André Thévand

Tandem mass spectrometry of poly(methacrylic Acid) oligomers produced by negative mode electrospray ionization.

Dissociation of small poly(methyl acrylic acid) (PMAA) anions produced by electrospray was characterized by tandem mass spectrometry. Upon collisional activation, singly, and doubly deprotonated PMAA oligomers were shown to fragment via two major reactions, dehydration and decarboxylation. The elimination of a water molecule would occur between two consecutive acid groups in a charged-remote mechanism, giving rise to cyclic anhydrides, and was shown to proceed as many times as pairs of neutral pendant groups were available. As a result, the number of dehydration steps, together with the abundance of the fragment ions produced after the release of all water molecules, revealed the polymerization degree of the molecule in the particular case of doubly charged oligomers. For singly deprotonated molecules, the exact number of MAA units could be reached from the number of carbon dioxide molecules successively eliminated from the fully dehydrated precursor ions. In contrast to dehydration, decarboxylation reactions would proceed via a charge-induced mechanism. The proposed dissociation mechanisms are consistent with results commonly reported in thermal degradation studies of poly(acrylic acid) resins and were supported by accurate mass measurements. These fragmentation rules were successfully applied to characterize a polymeric impurity detected in the tested PMAA sample.

Positive mode electrospray tandem mass spectrometry of poly(methacrylic acid) oligomers

The dissociation of small poly(methacrylic acid) (PMAA) cations produced by electrospray was characterized by tandem mass spectrometry. Similarly to PMAA ions produced in the negative ion mode, the two electrosprayed cationic forms, namely [PMAA+Na](+) and [PMAA-H+2Na](+), were shown to fragment via a major pathway consisting of successive dehydration steps. Elimination of a water molecule would occur between two consecutive acid groups in a charged-remote mechanism and was shown to proceed as many times as pairs of acidic pendant groups were available. As a result, comparing the number of dehydration steps observed in the MS/MS spectrum of two consecutive oligomers from the polymeric distribution reveals the degree of polymerization of the molecule. Secondary less informative reactions were shown to consist of losses of CO and/or CO(2), depending on the nature of the precursor ion. These fragmentation rules could be used to characterize PMAA-based copolymers, as successfully demonstrated for a polymeric impurity in the tested PMAA sample.

Tautomeric polymorphism in salicylideneamine derivatives: An X-ray diffraction and solid-state NMR study

Abstract The crystal structure of the N-(3-hydroxysalicylidene)-4-methoxyaniline has been studied by single-crystal X-ray diffraction and solid-state NMR spectroscopy. This is the first example of a Schiff base derived from 3-hydroxysalicylaldehyde which displays in the asymmetric unit, four distinct molecules linked together in the crystal lattice by two types of intermolecular O–H⋯O hydrogen bonds and formed by two independent tetramers. The 13C CPMAS NMR study corroborates the above results; the presence of different tautomeric equilibria in the same crystal structure is demonstrated and a qualitative estimation of the equilibrium mixture composition is given.

Determining chemical exchange rates of the uracil labile protons by NMR diffusion experiments

The exchange rates of the amido-protons of uracil with water were determined by NMR diffusion experiments and the results showed a factor 2 difference in lability between them, which was confirmed by more classical 2D-NMR exchange experiments.

Molecular Weight Determination of Block Copolymers by Pulsed Gradient Spin Echo NMR

Matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) is the technique of choice to achieve molecular weight data for synthetic polymers. Because the success of a MALDI-MS analysis critically depends on a proper matrix and cation selection, which in turn relates closely to the polymer chemical nature and size, prior estimation of the polymer size range strongly helps in rationalizing MALDI sample preparation. We recently showed how pulsed gradient spin echo (PGSE) nuclear magnetic resonance could be used as an advantageous alternative to size exclusion chromatography, to rationalize MALDI sample preparation and confidently interpret MALDI mass spectra for homopolymers. Our aim here is to extend this methodology to the demanding case of amphiphilic block copolymers, for which obtaining prior estimates on the Mw values appears as an even more stringent prerequisite. Specifically, by studying poly(ethylene oxide) polystyrene block copolymers of distinct molecular weights and relative block weight fractions, we show how PGSE data can be used to derive the block Mw values. In contrast to homopolymers, such determination requires not only properly recorded calibration curves for each of the polymers constituting the block copolymers but also an appropriate hydrodynamic model to correctly interpret the diffusion data.

NMR diffusion and nuclear Overhauser investigation of the hydration properties of thymine : influence of the methyl group

The absence of preferential hydration in thymine and its lowest water accessibility with respect to uracil were evidenced by NMR diffusion and HOESY experiments; the hydration differences observed between these pyrimidine bases were attributed to the electronic rather than steric properties of the methyl group.

