INTRODUCTION

In recent decades, atom transfer controlled radical polymerization has been one of the most urgent directions of polymer chemistry [1, 2]. The classical variant of this methodology is based on the reversible transfer of halogen atom between a growing radical (~\({\text{P}}_{n}^{ \bullet }\)) and a transition metal complex (Mn/L) (1):

(1)

Owing to the multiple activation/deactivation of a polymer chain a high control over the molecular weight characteristics and functionality of polymers can be achieved. The ability of a metal atom to easily and reversibly change the oxidation number under the action of light allowed a similar process be carried out under photoirradiation [3]:

(2)

(M is the complex of Ir or Ru).

However, considerable drawbacks of atom transfer processes are the need for isolation of a metal complex catalyst for subsequent regeneration and the mandatory purification of the final product from concomitant catalytic system components. In recent years, a key photocatalysis trend in synthetic organic chemistry is associated with the replacement of metal-containing catalysts with organic ones [4, 5]. Several photocatalysts were successfully applied in atom transfer polymerization processes for the synthesis of homo- and copolymers with a narrow MWD [6, 7]. This direction was coined metal-free atom transfer polymerization.

The activation/deactivation of macroradicals occurs by mechanisms with oxidative and reductive quenching cycles:

(3)

The catalysts of metal-free atom transfer polymerization with the oxidative quenching cycle of the photocatalyst (РС*) excited state are condensed aromatic compounds and heterocyclic derivatives of phenothiazine, phenoxazine, and phenazine series [8, 9]. The reductive type of quenching is typical for some type II photoinitiators and xanthene dyes [10, 11]. The possibility to obtain polymers of the predetermined MW and low polydispersity coefficients Đ that are capable of modification is maintained. The synthesis of polymers occurs at room temperature in the absence of transition metal complexes.

For similar processes to be effective photocatalysts should possess high molar extinction coefficients in the near UV and visible spectral ranges. Therefore, in terms of practical use, not only xanthene but also other types of commercial dyes and indicators are of undoubted interest. Little is known of their application as photocatalysts in organic synthesis [12, 13]. At the same time, dyes have proved themselves well as components of photoinitiating systems for the synthesis of crosslinked and linear polymers [14‒17].

The purpose of this work is to study the possibility of using 3-amino-7-dimethylamino-2-methylphenazium hydrochloride (Neutral Red) and 3,6-diamino-2,7-dimethyl-10-phenylphenazine chlorohydrate (Safranin Т) as photocatalysts, in particular, in the presence of air oxygen. The initiator was ethyl-2-bromoisobutyrate (EBIB):

EXPERIMENTAL

Neutral Red and Safranin Т (both of analytical grade) were used as received. Monomers methyl methacrylate, n-butyl methacrylate (BMA), glycidyl methacrylate (GMA), methyl acrylate (МА), acrylonitrile, EBIB, isopropylamine (IPA), tributylamine (TBA), and dimethyl sulfoxide were purified according to the general accepted techniques [18]. The physicochemical constants of all compounds corresponded to the published data.

The source of total white light was a LED strip; the number of light emitting diodes, 60 pcs/m; the diode size, 5360; and the strip length, 2 m. The strip was placed inside a 400 mL glass reactor. The emission spectrum of the LED strip was similar to that of sun light.

The absorption spectra of Neutral Red and Safranin Т under various conditions were recorded on an UV mini-1240 UV/VIS spectrophotometer. The initial catalyst concentration was 0.1 mg/mL. The organobromine initiator and amine were added in proportions similar to the composition of the catalytic system.

Polymerization solutions were prepared as follows. Weighed portions of the catalyst and initiator as well as amine were dissolved in preliminarily distilled monomer and solvent, and the resulting solutions were placed in glass ampoules. To remove oxygen the ampoule was degassed three times and frozen in liquid nitrogen and air was pumped out. After degassing the ampoule was sealed. To investigate the process in the presence of air oxygen (without degassing) access of air to the ready solution was blocked by a rubber septa.

