Kinetics and selectivity of the copper-catalysed oxidative coupling of 4-(2′,6′-dimethylphenoxy)-2,6-dimethylphenol

The kinetics of the copper/N-methylimidazole catalysed oxidative coupling reaction with the C–O coupled dimer of 2,6-dimethylphenol (DMP or monomer), viz. 4-(2′,6′-dimethylphenoxy)-2,6-dimethylphenol (dimer), as the substrate have been studied. The reaction was found to obey Michaelis–Menten kinetics. The dimer is more easily oxidised than the monomer, but the formation of a copper–substrate complex is more difficult. The reaction rates are higher than in the case of the monomer, and the amounts of diphenoquinone (DPQ) formed are much lower. With the dimer as the substrate, the order of the reaction in copper is 2, confirming that the formation of a dinuclear copper complex is an important step in the reaction mechanism. The amount of DPQ formed is proportional to the initial amount of the dimer. A slight, but clear preference for the dimer over the monomer as the substrate has been observed from experiments with mixtures of monomer and dimer. The amount of DPQ formed decreases exponentially with an increase in the fraction of dimer in the mixture, which can be ascribed mainly to a statistical effect.     

Catalyzed oxidation of 2,6-dimethylphenol in Triton X- 100 micellar solution

The oxidative coupling reaction of 2,6-dimethylphenol with H2O2 catalyzed by a copper(Ⅱ) Schiff complex in aqueous and Triton X-100 micellar solution under mild conditions was investigated. The kinetics of formation of 3,3′,5,5′-tetramethyl-4,4′-diphenoquinone (DPQ) was studied. Rate constant k2 were obtained. The optimum pH for DPQ generation reaction is 7.25. The main product was DPQ in aqueous buffer solution, but PPE and the oxidized products of PPE remained in Triton X-100 micellar solution.     

Catalysis of copper(II) chelate-amine complexes in the oxidative coupling of 2,6-dialkylphenols

The oxidative coupling reaction of 2,6-dimethylphenol and 2,6-di-tert-butylphenol with molecular oxygen was performed by using a series of copper(II) chelate complexes as a catalyst, derived from copper(II), β-diketone, and some Shiff bases. Under the applied reaction conditions, the reaction products of 2,6-dimethylphenol were poly(2,6-dimethyl-1,4-phenylene oxide) (C? O coupling product) and 3,3′,5-5′-tetramethyl-4,4′-diphenoquinone (C? C coupling product), and that of 2,6-di-tert-butylphenol oxidation was only 3,3′,5-5′-tetra-tert-butyl-4,4′-diphenoquinone (C? C coupling product). The catalytic activity has been shown to be dependent on the properties of the copper(II) chelates used as catalysts and the mole ratios of amine ligand to copper(II) chelate (ligand ratio). The basicity and the steric bulkiness of the amine used as a ligand for copper(II) β-diketonato catalysts were found to be two of the main factors that govern the oxidative coupling mode (C? O and/or C? C coupling) of 2,6-dimethylphenol. The oxidative coupling activity of 2,6-dialkylphenol is discussed in terms of both the stabilities of the copper(II) chelates and of the copper(II) chelate-amine adducts. The rate of oxygen absorption for 2,6-dimethylphenol catalyzed by the copper(II) acetylacetonato-piperidine system is first order in oxygen partial pressure and zero order in 2,6-dimethylphenol concentration, respectively. A Cu(II)-oxygen, as an intermediate is suggested on the basis of the results obtained.     

OXIDATIVE POLYMERIZATION BEHAVIOR OF 2,6-DIMETHYLPHENOL IN AQUEOUS MEDIA WITH POTASSIUM FERRICYANIDE

The effects of potassium ferricyanide,sodium n-dodecyl sulfate,sodium hydroxide and temperature on the molecular weight and the yield of poly(2,6-dimethyl-1,4-phenylene oxide)(PPO) synthesized in an aqueous medium were studied.It was found that oxygen in air had little influence on the oxidative polymerization of 2,6-dimethylphenol(DMP) in the aqueous medium,and potassium ferricyanide was only an oxidant during the oxidative polymerization of DMP.Sodium n-dodecyl sulfate could stabilize polymer particles...     

