Styrene derived radical

Fig. 52. EPR spectra recorded at 90 K. (a) TS-1 + aqueous H202. (b)-(d) TS-1 + H202+ styrene reacted at 333 K for 5, 10, and 20 min, respectively, and (e) TS-1 + H202 + allyl alcohol reacted at 333 K for 25 min. Asterisk represents signal caused by a styrene-derived radical formed during the reaction [from Shetti et al. (93)].

Examples of vinyl monomers for addition polymerization include acrylates, methacrylates, vinyl ethers and styrene derivatives. Radical, ionic, and group-transfer polymerizations are possible according to polymerizabil-ity of the monomers. Living polymerization is difficult because mesogenic monomers often contain bonds such as benzoate ester, which are easily attacked by growing ends. Cyclic and condensation monomers are less... 

Radical copolymerization is used in the manufacturing of random copolymers of acrylamide with vinyl monomers. Anionic copolymers are obtained by copolymerization of acrylamide with acrylic, methacrylic, maleic, fu-maric, styrenesulfonic, 2-acrylamide-2-methylpro-panesulfonic acids and its salts, etc., as well as by hydrolysis and sulfomethylation of polyacrylamide Cationic copolymers are obtained by copolymerization of acrylamide with jV-dialkylaminoalkyl acrylates and methacrylates, l,2-dimethyl-5-vinylpyridinum sulfate, etc. or by postreactions of polyacrylamide (the Mannich reaction and Hofmann degradation). Nonionic copolymers are obtained by copolymerization of acrylamide with acrylates, methacrylates, styrene derivatives, acrylonitrile, etc. Copolymerization methods are the same as the polymerization of acrylamide. 

The traditional means of assessment of the sensitivity of radical reactions to polar factors and establishing the electrophilicity or nucleophilieity of radicals is by way of a Hammett op correlation. Thus, the reactions of radicals with substituted styrene derivatives have been examined to demonstrate that simple alkyl radicals have nucleophilic character38,39 while haloalkyl radicals40 and oxygcn-ccntcrcd radicals " have electrophilic character (Tabic 1.4). It is anticipated that electron-withdrawing substituents (e.g. Cl, F, C02R, CN) will enhance overall reactivity towards nucleophilic radicals and reduce reactivity towards electrophilic radicals. Electron-donating substituents (alkyl) will have the opposite effect. 

Thermal insertion occurs at room temperature when R is XCH2CHAr-, at 40° C when R is benzyl, allyl, or crotyl (in this case two isomeric peroxides are formed), but not even at 80° C when R is a simple primary alkyl group. The insertion of O2 clearly involves prior dissociation of the Co—C bond to give more reactive species. The a-arylethyl complexes are known to decompose spontaneously into CoH and styrene derivatives (see Section B,l,f). Oxygen will presumably react with the hydride or Co(I) to give the hydroperoxide complex, which then adds to the styrene. The benzyl and allyl complexes appear to undergo homolytic fission to give Co(II) and free radicals (see Section B,l,a) in this case O2 would react first with the radicals. 

Meerwein Arylation Reactions. Aryl diazonium ions can also be used to form certain types of carbon-carbon bonds. The copper-catalyzed reaction of diazonium ions with conjugated alkenes results in arylation of the alkene, known as the Meerwein arylation reaction.114 The reaction sequence is initiated by reduction of the diazonium ion by Cu(I). The aryl radical adds to the alkene to give a new (3-aryl radical. The final step is a ligand transfer that takes place in the copper coordination sphere. An alternative course is oxidation-deprotonation, which gives a styrene derivative. 

The last decades have witnessed the emergence of new living Vcontrolled polymerizations based on radical chemistry [81, 82]. Two main approaches have been investigated the first involves mediation of the free radical process by stable nitroxyl radicals, such as TEMPO while the second relies upon a Kharash-type reaction mediated by metal complexes such as copper(I) bromide ligated with 2,2 -bipyridine. In the latter case, the polymerization is initiated by alkyl halides or arenesulfonyl halides. Nitroxide-based initiators are efficient for styrene and styrene derivatives, while the metal-mediated polymerization system, the so called ATRP (Atom Transfer Radical Polymerization) seems the most robust since it can be successfully applied to the living Vcontrolled polymerization of styrenes, acrylates, methacrylates, acrylonitrile, and isobutene. Significantly, both TEMPO and metal-mediated polymerization systems allow molec-... 

