Lignin valorization through polymer grafting by ring-opening polymerization and its application in health, packaging, and coating

The growing interest in the development of biodegradable polymers as a substitute for non-biodegradable ones has led researchers to look for alternatives for the development of sustainable materials. In this sense, the lignin - widely produced by biorefineries and paper industries - has been shown to be relevant, playing an important role during the preparation of polymers and composites by ring-opening polymerization (ROP). This is partly due to lignin's aliphatic and phenolic reactive hydroxyls that can be used as initiator of cyclic ester monomers, combined with its antioxidant and anti-UV properties, as well as biodegradability. Understanding the role of lignin during the preparation of such environmentally friendly materials can overcome the bottleneck that still remains in this approach. In this review, we address a critical review on the polymer grafting onto reactive hydroxyls from lignin surface by ROP, and the use of such grafted lignin with biodegradable polymer composites. Moreover, we offer a deeper understanding of the role of different types of lignin during such process. We also presented why the surface modification of lignin, as well as the ratio between monomer and initiator have a huge impact on the mechanical, thermal, antioxidant and UV absorbance properties of prepared materials. Applications mainly in biomedical fields were highlighted, but packaging and coatings applications were also covered.