| Abstrakt: |
Associative displacement and H-exchange chain transfer/termination mechanisms for the diimine−M(II)-catalyzed (M = Ni, Pd) ethylene polymerization have been studied using B3LYP and IMOMM methods. For unsubstituted diimine complexes the coordination of ethylene to the metal−olefin−hydride complexes L2M(C2H4)H+ and L2M(C3H6)H+ is exothermic and gives the five-coordinate complex 22. From 22, the following processes can take place: (a) the associative displacement, path E, corresponding to dissociation of propylene, (b) the dissociation of ethylene, reverse path D, (c) the H-exchange, path F, and (d) the reattaching of the hydrogen to the polymer chain, path G. For M = Ni, the associative displacement cannot compete with paths F and G and is unlikely to take place. For M = Pd, the energetics for the paths E−G are similar, and the chain transfer/termination via the associative dissociation path E is more likely. The H-exchange process, path F, is the most favorable chain transfer/termination mechanism for both metals and includes (a) oxidative addition of the β-agostic Cβ−Hagostic bond to the metal center to form the metal−olefin−hydride complex 21, (b) the coordination of ethylene to the metal center to form the five-coordinate complex 22, and (c) migration of the hydrogen atom from the metal to the ethylene molecule. The rate-determining steps are steps a and c for the diimine−Ni- and diimine−Pd-catalyzed reactions, respectively. The substitution of the imine hydrogens with bulky aromatic groups 2,6-C6H3(i-Pr)2 makes 22 thermodynamically unstable relative to C2H4 + diimine-M(C3H7)+. Therefore, all processes starting from 22 become unimportant. These results were compared with the previously studied β-hydrogen transfer and hydrogenolysis chain transfer mechanisms. |
