Esters of Tartaric Acid, A New Class of Potential âDouble Green

Research Article pubs.acs.org/journal/ascecg

Esters of Tartaric Acid, A New Class of Potential “Double Green” Plasticizers Krzysztof Zawada,† Andrzej Plichta,‡ Dominik Jańczewski,*,† Halina Hajmowicz,† Zbigniew Florjańczyk,‡ Magdalena Stępień,† Agnieszka Sobiecka,† and Ludwik Synoradzki† †

Laboratory of Technological Processes, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland ‡ Division of Polymer Chemistry and Technology, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland S Supporting Information *

ABSTRACT: A library of 13 esters and methylated esters, including entirely new molecules, derived from renewable natural tartaric acid, have been synthesized, characterized, and used as plasticizers of polylactide (PLA). Differential scanning calorimetry (DSC) studies of the blends consisting of PLA and tartaric acid derivatives reveal that all esters obtained exhibit plasticizing properties; however, their miscibility with PLA and ability to modify the thermal transition is dependent significantly on the structure of the ester group. It was found that esters bearing short alkyl groups (methyl, ethyl, n-butyl) may act as primary plasticizers that allow blends of elastomeric properties to exist at ambient temperature. The hydrolytic and enzymatic stability of plasticizers has been studied using dimethyl and diethyl tartrate, and both materials can be classified as biodegradable. KEYWORDS: tartaric esters, plasticizer, polylactide, polymer additive, double green plastic

■

INTRODUCTION The plastic industry utilizes large amounts of additives to modify the properties of polymeric products. Additives are used as fillers, antistatic agents, impacts strength modifiers, dyes, stabilizers, and, importantly, plasticizers. Since the additives content in some polymeric compositions can reach mass fractions of >50%,1 those materials are also subject to environmental concerns as much as the polymer base itself. A particular issue is a large volume of polymer material waste, resistant to the fast biodegradation.2 In this aspect, the problem of additives must be addressed, together with the biodegradability of the polymer matrix itself. Plasticizers are among the most popular chemicals produced by industry and represent the largest class of plastics additives by mass.1,3 This is a consequence of the particular importance of polymer blend elasticity in the context of the polymer processing and final plastic product characteristics. Plasticizers are used for a variety of polymers with a majority of the market devoted to the modification of poly(vinyl chloride) (PVC).4 Many biodegradable polymers, promoted nowadays as an environmentally friendly alternative to traditional plastics, also require the addition of a plasticizing agent to allow for the processing and adjustment of properties to specific applications.5 The plasticization process reduces the glass-transition © 2017 American Chemical Society

temperature (Tg) of a polymer blend and is typically required for polymers that are too stiff and brittle for specific applications.1 Plasticizers, which are used for the majority of polyesters, polyamides, and vinyl polymers, are typically viscous liquids with high boiling temperatures.3 Current efforts in the search for new plasticizers are devoted to chemicals meeting “double green” criteria. As such, an ideal plasticizer (i) should be available from renewable resources and (ii) undergo ready biodegradation in the landfill. At the same time, just as for other types of plasticizers, it must also be characterized by (iii) excellent miscibility with the polymer, eliminating the problem of migration and exudation of plasticizer, (iv) chemical stability in the polymer blend under processing conditions, and (v) low price and high availability. For the majority of applications, polymer additives should be nontoxic, colorless, and odorless. Plasticization reduces the Young’s modulus and increases the ability of the material undergo reversible deformations. It also affects crystallization and blend molecular structure. The prerequisite for reducing the Tg value is good mixing between the polymer matrix and the additive. Ideally, a single phase is Received: March 16, 2017 Revised: May 15, 2017 Published: May 19, 2017 5999

DOI: 10.1021/acssuschemeng.7b00814 ACS Sustainable Chem. Eng. 2017, 5, 5999−6007

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis of Derivatives 2a−g and 3a−3e

formed within the blend. The existence of the single phase can be detected with various methods but typical criteria is the presence of a single Tg value. As a consequence, blends with limited miscibility display two Tg values located typically between the values of the pure components. The presence of a plasticizer affects crystallization in blends with semicrystalline polymers. The melting temperature (Tm) of a blend is reduced more efficiently when components are well-mixed.6 Polylactide (PLA) is a popular biodegradable aliphatic polyester derived from natural resources such as corn starch, tapioca root chips or starch, or sugar cane.7 PLA has found a broad range of applications, such as use in packaging and textiles or various applications in agriculture and medicine.7−10 PLA is a brittle and stiff material11 typically with parameters of Tg = 58 °C, an elastic modulus of 3500 MPa, a maximum tensile strength of 50 MPa, and an elongation at break of 4%.12 Some applications of polymeric materials require other mechanical property profiles, especially higher flexibility and higher impact resistance.13,14 Those can be achieved by modification of the polymer structure with appropriate modifiers.15 Several compounds have been investigated as modifiers of PLA (e.g., lactide monomer,16 citrate esters,17 glycerol triacetate,18 polyethers,19,20 starch,21 and others22). PLA modification can be achieved using methods such as extrusion with additives or reactive extrusion.23,24 Derivatives of citric acidnamely, triethyl and tributyl estershave been used to modify PLA.17 Citrate blends display Tg values up to 14 °C lower than that of the pure PLA. Investigated citrates were crystallizing in the mixtures upon cooling, so the real composition of amorphous phase varied. Citrates were characterized by a decent miscibility with PLA up to 45% of the blend mass.25 However, heating of mixtures at 50 °C resulted in instability and phase separation. Another tested PLA additive was glycerol triacetate.18 This molecule was effective in reducing Tg of the blend; however, concentrations above 25% resulted in phase separation. Oligomeric polyethers, poly(ethylene glycol) (PEG),26−30 and poly(propylene glycol) (PPG),31−33 are probably the most popular molecules tested as PLA plasticizers. The addition of PEG reduces the Tg value over a broad range with low-molecular-mass oligomers being more effective then polymers with high molecular weight (Mw).25 When the mixing ability of those two polymers is considered, better results are achieved for PEGs with low molecular masses. PEG with Mw = 1500−3400 have a tendency to separate at a mass concentration of 20%, while those with higher masses phase-separate at concentrations of 10%.34 Modification of PLA properties can also be achieved via the addition of D-lactide, L-lactide, and racemic lactide monomers.35

To date, the tartaric acid (TA) derivatives have not been investigated as PLA modifiers. This class of compounds is meeting the requirements of origin and biodegradation to fulfill “double green“ criteria. TA derivatives play an important role as building blocks in the synthesis from the chiral pool. They have been successfully used in asymmetric synthesis, as chiral auxiliaries for separation, or as chiral ligands applied in numerous catalytic systems.36 There are only few reports on the use of TA derivatives for plasticization of other polymers. For example, dibutyl tartrate was used in combination with corn-based films, and diethyl tartrate was employed for plasticization of films made from keratin.37−40 In this work, a library of 13 chiral TA derivatives is synthesized and investigated as modifiers of PLA. Detailed synthetic protocols are discussed, followed by a systematic comparison of two series, namely, TA esters (2) and less-polar methylated TA esters (3). The ability of molecules to reduce the Tg value upon systematic variation in their structure is discussed across the series of compounds, providing important clues for future applications. Three chemical entities studied in this paper (3c−3e) are newly obtained structures and have not been reported previously. Selected molecules are investigated for biodegradation and their ability to modify the mechanical properties of PLA blends. Materials synthesis was performed at the large laboratory scale, which allows us to draw conclusions about the technological feasibility for each synthetic method.

