Unprecedented Mechanical Properties in Linear UHMWPE Using a Heterogeneous Catalytic System

Abstract

Several homogeneous postmetallocene complexes are known to produce weakly entangled ultrahigh-molecular-weight polyethylene (disentangled UHMWPE) under controlled polymerization conditions leading to ultimate mechanical properties. Major challenges in using a homogeneous catalyst system exist such as reactor fouling and uncontrolled polymer morphology, which could be addressed by heterogenization of the single-site complex. In such a scenario, a heterogeneous catalytic system that can synthesize ultrahigh-molecular-weight polyethylene with a reduced number of entanglements, to an extent that the mechanical properties are equivalent to those obtained using a postmetallocene catalyst in a homogeneous condition, remains a challenge. Herein, a magnesium chloride based in situ formed activator/support, MgClx/EtnAly(2-ethyl-1-hexoxide)z, is employed with a highly active bis[N-(3-tert-butylsalicylidene)pentafluoroanilinato] titanium(IV) dichloride (Cat. 1) for the synthesis of ultrahigh-molecular-weight polyethylene with a reduced number of entanglements. A novel route is adopted to make a nano-support that allows tailoring of the entangled state and control over the resultant morphology without reactor fouling and wall sheeting, thus providing the feasibility of pursuing the polymerization via a continuous process. The synthesized nascent polymer shows the formation of single crystals of linear UHMWPE, identifying the fold surface and crystal thickness and suggesting a low entangled state. The topological differences with the commercial entangled sample are identified by chain diffusion from the noncrystalline to crystalline region via solid-state NMR, following melting kinetics via DSC, and transformation of the nonequilibrium melt into the equilibrium state via rheology. Thus, the obtained disentangled crystals can be compressed in the solid state, without melting, to an extent that the macroscopic forces can be transferred to the molecular level. This allows the desired chain orientation for securing the ultimate tensile modulus (>200 N/tex) and tensile strength (>4.0 N/tex) in the synthesized low entangled linear polyethylene having a weight average molar mass exceeding a million grams per mole. These mechanical properties are equivalent to those perceived using a homogeneous catalytic system and are the first of their kind reported in a polymer synthesized using a heterogeneous catalytic system.