research article photoinitiated polymerization of 2...

Hindawi Publishing CorporationISRN PharmaceuticsVolume 2013 Article ID 958712 7 pageshttpdxdoiorg1011552013958712

Research ArticlePhotoinitiated Polymerization of2-Hydroxyethyl Methacrylate by RiboflavinTriethanolamine inAqueous Solution A Kinetic Study

Iqbal Ahmad1 Kefi Iqbal2 Muhammad Ali Sheraz1 Sofia Ahmed1 Tania Mirza1

Sadia Hafeez Kazi1 and Mohammad Aminuddin1

1 Institute of Pharmaceutical Sciences Baqai Medical University 51 Deh Tor Toll Plaza Super Highway Gadap RoadKarachi 74600 Pakistan

2Department of Dental Material Sciences Baqai Dental College Baqai Medical University 51 Deh Tor Toll PlazaSuper Highway Gadap Road Karachi 74600 Pakistan

Correspondence should be addressed to Muhammad Ali Sheraz ali sheraz80hotmailcom

Received 17 June 2013 Accepted 30 July 2013

Academic Editors F-R Chang D Kuzmich and R Veerasamy

Copyright copy 2013 Iqbal Ahmad et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Thepolymerization of 1ndash3M2-hydroxyethylmethacrylate (HEMA) initiated by riboflavintriethanolamine systemhas been studiedin the pH range 60ndash90 An approximate measure of the kinetics of the reaction during the initial stages (sim5 HEMA conversion)has been made to avoid the effect of any variations in the volume of the medium The concentration of HEMA in polymerizedsolutions has been determined by a UV spectrophotometric method at 208 nm with a precision of plusmn3 The initial rate ofpolymerization of HEMA follows apparent first-order kinetics and the rates increase with pH This may be due to the presenceof a labile proton on the hydroxyl group of HEMA The second-order rate constants for the interaction of triethanolamine andHEMA lie in the range of 236 to 867 times 10minus2Mminus1 sminus1 at pH 60ndash90 suggesting an increased activity with pH An increase in theviscosity of HEMA solutions from 1M to 3M leads to a decrease in the rate of polymerization probably as a result of the decreasein the reactivity of the flavin triplet state The effect of pH and viscosity of the medium on the rate of reaction has been evaluated

1 IntroductionAcrylic acid derivatives including 2-hydroxyethyl metha-crylate monomer (HEMA) [1] urethane dimethacrylatemonomer [2] proline modified acrylic acid copolymer [3]N-vinylcaprolactam-containing acrylic acid terpolymer [4]N-vinylpyrrolidone modified acrylic acid copolymer [5] andpolyurethane acrylate monomer [6] have been synthesizedfor dental cement applicationsThese derivatives are intendedto undergo polymerization on exposure to visible light andthus form a hardened mass (cement) Among these deriva-tives HEMA is widely used in glass ionomer cements (GICs)employed as dental restorative materials [7] Various types ofGICs containing HEMA have been developed as light curerestorative materials [8]

The photoinitiated polymerization of vinyl polymers hasbeen studied since the 1950s [9ndash12] The process involvesthe participation of photoinitiators absorbing in the visible

region Riboflavin (RF) absorbs at 444 nm and has been usedas a photoinitiator in the polymerization ofHEMAalongwithtriethanolamine (TEOHA) as a coinitiator to form a redoxpair involved in the process [13ndash16] RF is an efficient electronacceptor and meditates in numerous photochemical andbiological electron transfer reactions [17ndash22] The kineticsof polymerization reactions has been discussed by severalworkers [12 23ndash25] The medium characteristics ionizationbehavior of reacting species and efficiency of the photoini-tiators influence the rate of reaction [26] In most casesthe polymerization rates of HEMA have been determineddilatometrically [13ndash15] This method has been used to studythe effect of solvent on the rates of polymerization of HEMAphotoinitiated by azo compounds [26 27] camphorquinone[28 29] and pyrene derivatives [30] In this work we reporta study of the approximate measure of the kinetics of poly-merization of HEMA initiated by RFTEOHA system under

2 ISRN Pharmaceutics

visible light using a spectrophotometric method for the assayof HEMA during the reactions The effects of pH viscosityand TEOHA concentration on the initial rates of the reaction(sim5 HEMA conversion) have been evaluated These factorshave been found to influence the kinetics of the reaction andhence the efficiency of the polymerization process The studywould provide information on the photoinitiation reactionand interaction of HEMA and RFTEOHA system in the pHrange 60ndash90 and would facilitate the understanding of thepolymerization processes in light-cure GIC systems

2 Materials and Methods

21 Chemicals Riboflavin (RF) lumiflavin (LF) and lumi-chrome (LC) were obtained from Sigma and used as receivedTriethanolamine (TEOHA Sigma) and 2-hydroxyethylmethacrylate (HEMA Aldrich) were distilled under reducedpressure before use Water was purified using a MilliporeMilli-Q system

