Electron donors, propylene polymerization

Catalytic activities of these complexes for the polymerization of propylene oxide were tested. The pyridine complex had no catalytic activity, whereas the diethyl ether or dioxane complex had activity comparable to that of the mother organozinc compound. This fact indicates that a strong electron donor which cannot be replaced by propylene oxide inhibits the polymerization effectively. Thus, the co-... 

Tani,H., Oguni,N. Stereospecilic polymerization of propylene oxide by N,N-bis(ethylzinc)-t-butylamine electron donor complex of catalyst and its implication for polymerization mechanism. J. Polymer Scl B7, 769 (1969). 

The addition of a third component to the metal-alkyl is a widespread practice with MgCI2 catalysts in order to improve their performance and to control the polymer molecular structure. In ethylene polymerization the addition of modifiers (alkyl-halides, Lewis acids such as A1C13, halogens such as I2, and others) is rather limited and is principally used to modify the MWD (see Table 11 in Ref.53)). On the other hand, the addition of modifiers is almost indispensable to obtain satisfactorily stereoregular propylene polymers. The additives used for this purpose are generally electron donor compounds (Lewis bases) and a wide variety has been described in patent and scientific literature. 

For example, supported TiCl4/MgCl2 catalysts show a short period of acceleration, followed by a prolonged steady period 92,93). However, in the presence of electron donors, they may show the typical decay rate kinetics observed during propylene polymerization 93). Bulk catalysts prepared by interaction of TiCU with Mg(OR)2 show either a stationary rate, or a non-stationary rate, according to the titanium content 88,94). Bulk catalysts prepared by reduction of TiCl4 with organomagnesium compounds show a decay type rate 92-95>. 

As has been previously mentioned, electron donors are widely used as stereoregulating agents for the propylene polymerization with Mg/Ti catalysts. The addition of donors to highly active MgCl2/TiCl4—AlEt3 systems usually results in a remarkable increase... 

The existence of a further type of active centers was demonstrated by Pino and Rotzinger93> by polymerizing ethylene with a MgQ2-supported catalyst in the presence of an electron donor. A comparison of the ethylene and propylene kinetic curves shows that, while propylene polymerization is characterized by the well known rapid decrease in rate, the ethylene polymerization rate increases reaching a constant value after about 30 min. This has been attributed to the existence of active... 

The use of electron donors is of particular importance for propylene polymerization, where a severe control on polymer stereoregularity is required. 

Tacticity measurements can be correlated with reaction mechanisms and physical properties. For example, the incorporation of an electron donor into the polymerization catalyst formulation has been found to increase isotacticity in a propylene-1-butene copolymer [123], and the distribution of propylene and 1-butene contents as a function of molecular weight varied, depending on donor type. External donors, such as dimethox-ysilane, decrease the butene content more than internal electron donors (in this case, di-n-butyl phthalate). Mechanisms of new polymerization reactions, such as the group-transfer copolymerization of methyl methacrylate and lauryl methacrylate, can be elucidated by comparing NMR-derived structural details [124]. The presence of unanticipated peaks in the spectrum of poly(ethylene-co-norbomene) suggest the occurrence of epimerization... 

Electron donors play an important role in modem catalysts for propylene polymerization. Recent breakthroughs in polyolefin catalyst and process technology have largely come about through an increase in empirical understanding and use of Lewis bases in a right way. However, the functional mechanisms of the reactions are poorly known. 

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