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Cite This: ACS Omega 2019, 4, 15702−15710 http://pubs.acs.org/journal/acsodf

Carbamation of Starch with Amine Using Dimethyl as Coupling Agent Juho Antti Sirviö*,† and Juha P. Heiskanen‡

† ‡ Fibre and Particle Engineering Research Unit and Research Unit of Sustainable Chemistry, University of Oulu, P.O. Box 4300, FI-90014 Oulu, Finland

*S Supporting Information

ABSTRACT: A one-pot coupling of starch with alkyl amine was studied using (DMC) as the coupling agent. Although reaction occurred without a catalyst (24 h, 70 °C), different catalysts, namely, imidazole, tetramethylguanidine, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and combina- tions thereof were investigated to improve the reaction efficiency. When 20 mol % DBU was used as a catalyst, the degree of substitution (DS) could be improved from 0.05 to 0.15 compared to the noncatalyzed reaction. When the amount of DBU was decreased to 5 mol %, catalytical activity remained, albeit with a slightly lower DS (0.09). Temperature did not have a significant effect on the DS but it could be used to alter the solubility of the product. Based on chemical analysis, the alkyl group was attached to starch by the formation of a carbamate group. As the carbonyl carbon in the carbamate originated from DMC, which, in turn, can be produced from on an industrial scale, the current study provides a conventional way to utilize carbon dioxide-based chemicals in the functionalization of a natural polymer. DMC is also biodegradable and classified as a nonvolatile organic component, making it an environmentally desirable coupling agent.

■ INTRODUCTION potentially more desirable ethically and more sustainable 20 Nonrenewable resources have long been the main source for ecologically and environmentally. Starch has naturally high hygroscopicity and low thermoform- chemicals and materials utilized in science and industry. Due to fi concerns about the depletion of nonrenewable resources, along ability, and various chemical modi cations have therefore been with their possible toxic and polluting properties, a significant employed to improve these properties. For example, grafting amount of research has been focused on the use of renewable with a long carbon chain can be used to invert the polarity of materials instead. Natural polymers are among the most starch, and hydrophobic material has been obtained through methods such as etherification, esterification, and amidation. promising renewable sources for the production of green ffi chemicals and materials. Natural polymers, such as poly- Although many of these methods have good e ciency, they Downloaded via UNIV OF OULU on November 29, 2019 at 07:47:39 (UTC). saccharides, can be utilized in their natural form, for example, require the use of toxic and hazardous chemicals (e.g., halogenated alkyls for etherification21 and acyl chlorides for in the paper and pulp industry, or they can be converted into 22 1−4 esterification ) and multistep reactions (e.g., amidation of monomeric chemicals by hydrolysis. Natural polymers can 23 24 See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. also be converted into semisynthetic polymers by chemical succinylated or carboxylated starch). Consequently, there is − modification.5 7 Chemical modifications are generally intended a demand for more environmentally friendly methods of starch to improve inherited properties of natural polymers, such as alkylation. poor water tolerance, low solubility, and thermoformability. Recently, alkyl carbonate have been studied for the alkylation of polysaccharides, including cellulose25,26 and As a natural polysaccharide, starch is one of the most 27 investigated renewable raw materials to be used as a replacement starch. Alkyl , such as dimethyl carbonate for oil-based sources, for example, in films8 and composite (DMC), are formally esters of , although they are materials.9 Similar to cellulose, starch is a glucose polymer with traditionally produced using chemicals such as . Lately, abundant hydroxyl groups that can be used in chemical replacing the highly toxic phosgene has been investigated, and fi alkyl carbonates have been produced using carbon mon- modi cation to produce a bio-based material. Applications of 28,29 chemically modified starch include as flocculant10 and a wet-end oxide. An even more desirable route is the production of additive in papermaking,11,12 a carrier for drug delivery,13,14 and alkyl carbonates directly from carbon dioxide; DMC, for a tissue engineering scaffold,15 among others. Although starch is example, can be produced from carbon dioxide and largely present in food sources and its use in nonfood applications is not entirely ethical, it is also widely available Received: July 26, 2019 from many waste16,17 and nonedible resources.18,19 The Accepted: September 2, 2019 utilization of starch from waste and nonfood materials is Published: September 12, 2019

© 2019 American Chemical Society 15702 DOI: 10.1021/acsomega.9b02350 ACS Omega 2019, 4, 15702−15710 ACS Omega Article

