Biobased Polyamides: A Journey from the Biomass towards Cutting Edge Materials

1. Introduction

Polyamides (PAs), as the name suggests, constitute a class of polymer characterized by the presence of amide bonds between repeating units [1,2,3]. This structural feature enables the formation of hydrogen bonds between adjacent polymeric chains [1,4,5], thus, conferring to these materials outstanding mechanical properties as well as high chemical resistance [6,7,8,9]. For that reason, since the development of the aliphatic PA Nylon 6,6 and Nylon 6 [2,6,10], first synthetized by Carothers at DuPont and Paul T. Schlack, respectively in the 1930’s [11], these materials have found widespread use across a diverse range of applications. These include consumer products as well as high-performance smart materials in sectors such as electronic, textile, automotive or medicine [12,13,14]. However, PAs also present notable drawbacks. For instance, their tendency to absorb water considerably limits its dimensional stability [15]. In fact, the absorbed water acts as a plasticizer and, consequently, worsens both mechanical and thermal properties. On the other hand, the same strong intermolecular interactions that provide previously mentioned desirable attributes, such as high strength and chemical resistance, also lead to very high melting points, usually inaccessible, elevated transition temperatures and poor solubility in common organic solvents. These characteristics make the processing of certain PAs, particularly aromatic ones, extremely challenging and economically costly, thereby limiting their high scale production [3]. Consequently, over the last 20 years, many semi-aromatic PAs have been developed to combine the excellent thermal, mechanical, and chemical properties of aromatic PAs with the easier processing of aliphatic ones [4,15].

There are two common approaches for synthesizing PAs (Scheme 1): The first method is the step-growth polycondensation employing diamines and dicarboxylic acids, diesters or diacyl chlorides [16,17]. Alternatively, ω-amino acids or esters can be used as the only substrates in this polymerization. The second methodology, commonly employed to prepare PAs, is the ring-opening polymerization (ROP) of lactams [1,3,18]. For instance, ԑ-caprolactam is the most well-known lactam for making Nylon 6 through AROP (anionic ring-opening polymerization) [16].

In any case, the starting substrates used in both synthetic methodologies have traditionally been obtained from petrochemical industry [19]. However, the limited availability and the volatility of the prices of fossil resources are the main reasons that have encouraged industry and academia to look for other alternatives [3,20]. In fact, in recent years, real efforts have been made to develop polymers from more renewable feedstocks that maintain the excellent properties of their petro-based counterparts while minimizing the environmental impact [21,22,23]. Moreover, the incorporation of renewable feedstocks in polymer synthesis significantly enhances the biodegradability of the resulting materials, thereby mitigating their long-term persistence in the environment and reducing their contribution to environmental pollution [24]. Additionally, these materials often demonstrate a reduced carbon footprint over their entire life cycle, as the biomass used in their production sequesters atmospheric CO₂ during growth. This carbon uptake partially compensates for the greenhouse gas emissions associated with subsequent stages. such as manufacturing, processing, and transportation [8,25,26].

This review aims to provide a comprehensive overview of the path from the first steps in the synthesis of PAs that presented mediocre properties very far from the fossil-derived analogues to the current state in which promising innovative materials have been achieved with excellent properties, high proportions of biogenic carbon and great (bio)degradability.

2. Synthesis of PAs from Lignin-Derived Monomers

Lignin is the largest non-carbohydrate component of lignocellulosic biomass. In fact, after cellulose, this biopolymer constitutes the second most abundant renewable polymer. The oxidative coupling of three aromatic monomers, p-coumaryl, coniferyl, and sinapyl alcohols in different proportions depending on the source, origins the complex and amorphous highly branched network characteristic of lignin (Figure 1) [27]. Recently, lignin and its derivatives have emerged as potential components for diverse polymer applications, such as flame retardants [28], antimicrobial agents [29], stabilizing agents [30], coatings [31], and superabsorbent hydrogels [32,33], owing to the high carbon content, good thermal stability, biodegradability, and antioxidant/antibacterial activity [34].

Despite extensive research into controlled depolymerization of lignin, the efficient and economically viable production of aromatic compounds from this abundant biopolymer remains limited. To date, only a few aromatic compounds, such as guaiacol, eugenol, ferulic acid, vanillin and syringaldehyde, can be obtained from lignin using economical and effective methodologies (see Figure 2) [35,36]. These aromatic compounds have been employed as monomers for the synthesis of various biobased polymers, including PAs. Indeed, this review will specifically focus on the synthesis of PAs derived from the aromatic compounds, vanillin and eugenol.

2.1. Vanillin based PAs

The lignin derived aromatic compound vainillin holds significant potential as a precursor to prepare biobased monomers owing to the chemical versatility of its carbonyl and phenolic functional groups [37]. However, it is noteworthy that the polymers obtained from this type of monomer usually have relatively low molecular weights, mainly due to the low reactivity of this aromatic derivative [38].

In 2021, K. Yagura et al. reported a series of biobased PAs through polycondensation protocol using different diamines and divanillic acid (DVA) previously obtained through an enzymatic process (Scheme 2). In this case, the obtained polymers presented very high weight-average molecular weights (M_w) ranging from 50 to 110 kDa. They observed that the use of the dimeric starting compound not only enhanced the molecular weights of the polymers but also improved their solubility. In fact, the vast majority of the synthesized PAs were highly soluble in common organic solvents. These findings highlight DVAPAs as promising high-performance, bio-based plastics with superior thermal properties compared to many other renewable aromatic polymers. However, their mechanical properties still require improvement for broader applications [39].

