Evidence for several species of active sites in Ziegler-Natta catalysts

The kinetics of isoprene polymerization catalyzed by VCl₃ and Et₃Al were studied by measuring fractional conversions, polymer composition, and molecular weight distributions at a series of reaction times and temperatures. The rate of polymerization plotted against temperature shows an inflection point with a minimum and maximum in the 60–90°C range. The isomeric composition of the polymer changes with temperature but not with reaction time, while the molecular weight distribution undergoes substantial change with both of these variables. The rate of polymerization at sites producing low molecular weight polymers was measured, and the activation energy calculated to be about 10 kcal/mole. The active sites were found to deactivate at different rates. The results support the hypothesis that several species of active sites are present in the system and that these exhibit characteristic polymerization behavior.     

Ziegler-Natta催化体系中影响聚丙烯分子量分布的因素研究进展

分子量分布是聚丙烯产品的一个重要参数,直接影响材料的使用和应用,关于聚丙烯分子量和分子量分布的控制和影响因素的研究对于工业界和学术研究都有十分重要的意义。本文主要从催化剂组成、聚合条件等方面对Ziegler-Natta催化体系中影响聚丙烯分子量分布的因素进行了详细的综述。在催化剂组成方面,主要讨论了内给电子体、外给电子体及给电子体复合技术对聚丙烯分子量分布的影响;在聚合条件方面,主要讨论了聚合时间、聚合温度、助催化剂、聚合工艺对聚丙烯分子量分布的影响。     

Hexene-1 polymerization by homogeneous zirconocene and heterogeneous-supported TiCl3 catalysts

MgCl₂-supported TiCl₃ catalysts, with and/or without electron donor modifier (internal B_i or external B_e), were compared with rac-ethylenebis(indenyl)zirconium dichloride ( 1 ) activated with either MAO or the cation forming agent, triphenyl carbenium tetrakis(pentafluorophenyl)borate ( 2 ), with triethylalumium (TEA). The activities of the heterogeneous catalysts depend on the presence or absence of the Lewis base, were relatively insensitive to the temperature of polymerization, and produce poly(hexene) with molecular weights up to 10⁶. The 1 /MAO catalyst has about five times higher activity at 50°C but is almost inactive at ?30°C; the overall activation energy is 12.4 kcal mol^?1. In contrast, the activity for hexene polymerization by the 1/2 /TEA catalyst is actually slightly greater at lower temperature. The MW's of poly(hexene) obtained with the zirconocenium catalysts are only in the tens of thousands because of rapid β-hydride elimination by the electrophilic cationic Zr center. © 1993 John Wiley & Sons, Inc.     

Temperature dependence of stereospecificity of active sites in heterogeneous catalysts for propylene polymerization

Temperature dependences of the stereoregularity parameters of the most stereospecific active centres of α-TiCl₃-AlEt₃ catalytic system from ?25 to 120 and of VCl₃-AlEt₃ catalytic system from ?15° to 90° have been measured. It was found that this temperature dependence could be represented by a curve with a minimum at 20–50. The results could be explained by a two-step mechanism of isotactic chain growth (propagation) with preliminary co-ordination of monomer on the active centres.     

Studies on Ziegler-Natta catalysts. Part V. Stereospecificity of the active center

