| Since the Nobel-Prize-winning discovery of Ziegler and Natta catalysts, transition metal catalyzed olefin polymerization has enjoyed great successes both in industry and in academia. Another milestone discovery in this field is the study of α-diimine Pd(Ⅱ) and Ni(Ⅱ) catalysts by Brookhart et al. in 1990s. Since then, numerous efforts have been directed to this area, leading to the developments of various α-diimine based metal catalysts and a huge amount of functional polymers and copolymers. The most distinguishing feature of these catalysts is the chain walking mechanism, leading to the formation of highly branched polymeric materials. The branching density is an important parameter that greatly affects the polymer physical properties. In α-diimine Ni(Ⅱ) catalysts, the polymer microstructures can be modulated from highly linear to highly branched by varying the polymerization conditions, such as ethylene pressures or polymerization temperatures. In the Pd(Ⅱ) system, the topology and long chain branching distribution of the polymers can be controlled using ethylene pressure. However, the polymer molecular weight, branching density and the distribution of short-chain branches are relatively independent of polymerization conditions such as ethylene pressures or polymerization temperatures. Therefore it is proven to be highly challenging to tune α-diimine Pd(Ⅱ) catalysts, and most of my work is about modifications of the α-diimine Pd(Ⅱ) catalysts, realize the precise control of the α-diimine Pd(Ⅱ) catalysts and its application:1. A series of sterically demanding α-diimine ligands bearing electron-donating and -withdrawing substituents were synthesized based on an improved synthetic procedure at great yields. Subsequently, the Pd complexes were prepared and isolated using column chromatography. These Pd complexes demonstrated unique properties in ethylene polymerization, including high thermal stability and high activity, generating polyethylene with high molecular weight and very low branching density. Similar properties were observed in ethylene/methyl acrylate copolymerization. Because of the high molecular weight and low branching density, the generated polyethylene and ethylene/methyl acrylate copolymer are semicrystalline solids. The (co)polymers possess unique microstructures originated from the unique slow chain walking feature of these Pd complexes.2. The introduction of even a small amount of polar functional groups into the polyolefins could excise great control over important material properties. As the most direct and economic strategy, the copolymerization of olefin with polar functionalized monomers represents one of the biggest challenges in this field. The presence of polar monomers usually dramatically reduces the catalytic activities and the copolymer molecular weight (to the level of thousands even hundreds), making the copolymerization process and the copolymer materials far away from any potential industrial applications. In this contribution, we demonstrate that these obstacles could be potentially addressed through rational catalyst design. Copolymers with highly linear microstructure, high melting temperatures and very high molecular weights (close to or above one million) could be generated. The concept of direct synthesis of polar functionalized ultra-high-molecular-weight polyethylene was introduced, which has not been deemed possible previously.3. In the Brookhart type a-diimine palladium catalyst system, it is highly challenging to tune the polymer branching densities through ligand modifications or polymerization conditions. In this contribution, we described the synthesis and characterization of a series of a-diimine ligands and the corresponding palladium catalysts bearing dibenzhydryl moiety and with systematically varied ligand sterics. In this system and in ethylene polymerization, it is possible to tune the catalytic activities (0.77-8.85×105 g/(mol Pd·h)), polymer molecular weights (Mn:0.2-164.7 × 104), branching densities (25-116/1000C) and polymer melting temperatures (amorphous to 98 ℃) over a very wide range. In ethylene-methyl acrylate (E-MA) copolymerization, it is also 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 a-olefin polymerization. The tuning in polymer microstructures led to the significant tuning in polyethylene mechanical properties and the surface properties of the E-MA copolymer.4. A series of iminopyridyl Ni(Ⅱ) catalysts containing both the dibenzhydryl and the naphthyl moieties can polymerize ethylene with high activity and high thermal stability, generating polyethylene with molecular weight of up to one million in this chapter. In a-olefin polymerization, semicrystalline polymers with high melting temperatures are generated. |
