Synthesis and Ring-Opening Metathesis Polymerization of a New Norbornene Dicarboximide with a Pendant Carbazole Moiety

A new norbornene dicarboximide presenting a pendant carbazole moiety linked by a p-methylene benzyl spacer is synthesized. This carbazole-functionalized monomer is polymerized via ring-opening metathesis polymerization using Grubbs third-generation catalyst. Microstructural analysis of resulting polymers performed by Nuclear Magnetic Resonance (NMR) shows that they are stereoirregular. Wide-angle X-ray diffraction (WAXD) and thermal (DSC) analysis indicate that polymers are also amorphous. With respect to the fluorescence analysis, both solution and film polymer samples exhibit only “normal structured” carbazole fluorescence, while excimer formation by overlap of carbazole groups is not detected.


Introduction
In the latest recent years, carbazole-containing polymers have gained importance because of their use as organic photorefractive materials, photoconductors, light-emitting materials, etc. [1][2][3][4][5][6][7].From a structural point of view, carbazole-containing polymers can be divided into two groups: main chain-type and side chain-type carbazolecontaining polymers [1].As for the latter class of polymers, poly(N-vinylcarbazole) (PVK) was the first to be synthesized [8,9].It has excellent film-forming properties and a high glass transition temperature and has also been used in combination with other layers in the fabrication of white OLEDs [10][11][12].As for PVK and other side chain-type carbazole-containing polymers, their photophysical and photochemical properties were widely studied and it was found that their peculiar behaviour is due to the potential ability of these polymers to form two distinct excimers: a low-energy "sandwich-like" excimer and a high-energy "partially overlapping" excimer [13][14][15][16][17][18][19][20].Moreover, it was established that the photophysical properties and photoconductive behaviour of polymers can be modified by the conformation of the main chain as well as the nature and length of a spacer linking carbazole moiety to polymer backbone [1].
Side chain-type carbazole-containing polymers are generally nonconjugated polymers which, depending on the starting monomer, can be synthesized by using different polymerization techniques [1,[21][22][23].PVK is prepared by cationic or radical polymerization using a vinylcarbazole monomer [15][16][17][18].Higher homologues of PVK were synthesized by stereospecific Ziegler-Natta polymerization by using vinyl monomers [2-5, 24, 25].Styrene-based monomers were polymerized by anionic and Ziegler-Natta polymerization [23,26,27].Over the past two decades, the ringopening metathesis polymerization (ROMP) has emerged as a powerful polymerization technique for the synthesis of structurally precise polymers [28,29].With the development of ruthenium-based catalysts such as the commercial Grubbs catalysts [30][31][32] that possess an excellent functional group tolerance, a variety of functional polymeric materials with interesting architectures and useful properties have been synthesized via ROMP.Moreover, a high degree of control of both molecular weight and dispersity of polymers can be achieved [33].Norbornene and its derivatives are the most commonly utilized ROMP monomers due to their high reactivity and easy incorporation of substituents in their structure that allows to introduce appropriate pendant functional groups on the polymer backbone [28,29,[34][35][36][37].Therefore, ROMP appears to be an interesting method also for fabricating side chain-type carbazole-containing polymers.Some examples of ROMP of norbornene-based monomers containing carbazole groups promoted by Grubbs catalysts are described in the literature [38][39][40][41].Recently, Zeng et al. also reported the synthesis of highly efficient blue thermally activated delayed fluorescence polymers and copolymers based on side chain-type carbazole-containing polymers obtained by using Grubbs second-generation catalyst [42].In this work, we describe the synthesis of a new carbazole-functionalized norbornene dicarboximide (M, Figure 1(a)) and its polymer (Figure 1(b)) obtained via ROMP using the Grubbs third-generation catalyst G3 (Figure 1(c)) [43].A poly(5-norbornene-2,3-dicarboximide) backbone was chosen to impart high glass transition temperature, good mechanical properties, and high thermal resistance to the resulting material [44][45][46], while a p-methylene benzyl spacer between the main chain and the carbazole moiety was introduced to reduce flexibility of the pendant arm thus favouring the formation of carbazole excimers.An accurate 1 H and 13 C NMR microstructural characterization of obtained polymer samples, as well as the X-ray and thermal analyses, is reported.UV-vis and photoluminescence properties were also investigated.

Methods
2.2.1. 13C and 1 H Nuclear Magnetic Resonance Spectroscopy ( 1 H NMR and 13 C NMR).NMR spectra of monomer and polymers were recorded on a Bruker Advance 400 spectrometer ( 1 H, 400 MHz; 13 C, 100.62 MHz) operating at 298 K. Deuterated chloroform (CDCl 3 ) was used as the solvent to solubilize the samples, and tetramethylsilane (TMS) was used as internal chemical shift reference.

