Process of preparing dl-lactic acid-glycolic acid-copolymer

Abstract: A process for copolymerizing lactide and glycolide uses a specific amount of stannous octoate as a catalyst in the presence of a polymerization reg­ ulator at temperature of 200-230°C to prepare dl-lactic acid-glycolic acid-copolymers which contain 50-60 mol % of a dl-lactic acid unit and 40-50 mol % of a glycolic acid unit and having inherent viscosity of 0.4-0.6 in chloroform. The catalyst is used in an amount of 0.005-0.015 % by weight based on the aggregate amount of lactide and glycolide.

Description: This invention relates to a process of preparing dl-lactic acid-glycolic acid-copolymers. Such polymers can be used as matrices of molded slow-releasing drugs. Polylactic acid, polyglycolic acid and other similar compounds are bioabsorbable and hydrolyzable polymers. By applying these properties, these polymers are used for medical purposes such as for sutures or matrices of molded slow-releasing drugs in the form of pellet, film and bar which are administered by such methods as, for example, intravital imbedding. More particularly, polylactic acid has better processing ability and solubility in solvents as compared with polyglycolic acid, and is frequently used as a matrix in the form of various molded drugs. Polylactic acid, however, has a slow rate of decomposition in vivo, and hence is disadvantageous in that polylactic acid remains in the organism as long as one to several months after releasing the drugs. It has been known recently to use copolymers obtained by copolymerizing lactic acid with glycolic acid. According to the information of the present inventors in particular, a copolymer containing 50-60 mol% of a lactic acid unit and 40-50 mol% of a glycolic acid unit, has such physical properties as a weight average molecular weight of 30,000-100,000, good solubility in chloroform and an inherent viscosity of 0.4-0.6 in a 1 g/100 ml solution, and also has suitable absorbability in vivo and solubility. Therefore, it has already been known that the copolymer can be applied to various matrices of drugs in a wide field of medication forms and is particularly suitable for the slow-releasing drugs used for the intravital imbedding in such forms as pellet, film and micro capsule. A low molecular weight lactic acid-glycolic acid-copolymer having a relatively high content of the lactic acid unit, like the copolymer of this invention, and the process for preparing the same have been disclosed in Japanese Laid-Open Patent Nos. 64824/1987 and 46823/1981. Japanese Laid-Open Patent No. 64824/1987 discloses a low molecular weight and heterogeneous lactic acid-glycolic acid-copolymer containing 25-100 mol% of the lactic acid unit and 0-75 mol% of the glycolic acid unit, which is soluble in benzene and has an intrinsic viscosity of not more than 0.5 in a 1 g/100 ml solution of benzene. Also disclosed in this reference is a similar copolymer containing the same proportion of the above units, which is insoluble in benzene and has an intrinsic viscosity of not more than 4 in a 1 g/100 ml solution of chloroform or dioxane. As an illustrative example of the preparation process, the above patent describes that the desired copolymer is obtained by copolymerizing a lactide with a glycolide at 160°C in the presence of dl-lactic acid containing water as a polymerization regulator by the use of stannous octoate as a catalyst in an amount of 0.2 wt.% based on the sum of the lactide and the glycolide. In addition, Japanese Laid-Open Patent No. 46823/1981 discloses a lactic acid-glycolic acid-copolymer used for a matrix of molded slow-releasing drugs containing parasitic ascaricide and the process for preparing the same. This patent describes that a lactic acid-glycolic acid-copolymer can be derived from about 70-80 wt.% of lactic acid and about 30-20 wt.% of glycolic acid; the copolymer is said to have such properties as an inherent viscosity of about 0.13-0.23 in a chloroform solution and a weight average molecular weight of about 15,000-30,000. The polymer is obtained by conducting the copolymerization in the presence of a strongly acidic ion exchange resin. As stated above in Japanese Laid-Open Patent No. 64824/1987, the copolymer having the desired properties is obtained by using stannous octoate in an amount of about 0.2 wt.% based on the sum of lactic acid and glycolic acid. However, the tin content in the molded drugs administered in vivo must, of course, be minimized. It is difficult, however to remove tin from the copolymer. Although stannous octoate is known to have a remarkable effect as the catalyst, stannous octoate has a metallic tin content of 28-30 wt.