RAFT polymerization of MMA in channels of different mesoporous materials

A series of mesoporous silica (MS) materials, including rod-like and spherical SBA-15, spherical MCM-41, and MCM-48, was successfully synthesized and modified by silane coupling agent γ-aminopropyltriethoxysilane. The modified MS was utilized as a microreactor for solution reversible addition–fragmentation transfer (RAFT) polymerization of methyl methacrylate (MMA) using xanthate as a chain transfer agent and azodiisobutyronitrile as an initiator. Notably, modified MS had a low specific surface area and pore volume while maintaining the original morphological structure. Given to the confinement effects, poly(methyl methacrylate) (PMMA) prepared from internal mesoporous channels had higher molecular weight and initial thermal decomposition temperature than those from conventional RAFT polymerization. Moreover, PMMA obtained from spherical SBA-15 and modified MCM-48 exhibited the highest molecular weight (Mn = 8.78 × 104 and Mn = 7.43 × 104, respectively). This work provided a novel approach in designing a polymer microstructure for a more comprehensive application, such as in drug release.

In general, living radical polymerization is better than the traditional one under constant conditions because the latter limits polymer's molecular weight, polydispersity, and constructive multiplicity. However, living radical polymerization suffers from a series of weaknesses, including an expensive and noxious irreplaceable reagent, a few viable monomers, harsh polymerization conditions, and quite troublesome postprocessing of products (Corrigan et al. 2020).
In enhancing the efficiency and decreasing the cost, a novel type of living radical polymerization has emerged, namely, reversible addition-fragmentation transfer (RAFT) polymerization. The RAFT strategy exhibits a reversible transfer process between the active and dormant chains, which are controlled by a chain transfer agent with a functional group, yielding a powerful polymerization control. After immense exploration in reaction by the academia, RAFT polymerization seems to be the most versatile strategy regarding monomer selection, polymeric configuration, and reaction condition among a number of living radical polymerizations (Gregory and Stenzel 2012;Keddie 2014). Depending on the solubility of the monomer or polymer in the reaction medium, RAFT polymerization can be carried out either through homogeneous polymerization or heterogeneous polymerization (Zhang et al. 2014a, b;Pereira et al. 2020;Han et al. 2021).
MS provides an independent and restrictive space, which can lead free radicals to grow smoothly and control the termination of reaction in a better way (Nothling et al. 2020;Truong et al. 2021). An enormous deal of modifiable MS has been reported in the literature for diverse reversible deactivation radical polymerization to produce superb and relevant products. Xu et al. (2020) selected mesoporous SBA-15 with various pore sizes as microreactors to conduct a methyl methacrylate (MMA) reaction by RAFT polymerization using azodiisobutyronitrile as an initiator and ethyl xanthate ethyl propionate as a chain transfer agent. Chen et al. (2018) utilized mesoporous SBA-15 as a carrier for grafting functional monomer glycidyl methacrylate (GMA) via activators regenerated by the electron transfer atom transfer radical polymerization. The result shows that poly(glycidyl methacrylate) brushes grafted SBA-15 to serve as a talented reactive platform for more profound surface modification or functionalization. An extraordinary integration was given to the MS nanoparticles and RAFT agent (Joshi and Nebhani 2022). Zeng et al. (2020) selected three monomers (MMA, GMA, and styrene) with different polarities to react in the interior of SBA-15 and probe the confinement dimension of the resulting polymer properties. Moreover, an alternative stepwise approach for co-condensation in hexadecyl trimethyl ammonium bromide was applied, and MS nanoparticles containing RAFT agent were further used for the surfaceinitiated RAFT polymerization of several monomers. Begum and Simon (2011) modeled the confinement of MMA free radical polymerization and discovered that tighter confinement resulted in higher molecular weight and more other impacts in the created PMMA. These observations open a new gate and have attracted much attention to the effects of confined space on RAFT polymerization.
The morphology of MS is an important factor influencing their macroscopic application because of the shapeselective and confinement effects. Notably, each MS has unique advantages. In particular, spherical MS has great stability and ruggedness, whereas rod-like MS has an order characteristic. Du et al. (2020) designed a facile method to control and fabricate mesoporous carbon (MC) materials with different morphologies, and they confirmed that MC with a regular spherical morphology showed remarkable adsorption capacity, indicating its application potential in treating wastewater. Dan et al. (2016) prepared a class of SBA-15 to adsorb uranium from aqueous solution and confirmed that the platelet-like morphology of SBA-15 exhibited absolutely rapid and high capacity in uranium adsorption. Furthermore, the MS, as a microreactor; the type of monomer-wrapped internal; and the MS, which supplied an immune location for the former, are fairly important conditions for mesoporous materials during polymerization. Moritz and Geszke-Moritz (2020) synthesized three typical mesoporous materials (SBA-15, PHTS, and MCM-41) functionalized with a sulfonic acid derivative, which were applied successfully as carriers for a poorly watersoluble drug. However, RAFT polymerization in different MS is rarely reported. We are interested in how different MS interact with limited space to affect RAFT polymerization. Specifically, we want to comprehend how the confinement spaces affect the properties of the polymer.
Herein, a novel type of RAFT polymerization was demonstrated in a confined space such as various MS. The solution RAFT polymerization of MMA in different MS (rod-like and spherical SBA-15, spherical MCM-41 and MCM-48, and silane coupling agent functionalized SBA-15) was performed to comprehensively understand the effect of MS on polymerization. Furthermore, the polymers obtained from the internal channels of MS were characterized.

