RU2798656C1 - Method for obtaining oligomers of acrylonitrile and its co-oligomers in the presence of n-methylmorpholine-n-oxide - Google Patents

Abstract

FIED: chemical technology.

SUBSTANCE: invention relates to a method for producing low molecular weight composite homogeneous polymers of acrylonitrile and its copolymers by radical polymerization in the presence of N-methylmorpholine-N-oxide. A method for producing a compositionally homogeneous low molecular weight polymer based on acrylonitrile (AN) with a narrow molecular weight distribution by weight is described, characterized in that N-methylmorpholine-N-oxide is added to the polymerization system, which includes a monomer and, optionally, a radical polymerization initiator (MMO), optionally in the presence of an organic solvent; where the method is carried out in an atmosphere of inert gas when heated.

EFFECT: obtaining low molecular weight (co)polymers with a molecular weight of about 1 kDa (MM) and a narrow molecular weight distribution and a comonomer content of 15 to 20 mol%.

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Description

FIELD OF TECHNOLOGY

The present invention relates to chemical technology, in particular to a method for obtaining acrylonitrile (co)oligomers by radical polymerization in the presence of N-methylmorpholine-N-oxide. The proposed method makes it possible to obtain low molecular weight (co)polymers based on acrylonitrile with a molecular weight (MW) of the order of 1 kDa and a narrow molecular weight distribution.

BACKGROUND OF THE INVENTION

Acrylonitrile-based copolymers (PAN copolymers) are widely used as precursors for the production of carbon fibers, which are used primarily as a structural material in mechanical engineering and the sports industry. Suitable comonomers for acrylonitrile are, for example, methyl acrylate, acrylic, methacrylic and itaconic acids and their derivatives. The need to introduce comonomers is due to their plasticizing properties, as well as smoothing the exothermic thermal effect during fiber stabilization in the process of oxidation with atmospheric oxygen.

The main method for obtaining acrylonitrile copolymers is radical polymerization, which is carried out in an organic solvent (WO 2004060928 A1; KR 20190038322 A), in aqueous solutions of sodium thiocyanate or zinc chloride (GB 950878 A, EP 0824111 A1; JP 2004211047 A), emulsion (RU 2412966 C2; WO 2015040043 A1; US 7098280 B2; RU 2467021 C2) or by suspension methods (JPH 0310651B2), as well as by precipitation in water (RU 2017865 C1; RU 2088598 C1).

In the technology for obtaining acrylonitrile copolymers, aprotic solvents are most widely used: dimethylformamide (DMF), dimethylacetamide (DMAA), dimethyl sulfoxide (DMSO) and a hydrotropic solvent - 51.5% NaSCN aqueous solution. The most popular PAN solvent systems are DMF and DMSO. However, despite the fact that these solvents are more accessible, they are quite toxic. Moreover, the limiting concentrations of PAN spinning solutions in DMF and DMSO do not exceed 23–24% due to their high viscosity and phase instability. In turn, a 51.5% aqueous solution of NaSCN creates an alkaline environment during polymerization, which negatively affects both the quality of the product and the wear of process equipment.

In this regard, a highly polar donor solvent, N-methylmorpholine-N-oxide (MMO), is of great interest. This solvent is most effective for proton-donor polymers, in particular for cellulose, and allows you to transfer up to 45% of cellulose into solution (Head L.K. Chemical fibers. No. 1, 1996. p. 13-23). US Pat. for spinning, followed by extrusion into fiber.

The use of MMO in organic synthesis and for dissolving cellulose with subsequent production of fibers is due to the high polarity of the N ⁺ -O- bond, which is characterized by an increased electron density on the oxygen atom, which is the nucleophilic center, and the hydrophilicity of this compound. As a result, MMO is readily soluble in water and capable of forming hydrogen bonds. In addition, the NO bond is labile and is easily broken by a number of suitable reagents (Rosenau T., et al. // Prog. Polym. Sci. 2001, V. 26. P. 1763.).

MMO can exist in free and hydrated form. In the first case, MMO is thermally unstable, has a high melting point (182°C) and a high dissolving power. MMO forms the following types of hydrates - monohydrate with 13.3 wt.% water, 2.5 hydrate with 28 wt.% water and a series of sesquihydrates with lower melting points, which are often used in practice (Linton E.P. // J. Am. Chem. Soc. 1940 , V. 62. P. 1945.).