■

RESULTS AND DISCUSSION Synthesis of Plasticizers. Two series of tartaric acid derivativesnamely, esters (2) and dimethylated esters (3) were investigated as possible green polymer modifiers. The molecules were synthesized via acid-catalyzed esterification41−43 of TA 2a−2g (Scheme 1) in the presence of p-toluenesulfonic acid (TsOH), or by the alkylation using benzyl bromide 2h (see Scheme 2, as well as Table 1). In the subsequent step, hydroxyl groups of selected esters (2a−2e) were alkylated using dimethyl sulfate, yielding the methylated derivatives 3a−3e (see Table 2). Scheme 2. Synthesis of Benzyl Derivative 2h

6000


Research Article

ACS Sustainable Chemistry & Engineering Table 1. Synthesis Conditions and Specific Rotations for TA Esters 2a−2h product 2a 2b 2c 2d 2e 2f 2g 2h a

[α]25 D

TA/ROH 1/42.6 1/45 1/2.73 1/2.5 1/2.1 1/2.1 1/2.1 1/2

+21.0° (c 2.5, H2O) +7.8° (neat) +18.6° (c 2, acetone) +18.2° (c 2.2, acetone) +13.2° (c 1.8, acetone) +10.6° (c 1, MeOH) +5.5° (c 1, MeOH) +11.0° (c 2, acetone)

[α]lit D

reaction time, t (h)

yield, Ya (%)

24 24 10 24 24 24 40 6

98 97 89.5 85 83 98 97 75

46

+21° (c 2.5, H2O) +7.5° (neat)47 +18.6° (c 2, acetone)48 +14° (c 1.6, acetone)49

+11.7° (c 2, acetone)48

comments large excess of alcohol large excess of alcohol removal of water by azeotropic removal of water by azeotropic removal of water by azeotropic removal of water by azeotropic removal of water by azeotropic crystallization/TEA

distillation distillation distillation distillation distillation

Yield relative to TA.

Table 2. Synthesis Conditions and Specific Rotations for Methylated TA Derivatives 3a−3e [α]25 D

product 3a 3b 3c 3d 3e a

+71.0° +77.0° +71.1° +53.6° +46.5°

(c (c (c (c (c

2, MeOH) 1, EtOH) 5.1, EtOH) 2.1, acetone) 2.3, acetone)

[α]lit D

reaction time, ta (h)

yield, Ya (%)

purification method

12 12 12 12 12

85 71 98 83 87

distillation distillation distillation flash chromatography flash chromatography

50

+79° (c 2, MeOH) +77° (c 1.3, EtOH)51

Yield relative to the corresponding ester (2a−2e) used as a starting material.

further process development research. It was found that esterification progress can be easily followed by the measurement of water formed and removed from the reaction environment. At the stage of alkylation, it was observed that an excess of NaH limited the main reaction and resulted in the formation of elimination products. To probe the glass-transition temperature (Tg)52 and other phase transition effects, such as cold crystallization temperature (Tc) and melting temperature (Tm), series 2 and 3 compounds were subjected to differential scanning calorimetry (DSC) and modulated differential scanning calorimetry (MDSC) experiments. (See Figure 1 and Table 3, as well as Figures S1−S13 in

The methyl (2a) and ethyl esters (2b) were synthesized by employing longer reaction times (24 h) and excess of an alcohol to achieve higher yields. In the cases of esters 2c−2g, a smaller excess of alcohol (ROH) was used and water was removed via azeotropic distillation with toluene. As a result, the reaction time (t) was reduced and the product yield (Y) increased. Only the oligomeric alcohol derivative of PEG (2g) required a longer reaction time to achieve satisfactory yields. A lower reactivity of PEG alcohol is likely result of its polymeric character and related lowered mobility and accessibility of the reaction center. Importantly, across all known derivatives, no significant difference in specific rotation was observed between synthesized products and the literature data. As reported previously,44 the acid-catalyzed esterification of TA proceeds without detectable racemization of the tartrate molecule. Unlike products 2a−2g, ester 2h was synthesized through alkylation of TA with benzyl bromide in the presence of potassium iodide and triethyl amine (TEA). The major reaction improvement was achieved thanks to the replacement of DMF solvent with acetone and changes in separation protocol performed by crystallization in the presence of emulsifier. The proposed modification leads to a quantitative yield for the reaction and increased the combined synthesis and isolation yields from moderate levels45 to 75%. In the subsequent alkylation process, esters 2a−2e were treated with the stoichiometric amount of dimethyl sulfate. The process converts free OH groups to methoxy moieties with a high yield, substantially changing the hydrophilicity of the molecule. n-Butyl derivative 3c was synthesized with the highest yield while shorter methyl 2a and ethyl 2b esters were alkylated with somehow lower yields. We attribute this to the residual level of water remaining in highly hydrophilic molecules 2a and 2b. Also, in the cases of known products 3a and 3b, no significant difference in specific rotation was recorded, in comparison to the literature data. As such, no alteration at the chiral center can be concluded. All synthetic protocols were executed in large, multigram scale (∼20 g) to fulfill the requirements of plasticization experiments. Results collected during the large laboratory-scale synthesis allowed us to gather initial data and conclusions for

Figure 1. Glass-transition temperature (Tg) for pure TA derivatives.

the Supporting Information.) The Tg value, manifested as a change in the heat capacity of the amorphous phase below its melting temperature, can be used for evaluation if a given molecule exhibits sufficient ability to reduce the Tg value of a polymeric blend. The origin of the effect at molecular scale is probably not the same between investigated molecules; nevertheless, for simplicity of discussion, the observed change is referred as Tg. 6001


Research Article

ACS Sustainable Chemistry & Engineering

other investigated compounds (2a, 2c, 2g, 3b, 3e), clearly visible Tm and/or Tc temperatures were observed. Many investigated derivatives display high heat effects at elevated temperatures of ≳200 °C, this can be affiliated with thermal decomposition and evaporation or a combination of those effects. 1H NMR analysis of sample 2b after 4 h of heating in 200 °C also shows a contribution from the dimerization process of the molecule; this effect will become the subject of our further investigations. The molecules intended for use as a component of biodegradable plastics should decompose together with the polymeric matrix. The α-hydroxy and α-alkoxy esters are expected to undergo facile hydrolytic cleavage. Two tartrates used in our studymethyl (2a) and ethyl ester (2b)were selected to investigate this process. The esters degradation was performed in D2O using the phosphate buffer (pH 7) and progress was followed using the NMR technique (Figure 2).

