Alkyl substituent effects on the conductivity of polyaniline

The oxidative polymerization of aniline hydrochloride derivatives in water at low temperature is studied without lithium chloride. The resulting polymers have high molecular weight but the conductivity of the acid-doped films is strongly dependent on the alkyl-substituted chain at the 2-positions. The root cause of the alkyl-substitution effects is thoroughly investigated using density functional theory (DFT) methods (B3LYP using 6-1G(d,p) and 6-311++G(2d,2p) basis sets). Internal structural changes observed on substitution appear to be more significant than a variety of electronic parameters measured using the natural bond orbital (NBO) method. Interplanar angles steadily increase on substitution, whereas ring orbital properties and the amount of ring delocalization remain fairly constant. An investigation into the extent to which lone pair-sigma-orbital overlap is affected by substitution indicates that increasing the steric bulk of the substituent reduces the ability of the lone pair to delocalize into the ring orbitals. However, the amount of overlap between the two is not adversely affected until the dihedral between them is >30degrees, a situation that only occurs in i-propyl and s-butyl substitution. This finding is completely reflected in the experimental conductivity measurements.