Method for the preparation of a polymer lactide

Abstract: A polymer lactide is prepared by the polymerization of meso-3,6-­ dimethyl-1,4-dioxane-2,5-dione or mesodilactide and, if desired, other monomers. The mesolactide monomer has preferably a free acid content of less than 1 meq/kg dilactide and is then polymerized in a manner known per se. The compositions containing polymesolactide obtained according to this process do not posses the typical characteristics of poly-DL-lactide, poly-D-lactide or poly-L-­ lactide. A shorter degradation time of a substrate based on the present polymer is unofficial for several purposes. The present polymers have a good biocompatibility and are thus suited for bone plates, clamps and sutures and controlled release of biologically active compounds. The present polylactides and/or copolymers there­ of have the benefit of a good biodegradability, biocompatibility, non-toxicity and non-mutagenicity.

Description: The invention relates to a method for the preparation of a polymer lactide. Polymer lactides are generally known in the art and are used in surgery and pharmacy, for example, for bone plates, clamps, sutures and controlled release of biologically active compounds. The main advantage of the use of polylactides and/or copolymers made therefrom is lain in its biodegradability, biocompatibility, non-toxicity, non-mutagenicity etc. In US-A-2,703,316 a preparation of self-supporting thin films of polylactides is described. These polylactides are prepared by heating a lactide having a melting point above 120°C up to a temperature above its melting point but below 200°C in the presence of a polyvalent metal oxide until a polymer having a tack point of 60 to 120°C is formed. This type of polymers are also frequently termed "polylactides" as the empirical formula of these compounds is virtually the same as the formula of the lactide. These polylactides have an inherent viscosity of at least 0.45. The preparation of the starting material of US-A-2,703,316 is effected by concentrating commercially available lactic acid, which contains 20 to 50% water, by means of a distillation under atmospheric pressure at 120 to 130°C, resulting in the removal of virtually all the water. Then the concentrated acid is distilled under reduced pressure (for example 17 to 35 mm mercury pressure) for removing the water of esterification resulting in a low molecular weight syrupy polylactic acid. A further pressure reduction in the distillation system to 8 to 10 mm mercury pressure and an increase in temperature up to 200 to 250°C results in the formation of the cyclic dimer lactide from the polylactic acid. This dimer distills over at 130 to 140°C under a pressure of 10 to 13 mm mercury pressure. The "crude" product obtained can be purified by repeated crystallization from ethyl acetate. A polymer lactide can then be prepared from this material by polymerization under the influence of a polyvalent metal salt. Copolymers of L(-)-lactides and glycolides are known from US Patent 3,839,297. These are prepared by heating a mixture of the monomers to 200°C in the presence of a stannous octoate catalyst. The copolymers obtained in this way can be extruded to filaments, which are suitable as absorbable suture. In DE-A-1,595,085 a method for the preparation of polylactides is disclosed according to which L(-)-lactide, D(+)-lactide or mixtures thereof or optically inactive lactides, optionally in combination with comonomers like glycolide or δ-valerolactone are used as starting material. Polylactides having a high molecular weight (inherent viscosity of at least 0.4, preferably 0.7) are obtained when lactides of high purity (90%, preferably 99%) are employed. Further in NL-A- 6609613 a method for the preparation of cyclic ester anhydrides of α-hydroxy carboxylic acids like glycolide and lactide having an acid number of less than 10, preferably less than 0.5 is described. (The acid number is expressed in mg KOH per g of material). According to said method polymeric α-hydroxy carboxylic alkylesters are heated at 200-280°C, optionally under reduced pressure. The obtained cyclic ester anhydrides are suitable as starting material for the production of polymers having a high molecular weight. According to the Examples of NL-A- 6609613 the obtained minimum acid number is 1,51. A new method for the preparation of a polymer lactide has now been found according to which meso-3,6-dimethyl-1,4-dioxan-2,5-dione or mesodilactide monomer having a free acid content of less than 1 meq/kg is polymerized in a manner known per se, if desired, together with one or more dilactides having a free acid content of less than 1 meq/kg. These polymer lactides having high molecular weights (high intrinsic viscosity values) possess a considerably shorter degradation time than poly-DL-lactide, poly-D-lactide or poly-L-lactide. For use in surgery or pharmacy, the polymer preferably has an inherent viscosity of at least 0.5. As indicated above the