Radical-anions derived from styrene derivatives are nucleophilic in character. Electrochemical reduction of a-methylstyrene 5 gives the radical-anion intermediate, which in the absence of other electrophiles is sufficiently nucleophilic to attack dimethylfonnamide or acetonitrile [19], The radical-anion from styrene 6 under-... 

The resonance stability of the macroradical is an important factor in polymer propagation. Thus, for free radical polymerization, a conjugated monomer such as styrene is at least 30 times as apt to form a resonance-stabilized macroradical as VAc, resulting in a copolymer rich in styrene-derived units when these two are copolymerized. 

The SFRP or NMP has been studied mainly using the stable free radical TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy) or its adducts with, e.g., styrene derivatives. It is based on the formation of a labile bond between the growing radical chain end or monomeric radical and the nitroxy radical. Monomer is inserted into this bond when it opens thermally. The free radical necessary to start the reaction can be created by adding a conventional radical initiator in combination with, e.g., TEMPO or by starting the reaction with a preformed adduct of the monomer with the nitroxy radical using so-called unimolecular initiators (Hawker adducts). 

In a related approach, Padovani et al. prepared copolymers of styrene and a styrene derivative containing two pendant ester bonds using free-radical polymerization (Scheme 15) [108], Transesterification reactions were conducted with Novozym 435 as the catalyst and benzyl alcohol or (rac)-l-phenylethanol as the nucleophile. Interestingly, the ester bond closest to the polymer backbone (position A in Scheme 15) remained unaffected, whereas ester bond B reacted in up to 98% to the corresponding benzyl ester. The transesterification was not only highly chemoselective but also enantioselective. Conversion of (rac)-l-phenylethanol in the transesterification reaction amounted to a maximum conversion of 47.9% of the (/ )-alcohol, and only at the ester position B. 

Polymerization of isobutylene, in contrast, is the most characteristic example of all acid-catalyzed hydrocarbon polymerizations. Despite its hindered double bond, isobutylene is extremely reactive under any acidic conditions, which makes it an ideal monomer for cationic polymerization. While other alkenes usually can polymerize by several different propagation mechanisms (cationic, anionic, free radical, coordination), polyisobutylene can be prepared only via cationic polymerization. Acid-catalyzed polymerization of isobutylene is, therefore, the most thoroughly studied case. Other suitable monomers undergoing cationic polymerization are substituted styrene derivatives and conjugated dienes. Superacid-catalyzed alkane selfcondensation (see Section 5.1.2) and polymerization of strained cycloalkanes are also possible.118... 

A formal iron-catalyzed [3 + 2]-cycloaddition of styrene derivatives with benzoqui-none was reported by Itoh s group [96]. The process is believed to proceed via electron-transfer reactions mediated by a proposed Fe3+/Fe2+ couple, which generates a styrene radical cation and a semiquinone. These intermediates undergo stepwise addition to yield the benzofuran product 51 (Scheme 9.38). The reaction seems to be limited to electron-rich alkoxy-functionalized styrenes, as the Fe3+/Fe2+ redox couple is otherwise unable to transfer the electrons from the styrene to the quinone. 

Triphenylthiopyrylium salts have been employed as efficient electron transfer photosensitizers to promote the [4+2] cycloaddition between thiobenzophenone and substituted styrenes. A radical cation derived from the thiobenzophenone is involved in the formation of separable diastereoisomeric mixtures of 1,3,4-trisubstituted isothiochromans <2007OL3587>. Interest has been maintained in the photosensitizing properties of thiopyrylium salts for blood disinfection <2007BMCL4406>. Several new thioxanthylium salts have been reported <2007JOC2647, 2007JOC2690>. 

Another investigation along this line is the pulse radiolysis study of the electron transfer reactions from aromatic radical anions to styrene this type of reaction is commonly used to initiate anionic polymerization of styrene [35], The electron transfer rates from the unassociated biphenyl radical-anions to styrene derivatives in 2-propanol were found to increase along the... 