Table 3. DSC and MDSC Data for All TA Derivatives (40% Mass Concentration in PLA) 40% blend with PLA

Pure Tartrate compound

Tg (°C)

Tc (°C)

Tm (°C)

Tlit m (°C)

PLA

Tg (°C) 62.4

2a 2b 2c 2d 2e 2f 2g 2h

−30.2 −48.5 −65.9 −68.1 −66.8 −49.5 −58.7 −18.9

3a 3b 3c 3d 3e

−59.0 −74.7 −83.8 −86.8 −89.3

24.6

47−4855 1853 31.456

43.2

4157

−7.3 56.2

6258

46.5 −5.3

−24.2

33−3654 −19.3

12.0

−55.9

14.0

3.6 13.2 −8.5 −70.9, 36a −52.3, 30.9b −31.8, 6.4a −54, 57a 19.0 18.2 −35.7 −45.9, 33.4a −89, 38.9a −60.1, 60.2b

a

Second transition temperature (Tg). bThe results from MDSC analysis due to overlapping of Tg and Tc/Tm effects in normal DSC mode.

The observed Tg heat effect is not equal in value across investigated samples. The highest values are present for molecules of poor crystallinity (e.g., 2d or 3d) and samples that can form stronger intermolecular interactions (e.g., pegylated molecules 2f and 2g). Those derivatives present high potential as future plasticizers and, not surprisingly, have functional groups broadly used in commercial polymer modifiers such as PEG or 2-ethylhexyl alkyl chain. On the other hand, samples with a rather weak Tg transition effect form crystalline phases easily and typically have side groups that are able to provide a high degree of order in the solid phase, such as benzyl (2h) or n-octyl (2e, 3e). The Tg value decreases along both series with elongation of the ester group chain length (Figure 1). Tg changes from −68 °C to −30 °C for 2a−2e and from −89 °C to −59 °C for 3a−3e. Moreover, methylation of the hydroxyl groups results in a Tg decrease by an average of 20 °C across all samples 3a−3e, when compared to the 2a−2e series. The highest difference was observed for the 2a−3a couple. Melting effects, seen as Tc and Tm, were also manifested during the DSC experiments. For derivatives 2b, 2d, 2f, 3a, 3c, and 3d, Tm was not observed and it was impossible to determine Tc. Investigated substances do not have Tc and Tm values in the temperature range of the measurements, or the crystallization kinetics is too slow to observe the transition. Tm is not mentioned for those derivatives in literature reports, with the exception of compounds 2b and 3a, which are reported to have melting temperatures of 17 °C and 33−36 °C, respectively.53,54 Despite efforts in purification of 2b and 3a, using vacuum distillation, and use of crystallization-inducing DSC protocols, the previously reported phase transition temperature was not detected. Derivative 2b was reported for particular tendency to form an amorphous phase that was associated with traces of water.53 Such a behavior could be highly beneficial for polymer modifiers and benefit the enhanced disruption of blend crystallinity. Considering all

Figure 2. Hydrolytic degradation of 2a and 2b. The degree of hydrolysis is defined as a cleavage of ester groups (100% means complete transformation to tartaric acid).

Both 2a and 2b degraded faster in the presence of an enzyme and can be classified as biodegradable. Sample 2b hydrolyzed slower, compared to 2a in the pure buffer solution, but faster in the presence of proteinase K. Since 2a and 2b are very watersoluble, we associate the effect with a higher enzyme activity toward the diethyl derivative. Plasticization of PLA. In the search for environmentally friendly polymer additives, the synthesized TA derivatives were investigated as modifiers of PLA. Diesters 2 and 3 were blended with PLA at 40% concentration using the twin-screw counterrotating extruder equipped with the back flow channel at 175 °C (see Figure S14 in the Supporting Information). The time of mixing was 30 min, upon which the mixture was extruded out of the machine. Similar to the pure compounds 2 and 3, all blends were analyzed by means of DSC in order to estimate Tg (see Table 3, as well as Figures S15 and S16 in the Supporting Information), and to verify plasticization abilities of the additives. The gel permeation chromatography (GPC) was performed to analyze the average molar mass of blends and to observe the rate of PLA degradation in the presence of investigated compounds. DSC data indicates that the blends formulated using 2 and 3 display reduced Tg values, in comparison with pure PLA (Table 3). In some cases, there are two Tg values visible, one of which is similar to the Tg value of the additive. Therefore, it can be 6002


Research Article


their contaminations) were contributing to changes of the polymer properties through a polymer degradation. The most stable system is obtained using 2c and 2h. The result for 2c is important because it reduces Tg most effectively among the nonmethylated derivatives. In our experiments, to better examine the effect of degradation, the mixing process was conducted for a rather long time (30 min). The actual residence time of PLA with a modifier, at high temperature, under real manufacturing conditions, is considerably shorter. A correlation between the concentration of diethyl tartrate (2b) and the change of Tg was investigated to better understand the influence of representative biodegradable plasticizer on PLA properties. Ester 2b was chosen for further studies, since it can be derived entirely from renewable resources, meeting “double green” criteria, and displayed amorphous character with very clear Tg transition. The plasticizer concentration was varied in the range from 0 to 50% (see Table 5 and Figure 3).

expected that derivatives 2a−2c, 2h, 3a, and 3b act as primary plasticizers of PLA, while 2d−2g and 3c−3e have a limited or poor miscibility with the polymer. It may be concluded that good miscibility with PLA is linked to the shorter substituents, since, for both series, 2-ethylhexyl (2d, 3d) and n-octyl (2e, 3e) displayed limitations in this respect. In the cases for 2e and 3e, Tg was not visible at the DSC thermogram, because the expected value was overlapping with the temperature of melting phases of the additive. To established those Tg values, the MDSC analysis was performed. The results indicates that both 2e and 3e are secondary plasticizers. It can be concluded that the most effective plasticizer providing both good miscibility and decent reduction of Tg are the derivatives of dibutyl TA esters 2c (among diesters) and 3b (among methylated compounds). An important aspect of plasticizer quality is its influence on the polymer matrix stability. A particularly important parameter is water content introduced into the blend, since typically it strongly influences the PLA matrix degradation. It was noted that the low-molecular-weight derivatives were able to absorb more water. This can be explained by higher polarity of those molecules (e.g., 2a, 2b) in comparison to the higher alkyl tartrates or the methylated derivatives. Also derivatives 2f, 2g are characterized by high water content, which is related to the hydrophilicity of the PEG chains. In order to determine degradation of the polymer chains in the process of plasticization, samples of PLA upon mixing with respective additive were tested by the GPC to assess changes in the Mw of the polymer (Table 4). Polymer dispersity values (Đ) range from 1.9 to 3 and remain similar to that of the PLA.

Table 5. DSC and GPC Results for Various Concentrations of 2b in the PLA Blend mass % of 2b

Tg (°C)

Mw (kg/kDa)

Đ = Mw/Mn

0 5 10 20 30 40 50

62.4 48.8 41.7 39.9 32.3 13.2 7.8

155 133 118 123 122 111 104

2.4 3.09 3.21 3.10 2.78 3.28 3.43

Table 4. Content of Water and Changes in Mw for the 40% Blends with PLA H2O content (ppm)

Mw (kD)

Đ = Mw/Mn

2a 2b 2c 2d 2e 2f 2g 2h

1650 1300 131 100

105 111 129 83 70 85 49 133

2.6 3.3 2.5 1.9 2.0 2.4 3.0 2.3

3a 3b 3c 3d 3e

82 99

63 32 32 114 11

2.3 2.4 2.6 2.0 2.2

155

2.4

compound

PLA

1398 1010 129

Figure 3. DSC (Tg) results for various concentrations of 2b in the PLA blend. The baseline was arbitrarily normalized for a better data clarity. Complete nonmodified DSC data are available in the Supporting Information.