%. Therefore, the amount used as described in the above-noted patent inevitably results in a residue of tin in an amount of several hundred ppm, and this is expected to have adverse effects to some extent. As a result, the above Japanese Laid-Open Patent No. 46823/1981 is thought to have carried out the copolymerization of lactic acid with glycolic acid by using the strongly acidic ion exchange resin in the absolute absence of the metallic catalyst. Besides the slow-releasing drugs, no endotoxin must be contained in materials administered in vivo. Endotoxin is lipopolysaccharides existing in cell walls of bacteria and known to exert biological activities such as decrement in leucocytes and blood platelets, fever, hemorrahagic necrosis of bone marrow and reduction in blood sugar value. The above Japanese Laid-Open Patent No. 64824/1987 illustrates an example for conducting the copolymerization at 160°C. Polymerization at such a temperature causes a problem in the resultant polymer which enhances the possibility of its containing residual endotoxin derived from monomers and other raw materials. The object of this invention is to provide a process for preparing a dl-lactic acid-glycolic acid-copolymer which is harmless to man and has a specific range of properties. According to this invention, there is provided a process of preparing, from a lactide and a glycolide, a dl-lactic acid-glycolic acid-copolymer containing 50-60 mol% of a lactic acid unit and 40-50 mol% of a glycolic acid unit and having an inherent viscosity of 0.4-0.6 in chloroform which comprises conducting the copolymerization for 1.5 to 3.5 hours at a temperature of 200-230°C in the presence of a polymerization regulator, and by using stannous octoate as a catalyst in an amount of 0.005-0.015 wt.% based on the sum of the lactide and the glycolide. The inventors have intensively carried out the investigation with the intent of minimizing the amount of the tin catalyst used in the polymerization. As a result, it has been found that, in order to obtain the lactic acid-glycolic acid-copolymer having the specific range of properties by this invention, the amount of stannous octoate used as the catalyst is required at least 0.005 wt.% based on the sum of the lactide and the glycolide, but can be lowered to 0.015 wt.% or less. It has also been found that the copolymer obtained by the present method surprisingly contains substantially no endotoxin. The lactide has a slower rate of polymerization than that of the glycolide. Accordingly in the copolymerization of the lactide and the glycolide to obtain a lactic acid-glycolic acid-copolymer having a relatively high content of the lactic acid unit, it has hitherto been thought that a larger amount of the stannous octoate catalyst is required as compared with homopolymerization of the glycolide. However, it has unexpectedly been found that the copolymerization conducted within a specific range of temperature can remarkably reduce the conventional amount of stannous octoate catalyst in a usual polymerization time without causing either a decrease in the desired degree of polymerization or a variation of ingredient proportion on the resultant copolymer. Embodiments of the invention will now be described in more detail in the following non-limitative description. The raw material lactide can be prepared by a known process, for example, the process dislosed in the above Japanese Laid-Open Patent No. 64824/1987. The raw material glycolide can also be prepared by a known process, for example, the process described in US Patent No. 2668162. The proportion of these materials in charge is 40-50 mol% of glycolide to 50-60 mol% of lactide. Stannous octoate used as the catalyst in this invention is preferably prepared prior to use by distilling the commercially available grade under a vacuum of 10⁻³ mmHg (0.133 Pa). The amount of stannous octoate used is 0.005-0.015 wt.% based on the sum of raw materials, the lactide and the glycolide. When the amount of the catalyst is less than 0.005 wt.%, the polymerization degree does not increase even though the reaction temperature is raised. When the amount is more than 0.015 wt.%, the tin content in the polymer is unfavourably increased and the polymerization degree of resultant polymer is also lowered inversely by conducting the polymerization in the temperature range specified according to this invention. The generally preferred amount of stannous octoate used is in the range of 0.008-0.012 wt.