Synthesis of Novel Renewable Polyesters and Polyamides with Olefin

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Synthesis of Novel Renewable Polyesters and -Amides with Olefin Metathesis Annelies Dewaele, Lotte Meerten, Leander Verbelen, Samuel Eyley, Wim Thielemans, Peter Van Puyvelde, Michiel Dusselier, and Bert F. Sels ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00807 • Publication Date (Web): 31 May 2016 Downloaded from http://pubs.acs.org on June 2, 2016

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ACS Sustainable Chemistry & Engineering

Synthesis of Novel Renewable Polyesters and -Amides with Olefin Metathesis Annelies Dewaele,† Lotte Meerten,† Leander Verbelen,$ Samuel Eyley,ⱡ Wim Thielemans,ⱡ Peter Van †,*

Puyvelde,$ Michiel Dusselier,

Bert Sels†,*

†

Center for Surface Science and Catalysis, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium. Department of Chemical Engineering, KU Leuven, Campus Kulak Kortrijk, Etienne Sabbelaan 53, 8500 Kortrijk, Belgium. $ Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium. ⱡ

* [email protected]

Keywords: olefin metathesis, biomass, building blocks, methyl vinyl glycolate, α-hydroxyacids, diacids, biodegradable polymers, polylactic acid

ABSTRACT Unsaturated and hydroxyl-functionalized C6-dicarboxylic acids were successfully synthesized via olefin metathesis from methyl vinyl glycolate (MVG), a renewable α-hydroxy C4-ester product from Lewisacid carbohydrate conversion. Addition of a second-generation Hoveyda-Grubbs catalyst to neat MVG leads to a near quantitative yield of dimethyl-2,5-dihydroxy-3-hexenedioate (DMDHHD). Additional hydrolysis and hydrogenation steps form interesting polymer building blocks like 2,5-dihydroxy-3hexenedioic acid (DHHDA) and 2,5-dihydroxyadipic acid (DHAA). Their use in polyester and polyamide synthesis is demonstrated after determination of their physical and spectroscopic characteristics. Copolymerization of DHHDA with L-lactic acid for instance produces a crosslinked poly(L-lactic acid-co-DHHDA) polyester. Proof of crosslinks is ascertained by NMR and FT-IR. ACS Paragon Plus Environment

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Substantial impact on the melting, thermal and polar properties of PLA are observed already at low amounts of DHHDA (0.1 mol%) in accord with the presence of crosslinks in the polymer. Bio-based polyamides were also synthesized by equimolar reaction of DHHDA with hexamethylenediamine, producing a renewable polyamide analogue of the petroleum-based nylon 6,6. Interestingly, the assynthesized polyamide (α-bishydroxylated unsaturated polyamide, HUPA) possesses similar thermal stability as nylon 6,6 but shows different chemical properties as a result of the double bond and αhydroxy functionality.

INTRODUCTION From both an ecological and economic point-of-view, the interest in the production of energy, fuels, chemicals and materials from renewable feedstock is increasing.1 Supported by governmental directives dealing with the fossil-to-bio economy transition,2 biomass processing with chemo- and bio-catalytic technologies is emerging at high speed.3-9 Within such a bio-refinery approach, carbohydrates produced from (hemi)cellulose after removal of lignin but also from more classic starch or sugar-crops, offer great potential as renewable carbon source.7,10,11 Their unique chemical structure allows for synthesizing a range of highly functional chemicals, such as levulinic acid, sorbitol, 5-hydroxymethylfurfural, glycerol and lactic acid, among others.12 The rich functionality within the carbohydrate molecule (having indeed a high atomic O to C ratio) presents a great opportunity to particularly focus on novel synthesis routes to (drop-in or novel) chemicals with a variety of functional groups, as their atom economy is usually high.13-15 Moreover, in contrast to biofuels, such high-value chemicals and materials operate in niche markets with high profit margins.16 From this perspective, α-hydroxy acids, and lactic acid (LA) in particular, are interesting molecules as they can be derived very efficiently from sugary biomass, while they have exciting commercial value both in existing and new applications.17,18 LA is not only a highpotential platform molecule, it is a well-known building block for bioplastics, i.e. polylactic acid (PLA) polyesters.19,20 The high cost associated with the current fermentative production process of LA has stimulated research and development of new chemocatalytic routes to LA and its esters.21-29 The new ACS Paragon Plus Environment

