Synthesis and characterization of poly(ethyleneimine) dendrimers
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- Yemul, O. & Imae, T. Colloid Polym Sci (2008) 286: 747. doi:10.1007/s00396-007-1830-6
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Poly(ethyleneimine) (PEI) dendrimers up to the third generation (G3) were prepared by a divergent synthesis method from an ethylenediamine (EDA) core. The amine terminals were bonded with vinylbromide by a Michael addition reaction. Then, the bromide terminals were converted to amine groups using a Gabriel amine synthesis method. PEI dendrimers displayed pH-dependent luminescence, and their emission intensities at pH 6 increased over time. Fluorescence intensities also increased with increasing dendrimer generation from G1 to G3. Air-bubbling in aqueous solutions of dendrimers made to incorporate detectable amount of oxygen in dendrimers. EDA also behaved similarly in luminescence and oxygen incorporation.
KeywordsPoly(ethyleneimine) dendrimerSynthesisDivergent synthesisMichael addition reactionGabriel amine synthesisCharacterizationFluorescence
There has been growing interest among researchers about highly branched polymers such as dendrimers because of their unique properties and applications in various fields [1–4]. The physicochemical properties of dendrimers are sometimes in sharp contrast to the behavior of linear polymers because of their explicit symmetry and high density of functional terminals. Experimental works have reported various properties of dendrimers in connection with chemical structure, chemical character, size, and shape of dendrimers [5–11].
Examples of typical dendrimers are poly(amido amine) (PAMAM) dendrimers and poly(propyleneimine) (PPI) dendrimers. Because the former dendrimer has amide groups in spacer moieties but the latter dendrimer does not have them, the properties of dendrimers, such as solvent penetration and guest doping, are different from each other in dependence upon a different chemical structure [11, 12]. The properties were also compared between PAMAM dendrimes with a different spacer length . In comparison, the interest in physicochemical properties of PPI dendrimers inspired us to study poly(ethyleneimine) (PEI) dendrimers, which have a shorter alkyl spacer than PPI dendrimer. Poly(ethyleneimine) is one of the commercially available and valuable cationic polyamines and has been widely used as an efficient drug carrier, a gene delivery system , and an adhesive component for wood . However, there is no report on a synthesis of PEI dendrimers.
In this work, we synthesize PEI dendrimers and examine their characters. Although both divergent [16–18] and convergent [19, 20] synthetic methods have been utilized to produce well-defined nanostructures, we selected a divergent procedure from an ethylenediamine (EDA) core through a route of Gabriel amine synthesis, different from the synthesis of PPI dendrimer [21, 22]. The properties of a model compound, EDA, was also examined in comparison with those of PEI dendrimers.
EDA, vinylbromide, phthalimide, hydrazine hydrate, hydrochloric acid, and solvents were purchased from Aldrich Chemical Co., USA or Tokyo Kasei Kogyo Co. Ltd., Japan and were used without further purification.
Michael addition reaction (Gn Br)
Michael addition reaction was carried out as follows: EDA or amine-terminated PEI dendrimer (Gn PEI) was dissolved into 20 cm3 of methanol, and a methanol solution of vinylbromide was added dropwise. The mixture was stirred in the dark at room temperature for 2 days. The solvent and excess vinylbromide were removed under vacuum to give a light yellow oily product.
G1 Br: Reactants: EDA (0.6 g, 10 mmol), vinylbromide (4.24 cm3, 85 mmol). Yield: 80%. Fourier transform-infrared (FTIR; neat): 3,356, 3,287, 3,181, 2,932, 2,858, 1,602, 1,454, 1,041 cm−1. Elemental analysis (FW 484) (C10H20N2Br4): calculated C24.79, H4.13, N5.78; observed C25.08, H4.0, N5.70. Proton nuclear magnetic resonance (1H NMR; 500 MHz, CDCl3, δ): 1.21–1.39 (s, 12 H), 2.71–2.75 ppm (d, 8 H). G2 Br: Reactants: G1 PEI (0.92 g, 10 mmol), vinylbromide (3.5 cm3, 85 mmol). Yield: 70%. FTIR (neat): 3,354, 3,285, 3,180, 2,930, 2,859, 1,600, 1,453, 1,040 cm−1. Elemental analysis (FW 1087) (C26H52N6Br8): calculated C28.70, H4.82, N7.72; observed C29.00, H4.72, N7.80. 1H NMR (500 MHz, CDCl3, δ): 2.30–2.42 (m, 20 H), 2.84 (d, 16 H), 3.34 ppm (d, 16 H). G3 Br: Reactants: G2 PEI (0.57 g, 0.1 mol), vinylbromide (1.7 cm3, 0.16 mol). Yield: 65%. FTIR (neat): 3,353, 3,285, 3,180, 2,932, 2,859, 1,601, 1,455, 1,042 cm−1. Elemental analysis (FW 2288) (C58H116N14Br16): calculated C30.45, H5.11, N8.57; observed C31.10, H5.25, N8.60. 1H NMR (500 MHz, CDCl3, δ): 2.34–3.21 (m, 52 H), 2.80–2.90 (d, 32 H), 3.42 ppm (d, 32 H).
