Electroluminescence Properties

The CV results show that both HOMO and LUMO energy levels of the metallo-polymers do not match the work functions of indium-tin oxide (ITO) anode and Al cathode. Therefore, we

choose PEDOT and LiF/Al as the hole transporting layer and cathode, respectively, to overcome these large energy barriers. The electroluminescence (EL) properties of these polymers were investigated, except polymer 6a due to its poor solubility. The other polymers (6b-6d) were used as emitting layers in a double-layer light-emitting devices with configuration of ITO/PEDOT:PSS/Polymer/LiF/Al. The function of LiF/Al as a cathode is because that the electron injection capability from a high work-function cathode can be significantly improved by inserting polar or ionic species between a metal electrode and a light-emitting layer.^55b The electroluminescence properties of PLED devices (device structure: ITO/PEDOT:PPS/Polymer(6b-6d)/LiF/Al) with good external quantum yields between 0.36 and 1.02 and maximum brightnesses between 323 and 931 cd/m² (at 14 V) are listed in Table 3.4.

The emission colors of these devices (at a bias voltage around 10 V) were yellow to orange in Commission Internationale de l’Eclairage (CIE) coordinates, and the emission intensity was augmented by increasing bias voltages. The turn-on voltages of all devices were approximately 6 V, and the best power efficiency and brightness (in polymer 6c) were 0.33 cd A^-1 and 931 cd m^-2 (at 14 V), respectively. The current density-voltage-brightness characteristic curves of polymer 6c in the PLED device (device structure:

ITO/PEDOT:PPS/Polymer(6c)/LiF/Al) are shown in Figure 3.7, and similar turn-on voltages for both of the current density and brightness illustrate that a matched balance of both injection and transportation in charges was achieved.⁹⁰ Compared with the corresponding PL spectra of solid films in Figure 3.5 and Table 3.3, polymers 6c and 6d (excluding 6b) both showed red shifted emissions of λmax in EL spectra and polymer 6d exhibited a broader EL emission peak (see Figure 3.8 and Table 3.4). The different EL and PL emissions of polymers 6c and 6d may originate from different excited state or/and ground states.^63c The broader EL spectra of polymer 6d may be due to the recombination of excitons at wide interfaces of the emission layer and the hole-transporting layer in the PLED devices.⁹⁰

Figure 3.7 Current-voltage-brightness characteristics of the PLED device with the configuration of ITO/PEDOT:PSS/6c/LiF/Al.

Figure 3.8 Normalized EL spectra of the PLED devices with the configurations of ITO/PEDOTPSS/(6c or 6d)/LiF/Al.

3.5 Conclusion

In summary, a series of bis-tpyzinc(II)-based supramolecular polymers were obtained by self-assembled process. The formation of polymers 6a-6d was confirmed by the increased viscosities (up to 1.5-1.83 times) relative to those of their analogous monomers. Besides, the experiments of ¹H-NMR and UV-vis titration over the ratio of Zn²⁺/monomer = 1/1 confirmed the exact stoichiometric ratio of these metallo-supramolecular polymers. Various lateral substituents, such as methoxy (OMe), methyl (Me), and fluorine (F) units, were attached to the conjugated bis-tpy ligands, and thus to control the thermal properties and energy band gaps of the resulting metallo-polymers. Compared with the monomer counterparts, the film quality and the quantum yields of PL and EL were enhanced by the metallo-supramolecular design via introducing zinc(II) ions. These metallo-polymers gave green to yellow PL emissions (with good PL quantum yields) in solid films, and showed yellow to orange EL emissions. In general, the incorporation of lateral substituents into metallo-polymers reveals that the thermal and photophysical properties can be easily adjusted. Hence, with finely tuned structures integrated with coordination chemistry, metallo-polymers can provide a new protocol for the development of novel PLED materials.

