Coordination centres of Ziegler-Natta polymerizations

Ziegler et al. [181] discovered that the mixture of TiCl4 with R3 .A1C1 can polymerize ethylene at low temperatures and pressures. The product is linear, macromolecular polyethylene. 

Modern modifications of classical ZN systems, represented by the Solvay catalyst, are based on / -TiCl3. TiCl4 is mixed with Et2AlCl at 274 K, the reduction temperature is slowly increased up to 338 K and maintained at this value for 1 h. From the mixed TiCl3.] AlCl3 crystals, AICI3 is washed out with diisoamyl ether in this process, — recrystallization takes place, and ci-TiC  

According to Nielsen [187], the following Ti types are placed at the corners of TiCl crystallites Ti atoms with one vacancy and one pendant Cl atom (the pendant Cl is bound only to this one Ti atom, and to no other in the crystal lattice) Ti with two vacancies and two pendant Cl atoms and Ti with one vacancy and three pendant Cl atoms. From these atom groups, the active centres are generated (Cl atoms bound to other Ti atoms are only indicated by valence lines). 

With Solvay-type catalysts, Et AlCl is used as cocatalyst. 

It has not yet been unambiguously decided whether the active centre is formed by the transition metal atom with its ligands (monometallic centre), I, or by the bimetallic complexes II or III most authors now favour the idea that the centre is bimetallic. 

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PROPERTIES OF COORDINATION CENTRES OF ZIEGLER-NATTA POLYMERIZATIONS... 

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). 

The centres of coordination polymerizations should be able to form only a weak and reversible coordination bond with the monomer. Compounds irreversibly solvating the coordination centres (under the given conditions) act as catalytic poisons. Thus monomers with unscreened polar substituents and most heterocycles cannot be polymerized on Ziegler-Natta coordination centres (see Chap. 3 Sect. 4.1). 

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