Synthesis,characterization Of Bis(phenolate)Potassium Complexes And Their Application In The Ring-Opening Polymerization Of Lactides

Polylactide(PLA),as one significant biodegradable and biocompatible material,have been used in a wide variety of appliactions including the biomedicine and pharmaceutics fields.Ringopening polymerization of lactide catalyzed by metal complexes is the most efficient method for the preparation of PLAs with controlled molecular weight and narrow molecular weight.A number of metal complexes,such as aluminum,trivalent lanthanide,magnesium,calcium,lithium,sodium and potassium complexes,have attracted wide attention in recent years.Therefore,the metal residues are difficult to be removed completely from the resultant polymers,and will limit the application of PLA.Potassium and magnesium are abundant elements in human body and non-toxic,readily obtained.So potassium compounds supported by bisphenolate ligands,nontoxic/low toxic magnesium and lanthanide are excellent candidates to investigate the ring-opening polymerization of lactides.We report herein the synthesis,characterization,and catalytic properties of mononuclear potassium complex,multinuclear potassium complex,three potassium-magnesium bimetallic complexes and five potassium-lanthanide bimetallic complexes.Among these complexes seven complexes have been characterized by X-ray single crystal diffraction,and the effects of different metallic ions on the composition and reactivity for the ROP of lactides also have been investigated.The main contents are listed as below:1.Reaction of bisphenol ligand 2,2?-methylenebis(4,6-di(1-methyl-1-phenylethyl)phenol)(LH2)with one equiv or two equiv K[N(SiMe3)2],produced monometallic potassium complex [(LH)K(THF)](1)and multinuclear potassium complex [K12L6](2).Complexes 1 and 2 were characterized by 1H and 13 C NMR analysis,elemental analysis,and X-ray single crystal diffractions.The reactivities of complexes 1 and 2 towards the ring-opening polymerization of lactides to form polylactides(PLAs)were explored.Experimental results showed that complex 1 was an efficient catalyst for ring-opening polymerization of L-lactide(L-LA)and the conversion could be reached 99% within 2 h.Multinuclear potassium complex 2 catalyzed the ring-opening polymerization of lactides,yielding polymers with controlled molecular weight consistent with theoretical molecular weight.It was presumed that multinuclear potassium complex 2 had similar reactivity as monometallic potassium complex 1.Solvent effects: toluene > CH2Cl2 > THF,indicating the coordinating competition between THF and the monomer to the metal center.2.Reaction of [(L-H)K(THF)](1)with 0.5 equiv.or equivalent MgnBu2 produced potassiummagnesium bimetallic complex [(LK(THF)2)2Mg](3)and [(LK(THF))2Mg2nBu2](4).The complex 4 reacted with BnOH to produce bimetallic complex [(LK(THF))2Mg2(?-OBn)2](5).Complexes 3-5 have been characterized by 1H NMR and 13 C NMR,element analysis,and X-ray single-crystal diffractions.Experimental results showed that the potassium-magnesium bimetallic complex 3 had higher activity than complex 1.This may be contributed to the cooperative effects between metal ions.The ROP experimental results showed that a linear relationship between the number-average molecular(Mn)and the monomer-to-initiator ratio([M]0/[I]0)had been observed using compound 5 as the initiator.3.Reaction of [(L-H)K(THF)](1)with RE[N(SiMe3)2]3 with 3:1 molar ratio in THF produced potassium-lanthanide bimetallic complexes RE[K(L)]3(RE =Y(6),La(7),Nd(8),Sm(9),Gd(10)).Complexes Y[K(L)]3 and La[K(L)]3 were characterized by 1H and 13 C NMR analysis,elemental analysis,Y[K(L)]3 and Sm[(L)K(THF)]3 were also characterized by X-ray single crystal diffraction.Experimental results showed that these potassium-lanthanide bimetallic complexes 6-10 efficiently initiated the ring-opening polymerization of L-lactide in the presence of benzyl alcohol.The observed increasing order,La > Nd > Sm ? Gd > Y,was consistent with the order of ionic radii.It was also found that potassium-lanthanide bimetallic complexes showed higher activity than mononuclear potassium complex 1.The stereoselectivity of ROP of rac-lactide catalyzed by complex 6 has been demonstrated to a slightly heterotactic preference in toluene.