水介质中2,6-二甲基苯酚的氧化偶合聚合

聚2,6-二甲基苯醚(PPO)是重要的工程塑料,一般采用在有机溶剂中使2,6-二甲基苯酚(DMP)氧化聚合的方法合成,这就需要溶剂回收装置和防爆反应器,且污染环境.从绿色化学观点出发,以水作为反应介质,不仅对环境友好,而且PPO不溶于水,容易分离.近年来,一些学者研究了在油/水两相或全水介质中使DMP氧化聚合合成PPO的新方法.本文主要综述了该方法的研究进展,包括DMP氧化聚合的机理,油/水两相或水介质中对聚合速率、氧化偶合选择性及PPO分子量等的影响因素.     

The initial oxidative polymerization kinetics of 2,6-dimethylphenol with a Cu-EDTA complex in water

The initial oxidative polymerization kinetics of 2,6-dimethylphenol (DMP) catalyzed by a Cu(II)-EDTA complex in water was studied. The initial polymerization rate of DMP (R₀) increases with an increase in concentrations of DMP and catalyst. R₀ firstly increases with the molar ratio of N/Cu and then decreases. The reaction order with respect to oxygen is 0.1. R₀ increases with NaOH concentration and reaches its maximum value at a concentration of 0.50 mol/L. 1/R₀ is in direct proportion to 1/[DMP]₀, which indicates that the initial polymerization kinetics of DMP in water obeys Michaelis-Menten model. The dissociation rate constant of the intermediate complex (k₂) and Michaelis-Menten constant (K_m) at various temperatures are calculated. It is found that both k₂ and K_m increase with an increase in temperature.     

A novel synthetic method for preparing poly(2,6-dimethyl-1,4-phenylene oxide)/polystyrene alloy in reactor containing aqueous medium

Considering the defect of solution polymerization of 2,6-dimethylphenol (DMP), the low molecular weight of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) synthesized in water and difficulty in processing of PPO, a novel one-pot synthetic method for preparing PPO/PS alloy in reactor containing aqueous medium was proposed based on green chemistry. In the presence of styrene, DMP was polymerized to form PPO, and then styrene was in situ polymerized under the initiation of dibenzoyl peroxide (BPO) and dicumyl peroxide (DCP), finally thermodynamically compatible PPO/PS alloy was prepared. It was found that the introduction of styrene during the oxidative polymerization of DMP could increase the molecular weight of PPO. When styrene content was 50 wt%, for the synthesized PPO/PS alloy the yield and the weight-average molecular weight were determined to be 95% and 1.7 × 10⁵ for PPO, 93% and 2.0 × 10⁵ for PS, respectively.     

Phase transfer catalyzed polymerization of 4-bromo-2,6-dimethylphenol in the presence of either 2,4,6-trimethylphenol or 4-tert-butyl-2,6-dimethylphenol

The synthesis of poly(2,6-dimethyl-1,4-phenylene oxide) with one 2,6-dimethylphenol chain end (PPO–OH) and with well-defined molecular weight by phase transfer catalyzed polymerization of 4-bromo-2,6-dimethylphenol ( 20 ) in the presence of either 2,4,6-trimethylphenol ( 1 ) or 4-t-butyl-2,6-dimethylphenol ( 1 ′) as chain initiators is described. The range of controllable molecular weights and the mechanism of molecular weight control are discussed based on the differences between the reactivities of 20 , 1 , and 1 ′ and of the corresponding reactive species. The PPO–OH synthesized from 20 / 1 ′ has structural units derived from 1 ′ attached only at the chain end. PPO–OH synthesized from 20 / 1 contains structural units derived from 1 both internally and at the chain ends. Structural units derived from side reactions were identified by ¹H-NMR spectroscopy. A reaction mechanism is proposed to account for their formation.     

