Reversible addition fragmentation chain transfer polymerization of methyl methacrylate in the presence of lewis acids: An approach to stereocontrolled- living radical polymerization

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-fragmentation chain transfer controlled) molecular weight and molecular weight distribution (M̄_w/M̄_nn < 1.3 at > 95% conversion), Lewis acid-controlled tacticity (mnv.mr.rr = 12:44:44 for a mole ratio Sc(OTf)₃/MMA = 1:8.5 at 60 °C) and an enhanced rate of polymerization. The tacticities observed for poly(methyl methacrylate) (PMMA) are similar to those observed with scandium triflate in the absence of the RAFT agent. The stability of dithiobenzoate RAFT agents [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] and cyanoisopropyl dithiobenzoate [PhC(-S)SC(CH ₃)₂CN] are rapidly degraded and are therefore not appropriate for direct use in this application. These RAFT agents may, however, be used to synthesize stereoblock copolymers where the RAFT agent is first transformed to an oligomeric or polymeric RAFT agent in the absence of a Lewis acid. Cyanoisopropyl ] H-pyrrole-l-carbodithioate [(C₄H ₄N)C(=S)SC(CH₃)₂CN] is stable to Lewis acid-induced decomposition but slowly homolyzes at 60 °C (even in the absence of the Lewis acid) to initiate the RAFT polymerization of MMA.