Polypyrrole

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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 system of conjugated double bonds that extends over the entire molecule in connection with a “ doping ” with a negatively charged counterion enables conductivity . In the oxidized, conductive state, the charges of the defect electrons in the conjugated chains are compensated by embedded anions.

Structural formula of polypyrrole, doped with p-toluenesulfonic acid
Razor blade coated with polypyrrole

synthesis

Polypyrrole is made by the oxidative polymerization of pyrrole . In the chemical preparation it precipitates as an insoluble, black precipitate. A suitable oxidizing agent is iron (III) chloride ( FeCl 3 ). Water, methanol , ethanol , acetonitrile and other polar solvents can be used for the synthesis. During the electrochemical polymerization of pyrrole, polypyrrole is deposited as a black, electrically conductive layer on the anode . To chain n pyrrole units, ideally 2n-2 electrons are withdrawn. The polypyrrole is also oxidized during the polymerization because of its lower oxidation potential compared to the pyrrole. The degree of oxidation is usually between 0.25 and 0.4. As a result, every third to fourth monomer unit in the polymer chain carries a positive charge. To compensate for the charge, anions of the conductive salt are therefore incorporated during polymer formation.

Reaction mechanism

In the first reaction step , pyrrole is oxidized to the radical cation , which is an essential intermediate in oxidative polymerization. Two radical cations couple to the dimeric dication. It is possible that two hydrogen ions are split off simultaneously during this dimerization. Since the oxidation potential of the bipyrrole that is now formed is below that of the pyrrole, it is immediately oxidized to the radical cation. The polymerization only continues if the oxidation potential remains high enough to oxidize further pyrrole to the radical cation.

Side reactions

The polymerization of pyrrole is suppressed in an aqueous electrolyte solution with a pH buffer at pH 7. Hence, the presence of hydrogen ions is critical in the polymerization reaction. If the pH value is too high, the nitrogen atom of the radical cation deprotonates. The radical reacts to form a non-conductive polymer. The electrochemical polymerisation at too high a pH value results in thin, insulating layers. At very low pH values, pyrrole reacts to form 2,5-bis (2-pyrrolyl) pyrrolidine (pyrrole trimer). As a result of this competitive reaction, saturated pyrrolidine rings are incorporated, which interrupt the conjugation of the polymer chains and thus have a negative effect on the electrical conductivity of the polypyrrole.

application

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

Web links

Individual evidence

  1. ^ S. Machida, S. Miyata, A. Techagumpuch: Chemical synthesis of highly electrically conductive polypyrrole . In: Synthetic Metals . tape 31 , no. 3 , September 1989, pp. 311-318 , doi : 10.1016 / 0379-6779 (89) 90798-4 .
  2. ^ Jürgen Heinze: Electronically conducting polymers . In: Eberhard Steckhan (Ed.): Electrochemistry IV (=  Topics in Current Chemistry ). tape 152 . Springer, Berlin / Heidelberg 1990, ISBN 978-3-540-51461-9 , pp. 1-47 .
  3. ↑ Stephen V. Lowen, John D. Van Dyke: Mechanistic studies of the electrochemical polymerization of pyrrole: Deuterium isotope effects and radical trapping studies . In: Journal of Polymer Science Part A: Polymer Chemistry . tape 28 , no. 3 , February 1990, p. 451-464 , doi : 10.1002 / pola.1990.080280301 .
  4. ↑ Renyuan Qian, Yongfang Li, Baozhen Yan, Huiming Zhang: Electrochemical aspects of polypyrrole . In: Synthetic Metals . tape 28 , no. 1-2 , January 30, 1989, pp. 51-58 , doi : 10.1016 / 0379-6779 (89) 90498-0 .
  5. ↑ TF Otero, J. Rodríguez: Role of protons on the electrochemical polymerization of pyrrole from acetonitrile solutions . In: Journal of Electroanalytical Chemistry . tape 379 , no. 1-2 , December 12, 1994, pp. 513-516 , doi : 10.1016 / 0022-0728 (94) 87178-7 .