Features of Polymerization of Methyl Methacrylate using a Photocatalyst—the Complex Oxide RbTe1.5W0.5O6

Radical polymerization of methyl methacrylate in an aqueous emulsion was carried out using the complex oxide RbTe1.5W0.5O6 as a photoinitiator under visible light irradiation with λ = 400–700 nm. Study of the polymerization process and reaction products using methods of physical and chemical analysis (GPC, IR, NMR, etc.) has shown that there are several directions of monomer transformations at the same time. Polymethyl methacrylate with Mn ~ 140–145 kDa, produced in the organic phase, is a result of polymerization initiation by a hydroxyl radical formed due to complex transformations of electron–hole pairs during photocatalyst irradiation. Moreover, the interaction of the hydroxyl radical with OH-groups on the complex oxide RbTe1.5W0.5O6 surface and the subsequent formation of oxygen-centered radicals lead to grafting polymer macromolecules on the photocatalyst surface. In addition, methyl methacrylate is able to oxidize to a cyclic dimer with terminal double bonds and form a polymer with cyclic dimer links due to coordination by double bonds on the RbTe1.5W0.5O6 surface. The high activity of the hydroxyl radical allows to obtain the graft copolymer PMMA-pectin by grafting the polymer product on the surface of the natural polymer-pectin. Comparison of the sponge morphology of the graft copolymer PMMA-pectin and the initial pectin samples using the scanning electron microscopy has shown a noticeable difference in their structural and topological organization. It is especially interesting in terms of studying the properties of the graft copolymer as a material for the scaffolds.


Introduction
The creation of new efficient photocatalysts attracts the scientist's attention as a direction of research involving wastefree technologies. Heterogeneous photocatalysis is of interest due to wide field of application such as organic pollutant degradation and water splitting, however, inorganic photocatalytic materials can be also used as initiators for polymerization processes. Photoinitiators are a promising alternative to traditional material initiators because of the absence of initiator residues in the polymer, the weak dependence of the polymerization rate on temperature, since it is determined by the irradiation intensity, so it easy to regulate the polymerization time, etc. [1][2][3][4] The research focuses on the development of new biocompatible photoinitiators with high reactivity for use in biomedicine [1,3,4], as well as the use of photopolymerization to produce complex nanoobjects [2] and composite materials [5]. In particular, in the scientific work [5], the improvement of the thermomechanical properties of epoxidized natural rubber was achieved by grafting on it with an efficiency of more than 50%. This made it possible to increase the thermal stability and improve the dielectric properties of the material.
Recently we have obtained and investigated both crystal structure features and electronic properties of some representatives of β-pyrochlores [59][60][61]. Among them CsTeMoO 6 and RbTe 1.5 W 0.5 O 6 possess ability to organic decomposition under visible light irradiation according to preliminary tests [21]. Thus, RbTe 1.5 W 0.5 O 6 has been chosen to investigate photo-initiated polymerization of MMA.
The aim of this work is to identify the features of radical polymerization using a complex oxide photoinitiator RbTe 1.5 W 0.5 O 6 under visible light irradiation λ = 400-700 nm by the example of a water-emulsion process with a monomer -methyl methacrylate (MMA), and to characterize the most important properties of the resulting polymer products. The MMA has been chosen as a model monomer, because the polymerization process characteristics and the properties of the resulting polymer product are well studied. Comparison of the experimental results with the literature data allows to assess the processes occurring in this research correctly. Due to high reactivity of the hydroxyl radical formed in the mixture, there are competing reactions in the solution and on the surface. Thus, not only the properties of the resulting polymer, but also the possibility of grafting it on the RbTe 1.5 W 0.5 O 6 surface because of the covalent bonds' formation have been studied. Moreover, grafting on the natural polymer pectin surface, specially introduced into the aqueous phase, due to the separation of hydrogen atoms from the end groups, has been investigated. Such processes are of considerable practical interest for regenerative medicine.

Materials
Commercial organic solvents: chloroform (99.85%, Component-Reagent, Russia), toluene (99.5%, Ecos-1, Russia), tetrahydrofuran (99.5%, Component-Reagent, Russia) were used. MMA monomer (99.8%, Energoeffect, Russia) was previously cleaned from the stabilizer by washing the monomer with a 10% w/w alkali solution in a ratio of 1:1 at least 4 times, and then it was repeatedly washed with cold water to a neutral pH. Then the monomer was dried using calcium chloride for at least a day. At the end, the MMA was distilled under vacuum (1.33 Pa) at 40℃. Pectin Unipectine PG DS (Cargill, Germany, France) was dissolved in water. The initiator azobisisobutyronitrile (AIBN) was purified according to the well-known method by recrystallization from alcohol at 40℃ with further hot filtration and cooling with cold water. The crystals that precipitate out during cooling were filtered and dried under vacuum (1.33 Pa) at 25℃ to constant weight.

