Preparation of Non-Isocyanate Polyurethanes from Mixed Cyclic-Carbonated Compounds: Soybean Oil and CO2-Based Poly(ether carbonate)

This study presents the synthesis and characterization of non-isocyanate polyurethanes (NIPU) derived from the copolymerization of cyclic-carbonated soybean oil (CSBO) and cyclic carbonate (CC)-terminated poly(ether carbonate) (RCC). Using a double-metal cyanide catalyst, poly(ether carbonate) polyol was first synthesized through the copolymerization of carbon dioxide and propylene oxide. The terminal hydroxyl group was then subjected to a substitution reaction with a five-membered CC group using glycerol-1,2-carbonate and oxalyl chloride, yielding RCC. Attempts to prepare NIPU solely using RCC and diamine were unsuccessful, possibly due to the low CC functionality and the aminolysis of RCC’s linear carbonate repeating units. However, when combined with CSBO, solid NIPUs were successfully obtained, exhibiting good thermal stability along with enhanced mechanical properties compared to conventional CSBO-based NIPU formulations. Overall, this study underscores the potential of leveraging renewable resources and carbon capture technologies to develop sustainable NIPUs with tailored properties, thereby expanding their range of applications.


Introduction
Polyurethane (PU) possesses remarkable versatility in both properties and forms, which can be readily manipulated by adjusting the composition and types of its starting materials [1][2][3][4].This diverse nature translates to numerous applications across various fields, including automotive, construction, and pharmaceuticals, where PUs appear in various forms like foams, coatings, adhesives, and elastomers [5][6][7].Conventionally, PUs are synthesized through polyaddition polymerization of isocyanates and polyols, often aided by catalysts and additives [1].However, the use of isocyanates, derived from hazardous materials like phosgene, raises environmental and health concerns due to their inherent toxicity and moisture sensitivity [8][9][10].This has led to stricter regulations governing their handling and transport, as emphasized by the REACH (Registration, Evaluation, Authorization, and Restriction of Chemicals) regulation [11,12].
This study aims to address the limitation of low tensile strength in NIPUs derived from CSBO by incorporating CO 2 -based polyether carbonate (PEC) polyol.The rigid carbonate units in CO 2 -based PEC polyol promote strong entanglement between polymer chains, potentially leading to improved tensile strength [33][34][35][36].Compared to previous similar attempts that employed rigid polyamide components [37], PEC polyol in this study offers several advantages over petroleum-based alternatives.Life cycle assessment studies of the PEC polyol demonstrate reductions in global warming potential (11~19%) and fossil fuel depletion impacts (13~16%) [38].This translates to an environmentally friendly and industrially relevant NIPU with enhanced mechanical properties.We achieve this by employing a multi-step synthesis approach (Scheme 1).Firstly, PEC polyol is prepared via copolymerization of CO 2 and epoxide using a double metal cyanide (DMC) catalyst (Scheme 1a) [39,40].Secondly, five-membered CC-terminated poly(ether carbonate) (RCC) is synthesized by substituting the terminal hydroxyl group of PEC polyol with a CC unit (Scheme 1b).This RCC is then mixed with CSBO and cured with 1,3-diaminopropane (TDA) to form the NIPU (Scheme 2).
This study aims to address the limitation of low tensile strength in NIPUs derived from CSBO by incorporating CO2-based polyether carbonate (PEC) polyol.The rigid carbonate units in CO2-based PEC polyol promote strong entanglement between polymer chains, potentially leading to improved tensile strength [33][34][35][36].Compared to previous similar attempts that employed rigid polyamide components [37], PEC polyol in this study offers several advantages over petroleum-based alternatives.Life cycle assessment studies of the PEC polyol demonstrate reductions in global warming potential (11~19%) and fossil fuel depletion impacts (13~16%) [38].This translates to an environmentally friendly and industrially relevant NIPU with enhanced mechanical properties.We achieve this by employing a multi-step synthesis approach (Scheme 1).Firstly, PEC polyol is prepared via copolymerization of CO2 and epoxide using a double metal cyanide (DMC) catalyst (Scheme 1a) [39,40].Secondly, five-membered CC-terminated poly(ether carbonate) (RCC) is synthesized by substituting the terminal hydroxyl group of PEC polyol with a CC unit (Scheme 1b).This RCC is then mixed with CSBO and cured with 1,3-diaminopropane (TDA) to form the NIPU (Scheme 2).

