A New Role for CO2: Controlling Agent of the Anionic Ring-Opening Polymerization of Cyclic Esters

5 years ago

A New Role for CO2: Controlling Agent of the Anionic Ring-Opening Polymerization of Cyclic Esters

Kuo-Wei Huang, Jobi K. Varghese, Xiaoshuang Feng, Nikos Hadjichristidis, Yves Gnanou, Théo P. Gonçalves

Conventional anionic ring-opening of polymerization (AROP) of cyclic esters suffers from the nonselective and concomitant attack of the monomer and of the polymer chains by the growing active species, which results in polyester samples with uncontrolled molar masses and broad polydispersity due to the competition between propagation and transesterification reactions. In this report, we describe a new AROP system mediated by a controlled amount of CO₂ which prevents transesterification reactions from occurring. Using lithium monomethyl diethylene glycoxide (MEEOLi) as initiator and 1.5 equiv of CO₂, ε-caprolactone could be polymerized under truly “living” conditions in dichloromethane (DCM) at 70 °C, as evidenced by the control of molar masses, the narrow polydispersity indexes (M_n up to ∼40 kg/mol, Đ < 1.16), and also successful chain extension experiments. Lithium carbonate used as initiator in the presence of 0.5 equiv of CO₂ afforded similar polymerization results. Experiments carried out with other alkoxide salts and solvents demonstrate that CO₂ is indispensable as well as lithium and noncoordinating solvents for the suppression of transesterifications. A similar strategy was applied for the AROP of l-lactide (LLA). At −20 °C, LLA could be polymerized under living conditions with undetectable level of transesterification as demonstrated by MALDI-ToF analysis. To account for the polymerization mechanism occurring in the presence of a slight excess of CO₂, we resorted to computational studies. It appears that a fast equilibrium takes place between two tetrameric aggregates, one dormant comprising four carbonates (RCO₃Li)₄, and an active one involving three carbonates and one alkoxide (RCO₃Li)₃(ROLi). The latter is shown to selectively ring-open cyclic ester without indulging in transesterifications like (ROLi)₄ precursors.

Publisher URL: http://dx.doi.org/10.1021/acs.macromol.7b01425

DOI: 10.1021/acs.macromol.7b01425