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2022 Volume 5 Issue 5

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INEOS OPEN, 2022, 5 (5), 133–137 

Journal of Nesmeyanov Institute of Organoelement Compounds
of the Russian Academy of Sciences

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DOI: 10.32931/io2222a

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Condensation of α,ω-Oligodimethylsiloxanols in Ammonia: A New Method for the Synthesis of Polydimethylsiloxanes with the Low Content of Cyclosiloxanes

E. O. Minyaylo,a,b T. O. Ershova,a,b M. N. Temnikov,*a,b and A. A. Anisimov*a,b,c

a Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, str. 1, Moscow, 119334 Russia
b Tula State Lev Tolstoy Pedagogical University, pr. Lenina 125, korp. 4, Tula, 300026 Russia
c Moscow Institute of Physics and Technology (National Research University), Institutskiy per. 9, Dolgoprudny, Moscow Oblast, 141700 Russia

 

Corresponding authors:  M. N. Temnikov, e-mail: temnikov88@gmail.com; A. A. Anisimov, e-mail: anisimov.ineos@gmail.com
Received 16 May 2023; accepted 2 June 2023

Abstract

GA

A new efficient approach to the production of high molecular weight polydimethylsiloxanes (PDMSs) is developed based on the condensation of α,ω-oligodimethylsiloxanols in ammonia. This method provides control over the molar mass of the target PDMSs in a wide range (Mw = 11.0–82.0 kg/mol). The use of cyclic silazanes as dehydrating agents allows for suppressing the side cyclization processes.

Key words: polydimethylsiloxanes, polycondensation, ammonia, green chemistry.

 

Introduction

Modern materials science imposes increasingly stringent requirements for the quality of both new and already known polymers. Nowadays, one of the most important parameters is the absence of low molecular impurities in a polymer, the presence of which can change the properties of the target material for the worse.

One of the most popular organoelement polymers is polydimethylsiloxane (PDMS) which is widely used both at the household level and in high-tech industries. This is caused by a complex of valuable physicochemical properties: hydrophobicity, bioinertness, high thermal and thermooxidative stability, etc. [1, 2]. There is a large variety of approaches to the synthesis of PDMS based on the polymerization and polycondensation processes [3, 4]. However, one of the main drawbacks of the known methods for its synthesis is the presence of low molecular weight compounds in the target products—dimethylcyclosiloxanes, which deteriorate the physicochemical characteristics of materials. In this respect, a promising method for the synthesis of PDMS is the ring-opening polymerization of hexamethylcyclotrisiloxane in the presence of basic organocatalysts in a mixture of organic solvents [5–12]. This approach enables the production of PDMSs in a wide range of molar masses without the formation of cyclic by-products at almost complete monomer conversion. However, the use of organic solvents and catalysts leads to certain limitations of the mentioned method from an industrial point of view. The condensation processes are used less frequently than the polymerization ones. This is due to the actively occurring side cyclization processes during the growth of a macromolecule [13]. The search for new highly efficient approaches to the synthesis of PDMS with a low content of cyclic products that would comply with the principles of green chemistry is an urgent task.

Earlier our research group developed the methods for obtaining silicones based on the polymerization and polycondensation reactions in ammonia. In the first case, the ring-opening polymerization of hexamethylcyclotrisiloxane in the presence of water afforded narrowly dispersed telechelic PDMSs with terminal hydroxy groups in the ranges of molar masses from 3 to 10 kg/mol [14]. In the case of the polycondensation of cis-tetraphenylcyclotetrasiloxanetetraol, we have shown that it is possible to obtain ladder poly(phenylsilsesquioxanes) (L-PPSQs) with controlled molar masses (from 15 to 1000 kg/mol) by varying the synthesis temperature. A peculiarity of high molecular weight L-PPSQs obtained by this method is their lower brittleness, which along with good mechanical and thermal characteristics makes them promising objects for use in materials science [15]. In both cases, ammonia acts simultaneously as a catalyst and a solvent. The advantage of both processes is the isolation procedure of the target polymers: the removal of ammonia from the reaction mixture by decompression affords the final polymer product. Furthermore, the developed methods enable ammonia recycling and its use in subsequent reactions, which makes them consistent with the principles of green chemistry.

