Synthesis and characterization of a thermotropic liquid crystalline hyperbranched polyester

64

0

Research ArticleReceived: 5 August 2008 Revised: 10 November 2008 Accepted: 21 December 2008 Published online in Wiley Interscience: 20 April 2009

(www.interscience.wiley.com) DOI 10.1002/pi.2573

Synthesis and characterization of athermotropic liquid crystalline hyperbranchedpolyesterLiming Cai and Yafei Lu∗

Abstract

BACKGROUND: Hyperbranched polymers have received increasing attention in the fields of medicine, homogeneouscatalysis and materials science. Hydroxyl-functional aliphatic polyesters are one of the most widely investigated familiesof hyperbranched polymers. The research reported here is based on the preparation of a novel hyperbranched polyester andthe modification of its terminal hydroxyl groups by biphenyl mesogenic units.

RESULTS: 2,2,6,6-Tetramethylolcyclohexanol as a core and 8-[4′-propoxy(1,1-biphenyl)yloxy]octanoic acid as a mesogenic unitwere synthesized. A hyperbranched polyester (HPE) was synthesized in one step and subsequently substituted by reaction of itsterminal hydroxyl groups with the biphenyl mesogenic units to yield a novel liquid crystalline hyperbranched polyester (HPE-LC).The chemical structures of all compounds were confirmed using Fourier transform infrared, 1H NMR and 13C NMR spectroscopy.The thermal behavior and the mesogenic properties of the biphenyl mesogenic unit and HPE-LC were investigated usingdifferential scanning calorimetry, polarized optical microscopy and wide-angle X-ray diffraction. The results demonstrated thatthe degree of branching of the HPE is ca 0.63. Both HPE-LC and the biphenyl mesogenic unit exhibit mesomorphic properties,but HPE-LC has a lower isotropic transition temperature and a wider transition temperature range than the biphenyl mesogenicunit.

CONCLUSION: A novel liquid crystalline hyperbranched polyester was successfully synthesized, which exhibits mesomorphicproperties. This polymer has good solubility in highly polar solvents and good thermal stability.c© 2009 Society of Chemical Industry

Keywords: hyperbranched polyesters (HPEs); liquid crystals (LCs); synthesis; characterization

INTRODUCTIONDendrimers and hyperbranched polymers have attracted a greatdeal of attention since Kim and Webster reported hyperbranchedpolyphenylenes, which were found to exhibit properties differentfrom those of other linear analogues.1 Dendrimers have perfectmonodisperse architecture, but it is difficult to apply them sincetheir preparation is considered quite tedious. Hyperbranchedpolymers are imperfect and contain linear units as insufficientbranching but they can be conveniently prepared on a significantscale by one-step polycondensation with ABx-type monomers.They have many important features of dendrimers, specificallygood solubility, low viscosity and multiple end groups relativeto their linear analogues. Hyperbranched polymers have receivedincreasing attention in the fields of medicine, homogeneous catal-ysis and materials science.2 – 4 A large number of hyperbranchedpolymers have been reported in earlier literature including, forexample, polyesters, polyethers, polyamides, polyurethanes andpolysiloxysilanes.5 – 9 Hydroxyl-functional aliphatic polyesters areone of the most widely investigated families of hyperbranchedpolymers. The first aliphatic hyperbranched polyester (HPE)was reported by Hult and co-workers in 1995 synthesized us-ing 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) as AB2-typemonomer and trimethylolpropane as B3 core.5 Since then, manyaliphatic By core molecules and aliphatic ABx monomers have been

utilized to prepare new aliphatic HPEs, and a commercially avail-able aliphatic HPE named Boltorn series has been developed.10 – 17

These HPEs have many terminal hydroxyl groups at their periph-ery. In this article, we report the synthesis of a new aliphatic HPEthrough the reaction between 2,2,6,6-tetramethylolcyclohexanol(TMC) as a B5 core moiety and bis-MPA as an AB2 monomer.This HPE has more terminal hydroxyl groups at the periphery andbetter solubility than the polymer with a B3 core.

