Polypyrrole (PPy), a conjugated polymer, has been extensively investigated in recent years, due to its potential biocompatibility and promising electrical and optical properties. Recently, there has been tremendous interest in developing new and environmentally friendly procedures for the synthesis of conjugated polymers, such as polypyrrole. An enzymatic pathway to polyaniline, another conjugated polymer, was demonstrated nearly a decade ago; however a similar approach for the synthesis of polypyrrole has not been well established. Very recently, the use of redox mediators coupled with an oxidoreductase enzyme has shown promise in facilitating the enzymatic synthesis of polypyrrole. However a direct enzymatic route has not been developed to date.
With the objective of addressing the challenges involved in the direct enzymatic synthesis of polypyrrole and its derivatives, this thesis presents a generic methodology based on the use of the enzyme soybean peroxidase (SBP). Under appropriate conditions, the enzyme has been demonstrated to facilitate the oxidative polymerization of pyrrole and several of its derivative monomers in a mild, non-toxic, environmentally benign manner. This one pot synthesis is carried in a mild pH aqueous solution.
More importantly this research also explores the fundamental aspects pertaining to reaction conditions (temperature, reaction pH and time, dopant type) and reaction feasibility (reduction potential match) that can yield conjugated polymers with interesting electrical and optical properties. Appropriate control of the reaction conditions are shown to be important for the synthesis of a highly conductive polymer. A low synthesis temperature (2 °C) was shown to produce PPy that was more conductive and produced at a faster rate. Optimized reaction conditions produced PPy with a conductivity as high as 3 S/cm when doped with 10-camphor sulfonic acid. Additionally, the use of the SBP as a catalyst was observed to facilitate the synthesis of PPy which contains fewer structural defects than that prepared via traditional chemical and electrochemical polymerization techniques.
The role of redox potential in evaluating reaction feasibility and its influence on the initiation of polymerization has been investigated. Linear sweep voltametry was used to evaluate the reduction potentials of 10 pyrrole monomers containing a diverse array of substituents. It was observed that the enzyme SBP could oxidize or polymerize pyrrole monomers with reduction potentials equal to or less than 1.38 V (Ag/AgCl), with the exception of N-methylpyrrole. Pyrrole monomers with reduction potentials greater than 1.38 V could not be oxidized with SBP and hydrogen peroxide. SBP was shown to successfully catalyze the polymerization of 3-methylpyrrole and 3,4-diethylpyrrole to semi-conductive polymers.
This thesis also explores the enzyme catalyzed synthesis of fluorescent polypyrrole derivatives. The pyrrole derivative 4-(3-Pyrrolyl)butyric acid (3-BAP) was successfully polymerized using SBP as the catalyst. Under conditions previously optimized for the synthesis of PPy, poly(4-(3-Pyrrolyl)butyric acid) (P3-BAP) with an electrical conductivity as high as 10−2 S/cm was produced. When 3-BAP was polymerized in unbuffered deionized water, for the first time a fluorescent conjugated polypyrrole was produced enzymatically. Solutions of P(3-BAP) were observed to have a stable fluorescence that was effectively quenched by presence of metal ions in solution. Stern-Volmer constants for fluorescence quenching of P(3-BAP) were observed to be as high as 125,000 for metal ions such as cobalt(II). Scatchard plots suggest that P(3-BAP) exhibits cooperative binding with a wide variety of metals(II) salts.
Using polypyrrole and its derivatives this thesis seeks to extend the current understanding of the fundamental aspects involved in the oxidoreductase catalyzed synthesis of conjugated polymers. The electrically conducting and fluorescent polypyrrole derivatives synthesized and studied in this thesis provide new opportunities for exploring their possible use in applications, such as antistatic coating and sensors.