22 Polymerization Polymerization of HEMA (monomerwater ratio 121 879 242 758 and 363 637 vv corre-sponding to 1 2 and 3M) was carried out in the presenceof 1 times 10minus5M RF (absorbance of the solution at 444 nm waslow (0125) to avoid inhomogeneous free radical distribution)[31] and 00025ndash001M TEOHA at pH 60ndash90 (adjusted byHClNaOH solution) under anaerobic conditions at 25∘CThe solution was irradiated with a General Electric 15Wfluorescent lamp (emission in the visible region) fixed hori-zontally at a distance of 25 cm from the centre of the vessel

23 Thin-Layer Chromatography Thin-layer chromatogra-phy (TLC) of the polymerized solutions containing RFwas carried out on 250-120583m cellulose plate using the sol-vent systems (a) 1-butanol-acetic acid-water (40 10 50 vvorganic phase) and (b) 1-butanol-1-propanol-accetic acid-water (50 30 2 18 vv) [32] RF and photoproducts weredetected by comparison of their characteristic fluorescenceemission under UV (365 nm) excitation (RF yellow greenLC sky blue) with those of the reference standards

24 Spectral Measurement All spectral measurements onfresh and polymerized solutions of HEMA were carried outon a Shimadzu UV-1601 recording spectrophotometer usingquartz cells of 10mm path length

25 Light Intensity Measurement The measurement of theintensity of General Electric 15W fluorescent lamp wascarried out by potassium ferrioxalate actinometry [33] anda value of 285 plusmn 026 times 1016 quanta sminus1 was obtained

26 Assay of HEMA The assay of HEMA in fresh andpolymerized solutions was carried out by adjusting thepH to 70 (005M phosphate buffer) and measurement ofabsorbance at 208 nm after appropriate dilution At thisdilution the photoinitiator has negligible absorption at theanalytical wavelength The calibration data for the assay ofHEMA are reported in Table 1

Table 1 Calibration data for HEMA showing linear regressionanalysisa

120582max 208 nmConcentration range 01ndash10 times 10minus4 MSlope 7980SE (plusmn) of slope 00112Intercept 00010Correlation coefficient 09995Molar absorptivity (120576) 7980Mminus1 cmminus1aValues represent a mean of five determinations

27 Viscosity Measurements The viscosity of the HEMAsolutions was measured with a Brookfield RV viscometer(Model DV-II + Pro Essex UK)

3 Results and Discussion

Before a consideration of the polymerization process it isnecessary to evaluate the photostability of RF as an initiatorduring the reaction and whether there is any alteration in itsconcentration that might affect the rate of the reaction

31 Photoproducts of RF RF has been used by many workersas a photoinitiator in the polymerization of HEMA [13ndash16] Since it is sensitive to light [34] and may undergophotodegradation during irradiation with the visible lightit was felt necessary to examine if any photoproducts ofRF are formed during the irradiation of HEMA (up to60 s) The TLC of the aqueous solutions of RF (1 times 10minus5M)used in the photolysis of HEMA exposed to light for 5min(solvent systems a and b Section 2) indicated the presenceof lumichrome (LC) at pH 90 only In the alkaline solutionsthe photodegradation of RF is greater than that in theacid region [20 34] however under the present irradiationconditions (up to 60 s) no photoproducts of RFwere detectedin the irradiated solutionsThephotoproducts of RF observedin aqueous solution are known and have previously beenreported [20 34ndash36]

32 Spectral Characteristics of Photolysed Solutions of RFThe spectral characteristics of the photolysed solutions ofRF (pH 90) during irradiation (up to 60 s) showed no lossof absorbance at 444 nm in the visible region indicating thephotostability of RF during the period The formation of LCand other photoproducts of RF [20 32 37] would alter thespectral characteristics due to loss in concentration Since RFhas been used at a low concentration (1 times 10minus5M) in thepolymerization ofHEMA[13] its stability during the reactionis necessary to maintain its efficacy as a photoinitiator

33 Assay of HEMA A UV spectophotometric method hasbeen used for the assay of HEMA at 208 nm (pH 70) duringthe polymerization reactions This wavelength correspondsto the absorption maximum of HEMA involving the 120587-120587lowasttransition (Table 1) and on appropriate dilution would givethe concentration of HEMA monomer during the reaction

ISRN Pharmaceutics 3

The validity of Beerrsquos Law was confirmed in the desiredconcentration range prior to the assay and the contentof HEMA in polymerized solutions was determined using7980Mminus1 cmminus1 as the value of molar absorptivity at theanalytical wavelength (Table 1) The reproducibility of themethod was confirmed by the assay of known amountsof HEMA in the concentration range likely to be foundin polymerized solutions The values of RSD for the assayindicate the precision of the method to be within plusmn3This isa new rapid and convenientmethod for the determination ofHEMA in polymerized solutions since most of the previousworkers have used dilatometric method for this purpose [13ndash15] In some cases the rates of polymerization of HEMAhave been measured using gas chromatography [38] Ramanspectroscopy [28] ATR-FTIR spectroscopy [29] and differ-ential scanning calorimetry [39 40] The determination ofHEMA has only been carried out during the initial stages ofthe reaction (sim5 conversion) assuming that in this perioda negligible change in volume would occur in the mediumwhich does not affect the accuracy of the assay method withits defined reproducibility RF at the dilutions used for theassay ofHEMA inpolymerized solutions (more than 100 fold)exhibits negligible absorbance at the analytical wavelengthand hence does not interfere with the assay of HEMA

