Arenes oxidative polymerization

The polymerization of zwitterions with sufficient stability to be isolated and purified has been demonstrated. These reactions of stable zwitterions provide well-defined examples of the involvement of zwitterionic propagating species. The first example of the polymerization of a stable zwitter-ion used a tetrahydrothiophenium arene oxide zwitterion 555 [Eq. (95)] [340-342]. 

The above proposal is supported by the lack of reaction of electron-deficient arylamines, such as p-N02-aniline and p-Et02C-aniline, in the copper-mediated reactions with lead acetates because of the inability of the amines to reduce Cu(II) to Cu(I). This lack of reduction is indicated by the lack of a color change upon addition of Cu(OAc)2 to a solution of either of the amines. Further, the presence of Cu(III) species may be responsible for the formation of arenes and polymeric species during the A-arylation of anilines that are easily oxidized (Scheme 11). 

The reaction mechanism of oxidative polymerization of aniline has been a big controversy (Fig. 7). The dimerization step is generally proposed as (i), in which aniline is one-electron-oxidized to a cation radical, followed by coupling of two molecules of the cation radical to a dimer. The subsequent steps of chain extension are under discussion routes involving coupling of cation radicals such as (ii)-(iv) (137-140) and routes via electrophilic attack of a two-electron-oxidized quinodal diiminium ion (v) or nitrenium ion (vi) (141,142) have been proposed. The addition of electron-rich arenes does not inhibit the polymerization, and therefore the route through the nitrenium ion (vi) seems to be rejected (137). 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Table XXXVI is a list of some catalytic photochemical redox transformation of organic reactants by (Q or H)3PW 204o. In the presence of UV light, Q3PW12O40 reacts with paraffins, arenes, alcohols, alkyl halides, ketones, nitriles, thioethers, and water. Under either anaerobic or aerobic conditions, decarboxylation, dehydrogenation, dimerization, polymerization, oxidation, and acylation takes place.

To be effective as autoxidation inhibitors radical scavengers must react quickly with peroxyl or alkyl radicals and lead thereby to the formation of unreactive products. Phenols substituted with electron-donating substituents have relatively low O-H bond dissociation enthalpies (Table 3.1 even lower than arene-bound isopropyl groups [68]), and yield, on hydrogen abstraction, stable phenoxyl radicals which no longer sustain the radical chain reaction. The phenols should not be too electron-rich, however, because this could lead to excessive air-sensitivity of the phenol, i.e. to rapid oxidation of the phenol via SET to oxygen (see next section). Scheme 3.17 shows a selection of radical scavengers which have proved suitable for inhibition of autoxidation processes (and radical-mediated polymerization). 

Some electron-rich arenes or heteroarenes undergo SET even at room temperature when exposed to the air. Such compounds will usually darken quickly, even if only trace amounts of oligomers are formed by autoxidation, because these oligomers can absorb visible light very efficiently and tend to be oxidized even more readily than the monomer. Thus, older samples of aniline, alkoxyanilines, or aminophe-nols are usually dark or black, even if analysis by 1H NMR does not reveal any impurities. Particularly air-sensitive are five-membered heteroarenes (pyrroles, furans, thiophenes) with electron-donating substituents. Some of these compounds polymerize on oxidation to yield materials with good electric conductivity (Scheme 3.19). 

The idea that complex formation may be important in the catalytic process can be carried further. It has been found, for example, that bis-arene chromium complexes supported on silica-alumina are active for ethylene polymerization. These catalysts are prepared by activating the support alone in the usual manner and then impregnating with a hydrocarbon solution of the bis-arene compound at room temperature in the absence of air or other oxidizing agent. 

Partial oxidation of C2S4 proceeds with loss of sulfur and coupling to give the vinylidene dithiolate derivative of dmit2-. This planar dianion C4S62 is isolated as its blue-purple ELtN"1" salt (130). Treatment of this salt with metal halides affords di- and polymeric complexes, for example, [C4S6][RuCl(arene)]2 and the semiconducting [NK Seln (134) (Eq. 11). 

