syntheses of bisphenol-type oligomers having five-membered dithiocarbonate groups at the terminals...

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Syntheses of Bisphenol-Type Oligomers Having Five-MemberedDithiocarbonate Groups at the Terminals and Their Applicationas Accelerators to Epoxy-Amine Curing System

YUAN ZHANG, ATSUSHI SUDO, TAKESHI ENDO

Henkel Research Center of Advanced Technology, Molecular Engineering Institute, Kinki University, Iizuka,Fukuoka 820-8555, Japan

Received 24 May 2007; accepted 27 October 2007DOI: 10.1002/pola.22506Published online in Wiley InterScience (www.interscience.wiley.com).

Keywords: additives; curing of polymers; epoxide; gelation; oligomers

INTRODUCTION

Five-membered cyclic dithiocarbonate (DTC) is ex-pected to be a promising accelerator of many advan-tages for epoxy-amine curing system as a new class ofmonomers for polymer synthesis.1 It reacts exclu-sively with amine to undergo a rapid ring openingreaction accompanied by the formation of thiol group,which can be used for reactions with various electro-philes such as alkyl halide, acyl halide, isocyanate,and epoxide.2 The DTC-amine reaction also gives thecorresponding acyclic thiourethane quantitatively,3

which furthermore prompts the epoxy-amine curingreaction by intramolecular hydrogen bonding betweenthe thiol and thiourethane groups with deprotonationof the thiol to enhance its nucleophilicity, or intermo-lecular hydrogen bonding between the thiourethanegroup and the oxygen atom of the epoxide to enhanceits electrophilicity.4 These reactive features allowDTC to be applied as a new additive to epoxy-aminecuring system that is based on polyaddition chemistryof epoxide and amine.5 By addition of DTC, a part ofDTC component would be immediately transformedinto the corresponding thiol, and thus the epoxy-amine system would be endowed with various advan-

tages of epoxy-thiol system such as rapid curing andhigh adhesion strength.6

Besides this reactivity aspect, another attractivefeature of DTC is its straightforward and facile prepa-ration.7 DTC can be easily synthesized from the cor-responding epoxy precursor by its treatment withcarbon disulfide. This reaction proceeds efficiently atambient temperature by addition of neutral metalhalides such as lithium bromide. The reaction is com-patible with a wide range of functional groups suchas hydroxyl group, to allow facile derivation of variousepoxy precursors into the corresponding DTC com-pounds.

In this work, to develop some DTC derivatives ofbetter acceleration effect on the epoxy-amine curingsystem to prompt the industrial application of DTCs,we are attracted by an epoxy precursor for DTC syn-thesis, which is an oligomer obtained by polyconden-sation reaction of bisphenol A and epichlorohydrin,because a series of the oligomers with different molec-ular weights are available to permit us to adjustchemical and physical properties of the DTC-typemolecules. Another interest on the oligomer-type DTCis the presence of hydroxyl group in its side chain, ofwhich chemical modification would enable us todesign a wide range of derivatives. Herein, we reportsyntheses of a series of oligo(bisphenol A-co-epichloro-hydrin) having DTC moiety in the chain ends andtheir application as additives to epoxy-amine curingsystem.

Correspondence to: T. Endo (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 1907–1912 (2008)VVC 2008 Wiley Periodicals, Inc.

1907

RESULTS AND DISCUSSION

Syntheses of a Series of Oligomer-type Additives

Schemes 1 and 2 illustrate the structures of theoligomer-type additives used in the present work andthe reactions used for their syntheses. The precursor1 is a commercially available oligomer-type epoxyresin, which has been widely used in industry. Itshydroxyl group in the side chain underwent an effi-cient reaction with 3,4-dihydro-2H-pyran (DHP) inthe presence of pyridinium p-toluenesulfonate (PPTS)to give 2 (Scheme 1).

