Commercial polyurethanes (PURs) are produced on the industrial scale by reacting methylene diphenyl diisocyanate (MDI) with low MW polyols which are distributions of multiple individual diols. Thus, the resulting PUR oligomers consist of a mixture of structural isomers at each discrete mass in the distribution. The contribution of specific isomers can be studied by starting with monodisperse, structurally-pure polyols, however this approach becomes intractable for larger MW polyols, which are challenging to synthesize and purify. Here we demonstrate a straightforward synthesis approach for generating PUR oligomer distributions with unique numbers of polyol attachments. Four readily-available, low MW polyols (PEG, PPG, PTHF, and PBA) were reacted with phenyl isocyanate (PhNCO) to produce distributions of PUR oligomers exhibiting unique numbers of polyol attachments. Structural characterization by ion mobility (IM) and high resolution mass spectrometry (HRMS) confirmed these PUR oligomers are of a single measurable structure and unique mass, and tandem MS/MS results indicated the oligomer products follow known fragmentation mechanisms.

This article reviews the analytical tool chest used for characterizing alkoxylates and their associated copolymer mixtures. Specific emphasis will be placed upon the use of mass spectrometry-based techniques as rapid characterization tools for optimizing reaction processes in an industrial R&D setting. An initial tutorial will cover the use of matrix-assisted laser desorption/ionization-mass spectrometry and tandem mass spectrometry fragmentation for detailed component analysis (e.g., polyol and isocyanate) of a model polyurethane-based foam. Next, this critical feedback information will be used with the guidance of mass spectrometry to initiate the development of a new, more efficient, tris(pentafluorophenyl)borane (FAB) catalyst-based alkoxylation process for generating the next generation of glycerin-initiated poly(propylene oxide)-co-poly(ethylene oxide) copolymers. Examples will be provided for each step in the FAB-based optimization process that were required to generate the final product. Following this example, two-dimensional liquid chromatography, supercritical fluid chromatography, and ion mobility separations, along with their coupling to mass spectrometry, will be reviewed for their efficiency in characterizing and quantitating the components within these complex polyether polyol mixtures.

Polyurethanes are specialized polymeric materials with a diverse range of applications including foams, insulators, sealants, adhesives and elastomers. Analysis of these materials utilizing mass spectrometry is hampered by the polydispersity and the range of isomeric products produced during the polymerization process. To aid with the mass spectral studies of these materials we have developed methodologies to synthesize discreet polyether-based polyurethane oligomers that may be studied by mass spectrometry and used as standards in the analysis of more complex polymer products. We have synthesized oligomers consisting of polyethylene glycol and poly(tetrahydrofuran) adducts of methylene diphenyl diisocyanate (MDI) and characterized these by NMR and mass spectrometry. These oligomers represent both symmetrical and asymmetrical polyurethane derivatives.

Rationale Commercial-grade polymer synthesis is performed via melt polymerization, which leads to polydispersion. The work herein provides a synthetic strategy to produce mono-dispersive polyurethane oligomers and an analytical strategy to distinguish these oligomers, providing chemists with the tools necessary to synthesize and identify specific polymer structures that exhibit a desired property. Methods Three isomeric poly (ethylene glycol)-polyurethane (PEG-PUR) oligomers were synthesized and analyzed via flow-injection ion mobility-mass spectrometry (IM-MS). Each polymer oligomer was injected and run independently via flow injection at 100 μL•min-1 and analyzed in positive ion mode on a drift tube-QTOF instrument. Mobility measurements were determined using a single-field approach. For tandem MS experiments, the sodium adducted singly-charged precursor ion was isolated in the quadrupole and subjected to a range of collision energies. RESULTS: In MS experiments, both +1 and +2 sodium adducted species were observed for each oligomer at m/z 837.4 and m/z 430.2, respectively. When isolated and fragmented via MS/MS, the +1 precursor yielded distinct fragment ions for each of the three isomeric oligomers. Fragmentation generally occurred at urethane linkages via 1,3- and 1,5-H shift mechanisms. IM was also used to distinguish the three isomers, with greater IM separation observed for the +2 versus the +1 species. Conclusion Mono-disperse PEG-PUR oligomers were synthesized and analyzed. Although the polymeric oligomers analyzed in this study are quite small and structurally simple, this work serves as a model system for the synthesis and structural characterization of larger, more complex block copolymers.

