Use of a compound according to formula I shown below in the polymerisation of cyclic esters or cyclic amides: wherein: R 1 and R 2 are each independently alkyl; R 3 and R 4 are each independently hydrogen or alkyl, or R 3 and R 4 are linked such that, when taken in combination with the atoms to which they are attached, they form a 6-membered fused aromatic ring optionally substituted with one or more groups selected from alkyl, alkenyl, alkynyl, alkoxy, aryl, heteroaryl, carbocyclic and heterocyclic, wherein each aryl, heteroaryl, carbocyclic and heterocyclic group is optionally substituted with one or more groups selected from alkyl, alkenyl, alkynyl, alkoxy, halo, amino, nitro, cyano, alkylamino, [ alkyl] 2 amino and -S 2 alkyl; R 5 and R 6 are each independently hydrogen or alkyl, or R 5 and R 6 are linked such that, when taken in combination with the atoms to which they are attached, they form a 6-membered fused aromatic ring optionally substituted with one or more groups selected from alkyl, alkenyl, alkynyl, alkoxy, aryl, heteroaryl, carbocyclic and heterocyclic, wherein each aryl, heteroaryl, carbocyclic and heterocyclic group is optionally substituted with one or more groups selected from alkyl, alkenyl, alkynyl, alkoxy, halo, amino, nitro, cyano, alkylamino, [ alkyl] 2 amino and -S 2 alkyl; R a and R b are independently selected from alkyl, alkoxy, alkenyl, alkynyl, alkylamino, [ alkyl] 2 amino, aryl, halo, amino, nitro and cyano; X is selected from zirconium or hafnium; and each Y group is independently selected from halo, hydride, a phosphonated, sulfonated or borate anion, or a alkyl, alkenyl, alkynyl, alkoxy, aryl or aryloxy group which is optionally substituted with one or more groups selected from alkyl, halo, nitro, amino, phenyl, alkoxy, -C NR x R y, or Si[ alkyl] 3; and wherein R x and R y are independently alkyl. R 1 and R 2 are each independently alkyl; R 3 and R 4 are each independently hydrogen or alkyl, or R 3 and R 4 are linked such that, when taken in combination with the atoms to which they are attached, they form a 6-membered fused aromatic ring optionally substituted with one or more groups selected from alkyl, alkenyl, alkynyl, alkoxy, aryl, heteroaryl, carbocyclic and heterocyclic, wherein each aryl, heteroaryl, carbocyclic and heterocyclic group is optionally substituted with one or more groups selected from alkyl, alkenyl, alkynyl, alkoxy, halo, amino, nitro, cyano, alkylamino, [ alkyl] 2 amino and -S 2 alkyl; R 5 and R 6 are each independently hydrogen or alkyl, or R 5 and R 6 are linked such that, when taken in combination with the atoms to which they are attached, they form a 6-membered fused aromatic ring optionally substituted with one or more groups selected from alkyl, alkenyl, alkynyl, alkoxy, aryl, heteroaryl, carbocyclic and heterocyclic, wherein each aryl, heteroaryl, carbocyclic and heterocyclic group is optionally substituted with one or more groups selected from alkyl, alkenyl, alkynyl, alkoxy, halo, amino, nitro, cyano, alkylamino, [ alkyl] 2 amino and -S 2 alkyl; R a and R b are independently selected from alkyl, alkoxy, alkenyl, alkynyl, alkylamino, [ alkyl] 2 amino, aryl, halo, amino, nitro and cyano; X is selected from zirconium or hafnium; and each Y group is independently selected from halo, hydride, a phosphonated, sulfonated or borate anion, or a alkyl, alkenyl, alkynyl, alkoxy, aryl or aryloxy group which is optionally substituted with one or more groups selected from alkyl, halo, nitro, amino, phenyl, alkoxy, -C NR x R y, or Si[ alkyl] 3; and wherein R x and R y are independently alkyl.