There are a few examples of polymers based on vinylbenzofurans. Vinyldibenzofuran 324 has been patented for use in copolymer formulations with other vinyl arenes, used to prepare light-emitting devices <2004USP6803124>. Benzofuran 325 was developed as one of four polymerizable monomers that contain a built-in antioxidant. The polymerization process was transition metal catalyzed <2003MM8346>. Benzofuran 326 also contains the styrene substructure, but there are few examples of its polymerization. Poly(2,3-benzofuran) films were synthesized by anodic oxidation on stainless steel in the presence of boron trifluoride etherate. The films had good thermal stability and conductivity of lO Scm <2005MI1654>. 

Various bifunctional resins are based on acrylic epoxide monomers. Such systems can photopolymerize by the radical and/or cationic mechanism. With iron arene photoinitiators in the presence of an oxidant, radical as well as cationic photopolymerization of these monomers is possible . Onium -type photoinitiators form radical species upon photolysis, as shown in Figs. 3 and 4. The local radical concentration is, however, too low to permit the polymerization of such systems... 

Arene complexes have been found to be catalysts for the polymerization, hydrogenation, dismutation, and oxidative dimerization of olefins... 

Bisbenzene chromium has been found to be a catalyst for the polymerization of ethylene at 200°-250°C. Chromium metal was postulated as the active catalyst in the system 410, 411). The polymerization of ethylene by bisarene chromium(I) salts in the presence of (i-Bu)3Al has also been studied (406). The catalytic activity was found to be a function of both the arene and the anion present. When bisarene chromium complexes are air oxidized in water, hydrogen peroxide is produced ... 

Oxidation of arenes under nonaqueous conditions often results in polymerization [93], which has been observed for most of the simple arenes, such as benzene [94-97], naphthalene [98], pyrene [99], biphenyl [96], triphenylene [99], fluoranthene [99], and fluor-ene [99-101]. (See Chapter 32 for details.) Polymerization is often accompanied by severe electrode passivation [100]. 

See [6]. The following reaction types have been listed (a) Geometric isomerization of alkenes (b) Allylic [1,3] hydrogen shift (c) Cycloaddition of alkenes. Dimerization, Tri-merization. Polymerization (d) Skeletal rearrangments of alkenes and methathesis (e) Hydrogenation of alkenes (f) Additions to alkenes (g) Additions to C = X (h) Aliphatic substitutions (i) Aromatic substitution (j) Vinyl substitution (k) Oxidation of alkenes (1) Oxidation of alcohols (m) Oxidation of arenes (n) Oxidative decarboxylation (o) Oxidation of amines (p) Oxidation of vinylsilanes and sulfides (q) Oxidation of benzal-dehyde (r) Dehydrogenations. 

Triphenylenes provided with nonionic di(ethylene oxide) side-chains (25f)132 134 or with ionic alkyl chains (25g)135 form supramolecular polymers in water.136 The arene—arene interactions of the aromatic cores allow for the formation of columnar micelles . At low concentrations the columns are relatively short, and the solutions are isotropic. At higher concentrations the longer columns interact and lyotropic mesophases are formed.133 Computer simulations showed that in the isotropic solution the polymerization of the discotics is driven by solute-solute attraction and follows the theory of isodesmic linear aggregation the association constants for dimerization, trimerization, and etc., are equal and the DP of the column thus can easily be tuned by concentration and temperature.137 138 At higher concentrations the sizes of the columns are influenced by their neighbors, the columns align, and the DP rises rapidly. 

Polymerization of phthalocyanines in water occurs for derivatives substituted with oligo (ethylene oxide) side-chains (27c).167 168 In the lyotropic mesophases in water supramolecular polymers are present, and a comparative aggregation study between tetraphen-ylporphyrins and phthalocyanines proved the polymerization of the phthalocyanines to be stronger.168 The strong arene—arene interactions and the flatness of the aromatic core in the phthalocyanines causes them to aggregate more strongly, also mediated by the additional hydrophobic effect. 

A modern series of new plastics are based on transition metals (e.g. Fe, Ti, Cr, Zn, V) to form polymers and possess unusual properties such as variable oxidation states, and ligand exchange on the metal atom. They have reduced UV absorption and visible radiation and exhibit electrical conductivity. Examples include cyclopentadienyl and arene metal n polymeric complexes that act as electron rich aromatic system and are very reactive to a range of monomers to form polymers. 

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