NOVELTY - Self-supported polymer membrane, comprises: a dynamic polymer formed of repeating units (I).    USE - The self-supporting membrane is useful: for the selective separation of chemical species; for the selective separation of gaseous species contained in natural or industrial gases, preferably carbon dioxide; and as polymer electrolyte for solar cells and fuel cells (all claimed).    ADVANTAGE - The membrane can be manufactured using a simple and inexpensive method with high permeability thus providing easy and high selectivity of the chemical species without varying its chemical nature and total content of the monomers.    DETAILED DESCRIPTION - Self-supported polymer membrane, comprises: a dynamic polymer formed of repeating units of formula (-(D-G-T)-) (I).    D = ether repeating unit of formula (CH2=N-CH2-CH2-CH2(-O-CH2-CH2-CH2-CH2-)x-O-CH2-CH2-CH2-N=);    x = 10-20;    G = (hetero)aryl group of formula (a);    R1 = H or 1-4C alkyl;    Y1 = N or C-R2;    R2 = H or OH;    T = group of formula (=N-(CH(R3)-CH2-O)n-CH(CH2-(-O-CH2-CHR5-)p-N=CH2)2-(CH2-(-O-CH2-CHR5-)p-N=CH2)2);    R3-R5 = H or 1-6C alkyl; and    n, p = 1-20.    Provided that such sub-unit T is present at 20-45 mol.% relative to the number of moles of G subunits constituting the polymer. INDEPENDENT CLAIMS are included for:    (1) preparing self-supported polymer membrane comprising a dynamic polymer formed of repeating units (a), comprising: (a1) preparing a reaction mixture comprising at least one precursor of sub-unit D of formula (H2N-CH2-CH2-CH2-(-O-CH2-CH2-CH2-CH2-)x1-O-CH2-CH2-CH2-NH2), at least one precursor of sub-unit G of formula (g), and at least one precursor of sub-unit T of formula (H2N-(CH(R3)-CH2-O)n1-CH(CH2-(-O-CH2-CHR5a-)p1-NH2)-CH2-(-O-CH2-CHR4a-)p1-NH2) (t) in solution in at least an organic solvent, where the reaction mixture contains 20-45 mol.% of the precursors (t) relative to the number of moles of precursors (g) present in the reaction mixture; (b1) polycondensation of the precursors by heating the reaction mixture at reflux to obtain a dynamic polymer; and (c1) shaping of the membrane and evaporation of organic solvent; and    (2) process of separation of a chemical species comprising: passing a gas or liquid mixture containing the species that is desired to separate through the membrane.    R3a = H or 1-6C alkyl;    x1 = 10-20;    R4a, R5a = H or 1-6C alkyl; and    n1, p1 = 1-20.