Anthony P. Gies

MALDI-TOF/TOF CID study of 4-alkyl-substituted polystyrene fragmentation reactions.

MALDI-TOF/TOF CID experiments were conducted on a variety of hydrogen-terminated poly(4-methylstyrene), hydroxylated poly(t-butylstyrene), and polystyrene precursor ions: nu2009=u200910, 15, 20, 25, and 30, where the number of repeat units n corresponds to the oligomer mass number. The influences of structure, molecular weight, and effective collision kinetic energy on degradation mechanisms were examined to test the generality of our multi-chain fragmentation model developed for polystyrene. Each depolymerization mechanism is presented in detail with experimental and computational data to justify/rationalize its occurrence and effective kinetic energy dependence. These processes show the complex interrelationship between the various pathways along with preferred production of secondary radicals, which suppresses the appearance of primary radicals. Additionally, Py-GC/MS experimental data are presented, for comparison of the multimolecular free radical reactions in pyrolysis with the unimolecular fragmentation reactions of MS/MS.

MALDI-TOF/TOF CID study of poly(α-methylstyrene) fragmentation reactions

MALDI-TOF/TOF CID experiments are reported for hydroxylated poly(α-methylstyrene) precursor ions (PAMS: m/zxa01,445.9 (nu2009=u200910), 2,036.3 (nu2009=u200915), 2,626.7 (nu2009=u200920), 3,217.1 (nu2009=u200925), and 3,807.5 (nu2009=u200930), where the number of repeat units n corresponds to the oligomer mass numbers). The influences of structure, molecular weight, and kinetic energy on degradation mechanisms were examined to test the generality of our multi-chain fragmentation model developed for polystyrene. Our results indicate that poly(α-methylstyrene) free radicals are formed initially through multiple chain breaks and subsequently undergo a variety of depolymerization reactions to yield predominantly monomer and dimer species; the intensity of each species depends on the effective kinetic energy selected for the CID process. Each depolymerization mechanism is presented in detail with experimental and computational data to justify/rationalize the process and its kinetic energy dependence. These processes show the complex interrelationships between the various pathways along with preferred production of tertiary radicals, which suppresses the appearance of primary radicals. Additionally, Py-GC/MS experimental data are presented to allow a comparison of the multimolecular free radical reactions in pyrolysis with the unimolecular fragmentation reactions of MS/MS.

Collision induced dissociation study of ester-based polyurethane fragmentation reactions.

A combination of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) collision induced dissociation (CID) and ion mobility separations (IMS) was used to study a complex mixture composed of unreacted polyester starting material (polybutylene adipate) and polyurethane (PUR) end products. Collision induced dissociation fragmentation identified two primary fragmentation mechanisms of PURs, which were used to generate a general fragmentation model. Predicted fragment ions were used to distinguish: (1) linear and cyclic PURs, (2) hard-block and soft-block PURS, (3) the degree of blockiness within hard- and soft-block PURs, (4) the location of the MDI linkages within each PUR chain, and (5) the relative intensities of various isobars intermingled within a precursor mass peak. These results were consistent with the observed IMS separations.

Matrix-assisted laser desorption/ionization-time-of-flight/time-of-flight collision-induced dissociation study of poly(p-phenylenediamine terephthalamide) fragmentation reactions

MALDI-TOF/TOF collision-induced dissociation (CID) experiments are reported on model poly(p-phenylenediamine terephthalamide) (PPD-T) polymers, revealing a variety of synthesis reaction products. Diamine-terminated oligomers were the major product of synthesis using excess amine, and di-carboxylic acid oligomers were the major product for excess acid. Structures of major reaction products were confirmed by CID fragmentation studies, along with detailed studies of MS/MS decomposition pathways. Apparent fracture of the phenylcarbonyl bond was the major fragmentation pathway (independent of end groups), resulting from initial NHCO bond cleavage with subsequent CO loss. Hydrogen-transfer reactions play an important role in fragmentation, involving both cross-chain abstraction of NH hydrogen and long-range H-transfer. End-group and main-chain modifications produce fingerprint CID fragmentation patterns that can be used to identify end groups and branching patterns; the structure of an unanticipated synthesis product was established using CID. The effect of synthesis conditions on polymer composition was studied using the analysis of variance, specifically, the amine-to-acid ratio used and post-synthesis addition of CaO. Of particular interest is oligomer end-group modification by the solvent (N-methyl pyrrolidone) induced by addition of CaO.

MALDI-TOF/TOF CID study of poly(butylene adipate) fragmentation reactions

This study demonstrates the use of MALDI-TOF/TOF CID fragmentation for the identification of expected and “unexpected” side products in a complex mixture of melt polymerized poly(butylene adipate) (PBA), which aged at room temperature, unexposed to direct sunlight and extreme temperature fluxuations. Expected products include PBA structures terminated with butanediol, adipic acid and buteneol (due to dehydration during synthesis); as well as cyclic architectures with no terminal groups. Additionally, side products were observed containing “unexpected” terminal groups such as glycol, propenyl, methanol, and aldehydes. Low energy fragmentation pathways and computational data are presented to verify the structural assignments of the identified structures, followed by discussion of their probable origin. 1,5-Hydrogen shift reactions were identified as the major low-energy fragmentation pathway.

MALDI-TOF MS stability study of model poly(p-phenylene terephthalamide)s.

AbstractIn the present study, we address the possibility of matrix-assisted laser desorption/ionization (MALDI)–time-of-flight MS analysis-induced chain fragmentation in poly(p-phenylene terephthalamide) (PPD-T) by considering two possible sources: (1) grinding-induced fragmentation resulting from the evaporation–grinding MALDI sample preparation method (E-G method) and (2) in-source/metastable fragmentation induced by the MALDI laser. An analysis of variance (ANOVA) statistical study found, with a high probability, that obtaining MALDI spectra with the effective laser area as large as possible (the “fanned-out” setting) did not cause any chain fragmentation due to the E-G MALDI sample preparation method, even when three additional grinding steps were used. However, the effect of laser fluence was less clear. A significant effect of laser fluence was observed for lower mass oligomers (<1,400xa0Da), but there was essentially no effect for higher mass species up to our limit of ANOVA measurement (∼2,300xa0Da). Plausible explanations are presented to explain these observations. The most likely scenario is that “unexpected” end-group modifications occur during PPD-T synthesis, producing small quantities of low mass species, which are amplified by the MALDI-EG extraction procedure.n FigureMALDI-TOF MS stability studies indicate that analysis-induced fragmentation, due to sample grinding and laser-induced fragmentation, is not observed.