Propene polymerization with a magnesium chloride‐supported Ziegler catalyst, 2. Molecular weight distribution

Abstract

AbstractThe kinetic features of the molecular weight distributions (MWDs) of polypropylene produced with TiCl4/MgCl4/C6H5COOC2H5/Al(C2H5)3as catalyst are investigated as functions of the polymerization time and the concentrations of Al(C2H5)3, C6H5COOC2H5and hydrogen. The MWDs are dependent upon the concentrations of Al(C2H5)3and C6H5COOC2H5, and are correlated with a change of the isotactic index of the produced polymers. In contrast, both MWD and the number‐average molecular weight M̄nremain unchanged during the polymerization from 5 s to 3 h. When hydrogen is added as a chain transfer reagent, M̄ndecreases as anticipated but the polydispersity index (M̄w/M̄n) is independent of the hydrogen concentration. The MWDs obtained with this supported catalyst are rather narrow (3≤ M̄w/M̄n≤8)and are represented by a log‐normal form or Wesslau equation, which is different from the MWDs (6≤ M̄w/M̄n≤20) obtained with the traditional TiCl3catalysts, which are represented by the Tung equation. Broadening of the MWD obtained with heterogeneous catalysts is also discussed. The constancy of M̄w/M̄n, independent of the hydrogen concentration leads to the important conclusion that non‐uniform surface sites on heterogeneous catalysts are responsible for the broadening of the MWD. A surface heterogeneity of the propagation rate constant,kp, is demonstrated on the basis of the kinetic data, concerning the effect of CO‐inhibition on the polymerization rate. A surface distribution function ofkp, capable of accounting for the broad MWDs, is presented and discussed in terms of a distribution of the activation energy for the propagation step.