Sustainability of Nonisocyanate Polyurethanes (NIPUs)

Abstract

This work discusses the synthesis and properties of nonisocyanate polyurethanes (NIPUs) as an environmentally friendly alternative to traditional polyurethanes. NIPUs are made without the use of toxic isocyanates, reducing the environmental impact and safety concerns associated with their production. However, their synthesis reactions often require longer time and more energy to be completed. The sustainability of NIPUs is considered from various angles; the main methods for the synthesis of NIPUs, including rearrangement reactions, transurethanization, and ring-opening polymerization of cyclic carbonates with amines, are examined. Another part focuses on renewable sources, such as vegetable oils, terpenes, tannins, lignins, sugars, and others. The synthesis of waterborne and solvent-free NIPUs is also discussed, as it further reduces the environmental impact by minimizing volatile organic compounds (VOCs) and avoiding the use of harmful solvents. The challenges faced by NIPUs, such as lower molecular weight and higher dispersity compared to traditional polyurethanes, which can affect mechanical properties, were also addressed. Improving the performance of NIPUs to make them more competitive compared to conventional polyurethanes remains a key task in future research.

1. Introduction

The high toxicity of isocyanates and phosgene used for their synthesis from amines has inspired research toward more sustainable polyurethanes obtained without the use of isocyanates—nonisocyanate polyurethanes (NIPUs). Awareness of the dangers associated with isocyanate-derived polyurethanes has been growing since the 1960s [1,2], followed by the development of nonisocyanate PU synthesis routes [3]. The toxicity of phosgene and the isocyanates MDI and TDI, their CMR (Carcinogenic, Mutagenic, Reprotoxic) characteristics, and finally, aromatic amines are the main issues [4]. Reaction without the use of isocyanates has several benefits: less emission of toxic chemicals, lower sensitivity to water, lack of need to dry substrates, the possibility of conducting reactions in an aqueous environment, and reduced solvent emissions. When the focus is on omitting diisocyanates, the toxicity of amines should not be forgotten. Primary amines are used in the most sustainable route of synthesis of NIPUs, the aminolysis of cyclic carbonates, and, as the only source of nitrogen, they are unavoidable in this reaction. It should be noted that primary amines are more toxic than secondary and tertiary amines and can cause symptoms such as headache, dizziness, nausea, diarrhea, and damage to the nervous and cardiovascular systems [5,6,7]. Furthermore, amines present in the environment can act as precursors to potentially carcinogenic compounds, such as nitrosamines and nitramines [8]. The most concerning are the aromatic amines that cause lung, bladder, kidney, and other human cancers [9,10].

Until recently, the production of substrates for NIPUs was limited to fossil-derived ethylene oxide and dialkyl carbonates [11]; however, soon afterward, many substrates appeared from renewable sources, mainly plant sources such as tannins [12,13,14,15,16,17], lignin and lignans [18,19,20], starch [21], carbohydrates [15,22,23,24], vegetable oils [20,25,26,27,28,29,30,31,32,33], fatty acid diamines (FADs) [34,35,36,37,38], and fatty acid diesters [39,40,41,42]. These types of compounds are environmentally friendly for several reasons. First, NIPU is obtained from epoxy compounds by adding carbon dioxide, which consequently limits its emission. Second, plants provide an additional reduction in carbon dioxide from which substrates for synthesis are obtained. Therefore, it is a way to reduce CO₂ emissions. Third, renewable raw materials generally do not decompose into harmful compounds, which facilitates the compostability of NIPUs [21,43,44,45,46]. Although the current use of NIPU materials is still limited mainly to coatings and adhesives, their processing capabilities and applications are expanding [47,48]; for example, chemical-resistant coatings [49,50]; adhesives [17,51]; gas barriers [52]; thermoplastic NIPU [53]; flexible, self-blown polyurethane foams [54]; electrospun mats and fibers [55]; and self-healing materials [19] are being developed. Furthermore, due to the urethane and hydroxyl groups present in their structure, it is possible to form intermolecular hydrogen bonds, increasing the durability of NIPUs [21,56]. In classical polyurethanes, the urethane bond is the weakest link during thermal degradation and hydrolysis [57,58,59,60]. When intermolecular hydrogen bonds are formed, these materials are characterized by greater thermal and hydrolytic resistance [30,46,61]. Intermolecular bonding can also increase resistance to abrasion [62]. All of this indicates that the life of NIPU products should be longer than that of products made of classical polyurethanes.

However, the disadvantages of NIPUs are mainly the low molecular weights of the obtained polymers, which are still below the lower range of commercial polyurethanes. Another problem is the limited phase separation of soft and rigid segments, which is a key property of classical polyurethanes and thermoplastic polyurethane elastomers (TPU) that imparts good mechanical properties [63]. Unfortunately, the presence of hydroxyl groups in the segment from cyclic carbonates, which generate the intermolecular hydrogen bonds that improve the stability of the polyurethane structure, has the side effect of limiting the phase separation [64,65]. First, the disadvantage is the random formation of the primary or secondary OH group, which causes the formation of various types of interaction between the segments [64]. Another disadvantage is the lower reactivity of the raw materials, which requires long heating to complete the reaction at a high molecular weight level [4]. The large energy inputs that are required lower the sustainability markers. The same issue arises in the synthesis of cyclic carbonates. However, there has been progress in the selection of solvents and catalysts to increase the reaction rate and conversion rate. New and more effective methods for carbonate synthesis are also emerging [66].

The market for NIPUs is expanding, as many companies, such as Covestro AG, BASF SE, Evonik Industries AG, Wanhua Chemical Group Co., Ltd., Perstorp Holding AB, Asahi Kasei Corporation, Nippon Polyurethane Industry Co., Ltd., The Dow Chemical Company, Alberdingk Boley GmbH, Huntsman Corporation, as well as universities and research centers around the world, are in the process of developing new methods for the synthesis of CC and NIPUs [67]. For example, in the past decade, the research center Polymate Ltd. introduced various NIPU-based paints and flooring compositions, as well as new curing agents, foam, and UV-curable coatings [25]. In 2020, a large consortium was founded by the Horizon fund program, NIPU-EJD (Nonisocyanate Polyurethane, European Joint Doctorate), which involves a consortium of 15 international research-performing institutions from both academic and nonacademic sectors. Their focus is on developing sustainable and less hazardous polyurethane systems produced without isocyanates for rigid and flexible foams, coatings, elastomers, adhesives, vitrimers, and 3D printing [68]. In general, in 2022, the NIPU market was valued at US $1.09 billion and is expected to reach US $4.34 billion by 2032, with a compound annual growth rate (CAGR) of 9% during the forecast period [67], well above the benchmark for the chemical industry.

