1,3-Dioxolan-4-one (DOX) chemistry was exploited for the synthesis of poly(lactic acid) (PLA)-based copolymers. A new DOX monomer, bearing the naturally occurring carvacrol (CarvDOX) as a pendant, was synthesized and used as a model for the optimization of DOX-L-lactide, solvent-free, copolymerization reactions. A screening of conditions was carried out to find out the best catalytic system for both comonomers at the same time, as well as to maximize the growth of the molecular weight. The optimized copolymerization conditions were then applied to a second DOX derivative (CardDOX), bearing the naturally occurring 3-pentacylphenol (cardanol), as an appendage. Various copolymers between L-lactide and different amounts of CarvDOX or CardDOX were prepared and characterized through NMR and wide-angle X-ray scattering (WAXS), also assessing their thermal properties via differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). To exploit the potential use of such copolymers as functional additives in poly(lactic acid) (PLA), they were blended via solvent mixing with commercial PLA and the melt viscosity properties of the resulting blends were investigated through frequency sweep experiments.