Patent Publication Number: US-5633343-A

Title: High strength, fast absorbing, melt processable, gycolide-rich, poly(glycolide-co-p-dioxanone) copolymers

Description:
TECHNICAL FIELD 
     The field of art to which this invention relates is polymers, more specifically, biocompatible, absorbable copolymers; in particular, segmented copolymers of aliphatic polyesters of glycolide, and p-dioxanone. 
     BACKGROUND OF THE INVENTION 
     Polymers, including homopolymers and copolymers, which are both biocompatible and absorbable in vivo are known in the art. Such polymers are typically used to manufacture medical devices which are implanted in body tissue and absorb over time. Examples of medical devices manufactured from these absorbable biocompatible polymers include suture anchor devices, sutures, staples, surgical tacks, clips, plates and screws, etc. 
     Absorbable, biocompatible polymers useful for manufacturing medical devices include both natural and synthetic polymers. Natural polymers include cat gut, cellulose derivatives, collagen, etc. Synthetic polymers may consist of various aliphatic polyesters, polyanhydrides, poly(orthoester)s, and the like. Natural polymers typically absorb by an enzymatic degradation process in the body, while synthetic absorbable polymers generally degrade primarily by a hydrolytic mechanism. 
     Synthetic absorbable polymers which are typically used to manufacture medical devices include homopolymers such as poly(glycolide), poly(lactide), poly(e-caprolactone), and poly(p-dioxanone) and copolymers such as poly(lactide-co-glycolide), poly(e-caprolactone-co-glycolide), and poly(glycolide-co-trimethylene carbonate). The polymers may be statistically random copolymers, segmented copolymers, block copolymers, or graft copolymers. It is also known that both homopolymers and copolymers can be used to prepare blends. 
     U.S. Pat. Nos. 4,653,497, 4,838,267, 5,080,665 describe several biocompatible, absorbable, poly(p-dioxanone-co-glycolide) copolymers useful as biomedical devices. U.S. Pat. Nos. 4,838,267 and 5,080,665 additionally describe poly(p-dioxanone-co-glycolide) block or graft copolymers. 
     Furthermore, U.S. Pat. No. 4,838,267 describes (See FIGS. 1 and 2) the preparation of poly(p-dioxanone-b-glycolide) block copolymers by a two-step, two-reaction vessel process in which preformed, high molecular weight poly(p-dioxanone), that is substantially free of p-dioxanone monomer, is reacted with glycolide monomer at temperatures from about 140° C. to about 240° C. to yield block copolymers of the (A--B) n  type where A is a long block of repeating units of p-dioxanone (i.e., the homopolymer of poly(p-dioxanone)) and B is a long block of repeating units of glycolide (i.e., the homopolymer of poly(glycolide)). The repeating unit structure as well as a schematic representation of the block copolymer structure are shown in FIGS. 1 and 2. These copolymers are highly crystalline, due to their blocky structure, yielding materials with long breaking strength retention profiles (BSR), high strength and relatively high stiffness. It should be noted that breaking strength retention is a conventionally known standard method of measuring the strength of a device made of a bioabsorbable polymer, as a function of time under biological conditions in vitro or as a function of time after being implanted in vivo. 
     Additionally, U.S. Pat. No. 5,080,665 describes block or graft copolymers of poly(p-dioxanone-co-glycolide) prepared by a process in which the p-dioxanone monomer is reacted initially for a certain period of time, typically one hour at about 180° C., followed by reaction with glycolide at about 200° C. This process leads to block or graft copolymers which are useful due to their formation of a &#34;hard&#34; phase formed from the glycolide repeating unit blocks, and a &#34;soft&#34; phase formed from the p-dioxanone repeating unit blocks as illustrated in FIGS. 3 and 4. 
     Furthermore, U.S. Pat. No. 4,653,497 describes poly(p-dioxanone)-rich segmented copolymers comprising about 70 weight percent to about 97 weight percent polymerized p-dioxanone with the remaining small portion of the copolymer polymerized with glycolide as illustrated in FIGS. 5 and 6. 
     Although the above described copolymers yield materials with excellent properties such as high strength and stiffness and long BSR profiles as found with the block copolymers, or good strength, long elongations, low stiffness and shorter BSR profiles as found for the poly(p-dioxanone)-rich segmented copolymers, there is a need in this art for new copolymer compositions having characteristics of both the block copolymers and the segmented copolymers. 
     Accordingly, what is needed in this art are novel copolymer compositions which have the block copolymer characteristics of high strength and stiffness, and the segmented copolymer characteristics of shorter BSR profiles and absorptions rates. 
     In certain biomedical applications there is a strong need for these requirements, including sutures with strong tensile properties, like those of Vicryl® or Dexon®, but shorter BSR profiles and absorption rates. 