When all of these aspects are taken into account, it seems justified to assess the current stage at which isocyanate-free polyurethanes are on the path of sustainable development. In our work, we briefly summarize the methods of NIPU synthesis, present renewable raw materials for their synthesis, and describe the properties of NIPUs, including those obtained without solvents or water-based. Degradation routes, recycling, and the impact of decomposition products on the environment are discussed and, finally, future perspectives are outlined.

2. Methodology

During this study, we mostly used the Scopus™ and Web of Science™ databases, which are widely used by the scientific community. The first step of data extraction was the systematic search of titles, abstracts, and keyword lists, utilizing the specified terms “NIPU sustainability”, “NIPU green chemistry”, “NIPU water-based”, “NIPU recycling”, “NIPU self-healing”, “NIPU thermal properties”, and other related terms. After the preliminary search, we focused on the last ten years, which gave us about 130 original research articles and reviews that have been analyzed in terms of raw materials, synthesis methods, material properties, and potential applications. The results obtained gave us the idea for the main topics of the manuscript, and we continued the search of these topics for previously unnoticed articles.

3. Methods of NIPU and Substrate Synthesis

We can distinguish four main methods for the synthesis of isocyanate-free polyurethanes:

3.1. Rearrangement (Curtius, Hofmann, or Lossen)

The first method we point out is to obtain NIPUs by various rearrangements. Curtius from acyl azides, Hofmann from amides, and Lossen from hydroxamate esters can be used in this field; however, the isocyanate is still obtained during the process (Figure 1). In addition, they usually produce toxic waste containing halogens, such as bromine and chlorine. Nevertheless, attempts have been made in this field to reduce those drawbacks.

Recently, nonisocyanate polyurethane was synthesized in a different approach: the monomer was obtained by 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)-catalyzed Lossen rearrangement, which allowed it to bear two unsaturated bonds at both ends of the molecule. Then, polymerization was performed by thiol-ene ‘click’ chemistry [69]. The main by-product is urea, which can be used for polymerization. Lossen rearrangement took place one night at 100 °C and yielded 62% with respect to the monomer, and the polymerization of thiol-ene was performed in UV light with a radical initiator at room temperature. The highest molecular weights of such obtained polymers were 26 kDa; however, the dispersity index ranged between 1.9 and 4.2 [70]. The method is interesting because it does not imply polymerization with urethane formation; however, the presence of thiol moieties in the polymeric chain is inevitable in this case. The room-temperature and UV light-induced polymerization comply with the rules of sustainable energy use in chemical processes; however, the molar mass and insufficient yield of Lossen rearrangement products and the dispersity indices should be improved.

3.2. Transurethanization

To obtain nonisocyanate polyurethanes by transurethanization, polycarbamates and polyols react together (Figure 2). However, the method does not solve the problem of toxic intermediates since polycarbamates are synthesized with the use of phosgene. The need for a catalyst, the longer reaction time, the production of by-products of lower molecular weight, and the need for purification, as well as the use of toxic intermediates, have limited the expansion of polycondensation on a commercial scale [71].

In addition to the main method, some more sustainable methods have been investigated to obtain polycarbamates from urea [72], nitro compounds [73], and amine oxidation [63]. As an example of a more sustainable transurethanization reaction, we can distinguish reactions involving amines, halides, and carbon dioxide in the presence of Cs₂CO₃ and tetrabutylammonium bromide [74]. The polyurethanes obtained had thermal properties similar to those of conventional isocyanate-based polyurethane (PU). The molar mass reached only 22.400 g/mol; however, the dispersity indices were moderate (between 1.71 and 2.33). The reaction took 30 h in total with the use of 10 bar CO₂ pressure and a temperature of 80 °C, which are rather energy-consumptive parameters. The highest molecular weight was obtained when 1,6-hexanedicarbonate and polycarbonate diols were transurethanized; it reached 58.6 kDa, which fits the range of commercial polyurethane MWs [75].

3.3. Ring-Opening Polymerization

The other way to produce NIPU is the ring-opening polymerization of six- to seven-membered cyclic carbamates (Figure 3) [76,77]. The reaction is carried out at a rather high temperature, around 200 °C, and with the use of a catalyst, sodium hydride or N-acetyl caprolactam [74]. In newer reports, polyurethanes can be synthesized from aziridines in supercritical CO₂ at 100 °C for 24 h [78]. The extended reaction time is quite typical for the synthesis of nonisocyanate polyurethanes, which is not beneficial in terms of sustainability. Moreover, there is still no alternative route for aziridines that omits the use of phosgene.

3.4. Polyadditon of Bis-Cyclic Carbonates (CCs) and Diamines

The other method to obtain NIPUs is the polyaddition of bis-cyclic carbonates (CCs) and diamines (Figure 4). The ease of this method is due to the high solvency and high boiling points of cyclic carbonates. These compounds are not volatile, are biodegradable, and show low toxicity compared to isocyanates, and they may be produced from a variety of epoxy compounds by conversion with CO₂. The conversion reaction is easy to perform with high efficiency, without the formation of by-products or the necessity of using harmful reagents. Furthermore, CO₂ is readily available, non-flammable, non-toxic, bio-renewable, and chemically inert, so it fully complies with sustainability principles.

