CH484099A

CH484099A - Process for the preparation of optionally substituted glycolides

Info

Publication number: CH484099A
Application number: CH1632766A
Authority: CH
Inventors: Peter Dr Andratschke; Christoph Dr Boxan; Walter Prof Dr Voss; Wilfried Dr Vetter
Original assignee: Wolfen Filmfab Veb
Priority date: 1965-12-18
Filing date: 1966-11-14
Publication date: 1970-01-15
Also published as: DD53074A1; AT266068B

Description

  

  



  Verfahren zur Herstellung gegebenenfalls substituierter Glycolide
Die Erfindung betrifft ein Verfahren zur Herstellung von gegebenenfalls substituierter Glycolide mit Säurezahlen unter 10, die z. B. als Ausgangsstoffe für Polymere hohen Molekulargewichts verwendet werden.



   Es ist bekannt, technisch brauchbare Polymere aliphatischer a-Hydroxycarbonsäuren durch katalytische Polymerisation der cyclischen Esteranhydride dieser Säuren herzustellen, z. B. aus Glycolid   (1)    oder Lactid   (11)   
EMI1.1     

Die Gewinnung dieser cyclischen Monomeren erfolgt gewöhnlich durch einen zweistufigen Prozess. In der ersten Stufe werden die freien Säuren durch Erhitzen zu polymeren Säuren mit niedrigem Polymerisationsgrad von etwa 7 polykondensiert. In der zweiten Stufe werden diese Polymere bei höheren Temperaturen im Vakuum wieder depolymerisiert, wobei die gebildeten cyclischen Esteranhydride abdestillieren.



  Beide Stufen können katalytisch beeinflusst werden. Die cyclischen Esteranhydride sind bei Raumtemperatur kristalline Substanzen. Sie befinden sich nach der zweiten Stufe im Gemisch mit ebenfalls überdestillierten oligomeren Hydroxycarbonsäuren und haben nach mechanischer Abtrennung von letzteren noch eine Säurezahl über 15, ausgedrückt in mg Kaliumhydroxyd, die bei der Titration pro g Substanz verbraucht werden.



  Da für die angeführte katalytische Polymerisation zu Polymeren hohen Molekulargewichts jedoch Monomere mit sehr niedrigen Säurezahlen (kleiner als 0,5) notwendig sind, müssen die Rohprodukte nach dem bekannten zweistufigen Herstellungsverfahren aufwendigen Reinigungs- und Entsäuerungsverfahren unterworfen werden. Die Reinigungsoperationen müssen mehrmals durchgeführt werden, da eine einmalige Reinigung nicht zu den gewünschten Esteranhydriden niedriger Säurezahl führt, was zeitraubend und teuer ist.



   Zweck der Erfindung ist, den Aufwand für die Reinigungsoperationen bzw. deren Anzahl zu verringern.



  Der Erfindung liegt die Aufgabe zugrunde, cyclische Esteranhydride von a-Hydroxycarbonsäuren mit niedriger Säurezahl herzustellen.



   Erfindungsgemäss wird das dadurch erreicht, dass Alkylester von polymeren a-Hydroxycarbonsäuren auf 200 bis   2800 C,    vorzugsweise 200 bis   2400 C,    gegebenenfalls bei vermindertem Druck erhitzt werden.



  Geeignete Katalysatoren bestehen aus Metallen der vierten Hauptgruppe des Periodensystems oder deren Verbindungen, vorzugsweise Blei-II-stearat. Geeignete Verbindungen der Metalle sind ihre Oxide, Hydroxide oder Salze anorganischer oder organischer Säuren. Die polymeren a-Hydroxycarbonsäurealkylester werden vorteilhaft aus den entsprechenden monomeren a-Hydroxycarbonsäurealkylestern (Alkyl-:   Cl-C )    durch an sich bekannte Kondensation hergestellt. Es hat sich als günstig erwiesen, für beide Reaktionsstufen dieselben Katalysatoren zu verwenden und dadurch das Herstellungsverfahren nicht zu unterbrechen.