mesodilactide may be copolymerized with numerous other monomers. These comonomers can be used in an amount of at least 99% by weight, preferably at most 30% by weight. Examples of suitable comonomers are D(+)-dilactide, DL-dilactide, L(-)-dilactide, glycolide, β-propiolactone, β-butyrolactone, γ-butyrolactone, 2-keto-1,4-dioxane, γ-valerolactone, δ-valerolactone, ε-caprolactone, pivalolactone, α,α-diethylpropiolactone, 2,5-diketomorpholine, ethylene carbonate, ethylene oxalate, intermolecular cyclic diesters of α-hydroxybuteric acid, α-hydroxyvaleric acid, α-hydroxyisovaleric acid, hydroxycaproic acid and 2-hydroxyisocaproic acid. In the purification of the crude DL-dilactide use is made of conventional, inexpensive solvents, such as toluene, benzene and diethyl ether. The DL-dilactide crystallized, for example, in toluene can be separated simply from the mother liquor, mainly consisting of mesodilactide and toluene. After evaporating this mother liquor a liquid remains which consists of the mesodilactide, free acids and byproducts. After good cooling the mesodilactide can be removed from this mother liquor by crystallization and separating the crystals. This initial "crude mesodilactide" should be further purified in two or more crystallization steps, according to which finally a powdery, crystalline mesodilactide is obtained which is characterized by a free acid content of less than 1 meq/kg and a melting point of 43 to 44°C. The low free acid content is important for the preparation of polymesodilactide having a high molecular weight. When a mesodilactide having a free acid content larger than 1 meq/kg is used it is found that this does not result in polymers having a high molecular weight. The recrystallization of the "crude" mesodilactide can be effected from various solvents, such as ethanol, cyclohexane, toluene/ethanol (1:1) and methyl ethyl ketone. The use of isopropyl alcohol (IPA) is, however, preferred. It is also important to allow the recrystallization to proceed rapidly. This is in connection with the hydrolysis/decomposition of the mesodilactide. If the crystallization step lasts too long, the free acid content of the final mesodilactide will be too high, while the yield will be too low. An advantageous crystallization can be carried out as follows. 30 to 140% by weight of mesodilactide is dissolved in isopropanol at a temperature of preferably 30 to 40°C. Once the solid material has been dissolved, the solution is cooled rapidly to 0 to 10°C. The mother liquor and the mesodilactide are separated with the aid of a filter or centrifuge. The entire procedure is carried out under an inert atmosphere of preferably dry N₂ (in gas form) or argon (in gas form). This can be repeated once or several times, a mesodilactide with a free acid content of less than 0.1 meq/kg and a melting point of 43°C (after drying of the crystal under severely reduced pressure) being obtained. In view of the above it is remarked that the conventional method for the recrystallization of isomeric dilactides is not suitable in the case of the mesodilactide. Conventional solvents for the purification of dilactides are toluene, benzene (aromatic) and diethyl ether. However, these do not give satisfactory purification in this case. If the mesodilactide has to be kept or stored for a prolonged period this should preferably be carried out at lower temperature and under an inert gas atmosphere. For bulk homopolymerization of the mesodilactide use can be made of catalysts and reaction conditions customary for dilactide polymerization (see, inter alia, US-A- 3,839,297). To promote the comparability of the examples, only tin octoate (stannous 2-ethyl-hexanoate) dissolved in toluene has been used here; however, other catalysts may also be employed like SnO and ZnO. The polymerization temperature may vary from 60 to 180°C, but the polymerization is preferably carried out at a temperature of 80 to 120°C. The polymerizations are preferably carried out under an inert gas (dry N₂) atmosphere or under strongly reduced pressure (P<10⁻² mbar, P>10⁻⁷mbar). The reaction time range is preferably 3 to 24 hours. It is surprising that the polymesodilactides according to the invention do not, as was to some extent expected, possess the typical characteristics of poly-DL-dilactide, poly-D-lactide or poly-L-lactide. These three last-mentioned polymers are characterized by very long degradation times (longer than 200 days) in experiments carried out in physiological saline solutions at 37°C. Like poly-DL-lactide, polymesodilactide is an amorphous polymer. The great difference from the poly-DL-lactide is the appreciably faster degradation time of the polymesodilactide. The degradation times cited in the examples were determined in vitro at 37°C over a prolonged period (a maximum of 160 days) and in a medium of phosphate buffer, salt and water (physiological saline solution). The mesodilactide cannot only be polymerized in bulk but can also be polymerized under