Polymerization mechanism, 2,6-dimethylphenol

Polymerization Mechanism. The mechanism that accounts for the experimental observations of oxidative coupling of 2,6-disubstituted phenols involves an initial formation of aryloxy radicals from oxidation of the phenol with the oxidized form of the copper—amine complex or other catalytic agent. The aryloxy radicals couple to form cyclohexadienones, which undergo enolization and redistribution steps (32). The initial steps of the polymerization scheme for 2,6-dimethylphenol are as in equation 6. 

The study of the molecular weight of the intermediate course is an effective method for the classification of polymerization as chain or stepwise reaction. In Figure 3, the molecular weight of the obtained polymer is plotted against the yield, for the oxidative polymerization of dimethylphenol with the copper catalyst and for the electro-oxidative polymerization. The molecular weight rises sharply in the last stage of the reaction for the copper-catalyzed polymerization. This behavior is explained by a stepwise growth mechanism. 

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. 

Poly(phenylene oxide) (PPO) is a thermoplastic, linear, noncrystalline polyether commercially produced by the oxidative polymerization of 2,6-dimethylphenol in the presence of a copper-amine catalyst. PPO has become one of the most important engineering plastics widely used for a broad range of applications due to its unique combination of mechanical properties, low moisture absorption, excellent electrical insulation property, dimension stability and inherent flame resistance. This chapter describes the recent development of this polymer, particularly on the production, application, compounding, properties of its alloys and their general process conditions. The polymerization mechanism and thermal degradation pathways are reviewed and new potential applications driven by the increasing environmental concerns in battery industry, gas permeability and proton-conducting membranes are discussed. 

The oxidative polymerization of 2,6-dimethylphenol in the presence of a,o)-bis(hydroxyphenol) tetramethyl bisphenol-A polysulfone (PSUT) leads to an ABA triblock copolymer containing PPO segments as A blocks and PSUT as B blocks. The oxidative polymerization of 2,6-dimethyl-phenol takes place through a single electron transfer polymerization mechanism which differs from both step and chain polymerization reactions since the reactivity of the growing chains is molecular weight dependent. Heitz and co-workers labelled this type of polymerization process reactive intermediate polycondensation . ... 

Poly(2,6-dimethyl-l,4-oxyphenylene) (poly(phenylene oxide), PPG) is a material widely used as high-performance engineering plastics, thanks to its excellent chemical and physical properties, e.g., a high 7 (ca. 210°C) and mechanically tough property. PPO was first prepared from 2,6-dimethylphenol monomer using a copper/amine catalyst system. 2,6-Dimethylphenol was also polymerized via HRP catalysis to give a polymer exclusively consisting of 1,4-oxyphenylene unit, while small amounts of Mannich-base and 3,5,3, 5 -tetramethyl-4,4 -diphenoquinone units are always contained in the chemically prepared PPO. 

The following questions on the electro-oxidative polymerization arose. First, why various phenol derivatives were smoothly polymerized which could not occur by the oxidation with the copper catalyst or lead dioxide. Secondly, why the activated phenol was reacted preferentially through C-0 coupling to form the poly(phenyleneoxide). The mechanism of the electro-oxidative polymerization is discussed below by using the example of 2,6-dimethylphenol. 

The mechanism of the oxidative polymerization of 2,6-dimethylphenol (XOH) with an amine-Cu complex is represented by Eq. (20)145 147. ... 

Although redistribution and coupling can be observed separately, oxidative polymerization under ordinary conditions involves both reactions and redistribution of oligomers to form monomer followed by removal of the monomer by coupling is an important mechanism of polymer growth. Redistribution in dimethylphenol polymerizations is extremely rapid. Addition of monomer to a polymerizing solution causes an immediate drop in the solution viscosity almost to the level of the solvent, as redistribution of polymer with monomer converts the polymer already formed to a mixture of low oligomers. 

Dimethylphenol is oxidatively polymerized to poly(2,6-dimethyl-1,4-phenyl-ene ether) with a copper-amine complex by a laccaselike reaction. The activated phenols are coupled to form a dimer. The dimer is activated by a mechanism similar to that by which the polymerization proceeds. The effects of the amine ligands are to improve the solubility and the stability of the copper complex as well as the phenol-coordinated complex and to control the redox potential of the copper complex. 

