Synthesis and Radical Polymerization Properties of

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Synthesis and Radical Polymerization Properties of Thermal Radical. Initiators Based on ... tors such as benzoyl peroxide (BPO) as a diacyl peroxide,.
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Synthesis and Radical Polymerization Properties of Thermal Radical Initiators Based on O-Imino-Isourea: The Effect of the Alkyl Side Chain on the Radical Initiation Temperature Kyu Cheol Lee,1 Su Bin Jeong,1 Dong Yeon Kim,2 Tae Hee Lee,1 Soon Cheon Kim,1 Jin Chul Kim,1 Sang-Ho Lee,1 Seung Man Noh,1 Young Il Park 1 1

Research Center for Green Fine Chemicals, Korea Research Institute of Chemical Technology, Ulsan 44412, Republic of Korea Department of Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea Correspondence to: S. M. Noh (E-mail: [email protected]) or Y. I. Park (E-mail: [email protected])

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Received 12 March 2018; accepted 3 May 2018; published online in Wiley Online Library DOI: 10.1002/pola.29057

ABSTRACT: Four types of thermal radical initiators (TRIs) that are based on o-imino-isourea with cyclohexyl and isopropyl groups were successfully synthesized, namely, C-HexDCC, DiiprDCC, C-HexDIC, and DiiprDIC. The free radical polymerization and thermal properties of those synthesized TRIs were determined via differential scanning calorimetry (DSC) (using n-butyl acrylate) and thermogravimetric analysis (TGA), respectively. The TRI derivatives showed peak temperatures (Tmax) from 89 to 97 8C in n-butyl acrylate, and DiiprDIC, with isopropyl groups on both sides of the NAO group, showed the lowest peak temperatures. The rates of NAO bond homolysis (kd) of all the TRIs were calculated from their half-lives determined using real-time nuclear magnetic resonance (NMR) spectroscopy, and their theoretical bond dissociation energies (BDEs)

INTRODUCTION Conventional radical polymerization using

free radical initiators that can produce radical species is one of the most common, useful, and classical chemical reactions because of its mild conditions and facile synthetic methods. In particular, more than half of the polymers that are used in industrial polymer research are produced by radical reactions, providing plastic sources for a range of markets.1–3 This radical polymerization using radical initiators is mainly initiated by thermal or light energy. Therefore, radical initiators such as benzoyl peroxide (BPO) as a diacyl peroxide, azobisisobutyronitrile (AIBN) as an azo compound, and organotin hydrides play an important role in achieving efficient polymerization despite their minimal amounts.4,5 However, the traditionally used azo- or peroxide-based thermal radical initiators (TRIs) are notoriously difficult to store for long periods of time because their high exothermic reactivity can lead to explosions as well as gelation in coating formulations containing initiators. In addition, the release of CO2 or N2,

were calculated using density functional theory (DFT) calculations. The free radical polymerization of n-butyl acrylate using each TRI was efficiently determined from Tpeak of the DSC curves; conversions depending on polymerization temperature (80, 90, and 100 8C) were monitored to observe kinetic information of TRIs during polymerization. Furthermore, to investigate the use of TRIs in curing, we applied them to an automotive clear coating system and monitored the real-time evolution of the elastic modulus (G0 ) during thermal curing using a rheomeC 2018 Wiley Periodicals, Inc. J. ter for representative DiiprDIC. V Polym. Sci., Part A: Polym. Chem. 2018, 56, 1749–1756 KEYWORDS: DFT calculation; dissociation constant; free radical

polymerization; o-imino-isourea; thermal radical initiators

which always occurs after radical initiation, can adversely affect the film morphology and performance.6 To meet the requirements of environmental regulations and the coating industry, developing a new class of TRIs without azo or peroxide components is necessary. As new alternative TRIs used in curing processes, TRIs based on o-imino-isourea that contain nitrogen and oxygen single bonds have received much attention because of their low activation energy, facile synthesis, and non-explosivity.7–10 In addition, depending on their functionalized structures, they have different radical initiation temperatures; hence, the radical initiation temperature can be controlled using these TRIs. Recently, we reported the synthesis of new TRIs based on the o-imino group of linear and cyclic aliphatic types in an oxime group and demonstrated their use in the polymerization of n-butyl acrylate as well as their application in coating systems. Previous studies reported that the initiation

