Introduction

With a current global market volume of 25 million tons1, polyurethanes are an indispensable class of polymers with versatile applications in our daily life, such as coatings, foams and adhesives2. With regard to several hazards related to their production, including the use of toxic phosgene3 for the synthesis of isocyanates, which are hazardous themselves4, scientific effort within the last years has been put increasingly into the development of isocyanate-free routes to polyurethanes5,6,7,8,9. One possibility to access these non-isocyanate polyurethanes (NIPUs) is the ring-opening of cyclic carbonates with amines, as was reviewed extensively10,11.

Apart from the development towards less hazardous synthesis routes, the use of renewable feedstock for the production of polymeric materials, including polyurethanes, is highly desirable with regard to overall sustainable procedures12,13,14. Suitable monomers can be obtained from different renewable resources, such as oleochemicals15,16, tannin17 or terpenes18,19,20. As an example, cyclic carbonates derived from epoxidized soybean oil have been used for the synthesis of a variety of polymeric materials containing urethane units21,22,23.

Due to their structural diversity and their occurrence as waste products in the chemical industry24,25,26, terpenes have gained increasing research interest for the production of fine chemicals27, pharmaceutical products28 and polymeric materials29,30,31,32,33,34,35. In works by the group of Mülhaupt36,37 as well as by Della Monica and Kleij et al.38, the use of terpene-derived dicarbonates as AA monomers for the step-growth synthesis of NIPUs was investigated. However, in all cases, oligomers with limited molar masses were obtained due to viscosity reasons, thus requiring further reaction steps for the use of these pre-polymers.

The ring-opening of cyclic carbonates with amines can be catalyzed by the addition of Lewis acids39 and also by organocatalysts such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene40,41,42, which was also already applied in polymer synthesis43. Another possible compound group that can be used for the aminolysis of cyclic carbonates are thioureas40,42,44,45, which coordinate to carbonyl groups46 and thus can activate cyclic carbonates via hydrogen bonding47,48. For efficient hydrogen bonding, electron-withdrawing substituents are beneficial, further, aromatic groups can increase the activation by preorganization49,50. As such, the 3,5-bis(trifluoromethyl) phenyl group is often used in thiourea catalysts50, but can also be substituted by other electron-withdrawing groups51,34). Since compound 15 was not volatile enough to analyze the product mixture via GC-FID, the ratio can be determined with less accuracy due to overlap** signals in the NMR spectrum, however, the observations match the previous findings for 13 and 14.

Fatty acid-based amine

Considering the toxicity of allylamine as well as solubility issues in the case of the substrate 15, it is desirable to look for alternative amines containing a double bond. As such, fatty acid-based derivatives are desirable, being still simple in their structure. In previous work, it was shown that fatty acid-derived undecenoic acid can be used as starting material for the synthesis of decenyl amine (see Fig. 6) via esterification, substitution with hydroxyl amine, Lossen rearrangement and subsequent saponification65. The amine was thus synthesized according to the procedure described in the literature. The introduction of this moiety into the urethane monomers was first tried using limonene monocarbonate 6 (see Fig. 6). Analogous reaction conditions as in the case of allylamine were chosen (see Table 1, entry 1). Also in this case, two equivalents of the amine were used to enable higher conversion, and thiourea 9 was added to activate the endocyclic carbonate. The monomer 18 was obtained in a yield of 55%, the carbonate opening was thus less effective in comparison to the reaction with allylamine, which can be attributed to the lower reactivity of the amine due to its higher molecular weight66.

Fig. 6: Synthesis of fatty acid-based amine 17 and subsequent carbonate opening of 6 and 7 to urethane monomers 18 and 19.
figure 6

Only one of two formed regioisomers of 18 and 19 is shown for clarity, respectively.

Full size image

Further, limonene dicarbonate 7 was opened with 17 using analogous reaction conditions, yielding the diurethane monomer 19 in a yield of 15%. The low yield can be attributed to both the carbonate and the amine being less reactive than their respective counterparts, as discussed above.