The telechelica,(i -bis(2,6-dimethylphenol)-poly(2,6-dimethylphenyl-ene oxide) (PP0-20H) [174-182] is of interest as a precursor in the synthesis of block copolymers [175] and thermally reactive oligomers [179]. The synthesis has been accomplished by five methods. The first synthetic method was the reaction of a low molecular weight PPO with one phenol chain end with 3,3, 5,5 -tetramethyl-l,4-diphenoquinone. This reaction occurred by a radical mechanism [174]. The second method was the electrophilic condensation of the phenyl chain ends of two PPO-OH molecules with formaldehyde [177,178], The third method consists of the oxidative copolymerization of 2,6-dimethylphenol with 2,2 -di(4-hydroxy-3,5-di-methylphenyl)propane [176-178]. This reaction proceeds by a radical mechanism. A fourth method was the phase transfer-catalyzed polymerization of 4-bromo-2,6-dimethylphenol in the presence of 2,2-di(4-hy-droxy-3,5-dimethylphenyl)propane [181]. This reaction proceeded by a radical-anion mechanism. The fifth method developed was the oxidative coupling polymerization of 2,6-dimethylphenol (DMP) in the presence of tetramethyl bisphenol-A (TMBPA) [Eq. (57)] [182],... 

The oxidative dehydrogenation polymerization of 2,6-dialkylphenoles has been known for many years to be catalyzed by copper amine complexes [1] see figure for 2,6-dimethylphenol oxidation. The mechanism of action of this industrially very important reaction has been studied for some time by several groups. 

Definition Amorphous thermoplastic produced through oxidative coupling polymerization of 2,6-dimethylphenol good mechanical stability high heat distort., exc. impact str., flame resist., and... 

Another proposed mechanism of the polymerization is a two-electron transfer mechanism, which involvs phenolate-bridged dinuclear copper(II) complex as starting species. The complex generated phenoxonium cations and phenolate anion through a double one-electron transfer from a phenolate to both copper centres (step v) and form the quinone-ketal intermediate via nucleophilic attack (step vi). This reaction pathway is supported by theoretical calculations of atomic charges of monomeric and dimeric species of 2,6-DMP where phenoxonium cations are proposed as key intermediates. Ab Initio calculations on 2,6-DMP and 4-(2,6-Dimethylphenoxy)-2,6-dimethylphenol provided evidence of the phenoxonium cation in the copper-catalyzed oxidative coupling reaction which proposed that the selective C-O coupling was achieved via the nucleophilic attack of a phenolate on the para-carbon of a phenoxonium cation (25). Based on the experimental evidence currently reported, both... 

Heat resistant poly(oxyphenylene) coatings have also been pr ued by the electrochemical initiation of the polymerization of 2,6-dimethylphenol in the presence of Cu(OAc)2 in admixture with traces of Cua l While the inediation of cupric-cuprous couple in the oxidaticm rracticMi can be surmised, the details of the mechanisms of the polymerization are not e blished. 

The oxidative polymerization of 2,6-dimethylphenol (invented by Hay et al. [91], see also Chap. 8) and the polymerization of potassium 4-bromo-2,6-dim-ethylphenoxide (see Formula 16.6) have both the character of CCPs. The former CCP needs oxygen as reaction partner and a Cu amine complex as catalyst. Heitz et al. [92] observed that contrary to a normal polycondensation high oligomers were formed at low conversions, and he formulated a speculative radical-cation mechanism. The CCP of 4-bromophenoxide salts needs Cu " or other oxidizing metal ions as reaction partners and also involves a radical-cation mechanism [93]. When 4-methyl- or 4-tert.butylphenol are added as initiators, linear chains having one OH end group are formed, whereas addition of tetramethyl bisphenol yields telechelic polyethers (see Formula 15.6). 

Organic compounds having labile hydrogen, such as phenols, phenylenedi-amine, disulfides, and acetylene, are oxidatively coupled by metal complex to give polymeric materials as shown in Eqs. (73) and (74). These reactions are called oxidative polymerizations. Tsuchida et al. studied the oxidative polymerization of 2,6-dimethylphenol (XOH) as a redox reaction catalyzed by Cu(II)-polyvinylpyridine complexes, and have proposed a detailed mechanism [93-95]. 

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