Ambient Cross-linking System Based on the Knoevenagel Condensation Reaction between Acetoacetylated Sucrose and Aromatic Dicarboxaldehydes.

An ambient cross-linking system based on the Knoevenagel condensation reaction between acetoacetylated sucrose and aromatic dicarboxaldehydes was demonstrated. In this study, we use a rheological instrument to measure the gel time to predict and elucidate the likely reaction mechanism of the system, and we prepare films based on the mechanistic results. Acetoacetylated sucrose and 4,4'-biphenyldicarboxaldehyde were used as raw materials, piperidine was used as the catalyst, and nonvolatile dimethyl sulfoxide (DMSO) was used as the solvent. After mixing 4,4'-biphenyldicarboxaldehyde and piperidine for 30 min, the acetoacetylated sucrose was added, thus producing the shortest gel time. Then, the gel was characterized by Fourier transform infrared spectroscopy. In addition, three films were prepared by this approach with different aromatic dicarboxaldehydes, and the properties of the coatings were characterized by differential scanning calorimeter, dynamic mechanical analysis thermogravimetric analysis, and swelling ratio. It was found that these films have high Young's modulus, high glass transition temperatures, high pencil hardnesses, and low swelling ratios.

Introduction

Since the Knoevenagel condensation reaction was first reported by Emil Knoevenagel in 1894,[1] the Knoevenagel condensation reaction has been increasingly used and has become an important C–C bond formation reaction.[2−5] It has been used to obtain a variety of compounds, such as synthetic natural products,[6] drugs,[7] fine chemicals,[8] functional polymer materials,[9] and so forth.

In a Knoevenagel condensation reaction, in the dehydration condensation of compounds, compounds containing active methylene groups is used with aldehydes or ketones under weak base catalysis to produce α, β-unsaturated carbonyl compounds and their analogues.[10] Currently, the following two kinds of mechanisms consistent with the Knoevenagel condensation reaction are known (Scheme ): in the first mechanism, aldehyde compounds are first combined with catalysts and then react with acetylacetate to form an addition product, as was first proposed by Emil Knoevenagel[11](Scheme a); in the second mechanism, the catalyst first reacts with acetyl acetate and then attacks the aldehyde to obtain an addition product[12] (Scheme b). These two mechanisms are the subject of some agreement, but theoretical investigations of the mechanism of the Knoevenagel condensation reaction have been rare.[13,14] The Knoevenagel condensation reaction is a good cross-linking method because it can be carried out at room temperature with a high yield. Therefore, it is widely used in the synthesis of polymer materials, such as fluorescent materials,[15] conductive polymer materials,[16] and coatings.[17]

Scheme 1

Two Mechanisms of the Knoevenagel Condensation, (a) the Catalyst First Reacts with Aldehyde Compounds and (b) the Catalyst First Reacts with Acetyl Acetate

Paint has become an indispensable material for our society.[18] However, the production of paint is still based on petrochemical products. With the increasing importance of environmental issues, the need for sustainable resources and new approaches has gradually increased. As a renewable resource, sucrose is cheap, easily available, and has multiple hydroxyl groups, and it can be modified to produce a large number of coatings such as epoxy coatings,[19] polyurethane coatings,[20] and UV curing coatings.[21] However, there has been only one report in the literature on the preparation of the acetoacetylated sucrose coating which was used for the preparation of the coating for acetoacetylated sucrose and diamine.[22] Recently, our group[23] reported a Knoevenagel condensation reaction between plant groups and aromatic dicarboxaldehydes, but the possible reaction mechanism of the system was not studied.

In this work, we used the gel time to study the possible mechanism for the reaction between acetoacetylated sucrose and aromatic dicarboxaldehydes via the Knoevenagel condensation reaction at room temperature (25 °C) and elucidated the likely mechanism of the reaction. We then used this mechanism as guidance in the study of the coating properties.

Results and Discussion

NMR Characterization of Acetoacetylated Sucrose

The structure of acetoacetylated sucrose was confirmed based on the results of its 1H NMR and 13C NMR spectral analyses. Figure a shows the 1H NMR spectra of the acetoacetylated peaks at 3.58–3.54 and 2.29–2.24 ppm, while the 13C NMR spectra of acetoacetylated sucrose show peaks at 199, 165, 48, and 20 ppm[24] (Figure b). In addition, a significant decrease in the absorbance at 3380 cm–1 (−OH stretching frequency) and in the stretching vibration band of C=O of −COCH2COCH3 at 1746 and 1704 cm–1 is observed in Fourier transform infrared (FTIR) spectra (Figure ). These results demonstrate that sucrose and t-butyl acetylacetate undergoes a transesterification reaction.