34 Kinetics of Polymerization of HEMA An approximatemeasure of the kinetics polymerization at low conversion ofHEMAhas been studied spectrophotometrically as describedabove The assay data on HEMA during the initial stagesof the reactions (sim5 conversion) were subjected to anapproximate kinetic treatment and the photolysis of HEMA(a measure of polymerization) was found to follow an appar-ent first-order kinetics in the presence of various TEOHAconcentrations (00025ndash00100M) The polymerization ofHEMA at low conversion remains homogenous since thepolymer remains soluble inmonomer-rich aqueous solutions[23] The steady-state assumption of the rate of initiationbeing equal to the rate of termination is considered validonly at a low conversion of monomer [39 41] dependingupon the intensity of the radiation source (very low in thisstudy see Section 2) and is represented by the apparent first-order rate constant (119896obs) in this study The kinetic treatmentfor the steady state assumption of polymerization reactionhas been presented by Watts [24] During the initial stagesof the reaction (within sim5 conversion) the average degreeof polymerization as well as the viscoelastic properties ofHEMAwould almost be constant on changing the coinitiatorconcentration (00025ndash00100M) Under these conditionsthe shrinkage properties of the polymerized solution wouldnot be affected

35 Effect of pH The values of the determined 119896obs for thepolymerization of HEMA indicate that the rate of reactionis enhanced with an increase in pH from 60ndash90 Sincethe polymerization of HEMA has been carried out in thepresence of TEOHA a change in the ionization of TEOHA(p119870a 782) [42] from 985 to 61 at pH 60ndash90 would affectthe rate of polymerization since it would facilitate electron

00

20

40

60

80

100

40pH

50 60 70 80 90 100

1 M2 M3 M

k2times102

(Mminus1sminus1)

Figure 1 Rate-pH profiles for the polymerization of HEMA inpresence of RFTEOHAHEMAconcentration (e) 10M (◼) 20M(998771) 30M

pH 60

pH 70pH 75pH 80pH 90

00

20

40

60

80

100

40 50 60 70 80 90 100Viscosity (mPamiddots)minus1

k2times102

(Mminus1sminus1)

Figure 2 Plots of 1198962

for the polymerization of HEMA (1ndash3M) inpresence of RFTEOHAversus inverse of solution viscosity Symbolsare as in Figure 1

transfer from TEOHA to RF triplet state [14] resulting in thegreater polymerization ofHEMAThe rate-pHprofiles for thepolymerization of HEMA at 1ndash3M concentrations are shownin Figure 1 These indicate an enhancement in the rate withpH in the range of 60ndash90 as observed earlier byValdebenitoand Encinas [27] This increase in the rate of polymerizationwith pH is attributed to the presence of a labile proton onthe hydroxyl group of HEMA [39] The relative decrease inthe rate of polymerization of HEMA (60ndash90) from 1 to 3Mappears largely due to the viscosity effect as discussed below

36 Effect of Viscosity It has been observed that the rate ofpolymerization of HEMA in aqueous solution (pH 60ndash90)is affected by a change in the viscosity of the medium onincreasing the HEMA concentration (1ndash3M) The plots of119896obs at different HEMA concentrations versus the inverse ofviscosity of the solutions in the pH range 60ndash90 are linear(Figure 2) indicating that an increase in viscosity leads to adecrease in the rate of polymerization of HEMA probablyas a result of the flavin excited singlet and excited tripletstates quenching with an increase in viscosity [17] The rateconstants for diffusion controlled processes are a function ofsolvent viscosity [43] Therefore the rate of reaction wouldbe controlled by solute diffusion and hence the degree ofredox reactions occurring between the species involved in aparticular medium A small change in the viscosity of HEMA


R

C HHO

H2C

H3C

H3C

N

N

N O

O

NH

C HHO

H2C

H3C

H3C

N

N

N O

O

NH

RF

h isc

1[RFlowast]

Rlowast

R

HO

H2C

H3C

H3C

N

N

O

O

NH

H

TEOHA

e transfer

3[RFlowast]

