Role of Radical Species in Salicylaldiminato Ni(II) Mediated Polymer Chain Growth: A Case Study for the Migratory Insertion Polymerization of Ethylene in the Presence of Methyl Methacrylate

To date, an inconclusive and partially
contradictive picture exists on the behavior of neutral Ni­(II) insertion
polymerization catalysts toward methyl methacrylate (MMA). We shed
light on this issue by a combination of comprehensive mechanistic
NMR and EPR studies, isolation of a key Ni­(I) intermediate, and pressure
reactor studies with ethylene and MMA, followed by detailed polymer
analysis. An interlocking mechanistic picture of an insertion and
a free radical polymerization is revealed. Both polymerizations run
simultaneously (25 bar ethylene, neat MMA, 70 °C); however, the
chain growth cycles are independent of each other, and therefore exclusively
a physical mixture of homo-PE and homo-PMMA is obtained. A Ni–C
bond cleavage was excluded as a free radical source. Rather a homolytic
P–C bond cleavage in the labile aryl phosphine ligand and the
reaction of low-valent Ni­(0/I) species with specific iodo substituted
N^O (Ar–I) ligands were shown to initiate radical MMA polymerizations.
Several reductive elimination decomposition pathways of catalyst precursor
or active intermediates were shown to form low-valent Ni species.
One of those pathways is a bimolecular reductive coupling via intermediate
(N^O)­Ni­(I) formation. These intermediate Ni­(I) species can be prevented
from ultimate decomposition by capturing with organic radical sources,
forming insertion polymerization active [(N^O)­Ni­(II)–R] species
and prolonging the ethylene polymerization activity.