Main Chain–Type Block Copolymers through Atom Transfer Radical Polymerization from Double-Decker–Shaped Polyhedral Oligomeric Silsesquioxane Hybrids

In this study, we synthesized two main chain–type block copolymers featuring hydrogen bond donor and acceptor segments through atom transfer radical polymerization (ATRP) using a bifunctionalized polyhedral oligomeric silsesquioxane (POSS) nanoparticle as the initiator. Hydrosilylation of vinylbenzyl chloride at the two corners of a double-decker silsesquioxane (DDSQ) provided the bifunctionalized benzyl chloride initiator VBC-DDSQ-VBC, which we applied as a platform to prepare a main chain–type polystyrene homopolymer (PS-DDSQ-PS), the diblock copolymer poly(styrene–b–4-vinylpyridine) (P4VP-b-PS-DDSQ-PS-b-P4VP), and the diblock copolymer poly(styrene–b–tert-butoxystyrene) (PtBuOS-b-PS-DDSQ-PS-b-PtBuOS) through sequential ATRP. Selective hydrolysis of the tert-butoxyl units of PtBuOS-b-PS-DDSQ-PS-b-PtBuOS yielded the strongly hydrogen bonding diblock copolymer poly (styrene-b-vinylphenol) (PVPh-b-PS-DDSQ-PS-b-PVPh). We used Fourier transfer infrared spectroscopy, nuclear magnetic resonance spectroscopy, size exclusion chromatography, differential scanning calorimetry, mass-analyzed laser desorption ionization mass spectrometry, and transmission electron microscopy to investigate the chemical structures, thermal behavior, and self-assembled nanostructures formed by these main chain–type block copolymers based on DDSQ.


Introduction
The self-assembled nanostructures (e.g., cylindrical, lamellar, spherical, and double-gyroid structures) that form from immiscible diblock copolymers (BCPs) have received much attention because of their potential applications in, for example, nano-patterning, drug delivery, and photonic crystals [1][2][3][4][5]. The blending of a homopolymer or a BCP into another BCP matrix, to mediate the

Bis(4-Vinylbenzyl chloride) Double-Decker Silsesquioxane (VBC-DDSQ-VBC)
A mixture of DDSQ (5 g), toluene (5 mL), VBC (1.33 mL) and Pt (dvs) (0.01 mL) were added and then the mixture was heated at 75 °C with stirring for 48 h to complete hydrosilylation. The mixture was filtered through charcoal and then the filtrate was concentrated through vacuum distillation. The residue was dissolved in CH2Cl2 and extracted three times with a H2O/MeOH mixture. After drying (MgSO4, stirring for 30 min), the mixture was filtered to give a clear filtrate, which was dried in a vacuum oven for 24 h at 60 °C to give a white solid [Scheme 1d] (yield: 82%).

PS-DDSQ-PS
A mixture of VBC-DDSQ-VBC (0.380 g), CuBr (0.108 g), and THF (5 mL) was placed into a flask equipped with a reflux condenser and vacuumed by liquid N2 three times to deplete water. Styrene (12.0 mL) and PMDETA (0.162 mL) were added and then the mixture was stirred at 110 °C for 6 h. The solution was evaporated through vacuum filtration where alumina oxide was packed to filter Cu ions. The resulting clear solution was concentrated through vacuum distillation and re-precipitated from MeOH to obtain a white filamentous material. The mixture was evaporated through vacuum filtration to obtain a white powder, which was dried in a vacuum oven for 24 h at 60 °C to provide PS-DDSQ-PS as a white solid [Scheme 1e].

