Synthesis and stereochemical studies of chiral polyanilines

Author

Chinnarat Boonchu, University of Wollongong

Year

2002

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Recommended Citation

Boonchu, Chinnarat, Synthesis and stereochemical studies of chiral polyanilines, Doctor of Philosophy thesis, Department of Chemistry, University of Wollongong, 2002. https://ro.uow.edu.au/theses/1189

Abstract

The recent discovery of chiral polyanilines has opened up the possibility of their use as novel materials for the separation of enantiomeric chemicals or as chiral electrodes in electrochemical asymmetric synthesis. This thesis describes the synthesis and stereochemical characterization of several such chiral polyanilines. It focuses on three polymers, namely the parent un-substituted polyaniline (PAn), poly(2-methoxyaniline) (POMA) and poly(2-methoxyaniline-5-sulfonic acid) (PMAS). Circular dichroism (CD) and UV-Visible-near infrared spectral studies are extensively employed to establish the chiroptical properties of these polymers.

The thesis addresses a number of important fundamental questions that need to be answered in order for these chiral polyanilines to be employed in the preparation of enantiomeric chemicals. The approaches employed and the results achieved are summarized below.

Chapter 2 summarized the general experimental techniques employed, although later Chapters also detail more specific experimental aspects.

Chapter 3 examines the influence of the solvent (solvatochromism) on the chiroptical properties and conformation of optically active PAn.(+)-HCSA emeraldine salts. It was shown that the presence of 2.5 to 10.0 % (v/v) water an emeraldine base (EB) solution in NMP solvent before doping with (+)-HCSA leads to PAn.(+)-HCSA with the inverse CD spectrum to that formed in pure NMP solvent. A similar remarkable inversion of polymer configuration by a small amount of water co-solvent was observed for PAn.(+)-HCSA salts formed by doping EB in DMPU and DMF solvents, but not in DMSO and HCOOH. In contrast, the addition of water to PAn.(+)-HCSA solution after their formation organic solvents had little effect on their conformation.

When 25 to 50 % v/v of water was present in solutions of EB in each of the solvents, NMP, DMPU, DMF, DMSO and HCOOH before doping, optically inactive P An.(+)-HCSA salt was obtained. This may be due to the high water content causing a change in the EB structure from a helical cis-cisoid to a flat trans-transoid conformation. These chiroptical results in mixed water/organic solvents throw new light on the origin of optical activity in PAn.(+)-HCSA salts. They suggest that chiral supramolecular aggregates are not necessary for optical activity to be observed.

The influence of temperature (thermochromism) on the chiroptical properties and conformations of PAn.(+)-HCSA and POMA.(+)-HCSA emeraldine salt films is explored in Chapter 4. The electrochemically deposited PAn.(+)-HCSA and POMA.(+)-HCSA salts employed were shown to have different conformations for their polyaniline chains, assigned as "partial extended coil" and "compact coil" respectively. The PAn.(+)-HCSA salt may be transformed from its initial "extended coil" to a "compact coil" conformation by heating the film to 140 °C for 10 min. The rate of cooling the PAn.(+)-HCSA film down (rapidly or slowly) from the heating temperature of 140 ° C did not influence the final chiroptical properties.

No change in conformation occurred upon heating a POMA.(+)-HCSA film to 140 ° C. However, heating to temperatures higher than 200° C for 10 min resulted in thermal de-doping to form optically active POMA emeraldine base. This POMA (EB) film almost completely racemized when heated at a temperature of 240° C for 65 min.

Chapter 5 compares the ability of the parent PAn.(+)-HCSA and the ringsubstituted POMA.(+)-HCSA emeraldine salts to react with and discriminate between enantiomeric forms of selected amino acids. POMA.(EB) films could be made by the alkaline de-doping (1.0 M NH₄OH) of electrochemically deposited, optically active POMA.(+)-HCSA films. However, CD spectral study showed that these POMA (EB) films were largely racemic (Δε/ε ca. 0.02 %). UV-Visible and CD spectral studies showed that these films underwent slow (70 hr) partial doping when placed in 0.10 M aqueous solutions of a range of amino acids.

Upon removal from the amino acid solutions and standing for 3 weeks, the films developed very intense CD spectra (Δε/ε ca. 1.0 %). Mirror imaged CD spectra were observed for POMA (EB) films treated with enantiomeric amino acids such as D- and L-phenylalanine. This indicated strong chiral induction by the enantiomeric amino acids on the POMA (EB) chains caused by specific interactions such as H-bonding. These partially doped POMA (EB) films underwent a remarkable approximate inversion in their CD spectra after standing for a further 7 weeks. This was believed to arise from structural changes caused by the gradual loss of water molecules originally H-bonded to the emeraldine base chains.

Chapters 6 and 7 explore two potential new routes to chiral polyanilines. In Chapter 6, the ability to electropolymerize aniline in acid (HA = HC1 or (+)-HCSA) in the presence of aqueous hydroxypropyl-β-cyclodextrin (HP-β-CD) was investigated, as well as the influence on the conformation/structure of the emeraldine salt (PAn.HA) products. It was hypothesized that inclusion of the aniline monomer in the HPβP-CD cavity may hinder ortho- or meta-couplin1, which leads to undesirable branching in polyaniline products. Surprisingly, that the polymerization of aniline was found to be completely halted by the presence of HP-β-CD at concentrations > 0.04 M.

The UV-Visible-NIR and CD spectral studies shown that the conformation of PAn.HCl and PAn.(+)-HCSA films grown with [HP-β-CD] < 0.04 M were similar to those grown in the absence of the cyclodextrin. It was also found that the presence of up to 0.08 M a-cyclodextrin did not hinder the potentiodynamic deposition of PAn.HCl films. This is probably due to the smaller cavity opening for α-cyclodextrin compared to HP-β-CD, hindering the formation of a hostguest complex with aniline.

Finally, in Chapter 7 the synthesis of composites between the chiral biopolymer chitosan and PAn or PMAS was studied as a possible new route to chiral polyanilines. The chemical or electrochemical polymerization of aqueous aniline and 2-methoxyaniline-5-sulfonic acid (MAS) monomer in the presence of ca. 1% (w/v) chitosan and ca. 1 % acetic acid (v/v) successfully gave water soluble or dispersible PAn.CHI and PMAS.CHI composites. The observation of CD spectral bands for the chemically generated PMAS.CHI solution indicated that the chiral chitosan in the composite had induced chirality into the polymer chains of PMAS. This may result from the PMAS chains partially following the onehanded helical structure of the chitosan, due to acid-base interactions between PMAS sulfonate groups and the amine groups on the chitosan. The PAn.CHI and PMAS.CHI composites showed both similarities and differences in their pH and redox switching properties compared to those observed previously for unsubstituted parent polyaniline and PMAS(NH₄ ⁺), respectively.