%. In the process of this invention, the polymerization must be carried out in the temperature range of 200-230°C. When the temperature is lower than 200°C, it takes a long period of time for the polymerization and a relatively large amount of unreacted monomers remains when using stannous octoate in the specified range. Therefore, in this event it would be required to remove the residual monomers after terminating the reaction by such methods as reprecipitation; molar proportion of the ingredients in the copolymer obtained is also dependent upon the variation of polymerization conditions and always unstable; and the copolymer having the desired properties cannot always be obtained. Deleterious endotoxin existing in the reaction system is decomposed within a shorter period of time at higher temperatures and its biological activity is eliminated. However, if the reaction is conducted at a temperature below 170°C, it is difficult effectively to reduce the endotoxin content within the polymerization time. On the other hand, if the reaction is conducted at a temperature above 230°C, depolymerization results and severe coloring of the resultant copolymer occurs so excessive temperatures are unsuitable for the process of this invention. The preferred range of the temperature is 215-225°C and the temperature is properly determined on the basis of the amount of catalyst. The polymerization regulator is required also in the process of this invention. Typical examples of the regulator include alcohols such as lauryl alcohol, α-hydroxy lower fatty acids such as dl-lactic acid and glycolic acid, and water. dl-Lactic acid and glycolic acid are preferably used. When lactic acid is employed, the polymerization degree of the copolymer is apt to vary depending upon the moisture content of the lactic acid used because of a strongly hygroscopic property of the acid. Therefore it is more preferred to employ a commercially available grade of 90% dl-lactic acid which has a constant moisture content. The amount used is in the range of 0.25-0.50 wt.% based on total weight of the monomers and properly selected within the range according to the desired properties of the polymer to be obtained, amount of the catalyst and the polymerization temperature. In the preparation process of this invention, copolymer having desired properties can be obtained in a relatively short period of time as compared with usual processes. For example, a copolymer is prepared which contains about 55 mol% of the dl-lactic acid unit and about 45 mol% of the glycolic acid unit and has such physical properties as an inherent viscosity of about 0.5 in a chloroform solution. In this case, a reaction time of about 2.0-2.5 hours is sufficient when the polymerization is conducted at a temperature of about 220°C using 0.01-0.015 wt.% of stannous octoate. Examples of the invention will hereinafter be given. The properties of the copolymers were measured by the procedures described below. Weight average molecular weight Weight average molecular weight of the copolymer was measured by gel permeation chromatography in a 0.1 wt.% chloroform solution. The molecular weight was obtained on the basis of polystyrene standard. Inherent Viscosity Chloroform was used as the solvent. Measurement was carried out at 25 ± 1°C in a 0.5 g /dl solution. Inherent viscosity was calculated from the following equation. T₀ = reference measuring time T₁ = measuring time C = concentration of solution (0.5) Composition of copolymer A 1% solution of the copolymer in deuterated chloroform was added with a small amount of TMS and the ¹H-NMR spectrum was measured. The mole fraction of glycolic acid and lactic acid was calculated from peak intensities of methylene hydrogen in the glycolic acid unit and methyl hydrogen in the lactic acid unit. Content of tin metal The copolymer was decomposed with a mixture of sulfuric acid and nitric acid. Tin content was measured by absorptiometry. Endotoxin content The copolymer was dissolved in endotoxin free dimethyl-sulfoxide, diluted 10 times with distilled water and centrifuged. The resultant supernatant liquid was estimated by injection into a rabbit in an amount of 1 ml/kg or by LAL (Limulus Amberbocyte Lysate) test. The LAL test means the following method. LAL is respectively added to a sample solution and an aqueous solution having a known concentration of endotoxin. The resulting mixtures are reacted at 37°C for 30 minutes, added