Electrospray Ionization Mass Spectrometry (ESI-MS).
ESI-MS analysis was accomplished on a Waters spectrometer with an electrospray source.International Journal of Polymer Science were measured by gel permeation chromatography (GPC) at 30 °C, using tetrahydrofuran (THF) as a solvent, an eluent flow rate of 1 mL/min, and narrow polystyrene standards as reference.The measurements were performed on a Waters 1525 binary system equipped with a Waters 2414 RI detector using four Styragel columns (range 1,000-1,000,000 Å).

Gel Permeation Chromatography (GPC)
2.2.4.Thermogravimetric Analyses (TGA).TGA were performed on a TGA Q500 apparatus manufactured by TA Instruments, in flowing N 2 (100 cm 3 /min).Polymer samples of 5 mg were placed in platinum pans and heated in the range 20-1000 °C at a rate of 10 °C/min.

Differential Scanning Calorimetry (DSC).
Calorimetric measurements were carried out on a DSC Q20 apparatus manufactured by TA Instruments, in flowing N 2 .Monomer and polymer samples of 5-10 mg were placed in aluminium pans and heated/cooled at a rate of 10 °C/min.

Fourier Transform Infrared Spectroscopy (FTIR).
Infrared spectra were obtained at a resolution of 2.0 cm -1 with a Tensor 27 Bruker spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector and a Ge/KBr beam splitter.The frequency scale was internally calibrated to 0.01 cm -1 using a He-Ne laser.Thirty-two scans were signal averaged to reduce the noise.Samples were prepared by casting from a solution of polymer in chloroform.[48].A mixture of urea (1.11 g, 18 mmol) and 5-norbornene-exo-2,3-dicarboxylic anhydride (2.50 g, 15 mmol) was introduced into a 250 mL round bottom flask, equipped with a condenser and a magnetic stirrer.The flask was heated at 145 °C for 4 hours, then cooled at 90 °C, and the crude product was dissolved in water (60 mL).Upon cooling at room temperature, an offwhite solid precipitated.It was collected by filtration, washed with cold water, and dried in vacuo to obtain the desired dicarboximide S2 (1.5 g, 63%). 1 H NMR signals and 13 C NMR signals were congruent with those reported in the literature [48].

Polymer Synthesis and NMR Characterization.
As depicted in Figure 3, target polymer samples (poly-M) were synthesized by ring-opening metathesis polymerization of M in the presence of third-generation Grubbs catalyst [43].Polymerizations were performed under an inert atmosphere, at room temperature and using CH 2 Cl 2 as solvent.Before analysis, polymer samples were purified by passing through a silica gel column.Polymerization results are summarized in Table 1.
According to GPC analysis, polymers showed monomodal curves and molecular weight distribution values suggesting that ROMP polymerization reactions could proceed in a living way [33].
Polymer microstructures were evaluated by 1 H and 13 C NMR analysis.As expected, catalyst G3 produced polymers 4 International Journal of Polymer Science with a mixture of cis and trans double bonds.Figure 4 shows the 1 H NMR spectra of monomer M (A) and of its polymer (B, sample 2 in Table 1).In Table 2, the complete chemical shift assignments are also reported.As can be observed, the monomer olefinic signals at δ = 6 26 ppm (Figure 4(a)) were replaced by two new resonances centered at δ = 5 63 and 5.44 (Figure 4(b)), which correspond to the trans and cis double bonds of the polymer, respectively.The fraction of double bonds with cis configuration in the polymer backbone was estimated to be 45%.The 13 C NMR spectra of M and its polymer are displayed in Figures S2 and S8 of the Supplementary Material, respectively.The characteristic signals attributed to the olefinic carbons of the norbornene ring were observed at 138.02 ppm, while the resonances of the olefinic carbons of polymer chain were observed at 133.65-132.67 (cis) and 131.79 (trans).
3.3.X-Ray, DSC, and Optical Characterization.The X-ray diffraction pattern of both polymer samples (see Figure 5) shows an intense and broad amorphous peak centered at 2θ = 19 2 °with a weak shoulder at 2θ = 14 2 °.This pattern result is very similar to that recently reported for poly(4-(N-carbazolyl)methyl styrene) (PSK) which is a polyolefin leading similar side group of our polymer samples [26].For PSK, it was assumed that the most intense peak of the X-ray spectrum has to be associated with correlation distances among face to face arranged carbazole groups [26].Likewise, the maximum of the X-ray pattern of both our polymer samples should be associated with the stacking formation of carbazole units having an average relative distance of 4.5 Å.
To thermal analysis, both polymer samples show thermal stability with a decomposition temperature (T d , with 5% weight loss) of 423 °C (see Figure 6(a)).Consistent with the X-ray diffraction results, the DSC curves of both polymer samples show only the glass transition (T g ), ~189 and ~195 °C for samples 1 and 2 in Table 1, respectively (see Figure 6(b)).The lower T g value observed for sample 1 with respect to that for sample 2 is consistent with the lower molecular weight of 1 with respect to that of 2. It is worth noting that the thermal behaviour of amorphous polymers presenting the same main chain of our polymer samples (poly(5-norbornene-2,3-dicarboximide) but different N-substituted groups on N (i.e., phenyl and adamantyl) was already reported in the literature [45,50,51].
As expected, decomposition temperatures close to those of our polymers were detected, while slightly higher T g values were instead reported [45,50,51].This last result is expected taking into account not only the higher molecular weights of reported N-substituted poly(5-norbornene-2,3-dicarboximide)s with respect to our polymers (about ten times more) but also the minor ability of the N-substituted side group (9H-carbazole-9-yl)methyl)benzyl) of our polymer samples to decrease the chain mobility and to increase the rotational barrier of the polymer main chains respect to phenyl or more rigid adamantyl side groups.
UV-vis spectrum of polymer sample 2 in Table 1 is showed in Figure 7.The peaks which characterize the       1. 6 International Journal of Polymer Science It is worth nothing that bands attributable to "sandwichlike" or "partially overlapping" excimers usually formed by carbazole-containing polymers like poly(N-vinylcarbazole) (PVK) [15,18,54] or stereoregular higher homologues of PVK [2-5, 23, 24] are not observed.For solution emission spectra, this result can be easily explained assuming that the     International Journal of Polymer Science presumed main chain conformational freedom in solution prevents excimer formation.As for the lack of excimer emission in film spectra, it could be justified by the presence of the p-methylene benzyl spacer linking the poly(5-norbornene-2,3-dicarboximide) chain and the carbazole groups that increases the conformational freedom of carbazole groups preventing excimer formation.Very likely, the p-methylene benzyl spacer is not rigid enough to induce the stacking of carbazole groups and therefore excimer formation.Furthermore, the lack of excimer emission in the film spectra of our polymer samples could be also related to lack of conformational order of the main chain.In this regard, it is worth recalling that for carbazole-containing polymer, excimer formation can be induced by main chain stereoregularity [2-5, 15, 18, 24, 25, 54].As an example, both isotactic and syndiotactic PSK film fluorescence spectra present bands attributable to low-energy "sandwich-like" and/or highenergy "partially overlapping" excimers which are not observed in atactic PSK [26].Since our polymer samples do not present any stereoregularity degree due to the lack of stereospecificity of catalyst G3, their optical behaviour could be expected.

