Dynamic Light Scattering does only look at the fluctuations in the scattered light intensity. To determine to molar mass using light scattering you need at least a way to measure the absolute scattered intensity on a calibrated scale and the refractive index increment of your polymer in THF. See http://en.wikipedia.org/wiki/Molecular_mass#Static_light_scattering

I would start with a DLS analysis to see if your polymer does not give a broad distribution of relaxation times. If you have a (very) polydisperse sample your efforts may be wasted.

Agreed with Maxim. Actually, DLS is not an appropriate method for lignin MW determination. A lot of parameters you have to take into account, some of them are difficult to obtain. Besides, there is one more problem - lignin fluorescence which strongly interferes the scattered light measurements. This effect is significant even if NIR laser is used instead of VIS. 

Thank you guys