Synthesis and in Vitro Hydrolytic Degradation of Polyglycolide and Its L-Lactide Copolymer

Iranian Polymer Journal / Volume 10 Number 1 (2001) 1026.126512001

Archive of SID Synthesis and In Vitro Hydrolytic Degradation of Polyglycolide and Its l-Lactide Copolymer

Jamshid Mohammadi-Rovshandeh l (*) and Mohammad Nabi Sarbolouki2 (1) Department of Chemical Engineering, Vacuity of Engineering (Pardis-e-Chooka), University of Tehran, Rezvanshahr, Gilan, I .R. Iran (2) Institute of Biochemistry and Biophysics, University of Tehran, Tehran, I .R . Iran

Received 20 May 2000 ; accepted 3 January 2001

ABSTRACT

In this study homo- and copolymers of glycolide and !-lactide were synthesized and subjected to in vitro hydrolytic degradation . Glycolide and !-Iactide were prepared from glycolic and I-lactic acid, respectively . Stannous octoate was used as polymerization catalyst . Poly(!-lactide) with hh,,,=9 .4x105 and a polydispersity of 1 .89 was synthesized and characterized . Also copolymers of I-lactide and glycolide were prepared . Polyglycolide and its lactide copolymers with high proportion of glycolide do not dissolve in organic solvents. Degradation studies were conducted at 37 .0 and pH=7 .35 . The concentration of released glycolic acid was determined by chromotropic acid reagent. Purified polyglycolide powder shows a fast degradation profile in the first 10 days, after which it levels off. A copolymer obtained from 10 moi% flactide and 90 mol% glycolide shows different degradation pattern as polyglycolide, because of its amorphous nature it exhibits a higher degradation rate.

Key Words : polyglycolide, poly(I-lactide), in-vitro degradation, stannous octoate

INTRODUCTION and pharmaceutical applications. Lactic and glycolic acids are Kreb's cycle acids, and in body, they are Aliphatic polyesters derived from lactic acid (LA) and oxidized completely to carbon dioxide and water. glycolic acid (GA) were synthesized early in the 50's, Polymers derived from these a-hydroxy acids when most of the commercial polymers were launched are biocompatible and biodegradable, and their degrad- on the market . However, no attention was paid to these ation products are non-toxic . Since 1970, poly(glycolic polyesters because of their poor hydrolytic and thermal acid) has been commercially available as the surgical stabilities . But, in the years later their hydrolytic suture, Dexon [1], and the copolymer made of 92 mol% instability turned into an exciting advantage and made GA/8 mol% LA has had a limited application as the these materials suitable for a wide variety of surgical competitive suture, Vicrylwww.SID.ir [2], since 1975.

(.) To whom correspondence should be addressed .