other reaction conditions, such as in solution. In Example VI a polymerization in toluene under a nitrogen layer (in gas form) at elevated temperature is described. Examples VII to IX inclusive give several possibilities for the preparation of polymers in which the mesodilactide is one of the monomers. Other comonomers, such as already mentioned above, can, however, also be used. In addition to the true "chemical" copolymers, the polymesodilactide can also be processed into "physical" mixtures. These physical mixtures may contain a biologically active substance which is released as degradation of the polymer progresses; these physical mixtures may, however, also consist of a physical mixture with another polymer. An accelerated degradation of a substrate produced therefrom is still obtained with an amount of not more than 99% by weight of other polymer. Preferably, not more than 50% by weight of other polymer is used in a mixture of this type. It can be seen from Examples XII to XIV inclusive that the addition of polymesodilactides to the other polylactides D, DL and L has a great influence on the in vitro behaviour of the physical mixtures obtained in respect of degradation time in comparison with homopolymers. It is known that it is possible to obtain a good indication of the biocompatibility of polymers by carrying out in vitro and simulation tests. Use is made of cell cultures for these tests. In Examples XIV and XV these in vitro and simulation tests are carried out with 4 mesopolymers. It can be seen from these examples that the mesopolymers are well able to meet the requirements as biomaterials. It is also seen that a reduced cell growth occurs in the case of the materials according to the invention which degrade most rapidly. This is ascribed to the acidification which occurs during the degradation. A strong acidification inhibits cell growth. When choosing the material to be used, the aim will therefore have to be to obtain a good balance between the cell growth which occurs and the degradation of the material. This is needed to achieve optimum utilization of the great advantage of the material in question, which can be biologically degraded and can be replaced by new bone tissue. Tests carried out confirm the good biocompatibility of the polymers in question. It is true that from the morphological standpoint there are some odd deviations. Frequently cells are seen which contain very large numbers of small holes or are torn; however, this is not serious. This is probably caused by the adsorption of the cells on the underlayer, which can differ for the diverse materials. As a result, shrinkage can occur during drying. Example I Bulk homopolymerization. 30 g mesodilactide having a free acid content of less than 0.1 meq/kg are introduced with the aid of a small funnel under dry N₂ into a glass ampoule with a capacity of 50 ml. 0.23 ml of a 25.2% by weight solution of tin octoate in toluene is added, like-wise under dry N₂. The filled ampoule is then brought under strongly reduced pressure (P<0.1 mbar) and rinsed three times with dry and oxygenfree N₂. After this rinsing cycle the ampoule is brought under strongly reduced pressure (P<0.01 mbar) for one and a quarter hours. The ampoule is melted down under this pressure with the aid of a propane-oxygen burner. The polymerization is carried out for 2 hours at 123°C. On completion of the reaction a glass-clear polymer is obtained with an intrinsic viscosity of 2.58 and a glass transition temperature (Tg) of 27°C. After this time the monomer concentration in the polymer is less than 2% by weight, which corresponds to a conversion of more than 98%. The polymer is purified by dissolving the whole in distilled chloroform and then pouring into the precipitant (hexane). Ethanol or a mixture of hexane and ethanol is also suitable. After cooled drying of the fibrelike material under strongly reduced pressure a good free-flowing powder is obtained. Example II Bulk homopolymerization 26 g mesodilactide with a free acid content of less than 0.1 meq/kg is processed in the manner described in Example I except that 0.065 ml 25. 2% by weight solution of tin octoate is added and a reaction time of 18.25 hours is employed. The polymer obtained has an intrinsic viscosity of 3.62. Example III Bulk homopolymerization 27 g mesodilactide with a free acid content of less than 0.1 meq/kg are used in the manner described in Example I, except that 0.31 ml 5% by weight solution of tin octoate are employed (M/I 5000). A reaction time of 112 hours is employed. The polymer obtained has an intrinsic viscosity of 3.82. Comparison Example I Bulk homopolymerization (with a free acid content greater than 1 meq/kg). The procedure described in Example I is followed and 23.3 g mesodilactide with a free acid content of 1.2 meq/kg are employed. The following changes are made to the recipe: 0.18 ml 25.2% tin octoate (M/I 1,500). A reaction temperature of 120°C and a reaction time of 3 hours 5 minutes. A