Biocatalytic Synthesis of Epoxy Resins from Fatty Acids as a Versatile

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Biocatalytic synthesis of epoxy resins from fatty acids as a versatile route for the formation of polymer thermosets with tunable properties Susana Torron, Stefan Semlitsch, Mats Martinelle, and Mats Johansson Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.6b01383 • Publication Date (Web): 03 Nov 2016 Downloaded from http://pubs.acs.org on November 6, 2016

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Biocatalytic synthesis of epoxy resins from fatty acids as a versatile route for the formation of polymer thermosets with tunable properties Susana Torron,a Stefan Semlitsch,b Mats Martinelle,b Mats Johansson a* a.

KTH Royal Institute of Technology, Department of Fibre and Polymer Technology,

Division of Coating Technology, SE-100 44 Stockholm, Sweden. Email: [email protected]. b.

KTH Royal Institute of Technology, School of Biotechnology, Division of Industrial

Biotechnology, SE-106 91 Stockholm, Sweden KEYWORDS. Lipase catalysis, fatty acids, biobased, vegetable oils, epoxy resin, cationic polymerization, polymer thermoset

ABSTRACT. The work herein presented describes the synthesis and polymerization of series of biobased epoxy resins prepared through lipase catalyzed transesterification. The epoxyfunctional polyester resins with various architectures (linear, tri-branched and tetra-branched) were synthesized through condensation of fatty acids derived from epoxidized soybean oil and linseed oil with 3 different hydroxyl cores under bulk conditions. The selectivity of the lipases towards esterification/ transesterification reactions allowed the formation of macromers with up to 12 epoxides in the backbone. The high degree of functionality of the resins resulted in polymer thermosets with Tg values ranging from -25 to over 100oC prepared through cationic polymerization. The determining parameters of the synthesis and the mechanism for the formation of the species were determined through kinetic studies by 1H NMR, SEC and molecular modelling studies. The correlation between macromer structure and thermoset properties was studied through real time-FTIR measurements, DSC and DMA.

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INTRODUCTION Concerns regarding the environmental footprint of synthetic polymers in commodity plastics have driven research towards the development of renewable and biodegradable alternatives. Current strategies towards this purpose involve the utilization of bio-based feedstock, e.g. algae, sugars, terpenes or vegetable oils1, more benign synthetic schemes and controlled degradative routes. Vegetable oils (VO) are abundant and inexpensive and therefore one of the most employed renewable resources in the chemical industry. VO are mainly composed by triglycerides with varying length and functionalities on the fatty acids for the different species. Those intrinsic functionalities, in their majority unsaturations, provides an attractive and feasible source for the formation of bio-based materials. The number of unsaturations, as well as their positions within the aliphatic chain, affects strongly the physical and chemical properties of vegetable oils and the materials derived from them. In their inherent form, vegetable oils are commonly used as lubricants or plasticizers. However, their major industrial exploitation is based on chemical modifications of the double bonds, e.g. epoxidation. Epoxidized vegetable oils (EVO) have proven excellent for the formation of polyurethanes2, polycarbonates3 and polyamides4 through reaction of the epoxides and a co-reactant. Furthermore, the excellent thermal and mechanical properties, high adhesive strength and chemical resistance derived from them, encourage the utilization of EVO for epoxy resin purposes5. Nowadays, the production of epoxy resins account around 70% of the production of thermoset materials, which in turn accounts 20% of the total plastic production. This production is mainly from fossil fuel resources, being the highest production derivatives from bisphenol A


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(BPA). The controversial utilization of BPA motivates a global interest for finding more benign alternatives by using bio-based monomers. One of the main challenges when aiming to compete with petroleum-based epoxy polymers is to obtain stiff and strong materials, i.e. high glasstransition temperatures (Tg) and high modulus (E’), due to the lack of aromatic and cycloaliphatic components in fatty acids and the plasticizing effect of their long carbon chains. This issue was addressed by Zhou and coworkers who prepared epoxy resins containing a cycloaliphatic moiety by Diels-Alder coupling and their subsequent curing with an anhydride6. Matharu et al. shared this approach and prepared thermoset resins by crosslinking epoxidized linseed oil and a commercial diacid containing a hexane ring in their structure7. Webster and coworkers reported the epoxidization of commercial sucrose esters of fatty acids8 and their crosslink with anhydrides9 and natural acids10 to yield polymer thermosets with Tg’s up to 103oC and 96oC, respectively. In all cases, the introduction of “bulky” components and the attempts for increasing the degree of functionality in the resin benefited the rigidity and stiffness of the thermosets. The formation of highly functional resins from functional monomers requires orthogonal synthetic routes in order to preserve unaltered the initial intrinsic functionalities. Our group recently proved the versatility of lipase catalysis for the formation of multifunctional resins from epoxy-derived fatty acids11. However, one of the issues when aiming for highly functional polymers by lipase catalysis is the poor diffusion of the reactants to the active site of the enzyme when the molecular weight is increased as result of an increasing chain length. One way to address this limitation is to aim for branched architectures, as the viscosity will diminish with an increase in the degree of branching12. However, most examples found in literature dealing with branched structures synthesized by lipase catalysis focusses on the acylation of tri-ols, e.g.


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trimethylolpropane13. In order to understand the possibilities and limitations of lipase catalysis as a tool for the formation of branched epoxy resins from vegetable oil derived fatty acids are herein addressed. By combining specific epoxy-functional fatty acids obtained from epoxidized linseed oil and epoxidized soy bean oil and different hydroxyl-cores, resin structures with architectures ranging from linear to tri- and tetra- branched macromers and up to 12 functional groups have been prepared (see Figure 1). Control in the final structure of the resins leads to polymer thermosets with defined and tunable Tg values when the macromers are polymerized cationically. The high crosslink density possible to achieve through this route leads to polymer thermosets with Tg’s greater than 100oC proving the versatility and enormous possibilities of the method.