Preparation of rod-like SBA-15
Typically, 2.5 g of P123 was dissolved in 2 mol/L HCl at 35 °C and stirred at 300 r/min for at least 6 h. When the dissolution was completed, 5.5 g of TEOS served as a silica source was added dropwise within 10 min and stirred for 24 h. Subsequently, the solution was put into a 100 mL stainless steel hydrothermal reactor and crystallized at 100 °C for 24 h. The product was extracted, washed, and dried in a constant temperature drying oven at 40 °C. Finally, the white powder was put into a muffle furnace and calcined at 2 °C/min to 550 °C for 6 h. The template agent was removed by roasting to obtain the rodlike SBA-15 (R-SBA-15) (Gao and Duan 2015).

Preparation of spherical SBA-15
4.2 g of P123 and 3.0 g of KCl were dissolved in 2 mol/L HCl at 35 °C, and the solution was stirred for 6 h (300 r/ min). After the P123 was fully dissolved, the solution was cooled down to room temperature. Then, 3.0 g of TMB was added to the solution, and the reaction was continuously stirred for 12 h. Subsequently, 8.6 g of TEOS was slowly added drop by drop to the reaction. The white powder was finally calcined in a muffle furnace at 2 °C/min and held for 6 h at 550 °C. The spherical SBA-15 (S-SBA-15) was obtained (Zhao et al. 2000).

Preparation of spherical MCM-41
3.75 g CTAB was added to 71 mL of water and stirred at room temperature until completely dissolved, and then, 25.8 g of ammonia water and 90 g of ethanol were added and stirred for 30 min (500 r/min). After that, 7.5 g of TEOS was added dropwise within 10 min, and the mixture was maintained at 25 °C. After the reaction was stirred for 3 h (300 r/min), the product was dried in a thermostat at 60 °C for 24 h. Finally, the white product was calcined in a muffle furnace at 2 °C/min to 550 °C for 5 h to obtain spherical MCM-41 (Zhao et al. 1998).

Preparation of spherical MCM-48
2.4 g (6.59 mmol) of CTAB was dissolved in 50 g of deionized water, and then, 50 mL of ethanol and 12 mL of ammonia water were added and stirred at room temperature for 5 h (300 r/min). After that, 3.4 g (16.3 mmol) of TEOS was added dropwise within 10 min and stirred for 2 h (450 r/min). Subsequently, the mixture was crystallized for 24 h and filtered. The spherical MCM-48 was obtained by slowly increasing the temperature (2 °C/min) to 600 °C for 6 h and removing the template agent (Schumacher et al. 1999).

