*2.1. Lignin Graft Polymers and Copolymers*

The development of lignin graft polymers and copolymers is integral to the sustainability of modern civilizations and human life. However, there are critical constraints in the synthesis of lignin-based high-performance materials [40]. Most lignin-based materials have been used in the development of low-value products such as pesticides, animal feeds, surfactants, binders, dyestuff dispersants, concrete additives, and materials for dust control, which account for about 2% of the total fraction of lignin produced globally [40]; this means that there is a clear mismatch between the rates of lignin production and the development of value-added products. A feasible approach for the synthesis of value-added lignin products is the derivatives that entail the development of lignin graft polymers and copolymers.

Modern plastics are primarily sourced from petroleum byproducts, but there are valid and emerging concerns about the environmental implications, greenhouse emissions, and destruction of marine and terrestrial environments [41–43]. The growing environmental concerns have validated the need to develop sustainable methods for reducing the carbon footprint associated with plastics; this has, in turn, triggered the development of bio-based plastics for agricultural and packaging applications [44–46]. A fundamental constraint includes the shortage of suitable precursors for bio-based plastics. Chitosan and corn are

less abundant, pose a direct threat to commercial agriculture, and commercial viability is challenging [45,47], making it unfeasible to upscale operations. The highlighted challenges demonstrate the need for alternative and complementary sources of sustainable and biodegradable plastics; this informs the development of lignin graft polymers and copolymers for sustainable plastic materials. The focus on lignin in place of other forms of biomass is grounded on the natural abundance in plant and tree species [36,37]. The relative composition varies between 10 and 40 wt% [15,39], depending on the tree species, which is significant considering that about 70 million tons of pulp are generated [35]. The pulping byproducts are rich in lignin; this means that the precursor can be extracted using a scalable and low-cost method. Affordability is integral to the gradual phasing out of synthetic plastics with biodegradable materials in emerging and developed economies.

Lignin graft polymers have been synthesized using a variety of techniques, including blending natural lignin extracted from plants with commercial-off-the-shelf polymers [13] to augment the mechanical and thermal properties of the polymer blends. Copolymer blends have other desirable properties, including superior gene delivery properties, surfactant properties, UV absorber properties, super-plasticizer capabilities, and enhanced elasticity [16]. The realization of these properties is dependent on controlled polymerization and the selection of the backbone structure/polymers and the branch polymers [13,16,48]. The two are connected via covalent bonding.

The properties of the blends are predicted by a broad range of factors, including the polymerization techniques, click reactions, reversible-addition-fragmentation chain-transfer (RAFT) polymerization, and radical-mediated lignin-graft copolymerization [13,16,48]. Other feasible synthetic techniques include single-electron transfer living radical polymerization (SET-LRP), atom transfer radical polymerization (ATRP), macromolecular design via the interchange of xanthates (MADIX), and single-electron transfer-degenerative transfer living radical polymerization (SET-DTLRP) [49]. Even though there are multiple feasible copolymerization techniques, ATRP and RAFT are widely used.

The attainment of the desired properties during polymerization is contingent on graft density, the length of the grafts, and the functional groups on the graft polymers. The most commonly used grafting techniques (Figure 1) include grafting-through, grafting into, and grafting from [16]. The graft-from technique is characterized by the grafting of the polymers from active sites at the backbone polymer (lignin). *Polymers* **2021**, *13*, x FOR PEER REVIEW 10 of 33

> The grafted polymers emerge from the backbone as a result of radical polymerization, ring-open polymerization, RAFT, and atom transfer radical polymerization (ATRP) [16,48] (see Figure 2). Each polymerization technique has its benefits and constraints. For

> alyst. In contrast, RAFT polymerization is augmented by radicalization from the addition of a radical-generating species [13,16], while ROP is characterized by the opening of a cyclic molecule in the polymerization process; this results in the formation of a reactive species that undergo a chemical reaction with the next cyclic molecule [13]. The choice of either copolymerization process is predicted by a broad range of factors, including cost, the quality of the desired product. For example, Liu et al. [16] argue that RAFT is an ideal technique for newly made monomers, and ATRP is appropriate for commercially available acrylate monomers. In the former case, the lignin backbone is modified through the incorporation of an ester linkage to create a RAFT agent moiety; this procedure is subse-

