Modified Polyaniline and Other Conductive Polymers

 

The following is a proposal for an alternate synthesis of polyaniline (the references are below , at the end of the page) . This conductive polymer is prepared by the oxidative polymerization of aniline :

Note the nitrogen radical cations in the polymer . These cause it to have properties somewhat like Wurster Blue . The aminium radical cations were produced by the free radical oxidation of some of the amine nitrogens' lone pairs of electrons by the persulfate oxidizer . If the nitrogens all have a trigonal planar sp2 hybridization (similar to the carbons) , they can participate in the p bonding in the system . The radical cation centers will thus act as "positive holes" in the pi XO – the metallic bond (conduction band) – covering the entire polymer chain . Polyaniline should thus behave somewhat like a p-type semiconductor .

Primary amines can condense with aldehydes and ketones to produce aldimines and ketimines , respectively . Secondary amines can also condense with aldehydes and ketones , together with an acid , HX , to produce iminium salts with the anion "X" :

As an alternate approach to synthesizing polyaniline , it would be interesting to condense 1,4-benzoquinone (BQ) with p-phenylenediamine to obtain the polyimine . However , if the diamine was used in its neutral state , it would almost certainly add to the BQ olefin bond in an a,b manner to produce the small molecule hydroquinone derivative :

If the acidic salt of the diamine was used in the condensation , it might react at the BQ carbonyls instead of at its double bonds , as in the formation of the iminium salts :

I have indicated the use of perchlorate anions in the synthesis of the modified polyaniline . Leonard and Paukstelis considered perchlorate as the "best" anion in their experimental work with iminium salts :

“Both perchlorates and fluoborates are far superior to any other secondary amine salts having representative single anions , eg. chloride , bromide , nitrate , sulfate , which we have investigated” (p. 3022) .

They even preferred perchlorate over fluoborate anions :

“In general the fluoborate salts function less efficiently than the perchlorates but are probably handled with greater assurance of safety” (p. 3022) .

Speaking of handling perchlorate salts safely , some time ago I prepared one of the iminium perchlorate salts described by Leonard and Paukstelis . It was formed by the condensation of morpholine perchlorate and cyclohexanone in refluxing benzene , with azeotropic distillation of the water by-product . They obtained the iminium salt (m.p. 239-241 C) in 94% yield by this method . This proved to be a very lengthy process – something not mentioned by Leonard and Paukstelis – and required an overnight reaction period . Unfortunately , during the night the maintenance staff turned off the water in the Chemistry Building without notifying the researchers . The benzene apparently distilled off through the warm condenser , and the dry perchlorate salt overheated and detonated . When I returned to the laboratory in the morning , I was shocked and dismayed to see the apparatus disintegrated in a pile of shattered glass in the fume hood ! Undeterred I repeated the experiment , but this time substituting xylene (b.p. 138 C) for the benzene (b.p. 80 C) . Under my watchful eye this time , the reaction proceeded smoothly and was completed in about an hour and a half . After workup , a nearly quantitative yield of the tertiary iminium perchlorate salt was obtained .

Leonard and Paukstelis preferred the perchlorate anion because of its non-nucleophilic nature , and because its salts were nonhygroscopic and easily crystallizable from common solvents . An added advantage in our polymer case is the generally high solubility of perchlorates in water and in polar organic solvents . This elevated solubility would help to keep the growing polymer chains in solution for as long as possible , thus ensuring a higher molecular weight for the product . However , the bis-perchlorate salt of p-phenylenediamine would have to be prepared in advance , whereas p-phenylenediamine dihydrochloride is commercially available (eg. from the Aldrich Chemical Company) . It may be possible to synthesize the modified form of polyaniline sketched above with chloride rather than perchlorate anions . Different experimental techniques could also be tried , such as azeotropic distillation of the water by-product using various solvents , or the simpler method of warming the reactants in a suitable solution (water or one of the alcohols , THF , acetonitrile , etc.) . Unlike the long reaction time my experiment required , most of the iminium salts can be prepared very simply and quickly :

As an example of the ease with which conversion occurs , the mixing of pyrrolidine perchlorate with a slight excess of acetone liberates heat and produces crystalline isopropylidenepyrrolidinium perchlorate in a matter of seconds . The yield of pure product is 96% (p. 3021) .

I mentioned above the possibility of the a,b-addition of the amine to the benzoquinone double bond rather than to its carbonyl group . Leonard and Paukstelis investigated two a,b-unsaturated carbonyl compounds with respect to forming their corresponding iminium perchlorate salts . Cinnamaldehyde reacted at its carbonyl group , while mesityl oxide underwent an anomalous retroaldol reaction to produce its component acetone molecules which in turn formed the iminium salt . This is a hopeful indication that the amine acid salt will react at the benzoquinone carbonyls , rather than add to BQ at its double bonds in an a,b-manner .

