Studies on Ziegler-Natta catalysts. Part II. Reactions between α- or β-TiCl3 and AlMe3, AlMe2Cl,or AlEt3 at various temperatures

The following reactions, carried out in the absence of solvents, has been studied: α-TiCl₃ + Al(CH₃)₃ at 20°C., β-TiCl₃ + Al(CH₃)₃ at 65°C., α-TiCl₃ + Al(CH₃)₂Cl at 20 and 65°C., and α-TiCl₃ + Al(C₂H₅)₃ between 30 and 65°C. It appears that a general reaction mechanism, such as discussed in the preceding paper of this series, applies to all these reactions between TiCl₃ and aluminum alkyls. The differences in overall stoichiometry between some of these systems may be linked to differences in stability of the intermediate Ti? C bonds. In the case of α-TiCl₃ + Al(CH₃)₂Cl, alkylation is probably accompanied by fixation of the AlCH₃Cl₂ on the nonvolatile product.     

Vibrational spectrum of α- and δ-TiCl3

The vibrational spectrum of α-TiCl₃ is assigned on the basis of a normal coordinate analysis. The i.r. bands shift to higher frequency from α- to δ-TiCl₃. This is discussed in terms of a dispersion relation for k_c in α-TiCl₃ and force constants, where the former is carried out for examining effect on disorder to the c axis in δ-TiCl₃. The shift is explained in terms of a change in force constants rather than disorder.     

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 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 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.     

Crown effect in stereospecific polymerisation of dienes employing ziegler-natta heterogeneous catalysts

The formation dynamics and the structure of polymerisation active sites (AS) of the catalyst systems α-TiCl₃ (γ-, δ-TiCl₃, VCl₃), β-TiCl₃ with metal alkyls (R₃Al, R₂Be, R₂Mg, RLi, R₂Zn, R₂AlCl), as well as various models for growing polymeric chains, have been studied using the methods of “diatomic complexes” in molecules, CNDO/2 and “atom-atomic” potentials. The formation of AS is preceded by physical adsorption of an organometallic compound (the homogeneous component) on the surface of the crystals of Ti and V trichlorides (the heterogeneous component). At this stage, the composition and the relative numbers of active sites different in their stereoregulating functions are clearly defined. During the chemisorption step, active sites of polymerisation are formed. For the most part they have a specific bimetallic crown structure of the “tunnel” (the U'-site) on α-(γ-, δ-)TiCl₃ (VCl₃) or the “flower” type on β-TiCl₃ and are formed by the atoms of Ti, Al (Be, Mg, Li), R and Cl. The stereospecific structure of the AS of polymerisation determines the stereospecific action of catalysts and the polymerisation rate. Stereospecific polymerisation of dienes involves the participation of two metal atoms and the surrounding ligands (bimetallic catalysis), with the monomeric molecules activated via the “push-pull” mechanism followed by “stapling” at the TiR bond of the bridge bond TiRMe.     

Exafs structural parameters for some titanium halides

α-TiCl₃ and TiCl₂ were subject to an X-ray absorption spectroscopic study at the K threshold of titanium. Data analysis was performed by Fourier methods and curve fitting techniques. Using the theoretical phases and coordination numbers Ti-Cl distances of 2.46(1) and 2.42(1) Å are derived for α-TiCl₃ and TiCl₂, respectively, as compared to the crystallographic values of 2.46(1) and 2.50(?) Å. The observed discrepancy in case of TiCl₂ is discussed. The EXAFS results are confirmed by Rabe's evaluation procedure for the absorber-backscatterer interatomic distance (R, Å) as a function of the photoelectron wave vector k (Å^?1).     

Synthesis and characterization of polymers of 1,4-hexadienes

The homopolymerization of trans-1,4-hexadiene, cis-1,4-hexadiene, and 5-methyl-1,4-hexadiene was investigated with a variety of catalysts. During polymerization, 1,4-hexadienes undergo concurrent isomerization reactions. The nature and extent of isomerization products are influenced by the monomer structure and polymerization conditions. Nuclear magnetic resonance (NMR) and infrared (IR) data show that poly(trans-1,4-hexadiene) and poly(cis-1,4-hexadiene) prepared with a Et₃Al/α-TiCl₃/hexamethylphosphoric triamide catalyst system consist mainly of 1,2-polymerization units arranged in a regular head-to-tail sequence. A 300-MHz proton NMR spectrum shows that the trans-hexadiene polymer is isotactic; it also may be the case for the cis-hexadiene polymer. These polymers are the first examples of uncrosslinked ozone-resistant rubbers containing pendant unsaturation on alternating carbon atoms of the saturated carbon-carbon backbone. Polymerization of the 1,4-hexadienes was also studied with VOCl₃- and β-TiCl₃-based catalysts. Microstructures of the resulting polymers are quite complicated due to significant loss of unsaturation, in contrast to those obtained with the α-TiCl₃-based catalyst. In agreement with the literature, there was no discernible monomer isomerization with the VOCl₃ catalyst system.     

