EP1495063A1

EP1495063A1 - Method for producing melt-stable homo- and copolyesters of cyclic esters and/or diesters

Info

Publication number: EP1495063A1
Application number: EP03722283A
Authority: EP
Inventors: Gerald Rafler; Inna Bechthold; Horst Rieckert
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV; Schill and Seilacher AG
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV; Schill and Seilacher AG
Priority date: 2002-04-16
Filing date: 2003-04-11
Publication date: 2005-01-12
Also published as: US20050165206A1; DE10216834A1; WO2003087191A1

Abstract

The invention relates to a method for producing melt-stable homo- and copolyesters of lactic acid and comparable monomers by the ring-opening polymerization of cyclic esters or diesters in the presence of an initiator or stabiliser system. Conventional organotin initiators or catalysts are used for the ring-opening polymerization process. The melting property is stabilized by special organophosphoric additives with lower phosphor oxidation levels. The organophosphoric additives can be introduced both directly during the production of said polyesters and can be added to the polymers in a subsequent compounding process.

Description

Process for producing melt-stable homo- and copolyesters of cyclic esters and / or desters

The invention relates to a process for the preparation of melt-stable homo- and copolyesters by ring opening polymerization of the corresponding cyclic monomers, e.g. the cyclic diester of lactic acid, in the presence of an initiator / stabilizer system.

Homo- and copolyesters of L- and D, L-lactic acid are biodegradable polymer materials with thermoplastic processing and

Application properties can be used in a variety of ways as packaging plastic, in hygiene products, for disposable articles, but also as a surgical implant material or galenic auxiliary for parenteral drug delivery systems. An indispensable prerequisite for the use of these homo- or copolyesters in all the fields of application mentioned are constant product properties at the molecular level, such as molar mass and molar mass distribution for the homo- and copolyesters, maintenance of chirality in the case of poly-L-lactic acid or comonomer ratio and comonomer distribution in the Trap the copolyester. Under technical conditions, this constancy of product properties can only be achieved with synthesis processes that can be safely controlled or with efficient additives.

Due to the equilibrium constant of the ring-chain equilibrium, high molecular weight polyesters of lactic acid can only be obtained by ring-opening polymerization of the cyclic diester of lactic acid (L, L- or D, L-3,6-dimethyl-1,4-dioxane-2,5- dion, hereinafter referred to as L, L- or D, L-dilactide). Organometallic compounds of tin are preferably used to initiate or catalyze this polymerization reaction (see, for example, J. Dahlmann, G. Rafler: Acta Polymerica 44 (1993) 103 and cited therein). Technical process proposals, carried out as bulk polymerization in the molten phase at temperatures of 185-220 ° C., relate almost exclusively to tin ll octonoate, which is said to accelerate the ring-opening polymerization particularly efficiently (US Pat. No. 5,484,881). In addition to tin II octonoate, other compounds of di- and tetravalent tin are often described as initiators or catalysts (cf. US Pat. No. 5,484,881). However, other metal compounds, such as alkoxides of zinc, lead, magnesium, titanium or zirconium, are mentioned in principle as potentially usable catalytically active substances, without however describing technical processes based on these initiators or catalysts (S. Jacobson, Ph. Degee, H.-G. Fritz, Ph. Dubois, R. Jerome: Polymer Eng. Be. 39 (1999) 131 1; WM Stevels, PJ Dijkstra, J. Feijen, TRIP 5 (1997) 300.

The choice of initiator in the ring opening polymerization is also largely determined by the substrate to be polymerized. Cyclic monoesters, such as caprolactone, or cyclic carbonates, such as 1,3-dioxan-2-one (trimethylene carbonate), are far less sensitive to the initiator than, for example, dilactide or 1,4-dioxane-2,5-dione (diglycolide) ( G. Rafler, G. Dahlmann: Acta Polymerica 43 (1992) 91; G. Rafler: Acta Polymerica 44 (1993) 168), and they can therefore be used without problems in the presence of the initiators mentioned in US Pat. No. 5,484,881 or elsewhere can be polymerized (cf. for example also A. Löfgren, A.-C. Alberson, P. Dubois, R. Jerome: Rev. Macromol. Chem. Phys. C 35 (1995) 379) if the essential further boundary conditions for this polymer formation reaction How purity of the monomers, exclusion of water and minimization of thermal loads are taken into account when carrying out the process.

