Even–Odd Effect in Aliphatic Polycarbonates with Different Chain Lengths: from Poly (Hexamethylene Carbonate) to Poly (Dodecamethylene Carbonate)

Abstract

We have characterized a series of aliphatic polycarbonates synthesized by organocatalysis containing a variable number of methylene groups (nCH₂) in their repeating units ranging from nCH₂ = 6 to 12. The melting and crystallization behavior, and crystalline structures were studied by differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), and wide-angle X-ray scattering (WAXS). We found a clear even–odd effect in terms of thermal properties and crystalline structure, for nCH₂ = 6 to 9, and a saturation of the even–odd effect, for nCH₂ = 10 to 12. These results were independent of the crystallization conditions employed: nonisothermal, isothermal, and successive self-nucleation and annealing. The even–odd region showed that the even samples had higher melting temperatures than the odd ones and a monoclinic unit cell. On the other hand, the odd samples showed an orthorhombic unit cell. Both even and odd samples exhibited a trans-conformation, with a dilution of the impact of the carbonyl group as evidenced by the weakening of the crystalline memory effect as nCH₂ increases, independent of the even or odd nature of the samples. In the saturation region, methylene, instead of the carbonyl groups, dominated the behavior, resulting in thermal properties that changed almost linearly with nCH₂. The unit cells were all orthorhombic, and the strength of the memory effect was similar, as nCH₂ increased. Accordingly, the samples showed a shift of the FT-IR bands toward a PE-like dominated conformation.