Table 1. Catalysts and Temperatures Used During the Alkylation of Starch, and Nitrogen Content, DS of Carbamate Groups, and a Mass Recovery of the Product

b catalyst (mol %) c sampleb imidazole TMG DBU temperature (°C) nitrogen content (%) DS mass recovery (g)c 1 0 0 0 70 0.40 ± 0.007 0.05 ± 0.002 0.41 2 20 0 0 70 0.72 ± 0.014 0.09 ± 0.002 0.43 3 0 20 0 70 0.97 ± 0.028 0.13 ± 0.001 0.38 4 0 0 20 70 1.16 ± 0.028 0.15 ± 0.004 0.43 5 20 20 0 70 0.99 ± 0.085 0.13 ± 0.004 0.42 6 20 0 20 70 1.15 ± 0.042 0.15 ± 0.006 0.42 7d 0 20 0 70 0 0.45 8e 0 20 0 70 0 0.38 9f 0 20 0 70 0 0.4 10 0 15 0 70 0.89 ± 0.014 0.11 ± 0.002 0.41 11 0 10 0 70 0.84 ± 0.021 0.11 ± 0.003 0.47 12 0 5 0 70 0.72 ± 0.014 0.09 ± 0.002 0.45 13 0 10 0 100 0.65 ± 0.028 0.08 ± 0.004 0.43 14 0 10 0 90 0.74 ± 0.028 0.09 ± 0.004 0.46 15 0 10 0 80 0.73 ± 0.014 0.09 ± 0.002 0.44 16 0 10 0 60 0.72 ± 0.007 0.09 ± 0.001 0.4 aReaction time in all of the samples was 24 h. b5 wt % starch solution in DMSO with 6 times molar excess of both OA and DMC compared to starch. c0.5 g of starch was used as the starting material. dWithout OA. eWithout DMC. fWithout OA and DMC. on an industrial scale.30,31 DMC is a biodegradable chemical To improve the relatively low DS obtained without a catalyst, with low toxicity that was exempted from the United States three different basic catalysts, namely, imidazole, TMG, and Environmental Protection Agency’sdefinition of volatile organic DBU, were utilized. In addition, stoichiometric mixtures of compounds in 2009. These properties make DMC suitable for imidazole and either TMG or DBU were studied as protic ionic use in, for example, phosgene-free synthesis of polyurethanes32 liquid catalysts.37,38 When using imidazole, the DS of the and polysaccharide modification. product could be nearly doubled from 0.05 to 0.09. Further To date, DMC has been studied for the of improvement was observed using TMG (DS = 0.13), and the starch.27,33 In the current study, we investigate the possibility of highest was obtained using DBU (DS = 0.15). On the other executing a one-pot coupling of long-chain alkyl amine (octyl hand, the addition of imidazole together with TMG or DBU amine; OA) with starch, using DMC as a carbon dioxide-based showed no increase in the DS. DS obtained with OA and DMC coupling agent. The use of catalysts, namely, imidazole, is similar or even slightly higher than that obtained by the tetramethylguanidine (TMG), 1,8-diazabicyclo[5.4.0]undec-7- alkylation of starch nanocrystals with long alkyl chain cyclic acid ene (DBU), and mixtures of imidazole with either TMG or anhydride.39 DBU, was investigated. After identifying the most promising In the literature, DBU is a well-known catalyst, for example, in catalyst, the effects of catalyst loading and reaction temperature the transesterification of organic carbonates. A possible catalytic were investigated. The degree of substitution (DS) was mechanism in the DBU-catalyzed alkylation of starch is determined using elemental analysis of the nitrogen content of presented in Scheme 1b. The reaction involves the lone pair the products. The chemical characteristics of the products were of the DBU attacking the carbonyl carbon of the DMC, resulting 40,41 studied using diffuse reflectance Fourier transform infrared in the formation of a highly reactive intermediate. This spectroscopy, X-ray photoelectron spectroscopy, and nuclear activated carbonate then reacts with the amine group of the OA magnetic resonance spectroscopy. to produce methyl octyl carbamate. It is possible that similar activation of the methyl octyl carbamate is then repeated. ■ RESULTS AND DISCUSSION Activated octyl carbamate then reacts with the starch to form starch carbamate. DBU, as a strong base, can also improve The chemical modification of starch was performed using OA as reaction efficiency by deprotonating hydroxyl groups in the the alkyl amine, DMC as the coupling agent, and dimethyl starch, leading to the formation of starch alkoxide, which is sulfoxide (DMSO) as the . Without a catalyst, a DS of highly reactive in nucleophilic reactions.42 0.05 could be obtained (Table 1). It is postulated that the To investigate if the presence of nitrogen in the product was reaction took place via the formation of methyl octyl carbamate indeed due to the carbamation of the starch, the reaction was between the DMC and the OA, which were used in a molar ratio also performed catalytically (with DBU) using just DMC of 1:1, followed by reaction with the starch to form starch alkyl (sample 7 in Table 1) and just OA (sample 8), and without carbamate (Scheme 1a). Two moles of methanol are formed as a either chemical (sample 9). No nitrogen was observed in these byproduct. A similar, albeit two-step, reaction has been cases, indicating that both DMC and OA are needed for the previously utilized in the synthesis of natural-based polyur- alkylation of starch. ethanes.34 Another possible reaction mechanism is the The effect on the DS of the molar ratio of DBU was further formation of starch methyl carbonate, which then reacts with investigated. A small DS decrease from 0.15 to 0.11 was the OA. Either way, nucleophilic substitution reactions of observed when the catalyst loading was decreased from 20 to 15 carbonate are catalyzed many times by the base mol %. On the other hand, DS remained at 0.11 when a catalyst catalyst.35,36 loading of 10 mol % was used; a minimal decrease in nitrogen

