Polypyrrole (PPy) is an organic polymer obtained by oxidative polymerization of

pyrrole Pyrrole is a heterocyclic, aromatic, organic compound, a five-membered ring with the formula . It is a colorless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., ''N''-methylpyrrol ...

. It is a solid with the formula H(C₄H₂NH)_nH. It is an intrinsically conducting polymer, 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 heterocyclic compound, heterocycle. The parent PT is an insoluble colored solid with the formula (C4H2S)n.Strictly speaking, "polythiophene" is a misnomer, since the polymer consists of ...

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 ...

, 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 is ...

was awarded the

Nobel Prize in Chemistry The Nobel Prize in Chemistry () is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of chemistry. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outst ...

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 Radical cations are denoted M^. Salts of these species have been isolated in the cases of dibenzocyclooctatetraene, various tertiary amines, and some polymethylated derivatives of azulene. Radical cations, like radical anions, have one unpaire ...

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 In electrochemistry, cyclic voltammetry (CV) is a type of voltammetric measurement where the potential of the working electrode is ramped linearly versus time. Unlike in linear sweep voltammetry, after the set potential is reached in a CV expe ...

and chronocoulometry methods can be used for electrochemical synthesis of polypyrrole. Most recent micro and nano droplet researches have been conducted in the synthesis of polypyrrole microstructures using various fluid templates formed on different solid surfaces.

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). 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 In mathematics, a fractal is a Shape, geometric shape containing detailed structure at arbitrarily small scales, usually having a fractal dimension strictly exceeding the topological dimension. Many fractals appear similar at various scale ...

properties and ionic diffusion through them show anomalous diffusion 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 involves various methods and technologies designed to transport pharmaceutical compounds to their target sites helping therapeutic effect. It involves principles related to drug preparation, route of administration, site-specif ...

. The polymer matrix serves as a container for proteins. Polypyrrole has been investigated as a

catalyst support In chemistry, a catalyst support or carrier is a material, usually a solid with a high surface area, to which a catalyst is affixed. The activity of heterogeneous catalysts is mainly promoted by atoms present at the accessible surface of the ma ...

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