Preparation of the RbTe 1.5 W 0.5 O 6 Photocatalyst
Polycrystalline sample of the RbTe 1.5 W 0.5 O 6 was prepared by solid-state reaction using the method described in previous work [21]. Prepared powder sample was grounded in planetary mill for ~ 16-18 h with speed 30 rpm to reach minimal size of participles. Phase individuality of synthesized sample has been confirmed by powder X-ray diffraction (XRD) analysis. Figure 1a presents the RbTe 1.5 W 0.5 O 6 diffraction pattern, which can be indexed in cubic system with space group Fd − 3 m [61]. Comparison of the obtained X-ray diffraction pattern and the theoretical one, calculated from the single-crystal X-ray structural data, indicates the monophasic nature of the powder. Impurity phases have not been detected within the sensitivity of the method.
The morphology of RbTe 1.5 W 0.5 O 6 powder has been investigated by scanning electron microscopy (SEM) method and shown in Fig. 2a. The powder contains different size particles with average size of 736 nm (Fig. 1b). However, the particles size distribution has small maximum about of 1.4 µm that can be explained by formation of stable agglomerates consisted of nanosized particles, which hardly destroyed under ultrasonic condition.

Preparation of MMA-in-Water Emulsion
The MMA emulsion was prepared by mixing RbTe 1.5 W 0.5 O 6 powder and liquid components: water, MMA and emulsifiers. Water and monomer were in a ratio of 75:25 with the addition of "Ediscan" and tetraethylene glycol dimethyl ether (TEGDME) emulsifiers. The MMA emulsion with pectin was prepared similarly by mixing RbTe 1.5 W 0.5 O 6 powder and liquid components in a ratio of water: monomer: pectin = 70:25:5 with the addition of an ionogenic emulsifier "Ediscan". The completed emulsion was treated using ultrasonic dispergator UZDN-A 650 for 5 min. Argon was bubbled into the emulsion for 15 min with stirring before the reaction. The reaction was carried out in a glass vessel with a constant supply of argon current. The irradiation was carried out using a visible light LED lamp (white, 30 W LED, 6500 K) at a distance of no more than 10 cm from the reaction mixture under constant stirring (450 rpm) for 5 h. The emulsion was centrifuged for 30 min at a speed of 4000 rpm to separate the catalyst after the reaction, then the organic fraction of the emulsion was extracted with toluene, the aqueous part was dried under vacuum (1.33 Pa) at 50℃. The catalyst powder was washed by different ways: in tetrahydrofuran (THF), in chloroform in Soxhlet extractor and in water under ultrasonic condition. After washing the extracts were separated, the catalyst powder was dried in vacuum (1.33 Pa) at 50℃. The solid polymer was isolated from the solution by precipitators and dried. The polymer samples for SEM analysis were dried lyophilically.

Preparation of PMMA on AIBN
The MMA polymerization product obtained on RbTe 1.5 W 0.5 O 6 complex oxide has been compared with traditional polymethyl methacrylate (PMMA) by infrared spectroscopy (IR). Traditional PMMA was synthesized using AIBN as the initiator. The polymer was obtained by mass polymerization by mixing MMA and AIBN in a glass ampoule and holding it in vacuum (1.33 Pa) at 60℃ and until the block was cut off [62].

Characterization
Molecular weight characteristics were determined by gel permeation chromatography (GPC). Organic solutions in tetrahydrofuran were analyzed using a liquid chromatograph "Shimadzu Prominence LC-20VP" with "Tosoh Bioscience" columns (eluent flow rate 0.7 ml/min). Narrow disperse polystyrene standards were used for calibration; a differential refractometer was used as a detector. Water solutions were analyzed using a high-performance liquid chromatograph manufactured by Shimadzu CTO 20A/20A C (Japan) with the LC-Solutions-GPC software module. Separation was performed using a Tosoh Bioscience TSK gel g3000swxl column with a pore diameter of 5 microns and a low-temperature light-scattering detector ELSD-LT II. The eluent was a 0.5 M acetic acid solution, the flow rate was 0.8 ml / min, and narrow disperse dextran standards with a molecular weight (MW) range of 1-410 kDa (Fluca) were used for calibration.
The phase study of the prepared powder was carried out on a Shimadzu XRD-6100 diffractometer (CuKa, λ = 1.5418 Å) in the range of 2θ 10-60˚ with speed of 2 ˚/ min. The study of the surface of photocatalyst powder and polymer samples was performed using a scanning electron microscope JSM-IT300 (Jeolltd, Japan) with an electron probe diameter of 5 nm (operating voltage 20 kV), using detectors of low-energy secondary electrons and backscattered electrons in a low vacuum mode to avoid samples charging. The investigation of RbTe 1.5 W 0.5 O 6 elemental composition before and after polymerization was implemented on energy dispersive X-ray microanalysis method (EDXMA) with detector X-Max N 20 (Oxford Instruments) using characteristic X-ray lines K α (C, O) and L α (Rb, Te, W). Elemental composition of polymer samples was carried out by Vario EL Cube elemental analyzer. The absorption spectra were recorded using the spectrophotometer "IRPrestige-21" (Shimadzu, Japan), the wavenumber range is 4000-550 cm −1 , and the error did not exceed ± 0.05 cm −1 . Polymer films were prepared on a KBr reflection plate.
The particle size distribution of the prepared RbTe 1.5 W 0.5 O 6 powder sample by volume was determined by laser diffraction method using SALD-2300 analyzer (Shimadzu) (the particle size distributions were calculated by the Fraunhofer theory).

