L. A. Novokshonova

Kinetics of Olefin Polymerization and Active Sites of Heterogeneous Ziegler–Natta Catalysts

Kinetic investigations of olefin polymerization with Ziegler–Natta (ZN) catalysts provide information for understanding the mechanism of these reactions and are also necessary for development of industrial productions of polyolefins. In this chapter, the main kinetic features of olefin polymerization with heterogeneous ZN catalysts are considered as well as problems such as a deviation from the linear dependence of the rate of polymerization on monomer concentration, the hydrogen effect in ethene and propene polymerizations, and the comonomer effect, the natures of which are not yet completely clear and are discussed in the literature. For analysis of the kinetics and mechanism of olefin polymerization, data on the number of active centers and propagation rate constants are important. The main methods for determination of these kinetic parameters are discussed in this chapter. Data on the number of active centers in ZN catalysts of different composition are presented. On the base of these kinetic data, the hydrogen effect and the heterogeneity of active centers at propylene polymerization over ZN catalysts are analyzed.

Olefin polymerization on immobilized zirconocene catalysts containing alkylaluminoxanes synthesized on the support surface

Ethylene and propylene polymerization on immobilized catalysts of composition MMT-H2O/AlR3/Zr-cene (MMT = montmorillonite) and on the corresponding homogeneous catalysts of composition Zr-cene-MAO (Zr-cene = rac-Et(Ind)2ZrCl2, rac-Me2Si(Ind)2ZrCl2, rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2, rac-1-(9-η5-Flu)2-(5,6-Cp2-Me-1-η5-Ind)Et]ZrCl2) is considered. Here, the activating support for the zirconocenes is montmorillonite containing methylaluminoxane or isobutylaluminoxane synthesized directly on the monmorillonite surface by the partial hydrolysis of an alkylaluminum (AlMe3, Al(i-Bu)3, Al(i-Bu)2H) with the mobile water of the support (MMT-H2O/AlR3). The MMT-H2O/AlR3 supports are demonstrated to be effective activators for ansa-zirconocenes. The catalytic properties of the immobilized systems (process kinetics and efficiency, the molar mass of the resulting polymer, and the structure of the macromolecules) depend on the activating support and the zirconocene precatalyst. The complexes of the MMT-H2O/Al(i-Bu)3 support with all zirconocene precatalysts are more active in propylene polymerization than the same complexes of MMT-H2O/AlMe3. The zirconcenes immobilized on MMT-H2O/AlR3 afford polyethylene and polypropylene with a higher molar mass than the corresponding homogeneous systems. Furthermore, immobilization causes activesite heterogeneity. As compared to the homogeneous single-site catalysts of composition Zr-cene-MAO, the corresponding catalysts immobilized on MMT-H2O/AlR3/Zr-cene are more stereospecific in the case of rac-Me2Si(Ind)2ZrCl2 (C2 symmetry) and are less stereospecific in the case of rac-[1-(9-η5-Flu)2-(5,6-Cp-2-Me-1-η5-Ind)Et]ZrCl2 (Cs symmetry).

Heterogenized Methylaluminoxane and Isobutylaluminoxane as activators for metallocene catalysts

Heterogenized alkylaluminoxanes prepared in situ on the montmorillonite (MMT) surface by the partial hydrolysis of AIR3 with water of the support are effective activators for metallocenes. The thermal destruction of isobutylaluminoxane molecules in the MMT-H2O/Al(i-Bu)3 system has been studied by the temperature-programmed desorption method coupled with mass spectrometry (TPD-MS). The process begins at a lower temperature and is more complicated than the destruction of methylaluminoxane (MAO) in MMT-H2O/AlMe3. Isobutyl-substituted aluminoxane and the ansa-metallocene Me2Si(Ind)2ZrCl2 form metal-alkyl complexes that are more active in propylene polymerization than methyl-substituted aluminoxane. The TPD-MS study of the initial stages of gas-phase ethylene and propylene polymerization shows that the nature of the metallocene in the heterogenized metallocene catalysts is an essential factor in the distribution of active sites by the activation energy of the thermal destruction of active Zr-C bonds.