Preparation of DMC Catalyst [39]
The DMC catalyst was synthesized using ZnCl 2 (36 g, 0.26 mol) as the metal salt and K 3 [Co(CN) 6 ] (3.9 g, 12 mmol) as the metal cyanide salt with the help of complexing agents.EEA (45 g, 0.34 mol) served as the primary complexing agent, while P123 served as the co-complexing agent.In the preparation process, the solution was initially prepared by dissolving ZnCl 2 and EEA in 138 mL of water, followed by stirring at 50 • C for 10 min.Separately, another solution, comprising K 3 [Co(CN) 6 ] and 16 mL of water, was formulated and then combined with the previous solution, followed by stirring for 1 h.Subsequently, a mixture of EEA (40.5 g, 0.31 mol) and P123 (2.91 g, 0.5 mmol) was added to the solution, followed by stirring for an additional 5 min.Following centrifugation of the solution, washing was performed using two mixtures: a mixture of EEA (40.5 g, 0.31 mol)/distilled water (60 mL) and a mixture of EEA (40.5 g, 0.31 mol)/P123 (1.5 g, 0.3 mmol).The resulting precipitate was collected and washed repeatedly with distilled water and EEA.Finally, the DMC catalyst was obtained by vacuum drying at 80

Preparation of PEC Polyol [40]
A high-pressure reactor (Model No. 4568, Parr Instrument Company, Moline, IL, USA) was utilized for the copolymerization of PO and CO 2 in the presence of the synthesized DMC catalyst.For the synthesis of PEC polyol, PPG-1000 served as the initiator.The reactor was charged with 0.11 g of the DMC catalyst and 90 mL of toluene, followed by introducing CO 2 to pressurize the reactor to 20 bar at 115 • C.Then, a mixture of PO (74.7 g, 90 mL) and PPG-1000 (20 mL) was fed into the reactor using an HPLC pump over a period of 90 min, initiating the polymerization.The reaction continued for an additional 30 min before being terminated by cooling the reactor to room temperature and venting the CO 2 .The product mixture was first concentrated by evaporation to remove the solvent and unreacted PO monomer.Subsequently, the concentrated mixture was filtered and washed with chloroform to remove the DMC catalyst.Finally, the remaining mixture was thoroughly washed with distilled water to remove PC by-products and isolate the desired PEC polyol (61.3 g).

Preparation of Five-Membered CC-Terminated Poly(Ether Carbonate) (RCC)
A five-membered CC precursor was synthesized using GC and OC (Figure S2, see Supplementary Materials).In a 1000 mL three-necked round-bottom flask, OC (138 g, 1.1 mol) was dissolved in 170 mL of dichloromethane under a nitrogen atmosphere.Separately, GC (47 g, 0.4 mol) was dissolved in 200 mL of dichloromethane.The GC solution was then added dropwise to the flask containing OC with vigorous stirring.The reaction proceeded for 2 h at room temperature, followed by the removal of unreacted OC by vacuum in an ice bath.The resulting precursor was dried under vacuum at 80 • C overnight.RCC was then synthesized by reacting the PEC polyol with the CC precursor in a 1:3 molar ratio.The CC precursor was first dissolved in dichloromethane, followed by the addition of the PEC polyol in dichloromethane.The reaction mixture was stirred for 2 h at room temperature, followed by the isolation of the desired product by multiple washings with a 5% aqueous NaHCO 3 solution and distilled water.