The goal of this work was to show the possibility of producing PDMS with a low content of cyclosiloxanes by the condensation of α,ω-oligodimethylsiloxanols with controlled molar mass characteristics (MMCs) in ammonia.

Results and discussion

The condensation in ammonia was carried using commercially available α,ω-oligodimethylsiloxane (PDMS-(OH)2) according to Scheme 1.

sch1

Scheme 1. Condensation in ammonia.

For this purpose, PDMS-(OH)2 was placed in a high-pressure steel reactor (Fig. 1). Then the required amount of ammonia was loaded into the reactor through a gas flow regulator at –40 °C. The reactor was heated to the required temperature. The reaction mixture was agitated with a magnetic stirrer for a specified time.

fig1

Figure 1. General scheme for the reactions in ammonia.

Table 1 lists the data for three experiments (samples 1–3). As can be seen, an increase in the reaction time leads to an increase in the content of cyclic dimethylsiloxanes in the reaction mixture (exp. 2). The same pattern is observed with an increase in the process temperature from 70 to 90 °C and an increase in the reaction time to 24 h (exp. 3).

Table 1. Reaction conditions and molar mass characteristics of products 13a

Sample
T, °C
Time, h
Mp, kg/mol
Mn, kg/mol
Mw, kg/mol
PDI
HMWF/LMWF, %b
1
70
4
4.2
3.1
5.0
1.63
74:26
2
70
24
4.8
3.8
6.1
1.61
60:40
3
90
24
3.9
3.3
5.3
1.59
60:40
a PDMS-(OH)2 loading: 1 g (0.31 mmol); NH3 loading: 5 g;
b according to the SEC data.

Figure 2 shows the SEC curves for initial PDMS-(OH)2 and the experiments performed. From the presented diagrams, it is clear that the molar masses (MMs) of the resulting polymers slightly exceed that of initial PDMS-(OH)2. At the same time, a significant amount of cyclic low molecular weight products is formed in all experiments. This implies that, in addition to the condensation processes that lead to an increase in the molar mass of initial PDMS-(OH)2, it actively undergoes depolymerization. This can be caused by the formation of water as a result of the homocondensation of terminal Si-OH groups. The released water molecules interact with ammonia to form ammonium hydroxide which, under the reaction conditions, promotes the cleavage of the siloxane bond [14].

fig2

Figure 2. SEC curves of products 13 and initial PDMS-(ОН)2.

Therefore, it was decided to add a dehydrating agent in order to avoid the side processes of depolymerization. Octamethylcyclotetrasilazane (D4NH) was used as such an agent. The choice of this reagent was dictated by the fact that the interaction of D4NH with the terminal silanol groups leads to the cleavage of a silazane ring. The following hydrolysis results in the formation of siloxane and the release of ammonia, which ensures the absorption of released water in the system (Scheme 2). Furthermore, the choice of D4NH rather than the silazane rings of other sizes was due to the fact that this compound is crystalline and is formed in good yield (40–50%) upon ammonolysis of dimethyldichlorosilane (DMDCS) [16]. The suggested process utilized low loadings of D4NH (68 mg), so in the initial stage of the investigations, the addition of a solid compound seemed to be preferable for a more accurate adherence to the ratios of the cycle to the terminal Si-OH groups.

sch2

Scheme 2. Interaction of D4NH with the silanol group.

The experiments were carried out according to the scheme depicted in Fig. 1, except for the fact that, in addition to PDMS-(OH)2, the reactor was charged with the calculated amount of D4NH before ammonia injection. The first experiment with D4NH was carried out at the PDMS-(OH)2/silazane molar ratio of 3:2 in the presence of 5 g of ammonia (Table 2, exp. 4). The reaction time was 8 h. Figure 3 shows the SEC curves of initial PDMS-(OH)2, the reaction product, and octamethylcyclotetrasiloxane (D4Me2) used as an external standard.