Additional Supporting Information may be found in the online version of this article. C 2018 Wiley Periodicals, Inc. V

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FIGURE 1 Chemical structures of four kinds of thermal radical initiators (TRIs) with different alkyl group positions.

temperature could be controlled by cyclic TRIs blended with n-butyl acrylate by varying their ring sizes.7 TRIs based on the o-imino group were systematically investigated herein to more deeply explore the correlation between the chemical structure of aliphatic groups with NAO bonds and properties such as the initiation temperature, polymerization efficiency, and performance in coating. We synthesized four types of TRIs based on the o-imino group, containing isopropyl and cyclohexyl groups, as shown in Figure 1; these types are denoted as C-HexDCC, DiiprDCC, C-HexDIC, and DiiprDIC. Their rates of NAO bond homolysis (kd) were determined from their half-lives using real-time nuclear magnetic resonance (NMR) spectroscopy, and their thermal properties, such as initiation and degradation temperatures, were determined via differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The radical polymerization efficiency of TRIs in n-butyl acrylate, analyzed as poly(butyl acrylate), was determined by monitoring the number average/weight average molecular weight (Mn/Mw) and conversion over time to investigate the effect of the initiator on the polymerization rate. To explain the initiation temperatures of the TRIs, the bond dissociation energies (BDEs) were determined from density functional theory (DFT) calculations using Gaussian09, and these supported the experimental results. With regard to their industrial applicability in automotive coating processes, the real-time evolution of the rheological properties of clear coat resins formulated with the synthesized TRIs was monitored, indicating that an efficiently crosslinked polymer network formed during the thermal curing process. EXPERIMENTAL

The reagents as well as all starting materials were purchased from Aldrich Co., Alfa Aesar, and TCI Co. used without further purification. All solvents are ACS grade unless otherwise noted. In case of THF, solvent purification system was used. 1 H NMR and 13C NMR spectra were recorded on an UItrashied 300 MHz spectrophotometer (Bruker, Germany). FTIR spectra were recorded on Nicolet 6700/Nicolet Continuum (Thermo Fisher Scientific, Waltham, USA). Elemental

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analysis (EA) was measured from FLASH EA-2000 and Finnigan FLASH EA-1112 (Thermo Fisher, Scientific, USA). LC–MS were evaluated using Quadrupole LC/MS6130 (Agilent U.S. HPLC 1260). The thermal stabilities were performed by Thermogravimetric (TGA, TA Instruments, USA) in range from 25 to 400 8C at the heating rate of 5 8C/min and differential scanning calorimetry (DSC) curves were recorded by Differential Scanning Calorimeter (TA Instruments, USA) at different heating rate of 1–10 8C/min in between 25 and 160 8C according to the test condition. The molecular weight of polymers via radical initiator was determined by GPC (Agilent Tech 1260) equipped with a set of gel column (Agilent PLgel 5 mm MIXED-D column) using polystyrene standards in THF at 25 8C (1 mL/min). Bond dissociation energies (BDEs) and the geometry optimization of TRIs were prepared by DFT calculation using the Gaussian09.11 Elastic modulus data were recorded by oscillational rheometer (Thermo Scientific, Inc., MARS III) and pendulum period data were evaluated using RPT (A & D Co. Ltd, RPT-3000w). General Synthetic Methods The compound 2,4-dimethyl-3-pentanone oxime (2a) was prepared by a general oxime synthesis procedure that was reported in the literature,12 and C-HexDCC was synthesized by a procedure reported by our group.7 2,4-Dimethyl-3-Pentanone Oxime (2a) 1 H NMR (CDCl3, 300 MHz): d (ppm) 5 3.19 (septet, 1 H, J 5 6.9 Hz), 2.56 (septet, 1 H, J 5 6.9 Hz), 1.17–1.11 (m, 12 H); 13C NMR (CDCl3, 75 MHz): d (ppm) 5 168.06, 30.67, 27.52, 21.21, 18.69. LC–MS calc. for C7H15NO: 129.20; found: 130.3 (See Figure S1). (E)-Cyclohexanone O-(N,N0 -Dicyclohexylcarbamimidoyl) Oxime (C-HexDCC) 1 H NMR (CDCl3, 300 MHz): d (ppm) 5 5.24 (brs, 1 H), 3.46 (brs, 2 H), 2.52 (t, 2 H, J 5 6 Hz), 2.23 (t, 2 H, J 5 6 Hz), 2.20–1.14 (m, 26 H); 13C NMR (CDCl3, 75 MHz): 163.08, 148.37, 51.96, 33.98, 32.06, 26.72, 26.11, 25.99, 25.60, 25.51, 25.16. LC–MS calc. for C19H33N3O: 319.49; found: 320.6; element anal. calc. for C19H33N3O: C, 71.43; H, 10.41; N, 13.15; O, 5.01; found: C, 71.56; H, 10.28; N, 13.61; O, 4.90.

