Investigations of FeCl3 adducted N-heterocyclic carbene complex as curing-delayed action catalyst for polyurethane polymerization

Journal of Catalysis 382 (2020) 77–85

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Investigations of FeCl3 adducted N-heterocyclic carbene complex as curing-delayed action catalyst for polyurethane polymerization Hyeon-Jun Noh a,1, T. Sadhasivam a,b,1, Mingu Han c, Keundeuk Lee c, Ju-Young Kim d, Ho-Young Jung a,b,⇑ a

Department of Environment & Energy Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea Center for Energy Storage System, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea c 4th R&D Institute 2nd Directorate Agency for Defense Development, Yuseoung P.O. Box 35, Daejeon, 34186, Republic of Korea d Department of Advanced Materials Engineering, Kangwon National University, Samcheok, Kangwon 25913, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 13 October 2019 Revised 8 December 2019 Accepted 9 December 2019

Keywords: N-heterocyclic carbene Complex catalyst Polyurethane Action delay catalyst FeCl3 adduct catalyst Fast curing catalyst

a b s t r a c t A high performance of catalyst material for delaying polyurethane (PU) polymerization at room temperature and rapid PU polymerization at preferred temperature still remains a great challenge because of adverse activation. Herein, N-heterocyclic carbene (NHC)-FeCl3 complex catalyst is developed and optimized to control the PU polymerization. The effects of PU polymerization degree and reaction kinetic is remarkably altered by the NHC-FeCl3 catalyst. It is found that the as-synthesized NHC-FeCl3 shows efficient catalytic activity as very low viscosity changes in room temperature and fast curing (3 h) at 60 °C, which proves considerable catalytic activity than the commercial catalyst and none-catalyst system. Based on DSC measurement, an increase in reaction rate is observed to PU polymerization with NHCFeCl3. The PU polymerization reaction has excellently initiated at 60 °C by NHC-FeCl3 catalyst, where NHC act as Lewis acid and interacts with polyol, and FeCl3 reacts with isocyanate by electrophilic reaction. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction Polyurethane (PU) is a combination of polymers, which possess a unique advantage because of its structure arrangements, excellent tensile strength and chemical resistance [1–3]. As a polymer material, the PUs are widely used in many applications such as paint, adhesive, sealant, elastomer, fiber, and membrane [1,3–5]. In recent years, PUs based research has showed significant contribution in the industrial and military applications specifically as a polymer-bonded explosive (PBX) binder and solid propellant binder [6,7]. The PUs was initially made by Otto Bayer in 1937 and the importance of PUs polymerization is still most significant for many applications [8–10]. The PUs are synthesized by the polymerization of polyurethane process, where the polymerization reaction occurred between alcohol compound with hydroxyl (AOH) and isocyanate (AN@C@O) groups [11,12]. Generally, PU polymerization is controlled by introducing a specific catalyst, ⇑ Corresponding author at: Department of Environment & Energy Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea. E-mail addresses: [email protected], [email protected] (H.-Y. Jung). 1 These two authors (Hyeon-Jun Noh and T. Sadhasivam) contributed equally to this work and should be considered as co-first authors (shared equally the first authorship). https://doi.org/10.1016/j.jcat.2019.12.014 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

which can induces the reaction of isocyanate and polyol [3]. The catalyst materials were specifically used to PUs polymerization through rapid reaction, delayed reaction, or both based on the requirement under a specified condition. The desired temperature PUs polymerization reaction is most important to initiate the reaction and achieve the efficient curing performances in particular period and conditions. Among the various kinds of catalyst materials, the tertiary amine and organometallic catalysts such as 1,4-diazabicyclo [2.2.2]octane, N-alkylmorpholine, triphenyl bismuth and dibutyltin dilaurate has considerable attention for PU polymerization process [13–19]. By using the organometallic catalyst or tertiary amine catalyst, the PU polymerization occurred through increasing of nucleophilicity, where organometallic catalyst can activates isocyanate to makes electrophilic condition or tertiary amine catalyst activates alcohol [20–24]. The PU polymerization reaction varied by the applied temperature [25]. However, there is a need for a system that has high flowability including fast curing and long life time for reaction-injection molding (RIM) process using twocomponent polyurethane system (2K-PU) [26–28] as shown in Fig. S1 [29]. In this case, PU polymerization reaction is not effectively occurring in the room temperature reaction. Though, rapid PU polymerization reaction can occurred by applying the heat, UV, or ultrasonic waves [30,31]. In RIM process, organic-mercury

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polymerization delayed catalyst such as mercuric octoate, phenylmercury neodeocanoate, and phenylercric acetate has been investigated [14,20,22]. Apart from the numerous advantages, the commercialized catalyst materials possess certain disadvantages such as corrosion issue of the storage or the foaming machine and expose of heavy metals, which can severely affect the human health and environment [22,32,33]. Moreover, many of the commercial catalysts induces the PU polymerization at room temperature, which affects the curing process in the elevated temperature. It has been highly required to develop a new catalyst for PU polymerization at the elevated temperature. The new kind of catalyst material must delay the PU polymerization and increase the potlife of polymerization at room temperature [34–38]. In the viewpoint of searching an alternative and efficient catalyst system for PU polymerization process, N-heterocyclic carbene (NHC) can be an important candidate because of its structure, stability, coordination with metal salts and increasing the nucleophilic behavior [25,39–48]. Most interestingly, metal adducts of NHC catalysts can contribute the PU polymerization, where isocyanate activates by metal adducts and polyol reacts with NHC carbene. Noteworthy that the catalyst made with coordinate bond (NHC coordinate with metal salt adducts) considered as a stabilized catalyst system. Moreover, the reactivity and stability of the catalysts can be varied during PU polymerization based on the state of adducts and the amount of catalysts. In our previous report, NHC catalysts effect in delaying PU polymerization reaction was found [39]. By using the support materials for NHC-metal salt adduct catalyst, the catalyst showed delayed performance than the none-catalyst system because of the interaction between the support material and catalyst materials, and the presence of unreacted support materials with PU reactants. In the recent years, few efforts were made with NHC based adducts as a catalyst materials for PU polymerization [25,40,49–52]. To further development in NHC based catalysts and identify the promising catalyst material to PU polymerization, we synthesized the NHC-FeCl3 catalyst through easier preparation process. In this research, we investigate and optimize the effect and loading amount of NHC-FeCl3 catalyst for effective PU polymerization process. The detail investigation of structural and microstructural properties are examined for assynthesized NHC and NHC-FeCl3. The effect of NHC-FeCl3 catalyst loading amount is sincerely investigated by varying the catalyst during the curing process through viscosity measurements. Furthermore, DSC analysis is carried out with none-catalyst, commercial catalyst, and NHC-FeCl3 catalyst systems to identify the PU curing peak under identical conditions.

