Sangrama K. Sahoo

2D-NMR studies of a model for Krytox® fluoropolymers

Multiple two‐dimensional nuclear magnetic resonance (2D‐NMR) techniques have been used to study the structures of Krytox® perfluoro(polyalkyl ether) and its mechanism of polymerization. Model compound K4, containing four Krytox® fluoropolymer repeat units, was analyzed to interpret the multiplet patterns in the NMR spectra from the polymer model. 19F {13C}‐Heteronuclear single‐quantum correlation experiments, performed with delays optimized for 1JCF and 2JCF, provided spectra that permitted identification of resonances from individual monomer units. Selective, 19F‐19F COSY 2D‐NMR experiments were performed with different excitation regions; these experiments were combined with selective inversion pulses to remove 19F‐19F J couplings in the f1 dimension. The resulting COSY spectra were greatly simplified compared with standard 19F‐19F COSY spectra, which are too complicated to interpret. They give information regarding the attachments of monomer units and also provide insights into the nature of the stereoisomers that might be present in the polymer. Both infrared and NMR spectra show peaks identifying chain end structures. With the help of these studies, resonances can be assigned, and the average number of repeat units in the polymer chain can be calculated based on the assignments obtained. Copyright

VARIATION IN THE STRUCTURE OF CONDUCTING POLYANILINE WITH AND WITHOUT THE PRESENCE OF TEMPLATE DURING ENZYMATIC POLYMERIZATION: A SOLID-STATE NMR STUDY

ABSTRACT Polyanilines (PANI) were synthesized from peroxidase-catalyzed polymerization of aniline with and without the presence of a template in the aqueous buffer solution of pH 4.3. The template used was poly(vinylphosphonic acid) (PVP). Solid-state 13C and 15N CP/MAS NMR techniques were applied to characterize the structure of polyaniline in its self-doped conducting, dedoped base, and redoped conducting form of PANI. The charge distribution along the polymer backbone in the protonated form of the polyaniline is also discussed. The structural features exhibited by a template assisted enzymatically synthesized PANI is the result of linear 1,4-coupling of aniline moiety while the PANI synthesized in the absence of template contains both 1,2- and 1,4-coupled products. Solid-state NMR studies on the enzymatically-synthesized polyaniline with the aid of template (PANI-PVP) confirmed the formation of a linear polymer chain containing alternate benzenoid-quinoid repeat units. The dedoped form is in the emeraldine base state of the polyaniline. The structural features in the spectra of the PANI synthesized without a template are quite different. Solid-state NMR data suggest that the PANI synthesized without aid of the template contains structures having C-C and C-N-C coupled products indicating the presence of branching in the polymer chain.

ENZYMATICALLY SYNTHESIZED POLYANILINE IN THE PRESENCE OF A TEMPLATE POLY(VINYLPHOSPHONIC ACID): A SOLID STATE NMR STUDY

Template guided enzymatic synthesis of conducting polyaniline (PANI) is a one-step reaction and more importantly, it is an environmentally friendly process. Understanding of the reaction and coupling mechanism at the molecular level is of paramount significance to improve its processability and conductivity. Solid-state NMR techniques are useful to investigate molecular structures of enzymatically synthesized polyaniline (PANI). The PANI sample in three different forms i.e., (a) as-synthesized, self-doped conducting form; (b) dedoped, base form and; (c) redoped, conducting form, are investigated by solid-state 13C and 15N CP/MAS NMR techniques. Solid-state NMR data analysis shows that the structural features of enzymatically synthesized PANI are similar to that of chemically synthesized PANI. The solid-state 13C CP/MAS NMR spectrum of the base form of PANI confirmed that benzenoid-quinoid repeating units are present in the backbone of the PANI polymer chain. The poly(vinylphosphonic acid) (PVP) template provides charge compensation during the chain growth of linear polyaniline. After the completion of template-guided synthesis of PANI, it is now possible that the PVP template can be completely removed from the complex by dedoping with aqueous NH4OH. The detached PANI from the PANI-PVP complex can then be redoped to conducting form without the presence of the template. The conductivity of the PANI and PANI-PVP complex are of the same order of magnitude. The solid-state 15N CP/MAS NMR chemical shifts are sensitive to charge distribution on the nitrogens in the backbone. The solid-state 15N CP/MAS NMR spectrum of the base form of the enzymatically derived PANI sample showed the clear signature for benzenoid-quinoid repeating units in the polymer backbone. †Deceased.