# Effect of the formation of hydantoin in aspartate-based polyurea networks

Adrien Topalian, Françoise Méchin, Jannick Duchet-Rumeau, Rolf Klucker,

Jean-François Gérard

► To cite this version:

Adrien Topalian, Françoise Méchin, Jannick Duchet-Rumeau, Rolf Klucker, Jean-François Gérard. Effect of the formation of hydantoin in aspartate-based polyurea networks. Progress in Organic Coatings, 2025, 200, pp.109102. 10.1016/j.porgcoat.2025.109102. hal-04921789

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Submitted on 30 Jan 2025

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# Effect of the formation of hydantoin in aspartate-based polyurea networks

Adrien Topalian<sup>a</sup>, Françoise Méchin<sup>a</sup>, Jannick Duchet-Rumeau<sup>a</sup>, Rolf Klucker<sup>b</sup>, Jean-François Gérard<sup>a</sup>

## ABSTRACT :

The paper aims to give an understanding of the structure of crosslinked polyurea coatings based on aspartate esters and to link it with changes in mechanical properties observed over time. Model systems representative of the networks are synthesized and analyzed by NMR. It allows to put forward the formation of hydantoin by the reaction of the urea groups with the neighboring ester groups. It is seen that the presence of acid within the network accelerates this phenomenon. A feature of the work deals with the influence of hydantoin formation on mechanical properties. Depending on the presence of plasticizers, it appears that hydantoin formation leads to different mechanical properties changes.

Keywords : Polyurea, Hydantoin, Plasticizers, Toughness, Coatings

## 1 Introduction

Polyureas are complex polymers where molecular architecture plays a significant role on the resulting macroscale properties. ¹,² These polymers exhibit a very good resistance under heat humidity, even UV. According to the high reactivity of isocyanates with amines, their polymerization kinetics is very fast and can occur at room temperature. Resulting polymers combine stiffness and hardness which make them materials with good abrasion resistance. Nevertheless, the major disadvantage lies in the cost of such polyurea for coating applications.³

Polyurea coatings are synthesized from polyaddition polymerization of reactants bearing isocyanate and amine functions. This isocyanate-amine reaction, which can be instantaneous, is useful in hostile environments but the low "pot life" makes these reactive systems difficult to use. As a consequence, it is generally advised to use hindered secondary amines instead of primary ones according to their lower reactivity, i.e. allowing longer pot life⁴.

The latest advances report the use of polyaspartate esters as innovative hindered secondary amines. These are synthesized by aza-Michael addition of diethyl maleate to various difunctional primary amines. The low viscosity of these components allows to use lower amounts of solvents (VOCs) for spray deposition, which is commonly used for processing polyurea coatings. Such an advantage is crucial as the current trend is to reduce VOCs in coating applications.³⁻⁶

The Michael reaction or Michael addition leads to the creation of carbon-carbon bonds from a nucleophilic agent and a carbonyl compound called "Michael acceptor" containing an electron-deficient double bond. In the case of the Aza-Michael reaction, a carbon-nitrogen bond is created from a primary or secondary amine and a compound having an electron-deficient double bond (Michael acceptor)⁵⁻⁸ (Scheme 1a)

<footnote><sup>a</sup> Universite Claude Bernard Lyon 1, INSA Lyon, Université Jean Monnet, CNRS
UMR 5223, Ingénierie des Matériaux Polymères F-69621 Villeurbanne CEDEX, France</footnote>

<footnote><sup>b</sup> Vencorex France, 85 avenue Frères Perret, 69190 Saint Fons, France</footnote>
a)

<figure><img src="image_2.png" /></figure>

b)

<figure><img src="image_3.png" /><figcaption>Scheme 1: (a) Synthesis of a bis(aspartic acid ester) from a difunctional primary amine and diethyl maleate; (b) Aza-Michael reaction leading to the formation of the ester aspartate from cyclohexylamine and diethyl maleate, possibly implying isomerization to fumarate</figcaption></figure>

This reaction can take place at room temperature without any catalyst. Moreover, no additional product is formed, thus making this reaction widely used in the context of green chemistry.⁹

Nevertheless, the presence of the primary amine could catalyze the isomerization of diethyl maleate to diethyl fumarate. Diethyl fumarate also reacts with the primary amine to form an aspartate ester. However, this reaction is much slower (Scheme 1b).¹⁰

Diethyl fumarate formation also results in the yellowing of the final product and is harmful to human health. For these reasons, various studies focused on alternative synthesis routes of ester aspartates avoiding the presence of diethyl fumarate residues.¹¹⁻¹³

When reacted with a polyisocyanate (functionality > 2), those specific diamines lead to both chemically and physically crosslinked networks¹⁴. The chemical architecture and size of the amine and isocyanate, as well as the stoichiometric ratio, determine the structure of the resulting networks. Generally, long and linear amines lead to soft polymer networks with a rather low Tg, whereas a short amine leads to the formation of a more rigid polymer, i.e. having a higher Tg. The mechanical properties also depend on intermolecular Van der Waals and hydrogen bonds formation¹⁵⁻¹⁷. Within polyurea coatings, conventional -NH-CO-NH- urea units issued from the reaction of primary amines generate strong bidentate hydrogen bonds, whereas the reaction of secondary amines leads to units that can only form one hydrogen bond per substituted urea group. As a consequence, lower energy associations are formed. For those rather unusual chemical structures of amines combined with isocyanates, the presence of an ester group just nearby the substituted urea function can lead to the formation of hydantoin, i.e. a 5-atom cyclic compound with carbonyl functions⁴,¹⁸⁻²⁰ (Scheme 2)
<figure><img src="image_4.png" /><figcaption>Scheme 2: Cyclization of an aspartic ester urea leading to a substituted hydantoin moiety</figcaption></figure>

The first paper reporting hydantoin synthesis from the reduction of allantoin was published in 1861 by Baeyer.²¹ Since that time, many publications and reviews have traced different ways of synthesizing such compound¹⁹,²²⁻²⁴. Among these ones, the Biltz synthesis²⁵ involves 1,2-dicarbonyl compounds and unsubstituted or substituted ureas. The Bucherer-Bergs synthesis ²⁶ deals with the use of carbonyl compounds, potassium or sodium cyanide, and ammonium carbonate as a source of carbon dioxide for the synthesis of hydantoin. At last, the Read synthesis²³ consists of the reaction between amino acid derivatives and isocyanates. If the catalysis by the addition of acid agents has been extensively studied and demonstrated, the role of bases remains subject to discussion.²⁷⁻³⁰

The present work puts attention on the formation of hydantoin by reaction between an ester and a urea, both belonging to a single aspartic-based urea unit. In the past, various studies have been conducted on the synthesis of polyhydantoins from aromatic polyisocyanates and bisiminoacetates.³¹⁻³⁴ These works highlighted the role of the acidic agent in cyclization. In this work, the dependence of the physical properties of polyaspartic polyurea films is studied under acidic conditions. The consequences of hydantoin formation on the mechanical behavior are highlighted. The aim was to demonstrate that the formation of hydantoin led to a reorganization of the polymer network having consequences on the mechanical properties.

Firstly, model systems were considered to evidence the formation of hydantoin and to detail the chemical mechanisms. Subsequently, the synthesis and analysis of polyurea networks were performed to evidence hydantoin formation and to compare with the model systems.

In order to improve the properties of polymer materials and especially coatings, additives are usually used in reactive formulations³. Plasticizers are frequently added to increase flexibility or impact resistance.³⁵,³⁶ Thus, in this work, additives were intentionally added to determine the influence of hydantoin formation on the role of plasticizers and consequently on the mechanical properties of the resulting polyurea networks.