C∙

∙

N+ N(CH2CH2OH)3

R

HO

H2C

H3C

H3C

N

N

O

O

NH

H

C∙

+

minus

N∙

N(CH2CH2OH)3+ H+ transfer

3[RF∙minus+ TEOHA∙+]charged radical pair

R

HO

H2C

H3C

H3C

N

N

O

O

NH

H

H

C∙

N

+∙

N(CH2CH2OH)3

[RF∙] [TEOHA∙]neutral radicalneutral radical

OHH2C O

O

+∙

N(CH2CH2OH)3CH3

HEMA TEOHA radical

Polymer

R = ndash(CHOH)2CH2OH side chain

Figure 3 Scheme for the polymerization of HEMA initiated by RFTEOHA in aqueous solution


Table 2 Second-order rate constants (1198962

) for the interaction of TEOHA with HEMA at pH 60ndash90ab

pH 119896

2

times 10

2Mminus1 sminus1 plusmnSDMonomer water ratio(121 879 vv 10M)

Monomer water ratio(242 758 vv 20M)


60 332 plusmn 026 283 plusmn 022 236 plusmn 01870 495 plusmn 035 403 plusmn 034 350 plusmn 02875 629 plusmn 041 530 plusmn 040 449 plusmn 03780 761 plusmn 062 651 plusmn 052 567 plusmn 04590 867 plusmn 065 762 plusmn 054 669 plusmn 047a119873 = 3

bExperimental conditions Concentration of HEMA 1ndash3M concentration of TEOHA 00025ndash00100M wavelength visible radiation exposure time 60 stemperature 25 plusmn 1∘C

solutionswith an increase in pHhas been reported [27]ThustheHEMA concentration appears to be one of the controllingfactors in the rate of polymerization

37 Effect of TEOHAConcentration TEOHAplays an impor-tant role in the polymerization of HEMA and the reactiondoes not occur in the absence of the coinitiator RF radicalsinteract with TEOHA to initiate the polymerization reactionIt has been reported that the rate of polymerization of HEMAis maximum in the presence of 001M TEOHA [14] Inorder to observe the effect of TEOHA concentration (00025ndash00100M) and its interaction with HEMA in the initial stagesof polymerization the 119896obs values were determined for thereactions carried out at various TEOHA concentrations Aplot of 119896obs versus TEOHA concentrations was found to belinear and the slope yielded the second-order rate constant(1198962

) for the interaction of TEOHAwithHEMAThe 1198962

values(Table 2) have been found to increase with an increase in pHas explained above for the reactions at pH 60ndash90 Thus therate of polymerization is dependent on the reactivity of theamine radicals produced during the reaction

38 Mechanism of HEMA Polymerization A photoinitiationmechanism of HEMA polymerization by RFTEOHA inaqueous solution has been proposed by Orellana et al[14] It has been suggested that the free radicals producedin the photoinduced electron transfer from TEOHA toexcited RF lead to the polymerization of HEMA A similarmechanism for HEMA polymerization photoinitiated by RFin the presence of amines in aqueous solution has beensuggested by Encinas and Previtali [12] The free radicalsmay also be produced from other photoinitiators such asketones and add to HEMA to initiate the polymerization ofthe monomer [43] Based on the mechanisms proposed byprevious workers [12 14] a more elaborate scheme involvingthe structural considerations of the species participating inthe polymerization process is presented in Figure 3

The photoinitiator RF on the absorption of visible lightis promoted to the excited singlet state [1RFlowast] and by inte-rsystem crossing (isc) to the excited triplet state [3RFlowast] Anelectron transfer from TEOHA to [3RFlowast] leads to the for-mation of [RF∙minus] anion and [TEOHA∙+] cation radical pairThis is followed by rapid proton transfer between the species

leading to the formation of [RF∙] and [TEOHA∙] free radicalsThe [TEOHA∙] free radical interacts with HEMA and isadded to the monomer double bonds This would initiate thepolymerization process and thus lead to the formation of thepolymerThe rate and extent ofHEMApolymerizationwoulddepend on factors such as RF and TEOHA concentrationspH and viscosity of the medium and light intensity andwavelengths of irradiation

4 Conclusion

The polymerization of 1ndash3M HEMA in aqueous solutionsinitiated by RF in the presence of TEOHA has been studiedspectrophotometrically by its loss of absorbance at 208 nmThe rate of the reaction is increased with pH in the rangeof 60ndash90 In the initial stages of the reaction (sim5 HEMAconversion) the monomer undergoes change by an approx-imate first-order kinetics and the rate is dependent onTEOHA concentration The second-order rate constants forHEMA-TEOHA interaction decrease with an increase inHEMA concentration as a result of higher viscosity and lowerreactivity of the flavin radicals The apparent first-order rateconstants for HEMA polymerization are a linear function ofthe inverse of viscosity at pH 60ndash90 indicating a decrease inthe rate of the reaction with an increase in the viscosity of themedium

Conflict of Interests

The authors declare that there is no conflict of interests

References

[1] S B Mitra ldquoAdhesion to dentin and physical properties of alight-cured glass-ionomer linerbaserdquo Journal of Dental Rese-arch vol 70 no 1 pp 72ndash74 1991

