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Synthesis of methyl- and methoxy-substituted poly(p-phenylene vinylene) via a non-ionic precursor route
ELSEVIER
Synthetic
Metals 84 (1997)
399-400
Synthesis of methyl- and methoxy-substituted poly(p-phenylene
vinylene) via a non-ionic precursor route
M. Van Der Borght, J. Gelan and D. Vanderzande Institute for Materials Research - Division Chemistry Universitaire Campus - B-3590 Dieperlbeek - Belgium
Abstract Stable methyl- and methoxy-substituted soluble precursor polymers (5a and 5b) were synthesised via a non-ionic precursor route. These precursor polymers possess a sulphoxy substituent on the ethylene segment which, can be eliminated by means of a simple thermal treatment in order to yield the conjugated polymer. The synthesis of the monomers, the precursor and the conjugated polymers is discussed briefly together with the photoluminescence properties of the latter. Keywords:
1.
Poly(phenylene
vinylene)
and derivatives,
photoluminescence,
336, in H,O and toluene [5]. Then this sulphide is oxidised with H,O, and TeO, in MeOH to the sulphoxide 4 [6].
Introduction
In the last few years, the investigation of polymers which can be used in optical and electronic devices is strongly increased. One of the possibilities is the use of conjugated polymers in light emitting diodes (LEDs) [ 11. There are several methods to synthesise these conjugated polymers. The one commonly used is the Wessling route [2]. Unfortunately, this route possesses several drawbacks. One of the most important is the instability of the precursor polymers which leads to gel formation and eventually insoluble precursor polymers. To avoid these drawbacks, we generalised this route [3]. The monomer in this generalised route possesses a leaving group and a polariser, which differs from the former. The polariser stabilises the anion formed in the acid-base equilibrium and insures head-to-tail polymerisation. The leaving group insures that the 1,6-elimination will occur, forming the quinoid structure which polymerises via a radical mechanism [2]. In this way, it is possible to synthesise a stable and soluble precursor polymer. Given the large potential for tuning the electro-optical characteristics of conjugated polymers, it is of interest to explore the scope and limitations of this new route. The aim of this project is to vary the aromatic part of the monomer and to try to polymerise the synthesised monomers via this nonionic polymerisation route. The synthesis and polymerisation of two monomers substituted with methyl and methoxy groups will be presented. 2.
Synthesis
electroluminescence
and
polymerisation
of
the
monomers
The synthesis of these two monomers is shown in figure 1. In the first step, 2 is synthesised via a chloromethylation of 1 [4]. The sulphide 3 is formed by means of NaOH, Ibutanethiol (HS”Bu) and a phase-transfer catalysator, Aliquat 0379-67791971317.00 0 1997 Ekevier PII SO379-6779(96)03956-2
Science S.A All rig&s reserved
R
R
R = -CH3 -0CH3
finBu
(a) (b)
I
AT
)
NaOH HS”Bu
+
Fig 1. Synthesis of the monomers and polymers. The monomers are polymerised in different solvents under nitrogen atmosphere at room temperature. In all cases, Na+tBuO- is used as a base to generate the quinoid compound. The polymerisation is terminated by pouring out the reaction mixture in water and neutralised with a 0,l M HCl solution. The precursor polymer is extracted with chloroform, washed with water and precipitated in diethylether. In the case of the polymerisation of 5b in NMP, the precipitated polymer was
M. Yan Der Borght
400
et al. /Synthetic
redissolved in chloroform and again precipitated in hexane. All the molecular weights of the precursor polymers are determined by means of GPC relative to polystyrene. The solvent used is DMF. Polymer 5a
Solvent
M&05)
M,/M,
Yield(%)
MMF
6,2
239
Form/THF NMP
339 1,9
24 1,9
25 20
55 DMSO 2,4 1,9 IO 5b MMF 2,7 1,s 35 NMP 9 1,7 30 MMF = N-methylformamide; Form = formamide; THF tetrahydrofuran; NMP = N-methylpyrrolidone; DMSO dimethylsulphoxide Table 1. Molecular weights and yields.
analyses
and
84 (I997/
399-400
5a and 5b, and poly(2-methoxy-5-(2’-ethylhexoxy)-1,4phenylene vinylene) (MEH-PPV) were spincoated on a quartz substrate under similar conditions. In this way it is possible to compare the photoluminescence intensities of their conjugated polymers in some extent. The spectrum of 6a has a maximum intensity at 530 nm and 6b at 535 and 585 nm. 6a has a remarkably higher photoluminescence intensity than 6b or even MEH-PPV.