2.2. Preparation of N-heterocyclic carbine (NHC)-metal salt catalyst For synthesizing N-heterocyclic carbine-metal salt catalyst (NHC-FeCl3), initially, 1,3-dicyclohexylimidazolium tetrafluoroborate (NHC) was synthesized as shown in the Fig. 1a. The NHC preparation technique was followed from M. Hans et al procedure [54]. In a 3-neck round bottom flask, 100 mmol of cyclohexylamine and 100 mmol paraformaldehyde was added in the 100 ml toluene and stirred for 30 min at room temperature. The solution mixture was cooled to 0 °C by ice-bath. Then, 100 mmol of cyclohexylamine was included in the solution under continues stirring at 0 °C. After this, 125 mmol of fluoroboric acid solution was added. Then, 100 mmol of glyoxal was added drop by drop in the solution. The reaction temperature was raised to 60 °C and stirred for 15 h. After successful completion of reaction, the solution was evaporated at 70 °C using rotary evaporator. Remained solids were dissolved in methanol and cooled at –70 °C using dry ice/acetone bath. Then, necessary amount of diethyl ether was added. The final product (NHC) was filtered and washed with diethyl ether at 4 °C. Finally, NHC was dried for 12 h in a vacuum oven. The NHC-metal adduct preparation technique was inspired from the previous reports [39,52,55] and a different kind of 1,3dicyclohexylimidazolium tetrafluoroborate – metal salt adduct (NHC-FeCl3) prepared through facile synthetic route as shown in the Fig. 1b. For preparing NHC-FeCl3, 1 g of 1,3dicyclohexylimidazolium tetrafluoroborate, 0.011 g of potassium tert-butoxide, and 70 ml of THF were added together in a round bottom flask and it was continuously stirred for 12 h at room temperature. After complete the reaction, it was filtered to remove the unreacted reagents. Then, 0.516 g (3.18 mmol) of iron (III) chloride was added to the solution and reacted for 8 h at 60 °C. Then, the rotary evaporator was used to evaporate the THF solvent. The final product (NHC-FeCl3) was dried in a freeze dryer for 12 h. 2.3. Structural and microstructural analyses The functional group and chemical structure of as-synthesized materials were confirmed by the Fourier-transform infrared (FT-

2. Experiment 2.1. Reagent and material Toluene (99.8%), cyclohexylamine (99.5%), glyoxal (40 wt%), tetrafluoroboric acid (50 wt%), diethyl ether (99%), and tetrahydrofuran (THF, 99%) were purchased from Duksan Pure Chemicals. Paraformaldehyde (91–93%), and iron (III)- chloride (FeCl3, 98%) were purchased from Samchun Chemicals. Isophorone diisocyanate (IPDI, 98%), and bis(2-ethylhexyl) adipate (DOA, 98%) were purchased from Sigma-aldrich. Methyl alcohol (99.9%, Daejung Chemical & Metals), potassium tert-butoxide (98%, Acros Oranics), hydroxyl-terminated polybutadiene (HTPB, 99%, Samyang Fine Chemical Co., Ltd), and di-n-butyltin dilaurate (DBTDL, 95%, alfa aesar) were used in the present investigation. Triphenyl bismuth (TPB, 99%) was supplied by Korean Agency for Defense Development (ADD). All the reagents in the experiment were used without further purification.

Fig. 1. Schematic representation of (a) NHC, and (b) NHC-FeCl3 catalyst synthesis process.

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IR) spectroscopy (FT-Near Infrared Spectrometer, PerkinElmer, Spectrum 400) at the range of 4000 ~ 650 cm 1. For chemical structure confirmation, 1H NMR (500 MHz) and 13C NMR (125 MHz) (Nuclear Magnetic Resonance Spectrometer, Nutation) were used. For NMR analyses, the as-synthesized material dissolved in CDCl3 solvent. The X-ray photoelectron spectroscopy (XPS, VG Multilab 2000) was used to investigate the element composition and binding energy of the catalyst materials. The size, surface morphology and elemental composition of as-prepared catalyst materials were analyzed by Field Emission Scanning Electron Microscopy (FESEM) attaching EDS spectrometer (Jeol/Oxford, JSM-7500F+). 2.4. PU polymerization measurements The PU polymerization process was analyzed through two measurements: (i) viscosity analysis and (ii) differential scanning calorimetry (DSC). To determine the PU polymerization with respect to different catalyst system and different catalyst loading amount, the viscosity measurement was carried out in viscometer (LVDV-II + PRO model of BROOKFILE) at room temperature and 60 °C. The viscosity data was collected every 1 h reaction under identical conditions. To determine the PU polymerization degree, DSC (131 EVO model, SETARAM instrumentation) analysis was performed between 50 and 400 °C at different heating rates (5, 10, 15, and 20 °C/min) in nitrogen atmosphere. For DSC analysis, 10 mg samples were measured at 30 mL aluminum pan (LF255DR model of VIBRA) and DSC analysis was conducted under identical conditions. 3. Result and discussion