15703 DOI: 10.1021/acsomega.9b02350 ACS Omega 2019, 4, 15702−15710 ACS Omega Article

Scheme 1. Schematic Illustration of a Possible Reaction Mechanism in the Alkylation of Starch with OA by DMC (a), the Catalytic Activation of DMC with DBU (b), Formation of Acyclic and Cyclic Starch Carbonate in the Absence of OA

content was observed at 10 mol % compared to that at 15 mol %. solubility might be due to the different substitution patterns of In addition, the DS at a catalyst loading of 5 mol % was still the product obtained at 80 °C compared to that of other higher compared to the reaction without a catalyst (0.09 vs samples. However, further studies are requested to confirm this. 0.05). Diffusion reflectance infrared Fourier transform (DRIFT) Temperature had a minimal effect on the DS, and only a spectroscopy provided further insight into the reaction slightly lower DS was obtained when the reaction temperature mechanism. When the reaction was performed in the absence was increased from 70 to 90 °C, decreasing from 0.11 to 0.09. A of both OA and DMC, no apparent chemical changes were further increase in temperature to 100 °C resulted in a decrease observed (Figure 1). In addition, in the presence of OA but in of the DS from 0.09 to 0.08. At the high temperature, DMC is the absence of DMC, the product spectrum remained similar to presented in the gas phase of the reaction system, leading to that of pristine starch. On the other hand, with DMC but lower reaction efficiency at high temperature. The reaction without OA, a strong carbonyl vibration band was observed at a temperature, however, had an effect on the reaction mixture; at wavenumber of 1760 cm−1. This band is between cyclic (1790 temperatures of 70 °C and lower, a gel-like material was cm−1) and linear (1745 cm−1) carbonate esters,43 indicating that obtained after the reaction, most likely due to the poor solubility both are formed by the reaction of DMC with starch (Scheme of the product in the reaction mixture. In contrast, when a 1c). This is in contrast with previous results where methylation reaction temperature of 80 °C or higher was used, the reaction of the starch has been observed after its reaction with DMC.27,33 mixture was a clear, colorless solution. This difference resulted in However, linear carbonate esters of cellulose have been slightly different dissolution behavior of the product as the produced in homo-25 and heterogeneous44 reactions of DMC sample obtained at 80 °C was slightly soluble in pure DMSO, and other acyclic carbonates. The results here indicate that whereas other samples remained nonsoluble. This difference in DMC is the main requisite for a chemical reaction to take place.

15704 DOI: 10.1021/acsomega.9b02350 ACS Omega 2019, 4, 15702−15710 ACS Omega Article

Figure 1. (A) DRIFT spectra: (a) starch; (b) starch treated with OA (sample 1); (c) starch treated with DMC (sample 7); and (d) starch treated with OA and DMC (sample 1). (B) DRIFT spectra of region 1900−1500 cm−1 where the most distinguished peaks of carbonate and carbamate are indicated with dashed lines.

