Homo-and Block Copolymerizations Of ε-caprolactone With L-lactide Catalyzed By Novel N-heterocyclic Carbenes

In this study, the novel N-heterocyclic carbene (NHC) namedbenzo-15-crown-5-imidazole carbene (B-15-C-5-imY) wassynthesized and then used to catalyze homo-and blockcopolymerizations of ε-caprolactone (ε-CL) with L-lactide (L-LA) inthe presence of benzyl alcohol (BnOH). The data showed that theB-15-C-5-imY was an efficient origanic catalyst for the ring-openingpolymerization (ROP) of ε-CL and L-LA. The influences of reactionconditions on the polymerization were investigated systematically.The structure of the polymers and the polymerization mechanismwere supposed by1H NMR,13C NMR, DSC and IR.Homopolymer of ε-CL and L-LA has been synthesized by theB-15-C-5-imY/BnOH. The influences of concentration of monomer,the molar ratio of monomer and catalyst and the molar ratio ofmonomer and initiator, as well as temperatures and time on thepolymerization were investigated in detail. The optimum conditionof ROP of ε-CL by B-15-C-5-imY/BnOH can be described as:[ε-CL]=3.0mol/L,[ε-CL]/[C]=500(molar ratio),[ε-CL]/[I]=200(molar ratio),10℃,55min, in THF. The well-defined linear polycaprolactone (PCL)with high number-averaged molecular weight (Mn=50.8kg/mol) andrelative narrow polydispersity (PDI=1.51) was obtained in thiscondition. The kinetics study indicated that the polymerization rate was frst-order with respect to both monomer and catalystconcentration, which can be written as: Rp=kp [ε-CL][B-15-C-5-imY].The apparent activation energy of ROP catalyzed byB-15-C-5-imY/BnOH amounted to34.33kJ/mol. The DSC curve of thePCL showed a sharp melting peak at57.5℃. The optimum conditionof ROP of L-LA by B-15-C-5-imY/BnOH can be described as:[L-LA]=1.5mol/L,[L-LA]/[C]=300(molar ratio),[L-LA]/[I]=200(molar ratio),20℃,35min, in THF. The well-defined linear polylactide (PLLA) withhigh Mn and narrow PDI was obtained in this condition.1H NMR andIR data of the resulting PCL and PLLA demonstrated that thepolymerization of ε-CL and L-LA proceeded according to activatedmonomer mechanism.Block copolymerization of ε-CL with L-LA by B-15-C-5-imY/BnOHhas been carried out. It was found that the copolymer could only beprepared when ε-CL was first polymerized followed by the L-LA.Otherwise, PLLA will be obtained. The data showed that theoptimum conditions for the copolymerization of ε-CL and L-LA withB-15-C-5-imY/BnOH were as follows:[CL]=1.5mol/L,[L-LA]=2.0mol/L,[CL]/[I]/[C]=200:1:1, first stage of ε-CL polymerization:20℃,30min, second stage of L-LA polymerization:25℃,40min, in THF.The block copolymer (PCL-PLLA) with higher Mn and narrower PDIwas obtained in this condition. Analysis of the thermal properties ofthe PCL-PLLA by DSC showed that it had two melting temperatures(Tm=55.6and145.6℃).1H NMR and13C NMR spectral data ofPCL-PLLA showed the chemical structures without randomplacement.