LETTERS TO THE EDITORS 58;

The Thermal Degradation of 66 Nylon: Further Studies on the Pyrolysis of Di-n- butyl Adipamide

In a previous communication' it was shown that A',ili'-diakyl adipamides (alone among a series of amides examined) decompose on heating a t 330- 350 O producing substantial quantities of carbon dioxide, amines, and com- plex mixtures of products of (then) unknown constitution. 66 Nylon poly- mer, degraded a t temperatures of 290" and upward, has two notable properties resembling those of the pyrolysis products of dialkyl adipamides, viz. an ultraviolet absorption maximum a t about 280 mp (290 mp in acid solutionz) and a positive reaction for pyrroles with the Ehrlich reagent. An infrared absorption band at 1735 cm.-' also occurs in the dibutyl adip- amide pyrolysis product, and in degraded 66 nylon.

The resemblance between these systems prompted a detailed examina- tion of the pyrolysis of dibutyl adipamide, as a model of the 66 nylon system. The preliminary investigation' had led to the suggestion that certain of the above-mentioned phenomena were linked, being due to the possible presence of derivatives of indole in the pyrolysis residues. De- tailed examination has now shown that the Ehrlich color reaction and the ultraviolet absorption a t 280 mp are due to different molecular species; work on thermally degraded nylon shows a similar differentiation.

Dibutyl adipamide pyrolyzate has been separated by distillation into a number of broad cuts, and that boiling over the range 50-150"/0.1 mm. has been intensively studied. Fractional distillation gave a series of products which were examined by ultraviolet, infrared and mass spectrometry. Significant absorption a t 278 mp (increasing in intensity and peak wave length in acid solution) was concentrated in the higher boiling fractions while the Ehrlich reaction occurred over the whole distillation range. Infrared absorption at 1575 cm. --I tentatively assigned to an aromatic system was likewise concentrated in the higher boiling regions; but that a t 1735 cm.-' was widely spread throughout the fractions as was an absorp- tion due to substituted amide linkage. Mass spectral peaks a t 140 and 196 appeared particularly significant, and were correlated tentatively with the 1735 cm.-' absorption band as being possibly due to strained ring (e.g. , five membered) ketones of possible formulae C9HI60 and Cl3HP4O, respectively. Such ketones were subsequently isolated through their semicarbazones, and their structures were determined by degradation and synthesis.

Oxidation of the ketone CgH160 with potassium permanganate gave a ketomonocarboxylic acid CgH1603 identical with the known 8-ketopelargonic acid3; it must therefore have been 2-n-butyl cyclopentanone (111, R, = Bu, Rz = H). Ketone C13H240 was oxidized by nitric acid and ammonium metavanadate to valeric and n-butyl succinic acids; these products can arise only from 2,4- and 2,5-di-n-butyl cyclopentanones. The orientation was established by comparison of the isolated ketone and its semicarbazone with synthetic 2,5-dibutyl cyclopentanone (111, Rl = R2 = Bu) and its

588 LETTERS TO THE EDITORS

semicarbazone. This ketone and 2-n-butyl cyclopentanone were first synthesized by Mr. 0. B. Edgar of these laboratories by condensation of n-butaldehyde with cyclopentanone, followed by hydrogenation of the di- and monocondensation products, respectively.

Attempted hydrogenation of the aromatic material in the higher boiling fractions failed to remove the absorption at 278 mp or 1575 cm.-', but destroyed the substance responsible for the Ehrlich reaction. Examination of the products after hydrogenation revealed that the compound having these absorption characteristics was basic and present in only a small amount. Previous failure to recognize its basic character had been due to extensive resinification which occurred when the initially distilled pyrolysis products were treated with acid reagents. The relevant compound has been isolated and its formula has been shown (by mass spectrometric and chemical means) to be CI4Hl9N. Its structure has been determined by degradation with neutral potassium permanganate to (a) acetic, propionic, and butyric acids, and (b) nitrogen containing carboxylic acids, showing an absorption maximum at 280 mp. The latter products gave a positive Skraup reaction (orange with FeS04) suggesting the possible presence of pyridine-a-carboxylic acids. This clue to the nature of the nucleus was confirmed by decarboxylation to mixed alkyl pyridines. The probable dis- position of the residual carbon content about the pyridine nucleus was indicated by (1) the absence of an aromatic C-H bending-frequency in the infrared spectrum, (2) the formation of butyric acid on oxidation, and (3) the known absence of isolated olefinic bonds, leading to the struc- ture of 2-n-propyl-3 : 4: 5 : 6-bis-trimethylene pyridine (I).

