Article pubs.acs.org/IECR
Origin of Impurities Formed in the Polyurethane Production Chain. 1. Conditions for Chlorine Transfer from an Aryl Isocyanide Dichloride Byproduct June Callison,† Ruth Edge,‡ Kimberly R. de Cuba,§ Robert H. Carr,§ Joseph J. W. McDouall,‡ David Collison,‡ Eric J. L. McInnes,‡ Willem van der Borden,§ Klaas van der Velde,§ John M. Winfield,† and David Lennon*,† †
School of Chemistry, Joseph Black Building, University of Glasgow, Scotland G12 8QQ , U.K. School of Chemistry, The University of Manchester, England M13 9PL, U.K. § Huntsman Polyurethanes, Everslaan 45, 3078 Everberg, Belgium ‡
S Supporting Information *
ABSTRACT: Phenyl and 4-methylphenyl isocyanide dichlorides are models for byproduct that may be formed in the later stages of certain polyurethane production chains. Photochemical electron paramagnetic resonance (EPR) studies (λ > 310 nm), using the spin trap, N-tert-butyl-α-phenylnitrone, confirm a previously made suggestion that ArNCCl2 can behave as a chlorine radical source. EPR spectra recorded during and after irradiation and supported by simulations evolve over time and indicate formation of the short-lived spin trap−Cl• adduct and a longer lived benzoyl-N-tert-butylnitroxide radical. Photolysis of C6H5N CCl2, either alone or mixed with methylene diaryl isocyanate species, in o-C6H4Cl2, a polyurethane process solvent, led to the formation of mixtures containing dichloro- and trichlorobiphenyl isomers. products,1 although direct evidence on this point is lacking. These impurities are linked also with unfavorable coloration (darkening) of the product stream.2 A reasonable hypothesis is to assume that ArNCCl2 are sources of Cl• radicals or that they behave as Cl-atom-transfer agents. This hypothesis has led us to examine the behavior of C6H5NCCl2 and 4-CH3C6H4NCCl2 under photolysis conditions where homolytic C−Cl fission might be expected. Electron paramagnetic resonance (EPR) is selected as the method of choice to study the radical-based processes. Because of the technological importance of chlorocarbons and chlorohydrocarbons,6 considerable information is available regarding the presumed role of the chlorine radical in mechanistic studies. For example, Cl• has been implicated as a catalyst in the dehydrochlorination of simple hydrochlorocarbons7 and in chlorination of a variety of hydro- and hydrochlorocarbons.8−17 Although the lifetime of Cl• is short (for example at room temperature and low pressure its mean lifetime in glass is of the order of a few milliseconds18) the kinetics of its elementary reactions have been well-established and extensively reviewed.19 Its EPR spectrum in the gas phase has been observed,20 but spin-trapping methodology is necessary to obtain definitive evidence for the existence of Cl• in solution.21−23 This latter approach has been adopted in this work to demonstrate the Cl-atom-transfer behavior of ArNCCl2 species.
1. INTRODUCTION Methylene diaryl diisocyanate isomers and polymers (referred to here as MDI and polyMDI) are key feedstocks in the industrial production of polyurethanes.1,2 Despite recent interest in finding alternative routes,3−5 that most widely used involves the phosgenation of primary aromatic amines, a process that can lead to the formation of unwanted byproduct. One known side reaction is the formation of ureas via the reaction between MDI and the primary amine building block.2 Further reaction between ureas and OCCl2 can produce aromatic isocyanide dichlorides, ArNCCl2, as trace products (Scheme 1). The simplest of these molecules have boiling points close to the Scheme 1. Outline of Steps Leading to the Formation of Aryl Isocyanide Dichlorides
MDI main product, and thus they are not separable readily by distillation,1 while higher molecular weight homologues are not distillable at the industrial scale. The ArNCCl2 species have been proposed as the source of persistent Cl-containing impurities in the final polyurethane © 2011 American Chemical Society
Received: Revised: Accepted: Published: 2515
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2.3. Characterization. Samples of phenyl isocyanide dichloride and p-tolyl isocyanide dichloride were investigated by several methods to achieve identification and to determine sample integrity. Full details of their characterization are given in the Supporting Information (section S1), and some of their characteristic properties are summarized in Table 1. FTIR spectra
To explore this hypothesis further, complementary GCMS studies of the photolysis of C6H5NCCl2 in the presence of MDI or polyMDI have been undertaken. It might be expected that, under photolytic conditions, hydrogen would be abstracted from the -CH2- bridge of MDI or polyMDI with the formation of HCl and a delocalized carbon-centered free radical. This was not observed under the conditions used, however, but rather chlorination of the solvent and the formation of biphenyl species resulted. The implications of this finding are briefly stated.
