Various plastic products making use of the characteristics of plastics such as light weight, low price, excellent processability, and good resistance to corrosion and degradation have so far been put on the market. Owing to their convenience, plastic products have come to be found anywhere not only in our daily life but also in various industrial scenes.
Such a tendency is however accompanied with a yearly increase in the disposal amount of plastic products after use. Since they are resistant to corrosion or degradation, the disposal has come to be a serious problem of environmental pollution.
With the forgoing view, various biodegradable plastics easily degraded into water and carbon dioxide by organisms existing in the nature, particularly, by microorganisms in the soil or water have been developed as a substitute for the corrosion- or degradation-resistant plastic products as described above, and they are now attracting attentions as environmental protecting products.
Examples of such biodegradable plastics include resins produced by microorganisms, natural polymers and synthetic polymers.
Examples of the resins produced by microorganisms include hydroxybutylate polyesters produced by certain hydrogen bacteria.
Examples of the biodegradable natural polymers include natural polymers derived from plants (cellulose, starch and the like) and those derived from animals (chitin contained in the shell of shrimps or crabs)
In addition, polymers synthesized using the above-described natural molecules as a raw material, synthetic high-molecular weight polycaprolactone and pullulan each of which originally has biodegradability, and polymer alloys with a general-purpose plastic (non-biodegradable) can be given as examples.
Examples of the biodegradable synthetic polymer include biodegradable resins, for example, polylactic acid having a chemical structure and functional group which permits assimilation by microorganisms and recognition of a substrate by an enzyme.
A polyhydroxycarboxylic acid available by the polymerization of a hydroxycarboxylic acid such as lactic acid has already been prepared as a biodegradable resin in accordance with the following processes.
1) After synthesis of an oligomer of a hydroxycarboxylic acid, the resulting oligomer is depolymerized into a lactide. The lactide is purified by distillation, followed by open-ring polymerization.
2) After synthesis of a lactic acid oligomer, intramolecular crosslinking is carried out using a compound having a functional group such as isocyanate.
3) After dehydration of a hydroxycarboxylic acid , dehydrating condensation reaction using molecular sieves is carried out in the reaction mixture containing an organic solvent such as anisole or diphenyl ether.
The above-described conventional processes for producing a polylactic acid resin are each accompanied with such problems that since the direct addition of a catalyst to a hyderoxycarboxylic acid causes deactivation, a dehydration step is required in advance. Thus, in spite that a high molecular weight polymer is available, the reaction route is long and the reaction is complex so as to necessitate extra production equipment and many steps for the separation and recovery of the organic solvent, to thereby render the production cost high.
In JP-A-9-31182 (the term "JP-A" as used herein means an "unexamined published Japanese patent application), the present inventors have already disclosed a polyhydroxycarboxylic acid resin which can be prepared easily in one pot by adding as a polymerization catalyst 1,3-substituted-1,1,3,3-tetraorganodistanoxane to a hydroxycarboxylic acid such as water-containing L-lactic acid and heating and stirring the resulting mixture under a reduced pressure or in an organic solvent.
The above-described polyhydroxycarboxylic acid resin (biodegradable resin) which can easily be synthesized in one pot is however accompanied with such a problem as insufficient moldability or formability. There is accordingly a strong demand for the development of a biodegradable resin having good moldability or formability.