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Synthesis and characterization of new biodegradable thermosensitive polyphosphazenes with lactic acid ester and methoxyethoxyethoxy side groups
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Chinese Chemical Letters 21 (2010) 237–241 www.elsevier.com/locate/cclet
Synthesis and characterization of new biodegradable thermosensitive polyphosphazenes with lactic acid ester and methoxyethoxyethoxy side groups Yun Mei Bi a,*, Xiao Ying Gong a, Wen Zhao Wang a, Li Yu b, Min Qi Hu a, Li Dong Shao a b
a College of Chemistry and Engineering, Yunnan Normal University, Kunming 650092, China College of Chemistry and Chemical Engineering, Chonqing College of Science and Technology, Chongqing 40042, China
Received 10 June 2009
Abstract Two novel biodegradable thermosensitive polyphosphazenes with lactic acid ester and methoxyethoxyethoxy side groups were synthesized via the macromolecular substitution reactions of poly(dichlorophosphazene) with the sodium salt of lactic acid ester and sodium methoxyethoxyethoxide. Their structures were confirmed by 31P NMR, 1H NMR, 13C NMR, IR, DSC, and elemental analysis. The lower critical solution temperature (LCST) behavior in water and in vitro degradation property of the polymers was investigated. The results indicated that two polymers showed LCST phase transition over a range of concentrations from 0.13 to 15 wt% and pH-sensitive degradation properties. # 2009 Yun Mei Bi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Polyphosphazene; Lactic acid ester; Methoxyethoxyethoxy; Biodegradation; Thermosensitive polymer
Thermosensitive polymer solutions exhibit a phase transition known as the lower critical solution temperature (LCST), below which the polymers are soluble. When the temperature is raised above the LCST, the polymers precipitate from solution. These polymers have attracted much attention due to a wide range of potential applications, for example in drug-delivery systems [1,2] and for the preparation of thermoresponsive membranes [3]. Most of the investigations have been devoted to the homopolymer or copolymers of N-isopropylacrylamide [4,5]. Poly(Nisopropylacrylamide) (PNIPAM) shows a LCST around body temperature (at 32 8C), so it is a potential candidate as a biomedical carrier either as linear polymer, copolymer or hydrogel [6]. However, the selection of PNIPAM and its copolymers is limited because they are toxic and nonbiodegradable [7]. Polyphosphazenes have emerged as a relative novel family of biomaterials possessing a backbone of alternating nitrogen and phosphorus atoms with organic side groups attached to the phosphorus atoms. Polymers with a wide range of useful properties can be designed and attainable by nucleophilic substitution of poly(dichlorophosphazene) (PDCP) with various organic groups. Song et al. [8] have reported that thermosensitive polyphosphazenes bearing
* Corresponding author. E-mail address: [email protected] (Y.M. Bi). 1001-8417/$ – see front matter # 2009 Yun Mei Bi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2009.10.003
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Y.M. Bi et al. / Chinese Chemical Letters 21 (2010) 237–241
Scheme 1.