# 2 Materials and methods

## 2.1 Materials

Diethyl maleate (97%), heptyl isocyanate (97%), n-cyclohexylisopropylamine (98%), and triphenyl phosphate (99%) were purchased from Aldrich Co. and used without any further purification. Cyclohexylamine (98%) was purchased from ABCR Co. and was used without any further purification.

Tolonate HDT-LV (tHDI) (mainly composed of the trimer of 1,6-hexamethylene diisocyanate; average NCO equivalent 183 g.mol⁻¹) was purchased from Vencorex Co. and used as received.
The polyaspartic ester resin Feiyang F420 (Am1) (obtained from the reaction of 4,4'-methylene bis(cyclohexylamine) with diethyl maleate; average amine equivalent weight 277 g.mol⁻¹) was purchased from Feiyang Co. and used as received.

The polyaspartic ester resin Amicure IC-221 (Am2) (average amine equivalent weight 379 g.mol⁻¹) was purchased from Evonik Co. and used without any further purification. It is assumed to consist of the same diaspartic ester as the Feiyang F420 compound. However, according to the average amine equivalent weight, an unknown, presumably non-reactive additive representing 26% by wt. is also present.

### 2.1.1 Aspartate-based polyurea film processing

In order to synthesize aspartate-based polyurea networks, the reactive amine was first added to the isocyanate at room temperature under ambient humidity conditions. All systems are sufficiently fluid to be processed in bulk under such conditions. The mixtures were then homogenized using a planetary mixer for 2 minutes at 2,000 rpm. Finally, the mixtures were cast on a glass plate to make films using a bar-coater equipped with a 100 µm applicator. The films were then left under a ventilated hood at room temperature.

In this study, the network THDI(Am1) denotes aspartate-based polyurea networks synthesized by reacting tHDI with the amine Am1 at room temperature, maintaining a stoichiometry of NH:NCO = 1.3:1 to ensure no residual reactive isocyanate remains in the system. Similarly, the network THDI(Am2) refers to a network obtained by reacting the amine Am2 with THDI at room temperature, using a stoichiometry of NH:NCO = 1:1. Lastly, the network THDI-TPhP(Am1) was formed at room temperature by reacting tHDI with Am1 diamine in the presence of triphenyl phosphate (TPhP) at a stoichiometric ratio of NH:NCO = 1.3:1.

## 2.2 Methods

### 2.2.1 NMR analysis

High-resolution NMR spectroscopy analyses were carried out using an AVANCE III spectrometer operating at 400.13 and 100.6 MHz for ¹H and ¹³C analyses, respectively. The spectra were obtained using a 5 mm-BBFO+ probe at 298K. Other analyses were carried out using an AVANCE II spectrometer operating at 400.13 MHz and 100.6 MHz for ¹H and ¹³C analyses, respectively. In this case the spectra were obtained with a 10 mm-Selective ¹H/¹³C probe at 298K.

The analyses were carried out using deuterated chloroform as solvent and tetramethylsilane (TMS) as reference.

### 2.2.2 Thermal analysis

A DSC calorimeter TA Instruments Q10 was used with 4-7 mg specimens placed in aluminum pans. The tests were carried out under nitrogen. The heating rate was 10K.min⁻¹ while the coolings were run at 20K.min⁻¹.

The data were then analyzed on TA Universal software and plotted using Python software. The glass transition temperatures, Tg, were determined as the midset temperatures.

The thermogravimetric analysis (TGA) tests were carried out using a Q500 TGA device from TA Instrument under a constant nitrogen flow and at a temperature heating rate of 3K.min⁻¹. The results were analyzed using the TA Instrument software and the curves were plotted using the Python software.
Thermogravimetric analysis was occasionally coupled with infrared spectroscopy in order to analyze the gases emitted during degradation. These experiments were carried out using a Perkin Elmer TGA 8000 apparatus under nitrogen flow at 3K.min⁻¹. The results were analyzed using Time Base and Pyris Series softwares. The curves were plotted using Python software.

### 2.2.3 Spectrophotometric analysis

The chemical structures were analyzed by Fourier Transform infrared (FTIR) spectroscopy using a Thermo Scientific Nicolet iS10 spectrometer. Spectra were obtained in ATR (attenuated total reflectance) mode with a diamond crystal. The spectral acquisition range extends from 550 cm⁻¹ to 4,000 cm⁻¹ with a resolution of 4 cm⁻¹ and using 16 scans.

### 2.2.4 Mechanical analysis

The tensile tests were carried out using an Instron 33R-4469 tensile machine. Specimens were cut using an ISO 527-2 standard punch. The tests were carried out at a crosshead speed of 50 mm.min⁻¹. The dimensions of the samples were 20\*4 mm². The thickness varied from 0.1 to 0.4 mm depending on the spiral bar applicator chosen for processing coatings.

## 3 Results

### 3.1 Model system

For a better understanding and an easier characterization of the reaction mechanisms, a model system synthesized from lower functionality precursors was considered first. The latter allows to avoid the formation of a 3D network, i.e. enabling the possible solubilization of the reacted systems.

#### 3.1.1 Synthesis of a model N-modified aspartic acid ester: diethyl 2-(cyclohexylamino) succinate (CHAS)¹⁰

As shown before, the aza-Michael addition reaction between diethyl maleate (120 g, 69.7 mmol) and cyclohexylamine (6.912 g, 69.7 mmol) leads to the synthesis of diethyl 2-(cyclohexylamino)succinate (CHAS) (Scheme 2). The reaction was carried in bulk and without catalyst for 12h at 50°C under stirring.

The presence of the ethylene group in diethyl maleate results in a ¹H signal at 6.25 ppm. However, when the conformation changes to diethyl fumarate (DEF), this peak shifts to 6.85 ppm. The NMR spectra reported in the supporting information (Figure S1) show that traces of diethyl fumarate are observed after reaction, denoting that cyclohexylamine promotes the isomerization of diethyl maleate in the final mixture. The disappearance of the signal characteristic of diethyl maleate shows that it was completely consumed during the reaction. In the carbon NMR spectrum, the presence of diethyl fumarate can also be revealed by the presence of peaks at 165, 134, and 61 ppm (Figure S2).

The content of unreacted species present in the synthesized CHAS can be therefore estimated to be 8.2 wt%, including diethyl fumarate (DEF) (7.2 wt%) and cyclohexylamine (1 wt%).

#### 3.1.2 Synthesis of the urea-esters model compound: diethyl N-cyclohexyl-N-(heptylcarbamoyl)aspartate

The urea-esters model compound was obtained by reaction of heptyl isocyanate (0.52 g) with CHAS (1.11 g) at room temperature (Scheme 3a). The mixture was stirred manually for a few minutes at room temperature. After a few hours, a white solid was formed and directly analyzed (without purification).

The advantage of this model molecule lies in its functionality equal to 1. Thus, no network can be formed and the reaction products can be easily analyzed as they remain soluble. Moreover, the model molecule is not symmetrical which makes the NMR analysis easier.
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>Scheme 3 : (a) Synthesis of the model urea-esters; (b) Evolution with aging of urea-esters</figcaption></figure>

The resonance initially associated with the protons of heptyl isocyanate (3.27 ppm, alpha carbon) is not present in the ¹H NMR spectrum of the model urea-esters. Thus, the isocyanate groups have been entirely consumed (Figure S3).

