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

The degree-of-intramolecular-condensation, defined as the number of residua l silanol (Si-OH) groups per oligomer, for a variety of silsesquioxane poly mers was measured by matrix-assisted laser desorption/ionization time-of-fl ight mass spectrometry. Condensation of the Si-OH groups of a given silsesq uioxane oligomer leads to the formation of intramolecular Si-O-Si bridges a ccompanied by the loss of water. This elimination reaction is easily identi fied via high resolution time-of-flight mass spectrometry. Various polymeri c silsesquioxanes were prepared from the corresponding trialkoxyorganosilan es by hydrolysis-condensation reactions at moderate temperatures (25-65 deg rees C) in aqueous acetone with catalytic amounts of formic acid. An n-decy l-silsesquioxane was found to have the highest degree-of-intramolecular-con densation 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 a s well as the lowest overall molecular mass. Tn all cases the number of int ramolecular 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 i n isolated chains. (C) 1999 Elsevier Science Ltd. All rights reserved.