with azo colouring matter to develop colour and endotoxin content in the sample is measured by absorbance at 545 nm. Examples 1-6 and Comparative Examples 1-5 A thick-walled cylindrical stainless steel polymerization vessel equipped with a stirrer was charged with dl-lactide having a melting point of 124-125°C and glycolide having a melting point of 83.5-84.5°C in amounts indicated in Table 1. To the mixture were then added stannous octoate and a polymerization regulator in amounts indicated in Table 1, the resulting mixture was deaired for 2 hours under vacuum of 1-5 mmHg abs. (133-666 Pa) and the vacuum was replaced with nitrogen. The resulting mixture was polymerized under the temperature and time conditions indicated in Table 1 by heating in a mantle heater in a nitrogen atmosphere. The physical properties of the resultant copolymer is illustrated in Table 2. Weight average molecular weight of the copolymer obtained in Example 1 was 65000. Rabbit fever test of the copolymer obtained in Example 2 resulted in no fever of the rabbit. The amount of endotoxin determined by LAL test was 0.3 pg/ml. The value was remarkably smaller than the value of 173 pg/ml which was regarded as PD₅₀ (minimum amount to cause fever in 50% of rabbits). The merit of the process in this invention is clearly illustrated in the above examples. The dl-lactic acid-glycolic acid-copolymer containing 50-60 mol % of the dl-lactic acid unit and 40-50 mol % of the glycolic acid unit, having an inherent viscosity of 0.4-0.6 measured in chloroform and a tin metal content of 50 ppm or less can be prepared in a shorter period of time than conventional processes. In addition, when the reaction is carried out at the temperatures according to this invention, the noxious endotoxin existing in the reaction system is effectively decomposed within the polymerization time. Therefore, substantially no endotoxin is contained in the resulting copolymer. As a result, the dl-lactic acid-glycolic acid-copolymer having a high level of safety can be prepared more efficiently than conventional manufacturing processes, thereby providing a great contribution to the industry. Inherent viscosity (dl/g) Copolymer composition L/G Tin content (ppm) Color Example 1 0.51 54/46 27 pale yellow 2 0.49 52/48 33 colorless 3 0.50 53/47 44 pale yellow 4 0.40 55/45 14 colorless 5 0.50 53/47 30 pale yellow 6 0.53 53/47 29 colorless Comparative Example 1 0.28 38/62 10 colorless 2 0.32 40/60 29 colorless 3 0.35 50/50 30 brown 4 0.34 51/49 59 brown 5 0.26 43/57 591 colorless (Note) L/G: Mole proportion of lactic acid unit to glycolic acid unit in copolymer

Claims: A process for the copolymerization of a lactide and a glycolide to prepare a dl-lactic acid-glycolic acid-copolymer containing 50-60 mol % of a lactic acid unit and 40-50 mol % of a glycolic acid unit and having an inherent viscosity of 0.4-0.6 in chloroform when measured in a 0.5g/dl solution in chloroform at a temperature of 25° ± 1°C and according to the equation: T₀ = reference measuring time T₁ = measuring time C = concentration of solution (0.5) wherein the copolymerization is conducted for 1.5 to 3.5 hours in the presence of a polymerization regulator at a temperature of 200-230°C by using stannous octoate as a catalyst in an amount of 0.005-0.015 wt.% based on the sum of the lactide and the glycolide, the proportion of the lactide and glycolide monomers in charge being 50-60 mol % and 40-50 mol % respectively. A process according to claim 1, wherein the polymerization regulator is an α-hydroxy lower fatty acid. A process according to claim 2, wherein the α-hydroxy lower fatty acid is dl-lactic acid containing water and the amount thereof is 0.25-0.50 wt.% based on the sum of the lactide and the glycolide. A process according to claim 1, 2 or 3, wherein the catalyst is used in an amount of 0.008-0.012 wt.% on the aforesaid basis. A process according to any of claims 1 to 4, wherein the polymerization is conducted at a temperature in the range of 215-225°C. A process according to claim 1, wherein the polymerization regulator is selected from alcohols such as lauryl alcohol and α-hydroxy lower fatty acids such as dl-lactic acid and glycolic acid.

Inventors: KOBAYASHI TADASHI, MASUDA TAKAYOSHI, SHINODA HOSEI AZANISHIAKATSUCH, TAKAYANAGI HIROSHI, KOBAYASHI, TADASHI, MASUDA, TAKAYOSHI, SHINODA, HOSEI 45 AZANISHIAKATSUCHI, TAKAYANAGI, HIROSHI
Applicants: MITSUI TOATSU CHEMICALS, MITSUI TOATSU CHEMICALS, INC.
Application year: 1988
Main classification: C08G63
IPC codes: C08G63, A61K9