Synthesis and Characterization of Biodegradable Hyperbranched Poly

Biomacromolecules 2005, 6, 3181-3188

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Synthesis and Characterization of Biodegradable Hyperbranched Poly(ester-amide)s Based on Natural Material Xiuru Li,†,‡ Yali Su,§ Qingyong Chen,†,‡ Ying Lin,†,‡ Yuejin Tong,§ and Yuesheng Li*,† State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China, Graduate School of the Chinese Academy of Sciences, and College of Chemistry and Materials Science, Fujian Teachers University, Fuzhou 350007, P. R. China Received July 28, 2005; Revised Manuscript Received September 18, 2005

A series of novel AB3-type monomers were prepared from nontoxic natural gallic acid and amino acids. These monomers were then melt-polycondensed in the presence of MgO as a catalyst via a transesterification process at 170-190 °C to yield the hyperbranched poly(ester-amide)s bearing terminal acetyl groups. FTIR and NMR spectra confirmed the structures of all the monomers and polymers. The degrees of branching, estimated from 1H NMR and quantitative 13C NMR spectra, were 0.50-0.68. These hyperbranched polymers displayed moderately high molecular weights. Hydrolytic and enzymatic degradation studies were carried out in vitro at 37.5 °C in NaOH hydrotropic solution and in Tris-HCl buffer (pH ) 8.6) containing proteinase K, respectively. The results indicate that the hyperbranched poly(ester-amide)s are degradable hydrolytically as well as enzymatically, and the rate of hydrolytic degradation increases with the pH value of the solution. Introduction Recently, dendrimers and hyperbranched polymers (HBP) have gained widespread attention due to their unique globular structure and potential applications in coatings, modifiers and additives, catalysts, nanotechnology, self-assembled chemistry, analytical chemistry, biology and medicine, and so forth.1-5 Dendrimers attract the attention of many researchers due to their perfect monodisperse architecture, but their preparation is generally tedious because of various protection, deprotection, and purification steps, which limits their largescale application.6 In contrast, hyperbranched polymers can be conveniently prepared on large scales in one-pot procedures via self-polycondensation or addition polymerization reaction of ABx-type monomers,7,8 ring opening polymerization,9 self-condensation vinyl polymerization,10 atom transfer radical polymerization (ATRP),11 and so on. Biodegradable highly branched polymers are attractive more and more in biomedical and biomaterial areas.5 Dendrimers are excellent candidates as drug and gene deliveries for providing a well-defined molecular architecture.1,5b-g,12-14 Nevertheless, the commercial availability of dendrimers is limited. Currently, the only “commercially” available dendrimers are poly(amidoamine) and poly(propyleneimine). Both of them, however, are toxic in cells and biology due to their polycationic character.15,16 Hence, the searches for new systems with less toxicity for drug delivery purposes have received significant attention.5c-d,g,17-18 Although hyperbranched polymers contain linear units as insufficient branching, they still inherit the properties of * Corresponding author. E-mail: [email protected]. † State Key Laboratory of Polymer Physics and Chemistry. ‡ Graduate School of the Chinese Academy of Sciences. § Fujian Normal University.

dendrimers such as good solubility, low viscosity, and multifunctionality at end groups. Additionally, a well-defined structure can be obtained by improving synthetic methodology and technique. Therefore, hyperbrached polymers, especially hyperbranched polyesters, have been receiving more and more attention in the application of biomaterials. Kricheldorf and co-workers reported the synthesis of potentially biodegradable homo- and copolyesters from gallic acid, phloretic acid, and vanillic acid.19-21 Yan and his colleagues synthesized water-soluble and degradable hyperbranched polyesters based on AB and CD2 type monomers.22 van Benthem and co-workers reported the synthesis of hyperbranched poly(ester-amide)s via direct polycondensation of a cyclic carboxylic anhydride with di-2-propanolamine.23 More recently, we reported the facile synthesis of hyperbranched poly(ester-amide)s from commercially available aliphatic carboxylic anhydrides and multihydroxyl primary amines 24 and communicated the synthesis of hyperbranched poly(ester-amide) based on gallic acid and D,L-2-aminobutyric acid.25 In this paper, we report the synthesis, characterization, and degradation behaviors of a series of hyperbranched poly(ester-amide)s prepared from novel AB3-type monomers based on the nontoxic gallic acid and amino acids that occur in the metabolism of numerous plants and animals. Experimental Section Materials. Gallic acid (GA), glycine, D,L-alanine, D,L-2aminobutyric acid, and β-alanine were purchased from Aldrich and used as received. Proteinase K from Merck and tris(hydroxymethyl)aminomethane from ACROS were used as received. N,N-Dimethylacetamide (DMAc) and N,Ndimethylformamide (DMF) were purified by vacuum distil-

