Understanding structure–activity relationships: iron(ii) complexes of “Legacy Guanidines” as catalysts for the synthesis of polylactide†

In this work, eight novel iron(II) chloride complexes of well-known bisguanidine and N,N hybrid guanidine ligands are presented. Their activity in the synthesis of polylactide via a ring-opening polymerization was investigated under industrially relevant conditions with low catalyst loadings in the lactide melt. The conversion was monitored by in situ Raman spectroscopy to evaluate the reaction kinetics. The catalysts were investigated regarding their polymerization activity as well as their ability to maintain their polymerization activity over time. The most promising catalyst [Fe(TMGepy)Cl₂] (C6) polymerizes L-lactide at monomer-to-initiator ratios of 1000 : 1 and higher with a rate constant of propagation similar to the until now most active robust iron catalysts. Experiments on the influence of a co-initiator were carried out. Additionally, the experimental observations were further underlined with theoretical calculations explaining the stability and activity of the catalysts. Iron guanidines with rather simple ligands demonstrate a great potential for large-scale application in the industrial process. Finally, initial tests on the application of the compounds in the methanolysis of polylactide were conducted.