All oxygen- and/or moisture-sensitive procedures were performed in an Etelux glove box or under nitrogen protection using standard Schlenk techniques. Anhydrous toluene was purified using the Etelux solvent purification system. The rac-Et(Ind)₂ZrCl₂ complex was purchased from Strem Chemicals, Inc., America. Methylaluminoxane (MAO), 1.5 mol/L in toluene was purchased from Innochem Technology, Co., Ltd., China. Commercial ethylene (99.9%) was purchased from Liufang Gas, Co., Ltd., China, and was purified by oxygen-trap columns and molecular sieves prior to use. IUD was synthesized according to the procedures described in previous literature.^[

The copolymerization of ethylene and IUD was carried out in a Schlenk flask equipped with a mechanical stirrer with a total system volume of 120 mL. The flask was heated at 120 °C for 45 min under vacuum, then charged with nitrogen and evacuated to remove water and oxygen. After the flask was cooled to ambient temperature, it was placed in a water bath at a constant temperature of 25 °C. Ethylene gas was then pressurized to the flask (constant pressure of 0.1 MPa). IUD, toluene, and MAO were added sequentially. The rac-Et(Ind)₂ZrCl₂ complex (15 μmol) was dissolved in toluene (10 mL) and added to the flask to initiate polymerization. After stirring at 600 r/min for 20 min, the gas feed was stopped and the polymerization was terminated by air. The reaction mixture was rapidly poured into an acidic ethanol solution (5% HCl) for precipitation and the white solid was obtained after filtration. After full washing with ethanol, the resulting product was dried under vacuum at 80 °C to constant weight. The resultant copolymers were reacted with CH₃SO₃Na, CF₃SO₃K and LiTf₂N, respectively, in an ion-exchange reaction, and then the PE ionomers bearing CH₃SO₃⁻, CF₃SO₃⁻ and Tf₂N⁻ counterions, respectively were obtained.

The antibacterial properties of the copolymers and ionomers were demonstrated by the ability to suppress two of the most typical bacteria, Staphylococcus aureus (S. aureus ATCC 25923) and Escherichia coli (E. coli ATCC 25922). The sandwich method was used for examining the antibacterial properties, and the bacterial cultivation and testing procedures were detailed in published studies.^[

$$\mathrm{Bacterial}\mathrm{survival}=\frac{Q_{\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}}{Q_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}}}\times 100\text{\%}$$

1

The introduction of halogenated groups into non-polar polymer chains not only offered polar groups and structural diversity to PE, but also provided reaction sites for subsequent conversion to various functional groups. However, the halogenated monomers, i.e., ω-halo-α-olefins, may have a poisoning effect on the active centre. According to previous study, the intensity of the poisoning effect of halogen groups on the active centre followed the order: Cl > Br > I.^[

The rac-Et(Ind)₂ZrCl₂ is a typical bridged metallocene catalyst for olefin polymerization. In previous studies, this catalyst has been demonstrated to efficiently catalyze ethylene polymerization and copolymerization with various comonomers.^[

Scheme 1 shows the synthesis route to E/IUD copolymers and imidazolium-based PE ionomers. Activated by MAO, rac-Et(Ind)₂ZrCl₂ can efficiently catalyze the copolymerization of ethylene (0.1 MPa) with IUD at 25 °C to give a series of copolymers with high molecular weight (M_w), relatively narrow molecular weight distributions (MWDs), and different IUD incorporations. As the data presented in Table 1, the incorporation of IUD increases with IUD feed. The iodine atom in IUD acted as a reactive site in the subsequent reaction process, which underwent a nucleophilic substitution reaction with N-methylimidazole.

Fig 1  Synthesis of E/IUD copolymers and PE ionomers.

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Table 1  Polymerization results of ethylene and IUD. ^a

Entry	Sample	IUD b (mL)	Incorp. c (mol%)	Yield (g)	Activity d	Mw e (kDa)	MWD e	Tm f (°C)	ΔH f (J/g)	XC g (%)
1	PE	/	/	3.3	0.66	344	2.6	135.2	139.4	47.6
2	E/IUD (2.9)	0.9	2.9	5.1	1.02	279	2.5	128.3	78.9	26.9
3	E/IUD (6.1)	1.8	6.1	6.9	1.38	218	2.5	126.9	43.9	15.0
4	E/IUD (9.1)	2.7	9.1	8.3	1.66	176	2.2	124.6	31.6	10.8
5	E/IUD (12.2)	3.6	12.2	9.8	1.96	143	2.3	120.3	21.2	7.2
6	E/IUD (15.7)	4.5	15.7	10.4	2.08	125	2.2	114.5	5.9	2.0
7	E/IUD (16.6)	5.4	16.6	10.6	2.12	111	2.3	ND	ND	ND

