α-Polylysine (PLys) is a versatile, renewable, and biodegradable polymer with extensive amine functionality and water-solubility, making it an ideal candidate for environmental remediation, biomedical applications and beyond. However, conventional synthesis of linear PLys relies on toxic reagents for preparation of monomer L-lysine N-carboxyanhydride (Lys-NCA) and side-chain deprotection after polymerisation, raising environmental and safety concerns that hinder its commercial scalability and sustainability.
An environmentally friendly method for PLys production was adapted and modified in this study using thermal deprotection of fluorenylmethyloxycarbonyl (Fmoc)-protected PLys, eliminating the need for hazardous deprotection agents and offering a greener and more cost-effective alternative. Homopolymeric PLys and poly(ethylene glycol) -b-α-polylysine (PEG-b-PLys) block copolymers can be prepared using this method, showcasing their superior performance in removing Pb2+ ions from aqueous solutions.
Two types of PLys-based hydrogels, polylysine/glutaraldehyde (Glu) hydrogels (PLys/Glu Gels) and polylysine/genipin hydrogels (PLys/Gen Gels), were prepared by mixing PLys with varying mass ratios of Glu and genipin, respectively. The swelling behaviour, mechanical properties, and maximum uptake capacity of Reactive Black 5 (RB5), a water-soluble azo dye commonly used in the textile industry, were influenced by the degree of crosslinking: a lower crosslinker content resulted in enhanced RB5 adsorption and swelling but reduced mechanical performance. These findings highlight the potential of these PLys-based hydrogels as sustainable adsorbents for effective RB5 removal.
The ring-opening polymerisation of the 2,5-diketopiperazine (DKP) of Lys(cbz) was done as an alternative procedure to NCA ROP to create PLys. Lys(cbz) DKP monomer synthesis was achieved in glycerol, and polymerisation proceeded from hexylamine using an acid-base organocatalyst (MSA /DIPEA) system. Following side group deprotection to form PLys, genipin was employed as a natural crosslinking agent to yield chemical hydrogels, the mechanical properties of which could be altered by the extent of genipin crosslinking. These hydrogels were able to uptake and withhold diclofenac (DIC, a nonsteroidal anti-inflammatory drug widely used for pain relief and anti-inflammatory treatment), prior to prolonged payload release. PLys synthesis by DKP ROP offers a route to amino acid-based hydrogel materials that have potential as DIC enteric-coated tablets that enhance drug bioavailability.
Overall, the thesis demonstrates sustainable preparation and multifunctionality of PLys-based materials, ranging from synthesis methods with low toxicity, structural design and process optimisation. These applications in chapter 3-5 provide new perspectives for the advancement of PLys-based materials in environmental purification and controlled drug delivery.
