Can highly non-polar low molecular weight polyisobutylenes be measured by electrospray ionization mass spectrometry? Anion-attachment provides an appropriate method for their characterization

Polyisobutylenes (PIBs) with different end-groups including chlorine, exo-olefin, hydroxyl and methyl initiated from aliphatic and aromatic initiator were studied by electrospray ionization mass spectrometry (ESI-MS). Independently of the end-groups, presence or absence of aromatic initiator moiety, these PIB derivatives were capable of forming adduct ions with NO3 and Cl ions, and thus allowing the direct characterization of these compounds in the negative ion mode of ESI-MS. To obtain [PIB+NO3] and [PIB+Cl] adduct ions with appreciable intensities, addition of polar solvents such as acetone, 2-propanol or ethanol, to the dichloromethane solution of PIBs was necessary. Furthermore, increasing the polarity, i.e., increasing the acetone content in the acetone/dichloromethane mixtures and the ion-source temperature give rise to enhanced intensities for both [PIB+NO3] and [PIB+Cl] ions. Energy-dependent collision induced dissociation studies (CID) revealed that increasing the collision voltages resulted in the shift of the apparent molecular masses to higher ones. CID studies also showed that dissociation of the [PIB+Cl] ions requires higher collision energy than that of the [PIB+NO3] . In addition, high level quantum chemical calculations were performed to get deeper insight into the nature of the interactions between the highly nonpolar PIB chains and anions NO3 and Cl as well as to determine the Gibbs free energies for the formations of [PIB+NO3] and [PIB+Cl] adduct ions.