Polypyrrole (PPy) is an organic polymer obtained by oxidative polymerization of pyrrole. It is a solid with the formula H(C₄H₂NH)_nH. It is an intrinsically

conducting polymer Conductive polymers or, more precisely, intrinsically conducting polymers (ICPs) are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The biggest advantage of conductive polymers ...

, used in electronics, optical, biological and medical fields.

History

Some of the first examples of PPy were reported in 1919 by Angeli and Pieroni, who reported the formation of pyrrole blacks from pyrrole magnesium bromide. Since then pyrrole oxidation reaction has been studied and reported in scientific literature. Work on conductive polymers including polypyrrole,

polythiophene Polythiophenes (PTs) are polymerized thiophenes, a sulfur heterocycle. The parent PT is an insoluble colored solid with the formula (C4H2S)n. The rings are linked through the 2- and 5-positions. Poly(alkylthiophene)s have alkyl substituents at ...

polyaniline Polyaniline (PANI) is a conducting polymer and organic semiconductor of the semi-flexible rod polymer family. The compound has been of interest since the 1980s because of its electrical conductivity and mechanical properties. Polyaniline is one of ...

, and

polyacetylene Polyacetylene (IUPAC name: polyethyne) usually refers to an organic polymer with the repeating unit . The name refers to its conceptual construction from polymerization of acetylene to give a chain with repeating olefin groups. This compound ...

was awarded the

Nobel Prize in Chemistry ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...

in 2000 to Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa .

Synthesis

Different methods can be used to synthesize PPy, but the most common are electrochemical synthesis and chemical oxidation. Chemical oxidation of pyrrole: :n C₄H₄NH + 2n FeCl₃ → (C₄H₂NH)_n + 2n FeCl₂ + 2n HCl The process is thought to occur via the formation of the pi-

radical cation In organic chemistry, a radical anion is a free radical species that carries a negative charge. Radical anions are encountered in organic chemistry as reduced derivatives of polycyclic aromatic compounds, e.g. sodium naphthenide. An example of a ...

C₄H₄NH⁺. This electrophile attacks the C-2 carbon of an unoxidized molecule of pyrrole to give a dimeric cation C₄H₄NH)₂sup>++. The process repeats itself many times. Conductive forms of PPy are prepared by oxidation ("p-doping") of the polymer: :(C₄H₂NH)_n + 0.2 X → C₄H₂NH)_nX_0.2 The polymerization and p-doping can also be effected electrochemically. The resulting conductive polymer are peeled off of the anode.

Cyclic voltammetry Cyclic voltammetry (CV) is a type of potentiodynamic electrochemical measurement. In a cyclic voltammetry experiment, the working electrode potential is ramped linearly versus time. Unlike in linear sweep voltammetry, after the set potential is r ...

and chronocoulometry methods can be used for electrochemical synthesis of polypyrrole.

Properties

Films of PPy are yellow but darken in the air due to some oxidation. Doped films are blue or black depending on the degree of polymerization and film thickness. They are amorphous, showing only weak diffraction. PPy is described as "quasi-unidimensional" vs one-dimensional since there is some crosslinking and chain hopping. Undoped and doped films are insoluble in solvents but swellable. Doping makes the materials brittle. They are stable in the air up to 150 °C at which temperature the dopant starts to evolve (e.g., as HCl). PPy is an insulator, but its oxidized derivatives are good electrical conductors. The conductivity of the material depends on the conditions and reagents used in the oxidation. Conductivities range from 2 to 100 S/cm. Higher conductivities are associated with larger anions, such as

tosylate In organic chemistry, a toluenesulfonyl group (tosyl group, abbreviated Ts or Tos) is a univalent functional group with the chemical formula –. It consists of a tolyl group, –, joined to a sulfonyl group, ––, with the open valence on s ...

. Doping the polymer requires that the material swell to accommodate the charge-compensating anions. The physical changes associated with this charging and discharging have been discussed as a form of artificial muscle. The surface of polypyrrole films present fractal properties and ionic diffusion through them show

anomalous diffusion Anomalous diffusion is a diffusion process with a non-linear relationship between the mean squared displacement (MSD), \langle r^(\tau )\rangle , and time. This behavior is in stark contrast to Brownian motion, the typical diffusion process descri ...

pattern.

Applications

PPy and related conductive polymers have two main application in electronic devices and for chemical sensors and electrochemical applications.

Research trends

PPy is a potential vehicle for

drug delivery Drug delivery refers to approaches, formulations, manufacturing techniques, storage systems, and technologies involved in transporting a pharmaceutical compound to its target site to achieve a desired therapeutic effect. Principles related to dr ...

. The polymer matrix serves as a container for proteins. Polypyrrole has been investigated as a catalyst support for fuel cells and to sensitize cathode electrocatalysts. Together with other conjugated polymers such as polyaniline, poly(ethylenedioxythiophene) etc., polypyrrole has been studied as a material for "artificial muscles", a technology that offers advantages relative to traditional motor actuating elements. Polypyrrole was used to coat silica and reverse phase silica to yield a material capable of anion exchange and exhibiting hydrophobic interactions. Polypyrrole was used in the microwave fabrication of multiwalled carbon nanotubes, a rapid method to grow CNT's. A water-resistant polyurethane sponge coated with a thin layer of polypyrrole absorbs 20 times its weight in oil and is reusable. The wet-spun polypyrrole fibre can be prepared chemical polymerization pyrrole and DEHS as dopant.{{cite journal , last1 = Foroughi , first1 = J. , display-authors = etal , year = 2008 , title = Production of polypyrrole fibres by wet spinning , journal = Synthetic Metals , volume = 158 , issue = 3–4, pages = 104–107 , doi=10.1016/j.synthmet.2007.12.008

References