In recent years, there has been considerable interest indeveloping hyperbranched polymers. A significant number ofhyperbrached polymer structures have been reported to be liquidcrystalline (LC). In 1992, Kim reported the synthesis of lyotropic LChyperbranched aromatic polyamides.18 At about the same time,Percec and co-workers reported that a hyperbranched polymercontaining disc-like mesogens displayed a thermotropic columnarhexagonal mesophase. They also reported the first hyperbranchedthermotropic liquid crystals exhibiting a nematic mesophase.19 – 21

∗ Correspondence to: Yafei Lu, Beijing University of Chemical Technology, Box82, Beijing 100029, China. E-mail: [email protected]

Key Laboratory of Beijing City on Preparation and Processing of Novel PolymerMaterials, Box 82, Beijing University of Chemical Technology, Beijing 100029,PR China

Polym Int 2009; 58: 640–647 www.soci.org c© 2009 Society of Chemical Industry

64

1

Thermotropic liquid crystalline hyperbranched polyester www.soci.org

Ringsdorf and co-workers prepared a HPE composed of biphenylp-oxybenzoate type mesogenic structures and decamethylenespacers. This polymer was reacted with a chirally substitutedaromatic compound, and the resulting polymer revealed athermotropic cholesteric mesophase.22 In general, HPEs do notcrystallize, but in combination with mesogenic units they mayexhibit LC phases.23 Sunder et al. reported a LC HPE synthesizedfrom an aliphatic HPE containing terminal hydroxyl groups and aseries of biphenyl-type liquid crystals.24

DB = � dendritic units + � terminal units

� dendritic units + � linear units + � terminal units(1)

In the work reported in this paper, we synthesized andcharacterized a new HPE and its derivatives by reaction ofthe terminal hydroxyl groups with a biphenyl mesogenic unit.A significant increase of the glass transition temperature ofthe polymer was found after substitution with the mesogenic

HO OH + CH3CH2CH2Br OCH2CH2CH3HO

HOOC(CH2)7OOCH2CH2CH3HO OCH2CH2CH3

C2H5OOC(CH2)7Br

1) TABA, K2CO32) NaOH, THF

C3H7O OC7H14COO

Scheme 1. Synthetic routes to TMC, mesogenic unit, HPE and HPE-LC.

Polym Int 2009; 58: 640–647 c© 2009 Society of Chemical Industry www.interscience.wiley.com/journal/pi

64

2

www.soci.org L Cai, Y Lu

Figure 1. 1H NMR spectrum of 4-propoxy-4′-hydroxybiphenyl (mesogenicintermediate).

units, and the extent of substitution also affected the mesogenicproperties of the polymer.

EXPERIMENTALMaterialsParaformaldehyde, cyclohexanone, 4,4′-dihydroxybiphenyl, 1-bromopropane, 8-bromocaprylic acid ethyl ester, potassium hy-droxide, alcohol, potassium iodide, thionyl chloride, tetrabutylam-monium bromide (TBAB), chloroform, acetone, n-hexane, bis-MPA,p-toluenesulfonic acid (p-TSA), dimethylformamide (DMF), triethy-lamine (TEA), tetrahydrofuran (THF) and dimethylsulfoxide (DMSO)were of analytical grade and used as received from Beijing Chemi-cal Reagents Company.

Synthesis of TMCTMC was prepared by a procedure described in the literature.25

Paraformaldehyde (3.32 g, 110 mmol), cyclohexanone (1.96 g,20 mmol) and water were mixed in a three-necked flask, andcalcium hydroxide (0.93 g, 12.5 mmol) was gradually added during

30 min. After 1.5 h, formic acid was added to the reaction mixtureto adjust the pH to 6–6.5, and then the reaction mixture wasplaced under vacuum to eliminate the volatiles. The residue wascrystallized with methanol and a white solid was obtained in 58.3%yield.

