The diverse possibilities for chemi

The diverse possibilities for chemical compositions provide an opportunity for the integration of novel, bio-based feedstocks into polyurethane products. The total or partial replacement of fossil-derived polymer precursors (i.e., isocyanates, polyols, and other additives) with renewable, bio-basedfeedstocksisoneroutetoimprovethesustainabilityoftheseproducts[8]. Ligninsareaclass ofaromaticpolymersthatcompriseoneofthemajorcomponentsofthecellwallsofplantsandrepresent a large potential resource for renewable polymers. Lignins derived from chemical pulping processes and biorefining processes are promising candidates for use in polyurethane synthesis (generally as a polyol) if well-established challenges can be overcome. In addition to increasing the bio-based contentofthepolyurethaneproducts,ligninincorporationintovariouspolyurethaneproductshasbeen shown in some cases to provide performance advantages that include enhanced crosslinking density, improved biodegradability, increased ultraviolet (UV) stability, antioxidant properties, and improved mechanical strength and thermal stability of the final product [9,10]. Several challenges have been identified for the replacement of conventional polyols with lignins that have limited its utility in polyurethanes with a summary of these challenges outlined in Table 1. First, the reactivity of hydroxyl groups within lignin towards an isocyanate may be restricted due to the steric hindrance as a consequence of both the higher order structure [11] and, potentially, self-association of the lignin polymer that limits access to hydroxyl groups. Furthermore, alcohols within a lignin polymer have different intrinsic reactivities with an isocyanate group depending on whether the alcohol is a primary (1◦), secondary (2◦), or phenolic (Ph) hydroxyl group. The relative reactivities of these alcohols with an isocyanate group as determined by uncatalyzed reaction rate can be ranked as 1◦ > 2◦Ph with a phenolic hydroxyl group exhibiting up to a 1000-fold lower rate of reaction relative to a primary alcohol [12].