Figure 1

NMR characterization of acetoacetylated sucrose: (a) 1H NMR spectra of acetoacetylated sucrose and (b) 13C NMR spectra of acetoacetylated sucrose.

Figure 2

Infrared spectra of sucrose (blue) and acetoacetylated sucrose (red).

Characterization of Gel Time

Figure and Table show the different gel times of the system for different orders of the addition of acetoacetylated sucrose, 4,4′-biphenyldicarboxaldehyde, and piperidine in the Knoevenagel condensation reaction. At the gel point, the storage modulus (G′) is equal to the loss modulus (G″), and the viscosity (η) increases dramatically. As observed from Figure and Table , after mixing 4,4′-biphenyldicarboxaldehyde (B) with the catalyst piperidine (C) for 30 min prior to adding the mixture to acetoacetylated sucrose (A), the gel time is the shortest (Figure b and Table , entry 1). When acetoacetylated sucrose (A) and 4,4′-biphenyldicarboxaldehyde (B) are mixed for 30 min prior to the addition of piperidine (C) for the cross-linking system of the Knoevenagel condensation reaction, an intermediate gel time was obtained (Figure c and Table , entry 2). When acetoacetylated sucrose (A) was mixed with piperidine (C) for 30 min prior to adding the mixture to the 4,4′-biphenyldicarboxaldehyde (B), the gel time was the longest[25] (Figure d and Table , entry 3). The rheological complex viscosity results also prove that mixing 4,4′-biphenyldicarboxaldehyde (B) with the catalyst piperidine (C) followed by the addition of the mixture to the acetoacetylated sucrose (A) obtains the shortest gel time.[26]

Figure 3

Rheological properties of gel time for different orders of the addition of raw materials and catalysts:(a) raw materials and catalysts; (b) order of addition: mix B and C for 30 min, then add to A; (c) order of addition: mix A and B for 30 min, then add to C; and (d) order of addition: mix A and C for 30 min, then add to B.

Table 1

Gel Time for Different Orders of Addition of Raw Materials and Catalysts

entry	adding order	G′ = G″ (s)	η increase (s)
1	BC-A	2142	2131
2	AB-C	2600	2721
3	AC-B	4364	4461

An examination of the gel time data indicates that the mechanism of the Knoevenagel condensation reaction of acetoacetylated sucrose and 4,4′-biphenyldicarboxaldehyde under the piperidine catalyst may involve the reaction of piperidine with 4,4′-biphenyldicarboxaldehyde, followed by the reaction with acetoacetylated sucrose. The likely mechanism of this reaction is the mechanism proposed by Knoevenagel in Scheme a.

The FTIR spectra of acetoacetylated sucrose, 4,4′-biphenyldicarboxaldehyde, and the gel after reaction are shown in Figure . Significant peaks due to the −CHO stretching frequency at 2828 and 2753 cm–1 were observed to decrease after cross-linking, and the insoluble material showed the characteristic absorption peaks of benzene rings at 1510 and 1380 cm–1; these results indicate that the acetoacetylated sucrose has reacted with 4,4′-biphenyldicarboxaldehyde.[23,27]

Figure 4

FTIR characterization of acetoacetylated sucrose, the gel, and 4,4′-biphenyldicarboxaldehyde.

Characterization of the Films

After exploring the reaction mechanism by rheological testing, we prepared the coating using the optimum cross-linking time. According to the above-mentioned cross-linking system, as shown in Table , volatile THF was used as the solvent, and 4,4-biphenylboxaldehyde and piperidine (10 wt %) were mixed for 30 min; then, this mixture was added to the acetoacetylated sucrose to cure the film. We found that this system produced many insoluble substances after 10 min, but this method cannot form a uniform membrane. The main reason for this may be that the amount of catalyst is too large and the system cured too fast. When we add the mixture to the piperidine in 2.5 wt %, the film could be cured in 3 h, and the film was retracted and could not form a flat membrane. Encouraged by this result, we further decreased the catalyst amount and found that 1 wt % of piperidine resulted in the best performance with the curing time of 5 h and gel content of 96%.

