This invention relates to polymers that are inherently electrically conductive and to a method of making them. One of the applications for which the polymers may be used is in the control of electric fields in cable joints.

A variety of electrically conducting polymers have been discovered and a number of them are now finding useful applications. Among them are polyaniline, polypyrrole, polyacetylene, polythiophene, poly-p-phenylene, poly- {p-phenylene sulfide) and poly-N-vinylcarbazole .

All of these established conducting polymers have an essentially one-dimensional structure composed of resonant (aromatic and/or heterocyclic) rings connected to form a single chain in which conjugation extends from end to end of the molecule. A consequence of this is that they conduct primarily along the length of the molecule and have relatively small conductivities unless the molecules are aligned by orientation, and then conduct well in only one direction.

It is an object of the present invention to provide novel conductive polymers which have enhanced conductance in directions other than along the polymer chains and are potentially isotropically conductive. The polymers in accordance with the invention comprise chains composed of resonant (aromatic and/or heterocyclic) rings in which conjugation extends from end to end of the molecule and crosslinks between these chains and is characterised in that such crosslinks consist of resonant groups conjugated to the chains that they link.

Preferably the polymer chains are selected from polyaniline, polypyrrole, polythiophene, poly p-phenylene,

poly- (p-phenylene sulfide) and poly-N-vinylcarbazole.

The resonant groups constituting the crosslinks may be carbocyclic or heterocyclic aromatic groups, conjugated π-bonded aliphatic groups, or combinations of any of these. We prefer that (apart from any parts contributed by sidechains already present in the polymer chains) they are selected from p-phenylene groups, p-phenylene-vinylene groups and sequences of one or both of these.

In accordance with the other main aspect of the invention, a process for making such polymers comprises a conjugative condensation reaction catalysed by a palladium compound.

Three such condensation reactions are established and are named according to their respective authors as the Suzuki , Stille, and Heck reactions. Our current preference is for the Suzuki reaction.

The Suzuki reaction requires the prior introduction into one of the aromatic molecules to be linked of a leaving group selected from halides (preferably bromine) and the triflate group (also called trifluoromethane sulfonate) . Preferably these groups are introduced into the polymer chains. The other molecule to be linked (preferably the relatively small molecule that is to provide the crosslinking groups) is pre-reacted with an organoboron compound to introduce -B(OH) ₂ substituents. Practical catalysts for the condensation reaction between these groups to form notionally single bonds that bridge the resonant systems of the two molecules are dichloroJbis[1,1' -J is(diphenyl phosphino) ferrocene] - palladiu (II) , which we prefer on grounds of stability and ease of handling, and tetrakis(triphenylphosphine) palladium(0) . Other catalysts of the same class may be used

if available, but are not known to have any advantage. The Suzuki reaction requires aqueous base to be present, and we prefer the conventional use of potassium phosphate (triposassium orthophosphate) . The Stille reaction is closely similar and may use the same catalysts. The only fundamental distinction is that the Stille reaction introduces -SnR ₃ groups, instead of -B(OH) ₂ groups, into the second molecule to be linked.

The Heck reaction is a little different in that it requires a pendant alkene group in one of the molecules to be linked. This is reacted with an arylpalladium group introduced into the other molecule to be linked, and since this group is labile it is invariably generated in si tu . Four reactions may be used for this purpose: • reaction of a bromo- substituent with a palladium- triarylphosphine complex; reaction of an iodo- substituent with palladium acetate in the presence of a base; reaction of a substituent group selected from -HgR and -HgX (where R is an aromatic group and X is halide or triflate) with lithium palladium trichloride; and direct reaction of the unsubstituted aromatic molecule with palladium acetate (or metallic palladium and silver acetate) in acetic acid. The Heck reaction, rather than the others, is appropriate to forming p-phenylene-vinylene crosslinks.

If the crosslinking reaction is carried out in dilute solution, the product will normally be obtained as a powder which, because it is crosslinked, will be infusible, insoluble and generally difficult to use, except as a conductive filler in polymers and coating compositions.

