CONDUCTIVE POLYMERS AND PROCESSES FOR THEIR PREPARATION

Abstract: Novel highly conductive ordered polymers are prepared by polymerising a heteroaromatic monomer compound, for example a 3-alkylthiophene, in a reaction mixture in which a compound of nickel (II) or cobalt (II) is dissolved.

Description: CONDUCTIVE POLYMERS AND PROCESSES FOR THEIR PREPARATION This invention relates to conductive polymers and to processes for their preparation. Known conductive polymers include poly (heteroaromatic) compounds prepared by oxidative polymerisation in the presence of transition metal compounds. In some cases the metal compounds have been reduced to metallic islands. In other cases metal-containing anions increase conductivity by acting as dopants. Other mechanisms may also be at work. An important point is that in each case the polymer relies on the continued presence of the transition metal species for conductivity. In some cases the transition metal species is permanently bound into the polymer to the extent that it cannot be removed. In others it can be removed, by washing, but the conductivity of the polymer drops in line with removal of the transition metal species. We have now devised a process for making conductive poly (heteroaromatic) compounds, which process employs transition metal compounds. An ordered polymer may be produced by the process, which means that charge carrier mobility is high. Dopants are thus particularly effective in bringing about conductivity enhancement, and a polymer of very high conductivity may be produced. In accordance with a first aspect of the present invention there is provided a method of preparing a conductive polymer in a reaction mixture by the oxidative polymerisation of a heteroaromatic monomer compound in the presence of a compound of nickel (II) or cobalt (II) which dissolves in the reaction mixture, wherein nickel (II) or cobalt (II) species are removable from the polymer formed, and the polymer is such that it has greater conductivity (preferably at least 2x, more preferably at least 5x) than a corresponding polymer prepared in the absence of a said compound of nickel (II) or cobalt (II) . A polymer prepared by a process of the invention is believed to have a more ordered structure than a corresponding polymer prepared in the absence of a said compound of nickel (II) or cobalt (II) , and this is thought to be the reason for the greater conductivity. Suitably the presence of the nickel or cobalt compound during the polymerisation is responsible for enhanced conductivity (even through it may subsequently be removed from the polymer) , at least when doped by a suitable dopant, and preferably also when undoped. It should be noted that nickel (II) and cobalt (II) species are removable from the polymer formed but need not necessarily be removed. It may be acceptable to leave them in the polymer. In embodiments in which they are removed, their residual level in the polymer is then suitably no more than 100 ppmw, preferably no more than 20 ppmw, and most preferably no more than 10 ppmw. Thorough washing e.g. by Soxhlet extraction may give removal down to a content of a few ppmw. Complete removal may be achieved by use of a complexing agent such as EDTA. A conductive polymer as defined herein suitably has intrinsic conductivity of at least lxlO "2 Sm "1 , and this in itself is likely to be higher by a factor of at least 10 than that of a corresponding polymer prepared in the absence of said nickel or cobalt compound. By "intrinsic conductivity" we mean conductivity of a sample of the polymer which has a content of nickel (II) or cobalt (II) species in the range 0-100 ppmw, and no dopant, such as other transition metal species, or iodine. This level of conductivity means that a polymer in accordance with the present invention may find use e.g. as an anti-static coating even in the absence of any dopant. Suitably, however, polymers in accordance with the invention contain a dopant. Preferably, the conductivity of such a doped polymer is at least 1,000 Sm "1 , preferably at least 5,000 Sm "1 , most preferably at least 15,000 Sm "1 . Conductivities in excess of 50,000 Sm "1 are achievable in preferred embodiments. Such conductivities may be achieved even when the nickel or cobalt species are removed. In relation to possible dopants, any dopants conventionally used in the art may be employed. Iodine is a convenient dopant, primarily for laboratory assessment. When electrochemical processes are carried out in accordance with the invention examples of suitable dopants are fluoroborate, toluene sulphonate, trifluoromethane sulphonate and perchlorate species. When chemical processes are carried out in accordance with the invention suitable dopants may include transition metal oxidants such as FeCl 3 , and arsenic pentafluoride and antimony pentafluoride . Preferably the process is such that the presence of said compound of nickel (II) or cobalt (II) dissolved in the reaction mixture brings about a polymer of significantly higher molecular weight, (preferably at least double, whether assessed as weight average molecular weight or number average molecular weight) than that of a polymer prepared under identical conditions, but in the absence of said compound