TOF CID Study of Polycarbodiimide ... - ACS Publications

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MALDI−TOF/TOF CID Study of Polycarbodiimide Branching Reactions Anthony P. Gies,* William H. Heath, Richard J. Keaton, Jorge J. Jimenez, and Joseph J. Zupancic Department of Core R&D Analytical Sciences, The Dow Chemical Company, 2301 North Brazosport Boulevard, B-1219, Freeport, Texas 77541, United States S Supporting Information *

ABSTRACT: A combination of MALDI−TOF MS and TOF/TOF collision-induced dissociation (CID) experiments was conducted on toluene diisocyanate (TDI) based carbodiimide and phenyl isocyanate (PI) carbodiimide mixtures to examine their degradation mechanisms and identify “fingerprint” side products associated with each of their synthesis. Findings include the first observance of the 3-methyl-1-phenyl-2phospholene-1-oxide (MPPO)/carbodiimide (CDI) catalyst-attached intermediate and the use of CID fragmentation to verify its chemical structure and degradation processes. This work is significant for two reasons: (1) it identifies extensive branching as the root-cause of viscosity build-up in carbodiimides and (2) it presents a BF3initiated mechanism leading to polyguanidine formation.

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degradation mechansisms.4−10 Moreover, through the use of collision-induced dissociation (CID), additional structural information about polymers can be determined from the fragment peaks of the precursor ions.11−15 Additionally, most of the classical techniques are “relative” methods that rely on calibration standards, which in many cases do not possess properties similar to the specific polymer that is being studied, and inferences must be made to yield “estimated” characterization information. Alternatively, mass spectrometry is an “absolute” method which does not rely upon polymers standards for calibration; this is an important advantage when standards do not exist for the polymer of interest.16,17 In the present study, we combine the evaporation−grinding matrix-assisted laser desorption/ionization (MALDI) sample preparation method (E−G method)11−14 with TOF/TOF collision-induced dissociation (CID) to examine the side product formation of carbodiimides (CDIs). Our initial case study focuses on TDI-based carbodiimides, to examine their synthesis and side products formation. The model TDI-based carbodiimide displayed a distribution of side products: small quantities of urea linkages, predominance of guanidine-branched species, along with considerable quantities of uretone imine branched CDIs, and trace levels of urea-containing allophanates. It was hypothesized that extensive guanidine branch formation leads to a buildup of viscosity in the model TDI-based carbodiimides mixture. To test this hypothesis, a second model study was conducted using phenyl isocyanate based carbodiimiide (PI-based CDI). Initial experiments focused on the use of electrospray mass spectrometry to conduct time-dependent studies to examine the species formed during carbodiimide

INTRODUCTION Carbodiimides, the condensation products of two equivalents of isocyanate functional groups, are of interest both industrially and academically. Carbodiimides are commonly incorporated, in small quantities, into methylene diphenyl diisocyanate (MDI) to breakup crystallinity and reduce viscosity (e.g., Isonate 143L). Academically, researchers have studied the structures formed in polycarbodiimides.1−3 Carbodiimides can be prepared by simply heating isocyanates in the presence of catalysts such as 3-methyl1-phenyl-2-phospholene-1-oxide (MPPO) while liberating CO2 (Schemes 1 and 2). Synthesis of pure polycarbodiimides is essentially impossible due to the side reactions that result in branching and produce highly viscous or solid products. To avoid the formation of branched carbodiimide products, a common practice is to limit the concentration of carbodiimide groups and thus the rate of side product formation. This is achieved through deactivation of the catalyst with common Lewis acids such as BF3−etherate. However, this also leads to further side product formation, vide inf ra. Historically, size exclusion chromatography (SEC), vapor pressure osmometry (VPO), nuclear magnetic resonance (NMR), light scattering, infrared spectroscopy, and ultraviolet/ visible spectroscopy have been used for polymer characterization.4−6 However, these “classical” methods have the disadvantage of being averaging techniques, which provide general information about the “average” polymer mixture as a whole, instead of providing selective information about individual oligomers within the mixture.4−6 Further, these classical averaging techniques are rarely capable of providing information about the various oligomers and impurities that may be present within the polymer mixture. In contrast, mass spectrometry can be used for molecular mass determination, architectural elucidation, end group analysis, quantification at trace levels, analysis of complex mixtures, and determination of © 2013 American Chemical Society

Received: July 13, 2013 Revised: September 9, 2013 Published: September 23, 2013 7616

dx.doi.org/10.1021/ma401481g | Macromolecules 2013, 46, 7616−7637

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Scheme 1. Synthesis Mechanism for the Model TDI-Based Carbodiimide polymer

butyl etherate, initiates a series of side reactions that lead to polyguanidine formation. Findings include the first observance of the 3-methyl-1-phenyl-2-phospholene-1-oxide (MPPO)/CDI catalyst-attached intermediate and the use of CID fragmentation to verify its chemical structure and degradation processes. Furthermore, this study helps to broaden the knowledge base of isocyanate and carbodiimide chemistry and assist in troubleshooting problems associated with their unwanted side-product formation.

Scheme 2. Synthesis Mechanism for the Model Phenyl Isocyanate Carbodiimide

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EXPERIMENTAL SECTION

Materials. Toluene diisocyanate (TDI) was obtained from the Dow Freeport TDI facility as an 80:20 mixture of the 2,4- and 2,6-isomers, respectively. It was confirmed to be free of urea by FTIR analysis prior to use. Anhydrous n-hexanol (>90% pure) was obtained from SigmaAldrich. Phenyl isocyanate (PI) and BF3−butyl etherate were obtained from Sigma-Aldrich. 3-Methyl-1-phenyl-2-phospholene-1-oxide

synthesis. This was followed by the use of collision-induced dissociation (CID) fragmentation for elucidating the degradation mechanisms of the identified structures. Results of this study indicate that carbodiimide synthesis termination, using BF3−

Scheme 3. Low Kinetic Energy Fragmentation Pathways for the Model TDI-Based Carbodiimide Structure 1-1

