Two silicon-containing eugenol-based benzocyclobutene monomers were prepared by a simple solvent-free hydrosilylation reaction. After heat treatment, a series of materials were obtained by a free radical addition reaction, [4 + 4] cycloaddition, or [4 + 2] Diels–Alder cycloaddition. All polymers displayed high thermostability with a 5% weight loss temperature (T 5d) of over 410 °C and a glass transition temperature (T g) of up to 216 °C as well as a good transmittance of 90% varying from 500 to 800 nm. In particular, all polymers also exhibited good hydrophobicity with contact angles exceeding 100° and water absorption as low as 8‰ after immersing in boiling water for 72 h. It should be noted that P3 showed excellent dielectric properties with a dielectric constant (D k) of 2.48 and a dielectric loss (D f) of 6.38 × 10–3 at a high frequency of 2 GHz as well as good flame retardancy due to the high content of cyclotriphophazene compared with those of P1, P2, and previously reported biobased materials. The above advantages indicate that eugenol-based low-dielectric materials have great potential for application in encapsulation resins in the field of microelectronics.