hydroxyl-terminated hyperbranched aromatic poly(ether-ester)s: synthesis, characterization,...

Hydroxyl-Terminated Hyperbranched AromaticPoly(ether-ester)s: Synthesis, Characterization,End-Group Modification, and Optical Properties

THIYAGARAJAN SHANMUGAM, C. SIVAKUMAR, A. SULTAN NASAR

Department of Polymer Science, University of Madras, Guindy Campus, Chennai 600 025, India

Received 15 September 2007; accepted 5 May 2008DOI: 10.1002/pola.22861Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Novel AB2-type monomers such as 3,5-bis(4-methylolphenoxy)benzoic acid(monomer 1), methyl 3,5-bis(4-methylolphenoxy) benzoate (monomer 2), and 3,5-bis(4-methylolphenoxy)benzoyl chloride (monomer 3) were synthesized. Solution po-lymerization and melt self-polycondensation of these monomers yielded hydroxyl-ter-minated hyperbranched aromatic poly(ether-ester)s. The structure of these polymerswas established using FTIR and 1H NMR spectroscopy. The molecular weights (Mw)of the polymers were found to vary from 2.0 3 103 to 1.49 3 104 depending on the po-lymerization techniques and the experimental conditions used. Suitable model com-pounds that mimic exactly the dendritic, linear, and terminal units present in thehyperbranched polymer were synthesized for the calculation of degree of branching(DB) and the values ranged from 52 to 93%. The thermal stability of the polymerswas evaluated by thermogravimetric analysis, which showed no virtual weight lossup to 200 8C. The inherent viscosities of the polymers in DMF ranged from 0.010 to0.120 dL/g. End-group modification of the hyperbranched polymer was carried outwith phenyl isocyanate, 4-(decyloxy)benzoic acid and methyl red dye. The end-cap-ping groups were found to change the thermal properties of the polymers such as Tg.The optical properties of hyperbranched polymer and the dye-capped hyperbranchedpolymer were investigated using ultraviolet-absorption and fluorescence spectroscopy.VVC 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5414–5430, 2008

Keywords: degree of branching; end-group modification; functionalization of polymers;hyperbranched; hyperbranched aromatic poly(ether-ester)s; hyperbranched polymer;light-emitting materials; optical properties; polyesters

INTRODUCTION

Hyperbranched polymers (HBPs) are excellentcandidates in the family of dendritic and multi-branched polymers. They can be convenientlyprepared by one-pot synthetic procedures suchas self-polycondensation or an addition polymer-ization reaction of ABx-typemonomer,1,2 ring-

opening polymerization,3 self-condensation vinylpolymerization,4 atom transfer radical polymer-ization (ATRP),5 or reversible addition fragmen-tation chain transfer (RAFT) polymerization.6

Among these methods, the most generalapproach employed was one-pot self-polyconden-sation of ABx-type monomers to synthesize awide variety of HBPs. Thus HBPs are more ad-vantageous, as it is capable of rapid productionin large quantities of highly functionalized glob-ular polymer. Therefore, HBPs are better suitedto replace dendrimers in many applicationareas. Specific application of HBPs include

Correspondence to: A. S. Nasar (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 5414–5430 (2008)VVC 2008 Wiley Periodicals, Inc.

5414

crosslinking processes,7,8 toughening agents,7,9

coatings,10 additives,11 blends,12 nonlinearoptics,13 rheological modifications,14 and so on.Many reviews concerning the synthesis, modifi-cation, and applications of HBPs have been pub-lished during last years.7,15

The synthesis of highly branched aromaticpolyester copolymers was first reported by Kri-cheldorf in 1982 using 3,5-bis(trimethylsiloxy)-benzoyl chloride as an AB2-type monomer.16

Later, Frechet and coworkers in 1991 success-fully developed the thermal self-condensation ofthe same AB2-type monomer leading to the syn-thesis of hydroxyl terminated hyperbranchedaromatic polyesters.17 Bulk polymerization ofAB2-type monomers such as 3,5-diacetoxyben-zoic acid and 5-acetoxyisophthalic acid resultedin the formation of the hyperbranched aromaticpolyesters with the same backbones and differ-ent end groups (e.g. -COOH and -OAc).18 HBpolyesters containing coumarin groups were pre-pared from the acetylated AB2-type monomerand these polymers showed the property of bluelight emission.19 So far, all the reported hyper-branched aromatic polyesters required highertemperature to carry out the melt condensationreaction and thus resulted polyesters with veryhigh glass-transition temperature due to rigidityin the polymer backbones. The melt polymeriza-tion of 5-(2-hydroxyethoxy)isophthalic acid at190 8C in the presence of an organo tin catalystgave HB aromatic-aliphatic polyester.18(b) Intro-duction of alkyl chain in the backbone of themonomer was found to decrease the polymeriza-tion temperature and also the glass transitiontemperature. The new approach for the determi-nation of degree of branching (DB) by degrada-tive method was first established using thehyperbranched polyesters based on 4,4-bis(40-hydroxyphenyl)pentanoic acid as an AB2-typemonomer.20 Hawker first reported the polymer-ization of AB2 macro monomers to produce anew class of ion conducting HB poly(ethyleneglycols) through a trans-esterification reac-tion.1(c) Blencowe and coworkers reported thesynthesis of a rigid HB polyester using couplingagents such as 1,3-dicyclohexylcarbodiimide(DCC) and 1,3-diisopropylcarbodiimide with theAB2 monomer 3,5-bis (3-hydroxylprop-1-ynyl)-benzoic acid.21 Jen and coworkers reported a flu-orinated hyperbranched aromatic polyester thatwas prepared by mild one-step polyesterificationof an AB2-type monomer at room temperatureusing DCC and 4-(dimethylamino)pyridium 4-

toluenesulfonate as the condensing agents.22

Recently, Kumar and coworkers reported aseries of diethylmalonate-based HB polyesterscontaining flexible aliphatic spacers.23 The syn-thesis of hyperbranched aliphatic and aromaticpolyesters based on an AB2 monomer and multi-functional core moiety was also described in theliterature.1f,24 Apart from this, many reportsdealing with a variety of hyperbranched polyest-ers are well documented in the litera-ture.1(o),23,25–35 Despite numerous publications,only very few papers1(c),18(b),24(a),29,31 deal withthe presence of flexible ether group in the struc-ture of the hyperbranched polyester, and thereis no report in the literature on methylol-termi-nated hyperbranched poly(ether-ester).