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SCHEME 1 Synthetic route of thermal radical initiators. [Color figure can be viewed at wileyonlinelibrary.com]

(E)-2,4-Dimethyl-3-Pentanone O-(N,N0 Dicyclohexylcarbamimidoyl) Oxime (DiiprDCC) NaOH (110 mg, 2.85 mmol) was added to a solution of 2,4dimethyl-3-pentanone oxime (2a) (3.68 g, 28.5 mmol) in tetrahydrofuran (THF) (60 mL) at room temperature. After stirring for 5 min, dicyclohexylcarbodiimide (5.30 g, 25.7 mmol) was added to the reaction mixture, which was then stirred overnight. The reaction’s progress was monitored by NMR spectroscopy. Afterwards, dichloromethane (50 mL) was added to the product mixture, which was then filtered. The residue was extracted with hexane and acetonitrile three times and then evaporated to obtain a white solid (6.86 g, 72%). 1H NMR (CDCl3, 300 MHz): d (ppm) 5 5.23 (brs, 1 H), 3.48 (brs, 2 H), 2.96 (t, 1 H, J 5 6.9 Hz), 2.58 (t, 1 H, J 5 6.9 Hz), 1.88–1.11 (m, 32 H). 13C NMR (CDCl3, 75 MHz): d (ppm) 5 170.83, 148.65, 55.74, 52.74, 49.41, 35.29, 34.92, 32.85, 29.60, 26.02, 25.45, 25.01, 24.68, 20.62, 18.84. LC–MS calc. for C20H37N3O: 335.54; found: 336.6; element anal. calc. for C20H37N3O: C, 71.59; H, 11.12; N, 12.52; O, 4.77; found: C, 71.95; H, 10.89; N, 13.01; O, 4.51. (E)-Cyclohexanone O-(N,N0 -Diisopropylcarbamimidoyl) Oxime (C-HexDIC) This compound was prepared according to the synthesis method used for DiiprDCC. Cyclohexanone oxime (2b) (5.0 g, 44.19 mmol), diisopropylcarbodiimide (4.63 g, 36.67 mmol), and NaOH (180 mg, 4.42 mmol) were reacted to obtain a white solid (3.9 g, 40%). 1H NMR (CDCl3, 300 MHz): d (ppm) 5 5.01 (brs, 1 H), 3.84–3.75 (m, 2 H), 2.52 (t, 2 H, J 5 6 Hz), 2.23 (t, 2 H, J 5 6 Hz), 1.72–1.55 (m, 6 H), 1.21– 1.01 (m, 12 H); 13C NMR (CDCl3, 75 MHz): d (ppm) 5 163.25, 148.44, 44.01, 42.94, 32.07, 26.76, 26.18, 25.64, 25.53, 24.98, 22.76. LC–MS calc. for C13H25N3O: 239.36; found: 240.4; element anal. calc. for C13H25N3O: C, 65.23; H, 10.53; N, 17.56; O, 6.68; found: C, 65.58; H, 10.43; N, 18.68; O, 6.42.

