Systematic Investigations of Ligand Steric Effects on α-Diimine Palladium Catalyzed Olefin Polymerization and Copolymerization

Abstract

In the Brookhart type α-diimine palladium catalyst system, it is highly challenging to tune the polymer branching densities through ligand modifications or polymerization conditions. In this contribution, we describe the synthesis and characterization of a series of α-diimine ligands and the corresponding palladium catalysts bearing both the dibenzhydryl moiety and with systematically varied ligand sterics. In ethylene polymerization, it is possible to tune the catalytic activities ((0.77–8.85) × 105 g/(mol Pd·h)), polymer molecular weights (M n: (0.2–164.7) × 104), branching densities (25–116/1000C), and polymer melting temperatures (amorphous to 98 °C) over a very wide range. In ethylene–methyl acrylate (E–MA) copolymerization, it is possible to tune the catalytic activities ((0.3–8.8) × 103 g/(mol Pd·h)), copolymer molecular weights (1.1 × 103–79.8 × 103), branching densities (30–119/1000C), and MA incorporation ratio (0.4–13.8%) over a very wide range. The molecular weights and branching densities could also be tuned in α-olefin polymerization. The tuning in polymer microstructures leads to significant tuning in polyethylene mechanical properties and the surface properties of the E–MA copolymer.

Palladium‐Catalyzed Ethylene/Methyl Acrylate Co‐Oligomerization: The Effect of a New Nonsymmetrical α‐Diimine with the 1,4‐Diazabutadiene Skeleton

“…The same strong preference for the trans species was also found in the case of the neutral Pd complex with the nonsymmetrical ArBIAN ligand L4 , [Pd(CH 3 )Cl( L4 )] [ 10 (Cl)], which suggests that the preference for one isomer over the other is not affected by the ligand skeleton. Likewise, in the case of the neutral Pd complexes with L7 – L11 (Figure a) two isomers were observed in solution, but no indication of the nature of the major species was reported …”

“…As far as the synthesis of the nonsymmetrical ArDAB molecules reported up to now (Figure ) is concerned, all of these ligands share the presence of one aryl ring substituted in the 2‐ and 6‐positions with highly encumbered groups. The reaction of 2,3‐butanedione with the hindered 2,6‐disubstituted aniline led to the monoketoimine, which was isolated and then reacted with the other aniline …”

“…During the preparation of this manuscript, Pd catalysts for the ethylene/MA copolymerization with five nonsymmetrical ortho ‐substituted ArDAB ligands have been reported. They are characterized by one aryl ring that bears benzhydryl groups and the other that has various aliphatic or aromatic substituents of increasing bulkiness (Figure a) . As far as other nonsymmetrical ArDAB molecules are concerned, when we designed L1 , only three other examples were known, all of which featured iso ‐propyl groups on the ortho positions of one aryl ring (Figure b) .…”

Polar‐Functionalized, Crosslinkable, Self‐Healing, and Photoresponsive Polyolefins

“…The incorporation of polar comonomers is expected to efficiently modify the surface properties of polyolefin materials. Here, the surface properties of the polymer products were studied by measuring their water contact angles (WCAs) . The measurement samples were prepared by evaporation of about 5 wt % polymer toluene solutions on glass slides.…”

Highly resilient polyethylene elastomers prepared using α‐diimine nickel catalyst with highly conjugated backbone

“…[21][22][23] Owing to the sterically bulky ortho substituents on the aryls of the ligand that are positioned in axial sites of the central metal, the chain transfer reaction is greatly suppressed, and high-molecular-weight hyperbranched polyethylene (with ca 100 branches per 1000 carbons) can be prepared using typical α-diimine Ni(II) catalysts ( Figure 1, B-Cat). [26][27][28] In recent years there have been interesting research works concerning copolymerization of ethylene and α-olefins (1-hexene, 1-octene, 1-decene, 1-dodecene) catalyzed by α-diimine Ni(II) catalyst for the preparation of thermoplastic elastomers. [24] It is feasible that by tuning of the steric and electronic properties of ligands, specific α-diimine Ni(II) or Pd(II) catalysts can be designed for the preparation of high-molecular-weight polyethylenes with controlled branching [25] which can act as alternative materials to thermoplastic elastomers.…”