Trifunctional cross-linkers

Sinz, A., Kalkhof, S., and Ihling, C. (2005) Mapping protein interfaces by a trifunctional cross-linker combined with MALDI-TOF and ESI-FTICR mass spectrometry. /. Am. Soc. Mass Spectrom. 16(12), 1921-1931. 

Scheme 1 Synthesis of cross-linked PPEs by the palladium-catalyzed cross-coupling reaction of 2,5-diiodo-4-[(2-ethylhexyl)oxy]methoxybenzene, l,4-diethynyl-2,5-bis(octyloxy)ben-zene, and the trifunctional cross-linker 1,2,4-tribromobenzene. R =2-ethylhexyl, R =n-octyl...

Figure 192 The Wedekind trifunctional cross-linker can react with amine groups via its p-nitrophenyl ester to form amide bond linkages. The phenyl azide group then can be photolyzed to generate covalent bond formation with a second molecule. The biotin side chain provides binding capability with avidin or streptavidin probes.

Trifunctional cross-linkers contain three hetero or homo reactive groups for connecting three different or equal chemical species. 

Trifunctional cross-linkers are commonly employed to create dendrimers or dendritic layers on a surface [10,43,44]. The higher amount of receptor-molecules on the surface leads to a higher coupling density of capture probes [45]. An attractive application for trifunctional and multifunctional cross-linkers is the coupling of more specific biochemical species in one spot (Fig. 16). 

Fig. 16 Trifunctional cross-linkers for coupling different functionalities and enhancement of receptor density...

Numerous other cross-linkers may be used. For the trifunctional cross-linkers, the R group may be methyl, ethyl, vinyl, and several other groups, with methyl the most common. In some cases tetrafunctional and higher-functionality cross-linkers or polymeric cross-linkers may also be employed. The acetic add cure system should be avoided where substrates are subject to acid corrosion. 

Figure 20 Schematic illustration of thermally reversible coupling by PNA strands on a trifunctional cross-linker and the compiementaiy sequence on a PNA-polymer hybrid. Reprinted with permission from Wang, Y. Armitage, B. A. Berty, G. C. MacromoleculesUK, 38.5846. Copyright 2005, American...

Where a step polymerization is used, almost always it is for the first polymer synthesized in a sequential IPN. The reasons involve the slow diffusion into a preexisting network of most monomers used in step polymerization and the relatively high glass transition temperature of step-polymerized polymers. The latter reason is important because in order for diffusion and concomitant polymerization to occur rapidly, polymer network I should be above its glass transition at the temperature of polymerization of monomer mix II. Table 6.2 presents glycerol as a simple trifunctional cross-linker for step-polymerized materials, suitable for polyesters and polyurethanes. 

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