The ampoule with solution was placed in a photoreactor for the desired time. The kinetics of polymerization was monitored by gravimetry.

To isolate and purify polymers from the unreacted monomer, initiator, amine, and catalyst the samples were reprecipitated many times from methylene chloride solution into a mixture of petroleum ether and isopropyl alcohol. Polyacrylonitrile was reprecipitated from DMSO solution into water. After polymer precipitation the solution was decanted and the samples were dried under reduced pressure until a constant weight.

The molecular weight characteristics of polymers were analyzed by GPC [19] on a Knauer installation equipped with a cascade of linear columns 102‒103–105 Å (Phenomenex, United States). An RI Detektor K-2301 differential refractometer was used as a detector, and THF (25.0 ± 0.1°С) was used as an eluent. Calibration was done against narrowly dispersed PMMA standards.

The PAN samples were analyzed on a Knauer installation equipped with a cascade of linear columns 103‒105 Å (Phenomenex, United States). An RI Detektor K-2300В differential refractometer was used as a detector, and a 0.01 mol/L LiBr solution in DMF (70.0 ± 0.1°С) was used as an eluent. Calibration was done using narrowly dispersed PMMA standards.

The polymerization of MMA and other (meth)acrylic monomers in the presence of phenazine dyes was conducted in DMSO. The polar medium was used because of a low solubility of phenazine dyes in the monomers.

RESULTS AND DISCUSSION

Features of Methyl Methacrylate Polymerization Mediated by the Studied Catalysts

The data on the photopolymerization of MMA are presented in Table 1. It is found that the self-polymerization of MMA carried out under aerobic conditions under total white light (400–700 nm) irradiation promotes yields the trace amounts of PMMA with a high molecular weight and a broad MWD. In the absence of degassing, the self-polymerization of MMA affords no polymer within 2.5 h irradiation. The introduction of EBIB has almost no effect on polymer yield compared with the processes of MMA self-polymerization described above. The numerical values of MW are several hundreds, and the dispersity of PMMA samples remains high (at a level of ~3). In the presence of air oxygen, monomer conversion reached within 2.5 h is close to zero under degassing while conversion is not above 4%. The joint use of EBIB with Neutral Red or Safranin Т for polymerization initiation leads to a marked rise in the yield of PMMA both under degassing of the reaction mixture and in the presence of air oxygen. For both dyes under aerobic conditions the monomer conversion is higher than that under anaerobic conditions.

Table 1. Photopolymerization of MMA in the presence of systems phenazine dye‒EBIB‒amine (irradiation with total white light λmax ~ 400‒700 nm; MMA : DMSO = 1 : 1, vol/vol)
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The features of MMA polymerization mediated by the catalytic system phenazine dye‒EBIB depend not only on the dye structure but also on the presence of oxygen in the reaction system. Moreover, the process of PMMA synthesis is considerably affected by the introduction of amines. Figure 1 shows the monomer conversion as a function of the time of synthesis and Мn as a function of conversion for PMMA synthesized in the presence of the system Neutral Red ‒EBIB and Neutral Red‒EBIB‒amine under while light irradiation. It is clear that, in the case of the composition Neutral Red‒EBIB, regardless of the presence of oxygen in the reaction medium the yield of PMMA attained over 4 h is above 90% (Fig. 1a, curves 1, 2). Upon the introduction of primary amine IPA into the polymerization system polymerization also occurs to high conversions; however, the time of their achievement grows (curves 3, 4). The overall rate of the process catalyzed by Neutral Red‒EBIB‒IPA in the presence of oxygen is lower than that under preliminary degassing of the reaction medium. At the initial step, tertiary amine (TBA) contributes to a gain in the overall rate of MMA polymerization initiated by Neutral Red‒EBIB (curves 1, 5). Irrespective of the presence of oxygen the time needed to attain high conversions increases. The rate of polymerization carried out using the system Neutral Red‒EBIB‒TBA under degassing is lower than that without degassing. However, in both cases the conversion of MMA attains 80% or above.

