Poly phenylene oxide reactions

The backbone of poly(phenylene oxide)s is cleaved under certain extreme reaction conditions. Lithium biphenyl reduces DMPPO to low molecular weight products in the dimer and trimer molecular weight range (20) and converts poly(2,6-diphenyl-l,4-phenylene oxide) to 3,5-diphenylphenol in 85% yield (21) (eq. 4). 

Poly(phenylene oxide)s undergo many substitution reactions (25). Reactions involving the aromatic rings and the methyl groups of DMPPO include bromination (26), displacement of the resultant bromine with phosphoms or amines (27), lithiation (28), and maleic anhydride grafting (29). Additional reactions at the open 3-position on the ring include nitration, alkylation (30), and amidation with isocyanates (31). 

Halogen Displacement. Poly(phenylene oxide)s can also be prepared from 4-halo-2,6-disubstituted phenols by displacement of the halogen to form the ether linkage (48). A trace of an oxidizing agent or free radical initiates the displacement reaction. With 4-bromo-2,6-dimethylphenol, the reaction can be represented as in equation 10 ... 

Halophenols without 2,6-disubstitution do not polymerize under oxidative displacement conditions. Oxidative side reactions at the ortho position may consume the initiator or intermpt the propagation step of the chain process. To prepare poly(phenylene oxide)s from unsubstituted 4-halophenols, it is necessary to employ the more drastic conditions of the Ullmaim ether synthesis. A cuprous chloride—pyridine complex in 1,4-dimethoxybenzene at 200°C converts the sodium salt of 4-bromophenol to poly(phenylene oxide) (1) ... 

Verdet and Stille1 employed brominated poly(phenylene oxide) intermediates in an effort to synthesize more stable catalyst supports containing (cyclopentadienyl)metal complexes. Treatment of poly(oxy-2,6-dimethyl-l,4-phenylene) with N-bromosuccinimide under photolytic conditions produced only the bromomethyl derivative if the D.F. did not exceed 0.35. Subsequent treatment of the bromomethylated polymer with sodium cyclopentadienide afforded the cyclopentadienyl functionalized polymer, 5, but the reaction was accompanied by crosslinking and it was not possible to remove the bromomethyl substituents quantitatively. 

Copper-catalyzed oxidations of phenols by dioxygen have attracted considerable interest owing to their relevance to enzymic tyrosinases (which transform phenols into o-quinones equation 24) and laccases (which dimerize or polymerize diphenols),67 and owing to their importance for the synthesis of specialty polymers [poly(phenylene oxides)]599 and fine chemicals (p-benzoquinones, muconic acid). A wide variety of oxidative transformations of phenols can be accomplished in the presence of copper complexes, depending on the reaction conditions, the phenol substituents and the copper catalyst.56... 

Catalysts are also required in many stepwise polymerizations. For example, reaction of polycarboxylic acids and polyols (Reaction 7) is catalyzed by acids ester interchange, by metal compounds such as titanium alkoxides. On the other hand, polyurea synthesis (Reaction 6) generally does not require a catalyst. Metallic compounds are also useful in oxidative polymerization of phenols to give poly(phenylene oxides), illustrated in Reaction 14. 

Phenols are oxidized by NaBiO3 to polyphenylene oxides, quinones, or cyclohexa-2,4-dienone derivatives, depending on the substituents and the reaction conditions [263]. For example, 2,6-xylenol is oxidized in AcOH to afford a mixture of cyclohexa-dienone and diphenoquinone derivatives (Scheme 14.123) [264] and is oxidatively polymerized in benzene under reflux to give poly(2,6-dimethyl-l,4-phenylene) ether (Scheme 14.124) [265]. Substituted anilines and a poly(phenylene oxide) are oxidatively depolymerized by NaBiO, to afford the corresponding anils [266]. Nal iO, oxidizes olefins to vicinal hydroxy acetates or diacetates in low to moderate yield [267]. Polycyclic aromatic hydrocarbons bearing a benzylic methylene group are converted to aromatic ketones in AcOH under reflux (Scheme 14.125) [268]. 

There remain some problems to be solved for this electrolytic oxidation of phenols. (i) Considerable magnitude of overvoltage and low current efficiency. (ii) Poly(phenylene oxide) formed deposits on the electrode surface as a thin, insulating film passivating of the electrode. (iii) The side reaction which forms b iphenoquinone. 

Poly(phenylene oxide) (19) is obtained from free-radical, step-growth, oxidative coupling of 2,6-dimethylphenol (2,6-xylenol). This involves passing oxygen into a reaction mixture containing 2,6-xylenol, cuprous chloride, and pyridine. 

Poly(phenylene sulfide) (PPS) is the thio analogue of poly(phenylene oxide) (PPO) [57]. The first commercial grades were introduced by Phillips Petroleum in 1968 under the trade name Ryton. Other manufacturers also have introduced PPS (e.g., Tedur by Bayer). The commercial process involves the reaction of p-dichlorobenzene with sodium sulfide in a polar solvent. 

Around 1956, the oxidative coupling of 2,6 substituted phenols to yield high molecular products was discovered. Poly(phenylene ether) (PPE), also addressed as poly(phenylene oxide), was commercialized in 1964 by General Electric and AKZO," and eventually by several other companies. Remarkably, the oxidative coupling of phenols plays a role in certain biological reactions, e.g., in the formation of lignin or melamine. ... 

Discuss and illustrate with chemical equations the formation of poly(phenylene oxide) by the oxidative coupling reaction of 2,6-disubstituted phenols. 

Poly(phenylene oxides) are produced by the oxidative coupling of 2,6-disubstituted phenols. The polymers are also known as poly(oxyphenylenes) or poly(phenyl ethers), and, in the case of dimethyl compounds, also as poly(xylenols). Copper (I) salts in the form of their complexes with amines catalyze the reaction. Primary and secondary aliphatic amines must be used at low temperatures, since otherwise they are oxidized. Primary aromatic amines are oxidized to azo compounds, and secondary aromatic compounds probably to hydrazo compounds. Pyridine is very suitable. 

Poly(phenylene oxide)s can also be formed by oxidative displacement of bromides from 4-bromo-2,6-dimethylphenol [102, 105]. Compounds, like potassium ferricyanide, lead oxide, or silver oxide, catalyze this reaction ... 

YJ. Kim, I.S. Chung, S.Y. Kim, Synthesis of poly(phenylene oxide) containing trifluoromethyl groups via selective and sequential nucleophilic aromatic substitution reaction. Macromolecules 36 (11) (2003) 3809-3811. 

Other polyethers such as poly(phenylene oxide) are prepared by oxidation reactions using a transition metal complexes containing metal ions such as Cu" (see Eq. 2.64). 

Oxidative coupling polymerizations represent a general reaction for the preparation of high molecular weight linear polymers from many 2,6-di- and 2,3,6-trisubstituted phenols. When the ortho substituents on the phenols are relatively unhindered alkyl or aryl groups, the poly(phenylene oxide) is the chief product (40,41). Bulky ortho substituents, eg, fe/t-butyl, lead to the formation of... 

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