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The polymerization of AN at −75°C. in various systems with the participation of the organomagnesium compounds was studied. For the system AN–BuMgCI. Bu2O–toluene kinetic data are given and a reaction scheme including the deactivation of the catalyst is proposed. The initial period of the polymerization is characterized by first order with respect to the catalyst and second order with respect to the monomer. On the basis of: (a) the proportionality of the molecular weight of PAN to the conversion, (b) the independence of the polymer molecular weight on the catalyst concentration, and (c) narrow molecular weight distribution of the polymer obtained at relatively high conversion. the absence of the termination reactions of the growing chains is suggested. The maximum molecular weight of the polymer attained in the systems AN–RMgX–toluene was 435.000. The efficiency of the process in the hydrocarbon media increases sharply in the presence of catalytic quantities of dimethylformamide (DMF) and dimethyl sulfoxide. As it is shown for the system Bu3Mg2I–DMF–toluene, the interaction of the catalyst components at −75°C. leads only to the formation of a complex of the type RMgX DMF. The addition of RMgX to the carbonyl group of DMF can be observed only at a higher temperature.