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Research on Boron Polymers 1

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W. L. RUIGH and W. R. DUNNAVANT

Chemistry Research Branch, Aeronautical Research Laboratory, Wright-Patterson Air Force Base, Ohio 3

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F. C. GUNDERLOY, Jr., N. G. STEINBERG, MICHAEL SEDLAK, and A. D. OLIN

Downloaded by UNIV ILLINOIS URBANA-CHAMPAIGN on October 7, 2015 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch027

Department of Chemistry, Rutgers University, New Brunswick, N. J.

Research was undertaken to explore the possibility of developing inorganic or semi-inorganic polymers stable at high temperatures. The film-forming polymer resulting from the reaction of a bifunctional isocyanate and a boronic acid is a polyurea containing physically bound boron rather than a polymeric boronamide. This program led to the development of a new borazole synthesis based on the reaction of alkyl or aryl boron dichlorides and amines. The present effort is directed at forming linear polymers from boron-substituted borazoles. In recent work on B-tri-ß-chlorovinylborazole, the compound on refluxing with polar solvents gave a glassy product. This material, in addition to β-dichlorovinyl boronic acid and possibly other sub stances, gives an insoluble fraction which may be the desired linear polymer. Although the program has not developed heat-stable polymers, its by -products are of interest in the field of boron polymers and more generally in the synthesis and reactions of a number of unusual organoboron compounds.

The work reported here was initiated i n 1953, aimed at a study of possible inorganic polymers for high temperature applications. The initial concept was to prepare con densation polymers from the 2,4-dihydroxyborazene stated to have been prepared from dibromoborazene (26). Before any experimental work was undertaken, an extensive literature survey was made and the original borazene approach was abandoned because previous work seemed to indicate that the borazenes as a class were hydrolytically unstable. This literature survey covered the general field of inorganic polymers (1719-21). It was decided to concentrate the experimental work on possible polymers based on boron. The polyester type of polymer formed from boric acid, glycols, and glycerol dates 1

Present address, Division of Chemistry S R Q , Office of Aerospace Research, A i r Re search and Development Command, Washington 25, D . C . Present address, Chemical and Physical Research Division, Standard Oil Co., Cleve land 28, Ohio. Present address, Esso Research and Engineering Co., Linden, N . J . Present address, Merck Sharp & Dohme Laboratories, Rahway, N . J . Present address, Research Department, Socony M o b i l Oil Co., Paulsboro, N . J . Present address, Toms River Chemical Corp., Toms River, N . J . 8

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In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

