POLYBUTYLENE NAPHTHALATE

PROBLEM TO BE SOLVED: To obtain a polybutylene naphthalate having improved heat stability, hydrolysis resistance, and hue by effecting the solid phase polymerization of a specified polybutylene naphthalate(PBN). SOLUTION: This polybutylene naphthalate is obtained by effecting the solid phase polymerization of PBN satisfying the relationships: R1A/R1B>1.05 and r1A/r1B>1.05 (wherein R1A and r1A correspond to R1 and r1 of a PBN synthesized by using a Ti-containing composite catalyst CA; and R1B and r1B correspond to R1 and r1 of a PBN synthesized by using a Ti catalyst CB in the same molar concentration as in the composite catalyst, provided that R1 is the ratio of the jump height at the K absorption edge of Ti to the intensity of the main peak among the pre-edge peaks near 4.965-4.972 keV in the vicinity of the K absorption edge, and r1 is the ratio of the jump height at the K absorption edge of Ti to the difference between the maximum and minimum slopes of the main peak in a spectrum left after subtracting the background from the X-ray absorption near-edge structure in an X-ray absorption fine structure analysis of Ti) and having a terminal vinyl content of 10 eq/t or below and an intrinsic viscosity IV>=0.5 at 180-240 deg.C, desirably, 190-235 deg.C.