Centaurea solstitialis and Silybum marianum are considered to be weeds in agricultural fields and forests. These annual herbaceous plants can be converted into value-added thermoplastics by means of benzylation. Products can be utilized in different applications where plastics are used. Substitution reaction was carried out with benzyl chloride under different alkaline conditions, 15, 25 and 35 %, respectively. Alkali concentration affects the substitution reaction. The chemical structure of modified materials was remarkably changed. In order to detect changes, crystallinity index of the materials and their thermal properties were analyzed. In FTIR spectra, the peaks appeared at 695 and 736 cm(-1), indicating the aromatic C-C angular deformation. Multiple peaks appeared at 3050 cm(-1) which were indications of benzylation of the materials. A peak increase was observed at 1596 cm(-1) due to the aromatic substitution. Hydroxyl groups were replaced with benzyl groups which increased peak intensity. After modification, hydrogen bond energy changed and crystallinity of both materials was reduced. This substitution process on functional groups changed the decomposition temperature of the modified materials. It reduced to the decomposition temperature of the materials to a temperature range between 240 and 280 degrees C. Thermogravimetric analysis revealed that modified products were characterized by poorer thermal stability compared to raw materials.