Synthesis and characterization of liquid crystalline side-chain block copolymers containing luminescent 4,4'-bis(biphenyl)fluorene pendants

Side-Chain Block Copolymers Containing Luminescent

4,4’-Bis(biphenyl)fluorene Pendants

KUAN-WEI LEE, HONG-CHEU LIN

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, Republic of China

Received 18 March 2007; accepted 13 April 2007 DOI: 10.1002/pola.22167

Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A series of new liquid crystalline homopolymers, copolymers, and block copolymers were polymerized from styrene-macroinitiator (SMi) and methacrylates with pendent 4,40-bis(biphenyl)fluorene (M1) and biphenyl-4-ylfluorene (M2) groups through atom transfer radical polymerization (ATRP). The number-average molecular weights (Mn) of polymers P1-P4 were 10,007, 14,852, 6,275, and 10,463 g mol
1with polydispersity indices values of 1.21, 1.15, 1.31, and 1.22, respectively. All polymers exhibit the nematic phase. The thermal, mesogenic, and photoluminescent properties of all polymers were investigated.VVC 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4564–4572, 2007

Keywords: atom transfer radical polymerization; block copolymers; liquid crys-talline polymer; photoluminescence

INTRODUCTION

During the past decade, there are several types of liquid crystalline (LC) block copolymers pro-duced by living free-radical polymerizations.1–5

Matyjaszewski and coworkers6 and Sawamoto

and Kamigaaito7 independently developed

transition metal catalyzed living free radical polymerization known as atom transfer radical polymerization (ATRP), which is typically initi-ated by an alkyl halide (R-X) and catalyzed by a transition metal complex, such as CuX/bpy.

ATRP has been demonstrated to provide

controlled polymerizations of styrenes, metha-crylates, and acrylonitriles with variations of compositions and architectures.

Recently, ATRP were also used in the syn-thesis of side-chain LC block copolymers,8–12 in

which mesogenic pendent groups were grafted onto polymer backbones to form side-chain LC blocks. The interest in this type of materials resides in the cohesive properties of two (or more than two) completely different polymers that are chemically bonded to each other. The macrophase separation takes place because of the segregation of different polymer chains. To introduce luminescent properties into LC poly-mers, the combination of long conjugation rigid cores with flexible chains is required for the

molecular design of luminescent LCs.13–15

However, the microphase structures of side-chain LC polymers were influenced by the mesogenic groups, such as azobenzene16–18and biphenyl19–24 units, which were frequently used in LC monomers.

In our previous studies,25,26main-chain block

copolymers containing conjugated fluorene,

thiophene, and biphenyl backbones as well

as side-chain block copolymers containing

biphenyl-4-ylthiophene or cyanoterphenyl pend-ants possess the smectic A and columnar (Col_h

Correspondence to: H.-C. Lin (E-mail: [email protected]. edu.tw)

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 4564–4572 (2007)

V

and Col_r) phases, but side-chain block co-polymers consisting of biphenyl-4-ylfluorene pendants possess the nematic phase. Herein, a series of a new mesogenic homopolymer and block copolymer (P1 and P2), which were

com-posed of styrene-macroinitiators (SMi) and

methacrylates with pendent 4,40 -bis(biphenyl)-fluorene (M1) groups, were synthesized through ATRP. In addition, random copolymer P3 and block copolymer P4 consisting of pendent 4,40 -bis(biphenyl)fluorene units (M1) and biphenyl-4-ylfluorene units (M2) were also prepared by ATRP. Furthermore, thermal, mesogenic, and PL properties of all polymers were also investi-gated in this study.

