research article solvent effect on photoinitiator reactivity in the...

Hindawi Publishing CorporationAdvances in Physical ChemistryVolume 2013 Article ID 838402 7 pageshttpdxdoiorg1011552013838402

Research ArticleSolvent Effect on Photoinitiator Reactivity in the Polymerizationof 2-Hydroxyethyl Methacrylate

Iqbal Ahmad1 Kefi Iqbal2 Muhammad Ali Sheraz1 Sofia Ahmed1 Syed Abid Ali3

Sadia Hafeez Kazi1 Tania Mirza1 Raheela Bano1 and Mohammad Aminuddin1

1 Baqai Institute of Pharmaceutical Sciences Baqai Medical University 51 Deh Tor Toll PlazaSuper Highway Gadap Road Karachi 74600 Pakistan

2Department of Dental Material Sciences Baqai Dental College Baqai Medical University51 Deh Tor Toll Plaza Super Highway Gadap Road Karachi 74600 Pakistan

3HEJ Research Institute of Chemistry University of Karachi Karachi 75270 Pakistan

Correspondence should be addressed to Muhammad Ali Sheraz ali sheraz80hotmailcom

Received 26 September 2013 Revised 22 November 2013 Accepted 23 November 2013

Academic Editor Francesco Paolucci

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

Efficacy of photoinitiators such as riboflavin (RF) camphorquinone (CQ) and safranin T (ST) and triethanolamine as a coinitiatorhas been compared in carrying out the polymerization of 2-hydroxyethyl methacrylate (HEMA) in aqueous and organic solventsHEMA solutions were polymerized in the presence of RF CQ and ST using a low intensity visible radiation source HEMAwas assayed by a UV spectrophotometric method during the initial stages of the reactions (ie sim5 change) A comparison ofthe efficacy of photoinitiators in causing HEMA polymerization showed that RF is more efficient than CQ and ST The rate ofpolymerization is directly related to solvent dielectric constant and inversely related to the solvent viscosity RF is the most efficientphotoinitiator in the polymerization of HEMA and the highest rate of reaction occurs in aqueous solutions A general scheme forthe polymerization of HEMA in the presence of photoinitiators is presented

1 Introduction

The influence of solvent on the rates and mechanisms ofchemical reactions is of great importance and has been dis-cussed by many workers [1ndash5] 2-Hydroxyethyl methacrylate(HEMA) is a component of resin-modified glass-ionomercements used as restorative materials in dentistry It under-goes polymerization in the presence of a photoinitiatorduring the setting process on bonding to the teeth [6]The efficacy of photoinitiators in the polymerization ofHEMAmay be affected by medium characteristics includingthe polarity viscosity and the extent of radical formationinvolved in the reaction Several studies have been carriedout on the effect of solvent on the polymerization of HEMAusing dilatometry [7 8] gas chromatography [9] Ramanspectroscopy [10] ATR-FTIR spectroscopy [11] and differ-ential scanning calorimetry (DSC) [12] The primary pho-tochemical processes in polymerization may be dependent

on the solvent and therefore the dielectric constant of themedium could affect the initial quantum yield of the process[8] Most of the work on the polymerization of HEMA inaqueous solution has been carried out using water-solublephotoinitiators and information is lacking on their behaviorin organic solvents It would be worthwhile to evaluate theefficiency of these photoinitiators in the polymerization ofHEMA in organic solvents The present work is based on astudy of the effect of solvent dielectric constant and viscosityon the rate of polymerization of HEMA in aqueous andorganic solvents using a UV spectrophotometric methodRiboflavin (120582max = 445 nm) [13] camphorquinone (120582max =468 nm) [14] and safranin T (120582max = 532 nm) [15] have beenused as photoinitiators and triethanolamine as a coinitiator[7 8 16] in the reaction This work throws light on theeffect of solvent characteristics interactions and kinetics ofHEMA polymerization A comparative study of the reactivityof different photoinitiators in aqueous and organic solvents

