Makromol. Chem., Rapid Commun. 10,427-430 (1989) 427

A soluble polyacene precursor

Thomas Vogel, Karsten Blatter, Arnulf-Dieter Schluter *

Max-Planck-Institut fur Polymerforschung, Postfach 3 148, 6500 Mainz, F. R. G.

(Date of receipt: April 24, 1989)

Introduction

Despite various attempts to synthesize polyacenes 1, this intriguing, fully unsaturat- ed ribbon polymer is still unknown. The attempted syntheses of 1 or related systems furnished insoluble and intractable materials of unspecified structures I ) . Within our general interest in the area of ribbon polymer synthesis based on repetitive Diels-Alder (DA) additions 2-4), polyacene was selected as a challenging target structure. Poly- acenes are expected to be insoluble and instable. We, therefore, focussed on the synthesis of a soluble, fully characterizable precursor polymer which, in a second step, should be transformable to a polyacene bearing solubilizing groups. This paper reports on the synthesis of ribbon polymer 5, a potential precursor of polyacenes.

Experimental part

General: Apparatuses used and standard procedures were as described elsewhere'). Poly~4,9-cyclohexyl-5,8-epoxy-5,7-tetrahydro-l H,3 H-naphtho[2,3-cLfuran-1,3,6,7-tetraylj (5 b):

A suspension of 2 b (1,15 g; 1,00 mmol) and 4 b (378 mg; 1,00 mmol) in 10 ml of decalin under nitrogen was placed into a bath pre-heated to 180 "C. After 20- 25 min the formation of gas ceased and the remaining clear solution was cooled to room temp. The solvent was removed i. vac. and the resultant material was purified by precipitating it twice with methanol (200 ml) from a chloroform solution (200 ml). The colorless material obtained was lyophilized using benzene as solvent. Yield: 590 mg (84%).

'H NMR (CC1,D; 300 MHz): 6 = 0,9, 1,0- 1,7,2,2-2,9,4,5 and 5,3 (all signals are broad and unstructured; the intensity of the signal at 6 = 5,3 which has a shoulder at the high field onset, is three times that of the signal at 6 = 43).

13C NMR (CC1,D; 75 MHz): see Fig. 1.

0173-2803/89/$01 .OO

428 T. Vogel, K. Blatter, A.-D. Schluter

Results and discussion

Recently, Hart has shown that 1,4 : 8,l l-bis(oxomethano)-5,14: 7,12-diepoxy- 1,2,3,4,8,9,10,1 I-octaphenyl-I ,3,4a,7,8,10,1 la,14-hexadecahydropentacene (2 a) (Sche- me I) serves as an equivalent for 4H,BH-benzo[ 1,2-c : 4,5-c‘]difuran-4,8-diyl (3 a) 5) .

On the basis of this knowledge we proved that the hexyl substituted derivative 3b is a useful “monomer” for the synthesis of angular annulated ribbon polymers via cleanly proceeding DA polyaddition reactions3.4). We now show that “monomer” 3 can be utilized for the synthesis of linear systems as well. To achieve such an objective we chose the bisdienophile 4, whose mixture of syn/anti isomers (syn : anti = 1 : 1) is readily available3v6). Polymer 5, the expected product of a reaction of “monomers” 3 and 4, consists of linearly annulated six-membered rings. These rings are partially bridged with epoxy functions which should be at least partially removable by dehydration or deoxygenation chemistry. Both monomers bear flexible alkyl chains which ought to help keeping conformationally rigid species, as ribbon polymers are, soluble ’).

Scheme I:

4 L

5

In fact, heating of bisdiene 2 b with compound 4 b in a 1 : 1 mole ratio at 180 “C gives a soluble polymer to which we ascribe structure 5 b. The structure proof is based on its high resolution ‘H and I3C NMR spectra (cf. Fig. 1). In order to assign the signals in this spectrum we synthesized Fiesers diepoxide, 5,6,11,12-tetrahydr0-(5,12: 6,11)-di- epoxytetracene, as a model compound from isobenzofuran and 1 ,Cepoxynaphtha- line8). As he had already observed, isobenzofuran adds to the epoxynaphthaline in the ex0 fashion exclusively. Thus, only the two isomers exo,syn (6a) and exo,anti (6b), respectively, are formed. Their relevant j3C NMR shifts are given with the structures9).

A soluble polyacene precursor 429

801 51,l

&,s

\ / H H

/’

60

3

6b

On the basis of these shifts, as well as those of compound 4 b (C-4a, C-lOa: 6 = 145,9; C-9, C-10 6 = 126,9) all signals in Fig. 1 are easily assigned as indicated, thus establishing the proposed structure of polymer 5 b lo). The excellent agreement of the shifts of the models 4 and 6 a, b with those of polymer 5 b is evidence that “monomer” 3 adds to compound 4 in the exo-fashion. The ‘H NMR spectrum allows to deduce the ratio of exo,syn over exo,anti arrangements in 5 b. Comparison of the signal intensities of the hydrogens in a-position to the epoxy bridges which absorb at 6 = 5,31 (exo,syn) and 4,SO (exo,anti), respectively, gives a ratio exo,syn : exo,anti = 1.