Chapter 4

Metallo-homopolymer and metallo-copolymers containing light-emitting poly(fluorene/ethynylene/(terpyridyl)zinc(II)) backbones and 1,3,4-oxadiazole (OXD) pendants

4.1 Abstract

A series of novel metallo-polymers containing light-emitting poly(fluorene/ethynylene/(terpyridyl)zinc(II)) backbones and electron-transporting 1,3,4-oxadiazole (OXD) pendants (attached to the C-9 position of fluorene by long alkyl spacers) were synthesized by self-assembled reactions. The integrated ratios of ¹H NMR spectra reveal a facile result to distinguish the well-defined main-chain metallo-polymeric structures which were constructed by different monomer ligand systems (i.e. single, double, and triple monomer ligands with various pendants). Furthermore, UV-visible and photoluminescence (PL) spectral titration experiments were carried out to verify the metallo-polymeric structures by varying the molar ratios of zinc(II) ions to monomers. As a result, the enhancement of thermal stability (Td) and quantum yields were introduced by the metallo-polymerization, and their physical properties were mainly affected by the nature of the pendants. The photophysical properties of these metallo-polymers exhibited blue PL emissions (around 418 nm) with quantum yields of 34-53 % (in DMF). In contrast to metallo-polymers containing alkyl pendants, the quantum yields were greatly enhanced by introducing 1,3,4-OXD pendants but reduced by carbazole (CAZ) pendants. Moreover, electroluminescent (EL) devices with these light-emitting metallo-polymers as emitters showed green EL emissions (around 550 nm) with turn-on voltages of 6.0-6.5 V, maximum efficiencies of 1.05-1.35 cd A^-1(at 100 mA cm^-2), and maximum luminances of 2313-3550

cd/m² (around 15 V), respectively.

4.2 Introduction

Recently, the use of transition or rare earth metal complexes to build up polymeric light-emitting diode (PLED) devices has attracted much attention because of the enhancement in EL efficiency.28,48b,91-92

Chan and co-workers demonstrated that the construction of conjugated polymers made of ruthenium bipyridyl complexes can enhance the light-emitting performance by utilizing energy transfer from the triplet excited state.^55,93 A whole set of coordination polymers consisting of ditopic electro- and photo-active terpyridyl ligands complexed with zinc ions were recently published by Che et al.⁴⁴ These coordination polymers exhibit different emission wavelengths ranging from violet to yellow colors with high PL quantum yields, and these polymers were successfully applied to PLED devices. Hence, tuning electroluminescent (EL) properties could be achieved through the incorporation of different transition metal complexes into polymer main-chains.49,76-77,94

Moreover, it is confirmed that the phenomenon of intraligand charge transfer (ILCT) happens between terpyridine/zinc(II) complexes and chromophores even in fully conjugated metallo-polymers, due to the d¹⁰ zinc(II) species.^44,84,94 Therefore, the incorporation of terpyridine/zinc(II) moieties into the metallo-polymers with fine-tuned chromophores can provide good quantum yields and thermal stabilities, and thus to have the potential to become high-performance emissive or host materials in PLED applications.^53c,78,79

However, some important and fundamental challenges remain to solve, including the maximization of luminescence and power efficiency, the designs and syntheses of new materials for purer colors, and the modes of addressing devices for full-color displays with optimized resolutions. A major factor responsible for poor device performance is that the charge injection and transportation in emissive materials are generally unbalanced. This imbalance arises because the energy barrier between the indium tin oxide (ITO) anode and the highest-occupied molecular orbital (HOMO) level of an emissive material is different from that between the metal cathode and its lowest-unoccupied molecular orbital (LUMO)

level.⁹⁵In previous studies, molecular and polymeric 1,3,4-oxadiazole (OXD) derivatives are one of the most widely studied classes of electron injection and/or hole-blocking materials, mainly because of their electron deficiencies, high photoluminescence quantum yields, and chemical stabilities.^95-99Therefore, the introduction of electron-deficient OXD groups into the C-9 position of the fluorene units increase the electron affinities of resulting polymers and lead to more balanced charge injection and transporting properties as well as better recombination behavior.^65,67

In this context, various electron- and hole-transporting substituents, i.e. 1,3,4-oxadiazole (OXD) and carbazole (CAZ) pendants, were incorporated into poly(fluorene/ethynylene/(terpyridyl)zinc(II))-based metallo-copolymers (as shown in Figure 4.1). In addition, the ¹H-NMR, thermal, photophysical, and electrochemical properties were investigated as well. Furthermore, the PLED applications of metallo-copolymers as emitters in multilayer EL devices with two different hetero-junction

configurations of

ITO/PEDOT:PSS/Polymer/TPBI[2,2’,2”-(1,3,5-benzenetriyl)tris[1-phenyl-1H-benzimidazol

e]/LiF/Al and

ITO/PEDOT:PSS/Polymer/BCP(2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)/ALQ(tris(8 -hydroxyquinoline)aluminium)/LiF/Al were studied.

