Iron(III)-Catalyzed Chain Growth Reactions of Polymeric Methylene

Article pubs.acs.org/Macromolecules

Iron(III)-Catalyzed Chain Growth Reactions of Polymeric Methylene Diphenyl Diisocyanate Anthony P. Gies,* Zdravko Stefanov, Nathan J. Rau, Debashis Chakraborty, Praveenkumar Boopalachandran, and J. Paul Chauvel Department of Core R&D Analytical Sciences, The Dow Chemical Company, 2301 N. Brazosport Blvd., B-1820, Freeport, Texas 77541, United States S Supporting Information *

ABSTRACT: The overall goal of the present study was to identify the high molecular weight species formed during the synthesis and processing of polymeric methylene diphenyl diisocyanate (PMDI) and determine the root cause of the viscosity buildup in these materials. Initial studies focused on the use of MALDI-TOF mass spectrometry to examine the PMDI mixture and MALDI-TOF/TOF CID fragmentation for elucidating the degradation mechanisms of the identified compounds. The low molecular mass portion of the MALDI spectrum was observed to contain the expected PMDI and small quantities of carbodiimides (CDIs). The high molecular mass portion of the spectrum primarily contained 1,3-diazetidine branched carbodiimide dimers, uretonimine branched CDIs, and imino-s-triazine along with low levels of guanidine branched CDIs. The results of this study indicate that the root cause of the observed CDI defects was extensive branch formation through unexpected side reactions catalyzed by iron(III). These reactions lead to a buildup of viscosity which poses a significant challenge during the processing of these materials.

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INTRODUCTION On an industrial scale, polymeric methylene diphenyl diisocynate (PMDI) is prepared by first nitrating benzene to form nitrobenzene.1 The nitrobenzene is then catalytically hydrogenated to aniline and after purification is reacted with formaldehyde to yield a complex mixture of methylene dianiline (MDA) and its associated oligomers.1 Unreacted aniline is then removed, for recycling back into the process, and MDA is phosgenated to convert the amine groups into isocyanates, which produces methylene diphenyl diisocyanate (MDI) and its associated oligomers.1 When all goes well, a well-controlled PMDI and MDI product is formed. However, excessive side products are occasionally formed, which can lead to viscosity buildup or solids that can foul process equipment and reduce production. In these instances a fast and reliable analysis technique can effectively troubleshoot these manufacturing issues associated with PMDI and MDI production. For example, 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 could be caused by ppm levels of water entering the system, while (2) a predominance of carbodiimides, and its associated branched side products, would be produced by thermally induced or iron-catalyzed mechanisms that generate polycarbodiimide and CO2 gas. While both pathways generate insoluble solids, in order to choose the proper countermeasure, it is essential that the end products are thoroughly characterized. Historically, size exclusion chromatography (SEC), vapor pressure osmometry (VPO), nuclear magnetic resonance © XXXX American Chemical Society

(NMR), light scattering, infrared spectroscopy, and ultraviolet/ visible spectroscopy have been used for polymer characterization.2−4 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.2−4 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 degradation mechansisms.2−8 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.9−13 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 polymer standards for calibration; this is an important advantage when standards do not exist for the polymer of interest.14,15 Received: September 8, 2015 Revised: January 7, 2016

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DOI: 10.1021/acs.macromol.5b01973 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules In the present study, we combine the evaporation-grinding matrix-assisted laser desorption/ionization (MALDI) sample preparation method (E-G method)9−12 with TOF/TOF collision-induced dissociation (CID) to examine the side product formation when polymeric methylene diphenyl diisocyanate (PMDI) is heated with and without the presence of iron(II) and iron(III) chloride. For each PMDI mixture, the low molecular mass portion of the MALDI spectra displayed the expected levels of PMDI and carbodiimides (CDIs).

However, for the iron(III)-containing mixture, it was observed that the high molecular mass portion of the MALDI spectrum primarily contained branched CDI structures, which coincided with an increase in viscosity;16,17 this was not the case for the “neat” and iron(II)-containing mixtures. Collision-induced dissociation (CID) fragmentation mechanisms were developed for the low molecular mass linear and branched carbodiimides and used to elucidate the structures of the high molecular mass CDI structures. Based upon this information, synthesis

Table 1. Structural Assignments for the Peaks Observed in the MALDI-TOF Spectra of the PMDI and Associated Carbodiimide Side Productsa

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mechanisms are proposed to explain the root cause of the extensive branch formation,16,17 which led to increased viscosity.

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EXPERIMENTAL SECTION

Materials. Polymeric methylene diphenyl diisocyanate (PMDI) (structure 1-1 in Table 1) was obtained from the Dow Freeport PMDI

Table 1. continued

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Macromolecules facility. It was confirmed to be free of urea by FTIR analysis and free of Fe by atomic absorption spectroscopy, prior to use. All materials were used as received (“neat”). A portion of the sample was analyzed “neat” and the other portion was methanol capped to further assist in the characterization process. Methanol capping was performed by first dissolving 45 μL of PMDI in 500 μL of uninhibited THF (THF, Fisher). After the PMDI was observed to go into solution, 500 μL of HPLC grade methanol (Fisher) was added to convert the isocyanate end groups into methyl carbamates. The capping process was allowed to react overnight (∼24 h) in an effort to maximize methyl carbamate formation. Thermal Aging of PMDI Samples. The thermal aging of the PMDI material was performed in 9.5 mL stainless steel spherical vessels. Details of the experiments are displayed in Table S1 of the Supporting

Information. Briefly, polymeric methylene diphenyl diisocyanate (6.0 g) was added to one spherical vessel, and CaO (1.0 g) was added to another spherical vessel. The two vessels were connected with a U-shaped tube. The role of the CaO is to trap the released CO2 and replicate the conditions in an actual PMDI plant. This setup was executed with and without the addition of FeCl2 (Aldrich) or FeCl3 (Aldrich), respectively, using the rationale that HCl generated during PMDI synthesis can react with rust buildup (Fe2O3) to form FeCl3, in the reactor. The aging was done by alternating the temperature between the oven temperature and ambient temperature every 24 h. The samples were aged for multiple days. After the end of the aging temperature the samples were cooled to ambient temperature and analyzed. Viscosity measurements were taken for each mixture: “neat” (sample A (no added iron): 37 cps), iron(II) (sample B): 86 cps), and iron(III) (sample E): 149 cps).