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(MPPO) was obtained from Digital Specialty Chemicals Ltd. All materials were used as received. TDI-Based Polycarbodiimide Synthesis. Toluene diisocyanate (85.0 g, 0.489 mols) was transferred via cannula to an oven-dried 4-neck round-bottom flask equipped with overhead stirring, nitrogen inlet, and addition funnel. The TDI was heated to 65 °C and 1-hexanol (33.2 g, 0.325 mols) was added dropwise over 4 h (to minimize the formation of dihexyl carbamate). The mixture was cooled to room temperature and stirred overnight to ensure complete reaction of the 1-hexanol (Scheme 1A).5 GPC analysis (Figure S1, in the Supporting Information) of the intermediate product identified the following: (1) the major species were monosubstituted TDI and unreacted TDI; (2) trace levels of allophanate products were detected at 1.26 area%; (3) negligible amounts of dihexyl carbamate were observed. The following day, MPPO (40 mg) was added and the mixture was heated to 85 °C (to minimize side reactions known to occur above 140 °C).1 After 5 h, the reaction was not complete by FTIR. Additional MPPO (200 mg) was added and the reaction was heated for 24 h (Scheme 1B).1 Phenyl Isocyanate Carbodiimide Synthesis. Phenyl isocyanate (85.0 g, 0.489 mols) was transferred via cannula to an oven-dried 4-neck round-bottom flask equipped with overhead stirring, nitrogen inlet, and addition funnel. Next, MPPO (40 mg) was added and the reaction was heated to 90 °C. The synthesis reaction is shown in Scheme 3.1 Aliquots were removed for ESI−MS analysis, after 2 and 8 h of reaction. Additionally, a 2 h aliquot was reacted for 6 h in the presence of BF3butyl etherate, to examine the effects of catalyst termination. Gel Permeation Chromatographic Analysis. Gel permeation chromatography (GPC) analyses were performed using a High Performance Liquid Chromatography (HPLC) system equipped with an inline degasser (Agilent G1322A), a liquid autosampler (Agilent G1329A), a quaternary pump (Agilent G1311A), a thermostated column compartment (Shimadzu CTO-20A), a refractive index (RI) detector (Agilent G1362A), a variable wavelength UV detector (Agilent G1365A), a PLgel 5-μm guard column (50 mm ×7.5 mm), and a series of four PLgel 5-μm (300 mm × 7.5 mm) narrow porosity analytical columns (50, 100, 1000, 10000 Å). Uninhibited tetrahydrofuran (THF) was used as the mobile phase at a flow rate of 1 mL/min while the column and detector temperatures were set to 40 °C. The sample was dissolved in mobile phase (∼1%) and filtered through a 0.45 μm PTFE membrane. A 100-μL aliquot of the sample solution was injected and analyzed over a 50 min run time. The sample results were quantitated using Empower Pro software (Waters Corp., version V5.00) against a third order (cubic) standard curve comprised of 12 narrowly distributed PEG standards (Mp = 34900 to 100 Da; Agilent Technologies, Santa Clara, CA, part number: PL2070−0201). MALDI−TOF/TOF CID Measurements. All samples were analyzed using an Applied Biosystems 4800 Proteomics Analyzer MALDI− TOF/TOF MS (Applied Biosystems, Framingham, MA) equipped with a 355-nm Nd:YAG laser. Spectra were obtained in the positive ion mode using an accelerating voltage of 8 kV for the first source, 15 kV for the second source, and a laser intensity approximately 10% greater than threshold. Instrument voltages were optimized for each spectrum to achieve the best signal-to-noise ratio. External mass calibration was performed using protein standards from a Sequazyme Peptide Mass Standard Kit (Applied Biosystems) and a three-point calibration method using Angiotensin I (m = 1296.69 Da), ACTH (clip 1-17) (m = 2093.09 Da), and ACTH (clip 18−39) (m = 2465.20 Da). Internal mass calibration was subsequently performed using a PEG standard (Mn = 2000; Polymer Source, Inc.) to yield monoisotopic mass accuracy better than Δm = ± 0.05 Da. The instrument was calibrated before each measurement to ensure constant experimental conditions. The CID collision energy is defined by the potential difference between the source acceleration voltage and the floating collision cell; in our experiments this voltage difference was set to 1 kV. Air was used as a collision gas at pressures of 1.5 × 10−6 and 5 × 10−6 Torr (which will be referred to as “low” and “high” pressure, respectively). All spectra were acquired in the reflection mode with a mass resolution greater than 3000 full-width at half-maximum height (fwhm); isotopic resolution was observed throughout the entire mass range detected. MALDI spectra were run in a dithranol (Aldrich) matrix doped with sodium

trifluoroacetate (NaTFA; Aldrich). Polymer samples were prepared using the evaporation−grinding method (E−G method)11−14 in which a 2 mg sample of polymer was ground to a fine powder with 60 μL of distilled tetrahydrofuran (THF, Fisher) in an agate mortar and pestle. The molar ratios of matrix:NaTFA:polymer were 25:1:1. The mixture was then ground a second time to ensure homogeneity. A sample of the mixture was then pressed into a sample well, by spatula, on the MALDI sample plate. MS and MS/MS data were processed using the Data Explorer 4.9 software (Applied Biosystems). ESI−TOF MS Measurements. Phenyl isocyanate based samples were analyzed using a Waters Synapt G2 High Resolution Mass Spectrometer (Waters Corp., Milford, MA). Mass spectra were obtained in the positive ion mode with the capillary (3500 V), cone (30 V), source temperature (110 °C), desolvation chamber (250 °C), and TOF mass analyzer potentials optimized to achieve the best signal-to-noise ratio. A curtain of nitrogen drying gas was utilized to assist in the ESI process. All spectra were acquired in the reflectron mode (resolution “V” mode) of the TOF mass spectrometer at mass resolutions greater than 20,000 fwhm; isotopic resolution was observed throughout the entire mass range detected. External mass calibration was performed using sodium formate and a 15-point calibration method. Internal mass calibration was subsequently performed using the peptide leu-enkephalin (Tyr-GlyGly-Phe-Leu) to yield monoisotopic masses exhibiting a mass accuracy better than Δm = ±0.001 Da. The instrument was calibrated before each measurement to ensure constant experimental conditions. Sample solutions were initially prepared in THF (35 μg/mL) and introduced into the ESI interface by direct infusion using a Harvard Apparatus PHD Ultra syringe pump at a flow-rate of 10 μL/min. Mass spectral data were processed using Polymerix 2.0 software by Sierra Analytics (Modesto, CA).

