The Study On Ring-opening Polymerization (ROP) Of Cyclic Esters By Novel N-heterocyclic Carbene

Benzo15-crown-5imidazolium carbene was synthesized firstlyand catalyzed the ring-opening homopolymerization ofε-caprolactone (ε-CL) and2,2-dimethyltrimethylene carbonate (DTC)using benzyl alcohol (BnOH) as initiator. Benzo15-crown-5imidazole-carbene （B15C5imY） is effictive initiator for thesepolymerizations. Characteristics of these homopolymerization havebeen examined in detail. The structures and properties of thepolymers such as polycaprolactone (PCL) and poly (2,2-dimethyltrimethylene carbonate)(PDTC) were characterized andanalyzed thoroughly by IR and1H NMR.B15C5imY/BnOH system was applied to the polymerization ofε-CL. The influences of the monomer and initiator concentration,reaction temperature and time, monomer and catalyst/initiator molarratio on pilymerization were investigated, respectively. The optimumconditions of polymerization are as follows:[CL]=3.0mol/L,[CL]/[C] =500(Molar ratio),[CL]/[I]=200(Molar ratio),15℃,25min,tetrahydrofuran (THF) as solvent. PCL with molecular weight (Mn) of3.65×104and molecular weight distribution (PDI) of1.47is obtained.The kinetic study indicated that the ε-CL polymerization is first orderwith respect to monomer concentration and catalyst concentration.The overall activation energy of ε-CL polymerization amounts to35.8KJ/mol. The melting temperature is63.0℃from the DSC curve. A“monomer-activated” mechanism is presented for the ring-openingpolymerization of ε-CL by1H NMR and IR analysis.B15C5imY/BnOH system was applied to the homopolymerizationof DTC. The effects of different reaction conditions on thepolymerization of DTC were studied in detail. PDTC with molecularweight (Mn) of2.43×104and molecular weight distribution (PDI) of1.49can be easily obtained under the optimum conditions:[DTC]=2.0mol/L,[DTC]/[C]=300(Molar ratio),[DTC]/[I]=200(Molar ratio),15℃,25min, in THF. The kinetic study indicates that DTCpolymerization is first order with respect to monomer concentrationand catalyst concentration in the presence of BnOH. Results of1HNMR and IR indicate that the polymerization mechanism of DTCmonomer inserts into the growing chains with the acyl一oxygen bondscission.