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.
Highlights
NVP is categorized as a so-called less-activated monomer (LAM) and, according to the literature, the synthesis of well-defined PNVP homopolymers via reversible addition−fragmentation chain transfer (RAFT) polymerization is somewhat problematic.[7−10] In particular, aqueous formulations suffer from side reactions and hydrolysis that can lead to high dispersities and low blocking efficiencies.[11,12]
This means that the RAFT polymerization of NMEP in aqueous solution at 70 °C using a watersoluble PGMA63 macro-CTA should be an example of an aqueous dispersion polymerization formulation,[39] rather than a solution polymerization
NMEP was polymerized via RAFT solution polymerization in ethanol to obtain a series of PNMEP homopolymers with mean degrees of polymerization varying from 31 to 467
Summary
Poly(N-vinylpyrrolidone) (PNVP) is one of the most interesting and versatile water-soluble polymers; its non-ionic yet highly polar character, strong binding capacity, excellent film-forming ability, and non-toxicity have led to many commercial applications in both pharmaceutical and home and personal care products.[1−4] Well-known examples include the clarification of beer and wine, excipient binders for tablets, and hair spray formulations, as an anti-dye transfer agent in laundry formulations, and as a thickening agent in dental care products.[3,5,6]. In each case the solids content was selected to give the same molar concentration of NMEP as that used for the synthesis of PGMA63−PNMEPx diblock copolymer particles (see below) This enabled a meaningful comparison of any kinetic differences between these solution and dispersion polymerization formulations. Synthesis of PGMA63−PNMEPx Diblock Copolymer Particles via RAFT Aqueous Dispersion Polymerization of NMEP at 70 °C Using a PGMA63 Macro-CTA. A typical protocol for the synthesis of PGMA63− PNMEP480 diblock copolymer nanoparticles was as follows: PGMA63 macro-CTA (0.1008 g), NMEP (96% purity, 0.9573 g, 4.85 mmol; target DP = 500), and ACVA (0.0006 g, 2.14 μmol; macro-CTA/ACVA molar ratio = 4.0) were dissolved in deionized water (3.167 g, 25% w/w) in a 14 mL vial. Alternative diblock copolymer compositions were targeted by adjusting the NMEP/PMAA85 macro-CTA molar ratio
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