Fluorous polymerizations

To solve the issue of ligand leaching that was encountered in some of the examples above, fluorous polymeric phosphine ligands 15a-c [28] were developed. The rhodium complexes prepared from 15a-c using a 3 1 ratio of P Rh [28b, 29] displayed good turnover frequencies (TOFs) in the case of 15 a, but reaction rates for 15b,c were lower. The catalyst derived from 15 a was recycled seven times without loss of activity, although leaching was not studied quantitatively. 

The combination of supercritical CO2, Rh(CO)2(acac), and a fluorous polymeric phosphane 38 was found to be a highly effective catalytic system for the chemose-lective hydroformylation of usually unreactive alkyl acrylates (Equation 7.11) [120]. 

A fluorous biphasic system has been used to reduce the metal contamination arising in the copper-catalysed living radical polymerization of vinyl monomers. 

To our knowledge, there have been no previous attempts to develop a broad class of molecular catalysts that have temperature-dependent solubilities. When molecular catalysts are covalently bound to polymeric supports, they generally assume the solubihty properties of the host polymer. In the above fluorous catalysts, we Hke to think that a short segment of polymer is being grafted onto a molecular catalyst, hi other words, the ponytails can be viewed as pieces of Teflon , which impart more and more of the solubility characteristics of the polymer as they are lengthened. 

A series of pseudo-C or pseudo-C2 symmetric complexes 168-171 (Fig. 27) exhibited isotactic predominance P = 0.50-0.75) however, the isotacticity is compromised in solvent-free bulk polymerization at 130 °C [129]. Fluorous tertiary alcohol ligands with electron-withdrawing CF3 group are weakly basic and thus expected to reduce the possibility of catalyst deactivation by bridged species formation. Al complexes 172 and 173 offered highly isotactic-enriched stereoblock PLA (Pm = 0.87) from ROP of rac-lactide [168]. 

The application of perfluorous polyethers in biphasic catalysis was first described by Vogt (133), who also synthesized ligands based on hexafluor-opropene oxide oligomers to create metal complexes that are soluble in the perfluorous polyethers. The solvophobic properties of the fluorous solvent were successfully incorporated in the metal complexes catalytic oligomerization and polymerization reactions with nickel and cobalt complexes were demonstrated. 

Apart from the traditional solid supports (see above), several publications also report the successful use of microwave enhancement for supported transformations involving soluble polymers49-54, fluorous phase conditions55,56, and ionic liquids grafted onto polymeric supports57,58. 

The successful demonstration of the fluorous biphasic concept for performing organometallic catalysis sparked extensive interest in the methodology and it has subsequently been applied to a wide variety of catalytic reactions, including hydrogenation [59], Heck and Suzuki couplings [60, 61] and polymerizations [62]. The publication of a special Symposium in print devoted to the subject [63] attests to the broad interest in this area. 

This chapter explores the application of biocatalytic polymerization in exotic solvents. These solvents are often termed unconventional , in that they would not generally be considered as a polymerization media. However, their use over the previous decade has dramatically increased due to the international push for cleaner, greener reaction pathways in an effort to reduce volatile organic compounds (VOCs). The first solvents to be discussed (and by far the most fully investigated in the literature) are supercritical fluids. Within this field, supercritical C02 has been the most highly reported solvent. The second solvent class is ionic liquids. These have become increasingly popular over the last five years. Biphasic solvents will then be described and their application to biocatalytic polymerization. This section will be limited to biphasic solvents that are more unusual and, apart from a brief mention, will not encompass the broad field of emulsion polymerization in water. Finally, the use of fluorous solvents will be described. In all cases, the physical properties of the solvent imparts interesting,... 

Metal-mediated Polymerizations of Olefins in Fluorous Media... 

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