Synthesis of linear NIPUs

To demonstrate a possible application of the synthesized urethane monomers for polymer synthesis, the substrates 13–15, 18, and 19 were reacted with dithiols in a step-growth thiol-ene polymerization to obtain linear NIPUs. To promote a radical formation, 2,2-dimethoxy-2-phenylacetophenone (DMPA) was added as initiator and the reaction mixture was irradiated with UV light of 365 nm.

As promising substrate, limonene mono-urethane 13 was chosen as it showed good solubility in various solvents. Among commercially available dithiols, 1,10-decanedithiol 20 was chosen for first test reactions. It contains a linear spacer that is expected to keep the polar urethane moieties at a sufficient distance to enable a certain degree of solubility. Test reactions were performed in chloroform, a solvent shown suitable in previous studies59, and 2-methyl tetrahydrofuran (2-Me-THF), which represents a less hazardous and furthermore renewable solvent. A concentration of 0.5 M was found to be a good compromise between not using too much solvent and yet dissolving the monomers well enough to enable efficient stirring. The results of the polymerization reactions are shown in Table 3 and Supplementary Fig. 2, revealing that efficient formation of polymers with molecular weights >10 kDa takes place at the chosen conditions. Both solvents led to the formation of polymers (see Table 3, entries 1 and 2), with higher molecular weights being achieved in the case of the more sustainable 2-Me-THF.

Table 3 Thiol-ene polymerization of limonene-based urethane monomer 13 and 1,10-decanedithiol 19.
Full size table

To gain insight into the reaction taking place, control reactions were carried out in absence of either the diene monomer 13, the dithiol 20, UV irradiation or initiator. The results show that both monomers as well as irradiation with UV light are necessary for the formation of polymers. In absence of DMPA, oligomeric species with a molecular weight of Mn = 5.4 kDa were observed after two days of irradiation (see Table 3, entry 4), confirming that radical addition can also take place without an initiator. However, significantly higher molecular weights were achieved when adding DMPA to the reaction mixture. A reduction of the initiator concentration to 2.5 mol% also led to the formation of polymers with Mn > 10 kDa (see Table 3, entry 3). Yet, this molecular weight is lower than when using 5 mol% initiator.

The polymer could be precipitated from the crude mixture by drop** the solution into cold methanol. The 1H NMR spectrum of the precipitated polymer (see Supplementary Fig. 6) confirmed the presence of both the terpene moiety and the aliphatic chain of the dithiol within the material. Further, the IR spectrum of the precipitated polymer (see Supplementary Fig. 11) shows the presence of characteristic signals corresponding to the present urethane and thioether moieties. Together with the performed control reactions, this undermines the assumption of NIPU formation via thiol-ene polyaddition.

The formation of NIPUs from monomer 13 was monitored over time (see Supplementary Table 4 and Supplementary Fig. 1), showing that already after 1 h a molecular weight of 11.8 kDa was achieved. After 5 h, 14.1 kDa were observed, indicating that the polymerization can possibly be stopped earlier. Nevertheless, for a comparison between the different monomers, 24 h were kept as fixed reaction time to allow for slower reactions to still be observed.

After the first promising results, the determined reaction conditions were applied for the synthesis of linear NIPUs from all urethane monomers synthesized in this work. Besides the variation of the urethane monomer, a variation of the dithiol can be a possibility to achieve different properties. As renewable alternative to 20, dithiol 22 was synthesized from limonene. A summary of the used dithiols and of the results from the polymerization reactions is shown in Table 4 and Supplementary Table 5. SEC of the obtained polymeric materials was performed (chromatograms are shown in Supplementary Figs. 3–5) to determine Mn and Ð (see Table 4), revealing mostly high molecular weights and dispersities close to 2, as expected for a step-growth polymerization. In contrast to the results achieved with monomer 13, the use of monomer 14 (entry 4, Table 4) only led to the formation of oligomeric species, which can be attributed to the reduced reactivity of the double bond in α,β-position to the carbonyl group with respect to radical thiol-ene additions. The urethane monomers 15, 18 and 19 could successfully be used for the synthesis of NIPUs with Mn > 10 kDa. When compared to monomer 13, the polymer P6 derived from monomer 18 shows slightly higher molecular weight (entry 6, Table 4). The resulting polymer was precipitated and characterized (Supplementary Figs. 8 and 13). The observed Tg of 1 °C is significantly lower than that of polymer P1, which can be attributed to the longer alkyl chains and thus lower relative amount of urethane moieties. In the case of monomer 15, the molecular weight is limited (entry 5, Table 4), which could be related to an increased stiffness. The molecular weight was not sufficiently high for a successful precipitation; thus, the polymer could not be characterized for molecular and thermal analysis. Monomer 19 could also be successfully used for the synthesis of NIPUs. When using dithiol 20 (entry 8, Table 4), the polymer P8 with the highest molecular weight within this work of 31.2 kDa was obtained after precipitation. Its Tg of 23 °C is higher than that of P6, where the same amine was used for the carbonate opening. The higher Tg can be attributed to increased hydrogen bonding due to the presence of two urethane moieties per repeating unit. Structural characterization of P8 is found in Supplementary Figs. 9 and 14.

