The characteristics of reactions in the monomer phase, aqueous phase, and the interface of monomer/aqueous phase of soap-free emulsion polymerization of 4-vinylpyridine (4VP) and styrene (St) system were evaluated by using the different types of initiators, addition of organic solvents, and changes of agitation rate. The oil-soluble initiators 2,2′-azobis(2,4-dimethylvaleronitrile) (ADVN), benzoyl peroxide (BPO), and 2,2′-azobis(isobutyronitrile) (AIBN), the organic water-soluble initiator, 2,2′-azobis(2amidinopropane) dihydrochloride (V50), and the inorganic water-soluble initiator, potassium persulfate (KPS), were used. 1 H NMR and scanning electronic microscopy (SEM) were employed for the characterizations. As a result, the mechanism of interfacial particle formation was supported. Namely, the minimonomer droplets were generated by the disturbance in the interface of monomer/aqueous phase due to the agitation. The minimonomer droplets were stabilized by the adsorption of surface-active oligomer generated by the reactions in both the aqueous phase and the interface. The monomer transfer from the bulk monomer phase to the growing particles was via the coalescence of minimonomer droplets with particles. The role of reaction in the aqueous phase was proposed to just provide the surface-active oligomer for the stabilization of particles. The rapid reaction in the aqueous phase due to the high concentration of hydrophilic monomer produced longer hydrophilic chains and led to the coagulation of particles by a bridging-coagulation effect. On the basis of this mechanism, the coagulum-free stable latices with high monomer conversion were prepared by using KPS and AIBN and, theoretically, can be prepared by using any type of initiators at a high level of solid content and feed ratio of hydrophilic monomer.

The precipitation polymerization of acrylamide/methacrylic acid (AAm/ MAA) in ethanol (EtOH) was thoroughly investigated from detecting the homogeneity of the initial solution prior to polymerizations to the final products of the polymerizations. Dynamic light scattering and scanning electron microscopy were employed for the investigations. The solutions of AAm and AAm/acrylic acid (AAm/AA) were homogeneous. However, the solutions of AAm/MAA, AAm/poly(MAA) (PMAA), and AAm/ poly(AA) (PAA) were not homogeneous as they are usually considered to be: entities with size distributions of around 150, 40, and 17 nm, respectively, were detected at the polymerization temperature of 60°C. Accordingly, analogous to the entities that are similar to the structure of micelles formed in the solutions of AAm/PMAA and AAm/ PAA because of polymer-AAm interactions, it was suggested that the complexes of AAm/MAA stemming from the molecular interactions, particularly the (lypo-) hydrophobic interaction, aggregated to form minimonomer droplets at 60°C. The monodisperse microspheres were prepared only in the AAm/MAA-EtOH systems, whereas the microspheres were not prepared in the homogeneous AAm-EtOH systems despite the precipitation of PAAm. The results obtained from various polymerizations showed that the microspheres originated from the polymerization within the minimonomer droplets. A new mechanism was established that describes the processes for the formation of all products possibly generated in the AAm-MAA-EtOH polymerization system.

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A series of experiments were carried out on the soap-free emulsion polymerization mechanism of 4-vinyl pyridine (4VP)/styrene (St) and the solvent, ethyl acetate (EA), by using the cationic initiator, 2,2Ј-azobis(2-amidinopropane) 2HCl (V50). To investigate the mechanism of polymerization in detail, a particular quasi-static polymerization system-the reaction mixture in a sealed bottle allowed to stand without agitation during the whole polymerization period-was employed. The partition of monomers at ambient and solid content of the latex during the quasi-static polymerization in the presence of 8 wt % EA was measured by 1 H-NMR, and the variation of particle size was observed by scanning electron microscopy (SEM). The solubility of 4VP and St in water decreased as the amount of EA increased. Meanwhile, most of the EA was partitioned in the oil phase, even in the cases where the charged amount of EA was much lower than the solubility of EA (ϳ 8 wt %) in water at ambient. The solubility of EA in water was also affected by the composition of 4VP and St (i.e., increased as the fraction of 4VP increased, but decreased as the fraction of St increased). This observation, as well as the quantitative analysis of the quasi-static emulsion polymerization, indicated that the initial reaction in the soap-free emulsion polymerization was closely related to the disturbance of the interface of monomer/water phase, which triggered the generation of monomer droplets. Therefore, a new mechanism was proposed for the nucleation of soap-free emulsion polymerization, namely, the nucleation was performed in the droplets. The droplets absorbed the oligomeric radicals generated in the aqueous phase to continue the polymerization, as well as to promote the colloidal stability of the droplets.