10.1021/bm050531l CCC: $30.25 © 2005 American Chemical Society Published on Web 10/18/2005

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lation over phosphorus pentoxide. Dichloromethane and triethylamine were used after distillation over molecular sieves. Other reagents and solvents were of analytically pure grade and used without further purification. Triacetylgalloyl chloride 2 was prepared according to the procedure described by Kricheldorf.20, 26-27 Measurements. 1H and 13C NMR spectra (DMSO-d6) were recorded using a Bruker AV 300 or 400 MHz spectrometer with the residual 1H solvent peak as reference and the solvent carbon signal as standard, respectively. FTIR spectra were recorded on a Bio-Rad FTS-135 spectrophotometer. Glass transition temperatures (Tg) were measured by differential scanning calorimetry (DSC) on a Perkin-Elmer Pyris 1 DSC with the heating/cooling rates of 10 °C/min and the Tg was taken as the midpoint of the inflection tangent, upon the third or subsequent heating scan. TG measurements were performed with a Perkin-Elmer Pyris 1 Thermogravimetric Analyzer with a heating rate of 20 °C/min in nitrogen. Gel permeation chromatography (GPC) was performed with Waters 1525 fitted with two columns (Styragel HT3 and HT4 DMF or HT3 and HT4 THF 7.8 × 300 mm Column) connected in series and 2414 Refrative Index Detector with TEDIA dimethylformamide (DMF) containing 0.05 M LiBr or THF as the mobile phase. The inherent viscosities were measured with an automatic Ubbelohde viscometer thermostated at 25 °C. UV-vis spectra were taken on a Varian Cary 1E or Shimadzu UV-2450 UV-vis spectrophotometer. Synthesis of Monomers. (3,4,5-Triacetoxy-benzoylamino) Acetic Acid (3A). A 30 mmol amount of triacetylgalloyl chloride and a 10% excess of glycine (33 mmol) were dissolved in dry DMAc in a 100 mL round-bottomed flask with side tube. Triethylamine (36 mmol) was added dropwise with a syringe pump. The reaction mixture was stirred for 24 h at room temperature. After filtration, the filtrate was concentrated to 20 mL under reduced pressure and then poured into 500 mL of water. The mixture was extracted with ethyl acetate (3 × 50 mL), washed with saturated aqueous sodium chloride and water, and then dried over sodium sulfate. After filtration, the solvent was removed via revolving evaporator to give a yellow solid. Yield: 85%. mp 80-83 °C. FTIR (KBr): 3390 (N-H), 2941 (COOH), 1781 (CdO), 1653 (HN-CdO), 1598, 1550, 1493 (Ar), 1187 (C-O) cm-1. 1H NMR (DMSO-d6): δ 2.31 (d, 9H, CH3), 3.91 (d, 2H, CH2), 7.73 (s, 2H, Ar-H), 9.00 (t, 1H, NH), 12.64 (s, 1H, COOH). 13C NMR (DMSO-d6): 20.1, 20.9, 60.2, 120.6, 132.4, 137.7, 143.6, 164.8, 167.4, 168.5, 171.6. 2-(3,4,5-Triacetoxy-benzoylamino) Propionic Acid (3B). The synthetic procedure was similar to that of monomer 3A. Yield: 87%. mp 170-172 °C. FTIR (KBr): 3383 (N-H), 2943 (COOH), 1783 (CdO), 1645 (HN-CdO), 1593, 1554, 1494 (Ar), 1189 (C-O) cm-1. 1H NMR (DMSO-d6): δ 1.38 (d, 3H, CH3), 2.31 (d, 9H, CH3), 4.40 (m, 1H, CH), 7.74 (s, 2H, Ar-H), 8.80 (d, 1H, NH), 12.60 (s, 1H, COOH). 13C NMR (DMSO-d6): 17.3, 20.3, 20.7, 48.9, 121.0, 132.4, 137.8, 143.5, 164.2, 167.4, 168.6, 174.4. 2-(3,4,5-Triacetoxy-benzoylamino) Butyric Acid (3C). The synthetic procedure was similar to that of monomer 3A. Yield: 84%. mp 176-178 °C. IR (KBr): 3390 (N-H), 2940