Table 2

Optimization of the Curing Reaction Conditions

entry	piperidine (catalyst) (wt %)	cure timea (h)	pencil hardness	solvent-swelling(%)	Ggl content (%)
1	10
2	2.5	3
3	1	5	5H	22	96
4	0.5	8	5H	21	95

Dry through cure time of the film.

FTIR spectra of acetoacetylated sucrose, an acetone-swollen substance after curing (P1) and 4,4′-biphenyldicarboxaldehyde are shown in Figure . A comparison with Figure shows that the cross-linked insoluble material and the film insoluble material were similar. Both showed the −CHO stretching frequency decreases at 2828 and 2752 cm–1, and for both, the benzene ring peaks at 1510 and 1380 cm–1 were produced.[23,27] In summary, we have successfully prepared films using a Knoevenagel reaction of acetoacetylated sucrose and 4,4′-biphenyldicarboxaldehyde.

Figure 5

FTIR characterization of acetoacetylated sucrose, P1, and 4,4′-biphenyldicarboxaldehyde.

Under the above optimized conditions, we used 4,4′-biphenyldicarboxaldehyde, isophthalaldehyde, or 1,4-phthalaldehyde as cross-linkers (P1, P2, and P3, respectively) to prepare three films (Table ). The characterization data for these three films are presented in Table . The pencil hardness values of these coatings range between 4 and 5 H, and the pendulum hardness values were between 33.1 and 41.2 s. The swelling ratios in acetone were between 17 and 22%, and the gel content ranged from 91 to 96%.

Table 3

Films of Different Cross-Linking Aromatic Dicarboxaldehydes

	cure time (h)
sample	set to touch	tack free	dry hard	dry through	pencil hardness	pendulum hardness (s)	acetone-swelling (%)	gel content (%)
P1	1.2	2.5	3.8	5	5H	41.2	22	95
P2	0.8	2.1	3	4.5	5H	39.5	19	96
P3	1.3	2.8	4.2	6	4H	33.1	17	91

Cross-link Density Analysis

The cross-link density and the cross-link molecular weight Mc of these films were calculated by the Flory–Rehner equation,[28,29] and the results are shown in Table . It could be seen that the cross-link molecular weight increased gradually from 7579 to 18 425 g/mol, while the cross-link density of these films was reduced from 1.702 to 0.691 mol/cm.[3] This was due to the steric hindrance of benzene rings.

Table 4

Mc, Cross-link Density, and Density of These Films

	sample code
	P1	P2	P3
Mc (g/mol)	7579	12 731	18 425
ve × 10–4 (mol/cm3)	1.702	1.041	0.691
density (g/cm3)	1.290	1.325	1.273

Dynamic Mechanical Analysis

The stress–strain behaviors of these coating films are shown in Figure and Table . Compared to P1, P2, and P3, it is obviously seen that three films have higher Young’s modulus and the film of P1, the best extension ratio. The main reason may be, that is, the high cross-linking density of P1.

Figure 6

(a) Stress–strain curves for these films and (b) bar chart of Young’s modulus, stress at break, and elongation at break.

Table 5

Thermal and Mechanical Properties of the Three Filmsa

				TGA in nitrogen (°C)
codes	Young’s modulus (MPa)	stress at break (MPa)	elongation at break (%)	T10	T50	Tmax	DSC Tg (°C)
P1	1402 ± 26	3.05 ± 0.5	0.31 ± 0.02	183	275	612	72
P2	1229 ± 33	1.39 ± 0.3	0.17 ± 0.05	183	271	611	79
P3	1045 ± 39	1.37 ± 0.4	0.23 ± 0.01	183	268	608	71

T10, T50, and Tmax represent the temperatures at the mass losses of 10, 50 wt %, and maximum mass loss, respectively.

Differential Scanning Calorimeter Analysis

The glass-transition temperatures (Tg) were obtained from the differential scanning calorimeter (DSC) curves, as shown in Figure and Table . Three films have higher glass-transition temperature and their Tg values were in the 71–79 °C range. The main reason for this result is the high cross-linking density of P1.[30]

Figure 7

DSC curves indicating the glass transition temperature of three films.

Thermogravimetric Analysis

Figure a shows the thermogravimetric analysis (TGA) curves for three films; the DTGA curve is presented in Figure b, and the T10, T50, and Tmax data are summarized in Table . Based on Figure a, two distinct degradation stages were observed for all three films. The degradation at the first stage in the range of 169–320 °C can be attributed to the dissociation of the modified ester bonds and carbon groups to form CO2. At the second stage, when the temperature reaches 350 °C, the polymer skeleton is obtained.[31] According to the Table , the T10, T50, and Tmax decomposition temperatures of the three films were similar.

