Thermoplastic elastomers (TPE) are a class of high-molecular materials that can be easily molded, extruded, and reused like plastics, while also possessing the typical elastic properties of rubber. They are widely used in industries such as automotive and home appliances. The idea of synthesizing thermoplastic polyolefin elastomers from ethylene in a single step is both intriguing and challenging. In recent years, Professor Chen Changle's research group at USTC (University of Science and Technology of China) and some colleagues have successfully synthesized polyethylene thermoplastic elastomers via α-diimine nickel-catalyzed ethylene polymerization, achieving a series of significant results. Since the α-diimine palladium catalyst tends to have greater chain walking than nickel catalysts, it often results in highly branched polyolefins with inferior mechanical properties, making its use in the preparation and application of thermoplastic polyolefin elastomers very challenging. In this work, they have, for the first time, reported the direct synthesis of polyethylene thermoplastic elastomers catalyzed by an α-diimine palladium catalyst. With precise catalyst design and polymerization adjustment, high-performance polyethylene thermoplastic elastomers can be produced (with an elasticity recovery value reaching up to 83%). Most importantly and significantly, polar functionalized polyethylene thermoplastic elastomers can be prepared through the copolymerization of ethylene with biomass-derived comonomers, and the resultant polar functionalized polyethylene thermoplastic elastomers also exhibit excellent elastic properties (with an elasticity recovery value reaching up to 80%).

    In this study, the research team first prepared a diimine palladium catalyst substituted with a large steric hindrance tertiary butyl group, labeled 1-2. It demonstrates moderate polymerization activity in ethylene polymerization, producing polyethylene with a high molecular weight (Mn up to 45.58 x 10⁴) and a medium branching structure (60/1000C). At the same time, using the reported bulky palladium catalysts 3-7 under the same conditions, they synthesized a series of high molecular weight polyethylenes with varying branch structures (shown in

Figure 2). Additionally, they utilized catalysts 1-2 to prepare high molecular weight (Mn up to 34.09 x 10⁴) medium branched biomass-functionalized polyethylene, which retains high activity during copolymerization.

Figure 2. Diimine palladium catalysts prepared in this study with tertiary butyl substitution and a series of reported bulky diimine palladium catalysts.

    The polymers obtained from the aforementioned catalysts were first subjected to tensile strength tests (Figure 3). These samples showed mechanical properties characterized by relatively low break stress values (3.3-17.6 MPa) and very high strain-at-break values (452% - 1841%). Typically, under the same other conditions, the breaking stress and Young's modulus increase with increasing ethylene pressure. This indicates that increasing the melting point of the polymer or reducing its branching density will increase the material's ultimate tensile strength and tensile toughness. Meanwhile, under the same other conditions, polyethylene obtained from catalyst 1 has higher ultimate tensile strength and tensile toughness than that obtained from catalyst 2. This is mainly because the electron-rich catalyst 1 can produce polyethylene with a lower degree of branching and a higher melting point. The aforementioned experiments show that by merely altering the polymerization conditions or modifying the electronic effects of the ligands, the mechanical properties of the resulting polyethylene can be adjusted. For comparison, they examined polyethylene produced by catalysts 4-6, which displayed very high breaking stress values (19.6-26.0 MPa) and high strain-at-break values (500% - 860%). This suggests that polymers prepared by catalysts 1-2 have better elastomeric potential. They further studied the elasticity recovery rate of the resulting polymers (strain lag tests).

Figure 3. Stress-strain curves of the prepared polyethylene and polar copolymers.

    Polyethylene prepared using catalysts 1-2 had an elasticity recovery rate (SR) of 68% to 83%, comparable to previously reported polyethylene elastomers obtained through α-diimine nickel catalysts (Figure 4). In contrast, polyethylene produced by catalysts 5-7 displayed inferior elastic properties, with SR values of 22% -27% (Figure 4). The polymer's high molecular weight and the appropriate high branching microstructure (high entropic elasticity and suitable crystalline ratio) seem to be the primary factors contributing to their excellent elasticity. Thus, the catalytic system reported in this study offers an alternative effective route for the synthesis of thermoplastic polyethylene elastomers. Most importantly, to the best of our knowledge, this is the first time thermoplastic polyethylene elastomers have been prepared via an α-diimine palladium system, with most previous preparations coming from nickel systems. Interestingly, biomass-functionalized polyethylene prepared by catalysts 1-2 also has excellent elastic properties (SR = 72% -80%) (Figure 5). The above experimental results indicate that the structure of the catalyst plays a decisive role in the elastic properties of these polymer samples.

    The related research was published under the title "Direct Synthesis of Polar Functionalized Polyethylene Thermoplastic Elastomer" in the journal Macromolecules (2020, 53, 2539-2546). Professor Dai Shengyu of Anhui University and Professor Chen Changle of the University of Science and Technology of China were the corresponding authors. This work was funded by the National Natural Science Foundation of China  (NSFC 51703215, 21690071, U19B6001, and U1904212).

Reference link:

https://pubs.acs.org/doi/10.1021/acs.macromol.0c00083