Conjugated polymers based on poly (1, 4-phenylenevinylene) (PPV) have been extensively studied as the emissive layers in light-emitting diodes (LEDs). There have been a number of reports of methods to introduce small levels of non- conjugated sequences into PPV derivatives prepared via precursor polymers which in some cases has been shown to improve the emissive properties of the polymer. In conjugated polymers that are prepared via precursor polymers this has been achieved in two ways. First, by copolymerising two monomers of different reactivities it is possible to preferentially substitute the leaving group next to the more reactive monomer group with a less labile leaving group. On conversion the less labile moiety remains on the polymer backbone whereas the more labile leaving group is eliminated to give conjugated sequences. For example, with precursor copolymers containing phenylene and 2, 5-dimethoxyphenylene units prepared from bis- sulfonium monomers the sulfonium groups are substituted by methanol, which is used as a solvent, preferentially next to the 2, 5-dimethoxyphenylene moieties either during the polymerisation or subsequent purification and processing steps. On thermal conversion the sulfonium groups are eliminated leaving some of the methoxy leaving groups in place. The disadvantage of this route is that it can be difficult to control the ratio of the two monomers in the precursor polymer because of their different chemical reactivities. The alternate but similar route involves the substitution of some of the leaving groups of a homoprecursor polymer with less labile substituents before thermal or base catalysed elimination of one of the leaving groups. With both these methods the remaining groups on the ethylene linkages of the polymer backbone are still leaving groups and are therefore still liable to eliminate. This obvious source of polymer chemical instability could decrease the lifetime of LEDs based on these materials. It would therefore be advantageous to have a method of forming PPV based copolymers and the like which contain saturated ethylene linkages with no leaving groups attached.

According to the present invention a new method for the introduction of non-

conjugated sequences into conjugated polymers is disclosed. According to the present invention there is provided a process for preparing a precursor of a poly (arylenevinylene) wherein some of the vinylene groups are replaced by ethylene groups which process comprises subjecting a starting polymer which possesses recurring units of the formula -Ar-CHX-CHR'- wherein X represents a leaving group, Ar represents an arylene or heteroarylene group and R3 represents hydrogen or an alkyl group, to a radical reaction so as to substitute some of the X groups by hydrogen as well as the precursors obtained.

The issues which need to be addressed can be discussed more simply by reference to a particular precursor, namely the chloro precursor polymer i. e. leaving group X is chlorine to the insoluble poly [2- (2'-ethylhexyloxy)-5-methoxy-1, 4- phenylenevinylene] (I-MEHPPV).

The object of this invention is to remove a proportion of the chloro leaving groups from the precursor polymer to leave an ethylene linkage between adjacent phenyl rings. This can be achieved by radical dechlorination. However, for this method to work effectively a number of issues needed to be considered including the potential of thermal elimination during the reaction, the possibility of intra-or intermolecular crosslinking caused by the coupling of two carbon based radicals, the degree of control over the dechlorination reaction, and finally the possibility of polymer fragmentation.

The first issue that needs to be addressed is whether elimination of hydrogen chloride giving conjugated units occurs under the dechlorination reaction conditions.

We have found that hydrogen chloride is eliminated at 160°C. However provided that the reaction temperature is kept significantly below this there is little thermal elimination.

The second point is whether the polymers undergo either intra-or intermolecular crosslinking. Although regardless of this leaving groups would still be removed, there is a worry that if extensive intermolecular crosslinking occurred an

insoluble high molecular weight material could be formed. The molecular weight of the chloroprecursor polymer, as determined by gel permeation chromatography (g. p. c.), has been shown to be concentration dependent with the observed molecular weight decreasing on dilution. This has been attributed to the breakdown of polymer aggregates or physical networks. These observations are important in the dechlorination experiments. If concentrated solutions are used then gelling of the polymer solution, which can be attributed to intermolecular crosslinking, occurs regularly during the dechlorination reaction. However, if the polymer is diluted to the reaction concentration and disaggregated by sonication for 15 minutes then the gelling of the polymer could be completely prevented. Accordingly, it is preferred that the concentration does not exceed 10 mg per ml and also that the reaction mixture is sonicated.

Finally, we found that increasing the number of equivalents of hydrogen radical donor gave the expected increase in the level of polymer dechlorination.