EXPERIMENTAL

Measurements 1

H NMR spectra were recorded on a Varian unity 300 MHz spectrometer using CDCl3 and d6-DMSO solvents. Elemental analyses were performed on a HERAEUS CHN-OS RAPID ele-mental analyzer. Transition temperatures were determined by differential scanning calorimetry (DSC) (Perkin-Elmer, model: Diamond) with a heating and cooling rate of 5 8C/min. The meso-genic properties were studied using a polarizing optical microscope (POM) (Leica, model: DMLP) equipped with a hot stage. Thermogravimetric analysis (TGA) was conducted on a Du Pont Thermal Analyst 2100 system with a TGA 2950 thermogravimetric analyzer at a heating rate of 10 8C/min under nitrogen. Gel permeation chro-matography (GPC) analysis was conducted on a Waters 1515 separation module with chloroform as the eluant against a polystyrene calibration curve. High-resolution electron impact mass data were obtained on a Finnigan-MAT-95XL. UV-visible absorption spectra were recorded

in dilute THF solutions (10
6 M) on a HP

G1103A spectrophotometer. Photoluminescence (PL) spectra were obtained on a Hitachi F-4500 spectrophotometer.

Materials

Chemicals and solvents were reagent grade and purchased from Aldrich, ACROS, TCI, and Lan-caster Chemical. Dichloromathane and THF were distilled to keep anhydrous before use. Pyridine was dried by refluxing over calcium

hydride. The other chemicals were used without further purification.

Synthesis

ATRP is very versatile and is tolerant for a wide range of functional groups present either in the monomers, solvents, or initiators.25,27,28Schemes 1 and 2 summarize the steps involved in the synthe-ses of the macroinitiator (SMi), monomers (M1 and M2), and polymers (P1–P4), and the details of each step are given below.

4-Bromo-40-octoxybiphenyl (2)

1-Bromooctane (11.6 g, 60 mmol), 4-bromo-40 -hydroxybiphenyl (10 g, 40 mmol), and potassium carbonate (16.6 g, 120 mmol) were dissolved in butan-2-one (100 mL) and reacted under reflux for 24 h. After cooling to room temperature, the potassium salt was filtered off. The solvent was removed by rotavapor and the crude product was recrystallized from petroleum ether (bp: 35– 60 8C) to yield a white solid (13.5 g, 93%). 1H NMR (ppm, CDCl3), d: 0.89 (t, J ¼ 6.9 Hz, 3H), 1.29–1.47 (m, 10H), 1.80 (quintet, J ¼ 6.6 Hz, 2H), 3.98 (t, J ¼ 8.6 Hz, 2H), 6.99 (d, J ¼ 8.8 Hz, 2H), 7.40–7.54 (m, 6H).25

40-Octoxybiphenyl-4-ylboronic Acid (3)

4-Bromo-40-octoxybiphenyl (2) (5 g, 13.8 mmol) was dissolved in THF (200 mL) and then n-butyllithium (8.9 mL, 2.5 M, 22.1 mmol) was added dropwise to react at
78 8C. The reaction mixture was maintained under this condition for 1 h. Furthermore, it was added dropwise to tri-methyl borate solution (3.5 g, 33.2 mmol) at
78 8C. The solution was allowed to cool to room temperature overnight. The final solution was acidified with 100 mL of 10% HCl solution and stirred for 45 min at room temperature. The solution was washed with saturated sodium carbonate solution and water, and then THF was removed. The crude product was extracted by diethyl ether and the organic layer was dried over magnesium sulfate. After removing the solvent by rotavapor, the resulting solid was washed with petroleum ether and briefly dried on filter to obtain a white solid (6.0 g, 80%). 1

H-NMR (ppm, d6-DMSO), d: 0.85 (t, J ¼ 7.2 Hz, 3H), 1.24–1.41 (m, 10H), 1.71 (quintet, J ¼ 6.6 Hz, 2H), 3.98 (t, J ¼ 6.6 Hz, 2H), 6.99 (d,

