Polymerization of Bis( 2-Oxazoline) Compounds with Dicarboxylic Acids

YASUO SANO, Research Laboratories, Chemical Products Division, Take& Chemical Industries, Ltd., Osaka 532, Japan

Synopsis

A side reaction was found in the reaction of a 2-oxaaoline compound with a carboxylic acid. It is an oxazoline ring opening addition to an amide group formed by the main reaction. In addition, certain phosphites were found to act as catalyst for the side reaction. The rate constants of the main and side reactions in the system of 2-phenyl-2-oxazoline and n-octanoic acid were obtained through simulation of the reactions on an analog computer. The side reaction makes it impossible for a very high molecular weight polymer to form in the reaction of a bis-2-oxazoline with a dicarboxylic acid, but makes it possible for a new crosslinked polymer to form when excess bis-2-oxazoline and a dicarboxylic acid are heated in the presence of a certain phosphite.

INTRODUCTION

A 2-oxazoline compound reacts with a carboxylic acid to give an esteramide.' Accordingly, the reaction of a bis-2-oxazoline compound with a dicarboxylic acid must yield a linear polyesteramide. This polymer appears to be very interesting because it contains both ester and amide groups, each of which are the repeating units of widely used polymers. But only a patent' and a paper3 deal with this type of polymer. The former describes that heating an equimo- lar mixture of 23-( 1,4-phenylene)bis-2-oxazoline (abbreviated 1,4-PBO)

1,4-PBO

and adipic acid gives an elastic polymer which can be drawn to fibers. No further reference is made to the polymer. The latter states that a polymer was obtained by heating the same compounds as used in the patent in N,N- dimethylformamide. But as the product is reported to have had a small reduced viscosity of 0.07 at 35°C in N, N-dimethylformamide, it must have been an oligomer rather than a polymer.

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 27, 2749-2760 (1989) 0 1989 John Wiley & Sons, Inc. CCC 0360-6376/89/082749-12$04.00

2750 SANO

The initial aim of the present work was to make useful thermoplastics from bis-2-oxazoline compounds and dicarboxylic acids. In the course of the study we found some new features of the reaction between 2-oxazolines and car- boxylic acids.

RESULTS AND DISCUSSION

Reaction of Bis-2-Oxazolines with Sebacic Acid

First, the equimolar reaction of 1,3-PBO

n

O J 1,3-PBO

or 1,4-PBO with sebacic acid without any solvent was studied. The residual acid contents and viscosities of the polymerization products were measured under various conditions. Table I shows the results of the several experiments. If one end of a linear polymer is a carboxyl group, the number average molecular weight of the polymer with the acid content of 3.5% is about 6000. Accordingly, the products in Table I do not have very high molecular weights. For formation of higher molecular weight polymers, the reaction was exam- ined in more detail. When the reactants were heated at 18O-2OO0C, the acid content decreased rapidly, but did not decrease beyond a limit, mostly 3-4% of the initial acid content. Then, a 2-5% excess of the bis-2-oxazolines was

TABLE I Formation of Polymers from Bis-2-Oxazolines and Sebacic Acid

Temperature Time" Residual acid Atmosphere ("C) (W [ q ] in rn-Cresol at 30°C

1,4-PBO + Sebacic acid -Crystalline polymer Air 185 20 3.5 0.77 Air 190 6 3.5 0.95 N2 200 10 3.5 0.94 N2 200 20 3.6 0.83

1,3-PBO + Sebacic acid - Amorphous polymer

N2 190 30 4.7 not measured N2 215 70 4.9 not measured Air 240 30 3.4 1.21

N* 190 18 4.1 0.90

"The reaction time was measured from the point when the inner temperature reached 5°C below the reaction temperature.

BIS(2-OXAZOLINE) COMPOUNDS 2751

used to lower the residual acid content, and the products were found to contain a tiny part insoluble in hot rn-cresol. The amount of the insoluble substance increased with increasing amount of excess oxazoline and also with elongation of the reaction time. In other experiments small amounts of some phosphorus compounds were added to the reaction mixture to prevent air- oxidation. When triphenyl phosphite was used, the major part of the product was insoluble in hot rn-cresol. These phenomena have not been reported in the literature and cannot be explained by the knowledge of 2-oxazoline chern i~ t ry .~-~ It is known that 2-oxazoline compounds polymerize with a cationic catalyst.' In fact, liquid 1,3-PBO at 160°C polymerized and solidified the instant that 1% of methyl p-toluenesulfonate was added. But 1,3-PBO containing 1% of triphenyl phosphite did not polymerize when it was heated for an hour at 180°C. So we supposed that a 2-oxazoline ring reacts with the amide group formed by the interaction between an oxazoline ring and a carboxylic group, and phosphites act as catalysts.