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mechanistic insight and novel catalytic processes look promising and, importantly, some of them also led to the production of other novel α-hydroxy acids such as vinyl glycolic acid (VGA). Initial studies with soluble Sn-catalysts revealed the formation mechanism of VGA esters such as methyl vinyl glycolate (MVG) (Scheme 1) in alcoholic media from tetroses and glycolaldehyde.30,31 A solid Lewis acid-based catalytic system was investigated recently, obtaining MVG yields as high as 50%,32 while MVG was also found among the main products by directly converting cellulose in methanol applying Sn(II)-triflate catalysis.33 Interestingly, incorporating VGA in PLA polyesters (shown in Scheme 1) forms a novel polymer prone to a variety of postsynthetic modifications at the vinyl side–chains.31 The need for functional PLA - tunable to a broad range of properties and thus applications - together with the imminent availability of MVG as major co-product in some upcoming novel catalytic LA production processes,21 motivate further exploration of VGA (and its esters like MVG) as platform chemical. Given its three different functional groups in the 4-carbon skeleton, the potential of MVG is high. Herein, we explore the presence of the vinyl group to create novel building blocks and thereof derived renewable polyesters and polyamides. Terminal olefins are highly suited to undergo olefin metathesis, a reaction that has already demonstrated its commercial viability in the production of long-chain diacids from natural oils.34 This elegant reaction rearranges two unsaturated bonds in presence of an organometallic catalyst, mostly Ru-based. Upon adding such metathesis catalyst – here the so-called Grubbs complexes were used – to MVG, two MVG molecules are coupled to each other with formation of a new internal double bond and release of ethylene. As such, a symmetric unsaturated C6 di-ester with two α-hydroxyls (dimethyl-2,5-dihydroxy-3-hexenedioate, DMDHHD) is obtained, as well as the corresponding dicarboxylic acid (2,5-dihydroxy-3-hexenedioic acid, DHHDA) after hydrolysis (Scheme 1). Hydrogenation gives access to 2,5-dihydroxyadipic acid (DHAA) (Scheme 1). Dicarboxylic acids (diacids) have generally been recognized as high-value bio-based chemicals in many reports, like in the U.S. DOE list of chemicals from biomass.35 They may serve as (precursors of) plasticizers and lubricants, but most of all, they are used in the manufacturing of polyamides and polyesters.36 ACS Paragon Plus Environment

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biomass pyrolysis oil


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s te oli ze Sn e, lid a -h Sn

Scheme 1. The different reported synthesis routes to methyl vinyl glycolate and VGA from sugary biomass and their further valorization by olefin metathesis to new functional diacid or diester building blocks (this work). The reported co-polymerization of VGA with LA by polycondensation (PC) is also shown.21,25,30-33,37 Although literature describing the synthesis of diacids (mainly succinic and muconic acid recently) from biomass is numerous,15,38,39 synthesis routes via olefin metathesis are scarce and mostly limited to reacting unsaturated fatty acids (or esters) to obtain long-chain diacids (or esters).6,40-43 The synthesis of the above symmetric α-OH C6 diacids (and their esters) is, to the best of our knowledge, not reported. Metathesis of MVG, as proposed in this contribution, is therefore the first catalytic route to DMDHHD. Next to metathesis, reaction conditions to achieve high yields are illustrated for its hydrolysis and hydrogenation. Finally, the valorization potential of this new family of building blocks is illustrated in the synthesis of bio-based polyesters and polyamides. Use of DHHA as competent crosslinker in PLA polyesters is for instance demonstrated, as well as its use to create a polyamide with hexamethylenediamine is shown.

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EXPERIMENTAL SECTION Synthesis of dimethyl-2,5-dihydroxy-3-hexenedioate (DMDHHD). In a typical experiment, 0.2 g of methyl D,L-2-hydroxy-3-butenoate (racemic MVG, TCI Europe) was added neat or dissolved in toluene (VWR International) to 4.9 µmol of a Ru metathesis catalyst under nitrogen atmosphere in a 1 mL glass reactor vial. The mixture was stirred with a magnetic stir bar at 35°C for a certain time, dependent on the reaction progress, under a continuous flow of Ar to remove formed ethylene. During reaction, the liquid MVG was converted to solid DMDHHD. Prior to gas chromatography (GC) analysis, naphthalene (100 mg) was added as external standard as well as a known amount of acetonitrile (VWR International) to solubilize all compounds. MVG and DMDHHD were analyzed by GC on a HP-1 (30 m) to follow MVG conversion and obtain DMDHHD yield, and a CP-Sil-88 (100m) column to separate E/Z isomers of DMDHHD, both equipped with FID detector and ChemStation Software. The yield of DMDHHD and the conversion of MVG were calculated taking into account the respective response factors as determined by calibration curves with MVG and purified self-made DMDHHD. Isolation and purification of DMDHHD after a typical reaction was accomplished by filtering the reactor contents over a porous glass filter and rinsing with toluene to solubilize unconverted MVG and the Ru catalyst, but not DMDHHD, which remains as a fine white powder after drying. Synthesis of 2,5-dihydroxy-3-hexenedioic acid (DHHDA). DHHDA was synthesized by hydrolysis of DMDHHD. In a typical experiment, 1 g of purified MMDHHD was solubilized in 40 mL of Millipore water, and 1 g of Amberlyst 15, a strong acidic resin catalyst was added. The mixture was stirred at 60 °C for 48h. After complete conversion, as ascertained by 1H-NMR and GC analysis, the resin was filtered-off over a glass fritted filter and water was removed by rotary evaporation. DHHDA appeared as a hard yellow/white powder.