Figure 1. Schematic representation of the reactants used in the synthesis (upper left side and right side) and products formed by the combination of those (center). NEOPG: Neopentyl glycol, TMP: Trimethylolpropane, Di-TMP: Di-Trimethylolpropane, MS: Methyl Stearate, EMO: Epoxidized Methyl Oleate, EMLO: Epoxidized Methyl Linoleate, EMLEN: Epoxidized Methyl Linolenate. The acronyms for the products indicates which starting materials have been used for their synthesis, e.g. NEOPG-MS was formed as a combination of Neopenthyl glycol (NEOPG) and Methyl Stearate (MS)

EXPERIMENTAL Materials


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All materials were purchased from Sigma Aldrich and used as received unless noticed. Samples of epoxidized linseed oil and epoxidized soybean oil were kindly supplied by Ackross chemicals. Novozyme 435 (Candida antarctica lipase B immobilized on an acrylic carrier. Activity ≥ 5000 U/g, recombinant, expressed in Aspergillus niger) was purchased from SigmaAldrich. Neopentyl glycol (NEOPG), trimethylolpropane (TMP) and di-Trimethylolpropane were kindly donated by Perstorp AB, Sweden. The “photoiniator” Uvacure 1600 was supplied by CYTEC. Silica gel for purification steps was purchased from ICN SiliTech (ICN Biomedicals GmbH, Eschwege, Germany). Deuterated solvents (CDCl3) were provided by CIL (Cambridge Isotope Laboratories, USA). Instrumentation Nuclear Magnetic resonance (NMR). 1H NMR spectra were recorded on a Bruker AM 400 MHz using deuterated chloroform (CDCl3) as solvent. The solvent signal was used as reference. Matrix-Assisted Laser Desorption Ionization Time of Flight Mass Spectrometry (MALDI-TOF MS). Spectrum acquisitions were conducted on a Bruker UltraFlex MALDI-TOF MS with SCOUT-MTP Ion Source (Bruker Daltonics,Bremen) equipped with a N2-laser (337nm), a gridless ion source and reflector design. All spectra were acquired using a reflectorpositive method with an acceleration voltage of 25 kV and a reflector voltage of 26,3 kV. The detector mass range was set to 200-2500 m/z. A THF solution of DHB (2,5-dihydroxybenzoic acid) was used as matrix. The obtained spectra were analyzed with FlexAnalysis broker Daltonics, Bremen, version 2. Size Exclusion Chromatography (SEC) using DMF (0.2 mL min-1) with 0.01M LiBr as the mobile phase was performed at 50oC using a TOSOH EcoSEC HLC-8320GPC system equipped


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with an EcoSEC RI detector and three columns (PSS PFG 5µm; microguard, 100Å, and 300Å) (MW resolving range: 100-300000 g mol-1) from PSS GmbH. A calibration method was created using narrow linear poly(methyl methacrylate) standards (MW range: 800-160 000 g mol-1). Corrections for the flow rate fluctuations were made using toluene as an internal standard. Fourier Transform Infrared Spectroscopy (FTIR) analysis was carried out using a PerkinElmer Spectrum 2000 FT-IR instrument (Norwalk, CT) equipped with a single reflection (ATR: attenuated total reflection) accessory unit (Golden Gate) from Graseby Specac LTD (Kent, England) and a TGS detector using the Golden Gate setup. Each spectrum collected was based on 16 scans averaged at 4.0 cm-1 resolution range of 4000-600 cm-1. Data were recorded and processed using the software Spectrum from Perkin-Elmer. Real Time-FTIR analysis (RTIR). The RTIR analysis were made using a Perkin-Elmer Spectrum 2000 FT-IR instrument (Norwalk, CT) equipped with a single reflection (ATR: attenuated total reflection) accessory unit (Golden Gate) from Graseby Specac LTD (Kent, England). RT-FTIR continuously recorded the chemical changes over the range 4000-600 cm-1. Spectroscopic data were collected at an optimized scanning rate of 1 scan per 1.67 seconds with a spectral resolution of 4.0 cm-1 using Time Base® software from Perkin-Elmer. UV-Light sources. The light source used for the photo-RTIR measurements was a Hamamatsu L5662 equipped with a standard medium-pressure 200-W L6722-01 Hg-Xe lamp and provided with optical fibers. The UV intensity was measured using a Hamamatsu UV-light power meter (model C6080-03) calibrated for the main emission line centered at 365nm. The intensity of the lamp was 18.1 mW/cm2.The light source used for the crosslinking of the films was a Black Ray B-100AP (100 W, λ=365 nm) Hg UV-lamp with an irradiance of ~21mW/cm2 as determined


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with an UVICURE Plus High Energy UV integrating radiometer (EIT, USA), measuring UVA at 320 ≤ λ ≤390 nm. Differential Scanning Calorimetry (DSC). A differential scanning calorimeter (Mettler Toledo DSC 820 module) was used to analyze the thermal properties of the materials. The samples (5-10 mg) were encapsulated in 40µL aluminum crucibles. The samples were submitted to a first heating (I) 25oC to 150oC, (II) cooled to -60oC and (III) heated to 150oC with cooling and heating rates of 10oC/min under a nitrogen atmosphere. Dynamical Mechanical Analysis (DMA). The study of the physical properties of the polymerized resins was performed on a Q800 DMTA (TA instruments), equipped with a film fixture for tensile testing. The measurements were done between -40°C and 180°C, with heating rate of 5°C/min. The tests were performed in controlled strain mode with a frequency of 1Hz, oscillating amplitude of 10µm and forcetrack of 125% on rectangular geometries. Computer modeling on CalB was based on the PDB file 1LBT using YASARA14 (Yet Another Scientific Artificial Reality Application) version 16.4.6. The PDB file was preprocessed by adding all missing hydrogens and removing the N-Acetyl-D-Glucosamine on Asn74 and removing the inhibitor from the active site. All the substrates were built separately and energy minimized, followed by 10 ps molecular dynamics (force field: AMBER9915) and another energy minimization. In order to bring the substrate into the active site of the enzyme a docking approach was used: the substrate was added to the scene and a long covalent bond between the catalytic serine and the substrate was formed to achieve the tetrahedral intermediate as a model of the transition state for the acylation of the last free alcohol group of the polyols. The enzyme was fixed and the structure was minimized which caused the substrate to move into the active