2 Advances in Physical Chemistry

2-Hydroxyethyl methacrylate Riboflavin

Camphorquinone Safranin T

Triethanolamine

O

OOH

CH3

H2C

H3C N

OH

HO

NH

N

HO

OH

ClminusN+H2N

O

O

O

O

N

H3C N CH3

NH2

OH

OHN

HO

H3C

H3C CH3

H3C

Figure 1 Chemical structures of HEMA and photoinitiators

highlights the effect of solvent on the kinetics and mode ofpolymerization reactions The chemical structures of HEMAand the photoinitiators used in this study are shown inFigure 1

2 Experimental

21 Materials Riboflavin (RF) camphorquinone (CQ) andsafranin T (ST) were obtained from Sigma Triethanolamine(TEOHA Sigma) and 2-hydroxyethyl methacrylate (HEMAAldrich) were distilled under reduced pressure before useWater was purified using a Millipore Milli-Q system

22 Method of Polymerization Polymerization of HEMA(monomersolvent ratio 121 10 1M) was carried out in thepresence of photoinitiators RF CQ and ST (absorbance ofeach photoinitiator at the 120582max was not more than 0125to avoid inhomogenous free radical distribution) [17] and001M TEOHA as a coinitiator in aqueous and organicsolvents under anaerobic conditions at 25∘C The solutionswere irradiated with a low intensity General Electric 15W

fluorescent lamp (emission in the visible region) fixed hori-zontally at a distance of 25 cm from the center of the vessel

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

24 Fluorescence Measurements Fluorescence measure-ments of RF in various HEMA solutions were carried out atroom temperature (sim25∘C) using a SpectraMax 5 fluorimeter(Molecular Devices USA) in the end point mode using120582ex = 374 nm and 120582em = 520 nm [18] The fluorescence wasrecorded in relative fluorescence units using a pure 005mMRF solution as standard

25 Measurement of the Light Intensity Themeasurement ofthe intensity of General Electric 15W fluorescent lamp wascarried out by potassium ferrioxalate actinometry [19] and avalue of 285 plusmn 026 times 1016 quanta sminus1 was obtained

Advances in Physical Chemistry 3

Table 1 Apparent first-order rate constants (119896obs) for the polymerization of HEMA in water and organic solvents

Solvent Dielectric constanta (25∘C) Viscositya (mPasdots)minus1 119896obs times 104

(sminus1)b

RF CQ STWater 785 1000 505 404 298Acetonitrile 375 2898 371 302 210Methanol 326 1838 353 290 193Ethanol 243 0931 312 262 1711-Propanol 201 0514 mdash 245 1561-Butanol 171 0393 mdash 239 145aCRC Handbook of Chemistry and Physics 90th edition CRC Press Boca Raton FL 2010bThe values of rate constants are relative and depend on specific experimental conditions including light intensity

26 Assay of HEMA The assay of HEMA in fresh andpolymerized solutions was carried out by mixing a smallamount of the solution with 005M phosphate buffer pH 70and measurement of absorbance at 208 nm (molar absorp-tivity 8000Mminus1 cmminus1) At this dilution the photoinitiator hasnegligible absorption at the analytical wavelength The con-centration of the samples was calculated using the followingleast squares regression equation 119910 = 09920119909 + 00012 1199032 =09996 The validity of Beerrsquos law has been confirmed in theconcentration range of 01ndash10 times 10minus4M HEMA The RSD ofthe assay method is within plusmn 3 The method has previouslybeen used for the study of the photoinitiated polymerizationof HEMA by RFTEOHA system in aqueous solution [20]

3 Results and Discussion

31 Effect of Dielectric Constant on Polymerization The rateof the reactions between dipolar molecules is dependent onthe dielectric constant119863 of the medium [4] Consider

ln 119896 = ln 119896119863=infinminus 119870(

1

119863

) (1)

where 119896119863=infin

is the rate constant in a medium of infinitedielectric constant and119870 is a constant involving terms such asion charge and distance between ionsThe dielectric constantof the medium is approximately equal to the dielectricconstant of the solvent in dilute solutions A decrease indielectric constant of the medium tends to decrease the rateof reaction and conversely The effect of dielectric constanton the polymerization of HEMA has been evaluated in thepresence of different photoinitiators