> The choice between RAFT and ATRP entails a tradeoff of material properties and the synthesis process. Lai et al. [50] suggest that RAFT is highly suitable for commercial applications relative to ATRP and other synthetic techniques [50] due to the incorporation of acrylic derivatives and organic substances. The bio-applications of RAFT polymerization documented by Boyer et al. [49] corroborate the assessment of Lai et al. [50] of the key benefits associated with RAFT. For instance, RAFT features most of the desirable aspects of traditional free radical polymerization, namely facile reaction conditions, tolerance of most functionalities, site-specific functionality, control of molecular weight distribution and molecular weight, and compatibility with a wide array of monomers. In contrast to traditional living radical polymerization (LRP), RAFT features site-specific functionality

> The RAFT polymerization technique depends on the ability of RAFT agents to transport an ideal leaving group bonded to an S-atom to the reaction site; it is often challenging to attain such reaction specificity. In view of these constraints, the RAFT polymerization has low yields. There are also concerns about the synthesis of toxic compounds, namely Grignard reagents [50]. The inadequacy of RAFT and ATRP techniques demonstrates that new methods are required to meet the need for the synthesis of next-genera-

**Figure 1.** (**a**) An illustration of the graft-from synthesis of lignin copolymers; (**b**) a demonstration of graft-onto methods; (**c**) industrial lignin products; (**d**) sinapyl alcohol, coniferyl alcohol, and p-coumaryl alcohol units [16]. **Figure 1.** (**a**) An illustration of the graft-from synthesis of lignin copolymers; (**b**) a demonstration of graft-onto methods; (**c**) industrial lignin products; (**d**) sinapyl alcohol, coniferyl alcohol, and p-coumaryl alcohol units [16].

quently followed by the polymerization of the vinyl monomers.

[50]. The commercial significance is offset by synthetic requirements.

tion lignin graft polymers and copolymers.

The grafted polymers emerge from the backbone as a result of radical polymerization, ring-open polymerization, RAFT, and atom transfer radical polymerization (ATRP) [16,48] (see Figure 2). Each polymerization technique has its benefits and constraints. For example, ATRP relies on an alkyl halide initiator, which is then radicalized, resulting in the polymerization of monomers [13]; this process is augmented by a transition metal catalyst. In contrast, RAFT polymerization is augmented by radicalization from the addition of a radical-generating species [13,16], while ROP is characterized by the opening of a cyclic molecule in the polymerization process; this results in the formation of a reactive species that undergo a chemical reaction with the next cyclic molecule [13]. The choice of either copolymerization process is predicted by a broad range of factors, including cost, the quality of the desired product. For example, Liu et al. [16] argue that RAFT is an ideal technique for newly made monomers, and ATRP is appropriate for commercially available acrylate monomers. In the former case, the lignin backbone is modified through the incorporation of an ester linkage to create a RAFT agent moiety; this procedure is subsequently followed by the polymerization of the vinyl monomers. *Polymers* **2021**, *13*, x FOR PEER REVIEW 11 of 33

**Figure 2.** RAFT polymerization demonstrating: (**a**) acrylic acid and acrylamide graft-from polymerization; (**b**) RAFT polymerization of acrylamide; (**c**) soybean oil polymerization using the graft-from technique. The lignin compound is comprised of both phenolic hydroxyl and aliphatic hydroxyl groups [16]. **Figure 2.** RAFT polymerization demonstrating: (**a**) acrylic acid and acrylamide graft-from polymerization; (**b**) RAFT polymerization of acrylamide; (**c**) soybean oil polymerization using the graft-from technique. The lignin compound is comprised of both phenolic hydroxyl and aliphatic hydroxyl groups [16].

*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 The choice between RAFT and ATRP entails a tradeoff of material properties and the synthesis process. Lai et al. [50] suggest that RAFT is highly suitable for commercial applications relative to ATRP and other synthetic techniques [50] due to the incorporation of acrylic derivatives and organic substances. The bio-applications of RAFT polymerization documented by Boyer et al. [49] corroborate the assessment of Lai et al. [50] of the key benefits associated with RAFT. For instance, RAFT features most of the desirable aspects of traditional free radical polymerization, namely facile reaction conditions, tolerance of most functionalities, site-specific functionality, control of molecular weight distribution and molecular weight, and compatibility with a wide array of monomers. In contrast to traditional living radical polymerization (LRP), RAFT features site-specific functionality [50]. The commercial significance is offset by synthetic requirements.

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-The RAFT polymerization technique depends on the ability of RAFT agents to transport an ideal leaving group bonded to an S-atom to the reaction site; it is often challenging to attain such reaction specificity. In view of these constraints, the RAFT polymerization has low yields. There are also concerns about the synthesis of toxic compounds, namely

based polyurethane materials appropriate for high-strength applications.

Grignard reagents [50]. The inadequacy of RAFT and ATRP techniques demonstrates that new methods are required to meet the need for the synthesis of next-generation lignin graft polymers and copolymers.