Subsequently, these same authors synthesized fully DVA-PAs with molecular weights ranging from 49-60 kDa reacting RDVA monomer with RDVA-NH₂, a diamine monomer derived from RDVA (Scheme 3). Thermogravimetric analysis of the PAs found decomposition temperatures around 380 °C. The T_g values of fully divanillic DVAPAs ranged from 207 °C to 262 °C, depending on the side chain length. Notably, DVAPAs with shorter side chains exhibited higher T_g values than those with longer side chains [40]. The maximum tensile strength achieved with these materials was 40 MPa, which suggested that, although the thermal properties were good, further research was needed to improve the mechanical properties.

Similarly and more recently, Y. Zhao et al. described the synthesis of a vanillin-based poly(amide-imide) vitrimer. For this purpose, a NH₂-terminated PA was prepared through an acylation process. Subsequently, this prepolymer was reacted with previously synthetized DVA monomer (Scheme 4). This vanillin-based polymer presented excellent stability, and the amide and imine dynamic bonds conferred itself healing performance showing similar mechanic and thermal properties before and after the repair. In addition to the partially biobased origin, the authors also demonstrated the recyclability of the material, making it an even more attractive option for environmentally conscious applications [41].

2.2. Eugenol Based PAs

Another promising building block that can be obtained through lignin depolymerization is eugenol, although, currently, it is predominantly extracted from clove oil. This natural compound possesses suitable and reactive functional groups for chain growth polymerization, including allylic double bond, as well as additional reactive sites, such as the aromatic ring, hydroxyl and methoxy groups [42,43].

Although different polymers have been synthetized from this natural compound to the best of our knowledge, to date, the work recently published by our research group constitutes the only one related to the synthesis of PAs derived from this natural phenolic compound. In this paper, the synthesis of a wide series of semiaromatic sulfur containing PAs was presented. All the prepared materials resulted to be amorphous showing excellent thermal properties and dimensional stability. In addition to the bio-based nature of the materials, other aspects such as the solvent-free process and the employment of an ecofriendly catalyst are also noteworthy from an environmental point of view (Scheme 5) [44].

3. Furan Based PAs

In 2004, the US Department of Energy published the Platform Chemicals List, which was later updated in 2010. This is a list of organic compounds containing from 2 to 6 carbon atoms that can be obtained by catalytic or biocatalytic processes from the lignocellulosic part of biomass [45]. It is hoped that these biobased building blocks will provide an opportunity to replace fossil sources for obtaining energy and chemical products. Two furanic compounds, 5-hydroxymethylfurfural (HMF) and 2,5-furandicarboxylic acid (FDCA) (see Figure 3) were added to the revised list. In fact, it is believed that these heterocyclic aromatic compounds are the ones with the greatest synthetic potential. Moreover, the scientific community has accepted these derivatives as key elements in bridging the gap between a fossil fuel-based economy and a sustainable one, due to the similar or better properties of the biobased materials, showing similar thermal properties and crystal structures than petroleum-based polymers [46].

3.1. Furan Containing Aliphatic PAs

The use of FDCA as a monomer in the synthesis of furane-aliphatic PAs is not a novel synthetic challenge. In fact, as early as 1961 Hopff and Krieger described the synthesis of a series of PAs by condensation of linear amines with different heterocyclic dicarboxylic acids including FDCA. The PAs obtained in these works, unfortunately, decomposed either during the synthesis itself or during processing at high temperatures. Therefore, at that time the authors concluded that these materials would not have improved performance compared to their TPA-derived (terephtalic derived PAs) analogues [47,48]. Probably due to this negative conclusion, it took more than 10 years for another work related to FDCA based PAs to be described. In fact, in 1974 Heertjes and Kok reported the synthesis of a series of PAs by the condensation of FDCA or, alternatively, the methyl ester (DMFDCA) or the acyl chloride (FDCACl) with C4, C6 or C8 linear aliphatic diamines [49]. For this purpose, two different experimental protocols were employed: interfacial solution and melt polymerization. The main drawback described by these authors was the decarboxylation of the FDCA, which occurred above 195 °C. Indeed, the decarboxylation of the acid was the reason that led them to use the dimethyl ester or the acyl chloride derivative. In any case, both the yields of the polymerizations and the molecular weights of the obtained polymers were considerably better than those previously described by Hopff and Krieger. Synthesized crystalline PAs could be used to make fibers and films and had melting temperature (T_m) between 250 °C and 125 °C. However, it should be noted that these materials still had a long way to go before they could be considered analogous in performance to TPA derivatives. Again, it took a long period of time, until 2009, when O. Grosshardt et al. described the synthesis of PAs by condensation of FDCA and a series of linear aliphatic diamines through a melt polymerization protocol using Sn or Ti derivatives as catalysts. The obtained PAs, in this case all amorphous, presented complete decomposition temperatures between 350 °C and 450 °C and T_g between 70 °C and 100 °C [50].

Subsequently, in 2014 Gruter et al., in two successive patents, described that when DMFDCA dimethyl ester was used as starting product and subjected to high temperatures for polycondensation, extensive N-methylation of the obtained PA occurred. This undesirable secondary reaction dramatically worsened the thermal and mechanical properties of the materials. To avoid it, a two-step protocol was proposed based on a first stage, with a very strict temperature control, in which the DMFC was condensed with an excess of the corresponding diamine. In the second stage, the previously obtained mixture of oligomers was treated with a bifunctional linker to grow the chain [51,52].

