'THE THERMAL DISSOCIATION OF BLOCKED TOLUENE DIISOCYANATES GEORGE R . G R I F F I N AND LAWRENCE J. W I L L W E R T H Auco Gorp., Wilmington, Mass.
Unblocking temperatures for some blocked diisocyanates based on an 80/20 mixlure of toluene-2,4diisocyanate and toluene-2,6-diisocyanate were determined both by use of infrared spectroscopy and by employing a test based on carbon dioxide evolution resulting from reaction with water. Infrared studies showed that blocked isocyanates based on phenols, alkyl and aryl thiols, and P-dicarbonyl compounds dissociated when heated to produce a free isocyanate group. The adduct based on tert-butyl alcohol, however, produced only isobutylene, carbon dioxide, and the corresponding amine when heated. Thermal stability was decreased when the negative charge density was decreased on the bridge atom linking the adduct molecule to the toluene diisocyanaie, and lowest dissociation temperatures were observed in general with negatively substituted phenols. Dissociation temperatures were also reduced when a solvent-such as a polyalkylene glycol---containing a strongly nucleophilic group was employed.
ISOCYANATE is a compound which contains no free isocyanate groups, but which? though relatively inert at room temperature, wdl react a t elevated temperatures in a manner which is similar to that of a free isocyanate (73). The formation of such compounds from isocyanates may be illustrated by the equation
A
BLOCKED
RNCO
+ HZ
+ RNHCOZ
(1)
and the blocking agents (HZ) include such compounds as phenols and thiols ( 7 4 , tertiary alcohols ( g ) , secondary aromatic amines (70), sodium bisulfite, imides ( 1 4 , and 1,3dicarbonyl compounds (75). The reaction of a blocked isocyanate with a compound containing a n active hydrogen atom, such as a n alcohol or water, could conceivably involve either a two-step process in which there is an initial unimolecular thermal dissociation to free isocyanate, or a direct bimolecular displacement reaction. Gaylord and Sroog (7) have shown that the reaction of a n alcohol \vith an unsubstituted urethan proceeded by a n ester interchange or displacement mechanism, but that comparable reactions involving X-monosubstituted urethans proceeded through a primary dissociation to the free isocyanate. The ester interchange reaction has been used for preparing carbamates of higher alcohols (8). Since blocked isocyanates of the urethan or thiourethan classes are Ar-substituted, they would be expected to react through a n initial dissociation step. This is supported by the observation of Dyer and Newborn (5) that aryl biscarbamates derived from 1-butanol, 2,2,-dimethyl-l-propanol, and 1,6hexanediol dissociate to the corresponding isocyanate a t 300' C., though admittedly this is well above the temperature normally used with blocked isocyanate.
The purpose of this study is to establish the mechanism through which blocked isocyanates commonly function a t elevated temperatures and to show how such factors as structure and chemical environment relate to their reactivity. Experimental
Preparation of Blocked Diisocyanates. The diisocyanate upon which this work was based was a mixture of isomers containing 80% toluene-2,4-diisocyanate and 20% toluene2,6-diisocyanate. The blocked isocyanates were prepared by dissolving the blocking agent (HZ) in a suitable solvent containing a basic catalyst-3 to 570 on the weight of isocyanate -and adding a stoichiometric amount of toluene diisocyanate. After the exotherm had subsided, the solution was heated a t 60' to 100' C. for several hours. Upon cooling, the product usually separated as a solid, though in some instances a second liquid was added to effect precipitation. Where possible. the solid was purified by recrystallization. The preparations are summarized in Table I. Boric acid did not appear to form the expected mixcd anhydride with the isocyanate group, as the product had a substantially higher nitrogen content than the theoretical value of 13.0% which would result from simple addition of stoichiometric ratios of boric acid and toluene diisocyanate. Aries (7) reported that \\,hen a mixture of boric acid and toluene diisocyanate or other diisocyanate is heated to 90' C., either with or without a basic catalyst. condensaricn occurs with the loss of carbon dioxide, and a boron-nitrogen pol! mer is formed 300 chloride Boric acid TetrahydroNone None None >300 furan a Catalysts: A , triethylamine; B , triethylenediamine ( D A B C O ) ; C , sodium methoxide. Based on a stoichiometric ratio of 2 H 3 B 0 3to 3 C 9 H B 0 2 Wand 2 assuming loss of 3C02--see Erperimental.
