In the present invention, there are obtained aliphatic polyesters having a controlled thermal decomposing property, hydrolizability, and biodegradability, in which OH terminals and COOR terminals are decreased to 50% and 30%, respectively, through a ring-opening polymerization of a lactone and lactide by a mono alcohol having a high boiling point or a metal alkoxide, or by further decreasing both terminals through combining produced polymer terminals with a diisocyanate and, from a composition containing the polyesters, there is obtained a particle-state composition for agriculture and gardening in which a duration period of a fertilizing effect can be controlled, and which is not remained in soil by decomposition. From a resin primarily containing fatty acid cellulose ester derivatives, there are obtained a base film for a marking film which does not include a problem such as volatilization and migration of a plasticizer and a picture image formable heat-sensitve transfer recording material which is excellent in strippability, coloring concentration, and brightness, a conductive coating composition which is excellent in storage stability, adhesion, and conductivity, and single-liquid type coating an a moisture-curable graft copolymer, and a coating composition which have a non toxicity and nonirritating property and which are excellent in dryability. A copolymer in which average chain length in lactone and lactide units are controlled is excellent in heat resistance and impact resistance.