End-Group Functionalization And Influence On Poly(N-isopropylacrylamide)

Poly(N-isopropylacrylamide) (PNIPAM) is one of the most studied temperature responsive polymer that exhibit a coil-globule transition in aqueous solution at the lower critical solution temperature (LCST) of 32℃, which is near human body temperature. As such, it has become a promising main candidate for intelligent and advanced polymeric materials. However, PNIPAM modified by various functional groups is not always consistent with that of pure PNIPAM. End group is particularly a factor that drastically affects the thermal response of PNIPAM among all influences, because the phase transition is initiated at the end of a polymer chain due to its mobility. The studies of end group effects on the thermal response of narrow-disperse PNIPAM suggested specific affection of end groups on LCST, revealing that many potential applications were correlated with the modification of these polymers in tuning their thermal responsibility.In this paper, end group effects on the thermal response of narrow-disperse PNIPAM prepared by the combination of RAFT polymerization and click reaction was studied. Firstly, bifunctionalized chain transfer agent (CTA), EMPBA, was synthesized by coupling of 3,5-bis(propargyloxyl)benzoic acid with 2-hydroxyethyl 2-(ethylthiocarbonothioylthio)-2-methylpropanoate through Steglich esterification. Secondly, PNIPAM with bialkyne end group was prepared via RAFT polymerization of NIPAM monomer at 60℃, using EMPBA as a CTA and azobisisobutyronitrile (AIBN) as an initiator. Kinetic studies demonstrated the controlled characteristics of this polymerization process. MALDI-TOF MS analysis evidenced the living character and stability of polymer structure. The living character of the polymer was further confirmed by the facile synthesis of well-defined PNIPAM-PDMA block copolymers via RAFT polymerization, using an above-synthesized PNIPAM polymer as a macromolecular chain transfer agent.End group effects on the thermal response of narrow-disperse PNIPAM was studied. Firstly, click reaction between azido derivatives and PNIPAM were carried out to prepare bialkyne, biamino, bibenzyl, bihydroxyl and binoctyl linked PNIPAM with the same polymer chain. The phase transition of PNIPAM aqueous solution (0.5 mg/mL) was studied by dynamic light scattering (DLS) measurements. Results turned out that LCST was lowered or raised dependent on the introduced end groups. At the same time, obvious deviations were noted with lower molecular weight samples. Bialkyne and bioctyl linked PNIPAM with different molecular weight were then studied to determine the effect of molecular weight and end group. Results show that LCST of PNIPAM is determined by molecular weight and end group.