However, to achieve good control over the degree of dechlorination it is necessary to determine accurately the amount of polymer in solution and ensure that the quality of the hydrogen radical donor is known. The first can be achieved by taking a small known volume of precursor in a solvent such as tetrahydrofuran and rapidly removing most of the solvent under vacuum to leave a film. We did not dry the polymer extensively as this would have encouraged elimination of hydrogen chloride. The remaining material was then weighed and analysed for its composition by'H n. m. r., determining the ratio of polymer to remaining solvent, and hence calculating the mass of polymer present in the original solution. As the quality of tri- n-butyltin hydride, in particular, tended to be variable we standardised the reagent by carrying out a control reaction and calculated the active reagent by comparing the expected level of dechlorination and that actually achieved. When these procedures were used we gained excellent control over the dechlorination reaction with the theoretical ratio of ethylene to chloroethylene being close to that expected from the molar equivalents of tri-n-butyltin hydride used. In addition, the dechlorinated polymers were formed in reasonable to good yield. The level of dechlorination was determined by comparing the integrals of the'H n. m. r. signals of the methine of the

CHCI protons and methylene protons of the dechlorinated ethylene links which appear at 5. 65 and 2. 82 p. p. m. respectively. It is important to note that although the percentage of dechlorination was controlled the reaction occurs randomly along the polymer chain.

As indicated the starting polymer possesses recurring units of the formula -Ar-CHX-CHR3- Ar represents an arylene group, preferably a phenylene group or a heteroarylene group such as a pyridyl or thienyl group. The phenylene group is generally a 1, 4- phenylene group. The group can be substituted. All positions can possess substituents although it is preferred that there are 0, one or two substituents. For a phenylene group the substituents are preferably in the 2, 5- position or the 2, 3- position. The nature of the substituents and the nature of the leaving group X are inter related since the substituents can affect the ease with which the polymer is formed and the X group is eliminated. They will also have an effect on polymer solubility.

The leaving group X is preferably a halogen atom such as chlorine, which is particularly preferred, or bromine or S-alkylxanthate, O-alkylxanthate or O- arylxanthate group. The alkyl groups generally have 1 to 12, typically 1 to 6, atoms while the aryl group is preferably phenyl. Typically, X is an S-methylxanthate or an O-ethylxanthate.

If X is chlorine or bromine then it is desirable that the arylene group (Ar) possesses a substituent having at least 4 carbon atoms in order to enhance solubility while when X represents a xanthate group there is generally no lower limit on the number of carbon atoms in any substituent.

R3 represents hydrogen or a, typically C,-Cl2, alkyl group, preferably an alkyl group with 1 to 6 carbon atoms.

Thus the starting polymer preferably possesses recurring units of the formula

wherein each of R'and R, which may be the same or different, represents hydrogen or an optionally substituted alkyl, alkoxy, aryloxy, acetylenyl, alkyl and/or aryl silyl, aryl, heteroaryl, vinyl, or cyano, X represents a halogen atom or an S-alkylxanthate, O-alkylxanthate or an O-arylxanthate group and R'represents hydrogen or an alkyl group.

As indicated above preferably at least one of Rl and R2 is not hydrogen and preferably also at least one of R'and R2 represents alkyl or alkoxy, typically methoxy, 2-ethylhexyloxy, butyl or 2-ethylhexyl. The substituent (s), if any, on the various substituents R'and Ruz are not particularly critical but include alkyl, alkoxy, aryl and aryloxy. In general, the alkyl groups possess 1 to 12, preferably 1 to 8, carbon atoms whilst the aryl groups are preferably phenyl.

When R'and R2 are both substituents they can be the same or different.

In general, in this specification, the alkyl groups can be straight chain or branched. They typically have 1 to 12, generally 1 to 8, carbon atoms. Examples include methyl, ethyl, butyl and ethylhexyl.

In general, the radical reaction is carried out with a hydrogen radical source and a radical initiator in a compatible solvent i. e. a solvent in which the starting polymer is soluble.

A particular feature of the present invention is that the various substituents are chosen so that the precursor polymer which is obtained is itself soluble.

Although melt processing can be envisaged it is much preferred that the precursor

polymer is soluble so that it can be processed without difficulty.

Typical solvents which can be used to carry out the process include aprotic solvents some of which may be polar. Preferred solvents include ethers such as tetrahydrofuran, which is preferred, tetrahydropyran and substituted ethers and aromatic hydrocarbons such as benzene and toluene.

The radical initiator is generally a peroxide including an alkyl or aryl peroxide such as benzoyl peroxide or azobis (isobutyronitrile). Alternatively, initiation can be provided by a suitable light source.

