Effects of impurities on the control of anionic batch polymerizations

The homogeneity of the molecular weight distribution of polymeric material is, in general, described fairly adequately by the homogeneity ratio ¯xw/¯xn. In recent literature on anionic polymerizations, the ¯xw/¯xn ratio implicitly has been used also for the evaluation of the polymerization control attainable in these processes with regard to the reproducibility and predictability of molecular weights. For polymerizations in which the monomer is added gradually, this procedure is sufficiently indicative, but leads to wrong conclusions in the case of batch polymerizations. For the evaluation of the latter processes, in addition to the ¯xw/¯xn ratio, the predictability ratio ¯x i.e., the quotient of the number‐average degree of polymerization calculated for an impurity level α to the one predicted in the absence of impurities (α = 0), or a related quantity, has to be known for both mono‐ and difunctional initiator. As a result of this investigation, batch polymerization methods which have attracted considerable attention in the literature with respect to the monodispersity of the products attained are found to be unsatisfactory from the point of view of polymerization control.