Robust Guanidine Metal Catalysts for the Ring‐Opening Polymerization of Lactide under Industrially Relevant Conditions

The increasing awareness of sustainability has led to enormous growth of the demand for bio‐based and biodegradable polymers such as poly(lactide) (PLA). In industry, polymerization of lactide is currently carried out using tin catalysts (e. g., tin(II) ethyl hexanoate, Sn(Oct)2). Since the catalyst remains in the polymer, it can accumulate in the soil or in the human body after degradation and cause damage due to its toxicity. Therefore, a search for a suitable substitute for this catalyst has been going on for decades. Guanidine metal complexes prove to be excellent catalysts in the polymerization of lactide. They are not only convincing because of their activity and the synthesis of high molar mass polymers, but also show a high robustness against high temperatures, oxidation as well as residual protic impurities in the monomer. Herein, key zinc and iron guanidine complexes are discussed with respect to their apparent rate constant (kapp) and rate constant of propagation (kp), produced molar masses and the mechanism involved. Faster and better: This Minireview presents the latest and most rapid robust catalysts for the ring‐opening polymerization of lactide under industrial conditions. The focus is on the search for a substitute for the currently most commonly used and toxic catalyst Sn(Oct)2 in industry. Numerous ROP‐active guanidine metal complexes are presented, and their activity is compared by analyzing the rate constants of the polymerization.