The hydride radical source is generally an alkyl or aryl tin hydride such as tri- (n-butyltin) hydride or, alternatively, a silane such as tris (trimethylsilyl) silane.

The temperature at which the reaction is carried out is largely dependant on the initiator. It is typically 20 to 100°C, suitably 40 to 80°C. In general the time taken is 1 minute to 10 hours, generally 1 to 4 hours.

It will be appreciated that it is important that the reaction is carried out at a temperature well below the temperature at which the leaving group is eliminated to form the vinyl group. Generally, the elimination takes place at a temperature from 50 to 300°C more generally from 100 to 250°C, for example for 10 to 20 hours.

It will be appreciated that the aim of the present invention is to control the ratio of ethylene to vinylene links which in turn controls the semi-conductor band gap of the polymer so as to the control the optical properties of the copolymer. By introducing the ethylene groups into the final polymer the semi-conductor band gap is spatially modulated so as to increase the quantum efficiency of the polymer when excited to luminesce. One of skill in the art will appreciate that by spatially modulating the semi-conductor band gap it is possible to select the wave length of radiation emitted during luminescence and also to select the refractive index of the polymer. The leaving groups are typically removed in random fashion such that the precursor is a random copolymer.

In general, it has been found that it is desirable to replace a relatively low number of vinylene groups by ethylene groups although there is generally no difficulty in eliminating all the leaving groups present. Typically, 1 to 30% of the groups are eliminated and preferably 5 to 20%. In general, a random copolymer is

produced.

Preferably, at the end of the radical substitution reaction the precursor polymer is purified, particularly if it is intended that the precursor is used to form a layer in a light emitting device. The precursor polymer can generally be purified using a non-solvent, for example a hydrocarbon such as n-pentane or an alcohol such as ethanol. The precursors also form part of the present invention. Accordingly the present invention also provides a polymer which possesses recurring units of the formula : -Ar-CHX-CHR3-and-Ar-CH,-CHR'- wherein X, Ar and R 3are as defined above.

By appropriate selection of the substituent (s) the precursor polymer can readily be made soluble. Thus in making an LED it is a simple matter to form the polymer layer of the LED by spin coating from solution.

Desirably, the solvent used for the precursor polymer for its processing is the same as that used to prepare the precursor. However, it is possible to use other solvents such as halogenated solvents, in particular chloroform or chlorobenzene which would generally not be suitable for the preparation of the precursor.

The conversion of the precursor to the partially saturated poly (arylenevinylene) is typically carried out by heating, as indicated above. It is desirable that this step is carried out in vacuo e. g. at a pressure of 10-'to 10-6 millibar or in an inert or reducing atmosphere. Alternatively, conversion can be carried out using base where the leaving group is a halogen, provided that the polymer possesses sufficient solubilising groups, typically alkyl or alkoxy groups with at least 4 carbon atoms, so that it is sufficiently soluble. In any event it is important to exclude air during the conversion process to avoid degradation. These polymers form part of the present invention. Accordingly the present invention also provides a partially conjugated poly (arylenevinylene) which comprises recurring units of the formula : -Ar-CH,-CHR''-and-Ar-CH=CR''- wherein Ar and R3 are as defined above.

The polymer starting materials can generally be obtained simply by a non- nucleophilic base catalysed polymerisation of a compound of formula X'CH,-Ar-

CH, X where X'is as defined for X but can be the same or different. Typically, the polymerisation is carried out using a base such as potassium t-butoxide. A typical overall reaction sequence is shown in Figure 1 of the accompanying drawings. It will be appreciated other starting polymers can be prepared in an analogous manner.

As indicated above, the precursor polymers find particular utility in the preparation of LEDs where a film of the precursor can be converted in situ into the final polymer. In other words a layer of the precursor polymer can be spin coated over the appropriate adjacent layer of the LED from solution and then the conversion carried out on the resulting polymer film in the solid state. It is, of course, though, important to ensure that the layer or layers of the LED which have already been formed can withstand the heat of conversion without a change of their properties.

