SYNTHESIS OF POLY(PHENYLENE OXIDE)



|CHAPTER |

|12 |

|SYNTHESIS OF POLY (PHENYLENE OXIDE) |

12.1 INTRODUCTION

Poly(phenylene oxide)s [poly(phenylene)s] are useful materials for engineering thermoplastic applications because of their thermal, oxidative and chemical stability.The polymers are readilly prepared from phenols in good yields by a variety of oxidative coupling procedures and other types of step-growth polymerization reactions. Poly(2,6-dimethyl-1,4-phenylene oxide), PPO resin is the most important commercial poly(phenylene oxide) and is sold as a blend with polystyrene and other additives as Noryl® resin by the General Electric Co.

The oxidative coupling polymerization is often the preferred route to poly(phenylene oxide)s. The copper-catalyzed oxidation of 2,6- dimethylphenol in the presence of oxygen to PPO was first reported by Hay in 1959. This polymerization can also proceed by anodic oxidation, accompanying evolution of an equivalent of hydrogen.

[pic]

Scheme 1

It was found that the polymer (PPO) forms in high yield along with a small amount of a byproduct, 3,3',5,5'-tetramethyl-4,4-diphenoquinone. Many phenols can be used in oxidative polymerization reactions. These phenols usually have substituents in two ortho positions. Most phenols with ortho alkyl, aryl, chloro or bromo substituents produce poly(pheneylene oxide) in high yield. When the groups are bulky (e.g. t-butyl or isopropyl), the diphenoquinone becomes the main product and the yield of polymer is low. Phenols with only one ortho substituent and an open para position can be oxidatively coupled but the polymer is usually highly branched and colored.

Catalysts for oxidative coupling polymerization are usually composed of a transition metal salt (e.g. copper or manganese) and base. Molecular oxygen is normally the oxidizing agent. Most liquids that dissolve the polymer, such as benzene or toluene, can be used as solvents for the reaction. Copper-catalyzed systems often contain a copper halide and either an aliphatic or heterocylic amine (such as pyridine, N,N,N',N'- tetramethylethylenediamine, and N-methyimidazole ).

A variety of mechanism have been proposed for the oxidative coupling polymerization of phenols ranging from electrophilic and nucleophilic (both ionic) to radical coupling mechanism. The first synthetic method was the reaction of low molecularweight PPO with one phenol chain end with 3,3(,5,5(-tetramethyl-1,41diphenoquinone. This reaction occurred by radical mechanism. The second method was the electrophilic condensation of the phenyl chain ends of two PPO-OH molecules by formaldehyde. Third method consists of the oxidative copolymerization of 2,6- dimethylphenol with 2,2-di(4-hydroxy-3,5-dimethylphenyl)propane. 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-hydroxy-3,5-dimethylphenyl)propane. 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). For the oxidative coupling polymerization of DMP catalysed by Cu(II)- N-methylimidazol in toluene /isopropanol , Michaelis- Menten mechanism has been proposed as shown in scheme 2.

[pic] Scheme 2

Here, E is the Cu(II)- nmim-phenolate anion complex; E* is the Cu(I) complex . In the first step, the substrate phenol coordinates to the Cu(II) complex and electron transfers from substrate to Cu(II) ion . Then the activated substrate dissociates from the catalyst and the reduced catalyst (E*) is reoxidized to the original Cu(II) complex by the aid of oxygen.

12.2 PRECIPITATION POLYMERIZATION OF 2,6-DIMETHYLPHENOL (DMP)

12.2.1 MATERIALS REQUIRED

2,6-dimethylphenol (DMP) was recrystallized from n-hexane. All amine ligands were used as obtained. Chloroform, toluene, 2-propanol, and methanol were purified using conventional procedures.

12.2.2 FIRST PROCEDURE

A 1-L jacketed flask was thermostated at 40oC. A gas mixture of 80% N2 and 20% O2 was let through with a gas flow of 2 mL/s. Two grams (0.016 mol) DMP, 1.39 g (0.008 mol) N,N’-Di-tert-butylethylene diamine (DBEDA), and 10.44 g (0.08 mol) di-n-butylamine (DBA) were dissolved in 416 mL toluene/2-propanol (5% / 95%; v/v) mixture. When the reaction mixture was heated to 35oC, the reaction was started by the addition of 0.4 g (0.004 mol) CuCl to the reaction mixture. Then 48 g (0.39 mol) DMP in 250 mL toluene/2-propanol (5% / 95%; v/v) mixture was added slowly in 1 hour. After a reaction time of 5 hours, the precipitated polymer was filtered, washed with methanol, and dried in vacuum at 60oC. The polymer was precipitated twice from 250 mL chloroform in 3L methanol.

12.2.3 SECOND PROCEDURE

N,N,N’,N’-Tetra-ethylene diamine (TEED) is used as ligand in polymerization of DMP with the same solvent compositions. 8.7 g (0.051 mol) TEED is used instead of the DBEDA /BDA -ligand system as described above. All polymer yields are between 85 and 95%.

NOTES

1. Toluene and 2-propanol are solvent and nonsolvent for poly (phenylene oxide), respectively.

2. In order to prevent the formation of DPQ, a high amine / CuCl ratio was used and DMP was added slowly.

3. Characterization of polymer prepared was performed with 1H and 13C NMR spectroscopy and GPC.

4. The results of two series of polymerization using two different catalyst systems are as follows:

| | |Mn, g/mol | |

|Amine |1H NMR |GPC |Mw / Mn |

|DBA/DBEDA |3300 |3000 |1.75 |

|TEED |3400 |3400 |2.05 |

Please compare your results with the corresponding ones in the table.

REFERENCES

1. D. M. White, Poly (phenylene oxide)s in Comprehensive Polymer Science (G. Allen and J.C. Bevington, Eds.) Pergamon, Oxford, 1989, Vol. 5, pp. 473-480.

2. H. A.M. van Aert, R. W. Vanderbosch, M. H.P. Van Genderen, P. J. Lemstra, E. W. Meijer, J.M.S.- Pure Appl. Chem. A32, pp. 515-523 (1995)

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