Reactive & Functional Polymers 56 (2003) 45–57www.elsevier.com/ locate/ react

S tudies on photocrosslinkable polymers havingbromo-substituted pendant cinnamoyl group

a,b , a b*R. Balaji , D. Grande , S. Nanjundana ` ´Laboratoire de Recherche sur les Polymeres, UMR 7581, CNRS—Universite Paris XII, 2 Rue Henri Dunant, F-94320 Thiais, France

bDepartment of Chemistry, Anna University, Chennai 600 025,India

Abstract

New methacrylate monomers having a pendant photocrosslinkable bromo-substituted cinnamoyl group were synthesizedby reacting corresponding hydroxy chalcones with methacryloyl chloride. These monomers were homopolymerized in ethylmethyl ketone solution using benzoyl peroxide as a free radical initiator at 708C. The resulting polymers were characterized

1 13by UV, Fourier transform infrared, H-nuclear magnetic resonance (NMR) and C-NMR spectroscopy as well as bysize-exclusion chromatography. Thermal properties of the photoreactive polymers were assessed by thermogravimetricanalysis and differential scanning calorimetry. The photocrosslinking properties of the polymers were investigated in varioussolvents in the presence and absence of photosensitizers. The influence of solvent nature and concentration on the rate ofphotocrosslinking of the newly synthesized polymers were studied for using the latter as negative photoresist materials. 2003 Elsevier B.V. All rights reserved.

Keywords: Photoresist; Cinnamoyl group; Photocrosslinkable polymers; Bromochalcone

1 . Introduction display [6], liquid crystals [7,8], energy ex-change materials[9], etc. The synthesis and

Nowadays, there has been widespread re-photocrosslinking properties of polymers withsearch in the area of synthesis and application pendant photofunctional groups such as cin-of polymers bearing photosensitive groups[1,2]. namoyl esters of poly(vinyl alcohol)[10],Polymers containinga,b-unsaturated carbonyl poly(2-hydroxyethyl methacrylate)[11], poly(2-groups either in the backbone or as pendant vinyloxyethyl cinnamate) [12], poly(vinyl-4-group undergo crosslinking upon irradiation methoxy cinnamate)[13] and many other simi-with UV light or an electron beam, and are lar systems[14–16] have been reported.being used as photoresists in the field of print- Polymers with a combination of propertiesing technology[3], microlithography[4], non- such as high photosensitivity, good solubility,linear optical materials[5], holographic head-up thermal stability, ability to form films, resist-

ance towards solvents after crosslinking as well*Corresponding author. Laboratoire de Recherche sur les as resistance towards plasmas and etching

` ´Polymeres, UMR 7581, CNRS—Universite Paris XII, 2 Rueagents are required for practical use as commer-Henri Dunant, F-94320 Thiais, France. Fax:133-1-4978-1208.

E-mail address: balajirengarajan@yahoo.com(R. Balaji). cial negative photoresist materials. In continua-

1381-5148/03/$ – see front matter 2003 Elsevier B.V. All rights reserved.doi:10.1016/S1381-5148(03)00031-2

46 R. Balaji et al. / Reactive & Functional Polymers 56 (2003) 45–57

tion of our earlier studies[17–19] on polymers 2 .2. Photoreactivity measurementscontaining pendant photocrosslinkable groups,

A mercury lamp (Heber Scientific Photoreac-we report the synthesis, characterization andtor-UV, 6 W, 254 nm) was used for the irradia-thermal stability as well as photocrosslinkingtion of polymers in various solvents. The quartzproperties of polymethacrylates having a reac-cell (path length 1 cm) containing the polymertive pendant bromo-substituted chalcone groupsolutions was kept at a distance of 10 cm fromper monomeric unit. The effect of solvents,the UV lamp for different time intervals ofconcentration and photosensitizers on the rate ofirradiation. The UV spectra of the polymerphotocrosslinking of these polymers is assessedsolution were recorded immediately after eachfor using the polymers as negative photoresistexposure time and the rate of disappearance ofmaterials.theaC=Cb double bond of the chalcone moietyin the polymers was monitored using the fol-lowing expression:2 . ExperimentalExtent of conversion (%)5 (A 2 A )0 T

