Ziegler-type catalytic systems

Early polymerization work with Ziegler-type catalyst systems has shown (8,11,12,17,18) that both the solid and liquor phase of preformed R3Al-TiCl4 catalysts play an important role in determining the catalytic activity. In the second part of the present study we have attempted to correlate the differences in catalytic action encountered with the various alkylaluminum co-catalysts examined to either the solid or liquid portion of the preformed catalysts. 

At the first time ethylene and propylene copolymers were put into industrial production in 1959 in Italy due to Ziegler-Natta catalytic systems discovery [176,177]. At present various SKEP(T) types are produced in all leading in industry countries particularly in USA, Italy, Germany, Holland, Great Britain, Canada, Japan, France, etc. Nevertheless, there is a lack of ethylene-propylene rubbers at world markets at present and this fact determines advisability of their production expansion. 

In terms of the multisite nature of Ziegler-Natta catalytic systems [2, 3], each functioning type of active site generates polymer fractions with a certain... 

It should be stated first that, in spite, of the use of simole n -allyl model oatplexes discussed below for mechanistic studies, most of the synthetic work effected has called upon the new classical Ziegler-Natta type catalytic systems, v cii can be represented by the general formula ... 

Other examples of arene hydrogenation by Ziegler-type catalysts have been reported [35]. However, none of them is discussed at this point as they are generally poorly defined. Likewise, some hydrogenation catalytic systems in either oxo or water-gas-shift conditions are only reported in the list of references for sake of information [24, 36]. 

Ziegler-Natta Various types of Ziegler-Natta catalysts Polymerization mechanism and applications of molecular mechanics to the study of enantioselectivity of some stereospecific catalytic systems having chiral site stereocontrol Mechanism of enantioselectivity and its relevance to catalytic systems 56... 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

However, one should also be aware of possibilities that elements of carbocationic processes may directly influence Ziegler-Natta catalytic chemistry. For instance, polymers containing vinylidene end groups of the type CH2=CMe-polymer are often formed during Ziegler-Natta processes and might recoordinate to the metal cation in an fashion, as in I and J for monocyclopentadienyl and metallocene systems, respectively. intermediates of these types were to... 

The discovery of Ziegler-Natta catalysts led to many industrial and academic investigations on other kinds of metallic complexes for polymerization of different monomers. Several organometallic and coordination compounds have been synthesized and probed as catalytic systems. They have been classified based on generations or groups, transition-metal type, the chemical structure, the type of activator, and their applications in polymerization processes [2]. Currently, there are different groups of initiator systems based on early and late transition metals or lanthanide complexes, which have been studied in polymerization catalysis [3]. 

The effectiveness of Ziegler-Natta catalysts of the triethylaluminum-titanium tetrachloride type seems to be the subject of some controversy. One patent describes the formation of poly(vinyl fluoride) with such a catalytic system in THF in a bottle polymerization at 30 C and autogenous pressure for 6 hr [57]. A complex of triisobutylaluminum, vanadium oxytrichloride, and THF is said to be particularly effective at 30°C both for the homo- and copolymerizations of vinyl fluoride [58, 59]. The processes are said to resemble typical Ziegler-Natta systems and are independent of the THF concentration when the mole ratio of THF to VOCI3 was greater than 2.3 1. The use of triisobutylaluminum with tetraisopropoxytitanium at 30°C for 15 min is said to lead to a process with an ionic-coordination mechanism [60]. 

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