Disclosed is a polymer composition (I) 60 to 99.8 wt.% of an amorphous copolymer comprising (a) units derived from perfluoro-2,2-dimethyl-1,3-dixole and (b) units derived from at least one ethylenically unsaturated monomer selected from ethylene, tetrafluoroethylene, chlorotrifluoroethylene, and CF₂=CFOCF₃, and (II) 0.2 to 40 wt.% of a compound having a hydrocarbon group containing at least one fluorine atom and at least one functional group selected from -OH, -SR, -COOH, -SO-, -SO₂-, -CONH-, -COO-CO-, -CONHCO-, -COO-, -CN, -NCO, -CO-, -HCOO-, -NH₂-, -SO₃H, -NHNH₂, -CONH₂, -CH=CH₂, -NH-, (RO) n X 3-n Si-, wherein R is a C₁₋₅ alkyl group, n is 0, 1, 2 or 3 and X is a halogen atom or a C₁₋₅ alkyl group. The polymer composition is useful for a plastic cladding of an optical fiber.