Theoretical study on effects of substituent, ligand, and metal on Pd-catalyzed polymerization of norbornene

[(2-R-allyl)Pd(mesitylene)][SbF6] is a class of highly active catalyst for the polymerization of norbornene (NB). In this work, the detailed mechanism of this polymerization reaction was studied by density functional calculations (B3LYP) augmented with dispersion correction and PCM solvation model. The insertion of the second NB monomer is the initiation step which has the highest activation barrier in the overall cycle. The activation barrier of this step is 11.6 kcal/mol higher than that of polymer chain propagation. The stable intermediate before the second NB insertion is a cationic Pd complex with bidentate coordination of one NB monomer, which is consistent with previously reported crystal structure. However, the resting state of propagation is a cationic Pd complex with two coordinated NB monomers and the growing NB chain. The activation barrier of initiation decreases if electron withdrawing substituents are introduced into the ally moiety. Particularly, the 1,1,2-trichloro-allyl substituent decreases the activation barrier to +10.7 kcal/mol, which is lower than that of propagation. On the other hand, all the examined ligands such as pyridine, NMe3, PMe3 and N-heterocyclic carbene increase the activation barrier of initiation. Ni catalyst significantly decreases the activation barriers of both initiation and propagation. However, the barrier of initiation of Ni catalyst is still much higher than that of propagation.