The first attempt to transform 1 into the corre-sponding DTC 3 under the standard reaction condi-tions for our DTC synthesis7 turned out to be unsuc-cessful. However, this issue was overcome by a slight

modification of the reaction conditions with employinghigh concentrations of lithium bromide and carbondisulfide (Scheme 2). The obtained 3 inherited thehydroxyl group in the side chain from the precursor1, and two kinds of chemical modifications of thishydroxyl group gave the corresponding derivatives:Treatment of 3 with DHP in the presence of PPTSresulted in successful formation of the correspondingoligomer-type DTC 4, which had a less polar tetrahy-dropyranyl group (THP) in the side chain. The corre-sponding conversion of the hydroxyl group was 95%.Reactions of the hydroxyl group with phenyl isocya-nate and cyclohexyl isocyanate gave the correspond-ing DTC 5a and 5b, respectively. 1H-NMR analyses ofthese compounds revealed that the conversions of theefficiencies in the carbamoylation were 51 and 64% in5a and 5b, respectively.

Scheme 1. Modification of epoxy 1 with DHP.

Scheme 2. Synthesis of DTC 3 and its modifications.

1908 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 46 (2008)

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

Addition Effects of the Oligomers on Epoxy-AmineSystem: Acceleration Effect

To examine addition effects of the obtained oligomerson epoxy-amine curing system, a formulation com-prised of bisphenol A diglycidyl ether (BisA-DGE) andamine-terminated poly(propylene glycol) (PPG-dia-mine) was employed as a standard (¼ formulation A)(Scheme 3). In the formulation A, 10 mol % oligomerswere added to prepare a series of curable formula-tions B–G (Table 1). Although addition of theseoligomer-type additives increased the initial viscos-ities of the corresponding formulations, they werewithin a range for easy handling as liquid formula-tions. The processes of the curing reactions weremonitored by measurement of storage modulus (E0)with a thermal mechanical analyzer. Figure 1 showsthe time-dependences of E0 in the curing reactions ofthe curable formulations at 50 8C. In all cases, thecorresponding profiles had a plateau until the rapidincrease in E0, and the extrapolated onset time wasdefined as gelation time, at which growth of the net-worked polymer structure would have started.8

The formulation B containing the oligomeric epox-ide 1 underwent much faster curing reaction than thestandard formulation A, while the formulation C con-taining the epoxide 2 exhibited virtually same reactiv-ity as the formulation A. This difference in reactivitybetween the formulations B and C would be corre-lated to the difference in chemical structures between1 and 2, i.e., presence or absence of hydroxyl group inthe side chain. The good acceleration effect of 1 is ingood accordance with various reports describingenhancement of the reactivity of epoxide by alcohol,for which hydrogen bonding between them would bethe predominant reason.9

Compared with the epoxy-type additives, the DTC-type additives exhibited much higher accelerationeffects. By addition of DTC 3 having hydroxyl sidechain, the gelation time was remarkably reduced tonearly half of that of the standard formulation A. Onthe other hand, addition of DTC 4, which was derivedfrom DTC 3 by masking the hydroxyl side chain,resulted in a slower curing reaction. However, it wasstill faster than the formulation B that contained theoligomeric epoxide 1. These results suggested that thepredominant factor for the excellent accelerationeffect of DTC 3 was the intrinsically high reactivitiesof DTC with amine and the resulted thiol with epox-ide. The presence of hydroxyl group was an extra fac-tor to improve the performance as an accelerator.

Finally, acceleration effects of the oligomeric DTCs5 having urethane side chains were examined. DTC5a having phenyl urethane moiety showed the bestacceleration effect among the additives which wereused for the present study. On the other hand, theacceleration effect by DTC 5b having cyclohexyl ure-thane moiety was similar to that exhibited by DTC 4having less polar THP side chain. The difference inacceleration effect between 5a and 5b could be corre-

Scheme 3. Application of DTC additives to an epoxy-amine curing system.

Table 1. Effects of the Additives on the Epoxy-Amine Curing System

Formulation AdditiveInitial Viscosity

(Pa s)aGelation

Time (min)b

Thermal Properties AfterCuringc

Tgd Td5

e Td10e

A None 0.05 398 33 300 321B Epoxy 1 0.45 282 45 354 365C Epoxy 2 0.40 417 40 280 310D DTC 3 1.50 180 41 313 332E DTC 4 1.30 238 44 279 303F DTC 5a 3.30 138 43 278 300G DTC 5b 2.55 230 42 288 308

aMeasured with a viscometer.bMeasured with TMA.c The cured materials were obtained by the curing reactions of the formulations at 50 8C for 24 h.dMeasured with DSC.eMeasured with TG/DTA.

