Ring-expansion living cationic polymerization of vinyl ethers: Optimized ring propagation

Abstract

For the "ring-expansion" living cationic polymerization of vinyl ethers that has recently been achieved for the first time with a hemiacetal ester-embedded cyclic initiator (1), we investigated the effects of reaction conditions (Lewis acid catalysts/activators, solvents, temperature, and reagent concentration) on the selectivity and controllability for construction of ring chains. For example, the choice of the Lewis acid catalysts turned out crucial. Specifically, tin tetrabrommide (SnBr4) was suitable, with which an undisturbed ring-expansion propagation proceeded without irreversible side reactions, to selectively construct the ring topology, though accompanied by "ring fusion" into fused ring polymers of higher molelcular weights. The fusion event, however, could be suppressed by decreasing concentration of the initiator, namely, polymers to be formed therefrom. Under these optimized conditions, the propagation is controlled enough to allow sequential addition of monomers to give either chain-extended homopolymers or block copolymers. Thus, the ring-expansion cationic polymerization is potentially a powerful tool to construct ring polymer architectures with precision control similar to that for linear living polymers.