[52,53,56,57]. For FeCl3 adducted NHC, the dominant peaks were similar with the NHC. More interestingly, two new peaks were appeared for NHC-FeCl3 in the region of 1550–1775 cm 1. For NHC-FeCl3 catalyst, the new peaks detected at 1649 and 1595 cm 1 due to presence of FeCl3 with NHC, which confirms the coordination linkage of FeCl3 and NHC. The FT-IR data evidently proved the successful synthesis of NHC and NHC-FeCl3 catalyst. To further confirm the formation and chemical structure of NHC, NMR analysis was carried out. Fig. 3, represent the (a) 1H NMR and (b) 13C NMR of NHC. As a result of 1H NMR analysis (500 MHz, CDCl3) d = 8.91(A ,1H, CH2 Im), 7.43(B ,2H, CH4,5Im), 4.3(C, 2H, NCH), 2.16–2.15(D, 4H, Cy), 1.90–1.87(E, 4H, Cy), 1.70– 1.68(f, 6H, Cy), 1.46–1.24(G, 6H, Cy). The result of 13C NMR analysis (125 MHz, CDCl3) d = 133.1(A, CH2Im), 120.2(B, CH4, 5Im), 60.0(C, CH, Cy), 33.2(D, CH2, Cy), 24.9(E, CH2, Cy), 24.5(F, CH2, Cy). The NMR spectra is successfully confirmed with NHC material [54]. The FT-IR and NMR analyses were successfully confirmed the chemical structure of the as-prepared catalyst materials. The XPS technique was used to analyze the presence of elements in as-prepared materials and to further confirm the coordinate link between the NHC and FeCl3. The XPS spectra of NHC and NHC-FeCl3 has shown in the Fig. 4(a) and (f), respectively. As shown in Fig. 4(a), the major peaks are found at 284.6, 401.07, 193.05, and 685.33 eV, which are respectively assigned to C1s, N1s, B1s, and F1s peaks. The obtained deconvoluted peaks from NHC of N1s, B1s, C1s, and F1s are shown in the Fig. 4(b), (c), (d) and (e), respectively. The XPS data of NHC-FeCl3 is shown in the Fig. 4(f). As compared to the NHC spectra, the peaks of F1s (685.33 eV) and B1s (193.05 eV) is disappeared in the NHC-FeCl3 because of inclusion of metal adduct with the NHC. Most interestingly, an additional peaks are emerged with NHC, which are related

To confirm the successful preparation of catalyst materials, FTIR analysis was conducted to the as-prepared NHC and NHC-FeCl3 and the corresponding data has plotted as shown in the Fig. 2. In FT-IR spectra, most of the peaks were appeared in the region of 700–1750 cm 1. For the both the NHC materials, most of the characteristics peaks are appeared in the similar region. In NHC, the peak appeared at 3149 cm 1 correspond to the C H stretching vibration of imidazolium. Additionally, the peaks at 2935 and 2862 cm 1 existed due to presence of cyclohexyl symmetric stretching CAH bond in NHC. The distinct peaks of C@N stretching, C@C and amide CAN stretching in NHC was successfully detected at 1625, 1554 and 1269 cm 1, respectively. Furthermore, the presence of most important imidazolium was observed at 1160 cm 1. Thus, we confirmed that the NHC was successfully formed

Fig. 2. FT-IR spectra of NHC, and NHC-FeCl3 catalyst.

Fig. 3. (a) 1H NMR, and (b)

13

C NMR spectra of as-synthesized NHC.

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Fig. 4. XPS spectra and deconvoluted fitting peaks of (a) NHC (b) N1s in NHC, (c) B1s in NHC, (d) C1s in NHC and (e) F1s in NHC. (f) NHC-FeCl3, (g) N1s in NHC-FeCl3, (h) Fe2p in NHC-FeCl3, (i) C1s in NHC-FeCl3, and (j) Cl2p in NHC-FeCl3.

to the metal adduct of FeCl3. The new peak arises at 712.5 and 724. 7 eV corresponds to the Fe2p3/2 and Fe2p1/2, respectively. Moreover, an additional peak appeared at 198.87 eV, which is related to the presence of Cl2p with Fe2p in NHC-FeCl3. The existence of Fe2p and Cl2p confirms the presence of FeCl3 with NHC. The XPS deconvolated fitted curves of Fe2p and Cl2p has shown in the Fig. 4(h) and (j), respectively. As seen in Fig. 4(h), four fitted curves were obtained to the Fe2p, which are corresponds to the divalent (711.82 and 725.69 eV) and trivalent (715.60 and 729.93 eV) state of Fe [58,59]. In the Cl spectrum (Fig. 4j), the peaks positioned at 200.02 and 198.54 eV are respectively assigned to the C12P1/2 and Cl2p3/2 [16,20,52]. Moreover, N1s quaternary ammonium and tertiary ammonium peaks existed at 399.2 eV in NHC and 401.37 eV in NHC-FeCl3, respectively. The overall XPS results confirms the co-existence of FeCl3 with NHC and successful formation of coordinated NHC-FeCl3 complex catalyst system. The surface properties of the as-synthesized NHC and NHCFeCl3 was examined by the SEM analysis with different magnification. Fig. 5a and b corresponds to the SEM micrograph images of NHC and NHC-FeCl3, respectively. As seen in Fig. 5a, the surface morphology and size of the as-prepared NHC is uneven and inhomogeneous. However, the appearance of NHC surface is almost smooth. It can been seen (in Fig. 5a) that the maximum amount

of rod-like structure of NHC is observed without any aggregation. In addition, a minimum amount of sheet-like structure of NHC is observed with rod-like structure. The average size of rod-like structure NHC is ~30 lm. After coordination reaction between NHC and FeCl3, the surface morphology of NHC is significantly changed as shown in Fig. 5b. It clearly seen that the NHC-FeCl3 are aggregated together after complete removal of reaction solvents during the catalyst preparation process. Moreover, FeCl3 metall salt formed coordinate linkage with NHC, which may lead to form the amorphously condensed morphology. The EDS and EDS mapping analyses used to investigate the elemental composition and homogeneity of the catalyst. The EDS data of NHC and NHC-FeCl3 has represented in the Fig. 5c and d, respectively. In both the spectra, Pt elemental peak is observed with the element peaks of as-synthesized materials. For effective measurement in EDS, a thin layer of Pt was coated on the samples, which is the major reason for existence of Pt element peak with the assynthesized materials. As shown in Fig. 5c, the NHC encompasses the elements of boron (B), carbon (C), nitrogen (N) and fluorine (F). In NHC-FeCl3 (Fig. 5d), the peaks of B and F is not existed, which is replaced by the introduction of FeCl3 in NHC. In this case, an additional elemental peaks of iron (Fe) and chloride (Cl) was observed with the C and N element peaks, which is confirmed