311 Riboflavin as Photoinitiator The polymerization ofHEMA was carried out in aqueous and organic solvents(acetonitrile methanol and ethanol) containing RF as pho-toinitiator and 001M TEOHA as a coinitiator The reactionsin 1-propanol and 1-butanol could not be carried out due tothe insolubility of RF in these solvents The analytical datawere subjected to kinetic treatment and the reactions werefound to follow pseudo first-order kinetics in the initial stages(sim5 HEMA loss) using a low intensity radiation sourceThe steady-state assumption of the rate of initiation beingequal to the rate of termination in polymerization reactionsis considered valid only at a low conversion of monomer [14]

CQ

RF

ST

00

10

20

30

40

50

0 20 40 60 80Dielectric constant

kob

stimes104

(sminus1)

Figure 2 Plots of 119896obs for polymerization of HEMA againstdielectric constant times water e acetonitrile998771 methanol ⧫ ethanol◼ 1-propanol lowast 1-butanol

and is represented by the apparent first-order rate constant(119896obs) in this study The values of 119896obs for the polymerizationof HEMA in the presence of RF in water and organic solventsare reported in Table 1 In order to develop a correlationbetween 119896obs and the dielectric constant of the medium119863 a plot of 119896obs versus dielectric constant of the solventswas constructed (Figure 2) It was observed that the rateof reaction is dependent upon the solvent and is a linearfunction of the dielectric constant of the medium Since RFis used as a photoinitiator in this reaction it is necessaryto understand the behavior of RF on excitation This couldbe explained on the basis of the existence of a polar flavin(Fl) intermediate which would facilitate the polymerizationreaction with an increase in the polarity of the medium Astrong evidence for the presence of such an intermediate hasbeen presented by Ahmad and Tollin [21] who studied thesolvent effect on flavin electron transfer reactions using laserflash photolysis According to these workers the reductionof flavin triplet (3Fl) by the substrate (amine in this case)proceeds via a dipolar intermediate in water and organicsolvents and therefore the rate is increased with an increasein the solvent dielectric constant

3Fl+AH 997888rarr (Fl120575minus sdot sdot sdotH sdot sdot sdotA120575+

) (2)

The extent of solvent interaction with the dipolar inter-mediate would determine the degree to which it leads to the


Table 2 Fluorescence intensity of 1times 10minus5 MRF inwater and organicsolvents

Solvent Relative fluorescence intensity at 520 nmWater 1000Ethanol 871Methanol 867Acetonitrile 841

formation of radicals In these reactions the primary photo-chemical process is considered as being the electron transferbetween reactants In such a case the transition state is morepolar than the reactant and the rate of reactionwould increasewith the dielectric constant of the medium as observed inthe case of the photolysis of formylmethylflavin [22] Thusthe polarity of the medium in which the polymerization ofHEMA is being carried out would exert an effect on the rateof the reaction and the primary physical factor determiningthe observed dependence of 119896obs on 119863 is the electrostaticinteraction It should however be noted that an alternativeinterpretation to the interrelation between 119896obs and119863may begiven namely the increase in solvent dielectric may lead toincrease in the strength of solvophobic effect which is mainlyof entropic nature and originates from change in the solventstructure around the reactants on complexation [23] Thismay affect 119896obs on the state of HEMA polymerization andact together with the electrostatics Further discussion of thecontribution from these two factors to the dependence of 119896obson 119863 falls out of the scope of the present paper Howeverit is worth noting that 119863 is directly related to the strengthof electrostatic interactions and only indirectly accounts forthe solvophobic effect hence the linear interrelation between119896obs and solvent dielectric in Figure 2 presumably reflectsthe electrostatics as a major factor determining the observeddependence Anyway the obtained results suggest that waterwith the highest dielectric constant appears to be the bestmedium for carrying out polymerization of HEMA in thepresence of RF to obtain a greater yield than that in theorganic solvents

