Free radical homopolymerization of a vinylferrocene/cyclodextrin complex in water
We report the radical initiated homopolymerization of a soluble vinylferrocene cyclodextrin-complex in water. Uncomplexed vinylferrocene 1 and the corresponding homopolymer are hydrophobic and completely insoluble in water. Complexation of 1 with methyl-β-cyclodextrin 2 results in clearly water-soluble structures due to incorporation of the ferrocene moiety into the cyclodextrin cavity. After free radical polymerization of the water-soluble complexed monomer, corresponding to polyvinylferrocene (PVFc), the water-soluble polymer is obtained due to the host guest interactions. Those polymeric complexes are stable in water up to about 90 °C. Above this temperature the polymer precipitates due to decomplexation. The complex was investigated by 1 H NMR spectrometry, dynamic light scattering (DLS), differential scanning calorimetry (DSC), and lower critical solution temperature (LCST) measurements.
Introduction<!>Results and Discussion<!>Conclusion<!>Materials and Instrumentation<!>Homopolymerization of vinylferrocene 1<!>Copolymerization of vinylferrocene 1 and N-isopropylacrylamide 5
<p>Since its discovery in 1951, ferrocene [1] and its derivatives and their applications have been the subject of numerous papers [2][3][4][5][6][7]. One derivative, vinylferrocene 1, is of particular interest because of its anionic and radical polymerization behavior [8][9][10][11][12]. Due to its electrical semiconductivity properties the polymer of 1 has several practical applications. It is mainly used in biochemical sensors [13,14], batteries [15] and in the fuel industry [16]. However, the polymerization of the hydrophobic vinylferrocene in water has not been investigated.</p><p>Recently, we studied the radical polymerization of various vinyl monomers in water using cyclodextrin as the hydrophilic host component [17][18][19][20][21][22][23][24][25]. The hydrophobic part of the monomer molecule is included in the more hydrophobic cyclodextrin cavity yielding completely water-soluble complexes capable of polymerization. In this publication we wish to present our results of the aqueous free radical homopolymerization of vinylferrocene 1 as a guest in host methyl-β-cyclodextrin (methyl-β-CD) 2. The stability of the resulting complexed polymers in water is also reported.</p><!><p>Vinylferrocene 1 was added to an aqueous solution of methyl-β-CD 2 and the complex obtained was subjected to radical polymerization in water at 50 °C. During the reaction an increase in viscosity was observed, due to the formation of complexed polyvinylferrocene 3, which did not precipitate during the chain growth as a consequence of a strong host-guest interaction (Scheme 1). The complex remains in solution. From this data 1:1 complexation was indicated.</p><p>The DLS results showed the hydrodynamic radii in water for molecular dispersed cyclodextrin 2 to be 1.3 nm. By comparison, the radius of the complex of monomer 1 with methyl-β-CD 2 is slightly increased at 1.5 nm. However, the complexed polymer 3 shows a significantly larger hydrodynamic radius of 164 nm (Figure 1).</p><p>The complexed polyvinylferrocene 3 was thermally stable up to about 90 °C in water, as demonstrated by turbidity measurements. The complex between methyl-β-CD 2 and polyvinylferrocene 4 remains stable almost up to boiling point of water. Heating above 90 °C, results in destabilization of the complex and causes the polymer to precipitate. Furthermore, the reversibile redox behavior of complexed polyvinylferrocene 3 was evaluated by cyclic voltammetry. Oxidation of complexed polyvinylferrocene 3 to the polyvinylferricinium cation 6 takes place at 0.5 V and its reduction at −0.45 V (Figure 2). By contrast, irreversible oxidation occurred on heating the solution of complexed polyvinylferrocene 3 in aqueous hydrogen peroxide, which was accompanied with a change in color from yellow (Figure 3a) to green (Figure 3b). In a second step, the reduction of PVFc + 6 to PVFc 4 was exothermic, hence the complexed polyvinylferrocene 3 was destabilized and the  In addition, when potassium 1-adamantane carboxylate was added as a guest molecule, which competes with the complexed homopolymer solution, the polymer precipitated due to the hydrophobic character of the uncomplexed ferrocene. This uncomplexed polyvinylferrocene was investigated by MALDI-TOF which indicated a mass of 6172.7 [M+Na] + .