In the presence of both OA and DMC, two new distinct peaks The C−C/C−H bond peak was observed at a binding energy of were observed at 1702 and 1534 cm−1. These peaks are assigned 284.8 eV. When starch was allowed to react with OA in the to the carbonyl stretching and the NH deformation bands of the absence of DMC, no nitrogen was observed in the XPS carbamate group, respectively.45,46 The DRIFT results strongly spectrum. In addition, the only observable change in the XPS indicate that the alkylation of starch took place via DMC- spectra was a decrease in the intensity of the C−C/C−H bond mediated amine coupling to form carbamate linkage between peak at 284.8 eV. This can be attributed to the removal of 47 the OA and the starch. impurities that exist in natural starch. X-ray photoelectron spectroscopy (XPS) measurements were When starch was allowed to react with DMC in the absence of OA, a new peak was observed in both carbon and oxygen XPS conducted to further support the observations based on DRIFT. 44 Only the peaks relating to carbon (Figure 2) and oxygen (Figure spectra. The peaks at binding energies of 290.7 eV (Figure 2) and 534.2 eV48 (Figure S1) are related to the carbonate ester S1 in the Supporting Information) were observed in the pristine fi starch. Carbon peaks at binding energies of 286.4 and 287.7 eV moiety. This further con rms the formation of starch carbonate when starch reacts with DMC in the absence of amine. are associated with the C−O and O−C−O bonds, respectively. When both DMC and OA are presented in the reaction, no carbon peaks relating to carbonate ester were observed. Instead, new carbon peaks were seen at a binding energy of 289.6 eV, relating to the carbonyl carbon in the carbamate group (O− CO−N).49,50 The intensity of the peak relating to the C−C/C− H bond was observed to be higher compared to that of the sample prepared without DMC (i.e., regenerated starch), indicating the presence of an alkyl chain. Most importantly, and in contrast to other XPS spectra, a nitrogen peak was observed at a binding energy of 400.0 eV (Figure S2). The presence of these peaks supports the observations from the elemental analysis and DRIFT measurements that OA is coupled with starch via carbamate formation. Based on XPS, nitrogen content was found to be slightly lower compared to elemental analysis (0.8 vs 1.12 mmol/g). This difference is most likely due to the measurement techniques as the elemental analysis is a bulk method, whereas XPS only analyzes the surface of a sample. Due to the partial solubility of the sample produced at 80 °C (Table 1, sample 15, DS 0.09) in DMSO-d6, it was characterized using 1H and 13C NMR (Figure 3). The proton assignment of a starch moiety corresponds to the previous assignments.51 The signals of OH protons at positions 2, 3, and 6 together with Figure 2. High-resolution spectra of C 1s regions: (a) starch; (b) starch proton H-1 can be seen between 4.57 and 5.49 ppm. Also, the treated with OA; (c) starch treated with DMC; and (d) starch treated characteristic peaks of protons H-3 and H-5 can be clearly found with OA and DMC. at 3.65 and 3.58 ppm, respectively. The signals of protons H-2,

15705 DOI: 10.1021/acsomega.9b02350 ACS Omega 2019, 4, 15702−15710 ACS Omega Article

Figure 3. (A) 1H and (B) 13C NMR spectra of starch octyl carbamate (sample 15).

H-4, and H-6 cannot be assigned due to overlapping H2O signal. 3A) relates to a CH3 group in the octyl chain, and the signals at The successful attachment of an octyl carbamate to the starch 1.24 and 1.39 ppm (peaks b and c) can be assigned to six CH2 fi was also con rmed by the nuclear magnetic resonance (NMR) groups within the octyl chain. The CH2 group closest to the spectra. The signal at 0.86 ppm in 1H NMR (a peak in Figure nitrogen atom (peak d) is observed at 2.95 ppm, and most

