From CO2 to Polysiloxanes: Di(carbamoyloxy)silanes Me2Si[(OCO)NRR′]2 as Precursors for PDMS

Double insertion of carbon dioxide into the Si-N bonds of diaminosilanes of the type Me2Si(NRR')(2) gives di(carbamoyloxy)silanes Me2Si[(OCO)NRR'](2). The reactions proceed exothermically and quantitatively in most cases. A comprehensive analysis of the CO2-insertion products including single-crystal X-ray structure analyses was carried out. Quantum chemical calculations indicate an activation energy of about 124 kJ/mol for both the first and the second insertion and support the exothermal nature of the reaction. Investigation of the thermal decomposition of the di(carbamoyloxy)silanes Me2Si[(OCO)NRR'](2) reveals the formation of oligo- and polysiloxanes. Depending on the thermolysis parameters, isocyanates, amines, and/or ureas are formed in addition to the siloxanes. Various methods were applied to study the decomposition process and to identify and quantify the products, including thermal analyses, mass spectrometry, and FTIR and NMR (solution and solid-state) spectroscopy. The overall reaction scheme provides a novel route to polysiloxanes which uses carbon dioxide as an oxygen source.