(E)-2,4-Dimethyl-3-Pentanone O-(N,N0 Diisopropylcarbamimidoyl) Oxime (DiiprDIC) This compound was prepared according to the synthesis method used for DiiprDCC. 2,4-Dimethyl-3-pentanone oxime (2a) (4.0 g, 31.0 mmol), diisopropylcarbodiimide (3.53 g, 28.0 mmol), and NaOH (120 mg, 3.1 mmol) were reacted to obtain a colorless oil (7.26 g, 92%). 1H NMR (CDCl3, 300 MHz): d (ppm) 5 5.10 (brs, 1 H), 3.84 (brs, 2 H), 3.02 (septet, 1 H, J 5 7.2 Hz), 2.61 (septet, 1 H, J 5 7.2 Hz); 13C NMR (CDCl3, 75 MHz): d (ppm) 5 171.10, 148.66, 44.30, 43.07, 32.24, 29.46, 25.21, 22.87, 20.79, 18.98. LC–MS calc. for C14H29N3O: 255.41; found: 256.2; element anal. calc. for C14H29N3O: C, 65.84; H, 11.45; N, 16.45; O, 6.26; found: C, 64.21; H, 11.28; N, 17.47; O, 6.13. Polymerization of n-Butyl Acrylate A mixture of n-butyl acrylate (100 mmol) and TRI (1 mmol) without solvent under argon condition was stirred for 1 h, at 80, 90, and 100 8C, respectively. During polymerization, the conversion (%) of n-butyl acrylate was measured at intervals of 15 min to 1 h and was calculated by integration ratio of methylene unit next to the carbonyl and vinyl group in nbutyl acrylate in the NMR spectra as shown in Supporting Information Figure S10. RESULTS AND DISCUSSION

Synthesis of TRIs and Their Thermal Stabilities As shown in Scheme 1, a diisopropyl oxime (2a) was synthesized via a general oxime synthesis method using hydroxylamine and sodium acetate. The new oxime TRIs based on oimino-isourea contained either a diisopropyl (2a) or a cyclohexyl (2b) group and either N,N0 -dicyclohexylcarbodiimide (DCC) or N,N0 -diisopropylcarbodiimide (DIC); these were synthesized using NaOH with THF as a solvent, affording yields of 40–90%. After obtaining the products, C-HexDCC, DiiprDCC, and C-HexDIC were recrystallized from methanol

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FIGURE 2 DSC curves for (a) intrinsic properties of only TRIs and (b) TRIs with n-butyl acrylate. [Color figure can be viewed at wileyonlinelibrary.com]

to afford pure compounds. However, DiiprDIC with a diisopropyl group was purified by washing with acetonitrile and hexane several times at room temperature as it was a liquid. The chemical structures of the synthesized TRIs were characterized by NMR spectroscopy, liquid chromatography–mass spectrometry (LC–MS), and elemental analysis (See Figures S2-S9). To determine decomposition temperature of each TRI, DSC, and TGA was analyzed as shown in Figure 2(a), Supporting

Information Figure S12, and listed in Table 1. For TGA, the degradation initiation temperatures of C-HexDCC, DiiprDCC, C-HexDIC, and DiiprDIC were recorded at 109, 107, 94, and 85 8C, respectively. These results indicate that the alkyl group on the o-imino-isourea moiety could control for thermal stability and, particularly, the diisopropyl group on isourea part (DIC) was relatively more efficiently in regulating temperature. DSC analysis for C-HexDCC and C-HexDIC showed melting point transition at 50 and 45 8C,

TABLE 1 Various Molecular Properties of TRIs from Thermal Effect and Calculated Theory TRIs with n-Butyl Acrylate

