Macromol. Rapid Commun. 18,897-902 (1997) 897

Isolation and cationic ring-opening polymerization of 2,3,5,6-tetrahydroimidazo[2,1-b][ 1,3]oxazole to produce a thermally stable polyurea

Masatoshi Miyamoto *, Toshio Watanabe, Yoshiharu Kimura

Department of Polymer Science and Engineering, Faculty of Textile Science Kyoto Insti- tute of Technology, Matsugasaki, Sakyo, Kyoto 606, Japan

(Received: June 3, 1997)

SUMMARY A bicyclic pseudourea, 2,3,5,6-tetrahydroimidazo[2, I-b][ 1,3]oxazole ( l ) , was isolated

for the first time, and its cationic and spontaneous polymerizations were examined. Both polymerizations gave poly( 1,3-imidazolidin-2-one- 1,3-diylethylene) (2) in high yields. The polymer is a crystalline solid with glass transition and melting temperature of 5 1 "C and 286"C, respectively. Thermogravimetry showed that it is stable up to 450°C under nitrogen.

Introduction

Urea-formaldehyde resin is one of the oldest synthetic polymers in our history and is widely used for molding, laminating, and adhesives*). However, the investiga- tion on linear polyureas has been limited until the start of our recent research on the polymerization of cyclic pseudoureas*).

In the series of our work, we reported that double isomerization polymerization (DIP) of 1,4-bis(4,5-dihydro-1,3-oxazol-2-yl)piperazine (3) gave poly( 1,3-imidaz- olidin-2-one-1,3-diylethylene) (2) (Eq. ( l))3). This polymer 2 is of NJ"dialkylethy1- eneurea structure and may be considered as a polymer homologue of N,N'-dimethyl- ethyleneurea or N,N,N',N'-tetramethylurea, a representative aprotic polar solvent being capable of replacing toxic HMF?). Although the DIP of 3 is unique and very interesting from the view point of polymerization chemistry, it requires high dilution to avoid cross-linking and, therefore, it is difficult to prepare polymer in a large scale.

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3 2

In the present communication we report the isolation and the ring-opening poly- merization of a bicyclic pseudourea, 2,3,5,6-tetrahydroimidazo[2,l-b] [ 1,3]oxazole (l), for the first time (Eq. (2)): although the existence of 1 has been known, its isola- tion has failed so fa?7). The polymerization of 1 gave 2 of higher molecular weight than in the case of the DIP of 3, and the resulting polymer showed excellent thermal properties.

Macromol. Rapid Commun. 18, No. 10, October 1997

0 1997, Hiithig & Wepf Verlag, zug CCC 1022-1336/97/$02.50

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M. Miyamoto, T. Watanabe, Y. Kimura

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1 2

Results and discussion

Synthesis of 1

chloroethyl)-2-imidazolidinone with NaH in DME according to Eq. (3): The monomer 1 was prepared as a white waxy solid by the reaction of 1-(2-

The monomer is highly reactive toward polymerization, and the distillation of the crude product in the absence of a basic inhibitor, e. g., calcium hydride, gave a poly- meric product instead of the purified monomer, which is the reason why 1 has not been isolated until now. The structure of 1 was confirmed from 'H and 13C NMR and IR spectroscopic data, which are shown in the Experimental part.

The high reactivity of 1 is demonstrated, for example, from DSC measurements (Fig. 1 a). The sharp endothermic peak at 56.1 "C corresponds to the melting point of the monomer. The broad exothermic peak in the range of 60- 150 "C indicates that 1 polymerizes spontaneously upon heating above its melting point. The weak, broad peak at 290.0 "C is due to the T,,, of the resulting polymer.

Ring-opening polymerization of 1

The above DSC result was indicative that 1 could be polymerized without adding any initiator. In Tab. 1 are summarized the results of the polymerization of 1 in the presence and absence of a cationic initiator, methyl trifluoromethanesulfonate. The spontaneous polymerization of 1 proceeded either in bulk or in solution. The poly- mer yield in the bulk polymerization reached 75% at 60°C and 83% at 80°C after 24 h.

