Cascade approaches towards functional CO2-sourced cyclic carbonates and polycarbonates

Polycarbonates (PCs) belong to some of the world-leading polymers that are widely used in aircraft or automotive applications (windows, etc.), in safety equipment (helmets, bullet proof glass, etc.),… However, their industrial production requires the use of toxic compounds such as phosgene, high temperatures, and is not compatible with the introduction of some functional groups along the polymer backbone. Making plastics more sustainable by valorising CO2 as a cheap, inexhaustible and renewable feedstock imposes itself as a strategic driver for developing low carbon footprint materials. In 2017, our research group reported a new process for the preparation of a novel family of regioregular PCs (i.e. poly(oxo-carbonate)s) by the facile organocatalysed polyaddition of CO2-sourced bis(α-alkylidene cyclic carbonate)s (bisαCCs) with diols under ambient conditions. These bisαCCs were prepared by the organocatalysed carboxylative coupling of CO2 to bis(propargylic alcohol)s.
The goal of my PhD thesis work was to investigate the fundamentals to permit the synthesis of these new PCs in a one-pot process, wherein bisαCCs are produced in-situ and directly involved in polymerisation. Firstly, we designed various organic salts that were tested as catalysts for the carboxylative coupling of CO2 with propargylic alcohols. We investigated the influence of the structure of the organocatalyst (mainly the type and structure of the cation and anion) on the catalytic performances. Optimum activity resulted from the best compromise between ion-pair separation controlled by steric effects and the basicity of the anion. Organic salts with too basic anions increased the rate of the reaction but at the expense of the selectivity. Then we optimised the activity of the organic catalyst by incorporating a metal cocatalyst (CuI or AgI) that allowed to synthesise the bisαCCs with a high selectivity (> 95%) under mild conditions (25-40 °C, 15 bar). Eventually, we implemented the one-pot cascade approach to prepare oxo-alkylcarbonate scaffolds and poly(oxo-carbonate)s from CO2, propargylic alcohols, and mono-alcohols under moderate operating conditions (PCO2 = 1-15 bar, T = 40-80 °C). By varying the nature of the bis(propargylic alcohol) and the diol, various poly(oxo-carbonates) were successfully prepared. Lastly, we prepared new alkyne-1-n-diols and studied their reactivity for the carboxylative coupling with CO2. By carefully choosing the reaction conditions and the diols, we prepared diverse keto-cyclic carbonates, elusive tetrasubstituted carbonate scaffolds and poly(oxo-carbonate) oligomers. This thesis includes operando FT-IR/ATR and DFT computational studies that allowed to understand and explain the mechanism of the reactions investigated. In conclusion, this work contributes to the quest to develop simple catalytic systems for the transformation of CO2 into useful compounds (cyclic carbonates and poly(oxo-carbonate)s).