## 1 Introduction
The relationship between the molar polarizability \(P\), the dielectric constant, and the molecular polarization \(\alpha\) can be expressed in Equation [35]:  
\[P=\frac{\epsilon_{r}-1}{\epsilon_{r}+2}\times\frac{M}{\rho}=\frac{N_{A}\alpha }{3\epsilon_{0}}\]  
where \(P\) is the molar polarizability, \(\epsilon_{r}\) is the relative permittivity, \(\epsilon_{0}\) is the permittivity in vacuum, \(M\) is molecular weight of a repeat unit, \(\rho\) is density, \(\alpha\) is polarizability, and \(N_{A}\) is the Avogadro constant. Molecular polarizability refers to the proportionality constant for the formation of dipole under the influence of an electric field [35]. Based on the relation, all the molecular and structural parameters that enhance polarizability and density, such as the higher polarity of functional groups or atoms, higher molecular movability, inclusion of polar or conductive impurities, and less free volume, etc., will lead to a higher dielectric constant, and vice versa [35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45]. Dielectric loss is related to the energy dissipated due to the oscillating electric field. It is also related to the dielectric relaxation time \(\tau\), which is the time taken for the dipoles to return to their original position after the application of the electric field. When the rate of the electric field oscillate is much faster than the relaxation time, the polarization process in a molecule is unable to follow the rate of the change of the applied oscillating electric field and this results in energy absorption and dissipation as heat [35]. The molecular and structural parameters affecting polarizability usually have similar effects on dielectric loss. Synchronous changes of dielectric constant and dielectric loss have been observed in most studies. For thermosetting polymer systems crosslinked through free radical mechanisms, the type and concentration of initiator are the critical factors determining the dielectric properties by influencing the crosslinking density and creating low-polarity functional groups. [PERSON] et al. [46] studied the blends of 1,2-polybutadiene/styrene-butadiene-styrene triblock copolymer/ethylene-propylene-dicyclopentadiene (1,2-PB/SBS/EPDM) blends cured with two different organic peroxides, cumyl peroxide (DCP) and bis(1-(tert-butylperoxy)-1-methylethyl)-benzene (BIPb). The results showed that BIPb is more reactive and more effective in achieving high crosslink density and converting -C=C- double bonds with relatively high polarizability into -C-C- single bonds with low polarizability, which leads to low \(\mathrm{D_{k}}\) and \(\mathrm{D_{f}}\). Higher initiator concentration resulted in lower \(\mathrm{D_{k}}\) and \(\mathrm{D_{f}}\) regardless of initiator type through similar mechanisms, i.e., higher cross-link density that more effectively hindered the movement of polar groups, and further reduced the polarizability by replacing more -C=C- double bonds with new -C-C- bonds. In addition, the effects of the by-products from the decomposition of initiators on the dielectric properties cannot be ignored [47; 48]. [PERSON] [47] investigated the effects of initiator (DCP) content on the dielectric properties of crosslinked polyethylene (XLPE). The results showed that, with higher initial DCP concentration, more byproducts were produced which resulted in an increase in dielectric loss. Thermosetting PPO-based systems using TAIC as the crosslink agent have been widely used as the matrix of high-performance CCLs [13; 14; 15; 16; 49; 50; 51]. Usually, the PPO/TAIC ratio ranges from 1.0:1.0 to 4.0:1.0 and more typically from 1.2:1.0 to 2.0:1.0, which ensures excellent dielectric properties and good processability. A low PPO/TAIC ratio gives lower \(\mathrm{D_{f}}\)[32]; however, the prepreg starts to become sticky at room temperature which causes processing issues. In this work, a series of PPO/TAIC-based composites with ultralow dielectric loss were prepared with a fixed ratio of PPO/TAIC at 1.33:1.0 and a low loss glass fabric as the reinforcement.