Vinyl-Siloxane Cleavage: Applying Thermo-Analytical and Hybrid Calorimetry Techniques to Inform Process Safety and Design

The use of homogeneous and heterogeneous acid catalysts is widespread in the processing of siloxanes as they protonate Si-O bonds, enabling a vast array of rearrangement reactions. Therefore, developing a fundamental understanding how acid catalysts interact with various siloxane moieties is crucial to safe and sustainable operation. In this study, we present our findings on the interaction of acid catalysts with vinyl pendant siloxanes, specifically focusing on the exothermic acid-catalyzed cleavage of the C-Si bond with gas evolution. Thermo-analytical techniques including Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) were used to screen various moieties to evaluate gas and heat generation. Adiabatic Accelerating Rate Calorimetry (ARC) measurements were performed to quantify the kinetics of C-Si cleavage and evaluate effect of operational conditions on the kinetics of the cleavage reaction. Then, a hybrid technique, ARCxGC/MS was utilized to identify the various reaction products and the correlate them with the detected onset temperatures. Using molecular modelling and additivity techniques like secondary Benson Group estimation, the theoretical Heat of Reaction for the proposed reaction was determined and compared to predictions from the afore-mentioned calorimetry techniques. From the above effort, it was concluded that cleavage of the vinyl-silicon bond due to activation by the acid catalyst is an exothermic and gas-generating reaction. The findings were used to generate, for the first time, a detailed quantitative thermo-kinetic model of the vinyl-Si cleavage reaction pathway useful for design safety. Our findings were supported by a cursory review of available literature that qualitatively suggest that scission on the C-Si bond is activated in the presence of electrophiles and protic acids.