Table 1. continued

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Macromolecules Infrared Spectroscopy Analysis. The liquid- and solid-phase midinfrared spectra of the PMDI samples were collected on a Nicolet FTIR spectrometer equipped with a single bounce DuraScope diamond pike

ATR accessory. The spectrometer is equipped with a globar light source, a KBr beamsplitter, and deuterated L-alanine lanthanum triglycine sulfate (DLaTGS) detector. The samples were directly placed on the

Table 1. continued

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Macromolecules ATR crystal and scanned for mid-infrared experiments (4000−400 cm−1). Typically, 64 scans were collected using a resolution of 4 cm−1. MALDI-TOF/TOF CID Measurements. All samples were analyzed using a Bruker UltrafleXtreme MALDI-TOF/TOF MS (Bruker Daltronics Inc., Billerica, MA) equipped with a 355 nm Nd:YAG laser. Spectra were obtained in the positive ion reflection mode with a mass resolution greater than 20 000 full width at half-maximum height (fwhm), isotopic resolution was observed throughout the entire mass range detected, and the laser intensity was set 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 (Peptide Mix II) from a Peptide Mass Standard Kit (Bruker Daltronics) and a seven-point calibration method using Bradykinin (clip 1−7) (m = 757.40 Da), Angiotensin II (m = 1046.54 Da), Angiotensin I (m = 1296.68 Da), Substance P (m = 1347.74 Da), ACTH (clip 1−17) (m = 2093.09 Da), ACTH (clip 18−39) (m = 2465.20 Da), and Somatostatin 28 (m = 3147.47 Da) to yield monoisotopic mass accuracy better than Δm = ±0.05 Da. The instrument was calibrated before each measurement to ensure constant experimental conditions.

For CID fragmentation experiments, argon was used as a collision gas at pressures of 1.5 × 10−6 Torr, and the collision energy amounts to 20 keV.18,19 All spectra were acquired in the reflection mode with a mass resolution greater than 20 000 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)9−12 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 Polymerix 3.0 software supplied by Sierra Analytics (Modesto, CA). ESI-TOF MS Measurements. All 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

Table 1. continued

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Macromolecules optimized to achieve the best signal-to-noise ratio. A curtain of nitrogen drying gas was utilized to assist in the electrospray ionization (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 leuenkephalin (Tyr-Gly-Gly-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 3.0 software by Sierra Analytics (Modesto, CA). Electronic Structure Calculations. All calculations were performed by using Gaussian09 software.20 Unless stated otherwise, all electronic structure (DFT, density functional theory) calculations were performed using the B3LYP level of theory with 6-31+G* basis sets. All reported energies are for optimized geometries at 0 K and are corrected to account for zero point energies. Optimized geometries were confirmed as minima by ensuring no imaginary vibrational modes were present. For many of the structures, particularly the nearly linear carbodiimide moieties and their FeCl3 adducts, we had difficulty getting the selfconsistent field (SCF) calculations to converge. In these cases, we resorted to using ultrafine grids for numerical integration and increasing the maximum number of iterations to get convergence. While all structures were optimized in vacuo, some were also optimized in a simulated benzene solvent environment. To model solvation effects on molecular geometry we used density-based continuum solvation modeling (SMD).21 We chose to model solvation using benzene since it is a close approximation to the highly aromatic reaction environment (chlorobenzene). Geometry optimizations involving solvation modeling used the standard benzene parameters [scrf = (smd, solvent = benzene)] and started from the same initial guess structure as the vacuum

calculations. Optimized geometries are available in the Supporting Information. Nomenclature. 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 the letter “F” 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., H+); (ii) precursor ion backbone and side-group modification are labeled where XM = methyl side group and XP = a biphenyl linkage in the polymer backbone, where X = the structure number; (iii) f ragment ions are labeled with the letter “F” and are numbered in accordance with their respective fragmentation scheme; (iv) the number of repeat units (n) which corresponds to the calculated mass numbers found in Table 1. For example, a carbodimide dimer precursor ion peak labeled “1−3 H+” corresponds to structures 1−3 in Table 1 that is proton cationized (H+), and some undefined number of PMDI repeat units (n). The structures of the species identified in Figures 1−13, and Figures S1−S13 in the Supporting Information, are shown in Table 1, without the Na+ or H+ ion attached. The mass ranges are listed in the tables for ions having different “n” values. Additionally, it should be noted that calculated masses are listed in Table 1, while observed masses are reported in the figurestheir deviations being primarily linked to the MALDI ionization/calibration process.

