Mechanistic aspects in propene polymerization using mgcl 2-supported ziegler-natta catalysts: behaviour of silane as an external lewis base

Abstract

Silane compounds such as phenyltriethoxysilane (PTES) in the presence of triethylaluminium (TEA) generate a 1:1 complex which is stable under the concentration conditions (mmol 1 −1) and temperature (70°C) used in propene polymerization, when improved MgCl 2-supported Ziegler-Natta catalysts are employed. As demonstrated by 13C and 1 NMR studies, the complexation equilibrium is displaced fully towards the formation of the complex. Under these circumstances, the amount of free external Lewis base (ELB) is much less than in other systems, with the result that ELB is better able to promote the activation of isotactic specific sites on the solid catalyst. Once active sites have been formed via interaction of free or complexed alkylaluminium with titanium, further addition of silane, up to a 1:1 ratio with Al, does not lead to poisoning of the sites. A very stable ternary complex is formed between the titanium site and the silane-complexed alkylaluminium. This complex is strongly attached to the catalyst and the exchange equilibrium between the adsorbed ELB-TEA complex and the external solution, which is quite fast when aromatic esters are used as an ELB, is very slow in this case so that high dilution of the solution or addition of further TEA has no effect on the behaviour of the catalyst. Finally, it has been shown that PTES works exclusively through its complex with TEA.