- Email: [email protected]
Synthesis and properties of thermoplastic starch laurates
Carbohydrate Research 486 (2019) 107833
Contents lists available at ScienceDirect
Carbohydrate Research journal homepage: www.elsevier.com/locate/carres
Synthesis and properties of thermoplastic starch laurates Sascha Blohm, Thomas Heinze
T
∗
Institute for Organic Chemistry and Macromolecular Chemistry, Friedrich Schiller University of Jena, Center of Excellence for Polysaccharide Research, Humboldtstraße 10, D-07743, Jena, Germany
ARTICLE INFO
ABSTRACT
Keywords: Thermoplastic starch Esterification In situ activation Lauric acid Substitution pattern
Maize starch was allowed to react homogeneously in N,N-dimethylacetamide (DMAc)/LiCl with lauric acid activated with 4-toluenesulfonyl chloride, N,N′-dicyclohexylcarbodiimide/4-(1-pyrrolidinyl)pyridine, 1,1′-carbonyldiimidazole, and N,N-dimethylformamide (DMF) combined with oxalyl chloride. Characterization of the products by means of 13C NMR spectroscopy revealed different substitution patterns depending on the activation agent. The activation of lauric acid with DMF in combination with oxalyl chloride gave starch laurates of highest degree of substitution (DS), yield and reaction efficiency. The melting temperatures and the solubility of the thermoplastic starch laurates were found to depend on the DS, the substitution pattern, and on the molar mass of the starch esters.
1. Introduction
used to prepare starch acetates [12] and starch laurates [10]. Iminium chloride (ImCl), which is obtained by the reaction of N,N-dimethylformamide (DMF) with chlorinating agents like phosphoryl chloride, phosphorus trichloride, or oxalyl chloride [13] was applied for the conversion of cellulose with palmitic-, stearic-, 4-nitrobenzoic acid, and adamantane carboxylic acid and yielded the corresponding products with DS values of up to 1.89 [14]. In the present study, the conversion of starch with lauric acid applying different activation methods was carried out under comparable conditions. The goal was to figure out the most efficient in situ activation method for synthesis of starch fatty acid esters as well as to study the product properties in dependence on the synthesis method. Homogeneous reaction conditions could be realized using DMAc/LiCl as reaction medium that is more stable compared to dimethyl sulfoxide, which is often used in starch chemistry although it undergoes side reactions in a Swern-like manner.
Starch is one of the most abundant biopolymers and is applied in various areas like food, packaging, and cosmetics [1]. Starch may form gels that are not stable due to retrogradation/syneresis [2]. To improve the properties of starch products, the biopolymer is functionalized to a very low extent, yielding an enhancement of stability of suspensions and gels and adhesion and of film formation, on one hand [2,3]. On the other hand, derivatization of starch with carboxylic acids to a comparably high degree of substitution (DS) may lead to enhanced mechanical and thermal stability and may even yield thermoplastic material [1]. Starch fatty acid esters possess low water permeability, increased glass transition temperature, and increased tensile strength, which is improved with increasing DS [4,5]. Starch esters of monocarboxylic acids with DS values between 1.5 and 3.0 are hydrophobic and thermoplastic materials [1] and are still biodegradable [2,3]. They are used as coatings and polymers in hot melt adhesives [6]. For esterification of polysaccharides, various in situ activating agents are increasingly studied for the carboxylic acid. Sulfonic acid chlorides (in particular p-toluenesulfonic acid chloride, Tos-Cl) forming mixed- and symmetric anhydride as well as the chloride of the carboxylic acid of enhanced reactivity [7]. Tos-Cl was applied to prepare polysaccharide esters with carboxylic acids; even adamantane carboxylic acid led to products with DS values of up to 3 [8–10]. N,N′Dicyclohexylcarbodiimide (DCC) was used to synthesize cellulose propionate with DS values in the range from 0.1 to 2.5 [11]. Another promising activating agent is 1,1′-carbonyldiimidazole (CDI) that was
∗
2. Results and discussion 2.1. Synthesis of starch laurates applying different activation methods for the carboxylic acid Starch laurates were synthesized under homogeneous conditions in N,N-dimethylacetamide (DMAc)/lithium chloride applying different activating agents for lauric acid, namely p-toluenesulfonyl chloride (Tos-Cl), N,N′-dicyclohexylcarbodiimide (DCC) in combination with 4(1-pyrrolidinyl)pyridine (PP), 1,1′-carbonyldiimidazole (CDI), and
Corresponding author. E-mail address: [email protected] (T. Heinze).
https://doi.org/10.1016/j.carres.2019.107833 Received 26 June 2019; Received in revised form 2 October 2019; Accepted 8 October 2019 Available online 09 October 2019 0008-6215/ © 2019 Elsevier Ltd. All rights reserved.
Carbohydrate Research 486 (2019) 107833
S. Blohm and T. Heinze
Fig. 1. Reaction schema for the esterification of starch with differently activated lauric acid in N,N-dimethylacetamide/lithium chloride (DMAc/LiCl).
A product (5a) with a DS of 0.66 could be synthesized even within a short reaction time to 2 h and molar ratio to 1:1:1 of AGU: lauric acid: oxalyl chloride combined with DMF (ImCl). After 3 h, a product of high DS of 1.67 (5b) could be obtained that is even higher compared to the application of the other activating agents applying a reaction time of 16 h. The starch ester obtained after 6 h possesses a DS of 1.90 (5c). Products with DS values of up to 2.13 (5d) could be synthesized, which corresponds to an efficiency of 71%. To sum up, regarding the maximal DS achieved, it turned out that the activation of lauric acid with oxalyl chloride and DMF (ImCl) is most efficient. The DS of the starch laurates obtained under comparable conditions applying different activating agents increased in the order DCC/PP < CDI < Tos-Cl < ImCl and the efficiency decreases from 71% to 41% following the order ImCl > Tos-Cl > CDI > DCC.
Table 1 Reaction conditions for and results of the syntheses of starch laurates by activation of lauric acid with p-toluenesulfonyl chloride (Tos-Cl), N,N′-dicyclohexylcarbodiimide/4-(1-pyrrolidinyl)pyridine (DCC/PP),1,1′-carbonylimidazole (CDI), or iminium chloride (ImCl, obtained from oxalyl chloride and N,Ndimethylformamide) applying a molar ratio of anhydroglucose unit: lauric acid: activating agent of 1:3:3. Reaction conditions
Product
Activating agent
Time [h]
No.