2 .1. Instrumentation?100/(A 2 A )0 `

A Perkin-Elmer 2400 carbon–hydrogen ana- where A , A and A are the absorption0 T `lyzer was used for elemental analysis. UV intensities due toaC=Cb after irradiation timesspectra were recorded on a Shimadzu UV-1601, t50, t5T and t5`, respectively.UV–visible spectrophotometer. Infrared (IR)spectra were obtained from a potassium bro- 2 .3. Materialsmide pellet with a Nicolet Avatar 360 Fourier

1transform (FT) IR spectrophotometer. H-Nu- 2-Bromobenzaldeyde and 3-bromobenzal-clear magnetic resonance (NMR) spectra were dehyde from Fluka were distilled before use.run on a JEOL 400 MHz spectrometer at room Benzoyl peroxide (Merck) was recrystallizedtemperature with tetramethylsilane (TMS) as from a methanol–chloroform (1:1) mixture.the internal reference in CDCl solvent. The 4-Hydroxyacetophenone (Aldrich) was used as3

13proton decoupled C-NMR spectra were run on received. Methacryloyl chloride was synthesizedthe same instrument operating at 125.77 MHz at by reacting methacrylic acid with benzoyl chlo-room temperature and chemical shifts were ride according to the method of Stempel et al.recorded under similar conditions. The weight- [20]. All the solvents were purified by standardaverage (M ) and number-average (M ) molecu- procedures.w n

lar masses were determined with a Waters 501size-exclusion chromatography (SEC) ap- 2 .4. Preparationsparatus. Tetrahydrofuran (THF) was used as theeluent and polystyrene standards were employed2 .4.1. Synthesis offor calibration. Thermogravimetric analyses 4-(29-bromocinnamoyl)phenol (4,29-BCP)(TGAs) were performed with a Mettler TA In a three-necked flask equipped with a

213000 analyzer at a heating rate of 208C min mechanical stirrer, thermometer and droppingin an air atmosphere. Glass transition tempera- funnel, 4-hydroxyacetophenone (7.6 g, 0.05tures were determined with a Dupont 910 mol) and 2-bromobenzaldehyde (10.3 g, 0.05differential scanning calorimetry (DSC) system mol) were dissolved in 100 ml of ethanol and

21at a heating rate of 108C min under a cooled in an ice bath. An aqueous solution ofnitrogen atmosphere. sodium hydroxide (4.4 g in 40 ml of water) was

R. Balaji et al. / Reactive & Functional Polymers 56 (2003) 45–57 47

then added dropwise with constant stirring and recrystallized from ethanol. Yield: 9.84 gthe temperature was maintained at 18–208C. (55%); melting point (m.p.) 168–1698C.After stirring the reaction mixture for 16 h at Elemental analysis (%): C559.35 (found),room temperature, it was neutralized with dilute 59.42 (calculated); H53.61 (found), 3.65HCl to isolate the product (Scheme 1). The (calculated).

21brownish yellow coloured solid product was FT-IR (KBr, cm ): 3253 (OH), 1650filtered, washed with ice cold water, dried and (aC=O), 1606 (aliphaticaC=Cb stretching),

Scheme 1. Synthesis of 4,29-BCPMA and 4,39-BCPMA.

48 R. Balaji et al. / Reactive & Functional Polymers 56 (2003) 45–57

1562 (aromatic C=C stretching), 804 (C–H out- evaporator. The residue obtained was extractedof-plane bending), 570 (C–Br). with ether and the ether solution was washed

1H-NMR (CDCl , ppm): 10.06 (s, 1H; successively with 0.5% sodium hydroxide solu-3

O–H), 8.05–7.20 (m, 8H; Ar–H), 6.89 and 6.70 tion and distilled water to remove the unreacted] ]

(2d, 2H; –CH=CH–). contents. Finally the ether layer was dried over] ]

anhydrous sodium sulphate and then it was2 .4.2. Synthesis of evaporated using a rotary evaporator. The solid4-(39-bromocinnamoyl)phenol (4,39-BCP) product was recrystallized from an ethanol–

This chalcone compound was prepared by ethyl acetate (1:1) mixture to get yellow crystalsreacting 4-hydroxyacetophenone (10.4 g, 0.07 of 4,29-BCPMA (Scheme 1). Yield: 10.76 gmol) with 3-bromobenzaldehyde (14.2 g, 0.07 (66%); m.p. 89–908C.mol) in an ethanol–water mixture in the pres- Elemental analysis (%): C561.34 (found),ence of sodium hydroxide (6 g in 30 ml of 61.47 (calculated); H54.05(found), 4.07 (calcu-water) by employing a similar procedure that lated).

