Synthesis of paramagnetic polymers based on polyethyleneimine (PEI)

Abstract

Paramagnetic polymers based on PEI have been synthesized and characterized. Studies on the structure and property relationship showed that the magnetic property was affected by the molecular weight, ferric content and linear or branched structures of the resultant magnetic PEIs. Interestingly, the magnetic susceptibility had a linear relationship to the ferric content. The resultant polymers showed a magnetic response to an external magnetic field, and have great potential in magnetic separation, magnetic switching as well as controlled delivery.

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Introduction

Magnetic polymer materials are of increasing interest due to their extensive applications in magnetic resonance imaging,^1–3 hyperthermia therapy,⁴ drug delivery,^5,6 cell and DNA separations,^7–9 catalysis,^10,11 environmental remediation^12,13 and specific sensors.^14,15 However, the preparation of magnetic polymers is mostly based on the hybridization of polymers with magnetic nanoparticles such as inorganic ferric, nickelic and cobaltic compounds,^16,17 and the preparation of polymers with magnetic properties themselves remains a great challenge. The first organic ferromagnet was prepared by the polymerization of side-chain radical containing biacetylene in the 1980s.¹⁸ The magnetic properties of the resultant polymers were from the high-spin polyradicals because selected conjugated hydrocarbon polyradicals had strong ferromagnetic couplings.¹⁹ Theoretical studies on such conjugated polyradicals are of great interesting. With developing of polymer chemistry, magnetic polymers which complexed with transition metal or paramagnetic ions were reported.¹⁹ The metal containing (para)magnetic polymers have attracted great attention due to the facile design and synthesis of such polymers compared with the conjugated polyradicals.

Recently, paramagnetic ionic liquid containing tetrahalogen iron(III) anion, such as tetrachloroferrate (FeCl₄⁻), was obtained by Hayashi and Hamaguchi through mixing quaternized ammonium salt with iron(III) halide.²⁰ The paramagnetic ionic liquid showed magnetic response which has potential in magnetic control and separation.^21,22 Furthermore, paramagnetic poly(ionic liquid) containing FeCl₄⁻ and tetrabromoferrate (FeBr₄⁻) was synthesized and used as catalyst for Friedel–Crafts alkylation reactions by Döbbelin et al. in 2011.²³ The pyridine based magnetic block copolymer was prepared via post-modification of pyridine with FeCl₄⁻, and the nanostructured magnetic thermoset material was obtained by blending such magnetic block copolymer with epoxy resin.²⁴

Since quaternized nitrogen cation could be complexed with iron(III) halide, polymers with quaternizable nitrogen have the potential to become (para)magnetic materials. Polyethyleneimine (PEI), known as the polymer with the most abundant amino groups and the highest cationic-charge-density potential, could be the best candidate for preparing magnetic polymers. What's even better, PEI has attracted tremendous attention of researchers due to its unique physical and chemical properties. Boussif et al.²⁵ reported that PEI could function as a highly efficient vector for delivering oligonucleotides and plasmids both in vitro and in vivo, and hence could be an outstanding core for carriers in gene therapy. And PEI became the second polymer gene transfection reagent after poly-L-lysine.²⁶ Beyth et al. immobilized cross-linked quaternary ammonium PEI in medical composite resin and the antibacterial activities of such compound was studied.^27,28 Hu et al. grafted PEI to single-walled carbon nanotube, which was used as a substrate for cultured neurons and found that it promoted neurite outgrowth and branching.²⁹ By impregnating PEI in a series of mesoporous silica materials, Son et al. improved its CO₂ adsorption property and discovered that the CO₂ adsorbing performance of these materials was influenced primarily by their pore size.³⁰ Vinogradov et al. employed PEI as a structure-directing agent to obtain organized mesoporous CuO–Al₂O₃ composite.³¹ Although many impressive works have been done in the field of PEI related applications, to our best knowledge, the PEI related magnetic polymer is barely reported.

