Method for preparing a polyester

A method for preparing a polyester comprising providing a first cyclic ester having a ring size of from 12-40 atoms and subjecting the first cyclic ester to ring-opening polymerisation by contacting the first cyclic ester with a catalyst of formula I

• wherein
• M is a metal and selected from the group consisting of group 2 metals, aluminum, and group 12 metals;
• Z is selected from the group consisting of hydrogen, borohydrides, aluminium hydrides, carbyls, silyls, hydroxides, alkoxides, aryloxides, carboxylates, thiocarboxylates, dithiocarboxylates, carbonates, carbamates, guanidates, amides, thiolates, phosphides, hydrazonate, imide, cyanide, cyanate, thiocyanate, azide, nitro, siloxides and halides;
• X is selected from the group consisting of O, N, S, and P;
• R1 is an organic linking moiety and has a chain length of at least one, preferably at least two atoms;
• R2 is an organic moiety selected from the group consisting of hydrogen, C1-10 alkyl, silyl, C1-6 alkoxy, C3-8 cycloalkyl, C3-8 cycloalkoxy, aryl, aryloxy, C1-10 amine, C1-10 nitro, C1-10 cyano a halide (F, Cl, Br, I), and a 5- or 6 membered heterocycle containing from 1 to 4 heteroatoms selected from oxygen, sulfur, nitrogen, and phosphorous;
• R3 is an optional organic moiety and may be the same or different as R2;
• R4, R5, R6, R7 are organic moieties, may be the same or different and selected from the group consisting of hydrogen, C1-10 alkyl, silyl, C1-6 alkoxy, C3-8 cycloalkyl, C3-8 cycloalkoxy, aryl, aryloxy, C1-10 amine, C1-10 nitro, C1-10 cyano, a halide selected from the group consisting of F, Cl, Br, and I, and a 5- or 6 membered heterocycle containing from 1 to 4 heteroatoms selected from oxygen, sulfur, nitrogen, and phosphorous;
• and
• R8 is an organic moiety selected from the group consisting of hydrogen, C1-10 alkyl, silyl, C1-6 alkoxy, C3-8 cycloalkyl, C3-8 cycloalkoxy, aryl, aryloxy, C1-10 amine, C1-10 nitro, C1-10 cyano, a halide selected from the group consisting of F, Cl, Br, and I, and a 5- or 6 membered heterocycle containing from 1 to 4 heteroatoms selected from oxygen, sulfur, nitrogen, and phosphorous.