However, a very low intensity resonance can be evidenced in the 2.55-2.70 ppm region of the model urea-esters NMR spectrum, corresponding to a residue of CHAS amine. It is estimated that there are 7.3 wt% residual impurities, with among them the residual DEF contained in CHAS. (Figure S4)

The ¹H and ¹³C NMR analysis of the model urea allows to identify the resonances associated with each proton and carbon (Figure 1-Figure 2). In particular, the presence of the urea carbonyl (157 ppm) and the peaks corresponding to the carbonyls of the esters (171.8 and 172.1 ppm) can be found.
<figure><img src="image_7.png" /><figcaption>Figure 1 : $^{1}$H NMR spectra of a model urea-esters compound (298K, $CDCl_3$)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Figure 2 : $^{13}$C NMR analysis of the model urea-esters compound (298K, $CDCl_3$)</figcaption></figure>

### 3.1.3 Synthesis of hydantoin model compound: ethyl 2-(3-cyclohexyl-1-heptyl-2,5-dioxoimidazolidin-4-yl)acetate

The reaction of heptyl isocyanate (0.52 g, 3.69 mmol) with CHAS (1.02 g, 3.69 mmol) was carried out at room temperature. 0.029 g of acetic acid (i.e. 13 mol% with respect to NCO groups) was added to the reactive mixture under stirring. After 3 hours, a white solid was formed, which turned to a yellow viscous liquid after a day under room atmosphere. The model urea-esters compound, denoted as AA, was then formed and analyzed without any further purification. In what follows, the "model urea-esters AA compound 'fresh'" refers to the solid compound left less than a day after synthesis, while the "model urea-esters AA aged compound" refers to the liquid compound formed more than a day after synthesis due to the formation of hydantoin and ethanol (Figure 3 and Scheme 3b).
<figure><img src="image_9.png" /><figcaption>Figure 3: Model urea-esters, denoted as “fresh” (left) and model urea-esters AA, denoted as ‘aged’ (right)</figcaption></figure>

The comparison of the ¹H NMR spectra of heptyl isocyanate and of the model urea-esters AA shows that all of the isocyanate functions were consumed during the reaction.

In contrast, a comparison between ¹H NMR spectra of CHAS and of the model urea-esters AA shows that a CHAS/DEF mixture residue (about 11 wt%) remains.

<figure><img src="image_10.png" /><figcaption>Figure 4: ¹H NMR analysis of the ‘aged’ model urea-esters AA (298K, 400 MHz, CDCl₃)</figcaption></figure>

Many peaks evolved following the addition of acetic acid (Figure 4). First, proton 6 shifted in the 4.1-4.2 ppm region, therefore overlapping with protons 2. Proton 4 also shifted from 3.4 to 3.7 ppm. However, it is also the resonance zone of ethanol, thus explaining why the integration is equivalent to 1.9. Finally, proton 7 disappeared from the NMR spectrum while proton 9 shifted from 3.2 to 3.5 ppm.

The ¹³C NMR analysis confirmed the observations made previously (Figure 5). Some peaks shifted, such as that of carbon 19 (from 172.02 to 168.96 ppm) or carbon 20 (from 171.67 to 172.61 ppm). The resonances associated with the carbons CH₂ and CH₃ of one ester disappeared, therefore confirming the hypothesis of hydantoin formation. Two resonances appeared at 58.4 and 18.5 ppm corresponding to the generation of ethanol. One could consider the formation of urethane in the system from the reaction between the newly formed ethanol and isocyanate. However, in the carbon NMR spectrum of the aged model urea-esters AA, no new resonance peaks have been observed to confirm urethane formation. In the model systems, the presence of acetic acid catalyzes urea formation, leaving no residual isocyanate available to react and form urethane. Nonetheless, in a network with different mobility and kinetics, residual isocyanate could commonly be observed, and thus such a phenomenon cannot be ruled out.²⁰ The NMR assignments observed are close to the resonances reported in the literature.³²
<figure><img src="image_11.png" /><figcaption>Figure 5: ¹³C NMR analysis of the model urea-esters AA aged (298K, 100.6 MHz, CDCl₃)</figcaption></figure>

The possibility of a total conversion of the aspartic-based urea to hydantoin was therefore demonstrated by considering a soluble model system as polyurea networks cannot be solubilized for NMR analyses. Therefore, in this latter case, other ways to prove the hydantoin formation and possibly to quantify it must be found.

### 3.1.4 Spectrophotometric analysis

In the ATR-IR spectrum of the fresh model urea-esters molecules, the absorbance band at 3,350 cm⁻¹ indicates the presence of NH groups linked by hydrogen bonds. Two absorption bands at 1,609 and 1,548 cm⁻¹ confirm the presence of C=O and NH-C=O (amide II) groups. The absorption band at 1,732 cm⁻¹ corresponds to the C=O of the esters (Figure 6).

FTIR analyses were then carried out on the aged model urea-esters AA. First, it is possible to observe that a new absorption band centered at 3,500 cm⁻¹ appears and widens while the initial narrow peak at 3,350 cm⁻¹ is no longer detectable, suggesting that the hydrogen bonds were destroyed because the NH functions of the urea disappeared partially or totally while some hydroxyl groups appeared (that could originate from ethanol) (Figure 7). The absorption band intensity of the esters at 1,737 cm⁻¹ decreases while a new absorption band at 1,767 cm⁻¹ appears. Finally, the intensity of a new absorption band at 1,701 cm⁻¹ increases with time. Those two new absorption bands undoubtedly could be attributed to the hydantoin carbonyls.

Finally, the absorption bands corresponding to the NH-C=O (amide II) and the C=O (urea) disappear. This shows that FTIR is an efficient tool to determine whether ureas or hydantoins are present in the system.

<figure><img src="image_12.png" /><figcaption>Figure 6: FTIR spectra of the model urea-esters 'fresh' (blue) and the aged model urea-esters AA (red) (1,400 cm⁻¹-1,900 cm⁻¹) (room temperature, ATR-IR, 16 scans)</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Figure 7: FTIR spectra of the model urea-esters 'fresh' (blue) and the aged model urea-esters AA (red) (2,700 cm⁻¹-3,600 cm⁻¹) (room temperature, ATR-IR, 16 scans)</figcaption></figure>

### 3.1.5 Thermal analysis

During the synthesis of the model urea-esters AA compounds, after the reaction between CHAS and the isocyanate, the formation of hydantoin occurs very quickly. Therefore, to study the 'fresh' model urea-esters AA system, i.e. still containing urea groups, the analyses were made very shortly after the synthesis, but nevertheless after the entire consumption of the NCO groups. (Figure S5)

#### 3.1.5.1 Thermogravimetric analysis coupled with FTIR spectroscopy (TGA-IR)

When heating 'fresh' model urea-esters AA, a characteristic phenomenon is easily evidenced by TGA-IR. A large degradation accompanied by a non-negligible mass loss is observable at about 94°C (about 12 wt% loss at 130°C) (Figure 8).

The coupling of TGA with IR-spectroscopy confirms the simultaneous release of ethanol. It is therefore possible to correlate the degradation mechanism with the formation of hydantoin. One can notice a first small degradation at 70-75°C suggesting that some ethanol has already been present at the beginning of the analysis and that a few hydantoin groups presumably had already been formed.

However, in the case of a total conversion, the total amount of mass lost due to the ethanol evaporation in the model urea AA should be approximately equal to 10%, showing therefore that an additional degradation mechanism occurs. The identification of ethanol evaporation can be clearly confirmed by FT-IR at 100°C but another strong absorption band is visible at 2,300 cm⁻¹. This one could correspond to carbon dioxide as reported in Figure S6.

