Abstract
Three new zinc complexes with monoamine–guanidine hybridligands have been prepared, characterized by X-ray crystallography and NMR spectroscopy, and tested in the solvent-free ring-opening polymerization of rac-lactide. Initially the ligands were synthesized from camphoric acid to obtain TMGca and DMEGca and then reacted with zinc(II) halides to form zinc complexes. All complexes have a distorted tetrahedral coordination. They were utilized as catalysts in the solvent-free polymerization of technical rac-lactide at 150 °C. Colorless polylactide (PLA) can be produced and after 2 h conversion up to 60% was reached. Furthermore, one zinc chlorido complex was tested with different qualities of lactide (technical and recrystallized) and with/without the addition of benzyl alcohol as a co-initiator. The kinetics were monitored by in situ FT-IR or 1H NMR spectroscopy. All kinetic measurements show first-order behavior with respect to lactide. The influence of the chiral complexes on the stereocontrol of PLA was examined. Moreover, with MALDI-ToF measurements the end-group of the obtained polymer was determined. DFT and NBO calculations give further insight into the coordination properties. All in all, these systems are robust against impurities and water in the lactide monomer and show great catalytic activity in the ROP of lactide.
Highlights
Due to increasing environmental awareness, plastics based on petrochemical resources have to be replaced by plastics based on renewable raw materials
PLA is produced by the ring-opening polymerization (ROP) of lactide, the dimer of lactic acid, using a metal-based catalyst system
2-(3-amino-2,2,3-trimethylcyclopentyl)- 1,1,3,3-tetramethylguanidine (TMGca) [75] based on camphoric acid were prepared by the reaction of the corresponding Vilsmeier salt N,N0 -dimethylethylenechl or N,N,N0,N0 -tetramethyl-chloroformamidiniumchloride with33ofofthe diamine [76,77]
Summary
Due to increasing environmental awareness, plastics based on petrochemical resources have to be replaced by plastics based on renewable raw materials. Due to its resemblance to petrochemical polymers, PLA is utilized in the medical field, packaging, and microelectronics [5,6,7,8,9]. It is a biodegradable and biocompatible aliphatic polyester that can be produced from renewable raw materials such as corn and sugar beets [3,4,6,7,8,9,10,11,12,13,14,15,16]. Most catalysts produce atactic or heterotactic PLA but with isotactic PLA
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