Application Of Coordination Polymer-Based Catalysts For The Ring-Opening Polymerization Of ε-Caprolactone

Poly(ε-caprolactone)(PCL)is widely used in biomedical and envriomentally friendly materials due to its good biodegradability and biocompatibility.Metal-based catalysts for ring-opening polymerization(ROP)of CL to synthesis PCL is the most effective method.Organotin and organoaluminum compounds are common active catalysts for the preparation of PCL via ring-opening polymerization in industry.However,there exist some disadvantages such as some undesirable side reactions,broad molecular weight distribution(PDI)of PCL,lower catalyst stability and catalyst residue-related cytotoxicity,which limit its application in the biomedical field.Therefore,the design and synthesis of new low-toxic,high efficient and stable catalysts for ROP of CL has become an attractive research topic.Coordination polymers,a relatively new class of organic-inorganic hybrid materials featuring structural diversity,coordinatively unstatured metal centers and functionalized organic ligands,have demonstrated their potential for catalyzing a variety of organic transformations.Nevertheless,the application of coordination polymer-based catalysts in ROP reactions has rarely been explored yet.In this thesis,various metal ions involving main group metal,transition metal and rare earth metal in combination with nitrogen-containing heterocyclic compounds and carboxylic acids were used to design and construct several series of coordination polymers.Moreover,the influence of metal ions,organic ligands and guest molecules on the catalytic activity toward ROP of CL was investigated in detail.The main results are described as following:Reaction of main group metal salts with N/O-bridging ligands afforded four new one-dimensional(1-D),two-dimensional(2-D)and three-dimesnional(3-D)coordination polymers[Li(H2IDC)(H2O)]n(1),[Na2(H2IDC)2(H3IDC)2(H2O)4]n(2),[K(H2IDC)(H2O)]n(3)and ｛[Ca2L12(H2O)6]·H2O｝n(4),respectively.Among them,the coordination geometry of metal ion,coordination mode of ligands and net topology are also different due to the difference in metal ion radii.All these complexes could effectively catalyze the ROP of CL under solvent-free conditions,and the difference in catalytic acitivity is ascribed to the Lewis acidity of metal ions.In addition,by comparison of the catalytic performance over the as-synthesized and desolvated samples,it was found that the coordinated water molecules play a critical role in the catalytic process.Furthermore,on the basis of the MALDI-TOF MS analysis on the terminal group of the product,the catalytic mechanism involving the synergy of Lewis acid and Bronsted acid has been proposed.Reaction of transition metal salts with the flexible ligand Fbtx and aromatic dicarboxylic acids produced three new coordination polymers,namely,{[Co(NDC)(Fbtx)(H2O)2]·0.5H2NDC}n(5),{[Cu(NDC)(Fbtx)(H2O)]·1.5H2O}n(6)and[Zn5(NDC)4(μ3-OH)2(Fbtx)4]n(7),as well as four isostructural coordination polymers of M-MOF-74(M=Co,Ni,Cu,Zn).Complex 5 containing coordinatively saturated Co(Ⅱ)ions shows a 2-D layered structure,while the 2-D network of 6 contains coordinatively unsaturated Cu(Ⅱ)ions,whereas complex 7 is a 3-D coordination framework based on pentanuclear secondary building units(SBUs).Besides,the 3-D M-MOF-74 comprises the 1-D metal carboxylate chains as SBUs.Catalytic experiments revealed that all these metal-organic frameworks can efficiently catalyze the ROP of CL under solvent-free conditions.Among them,the Cu(Ⅱ)-based coordination polymer shows better catalytic activity and is a promising catalyst for ROP of CL.Solvothermal reaction of zinc salt with sodium formate yielded a novel coordination polymer[Zn3(HCOO)6]n(8).Complex 8 shows good chemical stability and thermal stability.By doping main group metals of ⅡA and ⅢA into 8,two series of metal-doped 8(8-M)were facilitated.Compared with 8-catalyzed ROP of CL,the bimetallic coordination polymers exhibit enhanced reactivity,giving high monomer conversion,high molecular weight of PCL.The doped-metal species,doped amount and the defect effect of metal ions have a profound influence on the efficiency of the ROP reaction.Among them,the In-doped 8(8-In)is a desired catalyst for ROP of CL.Hydrothermal reactions of polyoxometalate salts,transition metal ions and the flexbile ligand Fbtx led to the formation of four polyoxometalate-based coordination polymers,[Cu2(SiW12O40)(Fbtx)3(H2O)4]n(9),{[Cu2(SiW12O40)(Fbtx)2(H2O)2.75]·4.5H2O}n(10),[Co(Fbtx)(Mo3O10)(H2O)]n(11)and {[Co(Fbtx)(δ-Mo8O26)0.5]·TB A)} n(12),respectively.Both complexes 9 and 10 are 2-D[SiW12O40]4--based layered networks,while complex 11 displays a 3-D rod-shaped framework based on[Mo3O10]2-anions,whereas complex 12 features a novel 3-D anionic framework based on[δ-Mo8O26]2-anions.Of them,complexes 11 and 12 exhibit extremely catalytic activity in the ROP of CL,in which near quantitative conversion of monomer,high molecular weight of PCL,narrow PDI of PCL and high TOF value of 3800 h-1 were observed.In addition,the catalytic mechanism has also been investigated by the MALDI-TOF MS and 1H NMR technologies.Solvothermal reaction of ytterbium(Ⅲ)salt with 4,4’-(carbonylbis(azadiyl))dibenzoic acid afforded a urea-functionalized lanthanide coordination polymer[Yb2(L2)2(μ3-O)(DMF)3]n(13).Complex 13 is a 2-D double-layered coordination structure based on tetranuclear ytterbium(Ⅲ)oxo-clusters,which proved also effective for the ROP of CL in bulk.It was found that the active site is located at the urea group within the main backbone of 13 rather than the tetranuclear ytterbium(Ⅲ)oxo-cluster,where the urea group acting as a hydrogen-bonding donor catalyst could effectively avoid the the self-quenching effect of the urea molecules.In addition,by comparison of the catalytic acitivity of UiO-66 with FUiO-66,the results also highlighted the critical importance of ligand modification on the catalytic performance.