In this report, we present a facile synthesis ofnew benzyl alcohol-terminated hyperbranchedaromatic polyesters with flexible ether groupbased on 3,5-bis(4-methylolphenoxy)benzoicacid, methyl 3,5-bis(4-methylolphenoxy)benzoateand 3,5-bis(4-methylolphenoxy)benzoyl chlorideas novel AB2-type monomers. The polymeriza-tion reaction involved is an esterification (or)trans-esterification and this approach facilitatesa large scale production. These polymers can beconsidered as hyperbranched polyester polyolsand could be used as crosslinkers for manyapplications.

EXPERIMENTAL

Measurements

Infrared (IR) spectra were recorded by the KBrpellet method in a Thermo Mattson Satellitemodel FTIR spectrophotometer. 1H NMR and13C NMR spectra were recorded using a JEOLGSX 400 MHz and 100 MHz NMR instrumentrespectively. The EI mass spectra were recordedusing JEOL DX-303 mass spectrometer. Ther-mogravimetric analyses (TGA) were carried outin a ZETZSCH-STA 409C thermal analyzer from30 to 800 8C at a heating rate of 10 8C min�1

under nitrogen atmosphere with gas flow rate of90 mL min�1. Differential scanning calorimetry(DSC) was performed on a ZETZSCH-DSC 204instrument. The sample was heated from �50 to300 8C, cooled rapidly, and reheated under nitro-gen atmosphere. The heating rate was 10 8Cmin�1. Gel permeation chromatography (GPC)was performed on a Modular Build SEC system(Knauer, Berlin, Germany) equipped with a

HYPERBRANCHED AROMATIC POLY(ETHER-ESTER)S 5415

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

MALLS-detector (Dawn-EOS, Wyatt TechnologyCorporation, USA) and RI-detector (Knauer, Ger-many). THF solvent was used as an eluent at aflow rate of 1 mL/min. Columns of PL GEL 5 lmMixed C column, 300 mm 3 7.5 mm (PolymerLaboratories, UK) were used. Inherent viscositiesof the polymers were measured in THF (0.5 g/dL)at 30 8C using an Ubbelohde viscometer.

Materials

3,5-Dihydroxy benzoic acid (Aldrich), 4-fluoro-benzaldehyde (Aldrich), copper oxide (Lan-caster), benzoic acid (Merck), sodium borohy-dride (NaBH4) (Loba Chemie), dibutyltin diace-tate (DBTDA) (Lancaster), DCC (Merck), N,N-dimethylaminopyridine (DMAP) (Merck), phenylisocyanate (Merck), and methyl red dye ({2-[4-(dimethylamino)phenylazo]benzoic acid}) (Merck)were used as received. N,N-Dimethylsulfoxide(DMSO), thionyl chloride, tetrahydrofuran, ethylacetate, and petroleum ether were purified bythe standard procedures.36

Synthesis of 3,5-Bis(4-aldophenoxy)benzoic acid (1)

To a solution containing 3,5-dihydroxy benzoicacid (5.0 g, 32.46 mmol), 4-fluorobenzaldehyde(6.9 mL, 64.92 mmol), copper oxide (0.2 g), andDMSO (100 mL), anhydrous K2CO3 (17.9 g, 129mmol) was added. The reaction mixture wasstirred at 80 8C under N2 atmosphere for 48 h.After cooling, the solution was diluted withwater (1000 mL) and acidified with 1.0 N HCl(25 mL). The product was extracted with ethylacetate (2 3 250 mL) and washed with brine.The combined extract was dried over anhydrousMgSO4. Evaporation of the organic layer gavethe product as viscous oil, which was then puri-fied by column chromatography on silica gelusing 4:1 mixture of petroleum ether and ethylacetate as an eluent. A pale yellow crystallinesolid was obtained.

Yield: 7.45 g (63%). Melting point: 158–160 8C. IR (KBr, cm�1) 3066 (carboxylic ��OHstretching), 2829 (hydrogen-bonded ��OH), 1700(aldehyde C¼¼O), 1680 (carboxylic C¼¼O), 1582(aromatic), and 1227 and 1121 (C��O��C). 1HNMR [(acetone-d6), d, ppm]: 9.91 (s, 2H,��CHO), 7.92 (d, 4H), 7.41 (s, 2H), 7.22 (d, 4H),7.14 (s, 1H). 13C NMR [(acetone-d6), d, ppm]:191.48, 166.07, 161.61, 157.12, 135.0, 132.48,132.24, 118.78, 116.36, and 115.88. EI-MS: m/z362 (Mþ).

Synthesis of Methyl 3,5-Bis(4-aldophenoxy)benzoate (2)

Compound (2) was obtained by the reaction ofmethyl 3,5-dihydroxy benzoate and 4-fluoroben-zaldehyde in the presence of copper oxide andpotassium carbonate. The synthetic procedureadopted for compound (2) was similar to thatadopted for the compound (1).

Yield: 7.5 g (68%). Melting point: 100–102 8C. IR(KBr, cm�1) 1725 (ester C¼¼O), 1684 (aldehydeC¼¼O), 1578 (aromatic), and 1219 and 1003(C��O��C). 1H NMR [(acetone-d6), d, ppm)]: 9.92 (s,2H), 7.94 (d, 4H), 7.39 (s, 2H), 7.22 (d, 5H), 3.78 (s,3H). 13CNMR [(acetone-d6), d, ppm]: 191.74, 164.90,161.29, 157.0, 133.33, 132.30, 118.78, 118.70, 116.38,116.0, and 52.84. EI-MS:m/z 376 (Mþ).

Synthesis of 3,5-Bis(4-(hydroxymethyl)phenoxy)benzoic acid (3) (AB2-Type Monomer 1)

A 250-mL three-necked flask equipped withnitrogen inlet and stirrer was charged with the3,5-bis(4-aldophenoxy)benzoic acid (1) (3.0 g,8.28 mmol) and 100 mL of dry THF. This solu-tion was cooled to 0 8C. NaBH4 (1.57 g, 41.43mmol) was added portion-wise over a period of30 min. Then the ice bath was removed, and thestirring was continued at room temperature foranother 6 h. Excess NaBH4 was cautiouslyquenched with 3.0 N HCl (15 mL). THF wasremoved under vacuum, and the crude com-pound was extracted with ethyl acetate (2 3 125mL). The organic phase was washed with waterand brine, dried over anhydrous MgSO4, fil-tered, and concentrated in vacuo to afford theproduct as white solid. This product was furtherpurified by recrystallization from acetone.

