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Chem424 - Synthetic Polymer Chemistry


Poly(phenylene oxide)

Poly(phenylene oxide) or poly(phenylene ether) or PPO refers to an an important engineering thermoplastic material synthesized by an unusual mechanism and commercialized by General Electric Co. The polymers are formed by oxidative coupling of substituted phenols at the para position. Although many different choices of monomer exist, only 2,6-dimethylphenol has any practical importance.

PPO

The monomer is synthesized by reacting phenol with methanol in the vapor phase in the presence of a metal oxide catalyst. Naturally, it important that the phenol used in this reaction be very pure. Impurities in the monomer with blocked para and ortho positions are chain terminators, while impurities with open ortho positions can cause chain branching or crosslinking.

The polymerization reaction is accomplished by bubbling oxygen through a solution of the phenol plus a catalyst which is usually a copper salt plus an amine such as pyridine or dibutyl amine. The reaction is exothermic, and may require cooling to control.

The mechanism commences with creation of a phenoxy radical via hydrogen abstraction by O2. Two of these molecules couple, followed by tautomerization to create a dimer. The dimer and larger species can grow by a similar mechanism adding monomer. However, the chain extension reaction can also occur by reaction of two larger species. Therefore, this reaction has components of both step and chain polymerization chemistry.

PPO Mechanism

The final polymer is a stiff, tough, white plastic with a Tg of 205 °C. PPO can crystallize slightly under certain conditions (5-15%, Tm=267 °C), but because it is never used as a pure polymer, crystallinity is not important. The neat polymer has a melt viscosity that is too high for injection molding, so it is blended with other simpler polymers. For example, PPO is miscible with polystyrene in all proportions, and this blend is sold by GE as a family of materials under the trade name of "Noryl." The polystyrene not only reduces the melt viscosity without weakening the physical properties too much, but also it helps to reduce the price of the material. (Polystyrene is much cheaper than PPO.) PPO can also be blended with nlylons or other polymers with the inclusion of compatibilizing agents. PPO blends are used to make computer housings and frames for household appliances and electronic parts, and also to replace metal in automotive structural components. For example, a blend of PPO and nylon is used for the fenders in the Saturn automobile.


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