Chain walking polymerization of ?- olefins by ?- diimine Ni(ii) complexes

A milestone in the field of olefin polymerization was the discovery of ?-diimine Ni(II) and Pd(II) based catalysts by Brookhart in the mid-90s.[1] The peculiarity of these catalysts is attributed to the ability of the active growing site to «walk » on the polymer chain during the propagation step (chain-walking),[2] so that the new incoming monomer unit is assembled onto the polymer backbone rather than at the end, giving branched and hyperbranched polymers which may have interesting applications as thermoplastic elastomers (TPEs). TPEs are a class of polymers that combine the processing advantages and recycling potential of thermoplastics with the flexibility, low modulus, and soft touch of elastomers. Herein, as a part of our ongoing research in this field,[3,4,5] we report a systematic investigation on the polymerization of 1-octene, 1-decene, and 1-octadecene catalysed by differently substituted ?-diimine Ni(II) complexes (Fig.1). The effect of ligand steric and electronic perturbation on the catalytic behavior, monomer enchainment, and polymer properties is discussed. The polymers were characterized by 13C NMR for quantification of the total branching level and branch-type distribution: some unique features were identified and related to the chain-walking coordination/insertion mechanism. The tensile tests showed that the 1-octene and 1-decene polymers behave as elastomers exhibiting low modulus, high elongation at break, and good elastic recovery, while 1-octadecene polymers behave as plastomers. In addition, the crystal structures of Ni3-Ni6 are reported.