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back at least to 1866 {22). Polymeric glycol borates and esters of boronic acids have been investigated (24)- A remarkably careful study i n this field of condensation reac tions of orthoboric acid with diols was reported recently {12). The area, including polyamide-type polymers, was also surveyed i n 1952 {9). Upson {25) described very stable polymers, white film-forming solids, believed to be boronamides derived from the reaction of boronic acids with bifunctional isocyanates. The polymers had softening points up to 3 0 0 ° C , and a relatively high degree of hydrolytic stability and were soluble i n phenol and formic acid. The Upson boronamide synthesis was patterned on the reaction discovered i n 1906, where an isocyanate reacts with a carboxylic acid to form the amide and carbon dioxide (7, 15, 16). The reaction is markedly catalyzed b y tertiary amines. Upson's reaction was repeated using toluene-2, 4-diisocyanate and also hexamethylene diisocyanate i n benzene solution. The former gave insoluble powdery polymers, but the latter gave a mixture of powder and a tough film which could be peeled off the side of the reaction flask. A s Upson also found, the reaction products contained somewhat less than the theoretical percentage of boron. F o r example, the polymer from hexamethylene diisocyanate contained 4.06 boron (theoretical 5.36%). These materials, including the tough film-forming material, lost little boron when extracted with benzene. When methanol was used, almost a l l the boron was extracted (analysis 0.22% B ) after 70 hours but the film was left unimpaired. I t thus appeared that the product, instead of being a polyboronamide, was polyhexamethyleneurea, which was formed from water and the hexamethylene diisocyanate. The water would readily be formed from the cyclization of benzeneboronic acid to give water and the triphenylboroxine. The product was probably polyhexamethyleneurea prepared i n 1942 {10) and later studied for its infrared absorption {8). Infrared studies on the material as inter preted b y C . E . Erickson were i n accord with the polyhexamethyleneurea formula tion. Preparation of Boronic Acids The hydrolytic instability of polyesters from glycols and polyamides from diammines led next to an attempt to prepare a boronic acid i n which an odd 2- or 3-carbon chain was terminated with a strong donor group such as dimethylamine. I f such a compound could be prepared, one would have an internally quasi-chelate group and a quadricovalent boron atom with two hydroxyls as functional groups. These com pounds, for which the trivial name of "scorpion" boronic acids was coined, would be related to hydrolytically stable cage-structured triethanolamine borate ester {4, H)> The first attempts, using a preferential Grignard reaction on l-bromo-3-chloropropane {1) followed by treatment with methyl borate, failed to give any trace of the desired γ-chloropropylboronic acid. T h e Grignard on γ-dimethylaminopropyl chloride, following the general technique of Miesher and M a r x e r , also met with failure {13, 14, 20). Finally allylboronic acid was prepared by the Grignard procedure {19, 21). E a r l y attempts to isolate this acid met with failure, but b y working up the reaction mixture at low temperature under nitrogen and i n the presence of 0.1% inhibitor such as hydroquinine, success was attained and the acid melting at 75°C. was isolated. Light or traces of peroxide decomposed the acid into propylene (with a trace of hexadiene) and a mixed boric acid-boric anhydride mixture {18, 20). A l l attempts to add hydrobromic acid to allyl boronic acid to form the desired γ-bromopropylboronic acid met with failure due to its instability to both heat and peroxides. Interest then turned to repeating the work of Booth and K r a u s (3), who reported the formation of an oily polymer stable to boiling alkali and peroxide. They prepared the polymer from butyldichloroborane and liquid ammonia i n the presence of sodium and reported a


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boiling point of 100° to 110° at 5 microns. The authors' results gave a mobile liquid boiling at 34°C. at 5 microns, which was immediately identified as B-tributylborazene. B o t h this work and the preparation of the intermediate butylboron dichloride paralleled the effort of Buls (a). A t the time this work was done we were not aware of Buls' (S) classified reports, which are now declassified and available from A S T I A . A n i m proved synthesis of the alkyl halides was later described by Buls (6). When sodium was omitted, the reaction of alkylboron dichlorides and ammonia gave yields of up to 9 0 % of substituted borazenes.


Borazenes Efforts were next directed toward the formation of borazoles with α-unsaturated alkylene groups on the boron, which might be transformed to long-chain polymers through reduction of the borazene resonance. A n y tendency to fill the vacant orbitals of the boron from the π electrons or the a double bond should enhance the possibility of linear vs. cyclic polymer formation. The most readily available intermediate was β-cMorovinylborane dichloride, most readily prepared by passing a metered flow of acetylene and boron trichloride over a mercurous chloride-charcoal catalyst (2). A n apparatus was designed and put into operation, whereby acetylene was passed into a continuously recirculating flow of boron trichloride (21). When ammonia gas was passed into a solution of ^-chlorovinylborane dichloride in ether, a 7 0 % yield of crude tri-/?-chlorovinylborazene was obtained. Earlier the reaction was tried i n benzene solution, but the product was difficult to purify and gave insoluble materials, presumably polymers, when refluxed i n benzene. T h e ether-prepared material appeared stable when refluxed i n the polar solvents dioxane and acetone and the solvent when evaporated gave a colorless glass. This was presumed at first to be the desired polymer, i n spite of one negative viscosity determination i n dimethylformamide. T h e work was later repeated with acetone; only 1 0 % of the material was an ether-insoluble cream-colored solid. This solid may be a polymer but a Rast camphor molecular weight determination gave results open to question. I t was shown, however, that tri-/?-cMorovinylborazene i n refluxing ace tone rapidly picked u p moisture, and a hydrolysis product, β-cMorovinylboronic acid, was identified i n the ether-extractable portion of the dried product. The dryness of the acetone (dried over Drierite) was also an uncontrolled factor. Conclusions jB-tri-^-chlorovinylborazole under certain conditions gave products that appeared to be polymers. This conclusion is primarily due to their insolubility i n all solvents ex cept dimethylformamide. There is no direct structural evidence, however, of either ring or chain polymer formation. Further work on more stable borazoles lacking the β-halogen group will have to be undertaken before the problem can be solved. F u r t h e r more, the possibility of cross-linked polymers being formed through the v i n y l function of the side chain must also be borne i n mind. Acknowledgment The cooperation and encouragement of Peter A . V a n der Meulen and Charles E . Erickson, as faculty consultants, during the part of this work carried out at Rutgers University, are gratefully acknowledged, as is the active participation of the latter i n the interpretation of infrared spectra, and the editing of reports. Mass spectra were obtained from the Esso Laboratories, Linden, N . J . , through the courtesy of W . J . Sparks. In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

