Mechanism of Phenol Polymer Formation

As previously described, the function of the enzyme is to produce the phe-noxy radicals under mild reaction conditions. The resulting phenoxy radicals are able to form polymers via recombination processes if a suitable reaction medium (solvent mixture composition), pH value (buffer systems), and kind of substrate are chosen. Some phenols, for example, are known to react preferentially via oxidation to form orfho-diketones [68] or Pummerer ketones [52], and are therefore not suitable for polymerization. Other phenols are known to prefer dimerization reactions, for instance caffeic acid [71]. 

The oxidative polymerization of phenols is basically a polycondensation reaction. A model for the polymerization was developed, which divides the 

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Very recently, attempts have been made to develop PP/EOC TP Vs. In order to make TPVs based on PP/EOC blend systems, phenolic resin is ineffective because the latter needs the presence of a double bond to form a crosslinked network structure. Peroxides can crosslink both saturated and unsaturated polymers without any reversion characteristics. The formation of strong C-C bonds provides substantial heat resistance and good compression set properties without any discoloration. However, the activity of peroxide depends on the type of polymer and the presence of other ingredients in the system. It has been well established that PP exhibits a (3-chain scission reaction (degradation) with the addition of peroxide. Hence, the use of peroxide only is limited to the preparation of PP-based TPVs. Lai et al. [45] and Li et al. [46] studied the fracture and failure mechanism of a PP-metallocene based EOC based TPV prepared by a peroxide crosslinking system. Rajesh et al. 

The importance of crosslinked polymers, since the discovery of cured phenolic formaldehyde resins and vulcanized rubber, has significantly grown. Simultaneously, the understanding of the mechanism of network formation, the chemical structure of crosslinked systems and the motional properties at the molecular level, which are responsible for the macroscopic physical and mechanical properties, did not accompany the rapid growth of their commercial production. The insolubility of polymer networks made impossible the structural analysis by NMR techniques, although some studies had been made on the swollen crosslinked polymers. 

Based on the evidence obtained from the amount and nature of transformation products formed, a mechanism of melt stabilising action of tocopherol in PP and PE has been proposed, see Scheme 6 [34]. It is well known that, like other hindered phenols, a-tocopherol is rapidly oxidised by alkylperoxyl radicals to the corresponding tocopheroxyl radical (a-Toe, Scheme 6a). Further oxidation of the tocopheroxyl radical in the polymers leads to the formation of coupled and quinonoid-type products, e.g. SPD, TRI, DHD (see Figs. 8 and 9). Dimerisation of the intermediate o-quinone methide (QM) leads to the formation of the quinonoid-type dimeric coupled product, SPD (Scheme 6 reaction d). 

Write a mechanism for a step-growth polymerization, as in the formation of a polyester, polyamide, polyurethane, epoxy resin of phenol-formaldehyde polymer. 

Flame Retardance. The most important reason for phenolic foam being an excellent flame retarder is that the phenolic polymer is easily carbonized and the char part formed as a result is highly stabilized. This mechanism of char-formation is considered that of a multi-aromatic ring with chemically stabilized strong bond formed through a dehydrogenation reaction by heating and oxidation. 

Evans et al. (10) have shown that OH radicals produced by the Fenton reagent can initiate the polymerization of vinyl compounds and that the OH radicals, which in the first instance attack the double bond of the monomer, are built into the polymer chain. Stein and Weiss (45) have used the hydroxylation of benzene and of other simple aromatic compounds (e.g., benzoic acid and nitrobenzene) to detect these radicals. In the action of OH radicals on benzene in aqueous systems the formation of phenol and of diphenyl indicated a free radical mechanism of the following type ... 

AIBN-initiated oxidation of cumene and Tetralin in the presence of deuterated amines were unsuccessful. They proposed an alternative mechanism involving reversible formation of a complex of antioxidant with peroxy radical as the kinetically controlling process. We observed an isotope effect, dAh = 1.8, consistent with the hydrogen-donation mechanism in the retarded oxidation of SBR polymer with deuterated amines (7,8). Our results were confirmed by observation of significant isotope effects in the initial stage of oxidation of purified cfc-l,4-polyiso-prene with both hindered phenols and amines (9). Table I shows the effect of temperature and antioxidant concentration on the rates of oxidation and the observed deuterium isotope effects. 

Radicals RO play an important role in the oxidation mechanism of hydrocarbons and carbon-chain polymers. It was proved that phenoxyls are formed from phenols by their action44 49 85,143 144) followed by formation of phenolic and quinone methionide compounds in coupling reaction and by disproportionation. Also the reaction between RO and In and formation of alkoxycyclohexadienones of the type CIX may be presumed. Such compounds have not been isolated from the reaction mixtures yet. However, their formation was proved by spectra145, 179) and they probably appear only temporarily during the inhibited oxidation due to their low stability. According to1441, the following equilibrium reaction takes place ... 

Poly(p-phenylene) also fascinated many workers due to its cheap cost, good mechanical strength and high stability for use in electrochemical systems. In anodic oxidation and over-oxidation studies in aqueous acidic solutions of different nucleophilicity by cyclic voltammetry, two peaks were observed. The reversible first peak was attributed to poly(p-phenylene) oxidation and formation of some insertion compound, whereas the strong and irreversible second peak at higher potentials was attributed to over-oxidation of the polymer [291] and the polymer anode started degrading at acid concentrations below 13 M [292], Accordingly, the microanalytical, IR spectroscopic and acid-base titration studies showed evidence of the formation of aromatic dicarboxylic acid, phenolic compound and carbondioxide via a p-benzoquinone intermediate [291],... 

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