Depolymerization of poly(2,6-dimethyl-1,4-phenylene oxide) under oxidative conditions

Depolymerization of an engineering plastic, poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), was accomplished by using 2,6-dimethylphenol (DMP) under oxidative conditions. The addition of an excess amount of DMP to a solution of PPO in the presence of a CuCl/pyridine catalyst yielded oligomeric products. When PPO (M(n)=1.0x10(4), M(w)/M(n)=1.2) was allowed to react with a sufficient amount of DMP, the molecular weight of the product decreased to M(n)=4.9x10(2) (M(w)/M(n)=1.5). By a prolonged reaction with the oxidant, the oligomeric product was repolymerized to produce PPO essentially identical to the starting material, making the oligomer useful as a reusable resource. During the depolymerization reaction, an intermediate phenoxyl radical was observed by ESR spectroscopy. Kinetic analysis showed that the rate of the oxidation of PPO was about 10 times higher than that of DMP. These results show that a monomeric phenoxyl radical attacks the polymeric phenoxyl to induce the redistribution via a quinone ketal intermediate, leading to the substantial decrease in the molecular weight of PPO, which is much faster than the chain growth.     

Functional polymers and sequential copolymers by phase transfer catalysis. 18. Synthesis and characterization of α,ω-bis(2,6-dimethylphenol)–poly(2,6-dimethyl-1,4-phenylene oxide) and α,ω-bis(vinylbenzyl)–poly(2,6-dimethyl-1,4-phenylene oxide) oligomers

A novel and convenient synthetic method for the preparation of α,ω-bis(2,6-dimethylphenol)–poly(2,6-dimethyl-1,4-phenylene oxide) (PPO-2OH) is presented. It is based on the oxidative copolymerization of 2,6-dimethylphenol (DMP) with 2,2′-di(4-hydroxy-3,5-dimethylphenyl propane) (TMBPA) in a mixture of water–methanol or chlorobenzene–methanol. By using a 4/1 mole ratio of DMP to TMBPA and different solvent mixtures, it was possible to obtain bifunctional PPO-2OHs with number average molecular weights between 1000 and 5000. A phase-transfer-catalyzed etherification of PPO-2OH chain ends with a mixture of m- and p-chloromethylstyrene was used to synthesize α,ω-bis(vinylbenzyl)-poly(2,6-dimethyl-1,4-phenylene oxide)s (PPO-2VBs). The thermal polymerization of the PPO-2VBs was studied by differential scanning calorimetry, and has demonstrated a very high thermal reactivity for this new class of reactive oligomers.     

The kinetics of oxidative coupling of 2,6-dimethylphenol with a copper-diamine catalyst system

Reaction rate measurements show that a Michaelis-Menten model proposed earlier is inadequate to describe the full course of the polymerization of 2,6-dimethylphenol to poly(2,6-dimethyl-1,4-phenylene oxide). Modification of this model to include the effects of catalyst deactivation during the reaction and difference in reactivity between the monomer and other oligomers resulted in much greater accuracy. The kinetic constants in the modified model were influenced by reaction temperature, system composition, and method of catalyst component addition.     

Magnetic nanoparticle supported catalytic Cu(II)–poly(amindoamine) complexes for aerobic oxidative polymerization to form poly(2,6-dimethyl-1,4-phenylene oxide) in water

Poly(amindoamine) (PAMAM) was grafted onto magnetic Fe₃O₄ nanoparticles to produce PAMAM grafted Fe₃O₄ (shortened as Mag-PAMAM). Mag-PAMAM coordinated with Cu(II) to form the supported Cu(II)–PAMAM complex (shortened as Cu(II)/Mag-PAMAM). The stoichiometric ratio between amine groups in Mag-PAMAM and Cu(II) was found to be 4. The Cu(II)/Mag-PAMAM complexes were employed to catalyze the oxidative polymerization of 2,6-dimethylphenol (DMP) in water. The Cu(II)/Mag-PAMAM complexes demonstrated the excellent selectivity of C–O/C–C coupling and reactivity to form poly(2,6-dimethyl-1,4-phenylene oxide) (PPO). After polymerization, the Cu(II)/Mag-PAMAM complexes were recovered by an external magnetic field and used repeatedly in the next run with additional Mag-PAMAM and copper ions. After three runs of oxidative polymerization of DMP, the recovery ratio of the catalyst was about 95% and the yield of PPO maintained a relatively high value.     