TRIs

kd [s21]

BDE [kcal/mol]c

Tonset [8C]a

Peak Area [J/g]a

Td [8C]b

Tonset [8C]a

Tmax [8C]a

C-HexDCC

104

513.6

109

86

91

4.5

4.3 3 1025

24.36

DiiprDCC

108

322.8

107

93

97

6.0

3.2 3 1025

24.39

C-HexDIC

103

435.9

94

85

90

3.0

6.4 3 1025

22.27

1.5

24

21.99

DiiprDIC

102

639.6

85

84

a The onset and maximum DSC peak temperature (TRIs in n-butyl acrylate). Tonset of DSC for TRIs.

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Half-Life [h]

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1.3 3 10

Exothermic peak area in DSC curves. The initiation of the decomposition of TRI derivatives. c Bond dissociation energy of TRIs through DFT calculation. b

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FIGURE 3 (a) NMR spectra of TRIs standard and their half-life in CD6D6 at 70 8C. (b) Example of real-time NMR investigations of DiiprDCC. [Color figure can be viewed at wileyonlinelibrary.com]

respectively. However, DiiprDCC and DiiprDIC did not show melting point. Exothermic peaks of DSC are significant as the peaks exhibited thermal cleavage of NAO bond by decomposition of TRIs. All TRIs showed onset of decomposition range from 102 to 108 8C and, DiiprDIC was degraded at the lowest temperature (Tonset 5 102 8C) of the TRIs, which means radical can be initiated at the low temperature.13,14 In addition, the relatively broad exothermic domain of DiiprDIC (639.6 J/g) may have a positive effect to make radical species by activation energy. Decomposition Rate of the Synthesized TRIs The decomposition rate constant (kd) and half-life of the synthesized TRIs were measured by NMR spectroscopy in benzene at 70 8C, as shown in Figure 3 and Table 1. The NAO bond homolysis of the TRIs is ultimately associated with the initiation temperature. As shown in Figure 3(a,b), the halflives of each TRI were measured and real time, particularly, NMR spectroscopy of DiiprDCC was changed on 5.5 ppm in ANH group and on 1.0–2.0 ppm in aliphatic group on TRI chemical structure depend on time [see Fig. 3(b)].4,15,16 The half-lives of the synthesized TRIs were calculated from the half integration time compared with the initial integration at 5.5 ppm for the ANH group. The kd values of the TRIs were calculated according to the measured half integration time as per eq 1, and they are listed in Table 1. kd 5

ln2 t 12

(1)

While the half-lives of the TRIs were 6.0 h for DiiprDCC, 4.5 h for C-HexDCC, 3.0 h for C-HexDIC, and 1.5 h for DiiprDIC, respectively, the kd values calculated from the half-lives showed an opposing trend to those of the initiation and decomposition temperatures of the TRIs. Interestingly, the kd values of the TRIs showed that the diisopropyl group attached to the o-imino-isourea moiety has different effects depending on the alkyl chain position. In the DCC-type TRIs, the cyclohexyl group is slightly more effective than isopropyl in enhancing kd, whereas the isopropyl group on isourea

