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Palladium catalyzed synthesis of poly(1,4-phenylenevinylene)
Synthetic Metals, 41-43 (1991) 881-884
881
PALLADIUM CATALYZED SYNTHESIS OF POLY(I,4PHENYLENEVINYLENE), A. Greiner, H. Martelock, W. Heitz. Philipps-Universitllt Marburg, Institut tilt Polymere und Zentrum fur Materialwissenschaften, Hans-Meerwein-Strasse, W-3550 Marburg / Germany.
Abstract A wide variety of derivatives of poly(l,4-phenylenevinylene) can be synthesized by the Pd catalyzed arylation of ethylene with dihalogenoarenes. Model reactions of bromobenzene and ethylene to trans-stilbene indicated complete conversion of bromobenzene, few side products, and almost exclusively trans-stilbene. Many of the synthesized derivatives of PPV are soluble and fusible. The fusible derivatives are thermotropic. Although, thermogravimetry indicates good thermal stability of most of the fusible derivatives of poly(l,4-phenylenevinylene) irreversible changes were observed in the molten state combined with a loss of anisotropy, solidification, and insolubility. Introduction Poly(l,4-phenylenevinylene) (PPV) is a main-chain conjugated polymer with a wide variety of interesting physical properties. PPV has been shown to be a good electrical conductor after being doped. The polymer backbone of PPV is formed by an alternating sequence of phenylene and vinylene segments. The backbone is rigid rod-like if the vinylene segments are in the all-trans configuration. Since PPV is insoluble and infusible no liquid crystallinity can be observed. The purpose of our investigations is the synthesis of soluble and fusible derivatives of PPV with a rod-like main chain. Results Soluble and Fusible derivatives of PPV may be obtained by structural variations which result in reduced interchain interactions as there Elsevier Sequoia/Printed in The Netherlands
882
are lateral stiff substituents on the phenylene segments and twisted biphenylene segments in the main chain. More soluble and fusible derivatives may be obtained by copolymerization of monomers with different structures to result in less symmetric molecules. In order to realize these structures a synthetic route was needed which results in a high yield of trans isomers and tolerates a wide variety of different monomers. Incited by the investigations of Heck [1 ] on the palladium catalyzed arylation of olefins by halogeno compounds we tested this reaction for the synthesis of PPV and its derivatives. The synthesis of PPV and derivatives was accomplished by different monomer combinations, as there are the Pd catalyzed polymerization of p-halogenovinylarenes [2] or of p-dihalogenoarenes with p-divinylarenes [3] or of p-dihalogenoarenes with ethylene [ 4 ] . In order to estimate structural defects caused by side reactions and to optimize the synthesis we investigated the Pd catalyzed model reaction of bromobenzene and ethylene to trans-stilbene. Bromobenzene was completely converted. Benzene, 2-methylstilbene, biphenyl, cis-stilbene, l,l-diphenylethylene were detected as side products by gas chromatography. The side reactions can be eliminated (formation of benzene, biphenyl, and triphenylethylene) or limited (formation 2-methylstilbene and l.l-diphenylethylene) by lower amounts of catalyst and by lower reaction temperatures. The formation of cis- stilbene is hardly detectable by gas chromatography. Details of this study are given elsewhere [5]. The results of the model reaction were transferred to the polyreaction of p-dihalogenoarenes and ethylene. A number of soluble and fusible derivatives of PPV were synthesized and characterized. Some representative examples are given in table 1. Soluble and fusible derivatives of PPV were also synthesized by copolymerization of ethylene and dihalogenoarenes with different structures [6]. The formation of partially or totally anisotropic melts was observed with fusible derivatives synthesized here which is evident from thermotropicity of these materials. The best thermal stability was observed with phenyl substituted PPV by thermogravimetry. The 10 Z weight loss was detected at 420°C (heating 20°C/min). Polarizing microscopy indicates a much lower thermal stability since irreversible changes were observed on annealing in the molten state (220°C) combined with solidification and a loss of anisotropy. The polymers being soluble as polymerized became insoluble on annealing. All fusible derivatives of PPV showed similar irreversible changes on annealing in the molten state.
