Earlier work revealed that the internal plasticization of polyacrylonitrile by the higher n-alkyl acrylates or N-n-alkylacrylamides yielded only brittle copolymers. This difficulty was circumvented in the present work by starting with copolymers of acrylonitrile and ethyl acrylate, over the range of compositions, and further modifying these by incrementally displacing the ethyl acrylate in each recipe by n-octadecyl acrylate through terpolymerization. In this way, the stepwise small reduction in Tg for the base ethyl acrylate–acrylonitrile copolymers was greatly increased for each of the terpolymers. Compositions were obtained ranging from glassy, brittle terpolymers, with glass transitions above room temperature, to soft plasticized polymers having sufficient polar networks retained from the nitrile to confer useful properties. The decline in the glass temperature was shown to be dependent on the free volume conferred by the side-chain methylene groups of each acrylate ester. In contrast, the decline in tensile and flexural strengths and moduli for the terpolymers having glass transitions above room temperature was produced entirely by the presence of the methylene groups of the 18-carbon ester. The glass transition region corresponded to room temperature when the acrylonitrile content of the base copolymer had been reduced to 50 mole-%. Terpolymers of this nitrile content and lower had the low moduli and large elongations of plasticized compositions. An equation was developed which correlated empirically the glass transitions and the mechanical properties with the weight fraction of the acrylate esters for the glassy terpolymers.