NEW MATERIAL:A compound shown by formula I (R1-R3 are H, F or hydrocarbon group; Rf is 1-30C F-substituted hydrocarbon group linked to at least one F; n is 0-2).

USE: A comonomer for polymerizing a substance having oxygen permeability (e.g., hard contact lens) for medical use.

PREPARATION: Cyclopetadiene or dicyclopentadiene is reacted with an olefin compound shown by formula II in the molar ratio of (0.1-20):1 in an inert gas under pressure at 80-300°C to form a bicyclo compound shown by formula III first and further the reaction is continued to give a compound shown by formula I wherein n is 1 and then 2. Since the reaction product is a mixture of these compounds, the mixture is separated by a means such as distillation. In order to produce a compound shown by formula III, the amount of cyclopetadiene or dicyclopentadiene used is lessened based on the compound shown by formula II.