883
Table 1 Derivatives of PPV synthesized by the Heck reaction l~'-d~Br
+ CH2--CH 2
solubility a) R
Mw d)
H,F,NO 2 CF 3 CH 3 C6H 5
X
~
~500 3500 9300 7100
X
Tsb)/oc
+ + ++
infusible 175 180 155
+ CH2fffiCH 2 [Pd] J,
solubility a) M w d) NMPII50°C H
CH 3
> 500
-
10700
+
~, _ ~ ,
thermal behaviour
NMP/I50°C
X--Br, I
R
[Pd]
birefringent c) ++ +
+
~n
thermal behaviour Tab)/o C birefringent c) infusible 158
+
a) - insoluble + ~ 0.1 weight~g ++ ~ 1 weight~g b) Ts ffisoftening point determined by polarizing microscopy (heating rate: 10°C / min) c) -not birefringent, + partially birefringent, ++ totally birefringent as determined by polarizing microscopy d) Molecular weights were obtained by gel permeation chromatography calibrated by polystyrene standards. Several experiments were performed on the phenyl substituted PPV in order to check whether the observed irreversible changes are caused by the Heck reaction or whether this is an inherent property of fusible derivatives of PPV. As polymerized and annealed (220°C for 5h) phenyl substituted PPV was amorphous (check by X-ray). therefore crystallization out of the molten forming insoluble material
884
can be ruled out. Thermally induced crosslinking via vinyl endgroups is also an insuffcient explanation for the irreversible changes out of the molten state since phenyl substituted PPV with exclusively bromine end groups showed the same thermal behaviour. 0.5 ~g Pd residues were detected in as polymerized PPV (estimated by atomic absorption spectroscopy). The Pd residues were removed by chromatography with triphenylphosphine substituted polystyrene gels as stationary phase[7]. Still the same irreversible change was observed out of the molten state with the Pd free phenyl substituted PPV. Therefore crosslinking induced by Pd residues is also an insufficient explanation for the observed irreversible changes out of the molten state. We conclude from our studies on the thermal stability of fusible derivatives of PPV that the irreversible changes out of the molten state combined with a loss ofanisotropy, solidification, and insolubility is an inherent property of this class of polymers. ACKNOWLEDGEMENTS Parts of this work were supported by Max-Buchner foundation and Fonds der Chemischen Industrie. Thanks are due to Mr. M. Mayer performing X-ray experiments. References I P~ F. Heck, Org. React., 27 (1981) 345. 2 W. Heitz, W. BrUgging, L Freund, ~ Gailberger, A. Greiner, H. Jung, U. Kampschulte, N. NieBner, F. Osan, H. W. Schmidt, ]VL Wicker, Makromol. Chem., 189 (1988) 119. 3 Ashai Glass Co. Ltd., Jpn 57,207,618 (1981). 4 A. Greiner, W. Heitz, MakromoL Chem. Rapid Commun, 9 (1988) 581. 5 M. Brenda, A. Greiner, W. Heitz, MakromoL Chem., 191 (1990) 1083. 6 H. Martelock, A. Greiner, W. Heitz, Makromol. Chem. in press. 7 W. Heitz, R. Michels, Ansew. Chem., 84 (1972) 296
881
PALLADIUM CATALYZED SYNTHESIS OF POLY(I,4PHENYLENEVINYLENE), A. Greiner, H. Martelock, W. Heitz. Philipps-Universitllt Marburg, Institut tilt Polymere und Zentrum fur Materialwissenschaften, Hans-Meerwein-Strasse, W-3550 Marburg / Germany.
Abstract A wide variety of derivatives of poly(l,4-phenylenevinylene) can be synthesized by the Pd catalyzed arylation of ethylene with dihalogenoarenes. Model reactions of bromobenzene and ethylene to trans-stilbene indicated complete conversion of bromobenzene, few side products, and almost exclusively trans-stilbene. Many of the synthesized derivatives of PPV are soluble and fusible. The fusible derivatives are thermotropic. Although, thermogravimetry indicates good thermal stability of most of the fusible derivatives of poly(l,4-phenylenevinylene) irreversible changes were observed in the molten state combined with a loss of anisotropy, solidification, and insolubility. Introduction Poly(l,4-phenylenevinylene) (PPV) is a main-chain conjugated polymer with a wide variety of interesting physical properties. PPV has been shown to be a good electrical conductor after being doped. The polymer backbone of PPV is formed by an alternating sequence of phenylene and vinylene segments. The backbone is rigid rod-like if the vinylene segments are in the all-trans configuration. Since PPV is insoluble and infusible no liquid crystallinity can be observed. The purpose of our investigations is the synthesis of soluble and fusible derivatives of PPV with a rod-like main chain. Results Soluble and Fusible derivatives of PPV may be obtained by structural variations which result in reduced interchain interactions as there Elsevier Sequoia/Printed in The Netherlands
882
