João B. P. Soares

Polymerization reaction engineering — Metallocene catalysts

Abstract Metallocene catalysts are operative in all existing industrial plants that are presently used for polyolefin manufacture and have the potential to revolutionize the technology for the production of these polymers. A review of metallocene catalysis and its effects on polymer process engineering for the manufacture of polyolefins is provided. This review concentrates on the aspects of polymer reactor engineering, mathematical modelling of polymerization processes, and the characterization of polyolefins made with these novel catalysts.

Mathematical modelling of the microstructure of polyolefins made by coordination polymerization: a review

It is surprising that polymers such as polyolefins, composed only of carbon and hydrogen atoms in very simple configurations, can be used in so diverse applications as human bone prosthetic implants, gas pipelines, car bumpers, synthetic fibers, and plastic films. The reason for such versatility is that microstructural characteristics of polyolefins, such as distributions of molecular weight, chemical composition (or short-chain branching), and long-chain branching, have an enormous impact on their macroscopic properties and applications. Therefore, it is of utmost importance that these microstructural details be well understood and properly described with fundamental models. This manuscript reviews the state-of-the-art mathematical modelling techniques for describing the microstructure of polyolefins produced by coordination polymerization.

Polyethylene-clay hybrid nanocomposites: in situ polymerization using bifunctional organic modifiers

Abstract A hybrid polyethylene–clay nanocomposite was prepared using in situ polymerization with bifunctional organic modifiers. Morphological characterization of the product shows that a fraction of the polyethylene chains is chemically linked to the silicate surface. The chemical modification and intercalation of montmorillonite was carried out with alkylaluminum and vinylalcohol. The vinyl groups chemically linked to the silicate surface were copolymerized with ethylene inside the clay galleries using a coordination catalyst. The polymerization leads not only to effective exfoliation of the layered silicate but also to polyethylene chains that are chemically bonded to silicate surface.

Controlling molecular weight distributions of polyethylene by combining soluble metallocene/MAO catalysts

A critical look at the possibility of controlling the molecular weight distribution (MWD) of polyolefins by combining metallocene/methylalumoxane (MAO) catalysts is offered. Catalysts investigated were bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2), its titanium and hafnium analogues (Cp2TiCl2 and Cp2HfCl2), as well as rac-ethylenebis(indenyl)zirconium dichloride (Et(Ind)2ZrCl2). As observed by other researchers, the MWD of polyethylene can be manipulated by combining soluble catalysts, which on their own produce polymer with narrow MWD but with different average molecular weights. Combined in slurry polymerization reactors, the catalysts in consideration produce ethylene homopolymer just as they would independently. Unimodal or bimodal MWDs can be obtained. This effect can be mimicked by blending polymers produced by the individual catalysts. We demonstrate how a variability in catalyst activity translates into a variability in MWD when mixing soluble catalysts in polymerization. Such a variability in MWD must be considered when setting goals for MWD control. We introduce a more quantitative approach to controlling the MWD using this method.

Copolymerization of ethylene and α-olefins with combined metallocene catalysts. I. A formal criterion for molecular weight bimodality

Metallocene and other single-site catalysts can be combined to produce polyolefins with broadened distributions of molecular weight, chemical composition, and long-chain branching. These resins are finding increasing applications because of their enhanced properties compared to ones made with conventional Ziegler–Natta catalysts. Resins with bimodal molecular weight distributions (MWDs) have especially attractive mechanical and rheological properties. Although the use of these resins is expected to increase, there are very few studies available to quantify MWD bimodality or to decide a priori which combinations of metallocene catalysts will lead to the formation of polyolefins with bimodal MWDs. In this article, a necessary condition for the production of polymer with bimodal MWD using two single-site-type catalysts is derived. Additionally, a bimodality index is defined to quantify MWD bimodality.