With regard to the molecular weight of the polymer, tin-containing initiators, preferably the tin-ll-octonoate most commonly used in accordance with the prior art, cause a reaction profile which is difficult to master from a technical point of view, with an extremely steep increase at the start of the reaction, an undefined molecular weight maximum and a pronounced degradation of the polymer Running through the maximum (cf. E. Dahlmann, G. Rafler: Acta Polymerica 44 (1993) 107). This profile of the temporal development of the molar mass, which is unsuitable for an industrial process, is highly concentration-dependent, whereby in contrast to ionically and radically initiated polymerization processes of olefins, at least for the majority of the tin-initiated polymerizations, turnover and molar mass in the ring-opening polymerization are synchronous, i.e. H. high polymerization rate and high conversion also lead to high molecular weights. Ring-chain equilibrium and heterochain character of the polymers formed determine their molecular and thus also their deformation and application properties. Especially the

The equilibrium character of this special polymerization and the associated tendency to regress of the cyclic monomer by depolymerization is also initiated or activated by the initiator. This behavior of the initiators not only complicates the manageability of the synthesis process, but also leads to considerably disruptive depolymerizations with a corresponding reduction in the molar mass during the thermoplastic processing of the polymers. The monomer formed in the back reaction also leads to a considerably faster and uncontrollable hydrolysis of the polymer in Presence of moisture and thus an undesirable impairment of the possible uses of the polymers.

These undesirable side effects of the technically known polymerization initiators are intensified by the repeatedly described "back-biting" reaction, which leads to linear or cyclic products of lower molar mass (cf. for example HR Kricheldorf, M. Berl, N. Scharnagl: Macromolecules 21 (1988) 268). In addition to the reversible depolymerization or degradation processes of these polyesters caused by the reaction mechanism, irreversible chain cleavages due to thermal degradation reactions cannot be ruled out. These thermolysis processes lead to unspecific degradation products which remain on the polymer and, depending on the degree of this thermolysis, lead to discoloration of the polymer or even the formation of gel particles. While the reversible depolymerization with regression of the monomer or the comonomer is a function of the initiator type, initiator concentration and process temperature, the thermal degradation is determined almost exclusively by the temperature.

The cyclizing depolymerization during processing and processing is suppressed efficiently in the case of polyesters which are prepared in the presence of tin, titanium or zirconium initiators by chemical masking of the initiator by means of complexing agents. Tropolon and its derivatives are particularly suitable for tin compounds (DE-PS 195 37 365; US-PS 5 760 119). However, the technical implementation of this method is difficult because these complexing agents are only available to a limited extent and they only inhibit direct depolymerization. Tropolone does not prevent or delay thermally initiated, non-specific degradation processes.

Nonspecific thermooxidative and hydrolytic degradation reactions, preferably when these aliphatic polyesters are deformed, are inhibited by water-binding additives (hydrolysis), such as carbodiimides, activated acid derivatives or isocyanates (cf., for example, US Pat. No. 6,005,068). To inhibit degradation, the long-known phosphites (eg Ultranox RTM 626) or sterically hindered phenols are also used as antioxidants in US Pat. No. 6,005,068, preference being given to commercially available IRGANOX types. The ring opening polymerization should proceed faster in the presence of these antioxidants, and significantly higher molar masses should be achieved, as will be shown with the aid of Example 13. In addition, the polymer should be able to be stabilized against degradation reactions during extraction of monomer in vacuo, as in Example 11 shown. However, monomer regression cannot be achieved in this way when the polymer is processed: The addition of radical scavengers such as Irganox or Ultranox when remelting already polymerized samples from which the monomer had been extracted causes a new formation of monomer (see Table 13 in comparison to Table 12 of US Pat. No. 6,005,068).

Initiator combinations based on organotin and organotitanium compounds, which intervene differently in ring opening polymerization and cyclizing depolymerization, act in a very different reaction mechanistic manner, but with a very good effect. In the presence of such initiator combinations, the depolymerization can be suppressed under bulk polymerization conditions, the extreme character of the polymerization profiles largely overcome and the process can thus be made more reliable (see DE 101 13 302.2).