Preferably, therefore, the crosslinking reaction is carried out in a concentrated solution from which solvent may evaporate during and/or after the reaction to deposit the crosslinked product as a continuous coating. At least some of the polymers in accordance with the invention have been found to have markedly non-linear electrical properties, with conductivities at even 10 -20V in some cases two or three orders of magnitude higher than in the oh ic region at low voltages; thus polymers whose conductivity as measured in the ohmic region appears quite inadequate may be usefully conductive at voltages in the kV range.

In the jointing of high-voltage electric power cables with polymeric dielectric, electrostatic fields need to be precisely controlled to avoid local stress concentrations sufficient to risk electrical breakdown and degradation at transient peaks and eventual gross failure, and this requires careful attention to the place where the dielectric screen of the cable is terminated. One current practice is to chamfer the edge of the screen, but this entails the loss of at least about 0.5mm of the radial thickness of the dielectric where the screen has been removed from it, and this is increasingly unacceptable as cables are designed to operate at higher stress levels. Alternatively the screen is terminated in a plane normal to the cable axis, and as it is impracticable to make a cut with complete accuracy and total repeatability (and in most cases impossible to re-cut if anything goes wrong) this is achieved by removing the screen a little further than desired and restoring to the required position by applying a conductive coating based on carbon black. This coating is not of great strength and is at risk of damage

when other cable components (especially the "stress cone") are slid into place. In the nature of the problem, the coating is inaccessible and cannot be inspected for such damage after the stress cone is in place, since removing it would multiply the risk. It is suspected that such damage may be responsible for a significant fraction of joint failures in service. By forming in si tu in a coating on the joint the polymers of the present invention, a tougher conductive coating can be formed and the risk substantially reduced.

The invention will be further described, by way of example, with reference to the accompanying drawings in which each of equations 1-4 represents a step in the synthesis of one of the conducting polymers to be described and each of Figures 1-3 is a graph showing a current/voltage characteristic of one polymer in accordance with the invention.

Example 1

A copolymer of (α-thiophenediyl) benzylidene and its p-bromo derivative was synthesised from commercially available starting materials as follows (Equation 1) . Thiophene (25.2 g) , benzaldehyde (15.0 g) and p-bromobenzaldehyde (26.3 g) were dissolved in dichloromethane (120 ml) in a 3-necked flask under a nitrogen atmosphere. This solution was stirred and cooled to 0°C. Boron trifluoride etherate (14.3 g) was then added dropwise and the mixture refluxed for 17 hours. After this time the solvent was removed leaving a viscous dark brown liquid. This liquid was dissolved in dichloromethane (400 ml) , precipitated into methanol, washed with methanol and dried. The product, a copolymer of (α-thiophenediyl)benzylidene and (α-thiophenediyl)

p-bromobenzylidene, was a red/brown solid.

The above procedure and quantities give a polymer with about 30% of the phenyl rings brominated; the degree of bromination could be altered by varying the ratio of benzaldehyde to p-bromobenzaldehyde.

The copolymer of (α-thiophenediyl) benzylidene and (α-thiophenediyl) p-bromobenzylidene made as described (2.0 g) was dissolved in the volume of tetrahydrofuran indicated in Table 1 and 9 ml of 2M aqueous potassium phosphate added to form a first solution ("Solution A"). Dichloro istl,1' -Jis(diphenylphosphino) ferrocene] palladium(II) (0.02 g) as catalyst and 4,4' -biphenyl diboronic acid (in amount tabulated, calculated to produce the degree of crosslinking indicated on the basis of total reaction) to form a crosslinking group were dissolved in tetrahydrofuran (6 ml) to form a second solution ("Solution B") .

