Living Free-Radical Dispersion Polymerization of Styrene

Polystyrene latexes were synthesized via living free-radical chemistry with 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) in both alcoholic and aqueous alcoholic media using poly(N-vinylpyrrolidone) (PVP) as a steric stabilizer at 112−130 °C. Genuine dispersion polymerization was only achieved in ethylene glycol or ethylene glycol−water mixtures. In the absence of TEMPO, near-quantitative monomer conversions, high molecular weights, broad molecular weight distributions, and submicrometer-sized latexes were obtained. The addition of TEMPO had a profound effect on the polymerization chemistry: only moderate monomer conversions (as low as 20%, with a maximum conversion of 60% obtained after 71 h) and larger, typically micrometer-sized, latexes were obtained with TEMPO-mediated syntheses. Polydispersities (M w/M n's) as low as 1.11 were achieved in the presence of TEMPO, but only relatively low molecular weight polystyrene chains were obtained. All TEMPO-synthesized latexes had spherical particle morphologies and very broad size distributions, as evidenced by scanning electron microscopy (SEM) and disk centrifuge photosedimentometry (DCP), respectively. Surprisingly, high Γ values were calculated for the adsorbed PVP stabilizer, which suggests that it is not located exclusively on the outside of the polystyrene latex particles.