Wei Li

Heterogeneous catalysis is the most widely used process for ethylene polymerization in industry. Active site distribution is one of the crucial issues in the heterogeneous catalysis, which strongly influences the catalytic process and the microstructure of synthesized polyethylene. In this respect, aiming at an explicit understanding of the progress, the challenge and the possible future directions in the study of heterogeneous catalysis of ethylene polymerization, the recent strategies of designing heterogeneous catalysts are reviewed from the perspective of tailoring the active site distribution, where the regulation of chain structures, including molecular weight distribution, branch distribution and entanglement state, are highlighted. The in situ and operando characterizations are also reviewed for assisting the understanding of structural information in heterogeneous catalysts.

Linear low‐density polyethylene (LLDPE) is reinforced with low‐entanglement ultra‐high molecular weight polyethylene (UHMWPE) and formed using different melt processing methods. The results show that the low‐entanglement UHMWPE has good compatibility with LLDPE within a low addition range, which significantly improves the crystallinity and long period of LLDPE/UHMWPE composites. Furthermore, the shish‐kebab structure of the composite material is more perfect after injection molding and hot drawing. Due to the optimized structure, the mechanical properties of the composite are greatly improved. After hot drawing, the Young's modulus and tensile strength of the composites are increased by 350% and 565% respectively. Simultaneously, after injection molding and hot stretching molding, the thermal conductivity of the material is increased by 138% and 711% respectively. The thermal conductivity of the materials can be significantly improved by hot drawing. The current study opens a new avenue for the preparation of bulk polymers using the LLDPE matrix with simultaneously enhanced thermal conductivity and mechanical properties, with great potential for applications in thermal management. © 2022 Society of Industrial Chemistry.

By introducing low-entanglement UHMWPE, the mechanical properties of polyolefins are improved to varying degrees. For polypropylene, the lack of interaction between UHMWPE and polypropylene results in an unsatisfactory reinforcement effect, and the disentangled state makes it easier for the particles to form defects driven by a chain explosion. In contrast, regarding polyethylene and elastomer containing ethylene segments, low-entanglement UHMWPE plays a better role in reinforcement. A series of measurements including scanning electron microscopy (SEM), rheological measurements, differential scanning calorimetry (DSC), and mechanical measurement were used to investigate the mechanisms for the different enhancement effects. It originates from interdiffusion and entanglement forming of polyethylene segments across the interface, endowing the material with different aggregated and defect structures. For instance, EPDM possesses a higher optimal dosage of UHMWPE particles reflected in good interfacial interdiffusion with UHMWPE particles, leading to significant optimized mechanical performance.