Catalyst polymerisation siloxane

Polymers are formed via two general mechanisms, namely chain or step polymerisation, originally called addition and condensation, respectively, although some polymerisations can yield polymers by both routes (see Chapter 2). For example, ring opening of cyclic compounds (e.g., cyclic lactides and lactams, cyclic siloxanes) yield polymers either with added catalyst (chain) or by hydrolysis followed by condensation (step). Many polymers are made via vinyl polymerisation, e.g., PE, PP, PVC, poly(methyl methacrylate) (PMMA). It could be argued that the ethylenic double bond is a strained cyclic system. 

A prerequisite for the evaluation mentioned is knowledge about the reaction mechanism. Linear absorbance diagrams proved the photoisomerisation taking place as in solutions. However, the siloxane matrix has to be fresh. Different types of siloxanes were tested, some photochemically polymerised, others fabricated by a catalyst induced process. In the latter case the Pt-catalyst must not overcome a concentration limit otherwise it influences the azobenzene photoreaction. Approximate evaluations at low absorption (assuming a irradiation intensity independent of the volume element) do not offer appropriate results because of measurement problems. Therefore a transformation of the time scale has been used, discussed in Section 5.7.3. 

Bases that catalyse the polymerisation or organosilanols and cyclic siloxanes include hydroxides, alcoholates, phenolates and siloxanolates of the alkali metals quaternary ammonium and phosphonium bases, their siloxanolates and fluorides and organolates of the alkali metals quaternary ammonium and phosphonium bases, their siloxanolates and fluorides and organoalkali metal compounds. After polymerisation is complete, the activity of the catalyst must be destroyed. This can be done by neutralisation with chlorosilanes, CO2, etc. The liquid or solid polymers are subsequently cured to form expandable end products. 

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