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site. After 10 ps molecular dynamics, the enzyme was set free and the whole complex was minimized. Molecular dynamics were run for 10 ns with an AMBER99 force field. Simulation snapshots were saves every 10 ps and the change in the enzyme structure (backbone α-carbons) was followed via RMSD data. Pictures were taken after 10 ns. Synthetic strategies General procedure for the extraction of methyl esters from natural oils. All four methyl esters were obtained by transesterification reaction of commercial samples of epoxidized linseed oil (ELO) or epoxidized soybean oil (ESO) following procedures previously reported in literature16. 20 g of ESO or ELO were put in a round bottom flask and dissolved in 250 mL of 0.2M NaOH in MeOH. The mixture was kept at reflux temperature for 1h. The formed fatty acids methyl esters were extracted with 250 mL heptane and water and the organic phase dried over MgSO4. The desiccant was then filter out and the solvent eliminated under reduced pressure. The crude was purified through column chromatography using silica gel as stationary phase and a gradient mixture of Heptane: Ethyl acetate. Methyl stearate, MS. *Yield: 9%; 1H NMR (400 MHz, CDCl3); δ: 3.66 (3H, s, -OCH3), 2.29 (2H, t, -CH2-COOCH3, J=7.4), 1.61 (2H, m, -CH2-CH2-COOCH3, J=7.4), 1.3-1.2 (28H, m, aliphatic –CH2-), 0.87 (3H, t, CH3-CH2-, J=7.1) Epoxidized methyl oleate, EMO. *Yield: 11%; 1H NMR (400 MHz, CDCl3); δ: 3.58 (3H, s, OCH3), 2.81 (2H, bs, -CH-O-CH-), 2.22 (2H, t, -CH2-COOCH3, J=7.5), 1.55 (2H, m, -CH2-CH2COOCH3, J=7.4), 1.48-1.11 (24H, m, aliphatic –CH2-), 0.80 (3H, t, CH3-CH2-, J=7.1)


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Epoxidized methyl linoleate, EMLO. *Yield: 29%; 1H NMR (400 MHz, CDCl3); δ: 3.66 (3H, s, -OCH3), 3.19-2.89 (4H, m, -CH-O-CH-), 2.30 (2H, t, -CH2-COOCH3, J=7.6), 1.85-1.24 (22H, m, aliphatic –CH2-), 0.90 (3H, t, CH3-CH2-, J=7.1) Epoxidized methyl linolenate, EMLEN. *Yield: 12%; 1H NMR (400 MHz, CDCl3); δ: 3.66 (3H, s, -OCH3 ), 3.28-2.86 ( 6H, m, -CH-O-CH-), 2.30 (2H, t, -CH2-COOCH3, J=7.6), 1.89-1.23 (18H, m, aliphatic –CH2-), 1.05 (3H, m, CH3-CH2-) *Due to the varying composition of ELO and ESO from “batch to batch” it was difficult to assess the yield of the extraction. However, a total distribution of each component based on masses can be calculated in every case. The total reaction yield after extraction and evaporation of the solvent is ~60%. All yields for each monomer were calculated as a mass percent of this yield. General procedure for the synthesis of macromonomers by enzyme catalysis. The 12 different macromonomers were synthesized following the same procedure. The different mixtures of methyl ester fatty acids and polyols were put in a round bottom flask (1.33 eq of methyl ester per OH group) and heated up to 110oC under stirring. Once the two components were dissolved, molecular sieves (25% w/w of the total amount of reactants) and CALB (10% w/w of the total amount of reactants) were added to the reaction mixture. The reaction vessel was kept open for around 10min. After that time the flask was connected to a vacuum line. The esterification reaction using NEOPG occurs within 30min, while the reaction with TMP and Di-TMP occurs within 24h. All epoxy-functional macromonomers were purified through column chromatography on silica gel using Heptane/ Ethyl acetate mixtures prior polymerization.


