A possible two steps mechanism was proposed for crystallization during polymerization of cyclic butylenes terephthalate(CBT)(Figure 1).In such scenario(Figure 2),the catalyst was introduced into the end of the ring opened cyclic molecules during ring opening polymerization(ROP)and the propagated chains crystallized preferentially.The living end with the metal atom of the catalyst was excluded from the crystal cells of pCBT due to their incompatible lattice parameters.The freshly ring-opened chain segments were always connected with a living catalyst end,making the living ends always dangling on the surfaces of the crystal nuclei,termed as living crystal seeds.These seeds played a role of intelligent transfer arms for the subsequent lamellae growth and finally evolved into living lamellar crystal which could initiate ROP continuously unless the reaction was quenched.In this case,it was difficult for the ring-opened chains to move freely or penetrate and fold into several lamellae which led to the lack of intercrystalline tie-molecules(Figure 3a)and resulted in the brittleness: the main disadvantage of pCBT than conventional PBT.All of this was demonstrated further by the crystallization kinetics(Figure 4)on the basis of Avrami equation.