The third-order nonlinear optical susceptibility of regio-regular poly(3-dodecylthiophenes) is observed to be enhanced by a factor of 10 as compared to that of regio-irregular poly(3-dodecylthiophenes) when pumped at 1907 nm. The increase in the effective conjugation length from regio-irregular to regio-regular poly(3-dodecylthiophene) is estimated from linear absorption data and the reported absolute value measured at 1907 nm represents the nonlinear susceptibility of the most highly conjugated regio-regular polythiophene firm measured to date.