TECHNOLOGY FOCUS - POLYMERS - Preferred Elastomer: (E) preferably contains 20-40 mol.% of PSEPVE or PFSO2F and 80-60 mol.% of VDF, and includes at least one fluorinated olefinic hydrocarbon (O), preferably selected from vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene, bromotrifluoroethylene, hydropentafluoropropylene, hexafluoroisobutylene, trifluoropropene, 1,2-dichlorodifluoroethylene and 2-chloro-1,1- difluoroethylene, and/or a perfluoro vinyl ether 3-50 mol.%, selected from a perfluoroalkyl (especially perfluoromethyl or perfluoropropyl) vinyl ether, a perfluoroalkoxyalkyl vinyl ether selected from perfluoro(2-n-propoxy)propyl vinyl ether, perfluoro(2-methoxy)propyl vinyl ether, perfluoro(3-methoxy)propyl vinyl ether, perfluoro(2-methoxy)ethyl vinyl ether, perfluoro(3,6,9-trioxa-5,8-dimethyl)-dodeca-1-ene, and/or perfluoro(5-methyl-3,6-dioxo)-l-nonene.Preferred Solvent: The polymerisation is preferably carried out in the presence of a solvent selected from either esters of formula (I), fluorinated solvents such as perfluorohexane (preferably ClCF2CFCl2, n-C6F14, n-C4F10 and perfluoro-2-butyl-tetrahydro-furan), and solvents selected from methyl acetate, 1,2-dichloroethane, isopropanol, tert-butanol, acetonitrile and butyronitrile (preferably methyl acetate or acetonitrile).R-COOR' (I)R = H, 1-5C alkyl or OR'' (preferably H or CH3);R' = 1-5C alkyl or OR'' (preferably methyl, ethyl, iso-propyl or tert.-butyl); andR'' = 1-5 C alkyl.Preferred Conditions: The temperature of copolymerization is preferably 40-80 degrees C and the pressure is 20-40 bar. The polymerisation preferably takes place in an emulsion, a microemulsion, undiluted, in suspension, in a microsuspension or in solution.Preferred Olefin: Olefin (O) is preferably of formula (II).R1R2C=CR3R4 (II)R1-R4 = (per)fluoro alkyl.Preferred Initiator: The initiator is preferably of the type e.g. acetyl cyclohexane sulfonyl peroxide, benzoyl peroxide, diethyl peroxy dicarbonate etc. The ratio of initiator/monomer is 0.1-2 %.Preferred Composition: The copolymerization composition may also contain a surface active agent (1-3 wt.% of anionic, cationic or non ionic surfactant) and chain transfer agents. EXAMPLE - A vessel was charged with 75% t-butyl peroxypivalate (0.135 mmol), perfluoro(4-methyl-3,6-dioxaoct-7-ene) sulfonyl fluoride (PFSO2F) (2.66 mmol), and methyl acetate (26.4 mmol) and purged with He at 100 mm Hg for at least 5 cycles. Vinylidene fluoride (VDF) (0.007 mol +/- 8 mg) at a pressure of 0.28 bar, was then added to the tube, which had been cooled in liquid nitrogen. The mixture was stirred at 75 degrees C for 6 hours to complete polymerization. The conversion of VDF was 82% and the copolymer had a composition of 72/28 VDF/PFSO2F. The Tg of the copolymer was -34.8 degrees C.