Raman structural study of reactor blends of ultrahigh molecular weight polyethylene and random ethylene/1-hexene copolymers

For the first time we carried out a detailed Raman study of reactor blends of high-density ultrahigh molecular weight polyethylene (UHMW PE) with random ethylene/1-hexene copolymers (CEHs). The blends were produced by consecutive two-step polymerization in the presence of rac-Me2Si(Ind)2ZrCl2/methylaluminoxane catalyst. The blends differed significantly in the CEH content as well as in the 1-hexene content in the CEH. We revealed a strong dependence of the Raman spectra of the blends on their structure. We found out that an increase in both the CEH content in the blend and the 1-hexene content in the CEH causes a reduction of the blend crystallinity and the total content of trans-conformers, while an increase in the content of gauche-conformers is observed. To investigate the effect of molecular weight on the neat polyethylene (PE) structure and Raman spectrum, we analyzed three neat PE samples with molecular weights of 34 000, 750 000, and 1 000 000. In order to better understand general regularities in the spectra, Raman spectra of solid n-alkanes C18H38 and C36H74 were also studied.

Application of zeolites as supports for catalysts of the ethylene and propylene polymerization

The Na-form of zeolite ZSM-5 (SiO2/Al2O3=24) was studied as a support for organometallic catalysts of olefin polymerization. Zeolite-fixed aluminoxanes were prepared by partial hydrosysis of trimethylaluminium with the zeolites internal water. It was shown that aluminoxanes synthesized on the zeolite surface form heterogenized complexes with Cp2ZrCl2 and Et[Ind]2ZrCl2 which are active for a long time in ethylene and propylene polymerization without addition of another aluminiumorganic cocatalyst. Yields were up to 1200 kg PE/molZr. bar.h. and 4000 kgPP/molZr.bar.h. The activation energy of ethylene and propylene polymerization in the presence of Na-ZSM-5(H2O)/AlMe3-Et[Ind]2 ZrCl2 is equal to 32 and 48,5 kJ/mol, respectively. Molecular weight and melting point of polyethylene obtained with such zeolite supported Zr-cene catalysts are higher than those of polyethylene formed with the corresponding homogeneous metallocene systems.

Energy Distribution of Active Sites in Heterogeneous Ziegler-Natta Catalysts. Method of Study of Active Site Non-Uniformity

The new method of investigation of active site non-uniformity was developed. The method is based on mass-spectrometric study of temperature programmed desorption (TPD) products from the catalyst surface at initial stages of olefin polymerization (up to 10 – 15 monomer units in chain). The method allows to obtain the information concerning the energy non-uniformity of active sites in terms of a distribution of active sites over activation energy of active Mt-C bond thermal destruction. By the method, the initial stages of ethylene and deuteroethylene polymerization with Ti-supported catalysts of different structure were studied. It is shown that the view of energy distribution of active sites depends on catalyst structure. The values of activation energy of thermal destruction of Ti-C bonds in differing active sites were estimated and energy spectra of active sites for catalysts were obtained.

Study of the porous structure of TiCl3-based propylene polymerization catalysts☆

The method of capillary condensation of benzen and heptane has been employed to study the porous structure of highly active microspherical TiCl3 and a catalyst of TAS-101 grade. Study of the absorption of benzene and heptane vapours on the surface of the TiCl3 samples helped to determine the size of the specific surface, the total pore volume of the catalysts and the distribution of the volume of the pores over their effective radius. It has been shown that the difference observed in the kinetics of polymerization of propylene in the catalysts studied may be related to the special features of their porous structure.

Raman Spectroscopy of Novel UHMW Polyethylene-Based Nanocomposites with Nanographite and Nanoclay

We analyze the Raman spectra of nanocomposites based on ultrahigh-molecular-weight polyethylene with nanoclay, thermoexpanded graphite, and reduced graphite oxide fillers. We discuss the potential of Raman spectroscopy for quantitative analysis of the nanocomposite structure, the influence of the fillers on the phase and conformation compositions of the polymer matrix, as well as for the monitoring of dispersion of the nanographite fillers in the nanocomposites.

22-P-15-A new sorbent based on clinoptilolite-containing tuff modified by polyethylene

Publisher Summary This chapter discusses a novel sorbent based on clinoptilolite-containing tuff (CT) modified by polyethylene (PE). Clinoptilolite-containing tuff (CT) dust is modified by the catalytic polymerization of ethylene on the surface of CT particles. The compositions with 3-5wt.% of PE and 97–95 wt.% of CT are obtained. The thin PE coating formed on the CT particles is permeable to the filtrating water solutions. As a result of the encapsulation of the CT particles, the filter properties of this sorbent are improved. The ion-exchange characteristics of the modified CT (powder and pressed tablets) with respect to ammonium (NH 4 + ) and strontium (Sr 2+ ) cations are determined. CT dust–PE compositions retain the ion-exchange properties of initial CT.