MALDI-TOF MS investigation of the RAFT polymerization of a water-soluble acrylamide derivative

A matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) study of N-acryloylmorpholine (NAM) homopolymers, as obtained by reversible addition-fragmentation chain transfer (RAFT) polymerization technique, is reported. Polymerization of NAM was performed in dioxane using azobis(isobutyronitrile) (AIBN) as initiator and tert-butyl dithiobenzoate as RAFT chain transfer agent. Polymer samples of low molecular weights (< 10 000 g mol(-1)) and low polydispersity indices (PDI < 1.1) were obtained, which are essential requirements for such MALDI-TOF MS analysis. First, analysis of poly(NAM) samples in linear mode led to (M-n) over bar. values very close to both size exclusion chromatography/light scattering (SEC/LS) and H-1 NMR values as well as to the theoretical ones. Then, an accurate examination of chain end groups was performed using the reflectron mode. Two main chain populations were identified: (i) dormant chains (i.e., initiated by a tert-butyl and terminated by a dithiobenzoate group) together with sulfine and thioester-ended chains probably resulting from oxidation of dithiobenzoate chain ends during storage; (ii) proton-terminated chains mainly produced by fragmentation of the former chains in the spectrometer. In addition, some chains which could correspond to termination reactions onto the intermediate radicals involved in the RAFT equilibrium were suspected. Finally, comparison of polymer samples before and after aminolysis indicated that dithioester-ended chains constituted the majority of chains initially present in these samples. This study confirms that it is indeed possible to use MALDI-TOF MS to investigate the structure of polymer chains synthesized by the RAFT technique.