Production of meso-lactide, D-lactide, and L-lactide by back biting of polylactide

Abstract

Process for increasingly producing D-Lactide and meso lactide by depolymerizing by back biting polylactide (PLA) said process which comprises: (i) Depolymerizing polylactide into its corresponding dimeric cyclic esters by heating the polylactide in the presence of a catalyst system comprising a catalyst and a co-catalyst in a reaction zone at temperature and pressure at which the polylactide is molten; (ii) Forming a vapor product stream from the reaction zone; (iii) Removing the vapor product stream and optionally condense it; (iv) Recovering, either together or separately meso-lactide, D-lactide and L-lactide.

Claims

1. Process for increasingly producing D-Lactide and meso lactide by depolymerizing by back biting polylactide (PLA) said process which comprises: (i) Depolymerizing polylactide into its corresponding dimeric cyclic esters by heating the polylactide in the presence of a catalyst system which comprises MgO as catalyst and a cycloalkylalkoxysilane represented by the general formula QQ′Si(O-alkyle).sub.2, where the Q and Q′ are different and are alkyl or cycloalkyl radical containing from 1 to 8 carbon atoms as cocatalyst, or wherein the catalyst system comprises MgO as catalyst and cyclohexylmethyldimethoxysilane as cocatalyst in a reaction zone at temperature and pressure at which the polylactide is molten; (ii) Forming a vapor product stream from the reaction zone; (iii) Removing the vapor product stream and optionally condense it; (iv) Recovering, either together or separately meso-lactide, D-lactide and L-lactide.

2. Process according to claim 1, wherein the catalyst of the catalyst system is used in an amount comprised between 0.05 and 3% by weight of PLA and the co-catalyst is used in an amount comprised between 0.1 and 10% by weight of PLA.

3. Process according to claim 1, wherein the depolymerization is carried out at a temperature between 200 and 290° C. and at a pressure under lactide vapour pressure.

4. Process of mixing D-lactide and meso lactide obtained according to claim 1, with L-lactide to prepare a copolymer of P(L-D)LA by ring opening polymerization, the content of enantiomer D- of said copolymer not exceeding 35% by weight.

5. Process according to claim 3, wherein the depolymerization is carried out at a temperature between 210 and 260° C. and at a pressure under lactide vapour pressure.

Description

EXAMPLES

(1) The back biting of various PLA samples, all having an enantiomer L content of 99.4% by weight has been carried out.

(2) First the samples were ground and then deposited into a recipient (the reaction zone) which was introduced into the reactor. Then the catalyst and co-catalysts were added in the reaction zone. The Sn octanoate (Sn (oct).sub.2) used as catalyst was added in an amount of 1% by weight of the PLA. Regarding the co-catalyst, when fumaric acid was used (pKa.sub.1=3.03; pKa.sub.2=4.44), it was added in an amount of 5% by weight of the PLA. When triphenylphosphine was used as co-catalyst, it was added in an amount of 0.7% by weight of the PLA.

(3) The temperature was then raised up to 250° C. and maintained during 60 minutes.

(4) The pressure was adjusted to 10 millibar.

(5) During the period of time the reaction mixture was maintained at said temperature, a vapor product was formed and further extracted from the reaction zone while the vapor product was then subject to a condensation step.

(6) The condensed product was recovered and analysed by gas chromatography (GC) to determine its constituents and their respective contents in % by weight in L-lactide, D-lactide and meso-lactide. The lactide yield (%) represents the quantity of lactide recovered and condensed.

(7) The results are presented here below.

(8) TABLE-US-00001 Meso- lactide Lactide Impurities Ex. Catalyst Co-Cata yield (%) L-lactide (%) D-lactide (%) (%) (%) 1 Sn (oct).sub.2 None 90.7 83.3 1.7 14.0 1.0 2 Sn (oct).sub.2 Fumaric Acid 65.0 68.0 6.0 25.0 1.0 3 Sn (oct).sub.2 TPP 83.8 68 8.5 22.3 0.2

(9) A comparative example was conducted under the same conditions as those described for the examples according to the invention excepted that the sulfamic acid having a pKa of 0.99 was used as co-catalyst at 5.3% by weight of the PLA.

(10) TABLE-US-00002 Meso lactide L-lactide D-lactide lactide Impurities Ex. Catalyst Co-Cata Yield (%) (%) (%) (%) (%) 4 Sn (oct).sub.2 Sulfamic Acid 1.1 95 None 0.9 4.1

(11) Other examples within the process of the invention were conducted under the same operating conditions as those described above excepted that another catalyst than Sn octanoate and cocatalyst were used. MgO was used as catalyst in an amount of 1% by weight of the PLA and lactic acid (pKa: 3.86) was used as cocatalyst, it was added in an amount of 5% by weight of the PLA (see example 5). In example 6, MgO was used as catalyst in an amount of 1% by weight of the PLA and cyclohexylmethyldimethoxysilane was used as cocatalyst in an amount of 5% by weight of the PLA. In example 7, the same example was conducted without any cocatalyst (comparative example).

(12) TABLE-US-00003 D- Meso lactide L-lactide lactide lactide Impurities Ex. Catalyst Co-Cata Yield (%) (%) (%) (%) (%) 5 MgO Lactic Acid 81.0 70.3 11.0 18.2 0.5 6 MgO cyclohexylmethyl 90.0 37.4 30.7 28.5 4.4 dimethoxysilane 7 MgO None 60.9 92.3 1.4 6.1 0.2