Asymmetric Cationic Polymerization of Benzofuran through a Reversible Chain-Transfer Mechanism: Optically Active Polybenzofuran with Controlled Molecular Weights.

Benzofuran (BzF) is a prochiral, 1,2-disubstituted, unsymmetric cyclic olefin that can afford optically active polymers by asymmetric polymerization, unlike common acyclic vinyl monomers. Although asymmetric cationic polymerization of BzF was reported by Natta et al. in the 1960s, the polymer structure has not been clarified, and there are no reports on molecular weight control. Herein, we report dual control of the optical activity and molecular weight of poly(BzF) using thioether-based reversible chain-transfer agents for asymmetric cationic polymerization with β-amino acid derivatives as chiral additives and aluminum chloride as a catalyst. This asymmetric moderately living cationic polymerization leads to an increase in molecular weight and specific optical rotation with monomer conversion. In addition, asymmetric block polymers consisting of opposite absolute configurational segments were synthesized using both enantiomers sequentially as chiral additives. Finally, a comprehensive analysis of the polymerization products and the model reaction revealed that the optical activity of poly(BzF) originates from the threo-diisotactic structure, which occurs by regio-, trans-, and enantioselective propagation.