MALDI-TOF mass spectrometry analysis of TEMPO-capped polystyrene

Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry of polystyrenes prepared by 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)-mediated living free radical polymerization has been performed using two different matrices. A complete assignment of the observed peaks could be proposed, owing to the experimental resolution which allowed us to display the isotopic distribution. With the 1,8-dihydroxy-9(10H)-anthracenone (dithranol)/silver trifluoroacetate system, a major part of the charged chains undergoes gas phase fragmentation during the analysis. This phenomenon, only minor for conventionnally prepared polystyrene, is particularly enhanced when the chains contain a TEMPO-based alkoxyamine end group. In contrast, the dead chains with no alkoxyamine end group are properly detected. When using the 2,5-dihydroxybenzoic acid (DHB) matrix without added salt, only protonation occurs involving the alkoxyamine functionality. Only the TEMPO-capped polystyrene chains are observed and no fragmentation occurs; the dead chains which have no protonation site are not detected. This still allows determination of the nature of the polymer headgroup and gives an insight into the initiation mechanism. Thermally self-initiated polystyrene in the presence of TEMPO contains predominantly a 4-phenyl-l,2,3,4-tetrahydro-l-naphthyl headgroup. When dibenzoyl peroxide (BPO) is used as an initiator at 130 degrees C in the presence of TEMPO ([TEMPO]/[BPO] = 1.2), both benzoyloxy and 4-phenyl-1,2,3,4-tetrahydro-l-naphthyl headgroups are detected. When an alkoxyamine initiator is used, only the fragment derived from this initiator is observed and the extent of thermal initiation seems to be reduced by comparison with the use of dibenzoyl peroxide initiator. These results can be explained by the enhanced formation of 4-phenyl-1,2,3,4-tetrahydro- l-naphthyl radical in the presence of free TEMPO.