<figure><img src="image_14.png" /><figcaption>Figure 8: Thermogravimetric analysis of the 'fresh' model urea-esters AA (blue) and its mass loss derivative (red) (3 K.min⁻¹, under nitrogen)</figcaption></figure>

#### 3.1.5.2 Differential scanning calorimetry

The DSC thermogram of the 'fresh' model urea-esters AA displays an exothermic followed by an endothermic peak occurring from 75 to 150°C (Figure 9). This latter peak reminds of the degradation observed by TGA. The whole phenomenon may correspond to the cyclization reaction, where the exothermic peak corresponds to the hydantoin formation and the endothermic variation to the ethanol vaporization.
Therefore, during the second heating scan, the model system consists of hydantoin groups. And indeed the enthalpic variation is no longer visible, confirming that it corresponds to the formation of hydantoin.

<figure><img src="image_15.png" /><figcaption>Figure 9: DSC thermogram obtained for the ‘fresh’ model urea-esters AA (10 K.min⁻¹, under nitrogen)</figcaption></figure>

## 3.2 Analysis of an aspartate-based urea network – THDI (Am1)

As mentioned, different aspartate-based polyurea networks were synthesized using different ester aspartates. Polyureas THDI (Amx, x = 1 or 2) were obtained from the reaction of the secondary diamines Amx with tHDI.

In order to investigate the architecture of the resulting networks, the previous studies dedicated to hydantoin formation and its investigation on model urea-esters AA will be helpful.

### 3.2.1 Synthesis of THDI (Am1) network

The aspartate-based polyurea networks were synthesized by reacting tHDI (19.47 g -106 mmol NCO) with the amine Am1 (38.31g - 138 mmol NH) to have a stoichiometry NH : NCO=1.3 : 1, i.e. to avoid any residual reactive isocyanate left in the system. For a lower stoichiometric ratio, traces of residual isocyanates can indeed be found. No exothermic phenomenon has been observed during the synthesis.

In the following, two types of networks will be considered: "'fresh' THDI (Am1)" and "aged THDI (Am1)". THDI-fr (Am1) corresponds to the system right after synthesis. The network is still mainly composed of urea groups. THDI-ag (Am1) corresponds to the network consisting mainly of hydantoin groups.

### 3.2.2 Thermal analysis

#### 3.2.2.1 Thermogravimetric analysis

Similarly to what was observed on model compounds, hydantoin formation in THDI networks can be evidenced by coupled TGA-IR techniques (see Figure S7). An important degradation phenomenon is observable with a maximal rate around 162°C for the THDI-fr (Am1) ('fresh'). At 220°C, the total weight loss is estimated to be 8%. For the model urea-esters AA, a similar degradation at 94°C due to the hydantoin formation followed by the release of ethanol had been observed. Given the differences due to the acid presence and the system mobility, the temperature at which ethanol is formed and released varies. However, it is noted that no significant major degradation is visible at 160°C for the model system urea-esters AA. It can therefore be deduced that this phenomenon corresponds mostly to the release of ethanol following the hydantoin formation. Considering that one molecule of ethanol is formed for each urea group that converts into a hydantoin group, it is possible to determine the hypothetical mass of ethanol formed in case of a total urea conversion. It has been calculated that the
total amount of ethanol that could be released due to a total conversion of urea corresponds to 8.1% of the weight of the material.

### 3.2.2.2 Differential scanning calorimetry analysis

During the first DSC heating ramp, THDI-fr (Am1) should be mainly composed of aspartate-based ureas linked by hydrogen bonds (Figure 10). The DSC thermogram of THDI-fr (Am1) shows an exotherm at 139°C. This enthalpic event is no longer observable during the second ramp. As a consequence, this peak can be correlated with the degradation phenomenon already observed by TGA, i.e. the formation of hydantoin. The glass transition temperature is equal to 56°C during the first ramp but increases to 69°C during the second ramp. A simple explanation for this increase can first be found in hydantoin formation, with the release and more or less simultaneous evaporation of ethanol. However, the formation of hydantoin does not lead to changes in the molar mass between crosslinking nodes. Additionally, the loss of hydrogen bonds is expected to increase local mobility, consequently leading to a decrease in the glass transition temperature (Tg). It is also conceivable that carbonylureido links may form between two different chains, potentially influencing the crosslink density.²⁰ Moreover, considering that a dangling ester chain is not necessarily stabilized by a hydrogen bond, it is possible to make the hypothesis that its conversion to a more rigid cyclic group can reduce local mobility and finally lead to an increase in the glass transition temperature.

<figure><img src="image_16.png" /><figcaption>Figure 10: DSC thermogram obtained for THDI-fr (Am1) network (10 K.min⁻¹, under nitrogen)</figcaption></figure>

Both differential scanning calorimetry and thermogravimetric analyses thus confirm that hydantoin formation was possible in the THDI-fr (Am1) network.

### 3.2.3 Spectrophotometric analysis

Hydantoin formation can be catalyzed by acidic agents but can also be induced by annealing. Therefore, annealing at different temperatures was considered in order to study the hydantoin formation within networks.

To do so, the network THDI-fr (Am1) was heated at 50°C for seven days and its chemical structure was simultaneously monitored by FTIR spectroscopy. For the THDI-fr (Am1) network in blue in Figure 10, two absorption bands at 1,623 and 1,637 cm⁻¹ confirm the presence of C=O (urea) linked by hydrogen bonds and free C=O (urea), respectively. The absorption band at 1,530 cm⁻¹ corresponds to NH-C=O (amide II) groups. The absorption band at 1,731 cm⁻¹ corresponds to the C=O of the esters. The absorption band at 1,686 cm⁻¹ is due to the presence of specific carbonyl bonds introduced by the polyisocyanate (C=O, isocyanurate) (Figure 11).

Two absorption bands are found at 3,366 and 3,426 cm⁻¹ indicating the presence of NH groups, linked by hydrogen bonds or free, respectively (Figure 12).
<figure><img src="image_17.png" /><figcaption>Figure 11: Evolution of the FTIR spectra of THDI (Am1) networks before (blue) and after annealing at 50°C during 48 h (red) and 7 days (green), respectively (1,400-1,900 cm⁻¹ ATR-IR)</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>Figure 12: Evolution of the FTIR spectra of THDI (Am1) networks before (blue) and after annealing at 50°C during 48 h (red) and 7 days (green), respectively (2,700-3,600 cm⁻¹ ATR-IR)</figcaption></figure>

After annealing, two new absorption bands appear at 1,766 and 1,702 cm⁻¹ corresponding to the carbonyl groups of the hydantoin ring. The absorption bands at 1,623 and 1,530 cm⁻¹ considerably decrease, showing that the ureas have been transformed into hydantoins to a great extent. The absorption bands at 3,366 and 3,426 cm⁻¹ are no longer clearly visible, i.e. at least they decrease significantly. It can be assumed that most of the hydrogen bonds due to NH groups disappeared, as many NH groups are no longer present.

### 3.2.4 Mechanical properties of the networks

Tensile tests were conducted on THDI (Am1) networks to study the effect of hydantoin formation on the mechanical properties. Previously, it was seen via FTIR spectroscopy that THDI (Am1) network architecture integrated hydantoin after being heated at 50°C for seven days.

The networks were studied *i*/ right after their synthesis (presence of a majority of urea groups), *ii*/ after being heated to 50°C for 48 hours (mix of urea and hydantoin groups), and *iii*/ after being heated to 50°C for seven days (presence of a majority of hydantoin groups).