In a typical LED, the following layers are formed over a transparent substrate, typically glass, in order : a transparent electrode layer, a layer of the polymer, a charge transport layer and finally a second electrode layer. The transparent electrode layer is typically formed of indium tin oxide (ITO). The charge transport layer will normally be an electron transport layer or a hole blocking layer. It is often desirable to place one or more buffer layers between the ITO electrode and the polymer, particularly if the precursor polymer contains halogen leaving groups. This is to prevent the corresponding acid formed during the elimination procedure from reacting with the ITO. Typically, the buffer layer can be a conducting polymer or hole transport layer, for example, a PEDOT (poly (3, 4- ethylenedioxythiophene)/PSS (polystyrene sulphonate) layer. Obviously, the buffer layer should not be soluble in the solvent used to apply the precursor and must be able to withstand the conversion temperature.

In an alternative embodiment, in some circumstances with appropriate solubilising groups the final polymer, formed elsewhere, can be applied from solution by spin coating. In this case, the layer on which the final polymer is deposited must not be soluble in the same solvent. For this purpose the final polymer must be sufficiently soluble.

Again for some polymers it is possible to form the final polymer in solution in situ. This can be achieved with halo precursors where elimination can occur in

solution by a base catalysed reaction. Although such a reaction is not possible with the xanthates it is possible in some cases to make the precursor polymer sufficiently soluble in a solvent with a boiling point above the elimination temperature such that elimination can take place in solution.

In both cases the charge transport layer (s) can be deposited on top of the final polymer either by spin coating from solution, provided the final polymer is not soluble in the solvent used, or by evaporation.

Although the polymers are particularly useful in LEDs they are useful in other semi-conductor devices including photodiodes, solar cells, FET and solid state triodes.

The advantage of using the precursor polymer route to the insoluble polymer over the soluble form of the polymer is that it enables the easy preparation of multilayer LEDs which have improved efficiencies over single layer devices of the same configuration. We have tested the solubility of the converted films of 3a-f, (see Example 7 below) and thermally treated 3g by comparing the U. V.-visible spectra of the samples before and after being placed in tetrahydrofuran. All the polymers, including surprisingly 3g, were found to be essentially insoluble after thermal conversion. The largest decrease in optical density was ~20% on thin films with optical densities of order of 0. 1. Therefore, this gives rise to the possibility of preparing multilayer LEDs based on the copolymers by solution processing using a variety of solvents.

In conclusion, we have developed a new route for the introduction of saturated units into the backbone of PPV derivatives and the like prepared via precursor polymers. It has the advantage over the previously reported routes in that there are no remaining labile groups to affect device stability. The precursor polymers can be solution processible and can be thermally converted to give partially conjugated copolymers which have low solubility.

The following Examples further illustrate the present invention

Experimental NMR spectra were recorded on a Bruker AM-500 MHz spectrometer. IR spectra were recorded on a Perkin-Elmer Paragon 1000 FTIR spectrometer. Gel permeation chromatography separations were carried out with a Polymer Laboratories Plgel 20, um MIXED-A column (600 + 300 mm length) calibrated with polystyrene narrow standards (Mp 580-11. 6 x 106) in tetrahydrofuran with toluene as flow marker. The tetrahydrofuran solvent was pumped at lcm3min~'at 24°C and the UV detector was set at 257 nm. U. V.-visible spectra were recorded on a Perkin Elmer 14P spectrophotometer. Photoluminescence spectra were recorded on an Edinburgh Instruments FS900CDT spectrofluorimeter.

Example 1 Poly{[1,4-phenylene(1-O-ethylxanthylethylene)]0.87-co-[1,4- phenylene (ethylene) jOl3M, 4. Tri-n-butyltinhydride (13. 1 mg, 0. 044 mmol), O- ethylxanthate precursor to PPV (100 mg, 0. 45 mmol) and azo-bis (isobutyronitrile) (3.3 mg, 0. 02 mmol) were dissolved in dry tetrahydrofuran (11 mL) under argon and heated at reflux for 2 hours. The solution was concentrated and ethanol (20 mL) added. The mixture was cooled to-18°C for 10 minutes, to aid precipitation of the polymer. The polymer was then triturated with the supernatant. The mixture was then centrifuged (4500 rpm, 2 minutes), the supernatant decanted and the residue triturated with a second aliquot of ethanol (20 mL). The mixture was then centrifuged (4500 rpm, 2 minutes), the supernatant decanted, and the polymer was dried under high vacuum for 1 hour, giving a white solid 4 (87. 4 mg, 92%). Characterization : vmax (thin film)/cm~l 1218 and 1049 (SC=S) ; #max(thin film)/nm 288 ; bH (400MHz, CDC13) 1. 20-1. 45 (br m, CH3), 2. 76-2. 92 (br m, ArCH2CH2Ar), 2. 94-3. 21 (br m, ArCH2), 4. 5-4. 62 (br m, SCSOCH2), 4. 67-4. 95 (br m, CHSCS), and 6. 81-7. 16 (br m, ArH); Mw = 3.95 x 104, Mn = 1.55 x 104, pd = 2.0; Thermogravimetric analysis: 232 °C.