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Fig. 5. SEM micrograph images of (a) NHC and (b) NHC-FeCl3 catalyst. EDX spectra analysis of (c) NHC and (d) NHC-FeCl3 catalyst.

the existence of FeCl3 in NHC-FeCl3. As XPS confirmation, the EDX spectra also confirmed the successful formation of NHC and NHCFeCl3. To further extend, EDS mapping analysis was used to conclude the homogeneous dispersion of FeCl3 in NHC. As shown in the EDS mapping (Fig. S3), the uniform dispersion of Fe and Cl was observed, which is successfully proved the homogeneous formation NHC-FeCl3 complex. The PU polymerization process was investigated by viscometer and DSC measurements. The PU polymerization was conducted between the IPDI and HTPB with DOA plasticizer and catalyst/ none-catalyst system. Generally, the same amount of catalyst material can provide different performances (either fast or delay curing) by varying the PU reactants and using plasticizer under identical conditions. For curing measurement, the desired ratio of PU reactants was mixed together in a reactor as follows: 1.51 g of IPDI and 14.46 g of HTPB with 17.35 g of DOA. The anticipated quantity (0.056 g) of commercially available TPB and DBTDL catalysts were separately used and analyzed as reference catalyst materials to this study. Initially, the curing analysis was performed at 25 °C for 12 h to examine the effect and possibilities of curing process specifically delay of PU polymerization at room temperature. After 12 h reaction, the reaction temperature was raised to 60 °C to observe the PU curing behavior. The curing behavior

was measured through viscometer in each hour during the reaction process. To investigate the effect of as-synthesized NHC-FeCl3 catalyst in PU polymerization reaction at room temperature and 60 °C, a different amount (0.028, 0.042, 0.056, 0.07 and 0.084 g) of NHCFeCl3 used and the results were compared with the none-catalyst, TPB catalyst and DBTDL catalyst systems. The exact amount of reactants and catalyst materials were used in this study was tabulated in the Table 1. The viscosity analysis of PU polymerization with respect to time has shown in the Fig. 6. As represents in the figure, the PU polymerization process is varied depending on catalyst PU polymerization activity. For none-catalyst system (#1), there is no change in viscosity for PU polymerization reaction at room temperature and 60 °C because of insignificant activation at this specified conditions. After introduction of catalyst materials, the changes in viscosity has observed depending on reaction time and temperatures, which confirms the effectiveness of catalyst in PU polymerization. When using DBTDL catalyst (#3), the viscosity is rapidly increased to 12,000 cP at room temperature after for 3 h. This shows that DBTDL catalyst is normally used for fast curing catalyst, which has high PU polymerization reaction at room temperature. In contrast, the similar amount of TPB catalyst (#2) showed steady performance at room temperature, where the viscosity is not changed for 12 h reaction. In 60 °C PU polymerization process,

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Table 1 Catalyst loading amount in the polyurethane composition of HTPB/ IPDI/ DOA/ Catalyst.

#1 #2 #3 #4 #5 #6 #7 #8

HTPB(g)

IPDI(g)

DOA(g)

NHC-FeCl3Catalyst(g)

TPBCatalyst(g)

DBTDLCatalyst(g)

14.46 14.46 14.46 14.46 14.46 14.46 14.46 14.46

1.51 1.51 1.51 1.51 1.51 1.51 1.51 1.51

17.35 17.35 17.35 17.35 17.35 17.35 17.35 17.35

– – – 0.028 0.042 0.056 0.07 0.084

– 0.056 – – – – – –

– – 0.056 – – – – –

Fig. 6. Viscosity measurements of polyurethane curing analysis data for none catalyst, DBTDL, TPB, and different amount of NHC-FeCl3 catalyst.

the viscosity is significantly altered to TPB catalyst system and it reaches to high point of 12,000 cP after 4 h reaction. For NHCFeCl3 catalyst system, the change of viscosity was slightly observed at room temperature for 12 h. Here, the viscosity changes to approximately 1000 cP is for 0.028 (#4), 0.042 (#5), and 0.056 (#6) g of NHC-FeCl3 catalyst system. For 0.07 (#7) and 0.084 (#8) g, the observed viscosity is  2000 cP under identical conditions. In room temperature reaction, NHC-FeCl3 catalyst was partially activated due to change of heat by stirring of the reactants, which is the reason for slightly altering the viscosity changes in PU polymerization reaction. By changing the temperature to 60 °C after 12 h reaction, the viscosity changes was considerably observed to the NHC-FeCl3 catalyst system. The change of viscosity is varied in the order of introduction of NHC-FeCl3 catalyst amount. The increment of NHC-FeCl3 catalyst amount in PU reactants, the viscosity is increased rapidly owing to increase of rate of PU polymerization under similar conditions. The degree of viscosity increment and fast curing is #8 (0.084 g) > #7 (0.07 g) > #6 (0.056 g) > #5 (0.042 g) > #4 (0.028 g) for NHC-FeCl3 catalyst system. Here, the viscosity reached to 12,000 cP after 1, 2, 3, 6 and 7 h at 60 °C for #8, #7, #6, #5 and #4, respectively. As compared to TPB catalyst system, the NHC-FeCl3 catalyst showed similar performance at room temperature reaction for 12 h and it exhibited efficient performance than the TPB catalyst at 60 °C under identical conditions. Mainly, the fast or delay PU polymerization process can effectively occur with catalyst materials by activating the PU reactants based on the catalytic properties of the materials [17,25,35,39,49,60–62]. A probable catalyst reaction mechanism of NHC-FeCl3 during PU polymerization is shown in Fig. 7. Primarily, the NHC-FeCl3 complex can be separated as the NHC and FeCl3 by breaking the coordinate bond at a particular temperature, which can effectively participate in the PU polymerization reaction. Subsequently, the

Fig. 7. Possible catalytic activity mechanism during PU polymerization process by NHC-FeCl3 catalyst.

