Theoretical Studies On ROP Of Lactide Catalyzed By Rare Earth And Alkaline Earth Metal Complexes

Polylactide(PLA)is a biodegradable and biocompatible aliphatic polyester derived from biorenewable feedstock,It is of great interest for various potential applications.Because of the polymer microstructure significantly affectes its properties and determines its uses,developing highly active and high stereoselectivity catalyst has attracted wide attentions of researchers.In this context,rare-earth and alkaline-earth metalcomplexes often show high activity toward the ring-opening polymerization(ROP)of lactide and significant advances have been made.But the knowledge of mechanism on ROP of lactide catalyzed by their complexes has remained limited.This is mainly due to the difficulty in isolating and detecting intermediates and transition states experimentally and in elucidating the origin of different performance of various metal compelxes.Herein,for better understanding of the ROP mechanism of lactide catalyzed by rare earth and alkaline earth metal complexes,DFT studies have been carried out.The main results are as follows:1.In the catalytic system of bifunctional yttrium NHC catalyst,it has been found that the carbonyl of lactide is attacked by N(SiMe3)2 group which connected to the rare metal yttrium rather than NHC specie in chain initiation stage.The NHC moiety could accelerate chain initiation rate by promoting cleavage of Y-N(SiMe3)2 as a result of the NHC moiety serves a good electron donor and could decrease the Lewis acidity of the metal.The high selectivity of the system was investigated in the process of chain growth.The studies of the second monomer insertion in different assembly modes also demonstrate that the steric interactions between last enchained unit of monomer and the incoming monomer together with clash between the incoming monomer and ligand framework are the primary factors determining stereoselectivity of insertion via structural analysis.2.In the catalytic system of phosphasalen rare earth metal complexes,the yttrium complex with smaller metal atomic radius polymerize rac-lactide with iso-selectivity,but the lanthanum complex with larger matal atomic radius exhibited heteroselectivity.And,the kinetic analysis indicated that the yttrium complex showed lower catalytic activity than that of lanthanum complex,in good agreement with experimental results.According to the frontier molecular orbital analysis of catalysts,The lower chemical hardness of lanthanum complex than that of yttrium complex resulted in its higher catalytic activity,but yttrium complex is more stable.And the stabilities of yttrium complex has been proved through the structure analysis.In the polymerization process,the change in catalyst ligand structure could affact the activity and stereoselectivity.3.For the alkaline-earth metal magnesium complex system,in the presence of alcohol,the Mg-Cl bond could dissociate to give chlorine anion at the chain initiation stage.The ROP of lactide was therefore catalyzed by the resulting magnesium cationic species,where two molecules of alcohol were involved as an activator.The newly generated product as an alcohol continues to participate in the activation of incoming monomer.In the absence of alcohol as an initiator,the chain initiation process is relatively difficult to occur,studies on the sucssecive monomer insertions showed a decrease in the relative energies of the product and transition state,as anticipated for an addition polymerization pathway after the chain initiation process is completed.It has been found that the ROP of the fourth molecules insert is exergonic and the corresponding energy barrier is very low.This suggests that the chain growth is a feasible process.