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 influen ce of the coordination number and ligand type of model compounds in re levant reaction steps were studied. Thus, by using the MEHT method, in sertion 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 d o 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 transforma tions was found to be insignificant. While studying related conversion s 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 pronou nced pi-donor effect was observed. Methyl insertions in these cases sh owed a higher computed activation barrier than hydride migrations. An orbital basis for this phenomenon was provided and conclusions concern ing chain-length control in Ziegler-Natta catalysis were drawn.