Preparation of CSBO
A two-step process involving epoxidation and subsequent carbonation was employed for CSBO synthesis.Initially, soybean oil (100 g, 0.11 mol) was dried under vacuum at 80 • C overnight.In a three-neck flask equipped with a dropping funnel, the dried oil, acetic acid (15 g, 0.25 mol), Amberlite IRC-120H (25 g), and toluene (18) were combined.The reaction mixture was stirred at 45 • C while hydrogen peroxide (66.1 g, 1.94 mol) was added dropwise over 8 h.After the reaction, the mixture was filtered to remove Amberlite IRC-120H and washed with distilled water until a neutral pH was achieved.Purification was performed using magnesium sulfate, followed by solvent evaporation at 100~110 • C, yielding epoxidized soybean oil.For CSBO synthesis, epoxidized soybean oil (20 g), tetrabutylammonium bromide (112 g, 0.35 mol), and ascorbic acid (22.9 g, 0.13 mol) were charged into a high-pressure reactor.The reactor was purged with CO 2 gas for 10 min, then pressurized to 34.5 bar under a CO 2 atmosphere.The reaction mixture was stirred at 80 • C for 14 h.After completion, ethyl acetate (18 mL) was added to extract the product mixture.The extract was washed with distilled water and purified using magnesium sulfate.Finally, ethyl acetate was removed by vacuum evaporation at 70 • C for 2 h and then at 110 • C for 5 h, resulting in CSBO (M n ~1150 g/mol, CC functionality~4.3).

Model Reaction of CC with Amine
PC (2.00 g, 1.96 × 10 −2 mol) and HA (1.98 g, 1.96 × 10 −2 mol) were equimolarly mixed in a 50 mL round-bottom flask under a nitrogen atmosphere.The mixture was stirred at 80 • C for 24 h to induce the formation of a reaction product with a urethane linkage.The progression of the reaction was monitored using Fourier transform infrared (FT-IR) and proton nuclear magnetic resonance ( 1 H NMR) spectroscopy.FT-IR: 3622~3138 cm −

Preparation of NIPUs (RT and CRT)
A schematic representation of how to prepare NIPU with CSBO, RCC, and TDA (CRT) is shown in Scheme 2. RCC (0.2 g) and CSBO (1.8 g) were vigorously mixed in a Tefloncoated iron mold for 1 min at room temperature.Subsequently, TDA was introduced to the mixture and further mixed for 2 min at room temperature.The homogenized mixture was then poured into a Teflon mold conforming to ASTM D-638 [41] type V specifications and cured at 80 • C for 48 h to yield the desired reaction products (CRTs).For NIPU prepared with RCC and TDA without CSBO (RT), the aforementioned procedure was followed, albeit with a reduced reaction time of 24 h.The formulations to prepare RT and CRTs are summarized in Table 1.

Characterization
To investigate the reactivity of CCs towards amines and to characterize the structural attributes of PEC polyol and RCC, 1 H NMR analyses were performed using a Bruker 500 MHz AvanceIII HD500 spectrometer (Billerica, MA, USA).These analyses were conducted at room temperature, utilizing chloroform-d or DMSO-d6 as solvents.The conversion of PC via ring-opening reactions with HA was assessed by analyzing the area values under the 1  RCC were determined via volumetric titration according to ASTM D1957-86 [42], utilizing Equation (3) from the standard.

Hydroxyl value
where A is the volume (mL) of the potassium hydroxide solution used in the blank test, B is the volume (mL) of the potassium hydroxide solution used in the sample titration, N is the normality of the potassium hydroxide solution, and S is the weight (g) of the sample.Gel permeation chromatography (GPC) was conducted using a Shimadzu Nexera system (Kyoto, Japan) equipped with a PSS column (Styragel HR 2, 4, and 5) and a Shimadzu RID-20A refractive index detector (Kyoto, Japan).THF of high-performance liquid chromatography (HPLC) grade served as the eluent, flowing at a rate of 1.0 mL/min.Calibration of the system was executed employing linear polystyrene (PS) standards spanning a molecular weight range of 641 to 13.5 × 10 6 g/mol.FT-IR spectra were acquired using a Thermo Fisher Scientific Nicolet 380 (Waltham, MA, USA) spectrometer.For CRT samples, analysis was conducted in attenuated total reflection mode with a resolution of 4 cm −1 and 64 scans, covering a wavenumber range of 4000 to 650 cm −1 .Other samples were analyzed in transmission mode.The thermal degradation characteristics of the synthesized NIPU samples were examined through thermogravimetric analysis (TGA) using an SDT-650 instrument from TA Instruments (New Castle, DE, USA).Samples weighing 5 mg were subjected to heating from 30 • C to 700 • C under a nitrogen atmosphere, with a heating rate of 10 • C/min.The gel content of the NIPU samples was determined according to Equation (4).Initially, dry samples (W 0 ) were submerged in acetone for 48 h.Subsequently, the solvent was removed and evaporated under vacuum at 70 • C for 12 h to obtain the final weight (W 1 ).
The mechanical properties of the NIPU samples were assessed using a universal testing machine (AGS-5kNX, Shimadzu) under tensile testing conditions.Following ASTM D-638, standard type V dumbbell-shaped specimens were tested at room temperature with a crosshead speed of 10 mm/min and a gauge length of 7.6 mm.

