Soluble Olefin Polymerization Catalysts

Polymerization catalysis with soluble complexes of group IV transition metals, in particular with hydrocarbon-soluble titanocene complexes, was discovered in the 1950 s, shortly after the appearance of Ziegler s and Natta s reports on solid-state catalysts, and rather thoroughly studied from then on. Alkylalu-minium compounds, such as AlEt2Cl, are required to activate also these soluble catalysts. In distinction to their solid-state counterparts, however, early soluble catalysts were able to polymerize only ethylene, and not any of its higher homologues. After their activation by methylalumoxanes had been discovered (Section 7.4.1), soluble catalysts became as efficient as solid-state catalysts - in 

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Tian, J. and Huang, B., Soluble Olefin Polymerization Catalysts , in The Polymeric Materials Encyclopedia, CRC Press, Inc., Boca Raton, 1996, Vol. 6, pp. 4740-4749. 

Kallio, K. Kauhanen, J. Activation of Siloxy-substituted Compounds and Homopolymerization of Ethylene by Different Soluble Alumoxane Gocatalysts. In Organometallic Catalysts and Olefin Polymerization Catalysts for a New Millenium-, Blom, R., Follestad, A., Rytter, E., Tilset, M., Ystenes, M., Eds. Springer Berlin, 2001 p 14. 

Supported metallocene catalysts are an important field of research for the polymer industry and shall be mentioned here only briefly. Single-site olefin polymerization catalysts have been extensively investigated and are now achieving wide acceptance in the polyolefin industry. In order to achieve commercial significance, these soluble metallocene catalysts have to be immobilized on a carrier. The challenges of supporting these catalysts have been addressed in many creative ways and this topic has been taken up in an excellent review by Carnahan et al. [87]. This paper mainly describes anchoring techniques to silica, but these concepts may also be applied for other purposes. 

Interest in homogeneous olefin polymerization catalysts, especially group 4 metallocenes has caused a dramatic increase in the number of publications describing the synthesis of functionalized polyolefins by direct copolymerization. Many soluble metallocenes, such as bridged zirconocenes, have much better ability to incorporate higher a-olefins than do Ti-based Ziegler-Natta catalysts. This also makes them better suited for copolymerizations involving, often very bulky, functional comonomers. 

These siUca-supported catalysts demonstrate the close connections between catalysis in solutions and catalysis on surfaces, but they are not industrial catalysts. However, siUca is used as a support for chromium complexes, formed either from chromocene or chromium salts, that are industrial catalysts for polymerization of a-olefins (64,65). Supported chromium complex catalysts are used on an enormous scale in the manufacture of linear polyethylene in the Unipol and Phillips processes (see Olefin polymers). The exact stmctures of the surface species are still not known, but it is evident that there is a close analogy linking soluble and supported metal complex catalysts for olefin polymerization. 

As described in Section 9.1.2.2.3, several lanthanocene alkyls are known to be ethylene polymerization catalysts.221,226-229 Both (188) and (190) have been reported to catalyze the block copolymerization of ethylene with MMA (as well as with other polar monomers including MA, EA and lactones).229 The reaction is only successful if the olefin is polymerized first reversing the order of monomer addition, i.e., polymerizing MMA first, then adding ethylene only affords PMMA homopolymer. In order to keep the PE block soluble the Mn of the prepolymer is restricted to <12,000. Several other lanthanide complexes have also been reported to catalyze the preparation of PE-b-PMMA,474 76 as well as the copolymer of MMA with higher olefins such as 1-hexene.477... 

In contrast to the free-radical polymerizations, there have been relatively few studies on transition metal catalysed polymerization reactions in water. This is largely due to the fact that the early transition metal catalysts used commercially for the polymerization of olefins tend to be very water-sensitive. However, with the development of late transition metal catalysts for olefin polymerizations, water is beginning to be exploited as a medium for this type of polymerization reaction. For example, cationic Pd(II)-bisphosphine complexes have been found to be active catalysts for olefin-CO copolymerization [21]. Solubility of the catalyst in water is achieved by using a sulfonated phosphine ligand (Figure 10.5) as described in Chapter 5. 

Contents G. Henrici-Olive, S. Olive Oligomerization of Ethylene with Soluble Transition-Metal Catalysts. A. Zambelli, C. Tosi Stereochemistry of Propylene Polymerization. C.-D.S. Lee, W.H. Daly Mercaptan-Containing Polymers. Yu. V. Kissin Structures of CopolymerS of High Olefins. 

Poly (acetylenes) [16], There are several catalysts available for polymerization of substituted acetylenes. Whereas Ziegler-Natta catalysts are quite effective for polymerization of acetylene itself and simple alkylacetylenes, they are not active towards other substituted acetylenes, e.g. phenylacetylenes. Olefin-metathesis catalysts (Masuda, 1985 Masuda and Higashimura, 1984, 1986) and Rh(i) catalysts (Furlani et al., 1986 Tabata, 1987) are often employed. In our experience, however, many persistent radicals and typical nitrogen-containing functional groups serve as good poisons for these catalysts. Therefore, radical centres have to be introduced after construction of the polymer skeletons. Fortunately, the polymers obtained with these catalysts are often soluble in one or other organic solvent. For example, methyl p-ethynylbenzoate can be polymerized to a brick-coloured amorph- See the Appendix on p. 245 of suffixes to structural formula numbers. 

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. 

The published values of related to olefin polymerization with soluble and heterogeneous catalysts are in the range from several seconds to hours 7). It should be noted that the value of is strongly dependent upon the polymerization conditions since the rates of chain-terminating reactions Rt are functions of the temperature and concentrations of chain-terminating reagents. In a living polymerization the value of is infinite. 

Hydrous metal oxide powders, such as sodium titanate, NaT O, can be prepared by treating TYZOR TPT with sodium hydroxide in methanol solvent to form a soluble intermediate, which is hydrolyzed in acetone—water to form an ion-exchange material useful in treating radioactive waste (158). Exchange of the sodium ion with an active metal such as Ni, Pt, or Pd gives heterogeneous catalysts useful in olefin polymerization, coal liquification, and hydrotreating. 

Some half sandwich titanium compounds with cyclopentadienyl ligands have proven to be most active, but soluble tetraethyoxytitanium also shows a certain amount of activity. In contrast to olefin polymerization, titanocenes are more active than zirconocenes and fluoro ligands are better than chloro ligands. Table 24 [207] compares some catalysts for the polymerization of styrene. 

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