Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
2000-12-13
2002-10-22
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S357000, C525S415000
Reexamination Certificate
active
06469133
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
None.
REFERENCE TO A “MICROFICHE APPENDIX”
Not Applicable.
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a process for the direct synthesis of polymers from dimeric cyclic esters. In particular, the present invention relates to polylactic acid (PLA or polylactide) from racemic lactide or a polymandelide from mandelide. The process particularly relates to a racemic metal organic ligand catalyst such as racemic salbinap that catalyzes the polymerization of racemic dimeric cyclic ester monomers such as poly(L-lactide) and poly(D-lactide) to a polylactide stereocomplex.
(2) Description of Related Art
Lactides
Poly(hydroxybutyrate) and poly(lactide) herein “polylactide” (Chiellini et al., Adv. Maters. 8: 305-313 (1996)) are among the most widely studied degradable polymer systems, and polylactide is now being commercialized as a commodity polymer for high volume commercial applications such as fibers and packaging materials (Thayer, Chem Eng. News 75: 14-16 (1997)). Polylactic acid (PLA) is an attractive polymer because it can be derived from renewable resources and provides a biodegradable alternative to polymers obtained from petrochemical sources (Sinclair, Macromol. Sci.-Pure Appl. Chem. A33: 585-597 (1996)). PLA is prepared by the ring opening polymerization (ROP) of lactide, the cyclic dimer of lactic acid. Currently, enantiopure L-lactide is required for preparing crystalline materials. Therefore, considerable effort has been expended in preparing L-lactide via fermentation routes. To date there have not been reported any examples where crystalline PLA has been prepared from a racemic mixture of D- and L-lactide using an achiral catalyst.
Commercial polylactides usually are synthesized from lactide monomers prepared from a single lactic acid enantiomer, and because the resulting polymers are stereoregular, they have high degrees of crystallinity (Huang et al., Macromols. 31: 2593-2599 (1998)). The mechanical properties of crystalline polymers are stable to near the polymer melting point, and thus they have higher use temperatures than their amorphous analogs. For example, polymerization of L-lactide gives a semicrystalline polymer with a melting transition near 180° C. and glass transition (T
g
) of about 67° C. (Zhang et al., Macromol. Sci.-Rev. Macromol. Chem. Phys. C33: 81-102 (1993)), properties that make it useful for applications ranging from degradable packaging to surgical implants and matrices for drug delivery (Hollinger,
Biochemical Applications of Synthetic Degradable Polymers
; CRC Press: Boca Raton, Fla., 1995).
In contrast, the polymer derived from rac-lactide, a 1:1 mixture of D and L-lactide, yields amorphous polymers with glass transitions near room temperature. Although L-lactide can be prepared with relatively high enantiopurity from corn fermentation, the requirement for an enantiopure monomer places restrictions on the polymer synthesis.
Chiral catalysts have been employed to effect kinetic resolution of racemic lactide. For example, Spassky et al. (Macromol.
Chem. Phys.
197: 2627-2637 (1996)) reported kinetic resolutions of rac-lactide by employing a chiral Schiff's base complex of aluminum, (−)-1.
FIG. 1A
shows a scheme for the polymerization of L-lactide and D-lactide to isotactic-L-PLA by catalyst (−)-1. The structure for catalyst (−)-1 is shown in
FIG. 2
wherein R is a methyl group. At low conversions, high enantiomeric enrichment in the polymer was observed. In the kinetic resolution of rac-lactide by catalyst (−)-1, the enantiomeric excess at 20% conversion was 88% (Spassky et al., Macromol. Chem. Phys. 197: 2627-2637 (1996)). This indicated that the catalyst can override the tendency for syndiotactic placements that are typically favored by chain-end control (Thakur et al., Macromols. 31: 1487-1494 (1998)). At higher conversions, the enantiomeric enrichment in the polymer decreased. The drop in selectivity can be attributed to the fact that the relative concentration of the “wrong” isomer increases in the monomer pool as the desired enantiomer is incorporated in the polylactide.
