External alkoxysilane donors in Ziegler‐Natta catalysis. Effects on poly(propylene) microstructure

Correlations were sought between the structure of an external alkoxysilane donor and the microstructure of the polymer chain obtained by the polymerization of propene with a heterogeneous, high-activity Ziegler-Natta catalyst. Fourteen different alkoxysilanes of structure RnSi(OR′)4-n, where n = 1–3, R = n-alkyl or phenyl and R′ = C1–3-alkyl, were used as external donors. The polymers were fractionated by boiling heptane extraction. The microstructures of the polymers were studied by means of 13C NMR spectroscopy. The structure of the donor had a marked effect on the catalyst activity and isotacticity of the polymer. However, all of the alkoxysilanes tested produced qualitatively similar differences in the microstructure of poly(propylene): the size of the isotactic mmmm pentad peak varied with the donor efficiency, but the sizes of the non-isotactic pentad peaks were more or less constant relative to each other. In the boiling heptane soluble fraction, isotacticity and syndiotacticity increased and the viscosity-average molecular weight Mv decreased when a good external donor was used in the polymerization. Results indicate that all external alkoxysilane donors tested have the same qualitative effect on active centers, and only isotacticity, i.e., the number of mmmm sequences, can be controlled by an external alkoxysilane donor. The effectiveness and selectivity of deactivation strongly depend on the structure of the donor.