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lated with the difference in ability as a proton donor(¼ acidity), which activates epoxide by protonation:Phenyl group can enhance the acidity of urethanegroup by stabilization of the corresponding conjugatebase, while cyclohexyl group is intrinsically an elec-tron donating group to decrease the acidity.

Thermal Properties of the Cured Materials

The formulations A–G were cured by heating them at50 8C for 24 h. The glass transition temperatures (Tg)and the temperatures for 5 and 10% weight loss (Td5

and Td10) of the resulted cured resins are listed inTable 1. By using the oligomer-type additives 1–5, Tg

became slightly higher than that of the cured formu-lation A, suggesting that the incorporation of theintrinsically rigid segments of the oligomers into theformed networked polymers increased their rigidity.

Td5 and Td10 of the material obtained by the curingreaction of the formulation D, which contained DTC 3having hydroxyl side chain, were slightly higher thanthose of the cured formulation A. On the other hand,the materials obtained by the curing reactions of theformulations E-G containing DTC 4 and 5 started tolose their weights at lower temperatures, presumablydue to the presence of thermally labile acetal and ure-thane structures in the cured materials.

SUMMARY

A series of oligo(bisphenol A-co-epichlorohydrin)-derivatives having DTC moiety at the terminals werereadily prepared and applied as additives to epoxy-amine curing system. These DTCs exhibited remark-able acceleration effects, and in particular, the addi-tives having polar functional groups such as hydroxyland urethane in the side chain were found to behighly effective accelerators. Further development ofepoxy-DTC-amine ternary systems with rapid curabil-ity at low temperature will be pursued with the struc-tural optimization of oligomer-type DTCs.

EXPERIMENTAL

Materials and Instruments

Poly(propylene glycol)bis(2-aminopropyl ether) (PPG-diamine, Mn ¼ 400), poly(bisphenol A-co-epichlorohy-drin) glycidyl end-capped (1, Mn ¼ 1075), and carbondisulfide were purchased from Sigma-Aldrich and usedas received. The other reagents and solvents were pur-chased from Wako Pure Chemical Industries or TokyoChemical Industry Company, and used as received.

1H-NMR and 13C-NMR spectra were recorded with aJNM-AL300 NMR spectrometer (JEOL, Japan). Viscos-ity was measured with a Brookfield viscometer (ModelCap 2000+) at 50 8C. Storage modulus was measuredwith a Seiko TMA/SS (EXSTAR6000) thermal mechani-cal analyzer at 50 8C. Glass transition temperature(Tg), temperatures for 5 and 10% weight loss (Td5 andTd10) were measured with DSC6200 and TG/DTA6200apparatuses (Seiko Instruments), respectively.

Synthesis of Oligomer-Type Epoxide 2 HavingAcetal Side Chain

To a solution of oligomeric epoxide 1 (9.44 g, 8.78 mmol)and PPTS (0.22 g, 0.86 mmol) in dichloromethane(160 mL), DHP (2.50 g, 28.8 mmol) was added drop-wise. After stirring the solution at room temperaturefor 3 h, the volatiles were removed by evaporationunder reduced pressure. The residue was dissolved inethyl acetate (150 mL), filtered, washed with saturatedNaCl aqueous solution (150 mL3 3), and dried over an-hydrous sodium sulfate. The solution was filtered, con-centrated under reduced pressure, and dried in vacuofor 48 h to obtain 2 (yield: 79%). Conversion of hydroxylgroup was determined to be 80% by 1H-NMR analysis.1H-NMR (CDCl3, d, ppm): 1.53–1.83 [CH3C��;��OCH2CH2CH2CH2CH��], 2.72–2.75, 2.87–2.90 [2H,��OCH2CH��], 3.31–3.34 [1H, ��CH2CHCH2��], 3.91–4.00 [2H, ��CHCH2O��], 4.05–4.24 [��CHCH2O��,��OCH2CHCH2O��], 4.32–4.39 [1H, ��CH2CHCH2��],4.90–4.92 [1H, ��OCHO��], 6.80–6.83, 7.10–7.14 [4H,aromatic protons]; 13C-NMR (CDCl3, d, ppm): 19.48[��OCH2CH2CH2CH2CH��], 25.50 [��OCH2CH2CH2

