Engineering Nascent Disentangled Ultra-High-Molecular-Weight Polyethylene Based on Heterogeneous Catalytic Polymerization

Abstract

Ultra-high-molecular-weight polyethylene (UHMWPE) is a pivotal material in engineering and biomedical applications due to its exceptional mechanical strength, wear resistance, and impact performance. However, its extreme melt viscosity, caused by extensive chain entanglements, severely limits processability via conventional melt-processing techniques. Recent advances in catalytic synthesis have enabled the production of disentangled UHMWPE (dis-UHMWPE), which exhibits enhanced processability while retaining superior mechanical properties. Notably, heterogeneous catalytic systems, utilizing supported fluorinated bis (phenoxy-imine) titanium (FI) catalysts, polyhedral oligomeric silsesquioxanes (POSS)-modified Z-N catalysts, and other novel catalysts, have emerged as promising solutions, combining structural control with industrial feasibility. Moreover, optimizing polymerization conditions further enhances chain disentanglement while maintaining ultra-high molecular weights. These systems utilize nanoscale supports and ligand engineering to spatially isolate active sites, tailor the chain propagation/crystallization kinetics, and suppress interchain entanglement during polymerization. Furthermore, characterization techniques such as melt rheology and differential scanning calorimetry (DSC) provide critical insights into chain entanglement, revealing distinct reorganization kinetics and bimodal melting behavior in dis-UHMWPE. This development of hybrid catalytic systems opens up new avenues for solid-state processing and industrial-scale production. This review highlights recent advances concerning interaction between catalyst design, polymerization control, and material performance, ultimately unlocking the full potential of UHMWPE for next-generation applications.

1. Introduction

Ultra-high-molecular-weight polyethylene (UHMWPE) stands as a cornerstone material in modern engineering and biomedical applications due to its exceptional combination of mechanical, chemical, and tribological properties [1,2,3,4,5,6]. Its semi-crystalline structure endows it with unparalleled impact resistance, low friction, and wear resistance, making it indispensable for high-performance applications such as ballistic armor, artificial joint prostheses, marine cables, and high-strength fibers [7,8,9,10,11]. High molecular weight (MW) above 10⁶ g/mol reflects longer chains which enhance intermolecular interactions, leading to superior load-bearing capacity and durability [12,13,14,15,16,17]. Yet, this very attribute also presents a fundamental challenge, that is., the extreme chain length induces extensive physical entanglement, resulting in a melt viscosity that exceeds 10⁸ Pa·s, which is orders of magnitude higher than conventional thermoplastics [18,19,20,21,22,23]. This renders UHMWPE nearly intractable via standard melt-processing techniques like extrusion or injection molding, necessitating specialized methods such as gel spinning or powder sintering that are energy-intensive and limit design flexibility [22,23,24,25,26]. Despite its promise, the industrial utilization of UHMWPE has been constrained by the delicate balance between its desirable mechanical properties and the processing hurdles imposed by chain entanglement, driving the quest for innovative synthesis strategies to produce disentangled UHMWPE (dis-UHMWPE) with tailored molecular architectures [27,28].

Traditional UHMWPE production relies on Ziegler–Natta (Z-N) catalysts, which operate via heterogeneous polymerization to yield polymers with broad dispersity (Đ) and high entanglement densities [29,30]. While these catalysts are industrially robust, their randomly distributed active sites on supports like MgCl₂ promote overlapping chain growth, leading to dense entanglements that trap polymer chains in a rigid network [3,31,32,33]. The resulting material exhibits a critical shear rate as low as 0.1 s⁻¹, below which melt fracture occurs, severely limiting its processability [34,35,36,37]. To address this, early research turned to homogeneous single-site catalysts, such as metallocenes and fluorinated bis (phenoxy-imine) titanium (FI) catalysts, which offer precise control over chain structure, enabling the synthesis of dis-UHMWPE with a narrow dispersity (Đ < 3.0) and reduced entanglement [23,38,39,40]. However, these systems face scalability challenges, i.e., homogeneous catalysts often suffer from reactor fouling, limited thermal stability, and high costs, restricting their industrial adoption [25,41,42,43,44,45]. Concurrently, the discovery that dis-UHMWPE can undergo solid-state processing flowing plastically below its melting point (T_m) due to fewer entanglement nodes highlights the need to engineer nascent polymers with intrinsic low entanglement during polymerization, rather than relying on post-processing modifications [9,46,47]. This has prompted the development of hybrid heterogeneous catalytic polymerization to prepare dis-UHMWPE that combines the structural control of single-site catalysts with the practicality of common catalyst support for olefin polymerization, creating active sites that isolate growing chains and suppress interchain overlap (Figure 1) [48,49,50].

Characterizing the entanglement state of UHMWPE is critical to guiding catalyst design and optimizing polymerization conditions [51,52]. Melt rheological analysis, particularly dynamic time sweep tests, serves as a primary tool by monitoring the storage modulus (G′) during isothermal annealing at specific temperature such as 160 °C, which could be used to track the formation of entanglement networks [53,54]. In dis-UHMWPE, the slow build-up of G′ reflects fewer initial entanglements, indicating a metastable state where chains reorganize into entangled networks over time, in contrast to the rapid modulus increase in highly entangled commercial UHMWPE (C-UHMWPE) samples. Furthermore, thermodynamic annealing via differential scanning calorimetry (DSC) provides complementary insights, revealing dual melting peaks: a high-temperature peak (141–144 °C) from disentangled lamellae and a lower peak (134–136 °C) from re-entangled domains formed during cooling [55]. This bimodal behavior quantifies the fraction of chains that remain unentangled after melting, serving as a hallmark of dis-UHMWPE. Solid-state nuclear magnetic resonance (NMR) techniques, such as ¹³C cross-polarization magic angle spinning (CP/MAS) and low-field ¹H NMR, delve into the amorphous phase and identify monoclinic crystal signatures and heterogeneous chain dynamics indicative of reduced entanglement [56]. These methods collectively enable precise quantification of entanglement density, linking the structure of the catalyst to the polymerization conditions, and form the basis for iterative improvements in material design.

Heterogeneous catalytic systems have emerged as a transformative platform for dis-UHMWPE synthesis, leveraging nanoscale supports and innovative ligand engineering to create spatially isolated active sites [57,58,59,60]. Recent advances in heterogeneous catalytic polymerization techniques have opened up new pathways for synthesizing dis-UHMWPE, with supported FI catalyst systems demonstrating particular promise due to their well-defined molecular architecture and precisely controlled active centers that enable accurate regulation of polymer chain structure [61,62,63,64,65]. The immobilization of FI catalysts on optimized supports not only enhances catalyst dispersion but also effectively governs chain propagation and chain transfer reactions during polymerization, thereby significantly reducing chain entanglement while maintaining ultra-high MWs [66,67,68,69,70,71,72,73]. Meanwhile, polyhedral oligomeric silsesquioxane (POSS)-modified Z-N catalyst systems, incorporating POSS into conventional catalysts, leverage the unique nanostructure and chemical properties of POSS to modify active site distribution and the polymerization microenvironment, consequently directing polymer chain growth patterns and spatial arrangement to suppress entanglement formation and endow UHMWPE with distinctive microstructures and enhanced properties [4,74,75,76]. Current research further focuses on developing novel hybrid catalytic systems that integrate multiple catalytic advantages through molecular design and composite strategies, aiming to achieve high activity, superior selectivity, and precise chain structure control [11,77]. Along with catalyst development, meticulous optimization of polymerization conditions plays a crucial role in the degree of molecular chain entanglement [78]. For instance, temperature affects chain transfer reactions and short-chain branching, particularly in regulating the chain propagation kinetics and crystallization behavior to regulate molecular chain entanglement [31,79]. Secondly, moderate pressures can optimize monomer diffusion and reaction kinetics, and precise control of reactant concentrations prevents localized overconcentration-induced entanglement [3,23]. The synergistic combination of these advanced catalytic approaches, including supported FI catalysts, POSS-modified Z-N systems, and next-generation hybrid catalysts coupled with precisely tailored polymerization parameters establishes a robust framework for the efficient production of dis-UHMWPE [80,81], thereby not only addressing current processing challenges but also significantly expanding its potential applications in high-performance composites, medical implants, and advanced protective materials.

As the field progresses, the synthesis of dis-UHMWPE represents a convergence of catalysis, polymer physics, and materials engineering, offering solutions to long-standing challenges in both academia and industry. Current research highlights the critical role of heterogeneous catalysts in balancing MW, entanglement, and processability, with emerging strategies such as binuclear synergic effects and porous support design promising further breakthroughs. Future research will likely focus on optimizing catalyst activity and stability, exploring sustainable solvents and continuous processing techniques and integrating computational modeling to predict entanglement formation at the molecular level. By addressing these frontiers, the field can unlock the full potential of UHMWPE, enabling its use in next-generation applications, from high-performance composites to solid-state processing. The journey towards industrial-scale production of dis-UHMWPE not only enhances material performance but also redefines the boundaries of polymer synthesis, showcasing how targeted catalyst design and precise structural control can transform a historically intractable material into a versatile engineering marvel.

2. Entanglement Characterization of UHMWPE

The degree of entanglement in UHMWPE molecular chains has a significant impact on its processing properties (such as melt flowability and stretchability) and the mechanical properties of the final product (such as strength and modulus). Currently, a relatively mature quantitative analysis technology system has been established in this field, which primarily includes melt rheology methods, thermodynamic methods, and nuclear magnetic resonance technology [23]. Each of these methods has its own strengths and advantages, which together constitute the core technical basis for quantitative analysis of the degree of entanglement of UHMWPE chains and provide a crucial scientific basis for optimizing processing technology (such as low-temperature sintering and gel spinning) and improving material performance. Notably, scanning electron microscopy (SEM) has the potential to further evaluate the degree of molecular chain entanglement in prepared UHMWPE. Currently, researchers further utilize SEM technology to conduct auxiliary qualitative characterization of the prepared UHMWPE powder particles, aiming to analyze the appearance of solid product particles synthesized by different polymerization methods. Generally, UHMWPE powders with low entanglement chains are composed of aggregated micro particles and a typical stretched fibrous structure due to good flowability [27]. However, conventional high-entanglement UHMWPE powders may have severe entanglement of molecular chains, causing particles to easily adhere and aggregate, forming denser spherical or block structures with poor flowability, and they may lack this characteristic fibrous texture [82]. This discrepancy arises from the fact that low-entanglement structures afford greater molecular chain mobility, enabling more ordered arrangements and higher crystallinity [83]. This facilitates the formation of lamellar and fibrous crystalline structures. In contrast, high-entanglement structures, characterized by extensive molecular chain entanglement, disrupt the ordered fibrous morphology and result in relatively lower crystallinity.