</p><p>Very recently a paper was published, reporting the expected LCST properties of the copolymer consisting of N-allylferrocenecarboxamide and N-isopropylacrylamide (poly(NiPAAM/ FCN)) [26]. However, in this instance the copolymer was prepared by a classical synthesis in an organic solvent. Thus, we were encouraged to prepare a similar copolymer by free radical polymerization in water starting from cyclodextrin complexed vinylferrocene 1 and the water-soluble co-monomer N-isopropylacrylamide 5, the latter in a 20 fold molar excess. The copolymer obtained (PVFc-co-P(NiPAAM)) 7 was watersoluble, since the cyclodextrin is not covalently attached to the ferrocene moiety (Scheme 2).</p><p>The strong complexation was shown by correlations between methyl-β-CD 2 and polyvinylferrocene-co-poly(N-isopropylacrylamide) 7 at 4.16 ppm (cp) + 3.54 ppm (2) and at 4.17 (cp) + 3.52 ppm (2) in 1 H 1 H ROESY-NMR spectrum. The LCST of the copolymer in water was slightly increased from 17 °C to 19 °C (Figure 4) after the addition of cyclodextrin.</p><!><p>It can be concluded from the experiments described above that vinylferrocene can be easily polymerized in water after complexation with cyclodextrin. The strong polymer-CD complex obtained remained unchanged up to about 90 °C. The removal of cyclodextrin by the addition of a competing guest, potassium 1-adamantane carboxylate, was demonstrated. Additionally, the polymer shows redox behavior even in the presence of the cyclodextrin ring.</p><!><p>Methyl-β-cyclodextrin was obtained from Wacker-Chemie GmbH, Burghausen, Germany, and used after drying overnight with a vacuum oil pump over P 4 O 10 . Dimethyl-d 6 sulfoxide (99.9 atom % D) was obtained from Euriso-Top SA, France. 2,2-Azobis[2-(2-imidazolin-2-yl)-propane] dihydrochloride (VA-044) was obtained from Cayman Europe, Estonia. N-Isopropylacrylamide (NiPAAM) was obtained from Aldrich, Germany. Vinylferrocene (VFc) was obtained from Sigma-Aldrich, Germany. IR spectra were recorded with a Nicolet 5 SXB FTIR (Fourier transform infrared) spectrometer equipped with an ATR unit. The measurements were performed in the range of 4000-300 cm −1 at room temperature. 1 H NMR spectra were recorded with a Bruker AC 500 at 20 °C. Chemical shifts were referenced to the solvent value δ 2.51 for dimethyl-d 6 sulfoxide. Matrixassisted laser desorption/ionization-time-of-flight mass spectrometry (MALDITOF-MS) was performed on a Bruker Ultraflex TOF mass spectrometer. Cyclic voltammetry measurements were performed on a simplot instrument with a resolution of <100 pA. Dynamic light scattering (DLS) experiments were carried out with a Malvern HPPS-ET at 25 °C. The particle size distribution was derived from a deconvolution of the measured intensity autocorrelation function of the sample by the general purpose mode algorithm included in the DTS software. DLS measurements were performed in water; the concentrations of the substances were 10 mg × mL −1 . Each experiment was performed at least five times to obtain statistical information. Differential scanning calorimetry (DSC) measurements were performed on a Mettler DSC-30 instrument in a temperature range of −25 to 200 °C at a heating rate of 15 °C × min −1 as the average of five measurements using the midpoint method.</p><!><p>An aqueous solution of methyl-β-CD 2 in water (617.72 mg, 0.47 mmol) was prepared and purged with argon for 5 min. Vinylferrocene 1 (100 mg, 0.47 mmol) was added under an argon atmosphere and the mixture heated to 50 °C. VA-044 (1.53 mg, 4.7 × 10 −3 mmol) was added to the mixture followed by the addition of a similar amount of the initiator after 8 h. After a reaction time of 48 h, the reaction mixture was freezedried. No residual monomer was evident by thin layer chromatography. According to differential scanning calorimetry (DSC), the obtained polyvinylferrocene 3 had a glass transition temperature (T g ) of 145 °C.</p><!><p>An aqueous solution of methyl-β-CD 2 (617.72 mg, 0.47 mmol) was prepared and purged with argon for 5 min. Vinylferrocene 1 (100 mg, 0.47 mmol) was added under an argon atmosphere. After vinylferrocene 1 had completely dissolved, N-isopropylacrylamide (NiPAAM) 5 (1067.10 mg, 9.43 mmol) was added.</p><p>The mixture was heated to 50 °C and VA-044 (32.02 mg, 0.10 mmol) added. After 3 h an orange solid precipitated.</p><p>FT-IR (film) 7: 2970 cm −1 (-C-H), 1637 cm −1 (-C=C), 1533 cm −1 (-N-H), 1458 cm −1 (-CH 2 ), 1385 cm −1 (-CH 3 ), 1365 cm −1 (-C-N);</p>