Results and Discussion
PEC polyol was synthesized via the polymerization of CO 2 with PO, employing a DMC catalyst.The structural features of PEC polyol were elucidated through 1 H NMR analysis.As depicted in Figure 1a, the signals at 1.1~1.3ppm corresponded to protons of the CH 3 group in both the propylene ether and PC units.The signals at 3.3~3.8ppm corresponded to protons of CH and CH 2 in the propylene ether unit.Additionally, signals at 4.1~4.2ppm and 4.9~5.0ppm corresponded to protons of CH 2 and CH in the PC unit, respectively.The 1 H NMR analysis showed that the PEC polyol consisted of 73.7 wt% propylene ether units and 26.3 wt% propylene carbonate units.Peaks at chemical shifts of 3.9~4.0ppm were characteristic of the CH group adjacent to the terminal hydroxyl group.The 13 C-NMR spectrum of PEC polyol is provided in Figure S3a (see Supplementary Materials).The mass spectrum of PEC polyol is provided in our previous literature, demonstrating the co-existence of propylene ether and propylene carbonate units in the polymer chain [43].
The hydroxyl group of PEC polyol underwent substitution with a CC group using the product from the reaction between GC and OC to yield RCC (Scheme 1b).The chemical structure of RCC was investigated through 1 H NMR and FT-IR analyses (Figure 1b).Proton signals observed at 4.4~5.1 ppm confirmed the formation of the introduced CC units.Additionally, the peak at 4.1~4.2ppm of PEC polyol shifted to the c* peak at 5.2~5.3ppm.The 13 C-NMR spectrum of RCC (Figure S3b, see Supplementary Materials) also shows the presence of terminal CC units.In contrast to PEC polyol, the presence of a carbonyl peak at 1812 cm −1 in the FT-IR spectrum of RCC further confirmed the incorporation of CC units (Figure 2).The hydroxyl group of PEC polyol underwent substitution with a CC group using the product from the reaction between GC and OC to yield RCC (Scheme 1b).The chemical structure of RCC was investigated through 1 H NMR and FT-IR analyses (Figure 1b).Proton signals observed at 4.4~5.1 ppm confirmed the formation of the introduced CC units.Additionally, the peak at 4.1~4.2ppm of PEC polyol shifted to the c* peak at 5.2~5.3ppm.The 13 C-NMR spectrum of RCC (Figure S3b, see Supplementary Materials) also shows the presence of terminal CC units.In contrast to PEC polyol, the presence of a carbonyl peak at 1812 cm −1 in the FT-IR spectrum of RCC further confirmed the incorporation of CC units (Figure 2).The hydroxyl values of PEC polyol and RCC were determined using the titration method following ASTM D1957-86 (Table 2).The values were 54.89 and 12.37 mg-KOH/g for PEC polyol and RCC, respectively.These values translated to hydroxyl chain-end functionalities of 2.94 and 0.77 for PEC polyol and RCC, respectively, calculated from the hydroxyl values and their molecular weights.The substitution of the terminal hydroxyl group in PEC polyol with the CC unit in RCC corresponds to the simultaneous disappearance of a hydroxyl group and the formation of a CC unit.Therefore, the CC functionality of RCC (2.17, Table 2) was calculated by the difference in hydroxyl functionality values between PEC polyol and RCC.This value well agreed with the CC functionality determined using 1 H NMR (Figure 1) and Equation ( 2), which also yielded a CC chain-end functionality value of 2.17 for RCC.The hydroxyl group of PEC polyol underwent substitution with a CC group using the product from the reaction between GC and OC to yield RCC (Scheme 1b).The chemical structure of RCC was investigated through 1 H NMR and FT-IR analyses (Figure 1b).Proton signals observed at 4.4~5.1 ppm confirmed the formation of the introduced CC units.Additionally, the peak at 4.1~4.2ppm of PEC polyol shifted to the c* peak at 5.2~5.3ppm.The 13 C-NMR spectrum of RCC (Figure S3b, see Supplementary Materials) also shows the presence of terminal CC units.In contrast to PEC polyol, the presence of a carbonyl peak at 1812 cm −1 in the FT-IR spectrum of RCC further confirmed the incorporation of CC units (Figure 2).The hydroxyl values of PEC polyol and RCC were determined using the titration method following ASTM D1957-86 (Table 2).The values were 54.89 and 12.37 mg-KOH/g for PEC polyol and RCC, respectively.These values translated to hydroxyl chain-end functionalities of 2.94 and 0.77 for PEC polyol and RCC, respectively, calculated from the hydroxyl values and their molecular weights.The substitution of the terminal hydroxyl group in PEC polyol with the CC unit in RCC corresponds to the simultaneous disappearance of a hydroxyl group and the formation of a CC unit.Therefore, the CC functionality of RCC (2.17, Table 2) was calculated by the difference in hydroxyl functionality values between PEC polyol and RCC.This value well agreed with the CC functionality determined using 1 H NMR (Figure 1) and Equation (2), which also yielded a CC chain-end functionality value of 2.17 for RCC.The hydroxyl values of PEC polyol and RCC were determined using the titration method following ASTM D1957-86 (Table 2).The values were 54.89 and 12.37 mg-KOH/g for PEC polyol and RCC, respectively.These values translated to hydroxyl chain-end functionalities of 2.94 and 0.77 for PEC polyol and RCC, respectively, calculated from the hydroxyl values and their molecular weights.The substitution of the terminal hydroxyl group in PEC polyol with the CC unit in RCC corresponds to the simultaneous disappearance of a hydroxyl group and the formation of a CC unit.Therefore, the CC functionality of RCC (2.17, Table 2) was calculated by the difference in hydroxyl functionality values between PEC polyol and RCC.This value well agreed with the CC functionality determined using 1 H NMR (Figure 1) and Equation (2), which also yielded a CC chain-end functionality value of 2.17 for RCC.To evaluate the reactivity of the CC unit for the ring-opening reaction with amine, a model reaction between PC and HA was initially studied (Figure 3a).The formation of urethane linkages over time was monitored using both FT-IR and 1 H NMR spectroscopy.In 1 H NMR spectra, signals between 6.95 and 7.14 ppm were attributed to protons associated with the newly formed urethane bonds (Figure 3b).To optimize reaction conditions, the conversion of CC units to urethane linkages was calculated based on Figure 3b and Equation (1).The results demonstrated that at 80 • C, over 80% conversion was achieved within 30 min, with near-complete conversion observed after 24 h (Figure 3c).Similarly, Figure 4 reveals a gradual decrease in the cyclic carbonyl peak at 1790 cm −1 in the FT-IR spectra with increasing reaction time, indicating the consumption of CC units.This coincided with a slight increase in the peak at 3320 cm −1 , likely attributed to the newly formed O-H groups in the reaction product.functionality = hydroxyl value × molecular weight/56,100.d Cyclic carbonate functionality = OH functionality of PEC polyol-OH functionality of PECC.
To evaluate the reactivity of the CC unit for the ring-opening reaction with amine, a model reaction between PC and HA was initially studied (Figure 3a).The formation of urethane linkages over time was monitored using both FT-IR and 1 H NMR spectroscopy.In 1 H NMR spectra, signals between 6.95 and 7.14 ppm were attributed to protons associated with the newly formed urethane bonds (Figure 3b).To optimize reaction conditions, the conversion of CC units to urethane linkages was calculated based on Figure 3b and Equation (1).The results demonstrated that at 80 °C, over 80% conversion was achieved within 30 min, with near-complete conversion observed after 24 h (Figure 3c).Similarly, Figure 4 reveals a gradual decrease in the cyclic carbonyl peak at 1790 cm −1 in the FT-IR spectra with increasing reaction time, indicating the consumption of CC units.This coincided with a slight increase in the peak at 3320 cm −1 , likely attributed to the newly formed O-H groups in the reaction product.The preparation of NIPU using RCC and TDA was then attempted.TDA was employed as a diamine species in this study due to its good miscibility with RCC.The reaction, however, appeared to be unsuccessful, as shown in Figure S4 (see Supplementary Materials), where no solidification of the reaction mixture was observed.In addition to the low CC functionality (f~2.2) of RCC, the linear carbonate repeating units in RCC can also be cleaved through aminolysis reactions in the presence of amines [44][45][46].Despite attempts to control the amount of the amine to minimize aminolysis of linear carbonate units (RT-0.85~1.50,Table 1), the preparation of solid NIPU did not occur.Even with an increased amine content and an extended reaction time, RT-1.50 still exhibited a liquid state (Figure 5, RT-1.50).The preparation of NIPU using RCC and TDA was then attempted.TDA was employed as a diamine species in this study due to its good miscibility with RCC.The reaction, however, appeared to be unsuccessful, as shown in Figure S4 (see Supplementary Materials), where no solidification of the reaction mixture was observed.In addition to the low CC functionality (f~2.2) of RCC, the linear carbonate repeating units in RCC can also be cleaved through aminolysis reactions in the presence of amines [44][45][46].Despite attempts to control the