[2] M Atai M Ahmadi S Babanzadeh and D C Watts ldquoSyn-thesis characterization shrinkage and curing kinetics of a newlow-shrinkage urethane dimethacrylate monomer for dentalapplicationsrdquo Dental Materials vol 23 no 8 pp 1030ndash10412007

[3] A Moshaverinia N Roohpour J A Darr and I U RehmanldquoSynthesis of a proline-modified acrylic acid copolymer in


supercritical CO2

for glass-ionomer dental cement applica-tionsrdquo Acta Biomaterialia vol 5 no 5 pp 1656ndash1662 2009

[4] A Moshaverinia N Roohpour J A Darr and I U RehmanldquoSynthesis and characterization of a novel N-vinylcaprolactam-containing acrylic acid terpolymer for applications in glass-ionomer dental cementsrdquo Acta Biomaterialia vol 5 no 6 pp2101ndash2108 2009

[5] A Moshaverinia N Roohpour R W Billington J A Darr andI U Rehman ldquoSynthesis of N-vinylpyrrolidone modified acr-ylic acid copolymer in supercritical fluids and its application indental glass-ionomer cementsrdquo Journal ofMaterials Science vol19 no 7 pp 2705ndash2711 2008

[6] R Yaobin P Huiming L Longsi X Jianming and Y Yong-qiang ldquoSynthesis of polyurethane acrylate and application toultraviolet-curable pressure-sensitive adhesiverdquo Polymer vol45 no 4 pp 495ndash502 2006

[7] D C Smith ldquoDevelopment of glass ionomer cementsrdquo Bioma-terials vol 19 no 6 pp 467ndash478 1998

[8] Y-K Lee B Yu G-F Zhao and J I Lim ldquoEffects of aging andhema content on the translucency fluorescence and opales-cence properties of experimental hema-added glass ionomersrdquoDental Materials Journal vol 29 no 1 pp 9ndash14 2010

[9] G Oster and N-L Yang ldquoPhotopolymerization of vinyl mon-omersrdquo Chemical Reviews vol 68 no 2 pp 125ndash151 1968

[10] E A Lissi and M V Encinas ldquoPhotoinitiators for free radi-cal polymerizationrdquo in Photochemistry and Photophysics J FRabek Ed vol 4 pp 221ndash294 CRC Press Boca Raton FlaUSA 1991

[11] A Wrzyszczynski F Scigalski J Paczkowski and Z KucybalaldquoDyeing photoinitiators Electron transfer processes in pho-toinitiating systemsrdquo Trends in Photochemistry and Photobiol-ogy vol 5 pp 79ndash91 1999

[12] M V Encinas and C M Previtali ldquoExcited states interactionsof flavins with amines application to the initiation of vinylpolymerizationrdquo in Flavins Photochemistry and PhotobiologyE Silva and A M Edwards Eds pp 41ndash59 Royal Society ofChemistry Cambridge UK 2006

[13] S G Bertolotti C M Previtali A M Rufs and M V EncinasldquoRiboflavintriethanolamine as photoinitiator system of vinylpolymerization A mechanistic study by laser flash photolysisrdquoMacromolecules vol 32 no 9 pp 2920ndash2924 1999

[14] B Orellana A M Rufs and M V Encinas ldquoThe photoini-tiation mechanism of vinyl polymerization by riboflavintrie-thanolamine in aqueous mediumrdquoMacromolecules vol 32 no20 pp 6570ndash6573 1999

[15] M V Encinas A M Rufs S Bertolotti and C M Previ-tali ldquoFree radical polymerization photoinitiated by riboflavinamines Effect of the amine structurerdquoMacromolecules vol 34no 9 pp 2845ndash2847 2001

[16] G Porcal S G Bertolotti C M Previtali and M V EncinasldquoElectron transfer quenching of singlet and triplet excited statesof flavins and lumichrome by aromatic and aliphatic electrondonorsrdquo Physical Chemistry Chemical Physics vol 5 no 19 pp4123ndash4128 2003

[17] I Ahmad and G Tollin ldquoSolvent effects of flavin electron tra-nsfer reactionsrdquo Biochemistry vol 20 no 20 pp 5925ndash59281981

[18] I Ahmad M A Cusanovich and G Tollin ldquoLaser flash pho-tolysis studies of electron transfer between semiquinone andfully reduced free flavins and horse heart cytochrome crdquoProceedings of the National Academy of Sciences of the UnitedStates of America vol 78 no 11 pp 6724ndash6728 1981

[19] I Ahmad M A Cusanovich and G Tollin ldquoLaser flashphotolysis studies of electron transfer between semiquinoneand fully reduced free flavins and the cytochrome c-cytochromeoxidase complexrdquo Biochemistry vol 21 no 13 pp 3122ndash31281982

[20] I Ahmad and FHMVaid ldquoPhotochemistry of flavins in aque-ous and organic solventsrdquo in Flavins Photochemistry and Pho-tobiology E Silva and A M Edwards Eds pp 13ndash40 RoyalSociety of Chemistry Cambridge UK 2006