The PLA used in the study displays a glass transition at Tg = 62 °C, while, for pure diethyl tartrate 2b, Tg = −48.5 °C. The fabricated blends were examined using DSC and gel permeation chromatography (GPC), and the results for different concentrations are summarized in Table 5. It was found that any amount of 2b introduced into the PLA caused a reduction in Tg. An addition of 50% of the molecules results in a homogeneous amorphous mixture; therefore, one can expect that, in this range of concentrations, 2b acts as an external plasticizer. The slightly stronger reduction of Tg was achieved with the use of a structural analogue molecule derived from citric acid.17 Additions of triethyl citrate at 10, 20, and 30

The molecular weight values (Mw) of PLA blends show a decrease in value, from 155 kDa for pure processed PLA to 49− 130 kDa for nonmethylated tartrates 2 and 32−63 kDa for methylated tartrates 3. This noticeable decrease in weight may suggest that the use of methylated esters of tartaric acid results in a degradation of the polymer chains to a greater extent than the use of derivatives with free hydroxyl groups. The highest degradation of the PLA molecular weight can be observed for derivative 2g and is likely the result of the presence of residual primary oligo-alcohols. As such, one should take into consideration that, in some cases, investigated plasticizers (or 6003


Research Article

ACS Sustainable Chemistry & Engineering mass %, resulted in blends with Tg values of 42.1, 32.6, and 22.0 °C, respectively. With the increase of 2b concentration, Mw of the polymer matrix decreases from ∼150 kDa (processed PLA) to 104 kDa for 50% 2b. This effect can be associated with the hydrolysis of PLA due to the residual water content in 2b (0.135%), or with the processes of transesterification by free hydroxyl groups present in the 2b molecules. The PLA mixture with 30% 2b was selected for mechanical investigations of the blend properties. Because of the limited amount of material, elongation and tensile strength testing was performed using thin polymeric strands. Samples were mixed in the extruder for 5 and 25 min and compared to observe the effect of extrusion process. An each experimental point was calculated from at least 15 independent measurements. The addition of 2b clearly showed a plasticization effect on the PLA blend and substantially increased the elongation breakage of the sample (Figure 4). As expected, however, PLA has greater yield

those molecules is not limited to this material only and could be broadened to other polymers. Particularly interesting are prospects for application of hydroxyl esters 2 as modifiers of polymers, which are reported for preferential formation of hydrogen bonds within the matrix (e.g., starch).

■

MATERIALS AND METHODS

All reagents and solvents were used as received unless otherwise stated. Commercially available PLA was purchased from Nature Works 2003D (Mw = 158 kDa, PDI = 2.3). Thin-layer chromatography (TLC) was performed with TLC aluminum sheets coated with silica gel 60 RP-18 F254S. Spots were observed with UV irradiation. The DSC measurements were performed using the DSC Q200 V24.2 Build 107 apparatus (TA Instruments) in the temperature range from −100 °C to 200 °C with a cooling/heating rate of 10 °C min−1. Polymer mass distribution was analyzed using a LabAlliance GPC system equipped with a single DVB Jordi gel column (particle size = 5 μm, length = 25 cm, diameter = 1 cm, mass detection range = 100−107 D) and a refractive index detector (Viscotek, Model TDA305). Measurements were performed at 30 °C in CHCl3 and a flow rate of 1 mL min−1, using polystyrene as a standard (see Figure S18 in the Supporting Information). Water content was determined using an automated Karl Fischer titration method and Mettler Toledo C30 KF coulometric titrator. Infrared (IR) spectra were collected using the Bruker FT-IR Alpha spectrometer with ATR module. 1H NMR spectra were acquired at 400 and 500 MHz, and 13C NMR spectra were acquired at 125 MHz. Biodegradation experiments involving 2a, 2b were performed using 24 mL of 0.1 M phosphate buffer (pH 7), deuterated using two cycles of D2O addition (2 × 24 mL) and lyophilization. Dry residue was dissolved in 12 mL of D2O and 12 mg of sodium azide. Solution was used to prepare four different samples (2 mL) containing 2a or 2b (20 mg) in combination with or without of Proteinase K (0.6 mg). Samples were thermostated at 25 °C and measured using 1H NMR. The degree of hydrolysis, as a percentage of cleaved ester groups, was established by comparing the CH proton signal vicinal to the carbonyl ester group (see Figure S19 in the Supporting Information). The polymer blending experiments were performed using the Thermo Scientific Model Haake Minilab II extruder equipped with a counter-rotating screw bypassed system and rectangular die. Blending was performed at 175 °C, with the screw speed of 50 rpm for blending (corresponding shear rate 177 s−1) and 20 rpm for extrusion. Loading of PLA and plasticizer lasted up to 3 and 2 min, respectively. The blending process lasted 33 min, and 7 g of substrates were used in each experiment. Extrusion of strings, used in the mechanical properties testing, was performed using the same device. PLA and 2b (30%) were stirred for 30 min at 175 °C, and extruded in the form of strings for the next 25 min. The 8 cm sections were collected in two batches, namely, in the first 5 min of the extrusion and the last 5 min of the extrusion. Mechanical testing was performed using the Instron 5566 device. Synthesis. Dimethyl-L-tartrate (2a). Tartaric acid (50 g, 0.33 mol), methanol (575 mL, 14.2 mol) and p-toluenesulfonic acid (TsOH) (3.1 g, 5 mol % TA) were refluxed for 24 h and the reaction mixture was cooled to room temperature. Subsequently, the reaction mixture was neutralized with anhydrous NaHCO3 (1.5 g) and filtered, the methanol was evaporated, and the residue was distilled under reduced pressure to give a colorless product (57.6 g, 98% yield). Boiling point (bp) = 115°/1 hPa, Tm = 57−60 °C (melting point apparatus), Tm 46.5 °C (DSC), Tg −30.2 °C, [α]25 D + 21.0° (c 2.5, H2O), 1H NMR (DMSO, 400.1 MHz): δ 3.64 (s, 6H, CH3), 4.40 (s, 2H, CH), 5.67 (s, 2H, OH) (see Figure S20 in the Supporting Information). Diethyl-L-tartrate (2b). Tartaric acid (50 g, 0.33 mol), ethanol (875 mL, 15 mol), and p-toluenesulfonic acid (TsOH) (3.1 g, 5 mol % TA) were refluxed for 24 h and the reaction mixture was cooled to room temperature. Subsequently the reaction mixture was neutralized with anhydrous NaHCO3 (1.5 g) and filtered, the ethanol was evaporated, and the residue was distilled under reduced pressure to give a colorless

Figure 4. Elongation at break and tensile strength values for the strings fabricated from pure PLA and a PLA/2b 30% blend.

strength and greater tensile strength, when compared to the PLA/2b blend (see Figure S17 in the Supporting Information). Strings extruded after longer mixing periods displayed slightly worse mechanical properties; however, the observed differences were below statistical significance. We can conclude that 2b displays plasticization properties toward PLA; however, the exact composition of the mixture needs further optimization to improve performance (see Figure 4).