2.1. Solid NMR Method

Solid-state NMR spectroscopy has emerged as a powerful tool for characterizing chain entanglement in ultra-high-molecular-weight polyethylene (UHMWPE), particularly through the analysis of crystalline phase structures. Li et al. [32] demonstrated the utility of low-field solid-state NMR in probing UHMWPE’s entangled structure through chain dynamics. Their approach involved analyzing free induction decay (FID) curves, which were successfully deconvoluted into crystalline and amorphous components. The UPEN multi-exponential inversion program proved effective in evaluating relaxation time distributions, while Carr–Purcell–Meiboom–Gill (CPMG) echo measurements selectively isolated crystalline phase signals. Notably, dis-UHMWPE exhibited a heterogeneous transverse relaxation time (T₂) distribution (manifested as double peaks), contrasting with the single-peak profile of commercial highly entangled samples. Furthermore, the nascent dis-UHMWPE displayed a broad longitudinal relaxation time (T₁) distribution, indicative of diverse entanglement site distributions. Their work also revealed that thermal treatment below the T_m can induce chain rearrangement and recrystallization, with the extent of these structural modifications being strongly dependent on the initial entanglement configuration.

Complementing these findings, Yao et al. [84] employed cross-polarization magic angle spinning ¹³C nuclear magnetic resonance (CP/MAS ¹³C NMR) to investigate UHMWPE samples with varying entanglement densities (Figure 2). Their qualitative analysis revealed a distinctive monoclinic crystal signal at 32 ppm in low-entanglement samples, which intensified upon cooling, a phenomenon absent in highly entangled specimens (Figure 2b). This observation suggests that the reduced entanglement point density in low-entanglement samples promotes the formation of intermediate phase segments between crystalline and amorphous regions, thereby facilitating monoclinic phase development during thermal processing. For quantitative assessment, the researchers utilized single-pulse–magic angle spinning (SP/MAS) NMR to precisely determine the crystal phase composition, enabling more accurate entanglement degree analysis. Recent advances in solid-state NMR techniques have provided deeper insights into the chain dynamics and mobility of dis-UHMWPE. Zhao et al. [85] synthesized dis-UHMWPE via freeze-extraction and systematically investigated the enhancement in chain mobility using low-field solid-state NMR. By analyzing the completely refocused ¹H free induction decay (FID), they discerned distinct segmental mobility variations during the melting of UHMWPE with different entanglement densities. Their findings revealed that longer UHMWPE chains exhibited reduced mobility upon detachment from lamellar crystals, suggesting greater difficulty in reorganizing into mobile segments upon melting. To further probe residual dipolar interactions, the researchers employed ¹H double quantum (DQ) NMR. Notably, intramolecular-nucleated UHMWPE displayed an earlier DQ peak due to strong crystalline constraints prior to melting. Intriguingly, while dis-UHMWPE retained its disentangled state during melting, similar behavior was not observed in less entangled high-density polyethylene (HDPE). However, post-melting DQ studies revealed no significant differences between UHMWPE melts with varying entanglement levels, likely because entanglements contributed minimally to the overall residual dipolar interactions in the melt state. These results suggest that dis-UHMWPE can maintain its disentangled structure near the melting point for an extended duration, thereby facilitating improved processability.

2.2. Melt Rheological Method

The melt rheological characterization of UHMWPE is typically conducted using rotational rheometry in dynamic time sweep mode to probe the chain entanglement dynamics [86]. When heated to 160 °C, UHMWPE exhibits a gel-like behavior with restricted flowability due to its exceptional molecular weight and complex entanglement network [87]. During testing at a fixed frequency of 0.5 Hz within the linear viscoelastic regime, enhanced thermal energy promotes rapid molecular chain motion and re-entanglement. This process manifests as a progressive increase in storage modulus (G′) until reaching thermodynamic equilibrium, defined as the equilibrium modulus (G_N⁰) [67]. Two critical parameters are often used to quantify entanglement characteristics. The normalized entanglement modulus (G_N^t = G′_t/G′_max) reflects the instantaneous entanglement density, where higher values indicate higher entanglement density and lower entanglement molecular weight (M_e) (Figure 3). Concurrently, the equilibration time (t_m) required to attain 98% of G_N⁰ inversely correlates with initial entanglement levels, where prolonged t_m values signify slower structural reorganization due to fewer inherent entanglements.

Theoretical foundation is provided by the equilibrium modulus equation (G_N⁰ = g_nρRT/M_e), where g_n represents a numerical factor (1 or 4/5), ρ denotes density, and R and T represent the gas constant and absolute temperature, respectively. Notably, UHMWPE maintains a characteristic G_N⁰ value of ~2.0 MPa at specified temperatures, consistent with its stable entanglement network. Notably, methodological constraints arise from the sample preparation requirements. For instance, compression molding below the T_m preserves nascent entanglement structures but complicates processing of highly entangled systems, potentially limiting analytical precision. Despite this limitation, the technique remains predominant for entanglement characterization due to its ability to monitor real-time structural evolution through well-established viscoelastic parameters. Overall, this approach provides unique insights into polymer dynamics by correlating macroscopic rheological responses with microscopic chain interactions under controlled thermal conditions.

2.3. Thermodynamic Annealing Method

Differential scanning calorimetry (DSC) has proven to be an effective tool for evaluating chain entanglement in UHMWPE by precisely controlling its melting and crystallization behavior. This approach operates in two distinct modes, including heating–melting annealing and cooling–crystallization annealing [84]. Notably, in the heating–melting annealing mode, samples are annealed at 5–8 °C below the T_m. For instance, annealing at 136 °C could preserve the primary lamellae formed by disentangled segments, while chains in entangled and amorphous regions detach and melt from the crystal surfaces. Upon subsequent cooling, these molten segments recrystallize, generating two distinct melting peaks in the DSC thermogram. The relative area of the low-temperature peak, normalized against the total melting endotherm, serves as a quantitative indicator of entanglement density, i.e., a larger ratio corresponds to a higher degree of chain disentanglement [88]. Conversely, the cooling–crystallization annealing mode involves high-temperature annealing at 160 °C to induce chain entanglement, followed by crystallization annealing at ~128 °C [55]. This process yields two crystal populations, formed during cooling and isothermal annealing, respectively, which manifest as dual melting peaks in DSC. Notably, nascent UHMWPE with lower entanglement density exhibits enhanced crystallization during annealing, reflected by an increased high-temperature peak area.

The recent advancements by Ye et al. [89] have further refined this methodology by introducing a quantitative entanglement parameter (i.e., C_tan), derived from comparative DSC analysis of low-entanglement standards and commercial samples. The melting enthalpy data of high-molecular-weight standard samples obtained after DSC annealing as well as the influence of annealing time on crystallinity were also investigated via DSC to determine the C_tan (Figure 4). Significantly, C_tan exhibits direct proportionality to the rheological entanglement number (Z), validating its accuracy while circumventing limitations associated with rheological measurements, particularly the platform modulus uncertainty in the low-MW section. Notably, this technique not only enables entanglement assessment in high-MW resins (>1.5 × 10⁵ g/mol) but also eliminates the need for extreme temperatures, thereby mitigating thermal degradation risks.

2.4. X-Ray Method

Note that while DSC probes thermal transitions related to the crystalline structure, X-ray methods including wide-angle X-ray scattering (WAXS) also have potential for characterizing the crystalline–amorphous microstructure in UHMWPE during thermodynamic annealing. WAXS detects sharp Bragg peaks (e.g., (110), (200) for orthorhombic UHMWPE), indicating crystallinity and interplanar spacing, alongside a broad amorphous halo. Peak deconvolution quantifies crystallinity and provides structural parameters like crystallite size. For dis-UHMWPE, characterized by significantly reduced chain entanglement, WAXS offers critical insights. Enhanced chain mobility during solid-state processing (e.g., stretching, sintering) or melt-crystallization facilitates easier chain alignment and formation of highly oriented crystalline structures. Monitoring crystallinity evolution, crystal orientation, and phase transformations through WAXS (in/ex situ) indirectly reflects chain mobility and constraints, inferring entanglement state differences. Although WAXS cannot directly measure entanglement density, its sensitivity to crystalline behavior provides key microstructural evidence for understanding processing–structure–property relationships in low-entanglement UHMWPE, often requiring complementary techniques (e.g., rheology, NMR) for a comprehensive assessment.

Zhong et al. [90] used in situ small-angle X-ray scattering (SAXS)/ultrasmall-angle X-ray scattering (USAXS)/wide-angle X-ray diffraction (WAXD) to investigate the influence of retained shish crystals and stretching temperature on crystal structure evolution during hot stretching of high-entanglement UHMWPE films. The presence of shish crystals mitigated tilted lamellar stacking typically induced by high entanglement, acting as nucleation sites that facilitated the transformation of inclined lamellae into shish-kebab structures. Increasing the stretching temperature enhanced chain mobility through partial disentanglement, promoting crystallization and orientation, thereby accelerating the transition to shish-kebab crystals. Higher temperatures further yielded shish-kebab crystals with larger long periods and lateral dimensions. Moreover, stretching reduced entanglement and increased crystallinity by 37%, while irradiation increased entanglement and decreased crystallinity by 20%. Thus, there is a significant correlation between X-ray parameters and entanglement degree. This study provides key insights into shish-kebab formation mechanisms and guides processing optimization for enhancing the performance of industrial high-entanglement UHMWPE products. Chen et al. [91] employed in situ WAXD and SAXS to investigate the structural evolution of UHMWPE gel fibers containing a high paraffin oil plasticizer content during stretching. WAXD analysis revealed that the low-entanglement system (96 wt% plasticizer) achieved a higher orientation factor (0.9) compared to the high-entanglement system (88 wt% plasticizer). Furthermore, the low-entanglement system exhibited a significant increase in the crystallinity index upon stretching, reaching by ~60% at a stretching speed of 3.33 min⁻¹. Conversely, the high-entanglement system displayed a higher initial crystallinity but a smaller increase. The long period (L) and its distribution (Δq) were used to describe lamellar stacking. SAXS results indicated an initial L value of 24 nm for the low-entanglement system, which is larger than the 20 nm observed for the high-entanglement system. Upon stretching to 480% strain, the L value of the low-entanglement system decreased to 20 nm, while Δq decreased from 0.30 to 0.22. The L value of the high-entanglement system remained essentially unchanged. The low-entanglement system demonstrated superior orientation and a 35% increase in crystallinity when stretched at 3.33 min⁻¹, whereas the high-entanglement system showed better orientation at the slower speed of 0.33 min⁻¹. The structural changes decelerated significantly beyond a threshold strain of ~275%. Collectively, these data reveal distinct differences and evolution patterns associated with varying degrees of molecular chain entanglement.

