Phenyl acrylate is a versatile monomer for the synthesis of acrylic diblock copolymer nano-objects via polymerization-induced self-assembly

Abstract

Over the last decade or so, polymerization-induced self-assembly (PISA) has become widely recognized as a versatile technique for the rational synthesis of diblock copolymer nano-objects in the form of concentrated dispersions. However, there are relatively few examples of acrylic-based PISA formulations in the literature, partly because such copolymers typically possess relatively low glass transition temperatures (T_g) that preclude morphological characterization by transmission electron microscopy. To address this problem, we have selected phenyl acrylate (PhA) as a model monomer to generate the solvophobic block in three PISA formulations using reversible addition–fragmentation chain transfer (RAFT) polymerization. Thus, a poly(dimethyl acrylamide)-based chain transfer agent (CTA) is chain-extended using PhA via RAFT aqueous emulsion polymerization to produce a series of well-defined sterically-stabilized spheres whose mean diameter can be readily adjusted from 38 nm to 188 nm by varying the target degree of polymerization (DP). In contrast, RAFT alcoholic dispersion polymerization of PhA using a poly(acrylic acid) CTA leads to an evolution of copolymer morphology from spheres to worms to lamellae and finally vesicles as the target DP of the structure-directing PPhA block is increased. Similarly, RAFT dispersion polymerization of PhA in n-heptane also produces spheres, worms or vesicles depending on the target DP of the PPhA block. ¹H NMR studies indicate that >98% PhA conversion is achieved in all cases, while GPC analysis indicates high blocking efficiencies. However, relatively broad molecular weight distributions are observed (M_w/M_n = 1.37 to 2.48), which suggests extensive chain transfer to polymer in such PISA syntheses, particularly in the case of the RAFT aqueous emulsion polymerization formulation. Nevertheless, the relatively high T_g of PPhA (50 °C) enables characterization of the various copolymer morphologies using conventional TEM.