A careful study was made of the fractionation of GR-S (commercial poly(butadiene-co-styrene)) by stepwise precipitation, from solution in benzene, with the precipitant 50:50 (by volume) methanol-benzene. To determine the reproducibility of fractionation, particularly in the high-molecular-weight region, three runs were made with 2% solutions; and to determine the effect of concentration on efficiency, comparable fractionations were performed from solutions of three different concentrations—1, 2, and [Formula: see text] respectively. In each of the fractions the value of intrinsic viscosity, the viscosity slope constant β, and the viscosity-average molecular weight were determined. These provide conclusive evidence that in such a primary fractionation a much cleaner separation is accomplished from a dilute solution (1% or less).
