Emulsifier-free emulsion copolymerization of styrene and acrylamide using an amphoteric initiator

Emulsifier-free emulsion copolymerization of styrene (St) and acrylamide (A Am) has been investigated in the presence of an amphoteric water-soluble in itiator, 2,2'-azobis[N-(2-carboxyethyl)-2-2-methylpropionamidine]hydrate (V A057). The kinetics of polymerization and the colloidal properties of the r esulting latices were studied and compared with the cases using ionic initi ators. When adopting the amphoteric initiator at pHs lower than 10, stable amphoteric poly (St/AAm) latices, evidenced by the electrophoretic mobility , were prepared directly. Meanwhile, almost the same conversion versus time curves appeared and there were no apparent differences in the final partic le sizes for those polymerizations, whereas in the polymerization at pH 10, a much lower rate of copolymerization and a larger size of particles were observed. The surface charge density and the growth rate of latex particles produced with VA057 at pH < 10 were comparable to those of the particles w ith a cationic initiator, 2,2'-azobis(2-amidinopropane)dihydrochloride, but were apparently lower than those with an anionic initiator, potassium pers ulfate, when the polymerizations were carried out under corresponding condi tions. The number of initiator fragments incorporated onto the particle sur faces was independent of polymerization pH, except for pH 10. The abnormal performance of VA057 at pH 10 was attributed to its degradation due to hydr olysis.