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(COOH), 1780 (CdO), 1648 (HN-CdO), 1594, 1539, 1491 (Ar), 1186 (C-O) cm-1. 1H NMR (DMSO-d6): δ 0.94 (t, 3H, CH3), 1.80 (q, 2H, CH2), 2.31 (d, 9H, CH3), 4.27 (m, 1H, CH), 7.75 (s, 2H, Ar-H), 8.71 (d, 1H, NH), 12.64 (s, 1H, COOH). 13C NMR (DMSO-d6): δ 11.3, 20.3, 20.8, 24.5, 54.8, 120.9, 132.5, 137.9, 143.5, 164.8, 167.5, 168.5, 173.9. β-(3,4,5-Triacetoxy-benzoylamino) Propionic Acid (3D). A 30 mmol amount of triacetylgalloyl chloride and a 10% excess of β-alanine (33 mmol) were dissolved in dry DMAc in a 100 mL round-bottomed flask with side tube. Triethylamine (36 mmol) was added dropwise with a syringe pump. The reaction mixture was stirred for 24 h at room temperature. After filtration, the filtrate was concentrated to 20 mL under reduced pressure and then poured into 500 mL of water. The precipitate was isolated by filtrating, washed with water, and dried. Yield: 68%. mp 188-190 °C. FTIR (KBr): 3402 (N-H), 2940 (COOH), 1779 (CdO), 1655 (HN-CdO), 1593, 1554, 1492 (Ar), 1187 (C-O) cm-1. 1H NMR (DMSO-d6): δ 2.31 (d, 9H, CH3), 2.53 (t, 2H, CH2), 3.44 (m, 2H, CH2), 7.68 (s, 2H, Ar-H), 8.66 (t, 1H, NH), 12.25 (s, 1H, COOH). 13C NMR (DMSO-d6): δ 20.3, 20.7, 34.1, 36.3, 120.7, 132.9, 137.4, 143.5, 164.3, 167.3, 168.5, 173.2. General Procedure of Polycondensation. Monomer and a catalytic amount of magnesium oxide (MgO 0.2 wt %) were weighed into a cylindrical glass reactor equipped with a mechanical stirrer, with gas inlet and outlet tubes. The reactor was flushed with a slow stream of nitrogen and placed into an oil bath preheated at the desired condensation temperature. The homogeneous melt formed in a few minutes. The mixture was stirred at the desired condensation temperature for a period. Finally vacuum was applied for 1 h. The crude product was dissolved in DMAc, and the solution was filtrated to remove MgO and poured into water. The precipitate was isolated by filtrating and dried at 60 °C in vacuo. Hydrolytic and Enzymatic Degradation Test of the Polymers. Hydrolytic Degradation. A 5 mL dilute methanol solution of 4D was added dropwise, with constant stirring, into 100 mL of the hydrotropic solution of sodium dodecyl sulfate (SDS, ca. 0.5 cmc). Upon addition, methanol diffused and mixed with water quickly. The insoluble hydrophobic polymer chains collapsed and aggregated in water to form nanoparticles. In a typical degradation experiment, a described amount of dust-free NaOH aqueous solution (clarified by a 0.2 µm Millipore filter) was added to a dust-free suspension of 4D nanoparticles (clarified by a 2 µm Millipore filter). UV-vis absorption spectroscopy was simultaneously measured during the degradation. Enzymatic Degradation. A series of polymer 4D samples were exposed to the Tris-HCl buffer (pH ) 8.6) with proteinase K (concentration 1.00 × 10-6 g mL-1) or without proteinase K. The solutions were incubated at 37.5 °C. The polymer samples were removed from the solution at appropriate time intervals, weighed, and then dried to constant weights under vacuum. The enzymatic degradation experiments of the other polymers (4A-C) were also carried out via the same procedure.

