Method to manufacture PLA using a new polymerization catalyst

Abstract

The invention relates to a method for manufacturing polylactide, comprising the steps of mixing lactide and a metal-coordination compound as polymerization catalyst to obtain a reaction mixture, polymerizing the lactide in liquid phase at a temperature of at least 150 C. to form polylactide in liquid phase and allowing the polylactide to solidify, characterized in that the polymerization catalyst comprises a metal-ligand coordination compound whereby the parent ligand answers the formula (I), whereby R represents an H atom, an aliphatic group, a halide atom or a nitro group and the metal is at least one of Zr and Hf. The invented catalysts show kinetics which is comparable to the kinetics of the known Sn-octoate catalyst. ##STR00001##

Claims

1. A method for manufacturing polylactide, comprising the steps of mixing lactide and a metal-coordination compound as polymerization catalyst to obtain a reaction mixture, polymerizing the lactide at a temperature of at least 150 C. to form polylactide in liquid phase and allowing the polylactide to solidify, characterized in that the polymerization catalyst comprises a metal-ligand coordination compound whereby the parent ligand answers the formula (I), ##STR00004## whereby R represents an H atom, an aliphatic group, a halide atom or a nitro group, the metal is at least one of Zr and Hf, and the amount of metal originating from the catalyst ranges between 1 and 2000 ppm of the reaction mixture and the racemization rate of the lactoyl units within the polylactide during the method of manufacture is less than 2%.

2. The method according to claim 1, characterized in that the R group is a methyl group.

3. The method according to claim 1, characterized in that the metal is Zr.

4. The method according to claim 1, characterized in that a co-initiator is added to the reaction mixture.

5. The method according to claim 1, characterized in that the temperature of the liquid phase ranges between 160 C. and 220 C.

6. The method according to claim 1, characterized in that the liquid phase is subjected to a devolatilization step before solidifying the formed polylactide.

7. The method according to claim 1, characterized in that a catalyst deactivating agent is added to the liquid phase when at least 90% of the lactide is converted into polylactide.

8. A method for manufacturing polylactide, comprising the steps of mixing lactide and a metal-coordination compound as polymerization catalyst to obtain a reaction mixture, polymerizing the lactide at a temperature of at least 150 C. to form polylactide in liquid phase and allowing the polylactide to solidify, characterized in that the polymerization catalyst comprises a metal-ligand coordination compound whereby the parent ligand answers the formula (I), ##STR00005## whereby R represents an H atom, an aliphatic group, a halide atom or a nitro group, the metal is at least one of Zr and Hf and, wherein the metal coordination compound is represented by structure (II) ##STR00006##

9. Polylactide containing a Zr-containing compound, characterized in that the amount of Zr metal originating from the compound is 1-2000 ppm and the racemization rate of the lactoyl units within the polylactide during its manufacture is less than 2%.

10. The method according to claim 2, characterized in that the metal is Zr.

11. The method according to claim 2, characterized in that a co-initiator is added to the reaction mixture.

12. The method according to claim 3, characterized in that a co-initiator is added to the reaction mixture.

13. The method according to claim 2, characterized in that the temperature of the liquid phase ranges between 160 C. and 220 C.

14. The method according to claim 3, characterized in that the temperature of the liquid phase ranges between 160 C. and 220 C.

15. The method according to claim 2, characterized in that the liquid phase is subjected to a devolatilization step before solidifying the formed polylactide.

16. The method according to claim 3, characterized in that the liquid phase is subjected to a devolatilization step before solidifying the formed polylactide.

17. The method according to claim 2, characterized in that a catalyst deactivating agent is added to the liquid phase when at least 90% of the lactide is converted into polylactide.

18. The polylactide according to claim 9, wherein the amount of Zr metal originating from the Zr-containing compound is 1-1000 ppm.

19. The method according to claim 1, wherein the metal is at least one of Zr and Hf, and the amount of metal originating from the catalyst ranges between 10 and 1000 ppm of the reaction mixture.

Description

BRIEF DESCRIPTION OF THE INVENTION

(1) These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

(2) In the drawings:

(3) FIG. 1 shows the reaction sequence used in the preparation of the ligand tris(-3,5-dimethyl-2-hydroxybenzyl)amine,

(4) FIG. 2 shows several conversion curves indicating the reaction kinetics of the polylactide manufacture using the new catalyst system according to the present invention,

(5) FIG. 3 shows several additional conversion curves indicating the reaction kinetics of the polylactide manufacture using the new catalyst system according to the present invention, and

(6) FIG. 4 shows a conversion curve indicating the reaction kinetics of the polylactide manufacture using a catalyst system not according to the present invention.

