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 CH 3 CN ( M n in the range of 4.0 × 10 3 –6.0 × 10 3 ), 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 CH 3 CN, and the decreased T g values from 209.3 to 170.8 °C with decreasing M n values from 1.7 × 10 4 to 4.1 × 10 3 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.