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Biodegradable Hyperbranched Poly(ester-amide)s Scheme 1. Synthesis of Monomers and Polymers

Table 1. Reaction Conditions and Properties of Hyperbranched Poly(ester-amide)s solubility g polymer

Ta (°C)

timeb (h)

yield (%)

ηinhc (dL/g)

M h wd (kDa)

4A 4B-1 4B-2 4B-3 4C 4D-1 4D-2 4D-3 4D-4

170 170 180 190 180 190 190 190 190

7+1V 7+1V 7+1V 7+1V 7+1V 3+1V 5+1V 7+1V 9+1V

60 43 50

0.15 0.08 0.07

59 80 68 72 89

0.12 0.10 0.12 0.12 0.17

20.50 27.17 26.15 cross-link 11.24 41.51 46.25 47.63 54.09

Tge (°C)

Td5 f (°C)

0.68 0.57

186 165 172

279 249 273

0.50 0.65

178 141 142 150 150

268 305 304 298 305

PDI

DB

1.10 1.47 1.45 1.48 1.53 1.53 1.51 1.52

DMSO

DMAc

THF

CHCl3

++ ++ ++ +++ ++ ++ ++ ++

++ ++ ++ +++ ++ ++ ++ ++

+---++ -----

----------

a Reaction temperature. b Reaction time. V means vacuum was applied. c Measured at 30 °C with c ) 4 g/L in DMAc. d The molecular weight was measured by GPC with DMF containing 0.05M LiBr as eluent solvent. e From DSC measurements with a heating/cooling rate of 10 °C/min in nitrogen (second heating). f From TGA measurements with a heating rate of 10 °C/min in nitrogen. g ++ soluble, +- partly soluble, - - insoluble.

Results and Discussion Monomer Synthesis. A series of AB3-type monomers 3A-D bearing acetyl groups and carboxylic acid group were successfully prepared from gallic acid and amino acids via a procedure as shown in Scheme 1. The synthesis of triacetylgalloyl chloride (2) was performed by acetylizing gallic acid with an excess of acetic anhydride in refluxing toluene, according to the literature procedure,20,26-27 followed by acylating reaction with thionyl chloride in dichloromethane. The reactions of compound 2 with amino acids, in the presence of triethylamine, yielded the corre-

sponding AB3-type monomers 3A-D. All of the monomers were characterized by 1H and 13C NMR, and FTIR spectroscopies. Synthesis of Hyperbranched Poly(ester-amide)s. The self-polycondensations of monomers 3A-D were carried out in the presence of MgO as a transesterification catalyst under various conditions. The reaction conditions and results were summarized in Table 1. The data listed in Table 1 indicate that the polycondensations were sensitive to temperature and time. The polycondensations of monomer 3B were conducted in the temperature range of 170-190 °C for 7 h. It can be found that the suitable polymerization temperature for 3B is

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Figure 1. FTIR spectra of monomer 3D and polymer 4D.