Yield: 2.8 g (93%). Melting point: 162–164 8C.IR (KBr, cm�1) 3400 (carboxylic ��OH stretch-ing), 3237 (��OH stretching), 2938 (alkylstretching), 2878 (hydrogen bonded ��OH), 1688(carboxylic C¼¼O), 1594 (aromatic), and 1223and 1072 (C��O��C). 1H NMR [(acetone-d6), d,ppm]: 7.29 (d, 4H), 7.14 (s, 2H), 6.94 (d, 4H),6.71 (s, 1H), and 4.48 (s, 4H). 13C NMR [(ace-tone-d6), d, ppm]: 166.76, 160.34, 155.84, 139.68,134.44, 129.42, 120.43, 113.80, 113.05, and64.13. EI-MS: m/z 366 (Mþ).

Synthesis of Methyl 3,5-Bis(4-(hydroxymethyl)phenoxy)benzoate (4) (AB2-Type Monomer 2)

Compound (4) was obtained by the reduction ofthe compound (2) using NaBH4. The synthetic

5416 SHANMUGAM, SIVAKUMAR, AND NASAR


procedure adopted for compound (4) was similarto that adopted for the compound (3).

Yield: 2.3 g (76%). Melting point: 67–70 8C.IR (KBr, cm�1) 3416 (��OH stretching), 2919(alkyl stretching), 1725 (ester C¼¼O), 1590 (aro-matic), and 1219 and 1011 (C��O��C). 1H NMR[(acetone-d6), d, ppm]: 7.38 (d, 4H), 7.10 (s, 2H),7.07 (d, 4H), 6.90 (s, 1H), 5.24 (br, ��CH2OH),4.49 (s, 4H), 3.74 (s, 3H). 13C NMR [(acetone-d6), d, ppm]: 165.34, 159.27, 154.14, 139.24,132.56, 128.64, 119.73, 112.35, 112.0, 62.61, and52.72. EI-MS: m/z 380 (Mþ).

Synthesis of 3,5-Bis(4-(hydroxymethyl)phenoxy)benzoyl chloride (5) (AB2-TypeMonomer 3)

To a stirred solution of 3,5-bis(4-(hydroxyme-thyl)phenoxy)benzoic acid (3) (2.0 g, 5.46 mmol)in anhydrous THF (10 mL), freshly distilled thi-onyl chloride (0.39 mL, 5.46 mmol) was added at0 8C under nitrogen atmosphere. After 2.0 h,the solution was evaporated to dryness atreduced pressure. The traces of thionyl chloridewere removed from the product by stirring inhexane (2 3 15 mL), filtered, and dried in vacuoat room temperature. The compound was a palebrown pasty mass.

Yield: 2.0 g (100%). IR (KBr, cm�1) 3375(��OH stretching), 2975 (alkyl stretching), 1751(acid chloride C¼¼O), 1583 (aromatic), and 1219and 1011 (C��O��C). 1H NMR [(CDCl3), d,ppm]: 7.65 (s, 2H), 7.30 (d, 4H), 6.94 (d, 4H),6.83 (s, 1H), 5.24 (br, ��CH2OH), and 4.58 (s,4H). 13C NMR [(CDCl3), d, ppm]: 169.37, 158.36,156.31, 133.18, 132.47, 130.39, 119.35, 114.68,113.93, and 68.24. EI-MS: m/z 384 (Mþ).

Synthesis of Methyl 3,5-Bis(4-benzylbenzoateoxy)benzoate (6)

To a solution of diol (4) (2 g, 5.26 mmol) in 50mL of dry THF, benzoic acid (0.77 g, 6.31 mmol)was added followed by DCC (2.17 g, 10.52mmol) and 4-(dimethylamino)pyridine (DMAP)(1.28 g, 10.52 mmol). The reaction mixture wasstirred at room temperature under nitrogenatmosphere for 12 h, and then the solvent wasevaporated to dryness under reduced pressure.The crude product was extracted with ethyl ace-tate (2 3 250 mL), washed with 3.0 N HCl (50mL) and brine, dried over anhydrous MgSO4, fil-

tered, and then concentrated in vacuo. The com-pound obtained was further purified by columnchromatography on silica gel using 3.5:1.5 mix-tures of petroleum ether and ethyl acetate as aneluent. The product (6) was colorless viscous oil.

Yield: 0.077 g (25%). IR (KBr, cm�1) 2980 and2944 (alkyl stretching), 1790 and 1716 (esterC¼¼O), 1594 (aromatic), and 1211 and 1011(C��O��C). 1H NMR [(acetone-d6), d, ppm]:8.09–6.98 (m, aromatic 21H), 5.41 (s, 4H), 3.85(s, 3H). 13C NMR [(acetone-d6), d, ppm]: 167.74,166.54, 165.98, 159.52, 156.90, 133.88, 133.56,131.0, 130.30, 129.31, 127.94 120.07, 114.42,114.04, 66.50, and 52.62. EI-MS: m/z 588 (Mþ).

Synthesis of Methyl [3-(4-Methylolphenoxy)-5-(4-benzyl benzoateoxy)] benzoate (7)

Compound (7) was obtained as a white semisolidupon increasing the polarity of the eluent usedto isolate compound (6). Here, 3:2 mixture ofpetroleum ether and ethyl acetate was used asan eluent.

Yield: 1.01 g (40%). IR (KBr, cm�1) 3416(��OH stretching), 2985 and 2954 (alkyl stretch-ing), 1790 and 1716 (ester C¼¼O), 1594 (aro-matic), and 1210 and 1005 (C��O��C). 1H NMR[(Acetone-d6), d, ppm]: 7.93–6.78 (m, aromatic16H), 5.25 (s, 2H), 4.49 (s, 2H), 3.69 (s, 3H). 13CNMR [(acetone-d6), d, ppm]: 167.74, 166.54,165.98, 159.52, 156.90, 133.98, 133.38, 131.10,130.19, 129.42, 129.23, 120.25, 120.16, 113.96,113.81, 113.57, 66.57, 63.89, and 52.66. EI-MS:m/z 382 (Mþ).