DSb
Yield [%]
Efficiency [%]
Tos-Cl Tos-Cl Tos-Cl DCC/PP DCC/PP DCC/PP CDI CDI CDI ImCla ImCl ImCl ImCl
3 6 16 3 6 16 3 6 16 2 3 6 16
2a 2b 2c 3a 3b 3c 4a 4b 4c 5a 5b 5c 5d
0.68 1.31 1.59 0.94 1.20 1.18 0.74 0.81 1.37 0.66 1.67 1.90 2.13
73 97 99 100 100 98 94 99 95 73 91 96 93
23 44 53 31 41 41 25 27 46 66 55 63 71
a b
2.2. Structure characterisation of the starch laurates The starch lauryl esters obtained were characterized by elemental analysis and 13C NMR spectroscopy. Elemental analysis allows calculation of the DS values. Moreover, no impurities of nitrogen were revealed. Fig. 2 shows the 13C NMR spectrum of starch laurate 2a, which was synthesized using activation of lauric acid with Tos-Cl. The signal of unsubstituted position 6 at 60 ppm is very low indication a preferred conversion of this position. Additionally, there is only one signal at 102 ppm, which is caused by the C-1 of the AGU in neighbourhood to position 2 bearing a hydroxyl group. Thus, a preferred functionalization of position 6 of the AGU occurred by the reaction of starch with of lauric acid activated with Tos-Cl (products 2a-c). Surprisingly, there are further signals at 171 ppm and 21 ppm, which can be assigned to acetate moieties. It is known for cellulose tosylation applying Tos-Cl that acetate may be formed additionally by a side reaction as discussed in the literature for [17]. Obviously, a comparable side reaction may occur using Tos-Cl as in situ activation agent. A comparable substitution pattern was observed for products obtained with lauric acid activated by DCC/PP as may be concluded from the 13C NMR spectra. Again, only one signal at 100 ppm appears in the typical 13C NMR of sample 3a, e.g., which means that the position 2 of the AGU is not functionalized. Two signals appeared at 63 ppm and 59 ppm that can be assigned to carbon atoms of position 6 carrying an ester moiety (C-6s) and the non-modified OH group (C-6). Thus, position 6 reacts preferred. However, no complete substitution of this position occurred (3a-c). On the contrary, conversion of starch with lauric acid after activation with CDI yields products that are mainly functionalized at position 2 as can be concluded from the 13C NMR spectrum of 4b, e.g., displayed in Fig. 3. Two signals appeared for C-1 at 100 ppm and 96 ppm corresponding to the unsubstituted and substituted C-2 position; the latter causes a high-field shift of the C-1 signal. Only one signal for position 6 occurred at 60 ppm that can be assigned to the CH2OH moiety (4a-c). This distribution of ester substituents was already found for starch acetates, which were synthesized with acetic acid imidazolide previously [12,18].
Molar ratio of 1:1:1. Degree of substitution (DS) calculated by equation (1).
oxalyl chloride combined with N,N-dimethylformamide (DMF). The experiments were carried out at a reaction temperature of 60 °C and molar ratio of anhydroglucose unit (AGU) to lauric acid to activating agent of 1:3:3 were applied (Fig. 1). The reaction time was varied from 2 to 16 h. The products obtained are summarised in Table 1. The conversion of starch with lauric acid in presence of Tos-Cl forming the symmetric and mixed anhydride as well as the carboxylic acid chloride [7] yielded the starch laurate with a degree of substitution (DS) of 0.68 (2a, Table 1) after 3 h and of DS of 1.31 (2b) after 6 h reaction time. The DS of the products and hence the efficiency of the reaction applying the same molar ratio of anhydroglucose unit (AGU) to lauric acid to activating agent of 1:3:3 increased further with increasing reaction time; the maximum DS of 1.59 was achieved after 16 h (2c), corresponding to an efficiency of 53%. Reaction of carboxylic acids with DCC/PP forms the symmetric carboxylic acid anhydride [15], leading to loss of one half of the employed carboxylic acid in the further esterification. The DS achieved and efficiency of a conversion applying DCC/PP for activation was found to be 0.94 after 3 h (3a, reaction efficiency 31%) and could be increased by increasing the reaction time to 6 h; a DS of 1.2 (sample 3b, efficiency 41%) could be realized. After 16 h, a product with comparable DS of 1.18 was obtained (3c). The reactivity of carboxylic acid imidazolide formed with CDI [16] is described to be comparable to carboxylic acid chloride. Applying CDI to activate the carboxylic acid, product 4c with a maximum DS of 1.37 could be obtained, hence the efficiency is 46%. The DS values are lower (0.74, 4a and 0.81, 4b) at shorter reaction time of 3 h and 6 h. 2
Carbohydrate Research 486 (2019) 107833
S. Blohm and T. Heinze
Fig. 2. 13C NMR spectrum of starch laurate (2a, degree of substitution = 0.68, N,N-dimethylformamide (DMF)-d7, 62.90 MHz, 297 K) synthesized by conversion of starch with lauric acid activated by tosyl chloride.
Fig. 3. 13C NMR spectrum of starch laurate (4b, degree of substitution = 0.81, dimethyl sulfoxide (DMSO)-d6, 100.61 MHz, 297 K) synthesized by conversion of starch with lauric acid activated with 1,1’-carbonyldiimidazole in N,N-dimethylacetamide/LiCl.
3
Carbohydrate Research 486 (2019) 107833
S. Blohm and T. Heinze
Fig. 4. 13C NMR spectrum of starch laurate (5a, degree of substitution = 0.66, tetrahydrofuran (THF)-d8, 62.90 MHz, 297 K) synthesized by conversion of starch with lauric acid activated by oxalyl chloride/N,N-dimethylformamide.
Fig. 5. 13C NMR spectrum of starch laurate (5d, degree of substitution = 2.13, CDCl3, 62.90 MHz, 298 K) synthesized by conversion of starch with lauric acid activated by oxalyl chloride/N,N-dimethylformamide.
4
Carbohydrate Research 486 (2019) 107833
S. Blohm and T. Heinze
as by the SEC data (Table 2). DCC does not influence the molecular weight of the backbone, leading to products of higher melting temperature (Table 2). Increasing DS without cleavage of the polymeric backbone means increasing molecular mass, which causes increasing reduced viscosity with increasing DS. So sample 3a (DS of 0.94) shows a reduced viscosity of 37.740 cm3/g whereas sample 3b (DS of 1.20) has a higher reduced viscosity of 53.826 cm3/g. The samples are soluble in organic solvents. The solubility of the samples depends on DS values. 2a (DS 0.68) is soluble in the polar aprotic solvent DMF only, whereas the higher substituted samples 2b and 2c (DS 1.31 and 1.59) are soluble in less polar aprotic solvents like tetrahydrofuran (THF) or chloroform. Remarkably, the solubility depends on the activating method as well. The samples prepared with lauric acid activated with DCC (3a-c) were soluble in THF and chloroform. Sample 3a (DS 0.94) was additionally soluble in DMAc. Samples obtained by activation of lauric acid with CDI (4a-c) show solubility in DMSO, DMF and DMAc. 4a and 4b are samples that even dissolve in DMSO. That might be a result of the different substitution pattern. Sample 4c (DS 1.37) was soluble in chloroform. Products obtained after activation of the lauric acid by ImCl possess solubility in chloroform only except the sample of low DS (5a), which is soluble in polar aprotic solvents like DMF and DMAc. No clear correlation could be drawn between substitution pattern and solubility on the results obtained. Further studies are needed.