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RESULTS AND DISCUSSION In the initial study of TDI-based polycarbodiimides, the stoichiometry employed in the reaction was expected to produce an oligomer (n = 3 repeat units) with a molecular mass of 768 Da, but the highest MW peak observed in the GPC chromatogram corresponded to a mass of 5400 Da, relative to PEG standards (Figure S2, in the Supporting Information). Although it was expected that the rigid nature of these polymers would cause them to appear larger than they actually were, it was not anticipated that the result would be off by an order of magnitude. It is also worth noting that in addition to GPC analysis, the viscosity of the polymers was measured via parallel plate rheometry and found to be 6.34 × 105 cP at 50 °C. The combination of high viscosity and molecular weight justified further mass spectral characterization of this polymer to determine the true structure of the CDI oligomers. We will first define the nomenclature that will be used in this study. This will be followed by the use MALDI−TOF MS and TOF/TOF CID fragmentation to examine branching mechanisms in model TDI-based and PI-based polycarbodiimide systems to determine the root-cause of their formation. In the final portion of this study, we will propose carbodiimide degradation processes to explain the fragment species identified in the TOF/TOF CID mass spectra. Nomenclature. Terminology. All figures will show structures and peaks labeled according to the following key: (i) Precursor ion peaks are labeled in the x−y format (x = table number, y = structure number for precursor ions and species letters for fragment ions) and the table and structure number are followed by the ion which provides the charge added to the oligomers during the MALDI process (e.g., Na+). (ii) Precursor ion backbone and end-group modification are labeled where XA = amine end-group, XC = a 7618


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Table 1. Structural Assignments for the Peaks Observed in the MALDI−TOF Mass Spectrum of the Model TDI-Based Carbodiimide Polymer Mixture

1-3) are the predominant species, followed by the uretone imine branched CDI structures (structure 1-4). The last group of CDI oligomers we will discuss are the urea-containing allophanate branched CDI (structure 1-5), which are only observed at trace levels, in the high mass portion of the spectrum. Note that low molecular mass structures (structures 1-1 and 1-2) seem reasonable given that the CDI sample was prepared using a solution-based synthesis and any traces of water could lead to the production of low levels of urea linkages.18 However, the predominance of high molecular mass guanidine branched CDIs can only be rationalized through catalyst-initiated branching mechanisms occurring during synthesis, vide inf ra.2,3 It is also worth noting that our MALDI−TOF MS results are consistent with those observed by GPC; however, initial studies using ESI− TOF MS proved to be useless in analyzing these materials. Specifically, electrospray only allowed the observance of the low molecular mass structures (structures 1-1 and 1-2), along with considerable clustering due to hydrogen-bonding between carbamate linkages of the different molecules present in the CDI mixture. MALDI−TOF/TOF CID Fragmentation of the TDI-Based Carbodiimide Mixture. A. TDI-Based Carbodiimide Fragmentation Pathways. Structure 1-1. Figure 2 shows the MALDI−TOF/TOF CID spectrum for the protonated dihexanol capped CDI oligomer (structure 1-1: 509.3 Da) with n = 1 repeat units. Comparison studies of CID spectra identified a number of advantages for fragmenting protonated vs sodiated precursor ions: (1) protonated fragments display more intense

carbodiimide-linkage in polymer backbone, and XT = “trimer” formation by three isocyanate groups, where X = the structure number. (iii) The number of repeat units (n) correspond to the mass numbers found in Tables 1− 4. For example, a polycarbodimide precursor ion peak labeled “1− 1 Na+” corresponds to structure 1-1 in Table 1 that is sodium cationized (Na+), and some undefined number of polycarbodiimide repeat units (n). The structures of the species identified in Figures 1−15 are shown in Tables 1 through 4, without the Na+ or H+ ion attached. The mass ranges are listed in the tables for ions having different “n” values. MALDI−TOF Mass Spectrum of TDI-Based Carbodiimide Mixture. TDI-Based Carbodiimide Mixture. Figure 1 shows the overall MALDI spectrum for the TDI-based carbodiimide (CDI) sample used in the present study, for sodium cationization. The inset in the range of 740−1000 Da displays the complexity of the sample and indicates that the CDI species seen are independent of the cation. A total of five species are identified in the spectrum and their structures are given in Table 1, along with the masses of the individual oligomers cationized by the addition of a hydrogen atom or sodium atom. The major structures in the low mass portion of the spectrum, 370−1000 Da, are the linear dihexanol-capped CDI oligomers (structure 1-1). Second in intensity, in the low mass region, are the urea-containing CDI oligomers (structure 1-2), ranging from 0−11 CDI repeat units. In the high mass region of the spectrum, 880−2260 Da, the guanidine branched CDI oligomers (structure 7619


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Table 2. Structural Assignments for the Fragment Peaks Observed in the MALDI−TOF/TOF CID Fragmentation Spectra of the Model TDI-Based Carbodiimide Polymer Mixture

peaks in the low mass region under 381 Da (M − 128 Da) − this becomes very useful when fragmenting high mass precursor ions that yield less intense fragment peaks; (2) overall the protonated CID data present a much “cleaner” spectrum, primarily due to the greater S/N ratios obtained when performing CID fragmentation on protonated species; and (3) the protonated spectrum (Figure 2) shows higher end group loss of hexanol (the peaks in the range of 253−425 Da are the most intense in the spectrum) − this aids in the determination of end groups and major degradation pathways.

MS/MS spectra of the dihexanol capped oligomers were used as the general model for CDI fragmentation; MS/MS of oligomers having main-chain modifications were compared with them. For subsequent discussion of the “model” CDI oligomer, we consider only proton cationized spectra because they typically gave better signal-to-noise ratios than their sodium analogues. Depending on the specific oligomer studied, peaks were observed in the MS/MS range from n = 1 to n = 3. Inspection of Figure 2 reveals that three major fragment peaks are observed for H-cationized dihexanol capped CDI: 425.2 Da 7620


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Table 3. Structural Assignments for the “Fingerprint” Fragment Peaks Observed in the MALDI−TOF/TOF CID Fragmentation Spectra of the Model TDI-Based Carbodiimide Polymer Mixture

Table 4. Structural Assignments for the Peaks Observed in the ESI−QqTOF Mass Spectra of the Model Phenyl Isocyanate Carbodiimides

