A process for the polymerization of olefin monomers comprising contacting
ethylene or a mixture of ethylene and one or more C.sub.4-8 .alpha.
olefins with a catalyst composition comprising one or more Group 3-10
transition metal containing, Ziegler-Natta, procatalyst compounds; one or
more alkylaluminum cocatalyts; and one or more polymerization control
agents, said process being characterized in that at least one such
polymerization control agent is an alkyl or aryl, ester of an aliphatic
or aromatic (poly)carboxylic acid optionally containing one or more
substituents comprising a Group 13, 14, 15, or 16 heteroatom.