■

CONCLUSIONS The tartaric acid derivatives present an interesting opportunity as new plastic additives, because of their origin from natural resources and their ability to biodegrade together with a polymeric matrix. All investigated compounds, including three molecules that were not previously reported (3c−3e), were obtained with satisfactory yields at large laboratory scale. DSC experiments showed that all compounds, upon blending with PLA, reduced the Tg value of the resulting polymer mixture, in comparison to pure PLA. Not all derivatives were mixing effectively with PLA at 40% concentration. Considering the lowest obtained Tg value and good mixing criteria, the best results were achieved for dibutyl ester 2b, 2c, and methylated diethyl ester 3b derivatives. It was observed that extension of the chain of ester substituent in the additive causes a reduction of Tg values for PLA blends, but also limits miscibility of the molecule with PLA. Methylation of diesters resulted in reduced Tg temperatures across the entire series. The investigated molecules represent promising, biodegradable solutions available for the modification of PLA; nevertheless, potential use of 6004


Research Article

ACS Sustainable Chemistry & Engineering product (65.94 g, 97% yield). bp = 135 °C/0.133 hPa, Tg = −48.5 °C, 1 [α]25 D + 7.8° (neat), H NMR (CDCl3, 400 MHz): δ 1.24 (t, 6H, CH3), 3.58 (s, 2H, OH), 4.23 (m, 4H, CH2), 4.48 (s, 2H, CH) (see Figure S21 in the Supporting Information). Dibutyl-L-tartrate (2c). Tartaric acid (22.5 g, 0.15 mol), n-butanol (30 g, 0.41 mol), toluene (30 mL), p-toluenesulfonic acid (1.5 g, 5 mol % TA) were refluxed for 10 h. The water formed during the reaction was removed by azeotropic distillation and collected in a water separator. Subsequently, the cooled reaction mixture was neutralized with anhydrous NaHCO3 (1 g), filtered, n-butanol was evaporated, and the residue was distilled under reduced pressure to give a colorless product (35.2 g, 89.5% yield). bp = 147 °C/11 hPa, Tm = 20−22 °C (melting point apparatus), Tm = 24.6 °C (DSC), Tg = 1 −65.9 °C, [α]25 D + 18.6° (c 2, acetone), H NMR (CDCl3, 400 MHz): δ 0.89 (t, 6H, CH3), 1.35 (m, 4H, CH2), 1.63 (m, 4H, CH2), 3.42 (s, 2H, OH), 4.21 (m, 4H, CH2), 4.49 (s, 2H, CH). Bis(2-ethylhexyl)-L-tartrate (2d). Tartaric acid (22.5 g, 0.15 mol), 2-ethylhexanol (48.85 g, 0.375 mol), toluene (30 mL), ptoluenesulfonic acid (1.5 g, 5 mol % TA) were refluxed for 10 h. The water formed during the reaction was removed by azeotropic distillation and collected in a water separator. Subsequently, the cooled reaction mixture was neutralized with anhydrous NaHCO3 (1 g) and filtered, 2-ethylhexanol was evaporated, and the residue was distilled under reduced pressure to give a colorless product (33.7 g, 60% yield). bp = 196 °C/0.8 hPa, Tg = −68.1 °C, [α]25 D + 18.2° (c 2.2, acetone), 1 H NMR (CDCl3, 400 MHz): δ 0.891 (m, 12H, CH3), 1.28 (m, 16H, CH2), 1.63 (m, 2H, CH), 3.24 (s, 2H, OH), 4.18 (m, 4H, CH2), 4.51 (s, 2H, CH). Dioctyl-L-tartrate (2e). Tartaric acid (22.5 g, 0.15 mol), octanol (43 g, 0.165 mol), toluene (30 mL), p-toluenesulfonic acid (1.5 g, 5 mol % TA) were refluxed for 24. The water formed during the reaction was removed by azeotropic distillation and collected in a water separator. Subsequently, the cooled reaction mixture was neutralized with anhydrous NaHCO3 (1 g), filtered, octanol was evaporated, and the residue was distilled under reduced pressure to give a colorless product (46.6 g, 83% yield). bp = 200 °C/1 hPa, Tm = 43.2 °C (DSC), Tg = 1 −66.8 °C, [α]25 D + 13.2° (c 1.8, acetone), H NMR (CDCl3, 400 MHz): δ 4.49 (s, 2H), 4.21 (m, 4H), 3.32 (s, 2H), 1.64 (m, 4H), 1.29 (m, 20H), 0.85 (t, 6H). Dimethoxytriglycol-L-tartrate (2f). Tartaric acid (8.29 g, 0.055 mol), tri(ethylene glycol) monomethyl ether (19 g, 0.116 mol), toluene (100 mL), p-toluenesulfonic acid (1.05 g, 10 mol % TA) were refluxed for 24 h. The water formed during the reaction was removed by azeotropic distillation and collected in the water separator. Subsequently, the cooled reaction mixture was neutralized with anhydrous NaHCO3 (0.5 g) and filtered, and triethylene glycol monomethyl ether was evaporated. The remains were concentrated under vacuum to give a pale yellow to colorless crude product (24 g, 1 98% yield). Tg = −49.5 °C [α]25 D + 10.6° (c 1, MeOH), H NMR (CDCl3, 400 MHz): δ 4.60 (s, 2H), 4.35 (m, 4H), 3.70 (m, 4H), 3.62 (m, 14H), 3.52 (m, 4H), 3.34 (m, 6H). DiMPEG350-L-tartrate (2g). Tartaric acid (3.86 g, 0.026 mol), poly(ethylene glycol methyl ether), average molecular weight of MW = 350 (19 g, 0.054 mol), toluene (100 mL), p-toluenesulfonic acid (0.25 g, 5 mol% TA) were refluxed for 24 h and the produced water was separated by the water separator. The cooled reaction mixture was neutralized with anhydrous NaHCO3 (0.5 g), and filtered. Subsequently, unreacted poly(ethylene glycol methyl ether) was evaporated and the remains were concentrated under vacuum to give a pale yellow to colorless crude product (21 g, 97% yield). Tm = −7.3 °C 1 (DSC), Tg = −58.7 °C, [α]25 D + 5.5° (c 1, MeOH), H NMR (CDCl3, 400 MHz): δ 4.58 (s, 2H), 4.33 (m, 4H), 3.68 (m, 6H), 3.5921 (m, 66H), 3.50 (m, 6H), 3.32 (s, 6H). Dibenzyl-L-tartrate (2h). Triethyl amine (97.5 mL, 70.8 g, 0.7 mol) was added to the mixture of potassium iodide (7.03 g, 0.0423 mol), tartaric acid (50.02 g, 0.33 mol), and acetone (500 mL). Mixture was heated to reflux and benzyl bromide (79.5 mL, 114 g, 0.67 mol) was added dropwise. Heating was continued for the next 6 h. Subsequently, the mixture was filtered and the precipitate was washed four times with a small amount of acetone. The collected filtrate was concentrated to