Experimental results on Ziegler-Natta catalysts, based on observations made with the electron microscope, and a qualitative comparison of the stereospecificity of various catalyst combinations are given. The polymerization of olefin in these experiments is performed in the gas phase on dry catalysts in the absence of solvent or excess aluminium alkyl. The crystallographic structure of the lateral faces of α-TiCl₃ is established by electron microscopy and electron diffraction. The electron micrographs of α-TiCl₃–AlMe₃ catalysts show that the active centers, which are revealed by the dotwise formation of polymer, are located along the growth spirals, on lateral faces, and on surface defects. These regions of the surface are the only regions in which the surface titanium atoms are incompletely coordinated. The presence of chlorine vacancies and exposed titanium atoms is therefore an essential condition for the formation of active centers. However, the number of active centers is small in comparison to the number of incompletely coordinated titanium atoms, and hence it is concluded that the normally occurring α-TiCl₃ sites with one vacancy do not yield active centers on reaction with aluminum alkyl. It is proposed that the reaction with aluminum alkyl on such sites leads ultimately to a bimetallic complex which fills the original vacancy on the titanium atom. That the complexation is reversible and that the deblocked alkylated site, which is of the type proposed by Cossee, is an active center is not excluded. Such a center would, however, give atactic polymer. Similar complex formation on a TiCl₃ site having originally two vacancies would leave one vacancy on the titanium atom. This is believed to be the center of stereospecific polymerization. A model of this active center and a mechanism of polymer growth on it are proposed.     

Mechanism of olefin polymerization on supported Ziegler-Natta catalysts based on data on the number of active centers and propagation rate constants

The number of active centers (C _g) and propagation rate constants (k _g) for the polymerization of propylene and ethylene on highly active titanium-magnesium catalysts (TMCs) of different compositions at 70°C were determined using the method of ¹⁴CO inhibition of polymerization. In the polymerization of propylene on the TiCl₄/D₁/MgCl₂-AlEt₃/D₂ system (D₁ is dibutyl phthalate or 2,2-diisobutyl-1,3-dimethoxypropane; D₂ is a silane), the effects of D₁ and D₂ donors on the values of C _g and k _g were studied. It was found that the donors decreased the values of k _g for nonstereospecific centers, had no effect on the values of k _g for stereospecific centers, and increased the fraction of stereospecific centers, as well as the fraction of sleeping centers regardless of their stereospecificity. The rate constants of isotactic-chain transfer with C₃H₆, AlEt₃, H₂,and ZnEt₂ were determined. In the polymerization of ethylene, a number of TMCs exhibited strong diffusion limitations, which manifested themselves in a dramatic decrease in the determined values of k _g. It was demonstrated that diffusion limitations can be removed by decreasing the particle size and the concentration of active centers and by performing prepolymerization with propylene. The values of k _g in ethylene polymerization were similar for stereospecific and nonstereospecific centers.__________Translated from Kinetika i Kataliz, Vol. 46, No. 2, 2005, pp. 180–190.Original Russian Text Copyright © 2005 by Bukatov, Zakharov, Barabanov.     

Molecular mechanics and mechanisms of regulation of the stereospecificity in Ziegler-Natta catalysis

The model catalytic sites, proposed in our group for the homogeneous and heterogeneous stereospecific Ziegler- Natta polymerizations of olefins, are reviewed. For all the homogeneous metallocene-based as well as for the heterogeneous catalytic models, a common mechanism of enantioselectivity is indicated by the study of the non-bonded interactions. This mechanism of enantioselectivity, which involves a chiral orientation of the growing chain, is in agreement with a large number of experimental results for these catalytic systems. The model sites for the homogeneous isospecific polymerization of propene are also able to account for the observed enantioselectivities in the regioirregular placements. Recent calculations relative to a peculiar catalytic model site, which should present a polymerization mechanism involving a regular back-skip of the chain, to the starting position after each monomer insertion, are shortly reviewed. The relevance of this polymerization mechanism to the comprehension of the stereospecificity of some homogeneous and heterogeneous catalytic systems is also briefly discussed.     

