Abstract
Copper‐(N,N,N′,N′‐tetramethylethylenediamine) (tmed) complexes as catalysts in the oxidative‐coupling polymerization of 2,6‐dimethylphenol (DMP) give rise to poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO) in high yield, although the formation of the undesired 3,5,3′,5′‐tetramethyldiphenoquinone (DPQ) cannot be completely avoided.
At ligand‐to‐copper ratios lower than 10, the specificity for PPO formation can be raised by increasing the N/Cu ratio and by the addition of a strong base (potassium or lithium hydroxide). However, once a fivefold excess of tmed is used, neither further tmed nor hydroxide addition raises the specificity beyond 97% PPO. Using an N/Cu ratio of 10, neither the catalyst concentration nor the DMP concentration has an effect on the product composition.
On the basis of the experimental results included in this paper, together with those published in the literature, it is concluded that the specificity is not affected by the structure of the copper complex, provided that sufficiently high ligand‐to‐copper ratios are used. This results in a product formation which always consists of ca. 3% of DPQ and 97% PPO. The DPQ is formed mainly during the initial stages of the reaction.
However, final DPQ yields of up to 45 % are found for N/Cu < 4. High concentrations of mononuclear copper species Cu(tmed)~2~^2+^ appear to be present under these conditions as deduced from EPR experiments (in addition to polynuclear Cu__~x~Cl~y~__ species). Based on this observation, a radical mechanism for DPQ formation, using Cu(tmed)~2~^2+^ as oxidation catalyst, is proposed. There are thus two distinct reaction conditions: (1) At high N/Cu ratios, the highly active dinuclear complex yields 97% polymer and ca. 3% DPQ. In this case, however, factors which would influence the structure or concentration of the catalyst do not appear to affect the specificity. (2) At low N/Cu ratios, the presence of high concentrations of mononuclear Cu(tmed)~2~^2+^ results in high yields of DPQ and in this case, the hydroxide concentration strongly affects the specificity.