GB2298210A - Manufacture of polyanilines - Google Patents

Abstract

Aniline is polymerized by oxidation at a reaction temperature in the range from -10 to -40{C in aqueous acid solution containing sufficient inert solute to keep it from freezing and using a dichromate (preferably sodium dichromate) as the oxidising agent.

Description

Manufacture of Polyanilines This invention relates to the manufacture of polyanilines (homopolymers of unsubstituted aniline) by chemical oxidation in aqueous solution, and arises out of research work in the University of Durham.

In our prior application PCT/GB 95/00385 (W095/23822), we have described and claimed novel forms of polyaniline characterised by the substantial absence of chain branching as indicated by a nuclear magnetic resonance spectrum in the leucoemeraldine state showing only two 13C peaks, and have described prefered ways of making such polyanilines by lowtemperature oxidation with a persulfate.

We have now discovered that polyanilines with comparable and in some cases improved characteristics can be made with an alternative oxidizing agent. Dichromates have previously been used in the oxidative polymerization of aniline, but it has been reported (Yong Cao et al, Polymer 30 2305-2311 (1989) that they should be used at temperatures in the range from 0 to -50C and that lower temperatures are unsatisfactory. We93/09175 might be read as contradicting that teaching, but we think it preferable to understand page 6 lines 9-14 as indicating that the alternative polymerization agents listed can be used in forms of the invention in which the reaction temperature is suited to them.We have found, however, contrary to this teaching of Yong Cao et al, that it is possible to use much lower temperatures and our results to date lead us to believe that (at least in favourable cases) the resulting polyanilines have at least as good structural features as those made under comparable conditions with persulfates as oxidizing agent.

In accordance with the present invention, aniline is polymerized by oxidation at a reaction temperature in the range from -10 to -40 C in aqueous acid solution containing sufficient inert solute to keep it from freezing and using a dichromate as the oxidising agent. Sodium dichromate is preferred; lithium and ammonium dichromates are also considered suitable; potassium dichromate is less satisfactory because its low-temperature solubility is inadequate.

Preferably the polymerisation is effected at a temperature in the range -20 to -300C, more especially about -21 to -25 C. To achieve high molecular weights (which are usually but not in all cases desirable) the reaction time should be long; reaction times of the order of 6 to 60 hours are considered appropriate.

Preferably the oxidant is used in a molar ratio to the aniline in the range from 0.1:1 to 0.42:1 and most preferably between 0.30:1 and 0.35:1.

Acid is consumed in dichromate oxidation at a rate of 7 electron equivalents per mole of dichromate an an excess over this quantity is required to maintain acidity to the end of the reaction. On grounds of good low-temperature solubility and low-temperature buffering properties, hydrochloric acid is much preferred. It may be desirable for the dichromate to be added slowly over at least the first few hours of the reaction (preferably uniformly over at least six hours), and in such case preferably much of the acid required is added with it to reduce initial acidity and minimise pH changes during the polymerization.

Few suitable inert solutes are available; ionic solutes are desirable and we prefer to use one or more of the chlorides of lithium, magnesium and sodium, of which we think lithium chloride best.

Example 1 A solution of 58.2g of 35% hydrochloric acid (0.60 mole) was made up in water to a total mass of 300g and 12.96g (0.10 mole) of aniline hydrochloride and 21.62g (0.51 mole) of lithium chloride dissolved in it. The solution was cooled to -260C and a second solution of 12.42g (0.0417mole) of sodium dichromate dihydrate made up to 40g with water was added. The mixture was stirred for 45 hours and then filtered to separate the solid polymer which was washed ten times each time with 200ml of water and then stirred for 24 hours in 100ml of 33% aqueous ammonia to deprotonate it.The residue was filtered and rewashed, eight times each time with 200 ml of water and twice each time with 200ml of isopropanol, and then dried under vacuum at 600C for 24 hours to give a product in the form of a brown powder at a yield of 98. 1%.

