Charge-Transfer Interaction in Organic Polymers

COMMUNICATIONS EDITOR TO THE Downloaded by HENKEL KGAA on August 18, 2009 Published on November 1, 1964 on http://pubs.acs.org | doi: 10.1021/ja01076a070 5022 Purified reactants were condensed a t - 195' into a 130ml. nickel vessel. The vessel was allowed to warm to room temperature and the course of the reaction was followed by observing the change in pressure. I t was assumed t h a t the reaction had reached completion when no further pressure change was noted. The reaction products were separated a t the vacuum line by trapping a t different temperature and by gas chromatography. They were identified by their infrared, n.m.r , and/or mass spectra. In a typical experiment a mixture of XeFJ (0.75 mmole) and perfluoropropene (1.28 mmoles) was allowed to react in a nickel vessel a t room temperature. The products were removed from unreacted XeF4 in vacuo a t -78' and trapped a t -195'. They were shown to be perfluoropropane and xenon in stoichiometric amounts. Perfluoropropane was identified by comparison with an authentic sample. We observed that XeF6 is considerably more reactive than XeF4 while XeFz is less reactive. Under similar conditions, XeF6 reacts with perfluoropropane to give both C?F6 and CF4 as the major products, indicating extensive cleavage, while XeFz is unreactive even after six days. XeF4 + 2CF3CF=CF2 +. 2CFaCFaCF3 + Xe An interesting rearrangement occurs when xenon fluorides react with aliphatic olefins. At room temperature, ethylene reacts with XeF2 or XeF4 to give 1 , l - and 1,2-difluoroethane in 35 and 45% yields, respectively, together with a minor component which is not yet identified. However the reaction of propylene with XeF2 or XeF4 gives 1,l-difluoropropane as the main product (ca. 65% yield). Allylic fluorination may have occurred also to a certain extent as hydrogen fluoride was detected in the reaction mixture. XeF2.b XeF4: + - + + CH2=CH2 -* C H Z F C H ~ F C H ~ C H F Z Xe CH3CH=CH2 CH3CHzCHF2 The relative instability of secondary fluorides6 and the increasing stability of compounds with fluorine atoms attached on the same carbon7 suggest a possible pathway for formation of the gem-difluorides ; the vicdifluorides, presumably the initial products of the reactions, isomerize under the reaction conditions to give the gem-difluorides. IkTe detected a slow isomerization of 1,2-difluoroethane to 1,l-difluoroethane in CC14solution upon standing a t room temperature, which is in agreement with the suggested pathway. The reactions studied so far indicate the potential of the xenon fluorides as fluorinating agents in organic chemistry. The mechanism of the reactions and further studies extending the work to acetylenic and aromatic systems are in progress. (6) F o r example, see A . & . Lovelace, D. A. R a u s c h , a n d W. Postelnek, I " A l i p h a t i c F l u o r i n e Compounds." Reinhold Publishing C o r p . , N e w Y o r k , pi Y . . 1 5 5 8 , p . 12. (7) J H i n e , J . A m . Chem. Soc., 86, 3239 (1563) (8) A l f r e d P . Sloan F o u n d a t i o n Fellow, 1960-1964 DEPARTMEST CHEMISTRY OF USIVERSITY CHICAGO OF CHICAGO, ILLINOIS60637 ARGONNE NATIOXAL LABORATOR? XRGOSNE, ILLIKOIS RECEIVED SEPTEMBER1964 2, TSU-CHIA SHIEH S . C. Y A N G ~ C L. CHERNICK VOl. 86 Charge-Transfer Interaction in Organic Polymers' Sr i: The charge-transfer interaction between electron donors and acceptors has been a subject of both theoretical and experimental interest.z Certain types of charge-transfer complexes of simple organic compounds exhibit semiconducting properties, and charge-transfer interaction has been suggested as an intermediate stage in biological reactions. The present communication deals with the intramolecular interaction of electron-donating and electron-accepting groups in organic polymers. One major difficulty in the preparation of organic polymers containing both electron-donating and electron-accepting functional groups is that organic monomers containing strong electron-accepting groups, e.g., trinitrostyrene (TNS, I ) , are excellent inhibitors to both ionic and free-radical