In 2015, Y. Jiang et al. described the first enzymatic synthesis of a furane-based PA employing DMFDCA and 1,8-octanodiamine as starting building blocks. Novozym 435, an immobilized form of Candida antarctica Lipase B (CALB) on acrylic resin, was employed as biocatalyst. The choice of the diester as monomer was based on the higher solubility and lower T_m compared to the diacid. Regarding the amine, the better catalytic activity and the higher selectivity of lipases toward long-chain aliphatic amines was taken into account. The obtained molecular weights, up to 54 kDa, were much higher than those previously obtained through melt polymerization, with the great advantage that in the enzymatic alternative high temperatures were not required. The so-obtained PA8F resulted in a promising sustainable equivalent to terephthalic acid derived PA8T with similar T_g and thermal stability but with lower T_m. It is true that this work presented a challenge to improve. In fact, toluene and diphenyl ether were used as solvents that cannot be considered green alternatives. However, the high T_m of the starting diester derivative and the tendency of diamine to sublimate made the solvent-free process unfeasible [53]. Subsequently, these same authors described the reversible cross-linking of these materials through temperature regulated Diels-Alder reactions between the furanic rings and 1,1′-(methylenedi-4,1-phenylene)bismaleimide. Alternatively, to this reversible crosslinking, aromatization of the materials was shown to provide perpetual crosslinking (see Scheme 6) [54].

In 2017, M. Cao et al. described the synthesis of a new semiaromatic PA condensating terephthalic acid and FDCA with 1,10-diaminodecane through an existing industrialization process based on a first prepolimerization step followed by a solid-state polymerization (SSP) phase. The last step was on line controlled by a TG-IR equipment, monitoring the FDCA decarboxylation reaction. The exhaustive analysis of the structure of the obtained PA revealed that, probably due to the aforementioned decarboxylation process, the ratio of the furanic moieties incorporated into the polymeric chain was considerably lower than the feeding ratio of the monomers. The obtained copolymeric PA, PA10T/10F, presented a melting point higher than 280 °C and, to the delight of the authors, a stability comparable to the PA10T homopolymer. The authors attributed this excellent stability to the fact that not all the furanic rings incorporated in the polymeric chain had carboxyl groups, the cause of FDCA instability [55].

In the same year, a comprehensive comparative study was carried out by L. Cureton et al. In this work, a wide variety of PAs were synthetized using an interfacial polymerization protocol. For this purpose, the acyl chloride derivative of FDCA was condensed with structurally diverse aliphatic and aromatic diamines (Scheme 7). In addition, with comparative intentions, different aliphatic nylons and semiaromatic PAs were prepared through the same methodology. After properly characterizing the obtained materials by means of GPC, NMR and IR, some of their properties were compared in order to establish structure-property relationships. The first difference between furane based PA and those lacking this heterocyclic ring was the solubility. In fact, all furane derived PA were soluble in polar aprotic solvents such as DMF, DMP or THF, while all nylons or semiaromatic PA were completely insoluble in these solvents. This improved solubility of the biobased PA can facilitate their subsequent processing and expand their range of applications. Analogously, notable differences were detected in the thermal behavior of the polymers. The two fully aliphatic PAs synthesized in this work (nylon 4,6 and nylon 6,6) turned out to be crystalline polymers with T_m of 262 °C and 225 °C, respectively [56], which is completely consistent with the values previously reported in the literature and with computational predictions [57]. In contrast, the two semi-aromatic PAs exhibited very different thermal behavior depending on the aliphatic chain length. Thus, poly(hexamethylene terephthalamide) showed a very significant melting peak around 270 °C although the degree of crystallinity was lower than in the fully aliphatic PAs. In poly(butylene terephthalamide), however, no crystalline regions were detected. Therefore, it was concluded that the benzene aromatic rings disrupt the crystal packings that just remain with considerable aliphatic chain lengths. In contrast, all biobased furan PAs were found to be completely amorphous regardless of the aliphatic chain length. That is, furan moiety hinders the formation of crystalline areas more effectively than the benzene ring, probably due to the kinked architecture of the furane based polymeric chains that weakens and even prevents hydrogen bonds between amide groups [56]. In fact, Karam and colleagues demonstrated through computational analysis that the double bent conformation, in which both dihedral angles are 0° (see Figure 4), is the most stable. In this conformation, hydrogen bonds between the amidic hydrogen and the furanic oxygen are greatly favored, and, therefore, hydrogen bonds between the amide functions of different chains are minimized. Furthermore, this rigidity, which limits the degrees of freedom of the chains, was also reflected in the T_gs. Thus, furanic PAs presented the highest T_gs compared to aliphatic and poly terephthalamides. As expected, due to the major flexibility of the aliphatic segment, the longer the aliphatic chain, the lower the T_g. This trend was maintained in all three types of PAs [58].

More recently, M. Kamran et al. described the synthesis of a series of PAs employing DMFDC and aliphatic amines with different chain lengths. In this case, an environmentally benign protocol was employed based on a melt polycondensation technique and using very low concentrations of titanium isopropoxide (TIPT) as catalyst (see Scheme 8). The relatively high molecular weights of the synthesized PAs (M_n = 8–11 kDa), the amorphous character of most of them and the thermal stability comparable to the fossil-based analogues are the most relevant results of this work. It is worth mentioning that, in addition to the expected amines and ester, other end groups were detected by means of MALDI due to diverse secondary reactions promoted by the high temperatures necessary for the polymerization [59].