During synthesis, the tetrahedral intermediate is formed as a result of the nucleophilic attack of the amine on the carbonyl group in the first phase of the reaction [79]. In the second step, another amine deprotonates the tetrahedral intermediate, resulting in a break of the bond between carbon and oxygen and an opening of the ring [46]. In the case of the most popular 5CC, depending on which C-O bond is broken, primary or secondary hydroxyl groups are formed. Unfortunately, the high randomness of their formation disturbs phase microseparation, which contributes to the deterioration of the properties of polyurethanes. Moreover, the molar masses are lower than those of conventional polyurethanes, while the dispersity index is higher [11]. The selection of amines significantly influences the reaction; especially short, primary aliphatic amines are known for their high reactivity. The longer the alkyl chains, the lower the reactivity. Moreover, nucleophilicity, size, and the presence of electron-withdrawing groups affect the reactivity of primary amines [44]. On the carbonate side, the reactivity grows with the size of the cyclic carbonate. The solvent also plays a crucial role with protogenic solvents that accelerate deprotonation, whereas the aprotic solvent affects the nucleophilic attack. In the same way as that, increasing the electrophilicity or nucleophilicity or catalyzing the opening of CC may increase the reaction rate [46]. The electron-withdrawing groups attached to cyclic carbonate are also beneficial to their reactivity [80]. The reaction temperature must be compromised between increasing the reaction rate and undesired side reactions, resulting in emulsification and darkening. Practically, the optimal temperature range is determined between 105 and 120 °C [81]. Because of the lower reactivity of cyclic carbonates in comparison with that of isocyanates, NIPU polymerization reactions are associated with a lengthy reaction time that is neither sustainable in terms of energy nor acceptable for industrial developments. Therefore, we decided to examine these issues in more detail, as they are of key importance in the future sustainable design and production of NIPUs.

3.4.1. Reaction of Cyclic Carbonates

The cyclic carbonate synthesis process was industrialized in the 1950s and still suffers two drawbacks from a sustainability point of view: high pressure (5–8 MPa) and high synthesis temperature (180–200 °C) [41]. These harsh conditions that are needed for a satisfactory conversion rate are the main problems in the application of cyclic carbonates to the synthesis of NIPUs. However, over the years, scientific approaches have moved from high-pressure methods [82] and supercritical CO₂ [83] to reactions under continuous flow of CO₂ under normal pressure [84,85,86,87,88,89]. The current research focuses on new catalysts for the synthesis of cyclic carbonates to obtain high yields at normal pressure and low temperatures. The hopes of finding metal-free catalytic systems that facilitate the reaction at 50 °C and 1 bar CO₂ pressure seem to be scientifically justified [41]. The low reaction temperature is important not only for environmental reasons but also for thermodynamic reasons: since the cyclocarboxylation process is exothermic, the lowering of the temperature shifts equilibrium towards the products. The separability should also be expanded as, for the time being, it is necessary to use a high catalytic loading. Various catalysts have been designed: binary catalyst systems [90], ionic liquid catalysts [91], organocatalysts [92], low melting eutectic solvents (DES), and metal complex catalysts [93], and the reaction time to the complete conversion of epichlorohydrin was in the range between 6 h and 20 h; however, it increased with the complexity of the epoxide. The most well-known are onium salts, in which bulky tetrabutylammonium or -phosphonium ions enable high catalytic activity. The promising groups of catalytically active compounds are polyalkyl guanidines [94,95,96], enaminones [97], N-hydroxylamines [98], and amidoximes [99]. In addition, other methods of synthesis have been tested, such as synthesis in the microwave field, which reduced the conversion time of commercial epoxy resins by more than three times [66]. Furthermore, the idea of one-pot reactions, such as the production of cyclic carbonates from unsaturated cellulose products made directly from biobased fatty acid esters [100], ethylene carbonate from ethylene produced by low energy-demanding methods [101,102], and polyhexamethylenecarbonate (HMEC) from chlorinated biobased glycerol [103], seems to have attracted great interest recently. Finally, the method to obtain cyclic carbonates from 1,2 diols and dimethyl carbonate (DMC) is a viable alternative. DMC can be obtained by CO₂ capture, even direct air capture (DAC) [104], catalytic conversion [105], and amino acid-based [106] or non-aqueous CO₂-amine carbon capture [107]. Later, it may be used to obtain CCs using various diols [108,109,110].

3.4.2. Nonisocyanate Polyurethanes

Researchers often choose the path to NIPUs through the polyaddition of amines and cyclic carbonates due to the low toxicity of cyclic carbonates and their potential biodegradability, water insensitivity, the possibility of noncatalysts or solvents [111], and simple reaction conditions without the formation of by-products [112,113,114]. Here, we also focus on the energy consumption during polyaddition and the use of a catalyst. For the time being, most of the synthesized materials are cross-linked NIPUs that are used as coatings. An example may be biobased soybean oil, cyclic carbonate, and epoxidated sucrose soyate resins [83]. The reaction was carried out for 3 h at 120 °C; however, supercritical CO₂ was used to obtain cyclic carbonates. The catalyst was a widely used TBD, but also 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) has been tested, as it possesses only one active catalytic center. NIPUs were obtained from sebacic bis-carbonate and fatty acid-derived amines, and the reaction took 2 h at 75 °C [83]. However, the molar masses were also lower than those of conventional polyurethanes: 22 kDa instead of 40–80 kDa [42,112,115], and the dispersity indices were higher, ranging from 2.5 to 3.1 instead of 1.6 to 2.2 [42,112,115]. The presence of a solvent (methanol) can affect the formation of hydrogen bonds between the NIPU chains. The investigated NIPUs achieved higher molar masses when obtained by the same type of method under the influence of methanol (NIPU-1 and NIPU-2). The presence of methanol enhances the mobility of the chain and hinders hydrogen bonding between polyurethane chains. With amine deficiency, molecules attack the urethane linkage, leading to the formation of urea (IR band at 1665 cm⁻¹). The hydrogen bond is believed to improve stability and provide protection to the prepolymer in the presence of surplus amine, as methanol causes the said hydrogen bonding to weaken; however, the dispersity indices were in the range between 1.57 and 1.88, and the molecular weight, even with methanol, was only ~4 kDa.