Table 2 Melting temperature, reduced viscosity, number average molar mass, and solubility of starch laurates. No
2a 2b 2c 3a 3b 3c 4a 4b 4c 5a 5b 5c 5d
Melting area [°C]
Reduced viscosity [cm3/g]
M̅n [g/mol]
190–220 110–125 100–110 190–210 160–185 165–190 250b 230–240b 165–175 220b 175–200 160–170 150–165
4.827 28.433 77.086 37.740 53.826 48.806 78.797 34.355 47.312 / 404.798 177.527 291.836
11,203 / / 49,746 / / 53,305 63,012 / / 120,870 82,346 96,308
Solubilitya DMSO
DMAc
DMF
CHCl3
THF
+ + -
+ + + + + + -
+ + + + + -
+ + + + + + + + +
+ + + + + + + + + +
a DMSO Dimethyl sulfoxide, DMAc N,N-dimethylacetamide, DMF N,N-dimethylformamide, THF tetrahydrofuran, + soluble, - insoluble. b Melt occurs brownish.
The activation of lauric acid with oxalyl chloride/DMF and the subsequent reaction with starch yielding product 5a with DS of 0.66 gave a13C NMR spectrum that contains one signal for C-1 at 100 ppm and two signals for C-6 at 64 ppm and 61 ppm indicating preferred reaction at position 6 (Fig. 4). A representative 13C NMR spectrum of the highly substituted starch laurate 5d (DS = 2.13 acquired in CDCl3) is shown in Fig. 5. The signals at 96 ppm and in the range from 74 to 62 ppm are assigned to the carbon atoms of the modified AGU. The signal belonging to the carbon atom 4 of the AGU is covered by the solvent signal of chloroform-d. The signals at 173 ppm and in the range from 34 to 14 ppm can be assigned to the ester moiety and the aliphatic fatty acid chain of the substituent, respectively. The signals at 96 ppm and 62 ppm indicate the complete substitution of the positions 2 and 6 of the AGU due to the absence of signals of non-substituted positions, which would appear at 100 ppm or 60 ppm, respectively. Thus, it turned out that the starch esters obtained by the reaction of the biopolymer with the differently activated lauric acid derivatives show different selectivity regarding the three OH groups at position 2, 3, and 6 of the AGU.
3. Experimental section 3.1. Materials Maize starch FLOJEL 60 (Ingredion, Mn = 60,572 g/mol, DPn = 374) and lithium chloride (Sigma Aldrich, > 98%) were dried for 8 h at 110 °C in vacuo before use. N,N’-dicyclohexylcarbodiimide (DCC, > 99%) and 1,1‘-carbonyldiimidazole (CDI, 97%) were purchased from Fluka, N,N-dimethylacetamide (DMAc, 99.5% extra dry over molecular sieve), N,N-dimethylformamide (DMF, 99.8% extra dry), p-toluenesulfonyl chloride (tosyl chloride, > 99%) and lauric acid (99%) from Acros Organics. Oxalyl chloride was purchased from TCI Germany GmbH. All chemicals were used without further purification. DMSO‑d6 (99.8%), DMF-d7, THF-d8 and CDCl3 (99.8%) were purchased from Deutero and Euriso-Top. 3.2. Methods
2.3. Properties of starch laurates
3.2.1. Measurements Elemental analysis was performed with a VARIO EL III CHNS from Elementaranalysensysteme GmbH Hanau (Germany). Values obtained for carbon content were used to calculate the average degree of substitution (DS) by the following equation:
The starch laurates obtained possess thermoplastic properties. The melting areas depend mainly on the DS of the samples (Table 2). Starch laurate 2a with DS 0.68 melts at about 190 °C, whereas sample 2b (DS 1.31) has a significant lower melting temperature with ca. 110 °C as expected. Sample 2c with higher DS of 1.59 melts already at about 100 °C. This trend, i.e. decreasing melting temperatures with increasing DS, was found for all samples independent of the type of activating of the lauric acid except for products obtained with carboxylic acid imidazolide. These products possess significant higher melting temperature and become brownish while melting. Comparing samples with comparable DS values (2a, Tos-Cl and 4a, CDI, see Table 2), a difference in melting temperature of 60 °C was found. As already mentioned, the starch derivatives obtained by the different activation of the lauric acid have a different substitution pattern, which obviously influences the melting temperature, on the one hand. On the other hand, it turned out that 2b (DS 1.31, lauric acid activated with Tos-Cl) and 3b (DS 1.20, lauric acid activated with DCC/PP) with comparable DS show a difference in melting temperature of 55 °C. As known, the activation of a carboxylic acid with Tos-Cl forms HCl as by-product. The hydrogen chloride may cause chain cleavage of the polymeric backbone that is indicated by the comparably low viscosity of polymer solution as well
DS =
%C 100%
+ MAGU
MC *nC,Sub
nC,AGU *MC
%C *(MSub 100%
MH)
(1)
%C – carbon content, MAGU – molar mass of the anhydroglucose unit (162.14 g/mol), nC,AGU – quantity of carbon atoms in the repeating unit, MC – molar mass of a carbon atom (12.01 g/mol), nC,Sub – quantity of carbon atoms in the substituent, MSub – molar mass of the substituent (183.32 g/mol), MH – molar mass of a hydrogen atom (1.008 g/mol). The yield was calculated via the following equation:
Yield[%] =
mPS MAGU + DS*(MSub
nStarch
MH )
*100%
(2)
For calculation of the efficiency, the following equation was utilized:
Efficiency[%] =
5
DS *100% EquivalentsReagent
(3)
Carbohydrate Research 486 (2019) 107833
S. Blohm and T. Heinze
Reduced viscosities were calculated from one point measurements with 1 wt% solutions of the sample in DMSO or chloroform on an automatic capillary viscosimeter Lauda PVS 1/2. SEC measurements were performed in DMSO/LiBr (0.5%) with a sample concentration of 1 g*L−1 and pullulan as reference at 70 °C or in THF with a sample concentration of 1 g*L−1 and poly-styrene as reference at 30 °C. 13 C NMR spectra of the starch laurates were recorded with a Bruker Avance III (400 MHz, number of scans: > 10,000) in DMSO‑d6 or CDCl3. Chemical shifts are stated in parts per million (ppm).
33.48 (C-8), 31.42 (C-16), 29.14 (C-10-C-15), 24.35 (C-9), 22.17 (C17), 13.88 (C-18). 3.2.2.5. Conversion of starch with lauric acid and oxalyl chloride/DMF in DMAc/LiCl (5b, representative example). DMF (30 mL) was cooled to −30 °C and oxalyl chloride (3.17 mL, 37.01 mmol; 3.0 eq.) was added dropwise while the temperature was maintained between −30 °C and −25 °C. After gas formation was finished, lauric acid (7.41 g, 37.01 mmol; 3.0 eq.) was added and stirred at −20 °C for one hour. The suspension was added to the starch solution and stirred at 60 °C for 3 h. After cooling to room temperature the mixture was precipitated into 500 mL ethanol, filtered and the solid residue was washed twice with 200 mL 80% (v/v) ethanol and three times with 200 mL ethanol. The product was dried in vacuo at 40 °C. Yield: 5.18 g (91% modified AGU, 91% of theoretical yield), DS: 1.67 (55% efficiency). Elemental analysis found [%]: C = 67.06, H = 10.16. 13 C NMR (100.61 MHz, CDCl3, 297 K) [ppm]: δ = 173.34 (C-7), 95.93 (C-12s), 80-69 (C-4, C-3, C-2, C-2s, C-5), 62.73 (C-6s), 34.06 (C-8), 32.07 (C-16), 29.82 (C-10-C-15), 24.96 (C-9), 22.82 (C-17), 14.22 (C18).