polymer with an intrinsic viscosity of 0.294 is obtained in this way. It can be clearly seen from this that a less good polymer is obtained if a free acid content greater than 1 meq/kg is employed. Example IV Bulk hompolymerization The procedure described in Example I is followed and 27 g mesodilactide with a free acid content of less than 0.1 meq/kg is used. The following changes are made to the recipe described in Example I: reaction time 10 minutes (at a reaction temperature of 123°C); 0.30 ml 5% by weight stannous octoate is added. The polymer obtained has an intrinsic viscosity of 0.40. The conversion is greater than 95%. Example V Solution polymerization 100 g mesodilactide with a free acid content of less than 0.1 meq/kg are added to 300 ml toluene in a roundbottomed flask provided with a nitrogen inlet, a stirrer, a heating jacket and a condenser. Nitrogen is passed through the solution for 2 hours and during this time the temperature of the solution is gradually raised to 80°C. 1 ml of a 5% by weight solution of tin octoate and toluene is then added. After 7 hours' reaction the reaction is stopped. After pouring out the dissolved polymesolactide into hexane, separating off and drying, the viscosity is determined. A polymesodilactide with an intrinsic viscosity of 0.58 is obtained. Example VI Bulk Polymerization:copolymer The procedure described in Example I is followed and 7.74 g mesodilactide and 28.65 g L-lactide are used, both with a free acid content of less than 0.1 meq/kg. After a reaction time of 50 minutes at a temperature of 120°C and using a M/I = 1500 (ratio of monomer:initiator on molar basis), the polymer obtained has, after working up, an intrinsic viscosity of 4.22. The yield is 99%. Example VII Bulk polymerization: copolymer The procedure described in Example VI is followed and 11.21 g mesodilactide and 25.04 g L-lactide are used, both with a free acid content of less than 0.1 meq/kg. After a reaction time of 3.05 hours at 120°C and using a M/I = 2130, the polymer obtained has an intrinsic viscosity of 3.7. Example VIII Bulk polymerization: copolymer The procedure described in Example VI is followed and 15.31 g mesodilactide and 15.12 g glycolide are used, both with a free acid content of less than 0.1 meq/kg. After a reaction time of 17 hours at 120°C and using a M/I = 1500, a polymer is obtained which has an intrinsic viscosity of 1.16 when measured in hexafluoroisopropanol. Example IX Degradation tests on polymesolactide homopolymer A film is cast from a 25% by weight solution of polymesodilactide in chloroform and dried. The film is cut into pieces 15 by 6 cm. The pieces are kept for a prolonged period at 37°C in an aqueous physiological saline solution with a pH of 7.4 (provided with phosphate buffer). The thickness of the film was 25 µm. At fixed times a portion of the film was removed from the bath, rinsed carefully and dried. The intrinsic viscosity, the number of days, the molar mass (calculated using the Mark Houwink constants for poly-DL-lactide), mean viscosity molecular weight and the percentage of the initial mass of the film are shown in Table A. days intrinsic viscosity Mv (D.L.) % mass 0 0.79 41,100 100 15 0.42 18,100 100.2 49 0.18 6,000 81.7 84 0.04 800 18.1 116 0.00 0 0.5 169 0.00 0 0.4 Example X Degradation tests on polymesolactide hompolymer The procedure described in Example IX is followed but a 5% by weight solution in chloroform is used and a film with a thickness of 20 µm is produced. The measurements shown in Table B are obtained. days intrinsic viscosity Mv (D.L.) % mass 0 1.07 63,000 100 28 0.73 37,000 100.4 42 0.59 28,000 98.6 84 0.46 20,000 98.8 102 0.28 10,700 99.5 155 0.1 2,800 29.9 Example XI Degradation behaviour of physical mixtures of poly-L-lactide/polymesodilactide A film is cast in the manner described in Example IX from a 5% by weight solution of poly-L-lactide/polymesodilactide in chloroform. The poly-L-lactide/polymesodilactide ratios shown in Table C are used. The degradation tests are carried out in the manner described in Example IX. The mass percentages of mesodilactide in the physical mixture, the intrinsic viscosity and mass loss in % are given in Table C. Example XII Degradation behaviour of physical mixtures of poly-DL-lactide/polymesodilactide The procedure described in Example XI is followed. However, poly-DL-lactide is used as the second component in the physical mixture. The results are shown in Table D. Example XIII Degradation behaviour of physical mixtures of poly-D-lactide/polymesodilactide The procedure described in Example XI is followed. However, poly-D-lactide is used as the second component in the physical mixture. The results are shown in Table E. Example XIV: in vitro test Films are cast from a mesodilactide homopolymer (η intr = 3.6), a L-lactide/mesodilactide copolymer (50/50, η intr = 2.4), a physical mixture of polymesodilactide/ poly-L-lactide (50/50, η intr, meso = 1.0 intr, L = 7.8) and a mesodilactide/glycolide copolymer (90/10, η intr = 2.6) and dried. After drying the films are rinsed for 12 hours in tap water to remove any residual solvent (chloroform). The films are then dried and punched to give circles 3 cm in diameter. The base of TCPS (tissue culture polystyrene) culture dishes is then covered with these films, after which cell cultures of serially cultured rat middle ear epithelium are carried out at 37°C on these. Growth curves of the cultures were determined by determining the cell count after trypsination on days 1, 3, 6, 10 and 14. TCPS was included in the experiment as control. Figure 1 shows the number of cells as a function of the number of days. Example XV Extracts were made from the films mentioned in example XIV by exposing identical portions of the films to a pseudo-extracellular fluid for 62 hours at 115°C. The resulting salt solution later, after filtration, constitutes the basis for a culture medium in which cells are cultured. This medium is added on day 3 after the start of the culture in order to guarantee a good initial culture; the culture is carried out at 37°C. The cell counts are determined after 4, 6, 10 and 14 days. Melinex (positive control) and PVC (polyvinyl chloride: negative control) are included as control. In Fig. 2 the cell counts are plotted against the time (in days). Determination of free acid Principle The free acid in the sample is titrated to an end point with phenol red in a non-aqueous system using methoxide in methanol. Reagents and apparatus 1. Absolute methanol, reagent quality, must be kept dry. 2. Potassium methoxide - approximately 0.01 N solution in absolute methanol. (Obtained from Alpha Inorganic, Ventron, Beverly, Mass.). Approximately 0.34 g KOCH₃ is dissolved in absolute methanol in a volumetric flask with a capacity of 250 ml. 3. Benzoic acid, reagent quality. 4. Phenol red indicator, 0.05 % weight/vol in absolute methanol (a small quantity freshly prepared each day). 5. Balance with a sensitivity of 0.01 mg. 6. Burettes with a capacity of 10 ml. 7. Magnetic stirrer and a stirrer coated with Teflon. 8. Dry nitrogen, regulator and tubes. 9. Vacuum flasks with a capacity of 50 ml. 10. Weighing dishes with a capacity of 1 ml or equivalent. Standardization 1. Accurately weigh out approximately 5 mg benzoic acid as the first standard in each of two small weighing dishes (item 10). 2. Introduce 15 ml absolute methanol in the vacuum flask, which contains a rod stirrer, while a gentle stream of dry nitrogen flows through the side arm. 3. Add 5 to 6 drops phenol red indicator. 4. Titrate (2) above with absolute methanol/KOCH₃ solution to the end point with phenol red. 5. Add one dish with primary benzoic acid standard to the solution in step 4. 6. Titrate to the same shade of red as in step 4, with stirring. 7. Titrate the second primary standard in the same way. 8. Calculate the normality of the potassium methoxide. N = mg benzoic acid ml KOCH₃ solution x 122.1 9. Mean the value obtained in steps 6 and 7. Method 1. Proceed as in standardization steps 2, 3 and 4. 2. Accurately weigh approximately 0.4-0.5 g sample and introduce it into the vacuum flask. Expose the sample to the atmosphere as little as possible (the sample is difficult to dissolve in absolute methanol and dissolves slowly. It must be as free from lumps as possible). 3. Back-titrate to the same shade of red as in step 1 of the standardization, with stirring (the end point must be stable as long as a gentle stream of nitrogen is maintained). These values must be converted to meq/kg by multi-plying by a factor of 1,000. In this context it is also pointed out that it is very important to handle and store the monomer properly (i.e. avoid exposure to moisture from the air, reduce the air to 0.1 mm from half an hour before storage, keep at a temperature lower than 4°C in a plastic bottle in a plastic bag which contains dessicant).

Claims: Method for the preparation of a polymer lactide, characterized in that mesodilactide monomer having a free acid content of less than 1 meq/kg is polymerized in a manner known per se, if desired, together with one or more mono- or dilactides having a free acid content of less than 1 meq/kg. Method according to claim 1, characterized in that in addition to mesodilactide one or more monomers are used selected from the group consisting of D-lactide, DL-lactide, L-lactide, glycolide, β-propiolactone, β-butyrolactone, γ-butyrolactone, 2-keto-1 , 4-dioxane, γ-valerolactone, δ-valerolactone, ε-caprolacton, pivalolactone, α,α-diethylpropiolactone, 2,5-diketomorpholine, ethylene carbonate, ethylene oxalate, as well as intermolecular cyclic diesters of α-hydroxybuteric acid, α-hydroxyvaleric acid, α-hydroxyisovaleric acid, hydroxycaproic acid and 2-hydroxyisocaproic acid.

Inventors: MOL ARIE CORNELIS, NIEUWENHUIS JAN, MOL, ARIE CORNELIS, NIEUWENHUIS, JAN
Applicants: PURAC BIOCHEM BV, PURAC BIOCHEM B.V.
Application year: 1988
Main classification: C08G63
IPC codes: A61L31, A61L17, A61L27, C08G63, D01F6