Polyp [1, -phenylenevinylene]0.8-co-[1,4-phenylene(ethylene)]0.13}, 5. Thin films of 4 were heated at 205 °C for 16 hours under a dynamic vacuum to give 5.

Characterization : #max (film on KBr)/cm~ 962 (C=CH) ; amay (thin film)/nm 397, 381, 349 and 331.

Example 2 Poly 4-phenylene(1-O-ethylxanthylethylene)]0.5-co-[1,4-phenylene( ethylene)]0.5}, 6. Tri-n-butyltin hydride (65. 2 mg, 0. 22 mmol), O-ethylxanthate precursor to PPV (100 mg, 0. 45 mmol) and azo-bis (isobutyronitrilel) (7. 8 mg, 0. 04 mmol) were dissolved in dry tetrahydrofuran (5. 5 mL) under argon and heated under at for 2 hours. The solution was concentrated and n-pentane (20 mL) added. The polymer was then triturated with the supernatant and the mixture was then centrifuged (4500 rpm, 2 minutes). The supernatant was decanted and the residue triturated with a second aliquot of n-pentane (20 mL). The mixture was then centrifuged (4500 rpm, 2 minutes), the supernatant decanted, and the polymer collected by dissolution in dry tetrahydrofuran. However the polymer was found to be insoluble. After 15 minutes sonication, a soluble fraction was obtained, giving a clear solution (25 mg/mL, 10 mg, 14%). Characterization : Vmax (thin film)/cm-11219 and 1050 (SC=S) ; imay (thin film)/nm 288 ; on (400MHz, CDC13) 1. 23-1. 43 (br m, CH3), 2. 76-2. 94 (br m, ArCH9CH, Ar), 3. 06-3. 41 (br m, ArCH2) 4. 42-4. 62 (br m, SCOTCH2), 4. 78-4. 96 (br m , CHSCS), and 6.81-7.18 (br m ArH); Mw = 4.7 x 103, Mn = 2.7 x 103, pd = 1.7.

Poly{[1, 4-phenylenevinylene]0.5-co-[1,4-phenylene(ethylene)]0.5}, 7. Thin films of 6 were heated at 205 °C for 16 hours under a dynamic vacuum to give 7.

Characterization : Vmax (film on KBr)/cm~ 962 (C=CH) ; max (thin film)/nm 376, 343 and 327.

Example 3 Poly{[2-(2'-ethylhexyloxy)-5-phenylacetylene-1,4-phenylene(1 -S- methylxanthylethylene)]0.55-co-[2-(2'-ethylhexyloxy)-5-pheny lacetylene-1,4- phenylene(ethylene)]0.45, 8. Tri-n-butyltin hydride (17. 7 mg, 0. 06 mmol), O- methylxanthate precursor to poly (2'-ethylhexyloxy-5-

phenylacetylenylphenylenevinylene) (54 mg, 0. 12 mmol) and azo- bis (isobutyronitrile) (2 mg, 0. 01 mmol) were dissolved in dry tetrahydrofuran (5. 4 mL) under argon and heated at reflux for 2 hours. The solution was concentrated and n-pentane (20 mL) added. The polymer was then triturated with the supernatant and then mixture was then centrifuged (4500 rpm, 2 minutes). The supernatant was decanted and the residue triturated with a second aliquot of n-pentane (25 mL). The mixture was cooled to-18 °C for 10 minutes, to aid precipitation of the polymer and the mixture was then centrifuged (4500 rpm, 2 minutes). The supernatant was decanted, and the polymer dried under high vacuum for 1 hour, giving a yellow solid 8 (43 mg, 87%). Characterization : Vmax (thin film)/cm~t 1064, 1237 and 2209 (C=-C) ; #max (thin film)/nm 205, 232, 300 and 320 ; 8n (400MHz, CDCl3) 0. 7-0. 95 (br m, 2 x CH3) 1. 06-1. 40 (br m, 4 x CH2), 1. 44-1. 74 (br m. CH) 2. 45-2. 52 (br m, SCH3) 2. 93-3. 2 (br m, ArCH2CH,, 3. 2-3. 4 (br m, ArCH2) 3. 54-4. 0 (br m, ArOCH2) 6. 58- 6. 85 (br m, ArH and CHOCS) 7. 01-7. 43 (br m, ArH) and 7. 42-7. 55 (br m, ArH) ; Mw = 2. 2 x 10', Mn = 9. 6 x 104, pd = 3. 01 ; Thermogravimetric analysis : 141 °C.