Tensile tests performed on THDI (Am1) networks showed that the annealing at 50°C had a limited effect on mechanical behavior. The annealed networks still have a high Young's modulus (approximately 1,67 MPa), a high stress at break (57 MPa), and a low strain at break (6%) (Figure 13-Table 1).
<figure><img src="image_19.png" /><figcaption>Figure 13: Tensile tests carried out on THDI-fr (Am1) (blue), THDI (Am1)(50°C-48 h) (red), and THDI (Am1)(50°C-7 days) (green) networks (crosshead speed: 50 mm.min⁻¹, room temperature)</figcaption></figure>

<table><caption>Table 1: Mechanical properties of THDI (Am1) networks 'fresh' (Tg = 56°C) or after annealing at 50°C for various times (50 mm.min⁻¹, room temperature)</caption><thead><tr><th></th><th>THDI-fr(Am1)</th><th>THDI (Am1) (50°C - 48 h)</th><th>THDI (Am1) (50°C - 7 days)</th></tr></thead><tbody><tr><td>Young's modulus (MPa)</td><td>1,670 ±168</td><td>1,507 ±130</td><td>1,698 ±128</td></tr><tr><td>Stress at break (MPa)</td><td>53±2</td><td>57±4</td><td>57±3</td></tr><tr><td>Strain at break (%)</td><td>6±2</td><td>6±1</td><td>6±1</td></tr></tbody></table>

## 3.3 Analysis of plasticized aspartate-based urea networks THDI-TPhP (Am1)

### 3.3.1 Synthesis of THDI-TPhP (Am1) network

A THDI-TPhP (Am1) network was formed by reaction between tHDI (19.47 g -106 mmol NCO) and Am1 diamine (38.31g - 138 mmol NH) in which triphenyl phosphate (TPhP) was added (12.77g). The NH : NCO stoichiometric ratio chosen was once again 1.3 : 1 in order to avoid residual isocyanate in the resulting network (see above).

In this work, TPhP was intentionally added to determine the influence of hydantoin formation on the role of plasticizers and consequently on the mechanical properties of the resulting plasticized polyurea networks.

TPhP (which is solid at room temperature, mp 49-50°C) was first added to the Am1 diamine and the mixture heated up to 55°C for a few minutes to become homogeneous. A decrease in the viscosity of the mixture was noticed and the mixture remained liquid after cooling.

### 3.3.2 Spectrophotometric and thermal analyses

The THDI-TPhP (Am1) network was heated at 50°C for 7 days in order to accelerate hydantoin formation. FTIR spectroscopy analyses were carried out on this THDI-TPhP (Am1) network before and after the annealing to follow the reaction.

After the annealing stage, the absorption bands related to hydantoin formation appeared (1,702-1,766 cm⁻¹) while those corresponding to the C=O group of urea and amide II disappeared (Figure 14-Figure S8). It confirms that heating the films at 50°C for 7 days allowed to form hydantoin. Most of the ureas were converted to hydantoin during annealing. Additionally, a low intensity absorption band can be evidenced close to 1,590 cm⁻¹ corresponding to TPhP.
<figure><img src="image_20.png" /><figcaption>Figure 14: FTIR spectra of THDI-TPhP-fr (Am1) network (blue) and THDI-TPhP-ag (Am1) network (red) (room temperature, ATR-IR) (1400-1900 cm⁻¹)</figcaption></figure>

DSC analyses, reported in SI (Figure S9) were carried out on THDI-TPhP-fr (Am1) network containing urea groups. First, an exotherm at 139°C, corresponding to hydantoin formation, was evidenced only during the first heating. Then, a Tg of 33°C during the first heating but only of 20°C during the 2nd heating were measured. As a reminder, for the plasticizer-free THDI-fr (Am1) network, the Tg (1st heating) was higher, and increased again after the 2nd heating i.e. from 56°C to 69°C.

It can be concluded that using TPhP decreases the network Tg as TPhP acts as a plasticizer in the network during the 1st heating. However, during the 2nd heating, the network is supposed to be based on hydantoin moieties. For the network containing TPhP, a decrease in Tg following the formation of hydantoin can be therefore evidenced, unlike what occurs in the absence of plasticizer.

### 3.3.3 Mechanical properties of the TPhP-plasticized network

This decrease in Tg may have consequences on the room temperature mechanical properties of the network. Tensile tests were thus performed on THDI-TPhP-fr (Am1) and THDI-TPhP-ag (Am1) networks, i.e. before and after aging at 50°C. First, by considering the THDI-fr (Am1) network behavior, adding TPhP leads to a decrease in the stress at break combined with an increase in the elongation at break (Table 2). This behavior is in agreement with the fact that TPhP acts as a plasticizer.

<table><caption>Table 2: Influence of TPhP on the mechanical properties of THDI-fr (Am1) network (crosshead speed: 50 mm.min⁻¹, room temperature)</caption><thead><tr><th></th><th>THDI-fr (Am1)</th><th>THDI-TPhP-fr (Am1)</th></tr></thead><tbody><tr><td>Young's Modulus (MPa)</td><td>1,670 ±168</td><td>958+68</td></tr><tr><td>Stress at break (MPa)</td><td>53±2</td><td>18±1</td></tr><tr><td>Elongation (%)</td><td>6±2</td><td>68±7</td></tr><tr><td>Glass transition temperature, Tg (°C)</td><td>56</td><td>33</td></tr></tbody></table>

Then, it was observed that hydantoin formation did not influence the mechanical properties of the THDI(Am1) network (see above § 3.2.4). However, according to DSC analyses (Figure S10) of the THDI-TPhP-ag (Am1) network, after annealing this polymer is no longer in the glassy state at room temperature but in the Tg region, since Tg is equal to 20°C. Therefore, it now behaves like an elastomer with a very low yield stress and a high strain at break (Table 3-Figure 15).
<figure><img src="image_21.png" /><figcaption>Figure 15: Tensile tests performed on THDI-TPhP-fr (Am1) (blue) and THDI-TPhP-ag (Am1) (red) networks (crosshead speed: 50 mm.min⁻¹, room temperature)</figcaption></figure>

<table><caption>Table 3: Influence of TPhP on the mechanical properties of THDI (Am1) networks with or without plasticizer and aged at 50°C (crosshead speed: 50 mm.min⁻¹, room temperature)</caption><thead><tr><th></th><th>THDI-ag (Am1) (50°C-7 days)</th><th>THDI-TPhP-ag (Am1) (50°C-7 days)</th></tr></thead><tbody><tr><td>Stress at break (MPa)</td><td>57±3</td><td>18±2</td></tr><tr><td>Elongation (%)</td><td>6±1</td><td>138±5</td></tr></tbody></table>

## 3.4 Analysis of an aspartate-based urea network plasticized with an unknown plasticizer: THDI (Am2)

### 3.4.1 Synthesis of THDI (Am2)

As mentioned previously, the Am2 compound is not pure and contains about 26 wt% of a non-reactive additive. In a typical synthesis, 40 g of Am2 diamine (106 mmol NH) was mixed with 19.47 g of THDI (106 mmol NCO) at room temperature. Remarkably in this case, for a 1 : 1 NH : NCO stoichiometry, no trace of residual isocyanate was observed in the resulting fresh network. Indeed, in the presence of a plasticizer, as well as for our model systems, and as also observed in the work of Natour et al.²⁰ for their most mobile networks, complete conversion of NCO can be achieved. However, in systems with more reduced mobility (case of Am1 and of Natour's work for a similar aspartate ester), residual isocyanate was detected for a 1:1 NH : NCO ratio.

The denomination THDI-fr (Am2) refers to a 'fresh' network, i.e. just after synthesis, which contains mainly urea groups. The denomination THDI-ag (Am2) refers to an 'aged' network containing mainly hydantoin groups. Moreover, an exudation phenomenon occurring for those THDI-ag (Am2) networks was observed. Some large greasy and easily soluble stains appeared.