As stated above, the most abundant molecular weight of NIPU obtained by the polyaddition of five-membered cyclic carbonates is still lower than that of isocyanate-derived, ranging from ca. 20 kDa. The highest molecular weight of polyaddition-synthesized NIPU was obtained from seven-ring cyclic carbonate with 4,9-dioxa-1,12-dodecanediamine and reached ca. 35 kDa, which is close to the lower edge of the molecular weight for commercial polyurethanes. The reaction took 6 h at 70 °C; however, at 30 °C, the conversion of the CC groups was also complete, with slightly lower molecular weight and dispersity (2.7). Note that the reaction was catalyst-free. Catalyst and solvent-free synthesis is also possible; as an example, the reaction of ethylene carbonate and ethane diamine, diethylenetriamine, triethylenetriamine, and diethylenetriamine, leading to the formation of short segments [116], can be named. Then, the NIPU prepolymer was obtained by esterification with a fatty acid dimer and cured with epoxy resin; therefore, various chemistries were used here. In these reactions, the highest temperature used was 180 °C, and the total time reached was 24 h.

4. Renewable Substrates for the Synthesis of NIPUs

Despite chemical similarities, the direct replacement of conventional polyurethanes (PUs) with NIPUs has been proven to be challenging in practice, and new developments are required [117]. The advantage of NIPUs over classical polyurethanes lies in the tendency to avoid diisocyanate substrates and the large versatility of potential renewable resources that can be used in NIPUs. Most of the renewable resources proposed for bio-PUs—Figure 1—can also be used to produce NIPUs, as most are based on naturally occurring compounds with unsaturated bonds that can be epoxidized and transformed to glycols and react with isocyanates to produce PUs or used in the cycloaddition of CO₂ to form cyclic carbonates and produce NIPUs from amines (Figure 5). When comparing those two approaches, the more sustainable is the second, as it omits toxic intermediates and consumes CO₂ emissions.

A useful example of these two approaches may be materials from cashew nutshell liquid, an agrowaste product extracted from the soft honeycomb of cashew nut shells [118]. It contains anacardic acid, cardanol, and cardol derivatives, which contain unsaturated bonds and may be used to produce bisphenols for the synthesis of PUs or epoxy resins [119], glycols for PUs [120], and cyclic carbonates for NIPUs [62,121]. The epoxy resins and NIPUs derived from cashew nutshell liquid (CNSL) have been directly compared by Kathalewar et al. [121]. The addition of surplus urethane groups in the polymer structure has resulted in higher adhesion to the metal, flexibility, scratch hardness, impact resistance, and abrasion resistance.

A well-known topic is the use of unsaturated vegetable oils to synthesize polyols further applied in polyurethane production [32]. Similar to as described above, unsaturated bonds can be used for epoxidation and CO₂ cycloaddition to obtain cyclic carbonates. Soy oil was patented in 2006 [122]—the process took 70 h with TBAB as a catalyst and yielded a conversion of 90%. Carbonated soybean oil (CSFO) can be reacted with rigid aromatic amines such as m-XDA or p-XDA to produce polyurethanes with a remarkable tensile strength of 11.1 MPa and 129% elongation at break [123].

Furthermore, vegetable oils can be used to produce amines to further react with CCs; therefore, the process may be “green” from both types of substrates in the polyaddition reaction [30]. In this approach, various amines were obtained from castor oil and oleic acid in several steps, such as the extraction of triester acid (TEAc) from castor oil, the preparation of diacid (DA) from oleic acid, the extraction of diethyl ester (DEE) and tri-ethyl ester (TEE) from DA and TEAc, and finally, the extraction of diamide amine (DAA) and triester amide amine (TEAA) from DEE and TEE by amination. Furthermore, polyamine-polyol (PAPO) was obtained using ethylene diamine (EDA) from epoxidated sunflower oil (ESFO) (Figure 6).

The amines obtained were low-viscosity liquids derived from 100% renewable carbon, allowing ‘green’ solvent-free reactions to be carried out. These amines possess high flexibility and a low glass transition temperature as a result of their long alkyl chains. Furthermore, the additional long alkyl chains created a spacer between the urethane links and acted as a bulky solvent to hinder hydrogen bonding and obtain a high molecular weight of the synthesized NIPUs. The products—made of yellow transparent films—were prepared by reaction with CSFO at 90 °C and showed two glass temperatures, the first between −11 and 12 °C and the second in the range of 2 and 26 °C. The thermal stability, expressed as T_5%, was found to be 323–386 °C, depending on the amines used. The materials were also water- and environmentally resistant.

Fatty acid diamines (FDAs) are also available under the commercial name Priamine^TM, manufactured by CRODA (now Cargill Inc., Wayzata, MN, USA). Priamines have also been used in NIPU synthesis [34,35,37]. Similarly, fatty acid diesters can also be obtained, for example, from the fatty methyl ester of sunflower oil [124]. After transesterification with 1–5 pentane diol, the ester became unsaturated, having the C=C bond on the two ends of the molecule, which is an easy route to linear NIPU precursors after epoxidation and cycloaddition. In this way, internal epoxy fatty acid diesters (IEFADs) and terminal epoxy fatty acid diesters (TEFADs) were synthesized.

In addition to the oils mentioned above, soybean [20,27,29,125,126,127,128], sunflower [31] castor [28], linseed [36,125,129,130,131], and canola oil [132] have also been used. A potential topic for future exploration could be the application of grapeseed and nigella oils due to their high content of unsaturated bonds. On the other hand, the issue of using edible oils as chemical raw materials is debated due to the safety of the food market. Currently, plants of non-edible raw materials that could be grown in undemanding soils, such as radish oil or jatropha curcas [33], are being considered in the production of NIPUs.

Another approach to the transformation of vegetable oils is obtaining sucrose soyate that can be epoxidized and cyclocarbonized (Figure 7) [83]. The products of CC polyaddition on the basis of carbonated soybean oil (CSO) and carbonated sucrose soyate (CSS) were compared. Cyclocarbonation was carried out under high pressure of CO₂ air, with TBAB as a catalyst at 140 °C. Then, both obtained CCs were cross-linked with tris(2-aminoethyl)amine (TAEA) with DBU and TBD as a catalyst, LiOTf as a cocatalyst, and using various solvents. The usage of cocatalysts resulted in improved solvent resistance; the utilization of ethyl 3-ethoxypropionate (EEP) as a solvent with bulky tailings improved the coating appearance and led to the removal of haziness. Bulky and protic solvents were found to break the hydrogen bonding during the formation of hydroxyl groups, thus allowing for greater mobility of the polymeric chains and increased cross-linking. Finally, the higher functionality of sucrose soyate improved the coating properties, including the glass temperature. However, from a sustainability point of view, one should note the rather harsh environment for the synthesis of CCs.

