Polymerization of PPVs

Scheme 5.5 Synthesis of DOPO-Br-capped CdSe NCs and subsequent polymerization of PPV from the NCs surface.132 (Reprinted with permission from H. Skaff et al., J. Am. Chem. Soc. 2004,126,...

Alumina nanoporous templates have also been used in gas-phase growth of CPNTs. It has been reported that isolated nanotubes consisting of poly(p-phenylenevinylene) (PPV) and carbonized-PPV bilayers can be synthesized in an alumina template by chemical-vapor deposition (CVD) polymerization. In a smdy done by Kim and colleagues, CVD polymerization of PPV was carried out by passing monomer vapor through a pyrolysis zone at 625 °C to form precursor polymer nanotubes on the inner surface of the alumina nanochannels. The nanotubes were further thermally treated in vacuum at 270 °C for an extended time (14 h) to be converted into PPV nanotubes. In order to create PPV/ carbonized PPV bilayer nanotubes, the PPV nanotubes were then carbonized at 850 °C... 

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. 

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. 

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-naphthylenevinylenes) have been prepared by metathesis polymerization of benzobarrelenes [181,182] and the photoluminescence properties of homopolymers and block-copolymers have been studied in some detail [183]. PPV also has been prepared via ROMP of [2.2]paracyclophane-l,9-diene [184] and ROMP of a paracyclophene that contains a solubilizing leaving group [185]. The resulting polymer is converted to PPV upon acid catalysis at room temperature. ADMET of 2,5-dialkyl-l,4-divinylbenzenes using Mo or W catalysts has... 

The molecular weight of the polymers can be controlled (from ca. 50,000 to above 1,000,000) by changing the reaction temperature and time, the solvent, the concentration of the monomer, and the amount of base [39,40]. High molecular-weight polymers and a high content of trans double bonds are the reasons for the wide usage of Gilch polymerization in the synthesis of PPV homo- and copolymers. 

Polyacetylene has good inert atmospheric thermal stability but oxidizes easily in the presence of air. The doped samples are even more susceptible to air. Polyacetylene films have a lustrous, silvery appearance and some flexibility. Other polymers have been found to be conductive. These include poly(p-phenylene) prepared by the Freidel-Crafts polymerization of benzene, polythiophene and derivatives, PPV, polypyrrole, and polyaniline. The first polymers commercialized as conductive polymers were polypyrrole and polythiophene because of their greater stability to air and the ability to directly produce these polymers in a doped form. While their conductivities (often on the order of 10" S/m) are lower than that of polyacetylene, this is sufficient for many applications. 

Gibson and coworkers utilized the expected complexation between crown ethers and acrylonitrile for the preparation of poly(acrylonitrile-crown ether rotax-ane)s 94 [137]. Relative to that with the polystyrene backbone, the enhanced threading supported the intermediacy of the expected complex. The reaction intermediates, the cations 95 and 96 in the preparation of poly(phenylene vinylene) (PPV) also provided a source for interaction with crown ethers [70], The solution polymerization of precursor 95 in the presence of crown ethers followed by transformation of 96 produced polyrotaxanes 97. 

A histamine selective MIP chemosensor, based on impedimetric transduction, has been devised [136]. Its preparation involved immobilization of the histamine imprinted MIP particles in apoly[2-methoxy-5-(3,7-dimethyloctyloxy)-l-4-pheny-lene vinylene] (OCiCi0-PPV) film deposited on aluminium electrodes. Preparation of these particles comprised thermally induced co-polymerization of MAA (functional monomer), EGDMA (cross-linker) and AIBN (initiator) in the presence of histamine. This film efficiently rebound histamine in the presence of histidine and... 

Acyclic diene metathesis (ADMET) polymerization of divinyl benzene, shown in Scheme 37, using an extremely reactive tungsten alkylidene catalyst (Schrock s catalyst) yielded PPV oligomers with DP of 8 [153]. In this example ethylene is formed as a side product of metathesis, and its removal by the use of high vacuum helps drive the polymerization in the forward direction. 

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