Direct synthesis of low-molecular-weight poly(2,6-dimethyl-1,4-phenylene oxide) (LMW-PPO) by Cu(II)-ionic liquids

Although the low molecular weight poly(2,6-dimethyl-1,4-phenyl oxide) (LMW-PPO) has been extensively applied in the field of 5G communication, the complicated procedures involved in the synthesis of LMW-PPO and efficient recovery of the homogeneous catalysts are still challenging nowadays. In this study, five ionic liquids (ILs) have been successfully synthesized, and LWM-PPO is directly synthesized using different Cu(OAc)₂-ILs complexes as catalysts. Among them, Cu(OAc)₂-IL(a) not only exhibits the high yield of LWM-PPO (83.68 %, M_n = 3.6✕10³) with the low 3,3′,5,5′-tetramethyl-4,4′-diphenoquinone (DPQ) yield (0.12 %) and polydispersity index (PDI, 1.39) in methanol, but also maintains high stability after being recycled for 5 times. In addition, with increasing H₂O contents in the system from 1.0 % to 5.0 %, the M_n values of PPO decrease from 2.9✕10³ to 2.4✕10³ with the narrow PDI values in the range of 1.22–1.45, indicating that trace H₂O effectively promotes the formation of binuclear Cu(II) complex between Cu(OAc)₂·H₂O and ILs, which then polymerizes 2,6-dimethylphenol (DMP) to LMW-PPO. The synthesized LMW-PPO exhibits high thermostability (T_d5% = 427.07–430.52 °C), low glass transition temperature (T_g = 167.6–180.3 °C), and low dielectric constant (D_k = 2.14–2.64, 1 GHz) and dielectric loss (D_f = 2.83✕10⁻³-4.91✕10⁻³, 1 GHz), indicating that it is more appropriate to be used in the field of 5G communication. This study provides new insight on the importance of ILs in direct synthesis of LMW-PPO with high performances.