Polymerization by electrophilic aromatic substitution

Phenol-formaldehyde prepolymers, referred to as novolacs, are obtained by using a ratio of formaldehyde to phenol of 0.75-0.85 1, sometimes lower. Since the reaction system is starved for formaldehyde, only low molecular weight polymers can be formed and there is a much narrower range of products compared to the resoles. The reaction is accomplished by heating for 2 1 h at or near reflux temperature in the presence of an acid catalyst. Oxalic and sulfuric acids are used in amounts of 1-2 and <1 part, respectively, per 100 parts phenol. The polymerization involves electrophilic aromatic substitution, first by hydroxymethyl carboca-tion and subsequently by benzyl carbocation—each formed by protonation of OH followed by loss of water. There is much less benzyl ether bridging between benzene rings compared to the resole prepolymers. 

Since benzenesulfonyl peroxide was used as an initiator in polymerization reactions, it was thought that a free radical aromatic substitution of benzene by the benzenesulfonoxy radical takes place. A detailed study by Dannley and Knipple reveals that attachment of the sulfonoxy group derived from a bis(arylsulfonyl) peroxide to the aromatic ring occurs by electrophilic aromatic substitution (equation 5) °. 

Several chain transfer to polymer reactions are possible in cationic polymerization. Transfer to cationic propagating center can occur either by electrophilic aromatic substitution (as in the polymerization of styrene as well as other aromatic monomers) or hydride transfer. Short chain branching found in the polymerizations of 1-alkenes such as propylene may be attributed to intermolecular hydride transfer to polynier. The propagating carbocations are reactive secondary carbocations that can abstract tertiary hydrogens from the polymer ... 

The reaction of 151 with methanol to give dimethyl phosphate (154) or with N-methylaniline to form the phosphoramidate 155 and (presumably) the pyrophosphate 156 complies with expectations. The formation of dimethyl phosphate does not constitute, however, reliable evidence for the formation of intermediate 151 since methanol can also react with polymeric metaphosphates to give dimethyl phosphate. On the other hand, reaction of polyphosphates with N-methylaniline to give 156 can be ruled out (control experiments). The formation of 156 might encourage speculations whether the reaction with N,N-diethylaniline might involve initial preferential reaction of monomeric methyl metaphosphate via interaction with the nitrogen lone pair to form a phosphoric ester amide which is cleaved to phosphates or pyrophosphates on subsequent work-up (water, methanol). Such a reaction route would at least explain the low extent of electrophilic aromatic substitution by methyl metaphosphate. 

Similarly bis (3,5-dimethyl phenyl) disulfides yielded the corresponding polymer 209b in the absence of acid. In both cases it is suggested that ArSS+(Ar)SAr, formed in the reaction, effects electrophilic aromatic substitution by acting as an ArS+ donor. This same polymerization process can also be achieved by photooxidation of bis (3,5-dimethylphenyl) disulfide in the presence of 2,3-dicyanonaphthalene as sensitizer in acetonitrile containing trifluoro-acetic acid [486]. 

An aqueous Friedel-Crafts reaction has also been used in polymer synthesis. The acid-catalyzed polymerization of benzylic alcohol and fluoride functionality in monomeric and polymeric fluorenes was investigated in both organic and aqueous reaction media. Polymeric products are consistent with the generation of benzylic cations that participate in electrophilic aromatic substitution reactions. Similar reactions occurred in a water-insoluble Kraft pine lignin by treatment with aqueous acid. A Bisphenol A-type epoxy resin is readily emulsified in aqueous medium with an ethylene oxide adduct to a Friedel-Crafts reaction product of styrene and 4-(4-cumyl)phenol as emulsifier.Electrophilic substitution reaction of indoles with various aldehydes and ketones proceeded smoothly in water using the hexamethylenetetramine-bromine complex to afford the corresponding Z A(indolyl)methanes in excellent yields.InFs-catalyzed electrophilic substitution reactions of indoles with aldehydes and ketones are carried out in water.Enzymatic Friedel-Crafts-type electrophilic substitution reactions have been reported. ... 

Early work performed by Lenz et al. in the 1960s demonstrated this approach by using electrophilic aromatic substitution to produce poly(phenylene sulfide) [91] (Scheme 1.10). In this case, an aryl halide is the electrophile, which is substituted by the metal thiophenoxide nucleophile. In the monomer, the metal sulfide is a strong electron-donating group, which deactivates the para position where electrophilic substitution must take place. Conversely, the polymer chain end is only weakly deactivated by the sulfide bond, rendering the polymeric aryl halide more reactive than the monomeric aryl halide. Unfortunately, Lenz was unable to characterize molecular weight distribution due to the insolubility of the resultant polymers. 