Layer-lattices in Ziegler–Natta catalysts

Some layered inorganic compounds of interest in old and new polymerization processes by Ziegler-Natta stereospecific catalysis have been studied. The results are reported of an investigation concerning the crystallographic disorder phenomena in α-TiCl₃, VCl₃, FeCl₃, ball-milled α-MgCl₂, β-MgCl₂, and in a new catalytic system produced according to some recent trends in the field. Best fits are reported of the experimental x-ray diffraction patterns with patterns calculated for a theoretical model that accounts for structural disorder; the different fits indicate that disorder phenomena due to stacking faults are rather frequent in the examined compounds. The catalytic systems are more and more dispersed, and the maximum of dispersion in heterogeneous systems is reached for the recently proposed formulations.     

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.     

A crystallographic study of disorder in titanium trichloride,a component of Ziegler-Natta catalysts for stereospecific polymerization

The profiles of X-ray diffraction patterns have been examined for several powder samples of δ-TiCl₃, obtained by mechanical activation of γ-TiCl₃ and presenting different catalytic activities in the Ziegler-Natta stereospecific polymerization of propylene to isotactic polypropylene. Particular attention has been paid to the δ-TiCl₃ samples showing the best catalytic properties.On the basis of a previous investigation by Allegra, a mathematical treatment has been developed and a disorder function has been elaborated taking into account, in calculating the profiles of the X-ray diffraction intensities, not only the disorder effects but also the sizes of the δ-TiCl₃ crystallites. The selected experimental pattern (δ-TiCl₃, highly activated) has been well reproduced in this way and fitted by overlapping the calculated spectra of two disordered forms, labelled ?₁ and ?₂, which differ in the relative amounts cubic and hexagonal sequences of the Cl-Ti-Cl layers constituting the violet forms of TiCl₃. The best fit was achieved by introducing into the calculations crystallite sizes of about 70 Å.The results of the structural investigation are also discussed in terms of activity of the δ-TiCl₃ based catalyst for the stereospecific polymerization of propylene.     

Copolymerization of ethylene with higher linear α-olefins—reactivity ratios

Copolymerization of ethylene with mixtures of linear α-olefins C₆–C₃₆ in the presence of two heterogeneous Ziegler–Natta catalysts, δ-TiCl₃–AlEt₃ and TiCl₄/MgCl₂–AlEt₃, at 90°C was studied by the GC method, and reactivity ratios for all paris ethylene–α-olefin were estimated from the data on olefin consumption in the reactions. In the case of the δ-TiCl₃–AlEt₃ system, the r₂ value decreases from ca. 0.05 for 1-decene to ca. 0.02 for α-C₂₂H₄₄ and then remains approximately constant. This change is similar to the dependence of the modified steric parameter E_S^C of the olefin alkyl group on the size of the alkyl group. In the case of the supported TiCl₄/MgCl₂–AlEt₃ system a similar variation of r₂ with the length of the alkyl group were observed but the absolute values of r₂ were six to ten times lower than those for the first catalytic system.     

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.     

Homo-and copolymerization of vinylcyclohexane with α-olefins in the presence of heterogeneous and homogeneous catalytic systems

The polymerization and copolymerization of vinylcyclohexane with α-olefins in the presence of several heterogeneous and homogeneous catalytic systems were studied. It was shown that, with respect to activity in the polymerization of vinylcyclohexane, the tested catalysts can be arranged in the following order: α-TiCl₃ < titanium-magnesium catalyst < metallocene catalyst. Poly(vinylcyclohexane) prepared with heterogeneous catalytic systems is a solid semicrystalline polymer. The properties of polymers synthesized with homogeneous systems differ substantially depending on the type of the metallocene used. In the presence of metallocenes with a C ₂ symmetry, crystalline powderlike products arise, while in the case of metallocenes with C ₁ and C _s symmetries, polymerization yields amorphous viscous products. Molecular-mass distributions of poly(vinylcyclohexane) samples prepared using both heterogeneous titanium-magnesium catalysts and homogeneous metallocene complexes show a bimodal pattern, indicating the heterogeneity of active centers of these catalysts. Upon introduction of a comonomer (ethylene, propylene, and 1-hexene) into the reaction mixture, the activity of all studied catalytic systems increases. When Me₂C(3-Me-Cp)(Flu)ZrCl₂ and rac-Me₂SiInd₂ZrCl₂ are used as catalysts, the degree of crystallinity of the copolymers grows owing to the presence of ethylene or propylene units in poly(vinylcyclohexane) chains.     

