Abstract
The precise synthesis of polymers derived from alkyl lactate ester acrylates is reported for the first time. Kinetic experiments were conducted to demonstrate that Cu(0) wire-catalyzed single electron transfer-living radical polymerization (SET-LRP) in alcohols at 25 °C provides a green methodology for the LRP of this forgotten class of biobased monomers. The acrylic derivative of ethyl lactate (EL) solvent and homologous structures with methyl and n-butyl ester were polymerized with excellent control over molecular weight, molecular weight distribution, and chain-end functionality. Kinetics plots in conventional alcohols such as ethanol and methanol were first order in the monomer, with molecular weight increasing linearly with conversion. However, aqueous EL mixtures were found to be more suitable than pure EL to mediate the SET-LRP process. The near-quantitative monomer conversion and high bromine chain-end functionality, demonstrated by matrix-assisted laser desorption ionization time-of-flight analysis, further allowed the preparation of innovative biobased block copolymers containing rubbery poly(ethyl lactate acrylate) poly(ELA) sequences. For instance, the poly(ELA)- b-poly(glycerol acrylate) block copolymer self-assembled in water to form stable micelles with chiral lactic acid-derived block-forming micellar core as confirmed by the pyrene-probe-based fluorescence technique. Dynamic light scattering and transmission electron microscopy measurements revealed the nanosize spherical morphology for these biobased aggregates.
Highlights
Occurring lactic acid (2-hydroxypropionic acid) was first isolated from sour milk by the Swedish chemist Scheele in 1780
Single electron transfer living radical polymerization (SET-LRP) has gained great popularity as a facile tool for precision macromolecular engineering. 13, 14, 15, 16, 17, 18, 19 For example, when conducted in reaction media that facilitates disproportionation of Cu(I)Br into Cu(0) and Cu(II)Br220,21,22 this method enables the synthesis of vinylic polymers with nearly 100% chain end functionality at complete conversion. 23,24,25,26 This has been demonstrated to be feasible even in “programmed” biphasic single electron transfer living radical polymerization (SET-LRP) systems.[30,31,32,33]
It was demonstrated that ethyl lactate (EL) possesses interesting features related to SET-LRP, 41 expanding the myriad of academic/industrial applications found for this promising bio-sourced solvent
Summary
Occurring lactic acid (2-hydroxypropionic acid) was first isolated from sour milk by the Swedish chemist Scheele in 1780. It is worth to mention that with the recent developments toward the commercial production of bio-acrylic acid and the cost-competitive production of bio-ethanol, ELA may be prepared entirely from biomass derived platform chemicals.[3] the acrylic acid/EL acid-catalyzed esterification by azeotropic distillation in toluene was low-yield because extensive oligomerization of EL occurred at high temperature.[46] the use of propylphosphonic anhydride (T3P®) under milder conditions gave an excellent result.[52] This ester coupling promoter lacks the toxicity and shock sensibility associated with other coupling agents (e.g. DCC and EDC).[54] by-products from the coupling are H2O-soluble and separated from the reaction mixture.
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