Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Patent
1995-01-27
1996-08-06
Dodson, Shelley A.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
528357, 528361, 528480, 528501, C08G 6308, C08F 600
Patent
active
055434940
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of PCT/EP93/02033 filed Jul. 29, 1993.
The present invention relates to a process for the production of poly(lactic acid) with a high molecular weight from the lactic acid monomer.
It is generally known that polyesters of the poly(lactic acid) type do not achieve good properties until their molecular weights are at least 25,000. The term poly(lactic acid) "with a high molecular weight" as used in the present description thus means a poly(lactic acid) which has a molecular weight of at least 25,000.
It is known that free lactic acid can be subjected to direct polycondensation, but the polycondensation reaction is an equilibrium reaction which is difficult to shift towards the formation of the desired product properties. It is particularly difficult to shift the reaction because the water produced in the course of the reaction distills from the highly viscous melt very slowly. These difficulties persist even if antimony oxide is used as a polycondensation catalyst, as proposed in the literature.
For these reasons, in order to prepare polymers with high molecular weights, lactic acid is converted into lactide, the polymerization of which by ring opening is described extensively in the literature. The lactide is produced from lactic acid via the initial formation of oligomeric lactic acid by the dehydration of aqueous lactic acid, followed by a catalytic transesterification reaction known as a "back-biting" reaction.
The catalysts proposed for the above reaction include tin powder, tin halides, tin carboxylates (EP-A-0 261 572 and EP-A-0 275 581), tin alkoxides (GB 1,007,347) and zinc or tin (EP-A-0 264 926 and U.S. Pat. No. 4,797,468).
The polymerization of the intermediate lactide by the opening of its ring is carried out with the use of initiators such as Sn (II) salts which are sensitive both to hydrolysis reactions and to degradative oxidation reactions. Since the presence of only traces of water, lactic acid or other impurities in the intermediate lactide are sufficient to de-activate the initiator or the growing chain, a precise purification of the crude lactide is necessary and, as described in the literature, is carried out by repeated crystallizations, normally at least three, and by means of a final drying step at 45.degree. under vacuum for several hours.
The need of a purification of the lactide, which is done in batches, involves a substantial increase in the cost of producing the desired poly(lactic acid) with a high molecular weight.
It has now been found that, if the polycondensation of free lactic acid is carried out in the presence of iron salts, it is possible to produce poly(lactic acid) with a high molecular weight together with the cyclic dimer or lactide.
An object of the invention is therefore a process for the production of poly(lactic acid) with a high molecular weight by the polycondensation of the free monomer, characterized in that the polycondensation is carried out in the presence of a catalytically active quantity of iron ions to produce high molecular weight poly(lactic acid) and the cyclic dimer or lactide as the principal products.
In the process of the invention, the production yield of the poly(lactic acid) with a high molecular weight is not high and is generally of the order of from 15 to 20% with reference to the lactic acid monomer loaded into the reactor. However, the simultaneous production of lactide, in yields of the order of 55-65% with reference to the monomer, within the process for converting lactic acid into poly(lactic acid) as a whole, makes the process interesting from an economic point of view.
The method according to the invention is preferably carried out in successive steps, that is: solution at a concentration preferably of from 75 to 95% by weight, is dehydrated and pre-condensed at a temperature of the order of 120.degree.-220.degree. C. (preferably between 160.degree. and 200.degree. C.) and preferably at atmospheric pressure, and in which the water present initially and that produced by the reaction is distilled off, and t
REFERENCES:
patent: 4797468 (1989-01-01), De Vries
patent: 5041529 (1991-08-01), Shinoda et al.
patent: 5206341 (1993-04-01), Ibay et al.
patent: 5225129 (1993-07-01), van den Berg
patent: 5247059 (1993-09-01), Gruber et al.
patent: 5302694 (1994-04-01), Buchholz
patent: 5310865 (1994-05-01), Enomoto et al.
Chemical Abstracts, vol. 68, No. 24, Jun. 10, 1968, Abstract No. 68:105648 Kiyoshi Chujo et al. "Catalyst for Lactide Polymerization").
Bastioli Catia
Grzebieniak Karolina
Niekraszewicz Antoni
Perego Gabriele
Dodson Shelley A.
Ministero Dell'Univerita' E Della Ricerca Scientifica E Tecnolog
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