RAFT Aqueous Dispersion Polymerization of N-(2-(Methacryloyloxy)ethyl)pyrrolidone: A Convenient Low Viscosity Route to High Molecular Weight Water-Soluble Copolymers

摘要

RAFT solution polymerization of N-(2-(methacryoyloxy)ethyl)pyrrolidone (NMEP) in ethanol at 70 °C was conducted to produce a series of PNMEP homopolymers with mean degrees of polymerization (DP) varying from 31 to 467. Turbidimetry was used to assess their inverse temperature solubility behavior in dilute aqueous solution, with an LCST of approximately 55 °C being observed in the high molecular weight limit. Then a poly(glycerol monomethacylate) (PGMA) macro-CTA with a mean DP of 63 was chain-extended with NMEP using a RAFT aqueous dispersion polymerization formulation at 70 °C. The target PNMEP DP was systematically varied from 100 up to 6000 to generate a series of PGMA63–PNMEPx diblock copolymers. High conversions (≥92%) could be achieved when targeting up to x = 5000. GPC analysis confirmed high blocking efficiencies and a linear evolution in Mn with increasing PNMEP DP. A gradual increase in Mw/Mn was also observed when targeting higher DPs. However, this problem could be minimized (Mw/Mn < 1.50) by utilizing a higher purity grade of NMEP (98% vs 96%). This suggests that the broader molecular weight distributions observed at higher DPs are simply the result of a dimethacrylateimpurity causing light branching, rather than an intrinsic side reactionsuch as chain transfer to polymer. Kinetic studies confirmed thatthe RAFT aqueous dispersion polymerization of NMEP was approximatelyfour times faster than the RAFT solution polymerization of NMEP in ethanolwhen targeting the same DP in each case. This is perhaps surprisingbecause both ¹H NMR and SAXS studies indicate that thecore-forming PNMEP chains remain relatively solvated at 70 °Cin the latter formulation. Moreover, dissolution of the initial PGMA63–PNMEPx particles occurson cooling from 70 to 20 °C as the PNMEP block passes throughits LCST. Hence this RAFT aqueous dispersion polymerization formulationoffers an efficient route to a high molecular weight water-solublepolymer in a rather convenient low-viscosity form. Finally, the relativelyexpensive PGMA macro-CTA was replaced with a poly(methacrylic acid)(PMAA) macro-CTA. High conversions were also achieved for PMAA85–PNMEPx diblock copolymersprepared via RAFT aqueous dispersion polymerization for x ≤ 4000. Again, better control was achieved when using the98% purity NMEP monomer in such syntheses.