Distribution of active centres in Ziegler-Natta catalysts based on TiCl3 by their activity and stereospecificity: Catalyst poisoning

A fractional poisoning method has shown that all groups of Ziegler catalyst centres, both stereospecific and non-stereospecific, have a similar distribution of propagation rate constants. The most regular and highly molecular isotactic fraction is quite uniform by stereoregularity. Most of the poison-resistable part of active centres in δTiCl₃-AlEt₃ systems have properties similar to those of initial system TiCl₃-AlEt₂Cl, which may be caused by different adsorption abilities of co-catalysts AlEt₃ and AlEt₂Cl at TiCl₃ surface. Values n_p and $\text{k}\text{?}{}_{\mathrm{p}}$ have been estimated for δTiCl₃(TAC)-AlEt₃ system.     

Polarizer-Free Acousto-Optic Monochromators

Monochromators based on a noncollinear acousto-optic filter without external polarizers have been studied. As an element to select an operating diffracted light beam, (1) a spatial filter-telescope and (2) an output plane of a filter crystal rotated in a diffraction plane have been used. In the latter case, dispersion of the acousto-optic filter crystal is compensated using a correcting prism made from the same crystal and placed behind an output filter plane along a path of a filtered light beam. As a result, the transmission coefficient of the monochromator is increased upon an effective compensation of the angular drift of the filtered light beam, the monochromator design is simplified, and its sizes are decreased.

     

Stereospecific polymerization of propylene on systems TiX3-AlEt3

It has been shown that replacement of Cl atoms by I and Br atoms leads to decrease in the activity of the catalytis systems TiCl₃-AlEt₃ owing to decrease in the number of active centres. This decrease is probably due to the fact that a group of titanium atoms bound by halogen bridges, and not a single atom, is involved in the catalytis act. The most effective way of enhancing the isospecificity of the active centre is to induce the iodine-containing compound to act upon the TiCl₃ crystal surface. However, no simple correlation between the stereospecificity of the catalytis systems, ion radii of the anions and distances M-M and M-X was found.     

Hydrogen effects in propylene polymerization reactions with titanium‐based Ziegler–Natta catalysts. II. Mechanism of the chain‐transfer reaction

Hydrogen is a very effective chain‐transfer agent in propylene polymerization reactions with Ti‐based Ziegler–Natta catalysts. However, measurements of the hydrogen concentration effect on the molecular weight of polypropylene prepared with a supported TiCl₄/dibutyl phthalate/MgCl₂ catalyst show a peculiar effect: hydrogen efficiency in the chain transfer significantly decreases with concentration, and at very high concentrations, hydrogen no longer affects the molecular weight of polypropylene. A detailed analysis of kinetic features of chain‐transfer reactions for different types of active centers in the catalyst suggests that chain transfer with hydrogen is not merely the hydrogenolysis reaction of the Ti? C bond in an active center but proceeds with the participation of a coordinated propylene molecule. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1899–1911, 2002     

Study of the donor effect on the centres with different stereospecificities in propylene polymerization on the ziegler-natta catalytic system

A study was made of the effect of NEt₃ as a donor additive on the centres of different stereospecificities of the catalytic system δ-TiCl₃ (TAC)-AlEt₃. It is shown that the addition of amine, both in a free form (D) and as part of the complex NEt₃-AlEt₃ (C), produces a poisoning action on the catalytic centres. In the former case (D), the isospecific centres are also partially poisoned, leading to a drop in the system activity and a decrease in the $\text{M}$_v value of the polypropylene iso-fraction. In the latter case (C), at C/TiCl₃ = 5, one can observe the effect of system modification which shows up in increasing the molecular weight and amount of the iso-fraction and also in improving its stereoregularity parameters with the general catalyst activity remaining unchanged. The effect of the modifying additive is explained by the increase of iso-addition propagation constant.     

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.     

Polymerization of ethylene with the system (C5H5)2TiEtCl-AlEtCl2

The catalytic system (C₅H₅)₂TiEtCl-AlEtCl₂ in benzene and heptane was investigated. Only two species are formed at an equimolar ratio Al: Ti, viz. active (C₅H₅)₂TiEtCl.AlEtCl₂ (I) and inactive (C₅H₅)₂TiCl.AlEtCl₂ formed from (I). The rate constant of propagation is k_p^20° = 6.4 l/mole sec and is independent of the medium. The rate of polymerization decreases with time because of valence reduction. The bimolecular law is obeyed during a run but the apparent termination constant is inversely proportional to the initial catalyst concentration. The kinetic data with different ratios Al:Ti and the dependence of the number of polymer molecules/Ti atom show that AlEtCl₂ is a termination agent and a chain transfer agent.