Synthesis and in Vitro Hydrolytic Degradation of PG and Its 1-Lactide Copolymer

Poly(glycolicArchive acid) of (PGA) SID and poly(lactic acid) period, the tube was broken and the crude polymer (PLA) can be prepared by simple polycondensation was crushed and refluxed in ethyl acetate and dried in reaction of glycolic acid and lactic acid in the vacuum . Poly(l-lactide) was prepared analogous to presence of antimony trioxide. polyglycolide except that in the absence of lauryl However, because of thermal instability of the ' alcohol. For copolymer synthesis appropriate amounts polymer formed and the equilibrium nature of the of 1-lactide and glycolide were charged in a polymer- reaction as well as difficulties associated in removing ization tube and kept under high vacuum at 70 'C for 2 water from the viscous polymer mass, only low h. Catalyst solution (0.1 mL, 3% Sn(Oct) 2 in toluene) molecular weight polymers are obtainable which lack was then added to the reaction mixture and kept under the optimum properties for most applications. vacuum for 2 h to remove all volatiles . The tube was The preferred method for producing high mole- then sealed under vacuum and copolymerization carri- cular weight polymers is the ring-opening polymerized out at 110 'C for three days . Subsequently tube was ation of the cyclic dilactons, glycolide and lactide, in broken and the copolymer was cut to equal weight the presence of tin catalysts. specimens for in vitro degradation studies. In the work reported we prepared polyglycolide, polylactide and their copolymers and studied in In Vitro Degradation vitro hydrolytic degradation of polyglycolide and a For preparation of a calibration curve, one mL of each copolymer with 10 mol% 1-lactide and 90 mol% of the 5 different concentrations of pure glycolic acid glycolide (in the feed). (0 .2, 0.05, 0 .005, 0 .0005, 0 .0001 M) was mixed with 3 mL of analytical-grade chromotropic acid solution (to prepare chromotropic acid solution, 1 .0 g of the EXPERIMENTAL acid disodium salt was dissolved in 250 mL concentrated sulphuric acid). Materials The resulting absorbance of the most concentrated Glycolide was prepared from glycolic acid (Merck,, solution (0 .2 M) was determined by scanning the Darmstadt, Germany) and the crude extract was washed solution through the wavelength ranging from 200 to 900 with chloroform and recrystallized twice from ethyl rim in a Shimadzu-240A UV-visible spectrophotometer. acetate [3] . 1-Lactide was prepared from 90 % 1-lactic The wavelength at which the maximum absorbance was acid solution (Merck inc .) and recrystallized three observed was taken for the determination of the subse- times from ethyl acetate [4] . Due to instability of quent absorbance of the rest of the sample solutions . The glycolide and lactide they are used fresh just after calibration curve was constructed by plotting the observ- preparation . Tin-2-ethyl hexanoate, Sn(Oct)2 (Sigma, ed maximal absorbance (578 nm) vs . the corresponding St. Louis, USA) was purified by vacuum distillation. glycolic acid solution concentration. Chromotropic acid sodium salt (dihydrate, analytical Ten samples of polyglycolide powder with equal grade) was purchased from Merck. All solvents weights were immersed in 7 mL of phosphate buffer (Merck) were purified, if necessary, by the standard solution (PH-7.35) separately. The solutions were kept procedures [5]. at 37 'C for a predetermined period of time . One mL of each of the immersion solutions was withdrawn after Polymerizations the predetermined immersion period and immediately Glycolide was charged into a polymerization tube mixed with 3 mL of the chromotropic acid solution. together with 0.1 mL of the catalyst (3% solution in The absorbance maximum of each solution was toluene) and 0 .01 % lauryl alcohol as a catalyst determined in the manner described above . The corres- activator. The tube was kept under high vacuum (<0 .1 ponding glycolic acid concentration at the absorbance mmHg) at 80 'C for 2 h, and subsequently it was heat- maximum was determined by the use of the previously sealed and kept at 120 'C for three days. After this constructed calibration curvewww.SID.ir [61.

54 Iranian Polymer Journal / Volume 10 Number ! (2001)

Mohannrnadi-Rov,handeh J . et al.