3. Synthesis of Dis-UHMWPE via Heterogeneous Catalytic Polymerization

Notably, in the preparation of UHMWPE, a heterogeneous polymerization system can significantly reduce reactor fouling problems [92]. The catalyst (such as Z-N) exists as solid particles, and the generated polymer grows directly on the surface of the particles as loose particles, which are easily suspended in the reaction medium and reduce the risk of wall adhesion and fouling [93,94]. Due to the high viscosity of the solution and the ease of uniform precipitation of polymers, homogeneous systems are prone to form viscous sediment layers by stirring dead corners or heat transfer surfaces, leading to scaling [95]. The heterogeneous system maintains fluidity and heat transfer efficiency within the reactor through the self-supporting properties of particles; prolongs the continuous operation cycle; and reduces cleaning and maintenance costs. Moreover, polymerization methodologies aimed at synthesizing UHMWPE with augmented flow and drawability have garnered significant research interest [96,97]. Strategizing catalyst design and optimizing reaction conditions are imperative for developing the technology to produce nascent dis-UHMWPE. Specifically, during ethylene polymerization under conditions of isolated active sites or low crystallization temperatures, individual nascent chains can solidify with decreased entanglement, leading to the formation of dis-UHMWPE [98]. Additionally, the utilization of diluted catalysts provides adequate spatial separation for the independent crystallization of growing alkyl chains [96]. Notably, FI catalysts and POSS-modified Ziegler–Natta catalysts have emerged as focal points in dis-UHMWPE preparation, demonstrating promising potential (

Table 1. Summary of representative dis-UHMWPE prepared by different heterogeneous catalytic polymerization.

Catalyst	Support	Cocatalyst	Condition	Activity (106 g PE·mol−1[Ti]·h−1)	MW (106 g/mol)	Đ	Tm1 (°C)	Xc1 (%)	Tm2 (°C)	Xc2 (%)	Ref.
Fluorinated FI	GO	MAO	20 °C, n-hexane		1.34 (Mv)		143.1	63			[99]
Diallylamino modified FI		MAO	40 °C, 4 bar, toluene	3.44	4.1 (Mv)		140.8	84.6	136.2	47.3	[100]
FI	POSS/SiO2	MAO	30 °C, 10 bar, toluene	7.6	2.0 (Mw)	2.4	141.0	78.3	134.2	33.9	[101]
FI	SiO2	MAO	10 °C, 2 bar, toluene	1.24	11.8 (Mw)	8.3	141.1	79			[102]
FI	MgClx/EtnAly(2-ethyl-1-hexoxide)z	TEA	40 °C, 4 bar, toluene	2.01	4.6 (Mw)	3.3	140.3	75			[103]
FI	MgClx	R’nClmAly(OR)z	40 °C, 1.2 bar, toluene	2.41	4.4 (Mw)	3.8	139.9	80			[104]
FI	POSS/SiO2	MAO	30 °C, 10 bar, toluene	7.8	2.0 (Mw)	2.4					[105]
Z-N		TEA	85 °C, 6 bar, n-hexane, 1.89% POSS	14.2	2.8 (Mv)	6.0	136.09	53.89			[106]
Z-N		TEA	85 °C, 6 bar, hexane, 0.2% POSS	11.81	2.6 (Mv)	5.9	140.2	66.89	135.8	49.64	[107]
Z-N	POSS/MgCl2	TEA	60 °C, 3 bar, n-heptane	10.56	1.42 (Mw)	6.17					[108]
Z-N	POSS/MgCl2	TiBA	85 °C, 10 bar, n-heptane	2.72	2.25 (Mw)	11.8	143.8	66.9	137	51	[109]
Z-N	POSS/MgCl2	TEA	60 °C, 3 bar, toluene	3.92	2.05 (Mw)	9.5	144.2	65.5	137.2	53.3	[110]
Z-N	POSS/MgCl2	TEA	60 °C, 7 bar, n-heptane	6.052	2.46 (Mw)	5.0	144.3	66.9	136.8	50.4	[111]
Z-N	OH-POSS/MgCl2·(EtOH)1.28	TEA	85 °C, 6 bar, n-hexane	18.83	1.53 (Mv)	7.2	136.55	62.16	119.8		[112]
Z-N	POSS/MgCl2	TEA	60 °C/85 °C, 10 bar, n-heptane/n-hexane	3.429	0.184 (Mw)	5.7	136.3	60.5	135.3	62.5	[113]
PHENI* complexes	sMAO	TIBA	60 °C, 2 bar, n-hexane	7.438	2.087 (Mw)	5.2	133–135	68–83			[114]
[Ti(OEt)4]	[Ti(OEt)4]	MMAO12	40 °C, 1 bar, toluene	0.213	13.07 (Mw)	2.17	144.1	69.7			[115]
Z-N	SiO2/4%PS	TEA	70 °C, 7 bar, n-heptane	3.1	2.63 (Mv)		144.2	66.5	138.8	47.2	[116]
Z-N	MgCl2	TIBA	50 °C, 4 bar, pentane, ICA		5.45 (Mv)		142.0	66.1			[117]
Z-N	MgO	TEA	70 °C, 8 bar, n-heptane	9.78	5.3 (Mv)		142.9	75	135.4	58	[118]
Z-N	MgCl2		50 °C, 3 bar,	2.4	5.693 (Mv)	4.3	142.6	66.0	135.6	47.2	[119]

).

3.1. FI Catalyst-Based Heterogeneous Polymerization

Conventional Z-N catalysts produce entangled UHMWPE, necessitating carcinogenic solvents for processing. Therefore, there is a necessity to develop a heterogeneous catalytic system for synthesizing UHMWPE with reduced chain entanglements, enabling solvent-free solid-state processing into high-strength materials. FI catalysts demonstrate remarkable advantages in synthesizing dis-UHMWPE. Their single-active-site nature enables precise control over chain propagation while suppressing branch formation, producing polyethylene chains with exceptional linearity and narrow Đ (≈2) [120,121]. This characteristic significantly reduces interchain entanglement density compared to conventional Z-N catalysts. Notably, FI systems maintain high catalytic activity under mild conditions, facilitating the synthesis of UHMWPE with MWs exceeding 5 × 10⁶ g/mol [122,123]. The resultant low-entanglement architecture promotes regular crystalline alignment, endowing materials with enhanced tensile strength (>3.5 GPa) and impact resistance while addressing the inherent processing challenges of conventional UHMWPE through improved melt flowability. Recent advancements in supported FI catalysts further optimize polymer microstructure control. Immobilization on inorganic carriers (e.g., SiO₂, MgCl₂) ensures uniform spatial distribution of active sites, effectively mitigating reactor fouling and chain entanglement caused by catalyst agglomeration in homogeneous systems [25,124]. The supported configuration concurrently enhances catalyst thermal stability, enabling polymerization at elevated temperatures (e.g., 80–90 °C) through the carrier’s thermal buffering effect [64,125,126]. Mechanistically, the porous framework restricts chain diffusion pathways, guiding linear chain growth with improved spatial orientation. From an industrial perspective, this heterogeneous system demonstrates compatibility with gas-phase/slurry polymerization processes while offering the following practical advantages: (1) simplified product separation via sedimentation/filtration; (2) enhanced mechanical robustness to withstand reactor shear forces; and (3) direct integration into existing polyethylene production infrastructure with minimized retrofitting costs [65,127]. These synergistic effects make supported FI catalysts a transformative solution for scalable production of high-performance UHMWPE fibers and ballistic protection materials.

For example, Heidari et al. [99] explored an alternative strategy to integrate nanomaterial reinforcement with dis-UHMWPE synthesis by immobilizing a fluorinated bis(phenoxy-imine) titanium (FI) catalyst on graphene oxide (GO) nanosheets. The rationale centered on leveraging GO’s high surface area and functional groups (e.g., -OH, -COOH) to heterogenize the homogeneous FI catalyst while preserving its living polymerization characteristics. Methylaluminoxane (MAO) was first anchored to GO via the reaction with surface oxygen groups, followed by FI catalyst attachment, creating isolated active sites that minimized chain entanglement during ethylene polymerization (25 °C, 30 min, [Al]/[Ti] = 600). The resultant UHMWPE/GO nanocomposite (PE(FI)-GO) exhibited a lower-viscosity average molecular weight (M_v = 1.34 × 10⁶ g/mol) compared to pure UHMWPE from homogeneous FI catalysis (M_v = 2.45 × 10⁶ g/mol), attributed to chain transfer to GO-bound MAO. Despite this reduction, the nanocomposite retained partial disentanglement, achieving a draw ratio of 1600% (vs. 3300% for pure FI-derived UHMWPE), while Z-N-produced UHMWPE (PE(ZN)) reached only 600%. The DSC analysis showed a melting point depression (T_m = 143.1 °C for PE(FI)-GO vs. 146.6 °C for PE(FI)), likely due to GO-induced crystal disruption, with crystallinities (X_c) of 63% and 79%, respectively. X-ray diffraction (XRD) confirmed GO exfoliation (disappearance of the GO (002) peak at 2θ of 11.8°), while transmission electron microscopy (TEM)/SEM revealed oriented GO nanosheets between UHMWPE fibrils. Mechanical testing demonstrated a tensile strength of ~0.5 GPa for PE(FI)-GO, being intermediate between PE(FI) (~1 GPa) and PE(ZN) (~0.3 GPa), with GO acting as both a nucleating agent (increasing crystallization onset temperature by ~3 °C) and a reinforcing phase. The study highlighted the trade-off between nanofiller incorporation and chain entanglement, i.e., while GO immobilization enabled heterogeneous processing advantages (e.g., no reactor fouling) and improved thermal/mechanical properties, it partially compromised the ultra-high MW and extreme drawability intrinsic to dis-UHMWPE.

