Effect of the monomer hydrophobicity on the microemulsion polymerization. A comparison of the efficiency of water- and oil-soluble photoinitiators.

Polymerization rates in SDS/monomer/water oil-in-water microemulsions were measured employing a water-soluble (2,2'-azobis(2-amidinopropano), ABAP) an d an oil-soluble (2,2'-azobis(isobutyronitrile), A1BN) photoinitiators. Mon omers of widely different hydrophobicity, ranging from acrylamide and metha crylic acid to ethylhexyl methacrylate (EH MA) and styrene, were employed i n order to evaluate the role of the monomer distribution between the aqueou s and microaggregate pseudophases on the photoinitiation efficiency. ABAP w as more efficient initiator, irrespective of the monomer distribution. The lower efficiency of AIBN was explained in terms of intracage geminate recom bination. The initiation efficiency of AIBN relative to ABAP varied from 0. 35 (EHMA) to 0.02 (styrene). The high value for EHMA indicates that the ini tiation is due to radicals generated in the microaggegates, and that nearly one third of radicals avoids recombination in the original supercage. The low value of styrene is explained in terms of a fast addition to the monome r that reduces the radical exit rate.