Abstract: Methods of forming lignin prepolymers are provided. In an embodiment, such a method comprises adding an acid to an ozonized reaction mixture, the ozonized reaction mixture comprising ozonized lignin having a backbone, and aromatic monomers cleaved from a lignin, under conditions to react the cleaved aromatic monomers with the backbone of the ozonized lignin to form a lignin prepolymer. The methods may further comprise using the lignin prepolymer to form a lignin resin.