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Literature Cited


(1) (2) (3) (4) (5)

Allen, C. F. H., Wilson, C. V., Ball, W. L., Can. J. Research 9, 432-5 (1953). Arnold, H . R., U. S. Patent 2,402,590 (June 25, 1946). Booth, R. B., Kraus, C. Α., J. Am. Chem. Soc. 74, 1415-7 (1952). Brown, H . C., Fletcher, Ε. Α., Ibid., 73, 2808-13 (1951). Buls, V. W., et al., "Boron Compounds as Toxicants," Final Report, April 30, 1954, and Chemical Corp. Contract CML-4564, Project No. 4-08-03-001. (6) Buls, V. W., Davis, O. L., Thomas, R. T., J. Am. Chem. Soc. 79, 337-9 (1957). (7) Diekmann, W., Breest, F., Ber. 39, 3052-5 (1906). (8) Dyer, Elizabeth, Bartels, G. W., Jr., J. Am. Chem. Soc. 76, 591-4 (1954). (9) Gould, E. S., Urs, S. V., Overberger, C. C., Martinez, F., Brill, R., Final Rept., Signal Corps Project No. 32-2005-34 (1952). (10) Hanford, W. E., U . S. Patent 2,292,443 (Aug. 11, 1943). (11) Hein, F., Burkhardt, R., Z. anorg. allgem. Chem. 268, 159-68 (1952). (12) Laubengayer, A. W., Hayter, R. G., Watt, W. J., Abstracts, 132nd Meeting, ACS, New York, 1957, p. 10N. (13) Marxer, Α., Helv. Chim. Acta 24, 209E-25E (1941) (supplemental volume). (14) Miesher, K., Marxer, Α., U . S. Patent 2,411,664 (Nov. 26, 1946). (15) Naegli, C., Tyabji, Α., Helv. Chim. Acta 17, 931-57 (1934). (16) Ibid., 18, 142-60 (1935). (17) Ruigh, W. L., Erickson, C. E., WADC Tech. Rept. 55-26, Part I, ASTIA No. A D 63440, Office of Technical Services, PB 111,689. (18) Ruigh, W. L., Erickson, C. E., Gunderloy, F. G., Sedlak, M., First Regional Meeting, Delaware Valley Section, ACS, p. 60, 1956. (19) Ruigh, W. L., Erickson, C. E., Gunlerloy, F. G., Jr., Sedlak, M., WADC Tech. Rept. 55-26, Part II, AD 77231, PB 111,892 (1955). (20) Ruigh, W. L., Gunderloy, F. G., Jr., Sedlak, M., Van der Meulen, P. Α., Ibid., Part III, AD 100,806, PB 121,374 (1955). (21) Ruigh, W. L., Olin, A. D., Steinburg, N. G., Van der Meulen, P. Α., Ibid., Part IV, A D 110,403, PB 121,718 (1955). (22) Schiff, H., Bechi, Ε., Z. Chem. 9, 147 (1866). (23) Snyder, H . R., Cook, J. Α., Johnson, J. R., J. Am. Chem. Soc. 60, 105-111 (1938). (24) Stout, L . E., Chamberlain, D. F., WADC Tech. Rept. 52-192 (June 1952). (25) Upson, R. W., U. S. Patent 2,511,310 (June 13, 1950). (26) Wiberg, E., Bolz, Α., Ber. 73B, 209 (1940). RESEARCH done under contract for the Air Force, followed by further work in the Aero nautical Research Laboratory, Wright-Patterson Air Force Base.


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