Poly{[2-(2'-ethylhexyloxy)-5-phenylacetylene-1,4-phenylen evinylene)]0.55-co-[2-(2'- ethylhexyloxy)-5-phenylacetylene-1, 4-phenylene (ethylene) Jo_a ; J, 9. Thin films of 8 were heated at 140 °C for 16 hours under a dynamic vacuum to give 9.

Characterization : v,,,, (film on KBr)/cm^t 967 (C=CH) and 2207 (C_C) ; (thin film)/nm 300 and 454.

Example 4 Poly{[2-(2'-ethylhexyloxy)-5-phenylacetylene-1,4-phenylene(e thylene)]}, 10. Tri-n- butyltin hydride (14. 5 mg, 0. 05 mmol), 8 (20 mg, 0. 05 mmol) and azo- bis (isobutyronitrile) (1. 6 mg, 0. 01 mmol) were dissolved in dry tetrahydrofuran (5 mL) under argon and heated at reflux for 2 hours. The solution was concentrated and ethanol (25 mL) added. The mixture was cooled to-18 °C for 10 minutes, to aid precipitation of the polymer and the mixture and was then centrifuged (4500 rpm, 2 minutes). The supernatant was decanted and the residue triturated with a second aliquot of ethanol (25 mL). This was re-triturated and centrifuged (4500 rpm, 2

minutes). The supernatant was decanted, and the polymer dried under high vacuum for 1 hour, giving a brown solid 10 (12 mg, 70%). Characterization : Vmax (thin film)/cm~l 2207 (C-C) ; kmay (thin film)/nm 205, 244, 301 and 319 ; bH (400MHz, CDC13) 0. 65-0. 98 (br m, 6H, 2xCH3) 1. 14-1. 43 (br m, 8H, 4 xCH2), 1. 43-1. 74 (br m, 1H, CH) 2. 92-3. 2 (br m, 4H, ArCH2CH2Ar) 3.76-3. 98 (br m. 2H, ArOCH2) 6. 65- 6. 78 (br m, 1H, ArH) 7. 29-7. 45 (br m, 3H, ArH) and 7. 47-7. 55 (br m, 2H, ArH) ; Mw =3. 8x 104, Mn=2. 3 x 104, pd= 1. 65.

Example 5 Poly[2-butyl-5-(2'-ethylhexyl)-1,4-phenylene(ethylene)], 11. The bromide precursor to poly [2-butyl-5 (2'-ethylhexyl) phenylenevinylene] (Bu EHPPV) (47 mg, 0. 13 mmol) was dissolved in dry tetrahydrofuran (5. 7 mL) under argon and sonicated for 15 minutes. To this, tri-n-butyltin hydride (43. 3 mg, 0. 15 5 mmol), and azo- bis (isobutyronitrile) (4. 7 mg, 0. 02 mmol) were added and heated at reflux for 2 hours. The solution was concentrated and ethanol (20 mL) added. The polymer was then triturated with the supernatant and the mixture was then centrifuged (4500 rpm, 2 minutes). The supernatant was decanted and the residue triturated with a second aliquot of ethanol (20 mL) with the mixture being centrifuged (4500 rpm, 2 minutes).

The supernatant was again decanted, and the polymer dried under high vacuum for 1 hour, giving a green solid 11 (33 mg, 91%). Characterization : Vmax (thin film)/cm~l @ 899 (p-ArH) ; tmaY (thin film)/nm 208 and 326 ; 6H (400MHz, CDC13) 0. 79-1. 06 (br m, 9H, 3 x CH3) 1. 19-1. 47 (br m, 10H, 5 x CH2) 1. 60-1. 73 (br m, 3H, CH and CH2) 2. 43-2. 96 (br m, 8H, ArCH2 and ArCH2CH2Ar) 6. 95-7. 08 (br m, 2H, ArH) ; Mw = 1. 2 x 105, Mn = 4.0 x 104, pd = 2. 89.