Based on the initiator-related difficulties of the technical

Controllability of the ring opening polymerization, the unsatisfactory constancy of the product properties of polyesters synthesized in this way and the inadequate melt stability, it is the object of this invention to propose additives and processes which allow melt-stable homo- and copolyesters which are based on cyclic esters of L. - and allow D, L-lactic acid and other cyclic monomers, in particular further cyclic esters, to be polymerized, to be produced discontinuously or continuously in differently designed plant equipment and to be processed without monomer regression. Molecularly particularly uniform products should preferably result, regardless of the polymerization conditions.

It is proposed according to the invention that the ring opening polymerization is carried out in the presence of known organotin initiators, if appropriate in the presence of further initiators and / or stabilizers based on metals of subgroup IV, in particular based on titanium or zirconium. To avoid monomer regression and thermolysis, reducing agents may be added during production, preferably when or shortly before the desired degree of polymerization is reached, but above all during subsequent thermoplastic deformation, which largely or completely suppress the reversibility of the reaction. These substances are organophosphorus additives with lower oxidation levels of phosphorus, in particular phosphorus additives based on phosphinic acids, their salts or esters or amides of the general formula (I) (Ri) (R ₂ ) P (= 0) X (I)

wherein R 1 and R _{2 are} each independently hydrogen, alkyl, aryl or heteroaryl and X is -OR ₃ or -NR1R ₂ , where R ₃ is hydrogen, alkyl, aryl, M ¹ or AM ¹¹ with M ¹ is alkali metal ion and M ¹¹ is alkaline earth metal ion and the radicals Ri and R _{2 have} the meaning given above.

According to the invention, radicals R ₂ and R ₃ or radicals R ₁ and R ₂ together with the phosphorus and optionally with the nitrogen or oxygen atom can also form a saturated or unsaturated heterocycle, for example

Ri: H, alkyl, aryl, heteroaryl or

or

In principle, all cyclic starting compounds which can be polymerized to polyesters under the influence of tin-containing polymerization catalysts or initiators are suitable for the polymerization and stabilization process according to the invention. These can be, for example, cyclic esters, in particular mono- or diesters, such as dilactide or caprolactone. With regard to their chemical structure, their number and their quantitative proportions, these can be used in any way and may contain further components. The procedure according to the invention not only allows the ring opening polymerization to be reliably controlled by the adjustability of a stable molar mass level (cf.example 3 and FIG. 3), but also leads to a polymer of high thermal stability under production and processing conditions and defined narrow polydispersity (expressed by the ratio of the weight and number average molar masses M _w / M _n ). Both the reaction profiles which are difficult to reproduce under technical conditions and which are typical for tin ll-octonoate (Sn (oct) ₂ ) and which have a pronounced extreme character when the molar mass changes over time (cf. Example 2 and FIG. 2) and the regression of disruptive monomers as a result Thermal stress during work-up and thermoplastic deformation are avoided when using the initiator / stabilizer system according to the invention.

The average molar masses of the polymers in Examples 2-11 listed were extracted by gel chromatography in tetrahydrofuran and dried

Polymer samples determined. The gel chromatographic separation is carried out on Styragel with simultaneous determination of the concentration (refractive index) and molar mass (scattered light photometry) of the individual polymer fractions. This enables a very precise direct determination of the molar mass, which is clearly superior to the methods still frequently used with calibration substances of known molar mass for the calibration of the methodology or relative methods such as solution viscometry. The simultaneous determination of both mean values of the molar mass also allows a very exact determination of the molecular non-uniformity of the polymers on the basis of the quotient of the two mean values of the molar mass (M _w / M _n in Table 2). Molecular inconsistency is an essential product parameter for a polymer material, since at the molecular level, in addition to the molar mass, it decisively determines the application-relevant polymer properties. The combination of these polymer properties and the morphological properties of the solid polymer result in the deformation and material properties of a plastic. Here too, the superiority of the procedure according to the invention over the known prior art is evident. Not only is the technological controllability of the ring opening polymerization significantly improved, but also molecularly uniform products are obtained regardless of the process duration.

The stabilization of the polyester not only prevents the depolymerization due to equilibrium (avoidance of the extreme character in M _n , = f (t)) and the interchain exchange (M _w / M _n = f (t)) under synthesis conditions, but it also minimizes the annoying monomer regression in the thermoplastic processing of these aliphatic polyesters (see Table 3 in Example 11).

