Resequencing of Comonomer Units of Well-Defined Vectra Oligomers during MALDI-TOF Mass Spectral Measurements

To aid in the detailed characterization of the LCP polymer Vectra A950, 20 alternating dimers and 10 alternating tetrameric model oligomers were synthesized and subjected to detailed MALDI and ESI spectral characterization. The model compounds were synthesized using stepwise reactions on 4-hydroxybenzoic and 6-hydroxy-2-naphthoic acids using protection−deprotection strategies. MALDI-TOF spectra of these compounds in both the positive and negative modes were determined using dithranol, 2-(4-hydroxyphenylazo)-benzoic acid (HABA), and and its methyl ester (MHABA) as matrixes and in the presence or absence of silver trifluoroacetate. The “coolest” matrix, leading to least fragmentation, proved to be dithranol with silver trifluoroacetate, while the “hottest” was dithranol alone. On the positive side, the majority of ions detected other than the parent ions proved to be acylium ions resulting from cleavage of the unexpectedly fragile aryloxy-to-carbonyl bond. At higher applied laser power, the extrusion of oxybenzoyl groups as p-quinonemethide molecules from both the chain middles and ends, followed by recombination, led to extensive resequencing of the original monomer units. Similar behavior was observed in the negative ion spectra, allowance being made for the suppression of negative ions by carboxylic and phenolic functional groups. ESI spectra on the positive side, in contrast, gave much less fragmentation but more complicated spectra resulting from the occurrence of noncovalent molecular complexes. On the negative side, much fragmentation accompanied the complex spectra, reasonably resulting from the ready formation of carboxylate and phenoxide anions. In the ESI spectra, the original monomer sequences were cleanly maintained and identified. Our results demonstrate both the power and limitations of MALDI and ESI in the characterization of Vectra A950, which by implication extend to the characterization of other commercially important polymers based on 4-hydroxybenzoic acid as well.