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

Jobi K. Varghese, Theo Goncalves, Kuo-Wei Huang, Nikos Hadjichristidis, Yves Gnanou, Xiaoshuang Feng

Research output: Contribution to journalArticlepeer-review

4 Scopus citations

Abstract

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 (Mn 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.
Original languageEnglish (US)
Pages (from-to)6752-6761
Number of pages10
JournalMacromolecules
Volume50
Issue number17
DOIs
StatePublished - Aug 15 2017

Fingerprint Dive into the research topics of 'A New Role for CO2: Controlling Agent of the Anionic Ring-Opening Polymerization of Cyclic Esters'. Together they form a unique fingerprint.

Cite this