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Macromonomer from Neopentyl glycol and Epoxidized methyl oleate, NEOPG-EMO (1b). Yield: 40%; 1H NMR (400 MHz, CDCl3); δ: 3.88 (4H, s, -COO-CH2-C(CH3)2- ), 2.89 (4H, bs, CH-O-CH-), 2.31 (4H, t, -CH2-COOCH2-, J=7.6), 1.62 (4H, m, -CH2-CH2-COOCH2-, J=7.1), 1.55-1.11 (48H, m, aliphatic –CH2-), 0.96 (6H, s, -CH2-C(CH3)2-CH2-), 0.88 (6H, t, CH3-CH2-, J=7.0) Macromonomer from Neopentyl glycol and Epoxidized methyl linoleate, NEOPG-EMLO (1c). Yield: 40%; 1H NMR (400 MHz, CDCl3); δ: 3.85 (4H, s, -COO-CH2-C(CH3)2- ), 3.13-2.86 (8H, m, -CH-O-CH-), 2.29 (4H, t, -CH2-COOCH2-, J=7.5), 1.70 (4H, m, -CH2-CH2-COOCH2-), 1.59 (4H, m, -CH-O-CH-CH2-CH-O-CH-), 1.54-1.24 (36H, m, aliphatic –CH2-), 0.94 (6H, s, -CH2C(CH3)2-CH2-), 0.87 (6H, t, CH3-CH2-, J=7.0) Macromonomer from Neopentyl glycol and Epoxidized methyl linolenate, NEOPG-EMLEN (1d). Yield: 34%; 1H NMR (400 MHz, CDCl3); δ: 3.87 (4H, s, -COO-CH2-C(CH3)2- ), 3.25-2.85 (12H, m, -CH-O-CH-), 2.30 (4H, t, -CH2-COOCH2- , J=7.5), 1.86-1.22 (36H, m, aliphatic –CH2), 1.05 (6H, t, CH3-CH2-), 0.95 (6H, s, -CH2-C(CH3)2-CH2-) Macromonomer from Trimethylolpropane and Epoxidized methyl oleate, TMP-EMO (2b). Yield: 38%; 1H NMR (400 MHz, CDCl3); δ: 4.01 (6H, s, -CH2-COO-CH2- ), 2.89 (6H, bs, -CHO-CH-), 2.30 (6H, t, -CH2-COOCH2-, J=7.5), 1.61 (6H, m, -CH2-CH2-COOCH2-, J=7.0), 1.511.21 (74H, m, aliphatic –CH2-), 0.88 (12H, t, CH3-CH2-) Macromonomer from Trimethylolpropane and Epoxidized methyl linoleate, TMP-EMLO (2c). Yield: 44%; 1H NMR (400 MHz, CDCl3); δ: 4.01 (6H, s, -CH2-COO-CH2- ), 3.15-2.93 (12H, m, -CH-O-CH-), 2.30 (6H, t, -CH2-COOCH2-, J=7.5), 1.72 (6H, m, -CH2-CH2-COOCH2-), 1.60


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(6H, m, -CH-O-CH-CH2-CH-O-CH-), 1.57-1.23 (56H, m, aliphatic –CH2-), 0.88 (12H, t, CH3CH2-) Macromonomer from Trimethylolpropane and Epoxidized methyl linolenate, TMP-EMLEN (2d). Yield: 40%; 1H NMR (400 MHz, CDCl3); δ: 4.01 (6H, s, -CH2-COO-CH2- ), 3.25-2.88 (18H, m, -CH-O-CH-), 2.29 (6H, t, -CH2-COOCH2-, J=7.5), 1.88-1.23 (54H, m, aliphatic –CH2), 1.05 (9H, t, CH3-CH2-), 0.87 (3H, t, CH3-CH2-C-) Macromonomer from Di-Trimethylolpropane and Epoxidized methyl oleate, Di-TMP-EMO (3b). Yield: 20%; 1H NMR (400 MHz, CDCl3); δ: 3.98 (8H, s, -CH2-COO-CH2- ), 3.26 (4H, s, C-CH2-O-), 2.89 (8H, bs, -CH-O-CH-), 2.29 (8H, t, -CH2-COOCH2-, J=7.5), 1.65-1.20 (108H, m, aliphatic –CH2-), 0.92-0.80 (18H, 2t, CH3-CH2- and CH3-CH2-C-) Macromonomer from Di-Trimethylolpropane and Epoxidized methyl linoleate, Di-TMPEMLO (3c). Yield: 40%; 1H NMR (400 MHz, CDCl3); δ: 3.98 (8H, s, -CH2-COO-CH2- ), 3.26 (4H, s, -C-CH2-O-), 3.15-2.91 (16H, m, -CH-O-CH-), 2.29 (8H, t, -CH2-COOCH2-, J=7.5), 1.841.67 (8H, m, -CH2-CH2-COOCH2-), 1.60 (8H, m, -CH-O-CH-CH2-CH-O-CH-), 1.56-1.23 (76H, m, aliphatic –CH2-), 0.90 (12H, t, CH3-CH2-, J=6.5), 0.84 (6H, t, CH3-CH2-C-, J=7.5) Macromonomer from Di-Trimethylolpropane and Epoxidized methyl linolenate, Di-TMPEMLEN (3d). Yield: 34%; 1H NMR (400 MHz, CDCl3); δ: 4.0 (8H, s, -CH2-COO-CH2- ), 3.28 (4H, s, -C-CH2-O-), 3.25-2.92 (24H, m, -CH-O-CH-), 2.31 (8H, t, -CH2-COOCH2-, J=7.5), 1.911.26 (76H, m, aliphatic –CH2-), 1.08 (12H, t, CH3-CH2-, J=7.5), 0.86 (6H, t, CH3-CH2-C-, J=7.6) Kinetic studies. Samples of 10mg were extracted at different times for the kinetic study through 1H NMR and SEC. For the macromers formed from NEOPG, samples were taken after


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5, 10, 15, 20, 25 and 30 minutes. For the macromers from TMP and Di-TMP after 1, 3, 5, 7 and 24 hours. General procedure for thermoset formation. Mixtures containing 25mg of macromonomer, 2% (w/w) of photoinitiator (Uvacure 1600) and 400mg of chloroform were casted on a glass substrate and the solvent was allowed to evaporate completely prior exposure to the UV-lamp. RESULTS AND DISCUSSION Synthesis of epoxy resins through enzyme catalysis Various studies proving the difficulties for methanolysis of triglycerides with Candida antarctica lipase B (CALB) due to e.g. glycerol inhibition17 led us to the preparation of fatty acid methyl esters through alkaline methanolysis of either epoxidized linseed oil or epoxidized soybean oil. Condensation reactions between the four different fatty acids and the three different cores were performed using 1.33 eq fatty acid per OH group with Novozyme 435 (Candida antarctica lipase B immobilized on an acrylic carrier (CALB)) as a catalyst. All reactions yielded branched macromers with over 80% conversion, confirmed with 1H-NMR. Kinetic studies by 1

H-NMR and SEC revealed a “step-wise” type mechanism, where each OH group is reacted

subsequently, with reaction times of NEOPG, TMP and Di-TMP within 30 min, 7h and 24h respectively. As confirmed by 1H-NMR and Maldi-TOF, the epoxy groups remain unaltered during the whole process (see Figures S5-S20 of the SI). The distribution of products within the mixture, i.e. mono-ester, di-ester, tri-ester or tetra-ester, was assessed through a Gaussian fitting of the SEC curves at every time point (Figures S36-S47).