RESULTS AND DISCUSSION Archive0 of SID a 0 Glycolide and 1-lactide were prepared from corres- R R ponding a-hydroxy acids, glycolic acid and 1-lactic acid as shown in Scheme I. Sb203 Polyglycolide was obtained by ring-opening 250-270*C polymerization of glycolide in the presence of lauryl R-H, Glycolide alcohol as coinitiator and Sn(Oct)2 R-CH3, Lactide as catalyst . On the basis of the mechanism suggested by Kricheldorf [7- 8] and Duda [9] for polymerization of lactones in presence of Sn(Oct)2 catalyst and alcoholic coin iti- ators, the steps in Scheme II are proposed for the Scheme I formation of polyglycolide. According to Duda's recent works, polymeriz- Instruments and Measurements ation proceeds via "active chain end" mechanism, i .e., The concentration of glycolic acid released at different on the Sn-alkoxide bonds present at the chain ends. intervals was determined by a Shimadzu-240A UV-vis Polyglycolide is usually insoluble in all organic spectrophotometer, measuring the UV absorbance in solvents but dissolves in hexafluoroisopropanol to a the presence of chromotropic acid at 578 nm and extra- small extent . The TGA thermogram of polyglycolide polation through a calibration curve. Infra-red spectra (Figure 1) shows that the polymer begins to degrade were recorded by a Shimadzu-4300 FT-IR spectro- at 272 'C and the DSC studies show that the photometer. Differential scanning calorimetry (DSC) crystalline melting point of the polymer is 224 'C themograms were obtained with a Mettler Instrument (Figure 2) . Thus, the processing temperature range of DSC series PCII . Thermogravimetric analysis was polyglycolide is about 45 'C reflecting its extreme carried out on a Dupont 2000 V5 .1 A. sensitivity to the processing temperature, which calls Molecular weight and molecular weight distri- for accurately controlled process equipment . Figure 3 bution of poly(1-lactide) was determined by a shows the FT-IR spectrum of polyglycolide. Shimadzu-LC9A, Tokyo, Japan gel permeation Lactic acid exists in two stereoisomeric forms: chromatography on a 10° Styragel column, (refractive d and 1-lactic acid. In this study 1-and dl- lactide were index detector), using tetrahydrofuran as solvent, flow prepared from I- and dl-lactic acid in a manner rate 1 mL/min at 40 'C, and polystyrene of low analogous to glycolide . The melting points of I- and polydispersity as standards . dl-lactide crystals were 98 'C and 126 'C, respectively.

SnOct2 + R OH Oct2Sn- - -O-R 0

H i H 0 Oct Sn---O-R Oct2Sn O O— R 0 HO" j 0 `)•H O 0 0 /// 2n+2 Q)yo 0 Scheme II www.SID.ir

Iranian Polymer Journal / Volume 10 Number I (2001) 55

Synthesis and In Vitro Hydrolytic Degradation of PG and Its 1-Laclide Copolymer

Archive of SID 110 80 r 50 E

-10 0 100 200 300 400 Temperature (C) Figure 1. TGA Thermogram of polyglycolide. 4000 .0 2000 .0 1500.0 1000.0 500 0 Wavenumber (cm`') Poly(l-lactide) and poly (dl-lactide) were prepared by Figure 3. FT-IR Spectrum of polyglycolide. ring-opening polymerization of the respective monomers in the presence of stannous octoate at glycolide is higher than lactide in copolymerization temperatures slightly above their melting points. and this causes the copolymer to have microblock Films prepared by solution casting of poly(dl- structures [10] . A copolymer was made from a lactide) were transparent and this shows the amorphous monomer mix of 90 mol % glycolide and 10 mol % nature of the polymer, while the films prepared from l-lactide according to the above procedure. poly (1-lactide) were opaque because of semicrystalline According to Vert's acronyms [I1], this copoly- nature of the polymer . In contrast to polyglycolide, mer was designated as PLA I0/GApo. Because of the polylactides are soluble in some organic solvents, like copolymerization, the crystallinity of this copolymer is chloroform and solubility-dependent characterization lower than polyglycolide or poly(l-lactide) homopoly- were achieved readily . Poly(I-lactide) with M,,.°9 .4x mers, but it does not dissolve in most of the common 105 and a polydispersity of 1 .89 was synthesized with organic solvents. As mentioned, polyglycolide and its crystalline mp=l72 'C. lactide opolymers with high proportion of glycolide Various ratios of glycolide/lactide can be co- do not dissolve in organic solvents . Due to this polymerized in melt, in the presence of Sn(Oct)2. insolubility, the as-polymerized copolymer was used Figure 4 shows the FT-IR spectrum of poly(!-lactide). for in vitro degradation studies. It has been shown that the reactivity of For degradation studies the released glycolic

-11 0 100 200 300 U L . ~ Temperature (C) 4000.0 2000 .0 1500 .0 1000.0 500.0 Wavenumber (cm-') Figure 2 . DSC Thermogram of polyglycolide . Figure 4. FT-IR Spectrum ofwww.SID.ir poly(I-lactide) .