Since sucrose has already been mentioned, it is logical to move on to NIPU based on sugars and polysaccharides. Among them, starch [21] and glucose/sucrose [15,22,23] have been used. Starch was used as a biocomponent in the form of high amylose gelatinized starch (HAGS) and played the role of a physical, hydrogen-bonded cross-linker or was cross-linked by nonisocyanate urethane, which is a unique solution (Figure 8). Non-isocyanate urethanes were obtained with the use of ethylene carbonate and three different diamines: ethylene diamine (EDA), butane-1,4-diamine (BDA), and hexane-1,6-diamine (HDA), which resulted in tunable mechanical properties with break strain varying between 45 and 121% and tensile strength ranging between 1.7 and 3.2 MPa. The products were completely transparent without the typical yellowish tint characteristic of NIPUs, probably due to the low molecular masses of the nonisocyanate urethane cross-linkers.

NIPUs were also prepared as self-blowing foams and wood coatings [15,22,24,133,134]. Recently, fructose-based polyurethane hydrogels synthesized in this way were found to be efficient water purification agents [135].

One should also mention sugar alcohols, in which multiple hydroxyl groups can be converted into cyclic carbonate groups [136,137]. The facile method to obtain bis-cyclic carbonate from erythritol has recently been presented [138]. The synthesis of erythritol bis-carbonate (EBC) was carried out in a rotary evaporator with TBD as the catalyst; a high yield of 90% was achieved. Importantly, the precipitated product from the solution can be separated by filtration, and the remaining reaction mixture recycled in the new process. EBC can serve as a short building block for the further synthesis of NIPUs.

Another green substrate is polyphenolic tannins that can be obtained from the bark of mimosa (Acacia mearnsii). The extracts contain robinetinidin, fisetinidin, catehin, and delphinidin (Figure 9).

As a result of the abundance of hydroxyl groups, tannins may be used as a polyol source for polyurethanes, but to obtain NIPU, the tannins were hydrolyzed with an ammonia solution and dissolved in dimethyl carbonate. After raising the temperature to 90 °C, carboxymethylation occurs between the phenolic groups [14]. Then, condensation occurred with the aminated tannins obtained in the first step, and the mixture was gelled at room temperature for at least 24 h to obtain the oligomers that were soluble in the acetone/water mixture. Another part was fully cured in an oven at 103 °C for 24 h; the product was hard, solid, and insoluble. The MALDI-TOF analysis of the oligomers showed the highest molecular weight as 1465 Da. Synthesis is rather straightforward and has been applied for tannins obtained from, e.g., chestnut [12] or pine wood [13].

Lignin may be another source of raw materials for the production of NIPUs [139]. When reacted with CSO and 3-aminopropyltriethoxysilane, an elastomer with a tensile strength of 1.4 MPa was obtained [20]. In such materials, the biomass content is approximately 85 wt%, and they are considered for applications such as thermoplastic films, coatings, sealants, adhesives, paints, and elastomers. Depending on the method of extraction from wood, two main types of lignin can be distinguished: organosolv (OSL) and kraft lignin (KL), which vary with molecular weight and sulfur content. NIPUs obtained from lignin may be a solution that omits the issue of formaldehyde in wood panels [17].

Lignin Borregaard ultrafiltration may also be a source of various compounds and fractions that are interesting for the synthesis of NIPUs. Lignin may be a source of lignans, as well as vanillin [140]. The latter compound can be used in the synthesis of NIPUs after acetal coupling to produce diols that can be epoxidized and converted to cyclic bis-carbonates [19]. Importantly, polyurethanes with such spiro-biacetal structures can be degraded at room temperature, followed by regeneration with nearly full recovery of properties.

Most of the compounds were obtained from branched, multifunctional substrates, and thus the obtained polymers were insoluble, cross-linked thermosets and nonprocessable by methods other than obtaining them in situ. The first example of a thermoplastic NIPU was a polymer obtained from syringaresinol bis-cyclic carbonate (Figure 10), a lignan that can be obtained by a chemoenzymatic pathway from syringaldehyde [18]. The authors compared non-isocyanate polyurethanes obtained from three carbonates: syringaresinol; ethyl dihydroferulate; and bisphenol A, biscyclocarbonate, and various amines, including decane diamine—DA10, difurfurylamine—DIFFA, isophorone diamine—IPDA, and tris(2-aminoethyl)amine—TREN. The thermoplastics obtained were hard and brittle solids with advantageous thermal properties (Tg up to = 98 °C and T_5% = 280 °C); however, the molecular weights were quite low, between 4.6 and 5.4 kDa.

Another source of CCs may be terpenes, which can be used after oxidation and converted to cyclic carbonates [141,142]. Limonene and carvone have been used to synthesize five-membered CCs that react with allylamine in the presence of thiourea as a catalyst (Figure 11) [143]. In this way, a series of renewable monomers with one or two urethane linkages were obtained, which were polymerized by a thiol-ene click reaction with various dithiols. Polyurethanes obtained in this way have a glass transition ranging from 1 to 29 °C and molecular weights of up to 31 kDa.

Finally, the use of abundant waste or by-products can also be considered a sustainable way to produce NIPUs. Glycerol, a by-product of biodiesel production, is a promising alternative feedstock for glycerol carbonate, which may reduce the environmental impact of NIPU synthesis [144]. Using glycerol in the initial stage [29], linear waterborne polyhydroxy urethane can be synthesized from diglycerol dicarbonate (DGDC) by reaction with FDA (Priamine 1075) and 3,3′-diamino-N-methyldipropylamine (DMPA) [62]. Glycerol carbonate can be obtained by various methods, such as the carbonation of glycerol with carbon monoxide or carbon dioxide, glycerol reaction with urea, and transcarbonation with alkylene carbonate or dialkyl carbonates, among which the latter synthesis protocol is considered an eco-friendly and cost-effective approach [132]. However, glycerol carbonate has only one CC group; thus, it is not suitable for polymer production. The other reactive group is hydroxyl, which can react with carboxylic acid to produce esters or with isocyanate to produce urethane [145,146]. In the latter approach, isocyanate is still required; however, its amount is undoubtedly reduced. It is also possible to obtain glycerol tricarbonate in a four-step reaction of glycerol and acrolein. When searching for linear NIPUs, a diglycerol can be obtained, from which a diglycerol carbonate (DGDC) can be applied: one with five-membered CC rings (5,5BCC DGDC) [147] and one with five- and six-membered rings (5,6BCC DGDC) [145]. In the case of 5,6CC DGDC, no more than preliminary syntheses of polyurethanes and polycarbonates have been performed; however, it is interesting that the homopolymerization of six-membered rings was possible [147], which resulted in polycarbonate that carried multiple 5CC groups that could potentially be sites for grafting other molecules or acting as cross-linkers. Moreover, a reaction can be considered in which glycerol carbonate reacts with the amine, and the emerging hydroxyl group is utilized to produce a CC ring with the remaining OH group from glycerol carbonate (Figure 12). These three interesting routes will probably be tested in future research.

