Doctor of Philosophy
Department of Chemistry
Ashraf, Syed Aziz, Synthesis and characterization of chiral monomers and polymers, Doctor of Philosophy thesis, Department of Chemistry, University of Wollongong, 1997. http://ro.uow.edu.au/theses/1187
This thesis describes the synthesis and characterization of optically active monomers and their subsequent polymerization to optically active polymers (polyelectrolytes) and in one case a chiral hydrogel.
The novel chiral monomers (IS, 2R, 4S)- and (IS, 2S, 4S)-α-[2-amino- 7,7-dimethylbicyclo[2,2,l]-l-heptyl] methylsulfonic acid 4a and 4b were synthesised as two diastereomers (Chapter 2) via the reductive amination of (IS)-(+)-10-camphorsulfonic acid (HCSA). Reaction of these diastereomers with acroyl chloride yielded the corresponding optically active acrylamide monomers, 5a and 5b. These novel monomers incorporate a chiral auxiliary that also has an ionisable sulfonic acid group. Ultimately, the acrylamides were converted to novel, polyacrylamides la and lb, via free radical polymerization. These polyacrylamides possessed the useful features of both optical activity and polyelectrolyte behaviour. Hydrolysis of the polymer lb to give polyacrylic acid confirmed (via 1H NMR spectroscopy) that the polymer was ca. 72 % isotactic.The monomer 4a was also used to form crosslinked or interpenetrated networks that have chiral hydrogel properties and exceptional water intake (EWC=99.96%) properties.
The electrochemical syntheses of new polyaniline and polypyrrole salts were subsequently carried out in which the above mentioned monomers and polyelectrolyte were incorporated as a counterion (Chapter 2). Polypyrrole salts were successfully grown by both potentiostatic and galvanostatic methods and characterised by cyclic voltammetry. However, attempts to synthesise polyaniline salts incorporating the above monomers or polyelectrolyte were unsuccessful. This was probably due to the unsuitable pH conditions, since in order to grow polyaniline salts electrochemically, the pH of the reaction solution should normally be in the range 0-2.
Chemical doping of neutral emeraldine base (EB) with the novel monomers and polymers to generate chiral polyaniline salts was also examined (Chapter 3). A green film of optically active polyaniline salt was obtained by doping with the chiral acrylamide monomer 5b, as confirmed by uv-visible and C D spectroscopy. It was proposed that the polyaniline chain in this chiral polymer adopts a helical conformation maintained in one hand via H-bonding to the acrylamide (CO) group and ionic bonding to the SO3- group. In contrast, the monomer 4b did not dope EB, presumably because of the presence of a zwitterion structure for 4b, therefore restricting salt formation. Salt formation also did not occur upon the addition of polymer 1b to EB.
A detailed study was also carried out of the influence of various conditions on the chiroptical properties and conformations of the polyaniline salt generated by doping EB with (+)-HCSA under a variety of conditions (Chapter 3). The optical activity and conformational changes after doping were monitored in various solvents such as CHCI3, DMF, NMP , DMSO and THF by uv-visible and CD spectroscopy. Other conditions examined included the presence or absence of an inert argon atmosphere, the mode of mixing of EB and dopant (+)-HCSA, the concentration of EB (+)-HCSA, and the molecular weight of the EB. In each case, samples were examined over an extended period of time. CD and uv-visible spectra indicated that the PAn. (+)-HCSA salts generated in CHCl3 and NMP solvents in air had different initial conformations for their polyaniline chains, believed to be "compact coil", and "expanded coil" respectively. Interestingly, the CD spectrum observed in CHCI3 changed over 48 hr to that observed in NMP , indicating a change in polymer conformation. In contrast, analogous doping reactions in the presence of argon gave circular dichroism bands (at ca. 340, 405, and 460 nm) that were insensitive to the nature of the solvent. It is also significant that no optical activity was observed when small oligomers of EB (tetramer and sixteen-omer) were doped with (+)-HCSA in CHCI3 solvent, indicating the importance of the molecular weight of the EB substrate.
Spin-cast films of EB doped with (+)-HCSA in m-cresol, CHCI3 and 1,1,1,3,3,3-hexafluoropropanol solvents were also studied by CD and uvv-visible spectroscopy (Chapter 3). The CD spectrum of a spin-coated film of PAn. (+)-HCSA cast from CHCI3 onto quartz showed a localised polaron band at 767 nm. In contrast, a film of PAn. (+)-HCSA cast onto glass or quartz from m-cresol solvent showed that the localized polaron band had almost disappeared and was replaced by a free carrier tail in the near-infrared region. This suggests a change in conformation for the polymer chain from a "tight coil" to an "extended coil" on changing solvent from CHCI3 to m- cresol. Similar behaviour was found for a PAn. (+)-HCSA film spun cast from 1,1,1,3,3,3-hexaflouropropanol, suggesting a similar change in conformation for the polyaniline backbone.
A n alternative approach to chiral polyelectrolytes was explored in Chapter 4, namely the synthesis of vinyl monomers containing a 2- phenyl- or 2-tert-butyloxazolidin-5-one unit and their polymerization to the related polyvinyl species. Although polymerization was readily achieved via radical reaction (AIBN) in a quantitative yield, characterization was limited to solid state 13CMR and ir and uv-visible spectra, due to the insolubility of the polymers in a wide range of solvents. This insolubility also rendered the polymers inert to hydrolysis, even under severe conditions. Finally the synthesis and characterization of a related vinyl monomer in which the vinyl unit bears a thiophene substituent was also achieved.