Cationic Polymerization

Abstract

Cationic polymerization has a long history and various types of monomers have been examined. However, the inherently unstable propagating species causing a variety of side reactions had made it difficult to achieve controlled and/or living reactions in cationic polymerization, retarding the discovery of living cationic polymerization. The first part of this review focuses such historical and basic information about cationic polymerization and living cationic polymerization reactions including the HI/I₂ system and the strong Lewis acid systems by using additives, weak Lewis bases and salts. In the second part, recent extensive researches on living cationic polymerization are demonstrated. For example, Lewis acid catalysts with a weak base led to unprecedented living reaction systems, such as a wide range of available choices of metal halide catalysts, fast living polymerization within 1－2 sec, heterogeneous living polymerization with Fe₂O₃. The last part summarizes new classes of monomers and reactions: new naturally occurring monomers available, novel controlled polymerization systems including the RAFT cationic polymerization, the alternating copolymerization of aldehydes and vinyl ethers, the monomer-selective copolymerization, and the concurrent cationic vinyl-addition and ring-opening copolymerization of vinyl ethers and oxiranes. Especially, the quantitative synthesis of star-shaped polymers with a narrow MWD by the base-assisting living cationic polymerization was described in detail.