amount of the amine to minimize aminolysis of linear carbonate units (RT-0.85~1.50,Table 1), the preparation of solid NIPU did not occur.Even with an increased amine content and an extended reaction time, RT-1.50 still exhibited a liquid state (Figure 5, RT-1.50).The preparation of NIPU using RCC and TDA was then attempted.TDA was employed as a diamine species in this study due to its good miscibility with RCC.The reaction, however, appeared to be unsuccessful, as shown in Figure S4 (see Supplementary Materials), where no solidification of the reaction mixture was observed.In addition to the low CC functionality (f~2.2) of RCC, the linear carbonate repeating units in RCC can also be cleaved through aminolysis reactions in the presence of amines [44][45][46].Despite attempts to control the amount of the amine to minimize aminolysis of linear carbonate units (RT-0.85~1.50,Table 1), the preparation of solid NIPU did not occur.Even with an increased amine content and an extended reaction time, RT-1.50 still exhibited a liquid state (Figure 5, RT-1.50).CSBO is a type of soybean oil that has undergone a chemical reaction to incorporate five-membered CC units in its chemical structure.The process typically includes the reaction of epoxidized soybean oil with carbon dioxide, exemplifying "carbon capture and usage" technologies to provide environmentally friendly products.CSBO can further react with diamines to produce NIPU networks [25,37,47].Utilizing the reaction capability of CC units in CSBO, the preparation of NIPU (CRT) using a mixture of CSBO and RCC was thus attempted (Scheme 2).
The successful synthesis of solidified NIPU, i.e., CRTs, is demonstrated in Figure 5, resulting in brown ASTM D-638 Type V specimens.The successful preparation of CRTs was also evidenced by the appearance of characteristic peaks of urethane linkages in FT-IR spectra (Figure 6).In Figure 6b, the broad absorption peaks in the range of 3100~3600 cm −1 were attributed to the O-H and N-H stretching peaks originating from the ring-opening reaction of the CC with amine (Figure 6b) [48,49].Furthermore, the distinct appearance of the urethane carbonyl stretching peak at 1692 cm −1 and the N-H bending peak at 1532 cm −1 corresponded to the formation of urethane linkages (Figure 6c).The disappearance of the CC carbonyl peak at 1802 cm −1 provided clear support for the ring-opening reaction of CC units in CSBO/RCC mixtures to form NIPU (Figure 6c) [23,50].Additionally, the absorption peak of amide at 1648 cm −1 was barely noticeable, indicating that the aminolysis of ester groups of CSBO was not significant [23,50].
tion of epoxidized soybean oil with carbon dioxide, exemplifying "carbon capture and usage" technologies to provide environmentally friendly products.CSBO can further react with diamines to produce NIPU networks [25,37,47].Utilizing the reaction capability of CC units in CSBO, the preparation of NIPU (CRT) using a mixture of CSBO and RCC was thus attempted (Scheme 2).
The successful synthesis of solidified NIPU, i.e., CRTs, is demonstrated in Figure 5, resulting in brown ASTM D-638 Type V specimens.The successful preparation of CRTs was also evidenced by the appearance of characteristic peaks of urethane linkages in FT-IR spectra (Figure 6).In Figure 6b, the broad absorption peaks in the range of 3100~3600 cm −1 were attributed to the O-H and N-H stretching peaks originating from the ring-opening reaction of the CC with amine (Figure 6b) [48,49].Furthermore, the distinct appearance of the urethane carbonyl stretching peak at 1692 cm −1 and the N−H bending peak at 1532 cm −1 corresponded to the formation of urethane linkages (Figure 6c).The disappearance of the CC carbonyl peak at 1802 cm −1 provided clear support for the ring-opening reaction of CC units in CSBO/RCC mixtures to form NIPU (Figure 6c) [23,50].Additionally, the absorption peak of amide at 1648 cm −1 was barely noticeable, indicating that the aminolysis of ester groups of CSBO was not significant [23,50].The thermal stability of CRTs prepared with different CSBO/RCC ratios was evaluated using TGA (Figure 7a).The T10, T25, and T50 values correspond to 10%, 25%, and 50% weight loss of CRTs, respectively (Table 3).The T10 values, indicating the onset of decomposition, were all higher than the curing temperature of 80 °C, suggesting the formation of molecular network structures within CRTs during the reaction [51,52].Notably, the decomposition temperature values (T10, T25, and T50) for CRT8:2 and CRT7:3 were overall The thermal stability of CRTs prepared with different CSBO/RCC ratios was evaluated using TGA (Figure 7a).The T 10 , T 25 , and T 50 values correspond to 10%, 25%, and 50% weight loss of CRTs, respectively (Table 3).The T 10 values, indicating the onset of decomposition, were all higher than the curing temperature of 80 • C, suggesting the formation of molecular network structures within CRTs during the reaction [51,52].Notably, the decomposition temperature values (T 10 , T 25 , and T 50 ) for CRT8:2 and CRT7:3 were overall Polymers 2024, 16, 1171 11 of 15 higher than those of CRT9:1 (Table 3), implying that the CRTs with a higher RCC content exhibit enhanced thermal stability.