produces a significantly increased kd and is much more effective. From half-lives of TRIs, the kinetics of the reactions were calculated as shown in Supporting Information Figure S11 and DiiprDIC showed the highest slope, which results are consistent with the BDEs based on the DFT calculations.15,16 DFT Calculation of the Dissociation Energy of the NAO Bonds in the TRIs To more clearly understand the bond dissociation energy of the NAO bond in the TRIs, DFT calculations (xb97x-d/6– 31 1 G) were performed.17 First, we prepared optimized geometries of given TRIs, and all geometries were confirmed that there are no imaginary frequency through calculating second derivative of the SCF energy. As well, we calculated the BDE of TRIs by estimating the change in their thermal enthalpy (DH) of TRIs and two radical species. Thermal enthalpy for various TRIs and each radical species was calculated considering the zero point vibrational energy (ZPE) and thermal correlation term. Figure 4(a) shows the relative thermal enthalpy change (in kcal/mol) for various TRIs, whereas Figure 4(b) shows a typical initiation mechanism with an optimized radical initiator geometry based on oimino-isourea. In the schematic BDE profile, the solid and dotted lines of the thermal enthalpy changes indicate TRIs with Diipr – and C-Hex – on the imino group, respectively, and the black and blue lines indicate TRIs with – DIC and – DCC on the isourea group, respectively. In Figure 4(a), the variation in the BDE of the TRIs represents the effect of the alkyl chain position. That is, the change of the substituent in the imino group is represented by a variation in the shape of the line, and the change of the substituent in the isourea group is represented by a variation in the color of the line. Thus, we can see that changes in the isourea part have a more significant effect on the BDEs, because the variation in the BDEs between the black and the blue lines is higher than that between the solid and the dotted lines. For example, Table 1 shows the BDEs for various TRIs. The BDEs for DiiprDIC, C-HexDIC, DiiprDCC, and C-HexDCC are 21.99, 22.27, 24.39, and 24.36 kcal/mol, respectively. The difference

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FIGURE 4 (a) Schematic profile for calculated thermal enthalpy change (in kcal/mol) of TRIs. (b) Optimized imitator geometry and two radical species of DiiprDCC after initiation (gray color is carbon, red color is oxygen, blue color is nitrogen, and white color is hydrogen). [Color figure can be viewed at wileyonlinelibrary.com]

in BDE between DiiprDIC and C-HexDIC is about 0.3 kcal/ mol, whereas that between DiiprDIC and DiiprDCC is about 2.4 kcal/mol. As the substituents on isourea are doubly involved in the electronic structure of the NAO bond, unlike the substituents on imine, changes in the isourea moiety are more significant than those in the imine. Consequently, our data show that the – DIC isourea group TRIs have smaller BDEs than the – DCC isourea group TRIs. This indicates that the substituted isopropyl group is more suitable for use in low-temperature curing processes.

Free Radical Polymerization of n-Butyl Acrylate with the Synthesized TRIs To evaluate the polymerization efficiency of the synthesized TRI derivatives as radical initiators in n-butyl acrylate, the radical polymerization was monitored using gel permeation chromatography (GPC) and NMR spectroscopy. Before polymerization, curing processes of TRIs with n-butyl acrylate were measured to determine polymerization temperature using a DSC analysis at 1 8C/min.18 As shown in Figure 2(b), all maximum peak in DSC showed from 89 to 97 8C, so that

FIGURE 5 The conversion of TRIs (1 mmol) with n-butyl acrylate (100 mmol) using bulk polymerization depending on reaction temperature (80, 90, and 100 8C); (a) C-HexDCC; (b) DiiprDCC; (c) C-HexDIC; (d) DiiprDIC. [Color figure can be viewed at wileyonlinelibrary.com]

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TABLE 2 Summary of Molecular Weight and Conversion of Bulk Polymerization Using TRIs in n-Butyl Acrylate During Reaction Time Temp. [8C]

Reaction Time [h]

Mn [kDa]

Mw [kDa]

PDI

Conversion (%)

