Kinetics of Reversible Oligomerization of l-Lactic Acid with a SnCl₂·2H₂O/p-Toluenesulfonic Acid Catalyst

Abstract

An experimental and theoretical modeling study is presented on the kinetics of reversible oligomerization of L-lactic acid and the formation of L-lactide in closed reactors with SnCl 2 •2H 2 O/p-toluenesulfonic acid as a binary catalyst mixture. The experimental data and model simulations indicate that about 10 wt % L-lactide is produced by the cyclization of two repeating units of poly(L-lactic acid) at the chain end during the oligomerization process. The effects of the catalyst and reaction temperature at 160−200 °C on the discoloration of the reaction mixture have also been investigated, and it was observed that the discoloration of the reaction mixture was quite significant in the presence of the catalyst at high temperatures. The relevant kinetic parameters for the chain-growth reaction as well as cyclic L-lactide formation reaction have been estimated for both noncatalyzed and catalyzed reversible oligomerization reactions.

Size-Controlled Nanomicelles of Poly(lactic acid)–Poly(ethylene glycol) Copolymers with a Multiblock Configuration

“…Baick et al reported the reversible oligomerization of L-lactic acid in this binary catalyst system [24]. Miao et al applied this binary catalyst method to the synthesis of PLA-PEG-PLA triblock copolymers [25].…”

“…Miao et al applied this binary catalyst method to the synthesis of PLA-PEG-PLA triblock copolymers [25]. The reaction time, temperature, pressure and ratio of binary catalysts were studied to determine their effect on polymer decomposition, discoloring and racemization [14][15][16]24,25]. We previously reported that a reaction time longer than 20 h in melt polycondensation of PLLA was ineffective for further polymerization, but rather induced discoloration and racemization of PLLA [14].…”

Direct condensation of lactic acid in the presence or absence of supported zirconium sulfate

“…In the last years, medical applications of polylactic acid (PLA) have gained enormous interest due to the totally body absorbable nature of PLA. PLA can be synthesized by the melt condensation of lactic acid . Removal of water, formed by condensation reaction, from the bulk material is necessary either by evaporation under reduced pressure or by driving it away by a carrier gas (e.g., nitrogen).…”

“…Generally, maximum number average molecular weights (MWs) have been reached after 30–90 h at 180–200°C depending on the catalyst ( M n ∼ 2,300–90,000 g mol −1 ) . The use of Sn(II) catalysts activated by sulfonic acids can increase the MW of PLA . 1,3‐propanedisulfonic acid and 1,5‐naphthalene disulfonic acid have been found to successfully synthesize high MW PLA .…”

“…A problem faced during the melt condensation is the discoloration of the reaction mixture caused by the high reaction temperature, long reaction time, byproducts, and catalysts. The use of p ‐toluene sulfonic acid or triphenylphosphane as co‐catalysts are effective for hindering color formation and depressing racemization in the melt condensation process . An increasing degree of polymerization is strongly dependent on removal of water formed by condensation reaction.…”