[21] G Tollin ldquoUse of flavin photochemistry to probe intraproteinand interprotein electron transfer mechanismsrdquo Journal ofBioenergetics and Biomembranes vol 27 no 3 pp 303ndash3091995

[22] F Scigalski and J Paczkowski ldquoPhotoinitiating free-radical pol-ymerization electron-transfer pairs applying amino acids andsulfur-containing amino acids as electron donorsrdquo Journal ofApplied Polymer Science vol 97 no 1 pp 358ndash365 2005

[23] M V Encinas E A Lissi and C Martinez ldquoPolymerizationof 2-hydroxyethyl methacrylate induced by azo compoundssolvent effectsrdquo European Polymer Journal vol 32 no 9 pp1151ndash1154 1996

[24] D C Watts ldquoReaction kinetics and mechanics in photo-polymerised networksrdquo Dental Materials vol 21 no 1 pp 27ndash35 2005

[25] E Andrzejewska ldquoPhotopolymerization kinetics of multifunc-tional monomersrdquo Progress in Polymer Science vol 26 no 4 pp605ndash665 2001

[26] J Alvarez M V Encinas and E A Lissi ldquoSolvent effectson the rate of polymerization of 2-hydroxyethyl methacrylatephotoinitiated with aliphatic azo compoundsrdquoMacromolecularChemistry and Physics vol 200 no 10 pp 2411ndash2415 1999

[27] A Valdebenito and M V Encinas ldquoPhotopolymerization of 2-hydroxyethyl methacrylate effect of the medium properties onthe polymerization raterdquo Journal of Polymer Science A vol 41no 15 pp 2368ndash2373 2003

[28] Y Wang P Spencer X Yao and Q Ye ldquoEffect of coinitiatorand wafer on the photoreactivity and photopolymerization ofHEMAcamphoquinone-based reactant mixturesrdquo Journal ofBiomedical Materials Research A vol 78 no 4 pp 721ndash7282006

[29] X Guo Y Wang P Spencer Q Ye and X Yao ldquoEffects of watercontent and initiator composition on photopolymerization of amodel BisGMAHEMA resinrdquo Dental Materials vol 24 no 6pp 824ndash831 2008

[30] M V Encinas E A Lissi C Majmud and J J Cosa ldquoPhoto-polymerization in aqueous solutions initiated by the interactionof excited pyrene derivatives with aliphatic aminesrdquo Macro-molecules vol 26 no 23 pp 6284ndash6288 1993

[31] J Alvarez E A Lissi and M V Encinas ldquoEffect of the initiatorabsorbance on the transition-metal complex photoinitiatedpolymerizationrdquo Journal of Polymer Science A vol 36 no 1 pp207ndash208 1998

[32] I AhmadHDC Rapson P PHeelis andGO Phillips ldquoAlka-line hydrolysis of 78-dimethyl-10-(formylmethyl)isoalloxazineA kinetic studyrdquo The Journal of Organic Chemistry vol 45 no4 pp 731ndash733 1980

[33] C G Hatchard and C A Parker ldquoA new sensitive chemicalactinometer II Potassium ferrioxalate as a standard chemicalactinometerrdquo Proceedings of the Royal Society A vol 235 no1203 pp 518ndash536 1956

[34] I Ahmad Q Fasihullah A Noor I A Ansari andQ NM AlildquoPhotolysis of riboflavin in aqueous solution a kinetic studyrdquo


International Journal of Pharmaceutics vol 280 no 1-2 pp 199ndash208 2004

[35] I Ahmad and H D C Rapson ldquoMulticomponent spectropho-tometric assay of riboflavine and photoproductsrdquo Journal ofPharmaceutical and Biomedical Analysis vol 8 no 3 pp 217ndash223 1990

[36] I Ahmad T Mirza K Iqbal S Ahmed M A Sheraz and F HM Vaid ldquoEffect of pH buffer and viscosity on the photolysisof formylmethylflavin a kinetic studyrdquo Australian Journal ofChemistry vol 66 no 5 pp 576ndash585 2013

[37] I Ahmad S Ahmed M A Sheraz F H M Vaid and I AAnsari ldquoEffect of divalent anions on photodegradation kineticsand pathways of riboflavin in aqueous solutionrdquo InternationalJournal of Pharmaceutics vol 390 no 2 pp 174ndash182 2010

[38] K L Beers S Boo S G Gaynor and K Matyjaszewski ldquoAtomtransfer radical polymerization of 2-hydroxyethyl methacry-laterdquoMacromolecules vol 32 no 18 pp 5772ndash5776 1999

[39] J Jakubiak X Allonas J P Fouassier et al ldquoCamphorquinone-amines photoinitating systems for the initiation of free radicalpolymerizationrdquo Polymer vol 44 no 18 pp 5219ndash5226 2003

[40] E Andrzejewska M Podgorska-Golubska I Stepniak and MAndrzejewski ldquoPhotoinitiated polymerization in ionic liquidskinetics and viscosity effectsrdquo Polymer vol 50 no 9 pp 2040ndash2047 2009

