Synthesis of Ultra-High-Molecular-Weight Polyethylene by Transition-Metal-Catalyzed Precipitation Polymerization

Abstract

Ultra-high-molecular-weight polyethylene (UHMWPE) plays an important role in many important fields as engineering plastics. In this contribution, a precipitation polymerization strategy is developed by combination of highly active phosphino-phenolate nickel catalysts with polymer-insoluble solvent (heptane) to access UHMWPE (M _n up to 8.3 × 10⁶ g mol^-1) with good product morphology, free-flowing characteristics, and great mechanical properties. Compared with the academically commonly used aromatic solvent (toluene), the utilization of heptane offers simultaneous enhancement in important parameters including activity, polymer molecular weight, and catalyst thermal stability. This system can also generate polar functionalized UHMWPE with molecular weight of up to 1.6 × 10⁶ g mol^-1 in the copolymerization of ethylene with polar comonomers. More importantly, this precipitation polymerization strategy is generally applicable to several representative transition metal catalyst systems, leading to UHMWPE synthesis with good product morphology control.

Scheme 1. (1) Representative Phosphino-Phenolate Nickel Catalysts and (2) Transition Metal Catalysts for Precipitation Polymerization in This Work

Scheme 2. Phosphino-Phenolate Ligands for Polymerization

Figure 1

(A) Image of the isolated polyethylene product for L1+Ni(COD)₂ in toluene (Table 1, Entry 1). (B–D) Images of isolated polyethylene product for L1+Ni(COD)₂ in heptane (Table 1, Entries 2–4). (E) Time-dependence studies (yield vs time) of the nickel catalysts at 80 °C (1 μmol). (F) The stress–strain curve of the UHMWPE originally prepared with nickel catalysts at 20 °C from Table 1. (G) Image of the isolated polyethylene product for NN1-Ni in heptane. (H) Image of the isolated polyethylene product for NN1-Ni in toluene.