Synthesis of polymers from α-amino acids by polyaddition

Fur. Polym. J, Vol. 28, No. 2, pp. 183-186, 1992 Printed in Great Britain. All rights reserved

0014-3057/92 $5.00+ 0.00 Copyright © 1992PergamonPress pie

SYNTHESIS OF POLYMERS FROM ~-AMINO ACIDS BY POLYADDITION TSUYOSHI KIYOTSUKURI,t MINORU NAGATA,2. TETSUYAKITAZAWAl and NAOTO TSUTSUMI1 IDepartment of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan 2Junior Women's College of Kyoto Prefectural University, Shimogamo, Sakyo-ku, Kyoto 606, Japan (Received 3 June 1991) Abstract--Novel network polymer films were prepared from glutamic acid diethylester hydroehloride (2Et~Glu.HC1) and hexamethylene diisocyanate (film A) or lysine triisocyanate (film B). Prepolymers prepared by melt polyaddition were cast from dimethylformamide solution and then post-polymerized at 240° for various times. The resulting films were slightly yellow, transparent, flexible and insoluble in organic solvents. Infrared spectra showed that the post-polymerization reached equilibrium after 24-36 hr. Two broad but distinct peaks were observed in the X-ray diffraction curves for film A, suggesting the formation of some ordered structure. Heat distortion temperature measured by TMA was in good agreement with glass transition temperature measured by DTA; both values increased with increasing postpolymerization time and leveled out after 24-36 hr. The tensile strength, elongation and Young's modulus also increased with increasing post-polymerization time. However, prolonged post-polymerization caused decrease of tensile strength. Film B showed higher thermal stability and water absorption and lower alkali resistance than film A,

INTRODUCTION

EXPERIMENTAL PROCEDURES

We have investigated the application of ~-amino acids such as glutamic acid or lysine for the synthesis of polymers. The introduction of these ~-amino acids (or their derivatives) into polymers is expected to open the way to functional polymers which are biodegradable and water sensitive. Recently, we reported the copolymerization of ~-amino acids to nylons by melt-polycondensation [1], However, increase of -amino acid components in copolymers caused thermal degradation of polymers during the polycondensation and resulted in copolymers having much lower molecular weight. Therefore, it is necessary to use lower reaction temperature for the synthesis of polymers from ~-amino acids. Polyaddition usingpolyisocyanate is considered to be favourable, since polyisocyanate reacts with amino or hydroxyl compounds at lower temperatures to form polyureas or polyurethanes. There are a few papers on the use of glutamic acid or lysine for preparing polymers by polyaddition. Kasai et al. [2] prepared linear poly(acylsemicarbazide) from N-carbobenzoxyglutamic acid dihydrazide and various diisocyanates. Biodegradable poly(ester-urethane) network films were also synthesized from lysine diisocyanate or triisocyanate with polyester prepolymers [3]. We now report a synthetic approach to novel network polymer films from glutamic acid diethylester hydrochloride with hexamethylene diisocyanate or lysine triisocyanate using polyaddition. The structures and some properties of the network films are also investigated.

Monomers Structural formulae and codes for monomers used for polymer preparation are shown in Fig. 1. Hexamethylene diisocyanatefflMDI), lysine triisoeyanate(LTI) and glutamic acid diethylester hydrochloride(2Et-Glu.HCl) were used as received.

*To whom all correspondence should be addressed.

