*2.2. Lignin-Based Polyurethanes*

The synthesis of lignin-based thermoplastic polyurethanes has been demonstrated through copolymerization [40]. Lignin precursor is incorporated into the backbone of the polyurethane structure, but most of the products are rarely recyclable or processable due to extensive crosslinking and networking within the polymer structure [7,40,51]; this means that the polydisperse nature of lignin molecules is a critical impediment to the synthesis of thermoplastic polyurethanes. Saito et al. [40] suggested that the challenge could be offset through the incorporation of tie molecules to link the hard segments or crystals. The synthesis of recyclable thermoplastic lignin-based polyurethanes could also be augmented by bridging the soft segment (characterized by a low glass transition temperature Tg) with the hard segments containing lignin to ensure that the final product features the mechanical rigidity of lignin and the soft rubbery properties of the soft segments. The weight ratio of lignin in polyurethane has a direct impact on the material properties—a higher lignin content translated to better stress–strain behavior (see Figure 3) [40]. The higher stress and strain tolerance and other desirable mechanical properties make lignin-based polyurethane materials appropriate for high-strength applications. *Polymers* **2021**, *13*, x FOR PEER REVIEW 12 of 33

**Figure 3.** (**a**,**b**) Stress–strain behavior and storage modulus of lignin-based polyurethane thermoplastics [40]. **Figure 3.** (**a**,**b**) Stress–strain behavior and storage modulus of lignin-based polyurethane thermoplastics [40].

Saito et al. [40] reported the successful synthesis of lignin-based polyurethanes made of telechelic polybutadiene soft segments. Additionally, Da Silva et al. [51] used a kraft lignin precursor to synthesize vanillin and lignin-based polyurethanes, while Cateto et al. [7] synthesized lignin-based polyurethanes using 4,40-methylene-diphenylene diisocyanate (MDI), polycaprolactone (PCL). The successful synthesis of lignin-based polyurethanes from a wide variety of precursors demonstrates that existing synthetic routes can be modified to suit different commercial applications, with or without a catalyst. However, specific adjustments must be made in the synthesis process, including the chemical modification via etherification and esterification reactions [7]. Additionally, oxypropylated lignin materials have been proven useful relative to non-oxypropylated materials in the formation of rigid polyurethane foams. However, there are critical constraints associated with chemical modification. For example, the costs are higher after esterification and etherification; this means that the industrial production of high-value products from lignin precursors is often offset by cost constraints [7], which can be partly offset through the development of new synthetic routes [7,41,51]. The cost constraints reported by Cateto et al. [7] were also acknowledged by Da Silva et al. [51], who argued that cost was not the Saito et al. [40] reported the successful synthesis of lignin-based polyurethanes made of telechelic polybutadiene soft segments. Additionally, Da Silva et al. [51] used a kraft lignin precursor to synthesize vanillin and lignin-based polyurethanes, while Cateto et al. [7] synthesized lignin-based polyurethanes using 4,40-methylene-diphenylene diisocyanate (MDI), polycaprolactone (PCL). The successful synthesis of lignin-based polyurethanes from a wide variety of precursors demonstrates that existing synthetic routes can be modified to suit different commercial applications, with or without a catalyst. However, specific adjustments must be made in the synthesis process, including the chemical modification via etherification and esterification reactions [7]. Additionally, oxypropylated lignin materials have been proven useful relative to non-oxypropylated materials in the formation of rigid polyurethane foams. However, there are critical constraints associated with chemical modification. For example, the costs are higher after esterification and etherification; this means that the industrial production of high-value products from lignin precursors is often offset by cost constraints [7], which can be partly offset through the development of new synthetic routes [7,41,51]. The cost constraints reported by Cateto et al. [7] were also acknowledged by Da Silva et al. [51], who argued that cost was not the only constraint.

Other issues relating to the food-versus-fuel dilemma and technical know-how must

The physicochemical properties and kinetic properties affirm the suitability of lignin antioxidants in a broad range of engineering and non-engineering applications. However, the formation of the products is dependent on a broad range of factors, including the yield from dopamine polymerization and the polydopamine and copolymers. The antioxidant properties of lignin-based polyurethanes are linked to the presence of an aromatic ring with hydroxyl and methoxy functional groups [50,51]. The functional groups are integral to the oxidation propagation reaction, which is inhibited by hydrogen donation [51]. According to the literature, the performance of lignin polymers was comparable to 2,2-diphenyl-1-picrylhydrazyl (DPPH), and butylated hydroxyl-toluene, among other commercial

dustrial alternatives based on petrochemical resources" (p. 1273). The observations made by Cateto et al. [7], Da Silva et al. [51], and Saito et al. [40] illustrate that for lignocellulose feedstock (LCF), bio-refineries are preferred from a sustainability perspective. However, the future development of bio-based products would be catalyzed by conversion products, which offer competitive benefits relative to petrochemical products. This observation

is supported by the chemical properties of lignin materials depicted in Table 5.

only constraint.

antioxidant polymers [50].

Other issues relating to the food-versus-fuel dilemma and technical know-how must be resolved before the "novel systems become more competitive against the current industrial alternatives based on petrochemical resources" (p. 1273). The observations made by Cateto et al. [7], Da Silva et al. [51], and Saito et al. [40] illustrate that for lignocellulose feedstock (LCF), bio-refineries are preferred from a sustainability perspective. However, the future development of bio-based products would be catalyzed by conversion products, which offer competitive benefits relative to petrochemical products. This observation is supported by the chemical properties of lignin materials depicted in Table 5.


**Table 5.** Second-order kinetics of lignin-based materials [7].

The physicochemical properties and kinetic properties affirm the suitability of lignin antioxidants in a broad range of engineering and non-engineering applications. However, the formation of the products is dependent on a broad range of factors, including the yield from dopamine polymerization and the polydopamine and copolymers. The antioxidant properties of lignin-based polyurethanes are linked to the presence of an aromatic ring with hydroxyl and methoxy functional groups [50,51]. The functional groups are integral to the oxidation propagation reaction, which is inhibited by hydrogen donation [51]. According to the literature, the performance of lignin polymers was comparable to 2,2-diphenyl-1-picrylhydrazyl (DPPH), and butylated hydroxyl-toluene, among other commercial antioxidant polymers [50].
