Effect of internal electron donor on the active center distribution in MgCl2-supported Ziegler–Natta catalyst

Zhang

J. Polym. Sci. Part A: Polym. Chem.

et al. 2016

Self Cite

A series of ethylene, propylene homopolymerizations, and ethylene/propylene copolymerization catalyzed with rac‐Et(Ind)2ZrCl2/modified methylaluminoxane (MMAO) were conducted under the same conditions for different duration ranging from 2.5 to 30 min, and quenched with 2‐thiophenecarbonyl chloride to label a 2‐thiophenecarbonyl on each propagation chain end. The change of active center ratio ([C*]/[Zr]) with polymerization time in each polymerization system was determined. Changes of polymerization rate, molecular weight, isotacticity (for propylene homopolymerization) and copolymer composition with time were also studied. [C*]/[Zr] strongly depended on type of monomer, with the propylene homopolymerization system presented much lower [C*]/[Zr] (ca. 25%) than the ethylene homopolymerization and ethylene–propylene copolymerization systems. In the copolymerization system, [C*]/[Zr] increased continuously in the reaction process until a maximum value of 98.7% was reached, which was much higher than the maximum [C*]/[Zr] of ethylene homopolymerization (ca. 70%). The chain propagation rate constant (kp) of propylene polymerization is very close to that of ethylene polymerization, but the propylene insertion rate constant is much smaller than the ethylene insertion rate constant in the copolymerization system, meaning that the active centers in the homopolymerization system are different from those in the copolymerization system. Ethylene insertion rate constant in the copolymerization system was much higher than that in the ethylene homopolymerization in the first 10 min of reaction. A mechanistic model was proposed to explain the observed activation of ethylene polymerization by propylene addition. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 867–875

Section: Introductionmentioning

confidence: 71%

Kinetics and mechanism of metallocene‐catalyzed olefin polymerization: Comparison of ethylene, propylene homopolymerizations, and their copolymerization

Zhang

J. Polym. Sci. Part A: Polym. Chem.

et al. 2016

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“…Our group has previously reported the effects of alkylaluminum cocatalyst (TEA, TIBA and TEA/TIBA mixtures) on ethylene, propylene polymerizations and E/P copolymerization with Z-N catalyst. Stronger alkylaluminum cocatalyst effects on the active center distribution and molecular weight and molecular weight distribution of polyolefin have been found, and the effects were explained by the different alkylation, reduction and chain transfer properties of different diverse alkylaluminus [ 37 , 38 , 39 , 40 ]. According to the best of our knowledge, the effects of cocatalysts on fundamental polymer reaction stage have not been studied so far, despite the homogeneous metallocene catalyst systems based on borate/alkylaluminum derivatives demonstrated the most attractive combination of selectivity, activity and generality for a wide-ranging variety of olefins.…”

Section: Introductionmentioning

confidence: 99%

Kinetic and Thermal Study of Ethylene and Propylene Homo Polymerization Catalyzed by ansa-Zirconocene Activated with Alkylaluminum/Borate: Effects of Alkylaluminum on Polymerization Kinetics and Polymer Structure

et al. 2021

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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.

“…Although this famous catalyst attracts much attention from both academia and industry, the mechanism of the electron donors on the active site remains elusive, presumably due to the heterogeneity and multicomponent nature of the catalyst. Fortunately, great efforts have been carried out to understand the role of electron donors on the active site by experimental and theoretical methods. − Andoni et al observed a structure directing role of electron donors in the formation of MgCl 2 crystal morphology. , The presence of 1,3-diether led to the preferential growth of MgCl 2 crystals along the (110) surface, whereas diisobutyl phthalate or ethyl benzoate was less selective and allowed the growth of MgCl 2 crystals along both (110) and (104) surfaces. Furthermore, electron donors influence the distribution and amount of TiCl 4 in the ultimate catalyst.…”

Section: Introductionmentioning

confidence: 99%

Mechanistic Study on Effect of Electron Donors in Propylene Polymerization Using the Ziegler–Natta Catalyst

et al. 2021

TiCl 4 /MgCl 2 catalysts are still the main industrial catalyst for isotactic polypropylene at present. However, the mechanism of isotactic polymerization of propylene has not been fully understood. DFT calculations revealed that the bare Ti active site was regioselective and nonstereoselective in the absence of an electron donor. The role of electron donor ethyl benzoate (EB), diethyl-2,3-diisobutylsuccinate (DiBS), cyclohexylmethyldimethoxysilane (CMDMS), and dicyclopentyldimethoxysilane (DCPDMS) on the active site was investigated. The presence of EB, DiBS, CMDMS, or DCPDMS around the Ti active site can promote the activity and retain the regioselectivity. It is worth noting that in the presence of EB and DCPDMS, the stereoselective behavior can be promoted with the advantage of 1 kcal/mol. The copresence of AlEt 2 Cl species and external donors can increase both the stereoselectivity and regioselectivity.