Reactivity Ratios and
Mechanistic Insight for Anionic
Ring-Opening Copolymerization of Epoxides
- Publication date
- Publisher
Abstract
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 <sup>1</sup>H 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 <sup>1</sup>H NMR spectroscopy for the resulting copolymer. For the copolymerization
between EO and AGE, the reactivity ratios were determined to be <i>r</i><sub>AGE</sub> = 1.31 ± 0.26 and <i>r</i><sub>EO</sub> = 0.54 ± 0.03, while for EO and EGVGE, the reactivity
ratios were <i>r</i><sub>EGVGE</sub> = 3.50 ± 0.90
and <i>r</i><sub>EO</sub> = 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 transition-state energies agree qualitatively
with the observed relative rates for polymerization