Tuning the Temperature Response of Branched Poly(N-isopropylacrylamide) Prepared by RAFT Polymerization

Reversible addition-fragmentation chain transfer (RAFT) polymerization in the presence of a compound capable of both reversible chain transfer through a thiocarbonylthio moiety and propagation via a vinyl group led to highly branched copolymers by a method analogous to self-condensing vinyl copolymerization. An acryloyl trithiocarbonate prepared by copper-catalyzed azide-alkyne cycloaddition was copolymerized with N -isopropylacrylamide (NIPAM) in ratios selected to tune the distribution and length of branches in the resulting thermoresponsive polymers. The degree of branching increased with chain transfer agent (CTA) concentration, as proven by NMR spectroscopy, size exclusion chromatography, and viscometry. Retention of the thiocarbonylthio compound during the polymerization was evidenced by successful chain extension of a branched N -isopropylacrylamide (PNIPAM) macroCTA by RAFT polymerization of N, N -dimethylacrylamide. The branched polymers led to reduced lower critical solution temperatures as compared to linear PNIPAM, an effect attributed primarily to an increased contribution of hydrophobic end groups. End group cleavage by radical-induced reduction resulted in an increased transition temperature more similar to that expected for linear PNIPAM.