Convenient Cross-Linking Control of Lignin-Based Polymers Influencing Structure–Property Relationships

Lignin is an abundant and low-cost biomass polymer that has a high concentration of aromaticity. Therefore, lignin can be an excellent resource for new sustainable polymers. However, poorly defined excessive reaction sites of lignin lead to thermoset polymers due to uncontrolled cross-linking at its reaction sites. In this report, we present a convenient and efficient cross-linking control method in lignin-based copolymer synthesis. The new method uses a capping agent, acetic acid, to reduce the number of excessive reaction sites, hydroxyl groups, in lignin. Acetic acid deactivates hydroxyl groups on lignin by forming acetyl, which is inert to following covalent-bond-forming reactions. The present report demonstrates an initial example of lignin-graft-poly(ethylene glycol) (PEG). Although the polymer is not new, the new cross-linking control method enables a variety of property control with high convenience. The lignin-based polymer synthesis with the new cross-linking control method has two steps: (1) natural lignin modification with succinic acid and a capping agent (acetic acid) in a single pot and (2) graft copolymerization of PEG onto the modified lignin via carbodiimide-mediated esterification. We have prepared three modified lignins with carboxylic acid terminals of 28, 85, and 95% out of the entire reaction sites available with a convenient method, controlling the amount of acetic acid. In other words, 72, 15, and 5% of the available hydroxyl groups of natural lignin have been deactivated by acetic acid. We have integrated three different molecular weights of PEG-OH, having 1k, 2k, and 3k g/mol, onto lignin, followed by thermal property studies. The thermal property tests showed a significantly low glass transition temperature (Tg) of lignin-graft-PEG with low cross-linking (i.e., a large amount of the capping agent). The Tg of lignin-graft-PEG was −27.5 °C when 72% of the capping agent was used. The Tg increased to 2.6 °C with 17% of the capping agent under fixed conditions. This control of thermal properties showed consistent trends over diverse conditions such as lignin to PEG weight ratios and molecular weights of PEG. The new lignin modification method opens a new avenue to develop lignin-based thermoplastic polymers, which are highly valued in polymer thermal processing and recycling.