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3.1 Polymerization Scheme of Polypyrrole

Polypyrrole was the first conducting polymer to be synthesized electrochemically.⁷  A wide variety of other conducting polymers have been synthesized since that time.  The polymerization of pyrrole can be accomplished by chemical or electrochemical methods.  Each method involves the oxidation of the pyrrole monomer to initiate polymerization.  Illustration 3.2(a) shows the oxidation of pyrrole, which yields a radical-cation species.  In Illustration 3.2(b)-(c), radical-cation monomers can combine at the a positions to form a pyrrole dimer with the elimination of two protons.  This dimer can undergo further oxidation at even lower oxidation potentials⁵⁸ (as seen in Table 3.1) and form longer oligomers as seenin Illustration 3.2(d).

Illustration 3.1:Displayed are schematics of the following growth patterns (A) Frank-van der Merwe,(B) Stranski-Krastanov (C) Volmer-Weber (columnar) (D) Volmer-Weber (droplet-like).

Table 3.1:Oxidation potentials of pyrrole and thiophene oligomers of different chain lengths.⁵⁸

Illustration 3.2:The steps to the polymerization of pyrrole are a) the oxidation of the monomer to form a radical cation species, b) the formation of a pyrrole dimer before the elimination of hydrogen, c) the elimination of hydrogen, and d) the continued polymerization of polypyrrole.