### 3.4.2 Thermal and spectrophotometric analyses

A DSC analysis reported in SI (Figure S11) shows that the glass transition temperature measured during the first heating was 51°C. It is also interesting to notice that a large enthalpy relaxation was associated with this glass transition.

Before the second temperature ramp and during the temperature rise up to 200°C, the urea groups should have been converted to hydantoin groups and the formed ethanol has been removed from the network (no endotherm detectable in the second DSC scan, see Figure S11). However in this second ramp, a Tg at 29°C was measured. Therefore and similarly to what was reported for the THDI-TPhP (Am1) network, the Tg of the THDI (Am2) network decreases following hydantoin formation.

Concerning the THDI-fr (Am2) network, a degradation is evidenced by TGA at 151°C (Figure S12). This degradation phenomenon can be associated with the enthalpy change revealed by DSC at 160°C.
About 8% of the initial mass is lost during this degradation process. For this THDI-fr (Am2) network, an ethanol departure can also be evidenced by IR spectroscopy during heating at 165°C (Figure S13). Such an ethanol production confirms the hydantoin formation occurring during heating. A second degradation phenomenon begins at 210°C (maximum of 255°C). This degradation phenomenon can be correlated with the presence of additives in the Am2 compound.

In the FTIR spectrum of the THDI-fr (Am2) network, two absorption bands at 1,623 and 1,637 cm⁻¹ confirm the presence of C=O groups (urea) in interaction via hydrogen bonds (ordered and disordered). The absorption band at 1,530 cm⁻¹ corresponds to NH-C=O (amide II) groups. The absorption band at 1,731 cm⁻¹ corresponds to the C=O from esters and at 1,686 cm⁻¹ is related to the C=O of THDI (Figure 16).

The ‘fresh’ network based on Am2 was heated at 50°C for 7 days to accelerate the hydantoin formation. Similar FTIR analyses were then carried out for the aged THDI-ag (Am2) network. First, two new absorption bands appeared at 1,766 and 1,702 cm⁻¹ corresponding to the carbonyls of the hydantoin groups. Finally, the absorption bands at 1,623 and 1,530 cm⁻¹ considerably decreased, showing that the ureas were converted into hydantoins to a large extent. The IR spectroscopy analysis shows, therefore, that THDI-ag (Am2) network corresponds to a polyhydantoin polymer.

<figure><img src="image_22.png" /><figcaption>Figure 16: FTIR spectra of THDI-fr (Am2) (blue) and THDI-ag (Am2) (red) networks after being heated at 50°C for 7 days (room temperature, ATR-IR, 1,400 -1,900 cm⁻¹)</figcaption></figure>

### 3.4.3 Mechanical properties

As Am2 contains a large amount of additives (about 26 wt%), the resulting networks synthesized with this diamine should contain ~17 wt% of these additives. Although non-reactive, these additives are nevertheless supposed to play a role in the resulting network structure and in the physical properties of this network. It was shown that the additives may act as plasticizers and consequently affect the mechanical properties of the networks. Indeed, the previous tensile tests highlighted that the presence of additives led to an increase in the ability to sustain large deformation before fracture, and to a decrease in yield stress and stress at break. Namely, THDI-fr (Am1) network displays a higher Young's modulus (1,670 MPa) and stress at break (53 MPa) than THDI-fr (Am2) network (1,347 MPa and 33 MPa, respectively) (Figure 17). However, it should be noted that despite the presence of 26% additives, this plasticization effect remains limited. This could be attributed to the formation of hydrogen bonds between the network and the additives.
<figure><img src="image_23.png" /><figcaption>Figure 17: Tensile tests carried out on THDI-fr (Am1) (Tg = 56°C, red) and THDI-fr (Am2) (Tg = 51°C, blue) networks (crosshead speed: 50 mm.min⁻¹, room temperature)</figcaption></figure>

Once hydantoin groups are formed and the ethanol has been removed after heating the networks at 50°C for seven days, the Tg decreases for the THDI (Am2) network but increases for the THDI (Am1) network. At the same time, noticeable changes in mechanical properties can be noticed for the THDI networks synthesized from the Am2 diamine, i.e. increase of strain at break, decrease of stress at break, and decrease of the Young modulus (Figure 18). Meanwhile, only minor mechanical changes can be evidenced for the aged networks synthesized from the Am1 diamine which does not contain any additives (Table 1). Therefore, the properties of THDI networks differ more or less with aging depending on the nature and purity of the used amine.

<figure><img src="image_24.png" /><figcaption>Figure 18: Tensile tests carried out on THDI-ag(Am1) (red) and THDI-ag(Am2) (blue) (50°C – 7 days) networks (crosshead speed: 50 mm.min⁻¹, room temperature)</figcaption></figure>

# 4 Discussion

The presence of additives in the polyaspartic ester resin can be invoked to explain the softening of polymers over aging time. However, the question remains as to why the influence of these compounds acting as plasticizers is more significant once hydantoin is formed. According to the literature, two effects can be distinguished depending on the content of these plasticizers :

i) The anti-plasticization effect takes place at low concentrations. The plasticizers interact with the network chains directly and can hinder some molecular motions. This effect leads to an increase in the strain at break of the network ³⁷.

ii) The plasticizing effect takes place at higher concentrations. Some types of plasticizer molecules still interact with the network, but the rest of them phase separate, inducing cluster formation and increasing molecular mobility of the network. Therefore, the higher the amount of plasticizer, the higher the plasticizing effect³⁸,³⁹. However, when too much plasticizer is added, it can migrate to the surface.
Wang<sup>40</sup> studied the plasticization of a starch network by urea and water molecules. In small quantities, urea interacts with the network through hydrogen bonds, and an anti-plasticization effect is observed. However, the more urea is added, the more urea molecules form clusters, finally inducing a plasticization phenomenon. Beyond 30% of added urea, the mechanical properties do not vary anymore and the excess plasticizer migrates to the surface.

A comparison can be made with the THDI networks. For THDI-fr (Am2) network, the plasticization effect is already present on the ‘fresh’ networks. However, this effect is even more pronounced when the network contains hydantoin groups.

In a network without plasticizers, the loss of hydrogen bonds would be expected to increase local mobility, leading to a decrease in the glass transition temperature (Tg). However, the opposite phenomenon was observed, suggesting that a network restructuring via carbonylureido links is possible, resulting in an increased crosslink density, as proposed by Natour et al.<sup>20</sup> Further experiments such as swelling tests may be considered to confirm this hypothesis.

Conversely, in networks containing plasticizers, this loss of hydrogen bonds may have a significantly different impact. Once hydantoin is formed, the plasticizers present in the THDI-fr (Am2) network cannot interact anymore with the polar urea groups of the network since the latter have been replaced by hydantoins that do not bear any suitable hydrogen atom. By assuming that the hydrogen acceptors born by the plasticizer are e.g. C=O or P=O bonds, the latter can no longer form hydrogen bonds with the hydantoin species in the network.

Therefore, these free plasticizer molecules could gather and form larger clusters which would emphasize the plasticizing effect. The phenomenon also leads to the migration of the plasticizer at the surface, which would explain the exudation phenomenon mentioned earlier. Scheme illustrates the nature of the involved interactions.

However, it remains important to notice that without knowing the exact structure of the plasticizer(s), it is complicated to know precisely how they could interact with the network. Moreover, there are initially only a few donor sites within the network (only a single -NH per urea group). It would therefore be interesting to adjust the content of plasticizer in the networks to study its effects on their mechanical properties. Their influence may also be different whether the network chemical structure is constituted of urea or hydantoin groups.

