Kinetics and mechanism of ethylene and propylene polymerizations catalyzed with ansa-zirconocene activated by borate/TIBA

Ethylene and propylene polymerizations catalyzed with two ansa-zirconocenes (rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2 (M-I) and rac-Et(Ind)2ZrCl2 (Mt-II)) activated by (Ph3C)B(C6F5)4/triisobutylaluminum were respectively conducted under the same conditions for different duration ranging from 0.5 to 30 min, and quenched with 2-thiophenecarbonyl chloride for determining the active center fraction ([C∗]/[Zr]). Variations of polymerization rate, molecular weight, isotacticity and thermal properties with time were also studied. The [C∗]/[Zr] fraction gradually increased with time in the first 20 min and then reached steady stage in all polymerization systems. [C∗]/[Zr] value in the steady stage of Mt-II catalyzed ethylene polymerization was significantly higher than that of propylene polymerization, similar to the phenomena previously observed in ethylene and propylene polymerizations catalyzed with Mt-II/methylaluminoxane. When M-I/borate was used as the catalyst, both ethylene and propylene polymerization showed higher steady stage [C∗]/[Zr] levels than those of the Mt-II/borate systems, and the influence of zirconocene structure on [C∗]/[Zr] of propylene polymerization was much stronger than that of ethylene polymerization. Change in zirconocene structure also significantly influenced apparent chain propagation rate constant (kp) and its time-dependent variations. By taking into consideration the time-dependent changes of [C∗]/[Zr], polymer’s molecular weight distribution and chain structure, presence of multiple active centers in the polymerization system and late activation of the centers with lower intrinsic reactivity are proposed to be the main reasons for steep decay of kp with time. Structure-performance relationships of the ansa-zirconocene catalysts have also been discussed based on the results of kinetic study.