4.3 Experiment

4.3.1 Measurements

1H NMR spectra were recorded on a Varian Unity 300 MHz spectrometer using CDCl3

and DMSO-d6 solvents. Elemental analyses were performed on a HERAEUS CHN-OS RAPID elemental analyzer. Phase transition temperatures were determined by differential scanning calorimetry (DSC, Model: Perkin Elmer Diamond) with a heating and cooling rate of 10 °C/min. Thermogravimetric analysis (TGA) was conducted on a Du Pont Thermal Analyst 2100 system with a TGA 2950 thermogravimetric analyzer with a heating rate of 10

°C/min under nitrogen. Melting points were determined by a Buchi SMP-20 capillary melting point apparatus. Viscosity measurements were proceeded by 10 % weight of polymer solutions (in NMP) in contrast to those proceeded by the same condition of monomer solutions (with viscosity η = 6 cp) on a BROOKFILEL DV-III+ RHEOMETER system (100 RPM, Spindle number: 4) at 25 ℃ . UV-visible (UV-vis) absorption spectra were recorded in dilute DMF solutions (10^-5 M) on a HP G1103A spectrophotometer, and fluorescence spectra were obtained on a Hitachi F-4500 spectrophotometer. Fluorescence quantum yields were determined by comparing the integrated photoluminescence (PL) intensity of coumarin-1 in ethanol with a known quantum yield (ca. 5 x 10^-6 M, quantum yield = 0.73). Cyclic voltammetry (CV) was performed at a scanning rate of 100 mV/s on a BAS 100 B/W electrochemical analyzer, which was equipped with a three-electrode cell. Pt wire was used as a counter electrode, and an Ag/AgCl electrode was used as a reference in the CV measurements. The CV experiments were performed by solid samples immersed into electrochemical cell containing 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) solutions (in DMF) with a scanning rate of 100 mV/s at room temperature under nitrogen.

UV-vis and PL titrations were preformed by that 1.0 x 10^-5 M of monomer solutions in the solvent of CH3CN/CHCl3 (2/8 in vol.) were titrated with 50 µl aliquots of 3.9 x 10^-4 M of Zn(OAc)2solutions in the same solvent composition as described. The addition was done

stepwisely and the formation of Zn(II)-coordination polymers was monitored by UV-vis spectroscopy. Polymer thin solid films in UV-vis and PL measurements were spin-coated on quartz substrates from DMF solutions with a concentration of 10 mg/ml. A series of EL

devices with two device configurations of

ITO/PEDOT:PPS/Polymer/TPBI(2,2’,2”-(1,3,5-benzenetriyl)tris[1-phenyl-1H-benzimidazol e])//LiF/Al and

ITO/PEDOT:PPS/Polymer/BCP(2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)/AlQ(tris(8-hydroxyquinoline)aluminium)/LiF/Al were made, where TPBI and AlQ were used as electron transporting layers, and BCP was used as a hole-blocking layer collocated to AlQ. ITO substrates were routinely cleaned by ultrasonic treatments in detergent solutions and diluted water, followed by rinsing with acetone and then ethanol. After drying, ITO substrates were kept in oxygen plasma for 4 min before being loaded into the vacuum chamber. The solutions (10 mg/ml) of light-emitting materials in DMF were spin-coated on glass slides precoated with indium tin oxide (ITO) having a sheet resistance of ~20 Ω/square and an effective individual device area of 3.14 mm². The spin coating rate was 6000 rpm for 60 s with PEDOT: PPS, 4000 rpm for 60 s with resulting polymers, and the thicknesses of PEDOT: PPS and polymers were measured with an Alfa Step 500 Surface Profiler (Tencor). TPBI, BCP, and AlQ were thermally deposited at a rate of 1-2 Å/s under a pressure of ~2x10^-5 torr in an Ulvac Cryogenic deposition system. Under the same deposition conditions and systems, one layer of LiF was thermally deposited as a cathode at a rate of 0.1-0.2 Å/s, which was followed by capping with aluminum.

4.3.2 Materials

Chemicals and solvents were reagent grades and purchased from Aldrich, ACROS, TCI, and Lancaster Chemical Co. Solvents were purified and dried according to standard procedures.

