The copolymerization of styrene and methyl methacrylate in the presence of ethylaluminum sesquichloride in toluene yields alternating copoly-mers, independent of initial monomer ratio. The rate of polymerization is not influenced by the presence of an excess of either monomer, the conversion curves following a parallel course at a given total monomer concentration. When the concentration of the AlEt1.5Cl1.5and the S/MMA ratio are kept constant and the total monomer concentration is increased, the polymerization rate increases and reaches a limiting value at a S/MMA/Al ratio of 2:2:1. A similar result is obtained when the total monomer and the AlEt1.5Cl1.5concentrations are kept constant and the S/MMA ratio is varied. When the concentration of either monomer and the AlEt1.5Cl1.5concentration are kept constant and the concentration of the other monomer is varied, the polymerization rate reaches a limiting value at the same mole ratio, irrespective of which monomer is varied. The rate of polymerization is decreased in the presence of a small amount as well as a large amount of benzoquinone. However, the rate is higher than in the absence of the quinone when the Al/benzoquinone ratio is 2:1. The conductivity of a toluene solution of AlEt1.5Cl1.5increases only slightly upon the addition of methyl methacrylate, a further small increase occurring upon the addition of styrene. The results provide evidence for the participation of a comonomer complex in the polymerization, the optimum composition resulting either from the simultaneous interaction of several equilibria or the alignment of the complexes in the form of a matrix.