Reprocessable Aliphatic Polydithiourethanes Based on the Reversible

Aug 5, 2019 - Aliphatic polydithiourethanes (PDTUs) were successfully synthesized from diisothiocyanates and dithiols having different values of the c...
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Reprocessable Aliphatic Polydithiourethanes Based on the Reversible Addition Reaction of Diisothiocyanates and Dithiols Yoshiaki Yoshida,† Kenji Ohnaka,‡ and Takeshi Endo*,† †

Molecular Engineering Institute, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu-shi, Fukuoka 804-8550, Japan ‡ Materials Science Institute, Konishi Co., Ltd., 5-3-35 Nishibori, Sakura-ku, Saitama, Saitama 338-0832, Japan

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S Supporting Information *

ABSTRACT: Aliphatic polydithiourethanes (PDTUs) were successfully synthesized from diisothiocyanates and dithiols having different values of the carbon atom number. The thermal properties and hydrogen-bonding behavior of PDTUs were studied in detail by using differential scanning calorimetry, thermal gravimetric analysis, temperature dependence nuclear magnetic resonance, Fourier transform infrared-attenuated total reflection, and density functional theory and semiempirical molecular orbital methods. In this work, we demonstrated that the thermal decomposition of PDTUs happened from the cleavage of the C−S bond of the dithiocarbamate moiety in the main chain and also that the active isothiocyanate and thiol groups were reprepared in the terminal of the decomposed polymer (i.e., oligomer). Furthermore, we found that the thermal depolymerization of PDTUs and the repolymerization of dithiourethane oligomers at room temperature proceeded repeatedly a few times.

1. INTRODUCTION In the modern world supported by a variety of polymer materials, the reutilization and recycling of degraded and damaged polymers are one of the important challenges. During the first decade of this century, a lot of researchers have been interested in the self-healing and reprocessable polymers,1,2 which are known as the intelligent materials such as those with a response to external stimulus,3 molecular recognition,4 and shape memory5 in not only academic but also industrial fields. The self-healing polymers are classified broadly into the healing behavior by extrinsic and intrinsic mechanisms.6 In the case of an intrinsic self-healing process, the polymers exhibit the self-healing behavior based on the reversible chemical bond systems, which are divided into the noncovalent and covalent bonds.6 The self-healing based on the noncovalent bond system was provided with molecular interactions such as hydrogen bonds,7,8 electronic and ionic interactions,9,10 and π−π interactions.11 In particular, polyurethanes and polyureas can be applied as the self-healing and shape-memory materials because the hydrogen-bonding fashions can be tailored to the requirement by designing the molecular structure.12,13 On the other hand, the Diels−Alder reaction14 and bond exchange reaction of disulfide15 and ester moieties16−18 are applied for the intrinsic self-healing materials based on the dynamic covalent bond chemistry.19−21 Particularly, the Diels−Alder reaction between furan and maleimide components has been widely utilized to prepare the reversible cross-linked polymers.22 In the case of a covalent bond system, polyurethanes and polyureas have been also applied as the © XXXX American Chemical Society

reprocessable polymers, which give rise to the bond exchange reaction of carbamate and carbamide moieties, respectively, by an external stimulus such as high temperature and an addition of catalysts.23−25 Several research groups have demonstrated that a poly(urethane−urea) network derived from a hindered amine exhibited self-healing and shape-memory behaviors in a wide temperature range because the bond exchange reaction of the hindered urea derivatives happened under ambient temperature and catalyst-free conditions.26−29 Hence, polyurethanes and polyureas are attractive as the reprocessable materials from both perspectives of the molecular interaction due to hydrogen bonding and the dynamic covalent bonds due to the bond exchange reaction. However, there are two requirements in this system.30 First, the bulky substituents on the urea moieties are required to destabilize the urea bonds, which is one of the most stable chemical bonds against further reactions. Second, the reaction rate of isocyanates and hindered amines have to be more rapid than the degradation rate of isocyanates due to hydrolysis. Based on the previous research results, we hypothesized that the isothiocyanate−thiol system was suitable as the reprocessable material because the addition reaction of isothiocyanates and thiols, which are stable compounds in aqueous solution at room temperature,31,32 successfully proceeded with an amine catalyst at 25 °C, and the produced dithiocarbamate moiety was readily dissociated by Received: February 18, 2019 Revised: June 17, 2019

