Miniemulsion polymerization Conventional free radical

Besides normal ATRP, newly reported reverse ATRP and AGET ATRP are attracting much attention. Reverse ATRP is a convenient method to reduce the oxidation problems. [77] In a reverse ATRP, the transition metal complexes in higher oxidation state (e.g. Cu(II) complex) are initially added to the reaction system. Radicals are generated by decomposition of a conventional free radical initiator. The components of the initial system are less sensitive to air, so this technique is more compatible with commercial processes. [78] Simms et a/.[79] reported a successful reverse ATRP of nBMA in miniemulsion polymerization. The cationic surfactant CTAB was employed in the system with a concentration as low as 1 wt % with respect to monomer. 

Miniemulsion polymerization is a technique that, in principle, allows any water-insoluble monomer to undergo polymer reactions (not limited to the conventional free radical polymerization) inside the homogenized monomer droplets dispersed in the continuous aqueous phase. Thus, each miniemulsion droplet can be regarded as an ideal submicron scale reactor that is not controlled by the monomer mass transfer process for the synthesis of a variety of... 

Living free radical polymerization is a unique technique used to prepare a variety of polymers with well-controlled molecular structures such as polymers with narrow molecular weight distribution, multiblock copolymers, and star polymers, which cannot be achieved by conventional free radical polymerization [77, 78]. This polymerization technique was originally investigated in homogeneous bulk and solution polymerization systems, and it has been successfully applied to heterogeneous miniemulsion polymerization system in the last decade [62,63]. 

Claverie et al. [325] have polymerized norbornene via ROMP using a conventional emulsion polymerization route. In this case the catalyst was water-soluble. Particle nucleation was found to be primarily via homogenous nuclea-tion, and each particle in the final latex was made up of an agglomeration of smaller particles. This is probably due to the fact that, unlike in free radical polymerization with water-soluble initiators, the catalyst never entered the polymer particle. Homogeneous nucleation can lead to a less controllable process than droplet nucleation (miniemulsion polymerization). This system would not work for less strained monomers, and so, in order to use a more active (and strongly hydrophobic) catalyst, Claverie employed a modified miniemulsion process. The hydrophobic catalyst was dissolved in toluene, and subsequently, a miniemulsion was created. Monomer was added to swell the toluene droplets. Reaction rates and monomer conversion were low, presumably because of the proximity of the catalyst to the aqueous phase due to the small droplet size. 

In batch experiments, the solids were varied from 35 to 75% [10]. The primary surfactant was Aerosol A103 (disodium ethoxylated nonyl phenol half ester of sulfosuccinic acid) with HD as the cosurfactant. These were used in concentrations of 1 and 4 wt% on monomer, respectively. Two KPS concentrations, 1 and 2 wt% on water, were tried. The miniemulsions were produced by ultra-sonification. Parallel conventional emulsion polymerizations were conducted for comparison to the miniemulsion polymerizations (75 °Q. Coagulum-free latexes resulted from miniemulsion polymerizations up to 60% solids, while only 50% solids could be achieved for the cxmventional process. These differences were attributed to the resulting particle size distributions where the miniemulsion polymerizations produced latexes with larger particles, broader distributions and lower viscosities than their conventional counterparts. As in other studies, this difference in PSDs was explained by differing nucleation mechanisms. However, as in other studies, it was not possible to determine whether the nucleation in the miniemulsion systems was predominantly by radical entry into chxjplets. 

Chern et al. [33-39] used stearyl methacrylate or lauryl methacrylate as the reactive costabilizer to stabilize styrene miniemulsion polymerizations. Just like conventional costabilizers (e.g., hexadecane), long-chain alkyl methacrylates act as costabilizers in stabilizing the homogenized submicron monomer droplets. Furthermore, the methacrylate group (—C=C(CH3)COO—) of the polymerizable costabilizer can be chemically incorporated into latex particles in the subsequent free radical polymerization and thereby reduce the level of volatile organic compounds (VOC). As the polymerization proceeds, the reactive costabilizer concentration in the nucleated monomer droplets will decrease. The initial decrease of the costabilizer concentration should not cause any diffusional degradation because the hydrophobic polymer formed inside the nucleated monomer droplets can help stabilize the polymerizing miniemulsion. 

In general, conventional emulsion polymerization is faster in comparison with the miniemulsion polymerization because more latex particles (reaction loci) are nucleated and the rate of polymerization is linearly proportional to the number of latex particles per unit volume of water. Nevertherless, the rate of polymerization per particle is larger for miniemulsion polymerization, as evidenced by the higher concentrations of free radicals and monomer in the latex particles. 

---