   Die verwendeten Katalysatoren begünstigen sowohl die Kondensation der monomeren Alkylester von a Hydroxycarbonsäuren, bei der Alkohol abgespalten und destillativ aus dem Reaktionsgemisch entfernt wird, als auch die thermische Depolymerisation. Die Menge des eingesetzten Katalysators beträgt 0,1 bis   5 %,    bezogen auf das Gewicht der eingesetzten Alkylester von a Hydroxycarbonsäuren. Bei der Kondensation der monomeren Alkylester von a-Hydroxycarbonsäuren zu polymeren Säureestern wird bei Normaldruck oder gegebenenfalls unter Vakuum bis 2600 C erhitzt. Die Reaktionszeit beträgt etwa 1 bis 20 Stunden und ist ab hängig von der eingesetzten Menge der monomeren Ester.

   Der entstandene polymere Alkylester von a Hydroxycarbonsäuren wird anschliessend ohne weiteren Zusatz von Katalysatoren im Vakuum depolymerisiert, wobei lediglich die Temperatur bis auf maximal 2800 C gesteigert wird und die cyclischen Ester bei vermindertem Druck aus der Mischung abdestilliert werden.



  



  Process for the preparation of optionally substituted glycolides
The invention relates to a process for the preparation of optionally substituted glycolides with acid numbers below 10, which z. B. can be used as starting materials for high molecular weight polymers.



   It is known that industrially useful polymers of aliphatic α-hydroxycarboxylic acids can be prepared by catalytic polymerization of the cyclic ester anhydrides of these acids, e.g. B. from glycolide (1) or lactide (11)
EMI1.1

These cyclic monomers are usually obtained in a two-step process. In the first stage, the free acids are polycondensed by heating to form polymeric acids with a low degree of polymerization of about 7. In the second stage, these polymers are depolymerized again at higher temperatures in vacuo, the cyclic ester anhydrides formed being distilled off.



  Both stages can be influenced catalytically. The cyclic ester anhydrides are crystalline substances at room temperature. After the second stage, they are mixed with oligomeric hydroxycarboxylic acids which have also been distilled over and, after mechanical separation from the latter, still have an acid number above 15, expressed in mg of potassium hydroxide, which are consumed in the titration per g of substance.



  However, since monomers with very low acid numbers (less than 0.5) are required for the above-mentioned catalytic polymerization to polymers of high molecular weight, the crude products must be subjected to costly purification and deacidification processes according to the known two-stage production process. The cleaning operations have to be carried out several times, since a single cleaning does not lead to the desired ester anhydrides with a low acid number, which is time-consuming and expensive.



   The purpose of the invention is to reduce the cost of cleaning operations or their number.



  The invention is based on the object of producing cyclic ester anhydrides of α-hydroxycarboxylic acids with a low acid number.



   According to the invention, this is achieved in that alkyl esters of polymeric α-hydroxycarboxylic acids are heated to 200 to 2800 ° C., preferably 200 to 2400 ° C., if appropriate under reduced pressure.



  Suitable catalysts consist of metals from main group four of the periodic table or their compounds, preferably lead (II) stearate. Suitable compounds of the metals are their oxides, hydroxides or salts of inorganic or organic acids. The polymeric a-hydroxycarboxylic acid alkyl esters are advantageously prepared from the corresponding monomeric a-hydroxycarboxylic acid alkyl esters (alkyl-: Cl-C) by condensation known per se. It has proven advantageous to use the same catalysts for both reaction stages and thereby not to interrupt the production process.



   The catalysts used promote both the condensation of the monomeric alkyl esters of a hydroxycarboxylic acids, in which alcohol is split off and removed from the reaction mixture by distillation, and thermal depolymerization. The amount of catalyst used is 0.1 to 5%, based on the weight of the alkyl esters of a hydroxycarboxylic acids used. In the condensation of the monomeric alkyl esters of α-hydroxycarboxylic acids to polymeric acid esters, the mixture is heated to 2600 ° C. at normal pressure or, if appropriate, under reduced pressure. The reaction time is about 1 to 20 hours and is dependent on the amount of monomeric ester used.

   The resulting polymeric alkyl ester of a hydroxycarboxylic acids is then depolymerized in vacuo without any further addition of catalysts, the temperature merely being increased to a maximum of 2800 ° C. and the cyclic esters being distilled off from the mixture under reduced pressure.

Claims

The depolymerization and separation of the substituted or unsubstituted glycolides takes about 1 to 20 hours, depending on the amount used. The acid number of the compounds produced according to the invention is a maximum of 8. The process according to the invention has the advantage that the optionally substituted glycolides produced thereafter no longer have to be subjected to so many purification stages because of their low acid numbers. In addition, it allows the use of such catalysts which are suitable for both reaction stages and do not require any interruption of the production process. Furthermore, the corrosion of the reaction vessels used is reduced since their esters, which are known to not corrode, are used instead of the free acids.