     In addition, it would be highly desirable to have such polymers having little or no unreacted monomers present, since it is believed that the presence of certain levels of unreacted monomers can lead to problems such as adverse tissue reaction. 
     DISCLOSURE OF THE INVENTION 
     Surprisingly, it has been discovered that by preparing copolymers of poly(glycolide-co-p-dioxanone) rich in glycolide by a process in which a small proportion of p-dioxanone monomer is reacted at low temperatures from about 100° C. to about 130° C. followed by reaction with glycolide at higher temperatures of about 180° C. to about 220° C., segmented glycolide-rich copolymers with small proportions of p-dioxanone can be formed that have high strength and stiffness, but short BSR profiles and fast absorption rates, making them useful in a variety of biomedical devices such as suture anchor devices, staples, surgical tacks, clips, plates and screws. The copolymers of the present invention are especially useful for suture applications where Vicryl®-like sutures with excellent tensile properties but shorter BSR profiles than Vicryl®are needed. The copolymers of the present invention are, therefore, useful in plastic surgical applications, where faster absorption times tend to result in less tissue scarring. That is, for example, an absorbable suture which rapidly loses strength in the sections which have been implanted beneath a patient&#39;s skin can be more easily removed, by use of antitension skin tape, without the pain, discomfort and scarring typically suffered by patients who have had conventional sutures implanted. 
     Accordingly, novel, absorbable, biocompatible, poly(glycolide-co-p-dioxanone) segmented copolymers are disclosed. The copolymers have a major component comprising about 30 mole percent to about 95 mole percent of repeating units of glycolide, and a minor component comprising about 70 mole percent to about 5 mole percent of repeating units of p-dioxanone. 
     Yet another aspect of the present invention is a biomedical device made from the above described copolymers, especially implantable devices such as suture anchor devices, staples, surgical tacks, clips, plates and screws, and most especially for suture applications where Vicryl® or Dexon®-like sutures with excellent tensile properties, but shorter BSR profiles than Vicryl® or Dexon® are needed for plastic surgery. 
     An additional aspect of the present invention is a process for producing the segmented copolymers of the present invention. The initial step of the process is to polymerize p-dioxanone in the presence of a catalytically effective amount of catalyst and an initiator at a sufficient temperature and for a sufficient period of time to effectively yield a first mixture of p-dioxanone monomer and p-dioxanone homopolymer. Then, glycolide is added to the first mixture to form a second mixture. Next, the second mixture is polymerized at a sufficient temperature and for a sufficient amount of time to effectively produce a segmented copolymer comprising a major component comprising about 30 mole percent to about 95 mole percent of repeating units of glycolide and a minor component comprising about 70 mole percent to about 5 mole percent of repeating units of p-dioxanone. 
     Still yet a further aspect of the present invention is the copolymer of the present invention which is a product of the process of the present invention. 
     The foregoing and other features and advantages of the invention will become more apparent from the following description and accompanying examples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a synthetic process for the preparation of poly(p-dioxanone-b-glycolide) block copolymers as described in U.S. Pat. No. 4,838,267. 
     FIG. 2 illustrates a schematic representation of poly(p-dioxanone-b-glycolide) block copolymers as described in U.S. Pat. 4,838,267. 
     FIG. 3 illustrates a synthetic process for the preparation of poly(p-dioxanone-co-glycolide) block or graft copolymers as described in U.S. Pat. No. 5,080,665. 
     FIG. 4 illustrates a schematic representation of poly(p-dioxanone-co-glycolide) block or graft copolymers as described in U.S. Pat. No. 5,080,665. 
     FIG. 5 illustrates a synthetic process for the preparation of poly(p-dioxanone)-rich, poly(p-dioxanone-co-glycolide) segmented copolymers as described in U.S. Pat. No. 4,653,497. 
     FIG. 6 illustrates a schematic representation of poly(p-dioxanone)-rich, poly(p-dioxanone-co-glycolide) segmented copolymers as described in U.S. Pat. No. 4,653,497. 
     FIG. 7 illustrates a synthetic process for the preparation of the glycolide-rich, poly(glycolide-co-p-dioxanone) segmented copolymers of the present invention. 
     FIG. 8 illustrates a schematic representation of the glycolide-rich, poly(glycolide-co-p-dioxanone) segmented copolymers of the present invention. 
     FIG. 9 is a graph presenting BSR profiles of size 4-0 fibers (sutures) of Vicryl (90:10 (mol/mol) poly(glycolide-co-lactide)) or Dexon (poly(glycolide)), size 4-0 fibers of poly(p-dioxanone), size 5-0 fibers of a segmented copolymer of U.S. Pat. No. 4,653,497, and size 5-0 fibers of a 80:20 (mol/mol) segmented poly(glycolide-co-p-dioxanone) copolymer of the present invention. 
