Prog. Polym. Sci., Vol. 16, 331-360, 1991 0079-6700/91 $0.00 + .50 Printed in Great Britain. All rights reserved. © 1991 Pergamon Press plc STEREOCHEMICAL INVESTIGATION OF THE EFFECT OF LEWIS BASES IN HETEROGENEOUS ZIEGLER-NATTA INITIATOR SYSTEMS MARIA CARMELA SACCHI,* INCORONATA TRITTO and PAOLO LOCATELLI lstituto di Chimica delle Macromolecole del C.N.R., 20133 Milano, Via E. Bassini 15, Italy 1. INTRODUCTION Electron donors play a fundamental role in modern Ziegler-Natta initiator systems for propylene polymerization. They had previously been used with traditional TiCl3 based systems and their stereoregulating capability had already been extensively studied. ~ However, their function became extremely relevant with the discovery of highly active MgCl2 supported initiator systems] These initiator systems made it possible to develop high yield processes which are simpler and more economical than the traditional ones. However the stereo- regularity achieved in the absence of selected electron donors is poor (isotactic index 50-70%). Since the discovery of MgCl2 supported initiator systems many efforts have been made to find the most suitable donors to obtain satisfactory stereospecific initiator systems, at the same time preventing an unacceptable decrease in polymerization activity. A number of Lewis bases, differing in basic strength, steric hindrance and functionality, have been investigated both as components of the solid initiators (internal bases) and added to the metal-alkyl coinitiator (external bases). Among the internal bases, monoesters and diesters of aromatic acids have been shown to be the most effective. Among the external donors, aromatic monoesters like ethyl benzoate (EB), methyl para-toluate (MPT) and ethyl anisate (EA) have certainly been the most extensively studied. Another group of commonly used external donors are the bulky amines, like 2,2,6,6-tetramethylpiperidine (TMPip), At present the alkoxysilanes, whose outstanding capability of improv- ing stereospecificity was discovered in the early 1980s, have the widest industrial application. *Author to whom correspondence should be addressed. Editors' note: It was our editorial policy from the beginning, in putting together this issue, to refer to the metal complexes used in Ziegler-Natta polymerization uniformly as "initiators" or "initiator systems" rather than as "catalysts", and so the former term has been used in this paper and in the others in this issue. 331