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

Abstract

Abstract The kinetic features of the molecular weight distributions (MWDs) of polypropylene produced with TiCl 4 /MgCl 4 /C 6 H 5 COOC 2 H 5 /Al(C 2 H 5 ) 3 as catalyst are investigated as functions of the polymerization time and the concentrations of Al(C 2 H 5 ) 3 , C 6 H 5 COOC 2 H 5 and hydrogen. The MWDs are dependent upon the concentrations of Al(C 2 H 5 ) 3 and C 6 H 5 COOC 2 H 5 , and are correlated with a change of the isotactic index of the produced polymers. In contrast, both MWD and the number‐average molecular weight M̄ n remain unchanged during the polymerization from 5 s to 3 h. When hydrogen is added as a chain transfer reagent, M̄ n decreases as anticipated but the polydispersity index (M̄ w /M̄ n ) is independent of the hydrogen concentration. The MWDs obtained with this supported catalyst are rather narrow (3≤ M̄ w /M̄ n ≤8)and are represented by a log‐normal form or Wesslau equation, which is different from the MWDs (6≤ M̄ w /M̄ n ≤20) obtained with the traditional TiCl 3 catalysts, which are represented by the Tung equation. Broadening of the MWD obtained with heterogeneous catalysts is also discussed. The constancy of M̄ w /M̄ n , 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, k p , is demonstrated on the basis of the kinetic data, concerning the effect of CO‐inhibition on the polymerization rate. A surface distribution function of k p , capable of accounting for the broad MWDs, is presented and discussed in terms of a distribution of the activation energy for the propagation step.