To further study the thermal‐oxidative ageing mechanism of crosslinked polyethylene (XLPE), the terahertz frequency domain dielectric parameters and relaxation phenomena of thermal‐oxidative aged XLPE are analysed using terahertz time domain spectroscopy. It is found that the real part of the terahertz dielectric constant increases with the increase of thermal‐oxidative ageing duration, and the imaginary part shows an overall upward trend. Relaxation of the complex dielectric constant of the aged XLPE samples is obvious. Through the second‐order Cole–Cole equation fitting, it is found that there are two relaxation processes around 0.15 and 1.5 THz, and the relaxation strength gradually increases with the ageing time. To explain this phenomena, microstructure changes and crystallisation behaviours are characterised using Fourier transform infrared spectrum and differential scanning calorimetry, respectively. It is suggested that the two relaxations may be caused by elastic dipole polarisation originating from the angular deflection of polar groups and stretching rotation of hydrogen bond. In addition, by comparing the relaxation strength in different temperatures, the source of the relaxation process is further confirmed.

High-energy storable polymer dielectrics are highly desirable and applicable for compact and efficient power electronic devices. However, existing polymer dielectrics suffer from either a low dielectric constant or a low breakdown strength and exhibit an extremely low energy density. Here, the reversible addition−fragmentation chain transfer polymerization and polyaddition polymerization techniques are combined to create a dithioester-terminated polythiourea (PTU)-based copolymer dielectric with an extraordinarily high energy density of 10.7 J/cm³. The end-group functionalized copolymer significantly increases the dielectric constant from 6.0 to 7.6, which enhances the breakdown strength from 264 to 563 MV/m. The theoretical analysis from the proton nuclear magnetic resonance, atomic force microscope (AFM), and the dynamics of the polarization behaviors demonstrates conformational transitions between the two distinct trans/trans and cis/trans thioureas, along with the formation of polar PTU nanoregions. These result in a flexible polarization reorientation process and increased dielectric constant. Meanwhile, the density functional theory calculations for the electronic structures of the block copolymers indicate that the excellent breakdown strength of the end-group functionalized copolymer is attributed to strong charge trapping from the unique dithioesters. This work proposes a strategy to achieve a simultaneous high dielectric constant and breakdown strength toward excellent energy storage performances by end-group functionalization and composition modifications.