pb sa copolymer with a weight average molecular weight of 103 000 da raw materials succinic acid adipic acid and 1 4 butanediol are put into a reaction still with the molar ratio of the succinic acid to the adipic acid being 0 5 0 5 and the ratio of an added mole number of the 1 4 butanediol to a total mole number of the two carboxylic acids being 1 2 1 0 temperature in the reaction still is increased to 130 u00b0 c under inert gas atmosphere and raw materials are stirred for complete dissolution then in the reaction still the temperature is kept at 190 u00b0 c at atmospheric pressure the materials are dehydrated for one hour to yield an oligomer of the pbsa having a weight average molecular weight of 5 000 da then the temperature of the reaction system is decreased to 100 u00b0 c and a composite catalyst system the main catalyst being creatinine acetate the cocatalyst being tetrapropyl orthotitanate tpot and the molar ratio being 1 1 is added therein total catalyst amount is one hundred thousandth of the total weight fraction of the raw materials under inert gas atmosphere the catalyst and reactant are stirred and uniformly mixed then the reaction still is slowly vacuum pumped until an absolute pressure is under 200 pa in an hour and a half finally the temperature is increased to a 220 u00b0 c reaction temperature and the reaction lasts for 15 hours to yield the pbsa copolymer with a weight average molecular weight of 128 000 da raw materials succinic acid adipic acid and 1 4 butanediol are put into a reaction still with the molar ratio of the succinic acid to the adipic acid being 0 6 0 4 and the ratio of.