To tackle the challenges in conventional UHMWPE processing by focusing on controlling entanglement density during polymerization, Oleynik et al. [100] aimed to develop highly active titanium (IV) dichloride FI catalysts bearing diallylamino groups for synthesizing dis-UHMWPE with reduced chain entanglements, enabling solvent-free solid-state processing. The authors employed a family of 16 salicylaldarylimine titanium (IV) dichloride complexes bearing diallylamino groups (Figure 5), namely {2-[3- or 4-(CH₂=CH-CH₂)₂NC₆H₄N=CH]-6-R¹-4-R₂C₆H₂O₂TiCl₂ (R¹ = t-Bu, CMe₂(Ph); R² = H, Me, OMe, t-Bu) activated by MAO in a toluene-based heterogeneous polymerization system. These catalysts exhibited self-immobilization behavior, transitioning from homogeneous to heterogeneous states during polymerization, with high activity in the early stage of polymerization, followed by a rapid decrease in activity to a stable heterogeneous stage. The direct cause of this is the allyloxy or diallylamino groups on the complex catalyst reacting with the grown polyethylene chains, anchoring the catalyst molecules covalently to the macromolecules (each macromolecule binds to a catalyst molecule), causing the system to spontaneously transition from homogeneous to heterogeneous (without the need for an external carrier). After polymerization for 5 min, a yellow precipitate is precipitated in the system, and the titanium content in the supernatant is lower than 1% of the initial amount. This mechanism can inhibit reactor fouling (generating free-flowing powder without wall sticking), and after self-immobilization, the active center is adjacent to the surface of polyethylene crystals, reducing nucleation barriers and promoting rapid chain crystallization to reduce entanglement, achieving the synthesis of dis-UHMWPE. The resulting UHMWPE displayed ultra-high MWs (0.70–4.10 × 10⁶ g mol⁻¹), narrow dispersity (implied by pseudo-living polymerization kinetics), high melting temperatures (136.5–143.7 °C), and X_c (78–85%). The disentangled morphology was confirmed by DSC annealing experiments, demonstrating distinct dual melting peaks and reduced entanglement density. Compression molding of the nascent powders produced transparent films with high tensile modulus (187 GPa) and strength (3.87 GPa), validating the suitability of the synthesized UHMWPE for solid-state deformation processing. The study concluded that meta-substituted diallylamino groups and bulky R¹ substituents (CMe₂(Ph) > t-Bu) enhanced catalytic activity and MW, while low polymerization temperatures and rapid chain crystallization minimized entanglements. This work highlights the potential of self-immobilizing FI catalysts for industrial-scale production of high-performance dis-UHMWPE.

Hui et al. [101] investigated the influence of POSS-modified heterogeneous catalysts on fragmentation behavior and entanglement evolution during ethylene polymerization, leading to dis-UHMWPE. The study aimed to establish a reactor environment that isolates active species and growing chains by using POSS nanoaggregates (60 nm) immobilized on silica supports. A fluorinated bis(phenoxyimine)Ti complex (FI catalyst) was heterogenized on POSS-functionalized SiO₂ and activated by MAO in toluene. Polymerization occurred at 30 °C under 10 bar ethylene pressure, with Al/Ti = 500. The catalyst exhibited a maximum activity of 7.6 × 10⁶ g_PE mol_Ti⁻¹ h⁻¹ at 30 min, producing UHMWPE with MWs (M_v) ranging from 1.6 × 10⁶ to 4.2 × 10⁶ g mol⁻¹ and narrow Đ (2.0–2.5). The DSC analysis revealed high initial crystallinity (79.9–87.6%) and dual melting peaks (T_m1: 136.5–141.6 °C; T_m2: 132.7–134.9 °C), indicative of reduced entanglements. Rheological time sweep measurements demonstrated low initial storage moduli (0.31–0.62), confirming weakly entangled chains in early-stage polymers (≤30 min). SEM and XRD analyses linked morphological changes (spherical to petal-like particles) and crystalline phase transitions (monoclinic to orthorhombic) to catalyst fragmentation. Complete particle fragmentation at 30 min disrupted POSS-mediated isolation, increasing entanglement density in later-stage polymers. The study concluded that POSS nanoaggregates effectively reduced entanglements by spatially isolating active sites during early polymerization, while fragmentation-induced confinement in polyethylene fragments promoted entanglement formation post-30 min. These findings provide critical insights into designing tailored catalysts for entanglement control in UHMWPE synthesis.

In order to address the challenges of reactor fouling and uncontrolled polymer morphology in homogeneous catalytic systems for synthesizing dis-UHMWPE, Yang et al. [102] aimed to develop heterogeneous catalysts that enable scalable production while retaining the solid-state processability of dis-UHMWPE. They research employed silica-supported catalysts using MAO-activated mesoporous (S1: 8 nm pore size; S2: 4 nm pore size), spherical (S3: 130 µm; S4: 60 µm), and nano-sized (S5: nanopowder) silica particles to immobilize a bis(N-(3-tert-butylsalicylidene)-2,3,4,5,6-pentafluoroanilinato)titanium(IV) dichloride (FI) complex. Polymerizations were conducted at 10 °C in toluene under 2 bar ethylene for 60 min. None of the heterogeneous catalytic systems (UHMWPE-MAO-silicas/FI) exhibited reactor fouling or wall sheeting during or after polymerization, confirming their heterogeneous nature. Notably, catalytic activities varied significantly, i.e., homogeneous FI/MAO achieved 1941 kg·mol⁻¹·h⁻¹·bar⁻¹, while heterogeneous systems showed lower activities (MAO-S5/FI: 618 kg_PE mol_Ti⁻¹ h⁻¹ bar⁻¹; MAO-S4/FI: 127 kg_PE mol_Ti⁻¹ h⁻¹ bar⁻¹). MWs determined via rheology ranged from 5.1 × 10⁶ g/mol (MAO-S2/FI) to 11.8 × 10⁶ g/mol (MAO-S5/FI), with Đ between 5.4 (homogeneous) and 10.2 (MAO-S2/FI). All polymers exhibited high crystallinity (~80%) and T_m (~141 °C). Rheological and DSC analysis indicated that MAO-S5/FI yielded the least entangled UHMWPE, evidenced by the lowest initial storage modulus (G′ = 0.02 MPa) and slow entanglement formation kinetics. Moreover, the solid-state processing of MAO-S5/FI-derived UHMWPE produced tapes with a draw ratio of 260, achieving a tensile strength of 3.26 N/tex (2.48 GPa) and a modulus of 170 N/tex (129 GPa). Moreover, modifying silica with ethyltrimethoxysilane (small) or octadecyltrichlorosilane (large) reduced catalyst activity (e.g., MAO-smS5/FI yield: 4.0 g vs. MAO-S5/FI: 21.4 g) but did not significantly alter entanglement density. Notably, SEM confirmed the morphology replication of mesoporous and spherical silicas, while nano-silica yielded larger polymer particles (Figure 6). After MAO-grafting, all MAO-silicas (Figure 6a′–e′) displayed similar morphology and size to the starting silicas, indicating no fragmentation or aggregation during the reaction with MAO. There was no clear morphological duplication for the PE-MAO-nano silica (S5)/FI; the polymer particle size was orders of magnitude larger than the initial starting catalytic system MAO-S5/FI (Figure 6e″). The absence of particle replication is likely due to the very small size of MAO-S5/FI, which was unable to provide enough surface area for the polymer to nucleate and replicate the shape.

Gote et al. [103] addressed the challenge of synthesizing dis-UHMWPE using a heterogeneous catalytic system, aiming to overcome the limitations of homogeneous systems such as reactor fouling and uncontrolled polymer morphology while achieving mechanical properties comparable to those of homogeneous catalysts (Figure 7). Their research employed a bis[N-(3-tert-butylsalicylidene)pentafluoroanilinato] titanium (IV) dichloride (Cat. 1) supported on an in situ-formed MgCl_x/Et_nAl_y(2-ethyl-1-hexoxide)_z nano-activator/support, which ensured controlled polymer morphology and avoided reactor fouling. Polymerizations were conducted under 4 bar ethylene pressure at 40 °C in toluene or heptane, with triethylaluminum (TEA) as a cocatalyst. The system exhibited high activity (up to 1607 kg_PE mol_Ti⁻¹ h⁻¹ bar⁻¹), producing UHMWPE with weight-average molecular weights (M_w) of 4.6–14.7 × 10⁶ g/mol and dispersity (Đ) of 3.3–8.6, as determined by rheology. SEM images of the nascent dis-UHMWPE revealed distinct features compared to the sole lamella-like structure observed with the homogeneous Cat. 1/MAO catalytic system (Figure 7). The surface morphology of the nascent polymers consisted of clusters of small globules (∼0.5 μm in size), within which well-resolved lamellar structures were distinguishable. This globular morphology with discrete lamellar clusters corresponded to the low bulk density of the synthesized polymer (<160 g/L). Furthermore, electron diffraction analyses of crystals formed using a heterogeneous catalytic system confirmed that these aggregated crystals are single crystals (Figure 7d,e). Moreover, thermal analysis revealed T_m of 139–141 °C and crystallinity of 75–83%, while solid-state NMR and DSC demonstrated restricted chain dynamics in the noncrystalline regions, characteristic of disentangled polymers. Rheological studies showed a low initial storage modulus (G′) that increased over time, confirming the disentangled state. Solvent-free solid-state processing yielded uniaxial tapes with specific tensile strengths up to 4.4 N/tex (∼4 GPa) and modulus up to 207 N/tex (∼200 GPa), which matched homogeneous systems. Moreover, biaxial films were also produced, highlighting the material’s processability. The study concluded that the heterogeneous Cat. 1/MgCl_x/Et_nAl_y(2-ethyl-1-hexoxide)_z system uniquely combines high activity, controlled morphology, and low entanglements, enabling industrial-scale production of dis-UHMWPE with unparalleled mechanical properties. This breakthrough bridges the gap between homogeneous and heterogeneous catalysis, offering a sustainable and solvent-free route to high-performance UHMWPE for engineering applications.

