Physical properties, cyclic siloxanes

Although each of these cyclic siloxane monomers can be polymerized separately to synthesize the respective homopolymers, in practice they are primarily used to modify and further improve some specific properties of polydimethylsiloxanes. The properties that can be changed or modified by the variations in the siloxane backbone include the low temperature flexibility (glass transition temperature, crystallization and melting behavior), thermal, oxidation, and radiation stability, solubility characteristics and chemical reactivity. Table 9 summarizes the effect of various substituents on the physical properties of resulting siloxane homopolymers. The... 

TABLE 7. SOME PHYSICAL PROPERTIES OF CYCLIC SILOXANES... 

Rory (pages 5-9 of ref. 22) reported three types of experiments from which he deduced no evidence for structure (1) stress-temperature coefficients, (2) vapor pressure of a PIB-diluent system, and (3) ring-chain equilibrium constants between cyclic and linear siloxanes. In each case the systems were evaluated far above their respective T/ s. Such results are not pertinent to our present inquiry. We have searched sporadically but without success for physical measurements which span a temperature region across Tu in elastomers. Finally, we note that because elastomers tend to be flexible hydrocarbons, Tu should be weak and may not have a great influence on physical properties. The marked exception to this generalization is PIB with its stiff, stereoregular backbone. Tu in PIB has been discussed recently in great detail, with Tu 250 K, Tip 290 K. 

One of the basic properties of siloxenes is their general insolubility in organic solvents, a fact that strongly impedes physical and structural characterization. As a result, the question arose as to whether structurally better defined siloxene-like polymers with improved solubility can be assembled in a stepwise manner starting from appropriate molecular precursors, and whether the properties of siloxene, such as the intense photoluminescence, can be matched. We thus attempted to rebuild the proposed structure of Kautsky-siloxene by the controlled hydrolysis of cyclic or linear oligosilanes bearing hydrolytically labile substituents followed by the thermal condensation to polymeric siloxanes. The general route is outlined in Scheme 16.1. 

Table 6.11 shows the results of elementary analysis, physical chemical properties and the yield of ethylsiloxane oligomers containing cyclic fragments in the side chain. In the IR spectra of oligomers with cyclic fragments in the side chain one can observe the absorption bands characteristic of asymmetric valence oscillations of the =Si-0-Sis bonds in the region 1020 and 1080 cm characteristic of both linear siloxane links and cyclosiloxane rings. In the IR spectra one can also observe absorption bands for unreacted =Si-H bonds at 2160 cm. ... 

---