methoxy-poly(ethylene glycol) and amino acid easers are biodegradable. Qiu and co-workers [9] have synthesized thermosensitive amphiphilic graft polyphosphazene with oligopoly(N-isopropylacrylamide) and ethyl 4-aminobenzoate as side groups. Herein, we report the synthesis of two novel biodegradable thermosensitive polyphosphazenes with lactic acid ester and methoxyethoxyethoxy side groups. Poly(lactic acid) (PLA) is widely used as biodegradable sutures and drug-delivery devices [10]. However, it degrades slowly, limiting its use to certain application. Poly(bis(ethyl lactato) phosphazene) and poly[bis(benzyl lactato) phosphazene] hydrolyze at a faster rate than PLA [11]. So the LCST and hydrolysis studies of the novel polymers were also performed in this letter. The synthesis of the target polymers was depicted in Scheme 1. General procedure for the synthesis of the polymers 1 and 2 was as follows: poly(dichlorophosphazene) (PDCP) was prepared by the ring-opening polymerization of hexachlorocyclotriphosphazene in the presence of 2% AlCl3 as described by Sohn et al. [12]. Under nitrogen, PDCP (29 mmol) was dissolved in 200 mL of dry THF. Lactic acid ester (29 mmol) was added to a suspension of sodium hydride (29mmol) in 150 mL of dry THF. The resulting solution was added dropwise to the polymer solution. The reaction mixture was stirred and heated to 50 8C for 36 h. Methoxyethoxyethanol (72.5 mmol) was added to a suspension of sodium hydride (72.5 mmol) in 150 mL of dry THF. After the reaction was completed, this solution was added dropwise to the mixture containing the partially substituted polymer. The reaction solution was stirred at room temperature. Reaction progress was monitored by 31P NMR spectroscopy. After 48 h, the solvent was evaporated under vacuum and the final product was isolated by precipitation into petroleum ether. The polymer was further purified by repeated precipitations from THF into petroleum ether. After purification, the target polymers were characterized by the spectra, DSC and elemental analysis. Their molecular weights were determined by vapor pressure osmometry (VPO). Poly[(ethyl lactato)1.0(methoxyethoxyethoxy)1.0 phosphazene] 1: light yellow-colored solid; Yield: 69%; IR (KBr, cm 1): 2984, 2937 (C–H), 1745 (C O), 1229 (P N), 1102 (C–O); 31P NMR (CDCl3): d 8.27; 1H NMR (CDCl3): d Fig. 1; 13C NMR (CDCl3): d 170.65, 71.71 (overlapped), 70.23, 68.65, 64.22, 61.17, 58.74, 19.00, 13.94; Anal. Calcd. for C10H20NO6P: C 42.69, H 7.17, N 4.98, P 11.02; found: C 42.73, H 7.15, N 4.93, P 11.05; Tg: 49.9 8C; Mn: 6187. Poly[(propyl lactato)1.0(methoxyethoxyethoxy)1.0 phosphazene] 2: light yellow-colored solid; Yield: 66%; IR (KBr, cm 1):2979, 2931 (C–H), 1742 (C O), 1214 (P N), 1100 (C–O); 31P NMR (CDCl3): d 8.27; 1H NMR (CDCl3): d Fig. 1; 13C NMR (CDCl3): d 170.41, 71.61 (overlapped), 70.08, 68.53, 64.37, 61.46, 58.64, 21.59, 18.95, 9.98; Anal. Calcd. for C11H22NO6P: C 44.73, H 7.51, N 4.74, P 10.50; found: C 44.70, H 7.55, N 4.78, P 10.55; Tg: 44.0 8C; Mn: 4711. The spectral data were in agreement with the desired structure. The 31P NMR spectrum of polymer 1 showed one peak at 6.70 ppm. The existence of only one peak in the spectrum of the mixed-substituent polymer 1 is presumably a result of the close resonances of homogeneously substituted bis(methoxyethoxyethoxy)-substituted phosphorus [13] and bis(ethyl lactato)-substituted phosphorus [11], and the expected heterogeneously substituted phosphorus. Such a result and the data of elemental analysis implied that the chlorine atoms of poly(dichlorophosphazene) were completely substituted stepwise by ethyl lactato and methoxyethoxyethoxy. Polymers 2 and 1 showed the same chemical shifts because phosphorous environments were not seriously changed. Polymers 1 and 2 were found to be readily soluble in water at room temperature and both showed LCST behavior. The phase transition of each polymer was traced by monitoring the transmittance of a 500 nm light beam on a UV–vis spectrophotometer. The polymer was dissolved in deionized water at different concentrations. Turbidimetric experiments were performed from 20 to 95 8C with 2 8C increments every 10 min. The light intensity through the
Y.M. Bi et al. / Chinese Chemical Letters 21 (2010) 237–241
239
Fig. 1. 1H NMR of polymers 1 and 2.
