Tryptophan-based styryl homopolymer and polyzwitterions with solvent-induced UCST, ion-induced LCST and pH-induced UCST

Abstract

A new multi-stimuli responsive tryptophan-based styryl homopolymer exhibiting dual upper critical solution temperature (UCST)- and lower critical solution temperature (LCST)-type thermosensitivities under different conditions is presented. The monomer 4-vinylbenzyl N-[(tert-butoxy)carbonyl]-L-tryptophan (Boc-TrpVBz) is prepared by a simple substitution reaction followed by its subsequent RAFT polymerization to produce the poly(Boc-TrpVBz) homopolymer. Poly(Boc-TrpVBz) is not soluble in water, but is soluble in different organic solvents (such as DMSO, DMF and MeOH). An interesting upper critical solution temperature (UCST)-type turbid-to-transparent phase transition is detected for poly(Boc-TrpVBz) in mixed solvents containing any of these three solvents and a minimum percentage of water. The UCST-type cloud point is found to increase upon increasing the percentage of added water and the molecular weight of poly(Boc-TrpVBz). Furthermore, poly(Boc-TrpVBz) is soluble in water at above pH 7.5 and exhibits a lower critical solution temperature (LCST)-type transparent-to-turbid transition upon addition of tetrabutylammonium halide salts with a large organic cation. The LCST-type cloud point is variable with respect to the salt concentration and the nature of the halide ion. The deprotection of the Boc moiety of poly(Boc-TrpVBz) produces zwitterionic poly(TrpVBz) containing –NH₂ and –COOH groups, which exhibits dual pH- and thermo-responsiveness in aqueous solution below pH 2. In the cases of UCST- and LCST-type transitions, the turbidity is caused by the formation of spherical polymer aggregates as observed via TEM and FESEM.