of nickel (II) or cobalt (II) . The heteroaromatic monomer compound used in the process of the invention may be an asymmetric compound which can polymerise in one of two (or more) orientations. Preferably the process is such that the presence of said compound of nickel (II) or cobalt (II) brings about a polymer of higher regioregularity than that of a polymer prepared under identical conditions, but in the absence of said compound of nickel (II) or cobalt (II) ; preferably the incidence of regioregular couplings is at least 10% higher, preferably at least 20% higher. Preferably, the heteroaromatic monomer compound used in the process of the invention is a compound of general formula wherein X represents a sulphur or oxygen atom, or an -N-H group, R 1 and R 2 independently represent a hydrogen atom, or an optionally substituted alkyl group, or an optionally substituted alkoxy group, or R 1 and R 2 together represent an optionally substituted butylene chain or a chain of formula -O-Q-O- wherein Q represents an optionally substituted ethylene chain. R 1 and R 2 may be such that the compound is asymmetric about the hetero atom X. An optional substituent of an alkyl, alkoxy or alkylene group suitably includes halogen, especially fluorine, chlorine or bromine atoms, and nitro, cyano, hydroxyl, C M alkoxy, C haloalkoxy, (C M alkoxy) carbonyl, amino and C alkylamino groups. It is preferred, however, that alkyl, alkoxy or alkylene groups are unsubstituted. An optionally substituted alkyl or alkoxy group R 1 and/or R 2 suitably has 1 to 20 carbon atoms, preferably 1 to 12 carbon atoms, most preferably 1 to 8 carbon atoms. Any such group having more than 3 carbon atoms may be branched but is preferably linear. In preferred embodiments R 1 and R 2 represent independent moieties, rather than a chain. Those independent moieties may differ from each other, in preferred embodiments. Preferably, R 1 and R 2 independently represent a hydrogen atom or an alkyl group. Preferably, R 1 represents an alkyl group. Preferably, R 2 represents a hydrogen atom. The moiety X preferably represents an N-H group or a sulphur atom, most preferably a sulphur atom. Any nickel (II) or cobalt (II) compound which dissolves in the reaction mixture may be employed. Suitable organic salts may, for example, be sulphonates, for examples toluene sulphonate and trifluoromethane sulphonate. However inorganic salts are preferred, especially the chlorides and perchlorates. The reaction mixture preferably comprises an organic solvent and a small amount, suitably no more than 2% wt, and most preferably substantially no, water. A wide range of organic solvents may be employed and the choice of a solvent for the particular process is well within the compass of the person skilled in the art, using his ordinary skill and knowledge. Solvent optimisation may be achieved by the ordinary process of trial and error. However, by way of guidance, we can state that polar aprotic solvents e.g. propylene carbonate, ethylene carbonate and acetonitrile are suitable for electrochemical processes in accordance with the invention, and halogenated hydrocarbon solvents, for example C,^ chloroalkanes, or nitromethane or nitrobenzene are suitable for non-electrochemical processes. In general, amine, alcohol and ether solvents are preferably not used either for electrochemical or chemical processes. The temperature for the reaction is a matter of choice but most such reactions are best carried out at relatively low temperature, for example -20 to 50°C, preferably 0 to 40°C, to assist in the formation of an ordered polymer structure. The concentration of the heteroaromatic monomer compound in the reaction mixture may suitably be in the range 0.01 to 2 M, preferably 0.05 to 1 M, most preferably 0.1 to 0.3 M. The concentration of the compound of nickel (II) or cobalt (II) in the reaction mixture may vary particularly widely, suitably being in the range 0.0001 to 2 M, preferably 0.001 to 1. Electrochemical processes may require relatively lower concentrations, for example 0.0001 to 0.1 M, preferably 0.001 to 0.01 M. Non- electrochemical process may require relatively higher concentrations, for example 0.01 to 2 M, preferably 0.05 to 0.5 M. The relative proportions of the heteroaromatic monomer compound (A) and the compound (B) of nickel (II) or cobalt (II) may vary very widely, for example within the molar range 1000:1 - 1:100 (A:B) , preferably 200:1 - 1:10. Generally electrochemical processes may require higher proportions of heteroaromatic monomer compound to the compound of nickel (II) or cobalt (II) , for example within the molar range 1000:1 -10:1, preferably 200:1 - 20:1, and non-electrochemical processes may require lower proportions of heteroaromatic monomer compound to the compound of nickel (II) or cobalt (II) , for example within the molar range 100:1 - 1:10, preferably 20:1 - 1:5. Nickel and cobalt compounds may be used separately or together in processes of the invention and the above definitions relate to their combined amount, in embodiments in which they are used together. It is believed that the nickel (II) or cobalt (II) compounds used in the process of the invention dissolve in the reaction mixture to form Ni 2+ or Co + cations, and