Magnetism is very helpful in magnetic control, especially in separation, recycling of catalyst or agent via external magnetic field. Bringing in magnetism seems promising to obtain multifunctional materials. Since every third atom of PEI is a quaternizable nitrogen, it is feasible to prepare PEI based magnetic polymers through quaternization and iron halides complexation. Meanwhile, due to the high nitrogen density, PEI based magnetic polymers could display good paramagnetic property. Furthermore, PEI is commercially available and low-cost, which makes it possible for preparation of PEI based magnetic polymers in big scale. In this work, PEI based magnetic polymers were prepared via quaternization, ion exchange and FeCl₃ complexation. In attempt to obtain optimum paramagnetic property, molecular weight, linear or branched chain structures as well as FeCl₄⁻ (ferric) content were discussed in detail.

Experimental

Materials

Linear polyethyleneimine (PEI, 99%, M_w = 10k, 1.8k, 0.6k, Chengdu Micxy Chemical Company), branched PEI (99%, M_w = 10k, Alfa Aesar), methyl iodide (99%, Tianjin Bichenglan Chemical Reagent Factory), FeCl₃·6H₂O (99%, Tianjin Shuangchuan Chemical Reagent Factory), ion exchange resin (Amberlite IRA-400(Cl), Alfa Aesar). All the other chemicals were commercially available and used directly unless addressed.

Synthesis of quaternized polyethyleneimine with chloride ions (QPEI)

PEI (0.2709 g, 6.3 mmol of –CH₂CH₂NH– unit) was dissolved in 5 mL ethanol and added dropwise to methyl iodide (5.9 mL, 64 mmol) under stirring. The mixture was refluxed for 60 hours at the pH of ca. 8 by addition of NaOH aqueous solution. At the end of the reaction, the reaction mixture was centrifuged to give the crude product. Final product (QPEI-I) was obtained by precipitating the crude product in ethanol for 3 times followed by vacuum drying. QPEI-I was dissolved in distilled water and stirred with 5 times overdosed ion-exchange resin (IRA-400). The mixture was stirred for 12 h followed by filtration and clear resultant solution was obtained. After evaporating and vacuum drying, dry solid product (QPEI) was obtained. Yield: 98%. The quaternized PEIs with different ammonium content were prepared via similar process while changing reaction time.

Synthesis of magnetic quaternized ammonium compound (MPEI)

FeCl₃·6H₂O and QPEI was dissolved separately in methanol at the molar ratio of FeCl₃ : N⁺ of ca. 1 : 1, and the FeCl₃ solution was added into QPEI solution and stirred at 50 °C for 16 h. The resultant mixture was evaporated and redissolved in acetonitrile followed by precipitation in excessive amount of diethyl ether for three times. Brown colored solid product was obtained through centrifuge and followed by vacuum drying. Yield: 98%. All other groups of MPEI was synthesized likewise. All experiments were reproducible.

Characterization

¹H-NMR measurement was conducted on a 500 MHz Bruker Avance III NMR spectrometer. The Fourier transform infrared spectrometry (FTIR) was carried out on Perkin-Elmer System 2000 for wave number interval 500–4000 cm⁻¹ in which 5% w/w QPEI powder/KBr tablets were prepared and analyzed. Elemental analysis (EA) was conducted on a Elementar Vario cube (CHNS/CHN) analyzer. Raman measurements were carried out on a DXR Microscope at a wavelength of 780 nm. Magnetic measurements were conducted on a Superconducting Quantum Interference Device (SQUID)-Quantum Design PPMS-9 magnetometer from −10 000 Oe to 10 000 Oe at 298 K. Thermogravimetric analysis (TGA) was conducted on a Pyris 1 TGA (Perkin-Elmer System), ramping from 25 °C to 1000 °C at a rate of 20 °C min⁻¹ under the protection of nitrogen.

Results and discussion

Since PEI is the most nitrogen abundant polymer, which could be quaternized and complexed with iron(III) halides, the resultant polyelectrolyte contains the high spin FeCl₄⁻ as the anion and hence shows paramagnetic property. As is shown in Scheme 1, paramagnetic polymers based on linear and branched PEI were designed and prepared through quaternization, ion exchange and FeCl₃ complexation process. To study the relationship between magnetic property and the structure, the paramagnetic PEI based polymers (MPEIs) with different molecular weight, ferric (FeCl₄⁻) content and chain structure was designed. This is because the above mentioned factors might influenced the polymer chain and FeCl₄⁻ ordering and thus have a great effect on the magnetic property. PEIs with different molecular weight such as 0.6k, 1.8k and 10k were used because the molecular weight of MPEIs is dependent on the original PEIs. For PEIs with the same molecular weight, ferric content was controlled by the quaternization percent of PEI, which can be tuned easily by changing the reaction time, since the complexation of FeCl₃ with quaternized PEI (QPEI) is almost 100%. At the same time, MPEI prepared from branched PEI was synthesized to study the differences between linear and branched MPEIs.

