Living Cationic Polymerization of Silyl-Protected β-Methyl Vinyl Ethers (Propenyl Ethers): Synthesis of Hydroxy-Functional Polymers with High Tg and LCST-Type Thermoresponse

Hydroxy-functional poly(propenyl ether)s are promising thermoresponsive materials; here we establish a controlled synthesis via living cationic polymerization of silyl-protected monomers. Among the silyl protecting groups examined, only tert-butyldiphenylsilyl (TBDPS) enabled living cationic polymerization. The living cationic polymerization of tert-butyldiphenylsiloxybutyl propenyl ether (TBDPSBPE) afforded a high-molecular-weight polymer (poly(TBDPSBPE)) with a narrow molecular weight distribution (Mn = 12,900; Mw/Mn = 1.22). Additionally, chain propagation continued in monomer addition experiments, and the molecular weight increased further with a narrow molecular weight distribution, confirming the success of living cationic polymerization. Poly(TBDPSBPE) was successfully desilylated to afford poly(HBPE) with a narrow molecular weight distribution. Poly(HBPE) exhibited a glass transition temperature (Tg) of 44 °C, 82 °C higher than that of the corresponding polymer without β-methyl groups, poly(HBVE). The enhanced thermal properties of poly(HBPE) were attributed to the steric hindrance of the β-methyl group, which fixes the position of the hydroxy group and allows stronger hydrogen bonding. To investigate the aqueous thermoresponse, a hydroxylated analog with a shorter side-chain spacer (poly(HPPE)) was synthesized, and poly(HPPE) exhibited lower critical solution temperature (LCST)-type phase separation in water with a cloud-point temperature (Tcp) of 6 °C, showing reversible transitions with thermal hysteresis.