STRUCTURAL CHARACTERIZATION AND SEQUENCE DISTRIBUTIONS OF POLYSILOXANES USING PYROLYSIS MS MS/

Novel polysiloxanes, with 4-(dialkylamino)pyridine substituents, are c haracterized by pyrolysis tandem mass spectrometry. These polymers for m abundant cyclic oligomeric ions under both desorption electron ioniz ation (DEI) and desorption chemical ionization (DCI) conditions. Produ ct MS/MS spectra of the cyclic ions reveal characteristic fragmentatio ns under low energy collision activated dissociation. Protonated cycli c oligomers higher than the pentamer are mainly due to the proton boun d dimers of lower oligomeric units. The cyclic oligomers are shown to have proton affinities greater than 1000 kJ/mole. It is proposed that thermal depolymerization occurs through an intramolecular siloxane bon d rearrangement, which is in agreement with a previously proposed ''lo op mechanism''. Markovian statistical calculations are applied to the DCI mass spectral data in order to determine the sequence distribution of siloxane copolymers. Application of this method show that the mono mers in the copolymers examined are non-randomly distributed.