MMO is able to dissolve some polyesters and polyamides, forming concentrated solutions with them (Golova L., et al.// Polymer Science A. 2010. V. 52. No. 11. P. 1209 .; Golova L., et al. // Cellulose - Fundamental Aspects and Current Trends / Ed by Godbout L., de Ven T. InTechOpen, London, 2013. P. 303.), suitable for fiber spinning. However, acrylonitrile (AN) homopolymer has a low solubility in MMO (US 3447939 A). In contrast, AN copolymers with a low content of comonomers with carboxyl groups (1-2% mol.) easily form highly concentrated solutions with MMO (Kulichikhin V., et al.// Eur. Polym. J. 2017. V. 92. P 326.). The reason for this difference is the formation of a complex between the carboxyl group of the copolymer and the NO group of the solvent during the solid-phase activation of the mixture of polymer and MMO. Prolonged heating (1-3 h) of a concentrated solution of the copolymer AN and acrylic acid in MMO at 125°C leads to the transformation of the chemical structure of the copolymer associated with the formation of a system of conjugated bonds -C=N-C=N-. It is worth noting that in the copolymers of AN and acrylic acid, the processes of intramolecular cyclization in the absence of MMO become noticeable at significantly higher temperatures (Toms R.V., et al.// Polymer Science B. 2020. V. 62. No. 6. P. 660 .; Toms R.V., et al.// Polymer Science B. 2020. V. 62. No. 2. P. 102.). Therefore, MMO activates the low-temperature cyclization of nitrile groups.

The method for obtaining a solution of an acrylonitrile-based copolymer in MMO, disclosed in patent RU 2541473 C2, is based on solid-phase mixing of an AN copolymer containing acid groups with N-methylmorpholine-N-oxide hydrate. EFFECT: invention makes it possible to obtain a highly concentrated PAN solution when it is prepared in a highly environmentally friendly way.

The closest to the described invention in technical essence and basic objects is the radical (co)polymerization of acrylonitrile with vinyl comonomers in the presence of chain transfer agents based on mercaptans, which allow you to control the molecular weight of the copolymers and, in particular, obtain (co)oligomers (RU 2625314 C2, SU 378389 A1). However, mercaptans are toxic substances, and, according to the standards of the Department of State Sanitary and Epidemiological Supervision of the Ministry of Health of Russia of 2003, their MPC (maximum single value) for various mercaptans is less than 0.1 mg/m³. In addition, such polymerization is carried out in the presence of a solvent, which causes its subsequent purification and regeneration.

The main disadvantage of the method, according to the prototype, is the use of toxic substances and the need for cleaning and regeneration of the solvent.

SHORT DESCRIPTION

The method according to the invention makes it possible to obtain oligomers or co-oligomers based on acrylonitrile with a molecular weight of about 1 kDa and a narrow molecular weight distribution. The method is based on the use of radical polymerization, but differs from the known methods by the presence of additional regulation of the molecular weight of the resulting (co)oligomer by introducing N-methylmorpholine-N-oxide into the polymerization system.

The practical significance of low MW AN copolymers is that they can be used to produce PAN precursors by melt spinning, either by themselves or as a plasticizer additive to a higher molecular weight copolymer.

The proposed method makes it possible to obtain a product that does not require isolation of the polymer and its purification.

In the present invention, the polymerization is carried out at a comonomer content of 15 to 20 mole %. with the participation of N-methylmorpholine-N-oxide with the content of the latter from 10 to 50% wt. within 24 hours without the manifestation of the gel effect.

The objective of the invention is to develop a new environmentally friendly method for obtaining (co)oligomers of acrylonitrile of a given composition, molecular weight and chemical nature, applicable to any functional monomers that polymerize by a radical mechanism.

The technical result of the invention is to obtain low molecular weight (co)polymers based on acrylonitrile with a molecular weight of about 1 kDa, a narrow molecular weight distribution and a comonomer content of 15 to 20 mol%. The low molecular weight (co)polymers obtained in this way have a low transition temperature to the viscous flow state in the range from 70 to 80°C.

This result is achieved in the following way.

The reactor is purged with argon, loaded with monomers, optionally a radical initiator, N-methylmorpholine-N-oxide, optionally in the presence of an organic solvent, for example, DMSO, at the initial time or at high conversions, and filled with argon. An inert atmosphere is necessary to prevent inhibition of polymerization and to control the molecular weights of polymers. The reaction mixture is heated to 50-90 °C and polymerization is carried out for 24 hours without the appearance of a gel effect. Temperatures below 50 °C do not provide an acceptable polymerization rate; temperatures above 90 °C leads to a decrease in the limiting yield of the polymer. At temperatures of 80 - 90 °C, the initiation and polymerization of AN takes place without the participation of a material initiator with the achievement of a monomer-to-oligomer conversion of about 50% in 24 hours.

