Siloxane structures network formation

There are two technically important methods by which extended Si/0 systems can be formed from molecular precursors. The first is by reaction of chlorosilanes with oxygen at high temperatures, while the second is by hydrolysis and condensation reactions of chloro- or alkoxysilanes. Chapters 32 and 33 deal with the structural evolution of siloxane structures in such reactions from an experimental and theoretical viewpoint. M. Binnewies et al. compare the stepwise formation of Si-0 networks from SiCU for both the combustion and hydrolysis reactions. The stability and reactivity of intermediate chlorosiloxanes is an important issue in this work. Both the initial process in the reaction of SiCfi with O2 and the growth of larger siloxane cages are investigated theoretically in the contribution of K. Jug. 

Atomic Structure. The control of atomic structure is fundamental to any system, and an incomplete understanding of atomic structure can limit advancement. For example, our understanding of preceramic polymers, up through the formation of networks, is improving but the full exploitation of this chemistry is still limited by the lack of detailed knowledge of the structure of the resulting ceramic at the atomic level. Even with more familiar silicone polymer systems, synthetic barriers are encountered as polymers other than poly(dimethylsiloxane) are used. Stereochemical control is inadequate in the polymerization of unsymmetrical cyclic siloxanes to yield novel linear materials. Reliable synthetic routes to model ladder systems are insufficient. 

Silicone rubber has a three-dimensional network structure caused by cross-linking of polydimethyl siloxane chains. Three reaction types are predominantly employed for the formation of silicone networks (155) peroxide-induced free-radical processes, hydrosilylation addition cure, and condensation cure. Silicones have also been cross-linked using radiation to produce free radicals or to induce photoinitiated reactions. 

For the homologous dibutylsilanediol, a dimer is not able to form a space-filling aliphatic sheath around the hydrogen-bonded siloxane network, and so a potential columnar structure is not stabilized and mesophase formation does not occur. Other aliphatic substituents, for example, cyclohexane, are too bulky, thereby suppressing dimer formation, and again creation of a mesophase is not possible. 

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