,12 does not provide the ing species. Maintenance of a low steady-state control over the polymerization necessary for concentration of radicals ensures that termination functionalization of the chain-end toward block reactions are limited to nearly insignificant values copolymers. Using this mode of synthesis, sub- until very high monomer conversions are attained. stitution reactions on the phenyl rings of poly So far, ATRP has been demonstrated to provide (methylphenylsilylene) (PMPS) has resulted in graft controlled polymerizations of monomers such as copolymers with poly(tetrahydrofuran) and poly # 1998 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 12, 667–673 (1998) POLYMER SYNTHESIS FROM INORGANIC MACROINITIATORS 669 Table 1 Molecular-weight data for block copolymers containing polysilanes and vinyl-based carbon monomers a a Entry Type A Mn,A Mw/Mn B Mn,tot Mw/Mn b 1 AB Styrene 2400 1.06 (MePhSi)4 13,900 1.34 2 AB Styrene 4700 1.05 (MePhSi)4 9700 1.31 3 AB Styrene 5400 1.05 (MePhSi)4 26,400 1.24 4 AB Isoprene 3680 1.13 (MePhSi)4 13,360 1.31 a SEC relative to polystyrene standards. b 1,2,3,4-Tetramethyl-1,2,3,4-tetraphenylcyclotetrasilane. (methyl methacrylate).13 To date the best method of exists as a tight ion pair and, as a result, does not controlled polymerization of polysilanes has been possess sufficient nucleophilicity to open subse- the anionic ring-opening polymerization of 1,2,3,4- quent silicon rings. The presence of 12-crown-4 tetramethyl-1,2,3,4-tetraphenylcyclotetrasilane.14 generates a loose ion pair and a more reactive silyl Using this technique, the synthesis of block anion which facilitates ring-opening polymeriza- copolymers of PMPS with organic-based polymers tion of the remaining cyclotetrasilanes. In both of is illustrated in Scheme 1 for styrene and iso- the polymerizations, addition of the cyclotetrasi- prene.15 In the polymerizations the organic block is lane and crown ether to a solution of the organic polymerized first, since those processes obey polymer resulted in a clean shift of the size- ‘living’ characteristics. No transformation of the exclusion chromatography (SEC) trace to higher- active site is necessary in the reactions since both molecular-weight with only a small tail toward polystyryl-lithium and polyisoprenyl-lithium will lower-molecular-weight species. This indicates that initiate ring opening of the cyclotetrasilane. How- the efficiency of initiation of the first block toward ever, in benzene the resulting silyl-lithium species the second is quite high, e.g. greater than 70% in the Scheme 2 General method of synthesis of PDMS block copolymers using ATRP. # 1998 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 12, 667–673 (1998) 670 K. MATYJASZEWSKI, P. J. MILLER, E. FOSSUM AND Y. NAKAGAWA Table 2 Molecular-weight data for block and graft copolymers originating from poly(dimethylsiloxane) macroinitiators a a Entry Type A Mn,A Mw/Mn B Mn,tot Mw/Mn 1 BAB PDMS 4485 1.32 Styrene 17 020 1.33 2 BAB PDMS 9800 2.29 Styrene 20 700 1.61 3 BAB PDMS 4485 1.32 Isobornyl acrylate 13 610 1.64 4 A-g-Bb PDMS 6600 1.76 Styrene 14 800 2.10 a SEC relative to polystyrene standards. b Graft copolymer. case of polystyrene. Examples of block copolymers demonstrated that block copolymers, rather than a of polystyrene and polyisoprene with PMPS are mixture of homopolymers, had been produced. given in Table 1. The increase in polydispersity of The solid-state morphology of diblock copoly- the block copolymer relative to the macroinitiator mers of polystyrene and PMPS was probed by reflects the presence of residual homopolymer transmission electron microscopy (TEM) and which did not participate in block copolymeriza- scanning force microscopy (SFM).16 Microphase tion, as well as the non-living nature of the ring- separation
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