21was adopted for the preparation of 4,29-BCP. FT-IR (KBr, cm ): 1723 (esteraC=O),1663 and 1634 (ketoneaC=O), 1599 (aliphaticThe crude product on recrystallization fromaC=Cb), 1560 and 1501 (Ar, C=C), 785 andethanol gave 15.55 g (65%) of 4,39-BCP yellow749 (C–H out-of-plane bending), 577 (C–Br).crystals (Scheme 1); m.p. 159–1608C.

1H-NMR (CDCl ): 8.10–7.01 (m, 8H/2H;3Elemental analysis (%): C559.30 (found),Ar–H/CH =C), 6.28 and 5.73 (d, 2H;259.42 (calculated); H53.59 (found), 3.65 ]] ]–CH=CH–), 2.02 (s, 3H;a-CH ).3(calculated). ] ] ]13

21 C-NMR (CDCl ppm): 188.65 (keto C=O),3,FT-IR (KBr, cm ): 3250 (–OH), 1654 ]170.20 (ester C=O), 154.60–120.85 (Ar–C and(aC=O), 1610 (aliphaticaC=Cb stretching), ] ]–CH=CH–), 138.27 (=Cb), 111.25 (CH=),2] ] ]1560 (Ar, C=C), 812 and 760 (C–H out-of-18.50 (a-CH ).3]plane bending), 551 (C–Br).

1H-NMR (CDCl , ppm): 10.04 (s, 1H;3 2 .4.4. Synthesis ofO–H), 8.01–7.10 (m, 8H; Ar–H), 6.98–6.70 4-(39-bromocinnamoyl)phenyl methacrylate] ] ](2d, 2H; –CH=CH–). (4,39-BCPMA)] ]

The monomer, 4,39-BCPMA was prepared by2 .4.3. Synthesis of reacting methacryloyl chloride (94.2 g, 0.044-(29-bromocinnamoyl)phenyl methacrylate mol) with 4,39-BCP (12.1 g, 0.04 mol) under(4,29-BCPMA) the same conditions described in the preparation

4-(29-Bromocinnamoyl)phenol (12.1 g, 0.04 of 4,29-BCPMA. The crude product, 4,39-mol) and triethylamine (4 g, 0.04 mol) were BCPMA was recrystallized from ethyl acetate–dissolved in ethyl methyl ketone (EMK, 200 ethanol mixture (Scheme 1). Yield: 11.24 gml) in a 500 ml three-necked flask equipped (69%); m.p. 132–1338C.with a stirrer, thermometer and pressure equal- Elemental analysis (%): C561.32 (found),izer and the contents were cooled to 0 and 61.37 (calculated); H54.03 (found), 4.0725 8C. Methacryloyl chloride (4.2 g, 0.04 mol (calculated).

21in 30 ml of EMK) was then added dropwise FT-IR (KBr, cm ): 1736 (esteraC=O),over a period of 30 min with constant stirring 1660 and 1637 (ketoneaC=O), 1599 (aliphatic

aC=Cb), 1578 and 1505 (Ar, C=C), 819 andand cooling. The reaction mixture was stirredfor 1 h at 08C and for another 1 h at room 791 (C–H out of plane bending), 573 (C–Br).