Characterization of the combined molecular weight and composition distribution of industrial ethylene/α-olefin copolymers

Abstract The composition–molecular weight distribution, CD×MWD, of ethylene/α-olefin copolymers was measured quantitatively using two methods: (1) temperature rising elution fractionation (TREF) followed by size-exclusion chromatography (SEC) of each fraction and (2) SEC followed by Fourier-transform infrared spectroscopy (FTIR) of each fraction. TREF-SEC provides a rather complete, accurate, and quantitative representation of the CD×MWD. On the other hand, SEC-FTIR leads to loss of information on some details of the CD×MWD. The extent of the loss depends on the material examined. The main advantage of SEC-FTIR is the much shorter analysis time compared to TREF-SEC. We briefly discuss another method, TREF followed by measurements of the molecular weight of the fractions by light scattering, LS — either off-line or on-line. We point out that this technique also leads to some loss of information on the CD×MWD. However, for linear low density polyethylene-type materials, TREF-LS may be a more useful technique than SEC-FTIR while less time-consuming than TREF-SEC. Finally, we use deconvolution of the SEC-FTIR data into Flory–Stockmayer distributions in an attempt to recover all or part of the information loss inherent in the SEC-FTIR method. The attempt was not successful when applied to the data presented in this work. We discuss several reasons for this failure including limited instrumental resolution and intrinsic limitations of the SEC-FTIR technique.

Polymerization mechanism for in situ supported metallocene catalysts

The polymerization of ethylene was carried out with a novel in situ supported metallocene catalyst that eliminated the need for a supporting step before polymerization. In the absence of trimethyl aluminum (TMA), in situ supported Et[Ind]2ZrCl2 was not active, but the addition of TMA during polymerization activated the catalyst. Et[Ind]2Zr(CH3)2 was active even in the absence of TMA, whereas the addition of TMA during polymerization enhanced the catalytic activity. The polymerization-rate profiles of the in situ supported metallocene catalysts did not show rate decay as a function of time. A polymerization mechanism for the in situ supported metallocene catalysts is proposed for this behavior. During polymerization, the in situ supported metallocene catalysts may deactivate, but homogeneous metallocene species present in the reactor may form new active sites and compensate for deactivated sites.

Synthesis of tailor-made polyethylene through the control of polymerization conditions using selectively combined metallocene catalysts in a supported system

Tailoring of the molecular weight distribution (MWD) in ethylene polymerization was attempted by selectively combining different types of metallocene catalysts onto a single support. The catalyst produced by supporting Et[Ind]2ZrCl2 and Cp2HfCl2 onto a single MAO pretreated silica support was able to produce polymers with unimodal or bimodal MWDs. This approach permits the synthesis of polyethylene with different MWDs using the same catalyst as a function of the polymerization conditions.

Effect of operating conditions on the molecular weight distribution of polyethylene synthesized by soluble metallocene/methylaluminoxane catalysts

Investigations of the effects of polymerization conditions on the molecular weight distribution (MWD) of polyethylene synthesized with soluble metallocene/methylaluminoxane (MAO) catalysts have been performed. The following variables were investigated in this study: catalyst type, polymerization temperature, catalyst concentration, MAO concentration, chain transfer agent, ethylene partial pressure, as well as the substitution of MAO with trimethylaluminium (TMA), and of different catalyst activities of polyethylene. Similarities and differences with other published results are highlighted. In all cases, an effort was made to illustrate the significance of the effects by presenting replicate measurements. Catalysts investigated were bis(cyclopentadienyl)zirconium dichloride (Cp 2 ZrCl 2 (1)), its titanium and hafnium analogues (Cp 2 TiCl 2 (2) and Cp 2 HfCl 2 (3)), as well as rac-ethylenebis(indenyl)zirconium dichloride (Et(Ind) 2 ZrCl 2 (4)) and rac-ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride (Et(H 4 Ind) 2 ZrCL (5)). According to a 2 2 factorial experiment, independent increases in the concentrations of catalyst or MAO cause a decrease in average molecular weight, with no interaction between these two factors. Replacing MAO with TMA at constant overall aluminium concentration causes a drastic decrease in average molecular weight. Extremely high polymerization rates were observed to impart only a slight increase in the breadth of the MWD. The effects of ethylene partial pressure suggest that for the zirconium catalysts, transfer to monomer is the main chain transfer mechanism, while for hafnium catalysts, this is not the case.

Copolymerization of ethylene and α-olefins with combined metallocene catalysts. III. Production of polyolefins with controlled microstructures

The distributions of the molecular weight (MWD) and chemical composition of polyolefins can be controlled by the combination of two or more metallocenes from the knowledge of the behavior of each individual metallocene. Polyolefins with bimodal MWDs and a higher comonomer content in high molecular weight chains have physical properties suitable for advanced applications such as pipes for gas and water distributions. With conventional Ziegler–Natta catalysts, this type of polymer resin is produced only by tandem reactor technology in which two or more polymerization reactors are used in series. With combined metallocene catalysts, similar polymer resins can be produced in a single reactor. The versatility of these combined metallocene catalysts is investigated in this article.