close to its melting point (180 °C). When the reaction temperature was up to 190 °C, the resulting polymer was cross-linked. Therefore, the polycondensations of the other monomers were also conducted around their melting point. The molecular weights of the resultant polymers increased with reaction time. However, the reaction time cannot be lasted unlimitedly, otherwise a cross-linking side reaction would take place. For the polycondensations of monomers 3A-C, it was very difficult to carry through mechanical stirring when the reaction time was close to 7 h at 170 and 180 °C, respectively, for 3A and 3B/3C, and if the reaction time was longer than 8 h, the product became a gel and could not dissolve in any solvent. For monomer 3D, the suitable reaction time is about 9 h at 190 °C. Structure of the Polymers. The resulting polymers were characterized via FTIR and NMR spectroscopies. FTIR spectra provided evidence for the chemical structure of the polymers, showing that the characteristic absorptions of ester carbonyl and amide carbonyl groups for the polymers are at about 1770 and 1650 cm-1, respectively, as shown in Figure 1. Compared with the FTIR spectra of the monomers, the FTIR spectra of the corresponding polymers were broader and their characteristic peaks were absent from fine structures due to complicated repeat units and branched architectures. The 13C and 1H NMR spectra of polymer 4D were shown in Figures 2 and 3, respectively. Since the longitudinal relaxation time (T1) for the carbonyl carbons and quaternary carbons was measured to be approximately 2 s, the sum of the acquisition and delay time for the inverse-gated decoupling 13C NMR measurement was set to 11 s. The signal peaks in the NMR spectra of the polymers were assigned on the basis of the NMR signals of corresponding monomers. In the NMR spectra of polymer 4D, the proton signals of methyl a and b, and methylene c and d overlap (Figure 3), but the corresponding carbon signals display three peaks as a result of different structure units (Figure 2). The signals of Cg and aromatic protons (He) of the polymer also display three peaks at 137.50, 139.51, and 140.12 and 7.70, 7.75, and 7.90 ppm, respectively. Generally, there are possibly four different units, dendritic units (D), semi-dendritic units (sD), linear units (L), and terminal units (T) in the polymer prepared from AB3-type monomers as shown in Scheme 2.

Figure 2.

13C

Figure 3.

1H

NMR spectra of monomer 3D and polymer 4D.

NMR spectra of monomer 3D and polymer 4D.

It is very clear that the peak at 137.50 ppm should be assigned to the Cg in terminal units by comparison with the corresponding peak (137.30 ppm) of the monomer. Commonly, the assignment of another two peaks should have based on low molecular weight model compounds to determine which is D or L. However, the attempt to synthesize model compounds is unsuccessful. Fortunately, it was deducible and ascertained by the following method of assumption. The chemical environment of the Cg in the linear units is closer to that in the terminal units. Therefore, the peak at 139.51 ppm should be assigned to the Cg in the linear units. According to chemical reaction principle, a dendrtic unit should be resulted from the condensation reaction of a semi-dendritic unit with an AB3-type monomer. Generally, the formation of a dendritic unit is more difficult than that of a semi-dendritic unit due to steric effect. Thus, the other peak (140.12 ppm) should be assigned to the Cg in the semidendritic units. The degree of branching (DB) is an important molecular parameter of hyperbranched polymers. Frey have derived an expression for DB in the hyperbranched polymers from AB3-


Biodegradable Hyperbranched Poly(ester-amide)s Scheme 2. Possible Repeat Units in the Hyperbranched Polymers

type as the following:28 DB )

2D + sD 2 (3D + 2sD + L) 3

(1)

For the hyperbranched polymers with high molecular weight, the eq 1 can be simplified as DB )

T 1 2 T + L + sD 3 3

(2)