Additionally, based on positive feedback and research findings, Gotes et al. [104] continued to design a tunable MgCl_x/R’_nCl_mAl_y(OR)_z dual activator/support system combined with a bis[N-(3-tert-butylsalicylidene)pentafluoroanilinato] titanium(IV) dichloride complex to spatially isolate active sites and minimize chain overlap during polymerization. They further explored the impact of diverse alcohols (2-ethyl-1-hexanol, 3-methyl-1-butanol, n-butanol, n-pentanol, n-octanol, cyclohexanol, benzyl alcohol, phenol) and alkyl aluminum compounds on catalyst performance (Figure 8). Polymerizations were conducted in toluene at 40–50 °C under 1.2–4 bar ethylene pressure, with Al:Ti ratios of 600–3000. The system achieved high activities (357–2753 kg_PE mol_cat⁻¹ bar⁻¹ h⁻¹) and tailored UHMWPE with an M_w of 3.1–43 × 10⁶ g/mol and Ð of 3.3–38. Notably, using TMA at 4 bar ethylene pressure yielded an unprecedented M_w of 43 × 10⁶ g/mol (Ð = 38). The polymers exhibited high T_m (134–142 °C) and crystallinity (52–80%), as determined by DSC. SEM revealed lamellar structures in most samples, indicative of low entanglement density, except for polymers derived from cyclohexanol and n-pentanol, which showed dense morphologies and higher packing densities (up to 200 g/L). Rheological analysis confirmed the low-entangled state, with slow storage modulus (G′) build-up (<2 MPa equilibrium) during melt annealing. Solid-state processing of selected polymers (e.g., M_w = 3.1–7.1 × 10⁶ g/mol) yielded uniaxial tapes with draw ratios of up to 400×, achieving specific tensile strengths of 3.2–3.9 N/tex and specific moduli of 180–200 N/tex. However, the sample with M_w of 43 × 10⁶ g/mol had limited drawability (~225×) due to higher entanglements. Polymers with coarse morphologies (e.g., from cyclohexanol) failed to process, aligning with their rheological and thermal profiles. Advanced thermal analysis showed that samples with number-average molecular weight (M_n) < 0.8 × 10⁶ g/mol crystallized rapidly during isothermal annealing, while higher-M_n polymers exhibited entanglement-dependent crystallization kinetics. The study concluded that the MgCl₂-based activator/support system enables industrial-scale production of dis-UHMWPE with controlled morphology and mechanical properties rivaling homogeneous catalysts, while avoiding reactor fouling. Key innovations include the tunability of activators/supports and the correlation between polymerization conditions, entanglement density, and solid-state processability.

To address chain overlap and entanglement issues inherent in traditional supported catalysts, Li et al. [105] aimed to synthesize dis-UHMWPE using a heterogeneous catalyst system POSS with two hydroxyl groups adsorbed onto MAO-activated silica to immobilize fluorinated bis(phenoxyimine)Ti (FI) catalysts. The POSS acted as horizontal spacers, isolating active sites and hindering chain overlap during ethylene polymerization. The heterogeneous system achieved optimal performance at 10 wt% POSS loading, where the catalyst exhibited a polymerization activity of 7.8 × 10⁵ g_PE mol_Ti⁻¹ h⁻¹ bar⁻¹, comparable to homogeneous FI catalysts (10.6 × 10⁵ g_PE mol_Ti⁻¹ h⁻¹ bar⁻¹) and fourfold higher than POSS-free supported catalysts. The synthesized UHMWPE had a high molecular weight (M_w > 1 × 10⁶ g/mol) and a narrow Đ. Dynamic time sweep tests revealed the weakly entangled state of POSS-10-derived UHMWPE, showing delayed modulus build-up with the longest time and high crystallinity (80.7%) with a monoclinic phase fraction of 15.8%. Moreover, excessive POSS loading (20 wt%) caused pore blockage, reducing activity (2.3 × 10⁵ g_PE mol_Ti⁻¹ h⁻¹ bar⁻¹) and increasing entanglement. Moreover, characterization via X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDX) further confirmed uniform POSS distribution and isolated active sites at optimal loading. This strategy highlighted the role of spatial confinement in tailoring chain entanglement and crystallinity, offering a scalable route to processing UHMWPE with enhanced thermal and mechanical properties.

3.2. POSS-Modified Z-N Catalyst-Based Heterogeneous Polymerization

POSS nanoparticles, hybrid materials consisting of rigid silicon inorganic cores modified with organic groups [128], have attracted substantial attention in materials research. Commercially available in over 100 distinct forms, each bearing unique organic functional groups, these nanoparticles stand out for their diminutive size, measuring merely 1.5 nm, thereby ranking among the smallest silica particles [129]. Their physical dimensions, while small, are relatively significant when juxtaposed with polymer structures, closely approximating the sizes of most polymer segments and coils [76]. Previous reports have suggested that nanoparticles smaller than the entanglement mesh widths can enhance chain disentanglement and hasten monomeric relaxation rates [74]. In recent years, multiple studies have delved deeper into the impact of POSS on polymer molecular mobility, a factor intricately linked to disentanglement [75,130,131]. However, despite these efforts, achieving a more profound understanding of how POSS affects the entanglement of UHMWPE during Z-N polymerization remains a formidable challenge. This knowledge gap not only limits the optimization of polymerization processes but also hinders the development of UHMWPE-based materials with enhanced properties. Further research is thus warranted to elucidate the complex interactions between POSS and UHMWPE during Z-N polymerization, which could potentially unlock new opportunities for tailoring the performance of these advanced materials. In a study by Zhou et al., POSS was incorporated into UHMWPE composites via in situ ethylene polymerization at 85 °C with an Al:Ti ratio of 150 [106]. This research comprehensively investigated the impact of POSS integration on multiple properties of UHMWPE, including polymerization kinetics, melt viscosity, thermal transitions, annealing characteristics, and mechanical strength. By incorporating POSS at concentrations ranging from 0.5 to 2.16 wt%, the disentangling performance of UHMWPE was enhanced. This improvement was likely attributed to increased dispersion and reduced aggregation of POSS, which acted as a barrier, widening the Đ and ultimately improving the processability of the composites. Subsequent analysis indicated that the activation energy of pure UHMWPE was 550.31 ± 72.60 kJ/mol, while that of POSS/UHMWPE nanocomposites was 303.37 ± 27.71 kJ/mol. Regarding the melt viscosity, low concentrations of POSS (0.23–1.89 wt‰) led to a decrease, whereas higher concentrations caused an increase. The addition of POSS also significantly reduced the rubbery plateau modulus of UHMWPE, suggesting a dilatation effect on polymer entanglements. Notably, the in situ polymerization approach was found to be highly effective in minimizing the amount of additives required, providing a more efficient and potentially cost-effective method for fabricating UHMWPE-based composites with improved properties.

To investigate the effects of POSS on UHMWPE disentanglement, Zhou et al. [107] systematically investigated the disentanglement effects of octaisobutyl-POSS on UHMWPE through in situ polymerization using a TEA cocatalyst under optimized conditions ([Al]/[Ti] = 150, 85 °C). The synthetic strategy not only enhanced Z-N catalyst activation but also effectively regulated Đ while suppressing molecular aggregation. The identification of POSS in UHMWPE was investigated via SEM-EDS (Figure 9a,b). The dispersion of POSS within the UHMWPE matrix was analyzed using TEM, and the average particle size was determined (Figure 9c,d). Observations revealed that POSS was uniformly dispersed throughout the UHMWPE matrix. As indicated in the plot of average particle (domain) size in Figure 9e, the size of POSS domains increased with increasing POSS content. Notably, at low POSS contents (<0.3%), the particle size was smaller than the tube size of PE (d_t = 3.26 nm), which may exert specific effects on UHMWPE. Rheological characterization revealed three critical phenomena: (1) a progressive reduction in rubbery plateau modulus directly correlated with decreasing polymer chain entanglement; (2) minimum melt viscosity at 0.2 wt% POSS loading, indicative of molecular sliding facilitation and entanglement expansion through POSS-mediated lubrication; and (3) concentration-dependent inversion in entanglement parameters, manifested by 0.05–0.2 wt% POSS reducing both M_e and entanglement density (V_e), while ≥0.3 wt% loading paradoxically increased V_e despite further M_e reduction. The study further quantified POSS’s disentanglement efficacy through melt activation energy analysis, demonstrating a significant reduction in polymer chain annealing energy barriers, thereby mechanistically linking nanofiller incorporation to enhanced UHMWPE processability. To enable the synthesis of UHMWPE with optimized activity, processability, and mechanical performance, Chen et al. [108] developed a POSS-modified MgCl₂-based Z-N catalyst to synthesize ultra-pure, weakly entangled UHMWPE with controlled particle sizes and enhanced processability. The catalyst achieved a specific activity of 3.52 × 10⁶ g_PE mol_Ti⁻¹ h⁻¹ bar⁻¹ at 30 wt% POSS loading, representing a 3.2-fold enhancement over the POSS-free counterpart (Cat-POSS-0%), attributed to POSS coordination with the MgCl₂ (110) phase that formed nano-aggregates to prevent bimetallic deactivation. This structural modification simultaneously increased MW (142.5 × 10⁴ g·mol⁻¹) and reduced particle size, yielding UHMWPE/POSS composites with D₅₀ values of 171–178 μm. Mechanical characterization revealed a power law relationship between catalyst activity and entanglement reduction (i.e., G_N(0) = 2.77643 × A^−0.1575), where increased activity (A) correlated with a 21.4% improvement in impact strength (113.6 kJ/m²) and enhanced tensile properties; 13.7% higher tensile strength; an 8.2% greater Young’s modulus; and 11.2% increased elongation at break.

Chen et al. [109] revealed that POSS/MgCl₂ nanoaggregates could modify electronic environments through strengthened Mg 2p and Ti(IV) 2p binding energies, while their three-dimensional architecture exceeds chain folded lengths, creating permanent interchain barriers. A commercial mesoporous silica (955-SiO₂) modified with POSS was employed to immobilize the MgCl₂/TiCl₄ catalyst. The POSS moieties are capable of inducing the formation of POSS/MgCl₂ nanoaggregates, which act as horizontal spacers to isolate adjacent chains and active sites (Figure 10a–c). The catalytic system achieved sustained activity (2.7 × 10⁶ g_PE mol_Ti⁻¹ h⁻¹) across 60–85 °C when paired with a triisobutylaluminum (TiBA) cocatalyst. Notably, the critical importance of nanoaggregate separation could maintain particle integrity, thus diminishing the creation of entanglements. Quantitative analysis demonstrated an exponential decrease in entanglement density with increasing nanoaggregate concentration, which was reflected by G′ build-up curves (Figure 10d). The G′ increment equation for UHMWPE at 30 min was 1.052 × A^0.15609 (where A represents polymerization activity), whereas at 60 min, it was 2.44 × A^0.07117. Notably, at 10 wt% POSS loading, G′ exhibited exponential decay with activity enhancement, indicating effective suppression of thermally induced chain entanglement. These findings establish a methodology for controlling entanglement density through nanoaggregate-mediated spatial confinement prior to particle fragmentation, providing critical insights for high-temperature synthesis of dis-entangled UHMWPE with tailored chain architectures. Complementary research by Li et al. [110] developed a POSS-modified Z-N catalyst system that enabled high-temperature synthesis of weakly entangled UHMWPE with exceptional efficiency. Through combined experimental characterization and computational modeling, they identified that POSS integration forms ~48 nm nanoaggregates adhering to SiO₂ surfaces, creating dual active domains within the POSS/MgCl₂ nanoaggregates and δ-MgCl₂ matrix. Crucially, these POSS-derived nanoaggregates function as horizontal spacers that physically separate active TiCl₄ sites through steric hindrance mechanisms, effectively reducing chain overlap during polymerization. This structural configuration enables remarkable catalytic activity (i.e., 1.3 × 10⁶ g_PE mol_Ti⁻¹ h⁻¹ bar⁻¹) for producing dis-entangled UHMWPE at 60 °C. Subsequent optimization using a pore-restricted Grace-955 SiO₂ support enhanced both molecular weight (2.9 × 10⁶ g/mol) and activity (4.5 × 10⁶ g_PE mol_Ti⁻¹ h⁻¹ bar⁻¹) by preserving spatial separation functionality.

