Kinetics and mechanistic investigations of ethylene-propylene copolymerizations catalyzed with symmetrical metallocene and activated by TIBA/borate

A set of ethylene-propylene (E-P) has been copolymerized by asymmetric metallocene catalysts i.e. silylene-bridged rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2 (SiPh) and ethylene-bridged rac-Et(Ind)2ZrCl2 (EBI) and cocatalyzed with triisobutylaluminum(TIBA)/ borate [Ph3C][B(C6F5)4] under similar conditions for differ-ent desire time. Quenching reaction of active center ([C*]/[Zr]) with 2-thiophenecarbonyl chloride (TPCC) for determining the Sulfur content. The obtained E/P copolymers were characterized with GPC, FT-IR, 13C-NMR and sulfur analyzer techniques. Changes in reactivity ratios rE and rP (reactivity ratio of ethylene (rE) and propylene (rP)), chain propagation rate Rp (chain propagation rate for ethylene ethy-lene (RpE) and chain propagation rate for propylene RpP respectively) molecular weight (Mw), molecular weight distribution (MWD), and chain propagation rate constant kp (propagation rate constant for E (kpE) and propagation rate constant for propylene (kpP) respectively) also discussed. The results of the kinetic constants, changing in the polymerization time (tP) exerted evident effects on [C*]/[Zr] and kp. At initial polymerization time lower [C *]/[Zr] with higher kp was observed in both metallocenes. Initial propaga-tion rate constant kpE and kpP were quite large and quickly declined with polymerization time (tP). The fast decline of kpE and kpP with (tP) greatly attributing to serious diffusion restriction produced by the growing polymer chains. When (SiPh) was used in E/P copolymerization presented higher steady stage [C *]/[Zr] levels as compared to EBI/borate system, and the effect of (SiPh) catalyst structure on active cen-ter [C *]/[Zr] of E/P copolymerization was considerably stronger. By taking sight into the time-dependent variations of active caters [C*]/[Zr], chain structure, molecular weight distribution, and late initiation of the centers with lesser intrinsic reactivity are suggested to be the basic reasons for the steep decline of chain propagation rate constant with time. Based on kinetic data we also discussed the structure perfor-mance relationships of C 2-symmetric metallocene. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

The copolymerization of ethylene-propylene (E-P) using asymmetric metallocene catalysts under similar conditions was studied, showing significant effects of polymerization time on sulfur content and reactivity ratio constants. The use of different catalysts also influenced the active center levels in the E/P copolymers, with a notable decline in chain propagation rate constants over time.