Results and Discussion
The process of MMA polymerization was carried out in aqueous dispersion medium at 20-25 0 C. At the end of the process the organic fraction solvent was added; and the aqueous, organic phases and the catalyst powder have been analyzed separately.
A polymer (5-10% based on the initial MMA) was isolated from the organic phase using a precipitator, which was analyzed by GPC analysis in a tetrahydrofuran solution. The values of MW and the polydispersity index (PDI) possess the same order as for the MMA photopolymerization using titanium oxide powder [24] (Table 1).
*from the publication [24]: a-titanium oxide powder; b-titanium oxide on glass fiber.
However, the MMA polymerization process using RbTe 1.5 W 0.5 O 6 is characterized by the distinctive feature. SEM analysis and X-ray microanalysis showed that a significant number of polymer fibers are located on the catalyst surface (Fig. 2b).
In order to analyze the bond nature between the polymer and the catalyst to separate the polymer, the powder after reaction was heated at 50 0 C in a THF solution for 3 h. However, no noticeable changes of the catalyst surface have been observed; polymer fibers were still presented (Fig. 2c); and the polymer was not detected in the THF solution by the GPC method.
The catalyst after polymerization was subjected to chloroform extraction (temperature 61 0 C) in the Soxhlet extractor for 15 h, but polymer fibers have been still observed on the catalyst surface (Fig. 2d). Only using ultrasonic dispergator in water solution for 40 min with a water-cooling jacket connected to a flow thermostat Termex VT18 has been allowed to successfully remove the polymer from the powder surface.
Thus, a part of the polymer cannot be washed off from the oxide surface by organic solvents, and it is removed only by ultrasonic treatment due to polymer macromolecules destroying [63][64][65]. This fact indicates that the macromolecules are bound by a covalent bond to RbTe 1.5 W 0.5 O 6 , so the polymer is grafted on the oxide surface according to scheme (Fig. 3): After the catalyst extraction, a solid polymer was isolated in the Soxhlet extractor and studied by GPC, nuclear magnetic resonance (NMR) and IR spectroscopy. High-molecular products ( Table 1, line 3) with slightly larger values of MW polymer in comparison with the polymer isolated from the organic phase have been detected by the GPC method: Mn ~ 140-145 kDa and 200-210 kDa, respectively. According to NMR spectroscopy data for the polymer, two main signals were observed in the 1 H NMR spectrum (Fig. 4a): singlets at 4.69 and 8.10 ppm with an intensity ratio of 1:1. These signals can be attributed to the groups -CH 2 OC(O)and geminal protons at the double bond H 2 C = C, respectively. In addition, the weak signals were obtained in the region of 0.95-2.50 ppm and 3.65 ppm that belong to the MMA polymer. PMMA content according to NMR data does not exceed 10% of the main product. In the 13 C NMR spectrum (Fig. 4b) Apparently, the product isolated from chloroform contains not only PMMA, but also other compounds, which can be the result of polymerization of the MMA oxidation products on the oxide catalyst. The analysis of the chloroform   (Fig. 6), which is able to form a macromolecular chain due to carbon-carbon multiple bonds.
In the IR spectrum of both PMMA obtained during AIBN initiation and the product isolated from chloroform (Fig. 7a) characteristic absorption bands are observed in the region of 1720-1730 cm −1 , corresponding to the valence vibrations of the carboxyl group C = O. It confirms the presence of the MMA polymer structure, presented in the mixture, and the structure of the compound obtained because of polymerization of the product of MMA oxidative dimerization (Fig. 3).
The obtained experimental data allow us to conclude that the MMA transformations in the reaction mixture apparently take place in several directions: • PMMA formation in the emulsion due to the initiation of polymerization by a hydroxyl radical (the usual PMMA formation with Mn ~ 140-145 kDa): • PMMA formation by grafting on the RbTe 1.5 W 0.5 O 6 surface (Fig. 3), which can be explained by the following reasons. Metal oxides always have OH-groups on the surface, which can possess significantly different properties depending on the metal nature [66,67]. In this case, the hydroxyl radicals formed during photoinitiation can interact with the surface OH-groups that lead to appearance more stable oxygen-centered radical on the oxide surface, which can also initiate polymerization. The lifetime of the hydroxyl radical is very short, so the polymer formation path due to grafting on the oxide surface becomes preferred. The polymer formed on the surface prevents the release of radicals from the powder surface into the reaction mixture volume; • The monomer interacts with the complex oxide RbTe 1.5 W 0.5 O 6 and forms a coordination complex due  [68][69][70] is equiprobable, and MMA oxidation to a dimer (Fig. 6) by the catalyst RbTe 1.5 W 0.5 O 6 can be assumed. (Fig. 8) The products of the MMA conversion under complex oxide RbTe 1.5 W 0.5 O 6 irradiation partially allocated by extraction with chloroform in the Soxhlet extractor. Extracted products are mixture of PMMA and a polymer product of MMA oxidative dimerization, which is formed as shown in scheme (Fig. 9) according to GPC, NMR and mass spectrometry MALDI data.
The presented data confirm the well-known ideas that hydroxyl radicals are very active in the separation reactions of hydrogen atoms from C-H and O-H bonds. Accordingly, they should be highly active relative to all such bonds, for example, in natural polymers, proteins and polysaccharides.
The research of MMA graft copolymerization using the RbTe 1.5 W 0.5 O 6 photocatalytic material was carried out on the example of pectin polysaccharide.
MMA grafting on pectin was performed under conditions similar to MMA polymerization at 20-25 0 C. Analysis of the polymer product isolated from the aqueous phase after synthesis indicates the formation of a graft copolymer PMMA-pectin. The product weight from the aqueous phase after synthesis in comparison with the weight of the initial pectin increased and the grafting efficiency was 15-20%. The molecular weight parameters of the copolymer in comparison with the initial pectin have been noticeable changed ( Table 2).     pectin and PMMA indicates that all the characteristic bands of pectin and PMMA are observed, which is an additional confirmation of the graft copolymer formation. The SEM analysis revealed morphological differences between the initial pectin and the graft copolymer pectin-PMMA, indicating the inclusion of synthetic polymer fragments in the fibrillar organization of pectin. The pectin sponge has clear outlines of fibrils and formed pores (Fig. 2d), while the picture of the PMMA-pectin graft copolymer sponge clearly shows a more complex structural and topological organization between the pectin fibers (Fig. 2e).