3.2.2. Syntheses 3.2.2.1. Dissolution of starch in DMAc/LiCl. 2.00 g starch (12.34 mmol) were suspended in 50 mL DMAc and stirred at 120 °C for two hours. The temperature was decreased to 80 °C and 0.60 g LiCl were added. The heater was removed and after a few minutes, a clear solution was obtained. 3.2.2.2. Conversion of starch with lauric acid and tosyl chloride in DMAc/ LiCl (2a, representative example). A solution of lauric acid (7.41 g, 37.01 mmol) and Tos-Cl (7.05 g, 37.01 mmol) in DMAc (10 mL) was added to the starch solution. The mixture was stirred at 60 °C for 3 h and subsequently cooled to room temperature. It was precipitated into 500 mL 80% (v/v) ethanol, filtered and the solid residue was washed twice with 200 mL deionized water and three times with 200 mL ethanol. The product was dried in vacuo at 40 °C. Yield: 2.59 g (9.05 mmol modified AGU; 73% of theoretical yield), DS: 0.68 (23% efficiency). Elemental analysis found [%]: C = 59.53, H = 8.73. 13 C NMR (62.90 MHz, DMF-d7, 297 K) [ppm]: δ = 173.43–171.02 (C-7), 102.43 (C-1), 81.66 (C-4), 73.98 (C-3), 73.03 (C-2), 69.72 (C-5), 63.93 (C-6s), 34.20 (C-8), 32.37 (C-16), 30.07 (C-10-C-15), 25.39 (C-9), 23.07 (C-17), 14.28 (C-18).
4. Conclusions Starch laurates with DS values from 0.68 to 2.13 were synthesized by conversion of starch with the corresponding carboxylic acid activated with different in situ activating agents (Tos-Cl, DCC/PP, CDI, oxalyl chloride/DMF) under comparable conditions. Products synthesized with the carboxylic acid activated with oxalyl chloride/DMF possess highest DS values. The substitution pattern depends on the activating agent. Tos-Cl, DCC/PP and oxalyl chloride/DMF promote a reaction at the primary OH while CDI leads to starch esters substituted predominantly at position 2. It turned out that products with predominant substitution in position 2 degrade during melting. 6-substituted products showed thermoplastic behaviour. The reasons for the different substitution pattern and the influence on the thermal properties will be investigated in further studies.
3.2.2.3. Conversion of starch with lauric acid and DCC in DMAc/LiCl (3a, representative example). Lauric acid (7.41 g, 37.01 mmol; 3.0 eq.) and DCC (7.64 g, 37.01 mmol; 3.0 eq.) were dissolved in DMAc (10 mL), stirred for one hour at room temperature and subsequently the mixture was added to the starch solution. 4-(1-Pyrrolidinyl)pyridine (21 mg, 123 μmol; 0.01 eq.) were added and the reaction mixture stirred for 3 h at 60 °C. After cooling to room temperature it was precipitated into 400 mL methanol, filtered and the solid residue was washed three times with 200 mL methanol. The product was dried in vacuo at 40 °C. Yield: 4.11 g (12.32 mmol modified AGU; 100% of theoretical yield), DS: 0.94 (31% efficiency). Elemental analysis found [%]: C = 62.27, H = 9.30. 13 C NMR (62.90 MHz, CDCl3, 297 K) [ppm]: δ = 173.47 (C-7), 99.35 (C-1), 74-69 (C-3, C-2, C-5), 61.86 (C-6s), 58.77 (C-6), 34.11 (C8), 32.08 (C-16), 29.81 (C-10-C-15), 25.01 (C-9), 22.83 (C-17), 14.24 (C-18).
Contributors Sascha Blohm: concept development, syntheses, measurements and data evaluation, discussion of concept and results, manuscript preparation and discussion. Thomas Heinze: concept development, discussion of concept and results, manuscript preparation and discussion. Declaration of competing interest There is no conflict of interest. Acknowledgments
3.2.2.4. Conversion of starch with lauric acid and CDI in DMAc/LiCl (4a, representative example). Lauric acid (7.41 g, 37.01 mmol; 3.0 eq.) and CDI (6.00 g, 37.01 mmol; 3.0 eq.) were stirred at room temperature in DMAc (35 mL) overnight. The obtained suspension was added to the starch solution and the reaction mixture was stirred at 60 °C for 3 h. After cooling to room temperature, it was precipitated into 500 mL 80% (v/v) 2-propanol, filtered and the solid residue was washed three times with 2-propanol. The product was dried in vacuo at 40 °C. Yield: 3.43 g (11.55 mmol modified AGU; 94% of theoretical yield), DS: 0.74 (25% efficiency). Elemental analysis found [%]: C = 60.16, H = 8.95. 13 C NMR (62.90 MHz, DMSO‑d6, 297 K) [ppm]: δ = 172.81 (C-7), 100.40 (C-1), 95.60 (C-12s), 80-69 (C-4, C-3, C-2, C-5), 60.35 (C-6),
Financial support by the Federal Ministry of Food and Agriculture/ Fachagentur Nachwachsende Rohstoffe e.V. (project 22027414) is gratefully acknowledged. References [1] A.G. Cunha, A. Gandini, Cellulose 17 (2010) 1045. [2] A.O. Ashogbon, E.T. Akintayo, Starch-Stärke 66 (2014) 41. [3] M. Rinaudo, J. Reguant, E. Frollini, A.L. Leao, L.H.C. Mattoso (Eds.), Natural Polymers and Agrofibers Based Composites, Sao Carlos, 2000, pp. 15–39. [4] A.D. Sagar, E.W. Merrill, J. Appl. Polym. Sci. 58 (1995) 1647. [5] B.Y. Yang, R. Montgomery, Starch-Stärke 60 (2008) 146. [6] T. Liebert, M.C. Nagel, T. Jordan, A. Heft, B. Grünler, T. Heinze, Macromol. Rapid
6
Carbohydrate Research 486 (2019) 107833
S. Blohm and T. Heinze
[7] [8] [9] [10] [11] [12]
Commun. 32 (2011) 1312. T. Heinze, T.F. Liebert, K.S. Pfeiffer, M.A. Hussain, Cellulose 10 (2003) 283. T. Heinze, J. Schaller, Macromol. Chem. Phys. 201 (2000) 1214. D. Gräbner, T. Liebert, T. Heinze, Cellulose 9 (2002) 193. C. Grote, T. Heinze, Cellulose 12 (2005) 435. G. Samaranayake, W.G. Glasser, Carbohydr. Polym. 22 (1993) 1. T. Liebert, W.-M. Kulicke, T. Heinze, React. Funct. Polym. 68 (2008) 1.
[13] [14] [15] [16] [17] [18]
7
P.A. Stadler, Helv. Chim. Acta 61 (1978) 1675. M.A. Hussain, T. Liebert, T. Heinze, Polym. News 29 (2004) 14. M.C. Sheikh, S. Takagi, T. Yoshimura, H. Morita, Tetrahedron 66 (2010) 7272. H.A. Staab, Angew. Chem. 74 (1962) 407. C.L. McCormick, T.R. Dawsey, J.K. Newman, Carbohydr. Res. 208 (1990) 183. R. Hampe, T. Heinze, Macromol. Mater. Eng. 299 (2014) 1188.
