Silica-supported (ⁿ BuCp)₂ ZrCl₂ : effect of catalyst active center distribution on ethylene-1-hexene copolymerization

Abstract

Metallocenes are a modern innovation in polyolefin catalysis research. Therefore, two supported metallocene catalysts -silica/MAO/( n BuCp) 2 ZrCl 2 (Catalyst 1) and silica/ n BuSnCl 3 /MAO/( n BuCp) 2 ZrCl 2 (Catalyst 2), where MAO is methylaluminoxane -were synthesized, and subsequently used to prepare, without separate feeding of MAO, ethylene-1hexene Copolymer 1 and Copolymer 2, respectively. Fouling-free copolymerization, catalyst kinetic stability and production of free-flowing polymer particles (replicating the catalyst particle size distribution) confirmed the occurrence of heterogeneous catalysis. The catalyst active center distribution was modeled by deconvoluting the measured molecular weight distribution and copolymer composition distribution. Five different active center types were predicted for each catalyst, which was corroborated by successive self-nucleation and annealing experiments, as well as by an extended X-ray absorption fine structure spectroscopy report published in the literature. Hence, metallocenes impregnated particularly on an MAO-pretreated support may be rightly envisioned to comprise an ensemble of isolated single sites that have varying coordination environments. This study shows how the active center distribution and the design of supported MAO anions affect copolymerization activity, polymerization mechanism and the resulting polymer microstructures. Catalyst 2 showed less copolymerization activity than Catalyst 1. Strong chain transfer and positive co-monomer effect -both by 1-hexene -were common. Each copolymer demonstrated vinyl, vinylidene and trans-vinylene end groups, and compositional heterogeneity. All these findings were explained, as appropriate, considering the modeled active center distribution, MAO cage structure repeat units, proposed catalyst surface chemistry, segregation effects and the literature that concerns and supports this study. While doing so, new insights were obtained. Additionally, future research, along the direction of the present work, is recommended.

The Cytotoxic Effect of Small and Large Molecules of PMF Fraction Extracted from Camel Urine on Cancer Cells

“…Furthermore, NMR can be used for structural elucidation and to study molecular dynamics [17][18][19][20][21][22]. Consequently, NMR is generally used as a complementary technique to mass spectrometry (MS) for the identification and quantitative analysis of the chemical composition of natural products and biological samples, such as urine [12,[23][24][25][26].…”

Metallocene-catalyzed ethylene−α-olefin isomeric copolymerization: A perspective from hydrodynamic boundary layer mass transfer and design of MAO anion

“…Accordingly, different active site ion-pairs [Zr] + [MAO] n − are likely to prevail in the experimental zirconocene catalysts. This subject-metallocene active center distribution-has been particularly detailed in some of our recent publications [20,24].…”

“…We shall use the following two differing anions, unsupported [MAOCl 2 ] − and supported Si−O−[MAOCl 2 ] − , the synthesis of which has been illustrated through Scheme 1 that shows the major relevant reactions [6,20]. Note that the variation in the design of novel co-catalyst anions, which is much easier and less costly than synthesizing new metallocenes, widens the scope of industrial process development.…”

“…We shall discuss the influence of this comonomer effect on copolymer intra-chain compositional heterogeneity in Section 3.5. The above findings can be attributed to the following: The above factors variedly influenced the following chemically intrinsic phenomena: (i) the zirconocenium−MAO anion separation [20,[44][45][46][47][48] (in ethylene homopolymerization) and (ii) the activation of the dormant/sleeping Zr + −H catalytic sites [20,[49][50][51][52][53][54][55][56][57]] by 1-hexene and 4M1P (in ethylene copolymerization). Hence, we observe varying homopolymerization catalyst productivities and comonomer effects, respectively.…”

“…Hence, we observe varying homopolymerization catalyst productivities and comonomer effects, respectively. Note that the closer is the ion-pair separation, the higher is the energy barrier associated with each polymerization step; hence, the less is the catalyst productivity [20,[44][45][46][47][48].…”