Adhesives that function underwater are required in various fields, such as marine development, dental treatment, and surgical procedures. However, underwater adhesion is challenging compared to in air, as water molecules covering the surface of the adherend inhibit contact between the adhesive and the adherend surface. Turning our eyes to nature, the mussel can firmly adhere to reefs even underwater. Mussels adhere underwater by secreting an adhesive composed of proteins, which is rich in a special amino acid containing catechol groups. Taking a hint from this, the catechol-modified polymers have been investigated for underwater adhesives. While it is clearly shown that the catechol groups exhibit excellent underwater adhesion in previous studies, it is still under discussion whether the structure of catechol groups, which have two phenolic hydroxy groups in ortho positions, is best for underwater adhesion. Therefore, we systematically synthesized styrenic model polymers with different numbers of phenolic hydroxy groups and evaluated their underwater adhesion properties. As a result, we revealed a correlation between the number of phenolic hydroxy groups per monomer unit and underwater adhesion strength. Also, the adhesive strength of polymers containing phenolic hydroxy groups changed depending on the ion species and their ion strength in the aqueous solution. In developing underwater adhesives, it is required to design polymers by considering ion species present in the intended application environment and optimizing the balance between cohesive and adhesive forces.

In this study, we synthesized thermosets using the fran-protected maleimidobenzoxazine with a 2-ethylhexyl group as an N-substituent（fMB）and the thiol-modified silsesquioxanes（SQ107 and SQ183）．First, the adducts of fMB and SQ107 or SQ183 were prepared by the ring-opening reaction of the oxazine ring （COLBERT reaction） at room temperature, then the isolated adducts were heated at 140℃ for 1 h, 150℃ for 2 h, and finally 160℃ for 1 h under 2 MPa pressure conditions in order to obtain thermosets. The 5％ weight-loss temperatures, 50 ％ weight-loss temperatures, maximum decomposition temperatures, and residual weights at 800℃ were determined by the thermogravimetric analysis of the thermosets, and they were compared with the characteristics of the thermosets with the related chemical structures reported in previous papers. Furthermore, the effects of cross-linking structures on mechanical properties of the thermosets produced from fMB and SQ107 or SQ183 as the starting materials were discussed based on the results of viscoelasticity measurements and mechanical tensile tests.