Facile synthesis of acrylamide derivative copolymers with side urethane groups for systematic variation of the thermoresponsive behavior

Abstract

A simple method to finely control the thermoresponsive properties of polymers over a wide range of temperatures is to enhance the versatility of the thermoresponsive polymers. One such useful technique is the radical copolymerization of two types of monomers with similar copolymerization reactivities, which allows the hydrophilicity/hydrophobicity balance in the polymer structure to be easily tuned. In this study, we focused on a urethane-containing monomer as the key compound, which can be easily obtained by the reaction between an isocyanate and a hydrophilic precursor monomer containing a hydroxy group. A variety of urethane-embedded acrylamide monomers with different alkyl side chains (ethyl: EtUAAm, n-butyl: BuUAAm, and n-hexyl: HexUAAm) were synthesized from 2-hydroxyethylacrylamide (HEAAm) and alkyl isocyanates. Copolymers of HEAAm and EtUAAm with different compositions were prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization. The obtained copolymer with a high urethane composition (>67%) exhibited a lower critical solution temperature (LCST)-type thermoresponse in water due to the hydrophobic interaction and hydrogen bonding derived from the urethane side groups. The response temperature could be widely varied by altering the composition, molecular weight, end groups and alkyl side chains of the urethane monomer. Simple control of the thermoresponsive properties of polymers in water over a broad range is achieved by using a designed urethane-containing acrylamide monomer in combination with a hydroxy-containing precursor monomer, which forms a statistical sequence due to its similar backbone. The copolymers exhibited a lower critical solution temperature-type responsive behavior in water, and the effects of structural factors such as composition, molecular weight, end groups and side-chain structure in urethane monomers were systematically evaluated.