It has been reported that the rates of polymerization ofHEMA are decreased with a decrease in medium polaritythat is from water to acetonitrile as a result of singlet statequenching in organic solvents [7 8] The results obtainedin this study are in accordance with this behavior since thefluorescence of RF is reducedwith the polarity of themedium(Table 2)

312 Camphorquinone as Photoinitiator The results of theeffect of solvent dielectric constant on the rate of polymeriza-tion of HEMA in the presence of CQ as photoinitiator maybe considered on the basis of the data discussed above in thecase of RF as a photoinitiator The values of 119896obs for thesereactions are given in Table 1 The polymerization behaviorof HEMA in aqueous and organic solvents is similar to thatobserved in the presence of RF with respect to the effectof dielectric constant (Figure 2) However the values of 119896obsin this case are lower than those observed for RF and maybe due to a lower reactivity of the polar intermediate and

subsequent radical formation in this reaction In view of thestructural consideration (C=O groups) the polar character ofCQ would be lower than that of RF (a highly conjugated sys-tem) resulting in lower rate constants for the reactions Theeffectiveness of CQTEOHA system depends on the H-atomdonor ability of the amine in a particular environment andsubsequent interaction of the photoinitiator excited specieswith the monomer (HEMA) to undergo polymerization [11]

313 Safranin T as Photoinitiator The apparent first-order rate constants for the polymerization of HEMA inSTTEOHA system in aqueous and organic solvent arereported in Table 1 A plot of these rate constants as a functionof solvents dielectric constant is shown in Figure 2 Theseresults indicate that the reactivity of ST is lower than thoseof RF and CQ as photoinitiators Apart from a considerationof the excited state polarization behavior of this moleculeand polarity of the intermediate involved in this reaction thevisible absorption maximum (532 nm) of ST is higher thanthose of RF (444 nm) and CQ (468 nm) It would providerelatively less energy for the excitation of the molecule andwould have a lower efficiency compared with the other twophotoinitiators Thus the rates of polymerization of HEMAin this case are lower than those of RF and CQ

The slopes of the plots of 119896obs versus dielectric constant ofthe medium for the photoinitiators used are in the followingorder

RF gt CQ gt ST (3)

This indicates the magnitude of the solvent effect on the reac-tivity of these compounds in initiating the polymerization ofHEMA

32 Effect of Viscosity on Polymerization Another importantfactor that may influence the rate of a chemical reactionis the viscosity of the medium This appears to control thesolute diffusion and hence the rate of a reaction A perviousstudy has shown that the 3Fl quenching by a substrate isproportional to the inverse of solvent viscosity as expectedfor a diffusional process [21] The effect of viscosity onthe polymerization of HEMA in the presence of differentphotoinitiators has been discussed in the following sections

321 Riboflavin as Photoinitiator Polymerization reactionsof HEMA in water at 1 2 and 3M concentrations in thepresence of different photoinitiators have shown that therates are decreased with an increase in the viscosity of themedium [20] In order to confirm the effect of mediumviscosity on the rate of these reactions the values of 119896obsin organic solvents in the presence of RF were plotted as afunction of the inverse of solvent viscosity (Table 1) and alinear relationshipwas observed as expected (Figure 3)Theseobservations are also supported by the earlier data on the rateconstants reported by Valdebenito and Encinas [8] where adecrease in fluorescence quantum yields of the photoinitiatorin organic solvents compared to those in aqueous mediumwas observed The decrease in fluorescence intensity of RFin organic solvents (Table 2) indicates the effect of solvent


CQRF

ST

00

10

20

30

40

50

kob

stimes104

(sminus1)