(species 2-1: M − 84 Da), 407.2 Da (species 2-2: M − 102 Da) and 381.1 Da (species 2-3: M − 128 Da). CDI preferentially fragments at the C(O)O−CH2 bond through a 1,5-hydrogen transfer (McLafferty rearrangement16,17) to initially produce a hexanol-carbamic acid capped CDI fragment (species 2-1) and hexane (Scheme 3Ai). Because of its instability, the carbamic acid end group degrades further through the loss of CO2 to produce an amine end group (species 2-3), as shown in Scheme 3Aii.16,17 A second fragmentation pathway of the dihexanol capped CDI involves a single cleavage at the C(O)−O bond through a 1,3 hydrogen transfer from the carbamate nitrogen to its accompanying oxygen (Scheme 3B). This cleavage produces a hexanol-isocyanate capped CDI fragment (species 2-2) and hexanol. Note that these two fragmentation pathways account for

the three main fragment peaks in Figure 2: 425.2, 407.2, and 381.2 Da. Furthermore, various combinations of these pathways can be used to explain the remaining low mass fragment peaks: 341.1 Da (species 2-5), 323.1 Da (species 2-6), 305.1 Da (species 2-7), 297.1 Da (species 2-8), 279.1 Da (species 2-9), and 253.1 Da (species 2-10). For example, the fragment peak at 253.1 Da (species 2-10) is the product of two 1,5-H transfer reactions that resulted in the loss of two hexane and two CO2 molecules from structure 1-1. Similarly, the fragment peak at 323.1 Da (species 2-6) was produced by 1,3- and 1,5-H transfer reactions that lead to the loss of hexanol and hexene from the precursor ion. It is worth noting that, due to its rigid structure, the carbodiimde backbone was observed to remain intact during our CID studies (under 1 kV and 2 kV fragmentation conditions); 7621


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Figure 1. MALDI−TOF mass spectrum of the model TDI-based carbodiimide polymer.

Figure 2. Low effective kinetic energy (collision gas pressure: 5 × 10−6 Torr) Applied Biosystems (ABI) 4800 MALDI−TOF/TOF CID mass spectrum of a protonated dihexanol capped carbodiimide structure (structure 1-1) 509.3 Da precursor ion.

this proved useful during our studies of CDI main-chain modifications, vide inf ra. B. Urea-Containing Carbodiimide Fragmentation Pathways. Structure 1-2. Figure 3 displays the MALDI−TOF/TOF CID spectrum for the dihexanol capped CDI oligomer containing a urea linkage (structure 1-2: 787.4 Da), with n = 2

repeat units. This fragmentation spectrum displays the expected hexanol and hexene end group losses that were previously described in Scheme 3: 703.3 Da (M − 84 Da), 685.3 Da (M − 102 Da), and 657.3 Da (M − 130 Da). However this structure contains a weak link due to a urea linkage. Preferential mainchain cleavage of the urea NH−C(O) bond is accompanied by a 7622


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Figure 3. Low effective kinetic energy (collision gas pressure: 5 × 10−6 Torr) ABI 4800 MALDI−TOF/TOF CID mass spectrum of a protonated urea containing carbodiimide structure (structure 1-2) 787.4 Da precursor ion.

Scheme 4. Low Kinetic Energy Fragmentation Pathways for a Urea-Containing Model TDI-Based Carbodiimide Structure 1-2

CO2 loss to generate species 2-9 (149.1 Da, et seq.) (Scheme 4ARii). Inspection of the fragment peak intensities in Figure 3 reveals a preferential ionization for the hexanol-amine capped series (species 2-3). Assuming an equivalent proportion of species 2-2 and 2-3 are produced during the urea bond cleavage shown in Scheme 4A, it is reasonable to assume that the cationizing hydrogen atom would show preferential association with the amine end group of species 2-3 over the isocyanate end group of species 2-2. This fragmentation pattern also indicates that the cationizing agent prefers to associate with the nitrogen rich urea linkage over the carbamate linkage; if the carbamate linkage were

1,3-H transfer to produce two series of fragment peaks (Scheme 4A): (1) a hexanol-amine capped series (species 2-3: 251.2, 381.2, and 511.3 Da) and (2) a hexanol-isocyanate terminated series (species 2-2: 277.2, 407.2, and 537.3 Da). The hexanolamine capped series (species 2-3) can undergo additional fragmentation through a 1,5-H transfer and cleavage of the C(O)O−CH2 bond to produce species 2-8 (167.1 Da, et seq.) (Scheme 4ALi), followed by CO2 loss to yield species 2-10 (123.1 Da, et seq.) (Scheme 4ALii). Similarly, the hexanolisocyanate capped series (species 2-2) can undergo 1,5-H transfer reactions to produce the isocyanate-carbamic acid capped species 2-6 (193.1 Da, et seq.) (Scheme 4ARi) and 7623


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Figure 4. Low effective kinetic energy (collision gas pressure: 5 × 10−6 Torr) ABI 4800 MALDI−TOF/TOF CID mass spectrum of a sodiated guanidine branched carbodiimide structure (structure 1-3) 1041.6 Da precursor ion.

Scheme 5. Low Kinetic Energy Fragmentation Pathways for a Guanidine Branched Model TDI-Based Carbodiimide Structure 1-3

preferred there would be an equivalent distribution of cationized species 2-2 and 2-3. C. Guanidine Branched Carbodiimide Fragmentation Pathways. Structure 1-3. Figure 4 displays the MALDI− TOF/TOF CID spectra for the sodiated dihexanol capped CDI oligomer containing a guanidine branch (structure 1-3: 1041.6 Da) with n = 2 repeat units. Inspection of the high mass region of the CID spectrum identifies the expected fragment peaks at 939.5 Da (M − 102 Da) and 913.5 Da (M − 128 Da) produced by hexene and hexanol end group loss, respectively. Additional lower molecular mass fragment peaks are the result of main-chain

cleavages at the guanidine branch. For example, Scheme 5A displays a guanidine C-NH bond cleavage accompanied by a 1,3H transfer reaction. The result of this bond breaking reaction is the production of an amine-hexanol series (species 2-3: 251.2 Da, et seq.) and a dihexanol capped carodiimide series (species 31: 401.2 Da, et seq.). Additionally, species 3-1 can undergo 1,5-H transfer and CO2 loss to produce a low intensity sodiated hexanol-amine capped series (species 2-3) (Scheme 5B). It is worth noting that the fragment (species 2-3) with the amine end group prefers to be protonated and the dihexanol main-chain fragment (species 3-1) prefers sodium cationization, along with 7624


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Figure 5. Low effective kinetic energy (collision gas pressure: 5 × 10−6 Torr) ABI 4800 MALDI−TOF/TOF CID mass spectrum of a sodiated uretone imine branched carbodiimide structure (structure 1-4) 1067.2 Da precursor ion.