The stabilization of the molecular weight according to the invention by organophosphorus additives based on phosphinates can be carried out both in batch production, e.g. in stirred reactors or kneaders, as well as in continuous processes in vertical or horizontal reactors. Reactive extrusion processes in co-rotating twin-screw extruders are particularly efficient, in which the dosing of the melt stabilizer is particularly simple and the homogeneous distribution of the additive in the highly viscous polyester melt poses no difficulties. In the case of batchwise production, the additive is preferably incorporated at a point in time at which the reaction has reached the desired degree of conversion. In continuous production, the additive is preferably metered in at a location where the polymer is short of leaving the reactor, e.g. shortly before the discharge zone of a (screw) extruder.

Regardless of the process concept or type of plant, the additives according to the invention can be metered in either directly as a pure substance, in solution or in the form of a masterbatch with the polymer or else with the monomer.

According to the invention, the initiator / stabilizer system is also used for the synthesis of statistical and non-statistical binary or ternary copolymers by ring-opening polymerization. The statistical copolyesters are produced by simultaneous addition (discontinuous) or metering (continuous) of the monomeric esters or diesters. Non-statistical copolyesters are obtained with the gradual addition of comonomer or preferably by reactive compounding of the homopolyesters in reactors of high mixing intensity, such as kneaders or twin-screw extruders.

Due to the homogeneous kinetic nature of the ring opening polymerization of the cyclic esters and diesters, the choice of initiators is primarily determined by their solubility in the monomer or polymer melt and their compatibility with the selected melt stabilizer. Suitable tin compounds for the initiator / stabilizer system are, for example, tin ll carboxylates, tin IV alkoxides, dialkoxy tin oxides, trialkoxy tin hydroxides and tin IV aryls. Initiator combinations of tin with organo-soluble titanium or zirconium compounds can also be used. Suitable for this combination are, for example Alkoxides of titanium and zirconium, such as titanium IV acetylacetonate, zirconium octonoate or zirconium acetylacetonate.

The concentration of the initiator / stabilizer system according to the invention can be freely selected within wide limits, but the stabilizer must be used at least equimolar to the initiator. Otherwise, the concentration of initiator and stabilizer depends primarily on the technological requirements of the system and the application-specific material requirements, preferably material and deformation properties, which are largely determined by the molar mass and its distribution. The preferred concentration range for the

Polymerization initiator is 10 ^"5 - 10 ^{" 3} mol / mol monomer unit; the stabilizer is used in the stabilizer / initiator ratio of 2: 1 to 10: 1, preferably in concentrations of 0.01-0.1 mass%.

Above the melting temperature of the polymer, the polymerization temperature can also be varied within a relatively wide range. Temperatures of 180 ° C-225 ° C can be selected for the polymerization of the L, L-dilactide without disruptive degradation reactions. Due to the lower softening temperature, lower polymerization temperatures, starting at 125 ° C., can also be used for the polymerization of the D, L-dilactide. Also for the polymerization of other cyclic esters, such as caprolactone, 1,3-dioxan-2-one (trimethylene carbonate), 1,4-dioxan-2,5-dione (diglycolide) or 1,4-dioxan-2-one (Glycol ester of acetic acid), the reaction temperatures above the polymer melting temperature can be freely selected in a wide range in the presence of the initiator / stabilizer system according to the invention. Recommended polymerization temperatures are for caprolactone 130 ° C-200 ° C, 1, 3-dioxan-2-one 130 ° C-200 ° C, 1, 4-dioxane-2,5-dione 225 ° C-250 ° C and for 1,4-dioxan-2-one 120 ° C-180 ° C.

The invention will be explained in more detail below with the aid of examples.

Examples

example 1

The model studies on the degradation behavior of the polylactides were carried out in the aqueous phase. For this purpose, pressed test specimens measuring 10 * 10 * 1 mm were stored in phosphate-buffered solution and the mass was determined gravimetrically after drying and the molar mass was determined by gel chromatography after different times. The monomer content was determined for the poly-D, L-lactides by falling over from dimethylformamide / methanol and for the poly-L-lactides by extraction with methanol. 1 shows the in vitro degradation of poly-D, L-lactide as a function of the monomer content at 37 ° C.

