Termination by donors

When monomer coordination to the active centre is prevented, polymerization cannot occur. Coordination is a reversible reaction strongly solvating agents deactivate centres in a ratio very close to 1 1 (acetylenes, allene, ketene, tetrahydrofuran, ROH, H20, COS, CO, C02, R3N) but weaker donors must be present in some excess in order to cause total inhibition. It appears that isotactic and atactic centres of Ziegler-Natta polymerizations, particularly centres with TiCl3 and Et2AlCl, exhibit a different ability to coordinate donors (different acidity). 

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