Scheme 6. Low Kinetic Energy Fragmentation Pathways for a Uretone Imine Branched Model TDI-Based Carbodiimide Structure 1-4

with n = 2 repeat units. Because of the weak bonding at the uretone imine branch point, this fragmentation spectrum shows very little fragmentation in the high mass region from hexanol loss (939.4 Da: M − 128 Da). Instead, there is almost exclusive fragmentation at the C−N bonds of the uretone imine linkage to yield a dihexanol capped carbodiimide fragment series (species 3-1: 401.2 Da, et seq.) and a hexanol-isocyanate terminated CDI fragment series (species 2-2: 299.1 Da, et seq.) (Scheme 6). As noted for the guanidine branched CDIs, fragment species 3-1 only appears when branching is present in the CDI backbone and can be used as a diagnostic peak for “fingerprint” identification of uretone imine branched structures.

its species 2-3 degradation product. Presumably, this is due to the hydrogen cation preferentially associating with amine end groups during the initial fragmentation and the sodium cation preferentially associating with the carbamate linkages. It is also worth noting that fragment species 3-1 only appears in the CID spectra of guanidine branched carbodiimides and can be used as a “fingerprint” fragment for the identification of this backbone modification. D. Uretone Imine Branched Carbodiimide Fragmentation Pathways. Structure 1-4. Figure 5 shows the MALDI−TOF/ TOF CID spectrum for the dihexanol capped CDI oligomer containing a uretone imine branch (structure 1-4: 1067.2 Da), 7625


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Figure 6. Low effective kinetic energy (collision gas pressure: 5 × 10−6 Torr) ABI 4800 MALDI−TOF/TOF CID mass spectrum of a sodiated ureacontaining allophanate branched carbodiimide structure (structure 1-5) 1215.6 Da precursor ion. The overall CID spectrum is shown in part A and the expanded mass spectrum (390−1080 Da) is displayed in part B.

E. Urea-Containing Allophanate Branched Carbodiimide Fragmentation Pathways. Structure 1-5. Figure 6A displays the MALDI−TOF/TOF CID spectrum for the dihexanol capped CDI oligomer containing a urea linkage and an allophanate branch (structure 1-5: 1215.6 Da), with n = 3 repeat units. Initial inspection of the overall CID spectrum, shown in Figure 6A, shows the precursor ion with minor peak intensity and a significant amount of high mass fragment peaks within 70 Da of the precursor ion, which unfortunately proves useless in identifying the structural defects associated with this molecule. Therefore, we focus our discussion on the information rich mass region between 390−1080 Da (Figure 6B). Inspection of the peak series in Figure 6B identifies four main series associated with the fragmentation of two isobaric groups. Isobar 1 is

designated as having the urea linkage on the allophanate branch side arm and Isobar 2 contains the urea linkage in the main-chain. Scheme 7A illustrates the preferred fragmentation of Isobar 1 at the allophanate C(O)-N bond with an associated 1,3-H transfer. This cleavage produces fragment species 3-1 (401.2, et seq.) and species 3-3 (447.2 Da, et seq.). Isobar 2 undergoes similar fragmentation of the allophanate bond, as shown in Scheme 7B, to produce fragment species 2-2 (429.2 Da, et seq.) and species 3-2 (419.3 Da, et seq.). Note that all of the fragment series are sodiated. This is consistent with our previous observation that only the amine capped fragments show strong preference for protonation and carbamate containing fragments prefer sodium cationization. It is worth noting that this data indicates that the cleavage of allophanate linkages is a lower 7626


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Scheme 7. Low Kinetic Energy Fragmentation Pathways for a Urea-Containing Allophanate Branched Model TDI-Based Carbodiimide Structure 1-5

produce guanidine branches (structure 1-3; Scheme 8).18 Additionally, there is published data describing the reaction of carbodiimide linkages with isocyanates to produce uretone imine branches (structure 1-4; Scheme 9).18 The uretone imine branching pathway is preferred under 80 °C and reversed when the reaction mixture is heated to >160 °C (Scheme 8C).18 Finally, the allophanate branching mechanism has been widely published in the polyurethane literature: urethane linkages contain acidic protons capable of reacting with isocyanates to produce allophanate branches (structure 1-5; Scheme 10).18 Under the literature reported synthesis conditions, all of the previously described side-product mechanisms seem plausible. However, the synthesis conditions of the present study do not meet the criteria for a number of these mechanisms: (1) the dihexanol-capped carbodiimides were synthesized under dry conditions and (2) the reaction was monitored by IR spectroscopy until it was quenched with BF3-butyl etherate (80

energy fragmentation pathway than urea bond cleavage. This preferred fragmentation of the allophanate linkage is illustrated in the numerous “fingerprint” fragment peaks produced in the CID spectrum of structure 1-5: species 3-1, 3-2, and 3-3. Not only can these peaks be used for the identification of carbodiimide branch points and urea linkages, but they can also be used to differentiate the different isobars that are present in the carbodiimide mixture. Side-Product Formation in the TDI-Based Carbodiimide Mixture. When considering the origin of structures 1-2 through 1-5, there is literature precedence for their formation. For example, trace levels of H2O, present during synthesis, react with isocyanates to produce carbamic acid end groups, which will undergo loss of CO2 to yield amine groups.18 These amines undergo additional reactions with isocyanates to produce urea linkages (structure 1-2).18 Also, under acidic conditions, it has been reported that amines attack carbodiimide linkages to 7627


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Scheme 8. Mechanism for Formation of Guanidine-Branched Carbodiimides, under Acidic Conditions

Scheme 9. Mechanism for Formation of Uretone Imine Branched Carbodiimides

Scheme 10. Mechanism for Formation of Urea-Containing Allophanate Branched Carbodiimides

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Figure 7. ESI−QqTOF mass spectrum of the model phenyl isocyanate carbodiimide, taken 2 h after the initial reactions. No BF3−butyl etherate was used to terminate this reaction.