that these act as non-oxidising cations which do not initiate the polymerisation reaction, but which serve to catalyse or otherwise favour the formation of a more ordered polymer structure, and a polymer of higher molecular weight (perhaps by weak temporary complexation with the hetero atom) than would be achieved under corresponding conditions without the presence of the Ni 2+ or Co 2+ cations, and this is the cause of enhanced conductivity. It seems clear, because of the retention of conductivity after removal of the nickel or cobalt species, that they do not give rise to enhanced conductivity by known doping effects or by the formation of metal inclusions or by their permanent incorporation in or co-ordination with the polymer. However, whatever the mechanism is, it is not limiting upon the scope of the present invention, which is underpinned by the fact that there is a measurable improvement in polymer properties, rather than by its precise mechanism. The invention extends to a polymer made by a process in accordance with the present invention. Such a polymer is believed to be inherently new by virtue of its properties and not just by virtue of the process by which it is made and so in a further aspect the present invention provides a conductive polymer prepared by oxidative polymerisation of a heteroaromatic monomer compound, characterised by a conductivity when doped of at least 1000 Sm "1 (preferably at least 5000 Sm "1 , most preferably at least 15000 Sm "1 ) and a content of nickel (II) or cobalt (II) species in the range of 0-100 ppmw (preferably 0-20 ppmw, most preferably 0-10 ppmw) ; or a content of nickel or cobalt species in excess of 100 ppmw but which can be lowered to the range 0-100 ppmw (preferably 0-20 ppmw, most preferably 0-10 ppmw) with retention of conductivity when doped of at least 1000 Sm "1 (preferably at least 5000 Sm 1 , most preferably 15,000 Sm "1 ) . Preferably the number average molecular weight of the polymer is at least 10,000, most preferably at least 20,000. Preferably the weight average molecular weight of the polymer is at least 60,000, most preferably at least 100,000. Preferably the polymer contains no transition metal species, other than, optionally, the nickel (II) or cobalt (II) species. When the polymer is formed from asymmetric molecules which may polymerise in two (or more) orientations the proportion of identical couplings of monomers is preferably at least 65%, preferably at least 75%. As implied above by statements relating to the process the novel polymer is preferably a polythiophene, most preferably a poly (3-alkylthiophene) . Polymers described herein may find application as antistatic coatings for plastics products, but the conductivity is sufficiently high when doped that they may also be used as conductors in electronic circuits and devices, for example in printed circuit boards. Further applications include electromagnetic radiation shielding and electric wires and cables, with the polymer as an intermediate conductive interlayer between the conductor and the insulator. These may be wires and cables in which an insulating sheath surrounds a metallic core conductor. These may also be wires and cables, for example high tension power cables or other coaxial cables, in which the conductive layer is an outer sheath, around an insulator core. Used as an intermediate layer between the conductive sheath and the insulator core a conductive polymer of the invention will act as a field-smoothing layer. Polymers described herein may find application as such or may in known manner be blended or compounded with other polymers to obtain polymers with properties optimised for selected applications. The invention will now be described further with reference to the following illustrated examples. EXAMPLES Example 1 3-Methylthiophene (0.2 M) and dried tetraethyl- ammonium tetrafluoroborate (0.03 M) were dissolved in dry propylene carbonate. The solution was placed in a single compartment cell, and purged with nitrogen gas for 10 minutes. Using an indium-tin oxide anode, and a platinum cathode, it was then electrolysed at 5°C in a manner well known in the art, at a constant current density of 14 mA cm 2 . After 30 minutes' growth, the resulting black, shiny film on the anode had a conductivity of 7700 Sm "1 . Polymer films grown under identical conditions, but in the presence of 0.003 M of Ni(C10 4 ) 2 dissolved in the electrolyte had a typical conductivity of 21,000 Sm "1 . The polymer dopant was fluoroborate ions. Example 2 The same electrochemical polymerisation method as in Example 1, but with 0.003 M of anhydrous CoCl 2 dissolved in the electrolyte instead of Ni(Cl0 4 ) 2 , led to a film having a conductivity of 72,000 Sm 1 . Example 3 3-Methylthiophene (0.1 M) and anhydrous iron (III) chloride (0.2 M) were added to nitrogen-purged chloroform, with vigorous stirring at ambient temperature. The temperature was raised to 30°C and oxidative chemical polymerisation was allowed to occur over a period of 30 hours, the mixture being kept under a small excess pressure of inert gas. The product was filtered, rinsed with chloroform and vacuum dried. It was then Soxhlet- extracted for several hours with methanol, to remove FeCl 3 , and dried; at this