Synthesis of poly (2,6-dimethyl-1,4-phenylene oxide) by double-step polymerization in supercritical carbon dioxide

Oxidative polymerization of 2,6-dimethylphenol (2,6-DMP) is a convenient method to prepare poly (2,6-dimethyl-1,4-phenyleneoxide) (PPO), an important plastic in engineering. The polymerization of 2,6-DMP is carried out in organic solvents under oxygen; thus, a solvent-recovery system and an anti-explosive reactor are required for the industrial application. The use of scCO₂ instead of organic solvents is expected to make the polymerization of 2,6-DMP safer and environmentally more benign. We have studied the oxidative polymerization of 2,6-DMP in scCO₂. In this report, we have applied the 'double-step polymerization' method in scCO₂ to synthesize PPO with better polymer properties, i.e., higher average molecular weight (M_w) and narrower molecular weight distribution (M_w/M_n). In the first step, a prepolymer with a narrow molecular weight distribution (M_w/M_n = 1.4) was synthesized under a homogeneous-phase condition. The prepolymer was purified and dried to remove water. In the second step, the prepolymer was further polymerized to increase the molecular weight. The second-step polymerization was performed at 40°C and 20 MPa. The M_w was increased from 4.9 × 10³ to 9.0 × 10³, keeping the narrow molecular weight distribution (M_w/M_n = 1.4). We further optimized the polymerization condition in the second step to obtain higher molecular weight. The M_w was increased to 3.4 × 10⁴ (M_w/M_n = 2.3) at 80°C and 15 MPa in the presence of CuBr (0.0004 mol) and pyridine (0.019 mol).