The polymerization of acrylonitrile (AN) in aqueous nitric acid initiated by " cage " vanadyl polycarboxylate (P=VO)-thiourea (TU) complex was investigated. The overall rate of polymerization is Rp=9.7×105e-10

500/RT[AN][HNO3]1.5[P=VO]0.50[TU]0.76 The relationship between the induction period (τ) and the temperature of polymerization as well as the concentrations of reactants can be expressed as follows :1/τ=4.6×1012e-13

500/RT[HNO3]3[P=VO][TU]3/2=KτRi The molecular weight of polyacrylonitrile increases with increasing monomer concentration and decreases with increasing temperature of polymerization and concentrations of vanadyl polycarboxylate and thiourea Mn=KM1/T·[AN]/[HNO3]3/2[P=VO]1/2[TU]3/4=KM·1/T·Rp/Rt The polymerization mechanism was proposed and discussed.