A Radical Mechanism

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Science  24 May 2002:
Vol. 296, Issue 5572, pp. 1369
DOI: 10.1126/science.296.5572.1369c

Ziegler-Natta catalysts are used widely in the polymerization of ethylene and other unsaturated hydrocarbons, producing highly ordered polymers with a narrow molecular weight distribution. The mechanism by which they act remains incompletely understood because direct investigation of the active catalyst is difficult; most insights have been gained from studying reaction products. In particular, the role of the co-catalyst, usually an alkyl aluminum compound such as Al(CH3)3, remains speculative. Addition of Al(CH3)3 is believed to result in the reduction of Ti and formation of alkyl radicals, whereas for Al(CH2CH3)3 no radical formation is expected.

The limited knowledge of the catalytic mechanism has hampered efforts to design new catalysts or optimize existing ones. To address this problem, Risse et al. have studied a model Ziegler-Natta catalyst, TiCl4 anchored to a MgCl2 film, with in situ electron spin resonance. Activation of the catalyst by addition of Al(CH3)3 is shown to involve the formation of alkyl radicals, as expected. The authors conclude that radical formation likely occurs at the interface between the cocatalyst and TiCl4/MgCl2 after alkylation of the catalyst by ligand exchange. In contrast, activation with Al(CH2CH3)3 is not accompanied by radical formation, providing direct support for the previously proposed reaction mechanisms. — JU

Angew. Chem. Int. Ed.41, 1517 (2002).

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