Covalent Cross-linking Reactions

Figure 3.1 Covalent cross-linking reaction, (a) Cross-linking point between two macromoiecules. (b) Several tetrafunctional cross-linking points between macromo-lecular chains, which form a polymer network.

Covalent cross-linking reactions to form a compatibUizing copolymer may also be performed by addition of a coupling agent. Coupling agents react with the same... 

Two other important commercial uses of initiators are in polymer cross-linking and polymer degradation. In a cross-linking reaction, atom abstraction, usually a hydrogen abstraction, occurs, followed by termination by coupling of two polymer radicals to form a covalent cross-link ... 

In the second, complementary, approach the polycondensation of silica polymer is followed by the formation of an organic network made by cross-linking reaction of monomers covalently bound to silicon compounds (Scheme 4.1) resulting in polymeric materials with outstanding protective abilities, including thermal, mechanical and corrosion resistance. 

Cross-linking reactions, acid-catalyzed polymer, 15 168-169 Cross-linking resists, 15 172 Cross-linking systems, covalent dye fixation using, 9 489-491 Cross-finking treatments, in volumetric sweep efficiency, 18 620 Cross-finks... 

The most important reaction in blood clotting is the conversion, catalyzed by thrombin, of the soluble plasma protein fibrinogen (factor 1) into polymeric fibrin, which is deposited as a fibrous network in the primary thrombus. Thrombin (factor 11a) is a serine proteinase (see p. 176) that cleaves small peptides from fibrinogen. This exposes binding sites that spontaneously allow the fibrin molecules to aggregate into polymers. Subsequent covalent cross-linking of fibrin by a transglutaminase (factor Xlll) further stabilizes the thrombus. 

Some cross-linking takes place in the form of side reactions such as biuret and allophanate bonds. Deliberate introduction of covalent cross-linking can be carried out by the introduction of multifunctional agents (mainly triols) into either the prepolymer or chain extension system. 

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