Gabriel amine synthesis (Gn Ph)
Gabriel amine synthesis was carried out as follows: Gn Br was dissolved into 30 cm3 of dimethylformamide (DMF), and phthalimide was added. The mixture was heated in an oil bath at 80 °C for 6 h . After the reaction was completed, the product was precipitated as white solid in water. It was further purified by recrystallization from a mixture of ethylacetate/acetone and dried at 25 °C till constant weight.
G1 Ph: Reactants: G1 Br (1.21 g, 5 mmol), phthalimide (2.94 g, 20 mmol). Yield: 75%. FTIR (KBr): 2,932, 2,858, 1,671, 1,507, 1,438, 1,390, 1,258, 1,089 cm−1. Elemental analysis (FW 752) (C42H36N6O8): calculated C67.02, H4.78, N11.17; observed C67.0, H4.80, N12.0. G2 Ph: Reactants: G2 Br (1.0 g, 0.1 mol), phthalimide (1.17 g, 0.8 mol). Yield: 80%. FTIR (KBr): 2,931, 2,859, 1,670, 1,504, 1,440, 1,390, 1,259, 1,090 cm−1. Elemental analysis (FW 1617) (C90H84N14O16): calculated C66.82, H5.23, N12.12; observed C67.0, H5.40, N12.32. G3 Ph: Reactants: G2 Br (0.45 g, 0.1 mol), phthalimide (0.7 g, 0.16 mol). Yield: 78%. FTIR (KBr): 2,930, 2,859, 1,670, 1,505, 1,439, 1,390, 1,258, 1,089 cm−1. Elemental analysis (FW 3347) (C186H180N30O32): calculated C66.73, H5.42, N12.55; observed C66.85, H5.58, N12.68.
Synthesis of amine-terminated PEI dendrimer (Gn PEI)
Amine synthesis was carried out as follows: Gabriel Gn Ph was mixed with hydrazine hydrate and hydrochloric acid, and the reaction mixture was refluxed for 6 h and cooled. The solid product (phthalylhydrazide) was filtered off, and the amine product was isolated by neutralization and vacuum distillation.
G1 PEI: Reactants: G1 Ph (1.50 g, 10 mmol), hydrazine hydrate (2.0 cm3, 40 mmol). Yield: 85%. FTIR (neat): 3,361, 3,292, 2,943, 2,805, 1,576, 1,464, 1,375, 1,316, 1,184, 1,073 cm−1. 1H NMR (500 MHz, D2O, δ) 2.10–2.21 (s, 4 H), 2.32–2.50 (d, 8 H), 2.60–2.70 ppm (d, 8 H). G2 PEI: Reactants: G2 Ph (1.61 g, 0.1 mol), hydrazine hydrate (3.0 cm3, 0.1 mol). Yield: 90%. FTIR (neat): 3,356, 3,294, 2,946, 2,866, 2,804, 1,581, 1,469, 1,382, 1,314, 1,184, 1,072 cm−1. 1H NMR (500 MHz, D2O, δ) 2.10–2.45 (m, 20 H), 2.55–2.65 (d, 16 H), 2.70–2.90 ppm (d, 16H). G3 PEI: Reactants: G2 Ph (1.67 g, 5.6 mmol), hydrazine hydrate (3.0 cm3, 0.1 mol). Yield: 80%. FTIR (neat): 3,358, 3,288, 2,940, 2,866, 2,804, 1,581, 1,463, 1,382, 1,308, 1,185, 1,073 cm−1. 1H NMR (500 MHz, D2O, δ) 2.20–2.50 (m, 52 H), 2.60–2.85 (m, 32 H), 2.90–3.10 ppm (m, 32H).
Elemental analysis was performed on a LECO CHN-900C analyzer. Fourier transform-infrared absorption spectra were recorded using a Bio-Rad FTS 575C spectrometer equipped with a cryogenic mercury cadmium telluride detector. KBr pellets of powders were prepared. Fluorescence spectra were measured on a Hitachi F-4010 model. Solutions were prepared by dissolving EDA or dendrimer (0.7 mM) in water. Nuclear magnetic resonance spectra were recorded for a deuterated aqueous solution (1 mg cm−3) of EDA or dendrimer on a JEOL JNM-L500. All the measurements were carried out at room temperature.