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Synthesis of 2,5-dihydroxyadipic acid (DHAA). DHAA was synthesized by hydrogenation of DHHDA. Therefore, a solution of 0.4 g DHHDA in 6 mL Millipore water and 0.2 wt% (on DHHDA basis) of a 5 wt% Pd/C (Engelhard) catalyst were put together in a stainless steel reactor with a magnetic stir bar, and pressurized to 20 bar H2. After 8h of reaction at 25 °C, the solution was filtered over a PTFE syringe filter to retain the catalyst. After evaporation of water, DHAA appeared as a white powder. Polyester synthesis by polycondensation of L-LA and DHHDA. For the synthesis of poly(Llactic acid-co-DHHDA), the as-synthesized diacid DHHDA (1.19 x 10-4 – 1.43 x 10-3 mol, powder) was mixed with 1.19 x 10-2 mol of L-lactic acid (1.2 g of a 90 wt% aqueous solution, SigmaAldrich). As such, the monomer amount of DHHDA was varied between 1-12 mol%. To this, 0.2 wt% (on monomer basis) of SnCl2.2H2O (Acros) was added as a polycondensation catalyst, and finally p-xylene (Sigma-Aldrich) to obtain a 33 wt% monomer mixture. A magnetic stir bar was added and the reactor was equipped with a Dean-Stark trap filled with p-xylene. The mixture was refluxed in an oil bath at 160 °C for 72h.44 After reaction, the mixture was dissolved in CHCl3and precipitated in cold CH3OH. The polyester was filtered off, washed with CH3OH and dried under vacuum. Polyamide synthesis by polycondensation of HMDA and DHHDA. The polycondensation reaction between DHHDA and hexamethylenediamine (HMDA) was adapted from the literature.14 HMDA and DHHDA were both dissolved in CH3OH and mixed in a 1:1 molar ratio. The resulting solution was heated in a round bottom flask in an oil bath at 60 °C. After 30 min, a salt precipitated, which was filtered off, washed with methanol and dried under a flow of dry nitrogen for 1 hour. Then, the salt was mixed with Millipore water in a 0.86:1 mass ratio of salt: water and put in a porcelain crucible in a tube furnace and heated at a rate of 7.5 °C min-1 to 250 °C under pure


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nitrogen flow (30 mL min-1). After 30 min at 250 °C, the furnace was cooled to room temperature. Characterization of building blocks and polymers. 1H-, 1H-COSY-, 13C-, 1H-/13C-HSQC NMR were

recorded

in

DMSO-d6

(Sigma-Aldrich),CDCl3

(Sigma-Aldrich)

or

1,1,1,3,3,3-

hexafluoroisopropanol (for polyamide) on a Bruker Avance 400 MHz spectrometer. Fourier transform infrared spectra were measured with a Varian 670-IR spectrometer. UV-VIS spectroscopy was performed on a Shimadzu UV-1650PC spectrophotometer to determine the UV-VIS spectrum of HGII before and after reaction, and to quantify the extinction coefficients of DMDHHD, DHHDA and DHAA in water. After determination of the wavelength of maximal absorption (λMAX), the extinction coefficients were determined by measuring dilution series and applying the Lambert-Beer law. The melting points of DMDHHD, DHHDA and DHAA were determined on a Büchi M 560 set-up. Organic elemental analysis (C, H, N, S) was carried out on a Thermo-Scientific Flash 2000 elemental analyzer using 2,5-(bis(5-tert-butyl-2-benzooxazol-2-yl)thiophene (BBOT) as a calibration standard. Gel permeation chromatography (GPC) was performed using a Waters e2695 Separations Module and a Waters 2414 RI detector. Separation was performed on a Varian M-Gel 3 mixed column with a 1 mL min-1 flow of THF at 40 °C. Polystyrene standards were used for calibration. Thermogravimetric analysis was performed on the polymer powders, heating them at a rate of 10 °C/min under N2 atmosphere using a TGA Q500 (TA Instruments). Powder X-ray diffraction patterns were recorded on a STOE COMBI P diffractometer (monochromated CuKα1radiation, λ = 1.54060 Å) equipped with an IP-PSD detector in transmission geometry. Differential scanning calorimetry experiments were performed with a DSC Q2000 (TA Instruments) by cycling between 0-200 °C and 10-280°C for the polyesters and polyamides, respectively, at heating/cooling rates of 10 °C/min under N2 atmosphere. Static contact angle measurements were performed with double deionized water on polymer coated glass slides in a water-saturated chamber with a KSV


