Synthesis and characterization of thermo- and pH-responsive double-hydrophilic diblock copolypeptides

Synthesis of novel double-hydrophilic diblock copolypeptides (BCPs), poly(L-glutamic acid)-block-poly(N-isopropylacrylamide) (PLG(n)P(m)), and their thermoresponsive properties in aqueous solutions at different pH values are described. The diblock copolypeptides were synthesized by a combination of ring-opening polymerization (ROP) of gamma-benzyl-L-glutamate N-carboxyanhydrides (BLG-NCA) and reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NiPAM). A new class of RAFT agents (CTA-2 and CTA-3) with amino-functional groups was designed for this purpose. Two different strategies, i.e., macrochain transfer agent (CTA) and macroinitiator routes, were utilized and compared on the control of the chemical structures of the resulting BCPs. Their block ratios and lengths are broadly varied (n = 21-600 and m = 180-442). Their thermally switchable aggregation behaviors in aqueous solutions were investigated at the microscopic level by H-1 NMR spectroscopy and at the macroscopic level by turbidity measurements using UV/vis spectroscopy. The latter was also utilized for their lower critical aggregation temperature (LCAT) determination. The effects of block lengths and ratios as well as solution pH values on the collapse of NiPAM chain and aggregation process of BCPs were examined. This aggregation process was also followed by dynamic light scattering (DLS) measurements, and the thermally induced aggregate structures were investigated by transmission electron microscopy (TEM).