Abstract
The kinetics of ethylene and propylene polymerization catalyzed by homogeneous metallocene were investigated using 2-thiophenecarbonyl chloride followed by quenched-flow methods. The studied metallocene catalysts are: rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2 (Mt-I), rac-Et(Ind)2ZrCl2 (Mt-II) activated with ([Me2NPh][B(C6F5)4] (Borate-I), [Ph3C][B(C6F5)4] (Borate-II), and were co-catalyzed with different molar ratios of alkylaluminum such as triethylaluminium (TEA) and triisobutylaluminium (TIBA). The change in molecular weight, molecular weight distribution, microstructure and thermal properties of the synthesized polymer are discussed in detail. Interestingly, both Mt-I and Mt-II showed high activity in polyethylene with productivities between 3.17 × 106 g/molMt·h to 5.06 × 106 g/molMt·h, activities were very close to each other with 100% TIBA, but Mt-II/borate-II became more active when TEA was more than 50% in cocatalyst. Similarly, Polypropylene showed the highest activity of 11.07 106 g /molMt·h with Mt-I/Borate-I/TIBA. The effects of alkylaluminum on PE molecular weight were much more complicated; MWD curve changed from mono-modal in Mt-I/borate-I/TIBA to bimodal type when TIBA was replaced by different amounts of TEA. In PE, the active center fractions [C*]/[Zr] of Mt-I/borate were higher than that of Mt-II/borate and average chain propagation rate constant (kp) value slightly decreased with the increase of TEA/TIBA ratio, but the Mt-II/borate systems showed higher kp 1007 kp (L/mol·s). In PP, the Mt-I/borate presented much higher [C*]/[Zr] and kp value than the Mt-II. This work also extend to investigate the mechanistic features of zirconocenes catalyzed olefin polymerizations that addressed the largely unknown issues in zirconocenes in the distribution of the catalyst, between species involved in polymer chain growth and dormant state. In both metallocene systems, chain transfer with alkylaluminum is the dominant way of chain termination. To understand the mechanism of cocatalyst effects on PE Mw and (MWD), the unsaturated chain ends formed via β-H transfer have been investigated by 1H NMR analysis.
Highlights
On the way of synthesizing polyolefin with unique characterizations with narrow molar mass distributions and branching degree, academic and industrial researchers have devoted their consideration in polymer science and organometallic chemistry [1,2,3,4]
Ethylene polymerization at 0.1 MPa monomer pressure was conducted with Mt-MtII and I activated by different borate/alkylaluminum combinations (Borate-I, Borate-II, TEA and TIBA), respectively
The effects of alkylaluminum cocatalyst on propylene polymerization with Mt-I and Mt-II activated by different borate/alkylaluminum combinations were investigated
Summary
On the way of synthesizing polyolefin with unique characterizations with narrow molar mass distributions and branching degree, academic and industrial researchers have devoted their consideration in polymer science and organometallic chemistry [1,2,3,4]. To design new polymerization technologies based on transition metal compound, homogeneous metallocene single-site catalyst precursors and main-group organometallic alkylaluminum cocatalysts can be considered valuable strategies to achieve this goal [5,6,7,8,9]. The productivity of these polymerization catalysts has been significantly improved and produces polyolefin with narrow molecular weight distribution. Soluble Ziegler–Natta or homogeneous metallocene single-site catalyst can be used to synthesis new polymers without the need to remove the catalyst residues from the final polymer product. The activities of soluble catalysts are very encouraging for commercial purposes, but the kinetics behind these catalysts system have not been investigated extensively [14,15,16,17]
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