^a Polymerization conditions: Cat.Zr=15 μmol, ethylene pressure=0.1 MPa, n(Al)/n(Zr)=1000, V_total=120 mL, copolymerization at 25 °C for 20 min; ^b The feed of comonomer; ^c Incorporation of comonomer was determined by ¹³C-NMR; ^d Catalytic activity: 10⁶ g·mol_Zr⁻¹·h⁻¹; ^e The weight-average molecular weight and MWD were determined by high temperature GPC; ^f The melting temperature and melting enthalpy were determined by DSC; ^g The degree of crystallinity was calculated using the melting enthalpy. ND: Not determined.

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Fig. 1 shows the ¹³C-NMR spectrum of a representative E/IUD copolymer. All resonance signals were clearly assigned according to the literature.^[

Fig 1  ¹³C-NMR spectrum of the copolymer (IUD 2.9%).

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The copolymerization results of ethylene with IUD are summarized in Table 1. All obtained copolymers displayed narrow MWDs ranging from 2.2 to 2.5 with unimodal GPC curves. The values of M_w of E/IUD copolymers decreased from 279 kDa to 111 kDa with the increase of IUD incorporation (Fig. S3 in the electronic supplementary information, ESI). This was due to the steric effect of IUD located at the end of the propagating chain slowed down the monomer coordination, thus chain transfer was accelerated. The M_w of the copolymer decreased with increasing IUD feed, because of the increased ratio of chain growth rate to chain transfer rate. Such results demonstrated the high efficiency of the rac-Et(Ind)₂ZrCl₂/MAO catalytic system.

All copolymers showed a single melting temperature (T_m) in the DSC melting curves (Fig. S4 in ESI). With the increase of IUD content, the T_ms of E/IUD copolymers decreased to disappear, and the melting enthalpies (ΔH_ms) decreased to zero, which were determined by the position and area of the melting peaks, respectively. This was because the insertion of IUD interfered with the crystalline structure and disrupted the structural regularity of the main chains, leading to a decrease in the crystallinity (X_C) of the E/IUD copolymers. The glass transition temperatures (T_gs) of E/IUD copolymers gradually shifted to lower temperatures with the increase of IUD content (Fig. S5 in ESI). This was because of the decreased crystallinity and enhanced chain mobility caused by long side chains.

To investigate the effect of the ionic groups on copolymer properties, three typical counterions (CH₃SO₃⁻, CF₃SO₃⁻, Tf₂N⁻) were chosen because of their easy availability and different properties.^[

Fig. 2 displays the ¹H-NMR spectra of the representative copolymer (IUD 15.7%) and the corresponding ionomers bearing different counterions (CH₃SO₃⁻, CF₃SO₃⁻, Tf₂N⁻). For the copolymers, the characteristic peak of CH₂ protons adjacent to iodine appeared in the range of 3.0 ppm to 3.5 ppm. However, in the ¹H-NMR spectra of the ionomers, no signal peaks were observed in the range from 3.0 ppm to 3.5 ppm. Meanwhile, new resonance signals appeared at 3.9−4.5 ppm, which were attributed to the CH₂ (a in Fig. 2) and CH₃ (b in Fig. 2) protons adjacent to the nitrogens of imidazole group. The ionomers with different counterions showed different resonance signals of imidazole groups. The chemical shifts of the CH₂ proton (c in Fig. 2) between the two nitrogen atoms on the imidazole ring were quite different because of the counterions (CH₃SO₃⁻: 10.0 ppm, CF₃SO₃⁻: 9.3 ppm, Tf₂N⁻: 8.7 ppm). The resonance signals at 7.5 ppm were assigned to the rest CH₂ protons (d and e in Fig. 2) on the imidazole ring.

Fig 2  ¹H-NMR spectra of the copolymer (IUD 15.7%) and ionomers with different counterions.

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To examine the completeness of the ion exchange, silver nitrate solution was used to test for the presence of iodide ions in the ionomers.^[

The introduction of ionic groups in semi-crystalline polymers altered the values of T_m and X_C. In order to explore the thermal and crystalline properties, the ionomers were carefully characterized by differential scanning calorimeter (DSC), dynamic mechanical analyzer (DMA), and wide-angle X-ray diffraction (WAXD). To clearly demonstrate the effect of different counterion types on the thermal properties, the ionomers with a relatively high ionic content of 12.2% were selected for discussion. The copolymer (IUD 12.2%) exhibited a melting peak at 120.3 °C, while the corresponding ionomers with CH₃SO₃⁻, CF₃SO₃⁻, and Tf₂N⁻ counterion showed a melting peak at 123.9, 123.1, and 122.8 °C, respectively (Fig. 3). A similar trend was observed for other ionomers with different counterion contents.

