Cyclic thioacetal carbonates for dual-stimuli degradable poly(vinyl ether)s with cleavable thioacetal and carbonate bonds evenly distributed in the main chains by cationic degenerative chain-transfer copolymerization

Abstract

We report the synthesis of dual-stimuli degradable poly(vinyl ether)s with cleavable thioacetal and carbonate bonds evenly distributed in the main chains using cationic degenerative chain-transfer (DT) copolymerization of vinyl ethers with macrocyclic thioacetal carbonates (CTAC). The 22- and 26-membered cyclic thioacetal carbonates (22-CTAC and 26-CTAC) were initially synthesized by a cationic thiol-ene reaction between divinyl ether with a carbonate bond and dithiol with or without a carbonate bond under dilution conditions. These compounds were subjected to cationic copolymerization with vinyl ethers using the HCl-adduct of isobutyl vinyl ether as an initiator and ZnCl2 as a catalyst and were consumed much faster than the vinyl ethers by ring-opening reactions despite the large ring to introduce thioacetal and carbonate bonds in the main chains of the products. The in-chain thioacetal bonds subsequently served as dormant bonds for the cationic DT polymerization of vinyl ethers and enabled the synthesis of poly(vinyl ether)s with controlled total and segmental molecular weights between the thioacetal and carbonate bonds. The orthogonal degradations were successful when acid and base catalysts were used for the thioacetal and carbonate bonds, respectively; this resulted in low-molecular-weight products with controlled molecular weights. Furthermore, multiblock copolymers were synthesized by the one-time addition of the second monomer (B) to the cationic DT polymerization of the first monomer (A) and 22-CTAC and were selectively degraded into ABA and BAB triblock copolymers with acid and base catalysts, respectively.