1temperature. The precipitated quaternary am- H-NMR (CDCl ): 8.36–7.27 (m, 8H/2H;3

monium salt was filtered off and the solvent in Ar–H/CH=C), 6.30 and 5.92 (d, 2H;2]] ]the filtrate was removed using a rotary –CH=CH–), 2.04 (s, 3H;a-CH ).3] ] ]

R. Balaji et al. / Reactive & Functional Polymers 56 (2003) 45–57 49

13C-NMR (CDCl ppm): 188.85 (keto C=O), 458C for 12 h. Yield was 40%. Similarly,3, ]174.60 (ester C=O), 155.05–121.85 poly(4,39-BCPMA) was synthesized from 4,39-

](Ar–C and –CH=CH–), 138.12 (=Cb), BCPMA employing the same procedure. Yield

] ] ]110.60 (CH=), 18.65 (a-CH ). was 45%.2 3] ]

2 .4.5. PolymerizationThe newly synthesized monomer, 4,29- 3 . Results and discussion

21BCPMA was polymerized as 2 mol l EMKsolution using benzoyl peroxide (BPO) as a free 3 .1. Synthesisradical initiator at 7061 8C (Scheme 2). Thepredetermined quantities of 4,29-BCPMA and The methacrylate monomers, 4,29-BCPMABPO were dissolved in EMK, introduced into a and 4,39-BCPMA, having a bromo-substitutedpolymerization tube and the mixture was photosensitive chalcone unit were prepared influshed with a slow stream of oxygen-free two steps, i.e., a Claisen–Schmidt condensationnitrogen for 20 min. Then, the tube was tightly followed by a direct esterification (Scheme 1).closed and placed in a thermostated water bath The structures of the chalcones, 4,29-BCP andat 7061 8C. After the specified time (15 h), the 4,39-BCP as well as those of the new monomersreaction mixture was precipitated in excess were identified by elemental analysis, FT-IR,

1 13methanol to isolate the polymer, poly(4,29- H-NMR and C-NMR spectroscopy.BCPMA) (Scheme 2). The crude polymer was Poly(4,29-BCPMA) and poly(4,39-BCPMA)purified by redissolving in chloroform and having pendant photocrosslinkable chalconeprecipitating in methanol, filtering, then wash- units were obtained by free-radical solutioning with methanol and dried under vacuum at homopolymerization of the corresponding

Scheme 2. Synthesis of poly(4,29-BCPMA) and poly(4,39-BCPMA).

50 R. Balaji et al. / Reactive & Functional Polymers 56 (2003) 45–57

21monomers at 7061 8C (Scheme 2). The mono- 1501 and 1420 cm . The peak at 760–74021mer conversion was restricted to 50% in order cm may be due to aromatic C–H out-of-plane

to avoid any possibility of gel effect at higher bending vibrations. The polymers also exhibit21conversions. absorption bands at 552–550 cm corre-

sponding to C–Br stretching. The FT-IR spec-trum of poly(4,29-BCPMA) is shown inFig. 1,3 .2. Polymer characterizationas an example.

3 .2.1. Solubility1One of the important requirements for a 3 .2.4. H-NMR spectra1photosensitive polymer is its solubility in differ- The H-NMR spectra of poly(4,29-BCPMA)

ent organic solvents. The solubility of the and poly(4,39-BCPMA) (Fig. 2) show multipletpolymers obtained after different time intervals resonance signals at 8.15–6.58 ppm corre-of polymerization was tested. It was found that sponding to the aromatic protons overlappedthe polymers obtained up to 15 h of reaction with those of pendant olefinic protons. More-time and up to 50% yield were easily soluble in over, these spectra show groups of signals atpolar solvents including chloroform (CHCl ), 1.90–0.70 ppm due toa-CH protons, thus3 3methylene chloride (CH Cl ), dimethyl sulfox- indicating the presence of conformational tac-2 2

ide (DMSO), dimethyl formamide (DMF) and ticity.N-methyl-2-pyrrolidone but insoluble both in

13hydrocarbons, such as benzene, toluene and3 .2.5. C-NMR spectra13xylenes and in hydroxyl group containing sol- The proton decoupled C-NMR spectra of

vents like methanol, ethanol or 2-propanol. The poly(4,29-BCPMA) and poly(4,39-BCPMA)polymer was completely insoluble when the are presented inFigs. 3 and 4,respectively.

