A co-anchoring strategy for the synthesis of polar bimodal polyethylene

Abstract

Since polar groups can poison the metal centers in catalysts, the incorporation of polar comonomers usually comes at the expense of catalytic activity and polymer molecular weight. In this contribution, we demonstrate polar bimodal polyethylene as a potential solution to this trade-off. The more-polar/more-branched low-molecular-weight fraction provides polarity and processability, while the less-polar/less-branched high-molecular-weight fraction provides mechanical and melt properties. To achieve high miscibility between these two fractions, three synthetic routes are investigated: mixtures of homogeneous catalysts, separately supported heterogeneous catalysts, and a co-anchoring strategy (CAS) to heterogenize different homogeneous catalysts on one solid support. The CAS route is the only viable strategy for the synthesis of polar bimodal polyethylene with good molecular level entanglement and minimal phase separation. This produces polyolefin materials with excellent mechanical properties, surface/dyeing properties, gas barrier properties, as well as extrudability and 3D-printability.

Fig. 1. Expected properties of polar bimodal polyethylene by combining low-molecular-weight and high-molecular-weight fractions.

Below are listed the tensile properties of polar bimodal polyethylene products prepared by mixed homogeneous, mixed heterogeneous, and co-anchoring strategies.

Fig. 2. Synthesis of desired catalysts.

Synthesis of mixed homogeneous catalysts, mixed heterogeneous catalysts, and co-supported catalysts by the co-anchoring strategy.

Fig. 3. Comparison of bimodal polyethylene prepared in different systems.

a Tensile curves of polyethylene prepared by homogeneous polymerization. PE-Ni1 (Supplementary Table 1, Entry 1), PE-Ni3 (Supplementary Table 1, Entry 3), PE-Ni1/Ni3 (Supplementary Table 1, Entry 17). b Tensile curves of polyethylene prepared by heterogeneous polymerization. PE-Ni2-MgO (Supplementary Table 1, Entry 5), PE-Ni3-MgO (Supplementary Table 1, Entry 6), PE-Ni2/Ni3-MgO (Supplementary Table 1, Entry 16), PE-Ni2-MgO/Ni3-MgO (Supplementary Table 1, Entry 18). c Tensile curves of polar polyethylene prepared by homogeneous polymerization. PPE-Ni1 (Supplementary Table 3, Entry 1), PPE-Ni3 (Supplementary Table 3, Entry 3), and PPE-Ni1/Ni3 (Supplementary Table 3, Entry 4). d Tensile curves of polar polyethylene prepared by heterogeneous polymerization. PPE-Ni2-MgO (Table 1, Entry 5), PPE-Ni3-MgO (Table 1, Entry 6), PPE-Ni2/Ni3-MgO (Table 1, Entry 12), and PPE-Ni2-MgO/Ni3-MgO (Table 1, Entry 22). e Repetitive tensile curves of polymers PE-Ni2/Ni3-MgO (Supplementary Table 1, Entry 16) and PE-Ni2-MgO/Ni3-MgO (Supplementary Table 1, Entry 18). f DSC curve of PPE-Ni2-MgO (Table 1, Entry 5), PPE-Ni3-MgO (Table 1, Entry 6), PPE-Ni2-MgO/Ni3-MgO (Table 1, Entry 22) and PPE-Ni2/Ni3-MgO (Table 1, Entry 12). g Rheological curve of PPE-Ni2-MgO/Ni3-MgO (Table 1, Entry 22) and PPE-Ni2/Ni3-MgO (Table 1, Entry 12). h-1 SEM image of PPE-Ni2-MgO/Ni3-MgO (Table 1, Entry 22). h-2 SEM image of PPE-Ni2/Ni3-MgO (Table 1, Entry 12). The original SEM images were listed in Supplementary Fig. 50.

Fig. 4. Comparison of SEM images of polar bimodal polyethylene samples prepared by co-anchored catalyst and mixed heterogeneous catalyst after incorporation of polar monomer.

These bimodal polymers in the first row were prepared by co-anchored catalyst Ni2/Ni3-MgO, and those in the second row were prepared by mixed heterogeneous catalyst Ni2-MgO/Ni3-MgO. Incorp. (Incorporation) ratios of comonomers were determined from ¹H NMR spectra. The characterization data of these polymers are listed in Supplementary Information Table 4.

Fig. 5. Correlation diagram of mechanical properties with polar monomer incorporation of a series of bimodal polyethylene samples.

a Correlation diagram of tensile strength with polar monomer incorporation. b Correlation diagram of toughness with polar monomer incorporation. The characterization data of these polymers are listed in Supplementary Information Table 4.

Fig. 6. Polar properties and 3D printing of polar bimodal polyethylene.

a Tensile curves of samples from Table 1, Entry 7 (Incorp. 0.7%), Entry 2 (Incorp. 0.1%), and Entry 9 (Incorp. 0.7%). b Water contact angles of ethylene/tert-butyl acrylate copolymers from Table 1. Unimodal: unimodal polar polyethylene from Table 1, Entry 1, Entry 2, and Entry 7. Bimodal: polar bimodal polyethylene prepared by co-anchored catalyst from Table 1, Entries 8–11. c UV-vis absorption spectra of the dyed polymer products before and after acetone wash (a and a’: Table 1, Entry 9. b and b’: Table 1, Entry 7). d Oxygen permeability coefficient of polyethylene samples from Table 1 at 25 °C. The abscissa represents the relative entry in Table 1. Unimodal: unimodal polar polyethylene from Table 1. Bimodal: polar bimodal polyethylene prepared by co-anchored catalyst, Mixed: prepared by mixed heterogeneous catalyst. e Images of extruded samples of PE-Ni2-MgO (Supplementary Table 1, Entry 5), PPE-Ni2-MgO/Ni3-MgO (Table 1, Entry 22) and PPE-Ni2/Ni3-MgO (Table 1, Entry 12). The enlarged images were listed in Supplementary Fig. 53. f–i 3D-printed samples of commercial HDPE, PPE-Ni2/Ni3-APP (Table 1, Entry 19), HDPE: PLA 7:3 (prepared by blending commercial HDPE and polylactic acid in a ratio of 7 to 3) and PPE-Ni2/Ni3-APP: PLA 7:3 (prepared by blending PPE-Ni2/Ni3-APP and polylactic acid in a ratio of 7 to 3).