Fig. 1.
figure 1

Dependence of monomer conversion P on (a) synthesis time and (b) Mn of PMMA on monomer conversion. Molar ratio MMA : EBIB : Neutral Red : amine = (1, 2) 100 : 1 : 0.01 : 0 and (3–6) 100 : 1 : 0.01 : 0.5; (1, 2) in the absence of amine and in the presence of (3, 4) IPA and (5, 6) TBA; degassing: (1, 3, 5) present and (2, 4, 6) absent. Here and in Figs. 2‒5, synthesis conditions: white light irradiation and volume ratio MMA : DMSO = 1.

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The presence of amine in the composition also strongly influences the molecular weight characteristics of PMMA (Table 1; Fig. 1b). For example, the dependence of the number-average molecular weight Mn on conversion in the polymerization MMA mediated by the system Neutral Red‒EBIB under degassing of the reaction mixture has two sections with different tempos of increase in Mn (dependence 1). At the initial stage (below ~20% conversion) the Mn of PMMA samples increases at a higher rate than that in subsequent polymerization. The resulting samples are characterized by monomodal although broad MWD (Ɖ ~ 2.5–2.7). The numerical values of the molecular weight of PMMA synthesized under degassing conditions are lower than those without degassing. The Mn of PMMA samples synthesized using the system Neutral Red‒EBIB as an initiator in the presence of air oxygen somewhat decreases during polymerization (dependence 2). This change is related to the broadening of MWD curves in the course of time due to an increase in the proportion of low molecular weight fractions, while parameter Ɖ increases from 1.8 to 2.3.

Introduction of both primary and tertiary amines into the composition Neutral Red‒EBIB promotes to a reduction in the value of MW compared with processes proceeding without these compounds (dependences 3–6). At the same time, the linear dependence of Mn on MMA conversion is observed, as is typical for living chain processes [1, 2]. The numerical MW values of the polymers synthesized in the presence of the system Neutral Red‒EBIB‒amine under preliminary degassing are lower compared to those obtained without degassing regardless of the amine nature. Note that in the case of IPA both in the presence of oxygen and in its absence the values of Ɖ remain at a level of ~2.0. Upon the introduction of TBA the MWD curves narrow down during the process while the values of Ɖ of the polymers decrease in the range of 2.2–1.6 (degassing) and 2.8–1.5 (without degassing).

Similar features are observed for the polymerization of MMA mediated by Safranin Т‒EBIB. Specifically, regardless of the presence of oxygen the process proceeds to a conversion of ~90–95% over a short time (Fig. 2а, curves 1, 2). Under degassing of the reaction medium, the addition of amines promotes an increase in the time of synthesis to achieve high conversions (curves 1, 3, 5). The overall rate of polymerization initiated by the systems Safranin Т‒EBIB and Safranin Т‒EBIB‒TBA in the presence of air oxygen is higher than that in its absence (curves 1, 2, 5, 6). In the case of Safranin Т‒EBIB‒IPA, at the initial stage, the presence of oxygen lowers the overall rate of the process (curves 3, 4).

Fig. 2.
figure 2

Dependence of MMA conversion on (a) synthesis time and (b) Mn of PMMA on monomer conversion. Molar ratio MMA : EBIB : Safranin Т : amine = (1, 2) 100 : 1 : 0.01 : 0 and (3–6) 100 : 1 : 0.01 : 0.5; (1, 2) in the absence of amine and in the presence of (3, 4) IPA and (5, 6) TBA; degassing: (1, 3, 5) present and (2, 4, 6) absent. The dashed line depicts the theoretically calculated Mn for an EBIB concentration of 1 mol %.