Procedure for Solution Polymerization UsingCoupling Reagents

To a stirred solution of monomer (0.3 g, 0.8196mmol) in anhydrous THF, catalytic amount ofcoupling reagent DCC and DMAP was added.The reaction vessel was fitted with a calciumchloride guard tube, and the mixture wasstirred at room temperature or reflux tempera-ture for 24 h. Then the mixture was filtered,and the filtrate was concentrated in vacuo. Theresulting crude product was dissolved in DMFand added drop-wise into 100 mL of water con-taining 0.1% of lithium chloride to afford thehyperbranched polyester as a white precipitate.The polymer was then filtered and dried invacuo at 100 8C to a constant weight.



General Procedure for Melt Polymerization

In a typical experiment, a 25-mL round-bot-tomed flask charged with monomer (0.3 g,0.8196 mmol) was heated with stirring undernitrogen atmosphere in an oil bath at 80 8C.After 5 min, when an isotropic melt wasobtained, dibutlytin diacetate was added, andthen the reaction was continued under vacuum(8 3 10�3 Torr) to drive the condensation tohigher conversion. The other experimental con-ditions are given in Table 1. The solid obtainedwas then dissolved in DMF and recovered byprecipitation into excess of water containing0.1% of lithium chloride. The polymer was fil-tered and dried in vacuo at 100 8C to a con-stant weight.

Spectroscopic data for hyperbranched aro-matic poly(ether-ester)s: IR (KBr, cm�1) 3380–3441 (carboxylic ��OH stretching), 2919–2960(alkyl stretching), 1716–1720 (ester carbonyl),1585–1594 (aromatic), and 1211–1227 and 995–1007 (C��O��C). 1H NMR [(DMSO-d6), d, ppm]:7.47–7.07 (aromatic), 5.23 (br, ��OH), 5.04 (s,benzylic ��CH2), 4.75 (s, benzylic ��CH2), and4.48 (s, benzylic ��CH2).

End Group Modification of Polymer (Scheme 3)

Polymer Modified with Phenyl Isocyanate(HBPES-16)

About 0.2 g (0.018 mmol) of the polymer(HBPES-14) was dissolved in freshly distilledTHF (15 mL). Phenyl isocyanate (0.11 mL,1.07 mmol) and dibutyltin dilaurate (0.003 g)were added to this solution, which was thenstirred at room temperature under nitrogenatmosphere for 24 h. The solvent was removedunder reduced pressure. The crude productwas dissolved in 10 mL of DMF, and the endgroup modified polymer was precipitated in250 mL of water containing 0.1% lithium chlo-ride. The product was filtered, washed withwater, and dried in vacuo at 100 8C to a con-stant weight.

Yield: 0.18 g (90%). IR (KBr, cm�1) 3319(��NH stretching), 2925 (alkyl stretching), 1735(ester C¼¼O), 1724 (urethane C¼¼O), 1596 (aro-matic), 1560 (urethane NH bending), and 1220and 1064 (C��O��C). 1H NMR (DMSO-d6): d8.68 (CH2 ��OOCNH��), 7.42–6.95 (aromatic),5.16 (br, ��OH), 4.77 (benzylic ��CH2), 4.59(benzylic ��CH2), and 4.56 (benzylic ��CH2).

Polymer Modified with 4-(Decyloxy)benzoicacid (HBPES-17)

To a stirred solution of polymer HBPES-14 (0.2g, 0.018 mmol) and 4-(decyloxy)benzoicacid (0.29 g, 1.07 mmol) in anhydrous THF, cat-alytic amount of coupling reagents DCC andDMAP was added. The reaction vessel was thenfitted with a calcium chloride guard tube andstirred at room temperature for 24 h. The mix-ture was then filtered, and the filtrate was con-centrated in vacuo. The resulting crude productwas dissolved in DMF and added drop-wise to100 mL of water containing 0.1% of lithiumchloride to afford the end group modified poly-mer as a white precipitate. The product was fil-tered, washed with sodium hydrogen carbonateto remove the excess 4-(decyloxy)benzoic acid,and dried in vacuo at 100 8C to a constantweight.

Yield: 0.175 g (87%). IR (KBr, cm�1) 3380 (car-boxylic ��OH stretching), 2920 (alkyl stretching),1722 (ester ��C¼¼O), 1600 (aromatic), and 1218and 1007 (C��O��C). 1H NMR [(DMSO-d6), d,ppm]: 7.79–6.90 (aromatic), 3.95 (��CH2��), 1.68(��CH2��), 1.23 (��CH2��), 0.84 (��CH3��).

End Group Modification of Polymer Using MethylRed Dye (HBPES-18) (Scheme 4)

To a stirred solution of polymer HBPES-14(0.2 g, 0.018 mmol) and methyl red dye ({2-[4-(dimethylamino)phenylazo]benzoic acid}) (0.29g, 1.07 mmol) in anhydrous THF, catalyticamount of coupling reagent DCC and DMAPwas added. The reaction vessel was then fittedwith a calcium chloride guard tube, and themixture was stirred at room temperature for24 h. Then the mixture was filtered, and thefiltrate was concentrated in vacuo. The result-ing crude product was dissolved in DMF andadded drop-wise into 100 mL of water contain-ing 0.1% of lithium chloride to afford the dye-capped hyperbranched polymer as a dark greenprecipitate. The product was filtered, washedwith water, and dried in vacuo at 100 8C to aconstant weight.

Yield: 0.166 g (83%). IR (KBr, cm�1) 2927(alkyl stretching), 1720 (ester carbonyl), 1620(��N¼¼N��), 1577 (aromatic), and 1220 and 1011(C��O��C). 1H NMR [(DMSO-d6), d, ppm]: 8.4–6.6 (m, aromatic protons), 5.56 (benzylic ��CH2),4.84 (benzylic ��CH2), 4.4 (benzylic ��CH2), and3.1 (��N��CH3).



Table

1.

Synthesis

ofHyperbranch

edAromaticPoly(ether-ester)s

Mon

omer

Polym.

