Other Ways of Achieving Crosslinking

Apart fi-om the peroxide curing method as discussed above, there are several other ways to crosslink siloxane polymers. These rely on coupling reactions between reactive functional groups that have been deliberately introduced either as end-groups or in the side-chain of the pol3nner. Representative examples of such fimctional-group- containing polysiloxanes are shown in Fig. 6.19. 

Some of the important condensation reactions that have been very successful in bringing about crosslinking reactions are shown in Fig. 6.20. In each one of these reactions the main emphasis is on the fact that if any side-product emerges from the reaction it should not disturb the main-chain in terms of skeletal fragmentation or any other undesired reactions [1-2]. 

The most important reaction is the coupling between a Si-H functional group and a Si-CH=CH2 group to generate a Si-CH2-CH2-Si crosslink. This reaction is known as the hydrosilylation reaction and is catalyzed by 

Copolymerization of a mixture of [Me2SiO]4 and [MeHSiO]4 affords a polysiloxane containing Si-H side chains [27] (see Eq. 6.23). 

A similar approach can also be used for preparing vinyl side-group-containing polysiloxanes. These can then be condensed together in the presence of a catalyst to afford crosslinking (Fig. 6.21). 

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