The synthesis of high molecular weight (in excess of 10(6) million Daltons) poly(acrylonitrile) and poly(acrylonitrile-co-methylacrylate-co-itaconic acid) is described. An inverse emulsion polymerization formulation with AIBN as the initiator was used. However, polymer precipitation occurred early in the polymerization. In each case, the molecular weight distribution was surprisingly narrow ((M) over bar M(w)/(M) over bar M(n), similar to 1.5). Conversion vs. time plots with monomers containing the inhibitor had the "S" shape typical of emulsion polymerizations. The terpolymer composition and molecular weight were quite uniform throughout the polymerization. With inhibitor-free monomers, the initial molecular weights were very high (similar to 3 X 10(6) Daltons), but gelation occurred at ca. 50% conversion. There was an inverse relationship between the monomer inhibitor content and the polymer molecular weight. It is suggested that the growing polymer radicals are occluded in the precipitated polymer particles and are terminated by inhibitor diffusing into the particles, accounting for the narrow molecular weight distribution.