Utilizing eq 2, the DB of polymer 4D was determined to be 0.65 by calculating the integration ratio of the three peaks in 13C NMR, 137.50, 139.51, and 140.12 ppm assigned to the resonance of T, L, and sD units of Cg, respectively. In addition, if the three peaks (7.70, 7.75, and 7.90) of He protons in the polymer were assigned to T, L, and sD units, respectively, the DB calculated from the integration of them was 0.66, which was in consistent with the result from 13C NMR. Likewise, the DBs of polymers 4A-C were determined to be 0.68, 0.57 and 0.50 (see Table 1), respectively. Among the four polymers, polymer 4C displays the smallest DB due to the greatest stereo-hindrance. It is noteworthy that the DB values of polymers 4A-C are higher than Frey’s theory prediction (0.44). The possible reason is that the molecular weights of the polymers are not high enough to use eq 2 to calculate accurately DBs. The similar results have been reported by other research groups.29 The weight-average molecular weights (M h ws) and the polydispersity indices (PDIs) of the hyperbranched polymers were determined by Gel Permeation Chromatography (GPC) equipped with refractive index detector using narrowdispersity polystyrene as standards with DMF as solvent and eluent. The results were listed in Table 1. It is already known that this method has only limited suitability for hyperbranched polymers and usually underestimates the true molar

mass of high molar mass branched polymers due to their smaller hydrodynamic radii compared to linear analogues.30-32 On the other hand, the interaction of the large number of polar end groups with solvent and GPC columns can lead to strong overestimation of molecular weight,32b-c which is also a reason of the marked difference between the results determined with different solvents. Generally, the molecular weights measured with DMF were much higher than those measured with THF, which is likely due to the fact that stronger interactions of the polar end group with DMF than THF occurred. When the interactions of the polar end group with solvent are stronger, the measurement results deviate the true value more seriously (stronger overestimation of molecular weight). Moreover, the end-group effect plays a major role in the case of a molar mass of below 5000.32b In other words, the effect of the smaller hydrodynamic radius cannot overcome the end group effect. Therefore, the results determined with THF are more accurate. However, polymers 4A, 4B, and 4D have low solubility in THF and the GPC measurement with THF could not be carried out. The M h ws measurement from GPC with DMF listed in Table 1 may be higher than real values. Usually, the hyperbranched polymers prepared from self-polycondensation of ABx monomers, according to Flory’s prediction,7a display broad molecular weight distributions. However, the PDIs determined (see Table 1) are not higher than 1.55, which may be much less than real values. This underestimate resulted possibly from the GPC system, columns used and the precipitation process of the crude polymer products. The precipitation process will remove some polymers with low molecular weight, which causes a decrease in PDI of the polymers. The similar results have been reported by other research groups.29,33 Thermal Properties and Solubilities. The thermal properties and solubilities of the hyperbranched polymers obtained are also listed in Table 1. The glass transition temperatures (Tg) range from 141 to 186 °C. The Tgs of polymers 4D are much lower than those of the other polymers 4A-C, which can be attributed to the smaller block and more flexible chain segments. In other words, polymers 4D possess higher intrasegmental mobility than the other polymers. The thermogravimetric measurement revealed that these hyperbranched poly(ester-amide)s have good thermal stability. The thermal decomposition temperature at 5% weightloss of polymer is up to 305 °C. All polymers exhibit excellent solubility in DMF and methanol. Polymer 4C is also soluble in THF. Degradation Properties. The degradations of these polymers were investigated by UV-vis absorption spectroscopy. Figure 4 shows the absorption spectra of compounds GA, 1, and 3D and polymer 4D in methanol, which present the character of benzene derivatives. The spectra show the K band at about 200-220 nm and the B band at about 240300 nm. Both K and B band absorptions of compound 1 are smaller than those of GA, which shows that the absorptions of acetyl benzoic acid shift to short wavelength compared to phenol benzoic acid. The absorptions of compound 1 under neutral and basic conditions are almost the same, whereas a

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Figure 4. UV-vis spectra of GA, 1, 3D and 4D in methanol with a concentration of 1.05 × 10-5 g cm-3.

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Figure 6. Changes in UV-vis spectrum during degradation of SDSstabilized 4D nanoparticles at 37.5 °C, where initial nanoparticle concentration (C0) ) 2.05 × 10-5 g cm-3 and pH ) 10.8. The inset shows the degradation time dependence of the absorbance at 306 nm.

