Polypyrrole

Polypyrrole ( PPy abbreviation ) is a polymeric chemical compound that consists of 2,5- linked pyrrole units. It belongs to the class of conductive polymers. The extended over the entire molecular system of conjugated double bonds in combination with a "doping" with a negatively charged counterion permits the conductivity. In the oxidized conductive state, the charges of the defect electrons in the conjugated chains are compensated by intercalated anions.

Synthesis

Polypyrrole is produced by oxidative polymerization of pyrrole. In the chemical preparation, it precipitates as insoluble black precipitate. A suitable oxidizing agent is iron (III ) chloride ( FeCl3). Water, methanol, ethanol, acetonitrile and other polar solvents may be used for the synthesis. In the electrochemical polymerization of pyrrole is polypyrrole deposited as black, electrically conductive layer onto the anode. In order to concatenate n pyrrole, 2n -2, electrons are withdrawn ideally. The polypyrrole is also oxidized because of its lower oxidation potential of pyrrole compared to during the polymerization. The degree of oxidation is generally 0.25 to 0.4. Consequently, bears any third or fourth monomer unit in the polymer chain has a positive charge. For charge compensation therefore the supporting electrolyte are incorporated during the formation of polymer anions.

Reaction mechanism

Pyrrole is oxidized in the first reaction step to the radical cation, which is a key intermediate in the oxidative polymerization. Two radical cations couple to the dimeric dication. May simultaneously two hydrogen ions are removed already during this dimerization. Since the oxidation potential of bipyrrole now formed is below that of pyrrole, it is immediately oxidized to the radical cation. However, the polymerization is continued only if the oxidation potential remains sufficiently high to oxidize pyrrole further to the radical cation.

Side reactions

The polymerization of pyrrole is suppressed in an aqueous electrolyte solution with a pH buffer at pH 7. Consequently, the presence of hydrogen ions in the polymerization is critical. At an excessively high pH, the nitrogen atom of the radical cation is deprotonated. The radical reacts to a non- conductive polymer. The electrochemical polymerization at a high pH results in thin insulating layers. At very low pH values ​​pyrrole reacts to 2,5-bis (2- pyrrolyl) pyrrolidine ( Pyrroltrimer ). Through this competitive reaction saturated pyrrolidine rings are installed, which interrupt the conjugation of the polymer chains and thus have negative consequences on the electrical conductivity of polypyrrole.

Application

Polypyrrole is used in sensor technology, solar cell technology and microbiology. Special processes can even immobilize enzymes on the Polypyrroloberfläche.