Mechanism of soap-free emulsion polymerization of styrene and 4-vinylpyridine: Characteristics of reaction in the monomer phase, aqueous phase, and their interface

The characteristics of reactions in the monomer phase, aqueous phase, and t he interface of monomer/aqueous phase of soap-free emulsion polymerization of 4-vinylpyridine (4VP) and styrene (St) system were evaluated by using th e different types of initiators, addition of organic solvents, and changes of agitation rate. The oil-soluble initiators 2,2'-azobis(2,4-dimethylvaler onitrile) (ADVN), benzoyl peroxide (BPO), and 2,2'-azobis(isobutyronitrile) (AIBN), the organic water-soluble initiator, 2,2'-azobis(2-amidinopropane) dihydrochloride (V50), and the inorganic water-soluble initiator, potassiu m persulfate (KPS), were used. H-1 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 drop lets 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 t he aqueous phase and the interface. The monomer transfer from the bulk mono mer phase to the growing particles was via the coalescence of minimonomer d roplets with particles. The role of reaction in the aqueous phase was propo sed to just provide the surface-active oligomer for the stabilization of pa rticles. The rapid reaction in the aqueous phase due to the high concentrat ion of hydrophilic monomer produced longer hydrophilic chains and led to th e coagulation of particles by a bridging-coagulation effect. On the basis o f this mechanism, the coagulum-free stable latices with high monomer conver sion were prepared by using KPS and AIBN and, theoretically, can be prepare d by using any type of initiators at a high level of solid content and feed ratio of hydrophilic monomer.