AbstractIn contrast to 3‐buten‐1‐ynyldimethyl‐carbinol, 4‐penten‐2‐yn‐1‐ol gives on treatment with anhydrous ferric chloride an open‐chain dimer which readily polymerises on heating with benzoyl peroxide and 1: 1‐azobisisobutyronitrile as initiators to give resins which are insoluble in hot dioxan and coldcyclohexanone.Attempts to polymerise 1:1:3:3:6‐pentamethyl‐4‐isopropenyltsocoumaran and to copolymerise both this monomer and 1: 4‐di‐p‐chlorophenoxy‐2‐butyne with styrene by heating with benzoyl peroxide as initiator were unsuccessful. Similar attempts to polymerise and copolymerise 2‐methyl‐isopropenyl‐4‐methylo