Year

2001

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Abstract

This thesis describes the synthesis and characterisation of a range of novel, chiral conducting polymers formed from substituted anilines, which may have potential in future applications such as chirality assessment, asymmetric synthesis and chiral separations. Their redox and pH switching and ionochromic properties are also explored. A particular focus is the water-soluble sulfonated polyaniline, poly(2- methoxyaniline-5-sulfonic acid) (PMAS), where two ways of inducing main chain chirality in PMAS have been successfully developed.

In Chapter 3 it is shown that PMAS can be deposited in a highly optically active form via the potentiodynamic polymerisation of the monomer 2-methoxyaniline-5-sulfonic acid (MAS) in the presence of chiral amines (R)-(+)- or (L)-(-)l-phenylethylamine (PhEA). The enantiomeric amines induced intense, mirror-imaged CD spectra for the respective PMAS.(+)-PhEA and PMAS.(-)-PhEA films, suggesting the adoption of preferred one-handed helical structures by the polyaniline chains arising from enantioselective acid-base interactions between the amines and ionised sulfonate substituents on the polymer. The polymers were also characterised by cyclic voltammetry, molecular weight and electrical conductivity measurements. It was also found that chiral PMAS.(+)-PhEA could be immobilised on protonated poly(4- vinylpyridine) while retaining its optical activity and electroactivity.

Chapter 4 describes the alternative preparation of optically active PMAS films via mixing aqueous PMAS with a wide range of chiral amines and amino alcohols, followed by evaporative- or spin-casting. Chiral induction in the PMAS was believed to occur via a similar acid/base interaction to that proposed in Chapter 3. However, the CD spectra of the spin-cast PMAS (RNH2) films did not exhibit bisignate Cotton effects. This indicated that spin-casting causes the loss of exciton coupling between the responsible chromophores, suggesting that they are well separated either on the same or adjacent PMAS chains. The influence of the steric bulk and other structural features of the amines and amino alcohols on the extent of chiral induction in the PMAS was also explored. Although losing their optical activity when dissolved in water, it was found that the chiral PMAS.(+)-PhEA films could be crosslinked with poly(vinylalcohol), making them insoluble in water.

Dilute aqueous solutions of PMAS have been shown in Chapter 5 to exhibit remarkable and unprecedented ionochromism when 1.0 M alkali and alkaline earth metal salts were added, the colour changing from yellow/brown to blue over hours or days. A first rapid stage (few min) was attributed to replacement of protons from the "free" S03H groups on the PMAS chains, leading to changes in conformation/structure. The slow (hours/days) second step was believed to involve conversion of the PMAS from an "extended coil" to a "compact coil" conformation. The ionochromic effects were found to be strongly dependent on the nature of the metal ion in the added metal salts. The speed/extent of the rapid first step increased along the metal ion series: Li+ < K+ < Na+ < Ba2+ < Cs+ < Mg2+ < Ca2+; while the slower second step revealed a differing order: Cs+, Li+ < K+ < Ca2+ < Ba2+ < Na+. The second step also showed marked anion dependence, the speed of the conformational/colour change increasing along the series SCN-, S042- < Br- < CI- < I-

Calcium, magnesium and barium salts of PMAS have been precipitated when 1.0 M MX2 salts were added to more concentrated (1 %) aqueous PMAS. The PMAS (Ba 2+) salt had a higher electrical conductivity and molecular weight than the as-received PMAS (NH4 +), and atomic absorption analysis showed that ca. 75 % of the "free" sulfonate groups have Ba2+ ions attached. These ionochromic/conformational effects were reversed by the addition of 0.1 M HCl, regenerating PMAS in the original "extended coil" conformation.

PMAS was found in Chapter 6 to be remarkably resistant to alkaline dedoping, with no emeraldine base being formed even in 2.0 M NaOH. Instead, PMAS in alkaline solutions underwent very similar, but much more rapid, spectroscopic changes to those caused by adding metal salts. Its polymer backbone was therefore believed to undergo a conformational change from an "extended coil" to a "compact coil" conformation. During the titration of a PMAS (NH4+) solution with aqueous NaOH, spectroscopic changes in the first stage between pH 3.7 and 8.0 were attributed to deprotonation of "free" S03H groups on the PMAS chains. A second stage between pH 9 and 14 was believed to involve rearrangement from an "extended coil" to a "compact coil" conformation.

Like unsubstituted polyaniline, PMAS was found to be readily oxidised by ammonium persulfate at pH 3.7 to give the corresponding pernigraniline base form. However, chemical reduction of PMAS by hydrazine displayed distinctively different behaviour to that previously reported for unsubstituted polyaniline. At pH 9, the rapid formation of a novel polymer (Species I) was observed, characterised by an intense, sharp absorption band at 408 run, which was attributed to a conformer or isomer of leucoemeraldine(LB). This species then slowly converted to the leucoemeraldine base form of PMAS . In contrast, hydrazine reduction of PMAS at pH 13 or in 1.0 M KC1 produced only Species I. Acidification of this reduced solution resulted in aerial oxidation, leading reformation of the initial PMAS.

The synthesis and chiroptical properties of a number of other substituted polyanilines are described in Chapter 7, employing aniline monomers bearing methyl, phenyl or a chiral substituent on the nitrogen atom. Af-Methylaniline (NMA) was shown to form a chiral polymer PNMA.(+)-HCSA when potentiostatically oxidised in the presence of the chiral dopant acid (+)-camphorsulfonic acid. An electroactive poly(diphenylamine) (PDPA) polymer doped with (+)-HCSA was also electrodeposited from a water / acetonitrile mixed solvent. However, in this case the PDPA backbone was not found to form a one-handed helix in the presence of (+)-HCSA.

As an alternative approach, the chiral monomer, (S)-(+)-(anilinomethyl)pyrrolidine, bearing a chiral substituent on the nitrogen atom of aniline, was polymerised chemically and electrochemically in 1.0 M HC1. The product was optically active, as evidenced by its CD spectrum. However, this presumably oligomeric product was found to be soluble.

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