P4VP-b-PS-DDSQ-PS-b-P4VP
The mixture of PS-DDSQ-PS (0.5 g) and CuBr (0.0195 g) was placed in a flask equipped with a reflux condenser and vacuumed by liquid N2 three times to deplete water. 4-Vinylpyridine (8 mL) and PMDETA (0.03 mL) were added and then the mixture was stirred at 110 °C for 8 and 12 h,

Bis(4-Vinylbenzyl chloride) Double-Decker Silsesquioxane (VBC-DDSQ-VBC)
A mixture of DDSQ (5 g), toluene (5 mL), VBC (1.33 mL) and Pt (dvs) (0.01 mL) were added and then the mixture was heated at 75 • C with stirring for 48 h to complete hydrosilylation. The mixture was filtered through charcoal and then the filtrate was concentrated through vacuum distillation. The residue was dissolved in CH 2 Cl 2 and extracted three times with a H 2 O/MeOH mixture. After drying (MgSO 4 , stirring for 30 min), the mixture was filtered to give a clear filtrate, which was dried in a vacuum oven for 24 h at 60 • C to give a white solid [Scheme 1d] (yield: 82%).

PS-DDSQ-PS
A mixture of VBC-DDSQ-VBC (0.380 g), CuBr (0.108 g), and THF (5 mL) was placed into a flask equipped with a reflux condenser and vacuumed by liquid N 2 three times to deplete water. Styrene (12.0 mL) and PMDETA (0.162 mL) were added and then the mixture was stirred at 110 • C for 6 h. The solution was evaporated through vacuum filtration where alumina oxide was packed to filter Cu ions. The resulting clear solution was concentrated through vacuum distillation and re-precipitated from MeOH to obtain a white filamentous material. The mixture was evaporated through vacuum filtration to obtain a white powder, which was dried in a vacuum oven for 24 h at 60 • C to provide PS-DDSQ-PS as a white solid [Scheme 1e].

P4VP-b-PS-DDSQ-PS-b-P4VP
The mixture of PS-DDSQ-PS (0.5 g) and CuBr (0.0195 g) was placed in a flask equipped with a reflux condenser and vacuumed by liquid N 2 three times to deplete water. 4-Vinylpyridine (8 mL) and PMDETA (0.03 mL) were added and then the mixture was stirred at 110 • C for 8 and 12 h, respectively. The solution was evaporated through vacuum filtration where alumina oxide was packed to filter Cu ions. The resulting clear solution was concentrated through vacuum distillation and re-precipitated in hexane to obtain a filamentous material. The mixture was evaporated through vacuum filtration to obtain a powder, which was dried in a vacuum oven for 24 h at 60 • C to give P4VP-b-PS-DDSQ-PS-b-P4VP. The mixture of PS-DDSQ-PS (0.25 g) and CuBr (0.01 g) was placed in a flask equipped with a reflux condenser and vacuumed by liquid N 2 three times to deplete water. 4-tert-Butoxystyrene (6.0 mL) and PMDETA (0.015 mL) were added and the mixture vacuumed by liquid N 2 again. After stirring at 110 • C for 24 h, the solution was evaporated through vacuum filtration where alumina oxide was packed to filter Cu ions. The resulting clear solution was evaporated through vacuum distillation and re-precipitated from MeOH to obtain a filamentous material. The mixture was evaporated through vacuum filtration to obtain a powder, which was dried in a vacuum oven at 60 • C for 24 h, providing the block copolymer

Characterization
FTIR spectra were recorded at room temperature at a resolution of 4 cm −1 within the range from 4000 to 400 cm −1 from 32 scans, using a Nicolet 320 FTIR spectrometer and the typical KBr pellet method. NMR spectra were recorded using an INOVA 500 instrument with CDCl 3 or DMSO-d 6 as the solvent. The molecular weights of the DDSQ-based PS block copolymers were evaluated from their 1 H NMR spectra; their polydispersity indices (PDIs) were determined using GPC (Waters 510 apparatus) where the molecular weight calibration was used by PS standard and MALDI-TOF mass spectrometry (Bruker Daltonics Autoflex MALDI-TOF mass spectrometer). The thermal properties of the DDSQ-based block copolymers were determined through DSC using a TA Q-20 instrument; the sample was placed in an aluminum pan and heated at a rate of 20 • C min −1 from room temperature to 250 • C under a N 2 atmosphere (50 mL/min) after the sample was cooled quickly to −90 • C from the first scan. TEM images were recorded using a JEOL 2100 microscope (Japan) operated at 200 kV; ultrathin films were prepared using a Leica Ultracut microtome featuring a diamond knife and placed on a Cu grid coated with a carbon film. The P4VP-b-PS-DDSQ-PS-b-P4VP thin film was imaged after staining with I 2 to display its P4VP segments; the PVPh-b-PS-DDSQ-PS-b-PVPh thin film was stained with RuO 4 to display its PVPh segments.