= =
In all cases, high molecular weight polymers with a monomodal weight distribution are obtained. All the precursor polymers are soluble in chloroform, dichloromethane, NMP, DMF and MMF. Except for the polymer synthesised in DMSO, which is light yellow, all the precursor polymers are colourless. They can be stored in solid form during several months without occurrence of elimination. The ‘H- and “Cspectra show only pure precursor polymers. 3. Thermal spectra
Metals
photoluminescence
Figure 2 shows the thermal gravimetric analyses of the precursor polymers. The thermal elimination of the sulphoxy substituent of polymer 5a starts at 100°C and is completely finished at 330°C. The degradation of this conjugated polymer 6a starts at 490°C. The formation of the conjugated polymer 6b begins at 110°C and is finished at 280°C. The degradation starts at 380°C.
550
660
Wavelength (nm) Fig. 3. Photoluminescence polymers. 4.
measurements
650 ‘
7do
of the precursor
Conclusion
It is possible to polymerise 4a and 4b in several solvents to high molecular weight precursor polymers via this non-ionic route. These two electron rich precursor polymers are stable and easy to handle. They can easily be converted to their conjugated form. Taken together this is an excellent alternative for the Wessling route to these two substituted polymers. 5.
Acknowledgements
This work is supported by the Vlaams Instituut voor bevordering van het Wetenschappelijk-Technologisch onderzoek in de industrie (IWT) and Hoechst A.G., We thank J. Mullens for the thermal analyses and P. Adriaensens for the NMR measurements. 6.
0
Fig. 2. Thermal polymers.
100
200
300
400
Temperature (“C)
gravimetric
500
analyses
600
of
the
700
precursor
In figure 3 the results of the photoluminescence measurements are shown. Therefore, the precursor polymers
References
[l] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. MacKay, R.H. Friend, P.L. Burns and A.B. Holmes, Nature, 347 539 (1990) [2] R.A. Wessling, Journal of Polymer Science: Polymer Symposium, 72 55 (1985) [3] F. Louwet, D. Vanderzande, J. Gelan and J. Mullens, Macromolecules, u 1330 (1995) [4] Brunner, U.S. pat., 1887369 (1933) [CA. 272694 (1933)] [5] A.W. Herridt, D. Picker, Synthesis Comm., 447 (1975) [6] KS. Kim, H.J. Hwang, C.S. Cheong, C.S. Hahn, Tetrahedron Letters, 11 2893 (1990)
Synthetic
Metals 84 (1997)
399-400
Synthesis of methyl- and methoxy-substituted poly(p-phenylene
vinylene) via a non-ionic precursor route
M. Van Der Borght, J. Gelan and D. Vanderzande Institute for Materials Research - Division Chemistry Universitaire Campus - B-3590 Dieperlbeek - Belgium
Abstract Stable methyl- and methoxy-substituted soluble precursor polymers (5a and 5b) were synthesised via a non-ionic precursor route. These precursor polymers possess a sulphoxy substituent on the ethylene segment which, can be eliminated by means of a simple thermal treatment in order to yield the conjugated polymer. The synthesis of the monomers, the precursor and the conjugated polymers is discussed briefly together with the photoluminescence properties of the latter. Keywords:
1.
Poly(phenylene
vinylene)
and derivatives,
photoluminescence,
336, in H,O and toluene [5]. Then this sulphide is oxidised with H,O, and TeO, in MeOH to the sulphoxide 4 [6].
Introduction
In the last few years, the investigation of polymers which can be used in optical and electronic devices is strongly increased. One of the possibilities is the use of conjugated polymers in light emitting diodes (LEDs) [ 11. There are several methods to synthesise these conjugated polymers. The one commonly used is the Wessling route [2]. Unfortunately, this route possesses several drawbacks. One of the most important is the instability of the precursor polymers which leads to gel formation and eventually insoluble precursor polymers. To avoid these drawbacks, we generalised this route [3]. The monomer in this generalised route possesses a leaving group and a polariser, which differs from the former. The polariser stabilises the anion formed in the acid-base equilibrium and insures head-to-tail polymerisation. The leaving group insures that the 1,6-elimination will occur, forming the quinoid structure which polymerises via a radical mechanism [2]. In this way, it is possible to synthesise a stable and soluble precursor polymer. Given the large potential for tuning the electro-optical characteristics of conjugated polymers, it is of interest to explore the scope and limitations of this new route. The aim of this project is to vary the aromatic part of the monomer and to try to polymerise the synthesised monomers via this nonionic polymerisation route. The synthesis and polymerisation of two monomers substituted with methyl and methoxy groups will be presented. 2.