Japanese investigators reported that liquid sulfur dioxide polymerizes styrene derivatives (e.g., p-methyl styrene, a-methyl styrene) (19). Unfortunately, the experiments were not executed under rigorously anhydrous conditions (high vacuum) so that the possibility for proton (e.g., sulfurous or sulfuric acid) initiation exists although the authors seem to believe that S02 is the catalyst, probably by the following process 2S0a SO2 +SO e. The cationic nature of the mechanisms was proven by the facts that no polysulfones formed, that the polymerization was inhibited by bases, and that free radical inhibitors did not affect the reaction. These authors also claim that formaldehyde is polymerized by sulfur dioxide to a product which does not contain sulfur and whose infrared spectrum closely resembles that of a low temperature sample. 

Unfortunately, radicals derived from alkylmercuries are even more limited in what they will react with than radicals made from alkyl halides by the tin hydride method. Styrene, for example, cannot be used to trap alkylmercury-derived radicals efficiently because the radicals react more rapidly with the mercury hydride (which has an even weaker metal-H bond than Bi SnH) than with the styrene. 

The most important conclusion from stopped-flow studies is that the rate constants of propagation of several styrene derivatives are approximately kp 105-1 mol, L sec I at 0° C, which is relatively high compared with those of radical and anionic systems (average kp == 102 mol 1L-sec 1 at 0° C). Solvent effects are noticeable, with propagation slower in more nucleophilic 1,2-dichloroethane [17] than in CH2Cl2 [18] under comparable conditions. That is, the carbenium ion reactivity is apparently reduced by interaction with more nucleophilic solvents. However, such interactions do not result in formation of chloronium ions, whose spectra would be very different compared to those of the corresponding carbenium ions. 

Alkanesulfonyl chlorides are known to be a good source of alkanesulfonyl radicals or alkyl radicals with the aid of redox catalysts [3]. A series of studies using RuCl2(PPh3)3 as the redox catalyst have been carried out by Kamigata and coworkers (Scheme 13.6) [33-39]. Arenesulfonyl chlorides add to styrene derivatives to form the corresponding adducts, which undergo dehydrochlorination of EtsN to form the unsaturated sulfones [33]. When styrylsulfonyl chlorides are used as the precursor. 

Vinylsilanes as mono- and geminal disilyl-substituted C=C-double bonds like 1 [2] or 3 [3] afford, when brought to reaction with lithium metal in THE, the products of reductive dimerization, i. e., the 1,4-dilithiobutanes 2 and 4. This type of reaction is known as Schlenk dimerization [4]. Symmetrically tetrasilyl-substituted C=C-double bonds as in 5 on the other hand add lithium metal with formation of 1,2-dilithioethanes (Scheme 1) as stable intermediates in these reactions radical anions, like 7, can be observed, which are then reduced once again, here to the dianion 6 [5]. These two types of reaction are analogous to the reductions of the corresponding styrene derivatives... 

Furthermore, we have reported on the first examples of free radical homopolymerization and copolymerization of 2,3,4,5,6-pentafluorostyrene (3) with styrene (5) and its derivative 4-((V-adamantylamino)-2,3,5,6-tetrafluorostyrene (4) in aqueous solution via the host-guest complexation with Me-p-CD using water-soluble initiators (Fig. 5) [31]. The fluorinated monomers (3 and 4) and styrene (5) were complexed by Me-p-CD in water. The stoichiometries of the host-guest complexes were determined by NMR spectroscopy according to the Job method [32-34], It was clearly shown that styrene (5) forms a defined 1 1 complex while the fluorinated monomers (3 and 4) are encapsulated by two Me-p-CD molecules. In the case of fluorinated monomer (3) this result was not expected since 3 and 5 have the same molecular scaffold and molecular modelling showed that the fluorinated styrene derivative 3 is only slightly bigger than styrene itself (Fig. 6) [35],... 

An optically active polystyrene derivative, 40 ([a]25365 -224° to -283°), was prepared by anionic and radical catalyses.113 The one synthesized through the anionic polymerization of the corresponding styrene derivative using BuLi in toluene seemed to have a high stereoregularity and showed an intense CD spectrum whose pattern was different from those of the monomer and a model compound of monomeric unit 41. In contrast, polymer 42 and a model compound, 43,... 

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