The rate of hydrolytic degradation of amorphous poly-L-lactide obtained by melt quenching corresponds to that of racemate (Table 1).

Table 1 : . Rate constants of the hydrolytic degradation of amorphous poly-L- and poly-D, L-lactides (in vitro conditions)

Example 2

(Comparative examples without melt stabilizer with different types of stirrers)

72 g of L, L-dilactide (0.5 mol), purified by recrystallization and carefully dried, are melted in a cylindrical glass reactor using a cross bar stirrer or a screw stirrer under inert gas. When the desired temperature is reached, the initiator Sn (oct) _{2 is} added to the stirred monomer melt in the form of a 0.1% solution in toluene. To determine the course of the polymerization, samples are taken from the polymerizing melt, of which the mass (for the monomer conversion) and molar mass are determined by extraction or falling over after appropriate sample preparation. The extraction is carried out with methanol in a Soxhiet apparatus; for reprecipitation, the sample is dissolved in dimethylformamide and the polymer precipitated in methanol. Turnover (gravimetric) and molar mass (gel chromatography) are determined from the dried polymer samples. 2 shows the polymerization of L, L-dilactide in the presence of 7.5 * 10 ^"5 mol / mol Sn (oct) _2. The molar mass-time courses shown for the polymerization of the L, L-dilactide in Dependence on the mixing.

Example 3

72 g of L, L-dilactide (0.5 mol) cleaned and carefully dried by recrystallization are melted in a cylindrical glass reactor with a screw stirrer under inert gas and polymerized and worked up analogously to Example 2. The polymerization is carried out in the presence of 7.5 * 10 ^"5 mol / mol Sn (oct) ₂ as initiator, and 0.01% 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (UKANOL DOP 3) are added as soon as the desired degree of polymerization is approximately reached, and mixing is carried out in a screw stirrer at 195 ° C. The course of polymerization shown in FIG.

Example 4

Analogously to Example 3, L, L-dilactide is polymerized in the presence of 5 * 10 ^"5 mol / mol Sn (oct) ₂ at 195 ° C and with 0.01% by mass 9,10-dihydro-9-oxa-10- phosphaphenanthrene-10-oxide The course with a reduced initiator concentration is shown in Fig. 4. Stabilized poly-L-lactide has a high molecular uniformity, as shown in Table 2.

Table 2 Polydispersity of melt-stable poly-L-lactides as a function of the polymerization time

Example 5

72 g of D, L-dilactide (0.5 mol) cleaned and carefully dried by recrystallization are melted in a cylindrical glass reactor with a screw stirrer under inert gas and polymerized analogously to Example 3. The polymerization is in

Presence of 7.5 * 10 ^"5 mol / mol Sn (oct) ₂ as initiator and 0.01 mass% 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (UKANOL DOP). Die The polymer samples are worked up by falling over from dimethylformamide / methanol. After a polymerization time of 20 min, a poly-D, L-lactide with a number-average molar mass of M _π = 98,000 g / mol with a polydispersity of M _w / M _π = 2, 0 received.

Example 6

3600 g of L, L-dilactide (50 mol) cleaned and carefully dried by recrystallization are melted in a horizontal kneader with a discharge screw under inert gas. The kneader is equipped with a torque measurement to track the course of the polymerization. When the target temperature of 195 ° C. has been reached, the mixed monomer melt becomes the initiator Sn (oct) ₂ (5 * 10 ^"5 mol / mol in the form of a 0.1% solution in toluene) and after a further 7.5 min 0.36 g of the melt stabilizer 9,10-dihydro-9-oxa-10-phosphaphen-anthrene-10-oxide (UKANOL DOP) are added and the melt is thoroughly mixed in the kneader in a closed system at 195 ° C. for 25 minutes the polymer melt is discharged via the screw, cooled on a conveyor belt by blowing with cold air and granulated by means of a strand pelletizer. The polymer pellet is extracted with methanol and then dried in vacuo. After working up the samples, 3350 g of poly-L-lactide with a number-average Molar mass of M _n = 85,000 g / mol, a melting point of 174 ° C and an optical rotation of [α] ²⁰ = -156.2 ° obtained.