     FIG. 10 shows the compositional differences between the segmented poly(glycolide-co-p-dioxanone) copolymers of the present invention and the copolymers disclosed in U.S. Pat. Nos. 4,653,497, 4,838,267 and 5,080,665. 
     FIG. 11 displays the property differences between the segmented poly(glycolide-co-p-dioxanone) copolymers of the present invention and U.S. Pat. Nos. 4,653,497, 4,838,267 and 5,080,665, based upon differences in composition and structure. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The process of the present invention is a one-step, one-reaction vessel, two-temperature process in which a mixture of p-dioxanone monomer and p-dioxanone homopolymer is initially formed at low temperatures of from about 100° C. to about 130° C., preferably 110° C. The mixture is then reacted with glycolide at temperatures from about 120° C. to about 220° C. to form copolymers in which segments or sequences are composed of both p-dioxanone and glycolate moieties as illustrated in FIGS. 7 and 8. These segmented copolymers are substantially less crystalline than the block or graft copolymers of the prior art and, therefore, yield materials with good strength, but shorter BSR profiles, faster absorption rates, longer elongations and lower stiffness. 
     More specifically, the poly(glycolide-co-p-dioxanone) segmented copolymers of the present invention are prepared by a process in which p-dioxanone monomer is reacted in a conventional reactor vessel at low temperatures from about 100° C. to about 130° C., preferably about 110° C., for a sufficient time effective to cause polymerization, preferably about 4 hours to about 8 hours, followed by reaction with glycolide at higher temperatures of about 180° C. to about 220° C. for a sufficient time effective to cause copolymerization, preferably about 1 hour to about 4 hours. 
     Furthermore, the segmented poly(glycolide-co-p-dioxanone) copolymers will typically consist of about 30 mole percent to about 95 mole percent of glycolate moieties, more preferably about 30 mole percent to about 90 mole percent of glycolate moieties, and most preferably about 30 mole percent to about 50 mole percent of glycolate moieties. 
     The aliphatic segmented copolyesters useful in the preparation of the segmented copolymers of the present invention will typically be synthesized in a ring opening polymerization. That is, the aliphatic lactone monomers are polymerized in the presence of an organometallic catalyst and an initiator at elevated temperatures. The organometallic catalyst is preferably tin based, e.g., stannous octoate, and is present in the monomer mixture at a molar ratio of monomer to catalyst ranging from about 10,000/1 to about 100,000/1. The initiator is typically an alkanol, a glycol, a hydroxyacid, or an amine, and is present in the monomer mixture at a molar ratio of monomer to initiator ranging from about 100/1 to about 5000/1. The polymerization is typically carried out at a temperature range from about 80° C. to about 240° C., preferably from about 100° C. to about 220° C., until the desired molecular weight and viscosity are achieved. 
     Suitable lactone monomers may be selected from the group consisting of glycolide, lactide (l, d, dl, meso), p-dioxanone, delta-valerolactone, beta-butyrolactone, epsilon-decalactone, 2,5-diketomorpholine, pivalolactone, alpha, alpha-diethylpropiolactone, ethylene carbonate, ethylene oxalate, 3-methyl-1,4-dioxane-2,5-dione, 3,3-diethyl-1,4-dioxan-2,5-dione, gamma-butyrolactone, 1,4-dioxepan-2-one, 1,5-dioxepan-2-one, 1,4-dioxan-2-one, 6,8-dioxabicycloctane-7-one and combinations of two or more thereof. Preferred lactone monomers are selected from the group consisting of glycolide, and p-dioxanone. 
     More specifically, the segmented copolymers of poly(glycolide-co-p-dioxanone) useful in the practice of the present invention will typically be synthesized by a process in which p-dioxanone is polymerized in a ring opening polymerization in the presence of an organometallic catalyst and an initiator at elevated temperatures. The organometallic catalyst is preferably tin based, e.g., stannous octoate, and is present in the mixture at a molar ratio of monomer to catalyst ranging from about 10,000/1 to about 100,000/1. The initiator is typically an alkanol, a glycol, a hydroxyacid, or an amine, and is present in the monomer mixture at a molar ratio of monomer to initiator ranging from about 100/1 to about 5000/1. 
     The polymerization is typically carried out in a conventional reactor vessel at a temperature range from about 100° C. to about 130° C., preferably 110° C., for about 4 hours to about 8 hours, preferably about 5 hours to about 6 hours, yielding a mixture of p-dioxanone monomer and homopolymer. Then, glycolide monomer is added to the mixture of p-dioxanone monomer and homopolymer and the temperature is raised to about 180° C. to about 220° C., preferably from about 190° C. to about 220° C. until the desired molecular weight and viscosity are achieved. It should be understood that pure monomers and dry conditions are utilized to achieve such molecular weights. 