56 Iranian Polymer Journal / Volume 10 Number 1 (2001)

Mobumnam-Roo badd, I. 0 d-

Archive of SID SO3H HO3S H03S SO3H + HCHO

OH OH OH OH (A) (B)

CH OH (A) + (B) H2O OH

SO3H HO 3S (C)

HO (C) + 02 2–H O HO

SO3H HO 3S

Scheme i0

acid, by the action of the concentrated sulphuric acid, polymer sample . This is followed in the subsequent is converted to formaldehyde. stage by the slow degradation of the crystalline The resulting formaldehyde then reacts with regions. chromotropic acid through a condensation followed The as-polymerized copolymer, PLAidGA90 , by oxidation, and forms a p-quinoidal compound as was subjected to in vitro degradation in a manner shown Scheme III. analogous to polyglycolide and the concentration-time The conjugated double bonds in p-quinoidal profile is shown in Figure 6 . As seen, the degradation compound are responsible for the formation of colour [12] . 0.08 - The concentration of released glycolic acid during degradation can be determined from the 0.06 - corresponding calibration curve . Figure 5 shows the 0 molarity of the released glycolic acid versus 0.04 degradation time at 37 `C . This figure is analogous to the degradation pattern of polyglycolide suture (Dexon) [12] . However, since the polymer sample 0.02-.02 - used is in a powder form, its degradation is faster. The concentration-time profile obtained in this study 10 20 30 shows that in the initial 10 days there is a sharp Time (day) increase in the concentration of released glycolic acid, Figure 5. Molarity of releasedwww.SID.ir glycolic acid vs. time for poly- pointing to the degradation of amorphous regions of glycolide powder.

Iranian Polymer Journal / Volume 10 Number I (2001) 57

Symheais and In Vitro Hydrolytic Degradation of PG and ho t-Lactids Copolymer

Archive of SID pattern with a faster kinetics. 0.20 - ACKNOWLEDGEMENT 0.15 - The authors wish to thank the Research Council of m 0.10 - Tehran University for its financial support. 0.05 -

REFERENCES 10 20 30 40 Time (day) Figure 6. Molarity of released glycolic acid vs. time for 1. Ant . Cyanamid Co. Fr. Pat . 1563261, 1969. copolymer PLAIO/GAyo. 2. Wasserman D ., U S. Pat., 3839297, 1975. 3. Kulkarni R.K ., Moore E.G., Hegyeli A .F . and Leonard F., J. pattern is different from polyglycolide and the Biomed. Mater. Res ., 5, 169, 1971. degradation rate is faster than that, of the 4. Gilding D.K . and Reed A.M., Polymer, 20, 1459, 1979. polyglycolide. In this case the copolymer is mostly 5. Keese It., Muller R .K . and Toube T .P., Fundamentals of amorphous and as expected the degradation occurs at Preparative Organic Chemistry, Wiley, New York, 1982. a constant rate. 6. Mohammadi-Rovshandeh J., Farnia S .M.F., and Sarholouki M.N ., J.Appl. Polym. Sci ., 69, 627, 1998. 7. Kricheldorf H.R., Kreiser-Saunders I. and Boettcher C ., CONCLUSION Polymer, 36, 1253, 1995. 8. Kricheldorf H.R., Kreiser-Saunders 1. and Snicker A ., High molecular weight poly(a-hydroxy acid)s, suit- Macromolecules, 33, 702, 2000. able for biomedical applications can be prepared by 9. Kowalski A., Duda A., and Penczek S ., Macromolecules, 33, ring opening polymerization of dilactones . Degrada- 689, 2000. tion characteristics of these polymers are affected by 10. Grijpma D.W., Nijenhuis A.J. and Pennings A.J., Polymer, the physical form of the specimens . As expected, their 31, 2201, 1990. powder form degrades faster than their fiber form. II . Li S .M ., Garreau H . and Vert M ., J. Mater. Sci. Mater. Med., Due to its amorphous nature, the as-polymerized 1, 131, 1940. copolymer PLAIO/GA90 has a different degradation 12. Chu C. and Louie M., J. App!. Polym. Sci., 30, 3133, 1985.

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