This invention relates to aliphatic polyesters having a controlled thermal decomposition, hydrolizability, and biodegradability, in which OH terminals and COOH terminals are decreased through a ring-opening polymerization of a lactone and lactide by a mono alcohol or a metal alkoxide, or by decreasing both terminals through combining polymer terminals with diisocyanate. From a composition containing the polyesters, this invention provides a controlled release composition for agriculture. From a resin primarily containing fatty acid cellulose ester derivatives, this invention provides a base film for a marking film without volatilization and plasticizer migration problems; a picture image formable heat-sensitive transfer recording material with excellent strippability, coloring concentration, and brightness; a conductive coating composition with excellent storage stability, adhesion, and conductivity; a single-liquid type coating for a moisture-curable graft copolymer, and a non-toxic and non-irritating coating composition with excellent dryability. The copolymers of this invention also have excellent heat and impact resistance.