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NIMA CAM200 setup and KSV software. Hereto, the polymers were spin-coated from a 5 wt % solution in CHCl3 on a thin glass preparation slide for 1 min at a rate of 1000 rpm attained in 3 s, and dried overnight. Due to the lower solubility of the P(LA-co-DHHDA) with the highest degree of crosslinks (2.3 and 2.9%), these polymers were coated from a 1.7 wt% solution, in three successive spin-coat runs. Fifteen seconds after adding a water drop, 5 consecutive pictures were taken. The photographed droplets were fit according to the Young−Laplace equation, and the average static contact angle was calculated (average of 6 droplets). Standard deviations were calculated.

RESULTS Synthesis of new diester and diacid building blocks through metathesis. The terminal vinyl group makes MVG a potential metathesis substrate. Self-metathesis of MVG molecules was initially studied in toluene in presence of the 2nd generation Hoveyda-Grubbs catalyst (HGII), one of the most stable and active Ru metathesis complexes.45 The viability of dimethyl-2,5-dihydroxy-3hexenedioate (DMDHHD, Scheme 2) formation was quickly asserted (Table 1). Due to the fast reaction rate and co-formation of ethylene, the experiments were performed under mild argon flow, to avoid pressure built-up and to rapidly shift the reaction equilibrium to completeness. Reaction of 0.25 M MVG in toluene with only 0.25 mol% of HGII for instance gave an intermediate conversion of 65% (Table 1, entry 1) with high selectivity to DMDHHD. Increasing the amount of catalyst tenfold did not influence the final yield (entry 2). An increase of MVG concentration to 1 and 2 M increased the reaction yield, resp. to 72 and 86% DMDHHD, as long as sufficient amounts of HGII were added (1 mol%) (entry 3-5). This concentration effect, as well as the liquid aggregation state of MVG prompted us to perform metathesis under solvent-free conditions. Nearly complete conversion of MVG with > 99% selectivity to DMDHHD were attained in the neat conditions, even in presence


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of minute amounts of HGII catalyst (entries 6-8). For instance, with 0.25 mol% of HGII, a TON of 400 is attained (entry 8) in less than 30 minutes, corresponding to a turnover frequency of 800 h-1. The success of converting MVG to DMDHHD by self-metathesis is remarkable, as metathesis of allylic alcohol substrates is not at all trivial, since the presence of the hydroxyl group may impact the metathesis reaction positively or negatively.46-48 Here, the alcohol group seems to have a positive impact on the reactivity of MVG, which might be attributed to hydrogen bonding between the hydroxyl group and the chloride ligands, favoring subsequent reaction between the alkene and the carbene centers.49 Ethylene is instantaneously formed once MVG is added to the catalyst solution, while the mixture quickly solidifies as soon as DMDHHD is formed. A visual representation of the reaction is shown in Figure S1. An attempt to use even lower catalyst loadings, viz. 0.05 mol% HGII or only 0.17 mg of Ru metal for 200 mg of substrate, led to a mediocre yield of 51% (Table 1, entry 9), likely suggesting catalyst deactivation is at play since prolonged reaction times did not increase the conversion of MVG. This deactivation was visually seen by a fast green-to-brown color change of the catalyst, and verified by UV/VIS spectroscopy. Therefore, a solution of HGII in toluene before reaction was compared to HGII after reaction, which was isolated from the reaction mixture by first evaporating MVG, followed by dissolving the catalyst in toluene (a poor solvent for DMDHHD) and filtering of the solution. The UV/VIS spectrum clearly shows shifts to shorter wavelengths (blue shift) of maximal absorption of HGII before and after reaction (Figure S2). Next to HGII, other Ru metathesis catalysts (structures in Scheme S1), were therefore investigated, i.e. 1st generation Grubbs (GI), 1st generation Hoveyda-Grubbs (HGI) and 2nd generation Grubbs (GII). GI and HGI (entry 10-11) barely converted MVG in conditions where GII reached a moderate conversion of 60% (entry 12). The low activity of the first-generation complexes may be attributed to a faster


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destabilization of the active catalytic complex due to electronic differences in their ligand structures (compared to second-generation complexes).50,51 Though high turn-over numbers may be required to commercialize formation of DMDHHD from MVG by additional purification steps or new catalyst design, this contribution already proves a realistic potential. Because of the internal unsaturation, DMDHHD exists in different geometric isomers, here formed with an E:Z molar ratio of 40:1. E-isomers are generally thermodynamically favored due to the absence of steric effects.52 After the successful MVG-to-DMDHHD conversion in near quantitative yields (96%), the catalyst and remaining MVG were removed by rinsing the reactor content over a glass filter crucible with toluene. Toluene solubilizes HGII and MVG, but not DMDHHD. As such, with one simple step, DMDHHD can be obtained as a white powder with a purity of 99% (based on 1H-NMR). The ester was readily hydrolyzed to the corresponding diacid (2,5-dihydroxy-3-hexenedioic acid or DHHDA, Scheme 2).


