Moccia, Serena (2025) Following the transition from conventional plastics to biodegradable materials by a combined experimental and theoretical approach. [Tesi di dottorato]

Documento PDF SerenaMoccia-PhD-thesis-complete.pdf Visibile a [TBR] Amministratori dell'archivio Download (10MB) | Richiedi una copia

Abstract

The drive to develop bio-based, biodegradable, and chemically recyclable polymers aims to reduce dependence on finite feedstocks, minimize environmental impacts, and address end-of-life challenges for synthetic plastics. A robust strategy leverages bio-based, biocompatible, biodegradable, or even chemically recyclable-to-monomer (CRM) materials with thermal and mechanical properties comparable to polyolefins; polyesters and polythioesters are prime candidates. Within this context, I developed my Ph.D. project to advance a circular-economy vision across multidisciplinary work and I divided this thesis into three main Chapters: Chapter 1 deals with the stereoselective ring-opening polymerization (ROP) of lactide promoted by different catalytic systems, investigated using computational methods. PLA is characterized by different microstructures due to the presence of two stereogenic centers within the monomer. In particular, the racemic mixture of L-LA and D-LA (rac-LA) can yield highly isotactic stereoblock copolymers [PDLA-PLLA]ₙ with improved performance compared with the corresponding homopolymers. This makes stereoselectivity central, since tacticity strongly affects properties and processing. I used density functional theory (DFT) together with molecular descriptors, buried volume analysis (%VBur), and activation strain model (ASM) combined with Natural Energy Decomposition Analysis (NEDA) to elucidate the origin of stereocontrol and the intrinsic correlation between microstructure and properties. I focused in particular on achiral Al–salen systems discovered by machine-learning algorithms (Bayesian optimization), which have been shown experimentally to give highly isotactic stereoblock copolymers (Pm = 0.92) in the ROP of rac-lactide. The same methodology was also applied to chiral Al systems, elucidating key features involved in the stereoselective ROP of rac-LA. In addition, in collaboration with the University of Salerno, we initiated a systematic study comparing three different catalytic systems, Fe(II), Zn(II), and Ca(II) pyridylamido catalysts, in the ROP of L-LA, considered promising alternatives to the toxic systems still used in the industrial production of PLLA. This analysis involved DFT to examine free-energy reaction paths, combined with an assessment of steric and electronic factors. It is worth noting that, while there is experimental evidence for the Fe(II) and Zn(II) pyridylamido systems, Ca(II) is unprecedented, although other Ca-centered systems have proven to be very active in the ROP of lactide. In chapter 2 I extended my project to another green polyester, through an experimental approach, in collaboration with University of Salerno. Poly(3-hydroxybutyrate) (PHB) is a polyhydroxyalkanoate with thermal and mechanical properties comparable to those of polyolefins. Literature reports that stereoselective ring-opening polymerization (ROP) of rac-β-butyrolactone under chain-end control can be promoted by yttrium amido complexes supported by salan/salen ligands. Therefore, we synthesized and characterized phenoxyamine salan-like ligands with either N–CH₃ (Lig3 and respective complex →Complex 3) or NH (L4 → L4–Y) on the ethylene bridge, using Y[N(SiHMe₂)₂]₃(THF)₂ as the metal precursor. The isolated complex 3 affords syndio-enriched PHB, whereas the in situ-generated L4–Y yields isotactic PHB. Because these ligands differ only in the N-substituent on the ethylene bridge, it is crucial to understand how such a small change can strongly affect tacticity, and thus polymer properties. Moreover, conducting polymerization with an isolated complex versus an in situ-generated catalyst significantly influences catalytic activity, kinetics, and the thermal properties and crystallinity of the final material. Exploring these systems is therefore compelling, as tuning stereoselectivity and conditions a priori can guide the design of materials with targeted thermal and mechanical performance. In Chapter 3 I focused on the characterization of chemically recyclable polymers using a combined experimental/theoretical approach. We investigated an emerging class of poly(thioesters) produced via ring-opening polymerization (ROP) of the thiolactone 2-thiabicyclo[2.2.1]heptan-3-one, yielding poly(2-thiabicyclo[2.2.1]heptan-3-one) (PBTL). These materials exhibit exceptional thermal and mechanical properties. They are completely chemically recyclable to their monomers and exhibit tacticity-independent crystallinity, even in the presence of configurational disorder. Because crystallinity governs key physical properties, such as stiffness, barrier properties, and heat resistance, we used several techniques: molecular structure, thermal behavior, and morphology were characterized by NMR, DSC, and electron and optical microscopy (TEM/POM), while the crystal structure was elucidated using DFT in conjunction with electron and X-ray diffraction.

Downloads

Actions (login required)

Modifica documento