Electroinitiation

An electric field between two electrodes creates conditions suitable for initiation and/or modifications of other elementary polymerization steps. 

Electroinidated polymerization is induced by electron transitions between the electrodes and the molecules of the electrolyzed solution. The following classification is based on the features of the transfer reaction [309]. 

Generally, it is difficult to prove direct electroinitiation experimentally due to the instability and low concentration of the radical ions formed from the monomer. When the medium between the electrodes contains only the monomer which undergoes polymerization at high field intensities, direct electroinitiation is clearly involved [310] 

Indirect electroinitiation occurs by electrolysis of the supporting electrolyte in the presence of monomer when conditions for direct initiation are not fulfilled. Cerrai et al. [311] electrolyzed R4N + P and R4N + I3 in 1,2-dich-loroethane in the presence of isobutyl vinyl ether. They found iodine formed by the oxidation of anions 

The tetraalkylammonium cation is not reduced. The solvent is decomposed on the cathode, yielding H2, ethylene, vinyl chloride, and car-banions. Similar processes also occur in the cathode space with the electrolyte R4N + C104. HC104 is formed around the anode and the monomer is polymerized in the anode space so far it is not known whether this is only by the effect of HC104 or whether cations from the supporting electrolyte are also involved. 

Electrolytic or electroinitiated polymerization involves initiation by cations formed via electrolysis of some component of the reaction system (monomer, solvent, electrolyte, or other deliberately added substance) [Cerrai et al., 1976, 1979 Funt et al., 1976 Oberrauch et al., 

1978 Olaj, 1987]. Thus initiation in the presence of perchlorate ion proceeds by oxidation of perchlorate followed by hydrogen abstraction  

Some electroinitiated polymerizations proceed via monomer radical-cations (VII) formed by electron transfer 

---

Alternating equimolar copolymers of vinyl acetate and ethylene and alternating copolymers of vinyl acetate and acrylonitrile have been reported (127,128). Vinyl acetate and certain copolymers can be produced directly as films on certain metallic substrates by electroinitiation processes in which the substrate functions as one electrode (129). 

Mengoli, G. Feasibility of Polymer Rim Coating Through Electroinitiated Polymerization in Aqueous Medium. Vol. 33, pp. 1-31. 

Protonic acids are less suitable because the conjugate base is too active a nucleophile. HC1, for example, will not initiate polymerization because chloride ion adds immediately to the carbenium ion before the latter can propagate. Other initiators which have been studied include electroinitiation, photoinitiation and ionizing radiation. 

Toppare [84] and Ito [85] have studied the effect of ultrasound on electroinitiated chain polymerisation. Although more detailed discussion is reserved for Chapter 6, for the sake of completeness brief mention will be made here. 

In this section we will focus on electroreductive and electrooxidative synthesis and touch briefly on the electrosynthesis of selected organometallics and electroinitiated chain polymerisation, previously introduced in Chapter 5. 

There are two techniques employed to investigate electroinitiated chain polymerisation. They are potentiostatic control (constant potential) and the more usual galvano-static control (constant current). 

Conducting polymers are quite different systems to the electroinitiated chain polymerisations discussed above being formed by a step-growth mechanism involving stoichiometric transfer of electrons. 

The evidence for this mechanism is based on mass spectroscopy of the gas-phase radiolysis of isobutylene, which may not be applicable to the typical liquid-phase polymerization system. Initiation in condensed systems may follow the same course as electroinitiation— coupling of radical-cations to form dicarbocations. 

Electroinitiated polymerization proceeds by direct electron addition to monomer to generate the monomer anion-radical, although initiation in some systems may involve the formation of an anionic species by electrolytic reaction of some component of the reaction system (often the electrolyte) [Olaj, 1987]. 

Other experimental results, which more definitely contradict the electroinitiation mechanism, are found in the field influence on living anionic polymerizations. We come back to these problems later. 

We condude this section by stating that the field-accelerating effect on copolymerizations and the change of the monomer reactivity ratio with the field can be accounted for in terms of the interpretation proposed for cationic homopolymerizations, namely the field-facilitated dissociation of the growing chain ends. We should note that the observed field influence on the copolymerization excludes the possibility of the electroinitiated polymerization mechanism. 

---