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

  • Alexey Slobodinyuk ORCID logo , Vladimir Strelnikov ORCID logo , Dmitriy Kiselkov ORCID logo und Daria Slobodinyuk ORCID logo EMAIL logo
Veröffentlicht/Copyright: 16. August 2021
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Abstract

A method for the synthesis of oligotetramethylene oxides with terminal amino groups is presented. Its use as a hardener for urethane-containing oligomers has been demonstrated. The diamines were synthesized by a two-stage method based on oligotetramethylene oxide diol. The compounds can be used for the production of non-toxic, biocompatible and biodegradable segmented urethane-containing elastomers. The oligotetramethylene oxide diol with an average molecular mass of 1008 was chosen as a typical precursor component. Its dibromide was formed using a quasi-phosphonium reagent in various solvents. The corresponding amine was obtained by high-pressure amination. The compounds have been identified by 1H and 13C NMR spectroscopy, IR spectroscopy, and elemental analysis.

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).
Figure 1:

Synthesis of oligotetramethylene oxide dibromide (PTMO-dibromide).

Table 1:

Optimization of conditions for carrying out the bromination of oligotetramethylene oxide diol.

Solvent Brominating agent (equiv.) Yield (%)
1 DCM CBr4/Ph3P (2.1/2.1) 0
2 DCM CBr4/Ph3P (3/3) 45
3 DCM CBr4/Ph3P (4/4) 56
4 MeCN CBr4/Ph3P (2.1/2.1) 0
5 MeCN CBr4/Ph3P (3/3) 18
6 MeCN CBr4/Ph3P (4/4) 23
7 DCM NBS/Ph3P (2.1/2.1) 10
8 DCM NBS/Ph3P (3/3) 52
9 DCM NBS/Ph 3 P (4/4) 75
10 MeCN NBS/Ph3P (2.1/2.1) 8
11 MeCN NBS/Ph3P (3/3) 40
12 MeCN NBS/Ph3P (4/4) 45
  1. The bold values signify synthesis parameters that give the highest chemical yield.

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).
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.

Table 2:

1H NMR (400 MHz) and 13C NMR (75 MHz) data for PTHF, PTMO-dibromide and PTMO-diamine.

Structural fragment δH (ppm) δC (ppm)
OCH2CH2CH2CH2O PTHF (CDCl3) 1.51–1.62(br m, 51H)
OCH2CH2CH2CH2OH 1.51–1.62 (br m, 4H)
OCH2CH2CH2CH2OH 3.57 (t, J = 7.2 Hz, 4H)
OCH2CH2CH2CH2O 3.31–3.39 (br m, 51H)
OH 2.31 (br s, 2H)

OCH2CH2CH2CH2O PTMO-dibromide (CDCl3) 1.53–1.58 (br m, 47H),

1.64 (t, J = 7.6 Hz, 4H)
26.5, 28.3, 29.7
OCH2CH2CH2CH2Br 1.88 (t, J = 7.2 Hz, 4H) 33.6
OCH2CH2CH2CH2Br 3.32–3.38 (br m, 4H) 69.6
OCH2CH2CH2CH2O 3.32–3.38 (br m, 51H) 70.5

OCH2CH2CH2CH2O PTMO-diamine (CDCl3) 1.49–1.59 (br m, 51H) 26.4
OCH2CH2CH2CH2NH2 1.49–1.59 (br m, 4H) 26.4
OCH2CH2CH2CH2NH2 2.66 (t, J = 7.2 Hz, 4H) 70.4
OCH2CH2CH2CH2O 3.30–3.40 (br m, 51H) 70.4
NH2 2.87 (br s, 4H)

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.
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.
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

ln ( q / T m ) = E a / R T m ln ( A R / E a )

where q is the heating rate, Tm is the exothermic peak temperature, Ea 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/Tm2) versus 1000/Tm 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 5:

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

Figure 6: 
Linear plot of –ln(q/Tm2) versus 1000/Tm based on Kissinger’s equation [15].
Figure 6:

Linear plot of –ln(q/Tm2) versus 1000/Tm 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 Mn = 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 Mn = 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)