Table 1. Characteristic Data for Aryl Isocyanide Dichlorides compound
2. EXPERIMENTAL SECTION 2.1. General Methods and Instrumentation. As far as possible, operations were carried out either under dry N2 or in vacuo. However samples for spectroscopic and physical measurements were transferred quickly through ambient laboratory atmosphere. FTIR spectra were obtained using a Nicolet Avatar 360 FTIR spectrometer continually purged with dry air, from which the carbon dioxide had been removed (Donaldson Ultrapac MSD 0025 M). A Pike MIRacle ATR accessory with a diamond/ZnSe element (crystal diameter, 1.8 mm) provided a suitable method for analysis of solids and oils. In the latter case, a liquids plate accessory was used. 1H and 13C NMR spectra were recorded on a Bruker Avance spectrometer fitted with a Quattro nucleus probe (QNP) at 400 and 100 MHz, respectively, chemical shifts being reported using δH and δC scales. Mass spectra were obtained using a Varian CP-3800 gas chromatograph (Varian FactorFour VF-5ms column) integrated with a Saturn 2200 mass spectrometer. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out using a Q100 differential scanning calorimeter and a Q500 thermogravimetric analyzer (both from TA Instruments). A Sherwood chlorine analyzer was used to quantify the amount of free chlorine in the solution by argentimetry. Solutions for study by GCMS were photolyzed using a Photochemical Reactors, 3010, 125 W Hg lamp housed within a quartz immersion well. This arrangement produced broad-band UV−vis irradiation. 2.2. Synthesis of Aryl Isocyanide Dichlorides. Phenyl isocyanide dichloride (PID, also referred to as 1,1-dichloroN-phenylmethanimine) was prepared by the literature method from phenyl isothiocyanate and dichlorine.24−26 [PID is listed as a commercially available chemical from some sources, but export restrictions to the U.K. prevented acquisition of the chemical via this route; consequently, we were forced to prepare the arylisocyanide dichlorides ourselves.] A solution of phenyl isothiocyanate (1.41 g, 10.4 mmol, Sigma-Aldrich, 99%) in chloroform (25 cm3, Fisher Scientific, 99.8%), contained in a three-necked Pyrex round bottomed flask, was cooled in ice to below 276 K, while being purged with N2. Dichlorine gas (Linde, 99%) was flowed at 30 cm3 min−1 for 30 min, to ensure that the amount of Cl2 entering the system was greater than the 2:1 Cl2:C6H5NCS mole ratio required for a complete reaction. During this time the solution became saturated with Cl2 and was dark yellow/orange in color. The mixture was stirred under N2 for 2 h after which the solvent and sulfur dichloride were removed by rotary evaporation at 308 K to leave yellow oil identified from its properties as moisture sensitive, phenylisocyanide dichloride (yield, 75−85%). The reaction timings were chosen to balance consumption of C6H5NCS with minimization of ring chlorination.25,26 Similarly prepared was 4-methylphenyl isocyanide dichloride (tolyl isocyanide dichloride, TID) from 4-methylphenyl isothiocyanate (0.89 g, 5.94 mmol, Sigma-Aldrich, 97%). The product after removal of solvent and SCl2 was also isolated as a yellow oil.