5. Waterborne and Solvent-Free NIPUs

The avoidance of the use of moisture-sensitive isocyanates gives a promising opportunity to minimize volatile organic compound (VOC) emissions by adapting waterborne synthesis protocols. Waterborne nonisocyanate polyurethanes have been described in review work [148,149]; however, recent developments, including those using vegetable oils, deserve to be presented in the context of sustainability.

Along this line of interest, linseed oil is a useful raw material, as it is hydrophobic by nature and can be epoxidized and carbonated to CLSO (carbonated linseed oil) [129]. It was converted to linseed oil acid (LOA) by a thiol-ene reaction and carbonated to the cyclic carbonate of linseed oil acid (LOACC) [36]. Due to the presence of γ-linolenic acid with an unsaturated omega-6 bond, both carbonates obtained contained loose aliphatic chains that provide hydrophobic properties. The emulsifying properties were obtained in both cases by reaction with tertiary amines, such as DMPA; therefore, the NIPU prepolymer was able to form a salt with acetic acid. As the functionalities of the prepolymers were around six, the obtained coatings were highly cross-linked and exhibited performance comparable to those of commercial isocyanate-based polyurethane coatings and solvent-borne NIPUs.

Linear polyhydroxy urethanes have been synthesized from diglycerol dicarbonate (DGDC) by reaction with FDA (Priamine 1075) and DMPA [37]. The idea was to obtain two waterborne components: an epoxy chain extender and an amine-terminated NIPU, and to further react them to obtain a hybrid epoxy/NIPU coating that may be applied by water evaporation. NIPUs terminal on amino acids were obtained in three steps: prepolymer synthesis, neutralization, and dispersion. The synthesis included the reaction of DGDC and the varying composition of DMPA and FDAs with an amine-to-carbonate ratio of 1.2 in a nitrogen atmosphere. The prepolymers obtained were dissolved in methanol, and the DMPA was neutralized with acetic acid. Then, water was added, and methanol was removed with a rotary evaporator to obtain the NIPU liquid dispersion. Diglycerol carbonate was synthesized from diglycerol and dimethyl carbonate in the presence of sodium methoxide at 80 °C. The epoxy chain extenders were obtained similarly, but the precursors were trifunctional trimethylolpropane triglycidyl ether (TTE) and diethanolamine; therefore, the chain extender had two epoxy groups and a tertiary amine with three hydroxyl groups, and it should be noted that the polymer structure was linear. Waterborne 1K (one-package) hybrid epoxy-NIPU has also been synthesized from BADGE (bis-phenol A diglycidyl ether), FDA, DMPA, and the epoxy resin BADGE/TTE [150]. The coatings achieved a broad range of thermal and mechanical properties customized by the composition. The 1K waterborne formulation may facilitate practical application, especially since there was no visible phase separation.

The aforementioned works utilized the polyaddition of CC with amines. The recent literature also describes a reaction of 2-methylpentamethylene dicarbamate transcarbamoylation (DiNHBoc) with previously described DMPA, Priamine^® 1075, and poly(trimethylene ether) glycol (Velvetol H1000^TM—Allessa GmbH, Frankfurt, Germany) [151]. The dispersion was also obtained by obtaining a DMPA salt but with dl-lactic acid, which showed the best performance and was stable for up to 3 months. The resulting NIPU(U) prepolymers had hydroxyl groups at both ends, and tetramethylol diacetylene diurea (TMLAD) was used as a curing agent. The obtained elastic films showed high solvent, heat, light, hydrolysis, and hydrostatic pressure resistance, which promises a long service life.

In addition to water-based processes, solvent-free synthesis is also possible [30]. The loose, dangling side chains of biobased raw materials can provide a liquid state even for very large molecules. The solvent- and catalyst-free synthesis of NIPUs from ethylene carbonate and various amines has been presented in [116]. By solvent- and catalyst-free synthesis, the problem of harmful substances entering the environment is addressed in a sustainable way. In addition, it is a method of obtaining desired materials without extensive product purification and waste treatment. The noncatalyst and nonsolvent process was proposed using sebacic biscyclocarbonate (SB Bis CC) and two FDAs with average functionalities of 2.0 to 2.2 [111]. Carbonate was obtained by the reaction of sebacoyl chloride and glycerol carbonate in dichloromethane and triethylamine in an inert environment; therefore, the process is not fully solvent-free; however, the solvent was recovered under vacuum distillation. The synthesis of NIPUs was indeed solvent-free, which was possible because of the liquid state of the FDAs. It should be noted that it did not require high temperatures, as it was 75 °C, but it took 4 days. The best properties of the NIPUs obtained were found for stoichiometric proportions of CC to FDA; however, the MW oscillated around 20 kDa. All NIPUs obtained were amorphous and showed low Tg and high elasticity when the cross-linking degree was high. Another solvent-free and catalyst-free process was recently proposed by reacting ethylene carbonate and various amines (EDA, DETA, TETA, and TEPA) by a ring-opening reaction to obtain BHA (bis(hydroxyethyloxycarbonylamino) alkanes) under mild temperature conditions, between 60 and 100 °C. The reaction was carried out in bulk; however, acetonitrile was used to wash out impurities. Then, the obtained hard segments were connected with a little excess of fatty dimer acid by esterification carried out at 180 °C for 4 h under nitrogen. Subsequently, the reaction continued under reduced pressure with the removal of the water-side products, and finally, the products were dried at 60 °C. Finally, the prepolymers were cured with epoxy resin at mild temperatures of −60 to 90 °C for a total time of 14 h. Here, the highest degree of cross-linking resulted in the best thermal stability and highest Tg, which was mostly due to the high epoxy value and large secondary amine content.

