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 (M_n 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 (D_k = 1.96) and low dielectric loss factor (D_f = 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 M_n 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 T_g values from 209.3 to 170.8 °C with decreasing M_n values from 1.7 × 10⁴ to 4.1 × 10³ indicate the improved processability of the LMW-PPO while maintaining high thermal stability (T_d5% = 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.