po1ymerizations.j Apparently, the reactive intermediates in the polymerizations, ions or radicals, are rendered inactive by interaction with the electron-accepting groups. Several attempts made to prepare polymers and copolymers of T N S have been u n s u c ~ e s s f u l . ~ Although T N S could not be polymerized in previous attempts, i t might be copolymerized with monomers containing electron-donating groups. The electrondonating group may interact with the strong electronaccepting trinitrophenyl function, thus freeing the vinyl group for polymerization. The copolymerization of T N S with nitrogen-containing vinyl monomers was thus attempted with the nonbonding electrons of the nitrogen atom acting as the electron donor. The copolymerization of T N S and 4-vinylpyridine (4VP, 11) occurred exothermically upon mixing a t room temperature. After the mixture was dissolved in pyridine and treated with ether, copolymers of molecular weight ranging from 6000 to 8000 were obtained in about 60y0 yield. The presence or absence of oxygen made no difference. Apparently sufficient delocalization occurred during the charge-transfer process to initiate the polymerization.7 The relative proportion of the two monomers in the copolymers could be varied by adjusting the mole ratio of the two monomers in the polymerization mixture, e.g., a 1 : 1 mixture of the monomers reacted to give a 1 : 2 (TNSI-VP) copolymer while a 2 : 1 (TNS-I-VP) mixture gave a 1 : 1 copolymer. The copolymers formed were somewhat soluble in chloroform, acetone, or dimethylformamide but were insoluble in ether or aliphatic (1) T h e work is s u p p o r t e d in p a r t b y t h e U. S. Atomic E n e r g y C o m mission, C o n t r a c t No. A T ( l l - l ) - 1 0 4 3 . a n d b y t h e Alfred P. bloan F o u n d a tion. (2) F o r a review o n t h e interaction of electron d o n o r s a n d a c c e p t o r s , see R. S. Mulliken a n d W. B. P e r s o n , A n n . Rev. Phys. Chem., 13, 107 (1962). F o r applications of c h a r g e - t r a n s f e r complexes, see L. N . F e r g u s o n . " T h e M o d e r n S t r u c t u r a l T h e o r y of Organic C h e m i s t r y , " P r e n t i c e - H a l l , Englewood Cliffs, N.J . . 1963, p p . 122-126. 82, (3) D. S Acker, el a l . , J . A m Chem. SOL., 6508 ( 1 9 6 0 ) , a n d later p a p e r s , M. M. Labes. R . S e h r , a n d M .Bose, J Chem. P h y s . , 33, 868 (1960). (4) F o r discussions on t h e c h a r g e - t r a n s f e r interactions in biological syst e m s , see A. Szent-Gyorgyi, " I n t r o d u c t i o n t o a Submolecular Biology," Academic Press I n c . . N e w York. 1960, p . 7 6 ; E . Kosower, "Molecular Bioc h e m i s t r y , " McGraw-Hill Book C o . , I n c . , N e w Y o r k , N.Y . , 1962. p . 180. (5) R . H. W l e y a n d L . C B e h r , J . A m . Chem Soc , 72, 1822 (1950). (6) T h e molecular weights were determined with a Mechrolab v a p o r phase osmometer. (7) Polymerizations initiated h y charge-transfer interactions h a v e been reported b y H . S c o t t , G A. Miller, a n d M , M I.ahes. Tclvnhedron L e t t e r s . 1073 (1963) TABLE I Monomer ratio in the copolymer TNS4-VP 1:2 TNH-VP 1:l TNS-2-VP 1:l 460, 530 (6400, 4200) 464, 520 (6400, 4200) 460, 516 (5350, 3500) 464, 520 (5350, 3500) 452, 493, 583 (10,400, 4850, 4800) 460, 505, 585 (10,400, 5850, 5000 ) 445 (4400) --[ TNS-DMAS 2:1 omer as given in the monomer ratio) Solvent Acetone DMF Acetone DMF Acetone DMF Acetone hydrocarbons. We have also successfully prepared copolymers of T N S with 2-vinylpyridine (2-VP, 111) and with p-dimethylaminostyrene (DMAS, IV). Downloaded by HENKEL KGAA on August 18, 2009 Published on November 1, 1964 on http://pubs.acs.org | doi: 10.1021/ja01076a070 hv D-A-D-A-D-AI- +-[ D +A--D-A-D-A]t+ -[D-A-D+A--D-A]-; Xmax ( e per unit of mon- Copolymer 5023 COMMUNICATIONS EDITOR TO THE Nov. 20, 1964 etc. ( 1 ) Reactions of other types of vinyl monomers as well as the conductivity and the photoconductivity of these copolymers are being investigated. (8) Alfred P. Sloan Foundation Fellow. (9) The authors wish to express their appreciation t o Drs. K . D. Kopple, S. A. Rice, and S . R . Berry for many interesting and stimulating discussions and to Mr. Hanson Chen Ting and Miss Maria Tsong for their assistance in the laboratory. DEPARTMENT CHEMISTRY OF UNIVERSITY CHICAGO OF CHICAGO, ILLINOIS60637 N. C. YANG~~O YEHIEL A O N I ~ G RECEIVED SEPTEMBER 1964 23, Analysis of the Proton Nuclear Magnetic Resonance Spectrum of Benzene i a Nematic Liquid Crystal n Sir : An n.m.r. spectrum of benzene in a liquid crystal nematic phase' was recently published.2 The major features of that spectrum were attributed to direct magnetic dipole-dipole interactions of protons. We have reproduced that spectrum and wish to present an analysis based on computer simulations. Our proton n.m.r. spectrum of -15 mole yobenzene in the nematic phase of p,p'-di-n-hexyloxyazoxybenzenewas obtained a t 79' on a Varian DP-60 n.m.r. spectrometer. I t is displayed on the right side of Fig. 1. The spectrum is symmetrical. The charge-transfer interactions in these copolymers of T N S were subsequently investigated. Chargetransfer complexes from trinitrotoluene (TNT) and amines were selected as model systems to compare with these copolymers. The copolymers displayed two unusual and interesting properties when compared with the model complexes: (1) these copolymers exhibit strong and broad charge-transfer transitions near 450 mp which obey Beers Law, and (2) the extinction coefficients of the charge-transfer transitions are anomalously high. The complexes of T N T and amines dissociate rapidly upon dilution and their equilibrium constants for complex formation are low, e.g., T N T and 4-picoline form a 1 : 2 complex in solution [ X ~ ~ ~ c ' 2 474 mp (E 800), K = 0.13 1.2 mole-2], T N T and N , N dimethyl-p-toluidine form a 1 : 1 complex in solution Fig. 1.-Proton n.m.r. spectrum of benzene in nematic p,p'[ X ~ ~ ~ c " mp (E 700), K = 0.60 1. mole-'], and T N T 460 di-n-hexyloxyazoxybenzene: computer simulation on left, exforms a red solution in 2-picoline but the K for complex perimental on right. formation is too small to be determined by the variable Our program to simulate n.m.r. spectra3 was easily concentration method. The charge-transfer intermodified to incorporate the constants Dij, which repreactions in our copolymers were not perturbed by the sent the direct nuclear magnetic dipole-dipole intersolvent environment and exhibited no visible dissociaaction of nuclei i and j , into the spin Hamiltonian4 tion over a concentration range of 5 X to 1 X of eq. 1. Here yo is the spectrometer frequency, and M in acetone or dimethylformamide. This phenomenon may be rationalized by the explanation that the donating and accepting groups in our copolymers were held together by the polymer network and thus were unaffected by the solvent. In relation to the second property, the extinction coefficients of these copolymers per an appropriate unit of T N S and donor monomer were found to be of the order of the resonant frequency of an uncoupled proton in the 5,000-10,000, and several such examples are listed in (1) G. W. Gray, "Molecular Structure and the Properties of Liquid Table I. Intensification of the charge-transfer tranCrystals," Academic Press, New York, N . Y . , 1962. This is a compresitions in the copolymers may be due to the coupling hensive review on the occurrence and properties of liquid crystals. of adjacent donor-acceptor oscillators or to the de(2) A. Saupe and G. Englert, Z . N a l w f w s c h . , lS&, 172 (1964). These authors concluded in a footnote that J a r t h o and J,.ro are positive; see also localization of an excited donor-acceptor couple to the A . Saupe, i b i d . , lSa, 161 (1964). adjacent donor-acceptor couples (eq. 1). These ob(3) P. R . Story, L. C. Snyder, D. C. Douglass, E. W. Anderson, and R . L. Kronegay, J . A m . Chcm. Soc., 8 6 , 3630 (1963). servations suggest that we may have achieved the (4) J. A. Pople, W. G. Schneider, and H. J. Bernstein, "High-Resolution stacking of alternating donor (D) and acceptor (A) Nuclear Magnetic Resonance," McGraw-Hill Book Co., Jnc., New York, groups along the polymer chain. N. Y., 1959.