solution was measured as a function of temperature. The LCST was defined as the temperature at which the transmittance is 50% of value at room temperature. As shown in Fig. 2, polymers 1 and 2 showed an LCST phase transition over a range of concentrations from 0.13 to 15 wt% when dissolved in water and heated. And polymer 2, with more hydrophobic ester group, exhibited a lower LCST than polymer 1. In addition, it was found that the LCSTs of polymers 1 and 2 were independent in the range of 1.5–15 wt% of the polymer but increase at lower concentrations as, observed for other thermosensitive polymers [8]. The hydrolytic degradation of polymers 1 and 2 was studied in buffer solutions at pH 3.4, pH 7.4 and pH 10.8 at 37 8C by monitoring the molecular weight decline over a period of 3 days. As shown in Figs. 3 and 4, both polymers showed a significant decline in molecular weight, and their hydrolytic behavior was dependent on pH of the buffer solutions. The rate of polymers 1 and 2 hydrolysis in aqueous solution decreased in the order of base > acid > neutral solution. This was consistent with the hydrolytic behavior of lactic acid esters. The structure of ester groups of the lactic acids attached to the polymers affected the degradation of polymers. Polymer 1 with ethyl lactato as a side group showed a slightly faster hydrolysis than polymer 2 carrying a propyl lactato side unit.
Fig. 2. Concentration dependent LCST behaviors of polymers 1 and 2.
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Fig. 3. Time-dependent hydrolytic degradation of polymer 1 at pH 3.4, pH 7.4 and pH 10.8 at 37 8C.
Fig. 4. Time-dependent hydrolytic degradation of polymer 2 at pH 3.4, pH 7.4 and pH 10.8 at 37 8C.
In this letter, we have synthesized two new biodegradable thermosensitive poly(organophosphazenes) with lactic acid ester and methoxyethoxyethoxy side groups. Further studied of the hydrolysis mechanism of the new polymers and the salt and pH effects on the thermosensitivity of the polymers are in progress. Acknowledgments The authors gratefully acknowledge support for this work of the National Natural Foundation of China (No. 20364002) and the Natural Science Foundation of Yunnan Province (No. 2005B0027 M), China. References [1] [2] [3] [4] [5] [6]
D.C. Coughlan, O.I. Corrigan, Int. J. Pharm. 313 (2006) 163. D. Schmaljohann, Adv. Drug Deliv. Rev. 58 (2006) 1655. N.I. Shtanko, V.Y. Kabanov, P.Y. Apel, et al. J. Membrane Sci. 179 (2000) 155. I. Dimitrov, B. Trzebicka, A.H.E. Mu¨ller, et al. Prog. Polym. Sci. 32 (2007) 1275. J. Kopecek, Eur. J. Pharm. Sci. 20 (2003) 1. C.H. Ni, X.Y. Zeng, H. Huang, Chin. Chem. Lett. 16 (2005) 675.
Y.M. Bi et al. / Chinese Chemical Letters 21 (2010) 237–241 [7] H. Vihola, A. Laukkanen, L. Valtola, et al. J. Hirvonen. Biomater. 26 (2005) 3055. [8] (a) S.C. Song, S.B. Lee, J.I. Jin, et al. Macromolecules 32 (1999) 2188; (b) S.B. Lee, S.C. Song, J.I. Jin, et al. Macromolecules 32 (1999) 7820. [9] J.X. Zhang, L.Y. Qiu, Y. Jin, et al. Macromolecules 39 (2006) 451. [10] R. Jain, N.H. Shah, A.W. Malick, et al. Drug Dev. Ind. Pharm. 24 (1998) 703. [11] H.R. Allcock, S.R. Pucher, Macromolecules 27 (1994) 1. [12] Y.S. Sohn, Y.H. Cho, H. Baek, et al. Macromolecules 28 (1995) 7566. [13] H.R. Allcock, S.R. Pucher, M.L. Turner, et al. Macromolecules 25 (1992) 5573.