A

DOI: 10.1021/acs.macromol.9b00350 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules the external stimulus.33 Accordingly, we have demonstrated that polyaddition of various diisothiocyanates and dithiols was achieved with triethylamine in DMF at 25 °C to obtain the corresponding polydithiourethanes (PDTUs) in a higher yield and molecular weight.34 Recently, polythioureas are featured because the hydrogen bonding of their polymers is fairly weak and dynamic. Yanagisawa and coworkers have demonstrated that poly(ether−thiourea) exhibited self-healing behavior at ambient temperature because of the less-ordered hydrogen bonding in the amorphous polymer matrices.35 Moreover, Wagner and Mioskowski et al. have reported the synthesis procedure of N,N′-substituted thioureas by thermolytic cleavage of dithiocarbamate bonds in the solid state.36 Therefore, we have studied about the depolymerization of aliphatic PDTUs based on the thermal decomposition and the repolymerization of the decomposed polymers at room temperature to discover the reversible nature of addition and dissociation reactions of the aliphatic diisothiocyanates and dithiols.

2.4. Polymerization. Polymerization of diisothiocyanates and dithiols is as shown in Scheme 1.34 The results of polymerization are

Scheme 1. Synthesis of Polydithiourethanes from Aliphatic Diisothiocyanates and Dithiols

Table 1. Synthesis of Polydithiourethanes from Aliphatic Diisothiocyanates and Dithiols

2. EXPERIMENTAL SECTION 2.1. Materials. 1,3-Propanedithiol, 1,6-hexanedithiol, 1,10-decanedithiol, 1,12-diaminododecane, 1,6-hexanediol, hexyl mercaptan, and hexamethylene diisocyanate were purchased from the Tokyo Chemical Industry (Tokyo, Japan). 1,3-Propane diisothiocyanate and hexamethylene diisothiocyanate were supplied from Nippon Terpene Chemicals, Inc. (Tokyo, Japan). Triethylamine (TEA), dibutyltin dilaurate (DBTL), 1-[bis(dimethylamino)methylene]-1H-benzotriazolium 3-oxide hexafluorophosphate (HBTU), N,N′-dicyclohexylcarbodiimide (DCC), carbon disulfide (CS2), and all solvents were purchased from Wako Pure Chemical Industries (Osaka, Japan). 2.2. Measurements. NMR spectra were obtained using a JEOL ECS-400 spectrometer operating at 400 MHz for 1H and 100 MHz for 13C in chloroform-d (CDCl3) and N,N-dimethylformamide-d7 (DMF-d7) using tetramethyl silane (TMS) as an internal reference. FTIR spectra were recorded on a Thermo Scientific Nicolet iS10 spectrometer equipped with a Smart iTR diamond ATR sampling accessory in the range of 4000−650 cm−1. Gel permeation chromatography (GPC) was carried out on a TOSOH HLC-8320 system equipped with two consecutive polystyrene gel columns [TSKgel guard column SuperH-H (4.6 mm i.d. × 3.5 cm, 4 μm particle size) and TSKgel SuperHM-H (6.0 mm i.d. × 15 cm, 3 and 5 μm particle sizes)] and refractive index and ultraviolet (254 nm) detectors at 40 °C. The system was operated with tetrahydrofuran (THF) as eluent at a flow rate of 0.6 mL/min. Polystyrene standards were employed for calibration. Thermogravimetric analysis (TGA) was performed on Seiko Instruments Inc. TG-DTA 6200 with 1.5 mg of samples in an aluminum pan under a 100 mL/min N2 flow at a heating rate of 10 °C/min. Differential scanning calorimetry (DSC) analysis was conducted from 0 to 150 °C for the first heating scan and from −50 to 180 °C for the second heating scan on Seiko Instruments Inc. DSC-6200R with 1.5 mg of samples in an aluminum pan under a 50 mL/min N2 flow at a heating rate of 10 °C/min. The second scan was selected to determine glass transition temperature (Tg) values. Geometry-optimized structures for the model compounds were estimated by using DFT calculation with B3LYP/6-31G* and semiempirical molecular orbital (PM3) methods (Wavefunction, Inc., Spartan’06 Windows version 1.1.0).37 These model compounds were optimized by combination of DFT and PM3 methods. First, the unit structures of models 1 and 2 were optimized by DFT (B3LYP/631G*) calculation. Second, the conformation between the units was based on molecular interaction estimated by semiempirical molecular orbital calculation (PM3). 2.3. Monomer Synthesis. Dodecamethylene diisothiocyanate38 and hexyl diisothiocyanate39 were synthesized according to the literature. Procedures for monomer synthesis and characterizations of the monomer are shown in the Supporting Information.