J ¼ 8.8 Hz, 2H), 7.56–7.62 (m, 4H), 7.83 (d, J ¼ 8.8 Hz, 2H), 8.03 (s, 2H).25

Compound 5

Compound 4 (2.0 g, 5.3 mmol), compound3 (4.0 g, 12.3 mmol), and tetrakis(triphenylphosphine)-palladium(0) (307 mg, 0.27 mmol) were reacted in THF (120 mL) for 10 min, and then 80 mL of 2 M aqueous Na2CO3 solution was added. The mixture was reacted and refluxed for 48 h. After

reaction, the cooled solution was washed with dilute hydrochloric acid (10%) and water, and dried over magnesium sulfate. The final solution was purified by column chromatography (silica gel, CH2Cl2/hexane 1:1) to yield a white solid (3.3 g, 87%).1H NMR (ppm, CDCl3), d: 0.42 (t, J ¼ 6.9 Hz, 6H), 0.89 (t, J ¼ 6.9 Hz, 6H), 1.31– 1.61 (m, 20H), 1.84 (m, 4H), 2.14 (q, J ¼ 7.3 Hz, 4H), 4.01(t, J ¼ 6.0 Hz,4H), 6.98 (d, J ¼ 7.8 Hz, 4H), 7.57–7.80 (m, 18H).

Compound 6

Compound 5 (3.3 g, 4.5 mmol) was dissolved in dry chloroform (150 mL) under nitrogen and then boron tribromide (3.4 g, 13.6 mmol) was added dropwise to react at
78 8C. The mixture was allowed to warm up to room temperature and reacted for 24 h. The solution was washed with sodium hydroxide (1 M, 50 mL), and then the solution was acidified with 10% HCl and stirred for 4 h. Finally, the suspension was filtered off and purified by column chromatogra-phy (silica gel, ethyl acetate) to yield a white solid (2.4 g, 94%). 1H-NMR (ppm, d6-DMSO), d: 0.29 (t, J ¼ 7.2 Hz, 6H), 2.16 (q, J ¼ 7.2 Hz, 4H), 6.87 (d, J ¼ 8.4 Hz, 4H), 7.5 (d, J ¼ 8.7 Hz, 4H), 7.70 (m, 6H), 7.79 (m, 6H), 7.91 (d, J ¼ 7.8 Hz, 2H), 9.63 (s, 2H). Compound 7

Compound 6 (2.2 g, 3.94 mmol) and potassium carbonate (1.09 mg, 7.88 mmol) were dissolved in 50 mL of DMF and then 1-bromooctane (760 mg, 3.94 mmol) was added in solution to react for 24 h by reflux. After cooling to room temperature, the solution was extracted with dichloromethane and water, and the organic layer was dried over magnesium sulfate. The final solution was purified by column chromatog-raphy (silica gel, CH2Cl2) to yield a white solid (1.3 g, 54%).1H-NMR (ppm, CDCl3), d: 0.43 (t, J ¼ 7.2 Hz, 6H), 0.90 (t, J ¼ 6.6 Hz, 3H), 1.25– 1.48 (m, 10H), 1.82 (m, 2H), 2.14 (q, J ¼ 6.9 Hz, 4H), 4.01 (t, J ¼ 6.6 Hz, 2H), 4.90 (s, 1 H), 6.92– 7.01 (m, 4H), 7.53–7.67 (m, 12H), 7.73–7.80 (m, 6H). Compound 8

Compound 7 (1.2 g, 1.80 mmol), 6-bromo-1-hexa-nol (421 mg, 2.33 mmol), K2CO3 (740 mg, 5.4 mmol), and KI (20 mg) were dissolved in 80 mL of DMF to reflux overnight. The reaction mix-ture was then cooled and poured into 200 mL of water and stirred for 2 h. The crude product was extracted with ethyl acetate and the organic layer as washed with a saturated aqueous solu-tion of NaCl and water, and then the organic layer was dried over magnesium sulfate. After removing the solvent by rotavapor, the residue was recrystallized from absolute ethanol to give a colorless solid (1.1 g, 80%).1H-NMR (ppm, CDCl3), d: 0.43 (t, J ¼ 7.2 Hz, 6H), 0.88 (t, J ¼ 6.0 Hz, 3H), 1.31–1.86 (m, 20H), 2.15 (m, 4H),

3.67 (m, 2H), 4.01 (m, 4H), 7.00 (m, 4H), 7.57– 7.80 (m, 18H).

