Living Ring-opening Of ε-caprolactone Lactide Homopolymerization And Triblock Copolymerization

Precursor of carbene catalyst4-methyl benzo-12-crown-4imidazolium hexafluorophosphate was synthesized and reacted withsodium hydride in stu generating relative imidazolium-based carbene.In the presence of benzyl alcohol,4-methyl benzo-12-crown-4imidazole carbene catalyzed ring-opening homopolymerization ofε-caprolactone(ε-CL) and L-lactide(L-LA), respectively. In additional,the carbene catalyzed the block polymerization of ε-CL and L-LAinitiated by ethylene glycol to synthesize the triblock copolymers.The polymerization conditions and reaction kinetics of ring-openinghomopolymerization have been studied in detail. The experimentalresults demonstrated that this carbene was potent organic catalystsfor the ring-opening polymerization of ε-CL and L-LA. The structure ofthe polymers and the polymerization mechanism werecharacterized by1H NMR,13C NMR and IR.In the presence of benzyl alcohol, ring-opening homopolymeriza- tion of ε-CL catalyzed by4-methyl benzo-12-crown-4imidazolecarbene has been carried out. The influences of monomerconcentration, catalyst concentration, initiator concentration,polymerization time and temperature were investigatedsystematically, and the optimum conditions can be described as:[ε-CL]/[C]=600(molar ratio),[ε-CL]/[I]=200(molar ratio),[ε-CL]=3.0mol/L,10℃,40min, in THF. In this polymerization condition, a highmolecular weight (Mn=4.82×104) and relatively narrow molecularweight distribution polymer (PDI=1.49) was obtained. The kineticstudies indicated that the polymerization rate was first-order withrespect to monomer and catalyst concentration, which can bewritten as: Rp=Kp [ε-CL][C]. According to the Arrhenius equation,the apparent activation energy of the polymerization was42.94KJ/mol. The DSC curve of the PCL shows a sharp melting peak at56.8℃.1H NMR and IR spectral data indicated that the polymerizationmechanism was in agreement with the activated monomermechanism.The optimum conditions of the homopolymerization of L-LAcatalyzed by carbene/BnOH system can be described as:[L-LA]/[C]=300(molar ratio),[L-LA]/[I]=200(molar ratio),[L-LA]=1.5mol/L,20℃,30min， in THF. In this polymerization condition, a highmolecular weight (Mn=1.97×104) and relatively narrow molecular weight distribution polymer (PDI=1.51) was obtained. The DSC curveof the PLLA shows a sharp melting peak at163.7℃. The structure ofthe polymer has been characterized and the mechanism of thepolymerization has been speculated by1H NMR and IR.We also utilized this carbene as the catalyst and ethylene glycolas a difunctional initiator to synthesize the dihydroxyl functionalizedpoly (ε-caprolactone)(HO-PCL-OH). Then PLLA-PCL-PLLA triblockswere prepared in THF using dihydroxyl telechlic PCL as macroinitiatorfor the ring-opening polymerization of LLA. Effects of differentreaction conditions on the copolymerization have been examined indetail, such as monomer concentration, reaction time andtemperature. The data shown that the optimum conditions for thering-opening copolymerization of ε-CL and LLA with carbene/ethylene glycol were as follows:[ε-CL]:[LLA]=1:1in feed [ε-CL]/[C]=300(molar ratio)，[ε-CL]/[I]=200(molar ratio)，[ε-CL]=2mol/L, ε-CL:15℃,40min; LLA:25℃,40min. In this polymerization condition, ahigh molecular weight (Mn=3.92×104) and relatively narrowmolecular weight distribution polymer (PDI=1.30) was obtained.From DSC analyses, we observed that the glass transitiontemperatures of the triblock copolymer (Tm=52.6℃and160.4℃).The structure of the triblock copolymer was confirmed by1H NMR,13CNMR and IR.