Synthesis of oligotetramethylene oxides with terminal amino groups as curing agents for an epoxyurethane oligomer

1 Introduction

Polyester amines are an important class of oligomers which are widely used as curing agents for polymers for various purposes [1], [2], [3], [4]. For the synthesis of biodegradable and biocompatible polymers only polyether amines of aliphatic structure can be used due to the high toxicity of cyclic amines during decomposition [5]. The synthesis of new amines allows for further possibilities of obtaining elastomers with desired properties. To date, several methods for the synthesis of polyethylene glycol (PEG) diamines have been reported [6], [7], [8], [9], including also information about the preparation of polypropylene glycol (PPG) diamines [10].

A systematic analysis of the data presented allows us to distinguish three main approaches to obtain these polyether amines, namely, nucleophilic substitution reactions [6, 7, 11, 12], oxidation/reduction reactions [8], as well as catalytic amination [9, 10].

One of the strategies for the synthesis of polyether amines is a two-stage synthesis. It involves the initial substitution of the hydroxyl groups of the polyglycol by phthalimide substituents under the Mitsunobu reaction conditions, followed by the addition of hydrazine and obtaining the target polyglycol with terminal amino groups [6]. Reactions of nucleophilic substitution also include the interaction of polyglycol with thionyl halides, followed by treatment with ammonia at high temperatures and pressures [7]. The single-step catalytic amination of polyethylene glycol was studied under high pressure and temperature conditions [9, 10].

The synthesis of polyethylene glycol diamines which consists of the initial oxidation of PEG to the corresponding carbonyl compounds with their subsequent reductive amination was also described [8].

The most common strategy for the synthesis of polyether amines is a three-step strategy, which consists in the initial conversion of the hydroxyl groups of PEG into terminal azide groups via the corresponding sulfonate derivatives, followed by reduction to amino groups using triphenylphosphine (PPh₃) or zinc with ammonium chloride [11, 12].

Analysis of the literature indicates that there are currently no studies in the field of the synthesis of diamines based on oligotetramethylene oxide diol.

In this work, for the first time, a method for the synthesis of oligotetramethylene oxide with terminal amino groups is presented. The synthesized oligomer was used as a curing agent for an epoxyurethane oligomer.

2 Results and discussion

To obtain oligotetramethylene oxide with terminal amino groups (PTMO-diamine), a two-step synthesis is now proposed, which includes the initial nucleophilic substitution of the hydroxyl groups by bromine atoms.

Bromination of oligotetramethylene oxide diol was carried out through the formation of a quasi-phosphonium salt by the action of triphenylphosphine on the corresponding brominating agent.

The first stage of the synthesis of oligotetramethylene oxide diamines included the selection of the solvent, the brominating agent and its amount (Figure 1, Table 1).

Figure 1:

Synthesis of oligotetramethylene oxide dibromide (PTMO-dibromide).

It was found that the highest yield was achieved using dichloromethane (DCM) as a solvent and N-bromosuccinimide as a brominating agent. In this case the optimal ratio of oligotetramethylene oxide diol:brominating agent reagents was 1:4 (Table 1).

It should be noted that the low yields of the oligotetramethylene oxide dibromindes are primarily due to the formation of triphenylphosphine oxide which is rather difficult to separate from the reaction product. In most cases, multiple column chromatography has to be used for this process, which is time- and labor-intensive, especially for large volumes. In this regard, a convenient method proposed by the group of D. J. Weix was used for the separation of the oligotetramethylene oxide dibromides and triphenylphosphine oxide [14].

This method, which excludes carrying out column chromatography, consists of the precipitation of triphenylphosphine oxide with zinc chloride in polar solvents.

The further substitution of the halogen atoms by the amino group was carried out in a high-pressure reactor in ethanol medium by the action of concentrated aqueous ammonia on the oligotetramethylene oxide dibromides (Figure 2).

Figure 2:

Synthesis of oligotetramethylene oxide with terminal amino groups (PTMO-diamine).

The formation of the PTMO-diamine was confirmed by NMR and FTIR spectra.

Table 2 shows the NMR spectra of the intermediate product, as well as the starting oligotetramethylene oxide diol (PTHF) and diamine (PTMO). It was found that the nucleophilic substitution of bromine for the hydroxyl groups of the oligotetramethylene oxide diols lead to disappearance of the signal of the protons of the two hydroxyl groups (δ = 2.31 ppm). Also the signals of the protons of the methylene groups located near the halogen atoms were shifted. At the final stage of the preparation of PTMO-diamine, the ¹H NMR spectrum showed the appearance of a signal of the protons of two amino groups (δ = 2.87 ppm), as well as a shift of the signals of the protons of the methylene groups located next to the amino groups to higher field.