1H NMR (DMSO; δ, ppm): 1.00–1.48 (m, 6H, –CH2 –), 3.33–3.64 (s,8H, –CH2 –OH), 3.72 (s, 1H, –CH–OH), 4.40–4.67 (s, 5H, –OH). 13CNMR (DMSO; δ, ppm): 17.33 (–CH2CH2 –), 28.17 (–CH2C–), 43.81(–C–), 61.67–67.20 (–CH2OH), 75.99 (–CHOH). Fourier transforminfrared (FTIR) (KBr; ν , cm−1): 3375 (OH), 2927, 2878 (CH, CH2),1110(–C–O).

Synthesis of 4-propoxy-4′-hydroxybiphenyl (mesogenicintermediate)

4,4′-Dihydroxybiphenyl (9.41 g, 50 mmol) and alcohol were mixedin a three-necked flask, and a mixed alcohol–water solution ofpotassium hydroxide and potassium iodide was added dropwisewith stirring for 15 min. Then an alcohol–water solution of 1-bromopropane (7.38 g, 60 mmol) was added dropwise and themixture was reacted for 8 h. The reaction mixture was placeunder vacuum to remove solvent, the residue dissolved withheated sodium hydroxide solution and filtered while still hot. Theprecipitate, obtained from the cool filtrate, was acidified withhydrochloric acid, and recrystallized with alcohol. A white solidwas obtained in 55% yield.

1H NMR ((CD3)2CO; δ, ppm): 1.03–1.05 (t, 3H, –CH3), 1.78–1.82(m, 2H, –CH2CH3), 3.97–3.99 (t, 2H, –CH2O–), 5.14 (s, 1H,–OH), 6.89–7.53 (m, 8H, Ar–H). FTIR (KBr; ν , cm−1): 3401 (–OH),3034 (–Ar–H), 2934, 2857 (–CH2 –), 1605, 1500 (–Ar–), 1260(–Ar–O–C–).

Figure 2. (A) 1H NMR spectrum and (B) 13C NMR spectrum of the biphenyl mesogenic unit.

www.interscience.wiley.com/journal/pi c© 2009 Society of Chemical Industry Polym Int 2009; 58: 640–647

64

3


Figure 3. (A) 1H NMR spectrum and (B) 13C NMR spectrum of HPE.

Figure 4. FTIR spectra of (a) TMC, (b) HPE, (c) HPE-L1, (d) HPE-L2 and(e) HPE-LC.

Synthesis of 8-[4′-propoxy(1,1-biphenyl)yloxy]octanoic acid(mesogenic unit)TBAB (0.81 g, 2.52 mmol), potassium carbonate (4.43 g, 32 mmol),8-bromooctanoic acid ethyl ester (10.32 g, 41 mmol), 4-propoxy-4′-hydroxybiphenyl (6.84 g, 30 mmol) and acetone (100mL) weremixed in a three-necked flask and reacted for 24 h. The reactionmixture was acidified with hydrochloric acid after cooling, washedwith distilled water and crystallized with alcohol. The solid thusobtained was mixed with sodium hydroxide solution at 45 ◦C for30 min. The mixture was washed with hydrochloric acid at roomtemperature and then filtered. The cake obtained was washed

with distilled water, and crystallized with alcohol. A white solidwas obtained in 71% yield.

1H NMR (CDCl3; δ, ppm): 1.01–1.04 (t, 3H, –CH3), 1.27–1.83 (m,12H, –CH2 –), 2.25–2.29 (t, 2H, –CH2 –COO–), 3.93–3.95 (t, 4H,–CH2O–), 6.92–7.58 (m, 8H, Ar–H), 12.08 (s, 1H, –COOH). 13C NMR(CDCl3; δ, ppm): 10.53 (–CH3), 22.61–34.33 (–CH2 –), 67.95–69.57(–CH2O–), 114.72–158.19 (–Ar–O–), 179.04 (–COOH). FTIR (KBr;ν , cm−1): 3425 (–OH), 3035 (–Ar–H), 2934, 2857 (–CH2 –), 1701(–C O), 1606, 1501 (–Ar–), 1277(–Ar–O–C).

