Perfluorotetrahydro-2-methylene-furo[3,4-d][1,3]dioxole (monomer I) and perfluoro-2-methylene-4-methoxymethyl-1,3-dioxolane (monomer II) are soluble in perfluorinated or partially fluorinated solvents and readily polymerize in solution or in bulk when initiated by a free-radical initiator, perfluorodibenzoyl peroxide. The copolymerization parameters have been determined with in situ F-19 NMR measurements. The copolymerization reactivity ratios are r(I) = 1.80 and r(II) = 0.80 in 1,1,2-trichlorotrifluoroethane at 41 degrees C and r(I) = 0.97 and r(II) = 0.85 for the bulk polymerization. These data show that this copolymerization pair has a good copolymerization tendency and yields nearly ideal random copolymers. The copolymers have only one glass-transition temperature from 101 to 168 degrees C, depending on the copolymer compositions. Melting endotherms have not been observed in their differential scanning calorimetry traces, and this indicates that all the copolymers with different compositions are completely amorphous. These copolymers are thermally stable (the initial decomposition temperatures are higher than 350 degrees C under an N-2 atmosphere) and have low refractive indices and high optical transparency from UV to near-infrared. Copolymer films prepared by casting were flexible and tough. These properties make the copolymers ideal candidates as optical and electrical materials. (c) 2006 Wiley Periodicals, Inc.