Figure 8

(a) TGA curves for the three films and (b) DTGA curves for the three films.

Solvent Swelling

Figure shows the swelling ratio of the three films. After soaking for 30 h, the film swelling ratio reached the maximum values. Compared with the swelling ratio of the three coatings, P1 has the largest swelling ratio and P3 has the smallest swelling ratio. This result may be obtained because the aromatic dicarboxaldehyde cross-linking agent has the longest chain in the P1 structure.[32]

Figure 9

Acetone swelling of three films.

Conclusions

An ambient cross-linking system of acetoacetylated sucrose and aromatic dicarboxaldehydes was demonstrated. The possible mechanism for the synthesis of this system was explored by the rheological evaluation of gel time measurements. It was determined that the shortest gel time was obtained during the mixing of aromatic dicarboxaldehydes and the catalyst (piperidine) for 30 min, which was followed by the addition of acetoacetylated sucrose. In addition, three films were prepared by the procedure with the shortest gel time, and it was found that the films have high Young’s modulus, higher glass transition temperature, higher hardness, and lower acetone swelling ratio.

Experimental Section

Materials

Sucrose, 1,4-phthalaldehyde, 1,3-benzenedialdehyde, t-butylacetoacetate, and 4,4′-biphenyldicarboxaldehyde were obtained from Sigma-Aldrich, China. Piperidine dimethyl sulfoxide (DMSO) and tetrahydrofuran (THF) were purchased from Beijing Chemical Works. All the materials were used as obtained without further purification. The water used in this study was deionized and doubly distilled.

Preparation of Acetoacetylated Sucrose

The synthesis of acetoacetylated sucrose was carried out according to a previously reported study,[24] and the detailed synthesis method was as follows (Scheme ). In a 250 mL round-bottomed flask equipped with a magnetic stir bar and a condenser, a mixture of sucrose (40 g, 0.0538 mol) and t-butylacetoacetate (147.7 g, 0.936 mol) was stirred at 130 °C for 3 h, and the mixing was stopped when no more liquid evolved. Finally, 63 mL of the liquid was removed, and a yellow oil was obtained. 1H NMR (CDCl3, 400 MHz): δ (ppm) 5.61 (m, 1H), 5.49 (m, 1H), 5.29 (m, 1H), 5.12 (m, 1H), 5.06 (m, 1H), 4.99 (m, 1H), 4.89 (m, 1H), 4.39 (m, 6H), 4.28 (m, 1H), 3.58–3.54 (m, 16H), 2.29–2.24 (m, 24H). 13C NMR (CDCl3, 400 MHz): δ (ppm) 199.85, 165.34, 102.24, 89.18, 88.08, 73.67, 69.14, 69.32, 63.23, 52.58, 48.49, 29.13, 20.16. Calculated molecular weight = 1015; GPC: Mn = 931, Mw = 1121, PDI = 1.20.

Scheme 2

Preparation of Acetoacetylated Sucrose

Gel Time Study Using a Rheometer

Two sample bottles (30 × 50 mm) were used to study the gel time. Acetoacetylated sucrose (A, 0.25 g, 0.247 mmol) and DMSO (1.5 mL) were added to the first bottle, and 4,4′-biphenyldicarboxaldehyde (B, 0.156 g, 0.743 mmol) and DMSO (2.5 mL) were added to the second bottle for backup; the ratio of acetoacetylated sucrose and 4,4′-biphenyldicarboxaldehyde was 1:0.75, which was based on our previous report.

(BC-A)

Piperidine (52.5 μL, 0.478 mmol) was added to the dissolved sample bottle of 4,4′-biphenyldicarboxaldehyde (B) for 30 min; then, the dissolved acetoacetylated sucrose (A) was mixed by stirring for 2 min. Afterward, the mixing solution (20 drops) was added to the rheometer plate.

(AB-C)

The solution of acetoacetylated sucrose (A) and 4,4′-biphenyldicarboxaldehyde (B) was mixed for 30 min, and piperidine (52.5 μL, 0.478 mmol) was added with stirring for 2 min. Then, the mixing solution (20 drops) was added to the rheometer plate.