The molecular weight (Mp) of the product was estimated as 194,000 by gel permeation chromatography using poly-2vinylpyridine reference polymers in a solution containing 0.1% lithium chloride in N-methyl-2-pyrrolidone (NMP). The product dissolved in NMP noticeably more readily than a polyaniline made under similar conditions using ammonium persulfate as oxidant.

This polyaniline was dissolved in N-methyl-2-pyrrolidone at 7% solids concentration and homogenised in a centrifuge for an hour at 4000rpm before decanting the solution to remove any fine particles. The solution was coated on glass to give a coating weight of about 0.05g/cm2 and solvent partially removed by heating at 60"C under vacuum for about an hour. The coating was peeled from the glass to form a self-supporting film containing about 25% of N-methyl2-pyrollidone; samples around 2 by 3 cm were cut from it for orienting.

The longitudinal edges of these samples were folded twice over on themselves to strengthen the edges and resist fracture and then clamped at their narrow ends in metal-jawed clamps with paper liners; the two clamps were biassed apart while the temperature was raised from ambient to about 85"C and then slowly subjected to increasing tension until elongated by 700%. This produced a substantial degree of uniaxial orientation. The oriented film was then protonated by exposure to 15% aqueous methane sulphonic acid for 24 hours.

Examples 2-4 Following the general procedure of Example 1, three reactions were carried out at -30 C in solutions containing 19.1% by weight of lithium chloride. In Example 2, 0.1 mole of aniline was reacted with 0.0417 mole of sodium dichromate in a solution to which 0.4 mole of hydrochloric acid was added.

The initial acidity corresponded to a pH of about -0.3 at 20"C and the acidity at the end of the reaction to about +0.5. Polyaniline was obtained in a yield of about 93.5% and its molecular weight estimated (as above) at 103,000. In Example 3, the aniline and 0.1 mole of the hydrochloric acid was replaced by an equivalent amount of preformed aniline hydrochloride, thought to be of higher purity than the aniline used in Example 2. The final acidity in this case was estimated as equivalent to a pH of +0.8 at 20"C and the molecular weight of the product as about 158,000 but the yield was only about 88%. Example 4 was similar to Example 3 except that all the hydrochloric acid was replaced by 0.167 mole of sulfuric acid (so that the oxidant might be considered to be chromic acid).This resulted in an initial acidity corresponding to a pH of +1.0 at 20"C and a final one corresponding to about 0.0; yield was about 95% and molecular weight estimated as 121,000.

Example 5 This was basically a scale-up of Example 2 but with extra acid. 1.0 Mole of aniline was reacted with 0.42 mole of sodium dichromate in the presence of 6.85 mole of hydrochloric acid. The acidity range corresponded to a pH value at 20"C of -0.3 initially and was low enough throughout the reaction to avoid substantial ring chlorination (say < 0.0). Yield was over 98% and estimated molecular weight 145,000.

Example 6 This was a series of experiments differing in the reaction temperature and lithium chloride content, the latter being the minimum required to avoid freezing.

Aniline hydrochloride (0.1 mole) was dissolved in water with the tabulated amount of lithium chloride (Table 1) and 0.300 moles of hydrochloric acid added. 0.0417 Moles of sodium dichromate (as dihydrate) was separately dissolved in 27.6g of water. The first solution was placed in a 1 litre jacketed reaction vessel fitted with an anchor stirrer paddle and cooled to the tabulated reaction temperature by means of a Haake F3K circulating chiller. The stirrer was rotated at 200 rpm and the oxidant solution added at a rate of 5g/hour (total addition time 8 hours) using a peristaltic pump. The reaction mixture became dark blue in the first few minutes and the reaction was maintained for a total of 45 hours.The solid product was filtered and washed ten times, each time with 200 ml of water and was then treated with 200 ml of 35% ammonia solution for an hour to deprotonate it, rewashed ten times with water as before and then once with 200 ml of isopropanol. The filter cake was dried under dynamic vacuum at 60"C for 48 hours, giving a brown powder. The pH of the initial filtrate was determined at 20"C and the chlorine content of the product determined by standard analytical techniques; molecular weights were estimated as before.