In the same year, S. Xie et al. described the polycondensation between the diester derivative DMFDC and 1,10-decanediamine obtained from castor oil under different reaction conditions. The main difference with the previous work was that, in this case, the bulk polymerization was catalyzed by TBD, a versatile superbasic and no toxic organocatalyst (Scheme 8) [60]. Furthermore, this paper, together with a later one reported by the same authors, constitute, as far as we know, the only studies of the mechanical properties of this type of semi-aromatic PAs. In fact, for PA10F these authors described a tensile strength of 52.12 MPa, a tensile modulus of 1636 MPa and an elongation at break around 69% [61]. Subsequently, they carried out an exhaustive study on the influence of the aliphatic chain length of the diamine monomer on the thermal and mechanical properties of the prepared polymers [62].

The same diester was employed as monomer by Y. Feng et al. in 2024. In this case, an elegant and ecofriendly continuous flow technology was used to carry out the polycondensation between the furane-based diester monomer and a series of linear aliphatic diamines. The mild conditions employed prevented secondary reactions such as N-methylation. Moreover, in addition to the synthesis and the characterization of the PAs these authors described the chemical depolymerization of the materials to obtain the monomeric precursors, which is extremely important from an environmental point of view. In fact, chemical degradation opens the way to chemical recycling, a better alternative than the commonly carried out mechanical recycling as the raw monomers obtained can be reused to synthesize virgin polymers again [63].

In this point, it is important to mention the pioneering work recently published by J. Feng et al., which represents an important step towards more sustainable protocols (Scheme 8). These authors proposed the replacement of metallic or superbasic derivatives with liquid ions as catalysts. In fact, PA4F was efficiently synthetized in a two-step methodology. In the first stage, the prepolymerization took place employing different ionic liquids, all of them based on 1-butyl-3-methylimidazolium (Bmim) as catalyst and water, the greenest alternative, as solvent. In the second step, the melt polycondensation of the oligomers was carried out. The so obtained PAs showed satisfactory M_n (up to 27 kDa), elevated T_g (around 150 °C) and excellent elastic modulus (3.7 GPa) [64].

It has already been mentioned in this section that, due to the structure of the furan ring, hydrogen bonding between neighboring chains are hindered in favor of the corresponding intramolecular interactions. This fact influences the crystallinity of the materials and, consequently, their thermal and mechanical properties. Recently, Zhan et al. synthesized a series of PAs by melt polycondensation of DMFCA with long-chain aliphatic diamines containing oxalamide groups. Their strategy was to minimize intramolecular interactions by forming double hydrogen bonds between the oxalamide moieties of adjacent chains. Indeed, the crystallinity of the obtained materials increased and their properties were improved. Furthermore, the excellent chemical degradability of these PAs was demonstrated. In fact, when they were subjected to basic hydrolysis conditions, the starting monomers were recovered with excellent conversions (Scheme 9) [65].

3.2. Furan Containing Aromatic PAs

To the best of our knowledge, the first report regarding the synthesis of furan-aromatic PAs dates back to 1964. In this pioneering work, the synthesis of a series of PAs from FDCACl and various aromatic diamines was described. Unfortunately, the characterization of the materials was not very detailed. In fact, only the melting points, degradation temperatures, and inherent viscosities of some of them were given [66].

Later, Mitiakoudis and coworkers described the synthesis of an extensive series of furane-aromatic and all furane PAs. In this case, not only FDCA and the corresponding chloride, but also the 3,4-disubstituted regioisomers were used. After exhaustive optimization of the polymerization conditions in terms of yields and molecular weights of the obtained materials, these were characterized and some of their properties were compared. One of the main conclusions obtained by these authors was that furane-aromatic PAs presented, in general, better properties than wholly furane PAs [67,68].

Luo and coworkers described the direct polycondensation between FDCA and a series of aromatic diamines (Scheme 10). The so obtained partial biobased materials were properly characterized by means of FT-IR, NMR and GPC. In addition, mechanical properties of the prepared polymers were also analyzed and the influence of the structure of the diamine monomer on them was studied. Therefore, all the synthetized PAs presented adequate mechanical properties, highlighting the results obtained with the PA synthesized with para-phenylenediamine, presumably due to the rigidity of the polymeric skeleton and the high molecular weight. On the other hand, all of the PAs prepared in this work were completely soluble in common organic solvents, which can greatly facilitate processing compared with insoluble fossil-derived aromatic counterparts. However, furane derived PA prepared in this work failed to match the excellent thermal properties of traditional aromatic PA [69].

In 2020, Yu and coworkers reported the synthesis of an extensive series of furane containing full aromatic PAs through melt polycondensation reaction at low temperatures. The employed substrates were 2,5-furandicarbonyl dichloride (FDCACl), m-phthaloyl chloride (MPC), and terephthaloylchloride (TCL) and m-phenylenediamine (MPD), p-phenylenediamine (PPD), as diamine monomers (Scheme 11) and different combinations between them were performed obtaining eight furane containing PAs. An exhaustive characterization of all materials was accomplished by means of NMR, XRD, DSC and TGA. In addition, the solubility of the obtained polymers in different organic solvents was tested. The principal conclusion found in this work was that the furane containing materials presented similar thermal behavior and thermal stability to those of the traditional aromatic PAs. Regarding the solubility of these materials, these authors drawn the same conclusion previously described by Luo et al., in fact, these biobased PA presented the great advantage of being soluble in some organic solvents, which really facilitates their processing [70].

The same methodology was used few months later by Cao and coworkers, thus, a polycondensation at low temperature between FDCACl with the biobased 3,4’-diaminophenylether (Scheme 12). The so obtained polymer showed excellent solubility and spinnability, and a fiber with a smooth surface and uniform thickness was fabricated which, in addition, exhibited outstanding thermal and mechanical properties and flame-retardant quality [71].

