Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
2002-03-19
2003-12-02
Acquah, Samuel A. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S361000, C528S365000, C525S450000, C524S783000, C524S784000
Reexamination Certificate
active
06657042
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for manufacturing homo- and copolymers from lactic acid by ring scission polymerization of cyclic esters or diesters in the presence of an initiator/catalyst system which selectively accelerates chain growth. A mixture consisting of initiators is used. The process is thus particularly suitable for the continuous or discontinuous manufacture of these biologically degradable polyesters in high-capacity plants and with standard dwell time characteristics.
Homo- and copolyesters of L- or D, L-lactic acid can be employed as biologically degradable polymer materials with typical thermoplastic processing and application properties in a variety of ways as packaging plastic, in hygiene products, for non-returnable articles but also as surgical implant material or a galenic auxiliary agent for parenteral systems of dispensing medication. Consistent product properties at the molecular level, such as molar mass and molar mass distribution in the case of the homo- and copolyesters, retention of chirality with the poly-L-lactic acid or comonomer ratio and comonomer distribution in the case of the copolyesters, are the absolute precondition for the use of these homo- and copolyesters in the application areas mentioned. This consistency can be achieved under engineering conditions only with equally reliably manageable synthesis processes, which in addition must exhibit a high tolerance for unavoidable substrate-specific and process fluctuations.
As a result of the equilibrium constant of the ring-chain equilibrium, high molecular polyesters of lactic acid can be produced only by ring scission polymerization of the cyclic diester of lactic acid (L,L- or D,L-3,5-dimethyl-1,4-dioxan 2,5 dion, hereafter called L,L- or D,L-dilactide). In order to initiate or catalyze this polymerization reaction metal organic compounds of tins are preferably used (ref. for example, J. Dahlmann, G. Rafler:
Acta Polymerica,
44 (1993) 103 and the literature cited there). Proposals for engineering processes, performed as mass polymerization at the molten stage at temperatures of 185-200° C., involve almost exclusively tin-II-octanoate, which is supposed to accelerate ring scission polymerization especially efficiently (U.S. Pat. No. 5,484,881). In addition to tin-II-octonoate, other compounds of bivalent and tetravalent tin are frequently described as initiators or catalysts (c.f. U.S. Pat. No. 5,484,881). But additional metal compounds, such as alkoxyds of tin, lead, magnesium, titanium or zirconium, are also mentioned primarily as potentially usable, catalytically active substances, without technical processes based on these initiators or catalysts being described (S. Jacobson, Ph. Degee, H.-G Fritz. Ph. Dubois, R. Jerome:
Polymer Eng. Sci.
39, (1999), 1311; W. M. Stevals, P. J. Dijkstra, J. Feihen, TRIP 5 (1997) 300).
The choice of initiator in ring scission polymerization is additionally determined in great measure by the substrate to be polymerized. Cyclic monoesters, such as caprolactam, or cyclic carbonates, such as 3-dioxan-2-on (trimethylene carbonate), are far less sensitive with respect to the initiator than, for example, dilactide or 1,4 dioxan-2,5-dion (diglycolide) (G. Rafler, G. Dahlmann:
Acta Polymerica
43 (1992) 91; G. Rafler:
Acta Polymerica
44 (1993) 168), and they can consequently be polymerized without difficulty in the presence of the initiators named in U.S. Pat. No. 5,484,881 or in other places (c.f. for example, also A. Loefgren, A.-C. Alberson, P. Dubois, R. Jerome:
Rev. Macromol Chem. Phys
. C35 (1995) 379), if the essential additional constraints for this polymer formation reaction, such as purity of the monomers, exclusion of water and minimization of thermal loads, are observed when the process is being carried out.
Initiators containing tin, preferably tin-II-octonoate, which is the most used according to the prior art, cause a polymerization profile which is hard to manage technically with regard to molar mass of the polymer, with an extremely steep rise at the beginning of the reaction, a molar mass maximum undefined in its absolute amount and a pronounced degradation of the polymer after passing through the maximum (c.f E. Dahlmann, G. Rafler:
Acta Polymerica
44 (1993) 107). This polymerization profile, unsuited to a technical process, is heavily dependent on concentration, where in contrast to ionic and radically initiated polymerization processes, at least for the majority of tin-initiated polymerizations, conversion and molar mass run synchronously, that is, polymerization speed and high conversion also result in high molar masses.
Besides the technical problems with reaction in ring scission polymerization, which result primarily from the reaction profiles, from an ecotoxicological viewpoint the orientation also ought to be toward the lowest possible concentrations of tin in the polymers, since these aliphatic polyesters are intended primarily for biodegradable materials, with disposal by means of composting. Heavy metal compounds are introduced directly into the biosphere during disposal through composting.
In spite of the multiple problems that tin-bearing initiators cause, these compounds are favored for engineering process proposals because of their high activity in the polymerization process, independently of the substrate to be polymerized
Independently of the type of initiator and the initiator concentration and the particular cyclic esters or diesters to be polymerized, ring scission polymerization is characterized by high sensitivity to contamination in the initial substances and polymerization additives (initiators, co-initiators). The slightest contamination of the monomers or entry of humidity or air into the polymerization reactors and fluctuations in the process parameters cause considerable problems in the technical process for producing these aliphatic polyesters.
A series of measures that are intended to contribute to management of the technical process and to the reproducible formulation of product properties is therefore proposed for the technical management of the polymerization process, which is well worked out and unproblematic in execution for laboratory conditions. These measures involve all the aspects mentioned that make the process more difficult.
Most often, long-chain alcohols are used as co-initiators of the metal organic polymerization catalysts to initiate chain growth, in order not to mask or to relativize the non-defined influence of monomer or system contaminations containing hydroxyl groups on conversion and molar mass in ring scission polymerization (c.f. for example, EP-PS 0 778 304; U.S. Pat. No. 6,005,068). The addition of dodecyl alcohol or stearyl alcohol, for example, helps the reproducibility of molar mass. Besides that, in high proportions these additives containing hydroxyl groups can also be used to chemically modify the polyesters. Hydrophobic properties, for example, can be generated using stearyl alcohol (EP-PS 0 778 304). Through co-initiation with polyethylene glycols the polyesters become more hydrophilic (G. Rafler, M. Jobmann:
Pharm. Ind.
58 (1996) 1147.
In DE-PS 43 18 204 the sensitivity of ring scission polymerization as the result of contamination or byproducts from the monomers is overcome in a different way. In this patent, in addition to new, zirconium-based initiators silicon-organic additives are described, which, by reacting with monomer byproducts containing hydroxyl groups or water from the air or the inert gas, remove them from the system and thus increase process reliability.
Except for the effects of foreign substances, such as monomer byproducts or other contamination of the reaction system, reproducibility of the synthesis process and consistency of molecular product properties are determined to a great extent by the type and concentration of the initiators employed and process temperature. As a result of the balanced character of ring scission polymerization (ring-chain equilibrium of monomer esters or diesters and linear polymer mole
Bechthold Inna
Jobmann Monika
Lang Jacqueline
Rafier Gerald
Acquah Samuel A.
Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung
Young & Basile P.C.
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