<figure><img src="image_25.png" /><figcaption>Scheme 4: Influence of hydantoin formation on the plasticizer interactions within the network</figcaption></figure>

## 5 Conclusion

This work contributes to the understanding of the structure of polyurea coatings based on aspartate esters and the relationships with changes in mechanical properties observed over time.
For example, the networks synthesized from Am2 amine tended to show decreasing stiffness following hydantoin formation. The properties of these networks have been studied over aging time, and a decrease in Tg and Young's modulus was observed, as well as an increase in strain at break. It was put forward that Am2 amine contained 26 wt% of additives. However, the exact structure of these components remained difficult to confirm. Another reactive polyaspartic ester amine, Am1, which does not contain any additive, was thus considered. The network synthesized from this latter amine appeared to display a different behavior. Following the formation of hydantoin moieties in the network, an increase in the Tg was evidenced, while the mechanical properties did not present clear changes.

From these results, it has been shown that the changes in mechanical properties largely depended on the additive's presence. The interactions between these compounds, which act as plasticizers, and the network chains containing urea groups differ from those with the network containing hydantoin groups. In the latter case, the plasticizers form larger clusters increasing the plasticizing effect. It was also seen that impurities accelerate the formation of hydantoin groups within the network. Finally, it was seen that the presence of the additive has an interesting role during crosslinking. However, once the hydantoin groups are formed, migration of plasticizers at the surface can occur. Without knowing the nature of these plasticizers, it is challenging to draw conclusions about their interactions with the network. However, additional experiments could be conducted to further investigate the relationships between the network structure and the mechanical properties from creep tests or the studies of chain segment orientation using XRD before and after elongation. It may also be relevant to propose new plasticizers that preserve the interesting properties of the network but which do not have any influence after hydantoin groups are formed. It is therefore possible to consider the synthesis of polyisocyanate containing a plasticizer and use it with the amine Am1 to obtain new polyurea networks with tailored properties. It is also possible to propose the synthesis of systems with graftable plasticizers in order to avoid their migration to the surface of films.

## Corresponding authors

**Françoise Méchin** : Ingénierie des Matériaux Polymères (IMP) UMR CNRS 5223, INSA Lyon – Université de Lyon. Email : [francoise.mechin@insa-lyon.fr](mailto:francoise.mechin@insa-lyon.fr)

**Adrien Topalian** : Ingénierie des Matériaux Polymères (IMP) UMR CNRS 5223, INSA Lyon – Université de Lyon. Email : [adrien.topalian@yahoo.fr](mailto:adrien.topalian@yahoo.fr)

## Author contributions CRediT

Adrien Topalian : Validation, investigation, writing – original draft, writing – review and editing

Françoise Méchin : Validation, investigation, writing – original draft, writing – review and editing, supervision

Jannick Duchet-Rumeau : Conceptualization, Validation, investigation, writing – original draft, writing – review and editing, Supervision

Jean-François Gérard : Conceptualization, Validation, investigation, writing – original draft, writing – review and editing, Supervision, Project administration

Rolf Klucker : Conceptualization, Validation, investigation, Supervision
## Funding sources

This work was supported by the company Vencorex and by ANRT (Association Nationale de la Recherche et de la Technologie).

## Conflicts of interest

There are no conflicts of interest to declare.

## References

(1) Howarth, G. Polyurethanes, Polyurethane Dispersions and Polyureas: Past, Present and Future. *Surface Coatings International Part B: Coatings Transactions* **2003**, *86* (2), 111–118.
https://doi.org/10.1007/BF02699621.

(2) Chattopadhyay, D. K.; Raju, K. V. S. N. Structural Engineering of Polyurethane Coatings for High Performance Applications. *Progress in Polymer Science* **2007**, *32* (3), 352–418.
https://doi.org/10.1016/j.progpolymsci.2006.05.003.

(3) Wicks, D. A.; Yeske, P. E. Amine Chemistries for Isocyanate-Based Coatings. *Progress in Organic Coatings* **1997**, *30* (4), 265–270. https://doi.org/10.1016/S0300-9440(97)00002-7.

(4) Williams, C. T.; Wicks, D. A.; Jarrett, W. L. Hydrogen Bonding Effects on Aspartate Ester Reactions. *J Coat Technol Res* **2009**, *6* (1), 37–45. https://doi.org/10.1007/s11998-008-9139-z.

(5) Bosica, G.; Debono, A. J. Uncatalyzed, Green Aza-Michael Addition of Amines to Dimethyl Maleate. *Tetrahedron* **2014**, *70* (37), 6607–6612. https://doi.org/10.1016/j.tet.2014.06.124.

(6) Nozaki, K. Cis-Trans Isomerizations. II. The Mechanism of the Amine Catalyzed Isomerization of Diethyl Maleate. *J. Am. Chem. Soc.* **1941**, *63* (10), 2681–2683.
https://doi.org/10.1021/ja01855a049.

(7) Rulev, A. Y. Aza-Michael Reaction: Achievements and Prospects. *Russ. Chem. Rev.* **2011**, *80* (3), 197–218. https://doi.org/10.1070/RC2011v080n03ABEH004162.

(8) Genest, A.; Portinha, D.; Fleury, E.; Ganachaud, F. The Aza-Michael Reaction as an Alternative Strategy to Generate Advanced Silicon-Based (Macro)Molecules and Materials. *Progress in Polymer Science* **2017**, *72*, 61–110. https://doi.org/10.1016/j.progpolymsci.2017.02.002.

(9) Peyrton, J.; Avérous, L. Aza-Michael Reaction as a Greener, Safer, and More Sustainable Approach to Biobased Polyurethane Thermosets. *ACS Sustainable Chem. Eng.* **2021**, *9* (13), 4872–4884. https://doi.org/10.1021/acssuschemeng.1c00312.

(10) Bláha, M.; Trhlíková, O.; Podešva, J.; Abbrent, S.; Steinhart, M.; Dybal, J.; Dušková-Smrčková, M. Solvent-Free, Catalyst-Free Aza-Michael Addition of Cyclohexylamine to Diethyl Maleate: Reaction Mechanism and Kinetics. *Tetrahedron* **2018**, *74* (1), 58–67.
https://doi.org/10.1016/j.tet.2017.11.033.

(11) Greszta-Franz, D.; Drumm, C.; Morbach, J.; Witkowski, R. Polyaspartic Acid Ester Compositions and Method for Purification. US20200223787A1, July 16, 2020.
https://patents.google.com/patent/US20200223787A1/en?oq=US2020223787A1 (accessed 2023-02-21).

(12) Lal, G. S.; Fazel, S.; Boyce, S. M. Polyurea Compositions from Polyaspartic Esters and Secondary Heterocyclic Amines Derived Aspartic Esters. WO2019211127A1, November 7, 2019.
https://patents.google.com/patent/WO2019211127A1/en (accessed 2023-02-21).

(13) Podešva, J.; Hrubý, M.; Trhlíková, O.; Smrčková, M. D.; Chmelíková, D. Čisté aspartáty pro vysoce odolné nátěrové hmoty. *Chemické listy* **2018**, *112* (2), 117–121.

(14) *Thermosetting Polymers*; Pascault, J.-P., Ed.; Plastics engineering; Marcel Dekker: New York, 2002.

(15) Coleman, M. M.; Sobkowiak, M.; Pehlert, G. J.; Painter, P. C.; Iqbal, T. Infrared Temperature Studies of a Simple Polyurea. *Macromolecular Chemistry and Physics* **1997**, *198* (1), 117–136.
https://doi.org/10.1002/macp.1997.021980110.
(16) Cuvé, L.; Pascault, J. P.; Boiteux, G.; Seytre, G. Synthesis and Properties of Polyurethanes Based on Polyolefine: 1. Rigid Polyurethanes and Amorphous Segmented Polyurethanes Prepared in Polar Solvents under Homogeneous Conditions. *Polymer* **1991**, *32* (2), 343–352.
https://doi.org/10.1016/0032-3861(91)90024-D.

