Poly dehydrohalogenation

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... 

Dehydrochlorination of poly vinylidene chloride and chlorinated polyvinyl chloride was carried out. High chlorine content in the polymers (more than 60%) provides the formation of chlorinated conjugated polymers, polychlorovinylenes. The reactivity of chlorinated polyvinylenes contributes to the sp carbon material formation during heat treatment. Synthesis of porous carbon has been carried out in three stages low-temperature dehydrohalogenation of the polymer precursor by strong bases, carbonization in the inert atmosphere at 400-600°C and activation up to 950°C. 

Other degradation processes in addition to depolymerization can be initiated thermally. Thermal dehydrohalogenation of poly(vinyl chloride) is one such example [5]. 

Electrophilic nitration of olefins can also be carried out with nitronium salts in pyridinium poly (hydrogen fluoride) (PPHF) solution491 (which also acts as solvent) to give high yields of nitrofluorinated alkanes. In the presence of added halide ions (iodide, bromide, chloride) the related haloalkanes are formed, and these can be dehydrohalogenated to nitroalkenes492 [Eq. (5.183)]. 

The effects of silicon substitution on the luminescence properties are of interest in the field of polymer LEDs. Poly(2-dimethyloctylsilyl-l,4-phenylenevi-nylene) (DMOS-PPV) is completely soluble in common organic solvents. Hwang et al. reported synthesis and LED properties of DMOS-PPV [85]. Scheme 12 shows the synthetic route to DMOS-PPV by dehydrohalogenation. 

DBU and its salts have been patented and used as dehydrohalogenation agents for fluoropolymers (83JAP(K)219202), fluororubbers (78MI3), and poly(vinyl halide) in the preparation of polarizing films (83JAP(K)21929), and as dissociation catalysts for blocked isocyanates (83JAP(K)65764). 

One of the most innovative and useful techniques for the preparation of conducting polymers has been the synthesis of highly soluble precursor polymers that can be easily handled in solution, purified, and then later converted to the less tractable conducting polymer. The first example of such an approach was the dehydrohalogenation of poly(vinyl chloride) (102). This reaction, like most elimination reactions on polymers, rarely goes to completion and is not well suited for the synthesis of useful conducting... 

The product contains (besides some residual F and H) also inserted NBU4 , i.e. it is naturally n-doped. Even in the absence of BuOH, reactive nucleophiles like [(CH3)2NCHO] or (CH3)2N can be generated by preelectrolysis of dimethylformamide solutions [59]. Such a process allows electrochemical deposition of diamond-like carbon on an aluminum electrode [78] and it also produces polyyne from PVDF (cf. Eq. 4.22a). At similar conditions, PTFE shows no reactivity [59] and polyvinylchloride is dehydrohalogenated only to polyacetylene [79-82]. However, Zra 5-poly-acetylene, which was stereoregularly chlorinated by FeCh, is carbonizable by electrochemically generated BuO (cf. reactions 4.22 and 4.22a) [83] ... 

The synthetic methods for carbyne employed so far have involved the dehydrohalogenation of polyvinylidene halides [7], dehydrochlorination of chlorinated polyacetylene [8], defluorination of poly(tetrafluoroethylene) [9], and phase transition from carbon materials, such as graphite, under severe conditions. 

Theoretically, the chemical structure of original polymers can affect the structure of the dehydrohalogenation products. Indeed, complete dehydrochlorination of poly(l,l,2-trichlorobutadiene) could be expected to result in the formation of the polyyne form of carbyne because there seemingly is no other way of elimination except that shown in Scheme 12.2 (B = base). 

Finally, it should be noted that Raman spectra of profoundly dehydrohalogenated poly(vinylidene chloride) or poly(vinylidene bromide) reveal striking similarity to the spectra of carbyne-rich nanostructured carbon films produced by supersonic cluster beam deposition [24 26]. 

The dehydrohalogenation products of poly(ethylene-a/z-chlorotrifluor-oethylene) were also studied using solid-state NMR spectroscopy and the cross-polarization magic angle spinning (CP MAS) technique [18]. The elimination reaction induced by potassium ZerZ-butoxide (z-BuOK) in tetrahydrofuran (THF) was found to proceed slowly only 50% of hydrogen... 

The NMR results along with the IR spectroscopic data have shown that chemical dehydrohalogenation of poly(ethylene-aZz-chlorotrifluor-oethylene), by treating with potassium zerz-butoxide in THF, mainly yields halogen-substituted polyenyne structures containing randomly distributed isolated triple C=C-bonds. Based on the predominant structure of the original polymer, the polyene fragments can be described as head-to-head poly fluoro vinylenes (Scheme 12.8). 

Yet another serious problem that makes the preparation of perfect carbyne structures (long-chain polyynes or cumulenes) an almost hopeless endeavor is the presence of intrinsic defects in the original halogen-containing polymers. The polymers produced by common polymerization technologies always contain some defect structures such as head-to-head and "tail-to-tail links [32-34] as well as branching sites [35,36]. For instance, poly(vinylidene fluoride) was shown to contain up to 6 mol.% [32,33] and poly(vinylidene chloride) even 12.5% of abnormal links [34], which hamper exhaustive dehydrohalogenation (Scheme 12.9). 

Similarly, poly(trichlorobutadiene)s contain 3,4-connected links, along with predominating 1,4-links [37], that will afford allenic or acetylenic pendants during dehydrohalogenation (Scheme 12.10). 

Finally, poly(ethylene-a/z-chlorotrifluoroethylene) was found to have an appreciable proportion of the alternation defects, which makes complete elimination of hydrogen halides impossible regardless of the nature of the dehydrohalogenating agent (Scheme 12.11) [18]. 

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