phenyl isocyanide dichloride
FTIR νmax, cm−1 (assignment) 1654 (ν(CN)) 907 (νasym(C−Cl)) 846 (νsym(C−Cl))
4-methylphenyl isocyanide dichloride
1647 (ν(CN)) 859 (νasym(C−Cl)) 808 (νsym(C−Cl))
MS m/z (assignment) 173, 175, 177 (C6H5NCCl2)
DSC exotherm T, K 406 432
138, 140 (C6H5NCCl) 187, 189, 191 (CH3C6H4NCCl2)
416 427
152, 154 (CH3C6H4NCCl)
proved particularly useful, and Figure 1 presents the infrared spectra of the prepared compounds, which exhibit strong features
Figure 1. ATR infrared spectra of (a) phenyl isocyanide dichloride and (b) p-tolyl isocyanide dichloride.
due to CN (ca. 1650 cm−1) and C−Cl (900−800 cm−1) stretching modes.27−29 The agreement with reference spectra recorded in transmission mode is good,27 except for the attenuation of the C−H stretching modes, which is a consequence of the ATR sampling regime.30 There was no evidence in the spectra for isothiocyanate starting materials, since the intense band of the -NCS group at ca. 2050 cm−1 28 was not observed. It was noted that the aryl isocyanide dichlorides degrade over time. For example, after a few days storage in a sealed vessel, the 2516
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infrared spectrum shows the ν(CN) band intensity decreases, while concomitantly new bands at ca. 1751 and 1530 cm−1 grow in. These new features are respectively attributed to a C O stretch and an amide II mode of a primary hydrolysis product, ArNHC(O)Cl; spontaneous loss of HCl from this compound leads to ArNCO.29 Scheme 2 describes the slow
uniquely representative of the two spectral features observed (see section 3.1), with no overlap of adjacent bands. No change in the peak−peak width occurred (Figure 4), allowing the peak− peak height to be used as a relative measure of concentration. Simulation of selected spectra was carried out using the Bruker SimFonia software package (version 1.25). As a check on the experimental protocols, PBN was used to trap Cl• produced from the photolysis of hexachloroplatinate(IV)in solution. Details are given in the Supporting Information (section S.2). 2.5. Calculation of EPR Parameters To Support Radical Identification. Density functional calculations were carried out using the Gaussian03 suite of programs.38 Geometries for the structures examined were fully optimized using the B3LYP functional39 and the 6-311G(d,p)40 basis. All structures were confirmed as minima through vibrational analysis. Isotropic Fermi contact coupling constants were calculated at these optimized geometries. For the evaluation of hyperfine couplings the large EPR-III41 basis set was employed for all atoms except Cl. For Cl (since the EPR-III basis is not defined for this atom) the IGLOIII42 basis was employed. 2.6. Product Analyses from Photolysis of Phenyl Isocyanide Dichloride with MDI or polyMDI. The following solutions in o-dichlorobenzene (Sigma-Aldrich, 99%) were prepared under anhydrous conditions and were stored in closed Pyrex vessels: C6H5NCCl2 (0.05 mol dm−3), C6H5NCCl2 plus MDI (both 0.05 mol dm−3, the latter supplied by Huntsman Polyurethanes), and C6H5NCCl2 plus polyMDI (both 0.05 mol dm−3, the latter supplied by Huntsman Polyurethanes). Aliquots of the solutions were photolyzed by using a commercial photochemical reactor. Gas chromatography mass spectrometry (GCMS) was used to determine the species present; aliquots of the solutions before and after reaction were compared, and in every case solutions were diluted with chlorobenzene (1:10 volume ratio) before GCMS examination. The species identified on the basis of retention times and their characteristic m/z features are given in the Supporting Information (Table S.4). Due to the sensitivity of the GCMS method, trace impurities were observed in the mixtures examined. Notable were C6H5NCS, which was used to prepare C6H5NCCl2 (see section 2.1), and C6H5NCO, an end product of C6H5NCCl2 hydrolysis.29 The presence of these species did not obscure the interpretation of the photolytic reactions examined.
