Improving Thermal, Mechanical, and Crystalline Properties of Poly(butylene succinate) Copolyesters from a Renewable Rigid Diester

The introduction of rigid cyclic monomers into the poly(butylene succinate) (PBS) backbone is the most common way to elevate its low glass-transition temperature (Tg = −30.0 °C). However, the insertion of cyclic units always leads to very poor crystallinity and low molecular weight, which drastically hinder their industrial applications. Herein, a renewable rigid diester N,N′-trans-1,4-cyclohexane-bis(pyrrolidone-4-methyl carboxylate) (CBPC) was obtained via Michael addition. CBPC with linked rings had high spatial mobility, resulting in high reaction reactivity. A series of biobased PBCxBSy copolyesters were prepared by melt polycondensation of CBPC with succinic acid and 1,4-butanediol, achieving the high-number-average molecular weight of up to 44.5 kDa. The insertion of CBPC led to higher thermal stability and dramatically enhanced the Tg, such that the Tg of PBC80BS20 (87.5 °C) surpassed that of PBS (−30.0 °C) over 117.5 °C. Moreover, PBCxBSy showed an unexpected cocrystallization behavior, in which the rigid CBPC with a bulky tricyclic structure could be inserted into the crystal of PBS and formed a homogeneous crystalline structure. The cocrystallization was deeply analyzed by thermodynamic study and density functional theory calculation. Benefiting from the cocrystallization, PBCxBSy showed distinguished mechanical performances, which matched with or excelled those of the commercial polyesters of polyethylene terephthalate, polybutylene terephthalate, and polylactic acid. Accordingly, CBPC could be regarded as an effective biobased building block to spectacularly improve the thermal, mechanical, and crystalline performances of PBS at the same time.