are lateral stiff substituents on the phenylene segments and twisted biphenylene segments in the main chain. More soluble and fusible derivatives may be obtained by copolymerization of monomers with different structures to result in less symmetric molecules. In order to realize these structures a synthetic route was needed which results in a high yield of trans isomers and tolerates a wide variety of different monomers. Incited by the investigations of Heck [1 ] on the palladium catalyzed arylation of olefins by halogeno compounds we tested this reaction for the synthesis of PPV and its derivatives. The synthesis of PPV and derivatives was accomplished by different monomer combinations, as there are the Pd catalyzed polymerization of p-halogenovinylarenes [2] or of p-dihalogenoarenes with p-divinylarenes [3] or of p-dihalogenoarenes with ethylene [ 4 ] . In order to estimate structural defects caused by side reactions and to optimize the synthesis we investigated the Pd catalyzed model reaction of bromobenzene and ethylene to trans-stilbene. Bromobenzene was completely converted. Benzene, 2-methylstilbene, biphenyl, cis-stilbene, l,l-diphenylethylene were detected as side products by gas chromatography. The side reactions can be eliminated (formation of benzene, biphenyl, and triphenylethylene) or limited (formation 2-methylstilbene and l.l-diphenylethylene) by lower amounts of catalyst and by lower reaction temperatures. The formation of cis- stilbene is hardly detectable by gas chromatography. Details of this study are given elsewhere [5]. The results of the model reaction were transferred to the polyreaction of p-dihalogenoarenes and ethylene. A number of soluble and fusible derivatives of PPV were synthesized and characterized. Some representative examples are given in table 1. Soluble and fusible derivatives of PPV were also synthesized by copolymerization of ethylene and dihalogenoarenes with different structures [6]. The formation of partially or totally anisotropic melts was observed with fusible derivatives synthesized here which is evident from thermotropicity of these materials. The best thermal stability was observed with phenyl substituted PPV by thermogravimetry. The 10 Z weight loss was detected at 420°C (heating 20°C/min). Polarizing microscopy indicates a much lower thermal stability since irreversible changes were observed on annealing in the molten state (220°C) combined with solidification and a loss of anisotropy. The polymers being soluble as polymerized became insoluble on annealing. All fusible derivatives of PPV showed similar irreversible changes on annealing in the molten state.
883
Table 1 Derivatives of PPV synthesized by the Heck reaction l~'-d~Br
+ CH2--CH 2
solubility a) R
Mw d)
H,F,NO 2 CF 3 CH 3 C6H 5
X
~
~500 3500 9300 7100
X
Tsb)/oc
+ + ++
infusible 175 180 155
+ CH2fffiCH 2 [Pd] J,
solubility a) M w d) NMPII50°C H
CH 3
> 500
-
10700
+
~, _ ~ ,
thermal behaviour
NMP/I50°C
X--Br, I
R
[Pd]
birefringent c) ++ +
+
~n
thermal behaviour Tab)/o C birefringent c) infusible 158
+
a) - insoluble + ~ 0.1 weight~g ++ ~ 1 weight~g b) Ts ffisoftening point determined by polarizing microscopy (heating rate: 10°C / min) c) -not birefringent, + partially birefringent, ++ totally birefringent as determined by polarizing microscopy d) Molecular weights were obtained by gel permeation chromatography calibrated by polystyrene standards. Several experiments were performed on the phenyl substituted PPV in order to check whether the observed irreversible changes are caused by the Heck reaction or whether this is an inherent property of fusible derivatives of PPV. As polymerized and annealed (220°C for 5h) phenyl substituted PPV was amorphous (check by X-ray). therefore crystallization out of the molten forming insoluble material
884
can be ruled out. Thermally induced crosslinking via vinyl endgroups is also an insuffcient explanation for the irreversible changes out of the molten state since phenyl substituted PPV with exclusively bromine end groups showed the same thermal behaviour. 0.5 ~g Pd residues were detected in as polymerized PPV (estimated by atomic absorption spectroscopy). The Pd residues were removed by chromatography with triphenylphosphine substituted polystyrene gels as stationary phase[7]. Still the same irreversible change was observed out of the molten state with the Pd free phenyl substituted PPV. Therefore crosslinking induced by Pd residues is also an insufficient explanation for the observed irreversible changes out of the molten state. We conclude from our studies on the thermal stability of fusible derivatives of PPV that the irreversible changes out of the molten state combined with a loss ofanisotropy, solidification, and insolubility is an inherent property of this class of polymers. ACKNOWLEDGEMENTS Parts of this work were supported by Max-Buchner foundation and Fonds der Chemischen Industrie. Thanks are due to Mr. M. Mayer performing X-ray experiments. References I P~ F. Heck, Org. React., 27 (1981) 345. 2 W. Heitz, W. BrUgging, L Freund, ~ Gailberger, A. Greiner, H. Jung, U. Kampschulte, N. NieBner, F. Osan, H. W. Schmidt, ]VL Wicker, Makromol. Chem., 189 (1988) 119. 3 Ashai Glass Co. Ltd., Jpn 57,207,618 (1981). 4 A. Greiner, W. Heitz, MakromoL Chem. Rapid Commun, 9 (1988) 581. 5 M. Brenda, A. Greiner, W. Heitz, MakromoL Chem., 191 (1990) 1083. 6 H. Martelock, A. Greiner, W. Heitz, Makromol. Chem. in press. 7 W. Heitz, R. Michels, Ansew. Chem., 84 (1972) 296