Scheme 2. Hydrolysis of p-Tolyl Isocyanide Dichloride
hydrolysis process. Representative spectra exhibiting these temporal changes are presented in Supporting Information (Figure S.1). To maintain sample integrity, spectroscopic examinations were carried out, when possible, immediately after synthesis. 2.4. EPR Spectroscopy. EPR spectroscopic measurements were carried out using a Bruker EMX Micro X-band spectrometer (≈9.4 GHz) equipped with a digital temperature control system. Reactions performed at specific temperatures were carried out in situ in a cavity that could be heated to the required temperature or cooled using liquid nitrogen. The compound used to trap any Cl• formed in the reactions studied was N-tert-butyl-α-phenylnitrone (PBN), a species which has been used to identify the formation of Cl• in solution from a variety of chemical sources.21−23,31−36 Unfortunately PBN was thermally unstable at the elevated temperatures encountered in the industrial process, but it could be used to characterize photochemically initiated reactions. Consequently, the following section describes results connected with solutions that have experienced varying exposures of photoirradiation. Thermolysis reactions are more directly linked to the industrial process, and these will be explored in detail by other techniques in a follow-up publication.37 This communication seeks to use EPR to determine the lability of the C−Cl bond in the aryl isocyanide dichlorides under consideration. Solutions containing phenyl isocyanide dichloride (0.1 mol dm−3) with the spin trap PBN (0.01−0.1 mol dm−3, Sigma-Aldrich, ≥98%) in benzene (Sigma-Aldrich, ≥99%) were flushed with dinitrogen before an aliquot was added to an EPR tube. This was placed in the spectrometer cavity. For blank reactions, solutions of the separate components in benzene were examined. A xenon arc lamp was used to photolyze the mixtures in situ with a filter to cut off radiation with wavelength less than 310 nm. In all cases a scan was made before irradiation commenced to check that no radicals were present. Two different procedures were used to follow the reactions over time. The timer program was set either to record one scan every 30 s, or to scan constantly (one scan taking 14 s to complete). Acquisition of the spectra was carried out for up to 2 h. Samples were irradiated, either constantly while scanning took place or for 30 s followed by immediate scanning. To observe decay of the EPR-active species generated, the solutions were irradiated for 45 min with continuous scanning, the lamp was switched off, and scanning was continued for a further 45 min. The procedure was carried out at three different temperatures: 288, 298, and 308 K. To produce a reaction profile, the derivative peak to peak height of the resonance line equivalent to that labeled * in Figure 3 and the resonance line equivalent to that labeled # in Figure 3 were calculated and plotted against time. These peaks were selected as they were
3. RESULTS AND DISCUSSION 3.1. Identification of the EPR-Active Species Encountered. Separate solutions of phenyl isocyanide dichloride, C6H5N CCl2 (PID), and of N-tert-butyl-α-phenylnitrone (PBN), in benzene were irradiated at λ ≥ 310 nm to determine if radicals were produced by either compound alone in solution. The results are shown in Figure 2; they indicate that EPR-active radicals are not observed when PID is irradiated in the absence of the spin trap, Figure 2a, presumably due to the short lifetime of any chlorine radicals which may be formed. However, when PBN is irradiated in benzene a three line pattern is observed in the resulting EPR spectrum, Figure 2b, consistent with the formation of a 14N-containing radical. Splitting by a single 1H is also observed, the hyperfine couplings being aN = 14.31 G and aH = 1.96 G (Table 2). On this basis the signal is assigned to the hydroxyl adduct of PBN, hydroxybenzyl-N-tert-butyl nitroxide; this is formed from reaction with trace water in the solvent.43 It was observed also in low-temperature irradiations of PID plus PBN mixtures in benzene; simulations of spectra obtained over a range of times showed the radical to be present. Quantification of this spectral 2517
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Figure 3. EPR spectrum of C6H5NCCl2 plus PBN in benzene with irradiation at 289 K: (a) first scan after irradiation commenced, (b) scan after 50 min constant irradiation, and (c) simulation of b. Parameters from simulation are aN = 12.32, aCl‑35 = 6.25, aCl‑37 = 5.20, and aH = 0.80 G. For * and #, see text (section 2.4).
