Polyanilines boronate

Example 5 Polyaniline-Boronic Acid - Synthesis of Poly(aniline-co-3-amino-phenylboronic Acid)-without Fluoride (5a)... 

Freund has prepared polyaniline boronic acids by the electrochemical polymerization of 3-aminophenyl boronic acid [169, 170], The electrochemical potential of the polymer is sensitive to changes in the pK of the polymer as a result of boronic acid-diol complex formation. Fabre has also used polyaniline boronic acids as a con-ductiomeric sensor for dopamine [171]. 

Boronic acid-containing polyaniline has also been utilized in diabetes-related research. One such polymer (23) (Fig. 17) has been observed to exhibit a linear near-infrared optical response to saccharides.43 The polymer was prepared by the copolymerization of aniline with 3-aminophenylboronic acid using lOmM (NH4)S208 in 1M HC1. The films were observed to undergo changes in the absorption spectra on addition of saccharides at pH 7.2. 

Figure 17 Boronic acid-containing polyanilines (23 and 24) used in the detection of saccharides. (Adapted from refs. 43 and 44.)...

Boronic Acid Substituted Self-Doped Polyaniline... 

BORONIC ACID SUBSTITUTED SELF-DOPED POLYANILINE... 

Fabre et al. electrochemically polymerized PABA using a fluoride catalyzed reaction [31]. The electropolymerization of 3-aminophenyl-boronic acid in 0.5 M H2SO4 aqueous solution was found to be inefficient (Figure 3.3, A). However, addition of fluoride strongly enhanced the polymerization rate (Figure 3.3, B). This effect was not observed with Cl or with unsubstituted polyaniline in the presence of fluoride. 

Similarly, self-doped PABA can be prepared using excess of saccharide and one equivalent of fluoride to monomer. Complexation between saccharides and aromatic boronic acids is highly pH dependent, presumably due to the tetrahedral intermediate involved in complexation [25]. Because the pKa of 3-aminophenylboronic acid is 8.75, complexation requires pH values above 8.6. This pH range is not compatible with the electrochemical synthesis of polyaniline, which is typically carried out near a pH value of 0. However, Smith et al. have shown that the addition of fluoride can stabilize the complexation of molecules containing vicinal diols with aromatic boronic acids [23]. Based on this work, it was postulated that the electrochemical polymerization of a saccharide complex with 3-aminophenylboronic acid in the presence of one molar equivalent of fluoride at pH values lower than 8 is possible if a self-doped polymer is produced in the process. 

The redox behavior of chemically and electrochemically prepared PABA in acidic solution (0.5 M HCl) is reportedly similar to that observed for unsubstituted polyaniline [41]. Two sets of redox waves are observed, at 0.18 and 0.74 V, suggesting facile conversion of leucoemeraldine to emeraldine and subsequent conversion to pernigraniline oxidation states. These results suggest that the boronic acid substituent and polymerization conditions have no detrimental influence on the electronic properties of the polymer. This is in contrast to sulfonated polyaniline where the redox couples are more closely spaced than for polyaniline due to the electronic and steric effects of the -SOa" groups on the backbone of the polymer (for details see, Chapter 2, section 2.5.3). 

The conductivity of self-doped PABA without heat treatment was observed to be around 0.96 S/cm. This conductivity value is consistent with the 21 % doping suggested by NMR based on the conductivities of other forms of self-doped polyanilines [110, 113]. However, the conductivity was lower than HCl doped polyaniline, probably due to distortion of the polymer backbone by the presence of the boronic acid substituent [116-118]. After heat treatment at 100 and 500°C, a decrease in conductivity from 0.96 S/cm (without heat treatment) to 0.094 and 0.009 S/cm, respectively was observed. However, in the case of polyaniline, the conductivities of the heat treated polyaniline declined significantly compared with that of the self-doped PABA. The relative decrease in conductivity of heat treated PABA was less than that of HCl-doped polyaniline, probably due to the formation of a thermally stable boronic acid anhydride crosslink. In the case of polyaniline, the dramatic decrease in conductivity was a result of the decomposition of the backbone above 420 °C, as seen in the thermograms. In contrast, the process of crosslinking in the PABA polymer above 100 °C may make... 

Recently, Fabre et al. [31] and Freund et al. [7, 8] used electro-chemically deposited, self-doped, boronic-acid-substituted, conducting polymers for saccharide and fluoride detection. Freund et al. prepared a potentiometric sensor for saccharides using self-doped PABA [7, 8]. The transduction mechanism in that system is reportedly the change in pKa of polyaniline that accompanies complexation, and the resulting change in the electrochemical potential. Sensors produced with this approach exhibit reversible responses with selectivity to various saccharides and 1,2-diols (Figure 3.22) that reflect their binding constants with phenylboronic acid observed in bulk solutions. The sensitivity... 

Figure 3.24 shows the redox behavior of PABA thin films observed at neutral pH in the presence of NADH and NAD" ". The PABA film was redox inactive at neutral pH (Figure 3.24,a) due to deprotonation and loss of dopant as with polyaniline [150,151). However, in the presence of NADH (Figure 3.24, b) and NAD" " (Figure 3.24, c), PABA films became redox active due to complexation of boronic acid with cis-2,3-ribose diols and subsequent formation of self-doped polymer. In the presence of NADH, the cyclic voltammogram of PABA thin film exhibited a single redox couple at pa 0.05 and pc —0.10 V. In contrast, a second redox couple was observed in the presence of NAD+ at pa 0.34 and pc... 

The complexation of PABA with nucleotides was further studied using polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS). PM-IRRAS spectra of PABA films exhibit all the characteristic vibrations of polyaniline and boronic acid (Figure 3.27) [93, 94]. After complexation with NAD+ (Figure 3.27, b) and NADH (Figure 3.27, c), the disappearance of the free B-OH group vibration at 986 cm and increase in the intensity of the asymmetric B-O bond vibration at 1330 cm indicate the formation of boronate ester. The new vibrations at 1080 and 1470 cm have been attributed to ribose and adenine moieties, respectively [153]. The vibrations at 1218 (NAD+), 1245... 

As discussed above, complexation changes the nature of the boronic acid substituent and in turn the electrochemical potential of polyaniline. Therefore, in addition to voltammetric, NMR and PM-IRRAS... 

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