Structural characterization of a poly(methacrylic acid)–poly(methyl methacrylate) copolymer by nuclear magnetic resonance and mass spectrometry

Mass spectrometry (MS) and nuclear magnetic resonance (NMR) have been combined to achieve the complete microstructural characterization of a poly(methacrylic acid)-poly(methyl methacrylate) (PMAA-PMMA) copolymer synthesized by nitroxide-mediated polymerization. Various PMAA-PMMA species could be identified which mainly differ in terms of terminaisons. 1H and 13C NMR experiments revealed the structure of the end-groups as well as the proportion of each co-monomer in the copolymers. These end-group masses were further confirmed from m/z values of doubly charged copolymer anions detected in the single stage mass spectrum. In contrast, copolymer composition derived from MS data was not consistent with NMR results, obviously due to strong mass bias well known to occur during electrospray ionization of these polymeric species. Tandem mass spectrometry could reveal the random nature of the copolymer based on typical dissociation reactions, i.e., water elimination occurred from any two contiguous MAA units while MAA-MMA pairs gave rise to the loss of a methanol molecule. Polymer backbone cleavages were also observed to occur and gave low abundance fragment ions which allowed the structure of the initiating end-group to be confirmed.

Methylation of acidic moieties in poly(methyl methacrylate-co-methacrylic acid) copolymers for end-group characterization by tandem mass spectrometry

The complete structural characterization of a copolymer composed of methacrylic acid (MAA) and methyl methacrylate (MMA) units was achieved using tandem mass spectrometry. In a first step, collision-induced dissociation (CID) of sodiated MAA-MMA co-oligomers allowed us to determine the co-monomeric composition, the random nature of the copolymer and the sum of the end-group masses. However, dissociation reactions of MAA-based molecules mainly involve the acidic pendant groups, precluding individual characterization of the end groups. Therefore, methylation of all the acrylic acid moieties was performed to transform the MAA-MMA copolymer into a PMMA homopolymer, for which CID mainly proceeds via backbone cleavages. Using trimethylsilyldiazomethane as a derivatization agent, this methylation reaction was shown to be complete without affecting the end groups. Using fragmentation rules established for PMMA polymers together with accurate mass measurements of the product ions and knowledge of reagents used for the studied copolymer synthesis, a structure could be proposed for both end groups and it was found to be consistent with signals obtained in nuclear magnetic resonance spectra.

Analytical Strategy for the Molecular Weight Determination of Random Copolymers of Poly(Methyl Methacrylate) and Poly(Methacrylic Acid)

Molecular weight characterization of random amphiphilic copolymers currently represents an analytical challenge. In particular, molecules composed of methacrylic acid (MAA) and methyl methacrylate (MMA) as the repeat units raise issues in commonly used techniques. The present study shows that when random copolymers cannot be properly ionized by MALDI, and hence detected and measured in MS, one possible analytical strategy is to transform them into homopolymers, which are more amenable to this ionization technique. Then, by combining the molecular weight of the so-obtained homopolymers, as measured by MS, with the relative molar proportion of the MMA and MMA units, as given by 1H NMR spectrum, one can straightforwardly estimate the molecular weight of the initial copolymer. A methylation reaction was performed to transform MAA-MMA copolymer samples into PMMA homopolymers, using trimethylsilyldiazomethane as a derivatization agent. Weight average molecular weight (Mw) parameters of the MAA-MMA copolymers could then be derived from Mw values obtained for the methylated MAA-MMA molecules by MALDI, which were also validated by pulsed gradient spin echo (PGSE) NMR. An alkene function in one of the studied copolymer end-groups was also shown to react with the methylation agent, giving rise to MMA-like polymeric by-products characterized by tandem mass spectrometry and which could be avoided by adjusting the amount of the trimethylsilyldiazomethane in the reaction medium.

A "Magic-Angle" Sample-Spinning Nuclear Magnetic Resonance Spectroscopic Study of Interference Effects in the Nuclear Spin Relaxation of Polymers *

Abstract The proton-coupled, carbon-13 “magic-angle” sample-spinning nuclear magnetic resonance spin-lattice relaxation behavior of poly (cis-isoprene) and poly (cis-butadiene) as a function of temperature is reported. Analysis of the relaxation behavior of the methine carbons reveals strong temporal correlations between the 13CH dipolar and 13C anisotropic chemical shielding interactions. Equations are presented which enable the interpretation of these interferences for non-axially symmetric shielding tensors, subject to cylindrically symmetric reorientation. In the context of these expressions, the results indicate that the methine moiety in each of the two polymers executes relatively rapid, isotropic motions, on the time scale of the carbon-13 Larmor frequency. It is also shown that in certain polymeric systems, the sign of the interference term may change with frequency. This intriguing property provides additional insight into the relative reorientational anisotropies of the low- and high-frequency motions, and the specific case of the spin-lattice relaxation and differential line broadening of the methine carbons in poly (cis-isoprene) and poly (trans-isoprene) is considered in detail.