The final cross-linked polymer was produced by thoroughly mixing the whole of Solutions A and B and heating under reflux conditions for 24 hours. The product was obtained as a black insoluble powder, washed and dried; portions were pressed into circular plaques 20.1 mm ² in area and about 0.5 to 1 mm thick under a pressure of 25 tonnes; the plaques were sandwiched between electodes and current/voltage relationships plotted using a Keithley 617 Electrometer, each reading being taken after stabilising for 5 minutes. For initial comparison, the ohms law region of each plaque was determined (for accuracy by determining the part of the characteristic in which the gradient of log V vs log I was 1) and the resistivity calculated and tabulated for that region. This procedure will be more fully described

below, with reference to Figures 1-3

Table 1

Run no Solvent Amount of nominal % conductivity volume diboronic acid crosslinking (ohm cm) ^"1 (ml) ⁽ g)

1 40 0.75 30 3 x 10 ^~9

2 40 0.50 20 1 x lO ^"15

3 40 0.25 10 8 x 10 ¹²

4 40 0.25 10 5 x 10 ^"7

5 20 0.25 10 2 x 10 ^~8

6 10 0.25 10 1 x 10 ^"8

Example 2 This was substantially the same as Example 1 Run 1 except that the volume of solvent in each of solutions A and B was reduced until the solution became very viscous and the final polymer was obtained by mixing the solutions and applying the mixture immediately to a dry surface of an inert polymer material and then heating to 50°C for 24 hours to cure the coating and remove the residual solvent from it. The final cross-linked polymer was a black insoluble solid. Example 3 This was similar to Example 1 except that it was based on a hompolymer of p-bromobenzaldehyde instead of the copolymer; in the first stage of synthesis, the benzaldehyde was (of course) omitted and the amount of p-bromobenzaldehyde doubled to 52.6 g; subsequent procedure was as before and the results were as follows:

Table 2

Run no Solvent Amount of nominal % conductivity volume (ml) diboronic crosslinking (ohm cm) ^" acid (g)

7 40 0.75 30 8 x 10 ^"10

8 40 0.50 20 1 x 10 ^'15

9 40 0.25 10 5 x 10 ^~8

10 40 0.25 10 4 x 10 ^"8

11 20 0.25 10 3 x 10 ^-8

12 10 0.25 10 1 x 10 ^"8

To clarify the procedure for estimating conductivities and by way of illustration, Figures 1-3 show data for Run 10 of this Example, respectively in log/log, linear/linear and enlarged log/log format. Figures 1 and 2 clearly demonstrate the highly non-linear characteristics of this polymer for voltages of each polarity, while Figure 3 shows how a log-log plot (here restricted to the relevant part of the "positive" polarity side of the graph) more clearly identifies the Ohm's law region in which the gradient of Figure 2 is constant and that of figures 1 and 3 is unity. Having identified the linear region of Figure 2 in this way, its gradient and thus the resistance of the plaque can be determined and conductivity (and/or resistivity) calculated in the usual way.

Example 4

Poly (3 -phenyl-2 , 5-thiophene) was synthesised in three stages via 3 -phenyl-2 -chlorothiophene as follows (Equations 2-4 ) . 2 , 5-Dichlorothiophene (5 . 0 g) , dichloromethane (20 ml) and

benzene (8.8 ml) were mixed together in a 2-necked round bottomed flask, cooled to 0°C and stirred for 10 minutes. Aluminium chloride (4.4 g) was then added slowly over a period of 5 minutes. The mixture was stirred at 0°C for 30 minutes, then at room temperature for 1 hour and then refluxed for 30 minutes. After this time the mixture was poured into ice-water (120 ml) and the product, 3-phenyl- 2-chlorothiophene, extracted with dichloromethane (200 ml)and further purified by column chromatography. 3-Phenyl- 2-chlorothiophene (2.0 g) was dissolved in carbon tetrachloride (20 ml) and sulfuryl chloride (2.08 g) added. The mixture was refluxed for 60 hours then poured into ice- water (150 ml) . The product was extracted with carbon tetrachloride (200 ml) and purified by column chromatography. The product, 3-phenyl-2,5-dichlorothiophene, was a colourless oil.

A sample of the 3-phenyl-2,5-dichlorothiophene so produced was polymerised as follows: nickel chloride (0.155 g) , dipyridyl (0.187 g) , triphenyl phosphine (6.27 g) , zinc (4.84 g) and the monomer, 3-phenyl-2,5-dichlorothiophene (3.91 g) , were added to dry N, iV-dimethylacetamide (8 ml) under a nitrogen atmosphere. The mixture was stirred at 90°C for 3 hours then cooled and diluted with iV-methyl pyrollidone(30 ml). The reaction mixture was filtered and precipitated into 10% hydrochloric acid in methanol (1.5 1). The product was filtered, washed with methanol and dried giving the product, poly(3-phenyl-2,5-thiophene) , as a red/brown powder.

