Peculiar Even-Odd Effect in semi-crystalline poly(alkylene succinate)s with a wide range of repeating unit chain lengths

Abstract

A series of novel aliphatic polysuccinates was synthesized from dimethyl succinate and various linear diols, with methylene chain lengths (nCH2) ranging from 2 to 12 in their repeating units. The synthesis used an innovative and sustainable bulk polymerization method, greatly reducing solvent use and incorporating low-toxicity catalysts. This approach has enabled the production of a diverse array of polysuccinates for the first time, as evidenced by nuclear magnetic resonance. The crystallization behavior of these materials was systematically analyzed through a combination of Differential Scanning Calorimetry—encompassing non-isothermal, isothermal, self-nucleation (SN), and successive self-nucleation and annealing experiments—as well as in-situ synchrotron Fourier‐Transform Infrared Spectroscopy (FTIR), Wide- and Small-Angle X-ray Scattering (WAXS/SAXS), and Polarized Light Optical Microscopy (PLOM). An intriguing even-odd effect was discovered across the entire range of nCH2, with samples containing even chain lengths exhibiting superior property values compared to their odd counterparts. In the nCH2 < 5 region, the even-odd effect is stronger, reflected in marked differences in thermal transitions and crystallization kinetics, attributable to variations in intermolecular interactions and unit cell structures, e.g., orthorhombic versus monoclinic. For nCH2 > 5, the even-odd effect becomes less pronounced but does not reach saturation, presenting differences in thermal transitions, crystallization kinetics, and interplanar distance within the same unit cell. Remarkably, this study reports for the first time an even-odd effect in SN experiments, suggesting that the even-odd pattern in this series of polysuccinates lacks saturation, highlighting the complexity of these materials and their potential for further exploration in sustainable polymer chemistry.