Effect of styrene oligomers on syndiospecific polymerization of styrene

Styrene oligomers, preferentially consisting of styrene dimers and trimers, are formed by a free radical mechanism at the thermal polymerization of stabilizer-free styrene during storage and at higher polymerization temperatures. The identity of several dimer and trimer fractions formed in such a free radical polymerization, their influence on a coordinative polymerization reaction, the syndiospecific polymerization of styrene, as well as their effect on the properties of the resulting polymers has been investigated.Styrene dimers and styrene trimers reduce the polymerization activity of the transition metal catalyst significantly, especially at low amounts of oligomers added to the styrene. This behavior is discussed with respect to a proposed mechanism involving complexation of the active transition metal species with the specific oligomer instead of the styrene monomer, resulting in increased steric hindrance towards insertion of a styrene molecule to the active site.Both oligomers reduce the molecular weight of the syndiotactic polystyrene, by acting as chain-transfer agents. The constancy of the polydispersity over the whole concentration range of added dimer or trimer indicates that the uniformity of the active sites of the coordinative polymerization is not significantly influenced by the presence of the oligomers.The thermal properties of the polymers demonstrate that the oligomers do not affect the high syndiospecificity of the active catalytic sites, whereas the increase in crystallization temperature with increasing amounts of styrene dimer or trimer is comparable to effects observed by the addition of crystallization nucleators to semicrystalline polymers.     

Catalytic Activity of Polymer Anchored Cu-tren Complex in the Oxidation of 2,6-Di-t-butyl Phenol

Poly(styrene-co-dimethylaminoethyl methacrylate) and poly(methyl methacrylate-co- dimethylaminoethyl methacrylate) were prepared by solution polymerization. These polymers were quaternized by methyl iodide and n-hexyl bromide. The produced polymers were used as support in the aqueous oxidation of 2,6-di-tert-butylphenol (DBP) using hydrogen peroxide catalyzed by tris(2-aminoethyl)amine copper(II) complex “Cu(II)-tren complex” anchored on the prepared polymers. The products obtained from the reactions were 3,3′-5,5′-tetra-tert-butyldiphenoquinine (DPQ) and 2,6-di-tert-butyl-p-benzoquinone (BQ). No reaction products were obtained when the reaction was carried out in the absence of polymeric catalyst. The polymer composition and reaction medium greatly affect product distribution of the reaction. Polar organic solvent like DMF and methanol favor the formation of DPQ, while nonploar organic solvent like benzene and methylene chloride favor the formation of BQ. Hydrophobic branches of polymers 6 (PS-HexBr-Cu-TREN) and 8 (PMMA-HexBr-Cu-TREN) favor BQ formation as the weight of support increased. On the other hand, DPQ is favored in the presence of hydrophilic branches as observed for both polymeric catalysts 5 (PS-MeI-Cu-TREN) and 7 (PMMA-MeI-Cu-TREN).     