[41] S Beuermann M Buback P Hesse R A Hutchinson S Kuk-uckova and I Lacık ldquoTermination kinetics of the free-radicalpolymerization of nonionized methacrylic acid in aqueoussolutionrdquoMacromolecules vol 41 no 10 pp 3513ndash3520 2008

[42] A Albert and E P Serjeant Ionization Constants of Acids andBases Methuen London UK 1982

[43] N J Turro V Ramamurthy and J S ScaianoModernMolecularPhotochemistry of Organic Molecules University Science BooksSausalito Calif USA 2010

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MEDIATORSINFLAMMATION

of


visible light using a spectrophotometric method for the assayof HEMA during the reactions The effects of pH viscosityand TEOHA concentration on the initial rates of the reaction(sim5 HEMA conversion) have been evaluated These factorshave been found to influence the kinetics of the reaction andhence the efficiency of the polymerization process The studywould provide information on the photoinitiation reactionand interaction of HEMA and RFTEOHA system in the pHrange 60ndash90 and would facilitate the understanding of thepolymerization processes in light-cure GIC systems

2 Materials and Methods

21 Chemicals Riboflavin (RF) lumiflavin (LF) and lumi-chrome (LC) were obtained from Sigma and used as receivedTriethanolamine (TEOHA Sigma) and 2-hydroxyethylmethacrylate (HEMA Aldrich) were distilled under reducedpressure before use Water was purified using a MilliporeMilli-Q system

22 Polymerization Polymerization of HEMA (monomerwater ratio 121 879 242 758 and 363 637 vv corre-sponding to 1 2 and 3M) was carried out in the presenceof 1 times 10minus5M RF (absorbance of the solution at 444 nm waslow (0125) to avoid inhomogeneous free radical distribution)[31] and 00025ndash001M TEOHA at pH 60ndash90 (adjusted byHClNaOH solution) under anaerobic conditions at 25∘CThe solution was irradiated with a General Electric 15Wfluorescent lamp (emission in the visible region) fixed hori-zontally at a distance of 25 cm from the centre of the vessel

23 Thin-Layer Chromatography Thin-layer chromatogra-phy (TLC) of the polymerized solutions containing RFwas carried out on 250-120583m cellulose plate using the sol-vent systems (a) 1-butanol-acetic acid-water (40 10 50 vvorganic phase) and (b) 1-butanol-1-propanol-accetic acid-water (50 30 2 18 vv) [32] RF and photoproducts weredetected by comparison of their characteristic fluorescenceemission under UV (365 nm) excitation (RF yellow greenLC sky blue) with those of the reference standards

24 Spectral Measurement All spectral measurements onfresh and polymerized solutions of HEMA were carried outon a Shimadzu UV-1601 recording spectrophotometer usingquartz cells of 10mm path length

25 Light Intensity Measurement The measurement of theintensity of General Electric 15W fluorescent lamp wascarried out by potassium ferrioxalate actinometry [33] anda value of 285 plusmn 026 times 1016 quanta sminus1 was obtained

26 Assay of HEMA The assay of HEMA in fresh andpolymerized solutions was carried out by adjusting thepH to 70 (005M phosphate buffer) and measurement ofabsorbance at 208 nm after appropriate dilution At thisdilution the photoinitiator has negligible absorption at theanalytical wavelength The calibration data for the assay ofHEMA are reported in Table 1

Table 1 Calibration data for HEMA showing linear regressionanalysisa

120582max 208 nmConcentration range 01ndash10 times 10minus4 MSlope 7980SE (plusmn) of slope 00112Intercept 00010Correlation coefficient 09995Molar absorptivity (120576) 7980Mminus1 cmminus1aValues represent a mean of five determinations

27 Viscosity Measurements The viscosity of the HEMAsolutions was measured with a Brookfield RV viscometer(Model DV-II + Pro Essex UK)

3 Results and Discussion

Before a consideration of the polymerization process it isnecessary to evaluate the photostability of RF as an initiatorduring the reaction and whether there is any alteration in itsconcentration that might affect the rate of the reaction

31 Photoproducts of RF RF has been used by many workersas a photoinitiator in the polymerization of HEMA [13ndash16] Since it is sensitive to light [34] and may undergophotodegradation during irradiation with the visible lightit was felt necessary to examine if any photoproducts ofRF are formed during the irradiation of HEMA (up to60 s) The TLC of the aqueous solutions of RF (1 times 10minus5M)used in the photolysis of HEMA exposed to light for 5min(solvent systems a and b Section 2) indicated the presenceof lumichrome (LC) at pH 90 only In the alkaline solutionsthe photodegradation of RF is greater than that in theacid region [20 34] however under the present irradiationconditions (up to 60 s) no photoproducts of RFwere detectedin the irradiated solutionsThephotoproducts of RF observedin aqueous solution are known and have previously beenreported [20 34ndash36]