Results are shown in the Table: Table 1

Reaction LiCl pH of yield Cl Molecular temperature content filtrate (8) content weight ( C) (O ( ) 0 - +0.9 97 0.59 57,000 -15 11.2 +1.1 96 0.95 128,000 -20 13.5 +1.3 96 1.2 122,000 -23 14.8 +1.4 99 1.2 133,000 -25 15.7 +1.5 97 1.3 124,000 -27 16.5 +1.6 95 1.3 79,000 -29 17.4 +1.6 94 2.2 102,000 -31 18.3 +1.4 88 - 3.4 92,000 -33 19.2 +1.1 82 4.4 77,000 -35 20.2 +0.5 78 4.6 69,000 Example 7 This was similar to Example 6 except that aniline (0.1 mole) and hydrochloric acid (0.674 mole) were used instead of preformed aniline hydrochloride and that fewer temperatures were used. The results are in Table 2.

Table 2

Reaction LiCl content C1 content Molecular temperature ( ) (W) weight ( C) -20 13.5 1.6 136,000 -23 14.8 1.9 116,000 -26 16.1 2.4 122,000 -29 17.4 3.2 106,000 Example 8 The synthesis at 23"C in Example 6 was modified by reducing the amount of sodium dichromate to 0.383 mole and to 0.333 mole, and the resulting polyanilines had molecular weights estimated as 118,000 and 159,000 respectively.The polyaniline of molecular weight 159,000 was processed to doped film abbout 0.34 mm thick generally as in Example 1 using camphor sulfonic acid in a molar ratio to the polyaniline of 0.6:1. In its unstretched state, the conductivity of this film was 315 S/cm. A dumbell sample of this film with a parallel part 15 mm long and 3.0 mm wide was heated in an oven at 158"C for 20 minutes and then stretched at a rate of 5 mm/min until it length was 28.5 mm (90% elongation). The width and thickness after stretching were 1.9 and 0.026 mm respectively and the longitudinal conductivity was 1690S/cm (conductivities in this example were measured with a four-electrodes-in-line probe) Example9 The synthesis at 23"C in Example 6 was modified by using 19.2% sodium chloride instead of lithium chloride. This modification substantially reduced the chlorine content of the product, but at the expense of reduced yield and molecular weight.

Claims (11)

1 A method of polymerizing aniline by oxidation characterised by a combination of a reaction temperature in the range from -10 to -400C, an aqueous acid solution containing sufficient inert solute to keep it from freezing and the use of a dichromate as the oxidising agent.

2 A method as claimed in claim 1 in which the dichromate is sodium dichromate.

3 A method as claimed in claim 1 in which the dichromate is lithium dichromate or ammonium dichromate.

4 A method as claimed in any one of claims 1-3 in which the polymerisation temperature is in the range -20 too -300C.

5 A method as claimed in claims 1-3 in which the polymerisation temperature is in the range -21 to -250C.

6 A method as claimed in any one of claims 1-5 in which the reaction time is 6 to 60 hours.

7 A method as claimed in any one of claims 1-6 in which the oxidant is used in a molar ratio to the aniline in the range from 0.1:1 to 0.42:1.

8 A method as claimed in any one of claims 1-6 in which the oxidant is used in a molar ratio to the aniline in the range from 0.30:1 to 0.35:1.

9 A method as claimed in any one of claims 1-8 in which the reaction mixture contains hydrochloric acid.

10 A method as claimed in any one of claims 1-8 in which the reaction mixture contains one or more of the chlorides of lithium, magnesium and sodium.

11 A method of polymerizing aniline substantially in accordance with the Example.