Three-dimensional networks, polysiloxanes

As noted in Section 3.1, Si—0 bonds gain high strength from the large electronegativity difference between Si and 0, while remaining covalent and thus directional. Consequently, —Si—0—Si—0— (polysiloxane) chains are very stable indeed. Furthermore, since Si is almost always tetravalent, branched Si—0 chains and, indeed, sheets and three-dimensional networks are common both in naturally occurring minerals and in synthetic silicates, many of which are of primary economic importance (Chapter 7). 

The tensile strength of cross-linked polysiloxane elastomers is low, but can be markedly improved by reinforcement with a filler. The material of choice is fumed silica with high surface area, which can increase the strength by a factor of 20. It is thought that the silica particles agglomerate to form a three-dimensional network within the siloxane, greatly reinforcing the stmcture. 

Fig. 13. Modeling of a water-repellent silicone resin network on quartz (arbitrary assembly), viewed from the front (a) and from the side (b). Hie three-dimensional network is built up of two-dimensional monolayers (methyl derivative) for optical reasons (this leads to seemingly unsaturated oxygen atoms, which form the oxygen bridges in the three-dimensional case). A simplified silicone resin network on quartz could be composed of covalently bound polysiloxane chains, incompletely crosslinked polysiloxane, and embedded silsesquioxanes or homosilsesquioxanes (cages and ladders), going from the bottom (quartz) to the top [28, 42, 75, 78, 79]. The network has been optimized from graphical aspects. Total height approx. SO A.

It is well known that the useful properties of siloxanes stem from the nature of the siloxane bond (93), the structural characteristics of the polymer, the synthetic control over molecular weight and polydispersity, and the character of the framing groups (R). Polysiloxanes are presently available commercially as linear chains (93), rings, ladders, and three-dimensional networks (e.g., polyhedral oligomeric... 

Polysiloxanes gain more and more interest in recent years due to their excellent chemical and physical properties, such as thermal stability, hydrophobicity or electrical resistance (1). One synthesis route towards linear polymers is the ROP of cyclic monomers such as hexamethylcyclotrisiloxane (D3) (2). The functionalization of the substituents on the silicon atom in the polymer is performed via the modification of the cyclic monomers. Especially vinylic side groups are often introduced into the polymer backbone since they are active groups for cross-linking of polysiloxanes to three dimensional networks such as silicone elastomers (3). 

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