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Chinese Chemical Letters 21 (2010) 237–241 www.elsevier.com/locate/cclet
Synthesis and characterization of new biodegradable thermosensitive polyphosphazenes with lactic acid ester and methoxyethoxyethoxy side groups Yun Mei Bi a,*, Xiao Ying Gong a, Wen Zhao Wang a, Li Yu b, Min Qi Hu a, Li Dong Shao a b
a College of Chemistry and Engineering, Yunnan Normal University, Kunming 650092, China College of Chemistry and Chemical Engineering, Chonqing College of Science and Technology, Chongqing 40042, China
Received 10 June 2009
Abstract Two novel biodegradable thermosensitive polyphosphazenes with lactic acid ester and methoxyethoxyethoxy side groups were synthesized via the macromolecular substitution reactions of poly(dichlorophosphazene) with the sodium salt of lactic acid ester and sodium methoxyethoxyethoxide. Their structures were confirmed by 31P NMR, 1H NMR, 13C NMR, IR, DSC, and elemental analysis. The lower critical solution temperature (LCST) behavior in water and in vitro degradation property of the polymers was investigated. The results indicated that two polymers showed LCST phase transition over a range of concentrations from 0.13 to 15 wt% and pH-sensitive degradation properties. # 2009 Yun Mei Bi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Polyphosphazene; Lactic acid ester; Methoxyethoxyethoxy; Biodegradation; Thermosensitive polymer
Thermosensitive polymer solutions exhibit a phase transition known as the lower critical solution temperature (LCST), below which the polymers are soluble. When the temperature is raised above the LCST, the polymers precipitate from solution. These polymers have attracted much attention due to a wide range of potential applications, for example in drug-delivery systems [1,2] and for the preparation of thermoresponsive membranes [3]. Most of the investigations have been devoted to the homopolymer or copolymers of N-isopropylacrylamide [4,5]. Poly(Nisopropylacrylamide) (PNIPAM) shows a LCST around body temperature (at 32 8C), so it is a potential candidate as a biomedical carrier either as linear polymer, copolymer or hydrogel [6]. However, the selection of PNIPAM and its copolymers is limited because they are toxic and nonbiodegradable [7]. Polyphosphazenes have emerged as a relative novel family of biomaterials possessing a backbone of alternating nitrogen and phosphorus atoms with organic side groups attached to the phosphorus atoms. Polymers with a wide range of useful properties can be designed and attainable by nucleophilic substitution of poly(dichlorophosphazene) (PDCP) with various organic groups. Song et al. [8] have reported that thermosensitive polyphosphazenes bearing
* Corresponding author. E-mail address: [email protected] (Y.M. Bi). 1001-8417/$ – see front matter # 2009 Yun Mei Bi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2009.10.003
238
Y.M. Bi et al. / Chinese Chemical Letters 21 (2010) 237–241
Scheme 1.