Study of Model Reactions

The model reactions were carried out for confirmation of the supposition. 2-Phenyl-Boxazoline (I) and two kinds of esteramides, I1 and 111, were prepared.

0 @a] @ C O N H C H , C H , O ! ~

I I1

0

0

I11

Then an equimolar mixture of I and I1 was heated in an oil bath of 200°C for an hour. In the gel permeation chromatogram of the product (Fig. 1, the upper curve), there is a small peak which is absent in the chromatograms of the starting materials, and its elution volume is equal to that of 111.

Next, triphenyl phosphite was added to the mixture of I and 11, or I and 111, and the reactants were treated in the same way. The middle and lower curvs in Figure 1 are chromatograms of the products. They have several successive peaks besides those of the starting materials. These results prove that the oxazoline rings add to the amide group of I1 or I11 and also to the amide groups formed by the addition. The CH, regions of 'H-NMR spectra of I, 11, 111, and the reaction mixture after heating I and I1 in the presence of

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Solv : THF, Detector : R I VE

528 mQ

------- 45 1 ma

-_ ----- 0

a @ O N H C H . C H ~ O C ~ II 0

III 0 CONCHz CHz 0 (! 0 ------- 433 ma I C H ~ C H ~ N H C O ~

Reaction Condition B a t h Temp.: 200% 1 IFn Time : 1 hr

Elution Volume (d) Fig. 1. Gel permeation chromatograms of the products formed in the reaction of 2-phenyl-2-

oxazoline with esteramides.

triphenyl phosphite for an hour at 200°C are shown in Figure 2. The peaks of I11 are broad owing to hindered rotation of the C- N bond of the N, N- disubstituted amide. The spectrum of the product, D’ which was obtained by eliminating the spectra A and B from the spectrum D, reveals the presence of several ethylene groups sandwiched between two amide groups with different environments, and is consistent with the structures of the components.

From these results it is evident that the reaction of 2-phenyl-2-oxazoline with a carboxylic acid proceeds as shown in Scheme 1.

Catalytic Activities of Phosphites

Then, the catalytic activities of various phosphites were studied in the reaction of esteramide I1 with 2-phenyl-2-oxazoline. The amount of the

BIS(2-OXAZOLINE) COMPOUNDS 2753

b 4.42 a 4.05 I

A

b a

II

a b

- a

C

C

Fig. 2. 'H-NMR spectra (270 MHz).

catalysts was 1 wt % of the reactants. The decrease rates of the starting materials were measured from the gel permeation chromatograms of the reaction mixtures. Table I1 shows the result. The phosphites with phenoxy groups have much greater catalytic activity than the ones with alkoxy groups. Triphenyl phosphite has the greatest activity among the phosphites exam- ined. (The later experiments showed that diphenyl hydrogen phosphite has a greater activity than triphenyl phosphite, and phosphorous acid has a still

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CH,CH,NHCO

Scheme 1. Reaction of 2-phenyl-2-oxazoline with a carboxylic acid.

greater one.) Some other phosphorus compounds were tested. Triphenyl phosphine and triphenyl phosphate had no activity, and some kinds of phosphonates were slightly active.

Kinetics of the Reactions

Finally, the kinetics of the reaction between 2-phenyl-2-oxazoline and n-octanoic acid was studied. The reaction was carried out at several tempera-

BIS(2-OXAZOLINE) COMPOUNDS

TABLE I1 Effect of Catalysts on Reaction Between an Amide and 2-Phenyl-2-Oxazolinee

I I1

2755

Catalysts (1% of Reactants)

Decrease of reactants (%I

I I1

1 2 4

14

21

26

4 8 6

38

65

72

30 80

'Conditions: Bath temperature, 2 0 0 O C ; time, 1 hr.

100

90 O C

t c e, t c 0 0

130 O c *\

Q

[r

0 ' I I

0 20 40 60 80 100 120 140

T i m e ( m i n )

Fig. 3. Changes of the acid contents in the reaction of 2-phenyl-2-oxazoline with n-octanoic acid at various temperatures.