Analysis of FTIR spectra showed that after the bromination reaction (Figure 1) the band at 3300–3500 cm⁻¹ (hydroxyl groups) has completely disappeared while the presence of a band at 665 cm⁻¹ corresponds to the C–Br stretching band. After subsequent amination (Figure 2) the C–Br band has disappeared and the band for the amino groups appeared at 3300–3600 cm⁻¹. The other peaks of all intermediates and of the final PTMO-diamine were roughly the same as for oligotetramethylene oxide diol, which means that the basic oligomeric structure did not change, except for the transformation of the terminal groups.

The synthesized PTMO-diamine was used as a curing agent for an epoxyurethane oligomer (EUO) synthesized in a two-stage method. At the first stage, an oligodiisocyanate was obtained based on oligopropylene oxide diol with a molecular weight of 1000 and 2,4-toluene diisocyanate taken in a double molar excess. At the second stage, the resulting oligodiisocyanate was reacted with a twofold molar excess of 2,3-epoxypropanol. The synthesis has been already described in detail [13] (Figure 3).

Figure 3:

Synthesis of EUO.

The interaction of the newly synthesized PTMO-diamine with the above mentioned EUO is shown in Figure 4. The completeness of the reaction was monitored by FTIR spectroscopy by the disappearance of the absorption band of the epoxy groups.

Figure 4:

Synthesis of the polymer on the basis EUO-PTMO-diamine.

The kinetic parameters of this system were determined. The determination of the activation energy was carried out by the Kissinger method [15]. The activation energy could be obtained from the peak temperatures at different heating rates using the equation

where q is the heating rate, T_m is the exothermic peak temperature, E_a is the activation energy, R is the gas constant, and A is the pre-exponential factor.

Figure 5 shows DSC thermograms of the system curing process (Figure 4) at three different heating rates. A plot of dependence –ln(q/T_m²) versus 1000/T_m was built according to the Kissinger model and the data obtained are shown in (Figure 6) [16].

Figure 5:

DSC measurement of the EUO-PTMO-diamine curing system at different heating rates.

Figure 6:

Linear plot of –ln(q/T_m²) versus 1000/T_m based on Kissinger’s equation [15].

The activation energy of the system was calculated from the equation of the straight line, it was equal to 1.400 kJ mol⁻¹. Low activation energy is characteristic of products with aliphatic amines.

3 Conclusions

For the first time, a method has been developed for the synthesis of oligotetramethylene oxide with terminal amino groups, including the initial bromination of oligotetramethylene oxide diol using a quasi-phosphonium salt, followed by amination in a high-pressure reactor. The resulting oligoamine was shown to be effective as a curing agent for an epoxyurethane oligomer.

4 Experimental section

Oligotetramethylene oxide diol (mass average molecular weight M_n = 1008 g mol⁻¹, Sigma Aldrich), CBr₄, ammonia (Vitakhim), ZnCl₂ (Vitakhim). Epoxyurethane oligomer synthesized on the basis of oligopropylene oxide diol (mass average molecular weight M_n = 1000 g mol⁻¹ from Sigma-Aldrich), 2,4-toluene diisocyanate (mass fraction of isocyanate groups 48.28 wt% from Sigma-Aldrich), glycidol (mass fraction of epoxy groups 58.11 wt% from NIIPM) according to the described method [13].

¹H NMR and ¹³C NMR spectra were recorded on a Bruker Avance-Neo III HD spectrometer in CDCl₃ using tetramethylsilane (0.055 ppm) as an internal standard at room temperature. The proton signals are designated as follows: t (triplet), m (multiplet), br m (broadened multiplet). Elemental analysis was performed on a CHNS-932 analyzer LECO Corp. IR spectra of the starting and intermediate products, as well as of the synthesized amine, were recorded in the range 4000–400 cm⁻¹ on a Bruker IFS-66/S FTIR spectrometer at a resolution of 1 cm⁻¹. Amination of the PTMO-dibromide was carried out in a high-pressure Parr minireactor system 4561 with a Parr 4848 modular controller (Parr, USA) and with stirring (temperature range 20–350 °С, pressure range up to 200 atm)