Polyurethanes without Isocyanates (NIPUs)

Polyurethanes (PUs) were discovered in the 1930s by Otto Bayer and were mainly used to replace natural rubbers during World War II 4. Their global production accounts for 25.1 million tons in 2019.1 PU represent the 6th largest class of polymers 2 behind polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), and polyethylene terephthalate (PET).3 PU are mainly used in three main areas: flexible foams, rigid foams, and nonporous materials (Figure 1). 2 Flexible foams are mainly used for comfort applications, such as mattresses or sofas, rigid foams are used for insulation, especially in the building industry, and the last area includes all products used in coatings, adhesives, paints and elastomers for example.

Classically, polyurethanes are synthesized by polyaddition of diols (or polyols) and diisocyanates (or polyisocyanates) (Figure 1).4 The availability of polyols and polyisocyanates allows the generation of a very wide range of materials with diverse properties.4

Despite the many advantages of PUs, their synthesis can be problematic. Indeed, the phosgene at the origin of isocyanates is a deadly gas and isocyanates are classified as CMR (Carcinogenic, Mutagenic and Reprotoxic), thus very toxic for humans.2 Moreover, as for the vast majority of polymers, PUs are mainly derived from fossil resources. The synthesis of biosourced isocyanates is possible, but their hazardousness remains problematic.5 In order to limit the risks for producers and users, alternatives to the synthesis of PUs, by “isocyanate-free” process, have been studied and developed. They lead to the formation of polymers commonly known as non-isocyanate polyurethanes (NIPUs).

Poly(hydroxy-urethane)

The synthesis routes and properties of NIPUs have been studied for many years. Several scientific reviews have been published summarizing these different synthetic routes.2,4,6–11 They can be grouped into four types of polymerizations: by polycondensation, by polyaddition, by rearrangement reactions and by ring opening (Figure 2). Many efforts have been made to develop greener and less hazardous synthetic routes for PUs. Unfortunately, while many of these have eliminated the use of isocyanates, they have not eliminated the use of hazardous compounds. The use of phosgene, chlorides or azides is still necessary for the synthesis of PU precursors or for their synthesis.

In addition to limiting chemical risks, the interest in finding alternatives to PUs is also to produce materials of renewable origin. Promising synthetic routes, which have been studied extensively in recent years, involve the polycondensation of bis-carbamates and polyols or the polyaddition of cyclic polycarbonates (CCs) and pluri-amines (Figure 2).2,6 The latter synthetic route leads to poly(hydroxy-urethane)s (PHUs), which can be, to a large extent, bio-based. These polymers have the capacity to respond to the problems mentioned.

Cyclic carbonates and biosourced amines

Carbonates are rarely available naturally. A review by Zhang et al 12 refers to a few, however. These are mainly monocarbonates from plants (terpenoid carbonates), generated by bacteria or produced by fungi. Biobased cyclic polycarbonates are no more available naturally or synthetically, with a few exceptions. Recently, Wang et al 13 sought to reference biobased molecules that could lead to cyclic carbonates for PHU synthesis. The computer program developed by the research team was able to target nearly 40,000 potential CCs molecules, after one or two synthesis steps, from 15 biobased molecules. Numerous routes for the synthesis of cyclic carbonates have also been developed and very well described by different research teams, the main ones being described below.

A first family of cyclic polycarbonates can be synthesized from carboxylic acids or their derivatives and glycerol carbonate (GC) (Figure 3). These are the so-called “ester-activated” cyclic dicarbonates.

Specifically, cyclic ester-activated polycarbonates are obtained by derivatization of acids by reaction with glycerol carbonate.14–17 The acids can also be substituted by their derivatives, acyl chlorides, to react with glycerol carbonate.18–26 Currently, the synthesis of ester-activated CCs has several limitations in order to produce fully biobased polymers. On the one hand, glycerol carbonate is not fully biobased and on the other hand, the derivatization process of acid derivatives needs to be optimized in order to limit the use of toxic solvents or reagents and thus better comply with green chemistry principles.27

The second widely described route for the synthesis of biobased CCs is the carbonation of epoxides using CO2 (Figure 4).6,17,28–55 This synthetic route allows the valorization of carbon dioxide, which is generally fixed on epoxides by a catalyzed process at high temperature and high pressure.

