AbstractPentamethylstyrene was polymerized in high conversion to a high softening (>230°C.) solid using cationic initiators. Methanol insoluble polymer was not obtained under anionic, radical, or ZIEGLER‐NATTA conditions. Attempts to radically copolymerize this monomer with styrene were unsuccessful. Blends of poly(pentamethylstyrene) and polystyrene were incompatible although enhanced thermal properties were observed.Pentamethylstyrene oxide was polymerized to a high melting, colorless sol