PU polymerization reaction initiated by catalyst materials, where NHC act as Lewis acid and interacts with polyol, and FeCl3 reacts with isocyanate by electrophilic reaction. In this process, NHCFeCl3 catalyst material influences the PU polymerization process. Based on this reaction, the combination of polyol and isocyanate gradually forms as PU at desired temperature, which can be

Fig. 8. A comparative DSC curves of PU polymerization process with none catalyst, TPB catalyst, and NHC-FeCl3 catalyst systems at 5 °C/min.

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observed by increase in viscosity and change of phase transition. The control of pot life is an important parameter in PU polymerization process. DSC analysis was used to determine the reaction kinetics of PU polymerization process. For measuring the comparative performance, none catalyst, TPB catalyst and NHC-FeCl3 catalyst systems were taken in to the account. As shown in Fig. 8, DSC analysis were evaluated at the temperature range of 50–400 °C at heating rate of 5 °C/min. The obtained primary PU polymerization peaks are 168.1, 111.1, and 61.4 °C for none catalyst, TPB catalyst and NHC-FeCl3

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catalyst systems, respectively. Compared to none-catalyst and commercial catalyst system, the fast reaction is obtained to the NHC-FeCl3 catalyst systems, which reveals that the PU polymerization occurs at a relatively low temperature. Moreover, it confirms that the NHC-FeCl3 catalyst activated in the early stage due to heat from DSC, which resulted in low PU primary polymerization temperature leading to greater reaction kinetics. The DSC measurements were carried out in different temperatures (5, 10, 15, and 20 °C/min), resulting in DSC analysis curves are shown in Fig. 9 (a) none-catalyst, (b) TPB catalyst and (c) NHC-FeCl3 catalyst systems. As seen in the insert images in Fig. 9, the peak shift is observed for all the catalyst system by varying the heating rate. As heating rate increased, the peak shifted towards high temperature direction, which is due to time difference on the materials thermal balance depending on the heating rate. Thus, the primary PU polymerization reaction peak shifted to slightly higher temperature by increasing the heating rate. The DSC results confirms that the reaction kinetics of NHC-FeCl3 catalyst is considerably higher than the none-catalyst and commercial catalyst system. Based on the viscosity measurements and DSC analysis, it can be concluded that the NHC-FeCl3 catalyst showed stable performance at room temperature and it significantly improve the PU polymerization at 60 °C. In the view of the above, the NHC-FeCl3 catalyst can be considered as an efficient catalyst for PU polymerization and increase the pot-life of polymerization at room temperature. 4. Conclusion In summary, an efficient NHC-FeCl3 complex catalyst has been synthesized through chemical process. The structure and surface properties of the NHC-FeCl3 was confirmed through FT-IR, NMR, XPS, SEM, EDX and mapping techniques. Based on the structural and microstructural analyses, it was found that the FeCl3 metal salt coordinate with NHC and formed as a NHC-FeCl3 complex catalyst. The catalytic activity of the NHC-FeCl3 complex catalyst in PU polymerization process was measured through viscosity measurement and DSC analysis. In addition, the effect of catalyst loading in PU polymerization was optimized through introduction of different weight percentage of NHC-FeCl3 catalyst. Viscosity measurement test was used to examine the polyurethane polymerization delaying effectiveness. The use of NHC-FeCl3 catalyst (0.056 g), the PU polymerization reaction not occurred at room temperature for 12 h, where the viscosity is nearly 1,000cP. In contrast, a rapid PU polymerization reaction occurred by increasing the temperature to 60 °C, where the catalyst is effectively operated. The NHC-FeCl3 catalyst demonstrated the improved PU polymerization process. We consider, this research work can provides significant advantages and new opportunities in PU related applications. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements

Fig. 9. DSC analysis of PU polymerization process with (a) none catalyst, (b) TPB catalyst, and (c) NHC-FeCl3 catalyst systems at different heating rate.

This research work was funded by the 2016 Defense Acquisition Program Administration/Agency for Defense Development (No. 220160003), Republic of Korea. We gratefully acknowledge the financial support from the DAPA/ADD of Korea and Converged Energy Materials Research Center in Yonsei University, Republic of Korea. The authors also acknowledge the Defense Agency for Technology and Quality (DTaQ) -Defense Venture Center of Korea for the support to this study.