Table 2. Reaction conditions and molar mass characteristics of products 47a

Sample
NH3, g
Time, h
Mp, kg/mol
Mn, kg/mol
Mw, kg/mol
PDI
Ratio of linear/cyclic products, %b
HMWF
LMWF
cycles
4
5
8
72.4
48.1
69.7
1.44
70
22
8
5
5
16
73.9
43.5
67.1
1.54
70
12
18
6
2.5
16
46.8
29.3
45.8
1.56
88
9
3
7
1
16
33.0
22.0
32.8
1.51
96.5
3
0.5
a PDMS-(OH)2 loading: 1 g (0.31 mmol); D4NH loading: 0.068 g (0.23 mmol); temperature: 100 °C;
b according to the SEC data.

fig3

Figure 3. SEC curves of product 4 and initial PDMS-(ОН)2.

As can be seen from Fig. 3 and Table 2 (sample 4), the use of D4NH does result in a significant increase in the product MM (Mp = 72.4 kg/mol). However, the cyclization process cannot be completely suppressed. In addition, the resulting products contain a significant amount of the low molecular weight polymer (LMWF). This is likely to be due to the fact that all D4NH is spent for the formation of the high molecular weight product. The remaining part of PDMS-(OH)2 reacts according to the process of homofunctional condensation with the release of water. This leads to the formation of cyclic products, as we have seen in the above-mentioned examples.

At the next step, the conditions for the condensation of PDMS-(OH)2 in ammonia were optimized. For this purpose, the effect of the parameters such as the reaction time, ammonia concentration, temperature, molar ratio of PDMS-(OH)2 to the dehydrating agent, and the structure of the silazane itself (D4NH and D3NH) was explored. Recently we have established that a decrease in the amount of ammonia in the system intensifies the course of the condensation processes [14]. Based on this, we carried out the experiments with different ammonia contents at the constant PDMS-(OH)2/D4NH molar ratio of 3:2 (Table 2).

Figure 4 demonstrates the SEC curves of products 5–7.

fig4

Figure 4. SEC curves of products 57 and initial PDMS-(ОН)2.

From Table 2 and Fig. 4 it is obvious that an increase in the reaction time to 16 h in the presence of 5 g of ammonia does not lead to significant changes in the molar mass characteristics (MMCs) of the reaction product (exp. 4 and 5). However, the content of LMWF increases, which confirms our assumption that released water leads to an equilibrium in the system between the HMW and LMW fractions. The picture changes with a decrease in the amount of ammonia in the system from 5 to 2.5 g (Table 2, Fig. 4, exp. 6). The product MM decreases, although the content of the cyclic components in the system is only about 3%. The content of the LMW fraction almost does not change at that. It should be noted that the MMCs of the LMW fraction in exp. 6 is closer to those of initial PDMS-(OH)2 than in case of exp. 4 and 5. This indicates that a decrease in the amount of ammonia in the system leads to a reduction in the depolymerization processes. This is likely to be due to a decrease in the system polarity.

A further reduction in the ammonia content to 1 g (Table 2, exp. 7) confirms this hypothesis. Thus, the cyclic products were almost completely absent in exp. 7. The content of LMW also reduced to 3%.

Hence, it can be concluded that the optimal conditions for the production of PDMS with a minimum amount of the cyclic products at the PDMS-(OH)2/D4NH molar ratio of 3:2 are the reaction temperature of 100 °C and the addition of 1 g of ammonia.

The next stage was the use of another dehydrating agent, namely, hexamethylcyclotrisilazane (D3NH). This reagent is more available than D4NH, which is caused by its higher yield upon the ammonolysis of DMDCS (up to 80%) as well as easier purification by distillation [17].

The reactions were carried out according to the analogous scheme (Fig. 1). The results of the experiments are summarized in Table 3.