Poly[2-butyl-5-(2'-ethylhexyl)-1,4-phenylene(ethylene)], 12. O-Ethylxanthate precursor to BuEHPPV (32 mg, 0. 06 mmol) was dissolved in dry tetrahydrofuran (5. 3 mL) under argon. To this, tri-n-butyltin hydride (19. 7 mg, 0. 07 mmol), and azo- bis (isobutyronitrile) (2. 2 mg, 0. 01 mmol) were added and heated at reflux for 2

hours. The solution was concentrated and ethanol (20 mL) added. The polymer was then triturated with the supernatant and the mixture was then centrifuged (4500 rpm, 2 minutes). The supernatant was decanted and the residue triturated with a second aliquot of ethanol (20 mL) with the mixture being centrifuged (4500 rpm. 2 minutes).

The supernatant was again decanted, and the polymer dried under high vacuum for 1 hour, giving a white solid 12 (19 mg, 79%). Characterization : vax (thin film)/cm- 899 (p-ArH) ; ax (thin film)/nm 208 and 272 ; on (200MHz, CDC13) 0. 65-1. 03 (br m, 9H, 3 x CH3) 1. 11-1. 47 (brm, 10H, 5xCH2) 1. 50-1. 71 (br m, 3H, CH and CH2) 2. 32-2. 96 (br m, 8H, ArCH2 and ArCH2CH2Ar) 6. 89-7. 05 (br m, 2H, ArH) ; Mw = 1. 2 x 105, Mn = 5.6 x 104, pd = 2. 09 Example 6 Poly{[2-butyl-5-(2'-ethylhexyl)-1,4-phenylene(1-chloroethyle ne)]0.85-co-[2-butyl-5- (2'-ethylhexyl)-1,4-phenylene(ethylene)]0.15}, 13. The chloro precursor to BuEHPPV (70 mg, 0. 23 mmol) was dissolved in dry tetrahydrofuran (7 mL) under argon and sonicated for 15 minutes. To this, tri-n-butyltin hydride (11. 2 mg, 0. 04 mmol), and azo-bis (isobutyronitrile) (2 mg, 0. 01 mmol) were added and heated at reflux for 2 hours. The solution was concentrated and ethanol (25 mL) added. The polymer was then triturated with the supernatant and the mixture was then centrifuged (4500 rpm, 2 minutes). The supernatant was decanted and the residue triturated with a second aliquot of ethanol (25 mL) with the mixture being centrifuged (4500 rpm, 2 minutes). The supernatant was again decanted, and then dissolved in dry tetrahydrofuran (1 mL) giving a green solution of 13 (13. 7 mg/mL, 13. 7 mg, 20%). Characterization : #m (thin film)/cm~l 726 (C-Cl) ; max (thin film)/nm 209 and 286 ; OH (200MHz, CDCl3) 0. 65-1. 10 (br m, 3 x CH3), 1. 16-1. 75 (br m, CH and 6 x CH2), 2. 1-2. 9 (br m, ArCH2R and ArCH2CH2Ar), 5. 16-5. 40 (br m, ArCHCI) 6. 65-7. 5 (br m, ArH) ; Mw = 5. 6 x 10', Mn = 1. 8 x 10', pd = 3. 1.

Poly{[2-butyl-5-(2'-ethylhexyl)-1,4-phenylenevinylene)]0. 85-co-[2-butyl-5-(2'-

ethylhexyl)-1,4-phenyleneethylene)]0.15}, 14. Thin films of 13 were heated at 250 °C for 16 hours under a dynamic vacuum to give 14. Characterization : v (film on KBr)/cm~l 961 (C=CH) ; #max (thin film)/nm 216 and 411.

Example 7 <BR> <BR> <BR> <BR> <BR> Poly { [2- (2'-ethylhexyloxy)-5-methoxy-1, 4-phenylene (l-chloroethylene) o.,,-co- [2- (2'-ethylhexyloxy)-5-methoxy-1, 4-phenylene (ethylene)] 0.58} 2e : Tri-n-butyltin hydride (38. 6 mg, 0. 13 mmol) and azo-bis (isobutyronitrile) (4. 5 mg, 0. 02 mmol) were added to a solution of the chloro precursor to MEHPPV (70 mg, 0. 24 mmol) in dry tetrahydrofuran (7 mL). The reaction mixture was heated at reflux for 2 hours.