For example, direct fluorinations with elemental fluorine are kept imder control in this way, at very low conversion and by entrapping the molecules in a molecular-sieve reactor. As with some other aromatic substitutions they can proceed by either radical or electrophilic paths, if not even more mechanisms. The products are dif ferent then this may involve position isomerism, arising from different substitution patterns, when the aromatic core already has a primary substituent further, there may be changed selectivity for imdefined addition and polymeric side products (Figure 1.31). It is justified to term this and other similar reactions new , as the reaction follows new elemental paths and creates new products or at least new... 

As an example, this apply to enols or tautomeric enols such as maleic acid derivatives. While with a chemical reagent (cerium ammonium nitrate) the only process occurring is oxidative dimerization, when aromatic nitriles are used as the photochemical oxidant, selective trapping of the radicals by an electrophilic alkenes or by the nitrile itself occurs. Under these conditions, both the alkylation of alkenes and the oxidative alkylation/dimerization of dienes have been smoothly obtained (see Scheme 8) and side processes such as double alkylation or polymerization often occurring with other methods have been avoided. A three-component (Nucleophile-Olefin Combination, Aromatic Substitution) process is also possible. ... 

Certain polymers such as poly(arylene-ether)s are produced by nucle-ophihc aromatic substitution, which involves the addition of a nucleophile during polymerization. Nucleophiles typically have negative ions (anions) that are attracted to, and attach to, a positive charge. A nucleophile (nucleus-friendly) is an electron-rich ion or molecule that donates electrons to, and reacts with, an electron-poor specie. The positive nuclear charge of an electron-poor specie is an electrophile (electron-friendly) [16, 17]. Electrons always go from a nucleophile to an electrophile. The reaction forms a new covalent bond [16, 17]. 

Thiols react more rapidly with nucleophilic radicals than with electrophilic radicals. They have very large Ctr with S and VAc, but near ideal transfer constants (C - 1.0) with acrylic monomers (Table 6.2). Aromatic thiols have higher C,r than aliphatic thiols but also give more retardation. This is a consequence of the poor reinitiation efficiency shown by the phenylthiyl radical. The substitution pattern of the alkanethiol appears to have only a small (<2-fokl) effect on the transfer constant. Studies on the reactions of small alkyl radicals with thiols indicate that the rate of the transfer reaction is accelerated in polar solvents and, in particular, water.5 Similar trends arc observed for transfer to 1 in S polymerization with Clr = 1.4 in benzene 3.6 in CUT and 6.1 in 5% aqueous CifiCN.1 In copolymerizations, the thiyl radicals react preferentially with electron-rich monomers (Section 3.4.3.2). 

An additional important feature of this class of polymers lies in the fact that their polymerisation and doping processes may be driven by a single electrochemical operation which, starting from the monomer, first forms the polymeric chain and then induces its oxidation and deposition in the doped form as a conductive film on a suitable substrate. The polymerisation reaction may be basically described as an electrophilic substitution which retains the aromatic structure and proceeds via a radical cation intermediate ... 

Polysulfonylation. The polysulfonylation route to aromatic sulfone polymers was developed independently by Minnesota Mining and Manufacturing (3M) and by Imperial Chemical Industries (ICI) at about the same time (81). In the polymerization step, sulfone links are formed by reaction of an aromatic sulfonyl chloride with a second aromatic ring. The reaction is similar to the Friedel-Crafts acylation reaction. The key to development of sulfonylation as a polymerization process was the discovery that, unlike the acylation reaction which requires equimolar amounts of aluminum chloride or other strong Lewis acids, sulfonylation can be accomplished with only catalytic amounts of certain halides, eg, FeQ3, SbQ, and InCl3. The reaction is a typical electrophilic substitution by an arylsulfonium cation (eq. 13). 

The chemical reactivity of simple heterocyclic aromatic compounds varies widely in electrophilic substitution reactions, thiophene is similar to benzene and pyridine is less reactive than benzene, while furan and pyrrole are susceptible to polymerization reactions conversely, pyridine is more readily susceptible than benzene to attack by nucleophilic reagents. These differences are to a considerable extent reflected in the susceptibility of these compounds and their benzo analogues to microbial degradation. In contrast to the almost universal dioxygenation reaction used for the bacterial degradation of aromatic hydrocarbons, two broad mechanisms operate for heterocyclic aromatic compounds ... 

The formation of the C = O radical can be explained by considering the fact that pyrrole is a heterocyclic compound, considered as aromatic because of the delocalisation of the jr-electrons wich stabilize the ring. These delocalized 7r-electrons are very reactive and they can promote aromatic electrophilic substitution, wich produces to nitration reactions. Usually aromatic nitriles are obtained by means of nitrogen salts (N24), wich comes from aromatic amines, pyrrole in our case. The final step of the overall reaction is the production of the radical carbonyl. However, the polymerization of conductive PPy it must be avoided. Plays de role of TFB as electrolyte acts as activator of the reaction. 

---