Synthesis of Network Polymers Containing Si-Vinylene Units by Mizoroki-Heck Reaction

Network polymers containing Si-vinylene units have been synthesized by Mizoroki-Heck reaction of cyclic siloxane or cubic silsesquioxane compounds with vinyl groups, as joint molecules, and dibromo aryl compounds, as linker molecules, using a Pd catalyst. The reaction of 1,3,5,7-tetravinyltetramethylcyclotetrasiloxane (TVMCTS) or octavinyloctasilsesquioxane (PVOSS) with o, m, p-dibromobenzene (DBB) or 4,4'-dibromobiphenyl (DBBP) yielded the corresponding network polymers, except TVMCTS-oDBB system. Optical properties of the network polymers were studied by UV-vis and photoluminescence spectroscopy, and absorption and emission derived from σ-π conjugation of the Si-vinylene units were detected. Copolymerization of TVMCTS-DBB/bromobenzene yielded CHCl3 soluble fraction due to formation of the network fragments. The network polymers of TVMCTS or PVOSS with 2,7-dibromofluorene (BFl) or 9,9-dihexyl-2,7-dibromofluorene were synthesized by the same procedures. Scanning electron microscope image of TVMCTS-BFl network polymer indicated formation of porous structure. The network polymers have been also synthesized by the reactions of multifunctional aryl bromide, as the joint molecules, with divinyl or dially silane compounds, as the linker molecules. The molecular structure of the multifunctional aryl bromide affected the emission wavelengths of the network polymers. In the case of the 2,2’,7,7’-tetrabromo-9,9’-spirobifluorene-based network polymers, the emission spectra widely ranged from 450 to 500 nm, which were sensitive to molecular structure of the divinyl or dially silane compounds.


Introduction
Many kinds of π-conjugated polymers have been developed due to their characteristic photophysical features and application to organic light-emitting diodes (for example Burroughes, Bradley, Brown, Marks, Mackay, Friend, Burns, & Holmes, 1990).Among the π-conjugated polymers, poly(p-phenylene-vinylene), poly(thiophene), poly(p-phenylene), poly(fluorene), and poly(carbazole) are typical polymers that show high photoluminescence quantum yield.The copolymers based on these π-conjugated polymers have been synthesized to control the photophysical properties and/or increase solubility in organic solvents.

Synthesis of Network Polymers
Synthesis of the network polymers was carried out in a 10 mL ample tube equipped with a magnetic stirrer.A reaction of TVMCTS with DBB is described as a reference.DBB (1 mmol), P(o-Tol) 3 (0.12 mmol), and Pd(OAc) 2 (0.02 mmol) were added to the ample tube under a nitrogen atmosphere.After the addition of TVMCTS (0.5 mmol), DMF (2.0 mL), and NEt 3 (1.5 mmol), the ample tube was sealed by burning off.The reaction was conducted at 100°C for 24 h.The polymerization was terminated by adding a small amount of methanol.The polymer was precipitated in a large excess of methanol and recovered by filtration.The polymer obtained was washed by chloroform and dried in vacuo at 50°C for 6 h.Other network polymers were synthesized by the same procedures with the molar ratio of vinyl or allyl to Br was 1.0.

Analytical Procedures
FT-IR spectra of the network polymers were recorded on a Jasco FT/IR-410 spectrometer in pulse Fourier transform mode as KBr discs. 1 H NMR spectra of CHCl 3 soluble fractions of TVMCTS-DBB/BB polymers were recorded on a JEOL-JNM-LA300 spectrometer in pulse Fourier transform mode in CDCl 3 .The pulse angle was 45º and 32 scans were accumulated in 7 s of the pulse repetition.UV-vis absorption spectroscopy was conducted with SHIMADZU UV-2450.Photoluminescence (PL) spectroscopy was investigated with a SHIMADZU RF-5300PC.Scanning electron microscope (SEM) images were acquired by JEOL JSM-7400F.