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0 I I @-{I + C7HI5COOH CNCH2CH2OCC7H,,

X A XA

0 0

+ X

0 0 I1 II

- CNCH2CH,OCC7H,, I CH2CH2NCO Q

I CH,CH,NHC

Scheme 2. Reaction of 2-phenyld-oxazoline with n-octanoic acid.

tures and traced by titrating the samples with a standard solution of potas- sium hydroxide. The results are shown in Figure 3. Dilute solutions are generally used for the study of kinetics. Therefore, the reaction conditions of the present study were considered to be not very suitable. But preliminary examination indicated that the results could be analyzed kinetically. For the determination of the rate constants the reactions were simulated on an analog computer on the bases of some assumptions. Only three kinds of reactions were taken into account. Each reaction was assumed to be second order, and k , was assumed to equal to k,, though the latter assumption seemed to hold good only approximately (Scheme 2).

Table I11 shows the rate constants obtained, and Figure 4 is the Arrhenius plot of them. The activation energies and frequency factors were calculated by the method of least squares (Table 111). The activation energy for the side reaction is equal to that for the main reaction, which means that the ratio of the components in the final reaction product is constant regardless of the reaction temperature.

The progress of the reaction in the presence of triphenyl phosphite (Fig. 5) shows that it also accelerates the main reaction, though in a small degree. The rate constant in the presence of 1.0% of triphenyl phosphite is 1.3 times that without catalyst.

The mechanism of the catalysis has not been studied yet.

Further Development

The results obtained so far show that very high molecular weight polymers are not produced in the reaction of bis-2-oxazolines with dicarboxylic acids.

BIS(2-OXAZOLINE) COMPOUNDS 2757

TABLE 111 Rate Constants and Arrhenius Expressions

Rate constant Temperature (Lmol- 's-l)

("C) k , k ,

90 3.01 x 1 0 - ~ 110 1.20 x 1 0 - ~ 130 4.10 x lo-* 4.60 x

170 3.17 x 1 0 - ~ 3.55 x 150 1.24 x 1 0 - ~ 1.34 X 10

Activation energy (kcal mol-')

Main reaction Side reaction

19 19

Frequency factor (L mol-' s-l)

Main reaction Side reaction

6.1 X lo6 7.0 x lo4

But they suggest that a new type of crosslinked resin may be formed when excess bis-2-oxazoline and a dicarboxylic acid are heated in the presence of a certain phosphite. In fact, heating a mixture of 0.15 mol of 1,3-PBO, 0.1 mol of adipic acid and 0.5% of triphenyl phosphite gave a light yellow, transparent, hard solid, which was insoluble in any solvent and infusible. The details will be reported elsewhere.

T ('C)

170 150 130 110 90 I

-6 2.2 2.4 2.6 2.8

1 r x 10'

Fig. 4. The Arrhenius plot of k , and k,.

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> 3 ._

E4 .t V Q

L I I I

0 50 100 150 Time (rnin)

Fig. 5. The reaction of 2-phenyl-2-oxazoline with n-octanoic acid in the presence of triphenyl phosphite at 150°C.

EXPERIMENTAL

Materials

2-Oxazolines

1,3-PBO, 1,4-PBO, and 2-phenyl-2-oxazoline were prepared by the reaction of corresponding nitriles with 2-aminoethanol in the presence of cadmium nitrate as catalyst according to Witte and Seeliger? 1,3-PBO was recrystal- lized from ethanol; mp 149-151°C (lit? 143-146°C).

ANAL. Calcd for C,,H,,N,O,: C, 66.65; H. 5.59; N, 12.96. Found: C, 66.58; H, 5.62; N, 12.92. 1,4-PBO was purified by washing with hot iso-propanol; mp 248-249°C

ANAL. Found: C, 66.65; H, 5.52; N, 12.85. 2-Phenyl-2-oxazoline was purified by distillation under reduced pressure; bp

(lit.8 246°C).

115-116°C/ll XIUYI Hg.

Acids

Sebacic acid (purity: more than 99.5%. Hokoku Oil Co.) was used without further purification. n-Octanoic acid was distilled under reduced pressure before use: bp 136.0-136.5"C/22 mm Hg. Acid value: 389 (theoretical value: 389.0).

