Divergent Mechanistic Avenues to an Aliphatic Polyesteracetal or Polyester from a Single Cyclic Esteracetal

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 (T_g) ≈ -30 °C, whereas high molar mass PHPA had a T_g ≈ -30 °C and a melting temperature (T_m) ≈ 77 °C. When PHPA and PMDO were subjected to neutral or slightly acidic environments, PMDO exhibited expedited degradation as compared with PHPA.