Resequencing of Comonomer Units of Well-Defined Vectra Oligomers during MALDI-TOF Mass Spectral Measurements
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- 作者：El Hadj Elandaloussi ; Arpad Somogyi ; Anne Buyle Padias ; Robert B. Bates ; H. K. Hall ; Jr.
- 刊名：Macromolecules
- 出版年：2006
- 出版时间：October 3, 2006
- 年：2006
- 卷：39
- 期：20
- 页码：6913 - 6923
- 全文大小：185K
- 年卷期：v.39,no.20(October 3, 2006)
- ISSN：1520-5835
文摘
To aid in the detailed characterization of the LCP polymer Vectra A950, 20 alternating dimersand 10 alternating tetrameric model oligomers were synthesized and subjected to detailed MALDI and ESI spectralcharacterization. The model compounds were synthesized using stepwise reactions on 4-hydroxybenzoic and6-hydroxy-2-naphthoic acids using protection-deprotection strategies. MALDI-TOF spectra of these compoundsin 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 provedto be acylium ions resulting from cleavage of the unexpectedly fragile aryloxy-to-carbonyl bond. At higher appliedlaser power, the extrusion of oxybenzoyl groups as p-quinonemethide molecules from both the chain middlesand ends, followed by recombination, led to extensive resequencing of the original monomer units. Similar behaviorwas observed in the negative ion spectra, allowance being made for the suppression of negative ions by carboxylicand phenolic functional groups. ESI spectra on the positive side, in contrast, gave much less fragmentation butmore complicated spectra resulting from the occurrence of noncovalent molecular complexes. On the negativeside, much fragmentation accompanied the complex spectra, reasonably resulting from the ready formation ofcarboxylate and phenoxide anions. In the ESI spectra, the original monomer sequences were cleanly maintainedand identified. Our results demonstrate both the power and limitations of MALDI and ESI in the characterizationof Vectra A950, which by implication extend to the characterization of other commercially important polymersbased on 4-hydroxybenzoic acid as well.