Characterization and some insights into the reaction chemistry of polymethylsilsesquioxane or methyl silicone resins

Abstract

Structural characterization of a polymethylsilsesquioxane (PMSQ) and a DT-type methyl silicone resin (MeDT) has been carried out by various instrumental analyses including GPC, NMR, gas chromatography, and gas chromatography-mass spectrometry. Although the PMSQ had a Mw around 5000, the resin contained a significant amount of low molecular weight species consisting of T2 [MeSi(OH)O2/2] and T3 [MeSiO 3/2] units, ranging from T 3 4 T 2 3 to T 3 8 T 2 2 including many isomers. One isomer of T 3 6 T 2 2 was isolated of which structure was determined as a cage structure. The species are supposed to consist mainly of cyclotetra- and cyclopentasiloxanes, but presence of strained rings such as cyclotrisiloxane rings also was suggested. In MeDT, species in which the T 2 units in the molecules from PMSQ is replaced with D2 [Me2SiO2/2] were found, for example, T 3 6 D 2 2, suggesting that general silicone resins consist of similar structures as silsesquioxanes. The Mark-Houwink exponent for these methyl resins was 0.3, indicating the molecular shape to be compact. Investigation on the formation chemistry of the cubic octamers indicates that siloxane bond rearrangement is an important mechanism in the molecule build-up process. © 2012 Maki Itoh et al.