The distribution of comonomers in the copolymer is controlled by the rate at which monomers are fed into the reaction.

During the polymerization of AN in the presence of MMO, which is introduced into the reaction system at the initial moment of time or at high conversions, regularities appear: the molecular weight of the polymer in the presence of MMO is lower than in its absence, and the polymers themselves acquire color due to partial cyclization. Therefore, MMO is an active participant in the polymerization reaction. Upon irradiation or heating of MMO (120 °C), an EPR spectrum is recorded corresponding to the nitroxy radical formed upon ring rupture according to the following scheme:

The general scheme of the mechanism of AN polymerization with the participation of MMO can be represented as follows. During the polymerization of AN initiated by dinitrile azoisobutyric acid (DAA) in the presence of MMO, a chain of successive reactions (2) and (3) can occur, leading to the formation of a macronitroxide adduct. The higher the concentration of MMO and the temperature, the greater the concentration of the biradical and the faster the chain termination occurs according to the reactions

An increase in the monomer conversion with an increase in the proportion of MMO in the mixture suggests that the diradical is capable of initiating the polymerization of AN (reaction (4)), forming macronitroxide, which interacts with another growth radical according to reactions (2) and (3):

The initiation of the cyclization reaction is possible according to the reaction (5a), and the destruction - according to the reaction (5b):

The assumptions about the mechanism of AN polymerization with the participation of MMO do not contradict the experimental data.

The resulting low molecular weight polymers of acrylonitrile and its copolymers with vinyl and/or vinylidene monomers selected from the group consisting of acrylic acid; alkyl acrylates, for example methyl acrylate, ethyl acrylate, butyl acrylate, ethylhexyl acrylate; methacrylic acid, alkyl methacrylates, for example methyl methacrylate, ethyl methacrylate, butyl methacrylate, ethylhexyl methacrylate; styrene; vinyl acetate; acrylamide, 1-vinylimidazole, have a molecular weight of about 1 kDa, a narrow molecular weight distribution, the comonomer content is from 15 to 20 mol%.

Brief description of the drawings

Fig.1. Dependence of AN conversion on the mass fraction of MMO in the initial mixture after 24 h of AN polymerization initiated by 10 ^-2 mol/l DAA (1, 2) and in the absence of an initiator (3). Т=60 (1) and 80 °С (2, 3).

Fig.2. Dependence of the conversion (a) and the logarithm of the ratio of the current monomer concentration to the initial concentration (b) on the polymerization time of AN initiated by 10 ^-3 (1) and 10 ^-2 mol/l DAA (2) at 80 °C in the presence of 50 wt.% MMO.

Fig.3. MWD curves: a - PAN formed at 80°C after 24 h of AN polymerization in the presence of 10 ^-2 mol/l DAA (1-7) and in the absence of DAA (8) at a mass content of MMO 0 (1), 5 (2 ), 10 (3), 20 (4), 30 (5, 8), 40 (6), and 50% (7, 8); 6 - PAN formed at 60 °C after 24 h of AN polymerization in the presence of 10 ^-3 mol/l DAA at a mass content of MMO 2.5 (1), 5 (2), 7.5 (3), 10 (4) and 20% ( 5); c - copolymers of AN with acrylic acid (1), styrene (2), vinyl acetate (3), acrylamide (4), methyl acrylate (5) and 1-vinylimidazole (6), formed in the presence of 10 ^-2 mol/l DAA and 40 wt.% MMO.

Fig.4. Dependence of 1/M _n on the mass content of MMO for PAN obtained at different contents of MMO at 60 (1) and 80 °C (2).

Fig.5. ^1H NMR spectra in DMSO-d ₆ for PAN synthesized in the presence of 50 wt.% MMO (1) and in its absence (2).

Fig.6. ATR IR spectra: a - MMO (1) and PAN films synthesized at 80 ^° C in the absence of MMO (2) and in its presence (3-7) at a mass content of MMO 5 (3), 10 (4), 20 (5), 30 (6, 6) and 40% (7) in the presence of DAA (2-7) and in its absence (6); 6 - PAN films synthesized at 60°C in the presence of MMO at its mass content of 5 (1), 10 (2), and 20% (3).

Fig.7. DSC curves of AN polymerization products initiated by DAA at 80°C in the absence (1) and presence of MMO (2-5). The mass content of MMO in the initial mixture is 5 (2), 7.5 (3), 10 (4), and 40% (5).

