Controversies on the mechanism and kinetics of emulsion polymerization: An updated critical review.

Conventional emulsion polymerization (EP) is a process via free radicals whose driving force for its development has been its versatility to generate polymer colloids with ad hoc characteristics for a wide variety of applications, as well as its friendly character to the environment since the continuous medium is water. Although through decades of research, considerable progress has been made in understanding its mechanism and kinetics, some aspects are still not entirely clear. Furthermore, new ideas and experimental results have appeared in the literature that challenge the accepted knowledge about some aspects of EP. This work is a personal vision and an updated critical review on those controversial aspects whose precedent is the review with the same approach published by the author and collaborators almost 20 years ago (J. Macromol. Sci. Part C Polym. Rev., 2004;44:207-229). This review covers advances, aspects that are open to discussion or need improvement regarding what happens in the aqueous phase and in the interface (initiator decomposition, entry and exit of radicals, monomer transport) as well as in the polymer particles (free-radical propagation and termination, swelling, average number of radicals per particle). Special attention is paid to particle formation (nucleation) and its interrelation with colloidal stability and the evolution of the particle size distribution (PSD), which is one of the most fundamental and controversial issues of EP. The Smoluchowski collision rate coefficient to describe diffusion-controlled processes has practically become a paradigm despite the fact that there is evidence that questions its applicability. For this reason, this review also emphasizes this point and the alternatives that have been proposed to mathematically describe the diffusive stages of particle coagulation, the entry of radicals, and the termination reaction. Challenges in improving our understanding of the mechanism and kinetics of emulsion polymerization are pointed out.

Oil-in-eutectic mixture HIPEs co-stabilized with surfactant and nanohydroxyapatite: ring-opening polymerization for nanocomposite scaffold synthesis.

Mixtures of a nonionic surfactant and non-functionalized nanohydroxyapatite (NHA) enhanced the stability of oil-in-eutectic mixture high internal phase emulsions (HIPEs). Upon ring opening polymerization of the eutectic mixture composed of l-lactide and ε-caprolactone, biodegradable polyHIPEs with specific cavity sizes and selective interfacial functionalization with NHA are produced.

Super-enhanced particle nucleation in styrene emulsion polymerization in the presence of sodium styrene sulfonate.

The styrene (St) emulsion polymerization using Aerosol MA80 as surfactant and in the presence of sodium styrene sulfonate (NaSS) was studied. The effect of NaSS content was assessed using MA80 concentrations below and at the critical micellar concentration. It was found that at the higher NaSS and MA80 contents, the number of particles (N) reaches a maximum of the order of 1017particles/cm3 water, a huge value that has never been reported. In this work an explanation for this super-enhanced particle nucleation phenomenon is proposed. Such hypothesis is based on the role of St-NaSS oligomers formed in the aqueous phase and their synergy with MA80 molecules to provide colloidal stability to the system. The proposal seems to be consistent with the experimental data obtained for the evolution of monomer conversion, N, particles size distribution and the wideness of this latter as well as with a theoretical estimation of the N.