Figure 8. ESI−QqTOF mass spectrum of the model phenyl isocyanate carbodiimide, taken 8 h after the initial reactions. No BF3-butyl etherate was used to terminate this reaction.

ppm H2O) and no ureas were observed. This would certainly question the validity of H2O produced amine end groups as being the primary source of high molecular weight, guanidine branch formation (Scheme 8). A more plausible explanation would be catalyst-induced guanidine branch formation. To test this hypothesis, a model study was conducted using phenyl isocyanate and the MPPO catalyst. ESI−QqTOF Mass Spectra of Phenyl Isocyanate-Based Carbodiimide Mixtures. PI-Based Carbodiimide Mixture. Figure 7 shows the overall ESI mass spectrum for the PI-based

carbodiimide (CDI) sample, 2 h after the start of synthesis. This spectrum displays a predominant PI CDI peak at 195.0917 Da and a minor CDI dimer peak (∼10% of the 195 Da base peak) at 389.1761 Da. At 8 h (Figure 8), the only significant change is a tripling of the peak intensity of the CDI dimer (389.1761 Da) (∼30% of the 195 Da base peak). In our previous studies of TDIbased CDIs (vide supra), it was hypothesized that the termination reaction played a key role in guanidine branch formation. To test this hypothesis, a portion of the aliquot taken 8 h after synthesis was terminated with BF3-butyl etherate and examined by ESI− 7629


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Figure 9. ESI−QqTOF mass spectrum of the model phenyl isocyanate carbodiimide, taken 8 h after the initial reactions. Note that BF3−butyl etherate was used to terminate this reaction.

Figure 10. Expanded ESI−QqTOF mass spectrum (900−1800 Da) of the model phenyl isocyanate carbodiimide, taken 8 h after the initial reactions. Note that BF3−butyl etherate was used to terminate this reaction.

polyguanidines: (1) structure series 4-3n is for the catalystattached polyguanidines; (2) the low intensity peak series for structure 4-2n represents a polyguanidine terminated by an aniline group (vide inf ra), which will also be referred to as a “type 1” polyguanidine; and (3) the highest intensity peak series of this spectrum represents the “type 2” polyguanidine (structure 4-4n) that has been terminated by the addition of a PI-based diimine. Graphical depictions of the type 1 and type 2 polyguanidines are shown in Figure 10 and color coded as the green and blue peak series, respectively. Given that the catalyst-attached intermediate

MS; the mass spectral results are shown in Figures 9 and 10. The overall ESI spectrum in Figure 9 shows the first mass spectral observance of the catalyst-attached intermediate (structure 43n): 268.1250 Da (n = 0), 462.2094 Da (n = 1), et seq. The catalyst-attached intermediates are the predominant species observed in this spectrum, followed by the PI-based CDI unimer (195 Da) and dimer (389 Da). This spectrum also displays a guanidine-branched species at 288.1495 Da (structure 4-20). Inspection of the expanded high mass range of 900−1800 Da (Figure 10) reveals the presence of three different series of 7630


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Figure 11. ESI−QqTOF CID spectrum of the protonated 195.0917 Da phenyl isocyanate carbodiimide precursor ion (structure 4-1; n = 1).

Figure 12. ESI−QqTOF CID spectrum of the protonated 389.1761 Da phenyl isocyanate carbodiimide dimer precursor ion (structure 4-1; n = 2).

Scheme 11. High and Low Kinetic Energy Fragmentation Pathways of the Model Phenyl Isocyanate Carbodiimide Unimer (Structure 4-11)

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Scheme 12. High and Low Kinetic Energy Fragmentation Pathways of the Model Phenyl Isocyanate Carbodiimide Dimer (Structure 4-12)

Figure 13. ESI−QqTOF CID spectrum of the protonated 288.1495 Da guanidine-branched phenyl isocyanate carbodiimide precursor ion (structure 42; n = 0).

their chemical structures, and use their fragmentation information to develop a general CDI degradation model for identifying end-group and main-chain modifications in structurally related PI-based CDI oligomers. Inspection of the PI CDI unimer MS/MS spectrum (Figure 11) identifies four fragments of interest: 168.0808, 117.0447, 92.0495, and 77.0386 Da. Given that the PI CDI unimer (structure 4-11) is a very stable structure, its lowest kinetic energy fragmentation process most likely begins through charge-induced rearrangements. For example, the protonated structure 4-11 can undergo rearrangement to form the intrinsically charged isobar shown in Scheme 11; this can fragment at the C−N bond to form the charged aniline peak at 92.0495 Da (Scheme 11A) or undergo an internal loss of HCN to yield the intrinsically charged fragment at m = 168.0808

(structure 4-3) and significant quantities of polyguanidines were only observed after BF3-etherate termination, there is considerable evidence for presence of BF3-initiated side reactions. The next portion of this study used CID fragmentation to examine the decomposition pathways of structures 4-1 and 4-2, and then used this information to verify the chemical structures of the catalystattached polyguanidines: structure 4-3n. ESI−QqTOF CID Fragmentation Studies of Phenyl Isocyanate Carbodiimide. F. PI-Based Carbodiimide Fragmentation Pathways. Structure 4-1. Figures 11 and 12 show the ESI−QqTOF CID spectra of the protonated model phenyl isocyanate CDI unimer (structure 4-11: 195.0917 Da) and dimer (structure 4-12: 389.1761 Da), respectively. The purpose of examining these low molecular mass components was to verify 7632


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Scheme 13. Low Kinetic Energy Fragmentation Pathways of the Model Phenyl Isocyanate Carbodiimide Unimer Containing a Single Guanidine Branch Point (Structure 4-20)

Figure 14. ESI−QqTOF CID spectrum of the protonated 268.1250 Da catalyst-attached intermediate phenyl isocyanate carbodiimide precursor ion (structure 4-3; n = 0).

produce the lowest energy degradation pathways. This point is illustrated in Scheme 12, where the protonated structure 4-12 rearranges to form an intrinsically charged isobar. This isobar preferentially fragments through a 1,5-H transfer and loss of aniline to produce the intense fragment ion observed at 296.1182 Da (Scheme 12A). There is an additional low energy pathway of the intrinsically charged precursor ion that involves a 1,3-phenyl shift and loss of a diphenylamine groupits protonated peak is seen at m = 170.0964to yield the 220.0869 Da fragment (Scheme 12B). Surprisingly, the transannular bond cleavage pathway (Scheme 12C) appears to be the least dominant degradation process, and only produces a low intensity PI CDI fragment peak at 195.0917 Da (structure 1-11). Given that all of the precursor ions in this study are protonated, charge-induced degradation processes should be expected to occur when lower energy fragmentation pathways are not available. However, it should be noted that structural modifications to the carbodiimide backbone and end-groups will greatly affect the degradation pathways of these molecules, vide inf ra.

