Lipase-catalyzed syntheses of linear and hyperbranched polyesters using compressed fluids as solvent media
Graphical abstract
Research highlights
Lipase mediated synthesis of poly(δ-valerolactone) in compressed fluid media is achieved in relatively high molecular weights. ▶ Liquid 1,1,1,2-tetrafluoroethane enhances solubility of polar substrates with sustained lipase activity. ▶ Hyperbranched polyesters can be achieved using liquid 1,1,1,2-tetrafluoroethane.
Introduction
The development of biocompatible and biodegradable macromolecules with defined structures and properties aiming at food or biomedical applications leads to advanced polymer synthesis throughout non-toxic methodologies [1], [2], [3]. Enzymatic ring-opening polymerizations (eROP) of small and medium-sized (4-, 6-, and 7-membered), as well as large-sized (12- to 16-membered) lactones, macrolides and cyclic oligomers proved successful in low polar and hydrophobic solvents [4], [5], [6], [7], [8], [9]. Among them, eROP of the seven-member ring lactone, epsilon-caprolactone (CL) is widely reported using Candida antartica lipase B (CALB) biocatalyst. The linear polyester from the six-member non-substituted lactone, δ-valerolactone (VL), is also an interesting material but still relatively unexplored compared to its parent poly-ɛ-caprolactone (PCL). eROP of VL is reported in bulk and in common hydrophobic organic solvents with long reaction times (up to 10 days) mediated by lipase from Pseudomona fluorescens (lipase PF), lipase from Candida cylindracea (lipase CC), porcine pancreatic lipase (PPL), Rhizopus japonicus lipase (lipase RJ) and also by CALB to attain in all cases low molecular weight poly-δ-valerolactone (PVL) (<2000 g/mol) [5], [10], [11].
On the other hand, eROP to afford hyperbranched polyesters is a challenging field of research. The interest in hyperbranched structures arises from their compactness and their enhanced segment density and functionality [12], [13]. Properties of hyperbranched polymers are often affected by the nature of the backbone and the chain end functional groups, degree of branching (DB) or molecular weight distributions. The reported procedures for the synthesis of hyperbranched polyesters to date require temperatures exceeding 150 °C and the use of toxic catalysts or initiators [14]. There are several reports on enzyme-mediated polyesters with achieved branched structures in bulk reactions [15], [16]; however, reports on enzymatic synthesis of hyperbranched polyesters in the open literature are restricted to the work of Frey and co-workers [17]. These authors described an enzymatic method for the preparation of a hyperbranched polyester by co-polymerization of CL with 2,2-bis(hydroxymethyl)butyric acid (BHB) as AB2 core [17]. The main drawback of the reaction was the low solubility of the core molecule in low polar toluene. Addition of 1,4-dioxane, which is classified by the IARC as a Group 2B carcinogen, in order to increase substrate solubility had to be kept to minimum as it deactivates the lipase. Besides, enzymatic synthesis of a hyperbranched PVL had yet not been reported. Many polar polymers, such as hyperbranched polyester structures, or some of their monomer precursors present low solubility in the common hydrophobic solvents used in lipase-catalyzed polyester syntheses. Alternatively, we aim at reporting the use of the polar compressed 1,1,1,2-tetrafluoroethane (R-134a) as solvent to achieve novel polyester structures by enzymatic means. R-134a has nowadays extended use as refrigerant or propellant and its liquid state is easily reached at moderate pressures (<10 bar, at 25 °C) [18]. Compressed R-134a is regarded as green solvent since it is inert, non-toxic and non-flammable, it has no ozone depletion potential (ODP) and it is classified as generally recognized as safe (GRAS). R-134a is also manufactured to current good manufacturing practice (cGMP) standards for use in metered-dose inhaler applications in the pharmaceutical industry or other medical applications [19]. Further advantage relies on its polarity, which is comparable to tetrahydrofuran and dichloromethane, in which lipases present poor enzymatic activities. Instead, liquid R-134a is hydrophobic, thus allowing for the retention of water molecules on the active site of lipases to sustain activity [20], [21], [22]. Generally, compressed fluids technologies allow the complete re-use of the solvent media by recirculation in closed-loop systems, thus having the potential to be environmentally benign and economical alternative to VOCs.
