Water-soluble AIBN-initiated microemulsion

The conditions which have been defined for the formation of effective microemulsions (nature of the oil, Ro and HLB values) are also required for obtaining clear and stable microlatices after polymerization. Various water-soluble monomers have been polymerized by a free radical process in anionic and nonionic microemulsions, either under U.V. irradiation or thermally with AIBN as the initiator (11,14,22,29). Total conversion to polymer was achieved in less than 20 minutes (a few minutes in some cases). Series of experiments have been performed in various oils. Table III summarizes some of the results and emphasizes the importance of the formulation. A good chemical matching between oils and emulsifiers (G 1086 -t Arlacel 83, Isopar H) leads to stable latices, a poor matching (G 1086 Arlacel 83, heptane) leads to unstable latices which settle within a few hours to a few days (22). 

Leong and Candau (18) obtained inverse latices of small size (<50nm) via photopolymerization of acrylamide in a microemulsion system of acrylamide, water, toluene and Aerosol OT. They observed that rapid polymerization and total conversion was achieved in less than 30 minutes. The microemulsions remained transparent and stable during polymerization. Candau et al. (19) also reported the results of a kinetic study of the polymerization of acrylamide in inverse microemulsions. Both oil soluble AIBN and water soluble potassium persulfate initiators were used. The rate was found to depend on the type of initiator, but in both cases neither autoacceleration nor dependence on initiator concentration was observed. An excellent review of microemulsion polymerization was published recently by Candau (20). 

Polymerization of styrene in each of the three types of microemulsions was performed using a water soluble initiator, potassium persulfate (K2S208), as well as an oil-soluble initiator, AIBN. As desired, solid polymeric materials were obtained instead of latex particles. In the anionic system, the cosolvent 2-pentanol or butyl cellosolve separates out during polymerization. Three phases are always obtmned after polymerization. The solid polymer was obtained in the middle with excess phases at the top and bottom. GC analysis of the upper phase indicates more than 80% 2-pentanol, while Karl-Fisher analysis indicated more than 94% water in the lower phase. Some of the initial microemulsion systems have either an excess organic phase on top or an excess water phase as the bottom layer. GC analysis showed the organic phase to be rich in 2-pentanol. However, the volume of the excess phase is much less in the initial system than in the polymerized system. 

Preparation of iron oxide magnetic nanoparticles and their encapsulation with polymers in W/0, i.e. inverse microemulsion polymerization, was also applied by O Connor et al. [167]. Inverse microemulsion polymerization was used to prepare submicron hydrophilic magnetic latex containing 5-23 wt% iron oxide. AM and crosslinker MBA were added to an aqueous suspension of previously synthesized iron oxide nanoparticles (6 wt%) this aqueous phase was dispersed in a aerosol OT (sodium l,4-bis(2-ethylhexoxy)-l,4-dioxobutane-2-sulfonate) (AOT)-toluene solution to form a W/O microemulsion, followed by polymerization with AIBN or V-50 as initiator. The particle size (80-180nm)was controlled by tuning the concentration of the water-soluble crosslinker agent as well as the amount of surfactant with respect to water [168]. 

Candau et al. [41] later studied the polymerization of AOT-stabiUzed acrylamide inverse microemulsion by a thermal process using either oil soluble AIBN or water soluble K2S2O8 as the initiator and found the rate of polymerization to be first order with respect to initial monomer concentration in the presence of AIBN but 1.5 order in the presence of K2S2O8. An inverse relationship was found between polymer molecular weight and the surfactant concentration which suggested participation of the surfactant in the initiation reaction. This was further confirmed by the observed independence of the polymer molecular weight on the concentration of the initiator. 

---