Synthesis and characterization of waterborne pyrrolidone-functional diblock copolymer nanoparticles prepared via surfactant-free RAFT emulsion polymerization

Polymerization-induced self-assembly enables the facile synthesis of a wide range of block copolymer nano-objects in the form of concentrated dispersions. In this context, many surfactant-free reversible addition–fragmentation chain transfer (RAFT) aqueous emulsion polymerization formulations have been reported using various nonionic and polyelectrolytic water-soluble precursors for the steric stabilizer block. In the present study, we examine poly(2-(N-acryloyloxy)ethyl pyrrolidone) (PNAEP) as a new nonionic stabilizer block. A trithiocarbonate-based PNAEP precursor with a mean degree of polymerization of 67 was employed as the steric stabilizer for the RAFT emulsion polymerization of styrene, n-butyl acrylate (nBA), or statistical mixtures thereof. The RAFT emulsion polymerization of styrene using a VA-044 azo initiator at 80 °C and pH 7 led to essentially full conversion within 40 min, with induction times as short as 10 min, whereas gel permeation chromatography analysis confirmed efficient chain extension and relatively low dispersities (Mw/Mn < 1.30). Dynamic light scattering (DLS) studies indicated that systematically increasing the target degree of polymerization (DP) from 100 to 700 enabled the z-average diameter of the resulting kinetically trapped spherical nanoparticles to be varied from 55 to 156 nm. The same PNAEP67 precursor was then employed for the RAFT emulsion polymerization of nBA at 30 °C using a low-temperature redox initiator at pH 3. More than 99% conversion was achieved within 25 min, and efficient chain extension was observed up to a target DP of 700. However, relatively broad molecular weight distributions (Mw/Mn = 1.38–1.64) were obtained, presumably owing to side reactions such as chain transfer to polymer. DLS studies indicated that a series of kinetically-trapped PNAEP67–PnBAx spheres (where x = 100–700) exhibited z-average diameters ranging from 45 to 141 nm. Attempts to use this low-temperature initiator protocol for the homopolymerization of styrene led to essentially no conversion after 48 h at 30 °C. However, the statistical copolymerization of 45% styrene with 55% nBA could be achieved using this low-temperature redox initiator at 30 °C using the same PNAEP67 precursor. In this case, 1H nuclear magnetic resonance studies indicated a significantly longer induction period (95 min) compared to either homopolymerization. Nevertheless, once the statistical copolymerization commenced, essentially full conversion of both comonomers could be achieved within 45 min. Differential scanning calorimetry analysis indicated that these statistical copolymers exhibited intermediate glass transition temperatures compared to the two respective homopolymers. The film formation behavior of selected diblock copolymer nanoparticles was briefly explored