Gas phase conformations of synthetic polymers: Poly (methyl methacrylate) oligomers cationized by sodium ions

Gas phase conformations of a series of poly(methyl methacrylate) (PMMA) oligomers cationized by sodium ions were investigated experimentally and theoretically. Ion chromatography and molecular mechanics methods were used to measure the cross sections of Na⁺(PMMA)3 to Na⁺(PMMA) 11 (i.e. from the 3-mer to the 11-mer). Collision induced dissociation (CID) of Na⁺(PMMA) 10 is also reported. The dominant fragment peaks in the CID spectrum are solely in the low mass-to-charge range and correspond to the loss of 1-2 monomer units from each end of the oligomer. The fragments also retain the Na⁺ ion indicating that the ion interacts with both ends of PMMA. Molecular modeling of the Na⁺(PMMA) oligomers support this finding. The calculations show that the lowest energy structures are U-shaped with Na⁺ bound to multiple carbonyl oxygen atoms near both ends of the oligomer. The cross sections of these structures agree with those obtained from the ion chromatography experiments. Ion chromatography and molecular dynamics simulations also show that the cross sections are independent of temperature between 300 and 600 K, further supporting the indication that the Na⁺ ion strongly binds to both ends of the oligomer. A mechanism for the surprising CID results is suggested. (Int J Mass Spectrom 188 (1999) 121-130)