Method

Rea

ctionCon

ditions

DB

(%)

Mw

(g/m

ol)a

PD

g inh

(dL/g)b

Yield

(%)

Polymer

Cod

eTim

e(h)

Tem

perature

(8C)

Catalyst

Mon

omer-1

Soln.Polym.

HBPES-1

24

Roo

mtem.

DMAP/D

CC

82.6

3562

1.6

0.012

68

HBPES-2

24

Refl

uxtemp.

DMAP/D

CC

91.8

2089

1.3

0.010

74

MeltPolym.

HBPES-3

280

Bu2SnAc 2

Noreaction

HBPES-4

2100

Bu2SnAc 2

Noreaction

HBPES-5

1120

Bu2SnAc 2

Insoluble

material

Mon

omer-2

Soln.Polym.

HBPES-6

24

Roo

mtemp.

DMAP/D

CC

Noreaction

HBPES-7

24

Refl

uxtemp.

DMAP/D

CC

Noreaction

MeltPolym.

HBPES-8

280

Bu2SnAc 2

80.1

5700

2.8

0.013

59

HBPES-9

2100

Bu2SnAc 2

79.0

8900

2.8

0.015

86

HBPES-10

1120

Bu2SnAc 2

64.3

10600

2.2

0.070

81

Mon

omer-3

Soln.Polym.

HBPES-11

24

Roo

mtemp.

DMAP/D

CC

93.2

2746

1.9

0.011

64

HBPES-12

24

Refl

uxtemp.

DMAP/D

CC

84.4

3700

2.3

0.012

72

MeltPolym.

HBPES-13

280

Bu2SnAc 2

74.6

8400

2.8

0.015

77

HBPES-14

2100

Bu2SnAc 2

67.4

10600

2.3

0.061

81

HBPES-15

1120

Bu2SnAc 2

52.5

14900

2.5

0.120

89

aDetermined

bySEC

method

.bMea

suredataconcentration

of0.5

g/dLat308C

inDMFusingUbbeloh

deviscometer.



RESULTS AND DISCUSSION

Synthesis of AB2-Type Monomers

3,5-Bis(4-(hydroxymethyl)phenoxy)benzoic acid(3) (monomer 1) was prepared via two steps,starting from 3,5-dihydroxy benzoic acid asdescribed in Scheme 1. The first step was theUllmann ether synthesis using 3,5-dihydroxybenzoic acid and 4-fluorobenzaldehyde in thepresence of anhydrous potassium carbonate andcopper oxide to give 3,5-bis(4-aldophenoxy)ben-zoic acid (1). The second step was the reductionof the aldehyde functional group to the alcoholgroup using sodium borohydride as the reducingagent in dry THF at room temperature. Thisconversion yielded the AB2 monomer 3,5-bis(4-(hydroxymethyl)phenoxy)benzoic acid (3) (mono-mer 1) in an overall yield of 80%. Methyl 3,5-bis(4-(hydroxymethyl)phenoxy)benzoate (4)(monomer 2) was synthesized by the reductionof methyl 3,5-bis(4-aldophenoxy)benzoate (2),

which was obtained by the reaction of 3,5-dihy-droxy benzoic acid methyl ester and 4-fluoroben-zaldehyde in the presence of anhydrous potas-sium carbonate and copper oxide. The syntheticprocedure adopted for monomer 2 is similar tothat used for monomer 1, and the overall yieldwas 72%. 3,5-Bis(4-(hydroxymethyl)phenoxy)-benzoyl chloride (5) (monomer 3) was synthe-sized by reacting one equivalent of thionyl chlo-ride with one equivalent of (3) (monomer 1) indry THF at 0 8C, which afforded the product in100% yield. The structures of AB2 monomers(3), (4) and (5) and their precursors (1) and (2)were established by FTIR, 1H NMR, 13C NMR,and EI-Mass spectra. 1H NMR spectra of mono-mers 1 and 3 are given in Figure 1 and that formonomer 2 is given in Figure 2(c).

Synthesis and Properties of HyperbranchedAromatic Poly(ether-ester)s from ThreeDifferent AB2 Monomers

Large numbers of HBPs described in the litera-ture are polyesters, because they are most com-mon polymers, and large numbers of monomersare commercially available. The general proce-

Scheme 1. Synthesis of AB2-type monomers andcorresponding hyperbranched aromatic poly(ether-ester).

Figure 1. 1H NMR spectrum (400 MHz) of (a)monomer 1 in Acetone-d6 and (b) monomer 3 inCDCl3.



dure for constructing hyperbranched polyestersfrom ABx type (X � 2) monomers involves reac-tion between hydroxyl and carboxylic acid moi-eties either in ‘‘A’’ or ‘‘B’’ functionalities. In asimilar fashion, we have chosen the monomershaving carboxylic acid, methyl carboxylic ester,

and carbonyl chloride as ‘‘A’’ functionality andbenzyl alcohol as the ‘‘B’’ functionality. The syn-thesis of hyperbranched aromatic polyestersusing these monomers was performed via twodifferent routes (either solution or melt polymer-ization). In the melt polymerization of the

Figure 2. 1H NMR spectrum (400 MHz) of (a) dendritic (D) unit model compound,(b) linear (L) unit model compound, (c) terminal (T) unit model compound (monomer2), and (d) hyperbranched aromatic poly(ether-ester).



monomer 1 using Bu2SnAc2 catalyst under vac-uum, it was observed that no progression of thereaction at 80 and 100 8C for duration of 2 h.Upon increasing the temperature to 120 8C, thereaction afforded an insoluble material in 1 h.In the light of these observations, it was there-fore concluded that the direct esterification ofmonomer 1 at high temperatures is not possi-ble as the reaction undergoes cyclization orcrosslinking in the absence of solvent. Conse-quently, an alternative approach using couplingreagents to promote polyesterification in solutionpolymerization was examined. Blencowe et al.21

