Intermolecular cationic cross-links

Cross-linking reactions require an intermolecular Friedel-Crafts reaction between tertiary cations and positions of the phenyl group made less reactive by the presence of bulky groups on the indane structure. Electrophilic attack of benzylic cation 29 (Fig. 5) upon unsubstituted positions of the phenyl group of 15 is sterically inhibited. 

FIG. 3.23 Two possible cross-hnking routes (a) cationic growth via P—N=P hnk and (b) intermolecular condensation with formation of P—O—P cross-links. 

CL chemical cross-linking involves the formation of either intra- or intermolecular ionic or covalent bonds between the amino acid residues. CL cross-linking has been achieved through condensation of amino acid residues by both dehydrothermal treatment and photochemical cross-linking with ultraviolet light (Friess, 1998) or employing pol3walent cations, such as trivalent... 

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