Polymeric silsesquioxanes: degree-of-intramolecular-condensation measured by mass spectrometry

Abstract

The degree-of-intramolecular-condensation, defined as the number of residual silanol (Si–OH) groups per oligomer, for a variety of silsesquioxane polymers was measured by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Condensation of the Si–OH groups of a given silsesquioxane oligomer leads to the formation of intramolecular Si–O–Si bridges accompanied by the loss of water. This elimination reaction is easily identified via high resolution time-of-flight mass spectrometry. Various polymeric silsesquioxanes were prepared from the corresponding trialkoxyorganosilanes by hydrolysis–condensation reactions at moderate temperatures (25–65°C) in aqueous acetone with catalytic amounts of formic acid. An n-decyl-silsesquioxane was found to have the highest degree-of-intramolecular-condensation with lower degrees found in n-propyl and in 3-methacryloxypropyl silsesquioxanes. An n-propyl silsesquioxane synthesized without a catalyst was found to have the lowest degree-of-intramolecular-condensation of all as well as the lowest overall molecular mass. In all cases the number of intramolecular Si–O–Si bridges per oligomer was found to be a linear function of the number of repeat units in the oligomer. This linear relationship is discussed in light of theories of intramolecular interactions and bonding in isolated chains.