     Under the above described conditions, the segmented copolymers of poly(glycolide-co-p-dioxanone), will typically have a weight average molecular weight of about 20,000 grams per mole to about 300,000 grams per mole, more typically about 40,000 grams per mole to about 200,000 grams per mole, and preferably about 60,000 grams per mole to about 150,000 grams per mole. These molecular weights provide an inherent viscosity between about 0.5 to about 4.0 deciliters per gram (dL/g), more typically 0.7 to about 3.5 dL/g, and most preferably 1.0 to about 3.0 dL/g as measured in a 0.1 g/dL solution of hexafluoroisopropanol (HFIP) at 25° C. Also, it should be noted that under the above described conditions, the residual monomer content will be less than about 5 wt. %. 
     The segmented copolymers of poly(glycolide-co-p-dioxanone) will typically consists of about 30 mole percent to about 95 mole percent, more preferably about 30 mole percent to about 90 mole percent of glycolate moieties, and most preferably about 30 mole percent to about 50 mole percent of glycolate moieties. The lower limit of glycolate moieties in the copolymers is desirable because the addition of 30 mole percent leads to copolymers which have longer BSR profiles, but lower strength. The upper limit of glycolate moieties in the copolymers is desirable because the addition of 95 mole percent leads to copolymers which have shorter BSR profiles, but higher strength and stiffness. This leads to copolymers with a desirable range of strength, stiffness and absorption profiles for use in a variety of biomedical applications. One skilled in the art will appreciate that the number of moles of glycolate moieties and p-dioxanone moieties in the copolymer are equivalent to the number of moles of glycolide and p-dioxanone monomers needed to be added to the reaction to form the copolymer. 
     Articles such as medical devices are molded from the segmented copolymers of the present invention by use of various conventional injection and extrusion molding equipment at temperatures ranging from about 160° C. to about 220° C., more preferably 180° C. to about 220° C., with residence times of about 2 to about 10 minutes, more preferably about 2 to about 5 minutes. 
     The segmented copolymers of the present invention can be melt processed by numerous methods to prepare a vast array of useful devices. These materials can be injection or compression molded to make implantable medical and surgical devices, including wound closure devices. The preferred devices are suture anchor devices, staples, surgical tacks, clips, plates and screws. 
     Copolymers of the present invention with low levels of p-dioxanone have particular utility in the preparation of absorbable surgical devices that must be stiff and simultaneously tough. Of particular utility are those compositions having p-dioxanone at about 5 to about 30 mole percent. 
     Alternatively, the segmented copolymers can be extruded to prepare fibers. The filaments thus produced may be fabricated into sutures or ligatures, attached to surgical needles, packaged, and sterilized by known techniques. The materials of the present invention may be spun as multifilament yarn and woven or knitted to form sponges or gauze, (or non-woven sheets may be prepared) or used in conjunction with other molded compressive structures such as prosthetic devices within the body of a human or animal where it is desirable that the structure have high tensile strength and desirable levels of compliance and/or ductility. Useful embodiments include tubes, including branched tubes, for artery, vein or intestinal repair, nerve splicing, tendon splicing, sheets for tying up and supporting damaged surface abrasions, particularly major abrasions, or areas where the skin and underlying tissues are damaged or surgically removed. Especially, sutures with Vicryl®-like tensile properties but shorter BSR profiles than Vicryl® are needed, useful in plastic surgical applications where faster absorption times would lead to less tissue scarring. That is, a suture which rapidly loses strength in the portion which is implanted beneath the skin can be more easily removed, by use of antitension skin tape, without the pain, discomfort and scarring typically suffered by patients who have had conventional sutures implanted. Vicryl® is a trademark for sutres made from polyglycolide-polylactide (90-10) copolymers. 
     Additionally, the segmented copolymers of the present invention can be molded to form films which, when sterilized, could be useful as adhesion prevention barriers. Another alternative processing technique for the copolymers of the present invention includes solvent casting, particularly for those applications where a drug delivery matrix is desired. 
     Furthermore, the segmented copolymers of the present invention can be processed by conventional techniques to form foams, which are useful as hemostatic barriers and bone substitutes. 
