Reactive methoxy-functionalized carbosilane and carbosiloxane dienes can be either homopolymerized or copolymerized via acyclic diene metathesis (ADMET) polycondensation chemistry to produce reactive materials with mechanical behavior dependent on the molar ratios of the comonomers. The methoxy-functional group within the polycarbosilane repeat unit remains inert during the metathesis polymerization and can be triggered subsequently with water to generate crosslinks between polymer chains. In this way, linear, thermoplastic copolymers can be prepared with ADMET chemistry and concerted into crosslinked, thermoset copolymers upon exposure to moisture. Crosslinked films containing 5-10% of the crosslinked hard segment are soft and flexible materials.