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RESULTS AND DISCUSSION Polymeric Methylene Diphenyl Diisocyanate Aging Experiments. In an effort to study the effects of high levels of iron (e.g., rust (Fe2O3) and FeCl3) on PMDI side product formation, three experimental conditions were examined (Table S1): (1) PMDI heated in the presence of iron(III) chloride, (2) a similar “control” that used iron(II) chloride, and (3) a “blank” that consisted of PMDI with no iron added. Based upon these studies, iron(III) was found to catalyze the formation of branched carbodiimide dimers (structure 1-3) and

Table 1. continued

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Note that PMDI contains a mixture of ortho- and para-linkages. G


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Figure 1. MALDI-TOF mass spectrum of methanol-capped PMDI and iron(III) chloride (sample E) catalyzed PMDI side products, covering the mass range of 510−2600 Da.

Figure 2. MALDI-TOF mass spectrum of iron(III) chloride (1746 ppm Fe(III): sample E) catalyzed PMDI side products covering the mass range of 510−2600 Da.

methanol-capped PMDI (structure 1-1) as the predominant low molecular mass species with low levels of carbodiimides (structure 1-2), while the high mass region of the spectrum contains branched carbodiimide dimers (structure 1-3), uretonimines (a.k.a. one carbodiimide-containing dimers, “1CD”) (structure 1-5), imino-s-triazines (a.k.a., one carbodiimidecontaining trimers, “1CT”) (structure 1-6), guanidine branched PMDI (Str. 1−7), and imino-s-triazines containing biphenyl linkages (1−10). Figure 2, which displays the species not treated with methanol, does not display series peaks for PMDI.

uretonimines (structure 1-5), while only traces of these species were observed in the iron(II) “control” mixture, and none were observed in the “blank” experiment. The next section of this report details the characterization of these species based upon MALDI and CID fragmentation data (vide inf ra). MALDI-TOF Mass Spectrum of PMDI Mixtures. Iron(III) Chloride/PMDI Reaction Mixture. Figures 1 and 2 show the overall MALDI spectra for the thermally aged iron(III) chloride/ PMDI mixture with and without methanol capping of the isocyanate end groups, respectively. Inspection of Figure 1 identifies H


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Figure 3. Expanded MALDI-TOF mass spectrum of methanol-capped PMDI and iron(III) chloride (sample E) catalyzed PMDI side products, covering the mass range of 1030−1375 Da.

Figure 4. Expanded MALDI-TOF mass spectrum of iron(III) chloride (sample E) catalyzed PMDI side products covering the mass range of 900−1185 Da.

Instead, only the protonated carbodiimide-containing species are shown in this spectrum. By not treating the PMDI sample with methanol, all of the side-product peaks are essentially concentrated in this spectrum and not diluted by the PMDI species series. This has the advantage of greatly increasing the signal-to-noise ratios for these low level species, which aids in the identification of the peak mass and its selection for CID fragmentation (vide inf ra). It is interesting to note that only the methanol-capped PMDI species prefers sodium cationization, while all of the carbodiimidecontaining species series prefer protonation. It is hypothesized that the hydrogen cation can coordinate better between the branch points of the CDI-containing linkages. Closer inspection of the expanded mass spectra shown in Figures 3 and 4 reveals the complexity of the sample. A total of

11 species are identified in the expanded spectrum (900−1185 Da) shown in Figure 4. Their structures are given in Table 1, along with the masses of the individual oligomers cationized by the addition of a hydrogen atom. In the high mass region of the spectrum the carbodiimide branched dimers (structure 1-3) are the predominant species, followed by the uretonimine branched CDI structures (1CD) (structure 1-5), imino-s-triazines (1CT) (structure 1-6) and biphenyl-containing imino-s-triazines (structure 1-10). Additionally, there are low level peaks associated with each species series that represent the addition of “extra” methyl groups. For example the peak at 927 Da (structure 1-3M) represents the addition of one extra methyl group to structure 1-3, and the peak at 941 Da (1-3MM) results from the addition of two extra methyl groups. It is also worth noting that trace level species I


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Figure 5. MALDI-TOF mass spectrum of PMDI “blank” (sample A) side products, without the presence of iron, covering the mass range of 300−2600 Da.

Figure 6. MALDI-TOF/TOF CID fragmentation spectrum of the protonated 457.2 Da carbodiimide precursor ion (structure 1-2).

however, it should be noted that IR spectroscopy lacks the sensitivity of mass spectrometry and provides very little information about the side products which were being studied. Furthermore, 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. Heated “Blank” PMDI Reaction Mixture. Figure 5 shows the overall MALDI spectrum for the heated “blank” PMDI mixture, without methanol capping of the isocyanate end groups. This spectrum is much simpler than the complex iron(III)/PMDI spectrum shown in Figure 2. For example, the low mass series is predominately linear carbodiimide (structure 1-2), and the dominant high mass species series is imino-s-triazine (1CT)

include CDI branched dimers that have been chain extended with either an additional CDI linkage (structure 1-4) or a urea linkage (structure 1-9). Furthermore, in the very high mass region of Figure 2 (1600−4000 Da), chain-extended dimers of imino-striazine (1CT) (structures 1-13 and 1-14) were identified. Note that low molecular mass structures, PMDI and linear carbodiimides (structures 1-1 and 1-2), seem reasonable given that the PMDI sample was heated to 180 °C, and any traces of water (most likely introduced during the MALDI sample preparation) could lead to the production of low levels of urea linkages. However, the predominance of high molecular mass branched CDIs, specifically structures 1-3, 1-5, and 1-7, can only be rationalized through catalyst-initiated branching mechanisms occurring during synthesis (vide inf ra).16,17 It is also worth noting that our MALDI-TOF MS results are consistent with the predominant functional groups observed by IR (Figures S9 and S10); J


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Figure 7. MALDI-TOF/TOF CID fragmentation spectrum of the protonated 553.2 Da methanol-capped carbodiimide precursor ion (structure 1-2).