Synthesis of HPETMC (0.88 g, 4.0 mmol), bis-MPA (8.04 g, 60.0 mmol) and p-TSA(17.8 mg) were mixed in a three-necked flask equipped with anitrogen inlet, a drying tube and a stirrer. The flask was placed inan oil bath heated to 140 ◦C, and the mixture was reacted for 2 hunder nitrogen atmosphere. The nitrogen stream was then turnedoff and the mixture was evacuated for 1 h. When the pressurewas increased to atmospheric, bis-MPA (10.72 g, 80 mmol) andp-TSA (43.8 mg) were added and the nitrogen flow was restarted.The reaction was performed for 2.5 h at normal pressure, andthen evacuated for 2 h. The crude polymer was precipitated fromacetone using n-hexane. The cake obtained was dried undervacuum and HPE was obtained. FTIR (KBr) analysis showed noremaining carboxylic acid (1696 cm−l , carbonyl) but only ester(1740 cm−l , carbonyl).

Synthesis of biphenyl-based carboxylic acid-modified HPEs8-[4′-Propoxy(1,1-biphenyl)yloxy]octanoic acid, thionyl chlorideand DMF were mixed in a three-necked flask and heated for5 h. Excess thionyl chloride was removed under vacuum, andthe mixture was washed three times with petroleum ether, andthen dissolved with THF. This solution was added dropwise into


64

4


Figure 5. 1H NMR spectrum of HPE-LC.

a mixture of HPE, TEA and THF, and was reacted for 24 h atroom temperature. The reaction mixture was filtered and thefiltrate was distilled to remove THF. The solid thus obtainedwas dissolved in dichloromethane and the solution was washedwith sodium bicarbonate and water in turn. The crude productwas precipitated with methanol, and the cake was dried undervacuum after filtering. The resulting product was liquid crystallineHPE (HPE-LC).

The number of hydroxyl groups consumed by the mesogenicunit corresponded to 50, 75 and 100% (in theoretical molar ratio)and the corresponding specimens were designated as HPE-L1,HPE-L2 and HPE-LC, respectively. The hyperbranched polymers,prepared with different contents of mesogenic unit, were solublein highly polar solvents including THF, CDCl3, DMF, DMSO anddichloromethane. Acetone and ethyl acetate were non-solventsfor these polymers.

Characterization1H NMR and 13C NMR spectra were recorded with an Avance DMX-600 NMR spectroscope using tetramethylsilane as an internalstandard for chemical shifts at ambient temperature. FTIR spectrawere recorded using a Nexus-8700 spectrometer with samples inKBr pellets.

Gel permeation chromatography (GPC) was carried out usinga Waters liquid chromatography system equipped with a 600Emultisolvent delivery system and a 996 photodiode array detector,using a combination of two Styragel (HR2 and HR4) high-resolutioncolumns. The flow rate of the mobile phase was kept at 1.0 mLmin−1 and the molecular weight was calibrated with polystyrenestandards.

TGA was conducted using a TGS-2 instrument working with8–10 mg samples in aluminium pans, with a constant heatingrate of 10 ◦C min−1 in the temperature range 25–400 ◦C. DSC wasperformed with a Mettler DSC-30 thermal analyzer using samplesof ca 5 mg in a covered aluminium pan under nitrogen at a heatingor cooling rate of 10 ◦C min−1.

Thermo-optical studies were performed using a Jenapolpolarized optical microscopy (POM) instrument equipped witha Linkam THMS 600 heating/cooling stage and a Linkam TMS92 temperature control unit. The heating and cooling rates were0.5 ◦C min−1. Microphotographs were obtained at an interval of0.5 ◦C.

Wide angle X-ray diffraction (WAXD) was carried out using aBruker D8 advance X-ray diffractometer with a graphite monochro-mator using 1.5406 Å Cu Kα radiation at room temperature(scanning rate: 0.05◦ s−1; scan range: 5–60◦).