Contents lists available at ScienceDirect
Carbohydrate Research journal homepage: www.elsevier.com/locate/carres
Synthesis and properties of thermoplastic starch laurates Sascha Blohm, Thomas Heinze
T
∗
Institute for Organic Chemistry and Macromolecular Chemistry, Friedrich Schiller University of Jena, Center of Excellence for Polysaccharide Research, Humboldtstraße 10, D-07743, Jena, Germany
ARTICLE INFO
ABSTRACT
Keywords: Thermoplastic starch Esterification In situ activation Lauric acid Substitution pattern
Maize starch was allowed to react homogeneously in N,N-dimethylacetamide (DMAc)/LiCl with lauric acid activated with 4-toluenesulfonyl chloride, N,N′-dicyclohexylcarbodiimide/4-(1-pyrrolidinyl)pyridine, 1,1′-carbonyldiimidazole, and N,N-dimethylformamide (DMF) combined with oxalyl chloride. Characterization of the products by means of 13C NMR spectroscopy revealed different substitution patterns depending on the activation agent. The activation of lauric acid with DMF in combination with oxalyl chloride gave starch laurates of highest degree of substitution (DS), yield and reaction efficiency. The melting temperatures and the solubility of the thermoplastic starch laurates were found to depend on the DS, the substitution pattern, and on the molar mass of the starch esters.
1. Introduction
used to prepare starch acetates [12] and starch laurates [10]. Iminium chloride (ImCl), which is obtained by the reaction of N,N-dimethylformamide (DMF) with chlorinating agents like phosphoryl chloride, phosphorus trichloride, or oxalyl chloride [13] was applied for the conversion of cellulose with palmitic-, stearic-, 4-nitrobenzoic acid, and adamantane carboxylic acid and yielded the corresponding products with DS values of up to 1.89 [14]. In the present study, the conversion of starch with lauric acid applying different activation methods was carried out under comparable conditions. The goal was to figure out the most efficient in situ activation method for synthesis of starch fatty acid esters as well as to study the product properties in dependence on the synthesis method. Homogeneous reaction conditions could be realized using DMAc/LiCl as reaction medium that is more stable compared to dimethyl sulfoxide, which is often used in starch chemistry although it undergoes side reactions in a Swern-like manner.
Starch is one of the most abundant biopolymers and is applied in various areas like food, packaging, and cosmetics [1]. Starch may form gels that are not stable due to retrogradation/syneresis [2]. To improve the properties of starch products, the biopolymer is functionalized to a very low extent, yielding an enhancement of stability of suspensions and gels and adhesion and of film formation, on one hand [2,3]. On the other hand, derivatization of starch with carboxylic acids to a comparably high degree of substitution (DS) may lead to enhanced mechanical and thermal stability and may even yield thermoplastic material [1]. Starch fatty acid esters possess low water permeability, increased glass transition temperature, and increased tensile strength, which is improved with increasing DS [4,5]. Starch esters of monocarboxylic acids with DS values between 1.5 and 3.0 are hydrophobic and thermoplastic materials [1] and are still biodegradable [2,3]. They are used as coatings and polymers in hot melt adhesives [6]. For esterification of polysaccharides, various in situ activating agents are increasingly studied for the carboxylic acid. Sulfonic acid chlorides (in particular p-toluenesulfonic acid chloride, Tos-Cl) forming mixed- and symmetric anhydride as well as the chloride of the carboxylic acid of enhanced reactivity [7]. Tos-Cl was applied to prepare polysaccharide esters with carboxylic acids; even adamantane carboxylic acid led to products with DS values of up to 3 [8–10]. N,N′Dicyclohexylcarbodiimide (DCC) was used to synthesize cellulose propionate with DS values in the range from 0.1 to 2.5 [11]. Another promising activating agent is 1,1′-carbonyldiimidazole (CDI) that was
∗
2. Results and discussion 2.1. Synthesis of starch laurates applying different activation methods for the carboxylic acid Starch laurates were synthesized under homogeneous conditions in N,N-dimethylacetamide (DMAc)/lithium chloride applying different activating agents for lauric acid, namely p-toluenesulfonyl chloride (Tos-Cl), N,N′-dicyclohexylcarbodiimide (DCC) in combination with 4(1-pyrrolidinyl)pyridine (PP), 1,1′-carbonyldiimidazole (CDI), and
Corresponding author. E-mail address: [email protected] (T. Heinze).
https://doi.org/10.1016/j.carres.2019.107833 Received 26 June 2019; Received in revised form 2 October 2019; Accepted 8 October 2019 Available online 09 October 2019 0008-6215/ © 2019 Elsevier Ltd. All rights reserved.
Carbohydrate Research 486 (2019) 107833
S. Blohm and T. Heinze
Fig. 1. Reaction schema for the esterification of starch with differently activated lauric acid in N,N-dimethylacetamide/lithium chloride (DMAc/LiCl).
A product (5a) with a DS of 0.66 could be synthesized even within a short reaction time to 2 h and molar ratio to 1:1:1 of AGU: lauric acid: oxalyl chloride combined with DMF (ImCl). After 3 h, a product of high DS of 1.67 (5b) could be obtained that is even higher compared to the application of the other activating agents applying a reaction time of 16 h. The starch ester obtained after 6 h possesses a DS of 1.90 (5c). Products with DS values of up to 2.13 (5d) could be synthesized, which corresponds to an efficiency of 71%. To sum up, regarding the maximal DS achieved, it turned out that the activation of lauric acid with oxalyl chloride and DMF (ImCl) is most efficient. The DS of the starch laurates obtained under comparable conditions applying different activating agents increased in the order DCC/PP < CDI < Tos-Cl < ImCl and the efficiency decreases from 71% to 41% following the order ImCl > Tos-Cl > CDI > DCC.
Table 1 Reaction conditions for and results of the syntheses of starch laurates by activation of lauric acid with p-toluenesulfonyl chloride (Tos-Cl), N,N′-dicyclohexylcarbodiimide/4-(1-pyrrolidinyl)pyridine (DCC/PP),1,1′-carbonylimidazole (CDI), or iminium chloride (ImCl, obtained from oxalyl chloride and N,Ndimethylformamide) applying a molar ratio of anhydroglucose unit: lauric acid: activating agent of 1:3:3. Reaction conditions
Product
Activating agent
Time [h]
No.
