Hydraulicity in Organic Polymers Enabled by Hydrophobization Upon Water‐Triggered Curing of Hydrophilic Chains

ABSTRACT Water‐curing capability of cementitious materials, commonly known as hydraulicity, is key to their widespread use through simply mixing powder with water to form desired robust structures. However, hydraulicity has been difficult to achieve for organic materials due to the lack of a strategy for wet curing while ensuring water resistance after curing. Here, we report unprecedented hydraulicity in an organic polymer enabled by an unusual water‐induced polarity inversion, in which the reaction of a hydrophilic polymer with water induces hydrophobization via sol–gel reaction of silatrane. A model study using a low‐molecular‐weight silatrane revealed that water drives the sol–gel reaction, which is autocatalyzed by triethanolamine generated in situ. A silatrane‐containing polymer, prepared by conventional free‐radical polymerization, was obtained as a powder but rapidly plasticized upon addition of water. The formed sticky putty gradually transformed first into a soft hydrogel and finally into a rigid solid through hydrophobization associated with the formation of silsesquioxane structures. This unique behavior enabled not only wet molding into desired structures but also underwater adhesion, as demonstrated by repairing a broken vial and bonding substrates. These findings provide a versatile design strategy for organic materials exhibiting hydraulicity.