M1: Compound 8 (1.1 g, 1.4 mmol), triethyl-amine (1.4 g, 14.3 mmol), and 2,6-ditertbutyl-4-methylphenol (30 mg, as a thermal inhibitor) were dissolved in 100 mL of anhydrous THF under a nitrogen atmosphere and then metha-cryloyl chloride (448 mg, 4.3 mmol) was added dropwise. The reaction mixture was heated under reflux overnight and then cooled to pour into 200 mL of aqueous NH4Cl solution (10%). The crude product was extracted with CH2Cl2. The resulting organic layer was washed with a saturated solution of NaCl and water, and the organic layer was dried over magnesium sulfate. After removing the solvent by rotavapor, the resulting solid was purified by column chroma-tography using dichloromethane as an eluant to yield a colorless solid (1.1 g, 92%).

1

H-NMR (ppm, CDCl3), d: 0.41 (t, J ¼ 7.2 Hz, 6H), 0.88 (t, J ¼ 6.9 Hz, 3H), 1.31–1.82 (m, 20H), 1.94 (s, 3H), 2.12 (m, 4H), 3.98(m, 2H), 4.15 (m, 4H), 5.54 (s, 1H), 6.09 (s, 1H), 6.98 (m, 4H), 7.55– 7.79 (m, 18H). Elemental analysis for C59H66O4: Calc. C, 84.45; H, 7.93; Found C, 84.12; H, 7.74. LRMS (EI) m/z: Calc. 838.4; Found 838.4.

M2: The synthetic route has been reported in our previous study.26 Yield: 83%.

1 H-NMR (ppm, CDCl3), d: 0.36 (t, J ¼ 6.9 Hz, 6H), 1.45–1.56 (m, 4H), 1.67–1.83 (m, 4H), 1.94 (s, 3H), 2.06 (m, 4H), 4.01 (t, J ¼ 6.3 Hz, 2H), 4.16 (t, J ¼ 6.6 Hz, 2H), 5.54 (s, 1H), 6.09 (s, 1H), 6.98 (d, J ¼ 8.7 Hz, 2H), 7.32–7.36 (m, 3H), 7.55–7.77 (m, 10H). Elemental analysis for C39H42O3: Calc. C, 83.83; H, 7.58; Found C, 83.52; H, 7.54. HRMS (EI) m/z: Calc. 558.3134; Found 558.3126.

Polymerzation of Macroinitiator SMi

In a Schlenk flask, N,N,N0,N0

,N@-pentamethyl-diethylenetriamine (PMDETA, 3.46 mg, 0.5

mmol), CuBr (14.3 mg, 0.25 mmol), and styrene (6.8 g, 65 mmol) were added and stirred for 30 min. Ethyl 2-bromo-2-methylpropanoate (195 mg, 1 mmol) was added, and the mixture was immediately frozen in liquid nitrogen under vac-uum. After several freeze-thaw cycles, the flask was sealed under vacuum and put in an oil bath at 100 8C for 20 h. After the reaction, the con-tent was dissolved in chloroform. After being concentrated, the chloroform solution was pre-cipitated into methanol and the precipitation was repeated for three times. The final product

was dried at 50 8C under vacuum. Yield: 75%. The number-average molecular weight mea-sured by GPC is Mn ¼ 6196 g mol
1 with PDI (Mw/ Mn)¼ 1.11.

General Synthetic Procedures of All Polymers According to analogous procedures as shown in Scheme 2, P1–P4 were synthesized by utiliza-tion of different initiators.

