Heterogenized transition metal catalysts polymerization with

Despite the early use of phosphonium salt melts as reaction media [12, 18, 25], the use of standard ionic liquids of type 1 and 2 as solvents for homogeneous transition metal catalysts was described for the first time in the case of chloroaluminate melts for the Ni-catalyzed dimerization of propene [5] and for the titanium-catalyzed polymerization of ethylene [6]. These inherently Lewis-acidic systems were also used for Friedel-Crafts chemistry with no added catalyst in homogeneous [7] as well as heterogeneous fashion [8], but ionic liquids which exhibit an enhanced stability toward hydrolysis, i. e., most non-chloroaluminate systems, have been shown to be of advantage in handling and for many homogeneously catalyzed reactions [la]. The Friedel-Crafts alkylation is possible in the latter media if Sc(OTf)3 is added as the catalyst [9]. 

This chapter covers the polymerization of alkenes with homogeneous and heterogeneous catalysts based on group 4 metals, including the underlying reaction principles and the relationship between catalyst structure and polymer properties. Applications of related complexes in C-C bond-forming reactions in organic synthesis are covered in Chapter 00125. The use of transition metal catalysts in polymer synthesis is more widely discussed in chapter 11.06. 

Both homogeneous and heterogeneous Ziegler-Natta catalysts must be activated by a cocatalyst. Cocatalysts are alkyl aluminum compounds such as trimethyl aluminum (TMA) and triethyl aluminum (TEA). Cocatalysts are essential for polymerization with Ziegler-Natta, metallocene and late transition metal catalysts, as will be explained below. 

Syndiotactic polystyrene (SPS) can be readily polymerized using homogeneous or heterogeneous metallocene catalysts, based on group 4 metal compounds, especially titanium compounds like T1CI4, CpTiClj, and Cp Ti(OCH3)3 with methyl aluminoxane (MAO) as cocatalyst [1-3]. The recent developments of transition metal catalysts and reaction mechanisms are discussed in earlier chapters. This chapter will be focused on the quantitative aspects of SPS polymerization kinetics and related physical and chemical phenomena. 

It is necessary to note the limitation of the approach to the study of the polymerization mechanism, based on a formal comparison of the catalytic activity with the average oxidation degree of transition metal ions in the catalyst. The change of the activity induced by some factor (the catalyst composition, the method of catalyst treatment, etc.) was often assumed to be determined only by the change of the number of active centers. Meanwhile, the activity (A) of the heterogeneous polymerization catalyst depends not only on the surface concentration of the propagation centers (N), but also on the specific activity of one center (propagation rate constant, Kp) and on the effective catalyst surface (Sen) as well ... 

Bohm, L. L, Franke, R., Thum, G., The microreactors as a model for the description of the ethylene polymerization with heterogeneous catalysts, in Kaminsky, W., Sinn, H. (Eds.), Transition metals and organometallics as catalysts for olefln polymerization, pp. 391-403, Springer-Verlag, Berlin (1988). 

The polymerization of ethylene was carried out in an identical way with these heterogeneous catalysts as with the homogeneous systems. Typical results are given in Table XII and show that the Si-0 ligand enhances the activity of the transition metal site for polymerization. Some of the higher activities are minimum values since the concentration of ethylene in the diluent is well below equilibrium concentrations and with these conditions the process is diffusion controlled. 

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