Section snippets
General considerations
VL and CL were distilled under vacuum (30 mmHg, 90 °C) prior to use over CaH2 and stored over 3 Å molecular sieves at 5 °C. BHB (Aldrich, US) was used as received. The enzyme catalyst Novozym 435 was a kind gift from Novozymes (Mexico) and consisted of ∼10% by wt of CALB supported on an acrylic resin with a specific activity of 7000 PLU/g. Tetrahydrofuran spectrophotometric grade was purchase from J.T. Baker (Mexico). Carbon dioxide (research grade 99.8%) was provided by Infra-Air products (México)
eROP of VL in liquid R-134a and in scCO2
The relatively high ring-strain of the 7-member CL enhances de production of higher molecular weight polyesters by eROP (40,000–80,000 g/mol) than the six-member ring VL, which lipase-mediated polymerization has only been reported in low propagation (<2000 g/mol) using conventional hydrophobic solvents [5], [10], [11], [22]. However, PVL can also be achieved in relatively high molecular weight by eROP using liquid R-134a or scCO2 as solvent media and CALB biocatalyst (Fig. 1). The results on the
Conclusions
Lipase-mediated synthesis of relatively high molecular weight PVL is reported in supercritical carbon dioxide and liquid R-134a solvent media. scCO2 might be useful for green polymer synthesis whenever possible based on its low cost and availability, as well as good physical properties. However, new experimental evidences shown in this work demonstrate that the selected monomer substrates to achieve the hyperbranched polyesters present low solubility in compressed CO2 and good solubility in
Acknowledgments
We would like to thank PAPIIT IN200109 and CONACyT #48641 for financial support and scholarship (ALL). We also thank the valuable help provided by USAI (FQ, UNAM), Gerardo Cedillo and Miguel A. Canseco from IIM (UNAM) and Karla Grisel Calderón at the INMEGEN.
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Enzymatic reactions in subcritical and supercritical fluids
2018, Journal of Supercritical FluidsCitation Excerpt :Successively, Randolph et al. [12] examined the effect of aggregation of cholesterol on cholesterol oxidase activity and found that the addition of cosolvents (entrainers) promotes aggregation and thus resulted in an increase in the reaction rate proportional to the degree of aggregation. Since then several publications on oxidation [16–18], hydrolysis [19–38], transesterification [39,40], esterification [39,41–57], transesterification for synthesis of biofuels [40,55,57–63], enzyme synthesis of polymers [64–70] and enantioselective synthesis [20,28,35,46,71–76] have proved the feasibility of enzymatic reactions in SCFs. As one can realize from the above literature reviews, the main advantage of biocatalysis in SCFs is the tunability of solvent properties by variation of the main process parameters pressure and/or the temperature.
The phase behavior in supercritical carbon dioxide of hyperbranched copolymers with architectural variations
2016, Journal of Supercritical FluidsCitation Excerpt :The dynamic viscoelastic relaxation behavior and the molecular mobility for these polymers were investigated showing that the molecular mobility of three hyperbranched PCLs was higher than that of linear one, and was observed to enhance with decreasing lengths of oligo(ɛ-caprolactone) segments and increasing relative degree of branching (DB) [34]. Copolymers of bis-MPA with another cyclic ester—δ-valerolactone were obtained via lipase-mediated syntheses using compressed fluids: supercritical carbon dioxide or liquid 1,1,1,2-tetrafluoroethane [35]. 2,2-Bis(hydroxymethyl)propionic acid, glycolide, lactide, δ-valerolactone, ɛ-caprolactone, ω-hydroxydodecanoic acid, trimethylolpropane and trifluoroacetic anhydride were purchased from Aldrich Chemical (Poznan, Poland) and used as received.
Enzymatic syntheses of linear and hyperbranched poly-L-lactide using compressed R134a–ionic liquid media
2015, Journal of Supercritical FluidsCitation Excerpt :However, the relatively high polarity and low solubility of cyclic LA monomers discourage this approach [2,3]. In order to overcome this matter, alternative routes for PLA synthesis by eROP have been reported using ionic liquid (IL) media and compressed fluids [4–9]. ILs present negligible vapor pressures, which in addition to the interaction of the IL environment with the enzyme active site are responsible for the preserved activities of lipases even at temperatures near 373.15 K [10,11].
Phase behaviour of pseudo-binary systems of pressurized ((propane + l,l-lactide)) at different ethanol to l,l-lactide mole ratios
2014, Journal of Chemical ThermodynamicsEnzymatic synthesis of poly-l-lactide in supercritical R134a
2012, Journal of Supercritical FluidsCitation Excerpt :Industrial preparation of PLAs nowadays involves high bulk reaction temperatures (>150 °C) and organometallic catalysts by ring-opening polymerization of the cyclic dimers (i.e., the l-lactide (LLA)), therefore, post-reaction purifications are usually required to remove potentially toxic residues for practical utilization [1,2]. Alternatively, milder and less toxic syntheses via enzymatic ring-opening polymerization (eROP) have been extensively studied in common hydrophobic organic solvents [3] and as well, owing to the good preservation of enzymatic activities above atmospheric pressure, in compressed fluid (CF) media [4–8]. In this regard, Candida antarctica lipase B (CALB) immobilized on spherical polymeric supports in solvents such as hexane or toluene [3], scCO2 [4,5] and liquid 1,1,1,2-tetrafluoroethane (R134a) [6–8], have been mostly employed for successful eROP toward polyester-type products.