Ring-Opening Polymerization Of Cyclic Esters Catalyzed By N-hetercyclic Carbenes

Aliphatic polyester is a kind of biodegradable polymers with good physical and mechanical properties. Ring-opening polymerization (ROP) of cyclic esters is one of the most important methods in the synthesis of aliphatic polyester. At present time, catalysts used in the synthesis of aliphatic polyesters, especially in commercial manufacture, are mainly metal-containing compounds, e. g stannum, zinc, aluminum alkyl compounds and rare earth compounds. These metal-catalyst residues contaminated the polyesters, which limitted in the use of products. Therefore, research and development of metal-free catalysts draw more attention in the synthesis of aliphatic polyesters in recent years.In this study, seven imidazol-2-ylidene N-heterocyclic carbenes (NHCs) with different alkyl substituents attached to nitrogen atom (N) were synthesized in situ using imidazolium bromides as starting materials and then used to catalyze ROP of cyclic esters in the presence of an alcohol. The polymerization characteristics were investigated.It was found that catalytic activity of the carebenes had remarkable relationship with the their substitutes. The larger the substituent, the weaker the catalytic activity for both asymmetry and symmetry NHCs in the catalyst concentration of [C]=0.023mool/L. Among the asymmetry carbenes, the activity follows as 1-butyl-3-methylimidazol-2-ylidene carbene (bmim)>1-hexyl-3-methylimidazol-2-ylidene carbene (hmim) >1-octyl-3-methylimidazol-2-yildene carbene (omim). The symmetry carbenes also showed same rule, 1,3-bis(butyl) imidazol-2-ylidene carbene (bbim) >1,3-bis(hexyl)imidazol-2-ylidene carbene and l,3-bis(octyl)imidazol-2-ylidene carbene. Asymmetry carbenes were more active than symmetry carbenes, but showed less controllability in the polymerization. The reactivity of symmetry carbenes reduced more clearly at reduced catalyst concentration. The sterics effect of substituents can be explained from the activatedmonomer mechanism (AMM) and structure characteristics of carbene.However, 1-ethyl-3-methylimidazol-2-ylidene carbene (emim) didn't fit the law for its high reactivity and unstabiligy.1-Butanol, 2-butanol and tert-butyl alcohol were used as initiator in the ROP of 8-CL catalyzed by bbim in order to study the influence of hydroxyl group position on ROP. Butanol and 2-butanol did initiate the polymerization with close rate, but tert-butyl alcohol did not initiate the polymerization. It appears that both primary and secondary alcohol can initiate the ROP with bbim carbene as catalyst, but tert-alcohol can not. Poly(e-caprolactone)s with difunctional terminal hydroxyls were synthesized through ROP of ε-CL using ethylene glycol and 1,4-butanediol.The applicability of cabene/alcohol system of cyclic ester monomers was also examined. It has been found that ε-CL, L-LA and δ-VL can be ring-opening polymerized in the presence of a cabene/alcohol system, but the polymerization rate of L-LA was much slower than that of ε-CL. Unfortunately, β-BL can not be polymerized.In order to purify the polymer, the ROP of ε-CL was conducted in a biphasic system, using tetrahydrofuran (THF) as an organic phase and 1-ethyl-3-methylimidazolium tetrafluroborate (emimBF₄) as an ionic liquid (IL) phase. The ionic liquid was the precursor of the carbene and therefore a bank of the catalyst. NHC was synthesized in the IL phases and diffused into the organic phase to catalyze the polymerization. The side product, KBr, stayed at the IL phase. Polymers with molecular weight about 5000 were formed in the organic phase. After simple separation, precipitation and drying, pure polymer was obtained. The IL phase could be re-activated and used repeatedly and the ROP rate did not show clear decrease. However, it should be noted that the monomer conversion was lower than in solution polymerization.Kinetic characteristics of ROP of ε-CL in the presence of bbim/phenylcarbinol system under various temperatures, concentration of catalyst, initiator and monomerwere also studied. The kinetic results were analyzed according to the AMM.