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

Abstract

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 dimethacrylate impurity causing light branching, rather than an intrinsic side reaction such as chain transfer to polymer. Kinetic studies confirmed that the RAFT aqueous dispersion polymerization of NMEP was approximately four times faster than the RAFT solution polymerization of NMEP in ethanol when targeting the same DP in each case. This is perhaps surprising because both 1H NMR and SAXS studies indicate that the core-forming PNMEP chains remain relatively solvated at 70 °C in the latter formulation. Moreover, dissolution of the initial PGMA63–PNMEPx particles occurs on cooling from 70 to 20 °C as the PNMEP block passes through its LCST. Hence this RAFT aqueous dispersion polymerization formulation offers an efficient route to a high molecular weight water-soluble polymer in a rather convenient low-viscosity form. Finally, the relatively expensive PGMA macro-CTA was replaced with a poly(methacrylic acid) (PMAA) macro-CTA. High conversions were also achieved for PMAA85–PNMEPx diblock copolymers prepared via RAFT aqueous dispersion polymerization for x ≤ 4000. Again, better control was achieved when using the 98% purity NMEP monomer in such syntheses.

End-group ionisation enables the use of poly(N-(2-methacryloyloxy)ethyl pyrrolidone) as an electrosteric stabiliser block for polymerisation-induced self-assembly in aqueous media

“…Interestingly, higher CP values were observed for increasing PNMEP DP, contrary to normal behaviour. 15,49,50 However, Miasnikova et al observed a similar trend when examining endgroup effects on the LCST behaviour of methoxy-capped poly(diethylene glycol acrylate) (PDEGA) homopolymers. 55 More specifically, a series of PDEGA homopolymers were synthesised using a bistrithiocarbonate RAFT agent that conferred tert-butyl benzoate groups on both chain-ends.…”

“…a systematic reduction in cloud point with increasing DP) was observed by Cunningham and co-workers for a series of PNMEP hompolymers prepared using a non-ionic dithiobenzoate endgroup, which suggests that end-groups play an important role in determining cloud point. 15 At pH 7, thermal transitions were relatively broad for shorter PNMEPs (DP < 53), whereas homopolymers with higher DPs exhibited relatively sharp CP values. Similar effects were also observed by Miasnikova et al, 55 who suggested that dispersity effects (expected to be stronger for lower molecular weights) might lead to broader transitions for lower DPs.…”

“…17 Turbidimetry studies confirm that PNMEP exhibits inverse temperature solubility in aqueous solution. 15,26 Such behaviour has been used to prepare sterically-stabilised block copolymer nanoparticles with weakly hydrophobic PNMEP cores in water at 70 °C that subsequently dissolve to afford molecularlydissolved copolymer chains on cooling to ambient temperature. This strategy has enabled the synthesis of relatively high molecular weight PNMEP via RAFT aqueous dispersion polymerisation.…”

“…This strategy has enabled the synthesis of relatively high molecular weight PNMEP via RAFT aqueous dispersion polymerisation. 15 However, it also suggests that PNMEP does not have sufficient hydrophilic character to be used as a steric stabiliser block for PISA formulations conducted in aqueous media. In the present study, we show that this apparent limitation can be overcome by the judicious use of an appropriate RAFT agent.…”

New Insights into RAFT Dispersion Polymerization‐Induced Self‐Assembly: From Monomer Library, Morphological Control, and Stability to Driving Forces

“…In addition, N ‐(2‐(methacryloyloxy)ethyl)pyrrolidone (NMEP), another monomer to afford thermoresponsive polymer with LCST (≈55 °C) behavior, has been studied by Armes group in aqueous dispersion polymerization to target highly asymmetric BCPs with high molar masses. High‐molar‐mass PNMEPs were prepared in the form of colloidal dispersion at relatively low viscosities at 70 °C.…”