Undecanoic acid facilitates attachment of styrene to hydride-terminated Si nanocrystals at room temperature, avoiding polymerization of styrene, yielding freestanding styrene-terminated Si nanocrystals. The nanocrystals have diamond cubic crystal structure, with photophysical properties similar to typical alkene-capped Si nanocrystals, such as bright photoluminescence with relatively long radiative lifetimes. We propose a reaction mechanism for room temperature styrene addition in which the resonance form of undecanoic acid coordinates to surface Si-H and facilitates H- attack at terminal C=C of styrene.