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Scheme 2. Reaction scheme for the formation of DMDHHD, the unsaturated diacid (DHDHA) and the hydrogenated diacid (DHAA) from racemic MVG. Blue marked routes involve the addition of a Ru metathesis catalyst. Only the E-isomeric bonds are shown for DMDHHD and DHHDA as the E:Z ratio amounts to 40:1. // represents ethylene.

DHHDA is a structural analogue of an unsaturated C6-diacid, similar (but different) to itaconic acid, which was recently synthesized in its ester analogues from ethyl pyruvate, underlining its relevance.15 After hydrolysis with a heterogeneous Brønsted acid resin catalyst, DHHDA was recovered with a 93% yield (by weight) and appeared as a hard pale yellow powder in high purity (97%, based on 1H-NMR). The reverse reaction sequence to obtain DHHDA, i.e. hydrolysis of MVG followed by olefin metathesis to DHHDA, was not successful (Scheme 2). After MVG hydrolysis with a heterogeneous Brønsted acid resin catalyst, metathesis of VGA in toluene barely occurred. Different electron densities of the terminal vinyl chain of MVG and VGA may be responsible for their difference in reactivity, as the ability of an olefin to undergo metathesis is greatly determined by the functional group attached.53 However catalyst deactivation and solubility issues cannot be excluded.

Anyhow, the first synthesis route of DHHDA by self-metathesis of

MVG is the preferred one because MVG and not VGA is the dominant product in the reported biomass conversions (Scheme 1).30-32


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Table 1. Optimization reaction conditions for the homogeneous conversion of MVG to the diester DMDHHD by olefin metathesis.a

a

Entry

Catalyst Conc. Catalyst (mol%) (M)

Conversion Y b (%)b DMDHHD(%)

1 2 3 4 5 6 7 8 9 10 11 12

HGII HGII HGII HGII HGII HGII HGII HGII HGII GI HGI GII

67 65 68 76 88 96 96 93 53 5 25 60

0.25 2.5 0.25 1 1 1 0.5 0.25 0.05 0.25 0.25 0.25

0.25 0.25 1 1 2 neat neat neat neat neat neat neat

62 62 64 72 86 96 96 93 51 3 22 55

Reaction conditions: MVG dissolved in toluene or neat; 35°C; reaction time was 30 min (entry 6-8); 60 min (entry 9)

and 3h (entry 1-5 and 10-12); 0.05-1 mol% catalyst; mild Argon flow. Naphthalene was added as an external standard after reaction. b Conversion of MVG and yield of DMDHHD as determined by GC. DMDHHD exists as a mixture of diastereomers which are inseparable by GC analysis or 1H-NMR.

Additional hydrogenation of DHHDA was also successfully accomplished in a straightforward manner using a heterogeneous Pd/C catalyst under hydrogen pressure in water. The product, effectively 2,5-dihydroxyadipic acid (DHAA, Scheme 2) was recovered as a solid with a 92% isolated yield (99% pure, based on 1H-NMR). The above synthesis procedures, in accordance with the reactions in Scheme 2, demonstrate the practical availability to a set of unique and highly functional C6 building blocks. The compound DMDHHD for instance has never been reported until now, and can therefore rightfully be labeled as a truly new renewable building block. The corresponding diacid (DHHDA) was encountered in literature as a minor degradation product during alkaline treatment of pectin (< 1%).54 On-purpose synthesis routes were not reported. DHAA has often been detected during alkaline degradation of


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sugar polymers55-63 and humic acids,64 and in contrast with the other compounds, synthesis routes for DHAA have been reported, as it serves as an intermediate in the production of muscarine65 or muconic acid.66 DHAA may thus also be considered as a potential precursor of muconic acid, which in turn is a potential precursor for adipic acid or terepthalic acid, monomers for nylon and polyethylene terephthalate, respectively.67,68 The chemical structure of the three compounds (DMDHHD, DHHDA and DHAA) was confirmed by elemental analysis (C:H ratio, Table S5) and spectroscopic analyses such as 1H-NMR (Figure S3-S5),

