Design and synthesis of thermoresponsive aliphatic polyethers with a tunable phase transition temperature

Abstract

This paper describes a comprehensive study of the lower critical solution temperature (LCST)-type thermoresponsive properties of various poly(glycidyl ether) homopolymers with a varying side chain structure, molecular weight, and main chain tacticity, as well as their copolymers with a varying monomer composition and monomer sequence. For the initial screening, we prepared nine kinds of poly(glycidyl ether)s by the phosphazene base-catalyzed ring-opening polymerization of glycidyl methyl ether (MeGE), ethyl glycidyl ether (EtGE), glycidyl isopropyl ether (iPrGE), 2-methoxyethyl glycidyl ether (MeEOGE), 2-ethoxyethyl glycidyl ether (EtEOGE), 2-propoxyethyl glycidyl ether (PrEOGE), 2-(2-methoxyethoxy)ethyl glycidyl ether (MeEO₂GE), 2-(2-ethoxyethyl)ethyl glycidyl ether (EtEO₂GE), and 2-(2-(2-methoxyethoxy)ethoxy)ethyl glycidyl ether (MeEO₃GE). Among them, poly(MeGE), poly(EtGE), poly(MeEOGE), poly(EtEOGE), and poly(MeEO₂GE) (M_n = ca. 5000 g mol⁻¹) were found to exhibit a LCST-type phase transition in water at 65.5 °C, 10.3 °C, 91.6 °C, 41.3 °C, and 58.2 °C, respectively. Although the molecular weight and main chain tacticity had little impact on the phase transition temperature, the side chain structure, i.e., the number of oxyethylene units and terminal alkyl groups, significantly affected the transition temperature. The statistical copolymers composed of MeEOGE and EtEOGE revealed that the transition temperature of the polymer can be desirably customized in between those of the homopolymers by varying the monomer composition. On the other hand, we found that the block copolymer composed of MeEOGE and EtEOGE exhibited a complex thermoresponsive behavior due to its ability to form a micellar aggregate.