All three forms of the modified polyaniline sketched above would be very interesting to examine for their electrical conductivity . Unlike green Emeraldine hydrochloride , the modified forms have an alternating benzenoid-quinoid p bond structure . Perlstein has suggested that an aromatic wave passing through molecular metals such as TTF–TCNQ is the mechanism of electrical conductivity in those materials . We might therefore consider the possibility that a benzenoid-quinoid wave-like fluctuation in the modified polyaniline might act in a similar manner .

The observant reader will have noticed that the modified polyaniline proposed here for investigation is the oxidized form of green Emeraldine polyaniline ; or conversely , Emeraldine is the reduced form of modified polyaniline . That is , the modified form could be hydrogenated to Emeraldine . In this respect , the two polymer forms are comparable to the interrelated species , 1,4-benzoquinone and hydroquinone . The former compound is a mild oxidizer , and the latter aromatic molecule is its reduced form :

1,4-benzoquinone  + 2 H+  +  2 e-   ---------------->   hydroquinone ;  E0red  =  (0.6992  +  0.059 log [H+] ) V .

This is the half-reaction for the quinhydrone electrode , used for the measurement of pH in electrochemistry . We would also expect modified polyaniline to be a mild oxidizer , by analogy with the benzoquinone-hydroquinone couple . Treatment of a suspension of the free base form of modified polyaniline in a suitable solvent with a one-electron oxidizer should produce radical cation nitrogens in the polymer chains . The quinoid forms might have electrical conductivity properties resembling Wurster Blues . In effect , the oxidizers would create positive holes in the p XO (band) of the chains . We might reasonably expect a significant exhancement in the electrical conductivity of the polymer , as was observed in the case of doped polyacetylene . The "Wurster form" of modified polyaniline should also behave somewhat like a p-doped semiconductor , as conventional polyaniline probably does . Nitrosonium salts and antimony pentachloride could be examined as one-electron oxidizers for the polymer . In the latter case , the stoichiometry is :

2 [ polymer = N : ]  +  3 SbCl5  ------------------>  2 [ polymer = N.+ ] SbCl6- +  SbCl3

Nitrosonium cation is a much stronger oxidizer than antimony pentachloride :

NO+  +  e-  --------------->  NO (g)            E0red = 1.45 V

Sb (V)  +  2e-   --------------->  Sb (III)     E0red = 0.8 V approx.

It might be advisable to determine the nitrogen content of the polyaniline (free base form) by a conventional chemical analysis before carrying out doping trials in a controlled , quantitative manner with any one-electron oxidizers .



Poly(p-quino)phenylene



The "all-carbon" analogue of modified polyaniline would also be quite an interesting polymer to synthesize and study . It too would probably be an excellent electrical conductor , especially when doped with one-electron oxidizers .


Poly(p-quino)phenylene (PPQP) might be accessible using the Horner-Wadsworth-Emmons olefination reaction . This is the "phosphonate modification" of the Wittig olefination reaction . It is applicable to halide compounds with fairly labile carbon-halogen bonds ; in our case , a benzylic chloride compound ,
a,a'-dichloro-p-xylene . Both Horner and co-workers , and Wadsworth and Emmons , converted this difunctional compound into its corresponding tetraethyl phosphonate by heating it , probably to about 200 C or so , with two equivalents of triethyl phosphite . Two equivalents of ethyl chloride (b.p. 12 C) were expelled , with production of the bis-phosphonate . Treatment of this intermediate with a strong base (potassium tert-butoxide) , then refluxing the bis-phosphonate salt with an aldehyde or ketone , resulted in the formation of various polyolefin compounds in a good yield :



The successful condensation of cinnamaldehyde with the bis-phosphonate is especially encouraging in this present context , since it suggests that the phosphonate ylid reacts
preferentially with the carbonyl group , and not with the double bond , in the case of a,b-unsaturated substrates . This will be of critical importance in the following scheme , in which the substrate will be the highly reactive 1,4-benzoquinone , which is well-known for reacting with nucleophiles exclusively in a,b-additions at its double bonds :



The phosphonate modification of the Wittig reaction is noteworthy for its stereoselectivity , in that a trans (E) configuration of the product olefin is usually strongly favoured over the competing cis (Z) geometry . Of course , we want PPQP to have an all-trans configuration , since any cis double bonds will cause a pronounced kinking of the polymer chain , and its likely termination . Note that all the polymers discussed in this web page are of the
condensation variety , which are generally characterized by a relatively low degree of polymerization (DP) , and low molecular weights . An all-trans geometry of the polymer will be essential for obtaining a reasonable DP for it .


