Ring-Opening Polymerization Of ε-Caprolactone Catalyzed By Imidazolium Heterocyclic Carbene

In this study, three different imidazolium ionic liquids (precursors of N-heterocyclic carbenes), l-butyl-3-methyl imidazol bromide (bmimBr), l-hexyl-3-methyl imidazol bromide (hmimBr), and l,3-bis(butyl) imidazol bromide(bbimBr), were synthesized first and corresponding N-heterocyclic carbenes (bmim, hmim, bbim) were then in-situ generated via a reaction with t-BuOK. All these carbenes combined with phenylcarbinol (PI1CH2OH) were used as highly efficient catalyst/initiator systems for the ring opening polymerization (ROP) of E-caprolatone(ε-CL), among which bbim/PhCH₂OH system catalyzed/initiated the ROP of ε-CL in a living/controlled manner and thus biodegradable poly(E-caprolatone)s (PCL) with predetermined molecular weight and narrow molecular weight distribution were successfully synthesized and characterized. The living/controlled characteristics and kinetics of this polymerization system were investigated in detail and the polymerization mechanism has been discussed.N-heterocyclic carbenes were in-situ generated via the reactions between the corresponding imidazolium ionic liquids with t-BuOK, which was confirmed by carbene-trapping experiments. Thus obtained carbenes were stable enough at inert atmosphere and showed high catalytic activities toward the ROP of e-CL at room temperature when they were used together with PhCH₂OH initiator.The ROP of ε-CL catalyzed/initiated by bbim/PhCH₂OH system possessed characteristics of living polymerization: (1) the number average molecular weight of PCL (Mn) increased with monomer conversion in a linear manner and was inagreement with the theoretical relationship (DP)(2) the molecular weightdistribution was narrow; (3) the life of "active polymer chain" was long enough to ensure that the second-added monomer could be polymerized and the molecularweight of the resultant polymers grew sequentially when the first-added monomer had been consumed completely.It was shown from the polymerization kinetics that the effect of the concentration of the carbene catalyst on the polymerization rate was remarkable: the higher the catalyst concentration, the higher the polymerization rate and the apparent propagation rate constant. However, there was no obvious effect of catalyst concentration on the molecular weight of the resulting polymers. These results implied that N-heterocyclic carbene was a true catalyst in the polymerization system and it seemed that carbene participated in the propagation reaction in certain manner but was not concerned with the formation the active sites. Our experimental results also show that the initiator participated in the formation of the active sites but was not directly concerned with the propagation reaction. The molecular weight was determined with the molar ratio of the monomer concentration to the initiatorconcentration and the monomer conversion (DP_n =x), and therefore, it could befacilely tuned with the initiator concentration. The molecular weight distribution of the resulting polymers was relatively narrow and affected both by the concentration of the catalyst and initiator: low catalyst concentration and high initiator concentration both were favorable to narrow molecular weight. As a result, biodegradable PCLs with well-determined molecular weight and narrow molecular weight distribution were successfully synthesized by this new polymerization method.It was clear from the structural analysis of the polymers that one end of the resulting polymer chain was a hydroxyl and the other end was benzyl group, which implies that it was PhCH₂OH but neither the t-BuOK (a component needed to in-situ generates the carbene) nor the t-BuOH (the by-product in the in-situ generation of carbene) initiated the polymerization reaction. Based on the kinetic results and mechanism analysis, it is believed that the N-heterocyclic carbene played a true catalytic role in the polymerization reaction (it reacted with monomer to form "active monomer" which participated the propagation reaction) and PhCH₂OH was employed as an initiator indeed. Therefore, the polymer...