Catalytic applications of Cu-containing MOFs based on N-heterocyclic ligand in the oxidative coupling of 2,6-dimethylphenol

Two Cu^II complexes bearing a N-heterocyclic ligand, namely [Cu(SO₄)(pbbm)]_n (1) and {[Cu(Ac)₂(pbbm)] · CH₃OH}_n (2) (pbbm = 1,1′-(1,5-pentanediyl)bis-1H-benzimidazole) have been synthesized with the aim of exploiting new and potent catalysts. Single crystal X-ray diffraction shows that new polymeric complex 1 features 1-D double-chain framework. The catalytic studies on 1 and 2 indicate that they are efficient homogeneous catalysts for the oxidative coupling of 2,6-dimethylphenol (DMP) to poly(1,4-phenylene ether) (PPE) and diphenoquinone (DPQ) with H₂O₂ as oxidant and NaOMe as co-catalyst at room temperature. Optimal reaction conditions are obtained by examining the effects of solvent, the reaction time, temperature as well as the amounts of co-catalyst, catalyst and oxidant. Under the optimal conditions, the selectivity to PPE is almost up to 90% for both complexes, and the conversion of DMP is 85% for 1 and 90% for 2, comparable to those observed for highly active catalyst systems in the literature. Further comparison of their catalytic performances with those of the corresponding copper salt together with organic ligand, copper salt alone and free ligand reveals that the coordination of ligand to Cu^II ion plays a key role in generating the superior reactivities of complexes.     

Catalytic activity of CuI/CuII cyanide based phenanthroline-bicarbonate system for enhancing aerobic oxidation of 2,6-di-tert-butylphenol

The metal organic framework {[Cu₂(CN)₃(phen)₃]5H₂O} MOF1- bicarbonate system was investigated as an efficient catalyst for aerobic oxidation of 2, 6-di-tert-butylphenol (2,6- DTBP). The catalytic system showed very efficient catalytic behavior for the oxidation of selective coupling of 2,6- DTBP to 3,3′,5,5′-tetra-tert-butyl-4,4′-diphenoquinone (DPQ) in excellent yield. The influence of reaction parameters on the selective oxidation of 2, 6-DTBP to DPQ had been investigated. Photoluminescence probing technology of Disodium salt of terephthalic acid as well as scavenging experiments revealed the creation of the hydroxyl radicals as the main active oxidation radicals produced by the MOF1/O₂/basic bicarbonate system. The oxidation reaction mechanism was also discussed. The recycled catalytic system retained its activity for eight successive runs.     

The Synthesis of Poly(2,6-dimethyl-1,4-phenylene oxide)/Polymethylmethacrylate Alloy in Aqueous Medium via a One-Pot Method

An environmentally friendly one-pot synthetic method based on green chemistry was developed to prepare thermodynamically partially compatible poly(2,6-dimethyl-1,4-phenylene oxide)/poly(methylmethacrylate) (PPO/PMMA) alloy in water. The oxidative polymerization of 2,6-dimethylphenol in alkaline aqueous solution was firstly conducted and then methyl methacrylate (MMA) was added into the reactor before the end of polymerization. MMA could penetrate into PPO particles and then in situ reverse atom transfer radical polymerization (RATRP) of methyl methacrylate was initiated by 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride after the oxidative polymerization. Both the oxidative polymerization of 2,6-dimethylphenol and RATRP of methyl methacrylate were catalyzed by the complex of CuCl₂ and 4-dimethylaminopyridine. Finally, thermodynamically partially compatible PPO/PMMA alloy was successfully prepared which possessed a multi-layer core-shell structure with two polymers embedded in each other.     

Chemical oxidative polymerization of aminodiphenylamines

The course of oxidation of 4-aminodiphenylamine with ammonium peroxydisulfate in an acidic aqueous ethanol solution as well as the properties of the oxidation products were compared with those of 2-aminodiphenylamine. Semiconducting oligomers of 4-aminodiphenylamine and nonconducting oligomers of 2-aminodiphenylamine of weight-average molecular weights 3700 and 1900, respectively, were prepared by using an oxidant to monomer molar ratio of 1.25. When this ratio was changed from 0.5 to 2.5, the highest conductivity of oxidation products of 4-aminodiphenylamine, 2.5 x 10 (-4) S cm (-1), was reached at the molar ratio [oxidant]/[monomer] = 1.5. The mechanism of the oxidative polymerization of aminodiphenylamines has been theoretically studied by the AM1 and MNDO-PM3 semiempirical quantum chemical methods combined with the MM2 molecular mechanics force-field method and conductor-like screening model of solvation. Molecular orbital calculations revealed the prevalence of N prim-C10 coupling reaction of 4-aminodiphenylamine, while N prim-C5 is the main coupling mode between 2-aminodiphenylamine units. FTIR and Raman spectroscopic studies confirm the prevalent formation of linear N prim-C10 coupled oligomers of 4-aminodiphenylamine and suggest branching and formation of phenazine structural units in the oligomers of 2-aminodiphenylamine. The results are discussed with respect to the oxidation of aniline with ammonium peroxydisulfate, leading to polyaniline, in which 4-aminodiphenylamine is the major dimer and 2-aminodiphenylamine is the most important dimeric intermediate byproduct.     