Preparation of prepolymers Two prepolymers (A and B) were prepared as follows: A: A mixture of 2 mol HMDI and 1 tool 2Et--GIu.HCI was heated up to 220° at a rate of 3°/rain and held at 220° for 1 hr in a stream of N 2 gas. B: A mixture of 4 mol LTI and 3 tool 2Et-GIu,HCI was heated up to 1200 at 3°/rain and held at 120° for I hr in a stream of N~ gas. Film preparation The above prepolymer was cast on an A1 plate from ca 10% dimethylformamide solution at 80°. Post-polymerization of films The cast film was heated to 240° at a rate of 5°/min for cast film A and l°/min for cast film B, and was post-polymerized under a stream of N 2 gas at 240° for 2-36hr. After the

OCN-(CH2)~NCO (HMDI)

0 CNt CH2)4ClH-CO~CH2~2NCO (LTI) NCO C2H502C~CH2~2CH-CO2C2H5 (2Et-GIu-HCl) NH~HCl Fig. 1. Structural formulae and codes for monomers used for polymer preparation. 183

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TSUYOSHI KIYOTSUKURI et al.

post-polymerization, the A1 plate was dissolved off in 10% HC1. Characterization

Infrared spectrum was recorded on a Jasco model IRA-1 spectrophotometer using a thin film. X-ray diffraction intensity was measured with a Toshiba model ADG-301 X-ray diffractometer with Ni filtered CuK~ radiation. Thermomechanical analysis (TMA) was performed in the penetration mode under 5 kg/cm2 pressure at a heating rate of 10°/min in N2 using a Shimadzu model DT-30 thermomechanical analyzer. Differential thermal analysis (DTA) was carried out on a Shimadzu model DT-30 differential thermal analyzer at a heating rate of 10°/min in N2. Tensile properties were measured by a Shimadzu model IM-100 tensile tester. Thermogravimetry (TG) was performed on a Shimadzu model DT-30 thermogravimetric analyzer at a heating rate of 10°/min in N2.

RESULTS AND DISCUSSION

The network polymer films from 2Et-Glu.HC1 and HMDI (film A) or LTI (film B) were prepared using melt polyaddition followed by post-polymerization. The resulting films were slightly yellow, transparent and flexible. Increasing post-polymerization time caused them to be insoluble in organic solvents. The effects of post-polymerization time on the structure and properties of the network films were studied. Infrared spectra of films A and B post-polymerized at 240 ° for 24 hr are shown in Fig. 2. The absorption bands due to the urea grouping occur at 1650 cm -1 (C-----O stretching) and 1550 cm -I ( N - - H deformation). The absorption band due to the ester group is observed at 1770cm -1. A strong absorption at 1710cm -l may be assigned to the biuret grouping. Intensity ratios of these absorption bands relative to absorption of methylene group almost levelled out after post-polymerization for 24-36 hr, suggesting that the reaction reached equilibrium. Judging from these i.r. spectra, network polymer structure could be formed as follows: At first, the isocyanate group of HMDI or LTI reacts with the amino group in 2Et-Glu'HCI to form a urea. Then another isocyanate group reacts with the urea grouping to form a biuret. Thus linear prepolymer is formed. Network structure is formed by the addition of isocyanate group to the biuret grouping between linear polymer chains.

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Fig. 3. X-ray diffraction intensity curves of films A and B post-polymerized at 240° for 24 hr. Figure 3 shows X-ray diffraction intensity curves of films A and B post-polymerized at 240 ° for 24 hr. Two broad but distinct diffraction peaks are observed for film A. Similar X-ray diffraction intensity curves have been obtained for network polyester films, suggesting the formation of some ordered structure [4]. The intensities of these diffraction peaks changed a little during the post-polymerization. Two diffraction peaks are also observed for film B but the left peak is weak compared with that of film A. The intensities of these diffraction peaks for film B were almost unchanged during post-polymerization. Heat distortion temperature (T~) measured by TMA is shown as a function of the post-polymerization time for films A and B in Fig. 4. Th values of these films increase in two steps with increasing post-polymerization time. These step changes of Th suggest that network structure are formed during the postpolymerization by the reaction mechanism as shown above by i.r. analysis. Th values almost level out after postpolymerization for 24-36 hr for both films, corresponding to the change of i.r. spectra as described above. Th value of film B at the equilibrium is 170°, which is much higher than that of 104° for film A. This effect may be ascribed to the difference of crosslinking density, i.e. crosslinking density of film B is considered to be higher than that of film A because film B is prepared with the more reactive triisocyanate while film A is prepared with the less reactive diisocyanate. An endothermic transition due to the glass transition (Tg) was observed in DTA curves for films

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Fig. 2. Infrared spectra of films A and B post-polymerized at 240° for 24 hr.