a b

entry

R1

R2

yielda (%)

Mnb (kg/mol)

Mw/Mnb

1 2 3 4 5 6 7

C3 C3 C3 C6 C6 C6 C12

C3 C6 C10 C3 C6 C10 C6

98 98 94 99 98 98 92

55 43 50 42 42 42 15

1.82 1.92 2.07 2.52 2.24 2.10 1.98

Insoluble fraction in hexane/ethanol mixed solvent and methanol. Determined by GPC analysis (PSt, THF).

summarized in Table 1. Synthesis of polyurethane and polythiourethane is also shown in Scheme S2.33 Typical procedures for polymerization, 1H and 13C NMR spectra, and FTIR-ATR spectra of the obtained polymers are shown in the Supporting Information. 2.5. Reversible Reaction of the Model Compound. Hexyl isothiocyanate (287 mg) and hexyl mercaptan (237 mg) were mixed in bulk, and then triethylamine (21 mg) was added in the mixture. The mixture was held in an oven at 50 °C for 1 h. After cooling to room temperature, the crude product was held in an oven at 150 °C for 6 h. Before and after each step, the conversion of the isothiocyanate group was estimated by the FTIR-ATR method. The reversible reaction of the model compound was repeated one more time. 2.6. Depolymerization and Repolymerization. A solution of P(C6−C6) (Mn: 11,000 g/mol) in anhydrous DMF (1.0 mol/L) was stirred at 150 °C under a N2 atmosphere for 1 h. After cooling to room temperature, the mixture was stirred in the presence of triethylamine (11 mg) at 25 °C under a N2 atmosphere for 24 h. Before and after each step, the Mn and Mw of the polymer were estimated by GPC analysis. Depolymerization and repolymerization of P(C6−C6) were carried out five times.

3. RESULTS AND DISCUSSION The decomposition temperature for 5% weight loss (Td5) observed by TGA, glass transition (Tg), crystallization (Tc), and melting (Tm) temperatures observed by DSC, and chemical shift of the peak due to the NH moiety (δNH) observed by NMR are summarized in Table 2. 3.1. Thermal Property and Hydrogen-Bonding Behavior of Aliphatic Polydithiourethanes. Thermal properties of aliphatic PDTUs were observed by DSC analysis (Figure 1). The Tg values of the polymers decreased with the increasing value of the carbon atom number of diisothiocyanate and dithiol units in the main chain (Table 2 and Figure 1). Such behavior of Tg values suggests that the difference of the carbon atom number of diisothiocyanate and dithiol units in the main chain affects the hydrogen-bonding fashion between the polymer chains. In order to clarify the hydrogen-bonding behavior of aliphatic PDTUs, 1H NMR B

DOI: 10.1021/acs.macromol.9b00350 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 2. Td5, Tg, Tc, Tm, and δNH of PDTUs polymer

Td5a (°C)

Tgb (°C)

C3−C3 C3−C6 C3−C10 C6−C3 C6−C6 C6−C10 C12−C6

200.0 220.9 222.2 182.0 206.9 213.8 190.3

36.7 17.2 8.8 26.6 16.8 4.8 −8.7

Tcb (°C)

18.5

Tmb (°C)

δNHc (ppm)