methoxy-poly(ethylene glycol) and amino acid easers are biodegradable. Qiu and co-workers [9] have synthesized thermosensitive amphiphilic graft polyphosphazene with oligopoly(N-isopropylacrylamide) and ethyl 4-aminobenzoate as side groups. Herein, we report the synthesis of two novel biodegradable thermosensitive polyphosphazenes with lactic acid ester and methoxyethoxyethoxy side groups. Poly(lactic acid) (PLA) is widely used as biodegradable sutures and drug-delivery devices [10]. However, it degrades slowly, limiting its use to certain application. Poly(bis(ethyl lactato) phosphazene) and poly[bis(benzyl lactato) phosphazene] hydrolyze at a faster rate than PLA [11]. So the LCST and hydrolysis studies of the novel polymers were also performed in this letter. The synthesis of the target polymers was depicted in Scheme 1. General procedure for the synthesis of the polymers 1 and 2 was as follows: poly(dichlorophosphazene) (PDCP) was prepared by the ring-opening polymerization of hexachlorocyclotriphosphazene in the presence of 2% AlCl3 as described by Sohn et al. [12]. Under nitrogen, PDCP (29 mmol) was dissolved in 200 mL of dry THF. Lactic acid ester (29 mmol) was added to a suspension of sodium hydride (29mmol) in 150 mL of dry THF. The resulting solution was added dropwise to the polymer solution. The reaction mixture was stirred and heated to 50 8C for 36 h. Methoxyethoxyethanol (72.5 mmol) was added to a suspension of sodium hydride (72.5 mmol) in 150 mL of dry THF. After the reaction was completed, this solution was added dropwise to the mixture containing the partially substituted polymer. The reaction solution was stirred at room temperature. Reaction progress was monitored by 31P NMR spectroscopy. After 48 h, the solvent was evaporated under vacuum and the final product was isolated by precipitation into petroleum ether. The polymer was further purified by repeated precipitations from THF into petroleum ether. After purification, the target polymers were characterized by the spectra, DSC and elemental analysis. Their molecular weights were determined by vapor pressure osmometry (VPO). Poly[(ethyl lactato)1.0(methoxyethoxyethoxy)1.0 phosphazene] 1: light yellow-colored solid; Yield: 69%; IR (KBr, cm 1): 2984, 2937 (C–H), 1745 (C O), 1229 (P N), 1102 (C–O); 31P NMR (CDCl3): d 8.27; 1H NMR (CDCl3): d Fig. 1; 13C NMR (CDCl3): d 170.65, 71.71 (overlapped), 70.23, 68.65, 64.22, 61.17, 58.74, 19.00, 13.94; Anal. Calcd. for C10H20NO6P: C 42.69, H 7.17, N 4.98, P 11.02; found: C 42.73, H 7.15, N 4.93, P 11.05; Tg: 49.9 8C; Mn: 6187. Poly[(propyl lactato)1.0(methoxyethoxyethoxy)1.0 phosphazene] 2: light yellow-colored solid; Yield: 66%; IR (KBr, cm 1):2979, 2931 (C–H), 1742 (C O), 1214 (P N), 1100 (C–O); 31P NMR (CDCl3): d 8.27; 1H NMR (CDCl3): d Fig. 1; 13C NMR (CDCl3): d 170.41, 71.61 (overlapped), 70.08, 68.53, 64.37, 61.46, 58.64, 21.59, 18.95, 9.98; Anal. Calcd. for C11H22NO6P: C 44.73, H 7.51, N 4.74, P 10.50; found: C 44.70, H 7.55, N 4.78, P 10.55; Tg: 44.0 8C; Mn: 4711. The spectral data were in agreement with the desired structure. The 31P NMR spectrum of polymer 1 showed one peak at 6.70 ppm. The existence of only one peak in the spectrum of the mixed-substituent polymer 1 is presumably a result of the close resonances of homogeneously substituted bis(methoxyethoxyethoxy)-substituted phosphorus [13] and bis(ethyl lactato)-substituted phosphorus [11], and the expected heterogeneously substituted phosphorus. Such a result and the data of elemental analysis implied that the chlorine atoms of poly(dichlorophosphazene) were completely substituted stepwise by ethyl lactato and methoxyethoxyethoxy. Polymers 2 and 1 showed the same chemical shifts because phosphorous environments were not seriously changed. Polymers 1 and 2 were found to be readily soluble in water at room temperature and both showed LCST behavior. The phase transition of each polymer was traced by monitoring the transmittance of a 500 nm light beam on a UV–vis spectrophotometer. The polymer was dissolved in deionized water at different concentrations. Turbidimetric experiments were performed from 20 to 95 8C with 2 8C increments every 10 min. The light intensity through the
Y.M. Bi et al. / Chinese Chemical Letters 21 (2010) 237–241
239
Fig. 1. 1H NMR of polymers 1 and 2.