A sample of this poly(3-phenyl-2,5-thiophene) (4.0 g) was dissolved in carbon tetrachloride (90 ml) in a 2-necked round bottomed flask. The flask was covered with aluminium

foil to eliminate light from the reaction. Thallic acetate (0.124g) was added and the mixture stirred for a few minutes. Bromine (1.30 ml) was added dropwise and the reaction mixture refluxed for 3 hours. After this time the flask was cooled and the solution precipitated into methanol. The solid polymer was filtered, washed with methanol and dried. The product, poly(3- (p-bromophenyl) -2, 5-thiophene) , was a yellow solid.

The above procedure and quantities give a polymer with substantially all of the phenyl rings mono-brominated. The degree of bromination could be reduced by using smaller amounts of bromine and varying the reaction time.

Brominated poly(3-phenyl-2, 5-thiophene) (2.42 g) made as just described was dissolved in tetrahydrofuran (20 ml) and 9 ml of 2 aqueous potassium phosphate added together with 0.02g of dichloroJbis [1, 1' -Jis(diphenyl phosphino) ferrocene] palladium (II) and the tabulated amounts of 4,4' -biphenyl diboronic acid. The mixture was refluxed for 24 hours and the product (a brown solid) recovered and tested as in Example 1; results are given in Table 3 below, together with those for Examples 4 and 5. Example 4

This was substantially the same as Run 15 (or Run 16) of Example 3 except that instead of the reaction mixture being refluxed for 24 hours it was simply allowed to react at ambient temperature for the same length of time; it is suspected that the crosslinking reaction may not have been complete when the product was recovered. Example 5 This was also similar to Run 15 (or Run 16) of Example 3, except that in the polymeration reaction 10 mole % of the

3-phenyl-2,5-dichlorothiophene was replaced by 2,5-dichloro- thiophene.

Table 3

Run or Amount of Nominal degree conductivity

Example diboronic acid of crosslinking (ohm.cm) ^" no. ⁽ g ⁾ (%)

Run 13 0.125 5 2 x 10 ^"10 Run 14 7 x 10 ^'10

Run 15 0.25 10 4 x 10 ^'10

Run 16 3 x 10 ^'11

Run 17 0.375 15 3 x lO ^'15

Run 18 0.75 30 3 x lO ^"10

Example 4 0.25 10 4 x 10 ^"12

Example 5 0.25 10 1 x 10 ^-9

Example 6

This was similar to Run 18, except that in similar manner to Example 2 the reagents were prepared in two separate solutions and then the volume of solvent was in each reduced until the solution became very viscous. The final cross- linked polymer was produced by thoroughly mixing the two Solutions, applying the mixture to a dry surface of inert polymer and heating to 50°C for 24 hours to cure the coated polymer mixture and remove the residual solvent. The final cross-linked polymer was a brown insoluble solid. Examples 7-8

Tetrakis(triphenylphosphine)palladium(0) (0.02 g) as catalyst and 4,4 ' -biphenyl diboronic acid (0.334 g) to form crosslinking groups were dissolved in tetrahydrofuran (10 ml)

and the volume of the solvent reduced until the solution became very viscous and Examples 2 and 6 respectively were repeated except that this solution was used instead of the solution containing dichlorobis[1,1' -Jbis(diphenylphosphino) ferrocene] palladium(II) ; results were similar. Example 9

A 250kV power cable insulated with cross-linked polyethylene was prepared for jointing in the usual way and the dielecric screen trimmed normal to the cable axis some 5 mm beyond the design position of the screen end. At the latter position, an accurate square edge was defined by temporary application of a masking tape. The mixed solution from Example 2 was painted on the bare area of dielectric between the masking tape and the cut edge of the screen and for a few millimetres onto the screen. The coated area was heated to a temperature of about 50°C by a portable gas heater for 24 hours while work continued on other parts of the joint (the completion of a complete cable joint at such supertension voltages takes several days, and the reaction time is not unacceptable) .