A novel synthesis of linear high-molecular-weight poly(4-vinylphenol) (PVPh) and poly[styrene-co-(4-vinylphenol)] (STVPh, 2) via demethylation reaction is developed. The parent polymers, poly(4-methoxystyrene) and poly[styrene-co-(4-methoxystyrene)] produced by free-radical polymerization, are converted to PVPh and STVPh (2), respectively, by being treated with trimethylsilyl iodide (TMSI) at room temperature. Both H-1 NMR and C-13 NMR data show that methoxy is completely cleaved and converted to hydroxy after hydrolysis without crosslinking and other side reactions. In addition, size-exclusion chromatography data show that no chain scission occurs during group conversion.