6. NIPU Lifetime, Degradation, Recycling, and Environment Influence

When designing a sustainable polymeric material, several aspects must be taken into account. Previously, we focused on substrates, efficiency, catalyst, reaction temperature, and the solvents applied, i.e., on the chemistry of NIPU. However, there is another crucial aspect of sustainable material, that is, its useful life. It is important to consider how long the material may play its role efficiently and whether there is a need to synthesize a new one with environmental cost. Finally, when the life of the material ends, we need to consider whether the material obtained can be recycled to obtain the same material (materials recycling), recover raw materials (chemical recycling), or recover energy (energetical recycling).

6.1. Self-Healing

The property that may extend the shelf life of the product is its self-healing ability. Self-healing properties have been obtained in the development of bio-/CO₂-derived NIPUs that are recyclable and healable by thermohealing, moisture healing, and self-healing at room temperature, with the intrinsic moisture healing property being particularly unprecedented due to the hydroxyl functionalities in the structure of NIPUs [152]. Another approach is to obtain self-healing properties by ionic interactions between citric acid and waterborne PHU. In this model, the scratch is causing the breakage of ionic bonds, which increases the chain mobility in the scratch vicinity, and supramolecular interactions hinder the surface rearrangement (Figure 13) [153,154]. NIPUs with self-healing have also been obtained with excellent scratch repair performance and repeatable processability that contribute to sustainable development [155]. At this point, we should mention the vitrimers—unlike traditional linear or cross-linked PHU networks, vitrimers contain dynamic covalent adaptable bonds that allow them to be reprocessed or self-healed under external stimuli, such as heat or solvents. The recent work is summarizing this topic [156], but as an example, we can refer to the NIPU obtained from bis (six-membered cyclic carbonate of di(trimethylolpropane)) and α,ω-diamino liquid nitrile rubber [157]. Through transcarbonation, these polymers can be reprocessed with recovery of ca. 83–96% of initial tensile strength and 59–131% storage modulus. They also possess remarkable self-healing efficiency (~88%) and fracture strain (~1200%) when the monomer feeding ratio is optimized. It is noteworthy that such a combination of degradability and high mechanical performance is rarely found in conventional PU elastomers.

6.2. Thermal Stability

In polyhydroxyurethanes, the presence of numerous hydroxyl groups results in strong hydrogen bonds that, in some cases, contribute to improved thermal stability [50,158]. On the other hand, there are also results shown in the literature reporting lower thermal resistance of NIPUs [159], so the opinions are divided. Improvements in the thermal and chemical resistance of NIPU and its environmentally friendly nature were reported in ref. [32]; moreover, a very recent review about the thermal properties of NIPU was published in 2024 [159]. Therefore, we will be brief in discussing this field. When comparing commercial rigid PU foam and NIPU, the maximum temperature of thermal degradation can be delayed even by 100 °C, and the resulting foam is self-extinguishing, maintaining 90% of the residual mass (Figure 14) [133]. The materials obtained from succinic acid carbonate had good thermal stability (up to 245 °C) [160], while the thermoplastics from syringaresinol had T5% = 280 °C [18]. The chain extender plays an important role, such as 1,4-diaminobutane (DAB), isophorone diamine (IPDA), methylene bis(cyclohexyl amine) (H12MDA), and bis(aminomethyl) norbornane (AMNB). The use of the latter has resulted in higher flow temperatures and broad temperature ranges for tan δ ≥ 0.30 [161]. Furthermore, the selection of the type of diamine may influence the molecular weight of PHU and thermal properties, such as higher glass transition [116]. The highest temperature of 5% weight loss for PHU was achieved at approximately 350 °C for the most stable material; therefore, the PHU obtained can be considered a heat-resistant coating. In another approach, T50% was measured for NIPU obtained from various carbonates synthesized using epoxy resin: polypropylene glycol diglycidyl ether (Epoxy 2) and propoxylated glycerin triglycidyl ether (Epoxy 3). Then, it reacted with IPDA and epoxies, so multiple kinds of prepolymer polyols were obtained. Finally, the coatings were obtained by curing with hexa(methoxymethyl) melamine (HMMM). The prepared compositions achieved a 50% mass loss in the temperature range of 370 to 424 °C [162], which is a much higher temperature than that of conventional PUs, which begin to decompose around 240 °C [62].

6.3. Flame Retardancy

Flame retardancy is also connected with NIPU thermal properties and their life service extension. Certain formulations, such as those based on glucose or lignin, have demonstrated natural flame resistance [163,164]. The glucose-based NIPU foams catalyzed with phosphoric acid had an LOI value of 24.3%, which means that the material is self-extinguishing. In the case of tannin-based NIPU, the ignition test was performed by exposing a 3 cm cube facially on a stainless steel plate preheated to 1000 °C by a Bunsen burner, which was immediately extinguished after the sample ignition. Most of the samples were self-extinguishing after a maximum of 40 s, but in most cases immediately after the burner removal, and the ignition time was ~300 s [164]. Tannic acid converted into carbonate may also be a reagent to bind chitosan containing the amine group. In this case, fire retardancy is also obtained, as the foam exhibits an 80% higher residue mass after burning and a 75% reduction in afterburn time compared to commercial PU [133]. However, none of the mentioned studies had performed an analysis of the composition of the emitted gases. We believe that it would be beneficial to learn about the influence of natural raw materials on the toxicity of fumes during combustion. The potentially new application of NIPUs in this field may also be a deterrent coating for propellants that control the burn rate, reduce the sensitivity to heat, control the energy release, and reduce erosion and friction. UV-curable polyurethane acrylates are used in this field [165], and NIPUs appear to be a good competitor with their excellent adhesion and thermal properties.