used the commercially available coupling re-agents such as 1,1-carbonyldiimidazole (CDI), 1-{3-(dimethylamino)propyl]-3-ethylcarbodiimidehydrochloride (ECDI), and DCC for polymeriza-tion of AB2 monomer, 3,5-bis(3-hydroxyprop-1-ynyl)benzoic acid and reported that when using CDIas the condensation reagent, undesirable prod-ucts such as carbamates, carboxy-imidazoles,and number of anhydrides are obtained alongwith the oligomeric polyesters, which were con-firmed by 1H NMR, 13C NMR, and MALDI-TOFmass spectroscopic methods. The ECDI binds tothe focal point of each hyperbranched macromol-ecule during polymerization and retards the con-tinuation of polymerization, thereby resulting inthe low molecular weight polyesters. Polymer-ization of lactic acid polymerization also provedto be difficult using CDI as the coupling rea-gent.37 Based on these observations, DCC wasused for the polymerization of monomer 1 inthe presence of 4-dimethylamino pyridine in dryTHF and found that polymerization was medi-ated and afforded hyperbranched polyester. Thepurification procedure for the crude polyesteradopted was similar to that described by Blen-cowe and coworkers.21 The structure of the poly-mer was confirmed by FTIR and 1H NMR spec-troscopy. Compared to the FTIR spectrum ofmonomer, FTIR spectrum of polymer showed ashift in the carbonyl region (1716 cm �1) due tothe formation of ester linkages in the polymerchain. In addition, the peak intensification at2923 cm �1 due to the presence of a large num-ber of ��CH2�� groups in the polymer chain issustaining evidence. 1H NMR spectrum of themonomer showed only one peak at 4.48 ppm dueto the benzylic ��CH2�� proton, whereas the cor-responding polymer showed three distinct peaksat 5.04, 4.75, and 4.48 ppm, due to benzylic��CH2�� proton associated with dendritic, termi-nal, and linear units of hyperbranched polyester.

The parameters such as time, temperature,and catalyst were varied for polymerization ofall the three AB2 monomers (Table 1). The mo-lecular weights of the hyperbranched aromaticpoly(ether-ester)s were characterized by GPC-MALLS using THF as an eluent. Figure 3 showsthe GPC traces and Table 1 summarizes thecharacterization results. All the variablesshowed a significant effect on the molecularweight. Monomer 1, with the coupling agentDCC in the presence of DMAP, when stirred indry THF at room temperature for 24 h, affordedthe hyperbranched polyester with a molecularweight of 3562. The molecular weight wasreduced to 2089, when the contents were stirredat reflux temperature for 24 h.

No acceleration effect was observed in thecase of polymerization of monomer 2 using cou-pling agents, even though the methyl estergroup is expected to show higher reactivity to-ward alcohol in trans-esterification compared to

Figure 3. SEC trace of (a) HBPES-9, (b) HBPES-10,(c) HBPES-13, (d) HBPES-14, and (e) HBPES-15.



the carboxyl group. Consequently, an alternativeapproach of melt self-polycondensation wasadopted. Monomer 2, when heated at 80 8C fora time duration of 2 h, yielded the polymer hav-ing molecular weight 5700. Upon increasing thetemperature to 100 8C, a significant increase inthe molecular weight was observed (Mw, 8900).A further increase in temperature to 120 8C andfor the time duration of 1 h, monomer 2yielded the polymer with a molecular weight of10,600. Monomer 3, polymerized using the cou-pling agent DCC at room temperature for 24 h,yielded the polymer of molecular weight Mw,2746. A diminutive effect was observed in themolecular weight (Mw, 3700) when refluxed for24 h. Melt self-polycondensation of monomer 3at 80 8C for 2 h yielded a polymer with molecu-lar weight, 8400. When increasing the reactiontemperature to 100 8C, the molecular weightalso increased to 10,600. The molecular weightwas further increased to 14,900 when the poly-merization reaction was carried out at 120 8Cfor relatively short-time duration. The molecularweights of the polymers obtained from the meltself-polycondensation of monomer 3 are higherthan that of polymers obtained from monomer2. This is mainly due to the presence of carbonylchloride as the ‘‘A’’ functionality is highly reac-tive toward the ‘‘B’’ functionality of the mono-mer. The polydispersity values ranged from 1.3to 2.8. The inherent viscosities (ginh) (Table 1) ofthe polymers were plotted against the molecularweight, and the plot was a typical of HBP.

TGA of the polymers showed that they in-variably undergo decomposition in two distinctstages; the first stage starts around 200 8C, andthe second stage starts around 350 8C. The ini-tial decomposition temperatures of first and sec-ond stages for the individual polymer are givenin Table 2. The glass transition temperatures(Tg) of the polymers were determined usingDSC. The samples were quenched from 300 8Cto �50 8C and then was reheated to get clearTg; the values of which are given in Table 2.The low-molecular weight polymer showed Tg at176 8C, whereas this value was shifted to 256 8Cfor high molecular weight polymers. The DSCthermograms did not show any decompositionup to 300 8C. Both DSC and TGA results indi-cate that the polymer chains are flexible at tem-peratures substantially below their decomposi-tion temperatures. Solubility behavior of the pol-ymers is also summarized in Table 2. Thehyperbranched aromatic poly(ether-ester)s, pre-pared with different molecular weights, werefound to be soluble in highly polar solventsincluding THF. Some of them were found to bepartially soluble in DMAc. Acetone and ethylacetate were found to be nonsolvent for thesepolymers.

Degree of Branching

Model compounds for dendritic (D), linear (L),and terminal (T) units, which mimic exactly the

Table 2. Thermal and Solubility Properties of Hyperbranched Aromatic Poly(ether-ester)s

PolymerCode

Thermal Stability

Tg (8C)

Solubilityb

Tia of Istage (8C)

Tia of IIstage (8C)

DMF, THF,NMP, DMSO DMAc

Acetone,EtOAc

HBPES-1 203 421 210 þ þ �HBPES-2 193 375 176 þ þ �HBPES-8 214 413 NOc þ þ �HBPES-9 213 416 NOc þ 6 �HBPES-10 224 363 NOc þ 6 �HBPES-11 238 350 214 þ þ �HBPES-12 225 325 NOc þ þ �HBPES-13 239 376 NOc þ 6 �HBPES-14 218 405 245 þ � �HBPES-15 227 422 256 þ � �

a Ti, initial decomposition temperature measured by TGA.b þ, soluble; �, insoluble; 6, partially soluble.c NO, not observed up to 300 8C.