DSb
Yield [%]
Efficiency [%]
Tos-Cl Tos-Cl Tos-Cl DCC/PP DCC/PP DCC/PP CDI CDI CDI ImCla ImCl ImCl ImCl
3 6 16 3 6 16 3 6 16 2 3 6 16
2a 2b 2c 3a 3b 3c 4a 4b 4c 5a 5b 5c 5d
0.68 1.31 1.59 0.94 1.20 1.18 0.74 0.81 1.37 0.66 1.67 1.90 2.13
73 97 99 100 100 98 94 99 95 73 91 96 93
23 44 53 31 41 41 25 27 46 66 55 63 71
a b
2.2. Structure characterisation of the starch laurates The starch lauryl esters obtained were characterized by elemental analysis and 13C NMR spectroscopy. Elemental analysis allows calculation of the DS values. Moreover, no impurities of nitrogen were revealed. Fig. 2 shows the 13C NMR spectrum of starch laurate 2a, which was synthesized using activation of lauric acid with Tos-Cl. The signal of unsubstituted position 6 at 60 ppm is very low indication a preferred conversion of this position. Additionally, there is only one signal at 102 ppm, which is caused by the C-1 of the AGU in neighbourhood to position 2 bearing a hydroxyl group. Thus, a preferred functionalization of position 6 of the AGU occurred by the reaction of starch with of lauric acid activated with Tos-Cl (products 2a-c). Surprisingly, there are further signals at 171 ppm and 21 ppm, which can be assigned to acetate moieties. It is known for cellulose tosylation applying Tos-Cl that acetate may be formed additionally by a side reaction as discussed in the literature for [17]. Obviously, a comparable side reaction may occur using Tos-Cl as in situ activation agent. A comparable substitution pattern was observed for products obtained with lauric acid activated by DCC/PP as may be concluded from the 13C NMR spectra. Again, only one signal at 100 ppm appears in the typical 13C NMR of sample 3a, e.g., which means that the position 2 of the AGU is not functionalized. Two signals appeared at 63 ppm and 59 ppm that can be assigned to carbon atoms of position 6 carrying an ester moiety (C-6s) and the non-modified OH group (C-6). Thus, position 6 reacts preferred. However, no complete substitution of this position occurred (3a-c). On the contrary, conversion of starch with lauric acid after activation with CDI yields products that are mainly functionalized at position 2 as can be concluded from the 13C NMR spectrum of 4b, e.g., displayed in Fig. 3. Two signals appeared for C-1 at 100 ppm and 96 ppm corresponding to the unsubstituted and substituted C-2 position; the latter causes a high-field shift of the C-1 signal. Only one signal for position 6 occurred at 60 ppm that can be assigned to the CH2OH moiety (4a-c). This distribution of ester substituents was already found for starch acetates, which were synthesized with acetic acid imidazolide previously [12,18].
Molar ratio of 1:1:1. Degree of substitution (DS) calculated by equation (1).
oxalyl chloride combined with N,N-dimethylformamide (DMF). The experiments were carried out at a reaction temperature of 60 °C and molar ratio of anhydroglucose unit (AGU) to lauric acid to activating agent of 1:3:3 were applied (Fig. 1). The reaction time was varied from 2 to 16 h. The products obtained are summarised in Table 1. The conversion of starch with lauric acid in presence of Tos-Cl forming the symmetric and mixed anhydride as well as the carboxylic acid chloride [7] yielded the starch laurate with a degree of substitution (DS) of 0.68 (2a, Table 1) after 3 h and of DS of 1.31 (2b) after 6 h reaction time. The DS of the products and hence the efficiency of the reaction applying the same molar ratio of anhydroglucose unit (AGU) to lauric acid to activating agent of 1:3:3 increased further with increasing reaction time; the maximum DS of 1.59 was achieved after 16 h (2c), corresponding to an efficiency of 53%. Reaction of carboxylic acids with DCC/PP forms the symmetric carboxylic acid anhydride [15], leading to loss of one half of the employed carboxylic acid in the further esterification. The DS achieved and efficiency of a conversion applying DCC/PP for activation was found to be 0.94 after 3 h (3a, reaction efficiency 31%) and could be increased by increasing the reaction time to 6 h; a DS of 1.2 (sample 3b, efficiency 41%) could be realized. After 16 h, a product with comparable DS of 1.18 was obtained (3c). The reactivity of carboxylic acid imidazolide formed with CDI [16] is described to be comparable to carboxylic acid chloride. Applying CDI to activate the carboxylic acid, product 4c with a maximum DS of 1.37 could be obtained, hence the efficiency is 46%. The DS values are lower (0.74, 4a and 0.81, 4b) at shorter reaction time of 3 h and 6 h. 2
Carbohydrate Research 486 (2019) 107833
S. Blohm and T. Heinze
Fig. 2. 13C NMR spectrum of starch laurate (2a, degree of substitution = 0.68, N,N-dimethylformamide (DMF)-d7, 62.90 MHz, 297 K) synthesized by conversion of starch with lauric acid activated by tosyl chloride.
Fig. 3. 13C NMR spectrum of starch laurate (4b, degree of substitution = 0.81, dimethyl sulfoxide (DMSO)-d6, 100.61 MHz, 297 K) synthesized by conversion of starch with lauric acid activated with 1,1’-carbonyldiimidazole in N,N-dimethylacetamide/LiCl.
3
Carbohydrate Research 486 (2019) 107833
S. Blohm and T. Heinze
Fig. 4. 13C NMR spectrum of starch laurate (5a, degree of substitution = 0.66, tetrahydrofuran (THF)-d8, 62.90 MHz, 297 K) synthesized by conversion of starch with lauric acid activated by oxalyl chloride/N,N-dimethylformamide.
Fig. 5. 13C NMR spectrum of starch laurate (5d, degree of substitution = 2.13, CDCl3, 62.90 MHz, 298 K) synthesized by conversion of starch with lauric acid activated by oxalyl chloride/N,N-dimethylformamide.
4
Carbohydrate Research 486 (2019) 107833
S. Blohm and T. Heinze
as by the SEC data (Table 2). DCC does not influence the molecular weight of the backbone, leading to products of higher melting temperature (Table 2). Increasing DS without cleavage of the polymeric backbone means increasing molecular mass, which causes increasing reduced viscosity with increasing DS. So sample 3a (DS of 0.94) shows a reduced viscosity of 37.740 cm3/g whereas sample 3b (DS of 1.20) has a higher reduced viscosity of 53.826 cm3/g. The samples are soluble in organic solvents. The solubility of the samples depends on DS values. 2a (DS 0.68) is soluble in the polar aprotic solvent DMF only, whereas the higher substituted samples 2b and 2c (DS 1.31 and 1.59) are soluble in less polar aprotic solvents like tetrahydrofuran (THF) or chloroform. Remarkably, the solubility depends on the activating method as well. The samples prepared with lauric acid activated with DCC (3a-c) were soluble in THF and chloroform. Sample 3a (DS 0.94) was additionally soluble in DMAc. Samples obtained by activation of lauric acid with CDI (4a-c) show solubility in DMSO, DMF and DMAc. 4a and 4b are samples that even dissolve in DMSO. That might be a result of the different substitution pattern. Sample 4c (DS 1.37) was soluble in chloroform. Products obtained after activation of the lauric acid by ImCl possess solubility in chloroform only except the sample of low DS (5a), which is soluble in polar aprotic solvents like DMF and DMAc. No clear correlation could be drawn between substitution pattern and solubility on the results obtained. Further studies are needed.
Table 2 Melting temperature, reduced viscosity, number average molar mass, and solubility of starch laurates. No
2a 2b 2c 3a 3b 3c 4a 4b 4c 5a 5b 5c 5d
Melting area [°C]
Reduced viscosity [cm3/g]
M̅n [g/mol]
190–220 110–125 100–110 190–210 160–185 165–190 250b 230–240b 165–175 220b 175–200 160–170 150–165
4.827 28.433 77.086 37.740 53.826 48.806 78.797 34.355 47.312 / 404.798 177.527 291.836
11,203 / / 49,746 / / 53,305 63,012 / / 120,870 82,346 96,308
Solubilitya DMSO
DMAc
DMF
CHCl3
THF
+ + -
+ + + + + + -
+ + + + + -
+ + + + + + + + +
+ + + + + + + + + +
a DMSO Dimethyl sulfoxide, DMAc N,N-dimethylacetamide, DMF N,N-dimethylformamide, THF tetrahydrofuran, + soluble, - insoluble. b Melt occurs brownish.
