EHT MODEL STUDIES ON COORDINATION-SPHERE EFFECTS IN THE ELEMENTARY STEPS OF D0 TRANSITION-METAL-CATALYZED ZIEGLER-NATTA POLYMERIZATION

In a theoretical investigation on Ziegler-Natta catalysis, the influence of the coordination number and ligand type of model compounds in relevant reaction steps were studied. Thus, by using the MEHT method, insertion reactions of ethylene into Zr-CH3 and Zr-H bonds were analyzed in systems of the type [Zr(C2H4)R4R']-[Zr(CPXC2H4)R2R'], and [Zr(CP)(C2H4)R3R']- (R=R'=CH3, R=CH3, R'=H). It was found that all processes do not have significant kinetic barriers, whereas the reverse reactions in particular the beta-hydride elimination have relatively high ones. The influence of coordination geometry and number on these transformations was found to be insignificant. While studying related conversions starting from [Zr(L)(C2H4)R3R'], [Zr(Cp)(C2H4)RR'(L)]+, and [Zr(CP)(C2H4)R2R'(L)] (L=pi-donor, R=R'=CH3 or R=CH3, R'=H) compounds a pronounced pi-donor effect was observed. Methyl insertions in these cases showed a higher computed activation barrier than hydride migrations. An orbital basis for this phenomenon was provided and conclusions concerning chain-length control in Ziegler-Natta catalysis were drawn.