Example 7

1000 g of L, L-dilactide cleaned and carefully dried by distillation are premixed with 0.15 g of Sn (oct) ₂ and, with the exclusion of moisture and air, a twin-screw extruder (type Leistritz Micro 18) with a high proportion of kneading elements in the variably assembled screws supplied

(Transport elements / kneading elements = 4/1; LJD = 35; 7 heating zones). The melt stabilizer 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (UKANOL DOP) is continuously the polymer melt before the discharge zone of the extruder fed. With a temperature profile set via the extruder, starting at 100 ° C at the extruder feed, 190 ° C in the middle zones and 180 ° C at the outlet, and a speed of 100 min ^"1 , the average residence time of the polymerizing lactide melt in the extruder is approx. 10 The polymer melt is cooled on a conveyor belt by blowing with cold air and granulated by means of a strand granulator. The polymer granulate is extracted with methanol and then dried in vacuo. The poly-L-lactide obtained in a continuous manner by reactive extrusion has an average molecular weight of M. _n = 93,000 g / mol and the yield is 96.5%.

Example 8

Analogously to Example 3, 72 g of L, L-dilactide (0.5 mol) purified and carefully dried by recrystallization are melted in a cylindrical glass reactor with a screw stirrer under inert gas. The stirred monomer melt becomes 0.08 g (10 ^"4 mol / mol) of a reaction product of dibutyltin oxide (0.1 mol), titanium tetrabutylate (0.2 mol) and n-butanol when the target temperature of 200 ° C is reached (0.2 mol) and 0.0216 g (2 * 10 ^"4 mol / mol) 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide added. After 20 minutes, the polymerization is terminated by cooling and the polymer material is aftertreated by extraction with methanol to remove the monomers and vacuum drying. 65 g of poly-L-lactide with an average molar mass of M _n = 95,000 g / mol are obtained.

Example 9

Analogously to Example 8, 72 g of L, L-dilactide (0.5 mol) purified and carefully dried by recrystallization are polymerized and worked up. However, the melt is stabilized with 0.0332 g (2 * 10 ^"4 mol / mol) of 2-methyl-2- (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) -succinic acid 68 g of poly-L-lactide with an average molar mass of M _n = 89,000 g / mol are obtained.

Example 10

2700 g of L, L-dilactide cleaned and carefully dried by recrystallization are melted together with 1425 g of caprolactone (total amount: 50 mol) in a horizontal kneader with a discharge screw under inert gas. When the target temperature of 175 ° C. has been reached, the mixed monomer melt becomes 1.515 g (7.5 ^* 10 ^"5 mol / mol) Sn (oct) ₂ in the form of a 0.1% solution in toluene and after another 5 min 0.4125 g of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide was added. The melt is thoroughly mixed in the kneader at 175 ° C for 45 min. The poly (L-lactide (75) -co-caprolactone (25)) is demonomerized by falling over from dimethylformamide / water. After drying at 80 ° C. in vacuo, 3700 g of copolymer with an average molecular weight of 112,000 g / mol are obtained.

Example 11

Melt-stabilized polylactide produced in accordance with Example 3-9 is extracted exhaustively with methanol and, after drying to constant weight (residual moisture <0.02%), processed into test bars (shoulder bar, isostab) in an injection molding machine (type ARBURG Allrounder 270 M). The recovered monomer is determined gravimetrically from the test bars by extraction with methanol (Table 3).

Table 3 Extract content of melt-stabilized poly-L-lactides before and after injection molding

Claims

Expectations:

1. A process for the preparation of a homo- or copolyester which is obtainable from at least one corresponding cyclic monomer, the polymerization of the starting compound (s) in the presence of an initiator selected from organic tin compounds and tin carboxylates and tin alkoxides of oxidation levels II or IV, which may be can also contain hydroxyl groups, and optionally in the presence of organo-soluble metal compounds of subgroup IV, in particular titanium and / or zirconium compounds, characterized in that the mixture at the latest when the desired degree of polymerization is reached has a Phosphinic acid and / or a phosphine derivative of the formula (I)

(Rι) ( ₂ ) P (= 0) X (I)

is added, wherein R- ^* and R _{2 are} each independently hydrogen, alkyl, aryl or heteroaryl and X is -OR ₃ or -NRιR ₂ , where R ₃ is hydrogen, alkyl, aryl, M ¹ or M ¹¹ with M ¹ is equal to alkali metal ion and M ^{11 is} equal to alkaline earth metal ion and the radicals R ₁ and R _{2 have} the meaning given above.

