102 聚乙炔的發現聚乙炔最早是由一九六三年諾貝爾化學獎得獎人，義大利化學家納塔（Giulio Natta）在一九五八年製得不溶不融的粉末，當時並未引人注意。一九六七年，一個偶然的情況，使白川英樹教授幸運地得到薄的皮膜狀聚乙炔。經再實驗，才發現原來觸媒的濃度，比平常的設定值高了一千倍，而且反而是聚乙炔的不溶性有助於皮膜的生成。後來，由掃描式電子顯微鏡觀察到，膜是一種纏絡的纖維狀物。二○○○年諾貝爾化學獎得主，白川英樹教授（Hideki Shirakawa，圖中）、麥克戴阿密德教授（Alan G. MacDiarmid，圖左）、希格院長（Alan J. Heeger，圖右）。
108 Plastics that imitate metals Plastics are polymers, moleculesformed of many identical units boundto each other like pearls in a necklace.For a polymer to be electricallyconductive it must "imitate" a metal –the electrons in the bonds must befreely mobile and not bound fast tothe atoms. One condition for this isthat the polymer consists of alternatesingle and double bonds, termedconjugated double bonds.Polyacetylene is the simplest possibleconjugated polymer. It is obtained bypolymerisation of acetylene, shown tothe left.
109 A surprise with a silver lining... At the beginning of the 1970s Shirakawa wasstudying the polymerisation of acetylene. In hisreaction vessel polyacetylene appeared in theform of an unremarkable black powder. On oneoccasion a visiting researcher accidentally addedone thousand times more catalyst than usual.Imagine the researchers' surprise when abeautiful silvery film formed on the surface ofthe liquid in the vessel. The obvious questionwas: "If the plastic film shines like a metal, canit conduct electricity, too?"
110 ... and a Nobel medal in gold Although the polyacetylene film shone like silver, it was not an electricalconductor. Could it perhaps be modified in some way? In the mid-1970s the three Laureates began co-operating to investigate this andresults were quick to come. When they caused the films to react withiodine vapour, the conductivity increased by as much as ten milliontimes – a discovery that was eventually to give them a Nobel Prize inChemistry.Oxidation with iodine causes the electrons to be jerked out of thepolymer, leaving "holes" in the form of positive charges that can movealong the chain.
111 Doping raises molecule performance By analogy with semiconductor technology one speaks of doping thepolymer when it is subjected to oxidation with iodine vapour. Themore electrons are removed, the higher the degree of doping and thegreater the conductivity. While polyacetylene can be persuaded to conduct current as well asmany metals do, this material is unfortunately no good for practicaluse. Its conductivity drops rapidly in contact with air. This has led tothe development of more stable, conjugated polymers, e.g. polypyrrol,polyaniline and polytiophene.
113 Light-emitting diodes Just now the most intensivedevelopment is aimed atconjugated polymers in their un-doped, semiconductive state.This is because it was discoveredten years ago that someconjugated polymers exhibitelectro-luminescence, they glowwhen a voltages passes throughthem. Many applications arepredicted for luminescent plastic.We shall soon be seeing the firstpractical use in light displays inmobile telephones and oninformation boards. In a fewyears flat TV screens inluminescent plastic may havebecome a reality.
114 Solar cells The process giving rise to electro-luminescence can also be"run backwards". Absorption oflight creates positive and negativecharges that are picked up by theelectrodes, providing an electriccurrent. This is the principle ofthe solar cell. The advantage of plastic isthat large, flexible surfaces canbe made relatively easily andcheaply. Solar cell plastic could bespread out over large areas andgive us environmental friendlyelectricity in the not-too-distantfuture.
123 Delocalization of Charge Carriers in Polyacetylene Localized! Charge cannot go anywhere!!!Delocalized! Charge carriers can move around!!!
124 Delocalization and Transport of Charge Carriers
125 Polarons - doped carbon chains In the first of the above reactions, oxidation, the iodine molecule attractsan electron from the polyacetylene chain and becomes I3- . Thepolyacetylene molecule, now positively charged, is termed a radical cation,or polaron (fig. b below).The lonely electron of the double bond, from which an electron was removed,can move easily. As a consequence, the double bond successively movesalong the molecule. The positive charge, on the other hand, is fixed byelectrostatic attraction to the iodide ion, which does not move so readily.If the polyacetylene chain is heavily oxidised, polarons condense pair-wiseinto so-called solitons. These solitons are then responsible, in complicatedways, for the transport of charges along the polymer chains, as well as fromchain to chain on a macroscopic scale.
126 Brilliant applications Metal wires that conduct electricity can be made to light up when a strong enough current ispassing – as we are reminded of every time we switch on a light bulb. Polymers can also bemade to light up, but by another principle, namely electroluminescence, which is used inphotodiodes. These photodiodes are, in principal, more energy saving and generate lessheat than light bulbs. In electroluminescence, light is emitted from a thin layer of the polymer when excited by anelectrical field. In photodiodes inorganic semiconductors such as gallium phosphide aretraditionally used, but now one can also use semiconductive polymers.