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Molecular weight dispersity is not typically studied as a design parameter of block copolymer stabilizers but is often assumed to impact stabilization performance; low molecular weight dispersity is generally assumed to be associated with best performance. This is the first quantitative investigation of the effects of block copolymer molecular weight dispersity with regards to stabilization performance in an emulsion polymerization. Poly­(styrene)-b-poly­(acrylic acid) block copolymers were synthesized by nitroxide-mediated radical polymerization and employed as stabilizers in the emulsion polymerization of styrene. The effect of the stabilizing poly­(acrylic acid) block molecular weight dispersity on stabilization behavior was studied, independent of molecular weight and composition. Block copolymer stabilizers were evaluated in terms of critical aggregation concentration, dispersed phase particle size, distribution, and zeta potential. The molecular weight dispersity of the stabilizing block affected the aggregation number and final number of particles but displayed no negative effects on stability or size distribution.

Polystyrene-b-[polystyrene-r-poly(acrylic acid)] block-random copolymers have been synthesized at various molecular weights (7000−23 200 g/mol) and with compositions between 6 and 39 mol % acrylic acid by nitroxide-mediated polymerization. Emulsion polymerizations of styrene stabilized by block-random copolymers yielded stable latexes at stabilizer concentrations ≥3 wt % based on monomer. A series of emulsion polymerizations with varying stabilizer content suggests that a novel nucleation mechanism occurs in block-random copolymer-stabilized emulsion polymerizations, exhibiting distinctly different behaviors from block copolymers or conventional small-molecule surfactants. Moreover, alkaline aqueous dispersions of the block-random copolymers were prepared with ease up to concentrations of 300 g/L, whereas similar block copolymers are limited to ∼1 g/L. Analysis of the dispersions via dynamic light scattering and atomic force microscopy suggests that single-chain nanoparticles form via a self-folding process with hydrodynamic diameters between 2.4 and 5 nm. The novel stabilizer structures may be tuned for rapid dispersion through their anchoring block [polystyrene] and high stabilization efficiency through the stabilizing block [polystyrene-r-poly(acrylic acid)].

Poly­(sodium acrylate)-b-polystyrene block copolymers were employed as stabilizers in the emulsion polymerization of styrene. Previous work by our group has shown that the molecular weight dispersity of the stabilizing block is an important design parameter of block copolymer stabilizers; herein, the molecular weight dispersity of the anchoring polystyrene block, Đ PS, was investigated. Stabilization performance was evaluated by the critical aggregation concentration, aggregation number, and surface activity of the block copolymers and the size, distribution, and zeta potential of the polystyrene latex particles. It was observed that Đ PS had a strong effect on aggregation number, which led to a change in the number of latex particles in the seeded emulsion polymerization of styrene. Surface activity decreased with increasing Đ PS due to a greater diversity of copolymer compositions, supporting the idea that copolymers of different composition play different roles in the stabilization of an emulsion. The performance of block copolymer stabilizers, evaluated by the stability and size distribution of latex particles, was indistinguishable over the range of Đ PS studied; narrow stabilizer molecular weight distributions were not necessary for satisfactory performance.

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