Precursor route

Processing routes using a soluble, processible precursor which is then converted to the final CP are common. Some of the best known examples involve P(Ac) and P(PV), P(TV), P(PO), P(PPS) and related polymers. Reference is made to the sections on syntheses of these individual polymers in Chapters 13-14 in the sequel. 

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Poly(arylene vinylenes). The use of the soluble precursor route has been successful in the case of poly(arylene vinylenes), both those containing ben2enoid and heteroaromatic species as the aryl groups. The simplest member of this family is poly(p-phenylene vinylene) [26009-24-5] (PPV). High molecular weight PPV is prepared via a soluble precursor route (99—105). The method involves the synthesis of the bis-sulfonium salt from /)-dichloromethylbenzene, followed by a sodium hydroxide elimination polymerization reaction at 0°C to produce an aqueous solution of a polyelectrolyte precursor polymer (11). This polyelectrolyte is then processed into films, foams, and fibers, and converted to PPV thermally (eq. 8). 

Scheme 1-6. The Halo-precursor route to substituted PPVs a) NBS, CCl4, hv b) KOtBu, THF c) 160-220 °C, vacuum, 4 h.

The scope of Wessling route has been extended by Mullen and co-workers to develop a soluble precursor route to poly(anthrylene vinyiene)s (PAVs) [51]. It was anticipated that the energy differences between the quinoid and aromatic resonance structures would be diminished in PAV relative to PPV itself. An optical band gap of 2.12 eV was determined for 1,4-PAV 29, some 0.3 eV lower than the value observed in PPV. Interestingly, the 9, lO-b/.v-sulfonium salt does not polymerize, possibly due to stcric effects (Scheme 1-9). 

The strategy of Kaeriyama represents a so-called precursor route and was developed to overcome the characteristic shortcomings (insolubility, lack of process-ability) of previous PPP syntheses. The condensation reaction is carried out with solubilized monomers, leading to a soluble polymeric intermediate. In the final reaction step this intermediate is then converted, preferentially in the solid state allowing the formation of homogeneous PPP films or layers, into PPP (or other poly(arylene)s). 

Later on, Ballard et al. [42, 43] developed an improved precursor route starting from 5,6-diacetoxycyclohexa-1,3-diene (18), the so-called 1C1 route. The soluble precursor polymer 19 is finally aromatized thermally into PPP 1 via elimination of two molecules of acetic acid per structural unit. Unfortunately, the polymerization of the monomer does not proceed as a uniform 1,4-polymerization in addition to the regular 1,4-linkages ca. 10% of 1,2-linkages are also formed as result of a 1,2-polymerization of the monomer. 

The sulfonium precursor route may also be applied to alkoxy-substituted PPVs, but a dehydrohalogenation-condensation polymerization route, pioneered by Gilch, is favored 37]. The polymerization again proceeds via a quinomethide intermediate, but die syndicsis of the conjugated polymer requires only two steps and proceeds often in improved yields. The synthesis of the much-studied poly 2-methoxy-5-(2-ethylhexyloxy)-l,4-phenylene vinylene], MEH-PPV 15 is outlined in Scheme 1-5 33, 35]. The solubility of MEH-PPV is believed to be enhanced by the branched nature of its side-chain. 

Other poly(2,5-dialkoxy-1,4-phenylene vinylene)s have been prepared in a similar fashion [34, 35, 40, 41]. Alternatively, a soluble a-halo precursor polymer 17 may be obtained by using less than one equivalent of base (Scheme 1-6). This may then be converted into fully conjugated material 16 by thermal treatment. This halo-precursor route may be preferred if the fully conjugated material has limited solubility or if incomplete conversion is desired. 

Parent polyfthicnylene vinylene) has also been synthesized by an aldol precursor route [122]. In this method, 5-methyl-2-thiophenecarbaldehyde 76 is treated with a base and the monomer polymerizes yielding a precursor 77 which is soluble in water. Thermal treatment in an acidic solution at 80 nC yields the fully conjugated material. Alternatively, the solid polymer may be healed to 280 C to effect elimination of water. Fully conjugated material exhibits low conductivity (10 8 S cm"1) in its pristine stale. 

In 1992/1994, Grubbs et al. [29] and MacDiarmid et al. [30] described an improved precursor route to high molecular weight, structurally regular PPP 1, by transition metal-catalyzed polymerization, of the cyclohexa-1,3-diene derivative 14 to a stereoregular precursor polymer 16. The final step of the reaction sequence is the thermal, acid-catalyzed elimination of acetic acid, to convert 16 into PPP 1. They obtained unsupported PPP films of a definite structure, which were, however, badly contaminated with large amounts of polyphosphoric acid. 

Since the products often precipitate during the polymerization, a modification was reported by Swatos et al. [82] involving the use of only about one equivalent of f-BuOK. This method, the so-called chlorine precursor route , first gives a soluble non-conjugated precursor (66) which is then converted thermally in the film or in a high boiling solvent, e.g. cyclohexanone. In the latter case, homogeneous solutions of (soluble) PPV derivatives 63 can be obtained. 

Maeda, K., Terashima, H., Kase, K., and Domen, K. (2009) Nanoparticulate precursor route to fine particles of TaON and Zr02-TaON solid solution and their photocatalytic activity for hydrogen evolution under visible light. Applied Catalysis A General, 357 (2), 206—212. 

Increasing the size of PAHs makes their deposition on surfaces difficult because they can neither be sublimed nor made sufficiently soluble for solution processing. A precursor route has thus been designed according to which molecules are deposited on a surface and transformed into the final disc-type adsorbate structures in a thermal solid-state reaction with the substrate surface acting as a template.1261 An exciting example is the hexaether 41 (scheme 11) which is sublimed onto a Cu-(1U) sur-... 

SCHEME 2.1 The Wessling-Zimmerman precursor route to PPY. (From Wessling, R.A. and Zimmerman, R.G., Polyelectrolytes from Bis Sulfonium Salts, U.S. Patent 3,401,152, 1968.)... 

V.P. Conticello, D.L. Gin, and R.H. Grubbs, Ring-opening metathesis polymerization of substituted bicyclo[2.2.2]octadienes a new precursor route to poly(p-phenylene vinylene), J. Am. Chem. Soc., 114 9708-9710, 1992. 

P.L. Burn, D.D.C. Bradley, R.H. Friend, D.A. Halliday, A.B. Holmes, R.W. Jackson, and A. Kraft, Precursor route chemistry and electronic properties of poly(p-phcnylcnc vinylene), poly[(2,5-dimethyl-p-phenylene)vinylene] and poly[(2,5-dimethoxy-p-phenylene)vinylene], J. Chem. Soc. Perkin Trans., 1 3225-3231, 1992. 

T. Ahn, S. Ko, J. Lee, and H. Shim, Novel cyclohexylsiyl — or phenylsiyl-substituted poly(p-phe-nylene vinylene)s via the halogen precursor route and Gilch polymerization, Macromolecules, 35 3495-3505, 2002. 

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