Ring-Opening Polymerization Of Lactones Catalyzed Monourea And Bisurea

Aliphatic polyesters are a representative class of biodegradable,biocompatible,and environmentally friendly polymers,which have been widely used in packaging,agricultural film,and biomedical materials.Aliphatic polyesters are usually prepared by ring-opening polymerization(ROP)of lactone monomers.The catalysts for ROP of lactones mainly include metal catalysts and organocatalysts.The metal catalysts are usually sensitive to air and water,requiring harsh reaction conditions.In addition,the as-prepared polymers inevitably contain a small amount of metal ions that are difficult to be completely removed,which limits their applications of aliphatic polyesters in biomedical fields.Due to the ease of preparation,low cost,low toxicity,and high stability of organocatalysts,ROPs of lactones catalyzed by organocatalysts have drawn much attention in the recent years.First,the monourea catalyst was obtained by the reaction of isocyanate and amine.The structure was characterized byIH NMR,13C NMR,FTIR and EA.ROP of ?-CL and?-VL was also carried out under the room temperature and solvent-free condition with BnOH as an initiator and MTBD as a cocatalyst.PCLs and PVLs with different molecular weights were obtained by using monoureajMTBD as the catalyst of ROP of ?-CL and ?-VL,which were consistent with the theoretical molecular weights.Molecular weight distributions of obtained PVLs were narrow(D=1.10-1.14).Effect of the amount of monourea/MTBD on ROP of lactones was also studied.The monourea/MTBD-catalyzed ROP of the lactones was controllable even at a relatively low catalyst content around 0.25%-0.5%.Kinetic experiments further demonstrated that the monourea/MTBD-catalyzed ROP of lactones living and controllable.Possible mechanism for ROP of lactones catalyzed by monourea/MTBD was proposed,in which the monourea activated the carbonyl group of monomer through hydrogen bonding and the MTBD activated the initiator/chain alcohol and then attack monomer.In addition,thermal and mechanical properties of synthesized polyesters were investigated.Melting point of polyesters increased with increasing molecular weight,and the melting enthalpy and crystallinity decreased with increasing molecular weight.Tensile strength of PCL with molecular weight of 61.1 kDa was 21.4 MPa;its Young's modulus was 181.9 MPa and elongation at break was 740%.Second,the bisurea catalysts were prepared by one-step reaction of isocyanate and diamine,and their structures were characterized by 1H NMR,13C NMR,FTIR and EA.ROPs of ?-CL and ?-VL were carried out under room temperature and solvent-free condition with BnOH as an initiator and MTBD as a cocatalyst.Catalytic activity of the bisurea with phenyl group as linker(bisurea a)/MTBD catalyst system was the highest among the four catalyst systems.Effect of the molar ratio and amount of bisurea a/MTBD on ROP was studied to obtain a suitable molar ratio of bisurea a/MTBD of 1/1.Bisurea a/MTBD-catalyzed ROP of lactones was controllable,in spite of relatively low catalyst content(0.25%-0.5%).Similarly,solution polynerization of L-LA was carried out using bisurea a/Me6TREN at room temperature.The obtained PLLAs had narrow molecular weight distribution(D=1.11-1.13).Kinetics and chain extension experiments further demonstrated that the bisurea/MTBD was living/controllable for ROP of lactones.We analyzed and proposed a possible polymerization mechanism of the bisurea/MTBD-catalyzed ROP of lactones that bisurea a with enhanced catalytic performance by intermolecular hydrogen bonding activated the monomer and MTBD activated the initiator/chain alcohol and then attack monomer.Moreover,bulk ROPs of?-CL and 6-VL were carried out at room temperature using a bisurea a/MTBD catalyst system to obtain copolymers P(CL-co-VL)s with different molecular weights(20-30 kDa).Sequence structure of P(CL-co-VL)was analyzed by NMR.The results showed that the CL and VL units were randomly distributed in the copolrmer.Furthermore,thermal properties and crystallization behavior of P(CL-co-VL)were investigated.Crystallization and melting of P(CL-co-VL)exhibited eutectic behavior;the introduction of copolymerized units lowered the melting point and the copolymers had high crystallinity within the entire composition region.Crystalline structure of P(CL-co-VL)were analyzed by WAXD and SAXS.WAXD profile of P(CL-co-VL)was similar to PCL homopolymer when fCL?0.58,however,WAXD profile of P(CL-co-VL)was similar to PVL homopolymer when fCL ?0.28.SAXS results showed that the long period(LP)of P(CL-co-VL)s increased first and then decreased with the decrease of fCL.