The structure of the polymer was identified as poly( 1,3-imidazolidin-2-0ne- 1,3- diylethylene) from 'H, I3C NMR and IR spectroscopies. These spectra were essen- tially the same as the corresponding spectra of 2 prepared by DIP of 3. The molecu- lar weight of 2 prepared in the absence of the initiator reached 27000 (Run 7), while those prepared in the presence of the cationic initiator were generally low and decreased as the initiator concentration increased (Runs 1-3, 5-7, and 8-10). Moreover, the addition of sodium hydride almost completey inhibited the sponta- neous polymerization. These data show that the spontaneous polymerization of 1 is initiated by a small amount of acidic impurity contaminating the system.

Isolation and cationic ring-opening polymerization ... 899

56.1 'C

-?-- 51.2 'C

104.5% L 286.4 1

Temperature (%I Fig. 1. DSC curves of 1 (a) and 2 (b) measured under nitrogen. Heating rates were 2 "C/ min for 1 and 5 "C/min for 2

Tab. 1. Cationic and spontaneous polymerizations of la'

Run Solvent Temp. [IId[Mlo Polymer 2 in "C in %

yield - Mk) MJfi;) in %

1 2 3 4 5 6 7 8 9

10 11 12

nitrobenzene nitrobenzene nitrobenzene nitrobenzene nitrobenzene nitrobenzene nitrobenzene nitromethane nitromethane nitromethane none none

80 80 80 80 60 60 60 60 60 60 80 60

2 1 0.2 0 2 1 0 2 1 0.2 0 0

99 2.1 99 3.3 97 16 81 8

100 5.9 85 13 64 27 93 6.5 89 10 88 39 83 20 75 27

2.9 2.6 2.7 2.7 1.7 1.5 3.0 1.8 2.2 3.0 2.9 2.7

a) Initiator: MeOS02CF3. Polymerization time: 24 h. [ l I0 = 1 m o m in the solution poly- merization.

b, Determined from GPC with polystyrene standards.

900 M. Miyamoto, T. Watanabe, Y. Kimura

The solubility of 2 reflects the amphiphilic property of tetraalkylurea unit: it is soluble in both chloroform and water. It is also soluble in nitrobenzene, partly solu- ble in DMSO, but insoluble in other commodity solvents including toluene, 1,2- dimethoxyethane (DME), dichloromethane, nitromethane, acetonitrile, benzonitrile, DMF, NMP, methanol, and ethanol. The restricted solubility of 2 is due to its high crystallinity.

Thermal properties of 2 The polymer is highly crystalline, and the DSC trace of the quenched sample

(Fig. 1 b) shows that it has Tg at 51.2"C, T, at 104.5 "C, and T, at 286.4"C. Our pre- vious paper reported that the T, of 2 prepared from 3 was 210"C3). The difference in T, is due to the difference in the molecular weight between the samples: the mw of the sample subjected to DSC analysis was 14000 in the present work, while it was 2500 in the previous work. The T,,, of the sample raised to 292.4"C when 2 of Mw = 120000 was employed for the measurement.

In addition to the above DSC results, TGA analysis of the sample showed that it is significantly thermally resistant. Fig. 2 shows the TGA trace of 2 measured under nitrogen. The polymer is stable beyond its melting point, and the temperature of 5% weight loss is 450°C. A rapid decomposition starts above this temperature and becomes complete at ca. 500 "C.

The ring-opening polymerization of 2-alkyl-2-oxazolines has received consider- able attention, since it produces pol yamides with amphiphilic properties'). However, production of high molecular polymers from them is generally difficult since they have acidic hydrogens in 2-position, which causes chain transfer'). Although cyclic

Fig. 2. TGA curve of 2 measured at a heating rate of 15 "C/min under nitro- gen

100 200 300 400 500 600 700 Temperature ( C)

Isolation and cationic ring-opening polymerization ... 90 1

pseudoureas do not bear such acidic hydrogens, the preparation of high molecular polymer from them is also difficult because of the steric disturbance by their 2-sub- stituent during the polymerization"). On the other hand, 1 is free from these causes, which ensures the production of high molecular weight polymer. Since a though film could be prepared by solvent-casting of 2 of a,, = 39000, a wide variety of applications utilizing its amphiphilic property is expected.