More recently, the 4,4’-regioisomer of the diaminophenyl ether was subjected to the polycondensation reaction with FDCACl (Scheme 13). The so obtained biobased PA was properly characterized and compared with the isophthalic derived PA which presents a fossil origin. The furane containing polymer displayed analogous or even better thermal, mechanical and barrier properties. Moreover, the hydrolysis of the polymeric materials was performed under basic conditions and the starting monomers were recovered. Molecular dynamic simulations revealed that the presence of the heterocyclic ring lowered the energy barrier of the hydrolysis process [72]. Therefore, the furan ring not only increases the biogenic content of the materials but could also facilitate their chemical recycling, which is extremely important from an environmental perspective.

3.3. Multifuran Monomers

In addition to FDCA and its immediate derivatives like the dimethyl ester and diacyl chloride, other monomers have been developed that incorporate the furan ring into the polymer backbone. In fact, encouraged by the promising properties that the furan heterocycle confers on materials, different polymers have been prepared using monomers with two furan rings connected by different linkers. In this sense, Gharbi et al. synthetized a series of PAs containing a bifuranic structure condensating 2,2’-bis [2-(5-chloroformyl)furfuryl]propane with a number of structurally diverse aliphatic, cycloapliphatic or benzylic diamines (Scheme 14). A comprehensive study on the influence of the diamine structure on both, the process yields and the thermal properties of the obtained material, was conducted in this contribution [73].

Later, this protocol was extrapolated to aromatic ones with the principal objective of improving the properties of the materials due to the stiffness provided by the aromatic rings [74].

Interestingly, this type of monomers allows the incorporation of different functional groups into the spacer that joins the two furan rings, which also allows the modulation of the polymer properties. In this sense, recently, Shu and collaborators have described the synthesis of a series of bisfuran based PA with different spacer moieties employing furfuryl amine as starting material (Scheme 15). Moreover, in addition to studying the influence of the spacer on the thermal properties, crystallinity, solubility or water absorption, the obtained PAs were subjected to different postpolymerization modification processes achieving positive and negatively charged functionalized bioPAs [75]. Furthermore, these authors have demonstrated the high capacity of these cationic PAs as polyfluroalkyl (PFA) environmental contaminants adsorbents [76].

More recently, the bifuran moiety, in which two furan rings are directly linked through the 2-positions, has received more attention. In this structure, the π conjugation is extended mainly due to the complete coplanarity of the rings and the dihedral angle between them of 180°. Moreover, the rotational barrier between the heterocycles is relatively high which confers superior rigidity to this bifuran derived building block compared to those derived from biphenyl moiety. Taking into account these structural characteristics, and the possibility of synthesizing them from biobased furfural [77], different monomers with a bifuran structure have been used in the synthesis of polymeric materials, with the hypothesis of improving the thermal and mechanical properties, and, in consequence, the performance of the obtained materials [78,79]. In this context, regarding PAs, N. Miyawaka et al. described the polycondensation of bifuran derived dicarbonylchloride (see Scheme 16) with several aliphatic diamines though solution and interfacial methodologies. In fact, as expected, the prepared PAs exhibited higher T_m compared with the analogous PAs bearing a single furan ring. Moreover, PAs containing the bifuran structure also turned out to be more thermally stable [80].

In contrast, K. Arai et al. reported the condensation of bifurfurylamine (see Scheme 17), prepared starting from biomass derived furfurylamine, with seven structurally different diacyl chlorides. Newly, in this work, the superiority of polymers carrying the bifuran moiety compared to those presenting a single furan ring was demonstrated [9].

4. Terpene Derived PAs

Terpenes constitute a group of organic natural molecules, generally secondary metabolites, that exhibit remarkable functional and structural diversity. For example, some terpenes serve as defense agents in insects [81], while others act as growth regulators [82], among various other biological roles. Regarding the chemical structure, all terpenes share the common characteristic of the repetition of C5 isoprene units [6,83] although their architectures can vary. In fact, terpenes can be acyclic, mono, di or even polycyclic. Additionally, the number of repeating isoprenes is another criterion for classifying terpenes: monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), sesterterpenes (C25), triterpenes (C30) and so on[84]. Most terpenes are easily extracted from non-edible plant parts, and some of them are extremely abundant in nature. Therefore, their abundance and structural diversity make them excellent natural economically feasible building blocks with the potential to replace fossil sources as raw materials in polymer synthesis [14].

As previously discussed, the ROP of ε-caprolactam is the conventional methodology for the synthesis of Nylon 6 [84]. Similarly, the vast majority of terpene-based PAs are synthesized by ROP from the corresponding lactams. Indeed, the most widely used methodology for obtaining the lactam monomer is the Beckmann rearrangement from the oxime, which is usually prepared from the ketone derivative. In this context, Rieger and Winnacker proposed the use of terpenoids ketones, like menthone, for the synthesis of biobased PAs. For this purpose, two alkyl substituted lactams regioisomers were prepared via Beckmann rearrangement and subsequently polymerized through a ROP protocol in both acid and neutral conditions (Scheme 18). These initial studies laid the groundwork for the investigation of terpene-derived PAs with very interesting structures carrying pendant groups and stereocenters in the backbones [85].