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The pattern of Mn dependences on monomer conversion for the polymers synthesized in the presence of Safranin Т‒EBIB under degassing and without it is similar to that described above (Fig. 1b). For example, when polymerization is carried out in the absence of oxygen, the Mn of PMMA samples grows linearly (Fig. 2b, dependence 1). The values of Mn for PMMA samples synthesized under degassing using Safranin Т‒EBIB as an initiation system are lower than those without it (dependences 1, 2). The samples are characterized by a broad although monomodal MWD (Ɖ ~ 2.5). The molecular weight of the polymers synthesized under aerobic conditions decreases during the process, and the MWD curves broaden due to an increase in the contribution of low molecular weight fractions. Parameter Ɖ regularly increases from 1.7 to 2.1. It should be noted that the numerical MW values of the polymers synthesized using systems Neutral Red‒EBIB and Safranin Т‒EBIB in the presence of air oxygen are almost coincident. Upon the introduction of IPA the MW of PMMA samples are reduced compared with the polymer synthesized in its absence (dependences 1, 3). Under anaerobic conditions the dependence of Mn on monomer conversion has two sections with different tempos of increase in MW. During the process the Ɖ values of PMMA samples decrease from 2.3 to 1.8. In the presence of oxygen the combination Safranin Т‒EBIB‒IPA contributes to a linear increase in Mn with monomer conversion (dependence 4) and the MWD curves are monomodal. In the course of time they shift to high MW values and dispersity decreases from 2.8 to 1.8. When using composition Safranin Т‒EBIB‒Bu3N under preliminary degassing and without it, the MW of the resulting PMMA increases; however, regardless of the presence of oxygen the MWDs of the polymers are multimodal (Fig. 3). Furing polymerization modes shift to high MW values but their complete separation is absent.

Fig. 3.
figure 3

MWD curves of PMMA samples normalized to conversion (1) 8, (2) 29, (3) 70, and (4) 90. Molar ratio MMA : EBIB : Safranin Т : TBA = 100 : 0.01 : 1 : 0.5.

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Thus, the combination of Neutral Red or Safranin with EBIB enables one to initiate the polymerization of MMA under white light irradiation. The inconsistency between the Mn of the synthesized samples and the theoretically calculated values (Fig. 2b, dashed line), despite their linear rise with conversion, indicates a low activity of EBIB as an initiator under the used conditions. Amines considerably change the kinetic features of the process and the characteristics PMMA. The direction of their effect is determined not only by the nature of amine but also by the composition of the phenazine dye.

Polymerization of Other Monomers Mediated by Catalytic Systems Phenazine Dye‒Ethyl-2-Bromoisobutyrate‒Tributylamine

It is found that the catalytic systems phenazine dye‒EBIB‒TBA initiate the polymerization not only of MMA but also of a number of other acrylate and methacrylate monomers: BMA, GMA, МА, and AN. The results are presented in Table 2 and Fig. 4.

Table 2. Polymerization of monomers under aerobic conditions using the catalytic systems dye‒EBIB‒TBA (white light irradiation for 1 h, molar ratio of monomer : EBIB : catalyst : TBA = 100 : 1 : 0.01 : 0.5, and volume ratio of monomer : DMSO = 1)
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Fig. 4.
figure 4

(a) Mn and (b) Đ of (meth)acrylate polymers as a function of conversion. Molar ratio monomer : EBIB : catalyst : TBA = 100 : 1 : 0.01 : 0.5; aerobic conditions. Monomer: (1) МА, (2, 3) n-BMA, and (4) AN; catalyst (1, 3, 4) Neutral Red and (2) Safranin Т.

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Using the polymerization of BMA as an example, it is shown that the catalytic system based on Neutral Red is more efficient than a similar composition based on Safranin Т (Table 2). The overall rate of polymerization and the ultimate conversion of BMA synthesized in the presence of Safranin Т‒EBIB‒TBA are lower than those in the case of Neutral Red. The dependence of Mn on BMA conversion is linear for both dyes, and the MW values of the polymers prepared using Safranin Т are higher than those in the presence of Neutral Red. Parameter Đ diminishes with conversion in the case of both dyes.

The polymerization of GMA initiated by the system Neutral Red‒EBIB‒TBA proceeds at a high rate (Table 2). The yield of PGMA achieves ~80% already within 1 h, but the crosslinked polymer is isolated.