RESULTS AND DISCUSSIONSynthesis and characterizationThe synthetic routes to TMC, the mesogenic unit, HPE and HPE-LCare outlined in Scheme 1. Figure 1 shows the 1H NMR spectrumof the mesogenic intermediate (4-propoxy-4′-hydroxybiphenyl).Peaks at both 3.97–3.99 ppm (–OCH2 –) and 5.14 ppm (HO–Ar–)confirm the structure of 4-propoxy-4′-hydroxybiphenyl. The 1HNMR and 13C NMR spectra of the mesogenic unit (Fig. 2) indicatethat etherification was complete by the appearance of the peaksrelated to –O–at 158.19 ppm (–OAr–), 69.57 ppm (–OCH2 –) and67.95 ppm (–OCH2 –) and the peak at 179.04 ppm (–COOH) in the

Figure 6. DSC scans of (A) the biphenyl mesogenic unit (a, on heating; b, on cooling) and (B) HPE and HPE derivatives (a, HPE; b, HPE-L1; c, HPE-L2; d,HPE-LC on heating; e, HPE-LC on cooling).


64

5


13C NMR spectrum and the peak at 12.08 ppm (–COOH) in the 1HNMR spectrum.

GPC was used for the molecular weight characterization ofHPE and the HPE derivatives. The molecular weight of thehyperbranched polymers showed an increase with degree ofsubstitution, and the experimentally measured molecular weightswere systematically lower than the theoretical values obtainedfrom GPC measurements. The molecular weights of HPE andHPE-LC from GPC are 3012 and 5204 g mol−1, respectively.

The chemical structure of HPE was identified from the 1H NMRand 13C NMR spectra, as shown in Fig. 3. The 1H NMR spectrumshows the peaks of –CH2OH (3.68–3.72 ppm) and –COOCH2 –(4.06–4.28 ppm) of HPE, which confirms the chemical structureof HPE.

The 13C NMR spectrum exhibits four distinct groups of peaks.The methylene groups are found at 63–72 ppm. The signals of thequaternary carbons are in the region 40–52 ppm. Methyl groupsignals are found at the lowest chemical shifts, around 15–20 ppm.The repeating unit, bis-MPA, in the HPE could be incorporated intothe polymer in three main ways: dendritic (D; 46.75 ppm), terminal(T; 50.47 ppm) and linear (L; 48.63 ppm) repeating units.26 Theirsignificant chemical shifts are reported in Table 1. For an idealdendritic substance, the degree of branching (DB) is equal to 1. Ahyperbranched polymer has DB values between 0 and 1. Accordingto the Frechet method,27 the DB of hyperbranched polymers isgiven by

DB = D + T

D + L + T(2)

The degree of branching of HPE was calculated from the integralvalues established for the quaternary carbons and determined tobe 0.63.

The structure of HPE-LC was confirmed by FTIR and 1H NMRanalysis. Figure 4 shows the vibrational changes taking place forthe monomer and polymers at room temperature. The spectrum ofTMC shows a broad band in the region 3300–3400 cm−1, withoutthe appearance of the ester carbonyl peak. This band graduallydecreases with increasing degree of substitution, and is absentin the spectrum of HPE-LC (Fig. 4(e)). The absorption band in theregion 1740 cm−1 in Fig. 4(b) corresponds to the ester carbonyl. InFigs 4(c)–(e), the peak positions of phenyl at 1610 and 1500 cm−1

confirm the fact that the terminal hydroxyl groups are modifiedby biphenyl mesogenic units.

The biphenyl (6.93–7.45 ppm), –CH2OAr– (3.93–3.95 ppm) and–OOCCH2 – (2.27–3.35 ppm) proton signals appear in the 1H NMRspectrum of HPE-LC, as shown in Fig. 5. Compared with Fig. 3, the–CH2OH (3.68–3.72 ppm) signal is absent. This also indicates thatthe biphenyl-based carboxylic acid had reacted with the terminalhydroxyl of HPE and that no hydroxyl remained.

