A NEW SYNTHESIS OF SLUBLE POLY(1,4-PHENYLENEVINYLENE)S AND POLY(2,5-PYRIMIDINYLENEVINYLENE)S

Article abstract of DOI:10.1016/S0040-4039(00)93427-0

Alkoxy-substituted 4-methylbenzylideneanilines 2a-f and their pyrmidine analogues 2g,h show an efficient selfcondensation in the alkaline mediumKOC(CH3)3 / DMF.The totally stereoselective reaction yields 3a-h which are transformed by acidic work-up to 4a-h.A narrow distribution of the molecular weight can be achieved.

Full text of DOI:10.1016/S0040-4039(00)93427-0

Tetrahedron Letters, Vol.32, No.38, pp 5059-5062, 1991
Printed in Great Britain
0040-4039/91 $3.00 + .00
Pergamon Press plc
A NEW SYNTHESIS OF SOLUBLE POLY(1,4-PHENYLENEVINYLENE)S AND
POLY(2,5-PYRIMIDINYLENEVINYLENE)S
Holger Kretzschmann and Herbert Meier*
Institute of Organic Chemistry, University of Mainz
J.-J.-Becherweg 18-22, D-6500 Mainz, FRG
Abstract: Alkoxy-substituted 4-methylbenzylideneanilines 2a-f and their pyrimidine analogues 2g,h show an
efficient selfcondensation in the alkaline medium KOC(CH3)3/DMF. The totally stereoselective reaction
yields 3a-h which are transformed by acidic work-up to 4a-h. A narrow distribution of the molecular weight
can be achieved.
Poly(phenylenevinylene)s and their heterocyclic derivatives deserve great interest because of their electrical
and optical properties1). In the last years soluble PPV systems have been obtained by different methods:
McMurry reaction2), Hofmann elimination3), Wittig reaction4). However, these syntheses lead always in
longer chains to some failures like saturated carbon centres, (Z)-configurated double bonds, etc. Therefore we
have tried to use the condensation of azomethines and aromatic methyl compounds for this purpose. In con-
trast to the known Siegrist reaction5) both reactive groups were incorporated into the same molecule, in order
to provoke selfcondensation in an alkaline medium. The solubility in organic solvents like DMF has to be en-
hanced by flexible side chains as an "internal solvent"; otherwise the condensation reaction is immediately
stopped by precipitation. A repetition of earlier attempts with (E)-N-(4-methylphenylmethylene)benzen-
amine6) showed us that the reaction is already terminated after three condensation steps (n=3). Less than 5%
of the material possess a higher molecular weight (n---4,5...). Consequently, we have introduced alkoxy side
chains in order to get real PPV systems. The method can also be applied on heteroaromatics like pyrimidine
derivatives. Thus, we succeeded in the generation of poly(2,5-pyrimidinylenevinylene)s, the furst PPV
analogue materials with hetero atoms in every repetition unit. The incorporation of nitrogen atoms enhances
additionally the solubility and hence the average chain length.
The 2-alkoxy-4-methylbenzaldehydes la,c and the 3-alkoxy-4-methylbenzaldehyde lb were obtained in
yields of 70-90% by the reaction of the corresponding hydroxy compounds7,8) with iodoalkanes in the pre-
sence of K2CO3 in acetone9). The 2,5-dialkoxy-4-methylbenzaldehydes ld,e were prepared by a twofold
alkylation of the potassium salt of 2-methylhydroquinone with bromoalkanes in the presence of Aliquat 336
(yield 30-99%)1°) followed by a selective formylation11) with C12HC-OCH3/SnC14 in dichloromethane (yield
80-95%). 2,6-Dialkoxy-4-methylbenzaldehyde If can be generated by an analogous alkylation of 5-methylre-
sorcinol (orcinol) and treatment of the ether with C4H9Li/CtH5N(CH3)-CHO (yield 45-55%)12).
4,6-Dialkoxy-2-methylpyrimidine-5-carbaldehydes
lg,h can be obtained by nucleophilic substitutions 13) of
the chlorine atoms in 4,6-dichloro-5-(1,3-dioxolan-2-yl)-2-methylpyrimidine with sodium alkoxides and
subsequent hydrolysis of the dioxolane (yield 80-90 %). All aldehydes la-h react with one equivalent of
aniline at 40-60°C forming the azomethines 2a-h (yield 90-100 %). Polycondensation of 2a-h can be per-
5059

5060
R1,2
R1,2
la-h
2a-h
R1,2
X ~
R1,2
R1,2
H
H20
X
Y~'
(H+)
j_V
)n x
o
3a-h
X = OH,N
4a-h
R -- OAIkyl
Yield
Melting
Average chain length
Compound
X
%
range ['C]
nNMR nGpc
riMS
9
4
R
a
ct-OC6Ht3
~-OC6H13
tz-OC12H25
CH
CH
CH
CH
96
60
94
97
84
89
38
93
190...:280
195.....280
210.....280
190.....235
265.....320(Z)
190.....280
>250 (Z)
9
9
8
9
b
c
d
e
13
12
12
9
(9)
(8)
tx,~'-OC8H17
oq~'-O(CH2 )2OC2H5 CH
27
27
>30
>30
f
g
ct,tx'-OCsH17
ot,ct'-OC6H13
tx,tx'-OCsH17
CH
N
N
(8)
h
>250 (Z)
formed at 90°C in dry DMF with two equivalents of potassium tert-butoxide14). Work-up with hydrochloric
acid leads to the hydrolysis 3a-h 4a-h. The polycondensation reactions are in all cases a-h highly stereo-
~
selective. There are no hints for a (Z)-configuration at all. The investigated products correspond to a reaction
time of approximately 1 hour and are insoluble in acetone but soluble in chloroformiS). The singlet signals of
the terminal groups CH3 and CHO in the IH-NMR spectra allow a reliable determination of the average mole-
cular weight. In some cases a GPC analysis was made for comparison; however, the calibration by polymer
standards like polystyrene turned out to be complicated for n>10. Similarly, field desorption mass spectro-
metry gives only good results for n<10.
Normally, the yields of a Siegrist reaction5) are much lower. However, an intramolecular activation is impor-
tant in our systems 2a-h. The imino group enhances by its electronic effect the reactivity of the methyl group.
Consequently, this activation is decreased by an increasing chain length. Finally, the reactivity approaches to