13

C-NMR (Figure S6-S8), 1H/13C-HSQC NMR (Figure S9-S11) and FTIR spectroscopy

(Figure S12-S14 and Table S1-S3). Their extinction coefficients (ε) were determined at their λMAX values with UV-VIS spectroscopy (Figure S15 and Table S4). Solutions of the three compounds in ultrapure water are colorless, and as expected, their corresponding λMAX values occurred in the ultraviolet region (200-225 nm). Whereas rather low values of λMAX and ε are determined for DHAA, the higher values of DMDHHD and DHHDA, agree well with the presence of the double bond.69 In addition, melting points were estimated at 134 °C for the diester, 187 °C for DHHDA and 174 °C for DHAA. The value for DHAA is in good agreement with literature.65 The physical appearance of the three purified solids is illustrated in Figure S16.

Application of the building blocks in polymer synthesis: DHHDA as effective poly(L-lactic acid) crosslinker. DHHDA has a dual α-hydroxy carboxylic acid functionality, rendering it a potential polyester building block in poly-condensation chemistry. In particular, co-polymerization of this compound with other α-hydroxy acids (like lactic acid) could produce crosslinked polyesters, provided each DHHDA condenses with four α-hydroxy acid molecules (see inset Figure 1). As a proof-of-concept, the copolymerization of DHHDA and L-lactic acid was evaluated here, with the goal of producing P(LA-co-DHHDA) with improved properties over the traditional L-PLA. After


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all, crosslinking PLA has already demonstrated to improve thermal stability70,71 and toughness.72,73 While the most common crosslinking of PLA is performed by post-polymerization irradiation with crosslinking agents74-76 or with chain extenders,77-80 we here propose a bottom-up synthesis of a unique crosslinked PLA (from the monomer level). The copolymerization was successfully performed in p-xylene using an azeotropic distillation for water removal, according to a literature procedure for PLA.44 In the past, we have demonstrated this procedure to incorporate VGA monomer into PLA (as seen in Scheme 1).31 After recovery of the co-polymer, 1H-NMR (Figure 1), 1

H-1H-COSY NMR and

13

C-NMR (supporting information Figure S17-S18) proved the successful

incorporation of DHHDA in L-polylactic acid. Moreover, since none of the original proton signals of DHHDA are present in P(LA-co-DHHDA, all hydroxyl- and acid functionalities are esterified, proving the successful crosslinking of the polymer (Figure S19). The 1H signals of DHHDA in Figure 1 (left) (Hb and Hc) are situated between 5.6 and 6.4 ppm. The molar percentage of DHHDA in the copolymer was determined by integration, according to the following formula: ((Hc)/2)/(Ha+ (Hc)/2). Different molar amounts of DHHDA were incorporated in poly(L-lactic acid) by using different starting ratios of the monomers in the condensation reaction. As such, we succeeded in making a range of copolymers with DHHDA incorporation between 0.1 and 2.9 mol% (Figure 1, right). The data indicate a more or less linear uptake of DHHDA until 2.3 mol%, and a built-in efficiency of ca. 33%. At crosslinker concentrations over 8% (in total monomer), a deflection sets in, and only 2.9 mol% of DHHDA was effectively incorporated from a 12 mol% solution. These results reveal an expected lower reactivity of DHHDA, when compared to LA. Polycondensation with pure DHHDA was also attempted, but was unsuccessful, even after prolonged reaction times (96 h). Moreover, high concentrations of DHHDA remained as a solid in p-xylene, which likely prevent efficient polymerization. The liquid LA present during the co-


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polymerization thus seems necessary for DHHDA dissolution. As a result, incorporation of DHHDA is likely restricted by its solubility in lactic acid as well.

Figure 1. Left:

1

H-NMR of L-PLA and P(LA-co-DHHDA). Right: Degree of DHHDA

incorporation in poly(LA-co-DHHDA) versus that in the starting monomer solution.

Fourier-transform infrared spectra provided additional proof of the crosslinking of L-PLA by DHHDA, as alkene bending vibrations (absent in pure PLA) around 800-825 cm-1 appeared in P(LA-co-DHHDA), next to the traditional PLA vibrations around 1745 cm-1 (C=O stretching), 1350-1460 cm-1 (C-H bending) and 1000-1200 cm-1 (C-O stretching), see Figure S20.81 Upon crosslinking, poly(LA-co-DHHDA) becomes slightly more hydrophobic compared to pure L-PLA,82 as proven by static water contact angle measurements on thin polymer films (Figure S21). Polymer weights were determined by gel-permeation chromatography and the data are collected in Table 2. All polymers had an average molecular weight (MN) between 10-15 000 g mol-1, which is very comparable to that of pure L-PLA synthesized in exactly the same way. Therefore, incorporating DHHDA has only little effect on the size of the polyester.


