Manipulation of Pre-Equilibria in Olefin Polymerization Catalysis: Backbone Stiffening Converts Living into a Highly Active Salan-type Catalyst

A novel (ONNO)ZrBn2 precatalyst for olefin polymerization (n1-ZrBn2, Figure A.1-right; Bn = Benzyl) was prepared. Ligand n1 was stiffened compared with the Salan homologue o1 (Figure A.1-left) in order to inhibit the fac-fac/mer-mer isomerization of complex o1-ZrBn2, the primary reason for the poor activity of the catalyst originated from the latter.1 The performance of catalyst systems derived from n1-ZrBn2 significantly exceeds that of o1-ZrBn2 (Figure A.1) in propene homopolymerization and ethene/1-hexene copolymerization.2 Not only led this modification to a >100-fold more active catalyst, but also, remarkably, the polymer molar mass capability is greatly enhanced, both in propene homopolymerization as well as ethene/1-hexene copolymerization. Solution NMR spectroscopy and computational modeling by means of static Density Functional Theory (DFT) calculations confirmed a lower propensity to form the undesired mer-mer isomer, at least in the pre-catalyst. Finally, variation of the ortho-substituent was explored indicating that the tremendous effect on activity is most pronounced for catalysts heavily affected by unfavorable stabilization of the inactive mer-mer isomer and subsides for bulky ortho-substituents. This research forms part of the research programme of DPI, project #835.