The solution was concentrated and n-pentane (20 mL) was added. The mixture was then cooled at - 18 °C for 10 minutes, to aid precipitation of the polymer, and the precipitated polymer was triturated with the supernatant. The mixture was then centrifuged (4500 rpm, 2 minutes), the supernatant was removed, and the residue was triturated with a second aliquot of n-pentane (20 mL). The mixture was then centrifuged (4500 rpm, 2 min), the supernatant was removed, and the polymer was collected by dissolution in dry tetrahydrofuran (1. 5 mL), to give an orange/yellow solution of 2e (28 mg/mL, 42mg, 64%). #max(thin film)/nm 370, 305sh, and 207sh, AH (500MHz, CDC13) 0. 90 (br m, CH3), 1. 33-1. 51 (br m, CH and CH2), 2. 83 (br m, ArCH, CH2Ar), 3. 32 (br m, ArCHClCH2Ar), 3. 50-3. 91 (br m, OCH3 and OCH2), 5. 65 (br s, CHcl), 6. 46-6. 67 (br m, ArH) and 6. 98-7. 10 (br m, ArH) ; GPC, A"l = 4. 7 x 105, Mn = 1.65 x 105, polydispersity index 2. 9.

Both'H n. m. r and U. V.-visible spectroscopy confirmed that the chloroprecursor was thermally stable under the reaction conditions as there was only a small increase in conjugation after the dechlorination reaction.

Poly{[2-(2'-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylen e]0.42-co-[2-(2'- ethylhexyloxy)-5-methoxy-1, 4-phenylene (ethylene)". 5g]} : Thin films of 2e were heated at 165°C for 15 h under a dynamic vacuum to give 3e, Vmax (film)/cm-'965 ; v, (thin iilm)/nm 208, 301sh, 378 and 423.

Polymers 2a-d. f and g were obtained in a similar manner as were polymers 3a-d, f and g-see Table 1 for details.

An example of the effect of introducing the saturated units is described for the chloro precursor to I-MEHPPV. The Ms for the dechlorinated polymers 2a-g were determined by g. p. c. against polystyrene standards and found to be in the range 3. 1 x 105 _1. 1 x 10'when diluted and analysed immediately and there was less of a concentration dependence on the observed molecular weight. G. p. c also showed that there were no low molecular weight oligomers present after the reaction indicating that substantial polymer degradation was not occurring.

The conversion process was easily followed by U. V.-visible spectroscopy with an increase in absorption at longer wavelengths for polymers 3a-f (Figure 2).

The U. V.-visible spectrum of 3g was the same as for 2g as there were no leaving groups to be eliminated. The introduction of saturated linkages into the polymer backbone had two expected effects on the U. V.-visible absorption spectra. First, at high levels of saturated units there was a blue shift of the onset of absorption, and second, the ratio of the localised (214 nm) to delocalised (507 nm) 7 ! :-7t* transitions increased with the number of saturated units in the backbone. An example of the latter case is the comparison of the U. V.-visible absorption spectrum of I-MEHPPV and 3d which contains 25% of saturated units. Although both have a similar onset to the absorption the ratio of localised to delocalised z-7c* transitions increases in going from I-MEHPPV to 3d. Infrared spectroscopy was also used to follow the conversion process. However, once the level of dechlorination reached around 60% the absorptions associated with the vinyl units (for example, the absorption which occurs around 965 cm''which corresponds to the trans-vinylene CH out-of-plane bend) became difficult to observe.

The photoluminescence (PL) spectra of selected co-polymers are shown in Figure 3. The PL spectrum of I-MEHPPV is as previously reported. Vibronic peaks are seen at 600 and 640 nm, and a red tail extends to 800 nm. The spectra for 3c (not shown), 3d, and 3e have moderately blue shifted vibronic peaks compared with I- MEHPPV but differ little from each other. They do not however possess the long

wavelength tail seen for I-MEHPPV. Their full width half maximum linewidths are 95 nm for 3d compared to 129 nm for I-MEHPPV. It is believed that this difference may arise from differences in the relative contribution of excimer emission to the spectra. For I-MEHPPV excimer emission can be expected as already reported for the more common-MEHPPV. Excimer formation only occurs in the excited state and requires that neighbouring conjugated polymer segments can spatially relax to form a cofacial orientation with a typical separation of =3A. An excitation of one segment can then lower its energy by delocalisation over the pair of segments. In a partially conjugated polymer there is expected to be a higher degree of disorder and a corresponding larger average separation of conjugated segments. This could well act against the formation of excimers.