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Table 2. Characteristics of P(LA-co-DHHDA) polymers.a

a

Yield* (%)

MN TD,MAX (g mol-1) (°C)

Tg (°C)

Tm (°C)

∆Hm (J/g)

PLA

64

12 000

248

51

162

48

P(LA-co-DHHDA)(0.1%)

50

15 000

282

55

152

31


48

12 500

283

55

146

29


25

11 000

297

54

134

25


14

12 000

312

60

100

-


15

10 000

305

-

-

-

MN = number-average molecular weight; TD,MAX = temperature of maximal polymer degradation; Tm =

melting point; ∆Hm = melting enthalpy; Tg = glass transition temperature, which was determined in the cooling curve after the first heating cycle. * No attempts to optimize yields have been performed.

The thermal behavior of the novel bio-based polyesters was investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results are summarized in Figure 2 and Table 2. One might expect a higher thermal stability of the crosslinked polymers compared to their linear counterparts. And indeed, as more DHHDA is incorporated into L-PLA, the polymers


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Figure 2. Thermal analysis of L-PLA and P(LA-co-DHHDA) (0.1-2.3 mol%). (A) derivative weight change by degradation under nitrogen flow (TGA); (B) first heating cycle of differential scanning calorimetry.

degrade at a higher temperature, as is best indicated by the derivative weight change curve (%/°C) from TGA (in Figure 2A).While the temperature of maximal degradation (TD,MAX) was 248 °C for pure L-PLA, this increased with 40 °C for polymers with only 0.1% DHHDA. The additional gain in thermal stability by additional incorporation of DHHDA (> 2%) is substantial but limited. The DSC thermograms in Figure 2B confirm that crosslinking with DHHDA strongly impacts the polymer’s thermal behavior. Although glass transition temperatures (Tg) for all polymers were in the range of 50-60 °C, melting points (Tm) decreased substantially with higher DHHDA incorporation, up to 1.2%, showing ∆Tm > 50°C. In correlation, the observed melting enthalpies (∆Hm) were lower


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compared to that of pure L-PLA. When >2 mol% of DHHDA was incorporated, no melting point was observed. The above results suggest that poly(LA-co-DHHDA)s with up to 1.2 mol% of DHHDA are crystalline or have crystalline regions, but become amorphous when the crosslink density is too high (> 2%).83 These results are corroborated by the copolymers’ diffraction patterns (Figure S22): poly(LA-co-DHHDA)(1.2%) still shows the characteristic crystalline L-PLA reflection pattern,44 but these signals are absent with poly(LA-co-DHHDA) containing 2.3% crosslinker. The co-polymerizations clearly demonstrate the ease of improving the thermal stability of PLA by incorporating only small molar amounts of DHHDA, a crosslinker content of 0.1 mol% being sufficient. The lower melting point of poly(LA-co-DHHDA) together with the improved thermal stability of the as-synthesized polymer also offer a broader thermal processing window compared to pure L-PLA. While in-depth polymer analyses, as well as strategies to more efficiently incorporate the diacid crosslinker via ring-opening polymerization with respect to high MN,84,85 are ongoing, we have proven a great potential of the new diacid family in polyester chemistry.

Application of the new diacid building blocks: DHHDA as precursor for nylon 6,6-look-alike polyamides. Inspired by the synthesis of petrochemical nylon 6,6, formed by reacting adipic acid and hexamethylenediamine (HMDA), we here attempted to synthesize a similar bio-based polyamide from DHHDA and HMDA. After formation and drying of the 1:1 HMDA:DHHDA salt (yield of 82 wt%), polycondensation was performed at 250 °C under a flow of nitrogen.14 A yellow/brown, at least partially crystalline, hard solid was formed (XRD pattern in Figure S23). The polycondensation reaction to form the α-hydroxylated, unsaturated polyamide (denoted further as HUPA) is drawn in Scheme 3. Characterization of HUPA by FTIR spectroscopy revealed the


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characteristic amide bond signals, as present in nylon 6,6 ,86-88 together with additional vibrational bands from the alcohol and alkene groups (see annotations in Figure 3). Together with the elemental analysis data, giving molar ratios of C: 62.8; H: 7.5; N: 10.6 and a corresponding formula of [C13.4H19.0N1.9(O3.8)], only slightly deviating from the theoretical predicted [C12H20N2(O4)], the successful synthesis of the new polyamide is proven.

+

Scheme 3. Polyamide synthesis from poly-condensation of DHHDA and HMDA.

