Based Ziegler-Natta Catalysts

Compared to conventional heterogeneous Ziegler-Natta systems in which a variety of active centers with different structures and activities usually coexist, homogeneous metallocene-based catalysts give very uniform catalyt-ically active sites which possess controlled, well-defined ligand environments [37]. Consequently, the polymerization processes in homogeneous systems are often more simple, and kinetic and mechanistic analyses for these systems are greatly simplified [38]. 

The chiral anjo-metallocenes, that is, metallocenes with two Cp ligands arranged in a chiral way and connected together with chemical bonds by 

---

Fischer-Tropsch (FT) process is used for the production of hydrocarbon fuels. The process uses synthesis gases CO and H2O. It is shown that cobalt/alumina-based catalysts are highly active for the synthesis. The process is also used to convert coal to substitute or synthetic natural gas (SNG). The use of Fe-based catalysts is also believed to be attractive due to their high FT activity. HRTEM has played a major role in the study of phase transformations in Fe Fischer-Tropsch during temperature programmed reduction (TPR) using both CO and H2 (Jin et al 2000, Shroff et al 1995). TiClj/MgC -based (Ziegler-Natta) catalysts are used for polymerization of alkenes (Kim et al 2000) and EM is used to study the polymerization (Oleshko et al 2002). 

Ziegler-Natta Catalysts. All isotactic polymers of higher a-olefins are produced with the same type of heterogeneous, titanium-based Ziegler-Natta catalyst systems as that used for the manufacture of isotactic PP. 

Friebe, L., Nuyken, O. and Obrecht, W. Neodymium Based Ziegler/Natta Catalysts and their Application in Diene Polymerization. Vol. 204, pp. 1-154. 

Polybutadiene. Most polybutadiene is made by an emulsion process with a free radical initiator. If stereoregular cis-1,4-polybutadiene is desired, a titanium-based Ziegler-Natta catalyst is used. The catalyst is similar to those used for polyethylene and polypropylene in type and mechanism. 

Homogeneous alkylaluminium (alkylaluminoxane)-free olefin polymerisation catalysts are commonly referred to as catalysts containing group 4 metallocene cationic species. These catalysts resemble the structure and properties of the respective metallocene-based Ziegler-Natta catalysts but, by definition, are not included among the latter catalysts. The development of alkylaluminium-free metallocene-based olefin polymerisation catalysts is connected with discoveries... 

The Mw/Mn ratio is usually equal to 5-10 for polyethylene [49,64,66,67, 123,244-247], A much lower polydispersity is displayed by polymers obtained in polymerisation with homogeneous metallocene catalysts the Mw/Mn ratio usually does not significantly exceed a value of 2 [22,95,101,112,138,140], By polymerising propylene with soluble vanadium-based Ziegler-Natta catalysts at low temperature, a very narrow molecular weight distribution of the polypropylene has been found (the Mw/Mn ratio usually reaches values of 1.15-1.25) and a linear increase in its Mn with time has been observed, indicating a noticeable living character of the polymerisation [75,76,241],... 

Furthermore, studies of the microstructure of copolymers formed by the low-temperature copolymerisation of cis-1 -(2 H)-propene (or trans isomer) and perdeuteropropene in the presence of soluble vanadium-based Ziegler-Natta catalysts showed syndiospecific propagation to involve a monomer insertion of the cis type [27]. 

Syndiospecific Propagation Reaction Stereocontrol in the Presence of Soluble Vanadium-based Ziegler-Natta Catalysts... 

The syndiospecific polymerisation of propylene with soluble vanadium-based Ziegler Natta catalysts is not completely regiospecific [389 392], i.e. the monomer unit enchainment is not entirely head-to-tail. In addition to syndiotactic stereoblocks, the polymer also contains sterically irregular stereoblocks. The whole polymerisation can be thus described as a copolymerisation with four head-to-tail and tail-to-tail stages [2,379]. 

In view of the data concerning propylene polymerisation in the presence of homogeneous vanadium-based Ziegler-Natta catalysts, the syndiospecificity of the polymerisation is believed [387,395] to arise from steric repulsions between the last inserted monomer unit of the growing chain and the methyl group of coordinated propylene molecule, i.e. chain end stereocontrol is postulated to play the essential role in the stereoregulation. 

The same conclusion as in the case of propylene homopolymerisation has been drawn considering IR [396] and NMR [389,395] spectra of ethylene/propylene copolymers obtained with vanadium-based syndiospecific catalysts. The type of propylene insertion depends on the kind of last inserted monomer unit secondary insertion [scheme (40)] occurs more frequently when the last monomeric unit of the growing chain is propylene, while primary propylene insertion [scheme (39)] is more frequent when the last monomeric unit of the growing chain is ethylene [2]. The above explains the microstructure of ethylene/propylene copolymers obtained with vanadium-based Ziegler-Natta catalysts. These copolymers contain both m and r diads when the sequence of propylene units is interrupted by isolated ethylene units i.e. a propylene insertion after an ethylene insertion is substantially non-stereospecific [327,390,397], The existence of a steric interaction between the incoming monomer molecule and the last added monomer unit is also confirmed by the fact that the propagation rate for the secondary insertion of propylene in syndiospecific polymerisation is lower than for primary insertion in non-stereospecific polymerisation [398],... 

A nickel-based Ziegler-Natta catalyst, Ni(Acac)2—Al(/-Bu)3, yields a regular allene polymer consisting almost exclusively of head-to-tail linked (1,2-inserted) monomeric units [441],... 

Of particular interest are random copolymers of ethylene and propylene, which are obtainable with soluble vanadium-based Ziegler-Natta catalysts such as V(Acac)3—AlEt2Cl [453], VOCl3-AlEt2Cl [72], VOC13-A1(/-Bu)2C1... 

Alternating copolymers have been obtained by copolymerisation of ethylene and cycloolefins (using a large excess of cycloolefin) in the presence of vanadium-based Ziegler-Natta catalysts such as V(Acac)3-AlEt2Cl and VC14 AlEt2Cl ... 

Czaja, K. and Bialek, M., Vanadium-based Ziegler-Natta Catalyst Supported on MgCl2(THF)2 for Ethylene Polymerisation , Macromol. Rapid Commun., 17, 253-260 (1996). 

A scheme for the alternating copolymerisation of conjugated dienes and a-olefins has been proposed in the case of isoprene/propylene copolymerisation with V-based Ziegler-Natta catalysts [209] ... 

A flow scheme of m-1,4-polybutadiene production involving polymerisation with cobalt-based Ziegler-Natta catalysts in a solution process with the removal of catalyst residues from the polymer is presented in Figure 5.13 [227]. 

What are the advantages of Nd-based Ziegler-Natta catalysts over Ti-, Co- or Ni-based Ziegler-Natta catalysts in the manufacture of butadiene rubber (cis-1,4-polybutadiene). 

---