Synthesis and Characterization of Cis- and Trans-Trimethyltriphenylcyclotrisiloxane

2,4,6-Trimethyl-2,4,6-triphenylcyclotrisiloxane (P₃) exists as cis and trans-stereoisomers. Herein, we present the detailed synthesis and characterization of the cis- and trans-P₃ isomers. After preparation of the mixed cyclic methylphenylsiloxanes, it was found that vacuum distillation was the best method for separating P₃ from the mixture of P₃ and 2,4,6,8-tetramethyl-2,4,6,8-tetraphenylcyclotetrasiloxane (P₄) on a large scale and that pure cis-P₃ can be isolated by washing the mixed P₃ isomers with methanol. It is also shown that a selective crystallization method was successful for isolating cis-P₃ from the mixture of P₃ and P₄ 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 P₃ and P₄, respectively, are easily identified. The nuclear magnetic resonance (NMR) spectra, differential scanning calorimeter (DSC) thermograms, and infrared (IR) spectra with various ratios of cis-P₃ and trans-P₃ were also analyzed. A melting point depression of the cis-P₃ was observed with an increased amount of trans-P₃ for the mixed isomers, as expected. These P₃ stereochemical isomers provide valuable starting materials for the preparation of stereoregular siloxanes using ring-opening polymerization.