Cationization effect on the molecular weight distribution of an ethoxylated polymer: A combined theoretical and time-of-flight secondary ion mass spectroscopic study

The effect of metal cationization on the molecular weight distribution (MWD) of an ethoxylated polymer, Surfynol 465 (S465), is investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and a hybrid theoretical method combining ab initio density functional theory and molecular mechanics. The MWDs generated from sodium and from silver-cationized oligomers of S465 were measured by ToF-SIMS. The structure and bonding of the cationized complexes were calculated. The results suggest that upon cationization, the metal atoms are chelated by oxygen atoms and, in the case of Ag+, by the pi-orbitals of the C-C triple bond. Although the binding energy of both Na+ and Ag+ with the Surfynol molecules is very high for sufficiently long ethoxylate side chains, strong bonding preference is given to Ag+ over Na+ due to the orbital interaction between Ag+ and the Surfynol oligomer via 4d-pi*: and 5s-pi overlap and the ion-dipole interaction between the cations and the oxygen atoms in the ethoxylate chains with Na+ being of more ionic character. The theoretical results suggest that a minimum ethoxylate chain length is required for Na+ chelation and that in the high molecular weight region both cations will bind with the Surfynol oligomers strongly, consistent with our experimental observations. We demonstrate that ToF-SIMS is an effective technique for measuring the molecular weight distribution of a low molecular weight oligomer series.