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Bulk Organocatalytic Synthetic Access to Statistical Copolyesters from L-Lactide and #-Caprolactone Using Benzoic Acid Leila Mezzasalma, Simon Harrisson, Saad Saba, Pascal Loyer, Olivier Coulembier, and Daniel Taton Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.9b00190 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019
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Biomacromolecules
Bulk Organocatalytic Synthetic Access to Statistical Copolyesters from L-Lactide and ε-Caprolactone Using Benzoic Acid Leila Mezzasalma,† ‡ Simon Harrisson,§* Saad Saba,‖ Pascal Loyer,‖ Olivier Coulembier,†* Daniel Taton‡* †Center of Innovation and Research in Materials and Polymers (CIRMAP), Laboratory of Polymeric and Composites Materials, University of Mons, 23 Place du Parc, Mons B-7000, Belgium ‡Laboratoire de Chimie des Polymères Organiques (LCPO), CNRS, ENSCBP University of Bordeaux, UMR 5629, 16, av. Pey Berland 33607 Pessac CEDEX, France §Laboratoire des IMRCP, Université de Toulouse, CNRS UMR 5623, Université Paul Sabatier, 118 route de Narbonne 31062, Toulouse Cedex 9, France ‖Inserm, INRA, Univ Rennes, Institut NUMECAN (Nutrition Metabolisms and Cancer) UMR-A 1341, UMR S 1241, F-35000 Rennes, France KEYWORDS: organocatalysis, benzoic acid, lactide, caprolactone, copolymerization, bulk, statistical copolymers.
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ABSTRACT
The development of synthetic strategies to produce statistical copolymers based on L-lactide (LLA) and ε-caprolactone (CL), denoted as P(LA-stat-CL), remains highly challenging in polymer chemistry. This is due to the differing reactivity of the two monomers during their ring-opening copolymerization (ROcP). Yet, P(LA-stat-CL) materials are highly sought-after as they combine the properties of both polylactide PLA and poly(ε-caprolactone) (PCL). Here, benzoic acid (BA), a naturally occurring, cheap, readily recyclable and thermally stable weak acid, is shown to trigger the organocatalyzed ring-opening copolymerization (OROcP) of L-LA and CL under solvent-free conditions at 155 °C, in presence of various alcohols as initiators, with good control over molar masses and dispersities (1.11 > kLA-LA; Scheme S1).52 In these conditions, one can write the following relationships: kCL-CL≈ kCL-LA >> kLA-LA ≈ kLA-CL using equations (3):
(3) 𝑟𝐶𝐿 =
𝑘𝐶𝐿 ― 𝐶𝐿 𝑘𝐶𝐿 ― 𝐿𝐴
𝑟𝐿𝐴 =
𝑘𝐿𝐴 ― 𝐿𝐴 𝑘𝐿𝐴 ― 𝐶𝐿
Analysis by 13C NMR spectroscopy (Figure 4b) also confirmed the microstructure of the copolymer with the expected homo- and heterotriads, as previously reported.62 Kasperczyk and Bero described two distinct modes of transesterification reactions during the ROcP of LA and CL, as depicted in Scheme 2.63 Here, the “second mode of transesterification reactions”, i.e. giving rise to the anomalous CL-L-CL sequences (L representing one lactoyl unit) at 170.8 ppm, was barely observed. As the dispersity of the resulting copolymers remained low (1.11