Control experiments without enzyme

showed no esterification reaction taking place. The reaction kinetics evidenced an initial reaction


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rate dependent on the miscibility between components. Differences in polarity resulting from structural variations between reactants determine the degree of homogeneity of the mixture and thus the diffusion to the active site of the enzyme. The dependency of the initial reaction rate with the solubility was evaluated as the conversion of OH groups to esters at the beginning of the reaction. The initial premise was that for either of the reactants, the polarity increases with an increasing oxygen content, i.e. OH groups and epoxides. However, as summarized in Table 1, the initial reaction conversions presented inverted trends for the macromers using NEOPG or DiTMP as core and for the ones using TMP. According to Van Krevelen’s group contribution theory for the prediction of solubility parameters18, when polar groups are present on either side of one or more symmetrical planes the perceived polarity is reduced. This observation is coherent with the results obtained. The presence of two symmetrical planes in Di-TMP and one in NEOPG reduces the perceived polarity and thus the miscibility with the fatty acids containing lower number of epoxides is favored. Hence for the macromers synthesized from TMP, the reaction with the fatty acid of higher polarity, i.e. tri-epoxidized, was preferred. Table 1. Conversion of macromers at the beggining of the reaction calculated from 1H NMR spectra. %Conversion = ((Integral OH core peak) / (Integral OH core peak + Integral formed ester)) x 100 % Conversion at the beginning of the reaction (5 min)

% Conversion at the beginning of the reaction (1h)

% Conversion at the beginning of the reaction (1h)

NEOPG-MS

45

TMP-MS

12

Di-TMP-MS

28

NEOPG-EMO

43

TMP-EMO

23

Di-TMP-EMO

17

NEOPG-EMLO

35

TMP-EMLO

57

Di-TMP-EMLO

11

NEOPG-EMLEN

34

TMP-EMLEN

43

Di-TMP-EMLEN

3

As shown in Figure 2, whilst the fully-substituted species formed from NEOPG and TMP are formed at an early reaction stage, several hours (5h) are needed for the formation of the tetraester from di-TMP. The large amount of tri-ester formed before the tetra-ester starts to be built


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indicates the difficulties for its formation, most probably due to steric effects. In order to confirm whether those steric effects are caused due to the size of the hydroxyl-cores, the degree of branching or the number of epoxides, molecular dynamic studies were performed.

Figure 2. Superposed conversion from kinetic study through 1H NMR and distribution of products calculated from kinetic study through SEC

Molecular modelling of the synthesis of epoxy resins through enzyme catalysis The acylation of the polyols (NEOPG, TMP and Di-TMP) with epoxidized methyl linolenate (EMLEN) was modeled by building the second tetrahedral intermediate of the reaction, where the last OH-group of a partly acylated polyol is making a nucleophilic attack on the acyl-enzyme (see Figure 3). For all molecular dynamics simulations the important hydrogen bonds in the tetrahedral intermediate necessary for the catalytic activity of CalB19 were kept intact for all combinations of polyols and fatty acids. The RMSD of enzyme backbone (α-carbons) were also analyzed during the simulations of the acylation of NEOPG, TMP or Di-TMP, respectively and no significant differences were found. This shows that large polyols such as TMP and Di-TMP can be accommodated in the active site without large rearrangements in the enzyme structure.


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The simulations revealed the binding position of the different substrates inside the active site. Figure 3 shows that the polyol (Di-TMP) is entirely placed in the deep and narrow active site. This observation is coherent with the experimental data obtained from the kinetic study of the formation of tetra-esters from Di-TMP. The steric effects inside the cavity restrict the rate of formation of the fourth ester bond. The fatty acid comprising the acyl-enzyme binds up to 10 carbon atoms in the active site while the epoxide groups are placed outside or on the border of the cavity. Only the first epoxide of the fatty acid that acylates the enzyme binds inside the active site when the reaction takes place. However, the fatty acids on the acylated polyols are situated outside the active site and do not position any of their epoxides in the active site. This shows that differences in activity between the different fatty acids only to a minor extent could be explained with steric hindrance. However, as demonstrated experimentally, the number of epoxides in the fatty acids influence remarkably in the polarity of the mixture, affecting the miscibility of substrates and thus the enzyme activity (See Figures S48-S51 of the SI).

Figure 3. Snapshot from the molecular dynamics simulations of the tetrahedral intermediate of the acylation of Di-TMP (yellow) with epoxidized methyl linolenate (dark blue) catalysed by CalB (left side) and the schematic representation of the same (right side). The oxygen atoms are found in red. The rest of acyl chains are shown in different colors.

Thermoset formation through cationic polymerization In order to study the repercussion of the structural differences on the final material properties, all macromers were homopolymerized through cationic polymerization. The chosen


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photoinitiator, consisting on a diarylidonium salt20 (Uvacure 1600) triggers the formation of an oxiranium ion that subsequently initiates the ring opening of the epoxides. The change in structure produced when the strained cyclic ethers are transformed to their linear analogs can easily be followed spectroscopically through Fourier Transform Infrared Spectroscopy (FTIR) as the two types of ethers present differentiated peaks (νstr C-O: 1075 cm-1; νasym -C-O-C- ring deformation: 824 cm-1). The extent of the polymerization was assessed through real time-FTIR measurements performed while irradiating the samples at room temperature.

