AIBN initiator microemulsion polymerization

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]. 

Using just one surfactant without cosurfactant, it has been possible to prepare latex particles as small as about 20 nm in diameter [116]. They have used CTAB, or a mixture of anionic and nonionic surfactants. These systems have been more thoroughly studied as shown in a recent review by Antonietti [117]. This author [118] and later Vu [119] were able to develop a predictive model for styrene microemulsion polymerization with crosslinker, initiated by AIBN. This model shows that the size of the droplets is dependent on the ratio between the weight fraction of monomer and the total amount of monomer plus surfactant. This model is based on simple geometrical considerations, the monomer mixture being the core of a particle surrounded by a shell of surfactant. 

The photochemical UV radiation method was first employed by Leong and Candau [41] for the radical polymerization of acrylamide in inverse microemulsions stabilized by Aerosol OT. The polymerization was carried out using AIBN initiator and induced by UV irradiation. It was shown that the use of a microemulsion rather than an emulsion led to stable and clear microlatices d 50 nm) of uniform size, thus providing a way to overcome some of the problems of conventional inverse emulsion polymerization, such as instability of the latexes resulting in rapid flocculation and a broad particle size distribution. 

The polymerization of the microemulsion, in the above case, was carried out, with no apparent phase separation, by exposure to light in the presence of AIBN initiator. The solid polymer thus obtained was ground into powder, washed with methanol to remove surfactant and then dried. The final product was a porous polymer with active surfaces which had the ability to form complexes with Cu" displaying heterogeneous catalysis in the hydrolysis of p-nitrophenol diphenyl phosphate. 

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). 

Rabagliati et al. (14) studied the polymerization of styrene in a three phase system containing an anionic-nonionic surfactant mixture and brine. Both AIBN and potassium persulfate initiators were used. The system was reported to be microemulsion continuous and even multicontinuous. (14). No autoacceleration was observed and the authors concluded that the polymerization exhibits an inverse dependence of the degree of polymerization on initiator concentration, similar to bulk solution polymerization. 

Polymerization of the microemulsion systems was performed under nitrogen environment using both purified styrene and unpurified styrene containing inhibitor. The polymerization was initiated by the thermal decomposition of potassium persulfate or AIBN at 6Q0C. The polymerization duration for the purified styrene system under nitrogen environment was 18 hours, whereas it took 36 hours for the styrene with inhibitor. The mode and dynamics of polymerization were observed using both polarized light and enhanced video microscopy (22). 

Preparation and Polymerization of (0/W) Cetyltrimethylammonium Bromide Microemulsion (CTAB-yE) (5-7). An oil in water pE composed of 1.0 g of CTAB, 0.5 g of hexanol, and 1.0 g of 50% styrene-divinylbenzene in 50 mL of water was carefully prepared by slowly adding the water to a stirred mixture of the other components to yield a slightly bluish clear solution. A 0.1% solution (w/w) of initiator AIBN (based on monomer) was then solubilized in the system followed by removal of 02 (by gentle N2 bubbling for 5 min), and finally the system was heated in an oil bath (50°C) until complete polymerization was achieved as determined spectrophotometrically. Proper dilution with water was then made to give a 0.01 M CTAB-P-pE solution P-pE indicates polymerized microemulsion. 

In the case of thermal initiation of styrene [79,80], the polymerization rate was found to be proportional to [AIBN] and [KPS] , in good agreement with other data for three- or four-component microemulsions [66,81]. The dependence on AIBN concentration is consistent with the prediction of 0.40 based on the micellar nucleation theory in emulsion polymerization (Smith-Ewart case 2) (see, e.g.. Ref 129). The dependence on KPS concentration lies between this case and the value of 0.5 for solution or bulk polymerization. 

The overall activation energy, Ea, of polymerization was determined for both monomers as a function of the nature of the initiator [66,79,81,90,92]. For styrene polymerization in cationic microemulsions, Ea was found to be much higher for KPS systems (E 95 kJ/mol) than for AIBN systems (48 kJ/mol) in spite of similar decomposition energies. This difference was attributed to different radical capture efficiencies between the anion radicals of KPS and the uncharged AIBN radicals and the positively charged... 

The kinetics of polymerisation of styrene-in-water microemulsions is investigated using dilatometry. From plots of percentage conversion versus time, the rate of polymerization, Rp, is determined. From log-log plots of Rp versus styrene and initiator (2,2 -azobis(isobutyronitrile), AIBN) concentrations a relationship is estabhshed. The exponents are similar to those predicted by the theory of emulsion polymerisation. The results also show a rapid conversion in the initial period (interval 1) followed by a slower rate at longer times (interval 2). It is suggested that in interval 1, the main process in nncleation of the microemnlsion droplets, whereas in interval 2 propagation is the more dominant factor. The rapid polymerisation of microemnlsions is consistent with their strncture, whereby very small droplets with flexible interfaces are prodnced. 4 refs. 

The anionic surfactant, sodium bis (2-ethylhexyl sulfosuccinate) (AOT, represented as I) has been found to produce microemulsion without adding any cosurfactant. Thus, stable systems with cyclohexyl methacrylate monomer as the oil phase in a w/o microemulsion were obtained with AOT as surfactant [36]. Thermal polymerization of the microemulsion was carried out using AIBN as the initiator at 70 °C in a composition consisting of AOT (0.3 mol/L), water (4.5 mol/L), and AIBN (2 wt%). The microemulsions mmed opaque during the course of... 

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. 

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