Polymerization kinetics of rac-lactide initiated with alcohol/stannous octoate using in situ attenuated total reflectance-fourier transform infrared spectroscopy: An initiator study
2009
Moravek, Scott J. | Messman, Jamie M. | Storey, R. F. (Robson F)
rac-Lactide polymerization kinetics in THF at 72 °C were monitored in real-time using mid-infrared ATR-FTIR spectroscopy, with diamond composite insertion probe and light conduit technology. Monomer concentration as a function of time was acquired using the 1240 cm⁻¹ resonance associated with the ----CO----O----C---- stretch. Polymerizations were initiated with either n-propanol (PrOH), ethylene glycol (EG), trimethylol propane (TMP), or pentaerythritol (PENTA) with the coinitiator stannous octoate (Sn(Oct)₂). Polymerizations were found to be reversible at high monomer conversions, with a residual monomer concentration at 72 °C (345 K) of 0.081 M. The polymerizations were internally first-order with respect to monomer, indicating a constant concentration of propagating centers. For a typical reaction with [rac-LA]₀ = 1.0 M, [PENTA]₀ = 1.3 x 10⁻² M, and [Sn(Oct)₂] = 2.5 x 10⁻² M, the first-order rate constant, kapp was measured as 1.8 x 10⁻⁴ s⁻¹. First-order rate constants were determined to be independent of polymer architecture (i.e., initiator functionality) and proportional to [Sn(Oct)₂] for [Sn(Oct)₂]₀/[ROH]₀ [double less-than sign] 1, where [ROH]₀ represents the initial concentration of initiating hydroxyl groups.
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