Active centre determination in ziegler‐natta polymerization. An examination of the allene inhibition method

-Some recent developments in basic research on the polymerization of olefins to linear polymers are reviewed, including progress in preparation of catalytic systems, advances in knowledge about the mechanism of polymerization and attempts to control molecular weight distribution. For the catalytic systems of the type TiClq/MgC12/A1R /Zewis base indirect methods for the characterization of the acti4e centers are considered. Dissimilarities in the time dependence of the rate tion of the Lewis base-to-aluminum alkyl ratio indicate that ethylene and propylene polymerize, at least in a large part, on different active centers. However, productivity of the highly stereospecific centers synthesizing isotactic poly(a-o1efin)'s is of the same order of mgnitude for ethylene and propylene.of polymerization and changes in the productivity as a func-

Recent developments in the polymerization of olefins with organometallic catalysts

Pino

Rotzinger

1984

Adsorption of propylene and allene on TiCl₃·1/3 AlCl₃ and polymerization of allene with the system TiCl₃·1/3 AlCl₃/AlEt₂Cl

Kratochvíla

Mejzlík

1987

Adsorption of propylene and allene from the gas phase on TiC&.1/3 AlC& and Tic& .1/3 AlC&/AlEbCl catalyst was measured volumetrically. In the ranges of 2%-334 K and 1 -100 kPa, the adsorbed amounts of both gases was found to be approximately proportional to the gas pressure. T h e adsorbed amount of allene is about twice as high as that of propylene under the same conditions. Low heats of adsorption of both propylene and allene support the assumption of the physical nature of the adsorption. Addition of AlEbCl to the Tic& . 1/3 AlC& catalyst did not change substantially the sorption properties of the catalyst. Polymerization of allene with the system Tic& . 1 /3 AlC&/AlE~Cl in heptane was found to be first order in monomer and the rate decreases with increasing yield. A mechanism of retardation by the polymer formed is probably operative. Both heptane-soluble and heptane-insoluble polyallenes are produced. The soluble polymer has M, = lo3 and a very narrow molecular weight distribution (A!&/% = 1,2 f 0,2). Polyallene comprises mainly vinylidene units.

Retardation of the TiCl₃·1/3 AlCl₃/AlEt₂Cl catalyzed propylene polymerization by allene and its relevance to the determination of the number of active centers

Kratochvíla

Mejzlík

1987

The retardation of the TiClJAlEbCI catalyzed propylene polymerization by allene was studied. On the basis of the results, a mechanism of the competitive chemisorption on active centers was proposed. A non-linear dependence of the polymerization rate on the amount of allene consumed can be described in terms of the existence of two types of active centers, their numbers being 2 and 20 mmol . mol,i', respectively, differing very substantially in the adsorption coefficients. Copolymerization of propylene with allene was investigated, using the above catalytic system. Assuming identical active centers, the following copolymerization parameters were obtained: r1 = 0,66 rt 0,06 and r, = 40 f 30. Based on these results, the consumption of allene by copolymerization during the retardation of propylene polymerization was described in quantitative terms.