exhibit enhanced thermal stability.The derivative thermogravimetric (DTG) curves of the CRTs (Figure 7b) revealed distinct thermal decomposition stages.The first stage, occurring at 200~300 °C, corresponded to the degradation of urethane linkages within CRTs.The second stage, attributed to the degradation of the fatty acid chains of CSBO, occurred at higher temperatures [53,54].A final, minor stage above 400 °C was ascribed to the decomposition of carbonate units present in the RCC [43,55].
The tensile mechanical properties of the synthesized CRTs were evaluated using a universal tensile testing machine.Figure 8 illustrates the plastic deformation behaviors of the CRTs, which are characterized by tensile strengths ranging from 8 to 10 MPa and elongation at break values between 100 and 110%.Notably, previous studies focused on NIPU preparation using CSBO and diamine reported lower tensile strengths, typically ranging from 1 to 4 MPa, which obviously was a soft elastomeric behavior (Table S1, see Supplementary Materials) [25,30,56].In contrast, the CRTs synthesized in this study from CSBO/RCC mixtures exhibited a ductile plastic behavior with considerably higher tensile strengths (8~10 MPa), as presented in Figure S1 (see Supplementary Materials).This enhanced mechanical performance is likely attributed to the incorporation of RCC alongside CSBO in the NIPU structure.The relatively high molecular weight of RCC with stiff carbonate repeating units contributes to the formation of a denser and more rigid polymer network [54,57].As the CSBO content increased, a trend of increasing tensile strength and decreasing elongation at break was observed for the CRTs (Figure 8b,c).This phenomenon aligns with the gel content analysis (Figure 8d), which revealed a natural increase in crosslinking with higher CSBO content due to its multi-cyclic carbonate functionality [56,58].This increased cross-linking density contributes to the observed higher tensile strength and lower elongation at break values in the resulting CRTs [56].The derivative thermogravimetric (DTG) curves of the CRTs (Figure 7b) revealed distinct thermal decomposition stages.The first stage, occurring at 200~300 • C, corresponded to the degradation of urethane linkages within CRTs.The second stage, attributed to the degradation of the fatty acid chains of CSBO, occurred at higher temperatures [53,54].A final, minor stage above 400 • C was ascribed to the decomposition of carbonate units present in the RCC [43,55].
The tensile mechanical properties of the synthesized CRTs were evaluated using a universal tensile testing machine.Figure 8 illustrates the plastic deformation behaviors of the CRTs, which are characterized by tensile strengths ranging from 8 to 10 MPa and elongation at break values between 100 and 110%.Notably, previous studies focused on NIPU preparation using CSBO and diamine reported lower tensile strengths, typically ranging from 1 to 4 MPa, which obviously was a soft elastomeric behavior (Table S1, see Supplementary Materials) [25,30,56].In contrast, the CRTs synthesized in this study from CSBO/RCC mixtures exhibited a ductile plastic behavior with considerably higher tensile strengths (8~10 MPa), as presented in Figure S1 (see Supplementary Materials).This enhanced mechanical performance is likely attributed to the incorporation of RCC alongside CSBO in the NIPU structure.The relatively high molecular weight of RCC with stiff carbonate repeating units contributes to the formation of a denser and more rigid polymer network [54,57].As the CSBO content increased, a trend of increasing tensile strength and decreasing elongation at break was observed for the CRTs (Figure 8b,c).This phenomenon aligns with the gel content analysis (Figure 8d), which revealed a natural increase in crosslinking with higher CSBO content due to its multi-cyclic carbonate functionality [56,58].This increased cross-linking density contributes to the observed higher tensile strength and lower elongation at break values in the resulting CRTs [56].

