The effect of initiators on the emulsion polymerization of 2-hydroxyethyl methacrylate

The emulsion polymerization of 2-hydroxyethyl methacrylate (HEMA) was conducted at 80degreesC, using a series of initiators and using either sodium dodecyl sulfate (SDS) or a mixture of SDS and poly(vinyl pyrrolidone) (PVP) as the emulsifier. For the series of initiators (including 2,2'-azobisisbutyronitrile (AIBN), 2,2'-azobis-(2-amidinopropane) dihydrochloride (AAP), 4,4'-azobis(4-cyanopentanoic acid) (ACPA) and 1,1'-azobis(cyclohexane carbonitrile) (ACHC)), the aqueous (aq.) solubility sequence correlates well to the polymerization rate sequence, while the sequence of initiation coefficient correlates poorly to the polymerization rate sequence. There is significant aq. phase polymerization, and frequent particle nucleation and coagulation during the polymerization. Increasing the monomer/water ratio, the polymerization rate does not increase proportionally to the particle number. It is suggested that the dominant locus for the formation of radicals is in the oil phase, and single radicals are formed by desorption of counterpart radicals from the particles.