Structure of copper 4-(n,n- dimethylamino)pyridine complexes and their catalytic activity in the oxidative coupling of 2,6-dimethylphenol

The oxidative coupling of 2,6-dimethylphenol (DMP) by copper complexes of 4-(N,N)-dimethylamino)pyridine (DMAP) has been studied. Catalytic experiments were carried out in which the DMAP-to-copper ratio and the amount and nature of the copper counter ions were varied. Supporting UV and EPR experiments were performed, and it was concluded that both dinuclear and mononuclear complexes are catalytically active, the mononuclear species being the more active. In solution both species are in equilibrium with one another. The mono/di ratio can be increased by addition of extra DMAP ligands. An excess of coordinating counter-ions increases the amount of dinuclear species. However, a few coordinating counterions are inevitable, and the catalytically most active species was found to be ‘Cu(DMAP)₄Cl(OH)’, the role of Cl⁻ probably being that of a bridging counter-ion promoting the formation of dinuclear Cu(I) complexes for the reoxidation step. The DMAP ligands are coordinated to Cu(II) through the pyridine N-atoms, as was determined by X-ray analysis. The Cu(II)DMAP complexes are catalytically active even without initial hydroxide addition. It is believed that the strongly basic DMAP ligands produce some hydroxide from traces of water present in the reaction medium. The species ‘Cu(DMAP)₄Cl(OH)’ proved to be able to produce relatively high molecular weight polyphenylene oxide (PPO) in short time and with good specificity ( >95%).     

Homogeneous oxidative coupling catalysts: stoichiometry,characterization and kinetics of [(diamine)(μ-halo)copper(I)]2 oxidation by dioxygen in aprotic solvent

Copper(I) halides dissolve in deoxygenated methylene chloride and nitrobenzene solutions of equimolar N,N,N′-triethylethylenediamine (TriEED) to give air-sensitive colorless or pale yellow copper(I) dimers [(TriEED)(μ-X)Cu]₂, X = Cl, Br or I. Dioxygen uptake, analytical, cryoscopic and spectral data show that copper(I) dimers are oxidized to μ-oxo complexes, [(TriEED)₂(μ-X)₂(μ-O)Cu₂], which react with carbon dioxide to form μ-carbonato analogues, [(TriEED)₂X₂(μ-CO₃)Cu₂]. Both oxo and carbonato complexes are homogeneous oxidative coupling catalysts for oxidation of 2,6-dimethylphenol to mixture of diphenoquinone (DPQ) and polyphenyleneoxide (PPO). Kinetic data for oxidation of [(TriEED)(μ-X)Cu]₂ by dioxygen in nitrobenzene obey the third-order rate law d[[(TriEED)₂(μ-X)₂(μ-O)Cu₂]]/dt = k _D[[(TriEED)(μ-X)Cu]₂]²[O₂]. Comparison of the kinetic data with data for oxidation of [(TEED)(μ-Br)Cu]₂, TEED = N,N,N′,N′-tetraethylethylenediamine (the fully alkylated diamine), by dioxygen indicate that N–H in (TriEED) speeds the reaction by a factor of 220 due to an intermolecular attractive force between N–H of (TriEED) and the incoming dioxygen, helping to assemble the activated complex.