Interesting features are observed for the polymerization of acrylate monomers. For example, the system Neutral Red‒EBIB‒TBA allows the synthesis of poly(methyl acrylate) with a yield above 95%. In contrast to PMMA synthesized under similar conditions, the numerical values of the molecular weight of PMA samples are almost independent of conversion and only at MA conversions above 80% they decrease (Fig. 4а, dependence 1). A reduction in Mn is accompanied by the broadening of MWD curves and an increase in Đ (Fig. 4b, dependence 1).

The polymerization of AN in DMSO initiated by the system Neutral Red‒EBIB‒TBA proceeds to high conversions (~95%). PAN samples are characterized by a linear growth of Mn with monomer conversion (Fig. 4a, dependence 4). The MWD curves are monomodal, and the values of Ɖ decrease to ~1.4 (Fig. 4b, dependence 4).

Thus, under photoirradiation conditions the catalytic systems phenazine dye‒EBIB‒amine make it possible to effectively initiate the polymerization of (meth)acrylate monomers of various structure. For some of them MW can be increased with conversion and the relatively low values of Đ can be maintained.

About the Possible Mechanism of Polymerization of (Meth)Acrylate Monomers Mediated by the Studied Catalysts

According to our data, Neutral Red and Safranin Т combined with EBIB under white light irradiation can initiate the polymerization of vinyl monomers with the oxidative and reductive types of quenching of the catalyst excited state [6–11]. At the same time, the nature of amine influences features of a change in the molecular weight characteristics of the polymers only when using Safranin Т. It is reasonable to assume that the efficiency of the catalytic system phenazine dye‒organohalogen compound‒amine depends on reactions between its components both before and during light irradiation.

Actually, Neutral Red and Safranin Т are the cationic derivatives of phenazine and upon interaction with Н donor compounds via the sequential addition of electrons and H+ are transformed into dihydrophenazines [20–27]:

(4)

“Semireduced” forms generated at intermediate stages have an unshared electron, and their stability and tendency toward side reactions (e.g., disproprotionation) are determined by the initial dye composition [20–27]. Note that for transformation into the corresponding dihydrophenazine Neutral Red and Safranin Т need a proton and two electrons. At the same time, for reduction of the aromatic structure of the phenazine core Neutral Red‒0 should eliminate a proton while Safranin Т should attach an electron. To ascertain the sequence of interactions of the used dyes with amines and organobromine compounds the absorption spectra of solutions of the used catalyst were investigated.

The absorption spectrum of Neutral Red recorded in DMSO is characterized by two broad bands of different intensity in the visible spectral range with maxima at ~450 and 540 nm and the absorption band at ~280 nm in the UV range (Fig. 5а, curve 1). This shape of the Neutral Red spectrum is similar to those recorded in aqueous and water–alcohol solutions [27]. The combination of bands is typical for Neutral Red and suggests the presence of dye molecules in the protonated (λmax ~ 540 nm) and nonprotonated forms (λmax ~ 450 nm). Using tert-butyl bromide (TBB) as example, it is shown that the introduction of halogen-containing compounds causes a slight hypochromic effect of the peak with λmax ~ 450 nm and the hyperchromic effect of the peak with λmax ~ 540 nm but has no effect on the position and intensity of the peak with λmax ~ 280 nm (curve 2). Irradiation with total white light for 5 min causes a slight reduction in the intensity of the peak in the UV spectral range and smoothing of the peak with λmax ~ 450 nm (curve 3). This change in the spectrum shape may indicate that the interaction of Neutral Red‒0 with TBB occurs according to the oxidative scheme similar to Scheme 3. After introduction of amine into the solution of Neutral Red and TBB, it acquires a yellow color. In the absorption spectrum, the hypsochromic shift of the main peak to the 400–500 nm range of is observed and the intensity of the peak λmax ~ 280 nm increases somewhat (curve 4). It is evident that the protonated form of Neutral Red fully transforms into the Neutral Red‒0 form. Under white light irradiation the solution containing Neutral Red, TBB, and TBA becomes almost colorless. In the absorption spectrum, the intensity of all peaks decreases and a plateau appears in the range of ~350–410 nm (curve 5). Emergence of the signal in this range may suggest that “semireduced” forms of Neutral Red‒1, Neutral Red‒2, and Neutral Red‒3 are formed (Scheme 4) [20–24]. Similar changes in the spectrum are also traced in the presence of MMA.

