Biodegradable polymers, biopolymers, constitute a group of materials subject to continued development. Their uses include all kinds of applications ranging from packaging products to hygiene products, and also medical applications. Polyhydroxy acids are polymers the monomers of which contain both a carboxylic acid group and a hydroxyl group. Examples of such polymers include polylactic acid (polylactide, PLA) poly(hydroxybutyrate), polyglycolide, and poly(.epsilon.-caprolactone).
Polymers and copolymers of lactic acid have already for years been used for medical purposes, for example in the manufacture of surgical sutures, for degradable bone nails, and for controlled release of drugs. The use of polymers for packaging materials and for other bulk products has so far been limited by the high price of polymers and their susceptibility to degradation during processing. It has not been economically profitable to produce and treat in a manner intended for medical applications a polymer intended for bulk products. Interest in biodegradable polymers, i.e. biopolymers, has greatly increased in recent years, and many companies have made efforts to launch on the market packaging materials, hygiene products, sacks and films for agricultural purposes, and sacks for waste. In particular, various films have gained importance.
Polyhydroxy acids can be produced by polycondensation reactions, which are typical in the production of polyesters, or by ring-opening polymerization from cyclic dimers, lactones. Polyhydroxy acids are thermoplastic polyesters which resemble many conventional polymers.
Polylactides, i.e. condensation polymers based on lactic acid, are for many reasons an especially attractive group of biopolymers. Their principal degradation product, lactic acid, is a product common in nature, it is not toxic and is used widely in the food and pharmaceutical industries. A high molecular weight polymer can best be produced by ring-opening polymerization from lactide. Lactic acid is optically active, and thus its dimer appears in four different forms: L,L-lactide; D,D-lactide; L,D-lactide (mesolactide); and a racemic mixture of L,L- and D,D-lactides. By polymerizing these either as pure compounds or at different blend proportions, polymers are obtained which have different stereochemical structures affecting their resilience and crystallinity and, consequently, also their mechanical and thermal properties. The obtained polymers are usually hard and optically clear.
The ring-opening polymerization of cyclic lactones of hydroxy acids, such as lactide, glycols, .epsilon.-caprolactone, etc., constitutes technology known per se. The polymerization processes known are various, some examples being patent U.S. Pat. No. 5,378,801 relating to extrusion polymerization, patent publication EP0664309-A relating to two-step polymerization, and patent publication EP0499747-A describing polymerization in a mixing reactor. The feature common to these publications, as well as to most others, is that the catalyst is a tin compound, preferably tin octoate. Other catalysts have also been experimented with, but tin compounds have proven to be the best.
However, if the intention is to use the polymer in contact with food, for example, as a packaging material, and also for reasons of environmental protection, the use of tin compounds is not desirable.
It has now been observed, surprisingly, that by using certain types of iron compounds as catalysts (initiators) it is possible to produce polymers having competitive properties.
It was previously known to use an oxide complex of iron and propylene (JP68002948) or cyanide complexes of iron or other metals (JP04359014) as a catalyst in the polymerization of glycolides and lactides. In patent publication WO94/03522, iron compounds are used for the catalysis of the condensation polymerization of lactic acid. However, it was not possible to polymerize lactide, the other product of the condensation polymerization, with iron compounds, but a conventional tin catalyst was used. The molar mass of the polylactic acid was relatively low and the yield was low.