86.1

9.95 9.87 9.86 9.91 9.85 9.83 9.83

without depending on temperature (Figure S9b). The temperature dependence NMR spectra of P(C3−C3) and P(C6−C6) indicate that the chemical shift of the NH peak exhibits strength of hydrogen bonding between the polymer chains. Accordingly, the hydrogen bonding of P(C3−C3) is stronger than that of P(C6−C6) because the NH peak due to a strong hydrogen bonding is observed at a lower magnetic field.33 Moreover, the hydrogen bonding of the polymers containing C3-dithiol units was stronger than that of the polymers containing C6- or C10-dithiol units because the peak due to the NH moiety of the former polymers was observed at a low magnetic field compared to that of the latter polymers (Table 2 and Figure S10). Therefore, the Tg values of the polymers containing C3-dithiol units were significantly higher than those of the polymers containing C6- or C10-dithiol units. On the other hand, the NH peaks of the polymers containing C6- or C10-dithiol units exhibited almost the same chemical shift even though the Tg values of the polymers containing C6-dithiol units were higher than those of the polymers containing C10-dithiol units (Table 2 and Figure S10). These results mean that the Tg values of the polymers containing C6- or C10-dithiol units depend on the alkyl chain length (i.e., flexibility) in not only dithiol but also diisothiocyanate units because the hydrogen bonding of those polymers is fairly weak. The degree of polymerization also affects the hydrogen-bonding behavior of the polymers because the number of hydrogen-bonding moieties in the polymer chain depends on the degree of polymerization and alkyl length of the monomer units. Actually, P(C3−C3) has a higher molecular weight and shorter alkyl units than those of the other polymers, meaning P(C3−C3) has a higher number of hydrogen-bonding units (Table 1, entry 1). On the other hand, the number of hydrogen-bonding units of P(C12−C6) is decreased compared to that of the other polymers because of having the lowest molecular weight and the long alkyl chain in the monomer units (Table 1, entry 7). Therefore, P(C3−C3) exhibited the highest Tg value and P(C12−C6) exhibited a fairly lower Tg value. P(C12−C6) also exhibited the phase transition temperatures attributed to the crystallization and melting points due to the alkyl chain packing between the polymer chains. 3.2. Thermal Decomposition Behavior of Aliphatic Polydithiourethanes. Thermal decomposition behaviors of the aliphatic PDTUs were observed by TGA measurement (Figure 2). The Td5 values of polymers decreased with the increasing value of the carbon atom number in the diisothiocyanate unit and also increased with the increasing value of the carbon atom number in the dithiol unit (Table 2). In order to clarify the thermal decomposition behaviors of aliphatic PDTUs, those of polyurethane (PU) and polythiourethane (PTU) containing the alkyl chain units, which were the same as P(C6−C6), were also observed by TGA measurement (Figure 3a). The Td5 values of PU and PTU, which were 281 and 244 °C, respectively, were higher than those of PDTU. The thermal stability of these polymers depends on the strength of intermolecular hydrogen bonding.33 In fact, PU and PTU were insoluble in any organic solvents because of the strong hydrogen bonding between the polymer chains, whereas PDTU was readily soluble in polar organic solvents such as THF and DMF. More importantly, the bond energy can be directly related to the thermal stability of these polymers.40 Figure 3b,c shows the FTIR-ATR spectra of polymers before and after annealing, respectively. The peaks due to carbonyl

Measured by TGA under a flowing N2 gas. bMeasured by DSC under a flowing N2 gas. cObserved at 25 °C in DMF-d6 by NMR. a

Figure 1. (a) DSC curves of P(C3−C6), P(C6−C6), and P(C12− C6). (b) DSC curves of P(C3−C3), P(C3−C10), P(C6−C3), and P(C6−C10).

spectra were observed in DMF-d6. In the temperature dependence NMR spectra of P(C3−C3), the peak due to the NH moiety of the polymers was shifted to a high magnetic field with increasing temperature, although the other peaks never exhibited any chemical shift (Figure S9a). In the case of P(C6−C6), the temperature dependence NMR spectra exhibited a similar behavior of the chemical shift to that of P(C3−C3), whereas the NH peak of P(C6−C6) was observed at a high magnetic field compared to that of P(C3−C3) C

DOI: 10.1021/acs.macromol.9b00350 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. (a) TGA traces of P(C3−C6), P(C6−C6), and P(C12− C6). (b) TGA traces of P(C3−C3), P(C3−C10), P(C6−C3), and P(C6−C10).

groups of the carbamate moiety of PU, thiocarbamate moiety of PTU, and thiocarbonyl group of the dithiocarbamate moiety of PDTU were observed at 1680, 1635, and 1495 cm−1, respectively, in the FTIR-ATR spectra of polymers before annealing (Figure 3b). After annealing in bulk at 150 °C for 12 h, the peaks due to the diisothiocyanate group (−NCS) was observed around 2100 cm−1 in the FTIR-ATR spectra of PDTU, whereas the FTIR-ATR spectra of PU and PTU never changed before and after annealing (Figure 3c). These results indicate that the C−S bond of the dithiocarbamate moiety is readily cleaved under a high-temperature condition because the bond energy of the C−S bond is fairly lower than that of the C−O bond.33 Therefore, the thermal stability of PDTUs depends on not only the intermolecular hydrogen bonding but also the bond energy of the dithiocarbamate moiety. However, P(C3−C6) and P(C6−C3) exhibited different Td5 values even though the polymers were constructed with the aliphatic units of the same carbon atom number in the main chain. The model compounds of bisdithiocarbamate containing propane diisothiocyanate and propane dithiol units (models 1 and 2, respectively) were optimized by combination of DFT and PM3 methods (Figure 4). The optimized structures of models 1 and 2 showed the twisted and parallel conformations of the