Figure 5. UV-vis spectra of 1 and GA in water of pH ) 7 and pH > 7 with a concentration of 4 × 10-6 g cm-3.

marked difference is observed for GA, as shown in Figure 5. The blue shift is observed during the transition from neutral condition to middle basic condition (pH ) 10.8), and the red shift takes place under strong basic conditions. In the middle basic condition, the carboxyl was conversed to carboxylate, whereas the hydroxyl groups of phenols were unchanged, which made a pair of nonbonding electrons move off the conjugated system. In strong basic solution (pH ) 13), the OH groups of gallic acid would transform into phenolate anions, which made three additional pairs of nonbonding electrons available to the conjugated system, and the corresponding wavelengths of the absorption bands increased. Nevertheless, the wavelengths of the absorption bands of compound 1 are shorter than those of GA under neutral, middle basic and strong basic conditions (Figure 5). This deduces that the absorption band will shift to longer wavelength when the acetyl of compound 1 hydrolyzes to form hydroxyl or phenolate anion in the presence of base. Similarly, the red shift of the B bands occurred during the degradation of polymer 4D, as shown in Figure 6. This indicates that the degradation of polymer 4D are apt to form the staring materials, namely, gallic acid and amino acids, but not corresponding monomer (shown in Scheme 3). Upon the addition of base, the degradation of polymer 4D occurred immediately and finished within 24 h. As shown in Figure

Figure 7. pH dependence of degradation of 4D nanoparticles at 37.5 °C, where (C0) ) 1.05 × 10-5 g cm-3. Scheme 3. Possible Product after Degradation of Polymer 4D

6, the rate of degradation was very fast at the initial 2 h, and then decreased gradually. Figure 7 shows the time dependence of the absorbance intensity of the B band (306 nm) in the case of different pH values. It is clear that the rate of the degradation of polymer 4D increases with the pH value. The degradation behaviors of polymers 4A-C are similar to that of polymer 4D. In addition, the biodegradation behaviors of polymers 4A-D were also evaluated in Tris-HCl buffer (pH ) 8.6) in the presence of proteinase K as catalyst and absence of proteinase K at 37.5 °C. As shown in Figure 8, without proteinase K, 45 wt % of polymer 4D was degraded over a period of 300 min, whereas in the presence of proteinase K, over 80% of the polymer was degraded during this period. This result shows that polymer 4D is biodegradable. Furthermore, over 60, 85, 45 wt % of polymers 4A-C,


Biodegradable Hyperbranched Poly(ester-amide)s

(4) (5)

Figure 8. Degradation of polymer 4A-D, in Tris-HCl solution (pH ) 8.6) at 37.5 °C with and without proteinase K. The dry weight ratio (Wt/W0) is expressed as a function of digestion time (min). (W0, original weight; Wt, weight after different digestion time)

respectively, were degraded over a period of 300 min, in the presence of proteinase K, indicating that 4A, 4B, and 4C are also biodegradable. Figure 8 also shows that the degradation behavior of 4C in the presence of proteinase K is more like 4D without proteinase K, which indicates that the property of degradation of polymer 4D is much better than that of polymer 4C. This may be due to their structural differences. Polymer 4D are more hydrophobic than 4C because of the side groups of ethyl.

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Conclusion A variety of novel biodegradable hyperbranched poly(ester-amide)s have been synthesized via the self-polycondensation of a series AB3 monomers prepared from nontoxic natural gallic acid and amino acids. The hyperbranched polymers display high degree of branching, low inherent viscosity (0.05-0.17) and good solubility in DMF. The temperatures of 5 wt % mass loss (Td5) are above 249 °C, and Tgs are in the range of 141-186 °C, depending on the structure of the monomers used. In addition, these novel hyperbranched polymers are degradable hydrolytically under basic conditions. The structures of degradation products are close to starting material but not the corresponding monomers. Furthermore, the proteinase K can significantly catalyze the degradation of the polymers. Acknowledgment. The authors are indebted to the National Natural Science Foundation of China for financial support (No. 50473027).

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