Title: Tailoring of catalytic routes towards high performance polylactic acid polymers.
Other Titles: Op weg naar performante polymelkzuur polymeren met nieuwe katalytische syntheseroutes
Authors: Dusselier, Michiel
Issue Date: 30-May-2013
Abstract: Renewable polymers offer a promising alternative for certain fossil fuel derived plastics and harness potential as well in specialty applications. One of the top 3 polymers in this respect is polylactic acid (PLA). Next to its renewable origin Â# sugars, abundantly encountered in food crops but also in non-edible cellulose Â# PLA is biocompatible and biodegradable. These unique features render this polymer suitable for many custom applications, for instance in medicine (prostheses, drug delivery), next to its role as a suitable replacement for certain forms of polystyrene, polypropylene and polyethylene-terephthalate, e.g. in packaging, fibers and textiles. On the downside, two major bottlenecks threaten the worldwide megaton-scale breakthrough of PLA: i) its production cost and ii) - despite its overall decent properties Â# its brittleness, over-pronounced hydrophobicity and lack of reactive side groups. This doctoral work therefore focused on i) alternative catalytic routes for synthesizing existing PLA monomers in a more straightforward way, and ii) catalytic routes to novel alpha-hydroxy acid building blocks - preferably with reactive side groups - from available sugar resources. Moreover, a preliminary evaluation of the synthesis and reactivity of novel polyesters, obtained by copolymerization of such new monomers with commercial L-lactic acid, is presented. This approach should be seen as way of creating high performance PLA based polyesters. The alternative catalytic routes to existing PLA monomers, developed within this PhD, are not enclosed due to reasons of confidentiality. After a brief introduction and scope, the doctoral manuscript presents a compilation of three peer-review-journal articles. Each chapter can be read separately, but there is a clear connection between the chapters and a logic in their appearance. Chapter 1 offers a full literature review on, and introduction to, the world of lactic acid - the basic building block of PLA. The chapter discusses the role of lactic acid as platform molecule in future bio-refineries. Emerging chemocatalytic routes to lactic acid, a three-carbon alpha-hydroxy acid (AHA), are reviewed, as well as the catalytic conversion of lactic acid to a range of useful chemicals, materials (including PLA) and fuel intermediates. A note on the racemate separation of chemically derived lactates is presented as well, in relation to the requirement for enantio-pure building blocks of PLA chemistry.,, Chapter 2 deals with the synthesis of recently discovered four-carbon alpha-hydroxy acid based esters from tetrose sugars and the section unravels the delicate mechanism of their formation. The functionalized 2-hydroxybutyrates are potential intermediates for tailored solvents and monomers for PLA resembling polyesters. We discovered the unique catalytic activity of soluble tin metal salts for synthesizing methyl vinylglycolate and methyl-4-methoxy-2-hydroxybutanoate. In situ NMR spectroscopy, deuterium labeling and control experiments with intermediates revealed the individual pathways, in which the kinetic competition between a 1,4-nucleophilic addition Â# leading to a 4-methoxy-group Â# and a 1,2-hydride shift Â# transforming glyoxal intermediates to alpha-hydroxy-carboxylates Â# is key to the product outcome. Since tetroses are a rather expensive and Â# on a large scale Â# inaccessible raw material for building block synthesis, Chapter 3 reports on the one-pot transformation of glycolaldehyde into the four-carbon AHAs catalyzed by tin halides. Glycolaldehyde, Â#the smallest sugarÂ#, is encountered in significant amounts in bio-oils, which are obtained by high temperature pyrolysis of (mainly lignocellulosic) biomass. Insight into the multitude of reactions of the complex cascade network is delivered with a focus on identifying the rate determining steps. Knowledge of the mechanism allowed for tuning the optimal Brønsted to Lewis acid ratio of the catalytic system and for unraveling the pronounced effects of the solvent on the rate and selectivity. Moreover, a first proof of concept of the great potential of the AHAs for polymer chemistry was delivered by incorporating vinyl glycolic acid via copolymerization into poly-L-lactic acid based polyesters. The vinyl side group was preserved in the polymerization and the reactive double bond proved accessible to post-synthetic modification via radical thiol-ene chemistry. The versatility of this approach to boost the performance of PLA polymers was demonstrated by grafting thiols onto the vinyls. This proved useful for rendering PLA type materials with an increased (and tunable) hydrophilicity. Moreover, the unfunctionalized copolymers possessed a lower melting point than PLLA but a higher degradation temperature. These features give this copolymer a broader temperature range for thermal processing than classic PLA.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Centre for Surface Chemistry and Catalysis

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