Figure 2. EPR spectrum of (a) C6H5NCCl2 in benzene and (b) PBN in benzene after constant irradiation for 46 min at 288 K.
signature (triplet of doublets), via double integration of the simulated derived spectra, showed it to contribute up to 14% of the intensity of the spectra considered below. Its interference in those spectra could be accounted for and did not complicate the subsequent analysis of the mixtures of aryl isocyanide dichlorides with the PBN spin trap. A series of irradiations was carried out of C6H5NCCl2 (PID) with the spin trap, PBN in benzene. Figure 3 shows the spectra recorded initially and after 50 min irradiation. The first scan, Figure 3a, recorded immediately after irradiation was started, shows an eight line pattern consistent with the chlorine adduct of PBN, I in Scheme 3. The pattern of the lines (1:1:2:2:2:2:1:1) stems from three overlapping quartets
from 14N (I = 1), with further splitting due to 35,37Cl (I = 3/2). These features should be split further due to the single 1H at the Cl position, but this was resolved only after 50 min of constant irradiation, Figure 3b. These developing spectra, Figure 3a,b, indicate a temporal component to the adduct formation process. The hyperfine splitting constants determined for the chlorine adduct (I) in benzene are aN = 12.32 G, aCl‑35 = 6.25 G, aCl‑37 = 5.20 G, and aH = 0.80 G, Figure 3b, and are consistent with previous studies, shown in Table 2.31,36 To obtain information relating to the lifetime of the PBN−Cl adduct (I), a mixture of C6H5NCCl2 plus PBN in benzene was irradiated for 30 s, with the EPR spectra being monitored
Table 2. Hyperfine Coupling Constants of Radicals Observed Together with Literature Values a (G) radical PBN−chlorine adduct (I)
hydroxybenzyl-N-tert-butyl nitroxide
benzoyl-N-tert-butyl nitroxide (III)
a
solvent
N
DMSO C6H6 C6H5CH3 C6H6 C6H6 CCl4 CH3CN CH3CN C6H6 C6H6 H2O H2O CCl3F C6H6 C6H6 CH3CN CH3(CH2)4CH3 C6H6
12.56 12.32 12.43 12.28 12.12 12.22 12.70 12.33 12.31 14.31 15.46 13.40 14.00 8.00 8.01 8.08 7.89 7.67
35
Cl
6.28 6.25 6.26 6.18 6.05 6.08 6.20 6.20 6.36
37
Cl
5.20 4.92 5.20 4.88 5.00 5.12 5.12 5.00
H 0.80 0.78 0.75 0.80 0.89 0.83 0.75 1.96 2.72 2.00 2.80
ref present present present present 31 33 34 35 36 present 34 35 45 present present 35 36 46
worka workb workb workc
workd
workb workc
As derived from [PtCl6]2−. bAs derived from C6H5NCCl2. cAs derived from 4-CH3C6H4NCCl2. dAs derived from PBN. 2518
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It is proposed that the benzoyl-N-tert-butyl nitroxide radical can be formed from a reaction between the PBN−Cl adduct and additional PBN as shown in Scheme 3. The benzoyl radical (III) should be accompanied by a second product, C6H5CH NBut (II);44 evidence for the formation of this species is provided by the GCMS spectrum, m/z 162 (M+, C6H5CH NC(CH3)3, 9%), 105 (C6H5CHN, 41%), 77 (C6H5, 15%), of a sample of PID plus PBN, which had been irradiated for 45 min. The possibility of a reaction between excess PBN and the residue from PID after loss of Cl• to give radical IV is considered also in Scheme 3. However, formation of IV is inconsistent with the results of density functional calculations. A number of conformations exist for structures III and IV, but exhaustive searches of conformational space have not been attempted as the spin density is strongly localized on the N−O• unit; it is unlikely that the conformational environment will significantly affect the Fermi contact coupling (which is dependent on the spin density at the nucleus). Figure 5 shows the spin densities for III and IV. In IV the Mulliken spin densities on the N−O• unit are 0.45 (N) and 0.51 (O), the Fermi contact couplings are +12.2 G (N) and −14.8 G (O). The protons of the phenyl unit nearer to the radical center show very small spin densities (range of 0.0003−0.0023) with