Scheme 1 Synthetic route for PEI based paramagnetic polymers.

The QPEIs were studied via ¹H-NMR. As shown in Fig. S1,† compared with original PEI, the signals assigned for QPEIs were shifted to a lower field because of the electron withdrawing effect of nitrenium ion in quaternized ammonium unit. And, with increasing of quaternization percent (P_q), the signals down shifted a lot. However, it is difficult to calculate the P_q from ¹H-NMR. P_q of the QPEIs was calculated from elemental analysis (EA) by comparing the molar ratio of C/N according to eqn (1).

P_q = (r − m)/(M − m) × 100% (1)

where P_q stands for quaternization percent, r stands for the C/N ratio given by EA results, M stands for maximum C/N ratio (the C/N ratio of 100% quaternized PEI), m stands for minimum C/N ratio (the C/N ratio of original PEI). Since the linear PEI purchased was capped with amino group in both ends, the M and m value varies with the molecular weight.

Detailed characterization of QPEIs was shown in Table 1. For PEIs with the molecular weight of 10k, by changing the reaction time from 1 h to 100 h, the P_q can be controlled from 26% to 84% gradually, thus the QPEIs with gradient P_q were prepared. And, QPEIs with different molecular weight of linear PEI, branched PEI while maintain the similar P_q of ca. 60% were synthesized successfully.

Table 1 Synthesis and characterization of QPEIs

Code	Mn^a [×10^3]	Time^b [h]	C/N^c	Pq^d [%]
a Molecular weight of purchased PEI.b Reaction time for quaternization.c C/N molar ratio obtained from elemental analysis.d Quaternization percent calculated from EA analysis.e Quaternized QPEI based on linear PEI.f Quaternized QPEI based on branched PEI.
QPEI1^e	10	1	2.52	26
QPEI2^e	10	3	2.77	38
QPEI3^e	10	10	3.07	53
QPEI4^e	10	60	3.25	63
QPEI5^e	10	100	3.71	84
QPEI6^e	1.8	48	3.18	63
QPEI7^e	0.6	48	3.31	66
QPEI8^f	10	60	3.19	60

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The corresponding MPEIs were obtained by complexing QPEIs with FeCl₃ at the conversion of ca. 100% via the similar procedures reported.³² Since the unreacted FeCl₃ is soluble in diethyl ether, and the product can be purified by precipitating in diethyl ether for several times. Those MPEIs with linear and branched chain structure, different molecule weight (0.6k, 1.8k, 10k) and different ferric content were also synthesized following the similar procedures. Due to the paramagnetic property of FeCl₄⁻ anion, the MPEIs can not be characterized by ¹H-NMR. However, according to literature, Raman spectrometer at 300 K was used to examine the presence of FeCl₄⁻ anion in the magnetic polymers.²⁰ As shown in Fig. 1, all of the MPEIs showed signals at 341 cm⁻¹, which represents the Cl–Fe–Cl vibration in FeCl₄⁻ anion and demonstrates the successfully complex of FeCl₄⁻ to QPEI. The broadness of the signal in MPEI1 might be related to the low P_q of FeCl₄⁻. Table 2 shows the characterization of MPEIs, by controlling the P_q of QPEIs, the ferric content (C_Fe) of MPEIs could be tuned to study the effect of ferric content on the magnetism.

Fig. 1 Raman spectra of PEI based magnetic polymers. Code MPEI1–MPEI8 was corresponded to code MPEI1–MPEI8 in Table 2.