Functionalization of MS
The different R-SBA-15, S-SBA-15, MCM-41, and MCM-48 were modified with silane coupling agent KH550 in a typical synthesis (Wei et al. 2013). 1.0 g of MS activated by 2 mol/L dilute hydrochloric acid was dispersed in 15 mL toluene and treated by ultrasonication for 30 min at room temperature. Afterward, the solution was filtered and then washed 4-5 times with toluene in order to remove the silane coupling agent and dried at 60 °C for 24 h to obtain modified MS (denoted as M-MS).

RAFT polymerization of MMA in different MS
RAFT polymerization of MMA was conducted in different MS microreactor. MS (0.5 g) was added into a 50 mL welldried flask and treated by vacuum degassing at 150 °C for 5 h. Then MS was cooled down to room temperature under vacuum. The mixture composed of toluene (7 mL, solvent), MMA (5.2 mL, 0.049 mol, monomer), AIBN (0.0164 g, 0.1 mmol, initiator), and ethyl xanthate ethyl propionate (93 mL, 0.5 mmol, chain transfer) was added into the flask. The flask remained stirring in oil bath at 80 °C for 12 h under nitrogen protection. After the reaction, the product was cooled down to room temperature, isolated by filtration, and then extracted with THF for 12 h to remove the polymer

Etching of the composites with HF to obtain the internal PMMA
The composites MS/PMMA and M-MS/PMMA were treated by chemical etching in a 5% HF aqueous solution for 24 h, followed by centrifugal separation, washing with distilled water three times, and drying in a vacuum oven at 60 °C. Pure PMMA was obtained.

Characterization
A field-emission scanning electron microscopy (SEM, JSM-6380LV from Japan Electronics Co., Ltd) was used to observe the morphology of the mesoporous silica and MS/PMMA composites at 5-10 kV after spraying the specimens with gold. Nitrogen adsorption-desorption isotherms curves were obtained using a Micrometrics TriStar II 3020. Samples were degassed under vacuum at 200 °C (for silica) or 120 °C (for the composites) before adsorption measurements. The specific surface area was determined using the BET method. The total pore volume was calculated from the amount adsorbed nitrogen at a relative pressure of 0.99. The pore size distribution was calculated using the BJH method for cylindrical mesopores. The thermal decomposition behaviors of the MS, MS/PMMA, M-MS/PMMA composites, and PMMA obtained from MS were examined by means of thermogravimetric analysis (TGA) with a heating rate of 10 K/ min in nitrogen atmosphere on a TA Q500 (USA). The structures of the samples were characterized on a Nicolet-205 Fourier transform infrared spectrometer (FT-IR) from 400 to 4000 cm −1 by the KBr tablet methods and hydrogen nuclear magnetic resonance ( 1 H NMR) spectra were taken on an Avance 500 MHz spectrometer (Bruker, Switzerland) using CDCl 3 as solvent and tetramethylsilane (TMS) as internal standard. The molecular weight and molecular weight distribution (MWD) of the obtained pure PMMA were measured at 35 °C on a Malvern Model 270 gel permeation chromatograph (GPC) equipped with T6000 microstyragel columns, refractive detector, light scattering detector, and viscosity detector, THF was used as an eluent at a flow rate of 1.0 mL min −1 and polystyrene standards were used for calibration, pump pressure of 15 psi. The monomer conversion was determined by gravimetric method.

