Tunable “In-Chain” and “At the End of the Branches” Methyl Acrylate Incorporation in the Polyolefin Skeleton through Pd(II) Catalysis

The synthesis of
functionalized polyolefins through coordination–insertion
polymerization is a highly challenging reaction. The ideal catalyst,
in addition to showing a high productivity, has to be able to control
the copolymer microstructure and, in particular, the way of the polar
vinyl monomer incorporation. In this contribution, we modified the
typical Brookhart’s catalyst by introducing in the fourth coordination
site of palladium a hemilabile, potentially bidentate ligand, such
as a thiophenimine (N–S). The obtained cationic Pd­(II) complexes,
[Pd­(Me)­(N–N)­(N–S)]­[PF6], generated active
catalysts for the ethylene/methyl acrylate (MA) copolymerization leading
to the desired copolymer with a different incorporation of the polar
monomer depending on both the reaction medium and the N–S ligand.
Surprisingly enough, the produced copolymers have the inserted acrylate
both at the end of the branches (T­(MA)) and in the main chain (M­(MA))
in a ratio M­(MA)/T­(MA) that goes from 9:91 to 45:55 moving from dichloromethane
to trifluoroethanol (TFE) as a solvent for the catalysis and varying
the N–S ligand. The catalytic behavior of the new complexes
was compared to that of the parent compound [Pd­(Me)­(N–N)­(MeCN)]­[PF6], highlighting the fact that when the copolymerization is
carried out in trifluoroethanol, this complex is also able to produce
the E/MA copolymer with MA inserted both in the main chain and at
the end of the branches. Accurate NMR studies on the reactivity of
the precatalyst [Pd­(Me)­(N–N)­(MeCN)]­[PF6] with the
two comonomers allowed us to discover that in the fluorinated solvent,
the catalyst resting state is an open-chain intermediate having both
the organic fragment, originated from the migratory insertion of MA
into the Pd–Me bond, and the acetonitrile coordinated to palladium
and not the six-membered palladacycle typically observed for the Pd-α-diimine
catalysts. This discovery is also supported by both DFT calculations
and in situ NMR studies carried out on [Pd­(Me)­(N–N)­(N–S)]­[PF6] complexes that point out that N–S remains in the
palladium coordination sphere during catalysis. The open-chain intermediate
is responsible for the growth of the copolymer chain with the polar
monomer inserted into the main chain.