Synthesis Of Biodegradable Polyesters By Imidazolium-based Carbenes

Two precursors of carbene catalysts——1,3-dibenzylimidazoliumhexafluorophosphate （diBnimPF6）, 1-isopropyl-3-benzylimidazoliumhexafluorophosphate （ipBnimPF6）, were synthesized and reactedwith sodium hydride in stu generating relative imidazolium-basedcarbenes under dry and purified argon. In the presence of benzylalcohol, 1, 3 - dibenzyl imidazol - 2 - ylidene carbene （diBnimY）catalyzed ring-opening polymerization ofε- caprolactone.Ring-opening copolymerization ofε- caprolactone （ε- CL） and 2, 2 -dimethyltrimethylene carbonate （DTC） was carried out by using 1–isopropyl - 3 - benzyl imidazol - 2 - ylidene carbene （ipBnimY）. Theresults show that these two imidazol - 2 - ylidene carbenes werepotent organic catalysts for the ring-opening polymerization ofcyclic esters. The influences of reaction conditions on thepolymerization have been examined in detail. The structure of thepolymers and the polymerization mechanism were characterized by 1^H NMR and13C NMR.The homopolymer PCL has been prepared by usingdiBnimY/BnOH systems as organocatalysts. The influences ofdifferent conditions on the ring-opening polymerization ofε- CLhave been examined in detail, and the optimum conditions can bedescribed as: [CL]=3.0mol/L, [CL]/[I]/[C]=600:3:1, 25℃, 60min, in THF.The diBnimY/BnOH systems were highly effective catalyst forpolymerization ofε- CL, and produced high molecular weightpolymers （ Mn = 42,100 g/mol） with narrow polydispersities （PDI =1.50） at room temperature in 60min. The catalytic efficiency is 22.8kg PCL/mol initiator. The polymerization rate is first-order withrespect to monomer and catalyst concentration. It can bedemonstrated by the kinetic study with the overall reactionactivation energy amounts to 47.62 KJ/mol initiated by BnOH. FromDSC curve of the PCL, it has a melting temperature of 62.7℃.1HNMR end group analysis on the polymer demonstrates thering-opening polymerization ofε- CL proceeds according toactivated monomer mechanism with acyl-oxygen bond cleavage ofthe monomer.Random copolymer ofε- caprolactone with 2, 2 -dimethyltrimethylene carbonate has been synthesized by theipBnimY with BnOH systems. Effects of different reaction conditions on the copolymerization have been examined in detail,such as monomer, catalyst and initiator concentration, the molarratio of two monomers, as well as reaction time and temperature.The data show that the optimum conditions for the ring-openingcopolymerization ofε- CL and DTC with ipBnimY/BnOH are asfollows: [CL]:[DTC]=50:50 in feed, [CL+DTC]=3.0mol/L, [CL+DTC]/[I]/[C] = 200:1:0.4, 25℃, 60 min, in THF. And random copolymer couldbe preparep with molecular weight of 2.85×104and polydisperisityof 1.44. The monomer conversion is 95.10%. Different compositionsof copolymers were prepared for the purpose of determining thereactivity ratios ofε- CL and DTC. It could be found the more DTC bethrew in feed, the easier ring-opening copolymerization be carriedout. The values of the reactivity ratios are 4.21 for r_DTC and 0.25 forr_CL, and the value of r_DTC×r_CL for the copolymerization catalyzedby ipBnimY/BnOH systems is close to 1.0, indicating that thecopolymerization is nearly ideal. Analysis of the thermal propertiesof the copolymer by DSC show that single value obtained for all thecopolymers with various compositions, fall between the glasstemperatures of the homopolymers PCL （-65.5℃） and PDTC （27℃）.The random structure of the copolymer can be confirmed by ¹H NMRanalysis. The mechanism can be concluded that it involves anattack of the imidazol - 2 - ylidenes catalyst onto cyclic esters and the formation of a zwitterionic imidazolium alkoxide intermediate（activated monomer mechanism）.