Soluble Ziegler-Natta catalysts

Cyclopentadienyl compounds of Ti as polymerization catalysts were discovered by Natta et al. [202] and their properties were studied in greater detail by Breslow and Newburg [203]. The authors described the interaction of both catalytic components in the absence of monomer by the equations component complexation 

The polymerization rate reflected the time change of the concentration of complex II. This complex, or a very similar compound, was assumed to be the active centre. Complex II is also formed immediately and quantitatively on contact of the alkylated Ti component with EtAlCl2 [204]. 

Henrici-Olive and Olive [205] assume complex II to be octahedral with the metal atoms connected by two Al—Cl—Ti links, with one vacancy (suitable for ethylene coordination), and a reactive ethyl group in the cis position 

When only a shift of the centres of charge is considered, i.e. ionization instead of dissociation, then this hypothesis becomes very similar to the coordination mechanism. 

The active centres are greatly affected by the cocatalyst which, in the case discussed above, is the organo-aluminium component. As an acceptor part of the centre, it reduces the electron density on the donor (titanium). This facilitates monomer addition by insertion. Thus the activity of the centre is a function of the acidity of the aluminium component. The partly hydrolyzed organo-aluminium compound of general formula 

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Keli, T., Doi, Y. Synthesis of Living Polyolefins with Soluble Ziegler-Natta Catalysts and Application to Block Copolymerization. Vol. 73/74, pp. 201 —248. 

Catalysts such as HRuX(PPh3)3, where X is an optically active car-boxylate (e.g., / -mandelate) gave only 0.4% ee using 2-ethylhex-l-ene as substrate (124). Soluble Ziegler-Natta catalysts comprised of triiso-butylaluminum with the optically active alkoxide complex, titanium tetra-(-)-menthoxide, hydrogenated racemic terminal olefins such as 3,4-di-methyl pent- 1-ene, but with zero ee (323). 

Fig Growth step postulated for the polymerisation of ethylene by a soluble Ziegler-Natta catalyst. 

Soluble Ziegler-Natta catalysts can exhibit unique stereochemical properties. Group IV metallocenes in combination with methylaluminoxanes produce isotactic polypropylene with two different isotactic microstructures. The usual enantio-morphic site control is characteristic of enantiomeric racemic titano- and zirco-nocene complexes (e.g., ethylene-bridged indenyl derivatives279,349). In contrast, achiral titanocenes (e.g., [Cp2TiPh2]) yield isotactic polypropylene with microstructure 49, which is consistent with a chain end control mechanism 279,349-351... 

Synthesis of Living Polyolefins with Soluble Ziegler-Natta Catalysts and Application to Block Copolymerization... 

The first example of Iiving polyolefin with a uniform chain length was found in the low-temperature polymerization of propylene with the soluble catalyst composed of V(acac)3 and Al(C1Hi)2Cl. The mechanism of the living coordination polymerization is discussed on the basis of the kinetic and stereochemical data. Subsequently, some applications of living polypropylene are introduced to prepare tailor-made polymers such as terminally functionalized polymers and block copolymers which exhibit new characteristic properties. Finally, new types of soluble Ziegler-Natta catalysts are briefly surveyed in connection with the synthesis of living polyolefins. 

In conclusion, we indicate the future trends of research and development in the field of soluble Ziegler-Natta catalysts for the synthesis of living polyolefins and block copolymers. 

The first example of a living polyolefin with a uniform chain length was disclosed in 1979 by Doi, Ueki and Keii 47,48) who used the soluble Ziegler-Natta catalyst composed of V(acac)3 (acac = acetylacetonate anion) and A1(C2H5)2C1 for the polymerization of propylene. In this review, we deal with the kinetics and mechanism of living coordination polymerization of a-olefins with soluble Ziegler-Natta catalysts and the synthesis of well-defined block copolymers by the use of living polyolefins. 

While this review discloses the kinetic and stereochemical features of soluble Ziegler-Natta catalysts, we have little information on the structure of the active center. The steric environments of active centers must be very important in determining the monomer reactivity, regiospecificity and stereospecificity of soluble catalyst. The influence of ligands such as the aluminum components on the rates of chain propagation and chain-terminating steps should be correlated to the electronic structure of... 

Homogeneous Ziegler-Natta catalysts of the classic type, capable of olefin polymerisation, are usually derived from reactions of a hydrocarbon soluble transition metal compound with an alkylmetal compound in a hydrocarbon solution (heptane, hexane, toluene). However, the polymer formed in the presence of such soluble Ziegler-Natta catalysts is very often precipitated during polymerisation. 

Although low-temperature syndiospecific polymerisation of propylene with soluble Ziegler-Natta catalysts, based on soluble vanadium compounds and dialkylaluminium chlorides as activators, was first carried out successfully as... 

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