Chromatography was performed with Merck silica gel (mesh 70-230) and basic aluminum

oxide, which was deactivated with 4 wt% of water.

4’-[[(Trifuroromethyl)sulfonyl]oxy]-2,2’:6’,6”-terpyridines and compounds 1a, 4b (M2), and 4c (M3) were prepared and purified according to literature procedures.^56,84,94 The synthetic routes of monomer 4a-4c (M1-M3) and metallo-polymers P1-P4 are illustrated in Schemes 4.1-4.2.

Compound (2a). To a solution of compoumd (1a) (28 mmol) in 60 mL of THF/Et3N (1/1), 3-methyl-1-butyn-3-ol (84 mmol) was added. After the solution was degassed with nitrogen for 30 min, Pd(PPh3)2Cl2 (0.28 mol), PPh3(11 mol), and CuI (2.8 mmol) were added. The reaction was then refluxed at 70 ℃ under N2for 12 h. The solvent was removed under reduced pressure. The resulting solid was extracted with CH2Cl2/H2O then dried over MgSO4. The crude product was purified by column chromatography (silica gel, hexane/ethyl acetate = 4/1) to afford a white solid. mp 76-77 ℃ .¹H NMR (300 MHz, CDCl3): δ 8.01-8.05 then removed and crude product was purified by column chromatography (silica gel, hexane) to afford a white solid. mp 82-83 ℃ .¹H NMR (300 MHz, CDCl3): δ 7.99-8.04 (m, 8H), 7.64 (d, J = 7.8 Hz, 2H), 7.47-7.50 (m, 4H), 7.32 (d, J = 8.4 Hz, 4H), 6.95 (d, J = 9 Hz, 4H), 3.91 (t, J = 6.3 Hz, 4H), 3.16 (s, 2H), 2.44 (s, 6H), 1.97 (br, 4H), 1.59-1.66 (m, 4H), 1.13-1.20 (m, 12H). Yield: 77%. FABMS: m/e 882; C59H54N4O4 requires m/e 882.41.

Monomer 4a (M1). Compound (3a) (0.5 mmol) and

4’-[[(trifuroromethyl)sulfonyl]oxy]-2,2’:6’,6”-terpyidine (1.1 mmol) were dissolved in nitrogen-degassed benzene, then [Pd⁰(PPh3)4] (70 mg, 0.06 mmol) was added and followed by nitrogen-degassed ⁱPr2NH. The solution was then heated to 70 ℃ . After complete consumption of starting materials, the solvent was evaporated and the product was purified

by column chromatography (alumina, hexane/dichloromethane = 10/1 in vol.) to afford a zinc acetate (0.52 mmol) in NMP (10 ml) was added dropwisely. The resulting solution was heated at 105 ℃ under nitrogen atmosphere. After stirring for 24 h, excess KPF6 was added into the hot solution. The resulting solution was poured into methanol and the precipitate obtained was purified by repeated precipitations using NMP and ether. The polymers were dried under vacuum at 60 ℃ for 24 h and collected as a yellow solid. Yields: 78-82 %.

Metallo-alt-copolymer P2. To zinc acetate (1.25 mmol) in 20 ml of NMP (N-methylpyrrolidinone) solution, monomer 4b (M2) (0.61 mmol) in NMP (20 ml) was added dropwisely. After stirring at r.t. for 2 hr, then monomer 4a (M1) (0.64 mmol) was also added dropwisely. The resulting solution was heated at 105 ℃ under a nitrogen atmosphere.

After stirring for 24 h, excess KPF6 was added into the hot solution. The resulting solution was poured into methanol and the precipitate obtained was purified by repeated precipitations using NMP and ether. The polymers were dried under vacuum at 80 ℃ for 24 h and collected as a yellow solid. Yields: 74-80 %.

Metallo-alt-copolymer P3. The procedure is analogous to that described for P2. Yields:

80-84 %.

Metallo-copolymer P4. To zinc acetate (0.92 mmol) in 20 ml of NMP (N-methylpyrrolidinone) solution, monomer 4b (M2) (0.45 mmol) in NMP (20 ml) was added dropwisely. After stirring at r.t. for 2 hr, then mixture monomers 4a (M1) and 4c (M3)

(0.42 mmol, 4a (M1):4c (M3) = 1:1) was also added dropwisely. The resulting solution was heated at 105 ℃ under nitrogen atmosphere. After stirring for 24 h, excess KPF6 was added into the hot solution. The resulting solution was poured into methanol and the precipitate obtained was purified by repeated precipitations using NMP and ether. The polymer was dried under vacuum at 80 ℃ for 24 h and collected as a yellow solid. Yields: 80 %.