Scheme 1. Low Kinetic Energy Fragmentation Pathways of the Protonated 553.2 Da Methanol-Capped Carbodiimide Precursor Ion (Structure 1-2)

(structure 1-6), with low levels of the related biphenyl-containing imino-s-triazine (structure 1-10). Additionally, there is a very high mass region of Figure 5 (1600−4000 Da), where chainextended dimers of imino-s-trizine (1CT) (structures 1-13 and 1-4) are present. Note that structures 1-13 and 1-14 are the predominant species above 2000 Da. As previously stated, it is not surprising to observe linear carbodiimides (structure 1-2), given that the PMDI sample was heated to 180 °C. Moreover, the six-membered ring of the branched imino-s-triazine (1CT) is a very stable side product observed in carbodiimide mixtures. It is hypothesized that the biphenyl linkages observed in structure 1-10 are a side product of the PMDI synthesis process, along with the previously described “extra” methyl groups (e.g., structure 1-3M). Given that the only difference between the samples from Figures 2 and 5 is the presence of iron, it appears that iron(III) is catalyzing the formation of carbodiimide branched dimers (structure 1-3), uretonimine branched CDIs (structure 1-5), and low levels of guanidine branched species (structure 1-7) (vide inf ra). Similar results are observed when comparing the MALDI spectra of PMDI “control” (iron(II) chloride) with the iron(III) chloride-containing mixture (cf. Figure S11 and Figure 2). MALDI-TOF/TOF CID Fragmentation of the PMDI Linear and Branched Species. A. Linear CarbodiimideContaining PMDI Fragmentation Pathways: Structure 1-2. Figures 6 and 7 show the MALDI-TOF/TOF CID spectra for

Figure 8. MALDI-TOF/TOF CID fragmentation spectrum of the protonated 913.3 Da carbodiimide dimer precursor ion (structure 1-3).

the linear carbodiimide (structure 1-2) with n = 0 repeat units. Figure 6 is for the protonated structure (457.2 Da), and Figure 7 displays the protonated methanol-capped carbodiimide (553.2 Da) MS/MS data. Given the high strength of the carbodiimide linkages and lack of labile transferable hydrogens, K


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257 Da (F2) fragments observed in Figure 7. Likewise, the high kinetic energy pathway would cleave at a Ph−CH2Ph bond, with an associated 1,4-H transfer reaction, to generate the 402 Da (F3) fragment ion. B. 1,3-Diazetidine Branched Carbodiimide (“CDI Dimer”) Fragmentation Pathways: Structure 1-3. Figures 8 and 9 show MALDI-TOF/TOF CID spectra for the branched carbodiimide dimer (structure 1-3) with n = 0 repeat units. Figure 8 displays the protonated structure (913.3 Da), and Figure 9 depicts its protonated methanol-capped counterpart (1041.4 Da). The two spectra are presented to show the correspondence between their fragmentation patterns. Additionally, the MS/MS data of the branched CDI dimer will be used as the general model for examining the fragmentation pathways of branched carbodiimides, and MS/MS data of oligomers having main-chain modifications will be compared with them. Note that this discussion will consider only proton cationized spectra because they typically gave better signal-to-noise ratios than their sodium analogues. Beginning with Figure 8, this spectrum displays five fragment peaks of interest: (1) “F1” at 689 Da, (2) “F2” at 558 Da, (3) “F3” at 351 Da, (4) “F4” at 780 Da, and (5) “F5” at 457 Da. The branched CDI dimer preferentially fragments at the fourmembered linkage due to charge induced fragmentation from the hydrogen cation and a 1,4-hydrogen transfer to initially produce the “F1” fragment observed at 689 Da (Scheme 2Ai). Additional fragmentation, through a 1,4-H transfer reaction,

only high-energy fragmentations are observed in Figure 6. However, it was noted that treating the carbodiimide with methanol not only converted the isocyanate end groups into methyl carbamates but also added a methanol group to the carbodiimide linkage. Using the degradation pathways depicted in Scheme 1, the lowest energy pathway (Scheme 1A) would undergo a 1,4-H transfer reaction14,15 and cleave at the methoxylated CDI linkage to produce the 297 Da (F1) and

Figure 9. MALDI-TOF/TOF CID fragmentation spectrum of the protonated 1041.4 Da methanol-capped carbodiimide precursor ion (structure 1-3).

Scheme 2. Low Kinetic Energy Fragmentation Pathways the Protonated 913.3 Da Carbodiimide Dimer Precursor Ion (Structure 1-3)

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Scheme 3. Low Kinetic Energy Fragmentation Pathways the Protonated 1041.4 Da Methanol-Capped Carbodiimide Dimer Precursor Ion (Structure 1-3)

