wherein T= -CF2-COOH, T'= -CF2CF2-COOH; the m, n, q indexes ar integers, p is an integer, the indexes being such that the number average molecular weight is in the range 500-10,000, preferably 1,000-4,000; in structure (II) the m/n and p/m ratios are between 0.5 and 5; in structure (IV) the m/n ratio is between 0.1 and 10.