Selective 3,4-polymerization mechanism of isoprene catalyzed by rare earth alkyl complexes

The mechanism of selective 3,4-polymerization reaction of isoprene catalyzed by the rare earth lutecium(III) alkyl complexes [2,6-Me₂Ph-N-CH₂-C(CH₂SiMe₃)=N-PhMe₂-2,6]Lu(CH₂SiMe₃)₂(THF) was investigated by means of the M06/sdd method with solvation effects taken into account. The results show that the structure of the catalyst core remained almost unchanged as the isoprene molecules were alternatively inserted into the complex at two opposite sides. The Gibbs free energies of the coordination complexes, transition state and intermediates indicate that all the isoprene molecules prefer to insert into the complex with the 3,4-polymerization selectivity as catalyzed by the catalyst, which is consistent with the experimental observations. It is found that the insertion reaction of each isoprene is exothermic, which comes mainly from the coordination of the isoprene molecule to the lutecium(III) atom. The solvation effects were confirmed important in predicting the Gibbs free energies of the present reaction system.