An Example of Polymerization for Polymer P2 Four milligram (0.04 mmol) of CuCl, 63 mg (0.01 mmol) of SMi, and 251 mg (0.3 mmol) of M1 were mixed under nitrogen. 27 lL (22.9 mg, 0.1 mmol) of 1,1,4,7,10,10-hexamethyltriethyle-netetramine (HMTETA) in 3 mL of anisole was added through a syringe. The mixture was degassed three times using the freeze-pump-thaw procedure and sealed under vacuum. After stirring for 30 min at room temperature, the mixture was reacted at 1008C in a preheated oil bath for 12 h. The solution was passed through a neutral Al2O3 column with THF as an eluant to remove the catalyst. The white filtrate was concentrated under reduced pressure and repre-cipitated twice into methanol. The white product of polymer was collected by filtration and dried under vacuum. Yield: 125 mg (40%).

1_{H-NMR (ppm, CDCl}

3), d: 0.35 (broad), 0.88 (broad), 1.28–1.78 (broad), 2.01 (broad), 3.94 (broad), 6.56 (broad), 6.98 (broad), 7.55 (broad). The number-average molecular weight mea-sured by GPC is Mn ¼ 14,852 g mol
1with PDI (Mw/ Mn)¼ 1.15.

P1: Yield: 85 mg (34%). 1

H-NMR (ppm, CDCl3), d: 0.36 (broad), 0.88 (broad), 1.28–1.94 (broad), 2.03 (broad), 3.97 (broad), 6.94 (broad), 7.57 (broad). The number-average molecular

weight measured by GPC is Mn ¼ 10,007 g

mol
1with PDI (Mw/ Mn)¼ 1.21.

P3: (M1/M2/initiator ¼ 15/15/1). Yield: 75 mg (30%). 1H-NMR (ppm, CDCl3), d: 0.36 (broad), 0.86 (broad), 1.24–1.94 (broad), 2.04 (broad), 3.99 (broad), 6.96 (broad), 7.30 (broad), 7.62 (broad). The number-average molecular weight measured by GPC is Mn ¼ 6275 g mol
1 with PDI (Mw/Mn)¼ 1.31.

P4: (M1/M2/initiator ¼ 15/15/1). Yield: 121 mg (39%).1H-NMR (ppm, CDCl3), d: 0.35 (broad), 0.87 (broad), 1.28–1.79 (broad), 2.03(broad), 3.96 (broad), 6.55 (broad), 6.94(broad), 7.29 (broad), 7.55 (broad). The number–average molecular

weight measured by GPC is Mn ¼ 10,643 g

mol
1with PDI (Mw/Mn)¼ 1.22.

RESULTS AND DISCUSSION

Synthesis and Characterization

In this study, the styrene-macroinitiator (SMi),6 was used to copolymerize with mathacrylate monomers containing luminescent 4,40-bis (biphe-nyl)fluorene units (M1) to produce diblock copoly-mer P2 by ATRP. In addition, random copolycopoly-mer P3 and block copolymer P4 containing pendent 4,40-bis(biphenyl)fluorine units (M1) and bi-phenyl-4-ylfluorene units (M2) were synthesized by similar (ATRP) procedures. The number-aver-age molecular weights (Mn) and PDI values of all polymers (P1–P4) containing mesogenic 4,40-bis (biphenyl)-fluorene units and biphenyl-4-ylfluor-ene units are shown in Table 1. The number–av-erage molecular weight (Mn) of macroinitiators (SMi) is 6196 g mol
1with a PDI value of 1.11. Comparing NMR sprctra of random copolymer P3 and block copolymer P4, a broad peak of P4 at 6.56 ppm belonged to the chemical shift of poly-styrene block and a broad peak at 6.98 ppm incor-porated chemical shifts of M1 and M2 with that of polystyrene block, so P4 could be recognized as a diblock copolymer. However, because of the overlapped proton peaks around phenyl and alkyl groups of M1 and M2 in NMR spectra of polymers P3 and P4, the molar ratio of M1 and M2 in random copolymer P3 and block copolymer P4 could not be distinguished, so the values of x and y in the chemical structures of random co-polymer P3 and block coco-polymer P4 in Scheme 2 are undetermined.