produces the “F2” fragment at 558 Da (Scheme 2Aii), and further degradation with a 1,4-H transfer to yield the “F3” fragment at 351 Da. A second fragmentation pathway of the branched CDI dimer involves cleavage at the Ph−CH2Ph bond (Scheme 2Bi) to produce the “F4” fragment observed at 780 Da. Additionally, the CDI dimer precursor ion (913 Da) can undergo transannular bond cleavage of the four-membered linkage (Scheme 2C) to produce the 457 Da “F5” fragment ion. Side-by-side comparison of Figures 8 and 9 with Schemes 2 and 3 identifies a number of advantages for fragmenting the combination of “neat” and methanol-capped precursor ions. For example, the mass shifts associated with methanol capping of the isocyanate end groups can yield structural information about the number of isocyanates present as well as insight into the configuration of the branched species. Moreover, degradation mechanisms can be further validated through this process. These points can be illustrated in a comparison of Schemes 2 and 3. The degradation mechanisms are similar for each pathway. However, the branched CDI dimer precursor ion displays a 128 Da mass shift due to the addition of four methanol groups. Figure 9 shows the progressive loss of each of these four methanol end-caps from the precursor ion (M = 1041 Da, M-32 = 1009 Da, M-64 = 977 Da, M-96 = 945 Da, and M-128 = 913 Da). This reveals that the non-methoxylated parent structure (913 Da) contains four isocyanate groups and that the connectivity between the two carbodiimides must be between the carbodiimide linkages, to form a four-membered ring. Additionally, each fragment ion identified in Figure 8, and predicted in Scheme 2, is observed in Figure 9 and Scheme 3, with their respective mass shifts due to the methanol capping of the isocyanate end groups, forming a

self-consistent data set. Given the redundancy of describing the degradation pathways for both the “neat” and methanol-capped companion, future discussion will focus on the “neat” MS/MS data. If the reader is interested in examining the methanol-capped data, it can be found in the Supporting Information. C. Uretonimine Branched Carbodiimide (1CD) Fragmentation Pathways: Structure 1-5. Figure 10 displays the MALDITOF/TOF CID spectrum for the uretonimine branched

Figure 10. MALDI-TOF/TOF CID fragmentation spectrum of the protonated 707.2 Da uretonimine branched carbodiimide precursor ion (1CD) (structure 1-5). M


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Scheme 4. Low Kinetic Energy Fragmentation Pathways of the Protonated 707.2 Da Uretonimine Branched Carbodiimide Precursor Ion (1CD) (Structure 1-5)

carbodiimide (1CD) (structure 1-5: 707.2 Da), with n = 0 repeat units. This fragmentation spectrum displays four fragment peaks of interest: (1) “F1” at 483 Da, (2) “F2” at 352 Da, (3) “F3” at 574 Da, and (4) “F4” at 457 Da. These fragments are produced by degradation processes very similar to that previously described for the branched CDI dimer (structure 1-3), with preferential fragmentation at the four-membered linkage. Specifically, there is preferential charge-induced fragmentation initiated by the hydrogen cation and an accompanying 1,4hydrogen transfer that initially produces the “F1” fragment observed at 483 Da (Scheme 4Ai). This degradation pathway produces an additional fragment through a 1,4-H transfer reaction to yield the “F2” fragment at 352 Da (Scheme 4Aii). An additional fragmentation pathway involves cleavage at the Ph−CH2Ph bond (Scheme 4B) to produce the “F3” fragment observed at 574 Da. Transannular bond cleavage of the fourmembered linkage in the 707 Da precursor ion produces the 457 Da fragment (“F4”) (Scheme 4C). D. Imino-s-triazine Branched Carbodiimide (1CT) Fragmentation Pathways: Structure 1-6. Figure 11 displays the MALDI-TOF/TOF CID spectrum for the imino-s-triazine branched carbodiimide (1CT) (structure 1-6: 957.3 Da), with n = 0 repeat units. This fragmentation spectrum displays the expected fragment losses that were previously described in Schemes 1 and 4, such as 824 Da (M − 133 Da), 733 Da (M − 224 Da), and 602 Da (M − 355 Da). However, the sixmembered ring appears to be a weak link that is very susceptible to transannular fragmentation (vide inf ra). Preferential cleavage occurs through charge-induced fragmentation, accompanied by a 1,4-H transfer, to produce the 733 Da (“F1”) fragment ion (Scheme 5Ai). The “F1” fragment ion can undergo additional fragmentation through a 1,4-H transfer and cleavage of the Ph−CH2Ph bond to produce the 602 Da “F2” fragment ion (Scheme 5Aii). Additionally, the imino-s-triazine precursor ion can undergo cleavage of an Ph−CH2Ph bond to generate the “F5” fragment observed at 824 Da (Scheme 5B). Given the exaggerated peak intensity of the 457 Da “F4” fragment ion,

Figure 11. MALDI-TOF/TOF CID fragmentation spectrum of the protonated 957.3 Da imino-s-triazine branched carbodiimide (1CT) precursor ion (structure 1-6).

relative to the other fragment peaks, it appears that the sixmembered imino-s-triazine linkage is the weak link in the precursor ion, and it readily undergoes transannular bond cleavage to produce the “F4” fragment (Scheme 5Ci). Furthermore, the “F4” fragment ion can undergo additional degradation, with an associated 1,4-H transfer, to produce the 338 Da “F5” fragment (Scheme 5Cii). E. Guanidine Branched Carbodiimide Fragmentation Pathways: Structure 1-7. Figure 12 displays the MALDITOF/TOF CID spectrum for guanidine branched carbodiimide (structure 1-7; 681.3), with n = 0 repeat units. Inspection of the CID spectrum displays the expected fragment losses that were previously described in Schemes 2, 4, and 5, such as 548 Da (M − 133 Da), 457 Da (M − 224 Da), and 326 Da (M − 355 Da). However, the lack of a four- or six-membered ring branch point appears to eliminate the preferential charge-induced fragmentation. Presumably, the hydrogen cation was coordinated within the N


