Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
2000-11-08
2001-11-13
Acquah, Samuel A. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S358000, C528S359000, C502S155000, C502S159000, C502S200000, C502S324000, C502S340000
Reexamination Certificate
active
06316590
ABSTRACT:
TECHNICAL FIELD
The invention is directed at methods of making isotactic poly((R)-lactic acid), isotactic poly ((S)-lactic acid), heterotactic poly(lactic acid), syndiotactic poly(lactic acid), and atactic polylactic acid of narrow polydispersity. The invention is also directed at heterotactic poly(lactic acid) as a novel composition of matter. The invention is also directed at novel zinc centered single-site catalysts and at novel Mg centered catalysts.
BACKGROUND OF THE INVENTION
Poly(lactic acid)s (PLA)s are considered useful for medical, agricultural and packaging applications due to their biocompatibility and biodegradability. In view of this, it is desirable to provide new stereospecific forms of poly(lactic acid)s and new methods of preparing known forms of poly(lactic acid)s.
A convenient synthetic route to PLAs is the ring-opening polymerization of lactide, the cyclic diester of lactic acid. A range of metal alkoxide initiators have been reported to polymerize lactide with retention of configuration. For example, these initiators have been reported to polymerize optically active (R,R)-lactide or (S,S)-lactide to produce isotactic poly(lactic acid). Moreover, these initiators have been reported to polymerize rac-lactide to produce amorphous, atactic polymers. Polymerization of rac-lactide to produce heterotactic poly(lactic acid) or any other method of producing this PLA has not heretofore been known.
SUMMARY OF THE INVENTION
It has been discovered herein that isotactic poly ((R)-lactic acid) and isotactic poly ((S)-lactic acid) can be prepared using different catalysts than have heretofore been used for this purpose and that rac-lactide can be polymerized to form the novel stereospecific poly(lactic acid) heterotactic poly(lactic acid). It has also been discovered that syndiotactic poly(lactic acid) can be prepared from meso-lactide using a catalyst with a zinc center and two ligands. It has also been discovered that atactic poly(lactic acid) of low polydispersity can be prepared from rac-lactide using magnesium centered catalysts with two ligands in the presence of C
1
-C
4
alcohol.
One embodiment herein, denoted the first embodiment, is directed to a method of preparing isotactic poly ((R)-lactic acid) which comprises polymerizing (R,R)-lactide in the presence of a catalyst which has a zinc center or a magnesium center, and two ligands where one of the ligands is a propagating group and the other of the ligands is not a propagating group, or is dimer thereof
Another embodiment herein, denoted the second embodiment, is directed to a method of preparing isotactic poly ((S)-lactic acid) which comprises polymerizing (S,S)-lactide in the presence of a catalyst which has a zinc center or a magnesium center; and two ligands where one of the ligands is a propagating group and the other of the ligands is not a propagating group.
Still another embodiment herein, denoted the third embodiment, is directed to heterotactic poly(lactic acid).
Yet another embodiment herein, denoted the fourth embodiment, is directed to a method for preparing heterotactic poly (lactic acid) which comprises polymerizing rac-lactide in the presence of a catalyst which has a zinc center and two ligands where one of the ligands is a propagating group and the other of the ligands is not a propagating group, or is dimer thereof.
Yet another embodiment herein, denoted the fifth embodiment, is directed to magnesium centered catalysts having the structure:
where M is magnesium, R is selected from the group consisting of OAc, OR
2
where R
2
is selected from the group consisting of C
6
-C
20
aryl and C
1
-C
20
alkyl, N(SiMe
3
)
2
and C
1
-C
20
alkyl, and R
1
is selected from the group consisting of ethyl and isopropyl, or is dimer thereof.
Still another embodiment, denoted the sixth embodiment herein, is directed to a single-site catalyst for lactide polymerization containing a &bgr;-diiminate ligand which remains bound to a zinc center which is joined to a ligand which initiates polymerization.
Still another embodiment, denoted the seventh embodiment herein, is directed to a method for preparing syndiotactic poly(lactic acid) which comprises polymerizing meso-lactide in the presence of a syndiotactic poly(lactic acid) effecting catalyst which has a zinc center and two ligands where one of the ligands is a propagating group and the other of the ligands is not a propagating group.
Yet another embodiment, denoted the eighth embodiment herein, is directed to a method for preparing atactic poly(lactic acid) with polydispersity M
&ohgr;
/M
n
ranging from 1.15 to 1.40 as determined by gel permeation chromatography versus monodispersed polystyrene standards. The method comprises polymerizing rac-lactide in the presence of a catalyst which has a magnesium center and two ligands where one of the ligands is a propagating group and the other of the ligands is not a propagating group and the polymerization is carried out in the presence of 0.5 to 2 equivalents of C
1
-C
4
alcohol per mole of magnesium.
The term “heterotactic poly(lactic acid) is used herein to mean stereospecific polymer that has alternating pairs of stereogenic centers in the main chain. It may also be called syndiotactic poly(rac-lactide).
Herein, “Me” is used to mean methyl; “Et” is used to mean ethyl; “Bu” is used to mean butyl; “IPr” and “iPr” and “Pr” are used to mean isopropyl; “OAc” is used to mean acetate; and “BDI” is used to mean &bgr;-diimine.
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Chamberlain Bradley M.
Cheng Ming
Coates Geoffrey W.
Acquah Samuel A.
Cornell Research Foundation Inc.
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