Cyclic thioanhydride/episulfide copolymerizations catalyzed by bipyridine-bisphenolate aluminum/onium pair: approach to structurally and functionally diverse poly(thioester)s

Abstract

Facile synthesis of sulfur-rich polythioesters with high molecular weight via ring-opening copolymerization (ROCOP) of cyclic thioanhydrides and episulfides remains a great challenge because the highly nucleophilic RS⁻ chain end can competitively attack the episulfide and induce undesired transesterification side reactions. Herein, we report efficient ROCOPs of thioanhydrides and episulfides catalyzed by a binary catalyst consisting of a bipyridine bisphenolate aluminum ((BpyBph)Al) complex and an onium salt. The (BpyBph)Al/onium binary catalyst outperformed other common Lewis pairs and (salen)AlCl/onium pairs and could produce structurally and functionally diverse polythioesters with perfect alternating structures, with a rapid polymerization rate and good control over the molecular weight. High-molecular-weight polythioesters were facilely prepared under a high monomer/catalyst feed ratio, which provided opportunities to further investigate their thermal and optical properties. The glass transition temperature (T_g) and refractive index (n) of the polythioester could be modulated by using different thioanhydride/episulfide combinations. In particular, poly(CTAH-alt-PS) derived from carbic thioanhydride (CTAH) and propylene sulfide (PS) showed a T_g of 87.1 °C and an n of 1.61. In addition, poly(CTAH-alt-PS) with norbornene rings was readily cross-linked via a thiol–ene click reaction. The cross-linked poly(CTAH-alt-PS) showed improved T_g (up to 125 °C) and n (1.67), thanks to the suppressed chain mobility and the introduction of extra sulfur atoms and benzene rings. Fast reversible-deactivation chain transfers would occur by combining (BpyBph)Al/onium with thiol (chain transfer agent), affording polythioesters with tailored molecular weight and narrow distribution. This feature allowed for further synthesis of well-defined block polythioesters by the sequential addition method.