No abstract

Modified Smith-Ewart equations were used to quantitatively investigate compartmentalization effects in nitroxide-mediated radical polymerization in dispersed systems. Calculations were carried out for the specific system 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO)/styrene at 125 °C with thermal initiation included in the model, and accounting for compartmentalization of both propagating radicals and nitroxide. For particles with diameter (d) less than approximately 60 nm, a reduction in particle size led to lower rate of polymerization, higher degree of livingness, and a higher number of activation-deactivation cycles experienced per chain to grow to a given degree of polymerization. These effects have their origin in the pseudo first-order deactivation rate coefficient increasing in proportion to d -3 with decreasing particle size. There were no significant effects of compartmentalization for particles with d > 110 nm. The results show that it is important to consider compartmentalization effects also on the deactivation reaction, and not only bimolecular termination.

Effects of the kind and concentration of stabilizers on the nonspherical shape of polystyrene (PS)/poly(methyl methacrylate) (PMMA) composite particles prepared by release of toluene from PS/PMMA/toluene droplets dispersed in stabilizer aqueous solution were examined. In the case of poly(vinyl alcohol), the surfaces of the obtained particles always had a single dimple. In the case of sodium dodecyl sulfate (SDS), the shapes of the composite particles changed from the dimple, via acorn, to spherical with increasing SDS concentration. It was clarified that the dimple and acorn shapes of the PS/PMMA composite particles were caused by contraction of the PS phase after hardening of the PMMA phase in excentered core-shell and hemisphere morphologies, respectively, which were formed by phase separation during toluene evaporation.

Summary: Compartmentalization in atom transfer radical polymerization (ATRP) in dispersed systems at low conversion (<10%) has been investigated by means of a modified Smith–Ewart equation focusing on the system n‐butyl acrylate/CuBr/4,4′‐dinonyl‐2,2′‐dipyridyl at 110 °C. Compartmentalization of both propagating radicals and deactivator was accounted for in the simulations. As the particle diameter (d) decreases below 70 nm, the polymerization rate (Rp) at 10% conversion increases relative to the corresponding bulk system, goes through a maximum at 60 nm, and thereafter decreases dramatically as d decreases further. This behavior is caused by the separate effects of compartmentalization (segregation and confined space effects) on bimolecular termination and deactivation. The very low Rp for small particles (d < 30 nm) is due to the pseudo first‐order deactivation rate coefficient being proportional to d−3.Simulated propagating radical concentration ([P•]) as a function of particle diameter (d) at 10% conversion for ATRP of n‐butyl acrylate ([nBA]0 = 7.1 M, [PBr]0 = [CuBr/dNbpy]0 = 35.5 mM) in a dispersed system at 110 °C. The dotted line indicates the simulated [P•] in bulk at 10% conversion.magnified imageSimulated propagating radical concentration ([P•]) as a function of particle diameter (d) at 10% conversion for ATRP of n‐butyl acrylate ([nBA]0 = 7.1 M, [PBr]0 = [CuBr/dNbpy]0 = 35.5 mM) in a dispersed system at 110 °C. The dotted line indicates the simulated [P•] in bulk at 10% conversion.

Micrometer-sized silica-stabilized polystyrene latex particles and submicrometer-sized polystyrene−silica nanocomposite particles have been prepared by dispersion polymerization of styrene in alcoholic media in the presence of a commercial 13 or 22 nm alcoholic silica sol as the sole stabilizing agent. Micrometer-sized near-monodisperse silica-stabilized polystyrene latexes are obtained when the polymerization is initiated with a nonionic AIBN initiator. These particles are stabilized by silica particles that are present on the latex surface at submonolayer concentration. The total silica content is no greater than 1.1 wt %, which corresponds to a silica sol incorporation efficiency of less than 1.3%. Reduction of the initial silica sol concentration led to a systematic increase in the mean latex diameter. In contrast, submicrometer-sized polystyrene-silica nanocomposite particles are obtained when the polymerization is initiated with a cationic azo initiator. The silica contents of these nanocomposite particles are significantly higher, ranging up to 29 wt %. Zeta potential measurements, XPS, and electron spectroscopy imaging by transmission electron microscopy (ESI/TEM) studies reveal a well-defined core−shell morphology for these particles, whereby the core is polystyrene and the shell comprises the silica sol. After calcination, these nanocomposite particles can form hollow silica capsules. Variation of the initial silica sol and initiator concentration has relatively little effect on the final particle size and silica content of these polystyrene−silica nanocomposite particles, but indicates silica sol incorporation efficiencies up to 72%.