Conclusion
Herein, a new norbornene dicarboximide presenting a pendant carbazole moiety linked by a p-methylene benzyl spacer was prepared and utilized in polymerization by ROMP using Grubbs third generation catalyst G3.Consistent with a controlled living polymerization mechanism, the obtained polymer samples show narrow dispersities.They were fully characterized by NMR spectroscopy disclosing a stereoirregular microstructure of the main chain presenting random distribution of cis and trans double bonds.TGA analyses showed that polymer samples have high thermal stability with a decomposition temperature of 423 °C.The X-ray patterns of polymer samples present a broad amorphous halo centered at 2θ = 19 2 °and a not too well-defined hump at 2 θ = 14 2 °.The maximum of the X-ray pattern was tentatively associated to stacking formation of carbazole units having an average relative distance of 4.5 Å.According to the WAXD data, polymer DSC traces only present glass transition (T g ~190 °C).With respect to the optical analysis, both solution and film polymer samples showed similar fluorescence spectra with two weak shoulders in the high-energy region which are due to carbazole emission, while excimer emission was not observed.In particular, as for polymer film samples, the presence of a p-methylene benzyl spacer connecting carbazole with the main chain as well as the stereoirregularity of the polymer chain could avoid excimer formation.However, starting from this new monomer, stereoregular polymers with more interesting optical properties could be prepared by using stereoselective ROMP catalysts, thus leading to the development of attractive materials for optoelectronic applications.

Figure 4 : 1 H
Figure 4: 1 H-NMR spectra of M (a) and its polymer (b, sample 2) in CDCl 3 .The residual protio impurity of CDCl 3 is marked with an asterisk.

7
After stirring under reflux for 20 hours, the reaction mixture was cooled at room temperature, poured into water (200 mL), and extracted with CHCl 3 (3 × 100 mL).The combined organic phases were dried over Mg 2 SO 4 , and after removal of the solvent, the crude product was purified by silica gel column chromatography (petroleum ether/CH 2 Cl 2 2 : 1) to give a white solid (1.4 g, 62%).
[47]8.Ultraviolet-Visible (UV-vis) and Fluorescence Spectroscopy.UV-vis measurement was performed by a Varian Cary 50 spectrophotometer and photoluminescence recorded by a Varian Cary Eclipse spectrophotometer.UV-vis and fluorescence measurements in solutions were performed in THF.Thin polymer films have been prepared by spin coating on a quartz slide substrate.The film thickness and roughness were measured by a KLA Tencor P-10 surface profiler.Film thickness was about 100 nm.mixture was stirred for 4 hours[47].On completion of the reaction, the resulting brownish suspension containing the desired potassium carbazolate was filtered through a fritted glass funnel and immediately added dropwise to a THF solution of α,α ′ -dibromo-p-xylene (45 mL) previously prepared in a 250 mL three-necked round-bottom flask fitted with a condenser.

Table 2 :
1H NMR chemical shift assignments for M and its polymer.