Divergent Mechanistic Avenues to an Aliphatic Polyesteracetal
or Polyester from a Single Cyclic Esteracetal
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Abstract
The cyclic esteracetal 2-methyl-1,3-dioxane-4-one
(MDO) was polymerized
in bulk using diethyl zinc as the catalyst and benzyl alcohol as the
initiator to yield either the corresponding polyesteracetal (PMDO)
or the aliphatic polyester poly(3-hydroxypropionic acid) (PHPA) at
low and high catalyst concentrations, respectively. Spectral analysis
gave evidence for distinct propagating species in the two catalyst
concentration regimes. At low zinc concentrations ring opening by
attack of the initiating species at the acetal functionality, yielding
a zinc carboxylate, followed by propagation to yield pure PMDO was
implicated. At high zinc concentrations we propose that ring opening
via attack at the ester functionality produced a labile zinc hemiacetal,
which rapidly and irreversibly expelled acetaldehyde to form a propagating
zinc alkoxide and ultimately pure PHPA. Initial rate studies indicated
that the rate of PHPA formation had a second-order dependence on zinc
concentration; in contrast, the rate of PMDO formation was first order
in zinc concentration. High molar mass PMDO exhibited only a glass
transition temperature (<i>T</i><sub>g</sub>) ≈ −30
°C, whereas high molar mass PHPA had a <i>T</i><sub>g</sub> ≈ −30 °C and a melting temperature (<i>T</i><sub>m</sub>) ≈ 77 °C. When PHPA and PMDO were
subjected to neutral or slightly acidic environments, PMDO exhibited
expedited degradation as compared with PHPA