4.4 Result and Discussion

4.4.1 Synthesis and Characterization

The synthetic routes of monomers 4a, metallo-homopolymer P1 and metallo-copolymers P2-P4 are illustrated in Schemes 4.1-4.2.

Scheme 4.1 Synthetic Routes of Monomers 4a-4c (M1-M3)

Metallo-homopolymer P1 was obtained by refluxing monomer 4a (M1) with Zn(OAc)2 at a ratio of 1:1 in NMP solution and followed by subsequent anion exchange.^84,94The key steps in the syntheses of metallo-alt-copolymers were first to functionalize two end terpyridyl units of monomers 4b (M2) and 4c (M3) with Zn(OAc)2at the ratio of 1:2 to afford complexes 5b and 5c, respectively. Then, complexes 5b and 5c as initiators were coordinated with monomer 4a (M1) (see Scheme 4.2) at the ratio of 1:1 (as a sequential-coupling method), respectively, to obtain metallo-alt-copolymers P2 and P3.⁸⁴

Scheme 4.2 Synthetic Routes of Metallo-polymer P1-P4

Moreover, metallo-copolymer P4 was obtained by reacting the mixture of monomers 4a (M1) and 4c (M3) (at the ratio of 1:1) with complex 5b (monomer mixture: complex 5b = 1:1). In contrast to other polymerization methods, i.e. the Witting or Heck coupling reactions, there are three points worthy to be noted: First, the reactive liability of zinc(Ⅱ ) ions and the stability of six-coordinate bis-terpyridine zinc(II) moieties allow self-assembled reactions to take place under refluxing conditions.^28,44,48Second, the present procedure does not need any catalysts. Third, the chemical structures of metallo-copolymers can be controlled by proper stoichiometries of metals and monomers.^84,94

4.4.2 Structural Characterization of¹H NMR.

1H-NMR spectra of monomers 4a-4c (M1-M3), complexes 5b-5c, and metallo-polymers P1-P4 were recorded in DMSO-d6 as shown in Figure 4.1 Compared with ¹H peaks of monomers 4b (M2) and 4c (M3) in our previous study,⁸⁴ those in terpyridyl units of complexes 5b and 5c show the same downfield shift effect in proton peaks of (6,6”)-, (5,5”)-, (4,4”)-, (3’,5’)-, and (3,3”)-H. Furthermore, proton peaks of (4,4”)-H in terpyridyl units of complexes 5b and 5c overlap with¹H peaks of fluorene units. It appears that the formation of complexes 5b-5c can be respectively proven by the disappearance of original¹H peaks in terpyridyl units of monomers 4a-4b (M1-M2) in the mixture of zinc ions and monomers at the ratio of 2:1.⁸⁴ The formation of homopolymer P1 is clearly indicated by the appearance of a new set of¹H peaks and the absence of the original¹H peaks in terpyridyl units, which belong to the uncomplexed monomer 4a (M1).

Figure 4.1¹H-NMR spectra of monomers 4a-4c (M1-M3), complexes 5a-5b, and metallo-polymers P1-P4 in DMSO-d6.

(The assignments of ¹H peaks of the terpyridyl units for all polymers are made by asterisks with respect to 4-chloro-terpyridine Zn²⁺complexes). In terms of ¹H peaks of 1,3,4-OXD and carbazole (CAZ) pendants, there are no obvious changes in chemical shifts among monomers 4a (M1), 4c (M3) and polymers P3, P4. Therefore, the most up-shifted¹H peaks in the terpyridyl units of polymers P3 and P4 could be overlapped with the¹H peaks of these pendants. To distinguish the structural differences among these main-chain metallo-polymers P1-P4, it is feasible to compare the relative integrated ratios of the¹H peaks. As a result, the integrated ratios of the¹H peaks in the terpyridyl units (*A for P1, *A1 for P2, *A2 for P3, and *A3 for P4) and the¹H peaks of the alkyl chains (spacer –CH2–) attached to 1,3,4-OXD (B for P1, B1 for P2, B2 for P3, and B3 for P4) and CAZ units (C2 for P3 and C3 for P4) of these polymers are compared. It reveals that the relative integrated ratios are *A/B = 0.5,

*A1/B1 = 1, *A2/B2 = 1, and *A3/B3 = 2 for metallo-polymers P1, P2, P3, and P4, respectively, which suggests that the integrated ratios of polymers were consistent with the monomer amounts containing pendent 1,3,4-OXD units.⁸⁴ In contrast to the relative integrated ratio of B2/C2 (=1.1) in polymer P3, that of B3/C3 was 1 for polymer P4.