In the sketch immediately above , I have indicated the use of lithium hydride as the base to be used for generating the bis-phosphonate carbanions , and 1,2-dimethoxyethane (DME) as the preferred solvent , with an ambient reaction temperature . A study published in 1990 of the phosphonate stereochemistry by
Thompson and Heathcock concluded that a trans configuration of olefin products from the reaction could be optimized by the combined use of lithium cation , DME , and a higher (ambient) reaction temperature . Individually , each of these three factors had only a minor effect on the olefin geometry , but when all three conditions were present in the reaction , a trans configuration was almost exclusively favoured in the products , with only trace amounts of cis products detected .


Treatment of PPQP with one-electron oxidizers should produce a Wurster Blue-like polymer , whose electrical conductivity will undoubtedly be significantly enhanced compared to the virgin material .


Rathke and Nowak discovered that triethylamine could be used as the base in phosphonate olefinations . It is a much weaker base than lithium hydride or potassium tert-butoxide , but is cheaper and easier to handle than them . It was essential to have a small cation such as lithium or magnesium present in the reaction mix in the Rathke-Nowak procedure . While good yields of olefin could be obtained by their simplified method after a three hour reaction time , higher yields were generally favoured by 24 hour reaction periods at room temperature :

The Rathke-Nowak modification of the phosphonate olefination might thus be examined in the course of a study of PPQP and other such polyolefins .


Poly(p-phenylene-vinylene)


The phosphonate olefination reaction is fairly simple and easy to carry out , so much so that
Fieser included an example of it in his well-known organic chemistry laboratory textbook , used by many undergraduate science students (myself included) . His experiment , the preparation of all-trans 1,4-diphenylbutadiene , was originally described by Wadsworth and Emmons in their pioneer paper describing the phosphonate olefination reaction in 1961 . In this same report , the synthesis of several other interesting polyolefins were outlined , including the reaction of the difunctional bis-phosphonate with benzaldehyde to produce 1,4-bis(phenylvinyl)benzene :



The idea occurred to me : if benzaldehyde successfully reacts with the bis-phosphonate , could a structurally similar difunctional aromatic aldehyde replace it to form the analogous polymer ? The appropriate compound is indeed known , and is available commercially at a modest cost (eg.
Aldrich Chemical Company) : terephthaldicarboxaldehyde (also known as terephthalaldehyde) . A possible synthesis of the hypothetical material , poly(p-phenylene-vinylene) , is sketched below :



The related condensation of terephthaldicarboxaldehyde with 1,4-phenylenediamine , to produce the polyaldimine , poly(p-phenylene-vinylenimine) , is also sketched in the drawing . As with the other proposed conductive polymers discussed in this essay , these latter two materials might also benefit , in terms of their electrical conductivities , by a treatment with a one-electron oxidizer such as antimony pentachloride or a nitrosonium salt , to produce their corresponding Wurster Blue salt forms .

References and Notes

 

Useful Wikipedia review articles about conductive polymers : http://en.wikipedia.org/wiki/Organic_electronics and http://en.wikipedia.org/wiki/Conductive_polymers .

An excellent review article on the development of conductive polymers , Twenty-five Years of Conducting Polymers, by Nina Hall (Royal Society of Chemistry , UK , 2003) , can be downloaded (PDF , 2911 KB) for free from http://www.rsc.org/delivery/_ArticleLinking/DisplayArticleForFree.cfm?doi=b210718j&JournalCode=CC

C.B. Duke and H.W. Gibson , Polymers , Conductive, Kirk-Othmer Encyclopedia of Chemical Technology , 3rd ed. , vol. 18 , pp. 755-793 ; M. Grayson and D. Eckroth (eds.) , John Wiley , New York , 1982 .

polyaniline : J. Stejskal and R.G. Gilbert , “Polyaniline . Preparation of a Conducting Polymer”, Pure Appl. Chem. 74 (5) , pp. 857-867 (2002) . This definitive report on the preparation and properties of polyaniline (green Emeraldine hydrochloride) can be downloaded (PDF , 206 KB) for free from http://www.iupac.org/publications/pac/2002/7405/7405x0857.html .

a,b manner : The addition of primary and secondary amines (and other such nucleophilic molecules) to a wide variety of a,b-unsaturated aldehydes , ketones , esters , and nitriles is generally referred to as the Michael addition reaction : E.D. Bergmann , D. Ginsberg , and R. Pappo , Org. React. 10 , pp. 179-555 (1959) . Wikipedia : http://en.wikipedia.org/wiki/Michael_reaction .