Thermal and Mesogenic Properties

The thermal stability of polymers (P1–P4)

under an atmosphere of nitrogen was evaluated

Table 1. Molecular Weights and Polydispersity Indexes of Polymers P1-P4 Polymer Mn (g mol
1) Mw (g mol
1) PDI (Mw/Mn) P1 10,007 12,108 1.21 P2 14,852 17,137 1.15 P3 6275 8220 1.31 P4 10,463 12,984 1.22

by TGA, which indicates that Td (the degrada-tion temperature of 5% weight loss in nitrogen) 350 8C for all polymers (shown in Table 2). The mesogenic properties were characterized by polarizing optical microscopy (POM) and differ-ential scanning calorimetry (DSC). The phase transition temperatures and enthalpies of all polymers are summarized in Table 2. Regarding their mesomorphism, all polymers exhibit the nematic phase with mesophasic ranges wider than 200 8C. In our previous studies,25,26 block copolymers containing conjugated main-chain fluorene, thiophene, and biphenyl blocks as well

as block copolymers containing side-chain

biphenyl-4-ylthiophene or cyanoterphenyl blocks possess the smectic A and columnar (Col_h and Col_r) phases, but block copolymers consisting of side-chain biphenyl-4-ylfluorene blocks possess the nematic phase. Compared with the smectic A phase of the polymers in our previous studies, the side-chains of the rigid cores in polymers P1–P4 composed of pendent 4, 40 -bis(biphenyl)-fluorene and biphenyl-4-yl-bis(biphenyl)-fluorene units were separated by the diethyl groups on 9th position of fluorene units which cause the reduction of lateral interaction among rigid rods and thus to favor the nematic phase. To avoid thermal decomposition, these polymers were only heated up to about 320 8C (with a heating rate of 5 8C/ min) and all polymers (P1–P4) revealed isotrop-ization temperatures (Ti) around 299–316 8C. Figure 1 showed that a schlieren texture of the corresponding nematic phase of P1 observed by POM at 2808C (cooling).

Because of the unclear glass transition tem-peratures (Tg) of these block copolymers, Tg

val-ues were detectable in the first heating scans of DSC measurements (with a heating rate of 58C/ min) by quenching of polymers (from 200 8C) in liquid nitrogen. By this quenching process, the DSC results can reveal Tg values of all polymers more clearly (in the range of 56–90 8C as shown in Table 2). In Table 2, polymers P1 and P3 ex-hibit the glass transition temperatures (Tgs) at 60 and 56 8C, respectively. Tgs of P2 and P4 (around 908C) were mostly contributed from the polystyrene block.29,30

To elucidate the structures of the mesophases, X-ray diffraction (XRD) measurements were car-ried out at the temperature ranges of meso-phases, which were determined by DSC and POM, for polymers P1–P4. The nematic phase of all polymers characterized by the schlieren texture of POM is further confirmed by the results of no refraction peaks observed in the XRD patterns.

Photophysical Properties

The photophysical properties of polymers P1–P4 including UV-visible absorption spectra and PL spectra in THF solutions are summarized in Table 3. Because of to identical rigid cores of luminescent 4,40-bis(biphenyl)fluorene pendants (M1), the synthesized polymers P1 and P2 have almost the same maximum absorption wave-length around 343 nm in solutions and solid films. In addition, the maximum PL wavelengths (kmax,PL) were around 396 nm in solutions and 425 nm in films, respectively. Polymers P3 and

Table 2. Thermal Properties of Polymers P1-P4a,b,c

Polymer Transition Temp. (8C)b,c Td(8C)a Tg(8C) P1 N 299.5 (1.78) I 392.0 60.1 P2 N 316.1 (0.76) I 367.3 88.2 P3 N 308.4 (4.54) I 351.0 56.4 P4 N 298.2 (4.34) I 362.3 89.7

Transition temperatures (measured by DSC) of M1 and M2—M1: K 104.4 8C (31.5 kJ/g) N 266.0 8C (1.86 kJ/g) Ti and M2: K 60.4_{8C (3.4 kJ/g) N 98.0 8C Ti (Ti was} character-ized by POM).

a_{Temperature of 5% weight loss measured by TGA under}

nitrogen.

b_{Transition temperatures (8C) and enthalpies (in}

paren-theses, kJ/g)were determined by DSC (with heating rates of 58C /min).

c_N

¼ nematic phase; I ¼ isotropization temperature.