Thermal and mesomorphic behaviorThe thermal behavior of the mesogenic unit, HPE and HPEderivatives was studied using DSC, and the results are shown inFig. 6. Both the mesogenic unit and HPE-LC exhibit mesomorphicproperties. The melting temperature (Tm) of the mesogenic unitis 110 ◦C on heating and the corresponding LC phase to solidtransition temperature (Tk) is at 99 ◦C on cooling.28 The LC phaseto isotropization transition temperature (Ti) of the biphenyl-basedcarboxylic acid is 127 ◦C on heating and 123 ◦C on cooling. Thetwo transition temperatures of the mesogenic unit shift to lowertemperatures in cooling scans indicating supercooling of thecrystalline content. The glass transition temperature (Tg) of HPEand HPE derivatives tend to increase with increasing degree ofsubstitution of the terminal hydroxyl groups. Only Tg is visiblein the DSC curves of HPE, which indicates an amorphous state.HPE-LC might exhibit LC phases for the flexible chain of HPE incombination with rigid mesogenic units. Meanwhile, the contentof mesogenic unit may affect the LC properties of the compounds.Only in the scan for HPE-LC is an obvious endotherm detected. Ti

of HPE-LC is 123 ◦C on heating and 120 ◦C on cooling, but Tg is notvisible on cooling. Ti of HPE-LC is lower than that of the mesogenicunit due to the mesogenic carboxylic acid linked with flexible HPE,increasing the flexible spacer length on the mesogenic unit.

TGA was used to investigate the thermal properties of HPE-LC.The TGA results show that the synthesized polymer has goodthermal stability. The temperature at which 5% weight loss occurs(Td) is greater than 237 ◦C.

We also studied the LC properties of the biphenyl mesogenicunit and HPE derivatives using POM. The mesomorphic texturesof the biphenyl mesogenic unit and HPE-LC were observed, asshown in Fig. 7. The LC textures are clearer on cooling thanon heating. When the biphenyl mesogenic unit was cooledfrom its isotropic state, diminutive anisotropic rhombic entitiesemerge from the dark background of the isotropic liquid. Thefine textures grow into fan-like structures upon supplementarycooling (Figs 7(a)–(c)). HPE-LC exhibits a rod-shaped texture; therod expands to substantial domains when the sample is furthercooled. Continuously lowering the temperature encouragedsupplementary growth of the rod. Coalescence of the anisotropicdomains results in the formation of well-developed leaf-like texture(Figs 7(d)–(f)). The mesomorphic morphology of HPE-LC is superiorto that of the biphenyl mesogenic unit for formation of a perfect LCphase by the amorphous HPE chain inserted between the biphenylmesogenic units.

WAXD was used for ascertaining the LC textures. The polymersamples were heated into the liquid crystal phase, held at aconstant temperature for 30 min and then quenched with glacialsalt water. For the polymer sample synthesized, several sharpdiffraction peaks are observed on the WAXD pattern (Fig. 8). Three

Table 1. 13C NMR chemical shifts of model compounds

HO

HO

OO

T

HO

O

OO

L

O

O

OO

D

13C NMR (δ, ppm) 50.47 48.63 46.75


64

6


Figure 7. POM images (×50 magnification) of the mesogenic unit at (a) 111 ◦C, (b) 119 ◦C and (c) 126 ◦C; and of HPE-LC at (d) 78 ◦C, (e) 110 ◦C and(f) 121 ◦C.

Figure 8. WAXD pattern of HPE-LC quenched with glacial salt water fromits liquid crystalline states at 110 ◦C.

