Internal Lewis Base Research Articles

Magnesium chloride supported high‐mileage catalysts for olefin polymerizations. XVIII. Effect of hydrogen and lewis bases

AbstractHydrogen has been found earlier to increase the initial rate of polymerization by MgCl2/EB/PC/AlEt3/TiCl4‐AlEt3/MPT, CW‐catalyst (+Bi, +Be) (EB, ethyl benzoate; PC, p‐cresol; MPT, methyl‐p‐toluate), but decays more rapidly as compared to polymerizations in the absence of H2. In this study the effect of H2 was studied when either the internal Lewis base, EB Bi, or the external Lewis base, MPT Be, or both are deleted from the CW‐catalyst. H2 does not affect the stereospecificity of all the catalysts, but causes a slight increase of polymer yield, whereas the yield is virtually unchanged by H2 for the catalysts activated with Be. Unlike the catalyst (+Bi, +Be) where H2 increases active site concentrations [Ti*] about threefold, it affects [Ti*] negligibly when Be is absent. The rate constants of propagation is about the same with or without H2 for the CW‐catalyst (+Bi, –Be) or (–Bi, –Be); the same statement can be said about the rate constant of chain transfer with AlEt3 or with H2. Hydrogen increases the rate of catalyst site deactivation for the various catalysts in the order of(+Bi, +Be) > (–Bi, –Be) > (+Bi, –Be).

Magnesium chloride supported high‐mileage catalysts for olefin polymerization. XVII. Effect of lewis base on propylene polymerization

AbstractA systematic study has been made on the functions of external Lewis base (Be, methyl‐p‐toluate, MPT) and internal Lewis base (Bi, ethyl benzoate, EB) for the CW‐catalyst system MgCI2/EB/PC/AlEt3/TiCl4–AlEt3/MPT (PC, p‐cresol). Bi is a nonstereoselective modifier. It increases the active site concentrations and rate constants of propagation, kp, of both the isospecific and nonspecific sites, and thus the productivities of the stereoregular and irregular polypropylenes by five‐ to tenfold. It seems that Bi complexes with the MgCl2 support to lower the electronegativity of the surface Mg atoms. It also acts to lower the rate constant of chain transfer to aluminum alkyl, k, by two‐ to fourfold. The action of Be is highly stereospecific. The isotacticity index of polypropylene is ≫ 95% in the presence of Be but ≪ 68% without it. Addition of Be decreases nonspecific [Ti*]a by about (11 ± 2)‐fold; there is only about a twofold reduction of the isospecific [Ti*]i. It decreases kp,a about three times but has no effect on kp,i, so that the latter is (21 ± 4) times the former. Be decreases k for transfer with aluminum alkyl much more than it does to k; but it does not affect the rates of chain transfer with monomer and by β‐hydride elimination or the rate of catalyst deactivation. The number of active sites without Be is [Ti*]i = 15% and [Ti*]a = 55% for a total of 70%. In the presence of Be they are both about 6%. Optimum performance in propylene polymerizations requires both Bi and Be in the case of the CW‐catalyst.

The role of ‘internal’ and ‘external’ Lewis bases in controlling the stereochemistry of highly active Ziegler–Natta catalysts

Chiral discrimination of the enantiomers of racemic α-alkenes in polymerization with highly active MgCl2-supported Ziegler-Natta catalysts is observed when (–)-menthyl benzoate is used as internal Lewis base; using (–)-menthyl benzoate as an external Lewis base leads to opposite but lower stereoelectivity.