Facile preparation of the novel castor oil-based benzoxazine–urethane copolymer with improved high-frequency dielectric properties

The novel star shaped benzoxazine-co-urethane prepolymers are first synthesized from the natural resource castor oil, hydroxyl terminated benzoxazine and diisocyanate through the facile one-pot formulation method. Then the cured benzoxazine copolymers can be obtained by the ring-opening polymerization of oxazine groups of the prepared prepolymers. Both hexamethylene diisocyanate and 4,4′-methylenediphenyl diisocyanate are employed as aliphatic and aromatic diisocyanate, respectively, to clarify their effects on the chemical structure and high-frequency dielectric properties of copolymers. In comparison with the pure polybenzoxazine, the copolymer films occupy the improved high-frequency dielectric properties owing to the introduction of three branching aliphatic castor oil, leading to the reduction of molecular polarity and the improvement of free volume. It should be noted that the aliphatic diisocyanate not only decreases the polarity of the copolymer, but also enhances the intermolecular interaction and intermolecular entanglement between castor oil and benzoxazine for the copolymers. Accordingly, the aliphatic diisocyanate based poly(benzoxazine-co-urethane) occupies the minimum high-frequency dielectric constants (2.94, 5 GHz; 2.96, 10 GHz) and relatively high glass transition temperature (220 °C). Therefore, this work not only provides a simple path for improving high-frequency dielectric properties of benzoxazine, but also gives some insight into the effects of diisocyanates on the formation and high-frequency dielectric properties of poly(benzoxazine-co-urethane)s.