Aliphatic polycarbonates derived from epoxides and CO2: A comparative study of poly(cyclohexene carbonate) and poly(limonene carbonate)

The ring-opening copolymerization (ROCOP) of epoxides and CO2 provides an alternative approach towards polycarbonates and due to their aliphatic nature, they represent an interesting alternative to bisphenol-A based polycarbonates. A Lewis acidic β-diiminate (BDI)CF3-Zn-(SiMe3)2 complex 1 is used in the ROCOP of CO2 with cyclohexene oxide (CHO) and limonene oxide (LO), respectively. The knowledge gained from polymerizations monitored via in situ IR spectroscopy was used to upscale the reactions to a 1 L reactor. The two products poly(cyclohexene carbonate) (PCHC) and poly(limonene carbonate) (PLC) were then characterized via thermal analysis, a multiaxial pressure test, and dynamic mechanical analysis and compared with commercial polymers. While PCHC and PLC were both thermally stable at 150 °C for 20 min and only minor decomposition occurred at 180 °C, PLC is prone to cross-linking at elevated temperatures. This could be prevented by hydrogenation of the double bond or by the addition of an antioxidant. In the mechanical performance, the aliphatic polymers ranged between the highly impact resistant Durabio® and the brittle PMMA but broke without a splintering of the material. Overall, this study enabled a classification of CO2-based polycarbonates, especially of the novel PLC. Additionally, complex 1 was active in the terpolymerization of CHO, LO, and CO2. The formation of an actual terpolymer was confirmed via aliquot gel-permeation chromatography and diffusion-ordered NMR spectroscopy. High-pressure NMR techniques reveal an interesting kinetic feature. CHO gets copolymerized with CO2 exclusively, and LO incorporation only starts when CHO is fully consumed.