Catalysts and methods for polymer synthesis
The present invention provides unimolecular metal complexes having increased activity in the copolymerization of carbon dioxide and epoxides. Also provided are methods of using such metal complexes in the synthesis of polymers. According to one aspect, the present invention provides metal complexes comprising an activating species with co-catalytic activity tethered to a multidentate ligand that is coordinated to the active metal center of the complex.
1. A method for the synthesis of aliphatic polycarbonates comprising the step of contacting an epoxide and carbon dioxide with a metal complex comprising a metal atom coordinated with a salen ligand, the salen ligand comprising two halves, each half coordinated to the metal atom via an imine and an oxygen atom, wherein the metal complex is characterized in that one of the two halves of the salen ligand has two or more cationic activating functional groups tethered to it and the other half has no tethered cationic activating functional groups, wherein the half of the ligand having no tethered cationic activating functional groups comprises or is derived from a gamma dicarbonyl compound.
2. The method of claim 1 , wherein the cationic activating functional groups tethered to the ligand are independently selected from the group consisting of: onium salts, nitrogen-containing functional groups; phosphorous-containing functional groups; and arsenic-containing functional groups.
3. The method of claim 1 , wherein at least one cationic activating functional group comprises an ammonium salt.
4. The method of claim 1 , wherein at least one cationic activating functional group is selected from the group consisting of amidinium salts and guanidinium salts.
5. The method of claim 1 , wherein the gamma dicarbonyl compound comprises optionally substituted acetylacetone.
6. The method of claim 1 , wherein the metal complex contains a total of 2 to 4 cationic activating functional groups.
7. The method of claim 6 , wherein the metal complex contains a total of 2 cationic activating functional groups.
8. The method of claim 6 , wherein the metal complex contains a total of 3 cationic activating functional groups.
9. The method of claim 6 , wherein the metal complex contains a total of 4 cationic activating functional groups.
10. The method of claim 1 , wherein the epoxide is selected from the group consisting of: ethylene oxide, propylene oxide, butylene oxide, cyclohexene oxide, 1,2 octene oxide, 3-vinyl cyclohexene oxide, epichlorohydrin and mixtures of any two or more of these.
11. The method of claim 10 , wherein the epoxide is propylene oxide.
12. The method of claim 1 , wherein the CO 2 pressure is between about 50 and 800 psi.
13. The method of claim 1 , wherein a molar ratio of the metal complex to the epoxide is between about 1:1000 and about 1:100,000.
14. The method of claim 1 , further comprising the steps of letting the polymerization proceed until a desired polymer molecular weight has been achieved, quenching the polymerization reaction and isolating the polymer.