Esteramides

Esteratnide I1 was prepared by the reaction of 2-aminoethanol with benzoyl chloride, followed by neutralization with a sodium hydroxide solution, and recrystallized from ethanol; mp 89-90°C (lit.' 89-90OC).

ANAL. Calcd for C,,H,,NO,: C, 71.90; H, 5.68; N, 5.20. Found: C, 71.45; H, 5.61; N, 5.19.

BIS(2-OXAZOLINE) COMPOUNDS 2759

Esteramide I11 was prepared from N-2'-aminoethyl-2-a1ninoethanol and benzoyl chloride, and recrystallized from ethanol; mp 119-120°C (lit." 1 19 - 120" C). ANAL. Calcd for C2,H2,N204: C, 72.10; H, 5.81; N, 6.73. Found: C, 71.90, H, 5.68; N, 6.79. _. ~ ~

Other chemicals were obtained commercially and used without further purification.

Polymerization

The viscosities of the products were measured with an Ostwald viscometer in rn-cresol at 30°C at various concentrations, and the intrinsic viscosities were calculated from the data. The acid contents were measured by titrating the ethanol solutions of the products with a standard KOH ethanol solution.

Reaction of Esteramides with 2-Phenyl-2-Oxazoline

The contents of the starting materials were measured by use of a gel permeation chromatograph. Column: HSG-20 x 2 + HSG-15 x 3 (Shimadzu Corp.). Detector: refractometer.

The 'H-NMR spectra were recorded on a Jeol JNM-GX270 (270 MHz) spectrometer. These spectra were obtained in CDCl, solutions at room tem- perature, and with tetramethylsilane as internal standard.

Determination of the Rate Constants for the Reaction of 2-Phenyl-2-Oxazoline with n-Octanoic Acid

Acid contents of the reaction mixtures were traced by titration with a standard KOH solution (Fig. 2). The data at 90°C were taken over a period of 400 min, though they are not fully plotted in the figure.

From Scheme 2 the following equations were derived:

Here (X), C, and V mean, respectively, the total mole of X in the reaction mixture, the mole of X in the unit volume, and the volume of the reaction mixture. The volume of the initial reaction mixture and the final product at a certain temperature were obtained from Figure 6. The points marked with x

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I I I I I I 80 100 120 140 160 180

Tempera ture ( " c )

x Found va lue Fig. 6. Changes of the specific gravities.

in the figure was determined experimentally with a 20 mL measuring flask, whose volumes at higher temperatures were revised on the basis of the thermal expansion coefficient of the glass. The change of the volume during reaction was supposed to be proportional to the change of the acid content.

These equations, after being changed into the forms suitable to the com- puter, were put into an EAI 680 Analog/Hybrid Computer. We tried to set each potentiometer corresponding to the term including k, , k , , or k , (= k 2 ) a t such a value that the change of C on the computer would agree most closely with the curve of the acid content versus time. After some trials, a very good agreement was reached in every reaction, and k , and k , at different temperatures were calculated from the values of the potentiometers.

I am very grateful to Mr. Masahiro Yamada (then, research manager of Engineering Research Division, Takeda Chemical Industries, Ltd.) for teaching me how to simulate the process of the reaction on the analog computer.

References 1. E. M. Fry, J. Org. Chem., 15, 802 (1950). 2. A. Jaeger (Farbwerke Hoechst), German Pat., 1,050,540 (1953). 3. T. Kagiya, S. Narisawa, T. Maeda, and K. Fukui, J. Polym. Scz. B, 4, 257 (1966); see also

4. R. H. Wiley and L. L. Bennett, Chem. Reo., 44, 447 (1949). 5. W. Seeliger, E. Aufderhaar, W. Diepers, R. Feinauer, R. Nehring, W. Thier, and H.

6. J. A. Frump, C h m . Rev., 7 l , 483 (1971). 7. T. Kagiya and T. Matsuda, J. Macromol. Sci. Chem., AS, 1265 (1971). 8. H. Witte and W. Seeliger, Liebigs Ann. Chem., 1914, 996 (1974). 9. -V. Spiro and P. Madonia, Gazz. Chim. Ztal., 85,965 (1955).

US. Pat., 3,476,712 (1969).

Hellmann, Angew. Chem., 78, 913 (1966).

10. C. S. M. Stirling, J. Chem. SOC., 1962, 3676 (1962).

Received February 7,1988 Accepted December 21,1988