The mechanism of polymerization of methacrylonitrile initiated by triethylphosphine in dimethylformamide and tetrahydrofuran was investigated. The elemental analysis has shown that phosphorus is chemically bound to the polymeric molecules. Analysis by NMR spectroscopy, as well as conductivity measurements, prove that the zwitter-ion structure is formed in the initiation process. Monomer addition to the carbanion leads to macrozwitter-ion. The initiation reaction is slow and it is the rate determining step. Spontaneous monomolecular termination reaction is inherent to the system. During its life-time, the active centres can exist as either ion pairs or free ions. In the propagation step the free ion is by far the more reactive of the two species. In dimethylformamide the ion pairs play virtually no part in the propagation step and the polymer is formed through the propagation of free ions. The ion pairs-free ions intramolecular equilibrium constant depends on the length of the macrozwitter-ion. Consequently, the polymerization rate depends on the length of the growing chains. The paper presents experimental evidence for this and reports elementary constants for the macrozwitter-ion polymerization. It could have been expected that the zwitter-ion polymerization is strongly influenced by the polarity of the reaction medium. Therefore, we investigated the polymerization of methacrylonitrile initiated by triethylphosphine in tetrahydrofuran, as well as the influence of an external electric field on this system. In tetrahydrofuran polymerization is very slow and only oligomers are formed. Addition of dimetylformamide to tetrahydrofuran – monomer mixture, i.e. an increase in the dielectric constant of the reaction medium, increases the rate of polymerization and molecular weight of the polymer. In tetrahydrofuran, the presence of an external electric field was found to lead to a strong increase in both polymerization rate and molecular weight. These results suggest that an increase of dielectric constant of the reaction medium as well as the application of external electric field both enable the dissociation of low reactivity ion pairs into the highly reactive free ions.