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Macromolecules

Scheme 5. Low Kinetic Energy Fragmentation Pathways of the Protonated 957.3 Da Imino-s-triazine Branched Carbodiimide (1CT) Precursor Ion (Structure 1-6)

branched carbodiimide containing an “extra” biphenyl linkage (structure 1-10: 1033.3 Da), with n = 0 repeat units. Inspection of this fragmentation spectrum reveals a much more complex spectrum than that of structure 1-6 (cf. Figures 11 and 13). This is due to the additional fragmentation pathways that are available to structure 1-10 vs structure 1-6. Two degradation models will be presented to illustrate this point. In the first model (Scheme 7a), the 1033 Da precursor ion undergoes chargeinduced fragmentation, accompanied by a 1,4-H transfer, to produce the 733 Da (“F1A”) fragment ion (Scheme 7aAi), while losing the biphenyl-containing fragment. The “F1A” fragment ion can undergo additional fragmentation through a 1,4-H transfer and cleavage of the Ph−CH2Ph bond to produce the 602 Da “F2A” fragment ion (Scheme 7aAii), followed by a 1,4-H transfer initiated cleavage to yield the 469 Da “F3A” fragment (Scheme 7aAiii). Additionally, the 1033 Da precursor ion can undergo cleavage of an Ph−CH2Ph bond to generate the “F3A” fragment observed at 900 Da (Scheme 7aB). As stated for structure 1-6, given the exaggerated peak intensity of the 533 Da “F4A” fragment, relative to the other fragment peaks, it appears that the six-membered imino-s-triazine linkage is the weak link in the precursor ion, and it readily undergoes transannular bond cleavage to produce the “F4A” fragment (533 Da) (Scheme 7aCi), with further degradation, through a 1,4-H transfer reaction, to yield the 414 Da “F5A” fragment (Scheme 7aCii). In the second degradation model (Scheme 7b), the first degradation pathway initiates with charge-induced fragmentation of the 1033 Da precursor ion, and a 1,4-H transfer, to produce the 809 Da (“F1B”) fragment ion that retains the biphenyl linkage (Scheme 7bAi). The “F1B” fragment ion can undergo a 1,4-H transfer and cleave a Ph−CH2Ph bond to form the 678 Da “F2B” fragment ion (Scheme 7bAii). The additional degradation pathways (Schemes 7bB, 7bCi, and 7bCii) are identical to those previously described for pathway A and generate the 824 Da (“F3B”), 457 Da (“F4B”) and 338 Da (“F5B”) fragments, respectively. Side-Product Formation in the PMDI-Based Carbodiimide Mixture. When considering the origin of the branched carbodiimide dimer (structure 1-3), uretonimine branched CDI (structure 1-5), and guanidine branched CDI (structure 1-7),

Figure 12. MALDI-TOF/TOF CID fragmentation spectrum of the protonated 681.3 Da guanidine branched carbodiimide precursor ion (structure 1-7).

four- and six-membered ring and played an important role in their degradation processes. Instead, the predominant fragment peak at 457 Da is the result of a main-chain cleavage at the guanidine branch. For example, Scheme 6Ai displays a guanidine C−NH bond cleavage accompanied by a 1,3-H transfer reaction. The result of this bond breaking reaction is the production of the 457 Da diisocyanate-capped carbodiimide fragment ion (“F1”). Additionally, fragment ion “F1” can undergo 1,4-H transfer and Ph−CH2Ph bond cleavage to produce the 326 Da “F2” fragment (Scheme 6Aii) or the 338 Da “F3” fragment (Scheme 6Aii). Additionally, the guanidine branched CDI precursor ion can undergo cleavage of an Ph−CH2Ph bond to generate the “F4” fragment observed at 548 Da (Scheme 6B). It is worth noting that the 225 Da “F5” fragment is also produced by the Scheme 6Ai degradation process and could compete with the “F1” fragment for protonation. F. Biphenyl-Containing Imino-s-triazine Branched CDI Fragmentation Pathways: Structure 1-10. Figure 13 displays the MALDI-TOF/TOF CID spectrum for the imino-s-triazine O


Article

Macromolecules

Scheme 6. Low KE Fragmentation Pathways of the Protonated 681.3 Da Guanidine Branched Carbodiimide Precursor Ion (Structure 1-7)

the uretonimine branching pathway is preferred under 80 °C and reversed when the reaction mixture is heated to >160 °C.16,22 Additionally, it has been reported that under acidic conditions amines can attack carbodiimide linkages to produce guanidine branches (structure 1-7).16,22,23 Under the literature reported synthesis conditions, all of the previously described side-product mechanisms seem plausible. Inspection of the “blank” PMDI side products formed without the presence of iron (Figure 5) shows carbodiimide formation (structure 1-2) in the low mass region and a predominance of imino-s-triazine branched carbodiimides (structure 1-6; 1CT) in the high mass portion of the spectrum. Both of these species are expected based upon the literature reports described above. However, the PMDI side products formed in the presence of iron(III) chloride (Figure 1) display a very different distribution of species series; there is an overall predominance of fourmembered ring species: branched CDI dimer (structure 1-3) and uretonimine branched CDI (structure 1-5). Note that these reactions were conducted under very dry conditions, and no ureas were observed in the MALDI spectra shown in Figures 1 and 5. This would certainly question the validity of H2O produced amine end groups as being the primary source of high molecular weight, guanidine branch formation (structure 1-7). A more plausible explanation would be catalyst-induced guanidine branch formation. Such reactions have already been reported as known side products in carbodiimides made from MDI.22,24 Since these species exclusively appear in the iron(III) reacted PMDI, these results indicate that iron(III) is catalyzing the formation of branched carbodiimides. We will begin our explanation of the catalyst-induced branching mechanisms by first examining the known carbodiimide

