Facile Synthesis And Thermoresponsive Behaviors Of A Well-Defined Pyrrolidone Based Hydrophilic Polymer

Poly(N-vinyl pyrrolidone) (PVP) is a water-soluble polymer that has excellent biocompatibility, strong coordination ability, etc. It has been widely applied in various fields. However, pyrrolidone-based polymers are quite limited. only PVP polymer with narrow molecular weight distribution was well synthesized. Thermoresponsive behaviors of water-soluble polymers were extensively investigated in recent years. Clearly, developing a new type of pyrrolidone-based thermoresponsive polymers is desirable from academic and industrial standpoints. This will enhance the properties of pyrrolidone-based polymer for more fantastic and sophisticated applications.A well-defined pyrrolidone based thermoresponsive polymer, poly[N-(2- methacryl oyloxyethyl) pyrrolidone] (PNMP) was synthesized via reversible addition-fragmentation chain transfer radical polymerization or RAFT polymerization of N-(2-methacryloyloxyethyl) pyrrolidone monomer in methanol under a mild visible light radiation at 30 oC. The average molecular weights and polydispersity indices of PNMP polymers were characterized by gel permeation chromatography (GPC) and static light scattering analysis. The kinetic studies indicated that this RAFT polymerization exhibited a well-controlled behavior. The living character of this RAFT polymerization was confirmed by the facile synthesis of a series of well-defined PNMP-based block copolymers via RAFT polymerization under this mild visible light radiation at 30 oC, using an above-synthesized PNMP polymer as a macromolecular chain transfer agent. Temperature-variation 1H NMR unambiguously revealed that the PNMP polymer with weight-average molecular weight (Mw) of 105.4 kg mol-1 and polydispersity index (Mw/Mn) of 1.11 was molecularly dissolved in D2O at ambient temperature, e.g. 22 oC; upon elevating solution temperature, the dehydration process of this fully-hydrated PNMP polymer was triggered at 46.1 oC, leading to a dramatic decrease of integral ratios of proton resonance signals of PNMP to that of D2O; further elevating the solution temperature to 51.9 oC led to a sharp phase separation of PNMP polymer from aqueous solution. Laser light scattering analyses demonstrated that the cloud point of PNMP polymer decreased with molecular weight in Mw range of 20.6-105.4 kg mol-1. Moreover, this PNMP polymer exhibited a remarkably reversible thermoresponsive dehydration/hydration and phase transition behaviors in aqueous solution. Unlike what observed in PNIPAM aqueous solution, no hysteresis phenomenon was observed in PNMP aqueous solution during one heating-and-cooling cycle. Similar to what observed in PNIPAM aqueous solution, the cloud point temperature of PNMP aqueous solution were lowered by the addition of salt,e.g, Na2CO3, NaHCO3 and NaCl, and decreased linearly with salt concentration. However, theaddition of NaI tended to elevate the cloud point temperature of PNMP aqueous solution. At relatively high concentrations of NaI, the PNMP polymer was completely dissolved in water. No phase transition could be detected on elevating the solution temperature. The cloud point temperature of PNMP(20 mg mL-1, Mw = 105.4 kg mol-1, Mw/Mn = 1.11) in D2O was about 8.2 oC lower than that in H2O. This demonstrated that the hydrophobic interactions were the primary driving forces for the thermoresponsive behaviors.