In fact, most terpene-based PA have been prepared following an analogous synthetic pathway. Table 1 summarizes the most representative works on the synthesis of PAs using terpenes as starting substrates, focusing on the structure of the materials obtained and the most relevant aspects. In this context, recently, Stockmann et al. demonstrated that limonene oxide is a suitable monomer for the synthesis of PAs with two stereocenters per repeating unit and a pendant functionalizable isopropylene moiety (Table 1, Entry 2) [14] . Similarly, α- and β-pinene regioisomers have been used in the synthesis of PA employing both cationic and anionic protocols in the ROP reaction (Entries 3-5). Carene, a cyclic terpene found in the essential oils of various plants, has also been employed in the synthesis of PAs through β- and ε-lactam intermediates. For instance, in 2019, Sieber and co-workers [86] achieved (+)-3-carene derived PAs with particular interest because of the molecular weight exceeding 30 kDa. Besides, the high thermal stability of the synthesized PAs, DSC analysis also noticed the absence of T_m, which indicates a highly amorphous nature, advantageous for applications in optics and electronics [87].

To finish this section, in 2024 Kleybolte and Winnacker presented simple anionic ring-opening polymerizations (AROP) of β-pinene lactam in bulk and in solution, and they made the proof of different initiators, all of them green alternatives with the aim of using these polymers in biomedical applications. Thereby, good mechanical, thermal and transparent appearance renders them promising high-performance biomaterials [88].

Table 1. Synthesis of terpene-based PAs through ROP reaction.

Entry	Substrate	ROP	Properties of PAs	Structure	Ref.
1	Limonene oxide	Anionic	▪ Amorphous ▪ Tg= 120 °C		[14]
2	β-pinene	Cationic	▪ Tm= 400 °C ▪ Tg= 150 °C		[89]
3	β-pinene	Anionic	▪ Tm= 308–322 °C ▪ Tg= 150 °C		[87]
4	α-pinene	Anionic	▪ Amorphous ▪ Tg= 120 °C		[86]
			▪
5	β-pinene	Anionic	▪ Mn=28.9 kDa in bulk and 9.4 kDa in solution. ▪ Td= >440°C ▪ Tg= >195 °C		[88]
6	(+)-3-carene	Anionic	▪ Mn= 30 kDa ▪ Amorphous		[86]

Table 1. Synthesis of terpene-based PAs through ROP reaction.

Entry	Substrate	ROP	Properties of PAs	Structure	Ref.
1	Limonene oxide	Anionic	▪ Amorphous ▪ Tg= 120 °C		[14]
2	β-pinene	Cationic	▪ Tm= 400 °C ▪ Tg= 150 °C		[89]
3	β-pinene	Anionic	▪ Tm= 308–322 °C ▪ Tg= 150 °C		[87]
4	α-pinene	Anionic	▪ Amorphous ▪ Tg= 120 °C		[86]
			▪
5	β-pinene	Anionic	▪ Mn=28.9 kDa in bulk and 9.4 kDa in solution. ▪ Td= >440°C ▪ Tg= >195 °C		[88]
6	(+)-3-carene	Anionic	▪ Mn= 30 kDa ▪ Amorphous		[86]

As it has been mentioned, ROP is by far the most widely used methodology in the synthesis of PA from terpene or terpenoid substrates. However, a few examples of access to terpene-based PA via polycondensation processes can also be found in the literature. In fact, the presence of two double bonds in the limonene structure allows easy access from this cyclic monoterpene to difunctionalized monomers through epoxidation or hydration reactions, among others [90]. One of the pioneering authors in this context, was Trumbo in 1988, who employed 1,8-diamino-p-menthane (MDA), a readily available limonene derivative, to synthetize PAs via interfacial condensation with various diacid chlorides [91]. This study presented MDA as a renewable alternative to petroleum-based diamines in polymer chemistry. Although the polymers obtained in this pioneering work did not present satisfactory molecular weights, it laid the foundation for subsequent research in this area.

Some years later, in 2013, Firdaus and Meyer carried out a similar work using this terpene in the synthesis of various polymers including PAs (Scheme 19). The reactivity of the double bonds presented in the racemic limonene and the successful thiol-ene click reaction was an elegant combination for the access to difunctional monomers. In fact, the obtained enantiomerically pure limonene derived diamine monomers were reacted with various biobased diesters employing TBD as a catalyst, obtaining renewable PAs with molecular weights up to 12 kDa. Additionally, they observed that the presence of cycloaliphatic bulky groups prevented intermolecular interaction, leading to amorphous PAs with T_g values around 40 °C. The study revealed that the chirality of the employed monomers did not affect the thermal properties of the PAs [92]^.

5. Fatty Acid Derived PAs

Oils and fats of vegetable and animal origin have historically been, and continue to be, among the most important renewable feedstocks for the chemical industry. Among fatty acids, ricinoleic acid (from castor oil) [93,94], linoleic acid and oleic acid (from sunflower oil) and palmitic acid (from palm oil) are particularly notable as raw materials for chemical production. Fatty acids possess long aliphatic chains and functional carboxylic acid groups, making them excellent candidates for the development of bio-based monomers. Their structural versatility allows for a wide range of chemical modifications, enabling the synthesis of diamines, diacids, and other intermediates suitable for polycondensation reactions [95]. In particular, 10-undecenoic acid, oleic acid, and erucic acid stand out as ideal precursors for step-growth monomer synthesis for PA due to their bifunctional nature and high availability (Scheme 20) [93]. Thus, these three fatty acids can be regarded as key starting materials for sustainable polymer chemistry and will be the primary focus among fatty acids in this review.

An example of the use of undec-10-enoic acid derivatives was reported by J. F. Carpentier and coworkers. In this study, undec-10-enenitrile was hydroformylated using the catalyst (dicarbonyl)rhodiumacetoacetonate-biphephos [Rh(acac)(CO)₂-biphephos] to selectively produce the linear aldehyde. This intermediate was subsequently auto-oxidized upon exposure to air to yield the corresponding fatty acid, a precursor for nylon-12 (Scheme 21). Notably, this approach minimized by-products/waste formation, in part due to the effective recyclability of the rhodium catalyst, which maintains its activity over four to five catalytic cycles [96].

