Patent ID: 8241545

Claim:
A method for preparing polylactic acid, comprising: obtaining a raw material comprising a carbon dioxide adduct of N-heterocyclic carbene of formula (I) in a solid form and a lactide in a solid form; feeding the raw material into an extruder, wherein the lactide is melted so that the raw material becomes a molten mixture; raising a temperature of the mixture to a decarboxylation temperature of the N-heterocylic carbene; and obtaining polylactic acid through reactive extrusion of the mixture, wherein, in formula (I), the dotted line together with the solid line parallel to the dotted line represents a single bond or a double bond; X 1 is chosen from S and N; X 2 is chosen from C and N; and R 1 , R 2 , R 3 , and R 4 satisfy at least one of the following conditions: (I) R 1 and R 2 are independently chosen from hydrogen, alkyl groups having 1 to 10 carbon atoms, alkyl groups having 1 to 10 carbon atoms and one or more substituents chosen from halogens, a hydroxyl group, a phenyl group, and a cyano group, cycloalkyl groups having 3 to 6 carbon atoms, a halogen atom, an adamantane group, a phenyl group, and a phenyl group having one or more substituents chosen from halogens, a hydroxyl group, an alkyl group, and a cyano group; (II) R 3 and R 4 are independently chosen from hydrogen, a halogen atom, a cyano group, a hydroxyl group, alkyl groups having 1 to 4 carbon atoms, alkyl groups having 1 to 4 carbon atoms and one or more substituents chosen from halogens, a hydroxyl group, a phenyl group, and a cyano group, a phenyl group, and a phenyl group having one or more substituents chosen from halogens, a hydroxyl group, an alkyl group, and a cyano group; (III) R 3 and R 4 are connected to form cycloalkyl or cycloalkenyl rings having 3 to 8 carbon atoms fused to the five-membered ring that contains X 1 , X 2 , and nitrogen (in which case X 2 and the carbon atom in the 5-membered ring next to X 2 are also part of said cycloalkyl or cycloalkenyl rings); (IV) R 3 and R 4 are connected to form a benzene ring fused to the five-membered ring that contains X 1 , X 2 , and the nitrogen atom (in which case X 2 and the carbon atom in the 5-membered ring next to X 2 are also part of said benzene ring); and (V) R 2 and R 3 are connected to form 5-membered or 6-membered N-heterocyclic rings having no substituent, which are fused to the five-membered ring that contains X 1 , X 2 , and the nitrogen atom (in which case X 1 and the carbon atom in the 5-membered ring connected to R 3 are also part of said 5-membered or 6-membered N-heterocyclic rings).