Fig 3  DSC melting curves of the copolymer (IUD 12.2%) and the corresponding ionomers.

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The above results indicated that the introduction of ionic groups led to the increase of T_m, which was attributed to the additional ionic interactions and the improved intermolecular forces. The ionomers with the same ionic content but different counterion types displayed different T_m, among which those bearing CH₃SO₃⁻ counterion had the highest T_m due to their strongest ionic interaction.

The effect of different counterions on the crystalline behavior of the copolymers and ionomers was investigated by WAXD (Fig. S6 in ESI). Two crystalline peaks at 2θ=21.6° and 24.0° corresponding to 110 and 200 spacings and one amorphous peak at 2θ=19.6° were observed for all the copolymers and ionomers.^[

After ionization, the T_g values of the ionomers shifted from −27.8 °C of the copolymer (IUD 12.2%) to −4.9 (CH₃SO₃⁻), −13.4 (CF₃SO₃⁻), and −20.2 °C (Tf₂N⁻), respectively (Fig. 4). The introduced ionic groups could form ionic aggregates in polymer matrix. Such aggregation acted as physical cross-links, restricting the movement of the polymer segments, which in turn led to an increase in T_g value.^[

Fig 4  Typical DMA curves of the E/IUD copolymer and ionomers.

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The effect of ionic content on T_g of the ionomers bearing the same counterion was also considered. For the ionomers bearing CF₃SO₃⁻, T_g varied from −13.4 °C to −6.4 °C as the ionic content varied from 12.2% to 6.1%. The same trend was observed for the ionomers bearing CH₃SO₃⁻ and Tf₂N⁻ counterions. An increase in ionic content also meant an increase in side chain content, thus a greater number of side chains would lead to decreased crystallinity and enhanced chain mobility, and consequently decreased T_g.

To investigate the effect of the introduction of ionic groups on the mechanical properties, the stress-strain behaviors of the copolymers and ionomers were tested. For E/IUD copolymers, both the stress-at-break and elongation-at-break of the copolymers decreased with increasing IUD content. As presented in Fig. S7 (in ESI), the copolymer (IUD 2.9%) exhibited a stress-at-break of 20.5 MPa and an elongation-at-break of 900%. As the content of IUD reached 12.2%, the stress-at-break decreased to 2.3 MPa and the elongation-at-break decreased to 152%. On the one hand, the decrease in stress-at-break was due to the disruption of the crystalline structure of PE backbone by the incorporated IUD units. On the other hand, the decrease in elongation-at-break of the copolymers was attributed to the decreased M_w with IUD incorporation, because lower M_w weakened the chain entanglement that favored the ductility of polymers.

Fig. 5 shows the effect of introducing ionic groups on mechanical properties. Compared with the copolymer (IUD 6.1%), the stress-at-break values of the ionomers increased from 9.4 MPa to 25.6 MPa (CH₃SO₃⁻), 19.4 MPa (CF₃SO₃⁻), and 15.8 MPa (Tf₂N⁻), respectively. However, the elongation-at-break decreased from 808% to 529% (CH₃SO₃⁻), 611% (CF₃SO₃⁻), and 734% (Tf₂N⁻), respectively. The same trend was observed for the copolymer and ionomers with an IUD or ionic content of 2.9% (Fig. S8 in ESI). Compared with the copolymer (IUD 12.2%), the stress-at-break values of the ionomers significantly increased from 2.3 MPa to 23.0 MPa (CH₃SO₃⁻), 18.1 MPa (CF₃SO₃⁻), and 10.0 MPa (Tf₂N⁻), respectively. It is noteworthy that the elongation-at-break of the ionomers with ionic content of 12.2% displays a significant increase different from the ionomers with ionic contents of 2.9% and 6.1%. The elongation-at-break of the ionomers bearing CH₃SO₃⁻, CF₃SO₃⁻, and Tf₂N⁻ increased from 152% of the copolymer (IUD 12.2%) to 495%, 629% and 753%, respectively. The same trend was also observed for the copolymer and ionomers with an IUD or ionic content of 15.7% (Fig. S9 in ESI).

Fig 5  Stress-strain curves of (a) the copolymer (IUD 6.1%) and ionomers, and (b) the copolymer (IUD 12.2%) and ionomers.