The production of UHMWPE with a fine particle size and few entanglements is still a major challenge for both industry and academia. To address this issue, Guo et al. [111] addressed the persistent challenge of producing low-entanglement UHMWPE with controlled particle size through the development of a hetero Z-N catalyst (4POSS-Cat), synthesized via δ-MgCl₂-supported 4-OH POSS with 93.1% yield. Structural characterization revealed irregular block-shaped catalyst particles (0.1–10 μm diameter) incorporating POSS nanocrystals as active site insulators, which demonstrated exceptional chemical stability post-Ti modification. High-angle annular dark-field (HAADF) and TEM were used to verify the structural stability of 4POSS-Cat (Figure 11). The selected area electron diffraction (SAED) patterns revealed diffraction spots (Figure 11b), indicating a polycrystalline structure, further supported by lattice fringes reflected by TEM (Figure 11c). Moreover, measured fringe distances were 3.10 Å (green circles), corresponding to the Si₄O₄ face spacing of the POSS core, and 2.20 Å (red circles), matching the (015) plane of MgCl₂. EDX mapping showed uniform dispersion of Si (7.61%) and similar distributions of O and Mg, attributed to tetrahydrofuran (THF) covering abundant Mg²⁺ vacancies (Figure 11d). These results demonstrated that POSS nanocrystals are embedded within the MgCl₂ binder. The catalytic system exhibited optimal performance at 5.8% Ti loading, achieving an ethylene polymerization activity of 6.052 × 10⁶ g_PE mol_Ti⁻¹ h⁻¹, while lower Ti concentrations (1.0–3.1 wt%) substantially reduced activity due to rapid TiCl₄-POSS interactions. Temperature sensitivity studies showed linear increases in UHMWPE entanglement with rising temperatures, attributed to decoupled chain crystallization and propagation kinetics. Furthermore, the resulting UHMWPE products displayed superior structural characteristics, including small particle size attributes (diameters <120 μm), reduced G_N(0) with decreased entanglements versus C-UHMWPE, and enhanced mechanical properties encompassing tensile strength (39.4 MPa, +11.1%), Young’s modulus (383.5 MPa, +13.7%), impact strength (102.7 kJ/m², +24.6%), and elongation at break (940.5%, +39.3%). Notably, the catalyst generated UHMWPE with broader yet controlled Đ compared to C-UHMWPE, while maintaining sufficient thermal stability for pilot-scale processing. These findings collectively demonstrate 4POSS-Cat’s potential in balancing entanglement reduction with mechanical performance enhancement through precise catalyst design.

Notably, different dealcoholizing methods and the modification of active centers by POSS could also be performed to modify the distance between growing chains. To this end, Zhou et al. [112] systematically investigated the entanglement behavior and properties of UHMWPE synthesized via modified Z-N catalysts, addressing the critical challenge of reducing chain entanglement to improve processability while maintaining mechanical performance. The research motivation stemmed from the inherent limitations of UHMWPE, i.e., high chain entanglement due to its ultra-long molecular chains, which restricts chain mobility, crystallization kinetics, and melt processability, ultimately compromising industrial applications. To tackle this, the authors developed a heterogeneous polymerization system utilizing MgCl₂-supported catalysts modified through two distinct dealcoholization methods (physical and chemical) and functionalized with hydroxyl-terminated polyhedral oligomeric silsesquioxane (OH-POSS). Physical dealcoholization involved vacuum heating MgCl₂(EtOH)_3.4 at 60 °C for 6 h, yielding sup-1 with a specific surface area (SSA) of 30.47 m²/g, whereas chemical dealcoholization using TEA produced sup-2 (SSA: 16.38 m²/g). Four catalysts were synthesized: TiCl₄/sup-1, TiCl₄/OH-POSS/sup-1, TiCl₄/sup-2, and TiCl₄/OH-POSS/sup-2, with Ti loadings ranging from 2.94 to 5.20 wt% and an OH-POSS content of 8.68–8.73 wt%. Ethylene polymerization was conducted at 85 °C and 65 °C under 0.6 MPa pressure using TEA cocatalyst ([Al]/[Ti] = 150) in hexane, yielding UHMWPE with M_v exceeding 100 × 10⁴ g/mol and catalytic activities of 7.36–18.83 × 10⁶ g_PE mol_Ti⁻¹ h⁻¹, demonstrating industrial viability. Meanwhile, dispersity varied between 4.8 and 7.7, with broader distributions observed at higher temperatures (e.g., Đ = 7.7 for TiCl₄/sup-1 at 85 °C vs. 4.8 at 65 °C). Rheological analysis via dynamic time sweeps revealed that physically dealcoholized catalysts (e.g., TiCl₄/sup-1) produced UHMWPE with a lower initial storage modulus (indicative of reduced entanglement), attributed to enhanced SSA (33.78 m²/g vs. 20.88 m²/g for TiCl₄/sup-2) and improved active center dispersion, which increased interchain spacing. In contrast, OH-POSS modification paradoxically elevated entanglement due to restricted active site distribution and strong polymer–POSS interactions, as evidenced by higher storage modulus values (e.g., 18.83 × 10⁶ g_PE mol_Ti⁻¹ h⁻¹ activity for TiCl₄/OH-POSS/sup-1 vs. 12.68 × 10⁶ for TiCl₄/sup-1 at 85 °C). Thermal analysis showed T_m of 136.55–138.59 °C and X_c of 47.97–65.19%, with lower X_c correlating to higher MWs (e.g., X_c = 52.76% for UPE1-65, M_v = 421 × 10⁴ g/mol) due to delayed crystallization kinetics. Mechanical testing revealed tensile strengths of 39.4 MPa, Young’s modulus of 383.5 MPa, and wear resistance improvements of up to 24.6% for POSS-containing composites, though entanglement increases from OH-POSS that offset these gains. The study conclusively demonstrated that physical dealcoholization optimizes catalyst SSA and active site isolation, effectively reducing entanglement, while OH-POSS modification introduces competing effects of enhanced nucleation (reducing supercooling ΔT by 1.7–5.6 °C) but elevated entanglement via spatial confinement. These findings underscore the delicate balance between catalyst design, chain dynamics, and final material properties, providing actionable insights for industrial-scale production of disentangled UHMWPE with tailored mechanical and thermal characteristics.

Specifically, traditional physical blending methods limit the UHMWPE content to <3 wt% due to entanglement-induced agglomeration. In order to address the challenge of incorporating UHMWPE into HDPE matrices to achieve superior mechanical properties without phase separation, Chen et al. [113] proposed an in situ polymerization approach using a POSS-modified Z-N catalyst to synthesize dis-UHMWPE (30 wt%) within HDPE via a two-stage cascade polymerization. The heterogeneous catalytic system (POSS-10%-cat/TEA, [Al]/[Ti] = 100) operated at 60 °C (UHMWPE stage) and 85 °C (HDPE stage) under 10 bar ethylene pressure, achieving activities of 3429–4836 × 10³ g_PE mol_Ti⁻¹ h⁻¹. The resultant HDPE/UHMWPE blends exhibited bimodal dispersity (Đ = 3.4–16.7) and X_c of 52.7–62.1%. Rheological analysis confirmed reduced entanglement with an initial storage modulus (G′_(t=0)) of 0.28 MPa for dis-UHMWPE vs. 0.36 MPa for entangled). Notably, a large number of pre-extended UHMWPE chains will relax and function as tie molecules, connecting adjacent kebab structures and stacked lamellae, resulting in a significant increase in toughness (Figure 12a). 2D-SAXS and 2D-WAXD experiments were further employed to characterize the crystalline structures and chain orientation of HDPE/UHMWPE blends. The results revealed that the incorporation of UHMWPE can promote the formation of shish-kebab structures, which in turn enhance the strength, stiffness, and toughness of the blends (Figure 12b). Mechanical testing revealed remarkable enhancements in tensile strength (52.4 MPa, +97.7%), Young’s modulus (604.2 MPa, +43.6%), and impact strength (74.4 kJ/m², +675%) compared to pure HDPE. SAXS and WAXD demonstrated gradient-distributed shish-kebab structures with long spacing periods (L₁) up to 48.09 nm, which correlated with improved stiffness and toughness. However, excessive UHMWPE (>40 wt%) disrupted orientation, reducing mechanical properties. The study concluded that dis-UHMWPE can enhance the crystal structure and orientation, enabling industrial-scale production of high-performance polyethylene composites.

3.3. Other Novel Catalyst-Based Heterogeneous Polymerization

In order to address the challenges of processing UHMWPE caused by its high chain entanglement and melt viscosity, which restrict conventional thermoplastic techniques like injection molding, Collins et al. [114] developed a heterogeneous polymerization system using novel permethylindenyl-phenoxide (PHENI*) ansa-metallocene titanium complexes immobilized on inorganic supports to synthesize dis-UHMWPE with improved processability (Figure 13). These catalysts combined the MW control of single-site homogeneous systems with the industrial practicality of heterogeneous supports. PHENI* complexes, such as Me₂Si(^tBu₂ArO,I^∗)TiCl₂, were synthesized via a multi-step procedure involving silane intermediates and titanium dichloride complexation. These catalysts were then supported on solid polymethylaluminoxane (sMAO), MAO-modified silica (SSMAO), and MAO-modified layered double hydroxides (LDHMAO). Slurry-phase ethylene polymerizations were conducted in hexanes at 2 bar ethylene pressure, temperatures ranging from 30 to 90 °C, and tri-iso-butylaluminium (TIBA) as a co-catalyst/scavenger ([Al]/[Ti] = 1000). The sMAO-supported PHENI* catalyst (2_sMAO) exhibited exceptional activity (3.7 × 10⁶ g_PE mol_Ti⁻¹ h⁻¹ bar⁻¹ at 60 °C), significantly outperforming indenyl-PHENICS analogs (e.g., 6_sMAO: 0.1 × 10⁶ g_PE mol_Ti⁻¹ h⁻¹ bar⁻¹). The resulting UHMWPE displayed MWs up to 4.2 MDa (LDHMAO-supported catalyst at 50 °C) with narrow Đ (2.0–3.0), contrasting sharply with the bimodal distributions (Đ = 20.8–35.9) observed for traditional Z-N systems. DSC revealed T_m of 133–135 °C and X_c of 68–83%. Disentanglement was confirmed via annealing protocols, where rapid formation of dual melting peaks (135 °C and 142 °C) indicated reduced chain entanglements compared to commercial UHMWPE. Mechanical performance, inferred from the high MW and crystallinity, suggested suitability for solid-state processing, though specific tensile strength/modulus values were not explicitly reported. Support morphology significantly influenced catalytic performance, i.e., sMAO provided the highest activity, while LDHMAO yielded the highest MW (4.2 MDa) due to enhanced spatial confinement. The work underscored the superiority of PHENI* catalysts over indenyl-PHENICS in activity, molecular weight control, and entanglement reduction, positioning them as promising candidates for industrial dis-UHMWPE production.

