Density functional study of ethylene-norbornene copolymerization via metallocene and constrained-geometry catalysts

Single-site Ziegler-Natta catalysts allow a sterically distinct norbornene to copolymerize with ethylene. By performing density functional calculations on CP2ZrCH3+ and H2Si(CpNH)ZrCH3+, each representing metallocene and constrained-geometry catalysts (CGC), we have uncovered how the bulky norbornene competes with ethylene for the metal center during the insertion process. Instead of acting as a steric barrier to insertion, the cyclopentyl group of norbornene provides additional agostic sites and disposable ring strain, by which the monomer achieves its high reactivity at low temperature. This effect becomes more pronounced on the less sterically hindered catalyst, leading to a more discriminating catalytic activity of CGC toward the two monomers. The unfavorable endo orientation for insertion of norbornene results from a steric interaction of its bulky ethylene bridge facing the catalyst. A small geometric difference, as exemplified in a structurally similar bicyclooctene, can greatly affect the agosticity, steric hindrance, and ring strain of a cycloolefin.