Results and discussion
Synthesis of poly(ethyleneimine) dendrimers
Elemental analyses of EDA, Gn PEI, and their air-injected products
Compound FW (NH2 group, N atom)
Air-injected evaluated (composition formula)
EDA C2H8N2 (2, 2)
C19.10 H7.43 N20.70 (O52.77)
C2.1H9.8N2.0O4.3 (C2H8N2 + 2.1O2)
G1 PEI C10H28N6 232 (4, 6)
C51.72 H12.06 N36.20
C51.08 H12.20 N36.70
C25.06 H6.02 N18.10 (O50.82)
C9.7H27.7N6.0O14.7 (C10H28N6 + 7.4O2)
G2 PEI C26H68N14 576 (8, 14)
C54.13 H11.08 N33.99
C54.25 H11.20 N34.15
C27.12 H5.40 N17.13 (O50.35)
C26.0H61.7N14.0O36.2 (C26H68N14 + 18.1O2)
G3 PEI C58H148N30 1266 (16, 30)
C55.03 H11.78 N33.19
C55.00 H12.08 N33.25
C28.02 H6.00 N16.20 (O49.78)
C58.0H148.1N28.9O77.4 (C58H148N30 + 38.7O2)
The results of IR absorption and 1H NMR spectra confirmed the successful synthesis of dendrimers. Such verification was also obtained from elemental analysis. The observed values were in good consistency with calculated ones for all series of Gn Br, Gn Ph, and Gn PEI, as seen in the “Experimental section” and Table 1.
Recently, some research groups have discovered fluorescence emission from PAMAM dendrimers [24–31]. It has been confirmed that the fluorescence emission from the fourth generation PAMAM dendrimer was strongly dependent on pH [32, 33]. The strong fluorescence was detected for a PPI dendrimer and hyperbranched poly(amino ether)s as well [32–34].
The fluorescence behaviors of PEI dendrimers are alike previous observation for PAMAM and PPI dendrimers [32, 33]. It should be noticed that the fluorescence behavior is scarcely dependent on spacer groups and lengths like amide, propyl, and ethyl groups. It should be surprised that EDA also presents fluorescence. It can be focused that EDA has amine groups, although it is a small core molecule of PEI dendrimers.
Injection of air in dendrimers
Blue photoluminescence with high quantum yield has been achieved upon oxidation of PAMAM dendrimers with ammonium persulfate  or air-bubbling . Therefore, in the present work, the effect of air-bubbling for PEI dendrimers is compared to that for EDA. The injection of air in EDA and dendrimers was carried out as follows: EDA (0.60 g, 10 mmol) or Gn PEI (G1: 0.46 g, 20 mmol; G2: 1.14 g, 20 mmol; G3: 1.26 g, 10 mmol) was dissolved in 10 cm3 of water, air-bubbled for 5 min, and kept at room temperature. The solution turned yellowish after 1 week. The air-injected product was recovered by evaporating water, and the obtained white solid was further characterized.
The elemental analyses of air-injected PEI dendrimers (Gn PEI-Ox) and EDA (EDA-Ox) were carried out in comparison with neat compounds, as listed in Table 1. The contents of C, H, and N atoms of air-bubbled Gn PEI and EDA were only half of neat compounds corresponding. Supposing that the rest is oxygen, composition formula of each compound was evaluated and listed in Table 1. It can be noticed that the estimated composition formulas of air-bubbled compounds are in consistency with those of neat compounds including additional oxygen atoms. If oxygen in air-bubbled compounds is O2 molecules, the content of O2 is comparable to the number of N atom in EDA and is 1.3 times of N atom in Gn PEI.
As described above, the effect of air-bubbling drastically appeared on elemental analysis data. O2 comparable to or more than the number of amine was incorporated in dendrimers and EDA. It can be interpreted that O2 interacts with nitrogen in amine groups. It is known that amines after oxidation form heterocyclic organic compounds, and finally, they are decomposed into methane, nitrogen, and carbon dioxide gases [35, 36]. Otherwise, amines are oxidized to the corresponding carboxylic acid through aldehyde . Mechanism and rate of the decomposition depend on the type of catalyst and temperature. In the present procedure, only the air-bubbling was carried out, and no hard conditions like using of catalyst or heating were imposed. The situation of oxygen atoms in dendrimer and the relation of oxygen atoms with luminescence are less well on the present state, and the resolution of such subjects is in progress.
Amine-terminated PEI dendrimers up to third generation were synthesized using EDA as a core by a divergent synthesis method. A strong fluorescence from PEI dendrimers was observed under acidic conditions. Emission intensities increased with increasing dendrimer generation from G1 to G3 and time. To consider the effect of oxygen on fluorescence of PEI dendrimers, air-bubbling was carried out in the aqueous solutions. Detectable amounts of oxygen were incorporated in dendrimers. It was an amazing result that EDA also behaved similarly to PEI dendrimers on fluorescence properties and oxygen incorporation. The present results give us a clue on the clarification of the luminescence mechanism, which probably involves the interaction of oxygens with amine groups.
The authors are thankful to Dr. Y. Maeda in the Nagoya University for providing NMR spectra of samples. OY is grateful to the 21st Century COE Program (No. 14COEB01-00) for financial support of postdoctoral fellowship.