the largest Fermi contact coupling being 0.3 G. In III the spin densities on the N−O• unit are 0.27 (N) and 0.58 (O); the O atom of the carbonyl unit carries most of the remaining spin density (0.14). The carbonyl O atom pulls away significant spin density from the N atom, reducing the hyperfine coupling. The corresponding Fermi contact couplings are +5.5 (N), −14.8 (O), and −3.5 G on the carbonyl O atom. The spin density and the hyperfine coupling on the O atom are both very similar in III and IV, while those for N are significantly different. In III the phenyl protons again show very small spin densities, with the largest proton hyperfine being less than 0.1 G. Comparisons with experimental values in Table 2 suggest that the second species observed is III rather than IV. Fermi contact couplings are typically slightly underestimated in Gaussian orbital calculations41 due to the difficulty of treating the density at the nucleus. For species III experimental values in solution are 7.67− 8.08 G for the N hyperfine, Table 2. These values are substantially smaller than those calculated for IV but are a little larger than those calculated for III. Thus assignment to radical III is favored. As a further test of the analyses, Figure 6 compares experimental with simulated spectra for the two species, PBN−chlorine adduct (I), in Figure 6a,b, and benzoyl-N-tert-butyl nitroxide (III), in Figure 6c,d. The excellent agreement between the simulated and observed spectra reinforces the assignments for the octet and triplet splitting patterns. The spin trapping studies described thus far were also performed on solutions of 4-methylphenyl isocyanide dichloride (TID). It was found this compound behaved in the same way as PID; hyperfine values are given in Table 2. The temporal variations of the EPR signal intensities, on a peak-to-peak basis, have been determined for the formation and decay of I and for III at three temperatures: 288, 298, and 308 K. Data for the benzoyl nitroxide radical (III) are shown in Figure 7. The reaction profiles for I and III are very different; in particular the time scale of III is far slower than that of I. Table 3 presents the half-life values for both species at the three different temperatures. Although the decays for I approximate to single exponentials, the profile does not adhere to simple first-order kinetics over the full duration, implying a nonsimple mechanism for the loss of I. In contrast, the decay of III is well-described by zero-order kinetics, as represented by linear fits for plots of
Scheme 3. Reaction Chemistry Proposed for PID plus PBN
thereafter. Scanning was initiated immediately after the lamp was switched off and continued for 50 min. Specimen spectra, at 30 s and 5, 15, and 50 min are shown in Figure 4a−d.
Figure 4. EPR spectra of C6H5NCCl2 plus PBN in benzene at 288 K, the solution being irradiated for 30 s and then the irradiation stopped. Spectra after 30 s (a), 5 min (b), 15 min (c), and 50 min (d).
The PBN−chlorine adduct (I) initially produced decays to enable the observation of a second species, clearly visible after 15 min, Figure 4c. This is characterized by a three line pattern suggesting a species containing 14N with a hyperfine splitting of aN = 8.00 G. Several possibilities were considered, but the assignment proposed for the species contributing to Figure 4d is the benzoyl-N-tert-butyl nitroxide radical. It has been reported previously as a product from reactions involving PBN with dichlorine, dibromine, or a variety of other oxidizing agents.35,36 The spectra and splitting values are consistent with those obtained here (Table 2). 2519
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Figure 5. B3LYP/EPR-III(Cl: IGLO-III) spin densities (isosurface value, 0.004 au; t-Bu group shown in wireframe for clarity) for (a) IV and (b) III.