half volume (60 °C) and slowly poured into a vigorously stirred suspension of water (300 mL), Et2O (1 mL), and a drop of emulsifier (Rokopol 30P27). The formed solid was separated by filtration and washed with hexane, yielding dibenzyl-L-tartrate (82.1 g, 75% yield). Tm = 50−52 °C (melting point apparatus), Tm = 56.2 °C (DSC), Tg = 1 −18.9 °C, [α]25 D + 11.0° (c 2, acetone), H NMR (CDCl3, 400 MHz): δ 4.16 (s, 2H, CH), 5.31−5.23 (q, 4H, CH2), 7.38−7.34 (m, 10H, C6H5). Dimethyl(O,O′-dimethyl)-L-tartrate (3a). Dimethyl tartrate (3.56 g, 20.0 mmol) and dimethyl sulfate (3.89 mL, 41.0 mmol) were added to a cooled (0 °C) suspension of NaH (960 mg, 40.0 mmol) in diethyl ether (200 mL). The resulting mixture was stirred overnight at room temperature, filtered, and extracted with diethyl ether (3 × 50 mL), dried over MgSO4, and filtered. Solvent was evaporated under vacuum to give a colorless oil. Crude product was purified by distillation under reduced pressure to give a colorless product (3.5 g, 85% yield). bp = 1 112 °C/8 hPa, Tg = −59.0 °C, [α]25 D + 71° (c 2, MeOH), H NMR (CDCl3, 400 MHz): δ 3.38 (s, 6H, CH3), 3.73 (s, 6H, OCH3), 4.16 (s, 2H, CH). Diethyl(O,O′-dimethyl)-L-tartrate (3b). Diethyl tartrate (4.13 g, 20.0 mmol) and dimethyl sulfate (3.89 mL, 41.0 mmol) were added in parallel to a cooled (0 °C) suspension of NaH (960 mg, 40.0 mmol) in diethyl ether (200 mL). The resulting mixture was stirred overnight at room temperature, filtered, extracted with diethyl ether (3 × 50 mL), dried over MgSO4 and filtered. Solvent was evaporated under vacuum to give a colorless oil (3.7 g). Product was purified by distillation under reduced pressure to give a colorless oil (3.3 g, 71% yield). bp = 104°/ 0.5 hPa, Tm = 12.0 °C (DSC), Tg = −74.7 °C, [α]25 D + 77.0° (c 1, EtOH), 1H NMR (CDCl3, 400.1 MHz): δ 1.197 (t, 6H, CH3), 3.02 (s, 6H, OCH3), 4.102 (s, 2H, CH); 4.156 (m. 4H, CH2). Dibutyl(O,O′-dimethyl)-L-tartrate (3c). Dibutyl tartrate (5.25 g, 20.0 mmol) and dimethyl sulfate (3.89 mL, 41.0 mmol) were added to a cooled (0 °C) suspension of NaH (960 mg, 40.0 mmol) in diethyl ether (200 mL). The resulting mixture was stirred overnight at room temperature, filtered, and extracted with diethyl ether (3 × 50 mL). Subsequently, the solution was dried over MgSO4, filtered, and solvent was evaporated under vacuum to give a colorless oil (6.0 g). Distillation under reduced pressure resulted in a colorless product (5.7 g, 98% yield). bp = 162−163 °C/13.33 hPa, Tg = −83.8 °C, [α]25 D + 71.1° (c 5.1, EtOH), 1H NMR (CDCl3, 400 MHz): δ 4.16 (t, 2H), 3.39 (s, 3H), 1.64−1.59 (m, 2H), 1.39−1.30 (m, 2H), 0.88 (t, 3H), GC-MS: τret 9.35−9.59 min, C14H26O6 (M+): Calc. 290, Found 290 Da, Anal. Calc. for C14H26O6: C 57.91, H 9.03, O 33.06, Found: C 58.73, H 9.02, O 32.25, FT-IR (ATR) (ν, cm−1) 2959, 2932, 2874, 2833, 1756, 1731, 1149, 1110, 1061, 1H NMR (CDCl3, 500 MHz): δ 4.26−4.14 (m, 6H), 3.44 (s, 6H), 1.68−1.62 (m, 4H), 1.42−1.36 (m, 4H), 1.42−1.25 (m, 16H), 0.91−0.87 (m, 12H), 13C NMR (CDCl3, 125.7 MHz): δ 169.4, 81.3, 65.2, 59.7, 30.8, 19.2, 13.8 (see Figure S22 in the Supporting Information). Bis(2-ethylhexyl)(O,O′-dimethyl)-L-tartrate (3d). Bis(2-ethylhexyl) tartrate (27.4 g, 73.2 mmol) and dimethyl sulfate (13.88 mL, 143.3 mmol) were added to a cooled (0 °C) suspension of NaH (3.51 g, 146.3 mmol) in diethyl ether (200 mL). The resulting mixture was stirred overnight at room temperature, filtered, extracted with diethyl ether (3 × 50 mL), and dried over MgSO4 Upon filtration, solvent was evaporated under vacuum to give crude product (29.45 g). Product was purified by column chromatography (hexane/Et2O 95:5) to afford the title compound (24.5 g, yield 83%) as a colorless oil. Tg = −86.8 −1 °C, [α]25 D + 53.6° (c 2.1, acetone), FT-IR (ATR) (ν, cm ) 2958, 1 2927, 2860, 1758, 1733, 1149, 1112, H NMR (CDCl3, 500 MHz): δ 4.21 (s, 2H), 4.198−4.056 (m, 4H), 3.447 (s, 6H), 1.616 (m, 2H), 1.422−1.247 (m, 16H), 0.911−0.874 (m, 12H), 13C NMR (CDCl3, 125.7 MHz): δ 169.5, 81.3, 67.65, 67.62, 59.7, 38.93, 38.87, 30.5, 30.4, 29.04, 28.99, 23.8, 23.7, 23.1, 14.2, 11.1, 10.98 (see Figure S23 in the Supporting Information). Dioctyl(O,O′-dimethyl)-L-tartrate (3e). Dioctyl tartrate (7.7 g, 20.6 mmol) and dimethyl sulfate (3.89 mL, 41.1 mmol) were added to a cooled (0 °C) suspension of NaH (0.99 g, 41.1 mmol) in diethyl ether (50 mL). The mixture was stirred overnight at room temperature, filtered, extracted with diethyl ether (3 × 15 mL), and dried over 6005


Research Article

ACS Sustainable Chemistry & Engineering MgSO4. Subsequently, solvent was evaporated under vacuum to give crude product (8.22 g). The residue was purified by column chromatography (hexane/Et2O 95:5) to afford the title compound (7.23 g, 87.3% yield) as a colorless oil. Tm = 14.0 °C (DSC), Tg = −1 −89.3 °C, [α]25 D + 46.5° (c 2.3, acetone), FT-IR (ATR) (ν, cm ) 1 2954, 2924, 2855, 1758, 1734, 1149, 1112, H NMR (CDCl3, 500 MHz): δ 4.26−4.12 (m, 6H), 3.43 (s, 6H), 1.67−1.62 (m, 4H), 1.34− 1.24 (m, 20H), 0.85 (t, 6H), 13C NMR (CDCl3, 125 MHz): δ 169.4, 81.3, 65.6, 59.7, 31.9, 29.3, 28.8, 26.0, 22.7, 14.2 (see Figure S24 in the Supporting Information).

