Supported Catalysts for Polypropylene: Aluminum Alkyl-Ester Chemistry
The reactions of aluminum alkyl cocatalysts with the ethyl benzoate (EB) component of MgCl2 supported TiCl4 catalysts have a strong influence on both activity and stereospecificity. NMR investigations show that AlEt3 alkylates ethyl benzoate by the following reaction: PhCO2Et + 3 AlEt3 → 1/2 (Et2AlOEt)2 + Et3Al·Et2AlOCEt2Ph. The rate of ethyl benzoate consumption decreases with lower Al/PhCO2Et ratios. Atmospheric pressure polymerization studies reveal that the interaction of ethyl benzoate with the catalyst is responsible for achieving high isotacticity as measured by heptane insolubles (% HI). The aluminum alkoxide products from the alkylation reaction increase HI indirectly by complexing AlEt3, lowering the free [AlEt3] and the AlEt3/PhCO2Et ratio. However, lower free [AlEt3] also reduces polymerization rate. The introduction of steric bulk into the aluminum alkyl component minimizes the ester alkylation reaction while maintaining catalyst activity. Two types of cocatalysts have been found which give significantly better catalyst performance than the AlEt3 cocatalyst: (1) sterically hindered trialkyl aluminum cocatalysts, such as s-Bu2AlEt and t-Bu2AlEt and (2) certain aluminum dialkyl and diary1 amides such as Et2Al-2,2,6,6-tetramethylpiperidide and Et2AlNPh2.
Ethyl benzoate improves HI by inactivating the nonstereo- specific polymerization sites to a greater degree than the stereo- specific site. This has led to the finding that 2,2,6,6-tetra- methylpiperidine is highly selective in complexing the nonstereo- specific sites with minimal interaction toward stereospecific sites.
KeywordsTitanium Lithium Benzene Toluene Amide
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