Figure 7. Time profiles of the signal intensity of the second species produced by irradiation of C6H5NCCl2 plus PBN in benzene at different temperatures: (a) 288, (b) 298, and (c) 308 K.
Figure 6. EPR spectra of C6H5NCCl2 plus PBN in benzene at 288 K, the solution being irradiated for 30 s and then the irradiation stopped. Spectra (a) after 30 s and (c) after 100 min; (b and d) simulations of a and c, respectively. Parameters from simulation are (b) aN = 12.32, aCl‑35 = 6.25, aCl‑37 = 5.20 G, and (d) aN = 8.00 G.
Table 3. Half-Life Values for EPR Decay Profiles of PBN− Chlorine Adduct I and Benzoyl Radical III for Three Different Temperatures
([A]0 − [A]) versus time for all three temperatures for times > 3500 s. Such a correspondence to simple integer-order kinetics suggests decay of radical III to be a relatively straightforward process, e.g., by collisional deactivation, which is independent of reagent concentration. The spectral intensity due to PBN−Cl increases markedly up to a maximum value (not shown); at 288 K this occurs at ca. 500 s. After this point, formation and decay of I are both important. At higher temperatures the apparent rate of formation is slower, possibly indicating an increasing importance of the decay processes. After the irradiation is stopped, the decay of I is rapid, Table 3. The signal assigned to the benzoyl radical III is visible after ca. 3 min, Figure 7. Its behavior with time is similar to that of I, at least until the irradiation ceases. However, at this point, a second
t1/2 (s) temperature (K)
I
III
288 298 308
308 315 207
2516 2063 1161
phase of increase in concentration to a maximum followed by a very slow decay, as described above, is apparent. This behavior suggests that radical III is not formed solely via a reaction between I and PBN as described in Scheme 3. Although irradiation of PBN in benzene in the absence of C6H5NCCl2 does not lead to the benzoyl radical, a mixture of C6H5NCCl2 plus PBN in benzene, stored in the dark, yielded a very small EPR signal due to III after 2520
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significant at the industrial scale, e.g., in relation to recovery and reuse of the process solvent. It appears therefore that photolysis of C6H5NCCl2 can result in the formation of Cl•, consistent with the EPR studies reported above, but it is the solvent rather than MDI or polyMDI which is involved in the subsequent reactions. It appears that C6H5• is also formed. No strongly colored species were formed as a result of photolysis. Although development of color in MDI solutions has been tentatively linked to the formation of Cl-containing species,1 this work demonstrates that it is not a result of ring chlorination of MDI or polyMDI, since chlorinated species were, at best, formed in very small quantities. An hypothesis to account for formation of different radical species from thermolysis reactions will be developed in a subsequent publication.37
1 h, demonstrating the possibility of a nonphotochemical route. A possible additional route to the benzoyl nitroxide radical is shown in Scheme 4. It involves the formation of the radical cation Scheme 4. Proposed Reaction Chemistry Derived from PBN•+
4. CONCLUSIONS Phenyl and 4-methylphenyl isocyanide dichlorides have been synthesized and characterized by a variety of techniques. A consideration of issues relevant to the production of polyurethanes leads to the following points: (1) EPR studies of phenyl isocyanide dichloride (PID) irradiated in the presence of a suitable spin trap (PBN) yields an observed eight line spectrum that is assigned to the formation of the chlorine adduct of PBN (I). This outcome confirms the photolytic scission of PID to be a facile pathway, with PID acting as a source of chlorine radicals. (2) Inspection of the photoinitiated EPR spectra as a function of time indicated the observed eight line spectrum to evolve into a triplet, which is assigned to the benzoyl-N-tert-butyl nitroxide radical (III). Reaction schemes for the formation of III are postulated, which involve combination of the Cl−PBN adduct (I) or a PBN•+ (V) radical cation with a second molecule of PBN. (3) The decay of the chlorine adduct of PBN (I) is significantly faster than that observed for the benzoyl-N-tert-butyl nitroxide radical (III). (4) GCMS analysis of mixtures of PID with MDI or polyMDI solvated in o-dichlorobenzene that have experienced photolysis (UV−visible irradiation) indicate that photolysis leads to some chlorination and formation of phenyl radicals. This result indicates that photolysis can additionally induce electrophilic/radical chlorination processes.
