ELECTROCHEMICAL POLYMERISATION

Conjugated polymers may be made by a variety of techniques, including cationic, anionic, radical chain growth, coordination polymerisation, step growth polymerisation or electrochemical polymerisation. Electrochemical polymerisation occurs by suitable monomers which are electrochemically oxidised to create an active monomeric and dimeric species which react to form a conjugated polymer backbone. The main problem with electrically conductive... 

NB. Where only a current density was given in the original Ref. this is quoted in place of the polymerisation potential. c Counter ions are as incorporated during the electrochemical polymerisation process or by subsequent electrochemical doping unless suffixed with (chem.) which indicates the use of chemical doping. d Conductivities f-film, p-pressed pellet. 

The polymerisation appears to involve a nucleation process similar to that of the deposition of metals [182], and electrochemically-prepared polyaniline will form dense, non-fibrillar thin films [165, 173], but thicker films ( > 150nm) become less densely packed and more fibrous [176, 182]. This may be due a change in the deposition mechanism when the film becomes sufficiently thick to inhibit direct access to the platinum by unreacted monomer [176],... 

Many substituted thiophenes have also been electrochemically polymerised [19,54,399-405] (Table 4) as have thiophene dimers [21,37,55,251,400,406], trimers [21, 83,407], and tetramers [256,406], with the thiophene dimer giving rise to higher quality films than does the monomer [37, 395,408]. Several polycyclic monomers including a thiophene ring have also been polymerised [408-416], as have a series of compounds consisting of two thiophene rings linked by a polyene chain (Fig. 23c). The polymerisation of dithieno-thiophene (Fig. 23d) results in a polymer which shows remarkable similarity to polythiophene in its properties [409,410,414],... 

Since there is a close correlation between the specific conductivity of the catalytic solutions and the DP of the polymers formed in them, it follows that the electrochemical nature of the solutions must be largely unaffected by the polymerisation. Therefore at most a small fraction of the solute can be involved with the growing chain, and the remainder must be unaffected by the initiation of the polymerisation. This conclusion is strongly supported by the fact that in typical experiments the number of moles of polyisobutene formed was several powers of ten smaller than the number of moles of catalytic complex. 

An analysis is presented in this paper of the influence exerted on polymerisation kinetics by the complexing of carbocations with monomers. This had been brewing in the author s mind for a long time and had been mentioned in earlier works, and most other workers were aware of it to some extent. Curiously, few if any others had drawn the electrochemical conclusion that such a process would make meaningless the estimates of the population of paired cations in the reaction mixtures, because of the increase in the size of the cations resulting from such an association. 

Once again, in this paper, the electrochemical aspects of the ions and their equilibria are prominent. The Fuoss-Kraus equation is applied to the pairing of the carbenium ions with the anions. It is shown that since an increase in the solvent polarity reduces the propagation rate-constant, the increase in rate in changing from a less polar to a more polar solvent must be due to the increase in polarity augmenting the ratio of the concentrations of unpaired to paired cations, (here called Up and in later papers yip) on the assumption that as in anionic polymerisations, the unpaired ions propagate faster than the paired ions. 

Some Electrochemical Aspects of Cationic Polymerisation. P.H. Plesch, Journal of Polymer Science Symposium, 1976, No. 56, 373-382. 

An additional important feature of this class of polymers lies in the fact that their polymerisation and doping processes may be driven by a single electrochemical operation which, starting from the monomer, first forms the polymeric chain and then induces its oxidation and deposition in the doped form as a conductive film on a suitable substrate. The polymerisation reaction may be basically described as an electrophilic substitution which retains the aromatic structure and proceeds via a radical cation intermediate ... 

The electrochemical polymerisation offers several advantages over the... 

Investigations into the effect of ultrasound upon these polymerisation processes began in the mid 1980 s when Akbulut and Toppare [81] examined the potentiostatic control of a number of copolymerisations. In such copolymerisations initiation takes place once a potential in excess of the oxidation potential of either monomer has been applied. However, often potentials even higher than these are required due to the formation at the electrode of a polymer film. These films create a resistance to the passage of current in the bulk medium with consequent reductions in the possible electrochemical reactions and therefore reductions in the rate and the yield. The use of ultrasound has been rationalised in terms of its removal of this layer in a... 

Fig. 6.19. Effect of polymerisation potential (Epoi) on the isoprene content mol%) of electrochemically obtained isoprene-a methylstyrene copolymers o with ultrasound without ultrasound.

Other electroanalytical methods The use of h.v.t. in conjunction with electroanalytical techniques of the potentiometry-polarography type has been described in detail (Kesztelyi, 1984), so that it need not be discussed here. That author, however, ignores a very useful cell for electrosynthesis under vacuum (Schmulbach and Oommen, 1973) and the electrochemical techniques developed by Szwarc and his co-workers and others in the context of anionic polymerisation, which we have mentioned above. 

Up to 10 mM 30nAmM 1 2-3 min 2,4,7-Trinitro-9-fluorenone (TNF) was electrochemically polymerised on the carbon surface to avoid interferences. Flow-measurements. 

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