     In more detail, the surgical and medical uses of the filaments, films, foams and molded articles of the present invention include, but are not necessarily limited to knitted products, woven or non-woven, and molded products including: 
     a. burn dressings 
     b. hernia patches 
     c. medicated dressings 
     d. fascial substitutes 
     e. gauze, fabric, sheet, felt or sponge for liver hemostasis 
     f. gauze bandages 
     g. arterial graft or substitutes 
     h. bandages for skin surfaces 
     i. burn dressings 
     j. orthopedic pins, clamps, screws, and plates 
     k. clips 
     l. staples 
     m. hooks, buttons, and snaps 
     n. bone substitutes 
     o. needles 
     p. intrauterine devices 
     q. draining or testing tubes or capillaries 
     r. surgical instruments 
     s. vascular implants or supports 
     t. vertebral discs 
     u. extracorporeal tubing for kidney and heart-lung machines 
     v. artificial skin and others 
     w. stents 
     x. suture anchors 
     y. injectable defect fillers 
     z. preformed defect fillers 
     a1. tissue adhesives and sealants 
     b2. bone waxes 
     b3. cartilage replacements 
     d4. hemostatic barriers 
     EXAMPLES 
     The following examples are illustrative of the principles and practice of this invention, although not limited thereto. Numerous additional embodiments within the scope and spirit of the invention will become apparent to those skilled in the art. The examples describe new segmented copolymers of poly(glycolide-co-p-dioxanone), said copolymers being useful as biomedical devices. 
     In the synthetic process, the high molecular weight aliphatic segmented copolyesters are prepared by a method consisting of reacting p-dioxanone via a ring opening polymerization in a conventional reactor vessel at temperatures of about 100° C. to about 130° C. for about 4 hours to about 8 hours under an inert nitrogen atmosphere, followed by reaction with glycolide at temperatures of 180° C. to 220° C. until the desired molecular weight and viscosity are achieved. 
     In the examples which follow, the segmented copolymers and monomers were characterized for chemical composition and purity (NMR, FT-IR), thermal analysis (DSC), melt rheology (melt stability and viscosity), and molecular weight (inherent viscosity), and baseline and in vitro mechanical properties (Instron stress/strain). 
       1  H was performed on a 300 MHz NMR using CDCl 3  or HFAD as a reference. Thermal analysis of segmented copolymers and monomers was performed on a Dupont 912 Differential Scanning Calorimeter (DSC) at a heating rate of 10° C./min. A Fisher-Johns melting point apparatus was also utilized to determine melting points of monomers. Thermal gravimetric analysis was performed on a Dupont 951 TGA at a rate of 10° C./min. under a nitrogen atmosphere. Isothermal melt stability of the segmented copolymers was also determined by a Rheometrics Dynamic Analyzer RDA II for a period of 1 hour at temperatures ranging from 160° C. to 230° C. under a nitrogen atmosphere. Inherent viscosities (I.V., dL/g) of the segmented copolymers were measured using a 50 bore Cannon-Ubbelhode dilution viscometer immersed in a thermostatically controlled water bath at 25° C. utilizing chloroform or hexafluoroisopropanol (HFIP) as the solvent at a concentration of 0.1 g/dL. 
     Melt viscosity was determined utilizing a Rheometrics Dynamic Analyzer RDA II at temperatures ranging from 160° C. to 230° C. at rate of 1° C./min. to 10° C./min. at frequencies of 1s -1  to 100s -1  under a nitrogen atmosphere. 
     Fibers of copolymers of the present invention were prepared by a method as described in U.S. Pat. No. 4,643,191, which is incorporated herein by reference. The copolymers were melt extruded in a conventional manner using an INSTRON® capillary rheometer or single screw extruder. Rheometer packing temperatures ranged from about 100° C. to about 200° C. with dwell times of about 5 to about 15 minutes and ram speeds of about 1 to about 3 cm/min. Extrusion temperatures ranged from about 160° C. to about 230° C. 
     The extrudate was typically drawn at a draw rate of 4 feet per minute in a single or mulitstage drawing process with drawing temperatures of about 25° C. to about 75° C., giving a final draw ratio of about 4× to about 8×. 
     Fibers were also annealed under similar conditions as described in U.S. Pat. No. 4,643,191. Annealing temperatures were from about 70° C. to about 140° C., preferably 110° C., with annealing times of about 1 hour to about 10 hours, preferably about 4 to 7 hours. 
     In vitro studies were determined in a phosphate buffer solution (pH=7.27) at a temperature of 37° C. for periods of 4, 7, 14, 21, and 28 days. Cylindrical dumbbells (8 to 10 of a total weight of 2.4 to 3.0 grams) or fibers (8 to 10, 6 to 12 inches long) were placed in 100 ml of buffer solution. 
     Several synthesis examples will be described in the following few pages. Parts and percentages where used are parts and percentages as specified as weight or moles. 
     EXAMPLE 1 
     Synthesis of a 80:20 (mol/mol) poly(glycolide-co-p-dioxanone) segmented copolymer 
     To a flame dried 250 ml 2-neck round bottom flask equipped with an overhead mechanical stirrer, nitrogen inlet and glass stopper, 20.42 grams (0.20 moles) of p-dioxanone, 0.057 ml of diethylene glycol as an initiator, and 50.5 microliters of a 0.33M solution of stannous octoate (in toluene) catalyst were added. 