units present in the hyperbranched polymerwere synthesized (Scheme 2). The DB of thehyperbranched aromatic poly(ether-ester)s syn-thesized were conveniently determined by Fre-chet method using 1H NMR spectra of the poly-mer and the model compounds, because thespectra of polymers were reasonably simple, andit was possible to assign peaks for protons ofindividual unit clearly in the benzylic ��CH2��region. 1H NMR spectra of polymers showedfour discrete signals for ��CH2�� protons at d5.04 ppm, d 4.75 ppm, d 4.57 ppm, and d 4.48ppm [Fig. 2(d)]. In the 1H NMR spectra of modelcompounds, the peaks corresponding to the ben-zylic protons for dendritic (D) [Fig. 2(a)], linear(L) [Fig. 2(b)], and terminal units (T) [Fig. 2(c)],were observed at d 5.39 ppm, d 5.25 and 4.49ppm, and d 4.5 ppm, respectively. By comparingthe 1H NMR spectra of the model compoundswith that of the polymers, it was unambiguouslyconcluded that the peaks at d 5.04 ppm and d4.57 ppm in the hyperbranched polymers corre-spond to the dendritic (D) and terminal (T)units, respectively. This approach helps to differ-entiate linear (L) unit from terminal (T), anddendritic (D) units. Mole fraction of the individ-ual unit was calculated from the integrationvalue of corresponding peak. Based on the molefractions of D, L, and T units, the DB valueswere calculated and are given in Table 1. Theresults reveal that the DB is high (82–93%) forlow-molecular weight polymers (Mw 2089–3700)and is low (52–64%) for fairly high-molecularweight polymers (Mw 10,600–14,900). Account-ing for the error virtually associated with the1H NMR technique for this purpose, the DBvalue of 52–64% for polymers of relatively highmolecular weight is acceptable, becasue boththeoretical38 and experimental studies20 confirmthat this value is around 0.5 for HBPs basedon AB2-type monomers. The high-DB valueindicates that the low-molecular weight

polymers may have more number of terminalunits and less number of linear units. Thisinterpretation can be supported with topologicalconsiderations.

End-Group Modification of HyperbranchedAromatic Poly(ether-ester)s

HBPs prepared from AB2-type monomers have alarge number of end groups. The nature of theend groups influences the physical and chemicalproperties of the HBPs. In this study, the endgroups of hyperbranched aromatic poly(ether-ester)s are benzyl alcohols, and they are used asreactive sites for modification. The end-cappingreactions of HBPES-14 were carried out withphenyl isocyanate and 4-(decyloxy)benzoic acid,as shown in Scheme 3. The properties of theend-group modified hyperbranched aromatic poly(ether-ester)s are given in Table 3. As expected,physical and thermal properties of the modified

Scheme 3. End capping of hyperbranched aromaticpoly(ether-ester) using phenyl isocyanate and 4-(decy-loxy) benzoic acid.

Scheme 2. Synthesis of model compounds for thedetermination of degree of branching.



HBPs are found to vary with the chain ends. Theviscosity values of the modified hyperbranchedaromatic poly(ether-ester)s (HBPES-16 andHBPES-17) are higher than that of unmodifiedpolymer (HBPES-14). This may be due to theincorporation of bulky benzyl carbamates anddecyloxy benzoate groups at the polymer chainends.

The TGA thermograms of the unmodified andmodified polymers are shown in Figure 4. Likeunmodified polymer, modified polymers alsoundergo two-stage decomposition, and theirdecomposition temperatures are included inTable 3. The thermal stability of the modifiedpolymer is significantly high compared tounmodified polymer. As the end group of theparent hyperbranched aromatic poly(ether-ester)s (HBPES-14) was changed from benzylalcohol to benzyl carbamates (HBPES-16) anddecyloxy benzoate (HBPES-17), the glass tran-sition temperature (Tg) dropped drastically from245 8C to 155 8C and 118 8C, respectively, (Fig.5). The low glass transition temperature ofHBPES-17 is due to the incorporation of flexiblealkyl chain to the surface of the polymer. Modi-fied polymers were found to be partially solublein DMAc, whereas unmodified polymer was in-soluble in this solvent.

Optical Properties

HBPs have been intensively studied in therecent years due to their good solubility, process-ability, and tunable physical and chemical prop-erties. Moreover, their globular structure influ-ences the electron transporting39 and willdecrease intermolecular interactions,40 whichare advantageous for their application in light-emitting diode devices. Therefore, light-emittingHBPs are of current interest for developing effi-cient electroluminescent devices and other pho-tonic devices.41 There are some reports availablein literature dealing with the application ofHBPs as light-emitting materials.42

The optical properties of the hyperbranchedpolymer and methyl red dye-capped hyper-branched polymer were investigated with UV–vis and fluorescence spectroscopy. Initially, theoptical property of the hyperbranched polymerwas examined. The emission maximum (kmax)was observed at 345 nm, and it was redshifted.To enhance the optical property of the hyper-branched polymer, the end group was cappedwith the light-emitting dye. For this purpose,T

able

3.

Synthesis

andProperties

ofEndGroupMod

ified

Hyperbranch

edAromaticPoly(ether-ester)s

Polymer

Cod

eCapping

Rea

gen

tUsed

EndGroup

g inh

(dL/g)a

ThermalStability

Tg(8C)

Solubilityc

Tib

ofI

Stage(8C)

Tib

ofII

Stage(8C)

DMF,THF,

NMP,DMSO

DMAc

Acetone,

EtO

Ac

HBPES-16

Phen

yl

isocyanate

Ben

zyl

carbamate

0.140

250

389

155

þ6

�

HBPES-17

4-(Decyloxy)

ben

zoic

acid

Decyloxy

ben

zoate

0.09

288

416

118

þ6

�

aMea

suredataconcentration

of0.5

g/dLat308C

inTHFusinganUbbeloh

deviscometer.

bi,initialdecom

positiontemperature

mea

suredbyTGA.

cþ,

Soluble;�,

insoluble;6,partiallysoluble.



methyl red ({2-[4-(dimethylamino)phenylazo]ben-zoic acid}) dye, containing a strong donor andacceptor moiety with carboxylic acid group, wasused to condensate with the terminal benzylalcohol group in the hyperbranched polymer(Scheme 4).