Figure 3. (a) TGA traces, (b) FTIR-ATR spectra after purification, and (c) FTIR-ATR spectra after annealing in bulk at 150 °C for 12 h of polyurethane (PU), polythiourethane (PTU), and polydithiourethane (PDTU).

propane moieties, respectively. The twisted conformation of model 1 can give rise to a decreased bond energy of the C−S bond (Figure 5a). In contrast, the C−S bond of model 2 is more stable than that of model 1 because of the ordered structure of the propane bisdithiocarbamate moiety (Figure 5b). These results suggest that the C−S bond energy decreases with increasing alkyl length of the diisothiocyanate moiety and increases with increasing alkyl length of the dithiol moiety. D

DOI: 10.1021/acs.macromol.9b00350 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. (a) Model compounds of bisdithiocarbamate containing propane diisothiocyanate units. (b) Model compounds of bisdithiocarbamate containing propane dithiol units.

Figure 6. (1) The reversible reaction was confirmed by using dihexyl dithiocarbamate as the model compound (Figure 6a). (2) The reaction was completely achieved for 1 h because the peak due to −NCS was never observed in the FTIR-ATR spectra (Figure 6b-i). (3) The conversion ratio of the retroreaction was approximately 20% for 6 h, although the peak due to −NCS was observed around 2100 cm−1 in the FTIR-ATR spectra (Figure 6b-ii). (4) The FTIR-ATR spectra showed that the reaction of dissociated isothiocyanate and thiol was completely achieved in the presence of TEA at 50 °C for 1 h because the peak due to −NCS was never observed as well as the first reaction (Figure 6b-iii).

Figure 5. (a, b) Presumed models of the hydrogen bond (di and dii) and C−S bond (Di and Dii) attributed to alkyl length of diisothiocyanate and dithiol units.

Therefore, P(C3−C6) exhibited a higher Td5 value compared to P(C6−C3). 3.3. Reversible Reaction of Aliphatic Dithiocarbamate. The isothiocyanate group dissociated after annealing of PDTU suggested a possibility of the thermal depolymerization of PDTUs and the repolymerization of the dissociated monomers. The reversible reaction was confirmed by using dihexyl dithiocarbamate as the model compound (Figure 6). First, hexyl isothiocyanate and hexyl mercaptan were reacted in the presence of TEA at 50 °C. The reaction was completely achieved for 1 h because the peak due to −NCS was never observed in the FTIR-ATR spectra. Next, the retroreaction of the obtained dihexyl dithiocarbamate was carried out at 150 °C. The conversion ratio of the retroreaction was approximately 20% for 6 h, although the peak due to −NCS was observed around 2100 cm−1 in the FTIR-ATR spectra. Additionally, the conversion was almost the same ratio even after reaction for 24 h. This result suggests that the retroreaction of dithiocarbamate derivatives and the reaction of a part of dissociated residues proceed simultaneously during annealing at 150 °C. Furthermore, the activity of dissociated isothiocyanate and thiol groups was also confirmed by their addition reaction. The FTIR-ATR spectra showed that the reaction of dissociated isothiocyanate and thiol was completely achieved in the presence of TEA at 50 °C for 1 h because the peak due to −NCS was never observed as well as the first reaction.

3.4. Depolymerization of PDTU and Repolymerization of the Dithiourethane Oligomer. Based on the reversible reaction of the model compound, the depolymerization of PDTU was carried out in DMF at 150 °C for 1 h (Figure 7a), and then GPC and FTIR-ATR of the depolymerized PDTU were measured. After the depolymerization process, the Mn value estimated by GPC decreased from 11,000 g/mol to 1400 g/mol, and the peak due to −NCS was observed around 2100 cm−1 in the FTIR-ATR spectra (Figure 7b-ii,c-ii). Accordingly, the residue ratio of the −NCS group estimated by FTIR-ATR was 29.7% (Table 3). The repolymerization was also carried out with TEA in DMF at 25 °C for 24 h (Figure 7a). After the repolymerization process, the Mn value increased from 1400 to 8600 g/mol, and the peak due to the −NCS group was slightly observed around 2100 cm−1 in the FTIR-ATR spectra (Figure 7b-iii,c-iii). These results suggested that the dithiourethane oligomer was prepared via depolymerization based on the thermal cleavage of the C−S bond of PDTU, and active isothiocyanate and thiol groups were reprepared in the terminal of the obtained dithiourethane oligomer. The repolymerization of the obtained oligomer was achieved incompletely because the Mn value of the resulting polymer was slightly less than that of the original polymer and the −NCS moiety was not completely consumed E

