Molecular weight distribution of a low conversion polystyrene sample

The fractionation of a low conversion polystyrene sample has been carried out using the precipitation technique in the system methyl ethyl ketone–methanol. Six fractions were isolated and the cumulative and differential curves plotted according to the customary assumptions. Statistical analysis has also been applied on the basis of a Bernouillian distribution for the fractions. The parameters of the binomial have been calculated by means of Beall's method. The results show a considerable divergence with regard to the usual ones. The accumulation of the MW by the classicial procedure is too rapid from a DP of 5600, and is slightly lower than the binomial from zero to 5600. In addition, the classical procedure does not supply any information with regard to the high MW range of the distribution (DP up to 34,000), which amounts to nearly 18% of the whole sample. The distribution has been reproduced, using Bruns' series, from the successive MW averages, and the result agrees with that of the binomial distribution. On the other hand, the distribution has been worked out kinetically, assuming that combination and disproportionation are both the main mechanisms of termination. The results show that disproportionation occurred about twice as frequently as combination.