Synthesis and characterization of cis- and trans-trimethyltriphenylcyclotrisiloxane

2,4,6-Trimethyl-2,4,6-triphenylcyclotrisiloxane (P-3) exists as cis and trans-stereoisomers. Herein, we present the detailed synthesis and characterization of the cis- and trans-P-3 isomers. After preparation of the mixed cyclic methylphenylsiloxanes, it was found that vacuum distillation was the best method for separating P-3 from the mixture of P-3 and 2,4,6,8-tetramethyl-2,4,6,8-tetraphenylcyclotetrasiloxane (P-4) on a large scale and that pure cis-P-3 can be isolated by washing the mixed P-3 isomers with methanol. It is also shown that a selective crystallization method was successful for isolating cis-P-3 from the mixture of P-3 and P-4 directly and that large crystals may be obtained. Using gas chromatography and nuclear magnetic resonance (NMR) spectroscopy, the two different isomers and four different isomers in P-3 and P-4, respectively, are easily identified. The nuclear magnetic resonance (NMR) spectra, differential scanning calorimeter (DSC) thermograms, and infrared (IR) spectra with various ratios of cis-P-3 and trans-P-3 were also analyzed. A melting point depression of the cis-P-3 was observed with an increased amount of trans-P-3 for the mixed isomers, as expected. These P-3 stereochemical isomers provide valuable starting materials for the preparation of stereoregular siloxanes using ring-opening polymerization.