Results and discussion
Morphology Figure 1 shows SEM images of the different MS and MS/ PMMA composites. As shown in Fig. 1, different morphologies and types of MS are successfully prepared, namely, rodlike R-SBA-15, spherical S-SBA-15, spherical MCM-41, and MCM-48. The length of rod-like SBA-15 is above 1 μm, and the width is approximately 0.3-0.5 μm. In addition, the particle size of spherical SBA-15 is approximately 3 μm. On the contrary, the particle sizes   As shown in Fig. 2a, the hysteresis loop of R-SBA-15/ PMMA is smaller than that of R-SBA-15, and the adsorption-desorption temperature line belongs to type IV isotherms with an H1 hysteresis loop. By contrast, the adsorption-desorption isotherm curves of M-R-SBA-15 and M-R-SBA-15/PMMA belong to type-II isotherms with a decrease in specific surface area and pore volume. The results indicate that MMA was successfully adsorbed into the mesoporous pore channel and that polymerization occurred (Liu et al. 2016). In addition, the nitrogen adsorption/desorption curves shown in Fig. 2c demonstrate that the isotherms of S-SBA-15 and M-S-SBA-15 belong to type IV with H1 lagging loop, and the isotherms of S-SBA-15/ PMMA and M-S-SBA-15/PMMA belong to type IV with H1 hysteresis loop. However, the adsorption volume decreases, and the lagging loop becomes smaller, demonstrating that the mesoporous structure of S-SBA-15 and M-S-SBA-15 is retained, and some of the pores are filled with PMMA. Moreover, S-SBA-15/PMMA and M-S-SBA-15/PMMA composites show a decrease in specific surface areas and pore volumes, whereas the average porosity calculated based on the BJH model shows a slight increase, which is probably due to the generated polymer filling the micropores in S-SBA-15 (Serre et al. 2002).

Nitrogen adsorption-desorption isotherms
The isothermal adsorption curves for MCM-48 and MCM-41 are of type IV with a H4 lagging loop, which are due to the small pore size of these two MS and the presence of slight defects in the mesopore channels (Pérez-Quintanilla et al. 2009). Considering that the surfaces of the mesopore channels have been modified by organic functional groups, the specific surface areas and pore TG and DTG curves of PMMA in MS and C-PMMA by conventional RAFT polymerization are further investigated. Figure 4 shows the three stages of thermal weight loss peaks for C-PMMA. The first stage of weight loss occurs between 145 and 210 °C, which is due to double-bond breakage resulting from the termination of radical disproportionation; the second stage of weight loss occurs between 260 and 1 3 320 °C, which attributes to the decomposition of the shortchain PMMA generated by coupling termination, and the third stage of weight loss occurs between 320 and 420 °C, which results from the random chain breakage of the PMMA main chain. Compared with the C-PMMA, the initial thermal decomposition temperature of the PMMA obtained from internal pore polymerization is higher. In particular, the initial and termination thermal decomposition temperatures of the products from polymerization in M-MS are increased by approximately 80-100 °C. The thermal decomposition of the polymer backbone is primarily present, whereas the less weight loss of the short-chain PMMA generated by disproportionation termination occurs because the diffusion of free radicals is inhibited by the restricted effect of the pore channel, thereby suppressing the chain termination reaction of RAFT solution polymerization. The amount of termination in RAFT polymerization under normal conditions is minimal, and most chains have the RAFT chain end. The finding of significant amounts of termination may be due to high initiator concentrations. It indicates that the polymer with different thermal stability can be obtained by selecting different confined spaces.

FT-IR analysis
The structure of the obtained MS and M-MS and their composites were confirmed by FT-IR spectroscopy (Figs. 5 and 6). Figure 5a shows that the M-MS has new characteristic peaks at 3356 and 1577 cm −1 , which correspond to -NH 2 and -NH-compared with pure MS, indicating that KH550 successfully modified the MS (Kruk et al. 2000). As shown in Fig. 5(b, c), MS/PMMA and M-MS/PMMA composites not only retain the MS absorption peaks at 3430, 1080, 796, and 461 cm −1 of the MS corresponding to the stretching and bending vibrations of Si-OH, Si-O-Si, and Si-O bonds, but also reveal the presence of the SBA-15 framework (Choi et al. 2005;Zhang et al. 2014a, b). In addition, new peaks appear at 2953 and 1731 cm −1 , which represent the stretching vibration of -CH 3 and -C = O of the PMMA in the composite, indicating that the MMA has been adsorbed into the pore and has undergone polymerization (Chen et al. 2017).
After etching MS/PMMA and M-MS/PMMA composites, the resulting product PMMA shows the same characteristic peaks as the PMMA obtained by conventional RAFT polymerization of MMA (Fig. 6). In addition, the FT-IR spectra of polymers obtained from within and external of the pores show no absorbance at 1089 cm −1 , which is the

