Gas-phase conformations of synthetic polymers: Poly(ethylene glycol), poly(propylene glycol), and poly(tetramethylene glycol)

Effects of chain flexibility and cationizing agent on the gas-phase conformations of a series of polyethers were studied. Collision cross- sections of poly(propylene glycol) (PPG) and poly(tetramethylene glycol) (PTMEG) oligomers (from the 5-mer to 14-mer) cationized by sodium were measured using ion mobility methods and compared to earlier work on sodiated poly(ethylene glycol) (PEG). Molecular mechanics/dynamics calculations show that all three polyethers form a ring of oxygen atoms around Na⁺, coordinating additional oxygens to the cation from above and/or below this ring. However, the number and arrangement of these oxygens are system dependent. Up to 8 oxygens in PEG coordinate Na⁺, but that number drops to 7 for PPG and 6 for PTMEG. The difference is attributed to changes in the structural details of the ring as well as the position of the Na⁺ ion in that ring. Molecular mechanics/dynamics calculations were also used to investigate the structures of neutral PPG and PTMEG oligomers. In these cases, the oligomers are relatively compact below 200 K but they suddenly unfold into more extended structures between 200 and 300 K.