Figure 4. Final conversion (after 30 min) of the thermosets crosslinked through cationic polymerization (only UV-light). %Conversion (1075 cm-1 peak) = (At - A(t=0)) / (AF - A(t=0))

From the data obtained through the experiments was possible to calculate the conversion of the reaction at every time point (See Figures S52-S60 of the SI). It was observed that for most of the epoxy-functional macromers the conversion of cyclic to aliphatic ethers reaches a plateau before reaching full conversion (Figure S61). As can be seen in Figure 4, the final conversion of the photo-polymerization exhibits different trends for the different macromer architectures; whilst for the linear macromers (core: NEOPG) the reaction conversion increases with increasing number of epoxides, when branched architectures are used an inverted trend is observed. The increasing degree of branching and functionality causes restrictions in chain mobility when the


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network starts to form and thus hinders all epoxides of reacting. However, when the sample is heated small increases in molecular mobility allow the latent photo-initiator to trigger again the polymerization and thus reach full conversion without UV-light needed since no termination reactions occur in cationic polymerization with diarylidonium salts21. This well-known phenomenon, known as vitrification is produced due to the curing of the samples at temperatures under their glass transition temperature, Tg. The last was evidenced through DSC thermograms; while the first heating curve of the UV-cured samples (from 25oC to 150oC) is govern by an exotherm around 100oC as result of the curing reactions taking place, no exotherms are observed during the second heating (from -60oC to 150oC). Structure to properties relationship of polymer thermosets The main distinctive property of polymer thermosets with respect to other types of polymer networks, e.g. thermoplastics, is the curing process they are subjected to. By covalently attaching different branching points in the macromer or polymer, the so called crosslinking process leads to an irreversible network. The crosslink density and homogeneity of the thermoset will, in this terms, determine the final characteristics of the material. Other structural features as chain rigidity, secondary forces between chains, and the presence of non-covalently linked chain ends, will also have a determining effect on the final mechanical properties, commonly evaluated through the Tg. Structural differences arising from the different number of epoxides in the fatty acids, e.g. the length of the carbon chain outside the last epoxide, will, in this context, be determining factors in the final material properties. It can be foreseen that these “loose” ends will act plasticizing in a final thermoset network with increased softening effect with increased length.


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Figure 5. Tg of the cured thermosets determined at the second heating curve in the DSC thermograms as a function of the number of epoxides

It was observed that increasing number of epoxides in the macromer, led to the formation of polymer thermosets with higher Tg values (Figure 5). This can be ascribed to the increasing crosslink density resulting from higher functionality, which causes decrease in chain mobility and hence, increase in the Tg. Furthermore, the linear dependency between the Tg of the thermosets and the number of epoxides in the macromer (Figure 5), reveals a strong correlation between the mechanical properties and the number of crosslinking sites, i.e. epoxides. However, it should also be noticed that the distribution of the epoxides within each macromer also contributed to a variation in the Tg; whilst the NEOPG-EMLO macromer (4 epoxides distributed in 2 arms) exhibit a Tg of 45oC, the Di-TMP-EMO macromer (4 epoxides distributed in 4 arms) exhibited a Tg of 27oC. The same behavior was observed for the 2 macromers with 6 epoxides (NEOPG-EMLEN, Tg: 93oC and TMP-EMLO, Tg: 52oC) (see Table 2 and Figure S64). As aforementioned, the variations in the length of the carbon chain outside the epoxide going from 8, to 5, and 2 carbons in EMO, EMLO, and EMLEN, respectively, strongly affects the final mechanical properties, i.e. longer chains have more softening effect. At the same time, the differences in macromere architecture will also affect the properties of the networks, as


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increasing number of branches (for the same number of epoxides) will result in a network with higher mobility and thus lower Tg. The structural differences airing from the varying epoxy-content were also evidenced when compared networks with same architecture but different number of epoxides. As can be observed in Figure 6, for the macromers formed using TMP as core, increasing number of epoxides resulted in a decrease in Tan δ and broader Tg transitions. This behavior is a result of the increasing crosslink density which reduces the free volume by bringing adjacent chains close together and so raise the temperature of the glass transition. The transition region is greatly broadened, so that in very highly cross-linked materials, where motions of extensive segments of the main chain are not possible, there is no distinctive glass transition22. The decrease in the height of the Tan δ peak is, in the same terms, result from the increase of the storage modulus of the networks with increasing crosslink density.

Figure 6. DMA curves of the cured thermosets using TMP as hydroxyl-core

A summary of the Tg values can be found in Table 2. As can be noticed, no Tg values from DMA are reported for NEOPG-EMO and Di-TMP-EMLEN. The low degree of functionality of NEOPG-EMO did not allow to form polymer networks and therefore no measurements could be


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performed. On the contrary, the high concentration of functional groups in Di-TMP-EMLEN caused spontaneous microgelation as the functional groups started to react with themselves. In this case, the high heterogeneity of the sample did not allow DMA measurements to be performed. Table 2. Summary of the thermo-mechanical properties of the thermosets in increasing order of number of epoxides Nr epoxides

Nr branches

Tg DSC [oC][a]

Tg DMA [oC][b]

Tan δ

NEOPG-EMO

2

2

-25

n.d.

n.d.

TMP-EMO

3

3

-0,5

18

0,42

NEOPG-EMLO

4

2

12,5

45

0,36

Di-TMP-EMO

4

4

4,5

27

0,51

NEOPG-EMLEN

6

2

59,5

93

0,16

TMP-EMLO

6

3

39,5

52

0,26

Di-TMP-EMLO

8

4

41,5

70

0,34

TMP-EMLEN

9

3

92

115

0,13

Di-TMP-EMLEN

12

4

>100

n.d.

n.d.