15706 DOI: 10.1021/acsomega.9b02350 ACS Omega 2019, 4, 15702−15710 ACS Omega Article importantly, a weak amide proton signal can be seen at around 7 starch significantly alters the hydrophilicity of the material and, ppm.52 The degree of substitution (DS) was determined using therefore, its interaction with water. the following equation53 The starch octyl carbamate produced here could only be 4A dissolved in DMSO. Although the low solubility of products can DS = restrict their usability in many applications, hydrophobized + 3BA starch particles could, for example, be used to produce a where A is the integral value of methyl protons of the octyl chain Pickering emulsion. Hydrophobized microcrystalline cellulose and B is the integral value of proton H-1 and OH protons at has been studied for the production of an emulsion between positions 2, 3, and 6. Based on the integration of the 1H NMR water and toluene.59 Moreover, it is also envisioned that further spectrum, the DS of sample 15 is 0.090, which corresponds to optimization of reaction efficiency could be used to improve the the DS value received from elemental analysis (Table 1). The DS, which could then lead to better dissolution of the product. assignment of carbon atoms (C1−6) of the starch backbone In addition to the use of DMC as a coupling agent, carbon 54 (Figure 3B) corresponds to the previous assignments. The dioxide could be an even more sustainable reagent for starch assignment of carbon atoms (C1′−8′) of octyl chain is also carbamation. Several studies have been conducted for the direct presented in Figure 3B. However, the signal of carbon atom C8′ use of carbon dioxide for the synthesis of alkyl carbamates from 52 next to the nitrogen stands underneath the DMSO peak. The amines. However, instead of the use of , titanium60,61 or − signal of the carbonyl carbon could not be observed, most likely silicate62 64 esters have been used in these reactions. due to its long relaxation time.55 Previously, very high DS (around 3) of short carbon chain 56 ■ CONCLUSIONS carbamate has been obtained. However, previous studies have utilized very hazardous isocyanates as reagents.56,57 Thus, the Starch was alkylated with long-chain alkyl amine using DMC as a current study can be seen as a sustainable method to obtain coupling reagent. The use of basic catalysts improved the DS, starch carbamate when low DS is desired. The low DS in the and the highest reaction efficiency was obtained using DBU as current study might originate from the low solubility of octyl the catalyst. Reaction temperature did not have a significant carbamate starch in the reaction mixture. In addition, cross- effect on the DS of alkylated starch; however, the use of different linking of starch with DMC might take place. However, as the temperatures could be used to alter the dissolution of the reaction mixture remained clear (no formation of gel) when the product. The alkylation of the starch method introduced here DMC was used without OA, the amount of cross-linking might can be seen as environmentally friendly because DMC is be negligible. It is notable that DS obtained here is higher than biodegradable and not classified as a volatile organic component. alkyl starch having carbon chain lengths of 6 and 10 produced Furthermore, DMC can be produced from carbon dioxide, and using alkyl .58 this method could therefore be seen as a way to utilize waste The hydrophobization of starch due to the addition of a long carbon dioxide-based chemical for the modification of natural alkyl chain is apparent in Figure 4. Directly after mixing with polymers. Further studies should be conducted to improve the DS of the product and to investigate the possible applications of starch carbamate.

■ MATERIALS AND METHODS Materials. Starch from wheat, OA (99%), TMG (99%), DBU (98%), and deuterated dimethyl sulfoxide (DMSO-d6; 99.9 atom % D) were obtained from Sigma-Aldrich (Germany); dimethyl sulfoxide (DMSO; ≥99.5%) and ethanol from VWR (Finland); and DMC (>98.0%) and imidazole (>98.0%) from TCI (Belgium). All chemicals were used as received, without further purification. Chemical Modification of Starch. A reaction bottle was charged with 0.5 g of starch, 9.5 g of DMSO, 1.56 mL (18 mmol) of DMC, 3.06 mL (18 mmol) of OA, the desired amount (0− 0.15 mol %) of a catalyst (imidazole, TMG, or DBU), and a magnetic stirring bar. The reaction vessel was closed with a Figure 4. Starch (left) and starch octyl carbamate (right) in water (a) Teflon cap, placed in a heating block at a predetermined directly after mixing and (b) after mixing at 90 °C for 1 h (solid content temperature (60−100 °C), and allowed to react for 24 h under 1 wt %). stirring in a closed system. After reaction, the mixture was poured into 100 mL of ethanol under vigorous stirring. The water, the starch became a cloudy suspension, while starch octyl precipitated product was filtrated and washed with 500 mL of carbamate remained on the water’s surface. After intensively ethanol in 100 mL portion and, finally, dried in a 60 °C oven mixing at 90 °C for 1 h, the starch suspension became more gel- overnight. Reference reaction was conducted using both DMC like; starch is known to gelatinize or even dissolve in water. On or OA alone and without either of them. All of the used reaction the other hand, the starch octyl carbamate could not be conditions are presented in Table 1. efficiently dispersed in water as some of the material remained Elemental Analysis. The degree of substitution (DS) of the on the surface of the liquid, while the rest sank to the bottom of products was determined by analyzing their nitrogen content the container. In this case, the water layer remained more or less with an elemental analyzer (PerkinElmer 2400 Series II CHNS/ clear. This indicates that the introduction of long alkyl chains on O). The DS was calculated using the following equation

15707 DOI: 10.1021/acsomega.9b02350 ACS Omega 2019, 4, 15702−15710 ACS Omega Article

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15708 DOI: 10.1021/acsomega.9b02350 ACS Omega 2019, 4, 15702−15710 ACS Omega Article

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15710 DOI: 10.1021/acsomega.9b02350 ACS Omega 2019, 4, 15702−15710