Active Centers and Mechanisms

Active Centers and Mechanisms. Any discussion of the mechanism of coordination polymerization must necessarily recognize the complexity of the reaction. That is why, despite the enormous efforts and the extremely large number of papers published about this topic, the mechanism of coordination polymerization is not known in great detail. Two key steps, however, seem to be well established. First the complexation between the alkene and the active center takes place, which is followed by the insertion of the activated monomer to the growing polymer 

This generates a titanium-carbon a bond through ligand exchange. The active center was believed to be the bimetallic species 44 existing in equilibrium with different monometallic species. 

During propagation the alkene molecule forms a Jt-complex with the transition metal of the active center  

This results in strong polarization of the n bond and dissociation of the Ti—C bond, thus promoting insertion into the activator aluminum-alkyl bond. Repetitive insertions of alkene molecules result in lengthening of the polymer chain. This mechanism is also termed bimetallic after the growth center complex species 44. 

Variations of this mechanism included the suggestion of a partially bonded alkene molecule,299 the participation of a titanium-aluminum ion pair,300 and a concerted alkene insertion.301 The development of the activator-alkyl mechanism was probably strongly influenced by the Aufbau reaction, studied originally by Ziegle.102 He observed that Group I—III alkyl compounds such as Et3Al catalyzed the oligomerization of ethylene to terminal alkenes. Additional evidence of such mechanism comes from the fact that alkylaluminum compounds exist in dimeric 

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