According to these results, the input ratios (molecular ratios) of monomers for polymerization were very similar to the output ratios (the relative integrated ratios of

1H-NMR) of the metallo-polymers. Consequently, the amounts of monomer ligands incorporated in the monomer ligand-based metallo-polymers (i.e., P1: single monomer ligand system; P2 and P3: double monomer ligand systems; P4: triple monomer ligand system) can be confirmed by¹H-NMR.^54,57

4.4.3 Thermal and Viscosity Properties

The thermal and viscosity properties of monomers 4a-4c (M1-M3) and metallo-polymers P1-P4 were studied by thermogravimetric analysis (TGA) and rheometry as summarized in Table 4.1.

Table 4.1 Physcial Properties of Metallo-Polymers (P1-P4)

aThe decomposition temperatures (Td) (5% weight loss) were determined by TGA with heating rates of 20

℃min^-1 under N2atmosphere. The Td values were 221 ℃ for 4a (M1), 351 ℃ for 4b (M2), and 354 ℃ for 4c (M3), respectively.

bThe viscosities of metallo-polymers (10 % in weight) in NMP solutions at 25 ℃ (100 RPM, Spindle number:

4) were determined by rheometer system.

cSolutions of monomers 4a-4c (M1-M3) (10 % in weight) in NMP (with viscosities η = 6-7 cp, 25 ℃ ) were used as references to determine the viscosities of metallo-polymers.

cReduction peaks in N2-purged DMF, r in parentheses means reversible.

d LUMO energy levels were calculated from the measured reduction potentials versus the ferrocene/ferrocenium couple in DMF solutions.

eOptical band gaps were estimated from the absorption spectra in solutions by extrapolating the tails of the lower energy peaks.

The decomposition temperatures (Td) (5% weight loss measured by TGA) of monomers under nitrogen atmosphere were ranged from 221 to 351 ℃ , and those of polymers were ranged from 355 to 389 ℃ . In contrast to monomers, polymers exhibited slightly enhanced thermal stability due to the increased rigidity of the main-chain structures.⁵⁵As the bulky OXD and CAZ pendants are attached to the backbones of the metallo-polymers, it leads to reduced rigidity of the polymers.^31-32 Hence, compared with polymers P1, P3, and P4 containing more bulky OXD and CAZ pendants, P2 shows the highest Td value among these metallo-polymers due to its higher molar ratio of less bulky alkyl pendants. This behavior was also confirmed by that the Td value (422 ℃ ) of metallo-homopolymer containing M2 (with alkyl pendants) is larger than that (399 ℃ ) of metallo-homopolymer containing M3 (with CAZ pendants) in our previous report,⁸⁴ and both Td values are larger than that (355 ℃ as shown in Table 1) of metallo-homopolymer P1 containing M1 (with OXD pendants).

Moreover, the same trend of the pendent size effect on the Tg values were also observed in the monomers, i.e., 4b (M2 with alkyl pendant) = 103 ℃ > 4c (M3 with CAZ pendant) = 92

℃ > 4a (M1 with OXD pendant) = 67 ℃ , so molecular structures with larger pendant groups have lower Tg values. This obviously indicates that the presence of OXD and CAZ pendants in these metallo-polymers suppresses the crystallinity (and chain aggregation) of the polymers effectively. Similar results were also observed in poly(fluorene)-based copolymers containing various 1,3,4-oxadiazole dendritic pendants.^67a

To further investigate these polymers, molecular weight should be reported, however, these polymers showed the poor solubility in THF, CH3Cl, and alcoholic solvents. Therefore, the relative viscosities of polymer to monomer were carried out to support the polymers

To further investigate these polymers, molecular weight should be reported, however, these polymers showed the poor solubility in THF, CH3Cl, and alcoholic solvents. Therefore, the relative viscosities of polymer to monomer were carried out to support the polymers