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Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.12.014. References [1] K. Kato, M. Gon, K. Tanaka, Y. Chujo, Stretchable conductive hybrid films consisting of cubic silsesquioxane-capped polyurethane and poly(3hexylthiophene), Polymers (Basel) 11 (7) (2019) 1195, https://doi.org/ 10.3390/polym11071195. [2] T.E. Youssef, H. Al-Turaif, A.A. Wazzan, Investigations on the structural and mechanical properties of polyurethane resins based on Cu(II)phthalocyanines, Int. J. Polym. Sci. 2015 (2015), https://doi.org/10.1155/2015/461390. [3] J.O. Akindoyo, M.D.H. Beg, S. Ghazali, M.R. Islam, N. Jeyaratnam, A.R. Yuvaraj, Polyurethane types, synthesis and applications-a review, RSC Adv. 6 (115) (2016) 114453–114482, https://doi.org/10.1039/c6ra14525f. [4] M. Patri, S.K. Rath, U.G. Suryavansi, A novel polyurethane sealant based on hydroxy-terminated polybutadiene, J. Appl. Polym. Sci. 99 (3) (2006) 884–890, https://doi.org/10.1002/app.22815. [5] H.W. Engels, H.G. Pirkl, R. Albers, R.W. Albach, J. Krause, A. Hoffmann, H. Casselmann, J. Dormish, Polyurethanes: Versatile materials and sustainable problem solvers for today’s challenges, Angew. Chemie - Int. Ed. 52 (36) (2013) 9422–9441, https://doi.org/10.1002/anie.201302766. [6] A.G. Ajaz, Hydroxyl-terminated polybutadiene telechelic polymer (htpb): binder for solid rocket propellants, Rubber Chem. Technol. 68 (3) (1995) 481–506, https://doi.org/10.5254/1.3538752. [7] Q. An, S.V. Zybin, W.A. Goddard, A. Jaramillo-Botero, M. Blanco, S.N. Luo, Elucidation of the dynamics for hot-spot initiation at nonuniform interfaces of highly shocked materials 220101 Phys. Rev. B - Condens. Matter Mater. Phys. 84 (22) (2011), https://doi.org/10.1103/PhysRevB.84.220101. [8] O. Bayer, Das di-isocyanat-polyadditionsverfahren (polyurethane), Angew. Chem. 59 (9) (1947) 257–272, https://doi.org/10.1002/ange.19470590901. [9] J. Schmidt, R. Wei, T. Oeser, L.A.D.e.S. Silva, D. Breite, A. Schulze, W. Zimmermann, Degradation of polyester polyurethane by bacterial polyester hydrolases, Polymers (Basel) 9.2 (2017) 65, https://doi.org/10.3390/ polym9020065. [10] M.S. Rolph, A.L.J. Markowska, C.N. Warriner, R.K. O’Reilly, Blocked isocyanates: From analytical and experimental considerations to non-polyurethane applications, Polym. Chem. 7 (48) (2016) 7351–7364, https://doi.org/ 10.1039/c6py01776b. [11] G. Alva, Y. Lin, G. Fang, Synthesis and characterization of chain-extended and branched polyurethane copolymers as form stable phase change materials for solar thermal conversion storage, Sol. Energy Mater. Sol. Cells. 186 (2018) 14– 28, https://doi.org/10.1016/j.solmat.2018.06.023. [12] D. Bresolin, A. Valério, D. de Oliveira, M.K. Lenzi, C. Sayer, P.H.H. de Araújo, Polyurethane foams based on biopolyols from castor oil and glycerol, J. Polym. Environ. 26.6 (2018) 2467–2475, https://doi.org/10.1007/s10924-017-1138-7. [13] S. Mondal, Recent developments in temperature responsive shape memory polymers, Mini. Rev. Org. Chem. 46 (1) (2009) 114–119, https://doi.org/ 10.2174/157019309788167675. [14] J. Alsarraf, F. Robert, H. Cramail, Y. Landais, Latent catalysts based on guanidine templates for polyurethane synthesis, Polym. Chem. 4 (4) (2013) 904–907, https://doi.org/10.1039/c2py21006a. [15] S. Niyogi, S. Sarkar, B. Adhikari, Catalytic activity of DBTDL in polyurethane formation, Indian J. Chem. Technol. 9 (2002) 330–333. [16] H. Ni, H.A. Nash, J.G. Worden, M.D. Soucek, Effect of catalysts on the reaction of an aliphatic isocyanate and water, J. Polym. Sci. Part A Polym. Chem. 40 (11) (2002) 1677–1688, https://doi.org/10.1002/pola.10245. [17] S. Lee, J.H. Choi, I.K. Hong, J.W. Lee, Curing behavior of polyurethane as a binder for polymer-bonded explosives, J. Ind. Eng. Chem. 21 (2015) 980–985, https:// doi.org/10.1016/j.jiec.2014.05.004. [18] Y. Ou, Q. Jiao, S. Yan, Y. Zhu, Influence of bismuth complex catalysts on the cure reaction of hydroxyl-terminated polyether-based polymer bonded explosives, Cent. Eur. J. Energ. Mater. 15 (1) (2018) 131–149, https://doi.org/10.22211/ cejem/81176. [19] G. Sung, H. Choe, Y. Choi, J.H. Kim, Morphological, acoustical, and physical properties of free-rising polyurethane foams depending on the flow directions, Korean J. Chem. Eng. 35 (4) (2018) 1045–1052, https://doi.org/10.1007/ s11814-017-0328-2. [20] A.L. Silva, J.C. Bordado, Recent developments in polyurethane catalysis: Catalytic mechanisms review, Catal. Rev. - Sci. Eng. 46 (1) (2004) 31–51, https://doi.org/10.1081/CR-120027049. [21] R. Van Maris, Y. Tamano, H. Yoshimura, K.M. Gay, Polyurethane catalysis by tertiary amines, J. Cell. Plast. 41 (4) (2005) 205–322, https://doi.org/10.1177/ 0021955X05055113. [22] H. Sardon, A. Pascual, D. Mecerreyes, D. Taton, H. Cramail, J.L. Hedrick, Synthesis of polyurethanes using organocatalysis: A perspective, Macromolecules 48 (10) (2015) 3153–3165, https://doi.org/10.1021/ acs.macromol.5b00384. [23] S. Dworakowska, D. Bogdał, F. Zaccheria, N. Ravasio, The role of catalysis in the synthesis of polyurethane foams based on renewable raw materials, Catal. Today 223 (2014) 148–156, https://doi.org/10.1016/j.cattod.2013.11.054.