9.85 0.85 4.87 7.10 6.45 5.65 3.99 5.91
9.91: 9 91 0.35; 10.24 4.87; 4.89 6.97; 7.07 6.50: 6.56 5.87; 5.92 3.881 14.11 6.43; 16.30
A B B A
Ligroin None None Ligroin
Resorcinol Phloroglucinol 1-Dodecanethiol Benzenethiol Ethyl acetoacetate Diethyl malonate e-Caprolactam Ethyl carbamate
Table 11,
Unblocking Temperatures of Blocked Diisocyanates Based on Toluene Diisocyanate Unblocking Temperature, C. Infrared Carbon Dioxide In In methoxyTest parafin polyethylene UncataCataoil glycol lyzed lyzed
Blocking Agent
Ethanol 2-Methyl-2-propanol m-Cresol o-Nitrophenol p-Chlorophenol Guaiacol Resorcinol Phloroglucinol 1-Dodecanethiol Benzenethiol Ethyl acetoacetate Diethyl malonate e-Caprolactam Ethyl carbamate
No'NCO No'ACO up to 174 up to 174 105 70
152 125
85
80
...
...
81 84 NoNCO up to 154 No NCO up to 100 125
74 51 110
110 116 95 97
106 110 95 97
79
85
85
108 NoNCO up to 154
132
140
125 70
120 70
Boric acid
Table 111.
160 150
... .., ...
...
Solution Temperatures of Blocked Isocyanates in Polyester and in Polyether Temperature ofoComplete Solution, Toluene Diisocyanate Adduct
m-Cresol o-Nitrophenol p-Chlorophenol Guaiacol Resorcinol Phloroglucinol 1-Dodecanethiol s-Caprolactam Ethyl carbamate Boric acid
266
e.
In polyester
In polyether
65
69
90
83 85
70 63
122 Insol. 73 Partially .ol. Insol. Insol.
62 113
Insol. 73 Insol. Insol. Insol.
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
Toluene Toluene Toluene Chloroform carbon tetrachloride None None Ethanol None Acetone Methanol Toluene None
+
(16.3)5
16.57; 16.72
Dissociation Temperature of Blocked Isocyanates by Infrared Spectroscopy* Perkin-E1mer 221G infrared spectrophotometer equipped with a Model 21 horizontal cell assembly and hot-stage attachment was employed. T h e hot stage was calibrated to provide temperatures over a range of 25 to 200" =t 1 " C. T h e samples were prepared by abrading a suspension of the adduct in mineral oil or in methoxypolyethylene glycol (mol. wt. 550) between ground glass plates until completely dispersed, after which the mull was transferred to a sodium chloride window in the hot stage. T h e spectrophotometer was set to scan the region of 2500 to 1700 cm.?, using programmed slits set a t 2x and providing a resolution of 9.27 cm.?. T h e temperature of the sample was progressively increased in 5' increments and was held for 5 minutes at each level before readings were taken. T h e temperature a t ivhich the first detectable absorption at 2270 cm.-' (NCO band) appeared was taken as the unb1ockir.g temperature. A progressive increase in intensity of this isocyanate band was observed as the temperature \vas raised beyond this point. A typical set of curves representing the dissociation of the toluene diisocyanate-guaiacol adduct dispersed in methoxypolyethylene glycol is shown in Figure 1. Minimum Unblocking Temperature Based on Carbon Dioxide Evolution. T h e minimum temperature a t which a significant amount of reaction occurred between the blocked isocyanates and water was determined as follows.
A 175 X 22 mm. test tube was connected through a side arm to a fritted glass sparge tube which was immersed in a satu-
rated solution of barium hydroxide. Provision was made so that the system could be continuously sparged with a slow stream of argon. T h e test tube was charged with 10.0 grams of methoxypolyethylene glycol (mol. wt. 550), 0.01 equivalents of the blocked isocyanate, and 5.0 grams of molecular sieve (Linde 4X) which had been previously saturated with water vapor at room temperature and which contained approximately 217, water by weight. T h e use of the molecular sieve simplified the handling and weighing of the water and, by providing same element of contiol over the rate of its release, was designed to aid in the retention of the water in the system during the prolonged heating periods, particularly at temperatures above 100' C. I n parallel runs,
a similar charge was employed in which 0.06 gram of iVmeth>l morpholine was included to determine the effect of a basic catalyst on the unblocking temperature. The charge was heated a t the rate of 3' C. per minute, and that temperature was recorded a t which the first turbidity appeared in the barium hydroxide solution. Results