Fig.8. Dependence of the monomer conversion on the polymerization time of AN (20 wt.%) in DMSO at 80°C under the action of 5 × 10 ^-3 mol/l DAA (a), as well as the MWD of the polymerization products of AN 90 min after the start of reaction (1) and after the addition of MMO (20 wt.% in relation to AN) after 60 (2) and 120 min (3) after the introduction of MMO (b).

Fig.9. ATR IR spectra of films of AN polymerization products 60 min after the start of reaction (1) and after the addition of MMO (20 wt % with respect to AN) 30 (2) and 90 min (3) after the introduction of MMO at 80°C.

Fig.10. DSC thermograms of (1) MMO, (2) PAN, and (3–7) PAN-MMO films recorded in the dynamic mode in an inert atmosphere at a scanning rate of 10 deg/min. Mass content of MMO 5 (3), 10 (4), 20 (5), 30 (6) and 40% (7).

Fig.11. MWD curves of pure PAN and PAN subjected to heat treatment with MMO in DMSO at 135°C for 4 h. Mass content of MMO 0 (1), 5 (2), 10 (3), 20 (4) and 40% (5) .

Fig.12. IR spectra of PAN films with MMO heated at 135°C for 4 h. Mass content of MMO 5 (1), 10 (2), 20 (3), and 40% (4).

Fig.13. IR spectra of PAN-MMO films with an MMO content of 20 wt.%, recorded after isothermal treatment for 0 (1), 5 (2), 15 (3), 70 (4) and 240 min (5) in an argon atmosphere at 175 °C.

Fig.14. Time dependence of the conversion of nitrile groups during isothermal treatment in an argon atmosphere at 175°C of PAN-MMO films. Mass content of MMO 5 (1), 10 (2), 20 (3), 30 (4) and 40% (5).

Implementation of the invention.

Below are examples of specific implementation of the invention. These examples are not intended to limit the scope of the present invention and are provided to illustrate the invention.

Example 1

Freshly distilled AN 20 g (376 mmol), azoisobutyric acid dinitrile (AAB) to a concentration of 10 ^-2 mol / l, MMO 4 g (20 wt%; 34 mmol) are loaded into a 50 ml round-bottom flask and purged with dry argon (99.99 %) within 10 minutes. Next, the reaction mass is heated to 60 °C and kept for 24 hours. After the time has elapsed, the reaction mass is poured into an excess of cold water, the resulting polymer precipitate is separated by filtration, washed on the filter with a double volume of distilled water, dried in air at room temperature, then in a drying oven. vacuum oven at 70°C.

The structure of the resulting polymer was determined by NMR and IR spectroscopy. According to the analysis carried out by gel permeation chromatography, the resulting oligomer had a molecular weight of about 1 kDa and a narrow molecular weight distribution.

The polymerization process was carried out according to this method in various mass ratios of AN/MMO. The yield of the obtained polymers, depending on the ratio of AN/MMO, ranged from 10% at a ratio of AN/MMO of 90/10% wt. up to 55% at AN/MMO 50/50% wt.

Example 2

Polymerization of AN (20% wt.) is carried out at 80 °C in a DMSO solution at a DAA concentration of 5⋅10 ^-3 mol/l. After 1.5 h after the start of polymerization, MMO is introduced into the reaction mixture in an amount of 20 wt%. based on the starting monomer and continue the synthesis. During polymerization, before and after the addition of MMO, samples are taken for product isolation and analysis. The polymer yield is 65%.

Example 3

A 50 ml round bottom flask was charged with freshly distilled AN 10 g (188 mmol), MMO 6 g (40 wt%; 3 mmol) and purged with dry argon (99.99%) for 10 minutes. MMO is used as the polymerization initiator. The reaction mass is heated to 90 °C and kept for 24 hours. After the time has elapsed, the reaction mass is poured into an excess of cold water, the resulting polymer precipitate is separated by filtration, washed on the filter with a double volume of distilled water, dried in air at room temperature, then in a drying vacuum cabinet at 70°C, obtaining a polymer with a yield of 60%. During polymerization, samples are taken to isolate and test the product.

Example 4

Preparation of AN copolymers with vinyl or vinylidene monomers selected from the group consisting of acrylic acid; alkyl acrylates, for example methyl acrylate, ethyl acrylate, butyl acrylate, ethylhexyl acrylate; methacrylic acid, alkyl methacrylates, for example methyl methacrylate, ethyl methacrylate, butyl methacrylate, ethylhexyl methacrylate; styrene, vinyl acetate, acrylamide, 1-vinylimidazole, was carried out according to similar methods presented in examples 1-3, except for the step of combining AN monomers with vinyl or vinylidene monomers. Vinyl or vinylidene monomers are loaded in amounts of 15 to 20 mole %.