     The assembly was then placed in a high temperature oil bath at 110° C. The stirred p-dioxanone quickly began to melt. The low viscosity melt quickly increased in viscosity. Stirring of the high viscosity melt was continued for about 6 hours. 
     Then, 92.90 grams (0.80 moles) of glycolide was added in three equal portions while the temperature was raised gradually to 210° C. The glycolide quickly began to melt and the reaction mass slowly began to increase in viscosity. Stirring of the high viscosity melt was continued for another 0.5 hours for a total reaction time of 2 hours at 210° C. under nitrogen. 
     The 80:20 (mol/mol) poly(glycolide-co-p-dioxanone) segmented copolymer was removed from the bath, cooled to room temperature under a stream of nitrogen, isolated and ground. The polymer was then dried under vacuum at 80° C. for about 64 hours. The copolymer conversion was about 96.8%. The inherent viscosity was 1.76 dL/g as measured in a 0.1 g/dL HFIP solution at 25° C. The molar ratio of poly(glycolide) to poly(p-dioxanone) was found to be 83.2 to 16.8 by  1  H NMR. 
     EXAMPLE 2 
     Synthesis of a 70:30 (mol/mol) poly(glycolide-co-p-dioxanone) segmented copolymer 
     To a flame dried 250 ml 2-neck round bottom flask equipped with an overhead mechanical stirrer, nitrogen inlet and glass stopper, 30.60 grams (0.30 moles) of p-dioxanone, 0.057 ml of diethylene glycol as an initiator, and 50.5 microliters of a 0.33M solution of stannous octoate (in toluene) catalyst were added. 
     The assembly was then placed in a high temperature oil bath at 110° C. The stirred p-dioxanone quickly began to melt. The low viscosity melt quickly increased in viscosity. Stirring of the high viscosity melt was continued for about 6 hours. 
     Then, 81.30 grams (0.70 moles) of glycolide was added in three equal portions while the temperature was raised gradually to 210° C. The glycolide quickly began to melt and the reaction mass slowly began to increase in viscosity. Stirring of the high viscosity melt was continued for another 0.5 hours for a total reaction time of about 2 hours at 210° C. under nitrogen. 
     The 70:30 (mol/mol) poly(glycolide-co-p-dioxanone) segmented copolymer was removed from the bath, cooled to room temperature under a stream of nitrogen, isolated and ground. The polymer was then dried under vacuum at 80° C. for about 64 hours. The copolymer conversion was about 95.4%. The inherent viscosity was 2.25 dL/g as measured in a 0.1 g/dL HFIP solution at 25° C. The molar ratio of poly(glycolide) to poly(p-dioxanone) was found to be 75.2 to 24.8 by  1  H NMR. 
     EXAMPLE 3 
     Synthesis of a 60:40 (mol/mol) poly(glycolide-co-p-dioxanone) segmented copolymer 
     To a flame dried 250 ml 2-neck round bottom flask equipped with an overhead mechanical stirrer, nitrogen inlet and glass stopper, 40.84 grams (0.40 moles) of p-dioxanone, 0.057 ml of diethylene glycol as an initiator, and 50.5 microliters of a 0.33M solution of stannous octoate (in toluene) catalyst were added. 
     The assembly was then placed in a high temperature oil bath at 110° C. The stirred p-dioxanone quickly began to melt. The low viscosity melt quickly increased in viscosity. Stirring of the high viscosity melt was continued for about 6 hours. 
     Then, 69.64 grams (0.60 moles) of glycolide was added in three equal portions while the temperature was raised gradually to 210° C. The glycolide quickly began to melt and the reaction mass slowly began to increase in viscosity. Stirring of the high viscosity melt was continued for another 0.5 hours for a total reaction time of about 2 hours at 210° C. under nitrogen. 
     The 60:40 (mol/mol) poly(glycolide-co-p-dioxanone) segmented copolymer was removed from the bath, cooled to room temperature under a stream of nitrogen, isolated and ground. The copolymer conversion was about 95.7%. The inherent viscosity was 1.87 dL/g as measured in a 0.1 g/dL HFIP solution at 25° C. 
     As discussed above, U.S. Pat. Nos. 4,838,267, and 5,080,665 describe poly(glycolide-co-p-dioxanone) block or graft copolymers. U.S. Pat. No. 4,653,497 describes poly(p-dioxanone)-rich, segmented poly(p-dioxanone-co-glycolide) copolymers. 
     The present invention describes glycolide-rich, segmented poly(glycolide-co-p-dioxanone) copolymers. 