solution was measured as a function of temperature. The LCST was defined as the temperature at which the transmittance is 50% of value at room temperature. As shown in Fig. 2, polymers 1 and 2 showed an LCST phase transition over a range of concentrations from 0.13 to 15 wt% when dissolved in water and heated. And polymer 2, with more hydrophobic ester group, exhibited a lower LCST than polymer 1. In addition, it was found that the LCSTs of polymers 1 and 2 were independent in the range of 1.5–15 wt% of the polymer but increase at lower concentrations as, observed for other thermosensitive polymers [8]. The hydrolytic degradation of polymers 1 and 2 was studied in buffer solutions at pH 3.4, pH 7.4 and pH 10.8 at 37 8C by monitoring the molecular weight decline over a period of 3 days. As shown in Figs. 3 and 4, both polymers showed a significant decline in molecular weight, and their hydrolytic behavior was dependent on pH of the buffer solutions. The rate of polymers 1 and 2 hydrolysis in aqueous solution decreased in the order of base > acid > neutral solution. This was consistent with the hydrolytic behavior of lactic acid esters. The structure of ester groups of the lactic acids attached to the polymers affected the degradation of polymers. Polymer 1 with ethyl lactato as a side group showed a slightly faster hydrolysis than polymer 2 carrying a propyl lactato side unit.
Fig. 2. Concentration dependent LCST behaviors of polymers 1 and 2.
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Fig. 3. Time-dependent hydrolytic degradation of polymer 1 at pH 3.4, pH 7.4 and pH 10.8 at 37 8C.
Fig. 4. Time-dependent hydrolytic degradation of polymer 2 at pH 3.4, pH 7.4 and pH 10.8 at 37 8C.
In this letter, we have synthesized two new biodegradable thermosensitive poly(organophosphazenes) with lactic acid ester and methoxyethoxyethoxy side groups. Further studied of the hydrolysis mechanism of the new polymers and the salt and pH effects on the thermosensitivity of the polymers are in progress. Acknowledgments The authors gratefully acknowledge support for this work of the National Natural Foundation of China (No. 20364002) and the Natural Science Foundation of Yunnan Province (No. 2005B0027 M), China. References [1] [2] [3] [4] [5] [6]
D.C. Coughlan, O.I. Corrigan, Int. J. Pharm. 313 (2006) 163. D. Schmaljohann, Adv. Drug Deliv. Rev. 58 (2006) 1655. N.I. Shtanko, V.Y. Kabanov, P.Y. Apel, et al. J. Membrane Sci. 179 (2000) 155. I. Dimitrov, B. Trzebicka, A.H.E. Mu¨ller, et al. Prog. Polym. Sci. 32 (2007) 1275. J. Kopecek, Eur. J. Pharm. Sci. 20 (2003) 1. C.H. Ni, X.Y. Zeng, H. Huang, Chin. Chem. Lett. 16 (2005) 675.
Y.M. Bi et al. / Chinese Chemical Letters 21 (2010) 237–241 [7] H. Vihola, A. Laukkanen, L. Valtola, et al. J. Hirvonen. Biomater. 26 (2005) 3055. [8] (a) S.C. Song, S.B. Lee, J.I. Jin, et al. Macromolecules 32 (1999) 2188; (b) S.B. Lee, S.C. Song, J.I. Jin, et al. Macromolecules 32 (1999) 7820. [9] J.X. Zhang, L.Y. Qiu, Y. Jin, et al. Macromolecules 39 (2006) 451. [10] R. Jain, N.H. Shah, A.W. Malick, et al. Drug Dev. Ind. Pharm. 24 (1998) 703. [11] H.R. Allcock, S.R. Pucher, Macromolecules 27 (1994) 1. [12] Y.S. Sohn, Y.H. Cho, H. Baek, et al. Macromolecules 28 (1995) 7566. [13] H.R. Allcock, S.R. Pucher, M.L. Turner, et al. Macromolecules 25 (1992) 5573.
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