Spiroorthoesters as promising comonomers for degradable polymer synthesis by cationic copolymerization with vinyl ethers

Abstract

Endeavor to discover new copolymerizable monomers is of great significance for the creation of characteristic copolymers. Spiroorthoesters (SOEs) undergo cationic homopolymerization via the single ring-opening reaction of the cyclic ether ring or the double ring-opening reaction of both the cyclic ether and acetal rings, while copolymerization of SOEs with other monomers has been rarely investigated. In this study, we demonstrate that cationic copolymerization of vinyl ethers (VEs) and SOEs proceeds via very frequent crossover reactions to result in acidically degradable copolymers. 2,2-Dimethyl-1,4,6-trioxaspiro[4.6]undecane, which consists of seven-membered cyclic ether ring and five-membered cyclic acetal ring, underwent copolymerization with isobutyl VE (IBVE) by Ph₃CB(C₆F₅)₄ mostly via the single ring-opening reaction of the seven-membered ring. Homopropagation reactions of the SOE negligibly occurred during copolymerization. The average number of IBVE units per block was also very small (<1.5), indicating that the copolymers had pseudo-alternating sequences. The copolymers were degraded into low-molecular-weight compounds via the cleavage of acetal moieties that were generated by the crossover reactions from IBVE to the SOE. In addition, copolymerization of other VEs and SOEs was examined with a focus on the effects of monomer reactivities on the frequency of crossover reactions. Thermoresponsive, water-soluble copolymers with acid degradability were also synthesized from SOEs and 2-methoxyethyl VE.