Poly(p-phenylene vinylene)

Poly(p-phenylene vinylene) (PPV, or polyphenylene vinylene) is a conducting polymer of the rigid-rod polymer family. PPV is the only polymer of this type that can be processed into a highly ordered crystalline thin film. PPV and its derivatives are electrically conducting upon doping. Although insoluble in water, its precursors can be manipulated in aqueous solution. The small optical band gap and its bright yellow fluorescence makes PPV a candidate in applications such as light-emitting diodes (LED) and photovoltaic devices. Moreover, PPV can be doped to form electrically conductive materials. Its physical and electronic properties can be altered by the inclusion of functional side groups. PPVs can be synthesized by a variety of methods, the details of which determine purity and molecular weight. The most popular methods proceed via p-xylylene intermediates after a base induced elimination from α,α'-disubstituted para-xylenes. Although xylylene-based routes dominate the synthetic methodology, many other routes have been evaluated. PPV can be synthesized by Wittig-type couplings between the bis(ylide) derived from an aromatic bisphosphonium salt and dialdehyde, especially 1,4-benzenedialdehyde. Step growth coupling reactions, such as this Wittig condensation, usually yield low molecular weight oligomer with 5-10 repeat units. Incorporation of various side groups (alkyl, alkoxy, or phenyl) increases the solubility of the polymer and gives higher molecular weights. An advantage of the step-polymerization approach is that ortho-, meta-, and para-xylylene linkages can be incorporated in the main chain. Copolymers of defined stereoregularity can also be easily made in this way. PPV derivatives can be also produced via the Knoevenagel condensation between a benzylic nitrile and an aromatic dialdehyde. Since this method produces many side reactions, such as hydrolysis of nitrile group, careful optimization of the reaction conditions was needed.