Conclusion
It was found that several directions of monomer transformations occur simultaneously in the reaction mixture as a result of methyl methacrylate radical polymerization in an aqueous emulsion using the RbTe 1.5 W 0.5 O 6 complex oxide as a photoinitiator under visible light irradiation λ = 400-700 nm. Polymethyl methacrylate, characterized by Mn ~ 140-145 kDa, is formed in the organic phase because of the initiation of polymerization by a hydroxyl radical. Moreover, oxygen-centered radicals are formed on the RbTe 1.5 W 0.5 O 6 complex oxide surface, which grafts polymer macromolecules onto the photocatalyst surface by the interaction of hydroxyl radicals with OH-groups. At the same time, the monomer interacts with the RbTe 1.5 W 0.5 O 6 complex oxide and forms a coordination complex by the double bonds, while its oxidation occurs to a cyclic dimer with terminal double bonds.
The obtained data indicate that significant differences were revealed between radical transformations involving MMA using a complex oxide photoinitiator RbTe 1.5 W 0.5 O 6 under visible light irradiation λ = 400-700 nm from the usual radical polymerization of MMA in the presence of material initiators, when only the formation of PMMA with a set of certain characteristics is formed.
The high activity of the hydroxyl radical is confirmed by grafting the polymer product onto the surface of a natural polymer-pectin polysaccharide with the graft copolymer formation. A new structural and topological organization of the graft copolymer PMMA-pectin in comparison with the original pectin was established by the SEM method comparing the samples sponges. This morphological feature is the basis for conducting research on its properties as a material for the scaffolds. The absence of initiator fragments with organic nature, which is characteristic of polymers with organic material initiation, is important in this case.
Funding This work was supported by the Ministry of Education and Science of the Russian Federation (assignment 0729-2020-0053) on the equipment of the Collective Usage Center "New Materials and Resource-saving Technologies" (Lobachevsky State University of Nizhny Novgorod).

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Conflicts of interest
The authors have no conflicts of interest to declare that are relevant to the content of this article.