(17) Chiriac, C. I.; Tanasaă, F. Polyureas. In *Ullmann's Encyclopedia of Industrial Chemistry*; Wiley-VCH Verlag GmbH & Co. KGaA, Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2016; pp 1–5. https://doi.org/10.1002/14356007.d21_d01.pub2.

(18) Farrissey, W. J.; Alberino, L. M.; Sayigh, A. A. R. Polymers from Unconventional Reactions of Isocyanates. *Journal of Elastomers & Plastics* **1975**, *7* (3), 285–313.
https://doi.org/10.1177/009524437500700305.

(19) Podešva, J.; Smrčková, M. D.; Trhlíková, O. Role derivátů hydantoinu při syntéze polyaspartátů. *Chemické listy* **2022**, *116* (4), 215–222. https://doi.org/10.54779/chl20220215.

(20) Natour, S.; Gajdošová, V.; Morávková, Z.; Šlouf, M.; Hodan, J.; Sharma, A.; Šomvársky, J.; Dušková-Smrčková, M. Aspartate-Based Polyurea Coatings: Ambient Cure Process and Inevitable Transformation of Urea Groups into Hydantoin Cycles in Polyurea Networks and Their Impact on Film Properties. *Progress in Organic Coatings* **2024**, *192*, 108449.
https://doi.org/10.1016/j.porgcoat.2024.108449.

(21) Ware, Elinor. The Chemistry of the Hydantoins. *Chem. Rev.* **1950**, *46* (3), 403–470.
https://doi.org/10.1021/cr60145a001.

(22) Bogolubsky, A. V.; Moroz, Y. S.; Savych, O.; Pipko, S.; Konovets, A.; Platonov, M. O.; Vasylchenko, O. V.; Hurmach, V. V.; Grygorenko, O. O. An Old Story in the Parallel Synthesis World: An Approach to Hydantoin Libraries. *ACS Comb. Sci.* **2018**, *20*(1), 35–43.
https://doi.org/10.1021/acscombsci.7b00163.

(23) Kumar, V. Designed Synthesis of Diversely Substituted Hydantoins and Hydantoin-Based Hybrid Molecules: A Personal Account. *Synlett* **2021**, *32* (19), 1897–1910. https://doi.org/10.1055/a-1480-6474.

(24) Konnert, L.; Lamaty, F.; Martinez, J.; Colacino, E. Recent Advances in the Synthesis of Hydantoins: The State of the Art of a Valuable Scaffold. *Chem. Rev.* **2017**, *117* (23), 13757–13809. https://doi.org/10.1021/acs.chemrev.7b00067.

(25) Meusel, M.; Gütschow, M. Recent Developments in Hydantoin Chemistry. a Review. *Organic Preparations and Procedures International* **2004**, *36* (5), 391–443.
https://doi.org/10.1080/00304940409356627.

(26) Kalník, M.; Gabko, P.; Bella, M.; Koóš, M. The Bucherer–Bergs Multicomponent Synthesis of Hydantoins—Excellence in Simplicity. *Molecules* **2021**, *26* (13), 4024.
https://doi.org/10.3390/molecules26134024.

(27) Stella, V.; Higuchi, T. Kinetics of the Acid-Catalyzed Closure of Hydantoic Acids. Effect of 2-Aryl and 2-Alkyl Substituents. *J. Org. Chem.* **1973**, *38* (8), 1527–1534.
https://doi.org/10.1021/jo00948a017.

(28) Olimpieri, F.; Bellucci, M. C.; Volonterio, A.; Zanda, M. A Mild, Efficient Approach for the Synthesis of 1,5-Disubstituted Hydantoins. *European Journal of Organic Chemistry* **2009**, *2009* (35), 6179–6188. https://doi.org/10.1002/ejoc.200900868.

(29) Kaválek, J.; Macháček, V.; Svobodová, G.; Štěrba, V. Base Catalyzed Cyclization of Substituted Esters of Hydantoic and Thiohydantoic Acids. *Collect. Czech. Chem. Commun.* **1986**, *51* (2), 375–390. https://doi.org/10.1135/cccc19860375.

(30) Kaválek, J.; Macháček, V.; Svobodová, G.; Štěrba, V. Kinetics of Acid-Catalyzed Cyclization of Substituted Hydantoinamides to Substituted Hydantoins. *Collect. Czech. Chem. Commun.* **1987**, *52* (8), 1999–2004. https://doi.org/10.1135/cccc19871999.

(31) Tejero, R.; Lozano, Á. E.; Álvarez, C.; de Abajo, J. Synthesis, Characterization, and Evaluation of Novel Polyhydantoins as Gas Separation Membranes. *J. Polym. Sci. Part A: Polym. Chem.* **2013**, *51* (19), 4052–4060. https://doi.org/10.1002/pola.26806.

(32) Volonterio, A.; Ramirez de Arellano, C.; Zanda, M. Synthesis of 1,3,5-Trisubstituted Hydantoins by Regiospecific Domino Condensation/Aza-Michael/O→N Acyl Migration of Carbodiimides
with Activated α,β-Unsaturated Carboxylic Acids †. *J. Org. Chem.* **2005**, *70* (6), 2161–2170.
https://doi.org/10.1021/jo0480848.

(33) Imai, Y. Aromatic Polyhydantoins from Bisiminoacetate and Diisocyanates. *J. Polym. Sci. A-1 Polym. Chem.* **1967**, *5* (9), 2289–2296. https://doi.org/10.1002/pol.1967.150050908.

(34) Imai, Y. A New Class of Polymer Derived from N.N.N'-N'-Ethylenediaminetetraacetic Acid Dianhydride and Diamines. *Makromol. Chem.* **1970**, *138* (1), 293–297.
https://doi.org/10.1002/macp.1970.021380129.

(35) Immergut, E. H.; Mark, H. F. Principles of Plasticization. In *Plasticization and Plasticizer Processes*; Advances in Chemistry; American chemical society, 1965; Vol. 48, pp 1–26.
https://doi.org/10.1021/ba-1965-0048.ch001.

(36) Godwin, A. Plasticizers. In *Applied Plastics Engineering Handbook*; 2000; pp 157–175.
https://doi.org/10.1016/B978-008043417-9/50011-8.

(37) Bergquist, P.; Zhu, Y.; Jones, A. A.; Inglefield, P. T. Plasticization and Antiplasticization in Polycarbonates: The Role of Diluent Motion. *Macromolecules* **1999**, *32* (23), 7925–7931.
https://doi.org/10.1021/ma980205g.

(38) Mascia, L.; Kouparitsas, Y.; Nocita, D.; Bao, X. Antiplasticization of Polymer Materials: Structural Aspects and Effects on Mechanical and Diffusion-Controlled Properties. *Polymers* **2020**, *12* (4), 769. https://doi.org/10.3390/polym12040769.

(39) Liu, Y.; Roy, A. K.; Jones, A. A.; Inglefield, P. T.; Ogden, P. An NMR Study of Plasticization and Antiplasticization of a Polymeric Glass. *Macromolecules* **1990**, *23* (4), 968–977.
https://doi.org/10.1021/ma00206a013.

(40) Wang, J.; Cheng, F.; Zhu, P. Structure and Properties of Urea-Plasticized Starch Films with Different Urea Contents. *Carbohydrate Polymers* **2014**, *101*, 1109–1115.
https://doi.org/10.1016/j.carbpol.2013.10.050.