Synthesis and Characterization of Lignin-graft-poly(ethylene brassylate): a Biomass-Based Polyester with High Mechanical Properties

Lignin is an important biopolymer that can be used as a raw material to produce functional polymers due to its abundance, low price, sustainability, and high concentration of aromaticity. However, lignin modification is not well understood or characterized, limiting its potential as a new sustainable raw material. The biopolymer lignin can be integrated with aliphatic polyesters to produce new biomass-based biodegradable polymers. Among various aliphatic polyesters, poly(ethylene brassylate) (PEB) is a relatively unexplored polymer with many advantages. PEB is low in cost and made from castor oil, making it a sustainable resource. In this work, we have synthesized a new lignin-containing copolymer, lignin-graft-PEB. The biopolymer lignin was chemically modified by sebacic acid to introduce a carboxylic acid functionality. Lignin's abundant hydroxyl groups were used for its modification. Another precursor of the copolymer, PEB, was prepared by ring-opening polymerization of ethylene brassylate in the presence of the catalyst 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). The condensation copolymerization of the modified lignin and PEB occurred by reactions between the TBD terminus of the PEB and the carboxylic acid of the modified lignin. A decent melting temperature (78 °C) of the new polymer enables thermal processing. The mechanical properties of the new lignin-graft-PEB can be conveniently controlled by changing the mass ratio of lignin and PEB and the molecular weight of PEB. The highest modulus of lignin-graft-PEB is 471.99 MPa, which is 3-fold higher than that of homoPEB.