stage, an iron concentration of 250-300 ppm by weight was detectable in the polymer. After doping to saturation by exposure to iodine vapour, the powder was pressed into pellets, which had a typical (4-probe) conductivity of 100 Sm "1 . The same process repeated with 0.2 M nickel (II) chloride led to a nickel content below 5 ppmw after Soxhlet washing, but a conductivity averaging about 1600 Sm "1 after iodine doping. Example 4 The same process as in Example 3, but using 3- hexylthiophene as monomer instead of 3-methylthiophene led, after Soxhlet extraction, to a soluble polymer. This was dissolved in chloroform, shaken in a separating funnel with three successive portions of (0.1 M) aqueous ethylenediamine tetra-acetic acid, tetrasodium salt (sodium EDTA) to extract the metal compounds fully. After rinsing the chloroform solution with water and drying the solution with an activated molecular sieve, it was used to cast thin films (by evaporation of a small quantity of the solution spread on a horizontal glass plate) . The conductivity was 4000 Sm "1 after iodine doping; however, films of polymer synthesised under identical conditions except for the additional presence of 0.1 M of anhydrous NiCl 2 exhibited a conductivity of 38,000 Sm "1 when doped. Chemical analysis of the pure polymers showed that both materials could fairly be described as poly (3-hexyl-2 , 5- thiophenediyl) ; however, detailed examination of their nuclear magnetic resonance spectra and molecular weight distributions revealed a possible explanation for the difference in their properties. The structures of the peaks at about 5=7.0 ppm in the Η NMR spectra (due to vinylic protons) showed that the polymer synthesised in the presence of FeCl 3 alone had only a 50 to 60% incidence of head-to-tail, head-to-tail couplings between the thiophene rings, whereas the polymer synthesised in the presence of NiCl 2 was at least 25% more regioregular . Moreover, the number average relative molecular mass M„ of the former material was 5500 (M w = 50,300), while that synthesised in the additional presence of NiCl 2 was 23,900 (M„, = 125,000). The sodium EDTA treatments successfully removed all metal ions from both the polymers, and the difference in polymer quality using NiCl 2 is believed to be due to Ni 2+ ions acting in a catalytic role during the polymerisation reaction.

Claims: CLAIMS A method of preparing a conductive polymer in a reaction mixture by the oxidative polymerisation of a heteroaromatic monomer compound in the presence of a compound of nickel (II) or cobalt (II) which dissolves in the reaction mixture, wherein nickel or cobalt species are removable from the polymer formed, and the polymer is such that it has greater conductivity than a corresponding polymer prepared in the absence of a said compound of nickel (II) or cobalt (II) . A method as claimed in Claim 1, wherein the heteroaromatic monomer compound is such that the monomers can polymerise in a plurality of orientations, and the method is such that the polymer possesses higher regioregularity than that found in such polymers prepared under identical conditions but in the absence of a said compound of nickel (II) or cobalt (II) . A method as claimed in Claim 1 or 2 , wherein the heteroaromatic monomer compound is a compound of general formula wherein X represents a sulphur or oxygen atom or an -N-H group, R 1 and R 2 independently represent a hydrogen atom, or an optionally substituted alkyl group, or an optionally substituted alkoxy group, or wherein R 1 and R 2 together represent an optionally substituted butylene chain or a chain of formula -0- Q-0- wherein Q represents an optionally substituted ethylene group. 4. A method as claimed in Claim 3, wherein X represents a sulphur atom. 5. A method as claimed in Claim 3 or 4 , wherein R 1 represents an alkyl group and R 2 represents a hydrogen atom. 6. A method as claimed in any preceding claim, wherein the compound of nickel (II) or cobalt (II) is selected from NiCl 2 , CoCl 2 , Ni(C10 4 ) 2 and Co(Cl0 4 ) 2 . 7. A method as claimed in any preceding claim, wherein the conductivity of a said polymer, which polymer contains a dopant, is at least 1000 Sm "1 . A method as claimed in any preceding claim, wherein the nickel (II) or cobalt (II) species are removable from the conductive polymer to a content in the range of 0-100 ppmw. A conductive polymer prepared by a method as claimed in any preceding claim. 10. A conductive polymer prepared by oxidative polymerisation of a heteroaromatic monomer compound, characterised by electrical conductivity when doped of at least 1000 Sm "1 and a content of nickel (II) or cobalt (II) species in the range 0-100 ppmw; or a content of nickel (II) or cobalt (II) in excess of 100 ppmw but which can be lowered to the range 0-100 ppmw with retention of electrical conductivity of at least 1000 Sm "1 when doped.

Inventors: BUDD DARREN, DAVIS REGINALD, FOOT PETER JONATHAN SAMUEL, BUDD, DARREN, DAVIS, REGINALD, FOOT, PETER, JONATHAN, SAMUEL
Applicants: BAXENDEN CHEM LTD, BUDD DARREN, DAVIS REGINALD, FOOT PETER JONATHAN SAMUEL, BAXENDEN CHEMICALS LIMITED, BUDD, DARREN, DAVIS, REGINALD, FOOT, PETER, JONATHAN, SAMUEL
Application year: 1997
Main classification: C08G61
IPC codes: C08G61