5b

DEPI - 70.9 I 76.1

L L 80.0 /CDCI,

1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

150 100 50 6 in ppm

Fig. 1. 13C NMR spectrum of polymer 5b in CDCl, at room temperature and its DEFT spectrum supporting the following assignment: 6 = 50,2 (C-6, C-7), 76,l-80,O (C-1, C-3, C-5, C-8), 123,7 (C-4, C-9), 141,l (C-3a (C-9a) or C-4a (C-8a)), 144,5 (C-3a (C-9a) and/or C-4a (C-8 a))

430 T. Vogel, K. Blatter, A.-D. Schliiter

Thermogravimetric measurements (under nitrogen; heating rate 5 K/min) show that polymer 5 b is stable up to about 300 “C. Upon further heating to 550 “C a weight-loss in the order of 55 -60% is observed. From 550- 800 “C gradual decomposition takes place leaving 10% of the initial mass as a residue. The value of 55 - 60% corresponds to a complete loss of water, oxygen and all hexyl chains. Gel permeation chromato- graphy (THF, polystyrene ‘’1) shows an almost monomodal molecular weight distribu- tion (m,, = 16500; Mw = 29000, MW/M,, = 1,75). From viscosity measurements (room temp.; toluene) a Staudinger index of 1,18 dl/g is obtained. Polymer 5b forms transparent, self-standing, but brittle films from solution.

Presently, our interest is directed towards the development of techniques to remove the oxygen from polymer 5.

The authors express their thanks to Prof. G. Wegnec Mainz, for his interest in this work, and to the Bundesministerium fur Forschung und Technologie for financial support (BMFT-Project: “Planar systems”)

For example, see: G. A. Edwards, G. Goldfinger, J. Polym. Sci. 16,589 (1955); F. Bohlmann, E. Inhoffen, Chem. Ber. 89, 1276 (1956); H. A. Pohl, E. H. Engelhardt, J. Phys. Chem. 66, 2085 (1962); P. Teyssie, A. Korn-Girard, .I Polym. Sci., Part A, 2, 2849 (1964); E. Clar, Polycyclic Hydrocarbons, Vol. I, Acad. Press, New York 1964; S. Kivelson, 0. L. Chapman, Phys. Rev. B 28, 7236 (1983); Z. Iqbal, D. M. Ivory, J. Marti, J. L. Bredas, R. H. Baughman, Mol. Crysf. Liq. Crysf. 118, 103 (1985); M. Ozaki, Y. Ikeda, I. Nagoya, Synth. Met. 18, 485 ( 1987)

’) K. Blatter, A.-D. Schliiter, G. Wegner, J. Org. Chem. 54, 2396 (1989) 3, K. Blatter, A.-D. Schliiter, Chem. Ber. 122, 1351 (1989) 4, K. Blatter, A.-D. Schliiter, Macromolecules, in press ’) J. Luo, H. Hart, J. Org. Chem. 53, 1341 (1988) 6, The ratio of the syn/anti isomers of 4 was estimated by comparing the intensities of

corresponding signals in the 13C NMR spectrum of the mixture. This was fully confirmed by analytical HPLC of an analytically pure mixture.

’) M. Rehahn, A.-D. Schliiter, G. Wegner, W. J. Feast, Polymer30, 1054, 1060 (1989); K. Opitz, A.-D. Schliiter, Angew. Chem. 101, 513 (1989); see also: M. Ballauff, Angew. Chem. 101,261 (1989)

*) L. F. Fieser, M. J. Haddadin, Can. J. Chem. 43, 1599 (1965) ’) The assignment of the signals of 6 b is based on the assumption that the epoxy bridge at C-5,

C-12 causes slight downfield shifts of the carbons C-6-C-11, compared with the respective carbons C-1 -C-5, C-12, C-12a

lo) The shifts of the aromatic carbons C-3a (C-9a) and C-4a (C-8a), respectively, depend on the relative orientation of the two epoxy bridges adjacent to the respective pair of C-atoms. If these bridges are syn oriented, the two pairs of carbons are equivalent and absorb at B = 144,5; in case of the anti geometry C-3 a (C-9 a) and C-4a (C-8 a) are no longer equivalent and absorb at 6 = 144,5 and 141,l depending on whether or not an ether bridge is in close proximity

‘ I ) Presently, there is no appropriate standard for such rigid ribbon polymers available