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The results discussed above indicated that the introduction of ionic groups remarkably affected the mechanical properties of the copolymers. This can be explained by the ability of ionic groups to form ionic aggregates in the polymer matrix, which act as physical cross-links to strengthen the intermolecular forces.^[

The effect of counterion species on the mechanical properties of the ionomers has also been considered. The stress-at-break values of the ionomers with CH₃SO₃⁻ ranged from 17.9 MPa to 25.6 MPa, and the elongation-at-break values ranged from 445% to 536%. The stress-at-break values of the ionomers with CF₃SO₃⁻ ranged from 14.6 MPa to 24.2 MPa, and the elongation-at-break values ranged from 549% to 716%. The stress-at-break values of the ionomers with Tf₂N⁻ ranged from 7.8 MPa to 21.2 MPa, and the elongation-at-break values ranged from 681% to 847%. These results suggested that counterion type considerably impact the mechanical properties of the ionomers at the same ionic content. The stress-at-break values of the ionomers exhibited the following order: CH₃SO₃⁻ > CF₃SO₃⁻ > Tf₂N⁻, while the elongation-at-break values showed the opposite trend. The trend in stress-at-break was consistent with the previously reported binding intensity of the counterions with the imidazole cation.^[

Ionomers have been reported to play an important role in biomedical applications such as antibacterial material. To verify the antibacterial properties of the prepared copolymers and ionomers, the sandwich method in which a bacterial solution was coated in the middle of two polymer membranes was employed for testing. S. aureus, a typical Gram-positive bacterium, and E. coli, a typical Gram-negative bacterium, were selected for antibacterial test.^[

The antibacterial activities of the ionomers were also considered. For imidazolium-based ionomers with the same ionic content, the ionomers bearing Tf₂N⁻ displayed the best antibacterial activities against both S. aureus and E. coli (Fig. S12 in ESI). The ionomers bearing Tf₂N⁻ were chosen to discuss the effect of ionic content on the antibacterial properties of the ionomers. The ionomers with different ionic contents exhibited different antibacterial activities (Fig. 6a). As shown in Fig. 6(b), after 4 h of co-cultivation with the ionomers containing 6.1%, 9.1%, 12.2%, and 15.7% Tf₂N⁻ counterion, the corresponding survival rates for S. aureus were 3.8%, <0.1%, <0.1%, <0.01%, and for E. coli were 3.9%, 0.2%, 0.1%, <0.1%. The above results indicated that the antibacterial activities of the ionomers improved with the increase of ionic content. The imidazole cation group on the side chain can interact electrostatically with the bacterial surface carrying negative charges. The electrostatic interaction can cause cell rupture or change the permeability of cell membranes, resulting in the leakage of intracellular components of the bacterial cell which in turn led to the death of the bacterium.^[

Fig 6  (a) Photographs of bacterial colonies and (b) survival rates of S. aureus and E. coli after co-cultivation with the ionomers bearing Tf₂N⁻ for 4 h.

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The effect of counterion type on the antibacterial activities of the ionomers was further investigated. The ionomers containing 9.1% ionic group but different counterions were chosen for discussion. The ionomers with different counterion types exhibited different antibacterial activities (Fig. 7a). As presented in Fig. 7(b), the survival rate of S. aureus was 70.0% and that of E. coli was 71.6% after 4 h of co-cultivation with the copolymer (IUD 9.1%). After co-cultivation with ionomers bearing CH₃SO₃⁻, CF₃SO₃⁻, and Tf₂N⁻ for 4 h, the corresponding survival rates for S. aureus are 6.7%, 4.6% and <0.1%, and for E. coli are 4.8%, 3.4% and 0.2%. The ionomers bearing Tf₂N⁻ exhibited excellent antibacterial activities of more than 99% against both bacteria when the ionic content reached 9.1%. The above results indicated that the antibacterial properties of ionomers with the same ionic content but different counterions displayed the following order: CH₃SO₃⁻ < CF₃SO₃⁻ < Tf₂N⁻.

Fig 7  (a) Photographs of bacterial colonies and (b) survival rates of S. aureus and E. coli after co-cultivation with the copolymer (IUD 9.1%) and the corresponding ionomers for 4 h.

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Among three kinds of imidazolium-based ionomers, the ionomers bearing Tf₂N⁻ exhibited the best antibacterial activities against S. aureus and E. coli. The Tf₂N⁻ counterion had the weakest ionic interaction with the imidazole cation, thus the imidazole cation was more likely to interact electrostatically with negatively charged cell membranes rather than Tf₂N⁻. In contrast, the interaction between CH₃SO₃⁻ and imidazole cation was too strong, preventing the imidazole cation group from effectively interacting with the cell membrane. The above results demonstrated the advantages of imidazolium-based ionomers in efficient and broad-spectrum antibacterial activity and their potential application as antibacterial materials.