Figure 3. Infrared spectrum of HUPA with assignation of the different absorption bands. The HUPA polymer is insoluble in most common solvents and only slightly soluble in 1,1,1,3,3,3hexafluoro-isopropanol (HFIP). 1H-NMR of HUPA in HFIP shows a fairly pure spectrum where


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HMDA and DHHDA are built-in nicely in a 1 to 1 molar ratio (Figure S24). Thermal degradation of HUPA was very similar to nylon 6,6 89 with a TD,MAX value of 438°C as determined by TGA (Figure S25). Together with its high thermal stability, the polymer had a Tg around 140°C, but no melting point (≤ 280°C) was observed. The thermal stability of the polymer is consistent with a recently reported analogous polyamide synthesized from HMDA and 3-hexenedioic acid, having a comparable chemical structure (besides the α-hydroxyls) as our synthesized HUPA, showing a TD,MAX around 460°C.14 CONCLUSION Methyl vinyl glycolate (MVG), a functional and renewable α-hydroxy acid ester, derived from carbohydrate biomass and under increasing recent attention, was successfully transformed into a new family of di(α-hydroxy)acid (and ester) building blocks, viz. DMDHHD, DHHDA and DHAA. The key reaction is the coupling of two MVG molecules through its vinyl group by self-metathesis. The reaction proceeds highly selectively and nearly quantitatively in solvent-free conditions. For example, the conversion of neat MVG with 0.25 mol% of a 2nd generation Hoveyda-Grubbs catalyst produced the diester DMDHHD with a yield of 96%. The diester is easily hydrolyzed to DHHDA and hydrogenated to DHAA, both in high isolated yields (> 93%). Because of the lack of literature data, these novel building blocks were thoroughly characterized spectroscopically (IR, NMR, UVVIS) and physically (Tm). The significance of this new family of di(α-hydroxyacid)s was demonstrated in polymer chemistry by employing them as functional building blocks for new polyesters and polyamides. First, co-polymerization of DHHDA and L-lactic acid produced PLA-based polyesters wherein DHHDA acts as a true crosslinker. Crystalline materials were so obtained (as long as the crosslinker content remains below 2 mol%) with melting temperatures systematically lower than the pure L-


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PLA. Even low molar DHHDA content, e.g., 0.1 mol%, improved the thermal stability of the LPLA co-polymer significantly, allowing for a thermal processing within a broader temperature window. A second proof of the new molecules’ usefulness is demonstrated through the successful synthesis of a nylon 6,6-look-alike by reacting DHHDA with hexamethylenediamine (HMDA). The resulting polyamide has typical nylon characteristics, e.g. the thermal stability resembles that of the classic nylon 6,6 and recently reported hexenedioic acid-based nylons, but with the additional presence of α-hydroxyl groups. Overall, this study embodies and encourages the strategy of searching for sustainable synthesis routes to highly-functionalized chemicals from renewable (carbohydrate) resources, while testing their significance for instance as building blocks of novel polymeric materials.

ASSOCIATED CONTENT Supporting Information. A full characterization of DMDHHD, DHHDA and DHAA (NMR, FTIR, UV-VIS, elemental analysis) is given. Additional characterization data of P(LA-co-DHHDA) and the polyamide HUPA are shown. This information is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]


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ACKNOWLEDGMENT M.D. and B.F.S thank the Research Council of KU Leuven, IOF KP/14/003 for support. M.D. and A.D.W. acknowledge the Research Foundation Flanders (FWO) for postdoctoral and project funding respectively. W.T and S.E. thank the Research Foundation Flanders (FWO) for support (G.0C60.13N). ABBREVIATIONS DMDHHD: dimethyl-2,5-dihydroxy-3-hexenedioate; DHHDA: 2,5-dihydroxy-3-hexenedioic acid; DHAA: 2,5-dihydroxyadipic acid; L-PLA: poly(L-lactic acid); P(LA-co-DHHDA) (x%): poly(Llactic acid)-co-(2,5-dihydroxy-3-hexenedioic acid) with x% of DHHDA units in the copolymer; MVG: methyl vinyl glycolate; VGA: vinyl glycolic acid; HMDA: hexamethylenediamine; MeOH: methanol; DMSO: dimethylsulfoxide; HUPA: α-bishydroxylated unsaturated polyamide.

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For Table of Contents Use Only

Synthesis of Novel Renewable Polyesters and -Amides with Olefin Metathesis Annelies Dewaele, Lotte Meerten, Leander Verbelen Samuel Eyley, Wim Thielemans, Peter Van Puyvelde, Michiel Dusselier, Bert Sels Synopsis: Highly-functionalized chemicals from renewable resources were successfully transformed into new di(α-hydroxy)acids, and used as building blocks for new polymeric materials.


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Synthesis of Novel Renewable Polyesters and Polyamides with Olefin

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