CO/styrene copolymerization by α-diimine palladium catalysts is a promising method for direct synthesis of polyketones. The effect of the catalyst backbone structure on CO/styrene copolymerization has been studied with the aim of developing robust α-diimine palladium catalysts able to improve the polymerization productivity and controllability. Dibenzobarrelene derived α-diimine palladium catalysts without o-aryl substituents were designed and synthesized for CO/styrene alternating copolymerization. Introduction of the rigid and bulky dibenzobarrelene backbone enhanced the thermal stability and the productivity of palladium catalyst. The dibenzobarrelene ligand backbone also improved the polymerization controllability, and living CO/styrene copolymerizations were achieved at 15 °C using α-diimine palladium catalysts in CH 2 Cl 2 . The steric hindrance of backbone and the π−π stacking between the dibenzobarrelene backbone and the aniline played crucial roles in stereocontrol and productivity.
Combined
bulky 8-p-tolylnaphthylamine and bulky
dibenzo-/dinaphthobarrelene backbones, unprecedented “opening
box”-like α-diimine dibromonickel complexes were synthesized.
Their square-planar geometries and the Ni–phenyl interactions
in both solid and solution were identified by single-crystal X-ray
diffraction analysis, NMR analysis, and density functional theory
(DFT) simulations. Bulky α-diimine nickel complexes as ethylene
polymerization precatalysts exhibited enhanced activity and thermal
stability and produced unexpected lightly branched polyethylenes (PEs)
(32–42/1000C) with melting temperatures of 81–93 °C.
A combined experimental and theoretical study clearly showed how weak
attractive Ni–phenyl interactions could decrease PE branching
density in ethylene polymerization. The ethylene coordination as a
crucial step was promoted by Ni–phenyl interactions, causing
acceleration of linear chain growth.
To better understand the substituent
effects of vinyl arene, a
series of substituted styrenes with different groups and locations
(n-X-St, n = 2, 3, 4; X = H, Me,
t
Bu, MeO,
t
BuO,
F, Cl, and Br) were used as comonomers for palladium-catalyzed vinyl
arene/CO copolymerization. Dibenzobarrelene-based α-diimine
palladium catalyst Pd1 was capable of catalyzing alternating
copolymerizations of substituted styrene comonomers and CO in a living
fashion, which excluded the effect of the palladium catalyst. Electronic
effects of comonomer substituents were quantitatively examined by
Hammet constants (σ) of substituents and highest occupied molecular
orbital (HOMO) energies of comonomers. Experimental results clearly
showed that the turnover number (TON) of copolymerization, copolymer
molecular weight, and stereoregularity were greatly affected by the
inserted substituent. The steric effect of 4-substituents was presented
and clearly proved in addition to a widely acceptable electronic effect.
Strikingly, the unprecedented positioning effects of comonomer substituents
were initially discovered. Whatever substituents were located on 3-position,
vinyl arene/CO alternating copolymerization was greatly promoted because
of the neglectable steric effect of 3-substituents.
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