Matrix-assisted laser desorption ionization time-of-flight mass spectroscopy of polydimethylsiloxanes prepared via anionic ring-opening polymerization

Poly(dimethylsiloxane) prepared by the anionic ring-opening polymerization initiated by butyl-lithium in tetrahydrofuran has been examined by MALDI TOF mass spectrometry. The molecular weight distributions have been found to be different from those obtained by SEC, with the MALDI spectra underestimating the contribution of the higher mass macromolecules leading to an underestimation of the average molecular weights. Three distributions are observed in the mass spectra that correspond to the initiation and interchain-exchange mechanisms producing three different macromolecular species. The pre-dominant species is an integral sum of the repeat unit indicating that anionic ring-opening propagation is fast when compared to intermolecular chain transfer. This is in conflict with earlier studies and indicates that the relative rates of transfer by different processes are highly dependent upon reaction conditions.