Direct examination of initiation and propagation kinetics of zirconocene-catalyzed alkene polymerization

Zirconocene precatalysts with sterically bulky alkyl groups were designed in order to obtain models for the propagating species in zirconocene-catalyzed alkene polymerization. Alkyllithium reagents Li(CH2CEt3) and Li(CH2CMe2CH2Ph) were prepared and utilized in methyl alkylzirconocene synthesis. Dialkyl and methyl alkylzirconocenes of the form [(eta5-C5H5)(eta 5-C5Me5)Zr(R)2], [(eta5 -C5H5)2Zr(R)(CH3)], and [(eta5-C5H5)(eta5-C 5Me5)Zr(R)(CH3)] (where R = CH2CMe 3, CH2SiMe3, CH2CEt3, CH 2CMe2CH2Ph) were synthesized and fully characterized by NMR spectroscopy and in some cases X-ray diffraction. The molecular structures determined display the bent-sandwich coordination mode common for zirconocenes. The observed structural parameters are slightly perturbed by the steric influence of the bulky alkyl group.;A direct examination of propagation kinetics for alkene polymerization using the zirconocenium initiator, [(eta5-C5H 5)(eta5-C5Me5)Zr(CH 2CMe3)]+[CH3B(C6F 5)3]-, is reported. Propagation rate constants (kp) for the polymerization of propene and a series of 1-alkenes catalyzed by [(eta5-C5H 5)(eta5-C5Me5)Zr(CH2CHR) nCH2CMe3)]+[CH3B(C 6F5)3]- were measured by 1H NMR spectroscopy in toluene-d8 at low temperature. The kp obtained for propene and other alkenes decreases with increasing chain length and steric influence. The overall activation parameters for propagation determined from an Eyring analysis are Delta H‡ = 8.5(3) kcal·mol-1 and DeltaS‡ = -25(2) eu. The propagation rate was found to increase in the presence of [CH 3B(C6F5)3]- counteranion and in a polar toluene-d8-chlorobenzene- d5 solvent system. The experimental results are most consistent with propagation mechanism that does not involve the formation of outer-sphere ions for alkene polymerization by this catalyst system.;Propene initiation kinetics have been examined for a series of alkylzirconocene initiators, [(eta5-C5H5)(eta 5-C5Me5)Zr(R)]+[CH3B(C 6F5)3]- (R = CH3 (5), CH2CMe3 (1), CH 2SiMe3 (4)). Measurement of k i for the neopentyl initiator reveals that the rate of initiation is on the order of propagation for this catalyst. This initiator---with a polymeryl like alkyl group---serves as a model of the propagating species in propene polymerization. The catalyst initiation behavior has been investigated and the observed relative rates of propene initiation are not always predicted by ground-state (zirconium-carbon bond strength or extent of ion-pairing) considerations. The catalysts 4 and 5 are both poor initiators with ki << k p and show less than 50% initiation in the presence of excess propene at -60°C in toluene-d8.