Reactivity Ratios and Mechanistic Insight for Anionic Ring-Opening Copolymerization of Epoxides

Lee, Bongjae F.; Wolffs, Martin; Delaney, Kris T.; Sprafke, Johannes K.; Leibfarth, Frank A.; Hawker, Craig J.; Lynd, Nathaniel A.
, Macromolecules,
2012
, 45, 3772-3731

Reactivity ratios were evaluated for anionic ring-opening copolymerizations of ethylene oxide (EO) with
either allyl glycidyl ether (AGE) or ethylene glycol vinyl glycidyl ether (EGVGE) using a benzyl alkoxide initiator. The
chemical shift for the benzylic protons of the initiator, as measured by 1H NMR spectroscopy, were observed to be
sensitive to the sequence of the first two monomers added to the initiator during polymer growth. Using a simple kinetic
model for initiation and the first propagation step, reactivity ratios for the copolymerization of AGE and EGVGE with EO
could be determined by analysis of the 1H NMR spectroscopy for the resulting copolymer. For the copolymerization between
EO and AGE, the reactivity ratios were determined to be rAGE = 1.31 ± 0.26 and rEO = 0.54 ± 0.03, while for EO and EGVGE,
the reactivity ratios were rEGVGE = 3.50 ± 0.90 and rEO = 0.32 ± 0.10. These ratios were consistent with the compositional drift
observed in the copolymerization between EO and EGVGE, with EGVGE being consumed early in the copolymerization. These
experimental results, combined with density functional calculations, allowed a mechanism for oxyanionic ring-opening
polymerization that begins with coordination of the Lewis basic epoxide to the cation to be proposed. The calculated transitionstate
energies agree qualitatively with the observed relative rates for polymerization.