Scheme 1 .Scheme 1 .
Scheme 1. Schematic representation of the preparation of PEC polyol using CO2 as a feedstock (a) and the preparation of RCC through the end-group substitution reaction of PEC polyol with CC precursor (b).Scheme 1.Schematic representation of the preparation of PEC polyol using CO 2 as a feedstock (a) and the preparation of RCC through the end-group substitution reaction of PEC polyol with CC precursor (b).

Figure 3 .
Figure 3. Schematic representation of the preparation of urethane products by the reaction between PC and HA (a), 1 H NMR spectra of the urethane products (b), and conversion of CC as a function of reaction time (c).

Figure 3 .
Figure 3. Schematic representation of the preparation of urethane products by the reaction between PC and HA (a), 1 H NMR spectra of the urethane products (b), and conversion of CC as a function of reaction time (c).

Figure 4 .
Figure 4. FT-IR spectra of the urethane product prepared by the reaction between PC and HA as a function of time at 80 °C.

Figure 4 .
Figure 4. FT-IR spectra of the urethane product prepared by the reaction between PC and HA as a function of time at 80 • C.

Figure 4 .
Figure 4. FT-IR spectra of the urethane product prepared by the reaction between PC and HA as a function of time at 80 °C.

Figure 5 .
Figure 5. Visual appearances of CRTs in Teflon molds.Figure 5. Visual appearances of CRTs in Teflon molds.

Figure 5 .
Figure 5. Visual appearances of CRTs in Teflon molds.Figure 5. Visual appearances of CRTs in Teflon molds.
H NMR signals, following Equation (1)..06 and A 7.05 denoted the characteristic area values of the 1 H NMR signal at 4.06 ppm of HA and 7.05 ppm of the product, respectively.The CC functionality of RCC was also assessed using 1 H NMR spectroscopy, as described by Equation (2).
to the C-H peak of PEC polyol.Similarly, A 5.23 denotes the area value of the 1 H NMR signals at 5.23 ppm, corresponding to the C-H peak of RCC.13C NMR analyses were performed at room temperature using a Bruker 600 MHz AvanceIII 600 spectrometer (Billerica, MA, USA) with chloroform-d as the solvent.The hydroxyl values (mg-KOH/g) of PEC polyol and

Table 2 .
Hydroxyl and cyclic carbonate functionalities of PEC polyol and RCC.
a Determined by GPC.b Determined by volumetric titration according to ASTM D1957-86.c OH functionality = hydroxyl value × molecular weight/56,100.d Cyclic carbonate functionality = OH functionality of PEC polyol-OH functionality of PECC.