[24] A.A. Mahmoud, E.A.A. Nasr, A.A.H. Maamoun, The influence of polyurethane foam on the insulation characteristics of mortar pastes, J. Miner. Mater. Char. Eng. 5 (2) (2017) 49–61, https://doi.org/10.4236/jmmce.2017.52005. [25] B. Bantu, G.M. Pawar, K. Wurst, U. Decker, A.M. Schmidt, M.R. Buchmeiser, CO2, magnesium, aluminum, and zinc adducts of n-heterocyclic carbenes as (latent) catalysts for polyurethane synthesis, Eur. J. Inorg. Chem. 2009 (13) (2009) 1970–1976, https://doi.org/10.1002/ejic.200801161. [26] L.J. Lee, polyurethane reaction injection molding: process, materials, and properties, Rubber Chem. Technol. 53 (3) (1980) 542–599, https://doi.org/ 10.5254/1.3535053. [27] G.Y. Wang, Y.L. Wang, C.P. Hu, Interpenetrating polymer networks of polyurethane and graft vinyl ester resin: polyurethane formed with toluene diisocyanate, Eur. Polym. J. 36 (4) (2000) 735–742, https://doi.org/10.1016/ S0014-3057(99)00113-5. [28] A.J. Ryan, J.L. Stanford, R.H. Still, Thermal, mechanical and fracture properties of reaction injection-moulded poly(urethane-urea)s, Polymer (Guildf) 32 (8) (1991) 1426–1439, https://doi.org/10.1016/0032-3861(91)90423-G. [29] D.V. Rosato, D.V. Rosato, M.V. Rosato, Plastic Product Material and Process Selection Handbook, Elsevier, 2004, pp. 406–427, https://doi.org/10.1016/ b978-185617431-2/50015-3. [30] A.E. Polloni, A. Valério, D. De Oliveira, P.H.H. De Araújo, C. Sayer, Ultrasound assisted miniemulsion polymerization to prepare poly(urea-urethane) nanoparticles, Polimeros 28 (2) (2018) 155–160, https://doi.org/10.1590/ 0104-1428.02517. [31] H. Mao, S. Qiang, Y. Xu, C. Wang, Synthesis of polymeric dyes based on UV curable multifunctional waterborne polyurethane for textile coating, New J. Chem. 41 (2) (2017) 619–627, https://doi.org/10.1039/c6nj03159e. [32] D. Sridaeng, W. Jitaree, P. Thiampanya, N. Chantarasiri, Preparation of rigid polyurethane foams using low-emission catalysts derived from metal acetates and ethanolamine, E-Polymers 16 (4) (2016) 265–275, https://doi.org/ 10.1515/epoly-2016-0021. [33] S. Barman, B. Parasar, P. Kundu, S. Roy, A copper based catalyst for polyurethane synthesis from discarded motherboard, RSC Adv. 6 (79) (2016) 75749–75756, https://doi.org/10.1039/c6ra14506j. [34] O. Coutelier, M. El Ezzi, M. Destarac, F. Bonnette, T. Kato, A. Baceiredo, G. Sivasankarapillai, Y. Gnanou, D. Taton, N-Heterocyclic carbene-catalysed synthesis of polyurethanes, Polym. Chem. 3 (3) (2012) 605–608, https://doi. org/10.1039/c2py00477a. [35] S. Naumann, M. Speiser, R. Schowner, E. Giebel, M.R. Buchmeiser, Air stable and latent single-component curing of epoxy/anhydride resins catalyzed by thermally liberated N-heterocyclic carbenes, Macromolecules 47 (14) (2014) 4548–4556, https://doi.org/10.1021/ma501125k. [36] H. Kalita, Shape memory polymers: Theory and application, Walter de Gruyter GmbH & Co KG (2018). [37] S. Naumann, M.R. Buchmeiser, Liberation of N-heterocyclic carbenes (NHCs) from thermally labile progenitors: Protected NHCs as versatile tools in organoand polymerization catalysis, Catal. Sci. Technol. 4 (8) (2014) 2466–2479, https://doi.org/10.1039/c4cy00344f. [38] J. Alsarraf, Y.A. Ammar, F. Robert, E. Cloutet, H. Cramail, Y. Landais, Cyclic guanidines as efficient organocatalysts for the synthesis of polyurethanes, Macromolecules 45 (5) (2012) 2249–2256, https://doi.org/ 10.1021/ma2026258. [39] H.J. Noh, T. Sadhasivam, D.S. Jung, K.D. Lee, J.Y. Kim, H.Y. Jung, Poly (styrene)supported N-heterocyclic carbene coordinated iron chloride as a catalyst for delayed polyurethane polymerization, RSC Adv. 8 (65) (2018) 37339–37347, https://doi.org/10.1039/C8RA07677D. [40] V. Nesterov, D. Reiter, P. Bag, P. Frisch, R. Holzner, A. Porzelt, S. Inoue, NHCs in main group chemistry, Chem. Rev. 118 (19) (2018) 9678–9842, https://doi.org/ 10.1021/acs.chemrev.8b00079. [41] Ö. Karaca, M.R. Anneser, J.W. Kück, A.C. Lindhorst, M. Cokoja, F.E. Kühn, Iron(II) N-heterocyclic carbene complexes in catalytic one-pot Wittig reactions: Mechanistic insights, J. Catal. 344 (2016) 213–220, https://doi.org/10.1016/j. jcat.2016.09.029. [42] S.N. Riduan, J.Y. Ying, Y. Zhang, Solid poly-N-heterocyclic carbene catalyzed CO2 reduction with hydrosilanes, J. Catal. 343 (2016) 46–51, https://doi.org/ 10.1016/j.jcat.2015.09.009. [43] D.V. Espinosa, S. Martín, J.A. Mata, The non-innocent role of graphene in the formation/immobilization of ultra-small gold nanoparticles functionalized with N-heterocyclic carbene ligands, J. Catal. 375 (2019) 419–426, https://doi. org/10.1016/j.jcat.2019.06.009. [44] I. Misztalewska-Turkowicz, K.H. Markiewicz, M. Michalak, A.Z. Wilczewska, NHC-copper complexes immobilized on magnetic nanoparticles: Synthesis and catalytic activity in the CuAAC reactions, J. Catal. 362 (2018) 46–54, https://doi.org/10.1016/j.jcat.2018.03.015. [45] A.R. Hlil, S. Moncho, R. Tuba, K. Elsaid, G. Szarka, E.N. Brothers, R.H. Grubbs, M. Al-Hashimi, H.S. Bazzi, Synthesis and catalytic activity of supported acenaphthoimidazolylidene N-heterocyclic carbene ruthenium complex for ring closing metathesis (RCM) and ring opening metathesis polymerization (ROMP), J. Catal. 344 (2016) 100–107, https://doi.org/10.1016/j. jcat.2016.08.019. [46] W. Oberhauser, C. Evangelisti, A. Liscio, A. Kovtun, Y. Cao, F. Vizza, Glycerol to lactic acid conversion by NHC-stabilized iridium nanoparticles, J. Catal. 368 (2018) 298–305, https://doi.org/10.1016/j.jcat.2018.10.024. [47] F. Martinez-Espinar, P. Blondeau, P. Nolis, B. Chaudret, C. Claver, S. Castillón, C. Godard, NHC-stabilised Rh nanoparticles: Surface study and application in the

