Spectroscopic studies of a novel cyclic oligomer with pendant alkoxysilane groups

The structure and reactivity of a novel oligomer (degree of polymerization (DP) 4-8) made from the free radical polymerization of vinyltrimethoxysilane (VTMO) with butyl acrylate has been examined using ¹H, ¹³C, ²⁹Si NMR and FTIR spectroscopy. The oligomers contain on average two -Si(OCH₃)₃ groups per molecule. As opposed to typical silane functional oligomers, this oligomer is expected to contain pendant methoxysilane groups attached through a carbon polymer backbone. These classes of oligomers are low viscosity liquids as-supplied, but when exposed to ambient air with an acid catalyst quickly form hard films. The generally accepted route for cure is hydrolysis and condensation through the pendant silane groups. To gain better insight into the spectroscopic information, comparisons with ethyltrimethoxysilane (ETMO) and VTMO have been made. ²⁹Si NMR has revealed that in addition to the methoxysilane groups the as-supplied oligomer actually contains roughly 30% Si-O-Si condensation products. An analysis based on simple curing models would indicate that this level of condensation would lead to significant network formation and thus high viscosity which is not consistent with the observed low viscosity. Based on an analysis of the spectroscopic data in combination with molecular modeling the most plausible explanation for these condensation products is that they result from condensation of adjacent methoxysilanes on the same oligomer. This condensation product will not lead to molecular weight build and thus explains why the viscosity of the as-supplied oligomer remains low. This condensation reaction could have taken place from exposure to atmospheric moisture during preparation, from incorporation of a VTMO dimer during synthesis, or from internal condensation during synthesis. At this point we cannot differentiate between these routes; however, the latter appears to be the most consistent with the observed data. The spectroscopic data confirms that a significant amount of condensation occurs when the oligomer is exposed to ambient air with 0.1% p-toluenesulfonic acid catalyst. Little evidence is seen for the presence of silanol groups during the ambient air cure.