■

(11) Lebarbe, T.; Grau, E.; Gadenne, B.; Alfos, C.; Cramail, H. Synthesis of Fatty Acid-Based Polyesters and Their Blends with Poly(L-lactide) as a Way To Tailor PLLA Toughness. ACS Sustainable Chem. Eng. 2015, 3, 283. (12) Jacobsen, S.; Fritz, H. G. Plasticizing polylactideThe effect of different plasticizers on the mechanical properties. Polym. Eng. Sci. 1999, 39, 1303. (13) Ma, P.; Shen, T.; Xu, P.; Dong, W.; Lemstra, P. J.; Chen, M. Superior Performance of Fully Biobased Poly(lactide) via Stereocomplexation-Induced Phase Separation: Structure versus Property. ACS Sustainable Chem. Eng. 2015, 3, 1470. (14) Lebarbe, T.; Grau, E.; Alfos, C.; Cramail, H. Fatty acid-based thermoplastic poly(ester-amide) as toughening and crystallization improver of poly(L-lactide). Eur. Polym. J. 2015, 65, 276. (15) Kowalczyk, M.; Pluta, M.; Piorkowska, E.; Krasnikova, N. Plasticization of polylactide with block copolymers of ethylene glycol and propylene glycol. J. Appl. Polym. Sci. 2012, 125, 4292. (16) Sinclair, R. G. The Case for Polylactic Acid as a Commodity Packaging Plastic. J. Macromol. Sci., Part A: Pure Appl. Chem. 1996, 33, 585. (17) Labrecque, L. V.; Kumar, R. A.; Dave, V.; Gross, R. A.; McCarthy, S. P. Citrate Esters as Plasticizers for Poly(lactic acid). J. Appl. Polym. Sci. 1997, 66, 1507. (18) Ljungberg, N.; Wesslén, B. The Effects of Plasticizers on the Dynamic Mechanical and Thermal Properties of Poly(Lactic Acid). J. Appl. Polym. Sci. 2002, 86, 1227. (19) Sheth, M.; Kumar, R. A.; Davé, V.; Gross, R. A.; McCarthy, S. P. Biodegradable Polymer Blends of Poly(lactic acid) and Poly(ethylene glycol). J. Appl. Polym. Sci. 1997, 66, 1495. (20) Bao, R.-Y.; Yang, W.; Wei, X.-F.; Xie, B.-H.; Yang, M.-B. Enhanced Formation of Stereocomplex Crystallites of High Molecular Weight Poly(L-lactide)/Poly(D-lactide) Blends from Melt by Using Poly(ethylene glycol). ACS Sustainable Chem. Eng. 2014, 2, 2301. (21) Rodrigues, C. A.; Tofanello, A.; Nantes, I. L.; Rosa, D. S. Biological Oxidative Mechanisms for Degradation of Poly(lactic acid) Blended with Thermoplastic Starch. ACS Sustainable Chem. Eng. 2015, 3, 2756. (22) Eguiburu, J. L.; Iruin, J. J.; Fernandez-Berridi, M. J.; San Román, J. Blends of amorphous and crystalline polylactides with poly(methyl methacrylate) and poly(methyl acrylate): A miscibility study. Polymer 1998, 39, 6891−6897. (23) Persenaire, O.; Quintana, R.; Lemmouchi, Y.; Sampson, J.; Martin, S.; Bonnaud, L.; Dubois, P. Reactive compatibilization of poly(L-lactide)/poly(butylene succinate) blends through polyester maleation: From materials to properties. Polym. Int. 2014, 63, 1724. (24) Quintana, R.; Persenaire, O.; Lemmouchi, Y.; Bonnaud, L.; Dubois, P. Compatibilization of co-plasticized cellulose acetate/water soluble polymers blends by reactive extrusion. Polym. Degrad. Stab. 2016, 126, 31. (25) Baiardo, M.; Frisoni, G.; Scandola, M.; Rimelen, M.; Lips, D.; Ruffieux, K.; Wintermantel, E. Thermal and mechanical properties of plasticized poly(L-lactic acid). J. Appl. Polym. Sci. 2003, 90, 1731. (26) Martin, O.; Avérous, L. Poly(lactic acid): plasticization and properties of biodegradable multiphase systems. Polymer 2001, 42, 6209. (27) Lai, W.-C.; Liau, W.-B.; Lin, T.-T. The effect of end groups of PEG on the crystallization behaviors of binary crystalline polymer blends PEG/PLLA. Polymer 2004, 45, 3073. (28) Pluta, M.; Paul, M.-A.; Alexandre, M.; Dubois, P. Plasticized polylactide/clay nanocomposites. II. The effect of aging on structure and properties in relation to the filler content and the nature of its organo-modification. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 312. (29) Hu, Y.; Hu, Y. S.; Topolkaraev, V.; Hiltner, A.; Baer, E. Aging of poly(lactide)/poly(ethylene glycol) blends. Part 2. Poly(lactide) with high stereoregularity. Polymer 2003, 44, 5711. (30) Do, Y. S.; Soo, L. H.; Bae, J. Y. Bio-degradable plastic film for garbage bag made by using polyvinyl alcohol and phosphoric acid/ hydropropyl cross linking starch. Korean Patent KR 1020090032621, 2009.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00814. Original DSC results, GPC and spectroscopic data, additional mechanical testing data (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +48-22-234-5583. Fax: +48-22-625-5317. E-mail: [email protected]. ORCID

Dominik Jańczewski: 0000-0002-5466-6444 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are grateful to the National Centre for Research and Development (NCBiR), Poland for founding this research within the projects “BIOPOL” (POIG.01.01.02-10-025/09) and “CHIKADI” (PBS2/A1/14/2014).

■

REFERENCES

(1) Wypych, G. Handbook of Plasticizers, 2nd Edition; ChemTec Publishing: Toronto, Canada, 2012. (2) Sudesh, K.; Iwata, T. Sustainability of Biobased and Biodegradable Plastics. Clean: Soil, Air, Water 2008, 36, 433. (3) Cadogan, D. F.; Howick, C. J. Ullmann’s Encyclopedia of Industrial Chemistry; Wiley−VCH: New York, 2000. (4) Rahman, M.; Brazel, C. S. The plasticizer market: An assessment of traditional plasticizers. Prog. Polym. Sci. 2004, 29, 1223. (5) Tureckova, J.; Prokopova, I.; Niklova, P.; Simek, J.; Smejkalova, P.; Keclik, F. Biodegradable copolyester/starch blendsPreparation, mechanical properties, wettability, biodegradation course. Polimery 2008, 53, 639−643. (6) Bocqué, M.; Voirin, C.; Lapinte, V.; Caillol, S.; Robin, J.-J. Petrobased and bio-based plasticizers: Chemical structures to plasticizing properties. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 11. (7) Gupta, A. P.; Kumar, V. New emerging trends in synthetic biodegradable polymersPolylactide: A critique. Eur. Polym. J. 2007, 43, 4053. (8) Slomkowski, S.; Penczek, S.; Duda, A. PolylactidesAn overview. Polym. Adv. Technol. 2014, 25, 436. (9) Xu, S.; Yang, F.; Zhou, X.; Zhuang, Y.; Liu, B.; Mu, Y.; Wang, X.; Shen, H.; Zhi, G.; Wu, D. Uniform PEGylated PLGA microcapsules with embedded Fe3O4 nanoparticles for US/MR dual-modality imaging. ACS Appl. Mater. Interfaces 2015, 7, 20460. (10) Slomkowski, S.; Gadzinowski, M.; Sosnowski, S.; RadomskaGalant, I.; Pucci, A.; De Vita, C.; Ciardelli, F. Nanoparticles from polylactide and polyether block copolymers: Formation, properties, encapsulation, and release of pyreneFluorescent model of. J. Nanosci. Nanotechnol. 2006, 6, 3242. 6006