Polyurethane (PU) di-block copolymers are one of the most versatile polymeric materials, comprised of hard and soft segments that contribute to PU's broad range of applications. Polybutylene adipate (PBA) is a commonly used soft segment in PU systems. Characterizing the structure of PBA polymers is essential to understanding complex heterogeneity within a PU sample. In this study, ion mobility-mass spectrometry (IM-MS) and tandem mass spectrometry (MS/MS) are used to structurally characterize a PBA standard (Mn = 2250) adducted with a combination of monovalent alkali cations (Li, Na, K, Rb, and Cs). IM-MS profiles show unique trends associated with each cation-adducted PBA sample. Charge state trends: +1, +2, and +3 were extracted for cation-adducted PBA oligomers, and investigated to study gas-phase transitional folding. To quantitatively assess the gas-phase structural similarities and differences, a statistical test (ANOVA) was used to compare PBA oligomer-cation collisional cross sections (CCS). Fragmentation studies (MS/MS) identified the unique behavior of Li and Na for promoting 1,5 H-shift and 1,3 H-shift fragmentation, whereas the PBA precursor preferentially loses the larger K, Rb, and Cs cations as the ion activation energy is increased. The combination of adducted alkali cations, IM-MS, and MS/MS allow for unique structural characterization of this important PBA system.

This review covers the applications of mass spectrometry (MS) and its hyphenated techniques to characterize polyurethane (PU) synthetic polymers and their respective hard and soft segments. PUs are commonly composed of hard segments including methylene bisphenyl diisocyanate (MDI) and toluene diisocyanate (TDI), and soft segments including polyester and polyether polyols. This literature review highlights MS techniques such as electrospray ionization (ESI), matrix assisted laser/desorption ionization (MALDI), ion mobility-mass spectrometry (IM-MS), and computational methods that have been used for the characterization of this polymer system. Here we review specific case studies where MS techniques have elucidated unique features pertaining to the makeup and structural integrity of complex PU materials and PU precursors.

Matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) is used to characterize methylenedianiline (MDA) 3-ring and 4-ring species. Building on our previous MALDI-MS 2-ring MDA isomer study, here we compare 3-ring and 4-ring electrospray ionization (ESI) and MALDI results. In ESI, 3-ring and 4-ring MDAs each form a single [M + H]+ parent ion. However, in MALDI, each MDA multimer forms three unique precursor ions: [M + H]+, [M•]+, and [M - H]+. In this study, 3-ring and 4-ring MDA precursors are characterized to identify the unique fragment ions formed and their respective fragmentation pathways. In addition to the three possible precursors, the 3-ring and 4-ring species are higher-order oligomer precursors in polyurethane (PUR) production and thus provide additional insight into the polymeric behavior of these PUR hard block precursors. The combination of ion mobility-mass spectrometry (IM - MS) and tandem mass spectrometry (MS/MS) allow the structural characterization of these larger MDA multimers.

Characterization of methylenedianiline (MDA) 2-ring isomers (2,2'-, 2,4'-, and 4,4'-MDA) is reported using matrix assisted laser desorption/ionization-mass spectrometry (MALDI-MS), a common technique used for characterizing synthetic polymers. MDA is a precursor to methylene diphenyl diisocyanate (MDI), a hard block component in polyurethane (PUR) synthesis. This work focuses on comparing MALDI results to those of our previous electrospray ionization-mass spectrometry (ESI-MS) studies. In ESI, 2-ring MDA isomers formed single unique [M + H]+ (199 Da) parent ions, whereas in MALDI each isomer shows significant formation of three precursor ions: [M - H]+ = 197 Da, [M•]+ = 198 Da, and [M + H]+ = 199 Da. Structures and schemes are proposed for the MALDI fragment ions associated with each precursor ion. Ion mobility-mass spectrometry (IM-MS), tandem mass spectrometry (MS/MS), and computational methods were all critical in determining the structures for both precursor and fragment ions as well as the fragmentation mechanisms. The present study indicates that the [M - H]+ and [M•]+ ions are formed by the MALDI process, explaining why they were not observed with ESI.

In various polymerization processes, the formation of a wide variety of chains, not only in length but also in chemical composition, broadly complicates comprehensive polymer characterization. In this communication, we compare different stationary and mobile phases for the analysis of complex polymer mixtures via size-exclusion chromatography-mass spectrometry (SEC-MS). To the best of our knowledge, we report novel chromatographic effects for the separation of linear and cyclic oligomers for polyesters (PE) and polyurethanes (PUR). A complete separation for the different structures was achieved for both polymer types with a single-solvent system (acetonitrile, ACN) and without extensive optimization. Additionally, cyclic species were found to show an inverse elution profile compared to their linear counterparts, suggesting distinct physical properties between species.

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.