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Fig. 4.

P o s t - p o l y m e r i z a t i o n t i m e at 240 ° vs T h o f films A

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Synthesis of polymers from a-amino acids

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Fig. 5. Post-polymerization time at 240 ° vs Tg of films

Fig. 7. Post-polymerization time at 240 ° vs Young's

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modulus of films A and B,

A and B. Tg values are shown against the postpolymerization time in Fig. 5. Tg of these films increase in two steps with increasing post-polymerization time and level out after 24-36 hr for film A. These effects match those of Th described above. Tg is 105 ° for film A and 143 ° for film B at equilibrium, in good agreement with those of Th. Tensile strength, elongation and Young's modulus of films A and B are shown against the postpolymerization time in Figs 6, 7 and 8, respectively. Tensile properties of film B were measured only for those post-polymerized for 4 hr and for 12 hr, since it was difficult to prepare good films for others. Tensile strength and Young's modulus of film A increase with increasing post-polymerization time, matching the increase of Th and Tg described above, but treatment for 36 hr caused decrease of tensile strength. Elongation also increased with increasing postpolymerization time; that for the film treated for 36 hr is much larger than that post-polymerized for 24 hr, corresponding to the drastic decrease of tensile strength of the film post-polymerized for 36 hr. This effect may be explained by the fact that decrease of crosslinking density by thermal decomposition would increase the ease of slippage between polymer chains. Tensile strength and Young's modulus of film B are higher than those of film A, effects which may be

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Fig. 8. Post-polymerization time at 240 ° vs elongation of films A and B. ascribed to the higher crosslinking density of film B as described according to Th value. TG curves of films A and B post-polymerized at 240 ° for 24 hr are shown in Fig. 9. Weight loss at lower temperature and residue at 600 ° of film B are larger than those of film A. These differences may be ascribed to the difference in content of ~-amino acid components. Moisture regain, water absorption and alkali hydrolysis were measured for films A and B post-

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Fig. 6. Post-polymerization time at 240 ° vs tensile strength of films A and B.

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Fig. 9. TG curves of films A and B post-polymerized at 2 4 0 ° for 24 hr.

186

Tstrvosm KIYOTSUKURIel aL

polymerized at 240 ° for 4 hr. Moisture regain under standard condition (20 °, 65%) was 2.8% for film A and 2.6% for film B. Water absorption in water for 24 hr at room temperature was 11.6% for film A and 21.4% for film B. Weight loss of film A by alkali hydrolysis with 1% NaOH solution for 16 hr at room temperature was 1.8% but it exceeded 50% for film B treated for I hr at room temperature with 1% NaOH solution, and there was complete dissolution after 24 hr. The higher water absorption and lower alkali resistance of film B are probably due to film B being composed of ~-amino acid component only which is sensitive to water and alkali. Both films A and B are polymer films having ~t-amino acid components and so they are expected to be biodegradable. In addition, the degradation products of film B are all

non-toxic, which is essential for use as a feature degradable biomedical material. Acknowledgement--The authors are grateful to Kyowa Hakko Kogyo Co., Ltd for the gift of ~t-amino acids and their derivatives. REFERENCES

1. M. Nagata and T. Kiyotsukuri. Kobunshi Ronbunshu48, III (1991). 2. Y. Kasai and S. Yamazaki. Kobunshi Kagaku 29, 254 (1972). 3. P. Bruin, G. J. Veenstra, A. J. Nijenhuis and A. J. Pennings. Makromolek. Chem., Rapid Commun. 9, 589 (1988). 4. T. Kiyotsukuri, N. Tsutsumi and Y. H. Chen. J. Polym. Sci.; Part A: Polym. Chem. 28, 1197 (1990).