Oxidative coupling, phenolic monomers

This reaction has been actively studied since it was first reported by Hay in 1959 (I), but most of the extensive literature, which includes several recent reviews (2-8), deals primarily with the complex polymerization mechanism. Few copolymers have been prepared by oxidative coupling of phenols, and only one copolymer system has been examined in any detail. Copolymers of 2,6-dimethylphenol (DMP) and 2,6-diphenylphenol (DPP) have been prepared and the effect of variations in polymerization procedure on the structure and properties of the copolymers examined (4, 9) this work has now been extended to copolymers of each of these monomers with a third phenol, 2-methyl-6-phenylphenol (MPP). This paper presents a study of the DMP-MPP and MPP-DPP copolymers and a comparison with the DMP-DPP system previously reported. 

Sequential Oxidation of DMP and DPP. The usual approach to formation of block copolymers is by the sequential polymerization of two or more monomers or by linking together preformed homopolymer blocks. In view of the importance of the redistribution process in the oxidative coupling of phenols there can be no assurance that successive polymerization of two phenols will yield block copolymers under any conditions. It is certain, however, that block copolymers can be formed only if the conditions are such that polymerization of the second monomer is much faster than redistribution of the added monomer with the polymer previously formed from the first. The extent of redistribution is followed conveniently by noting the effect of added monomer on solution viscosity, as indicated by the efflux time from a calibrated pipet. 

Organic compounds having labile hydrogen atoms, such as phenols, anilines, and acetylenes, are also oxidatively polymerized by metal-complex catalysts (Eqs. 1-3). The oxidative coupling is a dehydrogenation reaction the polymer chain produced contains the dehydrogenated monomer structure as a repeating unit. As a remarkable example, poly(phenylene ether), one of the... 

The polymerization of compounds having active methyne groups has also been reported [81] (Eq. 8). The oxidative coupling polymerization of these monomers follows a mechanism similar to that of phenols. The catalytic cycle observed in the polymerization of / -phcnylcncdiaminc with Fe(edta) as the catalyst in an aqueous solution differs from that in the polymerization of phenols as follows The activation of monomers usually involves either electron transfer from the anion or elimination of a hydrogen atom from the monomer. The oxidative polymerization of phenols uses the former mechanism of the electron transfer. In contrast, in the case of the polymerization of aromatic diamines as monomers, the neutral amines are coordinated to the catalyst, followed by the subsequent electron transfer and dehydronation. The dehydronation proceeds by the reaction with 02. Another mechanism has also been proposed where dehydrogenation... 

Smejkalova D, Piccolo A. Rates of oxidative coupling of humic phenolic monomers catalyzed by a biomimetic iron-porphyrin. Environ Sci Technol 2006 40 1644-9. 

Fukuoka T, Uyama H, Kobayashi S (2004) Effect of phenolic monomer structure of precursor polymers in oxidative coupling of enzymatically synthesized polyphenols. Macromolecules 37 5911-5915... 

The oxidative coupling of 2,6-disubstituted phenols to poly-(arylene oxides) is a polycondensation reaction, in which polymer molecules couple with other polymer molecules as well as with monomer. Unstable quinone ketals formed by coupling of a polymeric aryloxy radical at the para position of the phenolic ring of a second radical are believed to be intermediates or the reaction. The ketals may be converted to polymeric phenols either by a series of intramolecular rearrangements or by disproportionation to aryloxy radicals, leading to a mobile equilibrium between polymer molecules of varying degree of polymerization. Both processes have been shown to occur, with their relative importance determined by the reaction conditions. 

The simplest explanation for the formation of high molecular weight polymer through oxidative coupling of aryloxy radicals involves the successive addition of monomer units to the radicals derived from polymer phenols (Reaction 4). 

Smejkalova D., Piccolo A., Spiteller M., Oligomerization of humic phenolic monomers by oxidative coupling under biomimetic catalysis. Environmental Science and Technology 2006 40(22) 6955-6962. 

In addition to the industrial apphcations, in Scheme 8.1, other reactions have been the focus of extensive research and development. For example. Chapter 12 surveys the research efforts directed toward Pd-catalyzed oxidative carbonylation of phenol affords the important monomer, diphenyl carbonate (Scheme 8.2a). Other reactions of potential industrial significance highlighted in this chapter include the oxidation of alcohols to aldehydes and ketones (Scheme 8.2b), oxidative coupling of arenes and carboxylic acids to afford aryl esters (Scheme 8.2c), benzylic acetoxylation (Scheme 8.2d), and oxidative Heck reactions (Scheme 8.2e). The chapter concludes by highlighting a number of newer research developments, including ligand-modulated catalytic oxidations, Pd/NO cocatalysis, and alkane oxidation. 

Polyethers are obtained from three different classes of monomers, namely, carbonyl compounds, cyclic ethers, and phenols. They are manufactured by a variety of polymerization processes, such as polymerization (polyacetal), ring-opening polymerization (polyethylene oxide, polyprophylene oxide, and epoxy resins), oxidative coupling (Polyphenylene oxide), and polycondensation (polysulfone). 

Preferred monomers for PPE are shown in Table 4.1 and in Figure 4.1. Alkylphenols are oxidized by air. They must be stored under nitrogen to prevent oxidation reactions. The oxidative coupling reaction is a general reaction, suitable for 2,6 disubstituted phenols. However, with bulky sub-... 

A generalized oxidative coupling reaction of phenolic monomers is shown in Scheme 1. Two kinds of couplings are possible, (a) C-C coupling to form phenylene units, and (b) C-O-C coupling to form oxyphenylene units. 

Various synthetic approaches have been demonstrated for the synthesis of PAEs since early days [35 0], PAEs were synthesized by Ullmann condensation between bisphenols and aryl fcis-halide monomers using Cu(I) salt/pyridine as catalyst [36], General Electric developed the first commercially successful PAE poly(2,6-dimethyl phenylene oxide) (PPO) [38], It was prepared by oxidative coupling of 2,6-dimethyI phenol. However, this process has its own restrictions, because it does not allow much structural variation or inclusion of any electron-withdrawing group into the polymer main chain. First attempts to synthesize polysulfones (PSF) were successfully done by Friedel-Crafts sulfonylation reaction of arylenedisulfonyl chlorides, for example, diphenyl ether-4,4 -disulfonyl chloride with diaryl ethers, for example, diphenyl ether, or by self-condensation of 4-phenoxy benzene sulfonyl chloride in the presence of FeCls [41], Whereas the former reaction involves side reactions (sulfonylation not only in the para- but also in the ort/io-position), the latter produces only the desired linear all-para products. 

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