Figure 1. The schlieren texture (the nematic phase) of P1 observed by POM at 2808C (cooling).

P4 both possess the biphenyl-4-ylfluorene pend-ants (M2), therefore, they had the same maxi-mum absorption wavelength around 336 nm in solutions and films. Figure 2 shows an example of PL spectra of the monomers (M1 and M2) and polymers (P1 and P2) in solutions. Com-pared with M2 (which was reported in our pre-vious study),26 M1 consisting of an additional biphenyl unit results in the longer conjugation lengths than M1. Hence, the maximum absorp-tion wavelengths and PL wavelengths of M1 are more red-shifted than M2. In Figure 3, because of the biphenyl-4-ylfluorene pendants of P3, the

maximum absorption wavelength of P3 was more blue-shifted than that of P1 (around 343 nm) in films and solutions. However, the kmax,PL values of P1 and P3 were almost the same (396 nm in solutions and 423 nm in films). This phe-nomenon may be originated from the energy transfer of biphenyl-4-ylfluorene pendants to 4,40-bis(biphenyl)fluorene pendants which can be confirmed by the overlapped UV-visible spectra of M2 and PL spectra of M1. However, Intermo-lecular aggregation of luminescent pendants could cause shoulders of P1 and P3 around 440 nm in Figure 3. In addition, the quantum yields of all polymers consist of 4,40 -bis(biphenyl)fluor-ene groups (70%) were better than other analo-gous polymers (around 20%) in our previous studies.25,26

CONCLUSIONS

Polymers (P1 and P2) containing 4,40 -bis(biphen-yl)fluorene pendants (M1) and random copoly-mers (P3 and P4) composed of 4,40

-bis(biphe-nyl)fluorene pendants (M1) and

biphenyl-4-ylfluorene pendants (M2) are polymerized by ATRP. Thermal and XRD investigations indicate that polymers exhibit the nematic phases which have little relationship with respect to the flexi-ble polystyrene block, but the glass transition temperatures (Tgs) of block copolymers P2 and P4 containing polystyrene blocks are higher than those of polymers P1 and P3 without the polystyrene block. In terms of PL and absorption wavelengths of all polymers in dilute solutions,

Table 3. UV-Visible Absorption and

Photoluminescence Spectral Data of Monomers M1-M2 and Polymers P1-P4

Sample

kmax,Abs(nm) kmax,PL(nm)

/(%)b Solutiona Film Solutiona Film

M1 344 – 392 – 74 M2c 322 – 385 – 23 P1 344 343 396 425 73 P2 344 340 397 424 70 P3 338 336 395 422 70 P4 337 336 394 423 72

a_{Absorption and PL emission spectra were recorded in}

dilute THF solutions at room temperature.

b_{9,10-Diphenylanthrance in THF is used as the reference}

of the quantum yield.

c_{UV-visible absorption and PL data of M2 in solutions}

was reported in our previous study.26

Figure 2. PL spectra of monomers (M1 and M2) and polymers (P1 and P3) in solutions.

Figure 3. UV-visible absorption and PL spectra of P1 and P3 in solid films.

P1 and P2 are a little red-shifted than P3 and P4, which might be due to longer conjugation lengths of the 4,40-bis(biphenyl)fluorene pend-ants (M1) in polymers P1 and P2.

The authors are grateful for the financial support pro-vided by the National Science Council of Taiwan (ROC) through NSC 94-2113-M-009-005. The powder XRD measurements were supplied by beamline BL17A (charged by Jey-Jau Lee) of the National Synchrotron Radiation Research Center (NSRRC), in Taiwan.

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