13conversion was above 75%. This might have C-NMR chemical shift assignments werebeen due to crosslinking. made from the off-resonance decoupled spectra

13of the polymers. The C-NMR spectrum of3 .2.2. UV spectra poly(4,29-BCPMA) shows resonance signal at

The polymers exhibit UV absorption bands at 188.42 ppm due to the ketone carbonyl carbon.299 and 304 nm due to the (p→p*) transitions The signal at 175.01 ppm corresponds to theassociated withaC=Cb of the pendant chal- ester carbonyl carbon. The aromatic carboncone moieties. attached to the ester oxygen atom exhibits a

3 .2.3. IR spectraThe FT-IR spectra of the polymers show an

21absorption peak at 3045–3040 cm charac-teristic of the aromatic C–H stretching vibra-

21tions. The bands at 2930 and 2840 cm are dueto the C–H stretching associated with methyland methylene groups. The strong absorption

21band at 1752 cm corresponds to the estercarbonyl stretching. The ketonic carbonyl

21stretching is observed at 1665 cm . The peaks21at 1604–1602 cm are assignable to the

stretching vibrations of the ethylenic doublebond of the pendant chalcone unit. The aromaticaC=Cb stretching vibration bands appear at Fig. 1. FT-IR spectrum of poly(4,29-BCPMA).

R. Balaji et al. / Reactive & Functional Polymers 56 (2003) 45–57 51

resonance signal at 155.11 ppm. The resonancesof other aromatic and ethylenic (aC=Cb) car-bons are observed between 143.83 and 121.90ppm. The backbone tertiary (aC–) and methyl-ene carbon signals are observed at 53.81 and46.92 ppm, respectively. Thea-methyl carbonsof the polymers appear between 19.55 and18.48 ppm, indicating the presence of tacticity.

13The C-NMR spectrum of poly(4,39-BCPMA)shows all the corresponding signals at closechemical shifts.

3 .2.6. Molar masses by SECThe M and M of poly (4,29-BCPMA)w n

determined by SEC are 40,300 and 20,800 g21mol and those of poly(4,39-BCPMA) are

21 2139,400 g mol and 20,600 g mol , respective-ly. The polydispersity index (M /M ) values ofw n

poly(4,29-BCPMA) and poly(4,39-BCPMA) are1.94 and 1.91, respectively. The theoreticalvalues of M /M for polymers produced via1 w nFig. 2. H-NMR spectrum of poly(4,39-BCPMA).radical combination and disproportionation are

13Fig. 3. C-NMR spectrum of poly(4,29-BCPMA).

52 R. Balaji et al. / Reactive & Functional Polymers 56 (2003) 45–57

13Fig. 4. C-NMR spectrum of poly(4,39-BCPMA).

1.5 and 2.0, respectively[21]. Usually, the The first stage of decomposition of the poly-polydispersity index (M /M ) of poly- mers may be expected to be the cleavage ofw n

(meth)acrylates[22] prepared by free-radical C–Br bond as its bond energy is lower than thatpolymerization depends, among other factors, of C–C bond. The decomposition of the bulkyon the chain termination mechanism. Hence, the ortho bromo substituted polymer [poly(4,29-polydispersity indices of the polymers suggest BCPMA)] is facilitated in comparison with thethat the tendency for chain termination by meta-bromo substituted homologue [poly(4,39-disproportionation is predominant with respect BCPMA)] due to release of steric strain. In theto chain coupling reactions. subsequent steps, the free-radical bearing pen-

dant groups as well as the backbone undergo3 .2.7. Thermal properties cleavage. Hence poly(4,29-BCPMA) shows

The thermal stability of the polymers was three decomposition stages, while poly(4,39-studied by thermogravimetric analysis in air BCPMA) shows only two decomposition stages.atmosphere. The thermogravimetry (TG) and Even though C–Br bond energy is lower thandifferential TG (DTG) traces of the polymers C–C bond, the backbone undergoes cleavageare shown inFig. 5. Table 1gives the differen- first followed by the side-chain groups, due totial thermogravimetric analysis data of the poly- the cluster effect of pendant groups. Evenmers. The initial decomposition temperature though the initial decomposition temperature of(IDT) of poly(4,29-BCPMA) is 2648C and that poly(4,29-BCPMA) is lower than that ofof poly(4,39-BCPMA) is about 2948C and 50% poly(4,39-BCPMA), its overall rate of decompo-weight losses of these polymers occur at 500 sition is lower than that of poly(4,39-BCPMA).and 4108C, respectively. These thermal studies The glass transition temperatures (T ) ofg

strongly indicate that the polymers have very poly(4,29-BCPMA) and poly(4,39-BCPMA) de-good thermal stability required for negative type termined by DSC are 111 and 1098C, respec-photoresists. tively. These highT values are essentially dueg