The absorption spectra of the hyperbranchedpolymer (HBPES-14), methyl red dye, andmethyl red dye-capped hyperbranched polymer(HBPES-18) were recorded in THF solution andare shown in Figure 6. Pure hyperbranchedpolymer displayed a broad absorption band at

301 nm in the ultraviolet region, attributed tothe p–p* (Table 4) transition contributed fromthe polymer backbone, and this pattern is con-sistent with the hyperbranched structure associ-ated with chromophoric groups. Pure dye dis-played a broad absorption band at 486 nm inthe visible region due to the presence of strongdonor–acceptor couple of the dimethylamino andcarboxyl groups in the same molecule. The dye-capped hyperbranched polymer displayed abroad absorption band at 413 nm in the visibleregion. This clearly shows that its absorptionmaximum was redshifted. The absorption at lon-ger wavelength region is due to the conjugation

Scheme 4. End capping of hyperbranched aromaticpoly(ether-ester) using methyl red dye to study theoptical properties.

Figure 4. TGA thermogram of (a) unmodifiedhyperbranched aromatic poly(ether-ester), (b) phenylisocyanate modified hyperbranched aromatic poly(-ether-ester), and (c) 4-(decyloxy) benzoic acid modifiedhyperbranched aromatic poly(ether-ester).

Figure 5. Second heating DSC thermogram of (a)phenyl isocyanate modified hyperbranched aromaticpoly(ether-ester), (b) 4-(decyloxy) benzoic acid modi-fied hyperbranched aromatic poly(ether-ester), and (c)unmodified hyperbranched aromatic poly(ether-ester).

Figure 6. UV–vis absorption spectrum of (a) hyper-branched polymer, (b) methyl red dye, and (c) dye-capped hyperbranched polymer.



of the dye molecules at the terminal groups ofthe hyperbranched polymer.

The emission spectra of the hyperbranchedpolymer (HBPES-14), methyl red dye, andmethyl red dye-capped hyperbranched polymer(HBPES-18) were recorded in THF solution andare shown in Figure 7. The samples wereexcited with a 300 and 413 nm light. Pure poly-mer and dye showed emission maxima at 345and 370 nm, respectively (Table 4). The dye-capped hyperbranched polymer showed emissionmaximum at 342 nm along with shoulders at327 and 358 nm. The excitation spectrum wasalso broad. This observation shows an efficientintramolecular energy transfer from the termi-nated dye molecule to the hyperbranched poly-mer. It is interesting to mention here that theemission maximum of the dye-capped hyper-branched polymer was blueshifted (Anti-Stokes

shift) nearly about 71 nm, whereas the emissionmaxima of the pure polymer was redshifted(Stokes shift) about 45 nm, and the pure dyewas blueshifted by 115 nm. This can be attrib-uted to the fact that the terminal benzyl alcoholgroups in the polymer and ��COOH group inthe dye form stable ester linkages, which conse-quently withdraws electrons from the ��NMe2group present at the para position of the azolinkages (��N¼¼N��). In addition, the intensityof the dye-capped hyperbranched polymer wasfound to be twice and thrice increased, whencompared with the pure polymer and the dye,respectively. This pattern further supports theformation of stable ester linkages between poly-mer and the dye. Moreover, the anti-Stokes shiftwas found to be broad due to the changes in themolecular structure of the dye-terminatedhyperbranched polymer upon excitation.43 A

Figure 8. Photograph of (a) hyperbranched polymer(b) methyl red dye, and (c) dye-capped hyperbranchedpolymer.

Figure 7. Photoluminescence (PL) spectrum of (a)hyperbranched polymer, (b) methyl red dye, and (c)dye-capped hyperbranched polymer.

Figure 9. DSC and TGA curves of the dye-cappedhyperbranched polymer.

Table 4. Absorption and Fluorescence Maxima(kmax) of Hyperbranched Polymer, Methyl Red Dyeand Dye-Capped Hyperbranched Polymer

ItemAbsorptionkmax (nm)a

Fluorescencekmax (nm)a

Hyperbranchedpolymer

301 345

Methyl red dye 486 370Dye-cappedhyperbranchedpolymer

413 342

a All the spectra were recorded in THF.



photograph of the hyperbranched polymer, puredye, and the dye-capped hyperbranched polymerin THF solution is shown in Figure 8. Thermalproperties of the dye-capped hyperbranchedpolymer were also investigated using thermog-ravimetric analysis (TGA) and DSC. As shownin Figure 9, TGA curve of the dye-capped poly-mer (HBPES-18) displayed thermal stability upto 214 8C and has shown no significant changescompared to the parent hyperbranched polymer(218 8C) (HBPES-14). Dye capping of polymerdecreased the glass transition temperature (Tg)from 245 8C to 218 8C (Fig. 9).

CONCLUSIONS

In conclusion, we have prepared three differentAB2-type monomers namely 3,5-bis(4-(hydroxy-methyl)phenoxy)benzoic acid (monomer 1),methyl 3,5-bis(4-(hydroxymethyl)phenoxy)ben-zoate (monomer 2), and 3,5-bis(4-(hydroxyme-thyl)phenoxy)benzoyl chloride (monomer 3)and demonstrated solution polymerization, andmelt self-polycondensation of these monomersled to the formation of hyperbranched aromaticpoly(ether-ester)s with terminal benzyl alcoholgroups. It was found that the molecular weightsof the polymers obtained by the melt self-poly-condensation method were higher than that ofthe polymers obtained by the solution polymer-ization method. Depending on the molecularweight, the DB was found to be varied from 52to 93%. These polymers have good thermal sta-bility and high Tg as measured by TGA andDSC, respectively. The benzyl alcohol groups atthe ends of the polymer chains were modifiedusing phenyl isocyanate, 4-(decyloxy) benzoicacid, and methyl red dye. The results of theend-group modification reveal that the endgroups change the physical, thermal, and solu-bility properties of the hyperbranched aromaticpoly(ether-ester)s. The PL properties of the purepolymer and the light-emitting dye-cappedhyperbranched polymer were investigated. Thedye-capped hyperbranched polymer is designedin such a way to have conjugation gradient intheir structure and is capable of light harvest-ing. Thus, the resulting polymers could be con-sidered as fluorescent materials, which wouldhave promising applications in molecular pho-tonics or fluorescent markers.

The authors thank Prof. Brigitte Voit, Liebniz Insti-tute for Polymer Research, Dresden, Germany, forSEC measurements, and T. Shanmugam thanks theCouncil of Scientific and Industrial Research, NewDelhi, for awarding Senior Research Fellowship.

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