Thermal decomposition of cyclolinear and cyclic methylsiloxane polymers

A series of branched-chain methylsiloxane polymers built up of four-membered siloxane cycles and linear segments of zero to four siloxy units, and another series consisting only of siloxane cycles of five to eight members were investigated. The samples were pyrolyzed at temperatures increased stepwise from 300 to 800°C. The pyrolysate was analyzed online by gas chromatography and mass spectrometry. In the pyrolysates, series of cyclic, bicyclic, and polycyclic methylsiloxane compounds were found. Quantitative analysis of the pyrolysates shows that their composition reflects the composition and structure of the pyrolyzed polymethylsiloxane. The amount of the cyclic dimethylsiloxanes is related to the length of the linear siloxane segments between the branching points. The amount of bicyclic and polycyclic dimethylsiloxane oligomers in the pyrolysates is related to the average frequency and vicinity of the branching points.