Process for the preparation of polyimides or of their biodegradable polypeptide hydrolysates

A process for the preparation of polyimides or their polypeptide hydrolysates which are at least 80% biodegradable, by thermal polycondensation of at least one amino acid, optionally followed by hydrolysis, the process comprising carrying out the thermal polycondensation step in the presence of at least one "acid catalyst" selected from the group consisting of

• a) sulfuric, nitric, phosphorus, sulfurous, hydrofluoric, silicic and sulphonic acids, organic sulphonic acids, containing less than 50 carbon atoms, amino acids bearing a non-carboxyl strong acid function, organic phosphonic acids, organic carboxylic acids having a pK.sub.a lower than or equal to 5;
• b) precursors of these acids;
• c) salts of the acids of step a)with an amino acid which is similar to or different from that or those used;
• d) salts of the acids of step a) with a polyimide derived from the polycondensation of at least one amino acid which is similar to or different from that or those used;
• e) hydrochloric acid and its precursors
• f) hydrochloric acid salts of an amino acid which is similar to or different from that or those used, when the amino acid to be polycondensed is other than glutamic acid;
• g) hydrochloric acid salts of a polyimide derived from the polycondensation of at least one amino acid which is similar to or different from that or those used;
• h) acid salts of sulfuric, phosphoric or phosphorous acids or their precursors;
• i) phosphoric acid precursors other than phosphoric anhydride or polyphosphoric acids;
• and
• j) Lewis acids;
• according to an "acid catalyst"/amino acid molar ratio of at least 0.001;
• said thermal polycondensation operation being carried out in bulk, with a reaction medium remaining in the pulverulent or friable form throughout the whole of the polycondensation operation, in which reaction medium the "acid catalyst" is distributed homogeneously.