Fig. 5.
figure 5

Absorption spectra of (a) Neutral Red and (b) Safranin Т in DMSO under while light irradiation. a: (1) Neutral Red initial solution, (2) after addition of TBB, (3) subsequent irradiation for 5 min, (4) after addition of TBA, (5) Neutral Red solution containing TBB and TBA after irradiation for 5 min. b: (1) Safranin Т initial solution, (2) after addition of Bu3N, (3) subsequent irradiation for 5 min, (4) after addition of TBB, and (5) Safranin Т solution containing TBB and TBA after irradiation for 5 min.

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The absorption spectrum of Safranin T in DMSO exhibits three weakly resolved peaks in the range of 450–560 nm and a single peak with λmax ~ 280 nm (Fig. 5b, curve 1). The introduction of TBA exerts no effect on the shape of the spectrum (curve 2). However, the irradiation of Safranin T solution containing TBA induces a strong hypochromic effect of all peaks and emergence of a broad absorption band in the range of 300–430 nm (curve 3). As in the case of Neutral Red, the signal in this range indicates formation of the “semireduced” radical forms of Safranin Т [25–27]. In fact, the addition of TBB returns the initial shape of the absorption spectrum of Safranin Т (curve 4). After irradiation of Safranin Т solution containing TBA and TBB the hypochromic effect is detected for all absorption bands and the band is returned to the 300–430 nm range (curve 5).

Thus, under irradiation TBA and TBB transform Safranin Т into the “semireduced” form. When irradiation is ceased, the initial shape of the spectrum of the Safranin Т mixture with TBA and TBB is returned in the course of time, although the intensity of solution color diminishes. Since the absorption spectra were studied without solution degassing, this finding may indicate that without irradiation the reduced and “semireduced” forms are oxidized to the initially cationic form of Safranin Т.

The data presented above unambiguously suggest that in the presence of amines of various structure Neutral Red and Safranin Т can interact with organobromine compounds under visible light irradiation. Sometimes not only the polymerization of MMA can be effectively initiated but also a linear rise in the Mn of polymer samples and a reduction in their dispersity Đ, which are typical for controlled synthesis, may be observed.

An analysis of the data obtained makes it possible to assume that Neutral Red and Safranin Т when combined with amines enable the photopolymerization of MMA according to the reductive mechanism (Scheme 2). Thus, interaction between the components of the tested catalytic system may be schematically presented as follows:

(5)

Here R and X are substituents corresponding to the structure of Neutral Red or Safranin Т; AmH is IPA or TBA.

Under the action of visible light the phenazine dye can interact with amine to generate a cation radical that abstracts a bromine atom from the initiator (or halogen-terminated polymer radical). Then, the phenazinyl radical undergoes disproportionation to the corresponding cationic forms of the dyes and dihydrophenazines while the bromine atom “comes back” to the growing radical. In the presence of air oxygen, dihydrophenazines are oxidized to the cationic form and are again involved in the reaction. The relative tolerance of the proposed catalytic systems against oxygen may be explained by both the “involvement” of oxygen in the cycle oxidation/reduction of the phenazine catalyst and its participation in side radical reactions occurring in the presence of amines [28].

Also note that the mechanism behind the reduction of Neutral Red or Safranin Т by amines under the studied conditions certainly calls for further studies, in particular, to enhance the efficiency of relevant catalytic systems.

CONCLUSIONS

Thus, phenazine dyes, Neutral Red and Safranin Т, are promising photocatalysts of the radical polymerization of (meth)acrylate monomers which may proceed to high conversions over a short time at a low concentration of the catalytic system. In practical terms, it is important that the proposed photocatalysts are very efficient even without preliminary degassing of the reaction mixture.