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Polyacetylene
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==Synthesis== ===From acetylene=== [[File:Ziegler natta scheme for polyacetylene.png|thumb|left|Ziegler–Natta scheme]] A variety of methods have been developed to synthesize polyacetylene. One of the most common methods is via passing acetylene gas over a [[Ziegler–Natta catalyst]], such as [[titanium isopropoxide|Ti(O''i''Pr)<sub>4</sub>]]/[[triethylaluminium|Al(C<sub>2</sub>H<sub>5</sub>)<sub>3</sub>]]. This method allows control over the structure and properties of the final polymer by varying temperature and catalyst loading.<ref>{{cite journal |last=Feast |first=W. J. |author2=Tsibouklis, J. |author3=Pouwer, K. L. |author4=Groenendaal, L. |author5=Meijer, E. W. |title=Synthesis, processing and material properties of conjugated polymers |journal=Polymer |date=1996 |volume=37 |issue=22 |pages=5017–5047 |doi=10.1016/0032-3861(96)00439-9|url=https://pure.tue.nl/ws/files/1422993/587897.pdf }}</ref> Mechanistic studies suggest that this polymerization involves metal insertion into the triple bond of acetylene.<ref>{{cite journal |last=Clarke |first=T. C. |author2=Yannoni, T. S. |author3=Katz, T. J. |title=Mechanism of Ziegler–Natta Polymerization of Acetylene: A Nutation NMR Study |journal=Journal of the American Chemical Society |date=1983 |volume=105 |issue=26 |pages=7787–7789 |doi=10.1021/ja00364a076|bibcode=1983JAChS.105.7787C }}</ref> [[File:Insertion mechanism for polyacetylene.png|thumb|Mechanism of polyacetylene synthesis from acetylene and a metal catalyst]] By varying the apparatus and catalyst loading, Shirakawa and coworkers were able to synthesize polyacetylene as thin films, rather than insoluble black powders. They obtained these films by coating the walls of a reaction flask under inert conditions with a solution of the Ziegler–Natta catalyst and adding gaseous acetylene resulting in immediate formation of a film.<ref name=Ito>{{cite journal |author1=Ito, T. |author2=Shirakawa, H. |author3=Ikeda, S. |title=Simultaneous Polymerization and Formation of Polyacetylene Film on the Surface of Concentrated Soluble Ziegler-Type Catalyst Solution |doi=10.1002/pola.1996.854 |journal=Journal of Polymer Science Part A |date=1974 |volume=12 |issue=13 |pages=11–20 |bibcode=1996JPoSA..34.2533I}}</ref> Enkelmann and coworkers further improved polyacetylene synthesis by changing the catalyst to a [[cobalt(II) nitrate|Co(NO<sub>3</sub>)<sub>2</sub>]]/[[sodium borohydride|NaBH<sub>4</sub>]] system, which was stable to both oxygen and water.<ref name=Feast/> Polyacetylene can also be produced by photopolymerization of acetylene. [[Glow discharge|Glow-discharge]], [[gamma radiation|gamma]], and [[ultraviolet]] irradiation have all been used. This method avoid the use of catalysts and solvents, but requires [[cryogenics]] to produce usable polymer. Gas-phase polymerization typically produces irregular cuprene, whereas liquid-phase polymerization, conducted at −78 °C produces linear ''cis''-polyacetylene, and solid-phase polymerization, conducted at still lower temperature, produces ''trans''-polyacetylene.<ref name=Saxman/> ===Ring-opening metathesis polymerization=== Polyacetylene can be synthesized by [[ring-opening metathesis polymerisation]] (ROMP) from [[cyclooctatetraene]], a precursor that is more expensive but easier to handle than the [[acetylene]] [[monomer]].<ref>{{cite journal |last=Klavetter |first=Floyd L. |author2=Grubbs, Robert H. |title=Polycyclooctatetraene (Polyacetylene): Synthesis and Properties |journal=Journal of the American Chemical Society |date=1988 |volume=110 |issue=23 |pages=7807–7813 |doi=10.1021/ja00231a036|bibcode=1988JAChS.110.7807K }}</ref> This synthetic route also provides a means for introducing solubilizing groups to the polymer while maintaining the conjugation.<ref name=Grubbs>{{cite journal |last=Gorman |first=C. B. |author2=Ginsburg, E. J. |author3=Grubbs, R. H. |title=Soluble, Highly Conjugated Derivatives of Polyacetylene from the Ring-Opening Metathesis Polymerization of Monosubstituted Cyclooctratetraenes: Synthesis and the Relationship between Polymer Structure and Physical Properties |journal=Journal of the American Chemical Society |date=1993 |volume=115 |issue=4 |pages=1397–1409 |doi=10.1021/ja00057a024|bibcode=1993JAChS.115.1397G }}</ref> Polymers with linear groups such as ''n''-[[octyl]] had high conductivity but low solubility, while highly branched ''tert''-[[butyl]] groups increased solubility but decreased [[conjugated system|conjugation]] due to polymer twisting to avoid [[steric]] crowding. They obtained soluble and conductive polymers with ''sec''-butyl and neopentyl groups, because the [[methylene group|methylene]] (CH<sub>2</sub>) unit directly connected to the polymer reduces steric crowding and prevents twisting.<ref name=Grubbs/> [[File:Modified Grubbs.png|thumb|center|upright 2.5|Grubbs route to polyacetylene]] ===From precursor polymers=== Polyacetylene can also be synthesized from other polymers. This method enables modification and processing of the polymer before conversion into the highly insoluble polyacetylene. Short, irregular segments of polyacetylene can be obtained by [[dehydrohalogenation]] of [[poly(vinyl chloride)]]:<ref>{{cite web |title=Conducting Polymers |url=https://www.ch.ic.ac.uk/local/organic/tutorial/steinke/4yrPolyConduct2003.pdf |work=ch.ic.ac.uk}}</ref> [[File:PVC base polyacetylene.png|thumb|300px]] More efficient methos for synthesizing long polyacetylene chains exist and include the Durham precursor route in which precusor polymers are prepared by ring-opening metathesis polymerization, and a subsequent heat-induced reverse [[Diels–Alder reaction]] yields the final polymer, as well as volatile side products.<ref name=Feast/> [[File:Durham precursor 3.png|thumb|upright=2.5|center|Durham precursor polymer (Reverse Diels–Alder) route to polyacetylene]]
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