Studies on Ziegler-Natta catalysts. Part IV. Chemical nature of the active site

The determination of the number of sites active in the polymerization of ethylene on the surface of α-TiCl₃–Al(CH₃)₃ dry catalysts leads to the conclusion that this number is small in comparison to the total surface of the catalyst. Qualitatively this conclusion is also reached by two other independent methods. Infrared spectra of the catalyst before and after polymerization do not show a change in the type of bonds present in the surface. Electron microscopy proves that no active sites are formed on the basal plane of the α-TiCl₃ which constitutes 95% of the total surface. The results strongly favor the lateral faces of α-TiCl₃ as the preferred location of active centers. The lateral faces contain chlorine vacancies and incompletely coordinated titanium atoms. This must then be the essential conditions for the formation of active centers. The propagation of the polymer chain has been repeatedly shown to follow an insertion mechanism. The active site, therefore, necessarily contains a metal–carbon bond. The study of catalysts derived from TiCl₃CH₃ leads to the conclusion that a Ti? C bond on titanium of incomplete coordination is the active species in these cases. The alkylation of surface titanium atoms was proven to be an intermediate step in the catalyst formation from TiCl₃ and AlR₃. Survival of titanium–alkyl bonds on the lateral faces, where titanium atoms are incompletely coordinated explains best, in the light of our data, the activity of Ziegler-Natta catalysts. Coordination of aluminum alkyl compounds in or around the active center probably complicates the structure of the active centers.     

Optimization of ethylene trimerization using catalysts based on TiCl3/half‐sandwich ligands

Catalytic trimerization of ethylene using three titanium‐based complexes [η⁵‐C₉H₆C(R)thienyl]TiCl₃ with various types of bridges (R = cyclo‐C₅H₁₀ ( C1 ), cyclo‐C₄H₈ ( C2 ) and (CH₃)₂ ( C3 )) has been successfully optimized and compared. First of all, three benzofulvene precursors, C₉H₆C(R), were synthesized. Then the corresponding indenyl‐based ligands were obtained via the reaction of the precursors with thienyllithium. The final titanium‐based catalysts display a distorted tetrahedral geometry, as expected for Ti(IV), with the ligand coordinated with a hemilabile behaviour. The structures of the compounds were confirmed on the basis of various analyses. The effect of catalyst concentration, ethylene pressure, reaction temperature and nature of the bridge as the significant factor affecting coordination and orientation of thienyl group relative to the metal centre on 1‐hexene (1‐C₆) productivity and selectivity was investigated. Results revealed that the bulky bridge groups such as cyclo‐C₅H₁₀ and cyclo‐C₄H₈ are appropriate for ethylene trimerization due to the closer coordination of sulfur atom with Ti, especially in cationic state, and catalyst C2 with cyclo‐C₄H₈ bridge exhibits moderate productivity equal to 785 kg 1‐C₆ (mol Ti)^?1 h^?1. According to the results, ethylene at a pressure of 10 bar, 50°C and 1.5 μmol of catalyst were selected as the best conditions for obtaining 1‐C₆ with high productivity and selectivity. The presence of indenyl enhances the thermal stability of the catalysts and preserves their activity in higher temperatures such as 50 and 80°C.     

高分子载体Lewis酸催化剂:聚苯乙烯、三氯化铝/四氯化钛复合物

聚苯乙烯交联白球与等摩尔的AlCL₃和TiCl₄混合物在CS₂中反应,可制得一种十分稳定的高分子-双金属盐复合物。此复合物含氧量为11.4％,Al/Ti=2/1。它在丙酮水溶液中显强酸性,且约在2小时接近平衡酸度值。紫外及红外分析证实了聚苯乙烯与AlCl₃/DiCl₄之间复合物的形成。     

The nature of active sites and stereospecificity of their action in diene polymerization catalysed by chromium-containing systems

