The Study On Ring-opening Polymerization Of Cyclic Esters By N-heterocyclic Carbene

1 - isopropyl - 3 - (4 - methoxyphenyl) limidazol hexafluorophosphate(ipBnimPF6) was synthesized and reacted with potassium tert-butoxidein stu (or with sodium hydride) to from relative 1 - isopropyl - 3 - (4 -methoxyphenyl) imidazol - 2 - ylidene (ipBnimY) by vacuum filtration.The obtained carbene was catalyzed the ring - opening polymerizationofε-caprolactone (ε-CL) and L - lactide (LLA) initiated by benzylalcohol (BnOH), respectively. It has been found that ipBnimY iseffective catalysts for these polymerizations. The effects ofpolymerization conditions, such as the monomer, catalyst and initiatorconcentration on the polymerization have been investigated. We havedemonstrated that the imidazol - 2 - ylidene were able to producepolycaprolactone (PCL) with controlled molecular weights, lowpolydispersities and well-defined end groups derived from the alcoholinitiator. The analyses of polymer ends by1HNMR reveal thepolymerization proceeds .IpBnimY/BnOH system was applied to the polymerization ofε-CL.Several factors which affecting the catalytic activities, such asmonomer and initiator concentration, polymerization time andtemperature were investigated systematically. It could prepare polycaprolactone (PCL) with molecular weight of 3.03×10~4g/mol andpolydisperisity of 1.46, the catalytic efficiency of 22.8 kg PCL/molinitiator. The optimum conditions ofε-CL polymerization are asfollows: [ε-CL] = 2.0 mol/L, [ε-CL]/[C] =300molar ratio, [ε-CL]/[I]=200molar ratio, 10℃, 30 min, tetrahydrofuran (THF) as solvent. Thekinetic studies indicate that the polymerization rate is first-order withrespect to both catalyst and monomer concentrations. The overallactivation energy ofε-CL polymerization amounts to 49.6 KJ/mol. Theanalyses of polymer ends by1HNMR and IR reveal that theε-CLpolymerization proceeds according to a monomer-activated process.IpBnimY/BnOH system was applied to the homopolymerization ofLLA. Effects of different reaction conditions on the polymerizationwere examined in detail. The optimum conditions for LLApolymerization can be described as: [LLA] =1.5mol/L, [LLA]/[C] =300molar ratio, [LLA]/[I] = 200molar ratio ,15℃, 30min, in THF. It couldprepare polycaprolactone (PCL) with molecular weight of 2.68×10~4g/mol,monomer conversion of 99.7% and the catalytic efficiency of 28.8 kgPLLA/mol initiator. The kinetic study indicates that the polymerizationrate is also first order with respect to both monomer and initiatorconcentration. The apparent activation energy of the LLApolymerization is 50.4KJ/mol. The Results of1HNMR indicatedmechanism of the ring-opening of LLA. It can be proposed that themonomer is activated by the carbene, with propagation occurring byacyl-oxygen cleavage and esterification with alcohol present in solution.Propagation through chain extension of the alcohol species affords highmolecular weight polymers.