Experimental part

Materials

1-(2-Chloroethyl)-2-imidazolidinone was prepared by the reaction of 1-(2-hydroxy- ethyl)-2-imidazolidinone with thionyl chloride". Other reagents and solvents were com- mercially available and were dricd by conventional methods and distilled under nitrogen. The solvents were stored over 3 A molecular sieves after distillation.

Instruments

'H and I3C NMR spectra were recorded on a Varian Gemini-200 NMR spectrometer. IR spectra were obtained on a Jasco IR-5300 infrared spectrometer. GPC analysis was performed on a Shimadzu LC-1OAD system using combined columns of Tosoh TSK gel G4000H8 and G2500H8 in chloroform at 35 "C. DSC measurements were performed on a Mac Science 3 100 thermal analyzer under nitrogen. TGA analysis was carried out on a Shimadzu DT-30 under nitrogen.

2,3,5,6-Tetrahydroimidazo[2,1 -b][l,3]oxazole (1)

All operations were carried out under nitrogen. A stirrer bar and 0.88 g of sodium hydride (60 wt.-% mineral oil suspension, 0.015 mol) was placed in a 50 mL three- necked flask equipped with a dropping funnel, a three-way stopcock, and a thermometer. Into the flask was added 15 mL of DME, and the flask was immersed into an ice bath with stirring. To the suspension was added dropwise 1.0 g of 1-(2-cNoroethyl)-2-imid- azolidinone dissolved in 10 mL of DME from a dropping funnel, keeping the temperature of the mixture below 5°C. After the addition was completed, the suspension was main- tained at 5 "C with stirring for 2 h and then at 30 "C for additional 14 h. The supernatant layer was decanted and concentrated in vacuum. Vacuum distillation of the residue in the presence of calcium hydride gave 0.48 g of 1 (62% yield); m.p. 56.1 "C; b.p. 60°C/1.9 Torr (Kugelrohr).

Hz and J5,6 = 7.6 Hz. Intensity ratio: 2 : 2 : 2 : 2. 'H NMR (CDC13): 6 = 3,22 (t; H3), 3,24 (t; H'), 3,98 (ti P), and 4.71 (t; H'); J2,3 = 6.7

I3C Nh4R (CDC13): 6 = 46.99 (C3), 51.11 (C'), 57.50 (C6), and 73.37 (C'). IR (CHCI,); 2973,2859,1680 (C=N), 1254,1211,1044,789 cm-'.

C5H8N20 (121,l) (hygroscopic) Calc. C 53,56 H 7,19 N 24,98 Found C53,04 H7,43 N24,62

Polymerization of 1

Typical procedure was as follows. In a test tube equipped with a three-way stopcock and a stirrer bar was placed 112 mg (0.999 mmol) of 1, 12 pL of methyl trifluorometh- anesulfonate (0.10 mmol) and 1 mL of nitrobenzene under nitrogen. The mixture was

902 M. Miyamoto, T. Watanabe, Y. Kimura

allowed to react at 60°C for 24 h. The content was poured into diethyl ether to obtain 2, which was purified further by reprecipitation from chloroform into diethyl ether.

‘H NMR (CDC13): 6 = 3.26 and 3.37. Intensity ratio: 4:4. 13C NMR (CDC13): 6 = 41.51,42.30, 160.95. IR (KBr): 3436,2863, 1688 (C=O), 1497, 1441, 1260,754 cm-I.

I ) B. Meyer, “Urea-Formaldehyde Resins”, Addison-Wesley, Menlo Park, Calif. 1979 *) M. Miyamoto, K. Aoi, “Double Isomerization Polymerization ’I, in: Polymer Materi-

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