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.     

Studies on Ziegler-Natta catalysts. Part I. Reaction between trimethylaluminum and α-titanium trichloride

The reaction between α-TiCl₃ and AlMe₃ at 65°C. in the absence of solvent was studied by a method which gives information about the early stages of the reaction. The results obtained give evidence for a sequence of consecutive reactions and show that the first of these is a fast partial alkylation of the α-TiCl₃ surface. The mechanism of formation of methane in the last step of the reaction is discussed in detail.     

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.     

Studies on Ziegler-Natta catalysts. Part III. Composition of the nonvolatile product of the reaction between titanium trichloride and trimethylaluminum or dimethylaluminum chloride

The surface product formed in the reaction between TiCl₃ and Al(CH₃)₃ has been studied. Stoichiometric data, CH₃/CD₃ exchange, and infrared spectra permit the conclusion that the surface product is essentially a compound having the formula A model structure is proposed for this compound, valid for the 001 face of α-TiCl₃. In it the titanium and chlorine atoms maintain the positions which they occupy in the α-TiCl₃ lattice. One of the methyl groups protrudes from the surface whereas the other occupies the chlorine vacancy created during the reaction in the chlorine surface layer. A different sterism of the methyl groups is compatible with the experimental result that half of the methyl groups are very easily exchanged whereas the other half are not touched by the exchanging agent. According to this model it has to be assumed that the titanium atoms in the 001 plane, by far the largest face of the α-TiCl₃ crystal, are not accessible. A similar model, loading to equivalence conclusions is proposed for β-TiCl₃. The infrared spectra of Al(CH₃)₃, Al(CD₃)₃, AlCl(CH₃)₂, AlCl(CD₃)₂, AlCl₂CH₃, AlCl₂CD₃, TiCl₃CH₃, TiCl₃CD₃, Hg(CD₃)₂, and Zn(CD₃)₂ are discussed. Spectra of surface products formed on interaction of some of these compounds with TiCl₃ are given.     

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.     

Study of Ziegler-Natta catalysts. Part II. The liquid phase and polymerization activity of the catalyst

The polymerization activity of the Ziegler-Natta catalysts is significantly affected by the aluminium alkyls in the liquid phase. These may be studied provided the solid phase of the catalyst remains unchanged. Part of these aluminum alkyls is adsorbed on the surface of the solid phase. It is possible to displace one type of alkyls by another one or to elute it from the adsorbed layer. This last affects the polymerization rate.     

Study of Ziegler-Natta catalysts. Part III. Effect of the structure of titanium trichloride on the polymerization of propylene

In addition to differences among the various modifications of TiCl₃ there may be certain structural differences even among α-TiCl₃ samples prepared by differences methods. Electron microscopic examination of two samples has revealed widely different free surfaces, in spite of the fact that both the specific surfaces (measured by adsorption) and the polymerization activities were identical. This might be explained by the finding that the surfaces of the free lateral planes and the quantities of the free edges are the same. This explanation is in agreement with the assumption of Rodriguez and his co-workers that the active centers of polymerization are situated on the lateral planes and edges of the TiCl₃ crystals.     

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.     

选择加氢催化剂结构与性能的研究进展

选择加氢催化剂是石油化工领域一类重要的催化剂,广泛应用于各类乙烯装置中C2、C3、C4等组分以及裂解汽油中炔烃和二烯烃的脱除等领域.本文从制备与处理方法、助剂、载体、活性中心结构形态、反应机理、非Pd催化剂6个方面,对近5年来选择加氢催化剂,特别是炔烃和二烯烃选择加氢催化剂结构与性能的研究进行了评述.     

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.     

Homogeneous vanadium-based catalysts for the Ziegler-Natta polymerization of alpha-olefins

Although their activity is often inferior to that of other systems, the use of vanadium-based catalysts in homogeneous Ziegler-Natta polymerizations allows the preparation of high-molecular-weight polymers with narrow molecular-weight distributions, ethene/alpha-olefin copolymers with high alpha-olefin incorporation, and syndiotactic polypropene. The main reason for the low activity of these catalysts is their deactivation during catalysis by reduction of active vanadium species to low-valent, less active or inactive species. We here present an up-to-date review of this area with particular emphasis on the attempts to improve catalyst performance and stability by the use of additives or ancillary ligands.     

Kinetics of olefin copolymerization with heterogeneous Ziegler-Natta catalysts

The kinetics of the ethylene/1-hexene copolymerization reaction with a Ti-based Ziegler-Natta catalyst has been studied. Kinetic analysis established the existence of several populations of active centers in the catalyst. The centers differ in two aspects: their ability to incorporate α-olefin units into copolymer chains (i.e., their reactivity ratios) and the average molecular weights of the polymer chains they produce. The centers of different populations are formed at different rates and have different kinetic stabilities. As a consequence, both the molecular weight distributions of the copolymers and their compositional distributions are relatively broad and change with in time. Two kinds of catalyst poisons were found. The poisons of the first type, arylalkoxysilanes, preferentially deactivate the centers which have the highest ability to copolymerize α-olefins with ethylene. These poisons decrease the average α-olefin content in the copolymers and the fraction of their olefin-rich components. The poisons of the second type, conjugated dienes, preferentially deactivate the centers which have the lowest ability to copolymerize α-olefins with ethylene. These poisons significantly increase the content of the olefin-rich components in the copolymers.     

Stereo- and enantioselective polymerization of hexa-1,5-diene on heterogeneous Ziegler-Natta catalysts obtained with the use of optically active internal donors

Pure racemates and individual enantiomers of diethyl 2,3-diisopropylsuccinate ((+,−)-DIPS and (−)-DIPS) and 2,3-diisopropyl-1,4-dimethoxybutane ((+,−)-DPDMB and (+)-DPDMB) were prepared. The synthesized compounds and diethyl phthalate were used as internal donors for the preparation of heterogeneous Ziegler-Natta catalysts. Polymerization of hexa-1,5-diene was carried out on these catalysts. The obtained samples of poly(methylene-1,3-cyclopentane) were studied by ¹³C NMR spectroscopy and polarimetry. The induction of optical activity to polymer is observed with (−)-DIPS as the internal donor. The induction value makes up 15–20% from the value recorded in the case of optically active metallocene catalysts. The emergence of induction may be associated with the presence of close contact between the titanium atom and the donor molecule within the active site of catalyst, as well as with the fact that the donor molecule either deactivates a part of the stereospecific titanium centers or influences the structure of the titanium centers during their formation and development.