R. Balaji et al. / Reactive & Functional Polymers 56 (2003) 45–57 53

radiation with a mercury lamp (6 W, 254 nm) asa UV source in air. The irradiation was carriedout in various polymer solutions in the presenceand absence of sensitizers.Figs. 6 and 7showthe changes in the UV spectral patterns of poly(4,29-BCPMA) and poly (4,39-BCPMA), respec-

21tively, in chloroform (21 mg l ) solution fordifferent time intervals of irradiation, at roomtemperature, in the absence of sensitizer. Thepolymers show the UV absorption band at 299or 304 nm due to the (p→p*) transitionsassociated withaC=Cb of the pendant chal-cone moieties. Upon irradiation, an isobesticpoint was observed at about 243 nm due totrans–cis isomerization of double bonds and the

Fig. 5. TG and DTG traces of (a) poly(4,29-BCPMA) and (b)poly(4,39-BCPMA).

to the rigid and bulky pendant bromo-substi-tuted chalcone units and to the presence ofmethyl side chains, which facilitate chain en-tanglement.

3 .3. Photochemical properties

Fig. 6. Changes in the UV spectral patterns of poly(4,29-BCPMA)The photosensitivity of poly(4,29-BCPMA) in chloroform solution upon irradiation. Top to bottom, after

and poly(4,39-BCPMA) were measured by ir- irradiation timet50, 5, 10, 20, 60, 120, 190, 300 and 600 s.

T able 1TGA data of poly(4,29-BCPMA) and poly(4,39-BCPMA)

a b,cPolymer DTR (8C) TWL (8C)

Stage 1 Stage 2 Stage 3 10 25 50 75 90

Poly(4,29-BCPMA) 264–348 350–480 482–654 312 350 500 575 602Poly(4,39-BCPMA) 294–455 460–630 – 324 372 410 542 594

a Decomposition temperature range.b Temperature corresponding to a given weight loss.c Figures in parameters indicate weight loss during the temperature range stated.

54 R. Balaji et al. / Reactive & Functional Polymers 56 (2003) 45–57

poly(4,39-BCPMA), photoconversions of about31, 63 and 80% are found after 5, 60 and 170 sof irradiation time, respectively. It is observedthat within 5 min of irradiation about 91%conversion takes place. This indicates that thepresence of bromo group at the 29-position ofthe chalcone moiety causes a decrease in thephotoconversion rate of theaC=Cb bond withrespect to that in the case of the compound withBr group at the 39-position. Although the photo-isomerization cannot be ruled out in solution,the question of disruption of the chromophoreaggregates does not arise since there is noordered arrangement of the chromophores insolution; therefore, photocrosslinking takes pre-cedence overtrans–cis isomerization.

The effect of various solvents on the rate ofphotocrosslinking of poly(4,29-BCPMA) and

Fig. 7. Changes in the UV spectral pattern of poly(4,39-BCPMA) poly(4,39-BCPMA) was studied. This rate inin chloroform solution upon irradiation. Top to bottom, after

chloroform is faster than that in the otherirradiation timet50, 5, 10, 20, 30, 60, 100, 120, 140, 200, 300and 600 s. solvents. The crosslinking rate associated with

polymers in various solvents is in the followingorder: CHCl .CH Cl .dioxane.DMSO.intensity of the band at 299 or 304 nm decreases 3 2 2

DMF (Fig. 9). This illustrated that the type ofvery rapidly with irradiation time, disappearingsolvents used play a significant role in thealmost completely within 10 min of irradiation.photocrosslinking rate associated with theThis is clearly due to crosslinking of polymerphotoreactive polymers in solution.chains through [2p12p] cycloaddition of the

The effect of polymer concentration on theaC=Cb group of pendant chalcone units, asrate of photocrosslinking was studied in chloro-shown in Scheme 3. Since the [2p12p]form with a concentration range 21–166 mgcycloaddition destroys conjugation in the entire21l . Fig. 10 shows the results concerning thep-electron system, the UV absorption intensity

rate of disappearance ofaC=Cb of chalconedecreases dramatically with irradiation time.Consequently, poly(4,29-BCPMA) and units in poly(4,39-BCPMA). In both the cases,poly(4,39-BCPMA) react photochemically ac- rate of crosslinking increases with increasingcording to a mechanism similar to that found for concentration because of proximity of morecinnamic acid and its derivatives[23,24]. photoactive chalcone moieties in concentrated