In a recent report, Coates et al. effected the syndiotactic polymerization of meso-lactide by using the isopropoxide catalyst (−)-2 (Ovitt et al., J. Am. Chem. Soc. 121: 4072-4073 (1999)). The scheme for this reaction is shown in FIG.
1
B. The structure for catalyst (−)-2 is shown in
FIG. 2
wherein
i
Pr is isopropoxide. Since meso-lactide possesses two stereocenters of opposite configuration, the concentration of D and L stereocenters remained constant and the intrinsic selectivity of the catalyst was not diminished by statistical depletion of the preferred stereocenter.
An interesting effect of stereoregularity on lactide properties was first reported by Tsuji and co-workers (Ikada et al., Macromols. 1987, 20: 904-906; Tsuji et al., Macromols. 24: 2719-2724 (1991); Brizzolara et al., Macromols. 29: 191-197 (1996)). As shown in
FIG. 1C
, upon mixing, L-PLA and D-PLA form a stereocomplex that has a T
m
230° C., which is 50° C. higher than the T
m
for either of the homochiral D- or L-PLA polymers. Preparation of this stereocomplex presently requires parallel ROP of D- and L-lactide with subsequent combination of the chiral polylactide chains. Despite the improved mechanical properties of the stereocomplex, practical applications of the stereocomplex have been prohibitive because of the requirement that separate pools of enantiopure lactide monomers must first be polymerized to enantiopure polymers before combining to make the stereocomplex.
Since lactic acid is commercially available in racemic form, it would be desirable that crystalline polymers similar in properties to the PLA stereocomplex made from enantiopure lactide monomers be prepared from a racemic mixture of lactides. This would provide a simple route to polylactide formation because it would eliminate the need for enantioselective fermentation routes for the synthesis of polylactide monomers.
Mandelides
While a broad range of physical properties is available from polymers consisting of Poly(hydroxybutyrate)s and poly(lactide), one unmet need is a glassy, degradable polymer with a high glass transition temperature (T). The backbone of polylactide is relatively flexible resulting in a T
g
near 60° C., but substituting an aromatic ring for the methyl group of polylactide should, by analogy to polystyrene, result in a polymer with a significantly higher T
g
. Thus, mandelide, the dimer of mandelic acid (2-hydroxy phenylacetic acid), is a particularly intriguing monomer for ring opening polymerization.
Prior art attempts at preparing polymandelides have produced polymers with number average molecular weights less than 5,000, too low for most practical applications. No glass transition temperatures were reported for these polymers. Direct condensation of 1-bromophenyl acetic acid in the presence of triethylamine (Pinkus et al., J. Polymer Sci. Part A-Polymer Chem. 27: 4291-4296 (1989)), transesterification of methyl mandelate (Domb, J. Polymer Sci. Part A-Polymer Chem. 31: 1973-1981 (1993)), and condensation of mandelic acid (Whitesell et al., Chem. Maters. 2: 248-254 (1990)) all provided low molecular weight polymers with degrees of polymerization near 30. Several indirect routes to polymandelide have also been reported. In the earliest synthesis of the homopolymer of mandelic acid, the trimethyltin ester of &agr;-bromophenyl acetic acid was pyrolyzed and the resulting viscous solid was identified as polymandelide (Okada et al., J. Organometal. Chem. 54: 149-152 (1973)). Deoxy-polymerization of phenylglyoxalic acid using cyclic phosphites yielded oligomers (Kobayashi et al., Polymer Bull. 3: 585-591 (1980)), and ring opening polymerization (with loss of CO
2
) of the anhydridocarboxylate of mandelic acid gave polymandelide with degrees of polymerization as high as 30(Smith et al., Macromol. Chem.-Phys. Makromol. Chem. 182: 313-324 (1981)). The latter metho
Baker Gregory L.
Smith, III Milton R.
Board of Trustees of Michigan State University
McLeod Ian C.
Wu David W.
Zalukaeva Tanya
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