A method is described for preparing regular copolymers with repeat unit [MRM], where M is a vinyl or diene monomer, and R is the ligand of an aliphatic dihalide. Over forty polymers of this type are reported and confirmation of their structures is given by n.m.r. analysis. Specific viscosities, measured as one per cent solutions in tetrahydrofuran at 25°C, of up to 0·25 have been obtained with these polymers, but no attempt has been made as yet to maximize the degrees of polymerization. Monomers which undergo this polymerization process include styrene, α-methylstyrene, butadiene and isoprene. © 1969.