AbstractAn exploratory survey of the polymerization and copolymerization of acenaphthylene has been carried out to see whether there are possibilities of its use as an industrial monomer. Polyacenaphthy‐lene been prepared by thermal polymerization with and without peroxide‐type catalysts, by Friedel‐Crafts polymerization and by the emulsion technique. The molecular weight of a thermal polymer exceeded 100,000.Some aspects of the copolymerization of acenaphthylene with styrene have been studied and it has been shown that a substantial elevation in the softening point of polystyrene results from the incorporation of acenaphthylene units in the polymer chain. The electrical properties of the copolymers are not significantly different from those of polystyrene.Copolymers of acenaphthylene andisobutylene have been obtained by low‐temperature polymerization with Friedel‐Crafts‐type catalysts. The products were of a relatively low order of molecular weight. Examination of the ultra‐violet absorption spectra of the copolymers has revealed a tendency for the acenaphthylene units to enter the macromolecules in groups rather than randomly in single units.Butadiene and acenaphthylene which, in certain conditions, undergo a Diels‐Alder condensation to give tetrahydrofluoranthene, furnish linear copolymers by emulsion polymerization. Attempts to prepare copolymers of acenaphthylene with ethylene proved unsuccessful, acenaphthylene showing a very pronounced inhibiting effect on the