Another example of the use of fatty acid for polymers synthesis was reported by S. Mecking and collaborators. In this case, oleic and erucic acids alkyl esters were used as renewable precursors for the synthesis of biobased polyesters (PEs) and PAs (Scheme 22). For this purpose, methyl oleate and ethyl erucate were firstly subjected to alkoxycarbonylation, and the so obtained diesters were subsequently converted via two parallel routes: (I) saponification to obtain the corresponding dicarboxylic acids, or, alternatively, (II) catalytic hydrogenation using a Ru-based catalyst to afford the corresponding diols. The obtained dicarboxyilic acids were condensed with the synthesized long-chain diols, as well as shorter diols, in order to produce tunable chain length polyesters. Complementarily, the obtained diols were converted into diamines, specifically, 19-nonadecanediamine and 1,23-tricosanediamine, which were further condensed with the previously synthetized dicarboxylic acids to yield long-chain PAs. These resulting PAs exhibited melting temperatures of 156 °C and 152 °C, respectively, and molecular weights of 10 kDa [97].

Similarly, A. R. Meier et al. employed methyl undec-10-enoate, methyl oleate and methyl erucate as precursors for the synthesis of homo-and co-PAs. In their approach, the double bonds present in these fatty acid esters were functionalized via thiol-ene reaction using cysteamine to produce amino-functionalized monomers (Scheme 23). These monomers were subsequently homopolymerized, copolymerized with each other and polymerized with adipic acid and 1,6-hexamethylenediamine by using TBD as an environmentally friendly and effective catalyst. The resulting renewable PAs exhibited molecular weights ranging from 3.7 kDa to 15.9 kDa. Differential scanning calorimetry (DSC) analysis revealed distinct thermal behaviors. So, while the homopolymers derived from undecenoate and erucate derivatives exhibited clear thermal transitions, no melting point was observed for the homopolymer of oleate derivative, indicating an amorphous nature. To modulate the broad melting range observed in these homopolymers, copolymerization of monomers was performed. The thermal properties of the copolymers obtained proved to be entirely dependent on the structure of the monomers used. Therefore, it is noteworthy the tunability of the thermal properties of these materials through the choice of monomers. Furthermore, copolymerization of these monomers with adipic acid and 1,6-hexamethylenediamine, key components of Nylon 6,6, showed that the melting temperatures of the resulting PAs could be finely modulate over a wide range. Notably, both homopolymerization and copolymerization reactions produced PAs with melting points ranging from 40 °C to 182 °C. Collectively, these findings indicated that selective monomer combinations enable the synthesis of PAs ranging from amorphous to high-melting semi-crystalline materials, offering a versatile platform for thermal property engineering [95].

Regarding green approaches, again Meier et al. designed an efficient and environmentally benign strategy for the oxyfunctionalization of fatty acid methyl esters (FAMEs) employing molecular oxygen as an oxidizing agent. For this purpose, they subjected methyl oleate and methyl erucate to a co-catalyst-free Wacker oxidation procedure in a dimethylacetamide/palladium(II)chloride solvent–catalyst system. The obtained unsaturated keto-fatty acid esters were further functionalized with amines via reductive amination. The prepared renewable monomers were polymerized with 1,6-hexamethylendimethylamine and dimethyl adipate in order to obtain the corresponding PAs (Scheme 24). The synthesized coPAs exhibited M_n ranging from 11.2 kDa to 21.4 kDa, with dispersities between 1.45 and 2.28. DSC analysis revealed melting points between 224 °C and 250 °C, alongside thermal stability up to 350 °C. Notably, the study concluded that increasing the content of amine-based renewable monomers led to a reduction in melting temperatures, attributed to the steric hindrance introduced by the long aliphatic side chains. These results further underscored the tunability of thermal properties in copolymerized Nylon 6,6 PAs. Moreover, the approach highlights another greener synthetic route for fatty acid-derived monomers, benefiting from low catalyst loadings, simplified product isolation, and effective recycling of solvent–catalyst systems [98].

Similar to their previous work, years later, the same authors introduced another innovative procedure to synthesize new dimer FAMEs monomers using methyl undec-10-enoate, methyl oleate, and methyl erucate as precursors. They employed again the Wacker oxidation to obtain the keto-FAMEs, which were further submitted to reductive amination using aliphatic, cyclic or aromatic diamines in order to obtain partial renewable dimer-monomers and they even employed amino-FAME in order to obtain fully renewable dimer-monomers. Interestingly, these diamines led to dimer FAMEs monomers which were further polycondensated with 1,10-diaminodecane using TBD as catalyst. The obtained PAs resulted in molecular weights ranging from 24.4 kDa to 33.5 kDa and melting temperatures from 70 °C to 161 °C. Overall, they developed an efficient synthesis strategy to prepare partially and fully renewable dimer fatty acid methyl esters (DFA) from mono-unsaturated fatty acids via catalytic oxyfunctionalization and reductive amination [99].