(AC-B)

Piperidine (52.5 μL, 0.478 mmol) was added to the dissolved sample bottle of acetoacetylated sucrose (A) for 30 min, and the dissolved 4,4′-biphenyldicarboxaldehyde (B) was mixed by stirring for 2 min. Then, the mixing solution (20 drops) was added to the rheometer plate.

The viscosity of the cross-linking system is used to determine the gel time. Parallel plate geometry was used and the gap between the plates was set to 400 μm. For the viscosity measurements, the sample shear rate was 10 rad/s, and the average values of three measurements of storage modulus, loss modulus, and viscosity monitored at room temperature (25 °C) were reported. The gel time was set as the time when the storage modulus was equal to the loss modulus or when the viscosity increased dramatically.

Coating Formulation

The method used for the preparation of the films is described in Scheme and Table , and the detailed experimental procedure was as follows: aromatic dicarboxaldehydes (2.958 mmol) and piperidine (1 wt %) were dissolved in THF (5 mL), and after mixing for half an hour, acetoacetylated sucrose (1 g, 0.986 mmol) was added to the mixture. Then, the mixture was stirred for 15 min and poured into a poly (tetrafluoroethylene) (PTFE) mould (8 cm × 8 cm × ss1.5 cm); then, the films were cured at ambient temperature to obtain a dry film (100–200 microns thick).

Scheme 3

Synthetic Route for Film Cross-Linking Structures

Table 6

Film Codes and the Ratio of Materials of the Knoevenagel Addition Reaction

sample code	acetoacetylated sucrose	cross-linker	acetoacetate/aldehyde ratio
P1	1 g (0.986 mmol)	4,4-biphenyldicarbaldehyde (0.621 g, 2.96 mmol)	1:0.75
P2	1 g (0.986 mmol)	terephthalaldehyde (0.396 g, 2.96 mmol)	1:0.75
P3	1 g (0.986 mmol)	M-phenylenedialdehyde (0.396 g, 2.96 mmol)	1:0.75

Coating Characterization

The viscosity of acetoacetylated sucrose was determined using a TA Discovery HR-2 rheometer. 1H NMR and 13C NMR spectra were obtained with a Bruker AV-400NMR instrument using tetramethyl silane as the internal reference. FT-IR spectra were obtained using a Bruker-Veretex70 spectrometer in the attenuated total reflection mode with the reported values obtained as averages of 32 scans for each sample in the 4000–500 cm–1 range. The drying times of the coatings were determined using a BK drying recorder, and the results were analyzed according to the ASTM D5895-2013 standard. The pencil hardness was measured according to the ASTM D3363 protocol. The pendulum hardness of these films was tested using an ASTM D4366 pendulum hardness tester (Sheen Instrument Ltd., UK).

Differential scanning calorimetry (DSC) employed a TA Instruments calorimeter (TA-Q200). The sample of films which at nitrogen atmosphere underwent the temperature raised from −60 to 100 °C at a rate of 10 °C/min.

TGA was performed using a TGA-Q50 system obtained from TA Instruments at a heating rate of 10 °C min–1 under a N2 atmosphere.

Dynamic mechanical analysis (DMA) was performed on a TA Instruments Q800 (New Castle, DE). A compression force of 3 N and a deformation of 5% were applied.

The gel content was measured by immersion of a film (2 cm × 2 cm pieces) with a known weight (W1); then, the dried film was dipped in acetone for 48 h and dehydrated for 48 h at 60 °C to obtain weight W2, and the gel content M was assessed according to the eq

Acetone swelling was performed by the immersion of a film with a known weight (W1) in an acetone bath, after which the towel-dried sample weight (W2) and the oven-dried sample weight (W1) were obtained; then, the acetone swelling (M %; amount of acetone absorbed by the film) of the films was calculated according to the eq

The cross-link density (ve) of the films was obtained by calculating the volume fraction of the swollen polymer with eq by the Flory–Rehner relation[33,34]where ve is the cross-link density, V2 is the volume fraction of the polymer in the swollen specimen, Vs is the molar volume of the solvent, χ is the interaction parameter between the polymer and the solvent, and Mc is the relative average molecular weight of the polymer.

The V2 from eq can be calculated according to eq where ρ1 and ρ2 are the densities of the solvent and the polymer, respectively.

The interaction parameter (χ) between the polymer and the solvent was determined by eq where R is the gas constant and T is the absolute temperature. δs and δp were the solvent and the solubility parameters of the polymer, respectively. The solubility parameter δp of the polymer could be determined by the swelling method.[35]