Gote et al. [115] addressed a critical challenge in polyolefin catalysis, that is, while heterogeneous Z-N catalysts dominate industrial polyethylene production, they typically yield highly entangled UHMWPE with broad Đ, whereas homogeneous single-site catalysts (e.g., metallocenes, FI catalysts) can produce dis-UHMWPE but face scalability limitations. To bridge this gap, the authors developed a novel pseudo-single-site heterogeneous catalyst system through controlled reduction of a MgCl₂-supported Ti(OEt)₄ precursor (Catalyst 1) via a two-stage activation strategy. Initial activation with diethylaluminum chloride (DEAC) yielded a multisite catalyst containing mixed Ti(IV), Ti(III), and Ti(II) states (XPS quantification: 25.7%, 56.8%, and 17.5%, respectively), whereas subsequent activation with excess modified methylaluminoxane (MMAO12; [Al]/[Ti] = 600) selectively generated Ti(III) species (>99% by XPS). This optimized catalyst exhibited remarkable ethylene polymerization activity (213 kg_PE mol_Ti⁻¹ h⁻¹ bar⁻¹ at 40 °C, 1 bar ethylene) and living characteristics, producing dis-UHMWPE with unprecedented MWs (up to 13.07 × 10⁶ g/mol) and narrow dispersity (Đ = 1.84–2.17) over 120 min. The polymerization temperature exerted a notable impact on both the activity and MW of the polymer. When the polymerization temperature was set at 0 °C, the M_w of the resulting polymer reached 3.3 × 10⁶ g/mol; however, this temperature corresponded to the lowest observed activity. In contrast, at a temperature of 60 °C, the M_w decreased to 1.35 × 10⁶ g/mol. This reduction in MW at 60 °C was attributed to the enhanced chain transfer reactions that occurred at elevated temperatures. The nascent dis-UHMWPE displayed characteristic thermal properties, including a high first melting peak (T_m1 = 141–144 °C) attributed to disentangled lamellae and a lower second melting peak (T_m2 ≈ 134–136 °C) reflecting re-entanglement after melting, as confirmed by DSC. Rheological analysis revealed a rapid elastic modulus (i.e., G′) evolution during isothermal time sweeps (48 h to equilibrium), consistent with metastable disentangled chains reorganizing into entangled networks. Compared to conventional Z-N systems (M_w ≈ 2.1 × 10⁶ g/mol, Đ ≈ 4.4) and homogeneous FI catalysts (M_w ≈ 5.2 × 10⁶ g/mol, Đ ≈ 3.1), this pseudo-single-site catalyst uniquely combined heterogeneous practicality with homogeneous-like control, achieving both high MW and low entanglement density critical for advanced fiber and film applications.

To address the challenge of POSS being unable to diffuse into sub-10 nm pores, Cao et al. [116] developed an innovative strategy for synthesizing dis-UHMWPE using a PS/Z-N catalyst system. The researchers immobilized TiCl₄ onto polystyrene (PS)-functionalized silica supports prepared through in situ radical polymerization of styrene within the pore structure. Pulsed-field gradient NMR and thermoporosimetry were utilized to characterize the self-diffusion coefficients of polymer chains and crystallization behavior of probe molecules within SiO₂ nanopores, revealing the swelling dynamics of incorporated PS blocks. This hierarchical catalyst design (SiO₂/X%PS/TiCl₄) enabled precise control over chain propagation during ethylene polymerization. These PS isolators confined the polyethylene chains, enabling the synthesis of dis-UHMWPE with high activity (3.68 × 10⁶ g_PE mol_Ti⁻¹ h⁻¹) at 70 °C and 7 bar ethylene pressure. Critically, they demonstrated sustained effectiveness in reducing entanglements within nascent UHMWPE over extended polymerization times even up to 4 h. The PS-functionalized matrix served dual functions: (1) preventing active site aggregation through spatial confinement and (2) physically segregating growing UHMWPE chains to suppress entanglement formation, even during prolonged polymerization. This architectural control enabled the production of dis-UHMWPEs (designated PE-SiO₂/X%PS, X = 0, 2, 4, 6) with improved MWs and reduced chain entanglements simultaneously. Moreover, the toughness/stiffness/strength balance of dis-UHMWPE was significantly improved. The SiO₂/6%PS-derived dis-UHMWPE demonstrated exceptional mechanical performance, exhibiting 114 kJ/m² impact resistance (+53.5% improvement), an 80 MPa increase in Young’s modulus (+188.2%), 31.4 MPa tensile strength (+44.2%), and 696% elongation at break (+16.6%) compared to control samples. These results established a critical balance between polymer chain mobility and mechanical reinforcement through precisely engineered catalyst supports.

3.4. Optimization of Polymerization Conditions

In preparing dis-UHMWPE, parameters such as temperature, pressure, polymerization time, and so on can have a multi-level effect on the degree of molecular chain entanglement by regulating polymerization kinetics and crystallization behavior. Firstly, the temperature regulation kinetics and the mechanism of crystallization competition can have an effect. When the temperature rises (usually controlled at 60–80 °C), it increases the diffusion rate of ethylene monomer and catalyst activity, accelerating the chain growth reaction [31,98]. At this stage, higher segment activity (an enhanced α relaxation process) is beneficial for the conformational adjustment of molecular chains and shortens the freezing time of physical entanglement. However, lower polymerization temperatures increase the crystallization rate of molecular chains and make it easier to prepare dis-UHMWPE. The next effect is the pressure-induced topological constraint. Low-pressure environments (1–3 bar) slowed polymerization by limiting the concentration of ethylene monomers, allowing molecular chains sufficient time to complete conformational relaxation and reducing the likelihood of topological entanglement [25]. High pressure (>5 bar) will increase the concentration of local monomers, causing explosive chain growth and creating highly entangled networks. Furthermore, the non-equilibrium control of polymerization time also affects the degree of entanglement of molecular chains [63]. Usually, short-term polymerization results in a low MW of the product phase, which reduces the mass transfer resistance of ethylene monomer and accelerates the polymerization rate, leading to the generation of highly entangled products. By extending the reaction time, the mass transfer resistance will increase, leading to a higher crystallization rate of the separated chain than the chain growth rate as the reaction time progresses, which helps to reduce the degree of molecular chain entanglement.

UHMWPE is a semicrystalline polymer with excellent properties, but it is difficult to process due to chain entanglements. The traditional gel-spinning process for producing high-performance UHMWPE fibers uses a large amount of toxic solvents (e.g., decalin), and other methods such as solution polymerization with unsupported single-site catalysts have problems like reactor fouling. Rosario et al. [117] focuses on gas-phase polymerization using a Z-N catalyst (TiCl₄/MgCl₂) with inert condensing agents (ICAs) such as pentane and hexane to produce dis-UHMWPE. Polymerizations were performed at 40–70 °C and ethylene pressures of 3–15 bar, yielding polymers with M_v of 2000–7000 kg/mol and X_c of 57–66%. The gas-phase process resulted in faster crystallization kinetics compared to slurry polymerization, as evidenced by lower activation energies (K_g = 2.1–2.5 × 10⁴ K⁻²) and thinner lamellar thicknesses (SAXS data), leading to reduced entanglement densities. As reflected by SEM characterization, gas-phase synthesis yielded finer powders with reduced agglomeration compared to slurry processes. Even when limited agglomeration occurs in gas-phase polymerization, the resulting aggregates maintain discrete primary particles with a narrow size distribution. In contrast, slurry-derived powders exhibited a higher agglomeration tendency with less defined particle morphology (Figure 14). Furthermore, the maximum draw ratios (λ_max) of the tapes ranged from 7 to 97, with tenacities reaching up to 4 N/tex, comparable to industrial standards. Both studies demonstrated the feasibility of heterogeneous catalytic systems for producing dis-UHMWPE with controlled morphology and mechanical properties, offering scalable alternatives to homogeneous catalysts and solvent-based processing. The findings underscore the importance of polymerization conditions (temperature, pressure, and ICA selection) in tailoring entanglement density and crystallinity, with gas-phase polymerization emerging as a promising route due to its faster crystallization kinetics and lower environmental impact. The data collectively suggest that optimizing catalyst design and process parameters can bridge the gap between laboratory-scale synthesis and industrial production of high-performance UHMWPE materials.

Reasonable control over polymerization conditions, such as temperature, can help prepare dis-UHMWPE. To verify if heterogeneous Z-N catalysts can synthesize dis-UHMWPE at low temperatures and break through the industrial bottleneck of homogeneous catalysts, Huang et al. [132] addressed the challenge of melt-processing UHMWPE, which is hindered by its high entanglement density and resultant extreme melt viscosity (Figure 15). The authors synthesized a linear dis-UHMWPE (M_v ≈ 3.4 × 10⁶ g/mol) using a single-active-site Z-N catalyst, enabling reduced chain entanglements. This heterogeneous catalytic system, involving TiCl₄ supported on MgCl₂ with a salicylaldehyde-derived electron donor, allowed polymerization at low temperatures to isolate growing chains and suppress entanglement formation. The polymer exhibited a linear structure (confirmed by ¹³C NMR) and superior melt processability, as evidenced by dynamic viscosity and torque measurements (equilibrium torque: 4–6 min at 220–260 °C), which outperformed C-UHMWPE blends. The oscillation shear injection molding (OSIM) induced self-reinforced structures, including interlocked shish-kebabs and oriented lamellae, to be confirmed by SEM, WAXD (the degree of orientation, 0.820–0.939 in the outer layers), and SAXS. These structures, along with reduced defects and higher crystallinity (65.0–67.0% vs. 60% for compression-molded samples), improved the mechanical properties, including strength increased by 75% (from 23.3 ± 0.2 MPa to 40.8 ± 1.3 MPa), ultimate tensile strength (UTS) by 71% (from 40.4 ± 0.6 MPa to 69.0 ± 0.8 MPa), and Young’s modulus by 40–45% (1215.6 ± 78.5 MPa). The impact strength (88.8 ± 2.4 kJ/m²) and fatigue resistance (UTS retention: 86.2 ± 3.8%) also improved significantly. The study demonstrated a breakthrough in fabricating high-performance UHMWPE through structural manipulation, leveraging disentangled chains and shear-induced crystallization.

