Polymerization, PPV

H Becker, H Spreitzer, K Ibrom, and W Kreuder, New insights into the microstructure of gilch-polymerized PPVs, Macromolecules, 32 4925-4932, 1999. 

Becker, H., Spreitzer, H., Ibrom, K., Kreuder, W., 1999. New insights into the microstructure of GILCH-polymerized PPVs. Macro molecules 32,4925-4932. 

The optoelectronic properties of dilute solutions of oligomeric and (broken-conjugation] polymeric PPV chains were studied using optical absorption and (time-resolved] emission spectrophotometry. The following properties were determined absorption and emission spectra, fluorescence quantum yields and decay times, exciton polarizabilities and dissociation probabilities, charge mobilities, and RC absorption spectra. The experimental results are compared with theoretical calculations of exciton polarizabilities, charge mobilities, and RC absorption spectra. ... 

The results of these lifetime and efficiency investigations have implied that the hahde-mediated defects must be considered whenever different synthesis conditions or different PPVs are to be screened. Furthermore, such results continue to cast doubt on the attention appHed to TBB defects in Gilch-polymerized PPVs. It must not be assumed, a priori, that any dehydrohalogenation achieved during the Gilch synthesis would be sufficient to exclude the impact of halide defects when, for example, the correlation between the extent of the TBB defects and the device hfetime is examined [49]. 

Electrochemical Polymerization. PPV has also been prepared electrochemically using different types of monomeric compounds [910,911]. 

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). 

Another interesting applications area for fullerenes is based on materials that can be fabricated using fullerene-doped polymers. Polyvinylcarbazole (PVK) and other selected polymers, such as poly(paraphcnylene-vinylene) (PPV) and phenylmethylpolysilane (PMPS), doped with a mixture of Cgo and C70 have been reported to exhibit exceptionally good photoconductive properties [206, 207, 208] which may lead to the development of future polymeric photoconductive materials. Small concentrations of fullerenes (e.g., by weight) lead to charge transfer of the photo-excited electrons in the polymer to the fullerenes, thereby promoting the conduction of mobile holes in the polymer [209]. Fullerene-doped polymers also have significant potential for use in applications, such as photo-diodes, photo-voltaic devices and as photo-refractive materials. 

By 1988, a number of devices such as a MOSFET transistor had been developed by the use of poly(acetylene) (Burroughes et al. 1988), but further advances in the following decade led to field-effect transistors and, most notably, to the exploitation of electroluminescence in polymer devices, mentioned in Friend s 1994 survey but much more fully described in a later, particularly clear paper (Friend et al. 1999). The polymeric light-emitting diodes (LEDs) described here consist in essence of a polymer film between two electrodes, one of them transparent, with careful control of the interfaces between polymer and electrodes (which are coated with appropriate films). PPV is the polymer of choice. 

A xylylene-fc/.v-phosphonium salt 11 gave films of PPV 1 upon clectropolymer-ization. The absorption and emission spectra of the resultant material were blue-shifted with respect to PPV produced by other routes, suggesting that the electro-polymerized material has a shorter effective conjugation length, possibly because of incomplete elimination of phosphonium groups [22]. 

A potential drawback of all the routes discussed thus far is that there is little control over polydispersity and molecular weight of the resultant polymer. Ringopening metathesis polymerization (ROMP) is a living polymerization method and, in theory, affords materials with low polydispersities and predictable molecular weights. This methodology has been applied to the synthesis of polyacctylcne by Feast [23], and has recently been exploited in the synthesis of PPV. Bicyclic monomer 12 [24] and cyclophane 13 [25) afford well-defined precursor polymers which may be converted into PPV 1 by thermal elimination as described in Scheme 1-4. 

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). 

Substituted soluble PPV derivatives may also be synthesized by step-growth polymerization methods. Arylene-fc/.v-phosphylidenes may be condensed with ler-ephthaldehydes in a Wittig fashion to yield alternating PPV copolymers [52]. An alkoxy-substituted PPV derivative 28 (Scheme 1-8) prepared in this fashion emits in the orange (2nmx=585 nm) region of the spectrum [52]. 

EL experiments showed that the yellow-emitting LEDs prepared from LPPP 12 exhibit quite remarkable characteristics (single layer construction ITO/LPPP 12/Ca quantum efficiency ca. 1.0%, applied voltage 4-6 V 135]). These figures are in the range of the best values described hitherto for polymeric emitters in a single layer arrangement, for example, poly(pcira-phenylenevinylene) PPV and PPV derivatives. 