Synthesis of the Homopolymer PS-DDSQ-PS
Scheme 1a-c displays the synthesis of the PS-DDSQ-PS homopolymer and the corresponding chemical structures of the DDSQ compounds, as confirmed using FTIR, 1 H NMR spectroscopy and MALDI-TOF mass spectrometry. Figure 1 provides the FTIR and 1 H NMR spectra of each compound obtained during the preparation of the PS-DDSQ-PS homopolymer. The FTIR spectrum of each DDSQ compound featured a strong signal for the Si-O-Si units at 1097 cm −1 , with weak signals at 1261 cm −1 for the SiCH 3 units of DDSQ [ Figure 1A,a] and VBC-DDSQ-VBC [ Figure 1A,b]. Furthermore, a signal at 2172 cm −1 , representing Si-H stretching, appeared for pure DDSQ [ Figure 1A,a], but it disappeared after the hydrosilylation of VBC with DDSQ [ Figure 1A,b], indicating the success of the reaction. The 1 H NMR spectrum of DDSQ [ Figure 1B,a] featured signals for the aromatic protons at 7.50-7.14 ppm, the SiCH 3 units at 0.36 ppm, and the SiH groups at 4.98 ppm. The signal for the SiH groups was     Next, we synthesized PS-DDSQ-PS from the initiator VBC-DDSQ-VBC through ATRP [Scheme 1e]. We performed this ATRP with CuBr as the catalyst and PMEDTA as the ligand at 110 • C for 6 h; the conversion of the styrene monomer was 78% and the degree of polymerization (DP) was 112. Figure 1c presents the FTIR and 1 H NMR spectra of the PS 56 -DDSQ-PS 56 hybrid polymer; signals for the aromatic ring in the former appeared at 3030 and 1600 cm −1 [ Figure 1A,c] while in the latter the aromatic protons appeared at 6.36-7.10 ppm and the main chain CH and CH 2 protons appeared at 1.85 and 1.41 ppm [ Figure 1B,c], indicating the successful ATRP yielding the PS-DDSQ-PS hybrid polymer. Figure 3A reveals shifts in the SEC distributions to lower retention times, indicating that the molecular weight increased upon increasing the reaction time, consistent with the FTIR spectra in Figure 3B of PS-DDSQ-PS hybrid polymers having PS segments of different molecular weights. Here, the intensity of the signal for Si-O-Si stretching of DDSQ was maintained, but the intensity of the corresponding absorptions of the PS segments at 3030, 1060, and 696 cm −1 all increased upon increasing the molecular weight of the PS segment [48].  Figure 2 displays the MALDI-TOF mass spectra of DDSQ and VBC-DDSQ-VBC, revealing their monodisperse mass distributions: [DDSQ + Na] + at 1175 g/mol and [VBC-DDSQ-VBC + Na] + at 1486 g/mol, consistent with their expected structures. Thus, the FTIR, NMR, and mass spectra confirmed the synthesis of the VBC-DDSQ-VBC initiator for ATRP.  Next, we synthesized PS-DDSQ-PS from the initiator VBC-DDSQ-VBC through ATRP [Scheme 1e]. We performed this ATRP with CuBr as the catalyst and PMEDTA as the ligand at 110 °C for 6 h; the conversion of the styrene monomer was 78% and the degree of polymerization (DP) was 112. Figure 1c presents the FTIR and 1 H NMR spectra of the PS56-DDSQ-PS56 hybrid polymer; signals for the aromatic ring in the former appeared at 3030 and 1600 cm −1 [ Figure 1A,c] while in the latter the aromatic protons appeared at 6.36-7.10 ppm and the main chain CH and CH2 protons appeared at 1.85 and 1.41 ppm [ Figure 1B,c], indicating the successful ATRP yielding the PS-DDSQ-PS hybrid polymer. Figure 3A reveals shifts in the SEC distributions to lower retention times, indicating that the molecular weight increased upon increasing the reaction time, consistent with the FTIR spectra in Figure 3B of PS-DDSQ-PS hybrid polymers having PS segments of different molecular weights. Here, the intensity of the signal for Si-O-Si stretching of DDSQ was maintained, but the intensity of the corresponding absorptions of the PS segments at 3030, 1060, and 696 cm −1 all increased upon increasing the molecular weight of the PS segment [48].   Table 1 summarizes the molecular weights and PDIs of our PS-DDSQ-PS hybrid polymers. The well-defined structure of the PS20-DDSQ-PS20 hybrid polymer was confirmed from its MALDI-TOF mass spectrum (Figure 4), which presented only one apparent mass distribution. The mass difference between each pair of adjacent peaks was 104.15 g mol −1 , equal to the expected mass of the repeat unit of a PS segment. Therefore, the NMR spectra, FTIR spectra, GPC traces, and mass spectra all confirmed the successful ATRP of the PS-DDSQ-PS hybrid polymers.   Table 1 summarizes the molecular weights and PDIs of our PS-DDSQ-PS hybrid polymers. The well-defined structure of the PS 20 -DDSQ-PS 20 hybrid polymer was confirmed from its MALDI-TOF mass spectrum (Figure 4), which presented only one apparent mass distribution. The mass difference between each pair of adjacent peaks was 104.15 g mol −1 , equal to the expected mass of the repeat unit of a PS segment. Therefore, the NMR spectra, FTIR spectra, GPC traces, and mass spectra all confirmed the successful ATRP of the PS-DDSQ-PS hybrid polymers.