2. The method according to claim 1, wherein the radicals R ₂ and R ₃ or the radicals Ri and R _{2 of} the formula I together with the phosphorus and optionally with the nitrogen or oxygen atom form a saturated or unsaturated heterocycle, and in particular formula I. has the following meaning:

3. The method according to any one of claims 1 or 2, characterized in that the molar ratio of initiator to the phosphinic acid and / or the phosphine derivative of the formula (I) is 1: 1 to 10: 1, preferably about 2: 1.

4. The method according to any one of the preceding claims, characterized in that the phosphinic acid and / or the phosphine derivative of the formula (I) is selected from the group of alkyl, dialkyl, aryl, diaryl or alkylarylphosphinic acids.

5. The method according to claim 4, characterized in that 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide is used as arylphosphinate, or that 2-methyl-2- (9,10-dihydro- 9-oxa-10-phosphaphenanthrene-10-oxide) succinic acid is used.

6. The method according to any one of the preceding claims, characterized in that the starting compound (s) is / are selected from cyclic esters, in particular from ε-caprolactone, 1,3-dioxan-2-one (trimethylene carbonate) and 1,4-dioxane -2-one (glycol ester of acetic acid), and cyclic diesters, in particular 1,4-dioxan-2,5-dione (diglycolide) and L, L-, D, L- or meso-3,6-dimethyl-1, 4-dioxane-2,5-dione (Dilactide), and mixtures thereof.

7. The method according to any one of the preceding claims, characterized in that the compound with the formula (I) is only added when the polymerization reaction is essentially complete.

8. The method according to any one of the preceding claims, characterized in that the compound with the formula (I) is added as a pure substance, in solution or in the form of a masterbatch.

9. The method according to any one of the preceding claims, which is a continuous process which takes place in an extruder, characterized in that the compound having the formula (I) is fed to the extruder shortly before the discharge zone.

10. Process for stabilizing the melt of a homo- or copolyester which is obtainable from at least one corresponding cyclic monomer, the polymerization of the starting compound (s) in the presence of an initiator selected from organic tin compounds and tin carboxylates and tin alkoxides of oxidation states II or IV, the possibly may also contain hydroxyl groups, optionally in the presence of organo-soluble metal compounds of subgroup IV, in particular titanium and / or zirconium compounds, characterized in that the melt comprises a phosphinic acid and / or a phosphine derivative of the formula (I)

wherein R-ι and R _{2 are} each independently hydrogen, alkyl, aryl or heteroaryl and X is -OR ₃ or -NRιR ₂ , where R ₃ is hydrogen, alkyl, aryl, M ¹ or Vz M ¹¹ with M ¹ is alkali metal ion and M ¹ 'equal

Alkaline earth metal ion and the radicals R-ι and R _{2 have} the meaning given above.

11. The method according to claim 10, wherein the radicals R ₂ and R ₃ or the radicals R-ι and R _{2 of} the formula I together with the phosphorus and optionally with the nitrogen or

Oxygen atom form a saturated or unsaturated heterocycle, and in particular formula I has the following meaning:

12. The method according to any one of claims 10 or 11, characterized in that the molar ratio of initiator to the phosphinic acid and / or

Phosphine derivative of the formula (I) is 1: 1 to 10: 1, preferably about 2: 1.

13. The method according to any one of claims 10 to 12, characterized in that the phosphinic acid and / or the phosphine derivative of the formula (I) is selected from the group of alkyl, dialkyl, aryl, diaryl or alkylarylphosphinic acids.

14. The method according to claim 13, characterized in that 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide is used as arylphosphinate, or that 2-methyl-2- (9,10-dihydro- 9-oxa-10-phosphaphenanthrene-10-oxide) succinic acid is used.

15. The method according to any one of claims 10 to 14, characterized in that the starting compound (s) is / are selected from the cyclic esters esters ε-caprolactone, 1,3-dioxan-2-one (trimethylene carbonate) and 1,4- Dioxan-2-one (glycol ester of acetic acid) or the cyclic diesters 1, 4-dioxane-2,5-dione (diglycolide) and L, L-, D, L- or meso-3,6-dimethyl-1, 4 -dioxane-2,5-dione (dilactide), and. Mixtures of these.