Leonard and Paukstelis : N.J. Leonard and J.V. Paukstelis , Direct Synthesis of Ternary Iminium Salts by Combination of Aldehydes or Ketones with Secondary Amine Salts, J. Org. Chem. 28 (11) , pp. 3021-3024 (1963) .

Perlstein : J.H. Perlstein , “Organic Metals – The Intermolecular Migration of Aromaticity”, Angew. Chem. Internat. Ed. Engl. 16 (8) , pp. 519-534 (1977) ; see especially Fig. 4 , p. 526 .

quinhydrone : Peter Keusch , Formation of Quinhydrone, discussion , photos , and video : http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/p15_chinhydr-e.htm

doped polyacetylene : C.K. Chiang et al. , “Conducting Polymers : Halogen Doped Polyacetylene”, J. Chem. Phys. 69 (11) , pp. 5098-5104 (1978) ; R.B. Kaner and A.G. MacDiarmid , Plastics That Conduct Electricity, Scientific American 258 (2) , pp. 106-111 (February , 1988) .

Wadsworth and Emmons : W.S. Wadsworth Jr. and W.D. Emmons , The Utility of Phosphonate Carbanions in Organic Synthesis, J. Amer. Chem. Soc. 83 (7) , pp. 1733-1738 (1961) .

preferentially with the carbonyl group : S. Trippett , The Wittig Reaction, Quart. Rev. 17 (4) , pp. 406-440 (1963) ; ........alkylidenephosphoranes normally attach at the b-position of a,b-unsaturated ketones only when the carbonyl group is highly hindered (p. 421) . But see the next reference .

in a,b-additions at its double bonds : F. Ramirez and S. Dershowitz , The Structure of Quinone Donor Adducts . I . The Action of Triphenylphosphine on p-Benzoquinone , 2,5-Dichloro-p-Benzoquinone , and Chloranil, J. Amer. Chem. Soc. 78 (21) , pp. 5614-5622 (1956) :

Thompson and Heathcock : S.K. Thompson and C.H. Heathcock , “Effect of Cation , Temperature , and Solvent on the Stereoselectivity of the Horner-Emmons Reaction of Trimethyl Phosphonoacetate with Aldehydes”, J. Org. Chem. 55 (10) , pp. 3386-3388 (1990) .

Rathke and Nowak : M.W. Rathke and M. Nowak , The Horner-Wadsworth-Emmons Modification of the Wittig Reaction Using Triethylamine and Lithium or Magnesium Salts, J. Org. Chem. 50 (15) , pp. 2624-2626 (1985) .

nitrosonium : W.J. Plieth , Nitrogen, Ch. 5 , pp. 321-479 in Encyclopedia of Electrochemistry of the Elements , Vol. 8 , A.J. Bard (ed.) , Marcel Dekker , New York , 1978 ; p. 325 ; G.A. Olah , G. Salem , J.S. Staral , and T.-L. Ho , “Preparative Carbocation Chemistry . 13 . Preparation of Carbocations from Hydrocarbons via Hydrogen Abstraction with Nitrosonium Hexafluorophosphate and Sodium Nitrite – Trifluoromethanesulfonic Acid”, J. Org. Chem. 43 (1) , pp. 173-175 (1978) .

antimony pentachloride : J. Holmes and R. Pettit , “Hydride Ion Abstraction with Antimony Pentachloride”, J. Org. Chem. 28 (6) , pp. 1695-1696 (1963) . Antimony pentafluoride has been used to greatly enhance the electrical conductivity of graphite , when doped into it up to 75% by weight . Such a graphite-SbF5 composite can have an extraordinarily high electrical conductivity , up to a million ohm-1-cm-1 at room temperature : J.-M. Lalancette and J. Lafontaine , “Intercalation of Antimony Pentafluoride in the Lattice of Graphite”, J.C.S. Chem. Comm. 1973 , p. 815 ; F.L. Vogel , “The Electrical Conductivity of Graphite Intercalated with Superacid Fluorides : Experiments with Antimony Pentafluoride”, J. Mater. Sci. 12 (5) , pp. 982-986 (1977) .

Fieser : L.F. Fieser , Organic Experiments , 2nd ed. , Raytheon Education Co. , Lexington (MA) , 1968 ; Ch. 24 , “Trans , Trans-1,4-Diphenyl-D1,2-Butadiene”, pp. 121-123 . This was the textbook I used as a chemistry student . The experiment is also included in the many subsequent editions of Fieser's excellent laboratory manual .

 

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