The minimum temperature a t which carbon dioxide was liberated in the reaction of blocked diisocyanates with water was determined for 14 compounds, and the results are summarized in Table 11. The lowest unblocking temperatures were observed with phenols, especially where negative substituents were present in ortho or para positions. T h e boric acid adduct probably contained free isocyanate groups as previously indicated from analysis. The lowered reactivity of this adduct, as indicated by the elevated unblocking temperature, could be caused both by the extreme insolubility of this compound and by steric hindrance of the residual free groups, since the unhindered groups in the para position would be expected to be more extensively involved in the condensation reaction. With the exception of adducts based on 2-methyl-2-propano1, o-nitrophenol, and p-chlorophenol, the catalytic effect of the AV-methylmorpholinewas not pronounced. When mulls of selected blocked diisocyanates were heated, and changes in the infrared spectra followed a t progressively increasing temperatures, all adducts based on phenols, alkyl-
2400
2300
2200
2100
2000
1900
1800
1700
WAVE NUMBER, C M - I
Figure 1 . Thermal dissociation of toluene diisocyanateguaiacol adduct in polyethylene glycol
and arylthiols, and 0-dicarbonyl compounds dissociated to free isocyanates (Table 11). Suspensions or solutions of most of the adducts in methoxypolyethylene glycol showed unblocking temperatures somewhat higher in the carbon dioxide test than in those tests based on infrared measurements, though exceptions are noted in the case of ethyl acetoacetate and ethyl carbamate adducts. Results from the tlvo test methods applied to the 2-methyl-2-propanol adduct are as might be expected, since in the discussion follo\ving it is shown that this compound does not dissociate to yield free isocyanate when heated. Dissociation temperatures measured by infrared \cere invariably lower in methoxypolyethylene gl)-col than in mineral oil. Indeed, adducts based on ethyl acetoacetate and on diethyl malonate showed detectable dissociation only in the glycol Lvithin the temperature range employed. Infrared studies indicated that the adducts based on ethyl carbamate and on 2-methyl-2-propanol did not show detectable dissociation even when heated to temperatures well in excess of those required for carbon dioxide evolution in the reaction with water. An analysis of the products of thermal decomposition of the adduct based on 2-methyl-2-propanol showed that the reaction followed quite a different path, for nearly stoichiometric amounts of isobutylene were recovered. Gaylord and Sroog (7) have reported that attempts to react ethyl urethan lvith 2-methyl-2-propanol in the presence of a n acid catalyst also produced isobutylene and not the expected tert-butyl urethan. Structure of Toluene Diisocyanate-Malonic Ester Adduct. The dissociation of the 1:3-dicarbonyl adducts (acetoacetic ester and malonic ester) to the isocyanate is particularly interesting since it could represent a C-C bond dissociation if the carbonyl substituent is in the methylene group. Infrared studies were therefore conducted to determine whether C-acylation (keto) or 0-acylation (enol) was involved. If 0-acylation \cere involved, cis-trans isomerism should exist, but no absorption \yas observed either in the 990 to 965 cm.-l region (trans) or in the 820 to 675 cm.-' region (cis) which would include any upward shift produced by conjugation Lvith the carbonyl group ( 3 ) . Strong absorption a t 3250 cm.-1 corresponding to S-H stretch did not distinguish the amide structure (C-acylation) from the urethan structure (0-acylation) but in the C=O stretch region. a strong absorption observed a t 1745 cm.-' corresponded to the ester linkage in the malonic ester rather than to a urethan linkage (1690 to 1736 cm.-l) such as jvould result from 0acylation. Furthermore, a second strong band a t 1660 cm.-] corresponded to the Amide I bond in primary and secondary amides, and a band a t 1540 cm.-' corresponded to the Amide I1 bond of a secondary amide ( d ) . The spectroscopic evidence thus points to a structure for the malonic ester adduct which \\.odd correspond to C-acylation. Solubility of Diisocyanate Adducts in Polyester and Polyether. A measure of the relative solubilities of selected blocked diisocy-anates in liquid polyester and in polyether was provided by determining the lowest temperature a t which these materials completely dissolved. \Vitco Fomrez R400 (\Vitco Chemical Co.), having a n hydroxyl number of 410, was selected as a representative polyester, and Atlas G-2410 (Atlas Powder Co.), ivhich is a propylene oxide adduct of sorbitol having a n hydroxyl number of 495, was selected as the polyether. Dilute dispersions of each adduct in the polyester and in the polyether were heated on a hot stage a t a rate of approximately 3' C. per minute, and that temperature was recorded VOL.
1
NO.