Research Article


Intramolecular Diels-Alder Strategy. J. Org. Chem. 1987, 52, 5704− 5714. (52) Debenedetti, P. G.; Stillinger, F. H. Supercooled liquids and the glass transition. Nature 2001, 410, 259. (53) Lowry, T. M.; Cutter, J. O. The rotatory dispersive power of organic compounds. Part X. The preparation and properties of pure ethyl tartrate. J. Chem. Soc., Trans. 1922, 121, 532. (54) Korth, H.-G.; Sustmann, R.; Merényi, R.; Viehe, H. G. Absolute rates for dimerization of capto-dative substituted methyl radicals in solution: absence of kinetic stabilization. J. Chem. Soc., Perkin Trans. 2 1983, 67. (55) Cope, A. C.; Mehta, A. S. Molecular Asymmetry of Olefins. II. The Absolute Configuration of trans-Cyclooctene. J. Am. Chem. Soc. 1964, 86, 5626. (56) Austin, P. C. The rotatory dispersion of derivatives of tartaric acid. Part III. Diacetyltartaric acid and its esters. J. Chem. Soc. 1928, 1825. (57) van Nunen, J. L. M.; Folmer, B. F. B.; Nolte, R. J. M. Induction of Liquid Crystallinity by Host−Guest Interactions. J. Am. Chem. Soc. 1997, 119, 283−291. (58) Hu, Y.; Yamada, K. A.; Chalmers, D. K.; Annavajjula, D. P.; Covey, D. F. Enantioselective Synthesis of Cyclothiazide Analogues: Novel Probes of the Stereospecific Actions of Benzothiadiazines at AMPA-Type Glutamate Receptors. J. Am. Chem. Soc. 1996, 118, 4550.

(31) Kulinski, Z.; Piorkowska, E.; Gadzinowska, K.; Stasiak, M. Plasticization of Poly(L-lactide) with Poly(propylene glycol). Biomacromolecules 2006, 7, 2128. (32) Piorkowska, E.; Kulinski, Z.; Galeski, A.; Masirek, R. Plasticization of semicrystalline poly(L-lactide) with poly(propylene glycol). Polymer 2006, 47, 7178. (33) Schroft, S.; Buechsel, M.; Heinrich, G.; Kausen, M. Biodegradable filter material for production of tea or coffee bags contains a cellulose derivative plasticized with a plasticizer, e.g. glycerol, or with a biodegradable polymer, e.g. aliphatic polyester. Ger. Patent DE1999131402, 2001. (34) Younes, H.; Cohn, D. Phase separation in poly(ethylene glycol)/poly(lactic acid) blends. Eur. Polym. J. 1998, 24, 765. (35) López-Rodríguez, N.; Sarasua, J. R. Plasticization of Poly-Llactide with L-Lactide. Polym. Eng. Sci. 2013, 53, 2073−2080. (36) Synoradzki, L.; Bernaś, U.; Ruśkowski, P. Application of tartaric acid and of o-acyl tartaric acids and anhydrides. Resolution of racemates. Org. Prep. Proced. Int. 2008, 40, 163. (37) Di Gioia, L. D.; Guilbert, S. Corn Protein-Based Thermoplastic Resins: Effect of Some Polar and Amphiphilic Plasticizers. J. Agric. Food Chem. 1999, 47, 1254−1261. (38) Lawton, J. W. Plasticizers for Zein: Their Effect on Tensile Properties and Water Absorption of Zein Films. Cereal Chem. 2004, 81, 1. (39) Ullah, A.; Vasanthan, T.; Bressler, D.; Elias, A. L.; Wu, J. Bioplastics from Feather Quill. Biomacromolecules 2011, 12, 3826. (40) Sun, W.; Howell, B. A. Tartaric Acid as a Useful Biosource for Polymer Additives. In Proceedings of 39th North American Thermal Analysis Society Conference, Des Moines, IA, USA, 2011; p 163. (41) Furuta, K.; Gao, Q.-Z.; Yamamoto, H. Chiral (acyloxy)borane complex-catalyzed asymmetric diels-alder reaction: (1r)-1,3,4-trimethyl-3-cyclohexene-1-carboxaldehyde. Org. Synth. 1995, 72, 86. (42) Buschhaus, B.; Bauer, W.; Hirsch, A. Synthesis and chiroptical properties of a new type of chiral depsipeptide dendrons. Tetrahedron 2003, 59, 3899. (43) Łukasik, B.; Mikołajczyk, M.; Bujacz, G.; Ż urawiński, R. Synthesis and the absolute configuration of both enantiomers of 4,5dihydroxy-3-(formyl)cyclopent-2-enone acetonide as a new chiral building block for prostanoid synthesis. Org. Biomol. Chem. 2015, 13, 807−816. (44) Gawroński, J.; Gawrońska, K. Tartaric and Malic Acids in Synthesis: A Source Book of Building Blocks, Ligands, Auxiliaries, and Resolving Agents; Wiley−Interscience: New York, 1999; pp 13−37. (45) Angelovski, G.; Keränen, M. D.; Eilbracht, P. Fluorescence screening of tartaric acid-derived azamacrocycles synthesized via sequential hydroformylation/reductive amination as potential ligands for asymmetric catalysis. Tetrahedron: Asymmetry 2005, 16, 1919. (46) Dang, H.-S.; Roberts, B. P.; Tocher, D. A. Optical Resolution and Absolute Stereochemistry of trans-2,5-Dimethyl-1-phenyl-1silacyclopentane. J. Chem. Soc., Perkin Trans. 1 1995, 2, 117−123. (47) Wang, Z.; Wang, Q.; Zhang, Y.; Bao, W. Synthesis of new chiral ionic liquids from natural acids and their applications in enantioselective Michael addition. Tetrahedron Lett. 2005, 46, 4657− 4660. (48) Abe, Y.; Shoji, T.; Kobayashi, M.; Wang, Q.; Asai, N.; Nishizawa, H. Enantioselective Distribution of Amino-Alcohols in a Liquid− Liquid Two-Phase System Containing Dialkyl L-Tartrate and Boric acid. Chem. Pharm. Bull. 1995, 43, 262. (49) Prelog, V. V.; Stojanac, Z.; Kovče vić, K. Ü ber die Enantiomerentrennung durch Verteilung zwischen flüssigen Phasen. 3. Mitteilung. Selektivität der lipophilen Weinsäureester für chirale Ammonium-Salze verschiedener Konstitution und Konfiguration. Helv. Chim. Acta 1982, 65, 377−384. (50) Haworth, W. N.; Jones, D. I. New reference compounds in the sugar group. The methylamides of D-, L-, and i-dimethoxysuccinic acids and of L-arabo- and i-xylo-trimethoxyglutaric acids. J. Chem. Soc. 1927, 2349. (51) Shishido, K.; Takahashi, K.; Fukumoto, K.; Kametani, T.; Honda, T. A Synthetic Approach to (−)-Quassimarin Based on 6007


Esters of Tartaric Acid, A New Class of Potential âDouble Green

Recommend Documents

Esters of Tartaric Acid, A New Class of Potential âDouble Green