In general, the photosensitivity of polymers solution[25].containing photoactive cinnamoyl group is mea- In order to observe the effect of sensitizers onsured in terms of rate of disappearance of the disappearance rate ofaC=Cb of the chal-aC=Cb with irradiation time.Fig. 8 compares cone units, the photochemical reactions ofthese rates associated with polymers which have poly(4,29-BCPMA) and poly(4,39-BCPMA)bromo substituent at the 29- or 39-positions of were carried out in the presence of tripletthe pendant chalcone moiety. In chloroform sensitizers such as benzoin, Michler’s ketone,solution, poly(4,29-BCPMA) displays photocon- etc. However, they are not effective in increas-versions of 10, 50 and 67% after 5, 60 and 190 s ing the photosensitivity further. This behaviourof irradiation, respectively. In the case of is similar to that reported for photosensitive

R. Balaji et al. / Reactive & Functional Polymers 56 (2003) 45–57 55

Scheme 3. Photocycloaddition reaction of poly(4,29-BCPMA) and poly(4,39-BCPMA) upon UV irradiation.

polymers having units such as a- solubility of the polymers after irradiation, thephenylmaleimide [26] and a-cyanocinnamic chloroform solutions at higher polymer con-ester[27], which have high photosensitivity but centrations were irradiated for 3–4 h and thecannot be sensitized. This strongly indicates that solvent was evaporated. The residue obtainedthe photocrosslinking of the polymers might not was found to be insoluble in all the organicbe taking place through the triplet (T) state, but solvents, in which these were soluble beforealternatively through the singlet state electrons irradiation. This results from the formation ofleading to a one-step, concerted [2p12p] rigid three-dimensional structures through cross-cycloaddition[28]. linking of polymer chains. This also confirms

The sensitivity of the negative-type polymers that the decrease in UV absorption intensities isfor image forming systems depends on the not due to the cleavage of pendant groups butefficiency of the photoinduced crosslinking, arises from the formation of cyclobutane ringsleading to insolubilization. To check the in- via the cycloaddition of pendant chalcone units

56 R. Balaji et al. / Reactive & Functional Polymers 56 (2003) 45–57

Fig. 10. Disappearance rate associated withaC=Cb of poly(4,39-Fig. 8. Disappearance rate associated with photoactiveaC=Cb ofBCPMA) in chloroform solution at different concentrations: (.)(m) poly(4,39-BCPMA) and (• ) poly(4,29-BCPMA) with irradia-

21 21 21 21166 mg l ; (3) 124.2 mg l ; (• ) 92.4 mg l ; (j) 44.2 mg l ;tion time in chloroform solution.21(m) 21 mg l .

polymers, even in the absence of sensitizer, itmight be expected that these polymers can beeffectively used as negative photoresist materi-als.

4 . Conclusions

Poly(4,29-BCPMA) and poly(4,39-BCPMA)having pendant photosensitive bromo-substi-tuted cinnamoyl groups were synthesized fromnewly prepared methacrylate monomers by freeradical solution polymerization. The structuresof the monomers and corresponding polymerswere confirmed by spectral and elemental analy-ses. The polydispersity indices of the polymersobtained from SEC indicate that there is a

Fig. 9. Disappearance rate associated with the double bondstronger tendency for chain termination by(aC=Cb) of poly(4,39-BCPMA) with irradiation time in different

solvents: (• ) CHCl ; (♦ ) CH Cl ; (m) dioxane; (3) DMSO; and disproportionation rather than by combination.3 2 2

(.) DMF solutions. The polymers were well soluble in chlorinatedsolvents and polar aprotic solvents. Thermo-

(Scheme 3). As these polymers with a pendant gravimetric analysis results clearly show thatbromo-substituted cinnamoyl group have a high the polymers possess very good thermal stabili-rate of crosslinking leading to insolubility of the ty as required for negative photoresists. The

R. Balaji et al. / Reactive & Functional Polymers 56 (2003) 45–57 57

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