Novel preparation of pure β-TiCl3 and its use in isoprene polymerization

Treatment of the reaction product of TiCl₄, Al, AlCl₃, and an aromatic compound with an ether and subsequently with TiCl₄ yields very pure β-TiCl₃. This material, when treated with small amounts of aluminum trialkyls, is a very active catalyst for the stereospecific polymerization of isoprene. If the above reaction is stopped after the ether addition, before the addition of TiCl₄, the product so obtained is largely TiCl₂. Reaction variables in the preparation of TiCl₃ are described as is the effect of various organoaluminum compounds as cocatalysts for polymerization.     

Experimental and calculated vibrational spectra and structure of Ziegler-Natta catalyst precursor: 50/1 comilled MgCl2-TiCl4

The vibrational Infrared and Raman Spectra of a MgCl₂-TiCl₄ Ziegler-Natta catalyst precursor with a 50/1 MgCl₂/TiCl₄ ratio have been recorded. The Raman spectrum of this catalyst precursor, in the range 50-500 cm⁻¹, shows clear scattering lines which can be assigned to the complex MgCl₂-TiCl₄, well separated from those of the initial species. Analogous, but less clear signals can be found in the infrared spectrum. Vibrational symmetry analysis and quantum chemical calculations of suitable models of MgCl₂-TiCl₄ complex have been made for the interpretation of the experimentally recorded spectra. The observed spectroscopic signals can be explained in terms of the existence of only one type of MgCl₂-TiCl₄ complex where the TiCl₄ molecules are complexed on the MgCl₂ along the (110) lateral cuts in a local C_2v symmetry with the Ti atoms in an octahedral coordination.     

Stereospecificity of catalytic systems MCl3AlEt3 in propylene polymerization reactions

Stereoregularity parameters are measured by different i.r. methods for samples of polypropylene, obtained with catalytic systems MCl₃AlEt₃ (M  Ti, V, Cr and Fe). The degree of stereoregularity of the fractions of these polymers insoluble in boiling n-heptane and n-octane decreases in the order α-TiCl₃ ? δ-TiCl₃ > CrCl₃ > VCl₃ > FeCl₃ which is close to the order of the decrease of MCl and MM distances in the MCl₃ lattices.     

Total synthesis of cis-shisool

This paper describes the total synthesis of cis-shisool through intermediate 3, which was derived from the Diels-Alder adduct 4 (or 5). The structure of the isomers produced by the Diels-Alder reaction of cyclohexadiene and crotonate has been elucidated by means of GC and NMR. Methylenation of 12 with Zn-CH₂Br₂-TiCl₄ or Ph₃P=CH₂ retains the stereochemistry of the carbon atom adjacent to the carbonyl group.     

Effect of the particle size of the TiCl<Subscript>4</Subscript>-Al<Emphasis Type="Italic">i</Emphasis>-Bu<Subscript>3</Subscript> catalyst on the contribution from mono- and bimetallic active sites to the polymerization of isoprene

The interaction between the coordinatively unsaturated surface of ß-TiCl₃ particles and a liquid phase in the TiCl₄-Ali-Bu₃ catalyst is responsible for the final particle size and the regularities of isoprene polymerization. The correlations of the catalyst activity and the molecular characteristics of polyisoprene with catalyst particle size in the course of catalyst formation and reactivation are indicative of the occurrence of two groups of active sites. “Surface” active sites correspond to the monometallic Cossee model, and they are characterized by low activity and low 1,4-cis specificity in the polymerization of isoprene. “Colloid” active sites have a bimetallic structure and produce polyisoprene at a high rate; the concentration of 1,4-cis units in the resulting polyisoprene is as high as 97%. The contribution from the colloid active sites to the polymerization of isoprene increases with the particle size of ß-TiCl₃.     

The magnetic properties of linear chains in β-TiCl3

Spin excitations in the linear chains of localized titanium 3d_z2 electrons are held responsible for the weak, temperature-independent static magnetic susceptibility of β-TiCl₃. Dynamic susceptibility measurements (ESR) fail to indicate any temperature-independent contribution to the susceptibility, and instead show a strongly temperature-dependent susceptibility. This discrepancy is attributed to the fact that in ESR only the unpaired electrons at titanium sites on the surface of β-TiCl₃, at the end of the one-dimensional chains, can be detected. The peculiar temperature dependence observed is ascribed to stress relaxation of two kinds of chain-end site. Both can occur in a ground-state nuclear configuration, in which ESR is observable, and in an excited nuclear configuration, in which it is not observable. Effects observed after adsorption of gases onto the surface are briefly discussed.