AbstractIn the polymerization of acrylonitrile initiated with α,α′‐azobisisobutyronitrile, triphenylphosphite exhibited two different effects on the rate of polymerization, a decreasing effect and an increasing effect. The decrease is due to the phosphoranyl radical formed by the addition of a 2‐cyanopropyl radical to triphenylphosphite. The increase is due to the formation of a weak n‐π complex between triphenylphosphite and acrylonitrile, which was confirmed from the UV absorption spectra. Triphenylphosphite had no significant effect on the molecular weight of the polymer. The phosphorus content in the polymer was small (0.02–0.06%), the number of phosphorus units in one polymer cha