A composition comprising (a) cross-linked polymers containing exchangeable pending links and exchangeable cross links, by boronic ester metathesis reactions; and (b) free monofunctional boronic esters, said boronic esters being chosen from among the following dioxaborolane and dioxaborinane rings of formulas (EB1) and (EB2) below: in which Rx, Rw and Rv are identical or different and each represent a hydrogen atom or a hydrocarbon radical or form together, as a pair, an aliphatic or aromatic ring Ry is a hydrocarbon radical linked to the boron atom of the dioxaborolane or dioxaborinane ring by a covalent bond through a carbon atom, characterised in that the composition (a) of cross-linked polymers is prepared by copolymerisation of the following compounds: (a') Precursor monomers to thermoplastic polymers comprising at least one pending boronic ester group, said pending boronic ester group not containing any polymerisable group; (b') Cross-linking agent comprising at least one boronic ester group enabling the formation of a network of cross-linked polymer containing pending functions and cross-links that are exchangeable by boronic ester metathesis reactions; said boronic esters being chosen from among the dioxaborolane and dioxaborinane rings of formulas (EB1) and (EB2) ; (c') if applicable monomers that are precursors to thermoplastic polymers that do not include a boronic ester group of formula (EB1) or (EB2). Rx, Rw and Rv are identical or different and each represent a hydrogen atom or a hydrocarbon radical or form together, as a pair, an aliphatic or aromatic ring Ry is a hydrocarbon radical linked to the boron atom of the dioxaborolane or dioxaborinane ring by a covalent bond through a carbon atom, characterised in that the composition (a) of cross-linked polymers is prepared by copolymerisation of the following compounds: (a') Precursor monomers to thermoplastic polymers comprising at least one pending boronic ester group, said pending boronic ester group not containing any polymerisable group; (b') Cross-linking agent comprising at least one boronic ester group enabling the formation of a network of cross-linked polymer containing pending functions and cross-links that are exchangeable by boronic ester metathesis reactions; said boronic esters being chosen from among the dioxaborolane and dioxaborinane rings of formulas (EB1) and (EB2) ; (c') if applicable monomers that are precursors to thermoplastic polymers that do not include a boronic ester group of formula (EB1) or (EB2). (a') Precursor monomers to thermoplastic polymers comprising at least one pending boronic ester group, said pending boronic ester group not containing any polymerisable group; (b') Cross-linking agent comprising at least one boronic ester group enabling the formation of a network of cross-linked polymer containing pending functions and cross-links that are exchangeable by boronic ester metathesis reactions; said boronic esters being chosen from among the dioxaborolane and dioxaborinane rings of formulas (EB1) and (EB2) ; (c') if applicable monomers that are precursors to thermoplastic polymers that do not include a boronic ester group of formula (EB1) or (EB2). <EOS>