The mechanism of the solution copolymerization of acrylonitrile and methylacrylate in DMF at 7$0^{circ}C$ initiated by azobisisobutyronitrile (AIBN) has been investigated in the presence of polyvinylpyrrolidone (PVP) as an additive. Data of fractional conversion of the copolymers, fractional polymerization rate, instantaneous and cumulative composition of the copolymer and instantaneous monomer feed composition as a function of the reaction time or conversion are presented. The monomer reactivity ratio determined by the Finemann-Ross method in the presence and absence of PVP (3 wt%) were $r_1=1.122,;r_2=0.858;and;r_1=0.995,;r_2=0.744$, respectively. The PVP was found to affect the polymerization rate, conversion, and degree of polymerization of polyacrylonitrile copolymer. The PVP in the polymerization solution was found to increase the initial polymerization rate and limiting conversion. The conversion and degree of polymerization at limiting conversion time increased.