The activation of lauric acid with oxalyl chloride/DMF and the subsequent reaction with starch yielding product 5a with DS of 0.66 gave a13C NMR spectrum that contains one signal for C-1 at 100 ppm and two signals for C-6 at 64 ppm and 61 ppm indicating preferred reaction at position 6 (Fig. 4). A representative 13C NMR spectrum of the highly substituted starch laurate 5d (DS = 2.13 acquired in CDCl3) is shown in Fig. 5. The signals at 96 ppm and in the range from 74 to 62 ppm are assigned to the carbon atoms of the modified AGU. The signal belonging to the carbon atom 4 of the AGU is covered by the solvent signal of chloroform-d. The signals at 173 ppm and in the range from 34 to 14 ppm can be assigned to the ester moiety and the aliphatic fatty acid chain of the substituent, respectively. The signals at 96 ppm and 62 ppm indicate the complete substitution of the positions 2 and 6 of the AGU due to the absence of signals of non-substituted positions, which would appear at 100 ppm or 60 ppm, respectively. Thus, it turned out that the starch esters obtained by the reaction of the biopolymer with the differently activated lauric acid derivatives show different selectivity regarding the three OH groups at position 2, 3, and 6 of the AGU.
3. Experimental section 3.1. Materials Maize starch FLOJEL 60 (Ingredion, Mn = 60,572 g/mol, DPn = 374) and lithium chloride (Sigma Aldrich, > 98%) were dried for 8 h at 110 °C in vacuo before use. N,N’-dicyclohexylcarbodiimide (DCC, > 99%) and 1,1‘-carbonyldiimidazole (CDI, 97%) were purchased from Fluka, N,N-dimethylacetamide (DMAc, 99.5% extra dry over molecular sieve), N,N-dimethylformamide (DMF, 99.8% extra dry), p-toluenesulfonyl chloride (tosyl chloride, > 99%) and lauric acid (99%) from Acros Organics. Oxalyl chloride was purchased from TCI Germany GmbH. All chemicals were used without further purification. DMSO‑d6 (99.8%), DMF-d7, THF-d8 and CDCl3 (99.8%) were purchased from Deutero and Euriso-Top. 3.2. Methods
2.3. Properties of starch laurates
3.2.1. Measurements Elemental analysis was performed with a VARIO EL III CHNS from Elementaranalysensysteme GmbH Hanau (Germany). Values obtained for carbon content were used to calculate the average degree of substitution (DS) by the following equation:
The starch laurates obtained possess thermoplastic properties. The melting areas depend mainly on the DS of the samples (Table 2). Starch laurate 2a with DS 0.68 melts at about 190 °C, whereas sample 2b (DS 1.31) has a significant lower melting temperature with ca. 110 °C as expected. Sample 2c with higher DS of 1.59 melts already at about 100 °C. This trend, i.e. decreasing melting temperatures with increasing DS, was found for all samples independent of the type of activating of the lauric acid except for products obtained with carboxylic acid imidazolide. These products possess significant higher melting temperature and become brownish while melting. Comparing samples with comparable DS values (2a, Tos-Cl and 4a, CDI, see Table 2), a difference in melting temperature of 60 °C was found. As already mentioned, the starch derivatives obtained by the different activation of the lauric acid have a different substitution pattern, which obviously influences the melting temperature, on the one hand. On the other hand, it turned out that 2b (DS 1.31, lauric acid activated with Tos-Cl) and 3b (DS 1.20, lauric acid activated with DCC/PP) with comparable DS show a difference in melting temperature of 55 °C. As known, the activation of a carboxylic acid with Tos-Cl forms HCl as by-product. The hydrogen chloride may cause chain cleavage of the polymeric backbone that is indicated by the comparably low viscosity of polymer solution as well
DS =
%C 100%
+ MAGU
MC *nC,Sub
nC,AGU *MC
%C *(MSub 100%
MH)
(1)
%C – carbon content, MAGU – molar mass of the anhydroglucose unit (162.14 g/mol), nC,AGU – quantity of carbon atoms in the repeating unit, MC – molar mass of a carbon atom (12.01 g/mol), nC,Sub – quantity of carbon atoms in the substituent, MSub – molar mass of the substituent (183.32 g/mol), MH – molar mass of a hydrogen atom (1.008 g/mol). The yield was calculated via the following equation:
Yield[%] =
mPS MAGU + DS*(MSub
nStarch
MH )
*100%
(2)
For calculation of the efficiency, the following equation was utilized:
Efficiency[%] =
5
DS *100% EquivalentsReagent
(3)
Carbohydrate Research 486 (2019) 107833
S. Blohm and T. Heinze
Reduced viscosities were calculated from one point measurements with 1 wt% solutions of the sample in DMSO or chloroform on an automatic capillary viscosimeter Lauda PVS 1/2. SEC measurements were performed in DMSO/LiBr (0.5%) with a sample concentration of 1 g*L−1 and pullulan as reference at 70 °C or in THF with a sample concentration of 1 g*L−1 and poly-styrene as reference at 30 °C. 13 C NMR spectra of the starch laurates were recorded with a Bruker Avance III (400 MHz, number of scans: > 10,000) in DMSO‑d6 or CDCl3. Chemical shifts are stated in parts per million (ppm).
33.48 (C-8), 31.42 (C-16), 29.14 (C-10-C-15), 24.35 (C-9), 22.17 (C17), 13.88 (C-18). 3.2.2.5. Conversion of starch with lauric acid and oxalyl chloride/DMF in DMAc/LiCl (5b, representative example). DMF (30 mL) was cooled to −30 °C and oxalyl chloride (3.17 mL, 37.01 mmol; 3.0 eq.) was added dropwise while the temperature was maintained between −30 °C and −25 °C. After gas formation was finished, lauric acid (7.41 g, 37.01 mmol; 3.0 eq.) was added and stirred at −20 °C for one hour. The suspension was added to the starch solution and stirred at 60 °C for 3 h. After cooling to room temperature the mixture was precipitated into 500 mL ethanol, filtered and the solid residue was washed twice with 200 mL 80% (v/v) ethanol and three times with 200 mL ethanol. The product was dried in vacuo at 40 °C. Yield: 5.18 g (91% modified AGU, 91% of theoretical yield), DS: 1.67 (55% efficiency). Elemental analysis found [%]: C = 67.06, H = 10.16. 13 C NMR (100.61 MHz, CDCl3, 297 K) [ppm]: δ = 173.34 (C-7), 95.93 (C-12s), 80-69 (C-4, C-3, C-2, C-2s, C-5), 62.73 (C-6s), 34.06 (C-8), 32.07 (C-16), 29.82 (C-10-C-15), 24.96 (C-9), 22.82 (C-17), 14.22 (C18).