(1) (11) (111)

This was confirmed by unambiguous synthesis of (I) from 2-cyclopentyli- dene cyclopentanone and n-butaldehyde ammonia followed by spectro- scopic comparison of the bases, and comparison of the ferricyanides and platinichlorides. The ultraviolet absorption spectra of the synthetic base in neutral and in acid solution, and its infrared absorption spectrum are reproduced in Figures 1 and 2. These were measured on a Cary photoelec- tric spectrophotometer and a Grubb-Parsons double beam infrared spec- trometer (Model S 4), respectively.

Investigation of the Ehrlich positive reaction component in the pyroly- zates by hydrogenation of the neutral portion as above described, followed by extraction of the newly formed basic products and dehydrogenation, provided evidence (mass spectroscopic and Ehrlich color spectra) for the presence of 2-methyl-N-butyl pyrrole (11) and possibly of a butyl homo- logue. Quantitative measurements of the Ehrlich colors have shown that these pyrroles occur to an extremely small extent.

LETTERS TO THE EDITORS 589

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WAVELENGTH ( m p )

Fig. 1. Ultraviolet absorption of synthetic 2-n-propyl-3,4,5,6-bis-tri- methylene pyridine. (-) Dissolved in methanol, (---) dissolved in methanol + concentrated HCI (10:90 v/v).

WAVELENGTH L) Fig. 2. Infrared absorption of synthetic 2-n-propyl-3,4,5,6-bis-trimethylene

pyridine (0.05 mm. liquid layer; NaCl prism).

Other minor products of the pyrolysis include N-butyl cyclopentylamine, dibutylamine, saturated and unsaturated hydrocarbons with the carbon skeleton of (III), n-valeronitrile, and appreciable quantities of N-butyl-n- valeramide.

590 LETTERS TO THE EDITORS

Direct evidence for the existence in thermally degraded nylon of species analogous to the principal ones for dibutyl adipamide has been obtained (1) by the isolation of pyridine bases analogous to (I) from zinc dust pyrolyzates, (2 ) from the ultraviolet absorption of degraded polymer, (3) by the occurrence of a carbonyl absorption a t 1735 cm.-', ( 4 ) from the formation of cyclopentene and cyclopentane derivatives on zinc dust fusion, and (5) from the development of the Ehrlich color.

Similar species are present in the high molecular weight material (MW up to 2000) which represents a large proportion of the dibutyl adipamide pyrolysis product. The formation of this, which resembles gel formation in the case of polymer, shows that the reactions are predominantly synthetic in character.

The close resemblance of the pyrolysis products of dibutyl adipamide and those of 66 nylon, and the determination of the named structures in the former case, shows that the substituted adipamide decomposition re- action is a highly complex process involving fission of each bond in and ad- jacent to the amide linkage, and the participation in the synthetic processes of the a-methylene group in the amide segment. The crosslinking which occurs in the case of 66 nylon involves a similar train of reactions. It is not reversible by hydrolysis, and the simple amide-linking hypothesis, dis- cussed, for example, by Hill,4 plays a t most only a minor part in its occur- rence; the crosslinking dehydration mechanism of Achhammer el aL5 would likewise seem to be excluded. It is hoped to publish a full account of this work, together with other investigations on the mechanism of the reaction a t a future date.

Grateful acknowledgment is made of the extensive assistance received in this work from Mr. J. H. Beynon and Mr. M. St. C. Flett of these laboratories, in the measure- ment and interpretation of mass and infrared spectra, from Dr. A. F. Wells in the com- parison of X-ray powder diagrams, and from Mr. 0. B. Edgar by the provision of samples and in valuable discussions.

References

(1) I. Goodman, J. Polymer Sci., 13,175 (1954). (2) A. M. Liquori, A. Mele, and V. Carelli, J. Polymer Sci., 10,310 (1953). (3) P. Dupont and G. Chavanne, Bull. soc. chim. Belges, 42, 537 (1933). (4) R. Hill, Chem. 4 Industry, 1954,1083. (5) B. G. Achhamrner, F. W. Reinhart, and G. M. Kline, J. Appl. Chem., 1,301 (1951).

ISAAC GOODMAN Research Department, Imperial Chemical Industries Ltd. Dyestuffs Division, Hexagon House Blackley, Manchester 9, England

Received May 5, 1955