Figure 13. MALDI-TOF/TOF CID fragmentation spectrum of the protonated 1033.3 Da biphenyl-containing imino-s-triazine branched carbodiimide precursor ion (structure 1-10).

there is literature precedence for their formation. For example, heating of isocyanates above 150 °C leads to the formation of asymmetric isocyanate dimers (i.e., uretdione), which can undergo loss of CO2 to produce carbodiimides (structure 1-2; CDI).16,22 Further, the generation of carbodiimides, in the presence of excess isocyanate groups, can lead to the formation of four-membered ring uretonimine (structure 1-5; 1CD), which can further react with isocyanates, or carbodiimides, in the presence of catalytic amounts of HCl, to produce the thermally stable six-membered ring species imino-s-triazine (structure 1-6; 1CT) and structure 1-11, respectively.16 It is worth noting that P


Article

Macromolecules

Scheme 7. Low KE Fragmentation Pathways of the Protonated 1033.3 Da Biphenyl-Containing Imino-s-triazine Branched Carbodiimide Precursor Ion (Structure 1-10)

(Scheme 9). For example, it is conceivable that two FeCl3 molecules could interact with two MDI isocyanate end groups to form an intermediate complex (Scheme 8A) that could undergo loss of CO2 and two iron(III) chlorides to produce a carbodiimide linkage (structure 1-2) (Scheme 8B). Next, FeCl3 could activate the formation of a complex between a carbodiimide linkage and isocyanate end group (Scheme 9A), which could lead to the formation of a uretonimine branched PMDI (structure 1-5) (Scheme 9B). However, if trace levels of water are present, the intermediate complex, shown in Scheme 9A, could undergo loss of CO2 and two iron(III) chlorides to produce a guanidine branched PMDI (structure 1-7) (Scheme 9C). Note that the trace levels of water are most likely introduced during the sampling and analysis of these materials. It is conceivable that trace levels of H2O in the THF, or simply from atmospheric exposure, could contribute to the production of small quantities of the guanidine linkages in the MALDI spectra of PMDI side products

branching mechanisms. Alberino et al. have reported a 3-methyl1-phenyl-3-phospholene-1-oxide (MPPO) (“phosphine oxide”) catalyzed carbodiimide synthesis mechanism involving a catalystattached intermediate that undergoes loss of CO2, before reacting with an isocyanate to yield a carbodiimide.17 Further, Bruce Novak’s group published extensive work on the synthesis of polyguanidine through the use of a titanium(IV) catalyst.22 The synthesis mechanism involves an initiation phase in which the catalyst reacts with a carbodiimide linkage to produce a catalyst-attached intermediate.22 The next step is the propagation phase, in which the intermediate species reacts with additional carbodiimides to yield a guanidine branched oligomer.22 While neither of the reported mechanisms by Ablerino17 and Novak22 applies to our PMDI reaction conditions, they provide insight into proposing new iron(III)-catalyzed mechanisms for carbodiimide formation (Scheme 8) and guanidine branching Q


Article

Macromolecules Scheme 8. Proposed Iron(III) Chloride Catalyst-Initiated Pathway for Carbodiimide Formation in PMDI

Scheme 9. Proposed Iron(III) Chloride Catalyst-Initiated Pathway for Formation of Uretonimine Branching and Guanidine Branching in PMDI-Based CDIs

Additional branching mechanisms involve carbodiimide reactions that form four-membered 1,3-diazetidine rings. Richter has reported the catalytic cyclodimerization of diphenylcarbodiimide, by tributylphosphine to form a 1,3-diazetidine (e.g., structure 1-3).25 Furthermore, Felhammer et al. reported

formed in the presence of iron(III). Again, it should be noted that urea linkages were not observed in any of the MALDI spectra, which would support the hypothesis that predominant source of guanidine branching can only be rationalized through Fe(III) catalyst-initiated reactions. R


Article

Macromolecules

Scheme 10. Proposed Iron(III) Chloride Catalyst-Initiated Pathway for the Formation of 1,3-Diazetidine Branched CDI Dimers

carbodiimides adding to metal carbon bonds in metal−organic compounds, such as cyclopentadienyliron dicarbonyl generating cycloadducts with diphenylcarbodiimide.26 Building upon the mechanisms reported by Richter25 and Felhammer,26 we propose a new mechanism in which iron(III), in this case iron(III) chloride, interacts with two carbodiimide linkages to form an intermediate complex (Scheme 10A) that can undergo loss of two iron(III) chlorides to produce the 1,3-diazetidine ring branched CDI dimer (structure 1-3) (Scheme 10B). In order to give mechanistic insight into iron(III)-catalyzed carbodiimide dimerization, we modeled how the carbodiimide functionality interacts with the iron(III) chloride, using density functional theory calculations. For these calculations, we chose to approximate the PMDI-based carbodiimide with toluene carbodiimide, and we used trigonal planar iron(III) chloride in place of higher order oligomers. Iron(III) chloride is not expected to dissociate in aromatic solvents and will exist as monomers in solution. Therefore, modeling with the iron(III) chloride monomer appropriately approximates the homogeneous catalysis scenario. However, the exact morphology of the iron(III) chloride catalyst in this system is not fully understood at this time. That being said, the energy-minimized structures of toluene carbodiimide, and its iron(III) chloride adduct, are shown in Figure 14. Focusing on the structures in Figure 14, it is not surprising to find that the optimized geometry in Figure 14A features highly symmetrical structure around the carbodiimide bond (Tables 2 and 3). In particular, the polar nature of the carbodiimide bond is evident in the atomic charges calculated using natural population analysis (NPA).27 These calculations show that both carbodiimide nitrogen atoms (N15 and N17) carry significant negative charge and flank the central

