Fractionation of acrylonitrile copolymers. I. Development of a method

The mechanism of precipitation of acrylonitrile polymers was investigated using different nonsolvents. The effect of the dielectric constant and the polarity of individual bonds in the molecule of precipitant is discussed briefly. A methyl methacrylate–acrylonitrile copolymer was successfully fractionated from dimethylformamide solution, using a two‐stage fractionation technique, and a mixture of (2 parts n‐hexane + 1 part ether) as a precipitant. The molecular weight distributions of primary fractions were skewed to the left and overlapped each other. A large, 100‐g. sample of the same copolymer was separated into eight primary fractions, which on refractionation gave a result substantially the same as that obtained by the small scale experiment. An experimental terpolymer of methylvinylpyridine–vinyl acetate–acrylonitrile, prepared by suspension polymerization, was carefully fractionated by the same method. The polymer was shown to have a double‐peak molecular weight distribution in agreement with the earlier suggestion that acrylonitrile in aqueous suspension polymerizes by two simultaneous mechanisms: (1) homogeneous polymerization of single radicals in solution, and (2) heterogeneous growth of radicals present in stable suspended polymer particles. The above conclusion was further supported by methylvinylpyridine analysis on secondary fractions. It was found that the low molecular weight fractions contained appreciably more of the water‐soluble monomer (M.V.P.), than the high molecular weight fractions.