Notably, the production of dis-UHMWPE faces a key challenge: while heterogeneous Z-N catalysts are industrially preferred, their crowded active sites on the support typically lead to highly entangled polymer chains. Conversely, homogeneous catalysts can produce dis-UHMWPE more effectively but suffer from practical limitations like reactor fouling, hindering their industrial adoption. This creates a fundamental dilemma in catalyst selection for dis-UHMWPE synthesis. Heidari et al. [133] explored whether a heterogeneous TiCl₄/MgCl₂ Z-N catalyst could achieve disentanglement under specific conditions, comparing results with a homogeneous bis[N-(3-tert-butylsalicylidene)-2,3,4,5,6-pentafluoroanilinato] titanium (IV) dichloride (FI catalyst). Polymerizations were conducted at low temperatures (0–40 °C) and low atmospheric pressure (1 atm) in hexane (heterogeneous) or toluene (homogeneous), with TEA or MAO as cocatalysts, respectively. The heterogeneous system used an [Al]/[Ti] ratio of 14, while the homogeneous system employed [Al]/[Ti] of 600. Key findings revealed that polymerization at 0 °C for 120 min with the Z-N catalyst (Sample 1) yielded dis-UHMWPE with properties comparable to FI-catalyzed polyethylene (Sample 5), evidenced by rheological and mechanical tests. The time sweep rheometry at 180 °C showed normalized storage modulus (G’/G’_max) trends indicating lower initial entanglements for Sample 1, similar to Sample 5. Frequency sweep tests confirmed molecular weights > 10⁶ g/mol for all samples. Solid-state drawability tests at 100–120 °C demonstrated draw ratios up to 1800% for Sample 1, which matched FI-catalyzed polyethylene, while Samples 3 and 4 (shorter time/higher temperature) exhibited poor drawability. Drawn tapes of Sample 1 achieved a tensile modulus of 6–8 MPa at 115 °C, with modulus increasing proportionally to draw ratio. XRD revealed X_c values of 68.3% (Sample 1), 52.7% (Sample 4), and 89.5% (Sample 5), with Sample 1 showing a hexagonal phase shoulder (2θ = 20.6°), suggesting extended-chain formation. SEM images confirmed morphological differences, with Sample 1 displaying agglomerated crystallites and Sample 5 a cobweb-like structure. The study concluded that low-temperature (0 °C) polymerization with Z-N catalysts slows the chain growth relative to crystallization, reducing entanglements. This was supported by modulus build-up kinetics, drawability, and crystallinity data, aligning with homogeneous systems. Moreover, the findings highlight the potential of tailored heterogeneous catalysis for industrial-scale dis-UHMWPE production, circumventing the drawbacks of homogeneous catalysis while maintaining desirable properties.

The dispersion state of the catalyst also affects the entanglement of molecular chains, and the nano-dispersed Z-N catalyst contributes to the formation of dis-UHMWPE. Chammingkwan et al. [118] addressed the challenge of synthesizing dis-UHMWPE to enhance its processability, which is hindered by excessive chain entanglements in conventional UHMWPE. Their study proposed a nano-dispersed MgO/MgCl₂/TiCl₄ core–shell catalyst system to achieve spatial isolation of growing polymer chains, preventing overlap and entanglement during polymerization (0.8 MPa, 70 °C, TEA co-catalyst), yielding UHMWPE with a M_v of 4.9–5.3 × 10⁶ g/mol and a narrow Đ. A mechanism was proposed for the formation of dis-UHMWPE in this study (Figure 16a). Owing to the nonporous nature of the nano-dispersed single catalyst particles, polymer chains growing from the catalyst surface can crystallize readily without interference from other chains confined within pores. Moreover, activation with bulky alkyl aluminum reduced the active-site density, increasing the distance between active sites and thereby minimizing chain overlap, which resulted in the production of a less entangled polymer. The DSC analysis revealed the nascent UHMWPE with high T_m (142.3–142.9 °C) and crystallinity (X_c = 74–76%), while melt-crystallized forms showed lower T_m (~135 °C) and X_c (57–59%). Annealing experiments at 138 °C demonstrated rapid chain detachment in synthesized UHMWPE (melt-crystallized fraction > 50% after 3 h), indicating reduced entanglement compared to C-UHMWPE (M_v = 2.7 × 10⁶ g/mol), which showed negligible chain mobility. Rheological time sweep tests at 180 °C revealed a modulus build-up from 1 kPa to 3 kPa over 3 h, attributed to entropic mixing of disentangled chains. A solid-state drawing achieved >20× elongation for synthesized dis-UHMWPE, contrasting with C-PE’s brittle failure (Figure 16b,c). Notably, co-catalyst modification with butylated hydroxytoluene (BHT) further reduced entanglement by increasing Ti site spacing, yielding UHMWPE with M_v of 6.5 × 10⁶ g/mol and slower modulus build-up (τ = 4373 s vs. 2731 s for unmodified catalyst) (Figure 16d). Key data included BET surface area (~30 m²/g), particle size (D₅₀ = 80–86 nm), and normalized modulus (G′_t/G′_max = 0.57–0.62), confirming weakly entangled structures. This work demonstrated that nano-dispersed catalysts and tailored co-catalysts enable industrially viable production of low-entanglement UHMWPE with enhanced processability and mechanical performance.

Polymerization conditions, including temperature, ethylene pressure, and nitrogen partial pressure, can also be used to prepare dis-UHMWPE. To enhance comprehension of solid-state sintering, Wang et al. [119] investigated sintering kinetics of UHMWPE by examining the role of initial entangled state and particle size in chain dynamics and mechanical performance (Figure 17). The high molecular weight (>1 million g/mol) of UHMWPE and entanglement density render conventional melt-processing infeasible, necessitating sintering techniques akin to metal powder processing. Their research aims to elucidate how sintering parameters, particularly particle size and entanglement state, influence interfacial chain diffusion, entanglement formation, and final material properties. Two types of UHMWPE particles, i.e., highly entangled (HE-PE) and Dis-PE, were synthesized using a Z-N catalyst (TiCl₄/MgCl₂) under heterogeneous slurry polymerization conditions. For HE-PE, polymerization occurred at 60 °C, 6 bar ethylene pressure, and an activity of 8.7 × 10⁶ g_PE mol_Ti⁻¹ h⁻¹ bar⁻¹, yielding an M_v of 607.4 × 10⁴ g/mol and Đ of 4.5. Moreover, dis-PE was synthesized at 50 °C, 3 bar ethylene pressure, and 3 bar N₂ partial pressure to reduce ethylene concentration, resulting in lower activity (2.4 × 10⁶ g_PE mol_Ti⁻¹ h⁻¹), an M_v of 569.3 × 10⁴ g/mol, and Đ of 4.3. Both polymers exhibited similar melting points (~142.9 °C for HE-PE, 142.6 °C for Dis-PE) and X_c (~62–66%) but differed in entanglement density, as confirmed by DSC analysis. Significantly, the initial entangled state and particle size play a crucial role in the sintering kinetics of UHMWPE. In contrast to conventionally synthesized C-UHMWPE, highly entangled UHMWPE contains a multitude of physically intertwined points within highly entangled regions. These points significantly impede the mobility of chain segments, thereby restricting interfacial chain fusion and the progression of entanglement formation. Sintering at 190 °C under 20 MPa pressure revealed distinct entanglement dynamics: small Dis-PE particles (95 µm) achieved an equilibrium storage modulus (G′ = 10 kPa) within 5 min due to rapid chain explosion and interfacial healing, while HE-PE required >30 min. Rheological analysis showed interfacial diffusion coefficients (D_n) of 2.33 × 10⁻¹⁴ m²/s (HE-PE-95) to 7.97 × 10⁻¹⁴ m²/s (HE-PE-275), with bulk-phase entanglement fractions (Φ_e,B) decreasing from 0.586 to 0.365 as particle size decreased. Mechanical testing demonstrated Dis-PE-95’s superior impact strength (98 kJ/m² vs. 67 kJ/m² for HE-PE-95) and stable tensile strength (~30 MPa) independent of sintering time, whereas HE-PE required prolonged sintering (30 min) to reach maximum tensile strength (35 MPa) and elongation (~400%). The frequency sweep tests confirmed higher entanglement density in small HE-PE particles (G’ = 10⁴ Pa at 100 rad/s) versus Dis-PE (G’ = 10³ Pa). The study concluded that small particle sizes and reduced initial entanglement synergistically enhance sintering efficiency by promoting interfacial chain diffusion and entanglement formation, enabling rapid processing without compromising mechanical integrity.

4. Conclusions

UHMWPE has emerged as a crucial component in engineering and biomedical applications owing to its outstanding mechanical strength, exceptional wear resistance, and superior impact performance. Nevertheless, its extreme melt viscosity resulting from extensive chain entanglements presents significant challenges for conventional melt-processing techniques. Recent breakthroughs in catalytic synthesis have facilitated the production of dis-UHMWPE, which combines improved processability with retained superior mechanical properties. In this context, heterogeneous catalytic systems have shown great potential, with supported FI catalysts and polyhedral oligomeric POSS-modified Z-N catalysts demonstrating an optimal balance between structural control and industrial scalability. These systems employ nanoscale supports and sophisticated ligand engineering to spatially isolate active sites, effectively suppressing interchain entanglement during polymerization. Furthermore, optimization of polymerization parameters has proven crucial in enhancing chain disentanglement while preserving ultra-high-molecular weights. Notably, advanced characterization techniques have further provided fundamental insights into entanglement density, revealing unique material behaviors including distinct reorganization kinetics and bimodal melting characteristics in dis-UHMWPE. Therefore, these innovative developments present new opportunities for both solid-state processing and industrial-scale manufacturing. In conclusion, this comprehensive review highlights the intricate relationships between catalyst design, polymerization control, and ultimate material performance, outlining a path towards fully realizing UHMWPE’s potential for next-generation advanced applications. The integration of these technological advances promises to overcome current processing limitations while expanding the material’s spectrum of application.