Oxidative Polymerization of 2,6-Dimethyl-phenol to Metal-free Poly(2,6-dimethyl-1,4-phenylene oxide) with Controllable Molecular Weight

Although the synthesis of low-molecular-weight poly(2,6-dimethyl-1,4-phenylene oxide) (LMW-PPO) has been widely studied, preparing metal-free LMW-PPO with high thermal stability and satisfactory dielectric properties is still challenging. In this study, a new method for synthesizing metal-free LMW-PPO by nonmetal catalysts has been developed. In the absence of Cu(II) catalysts, amines can catalyze benzoyl peroxide (BPO) to produce metal-free LMW-PPO in CH3CN (Mn in the range of 4.0 × 103–6.0 × 103), and N,N-dimethyl-p-toluidine (DMT) exhibits high reactivity in the yield of PPO (68.4%) with a low 3,3′,5,5′-tetramethyl-4,4’diphenoquinone (DPQ) yield (2.6 × 10–2%) and PDI (1.50). The low dielectric constant (Dk = 1.96) and low dielectric loss factor (Df = 1.57 × 10–3) of the obtained PPO indicate that amines are more appropriate for the synthesis of metal-free LMW-PPO with superior dielectric properties. Meanwhile, the Mn values of PPO can be successfully mediated by regulating the contents of the catalyst or mixing appropriate contents of toluene in CH3CN, and the decreased Tg values from 209.3 to 170.8 °C with decreasing Mn values from 1.7 × 104 to 4.1 × 103 indicate the improved processability of the LMW-PPO while maintaining high thermal stability (Td5% = 420.8–434.2 °C). Density functional theory (DFT) calculations further reveal the formation of oxidizing radicals from BPO by DMT, which then initiate H-abstraction from DMP to form the DMP radical. The produced DMP radicals then polymerize to LMW-PPO. This study provides new insight into synthesizing highly qualified LMW-PPO by metal-free catalysts.