Copper-catalyzed oxidative coupling of 2,6-dimethylphenol: A radicalar or an ionic polymerization?

The copper-catalyzed oxidative coupling of 2,6-dimethylphenol to produce poly(1,4-phenylene ether) (PPE), a valuable industrial thermoplastic, has been discovered in the late 1950s. Nevertheless, after almost 50 years of intensive research investigations, its mechanism is not yet entirely elucidated. Some groups advocate the involvement of phenoxyl radicals, while others claim for a carbocationic pathway. This account is meant to briefly summarize the past five decades of mechanistic studies on this polymerization reaction, with an emphasis on experimental data corroborating a heterolytic (ionic) pathway for the polymer chain growth.