MALDI-TOF/TOF collision-induced dissociation (CID) experiments were conducted on model aromatic polyester oligomers. CID fragmentation studies identified initial fracture of the ester bond and subsequent CO loss as a major pathway, consistent with the general fragmentation mechanism used to explain the origin of poly(p-phenylenediamine terephthalamide) (PPD-T) fragment ions. Specifically, both charge-remote and charge-site fragmentation were observed. Different parent-ion species were observed, the major ones being carboxyl-hydroxyl, di-carboxyl, di-hydroxyl, and phenyl-carboxyl terminated. One species observed was hydroxyl-diethylamine terminated caused by reaction of carboxyl groups with triethylamine added to the synthesis reaction mixture. Fragment ions reflected the end groups of the parent oligomers. Some MALDI fragment-ion spectra were obtained for species showing exchange between Li and H at the carboxyl end group. Bond energy calculations provide further insight into suggested fragmentation mechanisms.

Building on results from our previous study of 2-ring methylenedianiline (MDA), a combined mass spectrometry approach utilizing ion mobility-mass spectrometry (IM-MS) and tandem mass spectrometry (MS/MS) coupled with computational methods enables the structural characterization of purified 3-ring and 4-ring MDA regioisomers in this current study. The preferred site of protonation for the 3-ring and 4-ring MDA was determined to be on the amino groups. Additionally, the location of the protonated amine along the MDA multimer was found to influence the gas phase stability of these molecules. Fragmentation mechanisms similar to the 2-ring MDA species were observed for both the 3-ring and 4-ring MDA. The structural characterization of 3-ring and 4-ring MDA isomers using modern MS techniques may aid polyurethane synthesis by the characterization of industrial grade MDA, multimeric MDA species, and methylene diphenyl diisocyanate (MDI) mixtures.

Purified methylenedianiline (MDA) regioisomers were structurally characterized and differentiated using tandem mass spectrometry (MS/MS), ion mobility-mass spectrometry (IM-MS), and IM-MS/MS in conjunction with computational methods. It was determined that protonation sites on the isomers can vary depending on the position of amino groups, and the resulting protonation sites play a role in the gas-phase stability of the isomer. We also observed differences in the relative distributions of protonated conformations depending on experimental conditions and instrumentation, which is consistent with previous studies on aniline in the gas phase. This work demonstrates the utility of a multifaceted approach for the study of isobaric species and elucidates why previous MDA studies may have been unable to detect and/or differentiate certain isomers. Such analysis may prove useful in the characterization of larger MDA multimeric species, industrial MDA mixtures, and methylene diphenyl diisocyanate (MDI) mixtures used in polyurethane synthesis.

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.

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.

We report a combination of MALDI-TOF/TOF CID and Py-GC/MS to examine the fragmentation mechanisms of poly(phenyl-sulfidesulfone) (PPSS). Collision-induced dissociation (CID) fragmentation was initially used to examine the fragmentation pattern of PPSS and identify the preferred primary fragmentation pathways. On the basis of this information, a general fragmentation mechanism was developed and compared with the fragments generated by Py-GC/MS. These mechanisms were then compared with those of polysulfone (PSF) and poly(phenylsulfone) (PPSF), from our previous CID studies, to further refine our general fragmentation mechanism for poly(arylsulfone)s. Also, Py-GC/MS studies identified a total of eight sulfone-containing fragments; two of which have been observed in our previous study of PSF, while the remaining six represent new species which have not been previously reported for poly(arylsulfone)s. The identification of sulfone moieties in the Py-GC/MS of PPSS indicates that the phenyl-sulfide linkage, like that of the isopropylidene group found in polysulfone, may increase the stability of the polymer. Our study indicates that all three poly(arylsulfone)s preferentially cleave at the phenyl sulfone bond, followed by the phenyl-oxygen or phenyl-sulfide bond, depending on which is present.

A study involving the evaporation-grinding MALDI sample preparation method, MALDI-TOF/TOF CID, and Py-GC/MS is presented to examine the fragmentation mechanisms of poly(p-phenylene sulfide) (PPS). MALDI-TOF/TOF CID fragmentation studies yielded a wealth of information about the mass, structure (linear or cyclic), end-groups, and backbone modifications of the polymer. 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.(1,2) TOF/TOF CID results indicate that linear PPS undergoes random main chain Fragmentation along the polymer backbone and preferentially fragments at bonds adjacent to dibenzothiophene and phenyl end-ggroups. Cyclic species produce fragment ions similar to linear species. However, the MS/MS precursor ions for cyclic PPS are, by far, the most intense peaks, while the precursor ions for linear species show relatively low intensity. CID fragmentation results are supported by Py-GC/MS data and are consistent with the proposed degradation mechanisms.