(Scheme 11B). There is also a high kinetic energy degradation pathway that involves cleavage of the Ph−N bond (Scheme 11C). Both fragment ions produced by this process are intrinsically charged and observed at low intensities: 117.0447 and 77.0386 Da. Dimer formation during carbodiimide synthesis is a well documented phenomenon and it is not surprising that it would be observed during PI CDI synthesis.18 The most common practice for reducing the dimer concentration in the final CDI product is to raise the temperature above 160 °C; this pushes the equilibrium back toward the linear carbodiimide.18 This type of “degradation” process is comparable to transannular bond cleavage reactions, which are commonly observed during gasphase degradation processes.16,17 Therefore, one would expect this dimer to cleave down the middle to produce two PI CDI unimer fragments (structure 4-11). A quick inspection of Figure 12 reveals that this is not the lowest kinetic energy fragmentation pathway for the protonated structure 4-12. As observed in the PI CDI unimer, vide supra, charge-induced processes appear to 7633


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Figure 15. ESI−QqTOF CID spectrum of the protonated 850.3782 Da catalyst-attached polyguanidine trimer precursor ion (structure 4-3; n = 3).

Scheme 14. High and Low Kinetic Energy Fragmentation Pathways of the Catalyst-Attached Intermediate of the Model Phenyl Isocyanate Carbodiimide (Structure 4-30)

were “linear” CDI structures containing one guanidine branch point, unlike the ladder-like structures of the polyguanidines observed in the present study of phenyl isocyanate-based carbodiimides (e.g., structures 4-2n and 4-4n). Further, due to the ladder-like architecture of higher molecular mass polyguanidines (with n > 1), hydrogen transfer reactions are not the lowest energy degradation process, vide inf ra. H. Catalyst-Attached PI-Based Carbodiimide Fragmentation Pathways. Structure 4-3. Figure 14 shows the ESI− QqTOF CID spectrum for the catalyst-attached intermediate (structure 4-30); an associated polyguanidine oligomer (n = 3) is reported in Figure 15. Given that this study reports the first time that the catalyst-attached intermediate was observed by mass spectrometry, it was of the utmost importance that we confirm

G. Guanidine Branched PI-Based Carbodiimide Fragmentation Pathways. Structure 4-2. Figure 13 displays the ESI− QqTOF CID spectrum for the protonated guanidine-branched CDI oligomer (structure 4-2), with n = 0 repeat units. This spectrum shows that fragmentation preferentially occurs at the guanidine branch, through a 1,3-H transfer (Scheme 13), to produce two fragment ions: (1) the PI CDI unimer (m = 195.0917) and (2) an intrinsically charged aniline ion (m = 92.0495). In our previous study of TDI-based carbodiimides (vide supra), we reported a similar degradation process for guanidine-branched TDI-based CDIs and used this unique degradation pathway for “fingerprint” identification of these structural defects in carbodiimide oligomers with molecular masses up to 2000 Da. However, it should be noted that these 7634


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Scheme 15. Phosphine Oxide Catalyzed Mechanism for Carbodiimide Formation

using the top row of predicted fragment masses: 268, 462, and 656 Da. Again, all of these masses are observed in the MS/MS spectrum of structure 4-33. It should also be noted that the phosphorus-containing fragments (268, 462, and 656 Da) display higher peak intensities than the corresponding fragments containing an aniline end group. This is consistent with our hypothesis that the hydrogen cation preferentially localizes near the phosphorus-containing end group. Catalyst-Induced Mechanism for Polyguanidine Formation in Carbodiimide Mixtures. We begin our explanation of the catalyst-induced branching mechanisms by first examining the known reaction mechanism of isocyanates with 3-methyl-1phenyl-3-phospholene-1-oxide (MPPO) (“phosphine oxide”) to form carbodiimides. Alberino et al. reported a mechanism involving a catalyst-attached intermediate (structure 4-30) that reacts with an isocyanate to yield a carbodiimide (structure 4-10) (Scheme 15).2 Furthermore, Novak’s group published extensive work on the synthesis of polyguanidine through the use of a titanium(IV) catalyst.3 The proposed mechanism involves an initiation phase in which the titanium(IV) catalyst reacts with a carbodiimide linkage to produce a catalyst-attached intermediate.3 The next step is the propagation phase, in which the intermediate species reacts with additional carbodiimides to yield a guanidine branched oligomer.3 Building upon the mechanisms reported by Ablerino2 and Novak,3 we propose a new mechanism in which an MPPOattached intermediate CDI structure (structure 4-30) reacts with a carbodiimide linkage (structure 4-11) to produce a catalystattached guanidine branch (structure 4-31), which does not readily produce polyguanidinesunless the reaction is terminated by the addition of BF3−butyl etherate. Note that this catalyst (MPPO) has been previously reported as being unfavorable for producing oligomeric guanidine branches;2 our data is consistent with this finding. However, there is literature precedence for BF3 lowering the activation energy for chemical reactions19similar to the described MPPO/carbodiimide catalyst-attached side product formation. Our data supports

the chemical structure of this molecule and verify that all of its components are covalently attached. CID fragmentation was used for this purpose, and we used a series of examples to illustrate this point. We begin the discussion with the simplest form of structure 1-3 (n = 0): the catalyst-attached to aniline. Figure 14 displays a number of fragment peaks, but only the 200.0624 and 176.0749 Da fragments are of interest. The protonated Structure 4-30 ion has the ability to form an intrinsically charged isobar (Scheme 14), which can undergo cleavage at the N−P bond to form an aniline and a 176.0749 Da phosphorus-containing fragment (Scheme 14A). Given that the 176.0749 Da peak is the most intense fragment ion observed in the spectrum, it seems reasonable to assume that this is the lowest kinetic energy fragmentation pathway of this molecule. Note that this fragmentation by way of a self-charged isobar is consistent with degradation pathways reported above. The 200.0624 Da fragment is the product of a high energy degradation pathway involving transannular cleavages along the C−P bonds of the phosphorus-containing ring (Scheme 14B). Considering the amount of energy required to fragment this relatively small molecule, and that the observed fragments are consistent with connectivity between the phosphorus and nitrogen atoms, there is compelling evidence for identifying this precursor ion as the covalently bonded catalyst-attached intermediate, reported as structure 4-30 in Figure 14 and Table 4. Inspection of the MS/MS spectrum of the higher molecular mass oligomer shown in Figure 15 (structure 4-33) shows an overall preference for transannular fragmentation across the ladder-like backbone and regeneration of imide bonds. This fragmentation spectrum is significant in that all of the expected fragments are observed, allowing for backbone sequencing and end group analysis of this molecule. For example, using the structure and predicted fragment masses shown in the Figure 15 inset, we begin this explanation with the aniline end group and work from left-to-right. The bottom set of masses predicts fragment masses of 195, 389, and 583 Da, all of which are observed in the CID spectrum. We now work from right-to-left 7635