3.2.2. Syntheses 3.2.2.1. Dissolution of starch in DMAc/LiCl. 2.00 g starch (12.34 mmol) were suspended in 50 mL DMAc and stirred at 120 °C for two hours. The temperature was decreased to 80 °C and 0.60 g LiCl were added. The heater was removed and after a few minutes, a clear solution was obtained. 3.2.2.2. Conversion of starch with lauric acid and tosyl chloride in DMAc/ LiCl (2a, representative example). A solution of lauric acid (7.41 g, 37.01 mmol) and Tos-Cl (7.05 g, 37.01 mmol) in DMAc (10 mL) was added to the starch solution. The mixture was stirred at 60 °C for 3 h and subsequently cooled to room temperature. It was precipitated into 500 mL 80% (v/v) ethanol, filtered and the solid residue was washed twice with 200 mL deionized water and three times with 200 mL ethanol. The product was dried in vacuo at 40 °C. Yield: 2.59 g (9.05 mmol modified AGU; 73% of theoretical yield), DS: 0.68 (23% efficiency). Elemental analysis found [%]: C = 59.53, H = 8.73. 13 C NMR (62.90 MHz, DMF-d7, 297 K) [ppm]: δ = 173.43–171.02 (C-7), 102.43 (C-1), 81.66 (C-4), 73.98 (C-3), 73.03 (C-2), 69.72 (C-5), 63.93 (C-6s), 34.20 (C-8), 32.37 (C-16), 30.07 (C-10-C-15), 25.39 (C-9), 23.07 (C-17), 14.28 (C-18).
4. Conclusions Starch laurates with DS values from 0.68 to 2.13 were synthesized by conversion of starch with the corresponding carboxylic acid activated with different in situ activating agents (Tos-Cl, DCC/PP, CDI, oxalyl chloride/DMF) under comparable conditions. Products synthesized with the carboxylic acid activated with oxalyl chloride/DMF possess highest DS values. The substitution pattern depends on the activating agent. Tos-Cl, DCC/PP and oxalyl chloride/DMF promote a reaction at the primary OH while CDI leads to starch esters substituted predominantly at position 2. It turned out that products with predominant substitution in position 2 degrade during melting. 6-substituted products showed thermoplastic behaviour. The reasons for the different substitution pattern and the influence on the thermal properties will be investigated in further studies.
3.2.2.3. Conversion of starch with lauric acid and DCC in DMAc/LiCl (3a, representative example). Lauric acid (7.41 g, 37.01 mmol; 3.0 eq.) and DCC (7.64 g, 37.01 mmol; 3.0 eq.) were dissolved in DMAc (10 mL), stirred for one hour at room temperature and subsequently the mixture was added to the starch solution. 4-(1-Pyrrolidinyl)pyridine (21 mg, 123 μmol; 0.01 eq.) were added and the reaction mixture stirred for 3 h at 60 °C. After cooling to room temperature it was precipitated into 400 mL methanol, filtered and the solid residue was washed three times with 200 mL methanol. The product was dried in vacuo at 40 °C. Yield: 4.11 g (12.32 mmol modified AGU; 100% of theoretical yield), DS: 0.94 (31% efficiency). Elemental analysis found [%]: C = 62.27, H = 9.30. 13 C NMR (62.90 MHz, CDCl3, 297 K) [ppm]: δ = 173.47 (C-7), 99.35 (C-1), 74-69 (C-3, C-2, C-5), 61.86 (C-6s), 58.77 (C-6), 34.11 (C8), 32.08 (C-16), 29.81 (C-10-C-15), 25.01 (C-9), 22.83 (C-17), 14.24 (C-18).
Contributors Sascha Blohm: concept development, syntheses, measurements and data evaluation, discussion of concept and results, manuscript preparation and discussion. Thomas Heinze: concept development, discussion of concept and results, manuscript preparation and discussion. Declaration of competing interest There is no conflict of interest. Acknowledgments
3.2.2.4. Conversion of starch with lauric acid and CDI in DMAc/LiCl (4a, representative example). Lauric acid (7.41 g, 37.01 mmol; 3.0 eq.) and CDI (6.00 g, 37.01 mmol; 3.0 eq.) were stirred at room temperature in DMAc (35 mL) overnight. The obtained suspension was added to the starch solution and the reaction mixture was stirred at 60 °C for 3 h. After cooling to room temperature, it was precipitated into 500 mL 80% (v/v) 2-propanol, filtered and the solid residue was washed three times with 2-propanol. The product was dried in vacuo at 40 °C. Yield: 3.43 g (11.55 mmol modified AGU; 94% of theoretical yield), DS: 0.74 (25% efficiency). Elemental analysis found [%]: C = 60.16, H = 8.95. 13 C NMR (62.90 MHz, DMSO‑d6, 297 K) [ppm]: δ = 172.81 (C-7), 100.40 (C-1), 95.60 (C-12s), 80-69 (C-4, C-3, C-2, C-5), 60.35 (C-6),
Financial support by the Federal Ministry of Food and Agriculture/ Fachagentur Nachwachsende Rohstoffe e.V. (project 22027414) is gratefully acknowledged. References [1] A.G. Cunha, A. Gandini, Cellulose 17 (2010) 1045. [2] A.O. Ashogbon, E.T. Akintayo, Starch-Stärke 66 (2014) 41. [3] M. Rinaudo, J. Reguant, E. Frollini, A.L. Leao, L.H.C. Mattoso (Eds.), Natural Polymers and Agrofibers Based Composites, Sao Carlos, 2000, pp. 15–39. [4] A.D. Sagar, E.W. Merrill, J. Appl. Polym. Sci. 58 (1995) 1647. [5] B.Y. Yang, R. Montgomery, Starch-Stärke 60 (2008) 146. [6] T. Liebert, M.C. Nagel, T. Jordan, A. Heft, B. Grünler, T. Heinze, Macromol. Rapid
6
Carbohydrate Research 486 (2019) 107833
S. Blohm and T. Heinze
[7] [8] [9] [10] [11] [12]
Commun. 32 (2011) 1312. T. Heinze, T.F. Liebert, K.S. Pfeiffer, M.A. Hussain, Cellulose 10 (2003) 283. T. Heinze, J. Schaller, Macromol. Chem. Phys. 201 (2000) 1214. D. Gräbner, T. Liebert, T. Heinze, Cellulose 9 (2002) 193. C. Grote, T. Heinze, Cellulose 12 (2005) 435. G. Samaranayake, W.G. Glasser, Carbohydr. Polym. 22 (1993) 1. T. Liebert, W.-M. Kulicke, T. Heinze, React. Funct. Polym. 68 (2008) 1.
[13] [14] [15] [16] [17] [18]
7
P.A. Stadler, Helv. Chim. Acta 61 (1978) 1675. M.A. Hussain, T. Liebert, T. Heinze, Polym. News 29 (2004) 14. M.C. Sheikh, S. Takagi, T. Yoshimura, H. Morita, Tetrahedron 66 (2010) 7272. H.A. Staab, Angew. Chem. 74 (1962) 407. C.L. McCormick, T.R. Dawsey, J.K. Newman, Carbohydr. Res. 208 (1990) 183. R. Hampe, T. Heinze, Macromol. Mater. Eng. 299 (2014) 1188.