Synthesis of PtBuOS-b-PS-DDSQ-PS-b-PtBuOS and PVPh-b-PS-DDSQ-PS-b-PVPh
Scheme 1d-f summarizes the preparation of block copolymers based on DDSQ hybrids through ATRP. We used FTIR and 1 H NMR spectra ( Figure 5 Figure 5A,a], that of the hybrid block copolymer PtBuOS-b-PS-DDSQ-PS-b-PtBuOS [ Figure 5A,b] exhibited additional signals at 1243, 1163, and 905 cm −1 corresponding to the C-O and C-O-C stretching and C-H bending, respectively. Furthermore, the signal at 1.56 ppm in the 1 H NMR spectrum of the hybrid block copolymer PtBuOS78-b-PS91-DDSQ-PS91-b-PtBuOS78 [ Figure 5B,b], corresponding to the tert-butyl (C4H9) protons of the PtBuOS block segment, confirmed the successful ATRP of the tBuOS groups to give the PS-DDSQ-PS hybrid polymer. This signal at 1.56 ppm for the PtBuOS block segment essentially disappeared after treatment with N2H4, with a new peak appearing at 8.93 ppm representing the OH units of the PVPh block segment [ Figure 5B,c],. Furthermore, the hydrolysis of the tert-butyl groups was evidenced by the signals at 1163 and 905 cm −1 decreasing in intensity, with a signal representing the presence of phenolic OH units for the PVPh block segment at 3400 cm −1 , [ Figure 5A,c],due to the self-association of hydrogen bonding units [42], suggesting the successful hydrolysis and the formation of the hybrid block copolymer PVPh-b-PS-DDSQ-PS-b-PVPh.

