Abstract: The solution polymerization of 2,6-dimethylphenol (DMP) is widely used in industrial production of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), an important engineering plastics. However, both a solvent recovery process and an explosion- proof reactor are required. The use of water as the reaction medium is environmentally friendly and PPO can be separated easily due to its insolubility in water. Therefore, the oxidative polymerization of DMP to form PPO using water as the reaction medium would be of significance.
In this paper we report the first catalytic Cu(II)-PAMAM dendrimer complexes that catalyze aerobic oxidative polymerization of DMP in water with significantly high catalytic efficiency ([monomer]/[catalyst]=2000). The results are significant for three reasons as follows. First, it is the first example that Cu(II)-PAMAM dendrimer complexes are used as catalysts. Second, Cu(II)-PAMAM complex catalyzed aerobic oxidative polymerization of DMP under mild condition in water with a good selectivity to PPO. Third, the catalytic efficiency greatly enhanced compared with the catalyst reported in the literature.
The generation of PAMAM strongly influences the catalytic efficiency of Cu(II)-PAMAM dendrimer catalyst. Remarkably, the PPO yield remained still high (81.45%) at a copper concentration of 25μmol/L, which was even comparable with the results obtained through Cu(II)-G1, Cu(II)-G2, Cu(II)-EDTA at a much higher copper concentration (200 μmol/L). This clearly illustrates a distinct advantage of Cu(II)-G3 catalyst over other catalysts since almost the same high PPO yield was obtained at a extremely low catalyst concentration (25μmol/L).
Another important factor on catalytic efficiency is the ratio of copper ion to ligand. The low PPO yield at lower [Cu(II)]/[PAMAMG3] ratio could be ascribed to the dispersed state of copper complexes which do not have concentration effect, and might also be duo to inhibition of polymerization by uncoordinated nitrogen sites.
Keywords: 2,6-dimethyl; oxidative polymerization; water; PAMAM dendrimer; catalytic efficiency