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Scheme 16. Proposed BF3-Initiated Pathway for Guanidine Branching in Carbodiimides

through catalyst-initiated reactions. Furthermore, while catalystinduced formation of polyguanidines is not native to plugging issues in MDI or TDI plants,1 it does provide greater insight into their mechanism of formation and helps to broaden the knowledge base of isocyanate and carbodiimide chemistry and assist in trouble-shooting problems associated with their unwanted side-product formation.

this BF3-initiated pathway leading to guanidine formation. In our proposed mechanism (Scheme 16), BF3 forms a relatively weak bond with the MPPO catalyst, which lowers the activation energy for the MPPO/PI CDI reaction; this initiates the formation of the catalyst-attached intermediate (structure 4-30) (Scheme 16A), which reacts with a PI CDI unimer (structure 4-11) to form a catalyst-attached guanidine unimer (4-31) (Scheme 16Bi). The next step is the propagation phase involving repeated reactions with PI CDIs to form catalyst-attached polyguanidines (4-3n>1) (Scheme 16Bii). Note that the viscosity build-up in these materials is linked to guanidine branch formation. Polyguanidine synthesis termination occurs by two processes: (1) reaction of the catalyst-attached intermediate (structure 4-3n) with an H2O molecule cleaves off the MPPO catalyst to produce a polyguanidine type 1 structure (structure 4-2n) (Scheme 16Ci) or (2) an incoming phenylisocyanate can displace the MPPO catalyst to yield a polyguanidine type 2 structure (structure 4-4n) (Scheme 16Cii). This mechanism presents how the predominant source of the guanidine branching can only be rationalized

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CONCLUSIONS The results of this study indicate that a combination of MALDI− TOF mass spectrometry and CID fragmentation can be used to effectively trouble-shoot manufacturing issues associated with MDI and TDI production. Through the identification of “fingerprint” side-products, diagnoses can be made for the two most common cases: (1) a predominance of polyurea oligomers and trace levels of carbodiimides is caused by ppm levels of water entering the system, and (2) a predominance of carbodiimides and its associated branched side products is caused by a thermally induced mechanism that generates polycarbodiimide and CO2 7636


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gas. While both pathways generate insoluble solids, in order to choose the proper countermeasure, it is essential that the end products are thoroughly characterized.

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ASSOCIATED CONTENT

S Supporting Information *

GPC, FTIR, and NMR data are presented for the TDI-based polycarbodiimide. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(A.P.G.) Telephone: (979) 238-1778. E-mail: APGies@Dow. com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed under the auspices of The Dow Chemical Company. The authors would like to thank Manjiri Paradkar, Xiaoyun (Shawn) Chen, Mark Richard, and Anne Leugers, at The Dow Chemical Company, for their assistance with the NMR and FTIR data. Additional gratitude is extended to Prof. David M. Hercules, at Vanderbilt University, for his helpful discussions on characterization of polycarbodiimides, and Prof. David H. Russell, at Texas A&M University, for use of his MALDI−TOF/TOF instrument.

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REFERENCES

(1) Ulrich, H. Chemistry and Technology of Carbodiimides; John Wiley & Sons, Ltd.; West Sussex, England, 2007. (2) Alberino, L. M.; Farrissey, W. J.; Sayigh, A. R. J. Appl. Polym. Sci. 1977, 21, 1999−2008. (3) Kennemur, J. G.; Novak, B. M. Polymer 2011, 52, 1693−1710. (4) Murgasova, R.; Hercules, D. M. Int. J. Mass. Spectrom. 2003, 226, 151−162. (5) Gruendling, T.; Weidner, S.; Falkenhagen, J.; Barner-Kowollik, C. Polym. Chem. 2010, 1, 599−617. (6) Barner-Kowollik, C.; Gruendling, T.; Falkenhagen, J.; Weidner, S. Mass Spectrometry in Polymer Chemistry: John Wiley & Sons, Ltd.; West Sussex, England, 2012. (7) Montaudo, G.; Carroccio, S.; Montaudo, M. S.; Puglisi, C.; Samperi, F. Macromol. Symp. 2004, 218, 101−112. (8) Montaudo, G.; Samperi, F.; Montaudo, M. S.; Carroccio, S.; Puglisi, C. Eur. J. Mass Spectrom. 2005, 11, 1−14. (9) Montaudo, G.; Samperi, F.; Montaudo, M. S. Prog. Polym. Sci. 2006, 31, 277−357. (10) Pasch, H.; Ghahary, R. Macromol. Symp. 2000, 152, 267−278. (11) Gies, A. P.; Hercules, D. M.; Ellison, S. T.; Nonidez, W. K. Macromolecules 2006, 39, 941−947. (12) Gies, A. P.; Hercules, D. M. Macromolecules 2006, 39, 2488−2500. (13) Gies, A. P.; Kliman, M.; McLean, J. A.; Hercules, D. M. Macromolecules 2008, 41, 8299−8301. (14) Gies, A. P.; Geibel, J. F.; Hercules, D. M. Macromolecules 2010, 43, 943−951. (15) Crecelius, A. C.; Baumgaertel, A.; Schubert, U. S. J. Mass Spectrom. 2009, 44, 1277−1286. (16) Gross, J. H. Mass Spectrometry - A Textbook: Springer: Berlin, 2004. (17) McLafferty, F. W.; Turecek, F. Interpretation of Mass Spectra, 4th ed.; University Science Books: Sausalito, CA, 1993. (18) Rogers, M. E.; Long, T. E. Synthesis Methods in Step-Growth Polymers; John Wiley & Sons, Ltd.; West Sussex, England, 2003. (19) Chandra, M.; Roy, S. K. Plastics Technology Handbook, 4th ed.; CRC Press: Boca Raton, FL, 2006. 7637


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