Degradable Silyl Ether Polymers Synthesized by Sequence-Controlled Cationic Terpolymerization of 1,3-Dioxa-2-silacycloalkanes with Vinyl Ethers and Aldehydes

Polymers with silyl ether moieties have been gaining considerable interest due to both the properties derived from silicon–oxygen linkages and the acid-, base-, and fluoride ion-triggered degradability. In this study, 1,3-dioxa-2-silacycloalkanes were demonstrated to function as promising monomers for the synthesis of silyl ether polymers when combined with vinyl ethers and aldehydes in cationic terpolymerization with B(C6F5)3 as a catalyst. Importantly, all three monomers were indispensable for efficient terpolymerization via frequent crossover reactions. For example, a degradable terpolymer with an Mn of 29 × 103 was obtained from a six-membered 1,3-dioxa-2-silacycloalkane, 2-chloroethyl vinyl ether, and pivalaldehyde, which was superior to both the silyl monomer homopolymerization yielding a product with an Mn of 3.3 × 103 and the inefficient copolymerizations in the absence of the silyl monomer, vinyl ether, or aldehyde. Moreover, the terpolymers likely had ABC-type, pseudo-periodic sequences as a result of frequent and selective crossover reactions. The resulting terpolymers have acetal and silyl ether moieties in the main chain, thus exhibiting acid-, base-, or fluoride ion-triggered degradability. Indeed, the terpolymer with an Mn of 29 × 103 was smoothly degraded into a product with an Mn of 0.3 × 103 under acidic conditions. The key to the terpolymerization reaction was the reaction of an aldehyde with the silyl monomer-derived oxonium ion to generate a carbocation that can react with a vinyl ether because a vinyl ether does not react with the silyl monomer-derived oxonium ion.