Synthesis of PtBuOS-b-PS-DDSQ-PS-b-PtBuOS and PVPh-b-PS-DDSQ-PS-b-PVPh
Scheme 1d-f summarizes the preparation of block copolymers based on DDSQ hybrids through ATRP. We used FTIR and 1 H NMR spectra ( Figure 5 Figure 5A,a], that of the hybrid block copolymer PtBuOS-b-PS-DDSQ-PS-b-PtBuOS [ Figure 5A,b] exhibited additional signals at 1243, 1163, and 905 cm −1 corresponding to the C-O and C-O-C stretching and C-H bending, respectively. Furthermore, the signal at 1.56 ppm in the 1 H NMR spectrum of the hybrid block copolymer PtBuOS 78 -b-PS 91 -DDSQ-PS 91 -b-PtBuOS 78 [ Figure 5B,b], corresponding to the tert-butyl (C 4 H 9 ) protons of the PtBuOS block segment, confirmed the successful ATRP of the tBuOS groups to give the PS-DDSQ-PS hybrid polymer. This signal at 1.56 ppm for the PtBuOS block segment essentially disappeared after treatment with N 2 H 4 , with a new peak appearing at 8.93 ppm representing the OH units of the PVPh block segment [ Figure 5B,c],. Furthermore, the hydrolysis of the tert-butyl groups was evidenced by the signals at 1163 and 905 cm −1 decreasing in intensity, with a signal representing the presence of phenolic OH units for the PVPh block segment at 3400 cm −1 , [ Figure 5A,c],due to the self-association of hydrogen bonding units [42], suggesting the successful hydrolysis and the formation of the hybrid block copolymer PVPh-b-PS-DDSQ-PS-b-PVPh.  The glass transition temperature (Tg) of the hybrid polymer PS-DDSQ-PS (110 °C) was higher than that of a PS homopolymer because the inorganic DDSQ nanoparticles enhanced the thermal properties [49]. For the block copolymer hybrid PtBuOS-b-PS-DDSQ-PS-b-PtBuOS, the value of Tg was also 110 °C, consistent with the value of Tg of a PtBuOS block segment being similar to that of a PS block segment [43]. Two values of Tg appeared, at 110 and 192 °C, for the block copolymer hybrid PVPh-b-PS-DDSQ-PS-b-PVPh, implying that microphase separation existed as a result of chemical incompatibility between the PS and PVPh block segments. The value of Tg of the PtBuOS block (110 °C) increased significantly to 192 °C for the PVPh segment in the DDSQ block copolymer because strong OH···OH hydrogen bonding decreases the free volume of the PVPh block segment. Figure 7 displays a TEM image of the block copolymer hybrid PVPh-b-PS-DDSQ-PS-b-PVPh; the apparent short-range order of a lamellae structure is similar to that we observed for the pure diblock copolymer PS-b-PVPh in a previous study. Furthermore, the dark regions in the TEM images may have arisen from aggregation of DDSQ nanoparticles having dimensions of approximately 2-3 nm, indicating that the DDSQ units had dispersed well in the block copolymer matrix.  The glass transition temperature (T g ) of the hybrid polymer PS-DDSQ-PS (110 • C) was higher than that of a PS homopolymer because the inorganic DDSQ nanoparticles enhanced the thermal properties [49]. For the block copolymer hybrid PtBuOS-b-PS-DDSQ-PS-b-PtBuOS, the value of T g was also 110 • C, consistent with the value of T g of a PtBuOS block segment being similar to that of a PS block segment [43]. Two values of T g appeared, at 110 and 192 • C, for the block copolymer hybrid PVPh-b-PS-DDSQ-PS-b-PVPh, implying that microphase separation existed as a result of chemical incompatibility between the PS and PVPh block segments. The value of T g of the PtBuOS block (110 • C) increased significantly to 192 • C for the PVPh segment in the DDSQ block copolymer because strong OH···OH hydrogen bonding decreases the free volume of the PVPh block segment. Figure 7 displays a TEM image of the block copolymer hybrid PVPh-b-PS-DDSQ-PS-b-PVPh; the apparent short-range order of a lamellae structure is similar to that we observed for the pure diblock copolymer PS-b-PVPh in a previous study. Furthermore, the dark regions in the TEM images may have arisen from aggregation of DDSQ nanoparticles having dimensions of approximately 2-3 nm, indicating that the DDSQ units had dispersed well in the block copolymer matrix.