Synthesis
electroluminescence
and
polymerisation
of
the
monomers
The synthesis of these two monomers is shown in figure 1. In the first step, 2 is synthesised via a chloromethylation of 1 [4]. The sulphide 3 is formed by means of NaOH, Ibutanethiol (HS”Bu) and a phase-transfer catalysator, Aliquat 0379-67791971317.00 0 1997 Ekevier PII SO379-6779(96)03956-2
Science S.A All rig&s reserved
R
R
R = -CH3 -0CH3
finBu
(a) (b)
I
AT
)
NaOH HS”Bu
+
Fig 1. Synthesis of the monomers and polymers. The monomers are polymerised in different solvents under nitrogen atmosphere at room temperature. In all cases, Na+tBuO- is used as a base to generate the quinoid compound. The polymerisation is terminated by pouring out the reaction mixture in water and neutralised with a 0,l M HCl solution. The precursor polymer is extracted with chloroform, washed with water and precipitated in diethylether. In the case of the polymerisation of 5b in NMP, the precipitated polymer was
M. Yan Der Borght
400
et al. /Synthetic
redissolved in chloroform and again precipitated in hexane. All the molecular weights of the precursor polymers are determined by means of GPC relative to polystyrene. The solvent used is DMF. Polymer 5a
Solvent
M&05)
M,/M,
Yield(%)
MMF
6,2
239
Form/THF NMP
339 1,9
24 1,9
25 20
55 DMSO 2,4 1,9 IO 5b MMF 2,7 1,s 35 NMP 9 1,7 30 MMF = N-methylformamide; Form = formamide; THF tetrahydrofuran; NMP = N-methylpyrrolidone; DMSO dimethylsulphoxide Table 1. Molecular weights and yields.
analyses
and
84 (I997/
399-400
5a and 5b, and poly(2-methoxy-5-(2’-ethylhexoxy)-1,4phenylene vinylene) (MEH-PPV) were spincoated on a quartz substrate under similar conditions. In this way it is possible to compare the photoluminescence intensities of their conjugated polymers in some extent. The spectrum of 6a has a maximum intensity at 530 nm and 6b at 535 and 585 nm. 6a has a remarkably higher photoluminescence intensity than 6b or even MEH-PPV.
= =
In all cases, high molecular weight polymers with a monomodal weight distribution are obtained. All the precursor polymers are soluble in chloroform, dichloromethane, NMP, DMF and MMF. Except for the polymer synthesised in DMSO, which is light yellow, all the precursor polymers are colourless. They can be stored in solid form during several months without occurrence of elimination. The ‘H- and “Cspectra show only pure precursor polymers. 3. Thermal spectra
Metals
photoluminescence
Figure 2 shows the thermal gravimetric analyses of the precursor polymers. The thermal elimination of the sulphoxy substituent of polymer 5a starts at 100°C and is completely finished at 330°C. The degradation of this conjugated polymer 6a starts at 490°C. The formation of the conjugated polymer 6b begins at 110°C and is finished at 280°C. The degradation starts at 380°C.
550
660
Wavelength (nm) Fig. 3. Photoluminescence polymers. 4.
measurements
650 ‘
7do
of the precursor
Conclusion
It is possible to polymerise 4a and 4b in several solvents to high molecular weight precursor polymers via this non-ionic route. These two electron rich precursor polymers are stable and easy to handle. They can easily be converted to their conjugated form. Taken together this is an excellent alternative for the Wessling route to these two substituted polymers. 5.
Acknowledgements
This work is supported by the Vlaams Instituut voor bevordering van het Wetenschappelijk-Technologisch onderzoek in de industrie (IWT) and Hoechst A.G., We thank J. Mullens for the thermal analyses and P. Adriaensens for the NMR measurements. 6.
0
Fig. 2. Thermal polymers.
100
200
300
400
Temperature (“C)
gravimetric
500
analyses
600
of
the
700
precursor
In figure 3 the results of the photoluminescence measurements are shown. Therefore, the precursor polymers
References
[l] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. MacKay, R.H. Friend, P.L. Burns and A.B. Holmes, Nature, 347 539 (1990) [2] R.A. Wessling, Journal of Polymer Science: Polymer Symposium, 72 55 (1985) [3] F. Louwet, D. Vanderzande, J. Gelan and J. Mullens, Macromolecules, u 1330 (1995) [4] Brunner, U.S. pat., 1887369 (1933) [CA. 272694 (1933)] [5] A.W. Herridt, D. Picker, Synthesis Comm., 447 (1975) [6] KS. Kim, H.J. Hwang, C.S. Cheong, C.S. Hahn, Tetrahedron Letters, 11 2893 (1990)