A novel family of lignin-based thermosets that rely on acetal linkages and do not release hazardous compounds during degradation is proposed as future circular design materials. Poly(ethylene glycol) diisopropenyl ether (PDIP) was utilized as a soft segment to form the acetal linkage with the lignin hydroxyl groups via the addition reaction to the isopropenyl double bond. The use of PDIP instead of previously utilized poly(ethylene glycol) divinyl ether (PDV) prevents the release of harmful acetaldehyde during the acidic hydrolysis of the materials. In addition to the lower toxicity of the degradation products, thermosets with PDIP are more resistant to acidic hydrolysis. Characterization of the thermosets by thermal analysis revealed that the merits of this new lignin-PDIP thermoset extended to increased thermal stability, with T_d5%, T_d30%, and T_s values of 243–253, 349–363, and 152–155 °C, respectively. Furthermore, the developed materials demonstrated intrinsically lower flammability and reduced heat release potential, paving the way for safer materials with a reduced need for potentially harmful flame retardants. The ease of synthesis and high yields achieved encourage further work toward circular and safe materials solutions based on lignin and PDIP.