Table 3. Reaction conditions and molar mass characteristics of products 812a

Sample
D3NH, mg (mmol)
PDMS-(OH)2/
D
3NH ratio
NH3, g
T, °C
Mp, kg/mol
Mn, kg/mol
Mw, kg/mol
PDI
Ratio of linear/cyclic products, %b
HMWF
LMWF
cycles
8
51 (0.23)
3:2
2.5
100
10.8
8.3
11.4
1.37
95
5
9
68 (0.31)
1:1
2.5
100
21.0
15.7
23.3
1.48
95
4
1
10
100 (0.46)
2:3
2.5
100
42.6
26.6
40.6
1.53
76
17
7
11
100 (0.46)
2:3
1
100
53.3
32.6
51.2
1.57
95
4
1
12
100 (0.46)
2 : 3
1
120
89.0
54.9
82.0
1.49
95.8
4
0.2
a PDMS-(OH)2 loading: 1 g (0.31 mmol); reaction time: 16 h;
b according to the SEC data.

The intensification of the condensation processes with a decrease in the amount of ammonia in the system should lead to the release of a larger amount of water. Therefore, in a series of the experiments with D3NH, we also varied the PDMS-(OH)2/D3NH molar ratio. As can be seen from the results of these experiments, the variation of the amount of ammonia and the polymer/cycle ratio ensures the production of PDMS with controlled MM with almost complete suppression of the depolymerization processes which lead to the formation of cyclic products (Fig. 5).

fig5

Figure 5. SEC curves of products 812 and initial PDMS-(ОН)2.

Experimental section

General remarks

Anhydrous ammonia was purchased from Spectra Gases Inc. Silanol terminated polydimethylsiloxane (PDMS-(ОН)2) was purchased from Gelest. The molar mass characteristics of PDMS-(ОН)2 were as follows: Mp = 3.8 kg/mol, Mn = 1.7 kg/mol, Mw = 4.0 kg/mol, PDI = 2.35, viscosity at 25 °C = 45–85 cSt. Octomethylcyclotetrasilazane (D4NH) and hexamethylcyclotrisilazane (D3NH) were synthesized according to the published procedures [16, 17].

SEC analysis was performed on a Shimadzu chromatograph using a RID-20A refractive index detector, a PSS SDV analytical 103 Å column (300 × 8 mm), 104 Å column (300 × 8 mm), and toluene as an eluent.

Syntheses

The required amount of PDMS-(ОН)2 and D4NH or D3NH were loaded into an autoclave equipped with a magnetic stirrer. Then the autoclave was filled with the required amount of NH3 under chill-down using an IN-FLOW mass flow meter (Bronkhorst, Netherlands). The autoclave was thermostated at the required temperature. After the reaction completion, the decompression was performed at room temperature.

Conclusions

In this work, we have shown that the condensation of α,ω-oligodimethylsiloxanols in ammonia at temperatures of about 100 °C leads to dynamic equilibrium in the system between the processes of condensation and depolymerization. This occurs due to the release of water during the homocondensation of the terminal silanol groups. The depolymerization results in the formation of undesirable cyclic dimethylsiloxanes (up to 40%). The addition of a dehydrating agent, such as hexamethylcyclotrisilazane or octamethylcyclotetrasilazane, dramatically changes the situation. Thus, the optimized reaction conditions (initial polymer/dehydrating agent ratio of 3:2, 100 °C, 1 g of ammonia) allowed us to almost completely avoid the cyclization processes. Moreover, the variation of the ratio of the reagents and the content of ammonia in the system provides control over the molar mass characteristics of the target PDMSs, which makes this approach promising for industrial implementation.

Acknowledgements

This work was supported by the Grant Council of the President of the Russian Federation (project no. MK-3534.2022.1.3). The synthesis of octomethylcyclotetrasilazane (D4NH) and hexamethylcyclotrisilazane (D3NH) was performed with financial support from the Ministry of Science and Higher Education of the Russian Federation within the Grant for the development of youth laboratories as part of the Priority 2030 program of TSPU (agreement no. 073-03-2023-030/2), funds under supplementary agreement no. 073-03-2023-030/2 of February 14, 2023 to the agreement for federal subsidy for financial support of the state assignment for the provision of public services (Creation of the chlorine-free method for the synthesis of phenylalkoxysilanes and production of modern innovative materials on their basis) no. 073-00030-23-02 of February 13, 2023. The characterization of the compounds obtained was performed with financial support from the Ministry of Science and Higher Education of the Russian Federation using the equipment of the Center for Molecular Composition Studies of INEOS RAS (agreement no. 075-03-2023-642).

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