CH2CH��], 30.75 [��OCH2CH2CH2CH2CH��], 31.16[CH3CAr��], 41.80 [(CH3)2C(Ar)2��], 44.84 [��OCH2

CH��], 50.30 [��CH2CHCH2��], 62.51 [��OCH2CH2

CH2CH2CH��], 67.79 [��OCH2CHCH2O��], 68.82[��OCH2CHCH2O��], 73.55 [��CHCH2O��], 98.71[��OCHO��], 114.00–114.08, 127.80–127.87, 143.33–143.77, 156.37–156.70 [6C, aromatic].

Synthesis of Oligomeric DTC 3

To a solution of 1 (53.8 g, 50.0 mmol) and lithium bro-mide (1.30 g, 15.0 mmol) in THF (300 mL), carbon di-sulfide (21.6 mL, 360 mmol) was added dropwise, andthe resulted solution was heated to 40 8C. After 8 h,it was concentrated under reduced pressure. The cor-

Figure 1. Time-E0 relationships for the curing reac-tions of the formulations A–G at 50 8C. (FormulationsA–G are shown in Table 1).



responding residue was dissolved in ethyl acetate(300 mL), washed with saturated NaCl aqueous solu-tion (300 mL 3 3), and dried over anhydrous sodiumsulfate. The solution was filtered, concentrated underreduced pressure, and dried under vacuum for 24 h toobtain oligomeric DTC 3 (55.9 g, yield: 91%). 1H-NMR(CDCl3, d, ppm): 1.57–1.65 [21H, CH3CHCH3], 2.88[1H, ��OH], 3.58–3.72 [2H, ��SCH2CH��], 4.03–4.13[4H, ��OCH2CHCH2��], 4.20–4.27 [2H, ��OCH2CH��],4.30–4.34 [1H, ��CH��], 5.29–5.35 [1H, ��CH��], 6.78–6.81, 7.09–7.14 [4H, aromatic protons]; 13C-NMR(CDCl3, d, ppm): 31.06 [CH3CAr��], 36.20 [��SCH2

CH��], 41.71–41.82 [(CH3)2C(Ar)2��], 68.77 [��CHCH2

O��], 88.02 [��CH2CHCH2��], 113.98–114.11, 127.79–127.94, 143.43–144.39, 155.62–156.33 [6C, aromatic],211.55 [��SC¼¼SO��].

Synthesis of Oligomeric DTC 4 Having THPMoiety in the Side Chain

To a solution of 3 (24.6 g, 20.0 mmol) and pyridiniumPPTS (0.78 g, 3.1 mmol) in CH2Cl2 (160 mL), DHP(5.25 g, 62.4 mmol) was added dropwise. The resultedsolution was stirred at room temperature for 3 h, andthen concentrated under reduced pressure. Theresulted residue was dissolved in ethyl acetate (200 mL)and filtered to remove insoluble parts. The filtratewas washed with saturated NaCl aqueous solution(200 mL 3 3), dried over anhydrous sodium sulfate,filtered, concentrated, and dried under vacuum for48 h to obtain 4 (yield: 91%; conversion of OH: 95%).1H-NMR (CDCl3, d, ppm): 1.46–1.87 [CH3C��;��OCH2CH2CH2CH2CH��], 3.48–3.56, 3.93–4.01 [2H,��OCH2CH2��], 3.70–3.82 [2H, ��SCH2CH��], 4.05–4.24 [4H, ��CH2CHCH2��], 4.26–4.30 [2H, ��CHCH2

