Quantitative Screening of Ziegler-Natta: Catalyst Regioselectivity

High Throughput Experimentation (HTE) has gained importance in the screening of olefin polymerization catalysts. In particular, its recent integration with ‘fast’ 13C NMR spectrometers represents a most powerful tool for mechanistic investigations. This presentation introduces a quantitative HTE approach to the regioselectivity of Ziegler-Natta (ZN) catalysts for isotactic polypropylene (iPP), and its relationship with the so-called ‘hydrogen response’. The proposed protocol makes use of a state-of-the art HTE platform (Freeslate PPR48) and a Bruker Avance III 400 spectrometer featuring a 5 mm high-temperature cryoprobe. Quite surprisingly we found that, at odds with stereoselectivity, regioselectivity of these catalysts is primarily dictated by the lnternal Donor (ID), and only marginally affected by the External Donor (ED). ID’s belonging in the same chemical class (e.g. 1,3-dimethoxypropanes) end up with similar regioselectivities, almost irrespectively of their steric demand. The mechanistic interpretation of these findings is not obvious. On the other hand, the crucial relationship between regioselectivity and molecular weight capability of a given catalyst in the presence of H2 was confirmed; in most cases, the number average polymerization degree of the polymers obtained at practical p(H2)/p(C3H6) ratios coincides with the reciprocal of 2,1 unit concentration, fully supporting the ‘dormant site’ concept.