Dual stimuli-responsive copolymers with precisely arranged degradable units: synthesis by controlled alternating copolymerization of oxyethylene-containing vinyl ethers and conjugated aldehydes†
Thermoresponsive alternating, block, and gradient-like copolymers with acid-degradability were synthesized via controlled cationic copolymerization of conjugated aldehydes and vinyl ethers (VEs). The thermoresponsive behavior of the resultant copolymers in water was governed by the polymer sequence. For example, an alternating copolymer was soluble in water at low temperature and displayed a sharp one-step phase separation on heating. By contrast, a block copolymer consisting of a degradable alternating sequence and a VE homosequence exhibited two-step phase separation in water. The alternating copolymers contained acid-labile acetal linkages in the main chain at periodic intervals and thus decomposed rapidly into small molecules without residual oligomers under acidic conditions. The block and gradient-like copolymers were hydrolyzed into VE polymers and oligomers, respectively, due to the presence of non-degradable VE–VE linkages. The thermoresponsive patterns of the hydrolysis products differed from those of the original polymers.