DETAILED EMBODIMENTS OF THE INVENTION

(7) Methods of Analysis.

(8) Absolute molecular weights were determined using gel permeation chromatography (GPC) measurements in hexafluoroisopropanol (HFIP) using a triple detection system (Viscotek GPC Max VE2001), equipped with a light scattering detector, viscosity detector and refractive index detector. Relative molecular weights reported were measured using chloroform as the eluent, a light scattering detector (LALLS) and against narrowly disperse polystyrene standards.

(9) The stereochemical purity of the polymers was determined by a destructive method of derivatization to R- and S-methyllactates using an ion-exchange resin. The ratio of R- and S-lactates is subsequently detected using Gas Chromatography.

(10) Residual lactide levels are detected by a HPLC method after precipitation of the PLA fraction. To the person skilled in the art it is however evident that many other techniques can be used to determine the amount of lactide in PLA, for example FTIR, n-IR and .sup.1H-NMR.

(11) Catalyst Manufacture

(12) The ligands are manufactured according to the reaction sequence shown in FIG. 1. In this manufacture, hexamethylenetetramine (0.94 g, 6.66 mmol) is added to a mixture of 2,4 di-substituted phenol (80 mmol) and paraformaldehyde (3.00 g, 100 mmol). The solution is then refluxed for 48 hours and the resulting white powder recrystallized from methanol and ether.

(13) The Hf- and Zr-coordination compounds for use in the invented method are manufactured essentially according to the Experimental section of the article Isolation and characterisation of transition and main group metal complexes supported by hydrogen-bonded zwitterionic polyphenolic ligands in Chem. Commun., 2003, 1832-1833. In general, one reacts the metal isopropoxidefor example, Zr(OiPr)4.HOiPrin equimolar amounts with the ligand at room temperature for two hours and the product is obtained after (re)crystallization. Adaptation of the amounts of compounds, for example for producing the hafnium compounds, is well within the daily routine of persons skilled in this field of technology. The Hf- and Zr-coordination compounds obtained after recrystallization from hot toluene were used in the polymerization experiments. It has been demonstrated that the recrystallized compounds have excellent air stability.

(14) Polylactide Manufacture

Example 1

(15) In a 1 L stainless steel batch reactor, 500 g L-lactide (PuraLact L, Purac) was molten under nitrogen atmosphere and heated to 130 C.; a lactide melt sample of about 10 g was withdrawn for feed material analysis. Upon reaching 130 C., 0.15 g of Zr-catalyst complex II or 308 ppm was transferred into the reactor as a powder. The polymerizing melt was allowed to heat to 180 C. and the polymerization proceeded for 5 hours, while samples were taken after set time intervals to determine kinetics and the evolution of molecular weight. The absolute M.sub.w was determined to be 94 kg/mol at a conversion of 71%. M.sub.w versus PS was 256 kg/mol. The optical purity of the polymer was 99.4% L.

Example 2

(16) A polymerization was performed according to the procedure mentioned in Example 1, but the amount of Zr-catalyst complex II employed was 0.33 g or 676 ppm. The absolute M.sub.w of the final PLA was determined to be 167 kg/mol at a conversion of 93%. M.sub.w versus PS was 358 kg/mol. The optical purity of the polymer was 99.2% L.

Example 3

(17) Another polymerization was performed according to the procedure mentioned in Example 1, but the amount of Zr-catalyst complex II employed was 0.66 g or 1345 ppm. The absolute M.sub.w of the final PLA was determined to be 134 kg/mol at a conversion of 96%. M.sub.w versus PS was 276 kg/mol. The optical purity of the polymer was 99.0% L.

Example 4

(18) In a 1 L stainless steel batch reactor, 500 g L-lactide (PuraLact L, Purac) was molten under nitrogen atmosphere and heated to 130 C.; a lactide melt sample of about 10 g was withdrawn for feed material analysis. Upon reaching 130 C., 0.36 g 1-hexanol or 0.07 wt % was added as co-initiator. Next, 0.22 g Hf(.sup.tBuL) OiPr. HOiPr or 450 ppm was transferred into the reactor as a powder. The polymerizing melt was allowed to heat to 180 C. and the polymerization proceeded for 4 hours, while samples were taken after set time intervals to determine kinetics and the evolution of molecular weight. The M.sub.w of the final PLA was determined versus polystyrene standards to be 64 kg/mol at a conversion of 62%. The optical purity of the polymer was 99.2% L.