PBN•+ (V) and its subsequent reaction with PBN. Previous work indicates that PBN•+ may arise from PBN by either photochemical or chemical (thermal) oxidation;35,36,45 in the present situation we postulate that a chemical oxidizing agent could be dichlorine, resulting from C6H5NCCl2 decomposition. The intensity profiles of the EPR detectable species (species I and III) when illumination is stopped are complex and are inconsistent with a simple consecutive process. Because only the chlorine adduct of PBN (I) is directly connected to the isocyanate process chemistry, with the second stage benzoylN-tert-butyl nitroxide radical (III) merely an EPR-related event, the associated reaction kinetics were not explored further. That matter is beyond the scope of the present investigation. 3.2. Photolysis of Phenyl Isocyanide Dichloride with MDI or polyMDI. Having established that phenyl isocyanide dichloride can undergo a photoinduced Cl-atom-transfer reaction with PBN and thus that, consistent with an earlier suggestion,1 it can be regarded as a source of short-lived Cl• radicals, it was logical to examine its behavior on photolysis with the polyurethane feedstock poly(methylene diphenyl diisocyanate) and the low molecular weight compound 4,4′-methylene diphenyl diisocyanate, polyMDI and MDI, respectively. TGA/DSC experiments showed temperatures in excess of 400 K are necessary to thermally decompose the aryl isocyanide dichlorides (Supporting Information, section S1.4). Therefore, to be able to subsequently make measurements at these temperatures, o-dichlorobenzene (boiling point, 446 K) was selected as the solvent. It is also closely related to the process solvent, chlorobenzene.2 As mentioned in section 2.4, aspects of thermolysis experiments will be described in a follow-on paper.37 Photolysis of C6H5NCCl2 in o-C6H4Cl2 and analysis by GCMS (Supporting Information, section S.3) led to the unexpected observation of trichlorobiphenyl isomers as the main identifiable product; a small quantity of dichlorobiphenyl isomers was also formed. Photolyses in the presence of MDI or polyMDI resulted in very similar behavior, although formation of trichlorobiphenyls appeared to be enhanced. However, there was no evidence for the formation of chlorinated derivatives of MDI or polyMDI, nor chlorination of the o-C6H4Cl2 to form trichlorobenzene. Of course, this does not preclude formation of very low levels of such products at concentrations below the detection limits of the applied analyses which can still be
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ASSOCIATED CONTENT
S Supporting Information *
Text describing characterization of arylisocyanide dichlorides, FTIR spectroscopy, 1H and 13C{1H}-NMR spectroscopy, mass spectrometry, thermal analysis, available chlorine, and irradiation of hexachloroplatinate(IV) in DMSO, tables listing characteristic IR bands for arylisocyanide dichlorides (Table S.1), 1H and 13C{1H} NMR data for arylisocyanide dichlorides (Table S.2), mass spectra of arylisocyanide dichlorides (Table S.3), and product analysis from photolysis of phenyl isocyanide dichloride with MDI or polyMDI with components in solution before and after reaction identified by GCMS (Table S.4), and figure showing infrared spectra of p-tolylisocyanide dichloride recorded 1, 4, and 5 days after synthesis (Figure S.1). This material is available free of charge via the Internet at http:// pubs.acs.org.; 2521
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ACKNOWLEDGMENTS
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REFERENCES
Huntsman Polyurethanes and WestChem are thanked for the provision of a studentship (J.C.) and research support. The EPSRC is thanked for access to the EPSRC U.K. National Electron Paramagnetic Resonance Service at The University of Manchester. Mr. Michael Beglan (University of Glasgow) is thanked for assistance with the available chlorine measurements.
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