A study has been made of the nature of active sites, stereospecificity of their action and the regularities of diene polymerization catalysed by chromium-containing systems. All possible polymer structures with high stereospecificity can be produced for butadiene and isoprene with π-allyl chromium compounds. Tris-π-allyl chromium produces polybutadiene predominantly of 1,2-units. Cis-polybutadiene is formed when the electronegative group (Cl^?, CCl₃COO^?) is substituted for one or two π-allyl groups in Tris-allyl chromium or in the catalytic system (π-C₃H₅)₃CrAl₂O₃. A catalyst obtained through interaction of (π-C₃H₅)₃Cr with silica-alumina or silica gel produces 1,4-trans-polybutadiene and 1,4-trans-polyisoprene. The rate of butadiene polymerization in the presence of Tris-π-allyl chromium is given by k[Cr]², and in polymerization of isoprene with the catalytic system (π-C₃H₅)₃Cr-silica-alumina, by k[Cr].[M]². Polymerization of dienes catalysed by (π-C₃H₅)₃Cr-silica-alumina system or supported chromium oxide catalyst proceeds according to a type of living system. A study was made of copolymerization of butadiene and isoprene in the presence of supported chromium oxide catalyst and with that produced by the reaction of (π-C₃H₅)₃Cr with silica-alumina. The constants of copolymerization for the systems were equal. A conclusion has been drawn regarding the similar mechanisms for diene polymerization under the action of supported chromium oxide catalyst or of catalyst formed in the reaction of (π-C₃H₅)₃Cr with silica-alumina or silica gel.     

Study of Ziegler-Natta catalysts. Part I. Valence state and polymerization activity

Polymerization at temperatures lower than the temperature of catalyst formation induces no changes in the solid phase of the catalyst formed by Al(C₂H₅)₃ + TiCl₄. In polymerizations with catalysts of this class, maximum activity is observed when the titanium component of the catalyst is trivalent. Any different behavior indicates the presence of aluminum alkyl chlorides in the liquid phase of the catalyst.     

Determination of Mg, Ti and Cl in Ziegler-Natta catalysts by WDXRF

A method for determination of Mg, Ti and Cl in Ziegler-Natta (ZN) catalysts by wavelength dispersive X-ray fluorescence (WDXRF) spectrometry was developed. For comparative reasons, Ti was determined by spectrophotometry, Mg by complexometry and Cl by argentometric titration. Direct pressing was shown to be unsuitable for sample preparation due to catalyst decomposition. For Ti and Mg measurements, catalyst samples were calcinated at 1000 °C and pressed at 275 MPa. Their determination by the fundamental parameters based on the Ti Ka line measurement was shown to be equivalent to those results obtained by univariate calibration or by the classical methods. Cl was determined by aqueous extraction, followed by deposition on a support. Chloride loss was observed. Fixation of Cl as AgCl on polytetrafluoroethylene (FHLC) millipore membrane afforded the best results. Nevertheless, measurements by WDXRF were shown to be inferior to those obtained by argentometric titration.     

New evidence of the presence of internal electron donors in active sites of heterogeneous Ziegler-Natta catalysts

Poly(propylene) samples produced by heterogeneous Ziegler-Natta catalysts containing different internal electron donors were fractionated by using temperature rising elution fractionation; some key fractions were analyzed with ¹³C NMR. It was found that internal electron donors with different structures can convert aspecific active sites into different isospectrific ones. The employment of bis(2-ethylhexyl) phthalate as internal electron donor leads to chemically inverted structures in the atactic fraction. This suggests that an internal electron donor may also exist in the environment of aspecific active sites.     

Distribution of active centers on TiCl4/MgCL2 catalyst for olefin polymerization

1-Octene was polymerized with TiCl₄/MgCl₂—AlEt₃ and the polymerization was quenched with CH₃COCl to introduce a CH₃CO— group onto each propagation chain. The polymer was fractionated by fractional precipitation, and the number of active centers in each fraction was determined by measuring the CH₃CO— content of the fraction. Distributions of the number and reactivity of active centers among the fractions were determined and discussed based on these experiments. The active center distributions were also studied by fitting the molecular-weight distribution (MWD) curve from GPC with multiple Schulz-Flory most-probable distributions. The uncontinuous reactivity distribution of active centers reveals that there exist limited types of active centers on the catalyst. Five types of active center were distinguished by the MWD fitting treatment. The correlations between the active center distributions and catalyst structures are discussed. © 1996 John Wiley & Sons, Inc.