Table 4 Variation of diene and dithiol within the synthesis of NIPUs.
Full size table

For a variation of the dithiol, the urethane monomer 13 was chosen as starting point. The use of dithiol 21 with a shorter chain length led to a polymer with lower molecular weight (entry 2, Table 4), which might be attributed to a less favorable structure in which the terpene units are relatively close. The resulting polymer P2 was purified and characterized (Supplementary Figs. 7 and 12), showing a Tg of 16 °C that is similar to that of P1. The renewable dithiol 22 in combination with monomer 13 did not yield a polymer with high molecular weights (entry 3, Table 4), supporting the previous assumption of close terpene moieties hampering the formation of longer polymer chains. For this reason, dithiol 22 was tested for a polymerization with monomers 18 and 19, which contain one and two additional decenyl spacers, respectively. Indeed, already the use of monomer 18 (entry 7, Table 4) with an elongated chain length compared to monomer 13 (entry 3, Table 4) led to higher molecular weight, yet not high enough for a precipitation of the polymer P7. Extending this to monomer 19 with an additional C10 chain led to the formation of NIPU P9 that could be precipitated and characterized (see Supplementary Figs. 10 and 15). As in the NMR spectrum of P9 the double bond signals are still visible as end groups, their integration can be used to calculate the molecular weight of P9 (see Supplementary Fig. 10 and Supplementary Eq. (1)). The calculated molecular weight of 11.8 kDa is in a similar range as the value from SEC measurements (see Table 4, entry 9). The observed Tg of 29 °C is higher than that of P8 with a linear dithiol, corresponding to a higher stiffness of the terpene unit.

Discussion

This work showed the application of thiourea catalysis for the functionalization of terpene-based carbonates towards urethane building blocks. The presence of a thiourea catalyst significantly improved the opening of the endocyclic carbonate groups by allylamine, whereas no activation was necessary in the case of the exocyclic carbonate structures. This enabled the access to AA monomers for the synthesis of linear NIPUs as potential application in polymer synthesis. By thiol-ene polyaddition with dithiols, NIPUs with molecular weights of up to 31 kDa were obtained, strongly depending on the structure of the respective monomers. By elongating the carbon chains within the urethane monomers, it was possible to achieve higher molecular weights and further implement a renewable dithiol from limonene. The Tg values, ranging from 1 to 29 °C, are slightly higher than those of literature-described terpene-containing NIPUs of similar molecular weight37,59,67, which can be attributed to additional OH groups59,67 or higher terpene content, respectively37.

This approach complements previous strategies of introducing urethane moieties into polymers via thiol-ene reaction59,68. It should be noted that the obtained materials contain additional thioether linkages as well as hydroxy groups in contrast to industrially used PUs. However, other works also include thioether linkages, e.g. for self-blowing NIPU foams69,70, showing the potential of such new structures. Further, this work brings forward the use of thiourea catalysis for NIPU production71, as potential strategy to activate more hindered cyclic carbonates. Although several examples have shown the potential of implementing terpene structures into polyurethanes22,36,37,38,67,72,73, their number remains limited.

Methods

Materials and method procedures are provided in Supplementary Methods. Synthetic procedures and characterization details are provided in the Supplementary Notes 1–3.

For NMR spectra and GC-FID chromatograms of isolated compounds, see Supplementary Figs. 17–75.