NMR studies on the reaction mechanism of enzymatic polymerization and the conformations of poly(alpha-L-glutamic acid) polymers

Adopting NMR as the major approach, this dissertation focuses on: (1) the coupling mechanism of the enzymatic polymerization of 4-propylphenol at 273 K; (2) The effects of solvents on the conformation of poly(alpha-L-glutamic acid) (PLGA) and azobenzene-modified PLGA (AZOPLGA) in thin films; (3) The 1H and 13C spin-lattice relaxation study on PLGA and AZOPLGA in trifluoroacetic acid (TFA) and DMF.; Using 1D and 2D NMR techniques, the structure of the predominantly produced initial products in enzymatic polymerization of 4-propylphenol at 273 K were characterized completely. In addition, the possible mechanisms to produce these products have been inferred. A new reaction pathway which provides the possibility to synthesize ladder or semi-ladder type polyphenols was observed. The stereochemistry was found to play a crucial role in this reaction. Further in-situ variable temperature experiment was employed to examine the effect of temperature on the formation of Pummerer's type ketone, a major reaction pathway which inhibits further polymerization. The variable temperature experiment confirms that the formation of the Pummerer's type ketone is strongly temperature dependent, lowering the reaction temperature (273 K) alone can efficiently prevent its formation.; The conformations of PLGA cast thin films were investigated by CD, solid state NMR and FTIR spectroscopies. All spectral data suggest that casting solvents play an important role in determining the conformation of the PLGA backbones in cast films. In azobenzene-modified PLGA, both solvents and the azo content contribute to the backbone conformation in polymer films. However the effect of the azo content plays a dominant role over that of solvents. Interestingly, the side chains of AZOPLGA56 (AZOPLGA with 56% of functionalization) in films cast from TFA were demonstrated to have some degree of local orderness even though the polymer backbone is in random coil conformation.; Finally the spin-lattice relaxation experiments on PLGA, azo chromophore, and AZOPLGA were carried out in TFA and DMF solution at varied temperature. The relaxation activation energies for nuclei such as 1H and 13C in different functional groups were evaluated. The corresponding nuclei in the same functional groups show less activation energy in TFA solution than that in DMF solution suggesting all investigated molecules have a higher mobility in TFA solutions. Moreover the fundamental experiment demonstrates that introduction of azo chromophore in PLGA side chain will not only enhance the relaxation rate of the polymer side chains but will improve the backbone relaxation as well.