Synthesis using ionic techniques

It is hoped that this section will give the reader a better appreciation of the range of ionic liquids that have already been prepared, as well as a summary of the main techniques involved and the potential pitfalls. While the basic chemistry involved is relatively straightforward, the preparation of ionic liquids of known purity may be less easily achieved, and it is hoped that the ideas given here may be of assistance to the reader. It should also be noted that many of the more widely used ionic liquids are now commercially available from a range of suppliers, including some specializing in the synthesis of ionic liquids [53]. 

A question which has occupied many catalytic scientists is whether the active site in methanol synthesis consists exclusively of reduced copper atoms or contains copper ions [57,58]. The results of Szanyi and Goodman suggest that ions may be involved, as the preoxidized surface is more active than the initially reduced one. However, the activity of these single crystal surfaces expressed in turn over frequencies (i.e. the activity per Cu atom at the surface) is a few orders of magnitude lower than those of the commercial Cu/ZnO/ALO catalyst, indicating that support-induced effects play a role. Stabilization of ionic copper sites is a likely possibility. Returning to Auger spectroscopy, Fig. 3.26 illustrates how many surface scientists use the technique in a qualitative way to monitor the surface composition. 

Several synthetic strategies are used to produce block copolymers containing a cationic block. Because charged monomers are not polymerizable by ionic techniques, the synthesis of the required block copolymers can be carried out by free radical polymerization of ionic vinyl monomers using macroinitiators, by modifying one block of a block copolymer and by coupling of two readily synthesized blocks. 

In 1961, Natta reported one of the first examples of enantioselective catalysis using a transition metal catalyst. In this reaction, an optically active polymer was formed from 1,3-pentadiene using a chiral organoaluminum/VClj catalyst [62]. The optical activity of this polymer results from the main-chain chiraHty of polymer, where the methyl-substituted stereogenic centers are predominantly of one absolute configuration. Since this initial study, significant advances in the enantioselective synthesis of main-chain chiral polymers have been reported using ionic and metal-based techniques. 

There has been more work on the liquid/gas interface and surface tensions, and these have been recently reviewed [79]. These measurements, although relatively simple to make, can be very sensitive to impurities in the ionic liquids, particularly those that have a tendency to concentrate at the liquid/gas interface [80]. For example, secondary ion mass spectrometry (SIMS) of [C2Cjim][NTf2] showed the presence of poly(dimethylsiloxane) - commonly used to lubricate ground-glass joints [81]. This makes it difficult to compare across the work of different research groups, who have used different, often only partially described, synthesis and purification techniques. However, some general trends can be seen. 

The most frequendy used technique to shift the equiUbrium toward peptide synthesis is based on differences in solubiUty of starting materials and products. Introduction of suitable apolar protective groups or increase of ionic strength decreases the product solubiUty to an extent that often allows neady quantitative conversions. Another solubiUty-controUed technique is based on introduction of a water-immiscible solvent to give a two-phase system. Products preferentially partition away from the reaction medium thereby shifting the equiUbrium toward peptide synthesis. 

The first example of biphasic catalysis was actually described for an ionic liquid system. In 1972, one year before Manassen proposed aqueous-organic biphasic catalysis [1], Par shall reported that the hydrogenation and alkoxycarbonylation of alkenes could be catalysed by PtCh when dissolved in tetraalkylammonium chloride/tin dichloride at temperatures of less than 100 °C [2], It was even noted that the product could be separated by decantation or distillation. Since this nascent study, synthetic chemistry in ionic liquids has developed at an incredible rate. In this chapter, we explore the different types of ionic liquids available and assess the factors that give rise to their low melting points. This is followed by an evaluation of synthetic methods used to prepare ionic liquids and the problems associated with these methods. The physical properties of ionic liquids are then described and a summary of the properties of ionic liquids that are attractive to clean synthesis is then given. The techniques that have been developed to improve catalyst solubility in ionic liquids to prevent leaching into the organic phase are also covered. 

The same aplies to polymer brushes. The use of SAMs as initiator systems for surface-initiated polymerization results in defined polymer brushes of known composition and morphology. The different polymerization techniques, from free radical to living ionic polymerizations and especially the recently developed controlled radical polymerization allows reproducible synthesis of strictly linear, hy-perbranched, dentritic or cross-linked polymer layer structures on solids. The added flexibility and functionality results in robust grafted supports with higher capacity and improved accessibility of surface functions. The collective and fast response of such layers could be used for the design of polymer-bonded catalytic systems with controllable activity. 

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