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.     

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.     

Studies on the nature of hydrated zinc oxide. Part II

Summary Three samples of hydrated zinc oxide have been precipitated with (1) 10% deficient alkali (2) just equivalent and (3) 10% excess alkali. The adsorption of hydrogen and hydroxyl ions by these three samples have been studied at two different temperatures. It has been found that adsorption of hydroxyl ions is maximum with sample (1) whereas sample (3) adsorbs hydrogen ions preferentiably. Ageing and heating decreases the adsorption of hydrogen and hydroxyl ions. Ageing is more prominent for sample (3). The effect of ageing of the different samples has been explained from the amphoteric behaviour of the hydrated zinc oxide.

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Zusammenfassung Drei Proben von hydratisiertem Zinkoxyd wurden mit 10% Unterschu? (1) mit ?quivalenter Menge (2) und mit 10% überschu? (3) an Alkali ausgef?llt. Die Adsorption von Wasserstoff und Hydroxylionen wurde an diesen Proben bei zwei verschiedenen Temperaturen untersucht. Probe 1 adsorbiert die Hydroxylionen maximal, w?hrend Probe 3 bevorzugt den Wasserstoff adsorbiert. Altern und Erhitzen vermindert die Adsorption, Alterung ist besonders für die Probe 3 ausgepr?gt. Der Effekt der Alterung der verschiedenen Proben wurde wiederum durch den amphoteren Charakter des Zinkoxyds erkl?rt.

     

Mechanism of stereoregular polymerization of butadiene by homogeneous Ziegler-Natta catalysts. II. Effects of the nature of bases and halogens

The effects of the addition of various bases to metal acetylacetonate–diethylaluminum chloride catalyst systems were investigated. Generally speaking, the cis content of polybutadiene decreases as the basicity of bases in a homologous series increases. The effects were more prominent for the cobalt and the titanium catalysts than for the nickel and the vanadium catalysts. The effects of the species of halogens on the microstructure of polybutadiene were also discussed. In conclusion, the decrease of the cis content was ascribed to the decrease of the electron density of dπ orbitals of the transition metal atoms.     

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.     

The nature of the active site in heterogeneous metal catalysis

This tutorial review, of relevance for the surface science and heterogeneous catalysis communities, provides a molecular-level discussion of the nature of the active sites in metal catalysis. Fundamental concepts such as "Br?nsted-Evans-Polanyi relations" and "volcano curves" are introduced, and are used to establish a strict partitioning between the so-called "electronic" and "geometrical" effects. This partitioning is subsequently employed as the basis for defining the concept "degree of structure sensitivity" which can be used when analyzing the structure sensitivity of catalytic reactions.