Network Polymers from TVMCTS, PVOSS-DBB, DBBP
Mizoroki-Heck reaction of TVMCTS or PVOSS with DBB or DBBP has been investigated with Pd(OAc) 2 catalyst in DMF at 100ºC, as shown in Scheme 1. Polymerization results are summarized in Table 1.The reaction of TVMCTS with pDBB and DBBP yielded the polymers in good yields.The yield of the TVMCTS-mDBB polymer was much lower than that of the TVMCTS-pDBB polymer.Furthermore, the polymer was not obtained in the reaction of TVMCTS with oDBB.The sterically hindered o-and m-positions of Br in DBB should decrease the reaction conversion, which would cause the low polymer yields.The reactions with PVOSS yielded the polymers in good yields independent of the molecular structure of the dibromo aryl compounds used.Large number of vinyl groups, 8 groups per one molecule, should effectively form the network structure.
FT-IR spectra of TVMCTS-mDBB, pDBB, and DBBP are shown in Figure 1 (a), (b), and (c), respectively.The broad peaks derived from Si-O-Si structure of TVMCTS were detected at around 1050 cm -1 .The peaks derived from phenyl groups were detected at around 1500 cm -1 (C=C stretching of phenyl) and 750-800 cm -1 (C-H vending of disubstituted phenyl).The peaks derived from the unsaturated C=C units connected to Si, and Si-C units were detected at 1600 cm -1 , and 1275 cm -1 , respectively.These results clear the formation of the network polymers by Mizoroki-Heck reaction of TVMCTS with mDBB, pDBB, and DBBP.The network polymers with PVOSS showed similar spectral profiles derived from the corresponding network structure.
Absorptive and emissive features of the network polymers (solid state) were studied with UV-vis and PL spectroscopy.The UV-vis and PL spectra of TVMCTS-pDBB, TVMCTS-DBBP, and PVOSS-pDBB are shown in Figure 2. The λ max wavelength of the spectra are summarized in Table 1.The network polymers showed broad absorption ranged from about 300 to 400 nm.Excitation of the network polymers with the λ max wavelength of the absorption showed relatively sharp emission peaks.The network polymers with pDBB and DBBP showed the emission at longer wavelength than those with oDBB or mDBB.The red-shift of the network polymers with pDBB and DBBP would be due to the long and linear conjugated structure in the network derived from these p-substituted linker molecules.

Network Polymers from TVMCTS-pDBB/BB and TVMCTS/VTMS-pDBB Systems
Network polymers have been synthesized from TVMCTS-pDBB/BB systems.The results are summarized in Table 2.
The network polymers contained chloroform soluble fraction, and its amount slightly increased with increasing of the BB feed ratio.Molecular structure of the chloroform soluble fraction was studied by 1 H NMR spectroscopy.Figure 3 shows the 1 H NMR spectrum and chemical shift assignment of the chloroform soluble fraction of a TVMCTS-pDBB/BB network polymer (Run 10).The peaks derived from vinylene units and methyl group connected to Si were detected at 6.0, 4.7 and 1.5 ppm, respectively.These results indicate that the molecular structure of the chloroform soluble fraction is parts of the network structure.Incorporation of mono-functional BB prevents from formation of infinite network, and yields the small fragments.The network polymers were also synthesized from TVMCTS/VTMS-DBB systems.Although the chloroform soluble fraction of the network polymers was detected, the fractions were small.Absorptive and emissive features of the TVMCTS-pDBB/BB network polymers of the chloroform soluble fractions were studied with UV-vis and PL spectroscopy in chloroform, as shown in Figure 4. Intensity of the absorption peak at around 300-320 nm in UV-vis spectra increased with increasing of the BB feed ratio in the reaction system.The fragments derived from the defect structure, which shows the absorption at long wavelength, would increase with increasing of the BB feed ratio.shown in Scheme 2. The corresponding polymers were obtained in middle to good yields, 27-94% as summarized in Table 4. Figure 7 shows FT-IR spectra of 3BPBz-VMS, AMS, VTMS, MVSB network polymers.All the network polymers showed the peaks derived from stretching of C=C in vinylene, stretching of C=C in phenyl group, stretching of CH 3 -Si-CH 3 , and C-H vending of phenyl group, at around 1600 cm -1 , 1500 cm -1 , 1250 cm -1 , and 840-820 cm -1 , respectively.The spectrum of 3BPBz-AMS network polymer (Figure 7 (b)) or 3BPBz-MVSB network polymer (Figure 7 (d)) showed the peak derived from vending of Si-CH 2 -CH= or stretching of Si-aryl at around 1150 cm -1 or 1130 cm -1 , respectively.Molecular structure of the other network polymers was also confirmed by the FT-IR spectra.Emission spectra of the network polymers (solid state) are shown in Figure 8.The excitation wavelength was determined by fluorescence excitation spectrum.The averaged maximal emission wavelength of the emission spectra, which were exited with 280-380 nm at intervals of every 10 nm, was used as the excitation wavelength, as summarized in Table 4. 1,3,5-Triphenylbenzene, biphenyl, tetraphenylmethane, and 9,9-spirobifluorene, which are moieties of the network structure in the network polymers with 3BPBz, 4B2P, 4BPM, and 4B2F, showed the emission peak with λ max at 352 nm, 322 nm, 323 nm, and 310 nm, respectively.These wavelengths were different from those of the corresponding network polymers.These results indicate that the emission of the network polymers should be affected by the σ-π conjugation formed by Si-vinylene unit.Figure 8 (i) shows the emission spectra of 3BPBz-based network polymers.
The emission spectra showed maximal peaks at around 430 nm.In the case of the 4B2P-based network polymers, Figure 8 (ii), the polymer with VMS, VTMS, and MVSB showed the emission peaks with the λ max at around 410-420 nm.By contrast, 4B2P-AMS network polymer did hardly show emission, Figure 8 (ii)-(b).Methylene units between Si and vinylene in AMS should disturb formation of conjugation in the network, as previously reported in the similar liner polymers (Naga, Tagaya, Noda, Imai, & Tomoda, 2008).The 4BPM-based network polymers showed the emission peaks with the λ max at around 340 nm independent of the structure of divinyl or dially-silane compounds (Figure 8 (iii)).These wavelengths were shorter than those of the 3BPBz and 4B2P-based network polymers due to the short excitation wavelength.In the case of the 4B2F-based network polymers, the λ max of the emission spectra widely ranged from 450 to 500 nm, and depended on molecular structure of the divinyl or dially silane compounds (Figure 8 (iv)).One possibility to explain the results would be difference of the emission wavelengths, as summarized in Table 4.We studied the effect of the excitation wavelength on the emission wavelength of the 4B2F-based network polymers with fixed exaction wavelengths of 380 and 410 nm.The results cleared that the excitation wavelength did not affect the emission wavelength of those network polymers.The difference of the emission wavelengths of the 4B2F network polymers should be derived from the molecular structure of the divinyl or dially silane compounds.Substituent in 9,9-spirobifluorene molecule is sensitive to the emission wavelengths, as reported in some 9,9-spirobifluorene molecules (for example Abdel-Awwad, Luan, Messow, Kusserow, Wiske, Siebert, Fuhrmann-Lieker, Salbeck, & Hillmer, 2015).The molecular structure of the divinyl or dially silane compounds in the network structure with 4B2F should strongly affect the emission wavelengths.

