In-Situ Direct Mechanistic Transformation from RAFT to Living Cationic Polymerization for (Meth)acrylate−Vinyl Ether Block Copolymers

In-situ direct mechanistic transformation from living radical to cationic polymerization was investigated using a trithiocarbonate-type reversible addition−fragmentation chain-transfer (RAFT) agent and an azo-initiator for RAFT polymerization of (meth)acrylates followed by the addition of a Lewis acid catalyst for the sequential living cationic block polymerization of vinyl ethers. Prior to the mechanistic transformation reaction, the possibility of living cationic polymerization was examined via activation of the thioester bond by Lewis acids, such as SnCl4, EtAlCl2, and ZnCl2. The CH3CH(OiBu)SC(S)SEt/SnCl4-initiating system induced living cationic polymerization of isobutyl vinyl ether (IBVE) in the presence of ethyl acetate (EtOAc) as an additive in toluene at 20 °C to give polymers with controlled molecular weights and narrow molecular weight distributions (MWDs) (Mw/Mn = 1.18). The RAFT copolymerization of methyl acrylate (MA) or methacrylate (MMA) and IBVE was then first carried out using (CH3)2C(CN)SC(S)SEt in the presence of a low-temperature radical initiator [2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile); V-70] in toluene at 20 °C to result in controlled copolymers with a high incorporation of (meth)acrylate units along with unreacted IBVE monomer due to its radically non-homopolymerizable nature. On the addition of SnCl4 in EtOAc to the reaction mixture, a fast and quantitative consumption of the residual IBVE occurred to give unimodal block copolymers with controlled molecular weights (Mw/Mn = 1.3−1.4) consisting of the (meth)acrylate-rich copolymer and vinyl ether homopolymer segments. A more definite acrylate−vinyl ether block copolymer was also obtained by RAFT homopolymerization of MA followed by the addition of IBVE and then SnCl4 in EtOAc after an interval. 1H NMR analysis of the model reaction for the living cationic polymerization revealed that the thioester terminal is reversibly activated in the presence of Lewis acid to form a cationic species, whereas partial substitution of the trithiocarbonate moiety with the chloride in SnCl4 occurred.