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The crystallization and melting behavior of polymers is of theoretical importance.In this work,poly(butylene succinate) (PBS) was selected as an example to study such behavior at low supercooling via introduction of the extended-chain crystal (ECC) of the same polymer as nucleating agent.The crystallization of PBS with its ECC as nucleating agent in a wide temperature range (90-127 ℃) and the following melting behavior were studied.It is revealed that the melting point (Tm,for Tc-≥113 ℃) and the annealing peak temperature (Ta,for Tc=90-100 ℃) show similar asymptotic behavior.Both Tm and Ta approach to a value of ca.3.3 ℃ higher than the corresponding Tc when the crystallization time tc approaches the starting point.That is to say,the Hoffman-Weeks plot is parallel to Tm=Tc line.The crystallization line became parallel to the melting line when PBS was crystallized at Tc higher than 102 ℃.Based on these results,we propose that the parallel relationship and the intrinsic similarity between the Ta and the Tm observed at the two ends of the Tc range could be attributed to the metastable crystals formed at the beginning of crystallization.