Example 5

(19) In a 1 L stainless steel batch reactor, 500 g L-lactide (PuraLact L, Purac) was molten under nitrogen atmosphere and heated to 130 C.; a lactide melt sample of about 10 g was withdrawn for feed material analysis. Upon reaching 130 C., 0.37 g 1-hexanol or 0.08 wt % was added as co-initiator. Next, 0.32 g of Zr-catalyst complex II or 640 ppm was transferred into the reactor as a powder. The polymerizing melt was allowed to heat to 180 C. and the polymerization proceeded for 5 hours, while samples were taken after set time intervals to determine kinetics and the evolution of molecular weight. The absolute M.sub.w of the final PLA was determined to be 114 kg/mol at a conversion of 95%. M.sub.w versus PS was 234 kg/mol. The optical purity of the polymer was 99.5% L.

Example 6

(20) A polymerization was performed according to the procedure mentioned in Example 4, but the amount of co-initiator 1-hexanol employed was 0.72 g or 0.15 wt %. The absolute M.sub.w of the final PLA was determined to be 88 kg/mol at a conversion of 96%. M.sub.w versus PS was 182 kg/mol. The optical purity of the polymer was 99.6% L.

Example 7

(21) A polymerization was performed according to the procedure mentioned in Example 4, but the amount of co-initiator 1-hexanol employed was 3.54 g or 0.73 wt %. The absolute M.sub.w of the final PLA was determined to be 24 kg/mol at a conversion of 96%. M.sub.w versus PS was 47 kg/mol. The optical purity of the polymer was 99.8% L.

Comparative Example 1

(22) In a 1 L stainless steel batch reactor, 500 g L-lactide (PuraLact L, Purac) was molten under nitrogen atmosphere and heated to 130 C.; a lactide melt sample of about 10 g was withdrawn for feed material analysis. Upon reaching 130 C., 0.4 g 1-hexanol or 0.08 wt % was added as co-initiator. Next, 0.15 g tin octoate (Sn(C.sub.8H.sub.15O.sub.2).sub.2) or 300 ppm was transferred into the reactor as a powder. The polymerizing melt was allowed to heat to 180 C. and the polymerization proceeded for three hours, while samples were taken after set time intervals to determine kinetics and the evolution of molecular weight. The M.sub.w of the final PLA versus polystyrene was determined to be 242 kg/mol at a conversion of 96%.

(23) FIG. 2 shows a number of typical curves indicative of the reaction kinetics of the polylactide manufacture using the new catalyst system of Examples 1-4 according to the present invention. More particularly, this Figure shows the concentration c (in weight percentage) of lactide in the polymerization mixture as a function on time (t) at a reaction temperature of 180 C., all based on a series of analyses as described above. From these data it can be concluded that the dosing level of the catalyst determines the polymerization rate: the higher the loading level, the faster polymerization occurs. It is clear that within the chosen range of hundreds of ppm catalyst, high conversions in a matter of hours can be achieved.

(24) FIG. 3 shows an additional number of typical curves indicative of the reaction kinetics of the polylactide manufacture using the new catalyst system of Examples 2, 5, and 7 according to the present invention. More particularly, this Figure shows the concentration c (in weight percentage) of lactide in the polymerizing mixture as a function on time t (in minutes) at a reaction temperature of 180 C., all based on a series of analyses as described above. From these data it can be concluded that the use of a co-initiator further increases the polymerization rate. The higher the co-initiator loading, the higher polymerization rates are observed).

(25) From Table 1, it can also be concluded that the co-initiator may be used to control the molecular weight of the polylactide. Molecular weights can be reached that provide access to most polymer applications.

(26) TABLE-US-00001 TABLE 1 Amount of co-initiator M.sub.w Example (wt %) (relative to PS, kg/mol) 2 0 358 5 0.08 234 6 0.15 182 7 0.73 47

(27) FIG. 4 shows a typical polymerization conversion curve for a 300 ppm tin octoate catalyzed polymerization, according to the Comparative Example 1. From combined FIGS. 3 and 4 it is concluded that the new Zr-catalysts described in this application show kinetics which is comparable to the kinetics of the known Sn-octoate catalyst.

(28) While the invention has been illustrated and described in detail in the foregoing description, such description is to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments and experiments. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the disclosure and the appended claims.

(29) In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.