Patent ID: 6927275

Claim:
A process for producing a polyester resin comprising a dicarboxylic acid constitutional unit and a diol constitutional unit having a cyclic acetal skeleton, the process comprising: an oligomerization step of mainly producing oligomers by subjecting a diol (A) having a cyclic acetal skeleton and an ester (D) to transesterification reaction under conditions simultaneously satisfying the following requirements (i) to (iv): (i) a molar ratio, b/a, of 0 to 0.035, wherein a is a molar amount of the dicarboxylic acid constitutional unit in the ester (D) and b is a molar amount of free carboxyl group in the ester (D); (ii) a molar ratio, c/a, of 0.05 to 0.60 at initial charge, wherein a is the same as defined above and c is a molar amount of the diol (A) having a cyclic acetal skeleton; (iii) the following formula (I): 0≦( b/a )×( c/a )≦0.003 (I) wherein a, b and c are as defined above; and (iv) a water content of 0 to 0.5% by weight in the reaction system; and a step of mainly polymerizing the oligomers to the polyester.