Cyclosiloxane rings

Thermomechanical studies have indicated that the glass transition temperature of synthesized copo-lymers is decreased as the volume of cyclosiloxane ring in the chain is increased. The results of X-ray structural analysis indicate that the copolymers represent amorphous systems, and increase of cyclic fragment volume leads to an insignificant increase of the interchain distance. 

Keywords cyclosiloxanes, ring-opening polymerization, silyloxonium ions... 

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. ... 

While the measurements of the photoluminescence intensity under steady-state excitation as discussed above did not reveal a convincing systematic effect from the cyclosiloxane ring size, it was possible to achieve a classification of the emission characteristies from the phosphorescence excitation measurements. In this experiment, the emission intensity is recorded after a fixed delay time (A/ = 50 ps was selected) at a fixed emission wavelength (505 nm) as a function of the excitation wavelength. Measurements of the time dependence of this emission were also made at a fixed excitation wavelength Txc = 320 nm, and for two select emission wavelengths of = 380 nm and Ams = 458 nm. 

Rings Ha, b and c were found to polymerize in bulk with DMSO as an activator and various catalytic amounts of the anionic initiator, potassium poly(dimethylsiloxane)diolate (for polymerization of Ila and b, see (22)). The latter is well known as an efficient initiator of cyclosiloxane polymerization. Thus... 

Cyclosilazanes are found to be reluctant to polymerize by the ring-opening process, probably for thermodynamic reasons. On the other hand, six- and eight-membered silazoxane rings are able to undergo anionic polymerization under similar conditions to those which have been widely used for cyclosiloxane polymerization provided there is no more than two silazane units in the cyclic monomer. They can also copolymerize with cyclosiloxanes however, the chain length of the linear polymer formed is substantially decreased with increasing proportion of silazane units. 

Chojnowski, J. Ring-Opening Polymerization of Cyclosiloxanes. In Gelest Catalog Arkles, B., Larson, G., Eds. Gelest Inc. Morrisville, PA, 2004, pp 389-105. 

In contrast to the extensive homologous series known for cyclosiloxanes, monocyclic ring systems of the type (R2SiS) are limited to four- and six-membered rings n = 2, 3). The most common synthetic method involves the reaction of a diorganodichlorosilane with H2S in the presence of a base such as triethylamine or pyridine [eqn (10.34)]. ... 

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