DOI: 10.1021/acs.macromol.9b00350 Macromolecules XXXX, XXX, XXX−XXX

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Table 3. Mn, Mw/Mn, and Residue Ratio of −NCS of (i) Original PDTU, (ii) Depolymerized PDTU, and (iii) the Repolymerized Dithiourethane Oligomer polymer

Mn (g/mol)a

Mw/Mna

−NCS (%)b

i ii iii

11,000 1400 8600

2.44 1.48 2.10

0 29.7 5.4

a Determined by GPC analysis (PSt, THF). bResidue ratio of −NCS estimated by FTIR-ATR.

Table 4. Mn (Mw/Mn) Values of Depolymerized PDTU and the Repolymerized Dithiourethane Oligomer during Five Cyclesa Mn (g/mol) [Mw/Mn]b cycle run run run run run

1 2 3 4 5

depolymerization 1400 1100 1100 1200 1100

[1.48] [1.30] [1.32] [1.35] [1.30]

repolymerzation 8600 7500 6200 5100 4900

[2.10] [1.68] [1.61] [1.55] [1.75]

−NCS (%)c 5.43 7.42 9.34 11.4 11.6

a Initial Mn [Mw/Mn] value is 11,000 g/mol [2.44]. bDetermined by GPC analysis (PSt, THF). c Residue ratio of −NCS after repolymerization estimated by FTIR-ATR.

4. CONCLUSIONS Thermal properties of the aliphatic PDTUs synthesized from diisothiocyanates and dithiols were studied in detail by using DSC, TGA, temperature dependence NMR, FTIR-ATR, and computational methods. The DSC curves and 1H NMR spectra indicated that the Tg values of PDTUs increased with the decreasing value of the carbon atom number of diisothiocyanate and dithiol units because the hydrogen bonding of polymers containing the diisothiocyanate and dithiol units of short alkyl chains was significantly strong compared to that of polymers containing units of long alkyl chains. The TGA trace, FTIR-ATR, and DFT calculation also presumed that the thermal decomposition of PDTUs happened from the cleavage of the C−S bond of the dithiocarbamate moiety. Accordingly, the reversible reaction of isothiocyanate and thiol derivatives was confirmed by using the aliphatic dithiourethane carbamate as the model compound. Furthermore, the reversible reaction happened even in the case of the polymer as well as in that of the low-molecularweight compound. PDTU afforded the dithiourethane oligomer via depolymerization based on the thermal cleavage of the C−S bond. The obtained dithiourethane oligomer also afforded PDTU via repolymerization based on the addition reaction of the active isothiocyanate and thiol groups in the terminal, although the Mn value barely reached that of the original polymer.

Figure 7. (a) Depolymerization of PDTU and repolymerization of the dithiourethane oligomer. (b) GPC profiles and (c) FTIR-ATR spectra of (i) original PDTU, (ii) depolymerized PDTU, and (iii) the repolymerized dithiourethane oligomer.

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via repolymerization (Figure 7 and Table 3). Furthermore, depolymerization of PDTU and repolymerization of the dithiourethane oligomers proceeded repeatedly, although the Mn value of the repolymerized polymer decreased with the increasing number of repeated cycles (Table 4). Therefore, these results proved that the reversible reaction happened even in the case of the polymer as well as in that of the aliphatic dithiocarbamate derivative of the model compound.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00350. Experimental section including synthesis of monomers and polymers, spectral copies (1H and 13C NMR, and FTIR-ATR), and computational results (PDF) F

DOI: 10.1021/acs.macromol.9b00350 Macromolecules XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Takeshi Endo: 0000-0001-6903-8048 Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.macromol.9b00350 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.9b00350 Macromolecules XXXX, XXX, XXX−XXX