Mechanistic studies of the oxidative coupling polymerization of phenols. VI. Comparison of reactivities of DMP, PPO‐dimer, and ‐trimer in the copper‐catalyzed oxidative coupling polymerization

Abstract In the oxidative coupling polymerization, catalyzed by copper‐amine complexes, the oxidation rates of 2,6‐dimethylphenol (DMP) and its CO‐coupled dimer [4‐(2′,6′‐dimethylphenoxy)‐2,6‐dimethylphenol] and trimer [4‐(‐4′‐(2″,6″‐dimethylphenoxy)‐2′,6′‐dimethylphenoxy))‐2,6‐dimethylphenol] have been determined. The DMP concentration dependence shows a Michaelis–Menten‐type behavior. On the other hand, the dimer and trimer showed a first‐order rate‐dependence in the respective phenol concentrations. This indicates that the slow reaction step, following an equilibrium complex formation between DMP and copper complex, is relatively fast for both the dimer and the trimer. Therefore, coordination of dimer or trimer to the copper complex appears to be rate‐determining. Furthermore, the dimer and trimer gave overall reaction rates approximately eight times higher than found for DMP. Following the Flory principle of equal reactivity for functional groups of oligomers in polycondensations, all PPO oligomers can be assumed to have equally high oxidation rates as the dimer and trimer. The yield of undesired DPQ side product is strongly reduced when starting with the dimer (0.18%), or trimer (0.17%), compared to 3.3% for DMP. This is not unexpected, since DPQ can only be formed from two monomeric DMP residues. In fact, using a 1/10 molar mixture of dimer/DMP already results in a DPQ yield of only 1.7%. Furthermore, when starting from DMP, it has been observed that DPQ was predominantly formed during the first 30% conversion. Starting from dimer (or trimer) DPQ was formed at an almost constant very low rate during the whole course of the reaction. From these experiments it can be concluded that the most important polymerization reaction involves oxidation of copper‐coordinated DMP anion to its corresponding cations, followed by coupling with a copper coordinated PPO chain.