C-HexDCC

90

1h

29.0

120.0

4.1

77

DiiprDCC

90

1h

16.0

110.0

6.8

87

C-HexDIC

90

1h

25.0

120.0

4.7

75

DiiprDIC

90

1h

8.9

86.0

9.7

90

C-HexDCC

100

1h

20.0

74.0

3.6

76

DiiprDCC

100

1h

7.8

62.0

7.9

85

C-HexDIC

100

1h

23.0

84.0

3.6

80

DiiprDIC

100

1h

4.2

66.0

15.7

90

n-butyl acrylate was polymerized at 80, 90, and 100 8C, respectively. Besides, all polymers were prepared in bulk polymerization to realize similar to the DSC methods. Bulk(or mass) polymerization is still studied as industrial process due to many advantages of simple process. Figure 5(a–d) and Table 2 exhibited conversion values and molecular weights of poly-n-butyl acrylate after polymerization. Polymerization at 90 and 100 8C showed high conversion of 75– 90% within 1 h, whereas at 80 8C shown not occurred radical polymerization within 1 h. The bulk polymerization showed number average/weight average molecular weight (Mn/Mw) of Mn (4.2–29.0 kDa) and Mw (62–120 kDa), respectively. The polydispersity indices (PDI) of the polymerizations using synthesized TRIs after 1 h were showed 3.6– 15.7. DiiprDIC, which has lowest initiation temperature among TRIs, showed highest 90% conversion and wide PDI value. Automotive Applications Using Synthesized TRIs Recently, to commercialize lightweight automotive technologies, many researchers have tried to develop engineering plastics for automotive applications. Therefore, to realize low-temperature curing technologies for automotive coatings, a new curing agent or TRI needs to be developed to achieve free radical polymerization at low temperatures.7 To evaluate industrial coatings using the newly synthesized TRIs, a clear coat formulation was prepared with HFUMO

(DCR4265-60), which comprised 60% double bonds and 40 mg KOH/g of solid weight in its polymer backbone as the main resin, 1,6-hexanediol acrylate as a reactive diluent, and DiiprDIC as the TRI with the lowest initiation temperature among those synthesized as a radical initiator. The crosslinked network of the formulated clear coat sample was investigated using an oscillatory rheometer and RPT. The rheological properties of the clear coat were assessed using small-amplitude oscillatory shear (SAOS) mode of the rheometer. For the rheological test, the climatic test chamber at room temperature was heated from 30 to 150 8C at a rate of 5 8C/min for 24 min, and then it was maintained at 150 8C for 30 min.19,20 As shown in Figure 6(a), the clear coat containing DiiprDIC was evaluated with respect to its real-time rheological properties, and the storage modulus (G0 ) sharply increased at 1430 sec and 130 8C up to 107 Pa. This tendency is consistent with RPT data that showed the period sharply decreased at 91 8C and 850 sec as shown in Figure 6(b). These findings mean that the newly synthesized TRI was successfully demonstrated on the clear automotive coating system. CONCLUSIONS

A series of TRIs based on o-imino-isourea bearing cyclohexyl and isopropyl groups have been successfully synthesized. To

FIGURE 6 The measurement of automotive clearcoat curing process: (a) instrument of oscillatory rheometer, (b) plots of the period versus time obtained with RPT.

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demonstrate the experimental radical initiation temperatures, The TRI derivatives in n-butyl acrylate showed peak temperatures (Tmax) from 89 to 97 8C. Free radical polymerization for different temperatures (80, 90, and 100 8C) was conducted based on the DSC curves of TRIs with n-butyl acrylate; all the TRIs at 90 and 100 8C produced a high conversion of over 77–90% within 1 h. These results showed kinetic properties of TRIs in n-butyl acrylate depending on process temperature and means very efficient for low temperature curing process. Furthermore, to evaluate applications using TRIs, DiiprDIC, which showed the lowest initiation temperature among the synthesized TRIs, was applied to an automotive clear coating system based on HFUMO clear coat resin, and the real-time evolution of the elastic modulus (G0 ) was monitored during thermal curing using a rheometer. The result showed that the storage modulus (G0 ) sharply increased up to 107 Pa, which is consistent with the reverse data of RPT. Therefore, the clear coat that was formulated with DiiprDIC was successfully demonstrated on the clear coating system. These significant results based on the o-imino-isourea structure provide information on the relationship between the alkyl group and the radical initiation temperature for this series of TRIs. ACKNOWLEDGMENTS

This material was also based upon work supported by the Ministry of Trade, Industry & Energy (MOTIE, Korea), under Industrial Technology Innovation Program No. 10067706, “Development of automotive clearcoat and related coating process based on low temperature curing technology.

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JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2018, 56, 1749–1756

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