     As shown in FIGS. 1, 2, 3 and 4, block copolymers are copolymers where long blocks of repeating units of each of the homopolymers (i.e., the homopolymers of poly(p-dioxanone), or poly(glycolide)) are connected or linked at a single point. Segmented copolymers, as shown in FIGS. 5, 6, 7, and 8, are copolymers where short segments of repeating units composed of both monomeric units are connected or linked at many points. 
     The differences in the arrangement or sequences of the repeating units in the copolymer can lead to dramatic changes in the thermal, chemical, physical, and for absorbable polymers, biological properties. 
     For biocompatible, absorbable aliphatic poly(ester)s, the sequence arrangement of repeating units in the polymer chain has a strong effect on, for example, absorption rates, BSR profiles, strength, and stiffness (FIGS. 10 and 11). 
     Table 1 shows the dramatic changes in physical properties by comparing the glycolide-rich, segmented poly(glycolide-co-p-dioxanone) copolymers of the present invention, and examples 2 and 3 of the poly(glycolide-b-p-dioxanone) block copolymers of U.S Pat. No. 4,838,267. 
     For example, it can clearly be seen in Table 1 that the block copolymers (columns 3 and 5 of Table 1) have lower tensile fiber properties than the segmented copolymers (columns 2 and 4 of Table 1). Furthermore, the block copolymers have poor conversions. 
     
                                           TABLE 1                                 
__________________________________________________________________________
Properties of Poly(glycolide-co-p-dioxanone) segmented Copolymers and     
Poly(glycolide-b-p-dioxanone) Block Copolymers                            
           Segmented Copolymers                                           
                       Block copolymer                                    
                                Segmented Copolymer                       
                                           Block Copolymer                
           PGA-PDO     PGA/PDO  PGA/PDO    PGA/PDO                        
Properties Example 1*  Example 2**                                        
                                Example 2* Example 3**                    
__________________________________________________________________________
Initial Composition                                                       
           80/20       77.87/22.13                                        
                                70/30      67.24/32.76                    
(moles)                                                                   
Initial Composition                                                       
           81.98/18.02 80/20    77.62/27.38                               
                                           70/30                          
(weight)                                                                  
Fiber Diameter                                                            
           6.7         6.7      6.5        6.3                            
(mils)                                                                    
Fiber Tensile                                                             
           136000      47200    78000      57700                          
Strength (psi)                                                            
Fiber Knot Tensile                                                        
           111000      37000    69000      52600                          
Strength (psi)                                                            
Fiber Elongation (%)                                                      
           12.6        40       22.7       52.6                           
Fiber Young&#39;s                                                             
           2428        627      2007       551                            
Modulus (kpsi)                                                            
Polymerization                                                            
           97          89       96         76                             
Conversion (%)                                                            
Tensile Strength-                                                         
           11700                8600                                      
Inj. molded (psi)                                                         
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 **Block Copolymers are Examples 2 and 3 from U.S. Pat. No. 4,838,267     
 *Segmented Copolymers are Examples 1 and 2 from the present invention    
 
    
     The lower fiber properties and poor conversions of the block copolymers are caused by the sequence or arrangement of repeating units within the copolymers. That is, in the block copolymers, the polymers are composed of long sequences of both homopolymers, with some blocks of poly(p-dioxanone) on the ends of the chain (FIGS. 2 and 4). In contrast, the segmented copolymers, due to their more random sequence of repeating units, are composed mostly of glycolide end blocks (FIG. 8). Consequently, the glycolide end blocks lead to improved stability during polymerization, with less loss of p-dioxanone monomer, since it is not possible for the p-dioxanone segments to depolymerize at the end of the chain as can be found for the block copolymers. Hence, the segmented copolymers have higher conversions (i.e., less than 5 wt. % unreacted monomers, 95% or greater conversions), and better physical properties, especially fiber tensile properties. In addition, the higher conversions, and consequently, good thermal stability, found for the segmented copolymers, allows these materials to be injection molded with good tensile properties (Table 1). 
     In addition, the present invention requires only a single reactor process for polymerization, where the p-dioxanone monomer is polymerized to approximately 75% conversion, followed by reaction with glycolide to from copolymers with short segments of poly(p-dioxanone), poly(p-dioxanone-co-glycolide), and poly(glycolide) end blocks. In contrast, the block copolymers of U.S. Pat. No. 4,838,267, require a two-step, two reactor process, where the p-dioxanone is polymerized and then further processed by isolating, grinding, and drying to remove unreacted p-dioxanone monomer. Then, the poly(p-dioxanone) is reacted with glycolide to form block copolymers with long sequences of poly(p-dioxanone) and poly(glycolide), with some poly(p-dioxanone) end blocks. 
