Abstract
Highly accurate high-throughput experimentation (HTE) data for a set of 21 silicon-bridged C2-symmetric ansa-zirconocenes in propene homopolymerization were collected and were used to develop quantitative structure – activity relationship (QSAR) models for several performance indicators at high polymerization temperature (Tp = 100 °C) by using chemically meaningful descriptors. Most notably, stereoselectivity is well described by a two-descriptor model linking the quadrant model for stereoselectivity (sterics) with the chain epimerization model (electronics). The catalysts show widely varying temperature responses, most notably on stereoselectivity and molar mass capability, while the regioselectivity response is uniformly weak. Soft conformational locks lose their performance rapidly while hard conformational locks offer enhanced performance even at high temperatures. The quest for high-temperature stable, well-performing ansa-zirconocenes will unquestionably lead to systems with enhanced rigidity.
Highlights
In light of the results presented in this paper, we believe that further “stiffening” of ultrarigid metallocenes might offer avenues to high-performance, high-temperature stable zirconocenes
We have screened a library of 21 C2-symmetric zirconocenes for their performance in propene polymerization at a relatively high polymerization temperature of 100 °C
Together with earlier published data for 60 °C, this allowed for the first time for a broad systematic analysis of how temperature-sensitive substituent effects are at higher polymerization temperature
Summary
Known since the 1950s,1,2 metallocenes did not gain traction as a platform for the industrial production of polyolefin resins until the mid-1970s when Kaminsky and Sinn introduced a superior activator, methylaluminoxane (MAO).[3−6] The boost in activity observed with MAO started a metallocene research frenzy in both academia and industry that involved most polyolefin companies active at that time and hundreds of academic laboratories worldwide.[7−12] As single-center polymerization catalysts, metallocenes allow “easy” identification of structure/property correlations and precision tuning of the active pocket.[13−18] one of the most important contributions of metallocene research to the science of olefin polymerization catalysis came in the form of mechanistic understanding, from the origins of enantioselectivity[18−21] to factors determining molecular weight[22] or dormancy and catalyst activity.[15,23−28]. We have introduced QSAR models with predictive capability for the iPP performance indicators stereoselectivity, regioselectivity, and molar mass capability at 60 °C.51 These models are based on a set of seven descriptors that are intuitively related to simple electronic or steric properties of neutral LZrCl2 precatalysts (Figure 3 and Table 3). Optimized for performance at 60 °C, the catalyst represents the best-known balanced combination (Table 1) of stereoselectivity, regioselectivity, and molar mass capability.[51] This promise fails spectacularly to translate to higher operating temperatures (Table 2) To understand this failure, it is instructive to itemize the temperature response imparted by different substituent patterns (Table 5). A strong stereoselectivity temperature response often goes along with substituents that increase the electrophilicity of the central metal, like 4-(2-MePh), 4-(N-carbazolyl), 2-Et, 2-iPr, or 4-(2-thienyl), as judged by the NPA charge on the ZrCl2 fragment (Table 6) and the success of the previously introduced QSAR model. In light of the results presented in this paper, we believe that further “stiffening” of ultrarigid metallocenes might offer avenues to high-performance, high-temperature stable zirconocenes
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