Conformational Analysis of Poly(trimethylene carbonate) and Poly(butylene carbonate) and Structure-Property Relationships of Aliphatic Polycarbonates.

Conformational analysis of poly(trimethylene carbonate) (PTMC) and poly(butylene carbonate) (PBC) was conducted. The most stable conformations in the spacers, O(CH₂)_yO, of PTMC (y = 3) and PBC (y = 4) were found to be tg^±g^±t and tg^±tg^∓t, respectively. The former conformation leads to a long zigzag form for PTMC, while the latter extends the PBC chain along the molecular axis. The O-C and OC-CC bonds in the PTMC spacer prefer trans and gauche conformations, respectively, while the O-C, OC-CC, and CC-CC bonds of PBC show trans, gauche, and trans preferences, respectively. The characteristic ratios of PTMC and PBC in a nonpolar environment at 25 °C were evaluated to be 6.89 and 8.27, respectively, significantly larger than those of poly(ethylene carbonate) (PEC, 2.42) and head-to-tail isotactic poly(propylene carbonate) (PPC, 2.36). As the spacer length increases (PEC, PPC → PTMC → PBC), the negative charge on the carbonate group becomes delocalized, reducing interchain electrostatic repulsions. Consequently, PEC and PPC remain amorphous, whereas PTMC and PBC can only crystallize with difficulty. However, the weak interchain attractions in both crystals result in low enthalpies of fusion, and, correspondingly, relatively low melting points.