Sample

CONCLUSIONS The use of lipase catalysis for the synthesis of epoxy resins from fatty acids with varying degree of branching and functionalities is herein reported. The limiting parameters governing each reaction between the three hydroxyl cores and the four fatty acids used were scrutinized through kinetic studies by 1H NMR and SEC. These studies together with molecular modelling, help to elucidate reaction rates strongly dependent on the polarity of each component and the steric effects inside the active site of the enzyme. In order to get a better insight of how the structure of the macromers affect the final material properties the nine macromers containing epoxy-functionalities were homo-polymerized through


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cationic polymerization. The photo-polymerization process was followed through Real timeFTIR to prove different trends in the conversion to aliphatic ethers dependent on the structure of the macromer. In order to obtain fully cured thermosets was necessary to heat the samples to induce molecular relaxation. The high monodispersity of the resins led to the formation of polymer thermosets with defined Tg values. Those Tg values exhibited a linear dependence with the number of epoxides and so thermosets with Tg’s ranging from -25oC to over 100oC could be obtained. The broad range of Tg values obtained creates a “toolbox” for the formation of bio-based polymeric materials with customized properties that would enable new fields of applications to be targeted in terms of lipase catalysis and fatty acids. Although the purpose of this work was to study fully cured networks, the possibility of obtaining flexible and transparent “free-standing” films with unreacted epoxides by UV-curing opens up a broad range of possibilities and applications, e.g. composites. ASSOCIATED CONTENT Supporting Information. Characterization of all compounds (1H NMR and MALDI) together with the 1H NMR spectra and SEC traces from the kinetic studies for the synthesis of the same; conversions calculated from the kinetic study; snapshots from the molecular modelling; FTIR spectra from the kinetic study of the polymerization and characterization of the networks (DSC and DMA) as well as calculated conversions. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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*Mats Johansson. Email: [email protected] ACKNOWLEDGMENT The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme FP7/2007 2013/ under REA grants agreement no [289253] and Lantmännen Research Foundation. REFERENCES 1. (a) Belgacem, M. N.; Gandini, A.; Editors, Monomers, Polymers and Composites from Renewable Resources. Elsevier Ltd.: 2008; (b) Biermann, U.; Bornscheuer, U.; Meier, M. A. R.; Metzger, J. O.; Schäfer, H. J., Oils and Fats as Renewable Raw Materials in Chemistry. Angewandte Chemie International Edition 2011, 50 (17), 3854-3871; (c) Roesle, P.; Stempfle, F.; Hess, S. K.; Zimmerer, J.; Rio Bartulos, C.; Lepetit, B.; Eckert, A.; Kroth, P. G.; Mecking, S., Synthetic Polyester from Algae Oil. Angew. Chem., Int. Ed. 2014, 53 (26), 6800-6804. 2. (a) Petrovic, Z. S., Polyurethanes from Vegetable Oils. Polym. Rev. (Philadelphia, PA, U. S.) 2008, 48 (1), 109-155; (b) Pfister, D. P.; Xia, Y.; Larock, R. C., Recent Advances in Vegetable Oil-Based Polyurethanes. ChemSusChem 2011, 4 (6), 703-717. 3. (a) Poussard, L.; Mariage, J.; Grignard, B.; Detrembleur, C.; Jérôme, C.; Calberg, C.; Heinrichs, B.; De Winter, J.; Gerbaux, P.; Raquez, J. M.; Bonnaud, L.; Dubois, P., NonIsocyanate Polyurethanes from Carbonated Soybean Oil Using Monomeric or Oligomeric Diamines To Achieve Thermosets or Thermoplastics. Macromolecules 2016, 49 (6), 2162-2171; (b) Boyer, A.; Cloutet, E.; Tassaing, T.; Gadenne, B.; Alfos, C.; Cramail, H., Solubility in CO2 and carbonation studies of epoxidized fatty acid diesters: towards novel precursors for polyurethane synthesis. Green Chem. 2010, 12 (12), 2205-2213. 4. (a) Mutlu, H.; Meier, M. A. R., Unsaturated PA X,20 from Renewable Resources via Metathesis and Catalytic Amidation. Macromol. Chem. Phys. 2009, 210 (12), 1019-1025; (b) Pardal, F.; Salhi, S.; Rousseau, B.; Tessier, M.; Claude, S.; Fradet, A., Unsaturated polyamides from bio-based Z-octadec-9-enedioic acid. Macromol. Chem. Phys. 2008, 209 (1), 64-74. 5. (a) Auvergne, R.; Caillol, S.; David, G.; Boutevin, B.; Pascault, J.-P., Biobased Thermosetting Epoxy: Present and Future. Chemical Reviews 2014, 114 (2), 1082-1115; (b) Xia, Y.; Larock, R. C., Vegetable oil-based polymeric materials: synthesis, properties, and applications. Green Chem. 2010, 12 (11), 1893-1909; (c) Wang, R.; Schuman, T., Towards green: a review of recent developments in bio-renewable epoxy resins from vegetable oils. RSC Green Chem. Ser. 2015, 29 (Green Materials from Plant Oils), 202-241. 6. Huang, K.; Zhang, P.; Zhang, J.; Li, S.; Li, M.; Xia, J.; Zhou, Y., Preparation of biobased epoxies using tung oil fatty acid-derived C21 diacid and C22 triacid and study of epoxy properties. Green Chem. 2013, 15 (9), 2466-2475. 7. Supanchaiyamat, N.; Shuttleworth, P. S.; Hunt, A. J.; Clark, J. H.; Matharu, A. S., Thermosetting resin based on epoxidized linseed oil and bio-derived crosslinker. Green Chem. 2012, 14 (6), 1759-1765.


