Synthesis Of Biodegradable Polyesters By Benzo18-crown-6Modified Imidazolium-based Carbene

In this article, benzo18-crown-6modified imidazoliumhexafluorophosphate was synthesized and reacted with sodiumhydride in stu generating relative imidazolium-based carbene undernitrogen. In the presence of benzyl alcohol, benzo18-crown-6modified imidazolium-based carbene catalyzed ring-openingpolymerization of ε-caprolactone (ε-CL),2,2-dimethyl trimethylenecarbonate (DTC) and random copolymerization of ε-CL with DTC.The influence of various reaction conditions on the polymerizationas well as the kinetics of ring-opening polymerization ofε-caprolactone catalyzed by benzo18-crown-6modifiedimidazolium-based carbene was examined in detail. The structuralfeatures and polymerization mechanism of the polymers obtainedby benzo18-crown-6modified imidazolium-based carbene werecharacterized by1H NMR and IR.Benzo18-crown-6modified imidazolium-based carbene is highlyeffective catalysts for the homo-and copolymerization. Theinfluence of different conditions on the ring-opening polymerizationof ε-caprolactone catalyzed by B18C6imY/BnOH system have beenexamined in detail. The optimum conditions of polymerization aredescribed as:[ε-CL]/[C]=900,[ε-CL]=2.0mol/L,[ε-CL]/[I]=200,10℃, 60min, in THF. It could prepare polycaprolactone (PCL) withmolecular weight (Mn) of4.01×104g/mol and polydisperisity of1.48.The polymerization rate is first order with respect to monomer andcatalyst concentration which is demonstrated by the kinetic study.The overall activation energy of ε-CL polymerization amounts to39.73kJ/mol. The melting temperature of PCL is65.0℃from theDSC curve.1H NMR end group analysis on the polymer demonstratesthe ring-opening polymerization of ε-CL proceeds according to“monomer-activated” mechanism.B18C6imY/BnOH system was applied to the ring-openingpolymerization of DTC. Several factors affecting the catalyticactivities, such as monomer/catalyst molar ratio, monomer andinitiator concentration, polymerization time and temperature wereinvestigated intensively. The optimum conditions of polymerizationare as follow:[DTC]/[C]=400,[DTC]=2.0mol/L,[DTC]/[I]=200,15℃,50min, in THF. It could prepare PDTC with molecular weight (Mn) of4.11×104g/mol and polydisperisity of1.21. From the DSC curve ofthe PDTC, two peaks can be seen in the first heating process (a)which indicated that PDTC has two crystallizations. After quenchedof liquid nitrogen, only one crystallization is obtained in the secondheating curve.Random copolymer of ε-caprolactone with2,2-dimethyltrimethylene carbonate has been successfullysynthesized by the B18C6imY/BnOH system. Effects of differentreaction conditions on the copolymerization have been examinedsystematically. The optimum conditions of polymerization aredescribed as:[DTC]:[CL]=50:50,[DTC+CL]/[C]=400,[DTC+CL]=2.0 mol/L,[DTC+CL]/[I]=200,10℃,60min, in THF. The randomcopolymer [P(DTC-co-CL)] could be producted with molecularweight (Mn) of4.11×104g/mol and polydisperisity of1.44. Themonomer conversion is94.87%. Different compositions ofP(DTC-co-CL)s were produced for the purpose of determining thereactivity ratios of ε-CL and DTC. With the1H NMR analysis, thevalues of the reactivity ratios are18.42for rDTCand0.058for rCL, andthe value of rDTC×rCLis close to1.0, indicating that thecopolymerization is nearly ideal random copolymerization. Thethermal properties of the P(DTC-co-CL) can be seen in the DSCcurve which show that single value obtained for all theP(DTC-co-CL)s with various compositions and fall between the glasstemperatures of the PCL(-60℃) and PDTC(-28℃). The randomstructure of the copolymer can be confirmed by1H NMR analysis.The result show that the ring-opening copolymerization of ε-CL andDTC via “monomer-activated” mechanism.