The catalytic effect of trans-dichloro-bis(ethylenediamine)cobalt(III) chloride (trans-[Co(en)(2)Cl-2]Cl) complex on the microemulsion polymerization of acrylonitrile, in the absence of added emulsifier, was investigated. Polymerization was studied at varying concentrations of initiator, monomer, complex, and solvent over the temperature range of 30-70degreesC. The overall activation energy (E-a, 39.26 kJ/mol), energy of dissociation of initiator (E-d, 61.62 kJ/mol), number of micelles (0.122 x 10(18)), and viscosity average molecular weight of the polymer were computed. The distribution of particle sizes was determined by transmission electron microscopy (TEM). It was found that the oil-in-water microemulsion polymerization was stabilized by the presence of the Co(III) complex, reducing the particle size into the nano range. The average diameters of PAN nanoparticles, obtained by TEM, were in the range of 50-150 nm at maximum conversion. The experimental particle size was mainly dependent on the concentration of complex and temperature. (C) 2004 Wiley Periodicals, Inc.