Table 2 Synthesis and characterization of MPEIs

Code	Mn^a [×10^3]	CFe^b [wt%]	Susceptibility [×10^−6 emu g^−1]
a Molecular weight of purchased PEI.b Ferric content of MPEIs, that is the weight percent of Fe element in MPEI.c MPEI based on linear PEI.d MPEI based on branched PEI.
MPEI1^c	10	14	10.7
MPEI2^c	10	16	20.6
MPEI3^c	10	18	29.9
MPEI4^c	10	19	36.3
MPEI5^c	10	20	41.6
MPEI6^c	1.8	19	36.4
MPEI7^c	0.6	19	33.6
MPEI8^d	10	19	27.9

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All the synthesized MPEIs showed magnetic response to neodymium magnet. Take MPEI1 as example, the MPEI1 particle was moved under the direction of neodymium magnet in ethanol since MPEI is not soluble in ethanol (Fig. 2). Furthermore, a video of MPEI1 particles moved under the attraction of neodymium magnet was supplied in ESI† to illustrate the magnetic response of the resultant MPEIs at room temperature in detail. The MPEI “walked” under the attraction of neodymium magnet directly, and fell down to the bottom under the force of gravity when withdraw the magnet. Such polymers have great potential in magnetic switching, magnetic separation, magnetic control as well as controlled release.

Fig. 2 Magnetic response of MPEI1 polymer in ethanol solution under the attraction of neodymium magnet.

For further study of the synthesized MPEIs, the magnetic properties of MPEI were also examined using the SQUID method. The results were shown in Fig. 3 and Fig. 4. For all MEPIs with different molecular weight, ferric content and linear or branched structures, linear relationship between magnetic moment and magnetic field was observed, which showed a paramagnetic behavior for such MPEI polymers. The magnetic susceptibility of MPEIs was summarized in Table 2. For linear MPEIs at the PEI molecular weight of 10k, the magnetic susceptibility increased with increasing of C_Fe, as the ferric content changed from 14% to 20%, the susceptibility increased from 10.7 × 10⁻⁶ emu g⁻¹ to 41.6 × 10⁻⁶ emu g⁻¹ correspondingly. A positively correlated relationship between C_Fe and susceptibility of linear 10k PEI based MPEIs at 300 K was founded (inset in Fig. 3). The susceptibility of MPEIs increased linearly with the increasing of ferric content. This is probably because that MPEIs with higher ferric content had a larger magnetic unit density which led to higher magnetic susceptibility value under an applied external magnetic field. It seems that with a higher ferric content, the susceptibility should be higher. And by controlling the ferric content, the magnetic susceptibility could be tuned easily. However, for MPEIs based on QPEI, the maximum ferric content at P_q of 100% is just 21%. From this aspect, the polymers with higher weight percent of quaternized nitrogen should be a better candidate in design and synthesis of magnetic polymers.

Fig. 3 SQUID magnetization of magnetic polymers based on linear 10k PEI at 300 K. Code MPEI1–MPEI5 was corresponded to those in Table 2.

Fig. 4 SQUID magnetization of magnetic polymers with different molecular weight and chain structure at 300 K. Code MPEI4–MPEI8 was corresponded to those in Table 2.

Fig. 4 shows the relationship between magnetic moment and susceptibility for MPEIs with different PEI molecular weight (linear 0.6k, 1.8k, 10k and branched PEI) at similar quaternization percent (ca. 60%). The linear MPEIs showed similar paramagnetic property with very close magnetic susceptibility value (Table 2), which suggested that the molecular weight has less effect on the magnetic susceptibility. The magnetic susceptibility of branched MPEI (μ = 27.9 × 10⁻⁶ emu g⁻¹) was smaller than those of linear MPEIs with similar ferric content, which is probably because branched structure is not avail to forming ordered arrangement of the polymer chain and magnet unit FeCl₄⁻ and thus decrease the magnetic susceptibility.

Conclusions

Paramagnetic polyelectrolytes based on PEIs were prepared through quaternization, ion exchange and FeCl₃ complexation. Studies on the paramagnetic susceptibility and polymer structures showed that the susceptibility increased linearly with increasing of ferric content, and the susceptibility could be tuned easily. Branched MPEIs had smaller susceptibility compared with linear MPEIs at the similar ferric content. The prepared paramagnetic polymers showed magnetic response to external magnetic field. This magnetic property, when combined with other properties of PEI such as CO₂ absorbing ability or anti-bacterial ability, has great potential in preparing novel functional materials.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NNSFC 21304067), Tianjin Research Program of Application Foundation and Advanced Technology (14JCQNJC03400).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra18175e

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