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. 

There have been a number of different synthetic approaches to substituted PTV derivatives proposed in the last decade. Almost all focus on the aromatic ring as the site for substitution. Some effort has been made to apply the traditional base-catalyzed dehydrohalogenation route to PTV and its substituted analogs. The methodology, however, is not as successful for PTV as it is for PPV and its derivatives because of the great tendency for the poly(u-chloro thiophene) precursor spontaneously to eliminate at room temperature. Swager and co-workers attempted this route to synthesize a PTV derivative substituted with a crown ether with potential applications as a sensory material (Scheme 1-26) [123]. The synthesis employs a Fager condensation [124] in its initial step to yield diol 78. Treatment with a ditosylate yields a crown ether-functionalized thiophene diester 79. This may be elaborated to dichloride 81, but pure material could not be isolated and the dichloride monomer had to be polymerized in situ. The polymer isolated... 

As a final note, during the final stages of preparing this review the first example of a diphosphene-PPV was reported [111]. This exciting new polymer contains P=P bonds spaced by p-phenylenevinylene units in the main chain, has a degree of polymerization of approximately 6, and shows emissive properties. 

Poly(para-phenylenevinylene)s (PPVs) represent one of the most intensively investigated classes of rr-conjugated materials. Many synthetic procedures to generate unsubstituted and substituted PPVs have been developed. They include 1,6-polymerizations of 1,4-xylylene intermediates as well as several polycondensation methods. Parallel to the polymer syntheses, several series of PPV oligomers (OPVs) have been synthesized and characterized. Such model oligomers of different molecular size allow for a study of the dependence of electronic and optical properties on the length of the conjugated Ti-system. 

The general process involves polymerization of l,4-bis(dialkylsulfoniome-thyl)benzene dihalides (61) by addition of base. The immediately formed polyelectrolyte (62) is then converted thermally to the final PPV derivative (63). The process was first developed for the synthesis of unsubstituted PPV 60. The mechanism of the Wessling process is still not fully clear. First, Horhold et al. 

The polymerization of l,4-bis(halomethyl)benzenes to PPVs in the presence of a large excess of potassium f-butoxide is referred as the Gilch route [81]. The method was first described for the synthesis of unsubstituted PPV 60, but -unfortunately - this route produces the PPV as an intractable, insoluble powder. However, the adaptation of the Gilch route to the polymerization of l,4-bis(halo-methyl)benzenes possessing solubilizing side groups gives access to soluble PPV materials. 

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. 

The generation of PPV and corresponding derivatives via the dihalide approach is possible not only in solution reaction, but also - via the gas phase -in a so-called chemical vapor deposition (CVD) process. In this process, the vapor of a dichlorinated para-xylene (a,a or a,a) is pyrolyzed at moderately low pressures (0,1-0,2 torr) to form a chlorinated para-xylylene intermediate, which then condenses and polymerizes on a suitable, cooled substrate. The coating of the chlorinated precursor polymer can be heated to eliminate HCl, to form PPV 60 (or a PPV derivative) [88]... 

The polymerization methods to PPV and PPV derivatives described in the previous section involve 1,6-polymerization of an immediately formed 1,4-xylyl-ene derivative. Aside frome this polymerization approach, a broad spectrum of polycondensation procedures (step-growth methods) to PPV and PPV derivatives has been developed. The methods can be classified as follows ... 

Horhold et al. and Lenz et al. [94,95]. The polycondensation provides the cyano-PPVs as insoluble, intractable powders. Holmes et al. [96], and later on Rikken et al. [97], described a new family of soluble, well-characterized 2,5-dialkyl- and 2,5-dialkoxy-substituted poly(pflrfl-phenylene-cyanovinylene)s (74b) synthesized by Knoevenagel condensation-polymerization of the corresponding alkyl-or alkoxy-substituted aromatic monomers. Careful control of the reaction conditions (tetra-n-butyl ammonium hydroxide as base) is required to avoid Michael-type addition. 

Poly(l,4-naphthylenevinylene) (106) is accessible via the Wessling polymerization procedure. Lenz, Karasz, Wegner et al. have published the synthesis of PNV 106, starting from l,4-bis(chloromethyl)naphthalene [127, 128]. The poly(l,4-naphthylenevinylene) (106) displays an optical absorption energy of 2.05 eV, slightly red-shifted by about 0,3 eV relative to the parent PPV 60-system, due to the electronic effect of the annelated benzene ring. 

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