Conclusions
Mizoroki-Heck reaction of cyclic siloxane or cubic silsesquioxane compounds with vinyl groups, TVMCTS, PVOSS, as the joint molecules, and dibromo aryl compounds, as the linker molecules, gave the corresponding network polymers.The network polymers showed the emission derived from the σ-π conjugation.Molecular structure of the dibromo aryl compounds affected the emission wavelengths of the network polymers.Long conjugation structure with DBBP or BFl, and straight substituted structure with pDBB of the dibromo aryl compounds induced the long emission wavelengths.
Copolymerization with mono-functional BB produced the CHCl 3 soluble fraction in the resulting network polymers.
The SEM image of TVMCTS-BFl network polymer indicated formation of porous structure.The network polymers were also synthesized by the reaction of multifunctional aryl bromide, as the joint molecules, with divinyl or dially silane compounds, as the linker molecules.The emission wavelengths were depended on the molecular structure of the multifunctional aryl bromide.The emission wavelengths of 4B2F-based network polymers were sensitive to the structure of the divinyl or dially silane compounds, and widely ranged from 450 to 500 nm.Mizoroki-Heck reaction of multi-functional siloxane compounds having vinyl groups with dibromo aryl compounds, or multifunctional aryl bromide with divinyl or dially silane compounds is one of the useful methods to synthesize the network polymers containing Si-vinylene units.The network polymers synthesized by the present methods would be widely usable due to their features: porosity, photophysical properties, thermal stability, and chemical resistance.Such experiments are under way, and the results will be reported elsewhere.

Scheme 1 .
Scheme 1. Synthesis of network polymers by Mizoroki-Heck reaction of cyclic siloxane or cubic silsesquioxane compounds with vinyl groups-dibromo aryl compound systems

Table 1 .
Synthesis and optical properties of TVMCTS, PVOSS-DBB, DBBP network polymers a b Evaluated in solid state.c Excitation with λ max of absorption.

Table 4 .
Synthesis and optical properties of multifunctional aryl bromide-divinyl, diallyl silane network polymers a b Evaluated in solid state.c Excitation with λ max of absorption.