Figure 14. Energy-minimized structures of (A) toluene carbodiimide and (B) with its FeCl3 adduct.

electropositive carbon atom (C16). The CN bond length of 1.225 Å and the 90° twist about C16 are an indication of two orthogonal π-bonds centered at C16, which is characteristic of carbodiimides. The N15−C16−N17 bond angle is nearly linear at 170.02°, which is in line with previous experimental and computational observations.28 S


Article

Macromolecules Table 2. Selected Bond Lengths and Angles Calculated in Simulated Benzene Solvent at B3LYP/6-31+G* bond length (Å)

bond angle (deg)

bond

isolated carbodiimide and FeCl3

FeCl3 adduct

bond angle


FeCl3 adduct

C5−N15 N15−C16 C16−N17 N17−C18 N15−Fe32 Fe32−Cl33 Fe32−Cl34 Fe32−Cl35

1.407 1.225 1.225 1.407 N.A. 2.171 2.173 2.166

1.452 1.262 1.190 1.397 2.092 2.208 2.202 2.193

C5−N15−C16 N15−C16−N17 C16−N17−C18 Cl33−Fe32−Cl34 Cl34−Fe32−Cl35 Cl35−Fe32−Cl33 N15−Fe32−Cl33 N15−Fe32−Cl34 N15−Fe32−Cl35

132.76 170.02 132.76 119.45 121.36 119.19 N.A. N.A. N.A.

120.50 173.41 147.13 111.52 112.48 114.59 105.36 104.54 107.47

not appear to be the case, as the carbodiimide moiety (N15− C16−N17) in B only has 0.1 total calculated Mullikan spin density compared to A. The majority of the five unpaired electrons of B remain on the FeCl3 group.

Table 3. Selected Atomic Charges Calculated in Simulated Benzene Solvent at B3LYP/6-31+G* calculated NPA charge atom


FeCl3 adduct

Δ

C5 N15 C16 N17 C18 Fe32 Cl33 Cl34 Cl35

0.108 −0.505 0.675 −0.505 0.108 0.920 −0.305 −0.306 −0.309

0.117 −0.620 0.729 −0.367 0.081 0.670 −0.330 −0.316 −0.305

0.009 −0.115 0.054 0.138 −0.027 −0.250 −0.025 −0.010 0.004

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CONCLUSIONS The results of this study show that the low molecular mass portion of the iron(III) chloride/polymeric methylene diphenyl diisocyanate (PMDI) mixture contained the expected PMDI and small quantities of linear carbodiimides. The high molecular mass portion of the spectrum was observed to be dominated with branched structures: (1) 1,3-diazetidine branched CDIs were the predominant structures, (2) along with considerable quantities of uretonimine branched CDIs and (3) imino-s-triazine branched carbodiimides, and (4) low levels of guanidine branch CDIs. Experimental results indicate that iron(III) catalyst-induced reactions are the root cause of the extensive CDI branch defects observed in these materials. Without proper intervention, these unexpected branching reactions have the potential to form highly viscous CDI mixtures, which will pose a challenge during the processing of these materials.

Our computational results show that adduct formation is exothermic both in vacuum and simulated benzene solvent (ΔaddG = −22.5 and −19.7 kcal/mol, respectively). The ΔaddG was determined by directly calculating the difference between the sum of the individual carbodiimide and FeCl3 molecules and their adduct (see Supporting Information). To find the preferred geometry of the adduct (Figure 14B), we began with the optimized structure of toluene carbodiimide (Figure 14A) and placed a geometry optimized iron(III) chloride molecule about 2.5 Å from the carbodiimide bond in various orientations and used these as the starting geometries for adduct optimization. We found that the geometry optimization would always result in a structure with the iron coordinating to the nonbonding electrons of the carbodiimide nitrogen atoms (N15 and N17) for both the vacuum and benzene solvent conditions. Comparison of the structures in Figure 14 shows the strong correlation of iron(III) chloride with a nitrogen atom, and this interaction causes a significant perturbation of the molecular geometry of both A and (uncoordinated) iron(III) chloride. More importantly, the carbodiimide bonds in the adduct, B, are more polarized than in A. Table 3 shows that electron density shifts significantly toward the coordinating nitrogen (N15) in the adduct leaving both C16 and N17 more electropositive. Interactions between the chlorine atoms, and the carbodiimide bond, do not appear to have a pronounced effect. For instance, there is little difference in the charge on Cl34 between the adduct and the isolated FeCl3 molecule (Table 3). These calculations strongly suggest that in this chemical environment the action of iron(III) chloride on A is that of a Lewis acid which coordinates with a nitrogen atom, a Lewis base. Iron(III) chloride is well-known to act as a Lewis acid in many chemical reactions, often in a catalytic role. While a radical-based mechanism can be envisioned in adduct formation between open-shelled iron(III) chloride and the π-bonds in A, this does

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01973. Additional CID fragmentation spectra, FTIR, and computational data are presented for the PMDI-based polymeric mixtures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel (979) 238-1778; e-mail [email protected] (A.P.G.). 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 thank Helge Braun, Carla Schmidt, and Steven Horsch, at The Dow Chemical Company, for their assistance with the thermal aging studies and viscosity measurements. Additional gratitude is extended to Prof. David M. Hercules, at Vanderbilt University, for his helpful discussions on characterization of polycarbodiimides.

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REFERENCES

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Iron(III)-Catalyzed Chain Growth Reactions of Polymeric Methylene

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