The inert nitrogen microbubbles are incorporated during ethylene slurry polymerization to retard the formation of chain entanglements through the intermittent dormancy of living polymer chains. This dormancy effect is endowed by the highly frequent and intensive collisions of countless microbubbles on the growing polyethylene particles, which blocks the transfer channel of reactants on the particle surface. It is evidenced that the transfer barrier slows down the chain propagation and provides extra time for the propagated chains to be crystallized. Thus, the nascent polyethylene with the reduced entanglements and a considerable amount of monoclinic lamellae is achieved by the catalyst with the original ability to synthesize the highly entangled polyethylene. The synchronous increment of stiffness, toughness, and strength is found for the synthesized polyethylene owing to the reduced chain entanglements.

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The reduction in the entangled state of the nascent ultrahigh-molecular-weight polyethylene (UHMWPE) is of fundamental importance to enhance its processability and mechanical properties. However, it remains a challenge to retard the formation of entanglements during polymerization above 60 °C because the chain propagation rate is notably higher than the chain crystallization rate. Herein, we present a feasible method for synthesizing the weakly entangled polyethylene via nitrogen microbubble-assisted polymerization. The mass transfer of ethylene inside the growing polyethylene particles is found to dominate the chain propagation process, which is influenced by the nitrogen bubble size. The contact/interval time (10–3 s) between the microbubbles and the particles, observed by a high-speed camera, appears several orders less than the chain propagation time. Consequently, such frequent contact/separation instantly blocks/recovers the reactant transfer on the solvent–particle interphase, causing an intermittent “dormancy effect” to retard the chain propagation and facilitate the chain crystallization. Therefore, the less entangled polyethylene is synthesized at a relatively high activity.

Polyhedral oligomeric silsesquioxane (POSS) nanoaggregates can serve as a modifier of the MgCl 2 -supported Z-N catalyst, thus controlling the polymerization behavior and modifying the microstructure of the synthesized polymer. In this study, a series of POSS-modified MgCl 2supported Z-N catalysts were explored to reveal the relationship between the entanglement formation of ultrahigh-molecular-weight polyethylene (UHMWPE) and the adhesive effect control of hydroxy groups in POSSs. X-ray photoelectron spectroscopy (XPS) and CO in situ FTIR lowtemperature adsorption technology were employed to investigate the catalytic surface structure. It was evidenced that POSSs can coordinate with the MgCl 2 (110) plane, acting as adhesive of a cage structure to be in situ embedded into the MgCl 2 -based support. Therefore, the loaded TiCl 4 active sites were isolated in the molecular scale on the MgCl 2 -based support and the entanglements were greatly suppressed thereof. The ″adhesion effect″ of the MgCl 2 support can be controlled by adjusting the type (i.e., hydroxyl content) of POSS introduced, thereby suppressing entanglements to a greater extent. The MgCl 2 support modified with POSS with no hydroxy groups endowed it with the optimum entanglement reduction effect. As a consequence, the as-synthesized polyethylene exhibits an overall promotion of the impact resistance (+14.1%), tensile strength (+13.9%), Young's modulus (+16.8%), and break elongation (+10.5%), compared to those of UHMWPE synthesized from the POSS-free catalyst. In addition, the POSS-modified MgCl 2 -based catalyst shows more stable catalytic activity and responsiveness of molecular weight with hydrogen modulation, which exhibits great prospects of industrial application in polyethylene production.