4.1 Synthesis of PTMO-dibromide

40.92 g (0.156 mol) of triphenylphosphine dissolved in 150 mL of dichloromethane was placed in a three-necked flask equipped with a mechanical stirrer and thermometer. The resulting reaction mixture was cooled to 0 °C, after which 0.156 mol of brominating agent (CBr4 or NBS) was added at 0 °С. The resulting mixture was stirred for 0.5 h at 0 °С, after which 39.45 g (0.039 mol) of oligotetramethylene oxide diol was added and the reaction mixture gradually brought to room temperature and stirred for 12 h. Then the excess of dichloromethane was distilled off on a rotary evaporator. The dark, viscous residue was transferred to a crystallizer. The crude solid contains oligotetramethylene oxide dibromide, triphenylphosphine oxide and succinimide. In order to remove triphenylphosphine oxide, the reaction mass was dissolved in 250 mL of ethanol with heating to about 35–40 °С in a three-necked flask equipped with a stirrer, thermometer and reflux condenser. A solution of 42.53 g (0.31 mol) zinc chloride in 90 mL of ethanol was then prepared and was quickly poured into a stirred solution of a mixture of the product and triphenylphosphine oxide to give a clear, dark solution. After a few minutes, a white precipitate formed. The mixture was stirred for 12 h, after which the precipitate was filtered off and washed with ethanol. The excess alcohol in the filtrate was removed on a rotary evaporator (under vacuum). The residue was poured into water to remove excess zinc chloride and succinimide. Then the solution was extracted with dichloromethane. The product was a viscous white substance. Yield 33.3 g (75%). – 1H NMR (400 MHz, CDCl3, TMS): δH = 1.53–1.58 (br m, 47H, OCH2CH2CH2CH2O main chain), 1.64 (t, J = 7.6 Hz, 4H, OCH2CH2CH2CH2O main chain), 1.88 (t, J = 7.2 Hz, 4H, OCH2CH2CH2CH2Br), 3.32–3.38 (br m, 55H, OCH2CH2CH2CH2Br, OCH2CH2CH2CH2O main chain). – 13C NMR (75 MHz, CDCl3): δC = 26.5 (OCH2CH2CH2CH2O), 28.3 (OCH2CH2CH2CH2O), 29.7 (OCH2CH2CH2CH2O), 33.6 (OCH2CH2CH2CH2Br), 69.6 (OCH2CH2CH2CH2Br), 70.5 (OCH2CH2CH2CH2O). – FTIR: ν = 665 cm−1 (C–Br). – Elemental analysis (%): found C 58.41, H 9.87; calcd. C 58.20, H 9.70.

4.2 Synthesis of PTMO-diamine

PTMO-dibromide (24.9 g, 0.022 mol) was dissolved in 250 mL of ethanol and 4.16 mL of concentrated aqueous ammonia was added. The reaction mass was heated in an autoclave at 100 °C for 4 h. Then the autoclave was cooled and opened carefully. The excess solvent was distilled off on a rotary evaporator, after which ethanol was added to the residue and the solvent was distilled off again. The volatiles contained in the residue were evaporated. The result was a viscous yellow substance. Yield 7.75 g (35%). – 1H NMR (400 MHz, CDCl3, TMS): δH = 1.49–59 (br m, 55H, OCH2CH2CH2CH2O main chain, OCH2CH2CH2CH2NH2), 2.66 (t, 4H, OCH2CH2CH2CH2NH2, 3J = 7.2 Hz), 2.87 (br s, 4H, NH2), 3.30–3.40 (br m, 51H, OCH2CH2CH2CH2O main chain). – 13C NMR (75 MHz, CDCl3): δC = 26.4 (OCH2CH2CH2CH2O, OCH2CH2CH2CH2NH2), 70.4 (OCH2CH2CH2CH2NH2, OCH2CH2CH2CH2O). – FTIR: ν = 3353 cm−1 (–NH2). – Elemental analysis (%): found C 65.75, H 11.46, N 2.90; calcd. C 65.61, H 11.33, N 2.78.


Corresponding author: Daria Slobodinyuk, Institute of Technical Chemistry, Ural Branch of the Russian Academy of Sciences, Ac. Korolev Str., 3, 614130 Perm, Russia, E-mail:

Funding source: RFBR

Award Identifier / Grant number: 20-43-596010

  1. Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

  2. Research funding: The reported study was funded by RFBR and Perm Territory, project number № 20-43-596010.

  3. Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

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Supplementary Material

The online version of this article offers supplementary material (https://doi.org/10.1515/znb-2021-0085).


Received: 2021-06-15
Accepted: 2021-07-12
Published Online: 2021-08-16
Published in Print: 2021-10-26

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