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8. Pan, X.; Sengupta, P.; Webster, D. C., Novel biobased epoxy compounds: epoxidized sucrose esters of fatty acids. Green Chem. 2011, 13 (4), 965-975. 9. Pan, X.; Sengupta, P.; Webster, D. C., High Biobased Content Epoxy–Anhydride Thermosets from Epoxidized Sucrose Esters of Fatty Acids. Biomacromolecules 2011, 12 (6), 2416-2428. 10. Ma, S.; Webster, D. C., Naturally Occurring Acids as Cross-Linkers To Yield VOC-Free, High-Performance, Fully Bio-Based, Degradable Thermosets. Macromolecules 2015, 48 (19), 7127-7137. 11. Semlitsch, S.; Torron, S.; Johansson, M.; Martinelle, M., Enzymatic catalysis as a versatile tool for the synthesis of multifunctional, bio-based oligoester resins. Green Chemistry 2016, 18 (7), 1923-1929. 12. Johansson, M.; Trollsaas, M.; Hult, A., Synthesis, characterization, and curing of welldefined allyl ether-maleate functional ester oligomers: linear versus nonlinear structures. J. Polym. Sci., Part A: Polym. Chem. 1992, 30 (10), 2203-2210. 13. (a) Kulshrestha, A. S.; Gao, W.; Fu, H.; Gross, R. A., Synthesis and Characterization of Branched Polymers from Lipase-Catalyzed Trimethylolpropane Copolymerizations. Biomacromolecules 2007, 8 (6), 1794-1801; (b) Aakerman, C. O.; Gaber, Y.; Abd Ghani, N.; Laemsae, M.; Hatti-Kaul, R., Clean synthesis of biolubricants for low temperature applications using heterogeneous catalysts. J. Mol. Catal. B: Enzym. 2011, 72 (3-4), 263-269; (c) Uosukainen, E.; Linko, Y.-Y.; Lamsa, M.; Tervakangas, T.; Linko, P., Transesterification of trimethylolpropane and rapeseed oil methyl ester to environmentally acceptable lubricants. J. Am. Oil Chem. Soc. 1998, 75 (11), 1557-1563. 14. Krieger, E.; Darden, T.; Nabuurs, S. B.; Finkelstein, A.; Vriend, G., Making optimal use of empirical energy functions: Force-field parameterization in crystal space. Proteins: Struct., Funct., Bioinf. 2004, 57 (4), 678-683. 15. Wang, J.; Cieplak, P.; Kollman, P. A., How well does a restrained electrostatic potential (RESP) model perform in calculating conformational energies of organic and biological molecules? J. Comput. Chem. 2000, 21 (12), 1049-1074. 16. Samuelsson, J.; Johansson, M., A study of fatty acid methyl esters with epoxy or alkyne functionalities. J. Am. Oil Chem. Soc. 2001, 78 (12), 1191-1196. 17. Fedosov, S. N.; Brask, J.; Pedersen, A. K.; Nordblad, M.; Woodley, J. M.; Xu, X., Kinetic model of biodiesel production using immobilized lipase Candida antarctica lipase B. J. Mol. Catal. B: Enzym. 2013, 85-86, 156-168. 18. Van Krevelen, D. W.; Te Nijenhuis, K., Chapter 7 - Cohesive Properties and Solubility. In Properties of Polymers (Fourth Edition), Elsevier: Amsterdam, 2009; pp 189-227. 19. Takwa, M.; Larsen, M. W.; Hult, K.; Martinelle, M., Rational redesign of Candida antarcticalipase B for the ring opening polymerization of d,d-lactide. Chemical Communications 2011, 47 (26), 7392-7394. 20. Crivello, J. V.; Lee, J. L., Alkoxy-substituted diaryliodonium salt cationic photoinitiators. J. Polym. Sci. Part A: Polym. Chem. 1989, 27 (12), 3951-68. 21. Crivello, J. V.; Lam, J. H. W., Photoinitiated cationic polymerization with triarylsulfonium salts. J. Polym. Sci., Part A: Polym. Chem. 1996, 34 (16), 3231-3253. 22. Ward, I. M.; Sweeney, J., Relaxation Transitions: Experimental Behaviour and Molecular Interpretation. In Mechanical Properties of Solid Polymers, John Wiley & Sons, Ltd: 2012; pp 261-284.


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Table of Contents Graphic and Synopsis


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Schematic representation of the reactants used in the synthesis (upper left side and right side) and products formed by the combination of those (center). NEOPG: Neopentyl glycol, TMP: Trimethylolpropane, Di-TMP: Di-Trimethylolpropane, MS: Methyl Stearate, EMO: Epoxidized Methyl Oleate, EMLO: Epoxidized Methyl Linoleate, EMLEN: Epoxidized Methyl Linolenate. The acronyms for the products indicates which starting materials have been used for their synthesis, e.g. NEOPG-MS was formed as a combination of Neopenthyl glycol (NEOPG) and Methyl Stearate (MS) Figure 1 441x138mm (300 x 300 DPI)


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Superposed conversion from kinetic study through 1H NMR and distribution of products calculated from kinetic study through SEC Figure 2 546x191mm (300 x 300 DPI)


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Snapshot from the molecular dynamics simulations of the tetrahedral intermediate of the acylation of DiTMP (yellow) with epoxidized methyl linolenate (dark blue) catalysed by CalB (left side) and the schematic representation of the same (right side). The oxygen atoms are found in red. The rest of acyl chains are shown in different colors. Figure 3 192x73mm (300 x 300 DPI)


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Final conversion (after 30 min) of the thermosets crosslinked through cationic polymerization (only UVlight). %Conversion (1075 cm-1 peak) = (At - A(t=0) ) / (AF - A(t=0)) Figure 4 289x201mm (300 x 300 DPI)


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Tg of the cured thermosets calculated from DSC thermograms as a function of the number of epoxides Figure 5 200x162mm (300 x 300 DPI)


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DMA curves of the cured thermosets using TMP as hydroxyl-core Figure 6 289x201mm (300 x 300 DPI)


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Biocatalytic Synthesis of Epoxy Resins from Fatty Acids as a Versatile

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