Epoxides are generated by two main routes. The 1,2-diols can be dehydrated to form the corresponding epoxides34,47,53,56 and the unsaturations can also be epoxidized. As such, triglycerides (vegetable oils) and fatty acids are substrates of interest57,5 because they have a variable number of unsaturations (between 1 and 3)15,59–6263. Many types of PHUs can thus be synthesized, both thermoplastic and thermosetting

A final synthesis route allowing access to biobased cyclic carbonates involves the cyclization of diols (Figure 5), mainly 1,2 (for 5-membered cyclic dicarbonates) but also 1,3 (for 6-membered cyclic dicarbonates)6.

Many amines are produced by living organisms and are present in nature.64 We can cite 1,4-diaminobutane (putrescine) and its derivatives (musculamine, spermine, thermospermine, etc.),65 but also L-lysine, derived from L-aspartate, which can lead to the production of numerous diamines such as 1,5-diaminopentane (cadaverine) or homoarginine (Figure 6).66 Chitosan can also be cited, but its structure and properties make it a separate material. It is in fact a multi-amine polysaccharide of the glycosaminoglycan family (Figure 6).

Recent reviews identify the different synthetic routes for biobased amines.6,67 Amines can be generated by various processes from bioresources but are most often derived from carboxylic acid. The polyamide industry is an important pole of synthesis of biosourced pluri-amines68 such as 1,4-diaminobutane (BASF), 1,5-diaminopentane (BAYER, Covestro, Mitsubishi Chemical), 1,10-decanediamine (Evonik) or fatty diamine marketed under the brand name PriamineTM (CRODA) (Figure 6). The amines are also widely used for the synthesis of polymers such as polyamides.

Other synthesis routes can lead to the obtention of particular pluri-amines. Epoxides lead to the obtention of β-amino alcohols by fixing ammonia on them (Figure 1. 23).69 The “click” addition of thiols, such as cysteamine, can also be used to functionalize biosourced derivatives with unsaturations (Figure 6).70,71

Aminolysis of cyclic carbonates

Research on cyclic carbonates has greatly improved their reactivity towards aminolysis and reached values close to those of their isocyanate counterpart. This reactivity can be improved thanks to two main axes: the constraint of the rings of the CCs and the introduction of electron-withdrawing groups near the ring of the CCs (Figure 7).

Although 5-atom CCs are the most common, 6-atom and 7-atom CCs have also been described. The reactivity of CCs with respect to the aminolysis reaction is correlated to the number of atoms composing the carbonate rings and to the increase of the ring stress. The activation energy of the aminolysis of CCs is decreased by the increasing number of atoms composing their ring and the reactivity in the presence of amines is improved. 72–74

The increase in the stress of the rings, and thus the increase in their number of atoms, has the counterpart of decreasing the stability in time of the CCs (Figure 7).75-79 The reactivity of the cyclic carbonates, and thus in particular those of the 5CCs, can however be improved thanks to the presence of electrochemical activator close to the ring of the CCs.

The presence of electron-withdrawing groups in the α, β, or γ position of cyclic carbonates results in an increase in their reactivity toward the aminolysis reaction. This chemical activation has been extensively studied over the last twenty years to improve the reactivity of CCs in the presence of amines.6,77,78,80–8 In summary, the reactivity of cyclic carbonates is increased as the activating effect of the substituent increases (Figure 7). The choice of the chemical structure of the CCs thus has an important influence on the polymerization kinetics of PHUs. Carefully chosen, the CC can therefore lead to the obtention of PHUs at low temperatures (< 80°C) in a few hours. Similarly, the kinetics of the aminolysis reaction of CCs can be further improved by the choice of amine.

