Resin properties inorganic support

Recently, various Hoveyda-type carbenes were attached to different resins or insoluble inorganic supports preferentially via the 2-alkoxybenzylidene fragment. An alternative method involves the use of tagged catalysts, which can be easily separated from untagged products by a phase-transfer event (precipitation or hquid-hquid partition). One particularly innovative method involves a catalyst immobilization in ionic liquids (IL). Because of their interesting properties such as ease of reuse, nonvolatility, and good ability to dissolve... 

Ionomers are certainly not the only useful support for transition metals and ions. Indeed, inorganic oxides, such as silica, zeolites and aluminas, are the most widely used at present (13). Among the organic polymeric supports now used, the most closely related to the ionomers are the well known ion-exchange resins. While they are polyelectrolytic, as are the PFSA and PSSA ionomers, they are not thought to possess the potentially useful morphological properties of ionomers. 

In the incipience of the methodical development, after some tests with different types of supports, Merrifield had already selected from the gigantic palette of organic and inorganic polymers the commercially available beaded polystyrene (200-400 mesh, 80—20 /i diameter), cross-linked by 2% divinylbenzene as the most suitable up to that time for the purposes of peptide synthesis on solid phase. Today there are series of investigations to find better supports for use in Merrifield s synthesis (for comparison, see the review articles of Merrifield [16, 35] and Meienhofer [33]). But in all cases the improved properties of novel carriers or modified polystyrenes concern only one or two of the above-mentioned necessary parameters — e.g., mechanical stability or strengthened C-terminal bond of an amino acid to the carrier — whereas nearly all the other characteristics for a suitable solid phase turn out to be less favourable, compared to the original Merrifield resin. 

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