Copolymers of monocyclic esters and carbonates and methods for making same

1. A thermoplastic or crosslinked copolymer having, in polymerized form, units derived from a (a) monocydic ester or corresponding hydroxy acid or (b) a monocycic carbonate, or both (a) and (b), and units derived from a bicylic diester and/or carbonate having the structure

wherein each R is independently lower (C 1-4 ) alkyl or hydrogen, each Z is —O— or a covalent bond, each n and each o are independently zero or a positive integer, provided that the values of n and o, taken together, are such that the main ring contains 6 or 7 members when each Z is a covalent bond and 8 or 9 members when each Z is —O—, and Y is —(CR 2 ) m — where m is 1, 2 or 3, wherein the copolymer contains from about 0.05 to about 1.5 weight percent, based on the total weight of the copolymer, of units derived from a bicyclic diester and/or carbonate.

2. The copolymer of claim 1 , wherein the monocyclic ester is lactide.

3. The copolymer of claim 2 wherein the bicyclic diester is 2,5-dioxa-bicyclo[2.2.2]octane-3,6-dione.

4. The copolymer of claim 3 that has a number average molecular weight of from about 10,000 to about 500,000, as measured by the GPC/DV method.

5. The copolymer of claim 4 wherein the copolymer is semicrystalline and contains from about 98.4 to 99.9 percent of units derived from either the D or L isomer of lactic acid, based on the total moles of the lactic acid units, and from about 0.1 to about 1.6 percent of units derived from the other isomer, based on the total moles of the lactic acid units.

6. The copolymer of claim 5 which contains from about 0.3 to about 1.0 weight percent, based on the total weight of the copolymer, of repeating units derived from the bicyclic diester.

7. The copolymer of claim 4 wherein the copolymer contains up to about 98 percent of units derived from either the D or L isomer of lactic acid, based on the total moles of the lactic acid units, and about 2 percent or more of units derived from the other isomer, based on the total moles of the lactic acid units.

8. The copolymer of claim 7 that contains from about 0.3 to about 1.0 weight percent, based on the total weight of the copolymer, of units derived from the bicyclic diester.

9. A method comprising subjecting a mixture including lactide and a bicyclic diester and or carbonate to conditions sufficient to polymerize the mixture to form a copolymer having units derived from the monocyclic ester and/or carbonate and repeating units derived from the bicyclic diester and or carbonate, wherein the bicyclic diester and/or carbonate is 2,5-dioxa-bicyclo[2.2.2]octane-3,6-dione.

10. The method of claim 9 wherein the copolymer contains at least about 98 weight percent of units derived from either the D or L isomer of lactic acid, and up to about 2 weight percent of units derived from the other isomer, based on the total weight of the lactic acid.

11. The method of claim 10 , wherein the bicyclic diester and/or carbonate constitutes about 0.3 to about 1.0 weight percent, based on the total weight of the monomers.

12. The method of claim 9 wherein the copolymer contains no more than about 98 weight percent of units derived from either the D or L isomer oflactic acid, and at least about 2 weight percent of units derived from the other isomer, based on the total weight of the lactic acid.

13. The method of claim 12 , wherein the bicyclic diester and/or carbonate constitutes about 0.3 to about 1.0 weight percent, based on the total weight of the monomers.

14. The copolymer of claim 2 which has a melt flow rate at 210° C. and under a weight of 2.16 kg of from about 4–12 g/10 min has a melt tension of at least about 2 cN.

15. The copolymer of claim 14 which has a melt tension of at least 12 cN.

16. The copolymer of claim 14 which has a melt tension of at least 6 cN.