Considered to be of low risk to humans and the environment, biobased CCs offer a real alternative to poly(isocyanate)s.83,84 The toxicity and corrosivity of amines is more problematic, although they are present at a certain level in living organisms.64 Indeed, it depends mainly on their chemical structure. Cyclic and aromatic amines, for example, are dangerous or even lethal, whereas some linear amines are not very dangerous. If selected and handled with care, they are generally less toxic than isocyanates and can be biobased.

PHUs are thus intended to be an alternative to polyurethanes obtained by the “isocyanate” route. However, their hydroxyurethane chemical structure differs from that of PUs and their properties are impacted by this difference.



Considérés comme peu dangereux pour l’Homme et l’environnement, les CCs biosourcés présentent une vraie alternative aux poly(isocyanate)s.83,84 La toxicité et la corrosivité des amines est plus problématique, bien que présentes à une certaine teneur chez les organismes vivants.64 En effet, elle dépend principalement de leur structure chimique. Les amines cycliques et aromatiques sont, par exemple, dangereuses voire mortelles tandis que certaines amines linéaires sont peu dangereuses. Sélectionnées et manipulées avec précaution, elles sont globalement moins toxiques que les isocyanates et peuvent être biosourcés.

      

    

  







  

    

            

        

                    

            PHUs vs PUs          

        

        

          
The polymerization of PUs requires some precautions because isocyanates react mainly with alcohols but also with water, amines or acids (Figure 8). The aminolysis reaction of CCs presents a strong chemoselectivity. The polymerizations of PHUs can thus be carried out under less controlled conditions, with in particular the presence of water, without generating secondary reactions.

        

      

          

  












  

    

      

        
This first difference in reactivity between the monomers used for the polymerization of PUs and PHUs has a strong impact on the applications of these polymer families. Indeed, the secondary reactions of isocyanates can be condensation reactions and thus generate gases, such as CO2 in the presence of water (Figure 8). These gases are necessary and valuable for the main application of PUs, namely foam formation. PHUs cannot be used directly for this application.


The polymerization of PHUs is a little more different from that of PUs because it is a ring-opening polymerization of CCs in the presence of amine. This aminolysis of the CCs generates primary hydroxyl groups (OH)I (in position α of the urethane) or secondary hydroxyl groups (OH)II (in position β of the urethane) (Figure 9)

      

    

  












  

    

      

        
Although PHUS are rarely compared directly to their PUs counterpart because it is not easy to find similar CCs/amine monomer pairs to polyol/poly(isocyanate)s, the presence of hydroxyl groups along the backbone has been shown to impact the properties of PHUs compared to PUs. Thus, the thermal properties, such as glass transition and thermal degradation temperatures, of PHUs are higher than those of PUs of comparable structure.51,85 The solubility of PHU is similarly reduced in common solvents such as dichloromethane or chloroform, compared to PU. These properties are explained by the ability of the (OH)s to also interact via hydrogen bonding (Figure 10). PHUs are also sometimes presented as having better chemical resistance, especially to hydrolysis, due to the protection of the carbonyl of the urethane bond by hydroxyl groups. 8,86,87

      

    

  












  

    

      

        
All these properties induced by (OH)s make PHUs particularly suitable for coating or adhesive type applications, as it has been very well described in two recent reviews, a second one by Gomez-Lopez et al88 and one by Khatoon et al89. PUs already exhibit excellent properties of abrasion resistance and adhesiveness, thanks in particular to the formation of micro-aggregates induced by the strong electrostatic interactions of the hydrogen bond type between the urethane bonds.90 But PHUs, also composed of urethane bonds, are all the more adapted to uses for adhesives and coatings thanks to the (OH)s. Their adhesion properties to substrates are improved compared to their Pru counterpart with similar properties.85,89,91


The fields of application of PHUs are varied. The presence of hydroxyl groups in the core of the polymer chains allows PHUs to have numerous electrostatic interactions, particularly of the hydrogen bonding type. This makes them particularly suitable for applications requiring strong substrate/polymer affinities such as coatings or adhesives. PHUs also have the property of being derived from bioresources, as we have seen, which makes them materials of the future with complementary properties to PUs.

      

    

  







  

    

            

        

                    

                      

        

        

                  

      

              

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            Bibliographie          

        

        

          
 


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