GPC analysis
The molecular weight and molecular weight distribution (MWD) of the PMMAs obtained from the RAFT polymerization inside the pores of the MS and M-MS and the conventional RAFT polymerization were investigated. As shown in Fig. 7 and Table 2, the PMMAs obtained by the RAFT polymerization inside the channel of MS and M-MS exhibit a several times higher molecular weight than the C-PMMAs, which might be due to the lower chance of termination of free radicals within the restricted pore channels (Ng et al. 1997). The most significant increase in number-average molecular weight (M n ) is obtained within the pore channels of spherical SBA-15, which increased by 13.9 times. The molecular weight of the PMMAs obtained from the R-SBA-15 and S-SBA-15 pores was larger than those of the modified M-SBA-15 but featured smaller MWD, which might be due to the effect of the silane coupling agent on the surface of the modified SBA-15 (Lee et al. 2018). The PMMAs obtained inside the unmodified MCM-41 and MCM-48 pores have lower molecular weight and larger MWD than those of the modified MCM-41 and MCM-48, although the MWD remained in the narrow category.
In addition, if the monomer was fully polymerized, then the theoretical M n would be determined to be approximately 50.  Although the xanthate may not be the best option in this situation for polymerization, the confinement spaces were utilized to boost polymerization. Meanwhile, the kinetics of the evaluated systems in the channels of R-SBA-15 (Fig. 8) was investigated to confirm the activity of endgroups in mesoporous materials, and the results showed that the molecular weight and conversion increased over time. Early on, PMMAs with a high molecular weight can be obtained, and then, the molecular weight slowly grew. These findings indicate that the molecular weight is significantly influenced by polymerization occurring inside smaller pores in the early period, and as polymerization progresses, long polymer chains become more difficult to disperse for chain expansion as pore volume decreases.

H NMR analysis
Typical 1 H NMR spectra of C-PMMA and PMMA in R-SBA-15 are depicted in Fig. 9 and others are depicted in SI. The peaks at 0.81-1.27, 1.60-1.90, and 3.60 ppm are assigned to the chemical shifts of protons in the methyl, methylene, and methoxy group of PMMA, respectively. In addition, triplet peaks at 0.84-1.25 ppm are indexed to the proton of syndiotactic, atactic, and isotactic in α-CH 3 with the direction of the shift value increase, and stereoregularity data of all PMMA are summarized in Table 3. A relevant nanochannel effect on the polymer stereoregularity is observed in our polymerization system. Given the  three-dimensional interchanged pore structure of MCM-48, PMMA in MCM-48 has the most significant reduction in the proportion of syndiotactic structures, yielding the highest isotactic PMMA (Bandyopadhyay et al. 2017). On the contrary, the sequence distribution of PMMA in R-SBA-15 does not change much because of their large pore sizes and different wall thicknesses (Xu et al. 2020).
The result further confirms the confinement effect of MS on monomer polymerization.

Conclusions
RAFT polymerization of MMA inside the pore channels of MS or M-MS was successfully conducted. The resulting composites retain their original morphological structure, but the specific surface areas and pore volumes of the M-MS and its composites are reduced. Compared with C-PMMA obtained using the conventional RAFT solution polymerization, the molecular weight of the PMMA inside the pore channel is several times or even a dozen times higher than that of the conventional C-PMMA. Notably, the number-average molecular weight of the PMMA obtained inside the S-SBA-15 pore channel increased by 13.9 times. Moreover, the MWD remained at a narrow range, and the isotactic ratio of the polymer increased significantly. The initial thermal decomposition temperature was significantly increased. In particular, the decomposition temperature of PMMA obtained in the modified MS was increased by 80-100 °C. The RAFT polymerization in confined space not only controls the molecular weight of the polymer, but also improves the performance of the polymer. This work proves that different confined spaces exert a significant influence on RAFT polymerization and provides a novel method for designing and creating polymer microstructures for a variety of uses, such as in drug release.