Bio-based Low‑k Polymers at High Frequency Derived from Anethole: Synthesis and the Relationship between the Structures and the Properties

Bio-based polymers have been widely investigated as sustainable low dielectric (low-k) materials in past decades. Nevertheless, a few of the polymers with excellent comprehensive properties have been achieved to satisfy the requirements of high-frequency communication application. In this paper, two fluorinated monomers (BCB-F and 2BCB-F) have been designed and successfully prepared from biomass anethole. The thermal-cross-linkable benzocyclobutene and polyfluorobenzene groups were introduced in order to obtain low-k polymers with good comprehensive properties. A control monomer C1 was prepared from the estragole, the isomer of anethole, to study intensively the effect of structures on properties. Among the thermally cured polymers, cured BCB-F with higher fluoride content shows a comparable dielectric constant (D_k) of 2.62 and lower dielectric loss (D_f) of 1.31 × 10^–3 at a frequency of 10 GHz, as well as better hydrophobic properties with a water uptake of 0.18%. Such good hydrophobic properties enable it to maintain the good dielectric properties even after being soaked in boiling water for 96 h. Cured 2BCB-F with bifunctional benzocyclobutene groups displays excellent heat resistance with a high glass transition temperature (T_g) of 408 °C and a low coefficient of thermal expansion (CTE) of 52 ppm/°C in the temperature range 30–300 °C. Cured 2BCB-F also shows good dielectric properties with a D_k of 2.61 and a D_f of 2.60 × 10^–3 at a frequency of 10 GHz. The good comprehensive properties reveal that the anethole-based polymers are suitable candidates as matrix or encapsulation resins for application in electronics and microelectric fields.