H.-J. Noh et al. / Journal of Catalysis 382 (2020) 77–85

[48]

[49]

[50]

[51]

[52]

[53] [54]

[55]

catalytic hydrogenation of aromatic substrates, J. Catal. 354 (2017) 113–127, https://doi.org/10.1016/j.jcat.2017.08.010. A.C. Lindhorst, M. Drees, W. Bonrath, J. Schütz, T. Netscher, F.E. Kühn, Mechanistic insights into the biomimetic catalytic hydroxylation of arenes by a molecular Fe(NHC) complex, J. Catal. 352 (2017) 599–605, https://doi.org/ 10.1016/j.jcat.2017.06.018. B. Bantu, G.M. Pawar, U. Decker, K. Wurst, A.M. Schmidt, M.R. Buchmeiser, CO2 and Sn II adducts of N-heterocyclic carbenes as delayed-action catalysts for polyurethane synthesis, Chem. - A Eur. J. 15 (13) (2009) 3103–3109, https:// doi.org/10.1002/chem.200802670. M.N. Hopkinson, C. Richter, M. Schedler, F. Glorius, An overview of Nheterocyclic carbenes, Nature 510 (7506) (2014) 485, https://doi.org/ 10.1038/nature13384. K. Hara, K. Iwahashi, S. Takakusagi, K. Uosaki, M. Sawamura, Construction of self-assembled monolayer terminated with N-heterocyclic carbene-rhodium (I) complex moiety, Surf. Sci. 601 (22) (2007) 5127–5132, https://doi.org/ 10.1016/j.susc.2007.04.158. Y.H. Kim, S. Shin, H.J. Yoon, J.W. Kim, J.K. Cho, Y.S. Lee, Polymer-supported Nheterocyclic carbene-iron(III) catalyst and its application to dehydration of fructose into 5-hydroxymethyl-2-furfural, Catal. Commun. 40 (2013) 18–22, https://doi.org/10.1016/j.catcom.2013.05.014. S.M. Sadeghzdeh, M. Mogharabi, Metal complexes immobilized on magnetic nanoparticles, Licensee InTech (2016) 57–85, https://doi.org/10.5772/61585. M. Hans, J. Lorkowski, A. Demonceau, L. Delaude, Efficient synthetic protocols for the preparation of common N-heterocyclic carbene precursors, Beilstein J. Org. Chem. 11 (1) (2015) 2318–2325, https://doi.org/10.3762/bjoc.11.252. J.A. Przyojski, H.D. Arman, Z.J. Tonzetich, Complexes of iron (ii) and iron (iii) containing aryl-substituted N-heterocyclic carbine ligands, Organometallics 31 (8) (2012) 3264–3271, https://doi.org/10.1021/om300106u.

85

[56] H.C. Chang, D.T. Hsu, Interactions of ionic liquids and surfaces of graphene related nanoparticles under high pressures, Phys. Chem. Chem. Phys. 19 (19) (2017) 12269–12275, https://doi.org/10.1039/C7CP00978J. [57] A.H. Idehara, P.D. Gois, H. Fernandez, B.E. Goi, A.E. Machado, B.S. Lima-Neto, V. P. Carvalho Jr, Accessible ring opening metathesis and atom transfer radical polymerization catalysts based on dimethyl sulfoxide ruthenium (II) complexes bearing N-heterocyclic carbene ligands, Mol. Catal. 448 (2018) 135–143, https://doi.org/10.1016/j.mcat.2018.01.032. [58] B. Wang, M. Anpo, J. Lin, C. Yang, Y. Zhang, X. Wang, Direct hydroxylation of benzene to phenol on h-BCN nanosheets in the presence of FeCl3 and H2O2 under visible light, Catal. Today 324 (2019) 73–82, https://doi.org/10.1016/ j.cattod.2018.07.001. [59] A.P. Grosvenor, B.A. Kobe, M.C. Biesinger, N.S. McIntyre, Investigation of multiplet splitting of Fe 2p XPS spectra and bonding in i66ron compounds, Surf. Interface Anal. 36 (2004) 1564–1574, https://doi.org/10.1002/sia.1984. [60] J.O. Akindoyo, M. Beg, S. Ghazali, M.R. Islam, N. Jeyaratnam, A.R. Yuvaraj, Polyurethane types, synthesis and applications–a review, Rsc Adv. 6 (115) (2016) 114453–114482, https://doi.org/10.1039/C6RA14525F. [61] R.P. Subrayan, S. Zhang, F.N. Jones, V. Swarup, A.I. Yezrielev, Reactions of phenolic ester alcohol with aliphatic isocyanates - transcarbamoylation of phenolic to aliphatic urethane: a 13C-NMR study, J. Appl. Polym. Sci. 77 (10) (2000) 2212–2228, https://doi.org/10.1002/1097-4628(20000906) 77:10<2212::AID-APP15>3.0.CO;2-B. [62] H. Sardon, A.C. Engler, J.M.W. Chan, J.M. García, D.J. Coady, A. Pascual, D. Mecerreyes, G.O. Jones, J.E. Rice, H.W. Horn, J.L. Hedrick, Organic acidcatalyzed polyurethane formation via a dual-activated mechanism: Unexpected preference of n-activation over o-activation of isocyanates, J. Am. Chem. Soc. 135 (43) (2013) 16235–16241, https://doi.org/ 10.1021/ja408641g.