     Hence, the process of the present invention leads to the copolymers having end blocks of glycolide which prevents depolymerization and yields high conversions and excellent physical properties. Thus, the segmented copolymers of the present invention have a more elegant, simple, single reactor polymerization process than those of the block copolymers. This not only leads to advantageous properties, but is vital for large scale manufacturing development, being more cost effective and less time consuming. 
     Furthermore, the physical characteristics of the segmented copolymers of the present invention should allow for a variety of needs to be met for a wide range of medical devices. For example, there is a great need for absorbable polymers in plastic surgery wound closure, where high initial strength but very short BSR profiles are required. Currently, Vicryl® or Dexon® sutures are used (Table 2). Dexon® is a trademark for sutures made from braided poylcolide. 
     
                                           TABLE 2                                 
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Straight Tensile Strength of fibers as a function of days in-vitro        
DEXON (polyglycolide) VICRYL [poly(glycolide-co-lactide])                 
and Segmented Poly(glycolide-co-p-dioxanone) Copolymers                   
       Straight Tensile Strength (psi)                                    
       Mole %                                                             
Copolymer                                                                 
       PGA/PLA                                                            
             Zero-day                                                     
                  Four-day                                                
                       7-day                                              
                           14-day                                         
                               21-day                                     
                                   28-day                                 
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VICRYL*                                                                   
       90/10 117000                                                       
                  N/A  95000                                              
                           72000                                          
                               43000                                      
                                   0                                      
DEXON* 100/0 118000                                                       
                  N/A  96000                                              
                           85000                                          
                               44000                                      
                                   0                                      
       Weight %                                                           
       PGA/PDO                                                            
Example 1**                                                               
       80/20 136000                                                       
                  62000                                                   
                         0                                                
Example 2**                                                               
       70/30  80000                                                       
                  36000                                                   
                         0                                                
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 *SIZE 4.0, BRAIDED                                                       
 **SIZE 5.0, MONOFILAMENT                                                 
 
    
     However, as shown in FIG. 9, Vicryl® has a very long BSR profile. Consequently, this can lead to tissue scaring, an event which plastic surgeons want to avoid. The segmented glycolide-rich, poly(glycolide-co-p-dioxanone) copolymers of the present invention also have high zero-day fiber tensile properties. This would also allow the surgeon to suture the wounds securely. However, because of their very fast absorption rates and very short BSR profiles, less scarring will occur (Table 2, FIG. 9) than found with currently marketed, longer BSR sutures like Vicryl® and Dexon®. This behavior is caused by the unique structure of short poly(glycolide) end blocks found in the segmented copolymers of the present invention, which produces improved tensile properties and fast absorption rates. 
     The differences in properties between the copolymers of the present invention and U.S. Pat. Nos. 4,653,497, 4,838,267 and 5,080,665, due to differences in the structure and composition, are clearly indicated in FIG. 11. That is, surprisingly we have discovered that by moving towards compositions rich in glycolide where the copolymer structure is composed of short segments (i.e., segmented copolymer), and away from blocky structures, it is possible to obtain polymers with both high strength and short BSR profiles. That is, the block copolymer compositions yield high strength and longer BSR profiles or lower strength and longer BSR profiles. In addition, segmented copolymers, rich in p-dioxanone, yield polymers with lower strength and longer BSR profiles. 
     This surprising behavior is also clearly indicated in FIG. 9, which shows that the copolymers of poly(glycolide-co-p-dioxanone) of this invention have higher strength than either of the homopolymers of poly(glycolide) (i.e., Dexon®) or poly(p-dioxanone), but much shorter BSR profiles than even Vicryl®, Dexon®, poly(p-dioxanone) or the segmented p-dioxanone-rich, poly(p-dioxanone-co-glycolide) copolymers of U.S. Pat. No. 4,653,497. That is, the segmented copolymers of this invention lose all strength in less than 10 days, whereas the glycolide and p-dioxanone homopolymers and the copolymers of U.S. Pat. No. 4,653,497 maintain strength for 3 weeks or longer. 
     That is, it is unexpected, based upon composition, to obtain a copolymer of poly(glycolide-co-p-dioxanone) which has the tensile strength of either of the homopolymers of poly(p-dioxanone) or poly(glycolide), but a much shorter BSR profile than either homopolymer. 
     Therefore, it can be seen that glycolide-rich, poly(glycolide-co-p-dioxanone) segmented copolymers of the present invention possess a surprising and unexpected unique combination of high strength and stiffness, and very short BSR profiles and fast absorption rates. The copolymers of the present invention are useful in numerous applications requiring absorbable polymers which have high strength and stiffness and short BSR profiles. They are particularly useful, for example, in plastic surgical applications where the surgeon needs to secure the wound, but requires fast absorption to prevent tissue scaring. 
     Although this invention has been shown and described with respect to detailed embodiments thereof, it will understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed invention.