Organocatalytic Ring-Opening(Co) Polymerization Of Cyclic Esters

As one of the most important biodegradable and biocompatible polymeric materials,polyhydroxyalkanoates have been widely used as drug-controlled delivery systems,scaffolds for tissue engineering and packaging films,due to their outstanding properties.One of the most convenient and promising synthetic routes toward highly-controlled polyhydroxyalkanoates is ring-opening polymerization(ROP)of the corresponding cyclic esters,where the relief of ring-strain is the driving force for polymerization.Metal-based catalysis is the most common strategy used for promoting ROP of cyclic esters.However,the residual metal catalyst(and possibly ligands)are potentially(cyto)toxic and not desired in biomedical or microelectronic applications.Compared with metal-based catalysts,organic compounds,if suitably designed,can offer the potential for inexpensive and particularly environmentally friendly catalysts.To meet the challenge,we will utilize our core strengths in developing organocatalysts for the synthesis of advanced polymer materials.The results of this research are presented as follows:1)Novel bulky chiral and achiral N-heterocyclic carbenes(NHCs)with naphthyl groups substituted on the nitrogen were successfully synthesized.The organocatalytic stereoselective ROP of rac-LA by utilizing novel bulky chiral and achiral NHCs,1,3-bis-(1’-naphthylethyl)imidazolin-2-ylidene was fully investigated.The effect of polymerization conditions(e.g.solvent,temperature,alcohol initiator)on ROP behavior by these bulky NHCs has been studied,and the formation of isotactic-enriched stereoblock PLA(Pi=0.81)under optimized conditions with high activity(conv.=98%in 30 min)and narrow dispersity(D=1.05)was evidenced.The comparison of ROP reactios by chiral and achiral NHCs revealed that the stereoselectivity originates from a chain-end mechanism.2)The controlled synthesis of bio-based,biodegradable polylactide(PLA)-based diols was achieved by utilizing 1,8-diazabicyclo[5.4.0]undec-7-ene(DBU)as a highly active organocatalyst and without toxic organic solvents,which led to the production of polylactide diols of various molar mass values with narrow dispersities in a controlled manner.The structure of the diol was clearly confirmed by 1H and 13C NMR,FT-IR,MALDI-TOF MS and OHV.Kinetic study of the ROP demonstrates a pseudo-first-order kinetic model and a controlled "living" nature.In addition,various chain transfer agents and organocatalysts could also be used to successfully synthesize well-defined PLA diols,which offers a generally applicable method to synthesize various truly "green" bio-based diols for industrial applications.Furthermore,the high catalytic activity of DBU under solvent-free conditions towards the polymerization of Iactide and the role of 1,4-butanediol(BDO)as the chain transfer reagent was demonstrated in this work.3)The effective copolymerization of y-BL with two common cyclic esters monomers,such as ε-caprolactone(ε-CL)and δ-valerolactone(δ-VL),leading to a series of high Mn(up to 135 kg/mol)copolyesters with unprecedented levels of y-BL incorporations(up to 84.0 mol%)was achieved.This copolymerization was enabled by the judicious choice of organic catalysts which exhibit good kinetic and thermodynamic behavior.Kinetic studies and reactivity ratios for both copolymerization indicates random copolymers was formed,which are in agreement with the microstructure determined by 13C NMR analysis.In addition,the study thermal degradation behaviors of y-BL-based copolyesters has also been examined,which indicate incorporation of γ-BL into PCL accelerates the thermal degradation rate of PCL.The above results would be expected to provide basic knowledge on the application of high performance biodegradable polymer from renewable resources,and also provide some new strategies for design of organocatalysts in ROP.