It is also possible to line the reaction vessels and agitators with the metals to be used as catalysts and thus not to add any further catalysts for the 2-stage reaction.

example 1 100 g of lactic acid methyl ester are heated with 6 g of lead IT oxide in a 250 ml round bottom flask which is provided with an inlet tube and a 50 cm long packed column. The temperature of the heating bath in which the flask is located is increased from 160 to 2600 ° C. over 6.5 hours. During the condensation, a moderate stream of pure nitrogen is constantly passed through the mixture. 26 g of methanol are split off and distilled off. The residue (80 g) is a deep brown, soft resin.

The average degree of polymerization is about 7.

The depolymerization is carried out in a simple distillation apparatus equipped with an air cooler.

The temperature is increased from 1800 C at the beginning to 2800 C (temperature of the heating bath) at the end of the reaction. Initially, a small forerun distilled over at up to 1300 C in vacuo at 13 to 14 mm Hg. From 1300 C the main fraction is collected.

42 g of lactide with an acid number of 7.7 are isolated from this. A small amount of black resin remains as a residue.

Example 2 200 g of ethyl lactate are heated with 6 g of lead II stearate, as in Example 1. The maximum temperature reached is 2600 ° C. and the amount of alcohol split off is 66 g. The linear polyester (140 g, average degree of polymerization about 7) yields 93.7 g of lactide with an acid number of 6.1 on thermal depolymerization at 200 to 2800 ° C.

Example 3 200 g of ethyl lactate are condensed and depolymerized in the same way as described in Example 2. Lead-II-lactate serves as a catalyst (3 weeks, based on the weight of the ethyl lactate used). The yield of lactide is 65.5 g.

Example 4 200 g of ethyl lactate are, as described in Example 2, condensed. Lead IV oxide is used as the catalyst. 70 g of lactide with an acid number of 6.4 are obtained.

Example 5 400 g of ethyl lactate are heated with 12 g of lead II stearate, initially as in Example 1 to 2050 ° C. (bath temperature). The condensation is then continued under reduced pressure in such a way that it begins at a temperature of 160-1700 ° C. and a pressure of 100 to 150 mm Hg, then for three to four hours down to 2600 ° C. at 13 mm Hg is increased. The depolymerization gives 173.5 g of lactide with an acid number of 5.04.

Example 6 354 g of propyl lactate are initially heated in the presence of 10.5 g of lead (II) stearate at normal pressure of 155 to 2050 ° C. (bath temperature).

The condensation is then continued in vacuo, starting at 1300 ° C. and 60 mm Hg to 2400 ° C. and 15 mm Hg. The linear polyester (221 g, average degree of polymerization about 8) gives 78 g of lactide on depolymerization Example 7 400 g of isopropyl lactate are condensed according to Example 6. 3% lead II stearate is used as the catalyst. The polymeric product (245 g, average degree of polymerization about 8) gives the thermal depolymerization 131 g of lactide with an acid number of 1.51.

Example 8 180 g of methyl glycolate are heated with 1.8 g of lead (II) stearate while simultaneously introducing nitrogen. The temperature is increased from 150 to 2600 C (bath temperature) over a period of seven hours. The solid polyglycolic acid ester obtained (122 g) is depolymerized in vacuo (<1 mm Hg). 65 g (56 S) glycolide are obtained.

Example 9 200 g of ethyl glycolate are heated to temperatures of 155 to 2600 ° C. at normal pressure with 3% lead (II) stearate. The polyglycolic acid ester (120 g) is depolymerized at 210 to 2800 ° C. and 1 mm Hg. 50 g of glycolide with an acid number of 6.0 are obtained.

PATENT CLAIM Process for the production of optionally substituted glycolides with acid numbers below 10, characterized in that alkyl esters of polymeric α-hydroxycarboxylic acids are heated to 200 to 2800 C.

SUBCLAIMS 1. The method according to claim, characterized in that the polymeric α-hydroxycarboxylic acid alkyl ester is heated to 200 to 2400 C.

2. The method according to claim, characterized in that one works at reduced pressure.

3. The method according to claim or one of the preceding subclaims, characterized in that the heating of the polymeric a-hydroxycarboxylic acid alkyl ester takes place in the presence of catalysts which consist of metals of the fourth main group of the periodic table or their compounds, preferably lead II stearate.