Reversible Addition Fragmentation Chain Transfer Polymerization of Methyl Methacrylate in the Presence of Lewis Acids: An Approach to Stereocontrolled Living Radical Polymerization

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• 作者：Yen K. Chong ; Graeme Moad ; Ezio Rizzardo ; Melissa A. Skidmore ; San H. Thang
• 刊名：Macromolecules
• 出版年：2007
• 出版时间：December 25, 2007
• 年：2007
• 卷：40
• 期：26
• 页码：9262 - 9271
• 全文大小：185K
• 年卷期：v.40,no.26(December 25, 2007)
• ISSN：1520-5835

文摘

Reversible addition fragmentation chain transfer (RAFT) polymerization of methyl methacrylate(MMA) can be carried out in the presence of cyanoisopropyl methyl trithiocarbonate [CH₃SC(=S)SC(CH₃)₂CN]and scandium triflate [Sc(OTf)₃] to provide simultaneous RAFT-controlled (reversible addition-fragmentationchain transfer controlled) molecular weight and molecular weight distribution (_w/_n < 1.3 at >95% conversion),Lewis acid-controlled tacticity (mm:mr:rr = 12:44:44 for a mole ratio Sc(OTf)₃/MMA = 1:8.5 at 60 C) and anenhanced rate of polymerization. The tacticities observed for poly(methyl methacrylate) (PMMA) are similar tothose observed with scandium triflate in the absence of the RAFT agent. The stability of dithiobenzoate RAFTagents [ZC(=S)SR] in the presence of scandium triflate is strongly dependent on both the Z and R substituent.2-(ethoxycarbonyl)prop-2-yl dithiobenzoate [PhC(=S)SC(CH₃)₂CO₂CH₃] and the polymeric RAFT agent [PhC(=S)S-(CH₃)(CO₂CH₃)CH₂-PMMA] appear relatively stable. However, cumyl [PhC(=S)SC(CH₃)₂Ph] andcyanoisopropyl dithiobenzoate [PhC(=S)SC(CH₃)₂CN] are rapidly degraded and are therefore not appropriatefor direct use in this application. These RAFT agents may, however, be used to synthesize stereoblock copolymerswhere the RAFT agent is first transformed to an oligomeric or polymeric RAFT agent in the absence of a Lewisacid. Cyanoisopropyl 1H-pyrrole-1-carbodithioate [(C₄H₄N)C(=S)SC(CH₃)₂CN] is stable to Lewis acid-induceddecomposition but slowly homolyzes at 60 C (even in the absence of the Lewis acid) to initiate the RAFTpolymerization of MMA.