The output of the obtained polymers, depending on the ratio of AN+comonomer/MMO, is from 15% at a ratio of AN+comonomer/MMO of 90/10 wt%. up to 62% at AN/MMO 50/50% wt.

Example 5

To study the effect of MMO on the thermal behavior of PAN, films are formed by dissolving PAN synthesized in the absence of MMO in DMSO (15% solution). The calculated amount of MMO is placed in the solution (from 5 to 40 wt. % for the PAN-MMO mixture), the finished solution is applied to the glass and dried in an oven under vacuum at a temperature of 80 °C until the DMSO is completely evaporated. At the end of the process, films with a size of 50×50 mm and a thickness of 100 to 200 μm are obtained, which are separated from the glass and examined by ATR IR spectroscopy and DSC.

The molecular weight characteristics of PAN were studied by GPC on a GPC-120 chromatograph (Polymer Labs). The analysis was carried out at 50°С in DMF containing 0.1 wt % LiBr with a flow rate of 1 ml/min. Two columns PLgel 5 μm MIXED C (M=(5 (10 ² )-(1 (10 ⁷ )) were used for separation. _PAN =39.4 (10 ^-4 , (=0.75, K _PMMA =17.7 ×10 ^-4 , (=0.62 (Polymer Handbook/ Ed. by J. Brandrup, EH Immergut, EA Grulke. New York: Wiley, 1999.)) .

Thermal effects observed during dynamic heating of polymers were studied on a Netzsch DSC 204 differential scanning calorimeter (Netzsch, Germany) in an atmosphere of dried argon at a flow rate of 50 ml/min in the range from 30 to 500°C with a heating rate of 10 deg. /min For measurements, samples weighing from 4 to 8 mg were taken and placed in a standard aluminum crucible. The results were processed using Netzsch Proteus software.

The chemical structure of the polymers was studied by IR spectroscopy using a Spectrum Two FT-IR Spectrometer IR Fourier spectrometer (Perkin Elmer) in the region of 4000–600 cm– ¹ with a resolution of 0.5 cm– ¹ at room temperature in the ATR mode (crystal diamond).

To study the changes that occur in the structure of macromolecules during isothermal exposure, the sample was first placed in a Netzsch DSC 204 DSC cell (Netzsch) in an inert atmosphere and kept at a fixed temperature for a specified time, then IR spectra were recorded at room temperature.

The proportion of unreacted nitrile groups CN was determined by the equation (Collins G.L., et al. // Carbon. 1988. V. 26. I. 5. P. 671.)

where A _{2240 cmA} - ₁ - absorption intensity of nitrile groups -CN,

A _{1590 cmA-1} - absorption intensity of imine groups -C=N-,

- the ratio of the coefficients of molar absorption, equal to 0.29.

^1H NMR spectra were recorded on a BrukerDPX-500 FT-IR spectrometer in DMSO-d ₆ .

Claims (7)

1. A method for producing a compositionally homogeneous low molecular weight polymer based on acrylonitrile (AN) with a narrow molecular weight distribution by weight, characterized in that N-methylmorpholine-N-oxide is added to the polymerization system, including a monomer and, optionally, a radical polymerization initiator (MMO), optionally in the presence of an organic solvent; where the method is carried out in an atmosphere of inert gas when heated. 2. The method according to p. 1, where the method is carried out in an argon atmosphere at a temperature range from 60 to 90°C. 3. The method of claim. 1, where the resulting polymer has a molecular weight of the order of 1 kDa. 4. The method of claim 1, wherein the AN-based polymer is a copolymer of AN with a vinyl or vinylidene comonomer. 5. The method according to p. 4, where vinyl or vinylidene monomers selected from the group consisting of acrylic acid are used as comonomers; alkyl acrylates, for example methyl acrylate, ethyl acrylate, butyl acrylate, ethylhexyl acrylate; methacrylic acid, alkyl methacrylates, for example methyl methacrylate, ethyl methacrylate, butyl methacrylate, ethylhexyl methacrylate; styrene, vinyl acetate, acrylamide, 1-vinylimidazole. 6. The method according to paragraphs. 4, 5, where the comonomer content in the reaction system is 15-20 mol.%. 7. The method according to any one of paragraphs. 1-6, where the polymerization proceeds for 24 hours without the appearance of a gel effect.

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