The generation of dihydrogen as a possible side reaction in homogeneous single-site olefin polymerization.: A density functional theory study

Abstract

We present a theoretical study on a possible mechanism for deactivation of single-site olefin polymerization catalysts by unwanted dihydrogen generation and formation of an potentially inactive allyl complex during the polymerization process. In this study, six different catalyst systems have been investigated, they are (NPMe 3) 2TiR + ( 1); Cp(OSiMe 3)TiR + ( 2); Cp(NPMe 3)TiR + ( 3); Cp(NCMe 2)TiR + ( 4); (CpSiH 2NMe)TiR + ( 5); and Cp 2ZrR + ( 6); with R=propyl. It is found that the rate-determining step in H 2 generation is the β-hydrogen elimination starting from the propyl complexes. The barrier for this process is very similar for all the titanium–propyl complexes (15–17 kcal mol −1) and noticeable lower (11.0 kcal mol −1) for the zirconium complex. Substituting hydrogens with methyl groups on the γ and especially β-carbon of the propyl chain reduces significantly the barrier for the β-hydrogen elimination and thus increases the rate of H 2 generation. On the other hand β-hydrogen elimination and H 2 generation is suppressed by replacing the methyl groups on the ancillary ligands with more bulky t-butyl substituents. It was finally shown that the allyl complex formed from the H 2 generation can be reactivated by ethylene insertion with an estimated barrier of 13 kcal mol −1. It is argued that the reactivation might be more difficult with higher α-olefins.