Chung Research Group Sustainable and Biomedical Polymers
Lignin-based Sustainable Polymer Synthesis
Lignin is the second most abundant biopolymer derived from plants, and it is a major byproduct from pulp and paper production. However, lignin’s utility is limited because of its complex chemical structure, low reactivity, poor processability, and variability depending on source and extraction method. Therefore, new methods of lignin modification are required to use lignin in commodity materials. Unlike most other blending-based lignin materials, the Chung group uses clearly defined chemical covalent bonds to synthesize new lignin-based polymers focusing synthetic organic and polymer chemistry. The precisely characterized lignin modification method leads to excellent reproducibility, reliability, and durability which are difficult to achieve in a blending approach.
We developed a new low energy and environmentally friendly lignin modification method using visible blue light: photo-redox thiol-ene reaction (Figure 4). The lignin was modified to possess alkenes. Then the catalyst, Ru(bpy)3Cl2, was used to perform high conversion of 93−97% with diverse thiol compounds including poly(ethylene glycol). The developed photoredox catalyzed thiol-ene lignin modification occurred even by natural sunlight. With an ON/OFF light control, lignin modification can be precisely controlled without special facilities and/or treatments.
​
Figure 4. A new class of natural lignin modification; photoredox thiol-ene reaction.
Lignin-based functional polymers were synthesized by the covalent linking of natural lignin and synthetic polymers. The first example is a new self-healing polymer, lignin-graft-poly(5-acetylaminopentyl acrylate) (lignin-graft-PAA), which has been prepared by the copper-catalyzed click reaction between modified lignin and PAA, which is an end-group functionalized polymer (Figure 5). The new lignin-based polymer lignin-graft-PAA showed healing properties, recovering up to 93% of its original maximum stress before fracture. The second lignin-based functional polymer is a thermally responsive shape memory polymer that possesses polycaprolactone (PCL) and natural lignin (Figure 6). Lignin’s hydroxyl groups of its multibranched phenolic structure allows lignin molecules to serve as crosslinkers, whereas the aromatic groups serve as hard segments. The modified lignin containing alkene terminals is crosslinked with a thiol-terminal PCL via Ru-catalyzed photoredox thiol-ene reaction. The flat rectangular shaped lignin-crosslinked PCL sample demonstrates rapid thermal responsive shape memory behavior at 10 oC and 80 oC showing interconversion between a permanent and temporary shape.
Figure 5. Lignin-based self-healing polymers; lignin is covalently linked to an acetylamino group containing synthetic polymer.
Figure 6. Lignin-based shape memory polymers.
​​The third lignin-based functional polymer is lignin-graft-PEGs for solid electorlytes in lithium ion battery application. The new polymer is similar to the most studied solid polymer electolytes, polystyrene-block-PEG, because natural lignin also contains a high density of aromatic groups. In the newly synthesized lignin-graft-PEG, while lignin works as a rigid domain to enhance the mechanical and thermal properties, the graft copolymerized PEG provides a flexible domain to facilitate ion transport. Lignin-graft-PEG is thus a promising candidate for a sustainable solid polymer electrolyte.
​
The Chung group also studies synthesis methods of new lignin-based biodegradable plastics that are plant-based (lignin and non-food vegetable oil) and degrade without leaving behind fragments or harmful products in ocean water. The new plastics will substantially reduce marine debris in particular microplastics. The new lignin-based biodegradable plastics is prepared by polymerization of the lignin-based monomers and the vegetable oil-based monomer. The lignin segment provides strong mechanical properties and enhanced thermal properties due to the existing aryl groups in the lignin. Another monomer, vegetable-oil based monomer synthesized by ring opening polymerization, is a soft linear segment in the final product.