32 Spectral Characteristics of Photolysed Solutions of RFThe spectral characteristics of the photolysed solutions ofRF (pH 90) during irradiation (up to 60 s) showed no lossof absorbance at 444 nm in the visible region indicating thephotostability of RF during the period The formation of LCand other photoproducts of RF [20 32 37] would alter thespectral characteristics due to loss in concentration Since RFhas been used at a low concentration (1 times 10minus5M) in thepolymerization ofHEMA[13] its stability during the reactionis necessary to maintain its efficacy as a photoinitiator

33 Assay of HEMA A UV spectophotometric method hasbeen used for the assay of HEMA at 208 nm (pH 70) duringthe polymerization reactions This wavelength correspondsto the absorption maximum of HEMA involving the 120587-120587lowasttransition (Table 1) and on appropriate dilution would givethe concentration of HEMA monomer during the reaction





00

20

40

60

80

100

40pH

50 60 70 80 90 100

1 M2 M3 M

k2times102

(Mminus1sminus1)


pH 60


00

20

40

60

80

100


k2times102

(Mminus1sminus1)






R

C HHO

H2C

H3C

H3C

N

N

N O

O

NH

C HHO

H2C

H3C

H3C

N

N

N O

O

NH

RF

h isc

1[RFlowast]

Rlowast

R

HO

H2C

H3C

H3C

N

N

O

O

NH

H

TEOHA

e transfer

3[RFlowast]

C∙

∙

N+ N(CH2CH2OH)3

R

HO

H2C

H3C

H3C

N

N

O

O

NH

H

C∙

+

minus

N∙



R

HO

H2C

H3C

H3C

N

N

O

O

NH

H

H

C∙

N

+∙

N(CH2CH2OH)3


OHH2C O

O

+∙

N(CH2CH2OH)3CH3

HEMA TEOHA radical

Polymer






pH 119896

2

times 10













4 Conclusion




References





supercritical CO2
















































Volume 2014

ToxinsJournal of

VaccinesJournal of















AddictionJournal of






Autoimmune Diseases




Journal of


Pharmaceutics



of





00

20

40

60

80

100

40pH

50 60 70 80 90 100

1 M2 M3 M

k2times102

(Mminus1sminus1)


pH 60


00

20

40

60

80

100


k2times102

(Mminus1sminus1)






R

C HHO

H2C

H3C

H3C

N

N

N O

O

NH

C HHO

H2C

H3C

H3C

N

N

N O

O

NH

RF

h isc

1[RFlowast]

Rlowast

R

HO

H2C

H3C

H3C

N

N

O

O

NH

H

TEOHA

e transfer

3[RFlowast]

C∙

∙

N+ N(CH2CH2OH)3

R

HO

H2C

H3C

H3C

N

N

O

O

NH

H

C∙

+

minus

N∙



R

HO

H2C

H3C

H3C

N

N

O

O

NH

H

H

C∙

N

+∙

N(CH2CH2OH)3


OHH2C O

O

+∙

N(CH2CH2OH)3CH3

HEMA TEOHA radical

Polymer






pH 119896

2

times 10













4 Conclusion




References





supercritical CO2
















































Volume 2014

ToxinsJournal of

VaccinesJournal of















AddictionJournal of






Autoimmune Diseases




Journal of


Pharmaceutics



of


R

C HHO

H2C

H3C

H3C

N

N

N O

O

NH

C HHO

H2C

H3C

H3C

N

N

N O

O

NH

RF

h isc

1[RFlowast]

Rlowast

R

HO

H2C

H3C

H3C

N

N

O

O

NH

H

TEOHA

e transfer

3[RFlowast]

C∙

∙

N+ N(CH2CH2OH)3

R

HO

H2C

H3C

H3C

N

N

O

O

NH

H

C∙

+

minus

N∙



R

HO

H2C

H3C

H3C

N

N

O

O

NH

H

H

C∙

N

+∙

N(CH2CH2OH)3


OHH2C O

O

+∙

N(CH2CH2OH)3CH3

HEMA TEOHA radical

Polymer






pH 119896

2

times 10













4 Conclusion




References





supercritical CO2
















































Volume 2014

ToxinsJournal of

VaccinesJournal of















AddictionJournal of






Autoimmune Diseases




Journal of


Pharmaceutics



of




pH 119896

2

times 10













4 Conclusion




References





supercritical CO2
















































Volume 2014

ToxinsJournal of

VaccinesJournal of















AddictionJournal of






Autoimmune Diseases




Journal of


Pharmaceutics



of


supercritical CO2
















































Volume 2014

ToxinsJournal of

VaccinesJournal of















AddictionJournal of






Autoimmune Diseases




Journal of


Pharmaceutics



of
















Volume 2014

ToxinsJournal of

VaccinesJournal of















AddictionJournal of






Autoimmune Diseases




Journal of


Pharmaceutics



of

research article photoinitiated polymerization of 2...

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