O��], 4.32–4.40 [1H, ��CH2CHCH2��], 4.89–4.92 [1H,��OCHO��], 5.37–5.47 [1H, ��CH2CHCH2��], 6.78–6.85, 7.08–7.18 [4H, aromatic protons]; 13C-NMR(CDCl3, d, ppm): 19.48–19.84 [��OCH2CH2CH2CH2

CH��], 25.49–25.55 [��OCH2CH2CH2CH2CH��],30.75–30.78 [��OCH2CH2CH2CH2CH��], 31.11–31.15[CH3CAr��], 36.31–36.35 [��SCH2CH��], 41.77–41.91[(CH3)2C(Ar)2��], 60.47 [��OCH2CH2CH2CH2CH��],67.80–67.86 [��CHCH2O��], 73.55 [��OCH2COHCH2

O��], 87.98 [��CH2CHCH2��], 98.71 [��OCHO��],113.99–114.14, 127.78–128.03, 143.13–143.49, 156.34–156.74 [6C, aromatic], 211.43 [��SC¼¼SO��].

Synthesis of Oligomeric DTC 5a HavingPhenylurethane Moiety

3 (3.33 g, 2.70 mmol) and DBTDL (0.008 g, 0.01mmol) were dissolved in 50 mL tetrahydrofuran, andto this solution, phenyl isocyanate (0.86 g, 7.0 mmol)was added dropwise. The resulted solution wasstirred at room temperature for 24 h, and then con-centrated under reduced pressure. The residue wasdissolved in THF, and the solution was slowly added

to methanol (100 mL). The resulted precipitate wasseparated from the supernatant by decantation,washed with 20 mL methanol, and dried in vacuo for24 h to obtain 5a (3.28 g, yield: 79%; conversion ofOH: 51%). 1H-NMR (CDCl3, d, ppm): 6.96–6.99, 7.28–7.30, 7.45–7.50 [5H, aromatic protons of phenylur-ethane moiety]; 13C-NMR (CDCl3, d, ppm): 152.86[��OC¼¼ONH��].

Synthesis of Oligomeric DTC 5b HavingCyclohexylurethane Moiety

3 (4.00g, 3.25 mmol) and DBTDL (0.016g, 0.025 mmol)were dissolved in 50-mL tetrahydrofuran. To this solu-tion, cyclohexyl isocyanate (1.21 g, 9.67 mmol) wasadded dropwise. The resulted solution was stirred atroom temperature for 72 h, and then concentratedunder reduced pressure. The residue was dissolved inTHF, and the solution was slowly added to methanol(100 mL). The resulted precipitate was separated fromthe supernatant by decantation, washed with 20 mLmethanol, and dried in vacuo for 24 h to obtain 5b(4.06 g, yield: 81%; conversion of OH: 64%). 1H-NMR(CDCl3, d, ppm): 1.11–1.24 [10H, cyclohexyl]; 13C-NMR(CDCl3, d, ppm): 154.68 [��OC¼¼ONH��].

Curing Reaction

In 2,2-bis(4-glycidyloxyphenyl)-propane (BisA-DGE)(6.81g, 20.0 mmol), DTC-3 (2.46 g, 2.00 mmol) wasadded and dissolved by heating at 90 8C underreduced pressure for less than 30 min, and thencooled to room temperature. PPG-diamine (4.00 g,10.0 mmol) was added to the mixture followed with astirring and degassing step by an AR-100 conditioningmixer. The measurements of storage modulus varia-tion during curing reaction began at 25 min afterPPG-diamine was added, which was done with aSeiko TMA/SS (EXSTAR6000) thermal mechanical an-alyzer at a 0.01 Hz frequency and 50-lm amplitude ina sinusoidal mode when the mixture of reactants wascured at 50 8C in a protective atmosphere of nitrogengas. The addition experiments with other additivesfollowed the same procedures with DTC-3 as above-mentioned. Their gelation times were extrapolatedfrom the divergence point of the storage moduluscurve where it increased abruptly and confirmed withthe maximum point of the tangent delta curve.8

This work was supported by Henkel KGaA, Germany.

REFERENCES AND NOTES

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