5061
the behavior of a non-activated methylarene like toluene. Due to this effect, it is possible to generate products
with a relatively narrow distribution of the molecular weight16). We obtained for example 4a as a solid, in
THF soluble product which consists mainly of the decamer (n--9). The GPC diagram (figure 1) shows some
nonamer (n=8) and a trace of undecamer (n=10).
•50
110
.4D
>~. 30
. 2 0
Figure 1, GPC diagram of 4a
(THF, 22°C, standard: poly-
styrene, UV detection)
I1+1
9
11
.10
46
44
42
4Q
38
V e [ m l
]
The products 4a-h were well def'med by elementary analysis, UV-IR-, 1H-NM.R and 13C-NMR spectra. The
following chemical shift values ~ in CDC13 are characteristic for the PPV systems
4
a - f :
13C-NMR
1H-NMR
4a-f
end groups
2.25 - 2.35
9.9- 10.4
16 - 22
c n 3
ClIO
189 - 192
PPV - chain
side chains
CH (arom.,olef.)
CqO
7.0 - 7.7
109- 140
157- 158
4.0 - 4.2
68
-
70
O - CH2
CH2
1.2- 1.95
0.85 - 0.95
24 - 33
13- 16
CH3
Naturally, the N atoms in the pyrimidine systems 4g,h change the resonance signals, especially for the main

5062
chain. The characteristic 13C-shift ranges for the pyrimidine segments are 5 = 99-100 (C-5), 162-164 (C-2),
167-169 (C-4,6), and for the carbon atoms in the double bonds 5 = 126-132. The olefinic protons lead to an
AB spin pattern at 5 = 7.3-8.4. The coupling constant 3j = 16 Hz as well as the IR band at 970-990 cm1
confirm the (E)-configuration.
Acknowledgement: We are grateful to the Fonds of the Chemical Industry and the Hoechst AG, Werk
Kalle-Albert for financial support.
References and Notes
1)H6rhold, H. H.; Helbig, M.; Raabe, D.; Opfermann, J.; Scheff, U.; Stockmann, R.; Weil3, D.; Z. Chem., 27,
126(1987).
2)Rehahn, M.; Schliiter, A.-D.; Makromol. Chemic, Rapid Comrnun. 11, 375(1990).
3)Askari, S.H.; Rughooputh, S. D.; Wudl, F.; Heeger, A. J.; Polym. Prepr. 30(1), 157(1989).
4)Kol3mehl, G.; Samandari, M.; Makromol. Chem., 186, 1565(1985).
5)Siegfist, A. E.; Helv. Chim. Acta, 64, 662(1981).
6)KoBmehl, G.; Yaridjanian, A.; Makromol. Chem., 182, 3419(1981).
7)Casiraghi, G.; Casnati, G.; Puglia, G.; Sartori, G.; Terenghi, G.; J. chem. Sot., Perkin Trans. 1 1980, 1862.
8)Fukumi, H.; Kurihara, H.; Mishima, H.; Chem. Pharm. Bull., 26, 2175,(1978).
9)For a general experimental description see Allen, C. F. H.; Gates Jr., J. W.; Org. Synth., Coll. Vol. 11I,
140(1955).
l°)See Loupy, A.; Sansoulet, J.; Vaziri-Zand, F.; Bull. Soc. Chim. Ft. 1987(6), 1027.
ll)See Rieche, A.; Gross, H.; H6ft, E.; Chem. Ber. 93, 88(1960).
a2)See Adams, R.; Mathieu, J.; J. Am. Chem. Soc. 70, 2120(1948).
13)For related reactions see Kl6tzer, W.; Herberz, M.; Mh. Chem. 96(5), 1567 and 1573(1965) and Bredereck,
H.; Simchen, G.; Wagner, H.; Santos, A. A.; Liebigs Ann. Chem. 766, 73(1972).
141The reactivity of the methyl group is in the pyrimidine systems g,h much higher than in the benzene
systems a-f; even in the absence of catalysts lg,h show some selfcondensation on storage at room
temperature.
iS)The amount of product soluble in acetone as well as the amount of product insoluble in chloroform are very
small under these conditions. However, a much shorter reaction time increases the first one, whereas a
reaction time of 24 hours or more enhances the portion of totally insoluble product.
16)Usually an average rate constant is assumed in such polycondensation processes1). However, in our case we
have a much faster reaction at the beginning than towards the end. Hence, the curve for the average mole-
cular mass versus the reaction time should cross the "normal" curve. One can assume in the region of the
crossover a plateau which might be responsible for the high uniformity of the oligomer.
(Received in Germany 21 February 1991)