Active centre atactic

According to Cossee, the isotactic centre has one and the atactic two vacancies. According to Rodriguez, the organometal is coordinated into one of the two vacancies of the isotactic centre. The addition of donors (diethyl ether, triethylamine, pyridine, etc.) greatly affects the polymerization rate (catalyst activity) as well as the ratio of the stereoregular and atactic product components. Donors affect the structure of active centres and modify it. 

Propene and the higher 1-alkenes can be polymerized to chains with the required degree of tacticity from almost atactic up to very highly tactic structures. However, a syndiotactic polymer can only be obtained from propene, mostly on soluble catalysts. The main factors determining controlled tactic addition are complexation, cis or trans addition, and primary or secondary addition. Most authors agree on the point that the interaction of the alkene molecule with the transition metal atom of the active centre leads to complex formation immediately before monomer insertion into the metal—polymer bond. The assumed existence of the complex is based on indirect experimental evidence and on theoretical considerations. 

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 comparable values of the polydispersity index for both isotactic and atactic polymers led Keii to conclude that the same active centre distribution exists according to the ratio between propagation and transfer rate constants. Furthermore, the low values of the polydispersity index would suggest good active centre homogeneity. 

Rishina and Vizen observed that the MWD of the atactic fraction of polypropylene prepared with the 5-TiCl3—Al(C2H5)3 catalytic system narrows when polymerization temperature increased from 20 to 70 °C. This phenomenon was ascribed to a greater reactivity of the active centres for growing chain termination. 

Present views concerning the operation mechanism of ZN catalysts are not conclusive. Cossee [288, 289] assumes that, in the first step, donor-acceptor interaction occurs between the transition metal and the monomer. A a bond is formed by the overlap of the monomer n orbital with the orbital of the transition metal. A second n bond is formed by reverse (retrodative) donation of electrons from the orbital of the transition metal into the antibonding 7T orbital of the monomer. In the following phase, a four-centre transition complex is formed with subsequent monomer insertion into the metal-carbon bond. This, in principle, monometallic concept is criticized by the advocates of the necessary presence of a further metal in the active centre. According to them, the centre is bimetallic. Monometallic centres undoubtedly exist on the other hand, technically important ZN catalysts are multicomponent systems in which each component has its specific and non-negligible function in active centre formation. The non-transition metal in these centres is their inherent component, and most probably the centre is bimetallic. Even present ideas concerning the structural difference in centres producing isotactic and atactic polymers are not united. 

These catalysts were first introduced in the 1960s in slurry processes. The active centres of these catalysts are located at points of missing chlorine atoms in TiCls crystals. These catalysts have low yields (1 t/kg catalyst), produce 5 to 10 % atactic polypropylene and require de-ashing and atactic removal from the final product... 

This feature is due to the difference in reactivity ratios between comonomers which depend on the type of catalytic active site. The lower isospecific centres of traditional Ziegler-Natta catalysts are much more active toward ethylene than toward propylene or butene-1. Moreover, these less isotactic sites being the more reactive toward hydrogen, the atactic fractions will have lower mean molecular weight than isotactic ones. 

---