Polydiacetylene yellow solutions of the derivatives 2-, 3- and 4BCMU and 9PA in chloroform were prepared with a range of different molecular weights. The measured wavelength of the optical absorption spectrum peak maximum lambda(max) showed a continuous increase with increasing chain length up to lengths of nearly 5000 repeat units. A theory is proposed that this is due to the excluded volume effect. The persistence lengths of the polymer chains in the solutions were calculated from lambda(max), using small angle neutron scattering data from the literature. The persistance length is shown to obey a direct power-law relationship with physical chain length, which is in good agreement with the theory. Numerical parameters in equations derived from these results, using the theory, are in good agreement with those calculated from independent light scattering and intrinsic viscosity measurements in the literature for other polydiacetylenes and polymers. Furthermore, it is shown that realistic values for the expansion coefficient alpha(n) can be calculated from the derived equations.