Recycling and ring-opening polymerization optimization exploiting new catalysts systems for polyesters and polycarbonates

Polymers play a crucial role in modern society, serving diverse applications from packaging and textiles to medical devices and automotive components. The development of biodegradable polymers like polylactide (PLA) and polycarbonates is noteworthy for their sustainable advantages over traditional plastics. These materials can biodegrade and depolymerize under specific conditions, reducing environmental impact and supporting the circular economy through improved recycling and waste management practices. PLA, known as a "bioshopper" material, derived from starch fermentation, is esteemed as one of the greenest polymeric materials. Polycarbonates and PLA are exploited in Theranostics and medical fields. PLA slow biodegradability and biocompatibility make it ideal for drug delivery systems and pharmacokinetic control applications, whereas polytrimethylene carbonate well known biocompatibility made it one of the most exploited polymers in tissue repairing and engineering fields. Understanding its quantitative structure-property relationships (QSPR) is crucial to allow sustainable materials to competitively perform against fossil-fuel-based plastics. During my internship at IBM Research Almaden, I focused on synthesizing PLA and polycarbonates via ring-opening polymerization (ROP) of related 6-membered closed-ring monomers. This method can utilize organocatalysis, which is particularly appealing for medical applications due to the absence of inorganic contaminants in the final product. This thesis explores the synthesis of these materials using a new class of organic catalysts known as cyclopropenimines (CPIs), in combination with established (thio)ureas. Additionally, the chlorine salts of cyclopropenium cations were investigated for their potential as phase-transfer catalysts. Comparative studies with well-known catalyst systems and flow chemistry conditions were also conducted. The thermal and recycling properties, as well as the degradation behavior of the polymers synthesized were then evaluated. A still ongoing project aims to find the QSPR of PLA synthesised in automated flow chemistry using a supervised machine learning model. My contribution involved the creation of a large dataset for the ML model, needed to ameliorate its predictive performances. The ROP of lactides and carbonate monomers, facilitated by the CPI and (thio)urea catalyst system, led to high activity and living behavior with well-controlled dispersity. The correlation between CPI and (thio)urea pKa was found to significantly enhance synthesis performance. For PLA synthesis, the use of a solvent like dichloromethane, which promotes hydrogen bonding, was crucial. The stereo-selectivity of CPI was evidenced by improved thermal properties and wide-angle X-ray scattering (WAXS) analysis of the resulting stereo-block copolymer from racemic lactide mixture synthesis. These findings were recently published in the American Chemical Society's journal, MacroLetters. Attempts to use cyclopropenium cations chlorine salts as phase-transfer catalysts (PTCs) in ROP were unsuccessful, necessitating further investigation to understand the underlying failure mechanism. Thermal experiments and characterizations on the known and new materials investigated their degradation mechanisms, sometimes resulting in ring-closing depolymerization (RCDEP) with significant monomer yields. RCDEP occurred under non-ideal bulk conditions during isothermal heating without catalysis, confirming these materials' strong potential for chemical recycling.