Synthesis of P4VP-b-PS-DDSQ-PS-b-P4VP
Scheme 1h displays the synthetic process toward the hybrid block copolymer P4VP-b-PS-DDSQ-PS-b-P4VP through sequential ATRP. Figure 8A presents the FTIR spectrum of the block copolymer

Synthesis of P4VP-b-PS-DDSQ-PS-b-P4VP
Scheme 1h displays the synthetic process toward the hybrid block copolymer P4VP-b-PS-DDSQ-PS-b-P4VP through sequential ATRP. Figure 8A presents the FTIR spectrum of the block copolymer

Synthesis of P4VP-b-PS-DDSQ-PS-b-P4VP
Scheme 1h displays the synthetic process toward the hybrid block copolymer P4VPb-PS-DDSQ-PS-b-P4VP through sequential ATRP. Figure 8A presents the FTIR spectrum of the block copolymer P4VP-b-PS-DDSQ-PS-b-P4VP, revealing extra signals for pyridine stretching at 1590 and 993 cm −1 , corresponding to the P4VP block segment, when compared with the spectrum of PS-DDSQ-PS. The intensities of these two characteristic signals for pyridine rings increased upon increasing the reaction time (8 and 12 h) of the 4-vinylpyridine monomer. Furthermore, Figure 8B Figure 10a,b, with a minor phase (dark region) arising from the P4VP block segments, as displayed in Figure 10c, because the volume fraction of P4VP was approximately 31%. Increasing the molecular weight of the P4VP block segment, giving the hybrid block copolymer P4VP98-b-PS91-DDSQ-PS91-b-P4VP98, resulted in a lamellar structure, as displayed in Figure 10d,e; here, the volume fraction of P4VP block segments was approximately 52%, close to the 50% found in a typical lamellar structure [ Figure 10f].
Thus, in this study we could indeed synthesize block copolymers (PVPh-b-PS-DDSQ-PS-b-PVPh, P4VP-b-PS-DDSQ-PS-b-P4VP) featuring strongly hydrogen bonding donor and acceptor block segments. In the near future we will investigate the hydrogen bonding interactions and self-assembly of PVPh-b-PS-DDSQ-PS-b-PVPh/P4VP-b-PS-DDSQ-PS-b-P4VP block copolymer mixtures.  The dark regions correspond to the P4VP block segments; the bright regions, the PS block segments. We observed a cylinder structure for the hybrid block copolymer P4VP 62 -b-PS 91 -DDSQ-PS 91 -b-P4VP 62 , as displayed in Figure 10a,b, with a minor phase (dark region) arising from the P4VP block segments, as displayed in Figure 10c, because the volume fraction of P4VP was approximately 31%. Increasing the molecular weight of the P4VP block segment, giving the hybrid block copolymer P4VP 98 -b-PS 91 -DDSQ-PS 91 -b-P4VP 98 , resulted in a lamellar structure, as displayed in Figure 10d,e; here, the volume fraction of P4VP block segments was approximately 52%, close to the 50% found in a typical lamellar structure [ Figure 10f].

Conclusions
We have synthesized two DDSQ-functionalized hydrogen bond donor and acceptor hybrid block copolymers-PVPh-b-PS-DDSQ-PS-b-PVPh and P4VP-b-PS-DDSQ-PS-b-P4VP-through sequential ATRP, and characterized them using FTIR spectroscopy, NMR spectroscopy, GPC, DSC, and MALDI-TOF mass spectrometry. Both the hybrid block copolymers PVPh-b-PS-DDSQ-PS-b-PVPh and P4VP-b-PS-DDSQ-PS-b-P4VP exhibited two glass transition temperatures, indicative of microphase separation with lamellar or cylindrical structures, which we confirmed through TEM image analyses. This facile approach allows the preparation, through controlled living radical polymerizations, of main chain-type block copolymers featuring DDSQ NPs at their junction point.

Conclusions
We have synthesized two DDSQ-functionalized hydrogen bond donor and acceptor hybrid block copolymers-PVPh-b-PS-DDSQ-PS-b-PVPh and P4VP-b-PS-DDSQ-PS-b-P4VP-through sequential ATRP, and characterized them using FTIR spectroscopy, NMR spectroscopy, GPC, DSC, and MALDI-TOF mass spectrometry. Both the hybrid block copolymers PVPh-b-PS-DDSQ-PS-b-PVPh and P4VP-b-PS-DDSQ-PS-b-P4VP exhibited two glass transition temperatures, indicative of microphase separation with lamellar or cylindrical structures, which we confirmed through TEM image analyses. This facile approach allows the preparation, through controlled living radical polymerizations, of main chain-type block copolymers featuring DDSQ NPs at their junction point.