Bragg reflections in the wide-angle region indicate the existenceof a smectic mesophase in the polymer.29,30

CONCLUSIONSWe prepared a new core, TMC, with the biphenyl mesogenic

unit 8-[4′-propoxy(1,1-biphenyl)yloxy]octanoic acid. A novel HPE

was synthesized by melt polycondensation of TMC and bis-MPA,

and the hydroxyl groups at the ends of the polymer chains

were modified using the mesogenic unit to obtain a new liquid

crystalline HPE. The chemical structure of all compounds was

characterized using FTIR, 1H NMR and 13C NMR spectroscopy. The

degree of branching of HPE was found to be 0.63. HPE-LC had

good thermal stability as measured by TGA. Tg of the polymers

increased with increasing content of the mesogenic unit in the

polymer as measured by DSC. The mesomorphic textures of both

the mesogenic unit and HPE-LC were observed using POM. Ti of

HPE-LC was lower than that of the mesogenic unit, but HPE-LC

had a wider transition temperature range, from Ti to ambient

temperature. HPE was amorphous, but the polymer exhibited LC

phases when HPE was combined with the mesogenic unit, and

the degree of substitution of mesogenic units also affected the

mesomorphic properties. HPE-LC has potential for use as an optical

recording and storage material, and also in display materials.


64

7


ACKNOWLEDGEMENTThe authors are grateful to Mr Kent L McKinney for his carefulrevision of this paper.

REFERENCES1 Kim YH and Webster OW, J Am Chem Soc 112:4592 (1990).2 Frechet JM, Science 262:1710 (1994).3 Uhrich KE, Trends Polym Sci 5:388 (1997).4 Breton MP, US Patent 5266106 (1998).5 Malmstram E, Johansson M and Hult A, Macromolecules 28:1698

(1995).6 Percec V and Kawasumi M, Macromolecules 25:3843 (1992).7 Uhrich KE, Boegeman S, Frechet JMJ and Turner SR, Polym Bull 25:551

(1997).8 Kim YH and Webser OW, Macromolecules 25:5561 (1992).9 Spindler R and Frechet JMJ, Macromolecules 26:4809 (1993).

10 Kishore KJ, Raju KVSN and Prathab B, J Phys Chem B 111:8801 (2007).11 Sheth JP, Unal S, Yilgor E, Yilgor I and Beyer FL, Polymer 46:1018 (2005).12 Nunez E, Ferrando C, Malmstrom E and Claesson H, Polymer 45:5251

(2004).13 Asif A and Shi W, Eur Polym J 39:933 (2003).

14 Komber H, Ziemer A and Voit B, Macromolecules 35:3514 (2002).15 Zhu PW, Zheng S and Simon G, Macromol Chem Phys 202:3008 (2001).16 Malmstrom E, Hult A, Gedde UW, Liu F and Boyd RH, Polymer 38:4873

(1997).17 Malmstrom E, Liu F, Boyd RH, Hult A and Gedde UW, Polym Bull 32:679

(1994).18 Kim YH, J Am Chem Soc 114:4947 (1992).19 Percec V, Macromolecules 25:1164 (1992).20 Percec V, Chu P and Kawasumi M, Macromolecules 27:4441 (1994).21 Percec V, Cho CG, Pugh C and Tomazos D, Macromolecules 26:963

(1993).22 Bauer S, Fischer H and Ringsdorf H, Angew Chem Int Ed EngI 32:1589

(1993).23 Wang G and Wang X, Polym Bull 49:1 (2002).24 Sunder A, Quincy MF, Mulhaupt R and Frey H, Angew Chem Int Ed

38:2928 (1999).25 Cai LM, Xiang L and Lu YF, J Beijing Univ Chem Technol 35:34 (2008).26 Malmstrom E, Johansson M and Hult A, Macromolecules 28:1698

(1995).27 Hawker CJ, Lee R and Frechet JMJ, J Am Chem Soc 113:4583 (1991).28 Murali M and Samui AB, J Polym Sc. A: Polym Chem 44:3986 (2006).29 Miroslav H and Majda Z, Polymer 44:6187 (2003).30 Chovino C, Guillon D and Gramain P, Polymer 39:6385 (1998).


Documents

Synthesis and characterization of a thermotropic liquid crystalline hyperbranched polyester