Polydienes, catalyst systems

Figure 10. 4 f block elements catalyst systems for high cistactic polydienes. 

For example, these workers (103) found that polybutadiene can be successfully metalated by n-BuLi-TMEDA, and the product subsequently used to graft styrene monomer to form poly(butadiene-g-styrene). This result showed that the grafting efficiency was 100%. Catalyst efficiency was found to be 75-95%. Thus, the n-BuLi-TMEDA metalating and grafting system is particularly effective for polydiene backbones. In fact, Falk and Schlatt (104) showed that poly(butadiene-g-styrene) has excellent tensile strength. 

Many results obtained with diolefins can be explained in essentially the same way as for those with a-olefins. Nevertheless, there are some differences concerning observations made with T)3-allylnickel complexes and the influence of ligands on the results obtained by using Ziegler-type conditions (64). Some of these systems are far from being true Ziegler-type catalysts. Probably, the structural isomerism of polydienes depends essentially on the specific nature of the metal which forms a complex with the diene involved. 

Beyond this exclusive lanthanide Ziegler-Natta model, Ziegler-type multicomponent systems ( Mischkatalysatoren ) represent the only class of homogeneous rare-earth metal catalysts of considerable commercial relevance [40-43]. High-czs-1,4-polydienes are industrially produced from 1,3-dienes (butadiene and isoprene) in aliphatic or aromatic hydrocarbons by a number of Mischkatalysatoren based on the transition metals titanium, cobalt, and nickel, and the lanthanide element neodymium [40-47]. The... 

The first Ziegler-Natta systems used for the production of several elastomers, and crystalline polydienes, were based on Ti, Co, V, Ni, or Cr halide or nonhalide catalyst... 

The distinctive feature of lanthanide and in particular Ziegler-Natta catalysts is that they allow one to synthesise polydienes with a high content of c/s-1,4-units. In most cases, lanthanides are used in the form of blends and concentrates. About 50% of lanthanides consumed worldwide are used for the production of catalysts for various chemical processes. Using lanthanide catalysts in the manufacture of synthetic rubbers can increase the number of these processes. A large number of studies have been devoted to polymerisation of dienes with lanthanide catalytic systems. Many of these studies are concerned with the problems related to the mechanism that controls the microstructure of polydienes. Although not all aspects of stereoregulation have been clarified, many problems have been solved and possible explanations offered for some of the others [1-4]. 

Commercial poly(butadiene), which is mainly the 1,4 isomer, is also used to improve the impact resistance of polystyrene (Chapter 1). Polydienes also increase the rate of physical disintegration of polyblend containing them. The addition of a styrene-butadiene block copolymer e.g. SBS, page 9 et seq.) to polyethylene also accelerates the peroxidation of the latter. However, this system also requires a polymer-soluble transition metal ion catalyst e.g. an iron or manganese carboxylate) to increase the rate of photooxidation in the environment by the reactions shown in Scheme 5.3. The products formed by breakdown of alkoxyl radicals (PO ) (Scheme 3.4) are then rapidly biodegradable in compost (page 107 et seq.). 

Polydiene microstructure is also dependent on the ratio of concentrations of monomer to organolithium. This is true for both non-polar and polar media. As catalyst levels are reduced in a non-polar system, cis-l,4-values as high as 86% have been observed in high molecular weight polybutadienes prepared in the absence of any solvents. This compares with 96% cw-l,4-content in similarly prepared polyisoprenes. In the case of high-vinyl BRs (HVBRs) prepared with a polar modifier, such as DIPIP, a decrease in alkyllithium (RLi) concentration at constant ratio of DIPIP/RLi produces smaller amounts of vinyl structure. 

Data from table 5, which summarize the effect of a halogen atom in the NdX3-AlEt3 EtOH system on the polymerization of dienes, shows that the rare earths are capable of producing 96-98% cis PBd with all four halides whereas, for Ti, Co, and Ni catalysts, in order to prepare high cis PBd, a suitable electronegativity of the halide is required, as shown in table 6. In the Nd(Oalk)3 Cl -AlEt3 system (Shan et al., 1983), the variation of n has an obvious effect on the microstructure of polydienes, as shown in table 7. 

A series of catalytic systems was also developed, in which the mechanism of action is similar to Ziegler-Natta catalysts. These are the oxides of nickel, cobalt, vanadium and molybdemun deposited on the surface of aluminum and of chromium oxide on silica gel. Such catalysts contain different promoters in the form of metal alkyl. They can be also used for the synthesis of polydienes. 

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