End-group characterization of poly(methylphenylsilane) by alkali metal salts doped MALDI-TOF mass spectra

The end groups of a sample of poly(methylphenylsilane) (PMPS) obtained by the Wurtz coupling reaction of diorganodichlorosilane were characterized by MALDI-TOF mass spectrometry. Three fractions were obtained by fractionation of a PMPS sample with methanol, in order to get information on the distribution of end groups as a function of the molecular weight. The MALDI mass spectra of these fractions showed well-resolved signals up to 2500 Da corresponding to open chain oligomers of PMPS bearing four different types of end groups: -Si-(CH3)(3), -O-Si-(CH3)(3), -O-CH3, -H. Six different types of linear oligomers were identified together with a sizeable amount of cyclic oligomers, most likely generated during the polymerization step. The problem of isobar mass series in the MALDI spectra due to clusters of oligomers with Li+, Na+, K+ was solved by comparing the MALDI spectra of PMPS fractions with and without the addition of a specific alkaline salt (KCl or NaI) to the solution containing the sample and matrix. The intensity of the oligomer clusters with the specific cation were enhanced with respect to the others. The mass shifts observed in the MALDI-TOF doped spectra with respect to the undoped ones allowed the unambiguous assignments of the isobar peaks corresponding to PMPS oligomers.