4 D E C E M B E R 1 9 6 2 267
a t which solution was complete. The results are summarized in Table 111. Heating was terminated a t 140’ C. if the material was insoluble. Discussion and Conclusions
0
I1
The ease with which a blocked isocyanate l i N H C Z will dissociate to the isocyanate will depend to a large extent upon the strength of the bond between Z and the carbonyl group. Since the carbonyl carbon atom bears a partial positive charge, the negative charge density on the atom in group Z attached to it will influence the extent of ionic character of this bond. The greater this charge difference the greater the expected strength and, therefore, the higher the unblocking temperature. This is supported by the observation that when the oxygen bridge atom in an adduct alcohol molecule such as ethanol is replaced by the less nucleophilic sulfur atom, the unblocking temperature is substantially lowered. When the alkyl group in the alcohol is replaced by a n aryl group (phenols), the reduction in negative charge density through resonance again lowers the temperature of dissociation. I n further support of this? Mukaiyma and Iwonami (72) have shown that in a series of substituted N-phenylbenzylcarbamates, the rate of thermal dissociation increases with increasing electron attracting power of the substituent on the ring, excluding steric effects. The thermal decomposition of the blocked isocyanate based on tert-butyl alcohol produces no free isocyanate and may involve an initial dissociation to a tert-butyl carbonium ion, the driving force of the reaction possibly being the resonance stabilization of this ion through hyperconjugation. This dissociation would be followed by deprotonation of the ion to produce isobutylene and decarboxylation of the residual aryl carbamic acid to produce carbon dioxide and the corresponding amine. As has been previously noted, the dispersing medium strongly affects the dissociation temperature, and values were invariably lower in methoxypolyethylene glycol than in mineral oil. Though the adducts are in general more soluble in the glycol than in mineral oil, this phenomenon does not appear to be simply a solution effect, since a solution of guaiacol-toluene diisocyanate adduct was prepared in diphenyl. and the unblocking temperature was 107’ C., which is of the same order of magnitude as was observed for the paraffin oil mull, and by no means approaches the value of 60’ C. observed in methoxypolyethylene glycol. The solvent can have a pronounced effect on the reaction rates of isocyanate systems. Thus, Baker and Gaunt ( 2 ) observed that both methanol and ethanol reacted more rapidly with phenyl isocyanate in benzene than in di-n-butyl ether. Mesrobian and coworkers ( 6 ) conducted extensive studies on
268
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
the effect of solvents on the rates of reaction of phenyl isocyanate with selected alcohols, particularly methanol, and reported that the reaction was substantially more rapid in relatively nonpolar solvents such as benzene and toluene than in strongly nucleophilic solvents such as methyl ethyl ketone and dioxane. The present study suggests that a solvent may actively participate in the thermal dissociation process through aiding in the deprotonation of the blocked isocyanate. The more strongly nucleophilic the solvent the greater this effect would be, and in the polyglycol, strongly nucleophilic oxygen atoms are present both in the ether linkages and in the hydroxyl groups. A solution of the guaiacol-toluene diisocyanate adduct in a 73:27 mixture of diphenyl ether and biphenyl also showed an unblocking temperature a t 107’ C. This might be expected since the oxygen atom in diphenyl ether would be appreciably less nucleophilic than in an aliphatic ether because of resonance in the aromatic structure. Support of this is found in studies reported by Mukaiyma and Hoshino (77) in which the rate of dissociation of cinnamyl-ATphenylcarbamate a t 165’ C. to the corresponding isocyanate was increased by the addition of amines, which are strong deprotonating agents. Furthermore, the rate of dissociation increased as the basicity of the amine increayed. Acknowledgment
The authors are indebted to Patrick L. Sciaraffa and to Roland J. Bergeron for assistance in the preparation and evaluation of the blocked isocyanates, and to Jean P. Phaneuf for assistance in the infrared analytical studies. literature Cited
(1) Aries, R. S., U. S. Patent 2,945,841 (July 19, 1960). (2) Baker, John W., Gaunt, J., J . Chem. SOC.1949, 27-31. (3) Bellamy, L. J., “The Infrared Spectra of Complex Molecules,” 2nd Ed., pp. 36-51, Wiley, New York, 1958. (4) Zbid., pp. 179, 205, 222. (5) Dyer, E., Newborn, G. E., Jr., J . A m . Chem. Soc. 80, 5495-8 11958). (6)’ EpLriam, S., Woodward, E., Mesrobian, R. B., Zbid., 80, 1326-8 (1958). (7) Gaylord, N. G., Sroog, C. E., J . Org. Chem. 18, 1632-7(1953). (8) I. G. Farbenind, A. G. (Wilhelm Muhl), Ger. Patent 565319 (April 25, 1931). (9) Mastin, T. G., Seeger, N. V. (to Goodyear Tire and Rubber Co.), U. S. Patent 2,683,728 (July 13, 1954). (10) Zbid., 2,683,727. (11) Mukaiyma, T., Hoshino, Y., J . A m . Chem. SOC.78, 1946-8 (1956). (12) Mukaiyma, T., Iwonami, M., Zbid.,79, 73 (1957). (13) Petersen, S., Ann. Chem. 562, 205-29 (1949). (14) Seeger, N. V., Mastin, T. G. (to Goodyear Tire and Rubber Co.), U. S. Patent 2,801,990 (Aug. 6, 1957). (15) Zbid.,2683,729 (July 13, 1954). \
,
I .
RECEIVED for review March 12, 1962 ACCEPTEDOctober 10, 1962