6.4. Hydrolytic Resistance [163,164]

However, a great number of -OH groups in the NIPU structure can decrease its water stability. To enhance the stability of nonisocyanate polyurethanes (NIPUs) against hydrolysis, particularly in water environments, several modifications can be considered. A viable approach involves the introduction of hydrophobic groups to reduce water uptake. For example, the incorporation of silane-modified emulsions, as described in [166], can improve water resistance due to the hydrophobic nature of silane. Furthermore, the use of bulky substituents, such as benzoyl groups, has been shown to improve chlorine resistance in cellulose esters, which could be extrapolated to NIPUs for enhanced stability [149]. Another strategy is to incorporate chemical structures that enhance resistance to hydrolysis and oxidation. The carbon dioxide-based amino alcohol compound (CO₂-AA) used as a chain extender in the preparation of waterborne polyurethanes (CO₂-WPU) has been found to improve mechanical properties and resistance to water, solvents, and hydrolysis/oxidation [167]. Similarly, the use of diglycerol glycerol ether (GDE) as an enhancer in tannin-based NIPU adhesives has been shown to improve bonding performance and thermal stability, which can contribute to hydrolytic stability [16].

6.5. Recyclability

A truly sustainable polymeric material should be designed following the concept of circular economy. However, the recyclability of NIPUs presents certain challenges [139]. A linear thermoplastic NIPU is rare, and most cross-linked polyhydroxyurethane networks have permanent cross-links that hinder their recyclability for high-value applications [156]. Furthermore, despite significant academic research, the market size of NIPUs remains small, suggesting that the transition from laboratory to industry, including the aspect of recyclability, is still limited [168]. To address these issues, various strategies have been explored. One approach involves the design of NIPUs with dynamic covalent bonds that allow reprocessing or self-healing under external stimuli, such as heat or solvents, thus improving recyclability [156]. Another solution is the development of closed-loop recyclable NIPUs derived from renewable resources such as lignin and CO₂, which can be degraded and regenerated with almost complete recovery of properties [19]. These strategies demonstrate the potential for creating NIPUs that are sustainable and align with current environmental goals.

6.6. Biodegradability

The other way to design the shelf life of NIPU products is through their biodegradability, which is presented as a key advantage [19,32]. The use of biobased feedstocks, such as vegetable oils and carbon dioxide, essentially contributes to the development of this sustainable approach, as it leads to non-toxicity and biodegradability. However, the literature presents a nuanced picture. Although NIPUs are designed to be more environmentally friendly, their biodegradation is not universally guaranteed and can be influenced by their chemical structure and the presence of biobased content. For example, the use of cyclic carbonates derived from vegetable oils has been shown to produce NIPUs with a certain degree of biodegradability [43]. However, the specific conditions under which NIPUs biodegrade, such as the presence of microorganisms, environmental conditions, and the inherent susceptibility of the polymer to biodegradation, are not extensively detailed in the provided context. Another NIPU that biodegrades in the presence of Gordonia alvanivorans was synthesized from urea, phenol sulfonic and benzoic acid, formaldehyde, and long polyols such as polytetramethylene glycol (PTMG 1000) or oligooxypropylene triol (G1000) [169]. The study evaluated emulsifying activity, protein concentration, pH, tensile strength, and deformation at break as a function of biodegradation time and formaldehyde content. The results showed that the tensile strength of the NIPU based on PTMG 1000 and a 50% excess of formaldehyde decreased significantly after 10, 21, and 90 days of biodegradation, while the decrease was less pronounced for the NIPU prepared from G1000. The analyses demonstrated that biodegradation occurred mainly in hydrolysis-sensitive hard segments and, to a lesser extent, in flexible oligomerol segments.

7. Summary and Future Insights

The development of non-isocyanate polyurethanes (NIPUs) is an important area of research driven by the need for more sustainable and environmentally friendly polymer materials. This review summarizes the current state of NIPU synthesis, focusing on key methods and challenges. The polyaddition of cyclic carbonates and amines is the most widely studied approach, offering advantages such as mild reaction conditions, low toxicity, and reduction in CO₂ emissions with a wide variety of renewable resources. However, the relatively low reactivity of cyclic carbonates compared to isocyanates remains a challenge, often requiring high temperatures, long reaction times, and catalysts. Ongoing research is exploring new catalysts and reaction conditions to improve the efficiency of this route. It is noteworthy that NIPUs often have lower molecular weights and higher dispersity compared to traditional PUs. This can lead to differences in mechanical properties, such as tensile strength and elasticity. The MW oscillates close to 20–26 kDa in the case of CC reacting with primary amines, 30 kDa [143] in the case of the click reaction of unsaturated urethane thiol-ene, and 50 kDa in the case of transurethanization [75]. However, transurethanization and ring-opening polymerization face issues such as the use of toxic phosgene intermediates or high reaction temperatures. Renewable and biobased raw materials, such as vegetable oils, sugars, and lignin, offer promising opportunities to develop truly sustainable NIPU systems. Overall, significant progress has been made in NIPU synthesis, but further improvements are still needed to achieve industrially viable processes that meet the demands of sustainability. Despite lower molecular weights, NIPUs tend to show improved thermal stability because of strong hydrogen bonding in their structure, which makes them promising for applications where thermal resistance is crucial and, therefore, they are found in such applications as coatings. In addition, the first approaches to obtaining elastomers, foams, and mats by NIPU electrospinning have been successful. Biomaterial research is also beginning, as a new heart valve was recently made from NIPU materials.

Finally, continued systematic research on novel (bio)monomers, catalysts, and reaction/processing conditions will be crucial to unlock the full potential of NIPUs as a sustainable replacement for conventional polyurethanes in selected applications. Quantifying the environmental impacts of a NIPU product from its ‘cradle’ (raw material preparation) to the end-of-life ‘grave’ life cycle assessment (LCA) should be performed, including the global warming potential (GWP) and end-of-life analysis (EOL). As LCA aims to provide a comprehensive and quantitative analysis of the environmental impacts of a product or process throughout its entire life cycle [170], it is an indispensable tool for assessing the sustainability of NIPU materials and technologies. However, various NIPU synthesis/processing and disposal routes add to the complexity of this important environmental issue. Future environmentally friendly solutions will probably include developments in the field of ‘all-bio’ recyclable thermoplastic NIPU composites reinforced with natural fibers that follow the circular economy concept, as well as new solutions in the promising area of covalently bonded organic–inorganic hybrid materials with superior thermal and mechanical properties.