Nonhydrolytic synthesis and structural study of methoxyl-terminated polysiloxane D/Q resins

Abstract

Low-viscosity, methoxylated polysiloxane resins incorporating Me 2 SiO 2/2 (D) and SiO 4/2 (Q) units were prepared using nonhydrolytic condensation between Si-Cl and Si-OMe groups with the formation of MeCl, catalyzed by a Lewis acid. With the commonly used catalysts, condensation between two Si-OMe groups, with formation of Me 2 O, also took place to a large extent, hindering the control of the degree of condensation of the resins. Several catalysts were tested by monitoring the formation of MeCl and Me 2 O using sealed NMR tubes and 1 H-NMR spectroscopy. The best compromise between reactivity and selectivity was obtained with ZrCl 4 . Resins with various compositions were prepared in the absence of solvent by condensation between Me 2 SiCl 2 and Si(OMe) 4 at 130°C, catalyzed by 1 mol % ZrCl 4 . They were characterized using viscosimetry, gas chromatography coupled with mass-spectrometry (GC-MS), and quantitative 29 Si-NMR spectroscopy. The resins consisted of a complicated mixture of oligomers, linear or branched (n > 1) and cyclic (n > 3), with a high degree of D/Q bonding. The distribution of Si-OMe and Si-OSi bonds and the bonding between D and Q units were found to be nearly random. This was ascribed to the occurrence of Si-OSi/Si-OMe and Si-OSi/Si-OSi redistribution reactions that reached equilibrium during the synthesis.