Dual thermoresponsive homopolypeptide with LCST-type linkages and UCST-type pendants: Synthesis, characterization, and thermoresponsive properties

Abstract

Water-soluble homopolypeptides bearing di(ethylene glycol) (DEG) linkages and various ionic liquid (IL) pendants with both lower and upper critical solution temperature (LCST and UCST) or a UCST were prepared and systematically investigated. Poly (γ-2-(2-chloroethoxy)ethyl l-glutamate) (PCEELG) with DEG linkage and reactive chloro groups can be readily prepared from ring-opening polymerization of γ-2-(2-chloroethoxy)ethyl l-glutamic acid based N-carboxyanhydride. Homopolypeptides were conjugated with various IL pendants and counteranions by nucleophilic substitutions and subsequent ion-exchange reactions. Homopolypeptide with DEG linkages and tributylphosphonium iodide (PEELG-TBP-I) showed interesting LCST < UCST (i.e., soluble-insoluble-soluble) dual thermoresponsive behavior in DI-H2O and NaI aqueous solutions. The LCST-type and UCST-type phase transition temperatures (Tpts) were tunable by polymer concentration, molar mass, and NaI concentration. Homopolypeptide with DEG linkages and tributylphosphonium tetrafluoroborate showed a UCST both in DI-H2O and PBS buffer while homopolypeptide with DEG linkages and triphenylphosphonium iodide (or tetrafluoroborate) or with alkyl linkages and tributylphosphonium iodide (or tetrafluoroborate) showed a UCST in PBS buffer. The UCST-type Tpt in PBS buffer was in a broad temperature range of 20–90 °C which was tunable by adjusting the polymer concentration, molar mass, IL moieties, and counteranions.