Single to Multiple Site Behavior of Metallocenes through C–H Activation for Olefin Polymerization: A Mechanistic Insight from DFT

• Published in: ACS catalysis 2020-02, Vol.10 (3), p.1704-1715
• Main Authors: Kumawat, Jugal, Gupta, Virendra Kumar
• Format: Article
• Language: English
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.9b03741

Density functional theory has been used to study single to multiple site behavior of metallocene catalysts for olefin polymerization using (CpPr)2Hf­(R)2 and (CpPr)2Zr­(R)2 (here, R = Me and n-butyl group) and boron activators B­(C6F5)3 and [CPh3]+[B­(C6F5)4]−. Detailed pathways were investigated for two steps: (i) catalyst activation and (ii) ethylene and 1-hexene polymerization. For the catalyst activation step, the C–H activation of Cp-substituents (here, the n-propyl group) has also been studied, which leads to another active site named “metal cyclic active site” in comparison to the conventional active site. Among these sites, the formation of a metal cyclic active site is slower with respect to the conventional active site. Additionally, two different routes have been explored for the catalyst activation, depending on the nature of activators, and observed that [CPh3]+[B­(C6F5)4]− is a better activator with respect to B­(C6F5)3. Moreover, complete insertion (primary and secondary) and termination (chain transfer to monomer and β-H elimination) steps have also been studied for both the active sites. These results suggest that the metal cyclic active site can also produce the ethylene and 1-hexene polymer, which defines the multiple site behavior of metallocene catalysts, which explains the broadening of molecular weight distribution of linear low-density polyethylene-produced metallocene catalysts. Along with hafnocene and zirconocene, the toluene-solvated hafnocene catalyst system has also been investigated for catalyst activation and ethylene polymerization for comparison study. These results indicate that the toluene-solvated hafnocene catalyst system is energetically more facile in comparison to other catalysts. Furthermore, the effect of solvents as well as dispersion interaction on catalyst activation steps has been studied for the (CpPr)2M­(n-butyl)2 (M = Hf and Zr) catalyst systems and observed that the dispersion interaction played an important role in the catalyst activation process. In addition to this, the regio- and stereoselective behavior of 1-hexene monomer for the (CpPr)2Hf­(Me)2 catalyst has been investigated, and it was found that the metal cyclic active site improves the stereoselective nature of the hafnocene catalyst. Therefore, the current chemical calculations provide insights into the nature of active sites in metallocene catalysts for olefin polymerization.