On the other hand, olefin metathesis has shown to be a remarkably effective method for the synthesis of fatty acids derived monomers (Scheme 25). In this context, the same FAMEs as in previous works were protected via catalytic Lossen rearrangement. The resulting carbamates were subsequently subjected to cross-metathesis with methyl acrylate, and, following deprotection, amino-end-functionalized unsaturated FAMEs were obtained and employed as monomers to yield the corresponding PAs which exhibited M_n ranging from 14.9 kDa to 22.6 kDa and dispersities between 1.73 and 2.20. DSC analysis revealed melting points between 169 °C and 186 °C, comparable to those of commercial PAs. The study also explored the mechanical properties of the obtained materials, reporting that PAs with longer aliphatic chains exhibited lower Young’s modulus, attributed to a reduced frequency of amide groups and, consequently, diminished hydrogen bonding interactions. Overall, the study demonstrated the potential use of cross-metathesis strategy for the preparation of new bio-sourced PAs with good properties [100].

At this point, it’s worth mentioning that obtaining pure methyl oleate is neither easy nor cheap. This makes extrapolation to an industrial scale of procedures developed at the academic level with pure methyl oleate economically unviable. Recently, the use of technical-grade methyl oleate in the synthesis of different polymers has been described by Ortiz and coworkers with the aim of increasing industrial feasibility. In this work, the selective self-cross-metathesis of technical grade methyl oleate, which implies significant percentages in weight, around 20%, of methyl linoleate was reported. The so-obtained diester monomer, was subsequently reacted with diols or diamines to yield PEs and PAs. To facilitate this transformation, two novel ruthenium-based catalysts were developed. One featuring an N-heterocyclic carbene ligand in place of the traditional phosphine, and another incorporating an o-isopropoxybenzylidene moiety. These catalysts enabled efficient metathesis to afford selectively dimethyl octadec-9-enedioate. When this diester monomer was subjected to polycondensation with 1,8-diaminooctane a semicrystalline PA with a melting point of 152 °C was achieved (Scheme 26) [101].

Another interesting fatty acid-based PA synthesis starting from methyl undec-10-enoate was reported by C. Tang and coworkers. In this study, the authors reported the synthesis of biomass-derived long-chain PAs bearing pendant polar hydroxyl moieties or, alternatively, non-polar butyrate groups in order to regulate the crystallization behavior of the obtained materials. In this case, the presence of two terminal olefinic double bonds in the co-monomer allowed the polymerization reaction to be carried out by a thiol-ene click reaction to yield the corresponding functionalized PAs (Scheme 27). The obtained semicrystalline PAs displayed melting temperatures ranging from 49.5 °C to 122.3 °C. As expected, compared to conventional PAs, these long-chain PAs exhibited programmable supramolecular architectures with tunable crystallinity and mechanical performance, readily adjustable by varying the co-monomer composition. To further enhance mechanical properties, they decided to introduce metal–ligand coordination into the PA. Therefore, by coordinating cuprous ion with the sulfur atoms, the mechanical strength of these PA was significantly increased. Interestingly, the metal-coordinated PAs also provided strong luminescence, in fact the coordinated PAs revealed photoluminescence with emission peaks around 418 nm and UV–visible maximum absorption near 210 nm [102].

In the same year, A. Ullah and R. Ahmadi synthesized two bio-based PAs from dimeric FAME precursors using both conventional and microwave-assisted methods. Specifically, dimethyl 9-octadecenedioate was reacted with either p-xylylenediamine (PXDA) or diethylenetriamine (DETA), employing DBU as the catalyst (Scheme 28). In all cases, semi-crystalline PAs with high melting temperatures ranging from 45 °C to 201 °C were obtained. The obtained materials revealed degradation temperatures between 124 °C and 373 °C . Notably, the PA synthesized with DETA under microwave-assisted conditions exhibited significantly higher melting temperatures compared to its conventionally synthesized one. Further wide-angle X-ray scattering (WAXS) analysis revealed that microwave irradiation facilitated the formation of γ-crystalline phases, while conventional heating predominantly yielded amorphous or α-crystalline structures [103].

To transition away from the traditional “take, make, dispose” model, the circular economy proposes a regenerative system in which products, materials, and resources are maintained in use for as long as possible being recovered, recycled, or regenerated at the end of their life, reducing in consequence environmental impact and material wastes. Addressing this challenge, Greiner and Rist reported the synthesis and chemical recycling of linear aliphatic PAs derived from plant oil-based monomers (Scheme 29). Specifically, 1,19-nonadecanedioic acid, obtained from oleic acid, was polycondensed with various aliphatic diamines containing 4, 6, 8, or 12 methylene units to yield semi-crystalline PAs. Size exclusion chromatography (SEC) revealed high molecular weights ranging from 26 kDa to 51 kDa, with narrow dispersities between 1.6 and 1.9. Thermal and mechanical characterization via DMA, DSC, and TGA confirmed the formation of semi-crystalline materials with glass transition temperatures between 44 °C and 62 °C, melting points from 172 °C to 208 °C, and thermal degradations up to 448 °C. Remarkably, the authors also demonstrated chemical recycling of the PAs through microwave-assisted hydrolysis in aqueous hydrochloric acid. This process enabled the recovery of starting diacid quantitative yields which was newly used to synthesize a virgin PA. SEC analysis of the new polymer showed a M_n of 38 kDa and enhanced tensile strength and Young’s modulus compared to the original polymer. Overall, these results underscored the potential for closed-loop recycling, providing a significant advance toward circular bio-based polyamide materials [94].

Overall, the incorporation of fatty acid-based monomers into PA structures not only reduces the reliance on fossil resources but also provides opportunities to tailor polymer performance by varying chain length, degree of unsaturation, and branching. These properties can influence crystallinity, flexibility, and hydrophobicity, making fatty acid-derived PAs suitable for applications ranging from packaging and textiles to automotive and biomedical fields. Additionally, the position of the double bond, the length of the carbon chain, and whether the double bond is internal or terminal can significantly influence the material properties of the resulting polymeric materials [93].