Biodegradable polyesters

A process for preparing biodegradable polyesters, which comprises, in a first steps polyester A containing as monomeric building blocks

• a.sub.1) an acid component comprising
• a.sub.11) 20-95 mol % of at least one aliphatic or cycloaliphatic dicarboxylic acid or its ester-forming derivative and
• a.sub.12) 5-80 mol % of at least one aromatic dicarboxylic acid or its ester-forming derivative, the proportions of a.sub.11 and a.sub.12 being based in each case on the total amount of a.sub.11 and a.sub.12, and
• a.sub.2) at least one dihydroxy compound or at least one amino alcohol or their mixtures
• being prepared and, in a second step, being reacted with a mixture B comprising
• b.sub.1) 45-80% by weight of at least one mononuclear isocyanurate (b.sub.11) or at least one compound (b.sub.12) which contains two functional groups capable of reacting with the end groups of polyester A or mixtures of b.sub.11 and b.sub.12,
• b.sub.2) 13-25% by weight of at least one binuclear isocyanurate (b.sub.21) or at least one compound (b.sub.22) which contains three functional groups capable of reaction with the end groups of polyester A or mixtures of b.sub.21 and b.sub.22,
• b.sub.3) 5-12% by weight of at least one trinuclear isocyanurate (b.sub.31) or at least one compound (b.sub.32) which contains four functional groups capable of reaction with the end groups of polyester A or mixtures of b.sub.31 and b.sub.32 and
• b.sub.4) 2-18% by weight of at least one tetra- or higher-nuclear isocyanurate (b.sub.41) or mixtures of different isocyanurates (b.sub.41),
• from 95 to 99.9 mol % of polyester A and from 0.01 to 5 mol % of mixture B being reacted with one another.