Polymerization catalysts

1. In a process for the polymerization or copolymerization of heterocycles, the improvement comprising using the polymerization catalyst a compound of the formula

wherein M is a lanthanide

R M is selected from the group consisting of hydrogen, halogen, alkyl, cycloalkyl, aryl, alkoxy, cycloalkoxy, aryloxy, alkylthio, cycloalkylthio, arylthio, amino, alkylamino, dialkylamino, cycloalkylamino, di(cycloalkyl)amino, alkyl(cycloalkyl)amino, arylamino, diarylamino, alkylarylamino and (cycloalkyl)arylamino;

A and B are, independently, a carbon chain of 2 to 4 carbon atoms, optionally substituted by at least one member of the group consisting of substituted or non-substituted alkyl, cycloalkyl and aryl, the substituents being selected from the group consisting of halogen, alkyl, nitro and cyano;

L 1 , L 2 and L 3 are, independently, -E 15 (R 15 )— in which E 15 is an element of Group 15 of the Periodic Table and R 15 is selected from the Group consisting of i) hydrogen, ii) substituted or non-substituted alkyl, cycloalkyl and aryl, in which said substituent is selected from the Group consisting of halogen, alkyl, nitro and cyano, iii) -E 14 RR′R″ in which E 14 is an element of Group 14 of the Periodic Table and R, R′ and R″ are independently selected from the group consisting of hydrogen, substituted or non-substituted alkyl, cycloalkyl, aryl, alkoxy, cycloalkoxy, aryloxy, alkylthio, cycloalkylthio and arylthio, in which said substituent is at least one member of the group consisting of halogen, alkyl, nitro and cyano; and iv) SO 2 R′ 15 in which R′ 15 is selected from the group consisting of halogen, alkyl, haloalkyl and aryl optionally substituted by at least one member of the group consisting of alkyl, haloalkyl and halogen.

2. The method of claim 1 , wherein the compound is in the form of a monomer or a dimer.

3. The method of claim 1 , wherein

R M is halogen;

A and B are, independently, a carbon chain with 2 to 4 carbon atoms;

L 1 , L 2 and L 3 are, independently, -E 15 (R 15 )— in which E 15 is nitrogen or phosphorus and R 15 is alkyl or -E 14 RR′R″, E 14 is carbon or silicon and R, R′ and R″ are, independently, hydrogen or alkyl.

4. The method of claim 1 , wherein M is samarium.

5. The method of claim 1 , wherein R M is chlorine; A and B are, independently, a carbon chain with 2 carbon atoms; L 1 , L 2 and L 3 are, independently, E 15 (R 15 )— in which E 15 is nitrogen and R 15 is selected from the group consisting of methyl, ethyl, propyl, isopropyl and -E 14 RR′R″ in which E 14 is silicon and R, R′ and R″ are, independently, selected from the group consisting of hydrogen, methyl, ethyl, propyl and isopropyl.

6. The method of claim 1 wherein the compound is

[(Me 3 SiNCH 2 CH 2 ) 2 NMe]SmCl.

7. The method of claim 6 wherein the compound is in the form of a dimer.