00 05 10 15 20 25 30Viscosity (mPamiddots)minus1

Figure 3 Plots of 119896obs for polymerization of HEMA against inverseof solvent viscosity Symbols are the same as those in Figure 2

viscosity (Figure 3) on the reaction This may be explainedon the basis of Fl singlet quenching in organic solvents asa result of change in viscosity The radical-radical reactionsas in the case of polymerization of HEMA are sensitive tosolvent viscosity [5] Moreover the decrease in the rate ofpolymerization has also been ascribed to the combination ofa monomer viscosity effect [24]

322 Camphorquinone as Photoinitiator Theeffect of viscos-ity on the rate of polymerization of HEMA using CQ as aphotoinitiator shows a similar behavior as observed in thecase of RF A plot of 119896obs versus the inverse of solvent viscosityshows a linear relationship and the rates tend to decrease withan increase in the viscosity of the medium This appears tobe due to a decrease in solute diffusion processes with anincrease in solvent viscosity The slope of the plot (Figure 3)indicates that viscosity exerts a lower effect on the rates in thepresence of CQ compared to that of RF

323 Safranin T as Photoinitiator The results obtained withST as a photoinitiator in the polymerization of HEMA aresimilar to those of RF and CQ A plot of 119896obs versus inverse ofviscosity shows a linear relationship (Figure 3) and the ratesare further lower than those observed in the case of CQ Theeffect of viscosity on the rates of polymerization of HEMAusing ST as a photoinitiator is lower than those of RF and CQThus viscosity appears to play a significant role in the efficacyof polymerization processes

33 Spectral and Structural Characteristics of PhotoinitiatorsIn order to provide further explanation of the reactivity of thethree photoinitiators (RF CQ and ST) used in this study aconsideration of the spectral and structural characteristics ofthese compounds is necessary

RF exhibits an absorption maximum at 445 nm andundergoes 120587-120587lowast transition resulting in high molar absorptiv-ity (12500Mminus1 cmminus1) [25] It is a polar compound (p119870

119886

19102) and exists as a dipolar molecule in aqueous solution Itis known to produce a polar intermediate on light absorption[21] which would further lead to the formation of freeradicals and then efficient interaction with the amine to

PI 1PIlowast 3PIlowast

Am

PI

H+ transfer

Polymer PIH∙+ Am∙HEMA

h isc

3(PI∙minus + Am∙+)

Figure 4 A general scheme for the polymerization of HEMA in thepresence of photoinitiators

initiate polymerization Compared with RF CQ possessesweakly ionizable C=O groupsThe light absorption at 468 nmwould lead to 119899-120587lowast transition of the dicarbonyl group inthe molecule which has low molar absorptivity Hence itsefficiency in water would be lower compared to that of RF asobserved ST (p119870

119886

40) is an ionizable compound and exhibitsan absorption maximum at 532 nm that results from 120587-120587lowasttransition All these photoinitiators on excitation produceradicals which interact with the amine and thus initiate thepolymerization of HEMA The degree of interaction of thesephotoinitiators with TEOHA would depend on the yield oftheir radicals in aqueous and organic solventsThe rate of thereaction would depend on the viability of the radical pair ina specific medium leading to polymerization

It needs to be emphasized that under the assay conditionsof HEMA on the dilution of photolysed solutions themaximumTEOHAconcentration used (001M)would be toolow to undergo complexation with HEMA in a molar ratioThe UV spectra of HEMA at such a dilution did not showany change inUV absorption in the presence of TEOHAThissuggests that there is no possibility of interaction betweenthese compounds to affect the rate constants

34 Mechanism of Polymerization Themechanisms of poly-merization of HEMA using RF [26] CQ [14] and ST [7]as photoinitiators have previously been reported and involvesimilar steps in radical formation and further interactionsto yield the polymer Based on these mechanisms a generalscheme for the polymerization of HEMA in the presence ofdifferent photoinitiators is presented in Figure 4

The photoinitiator (PI) on the absorption of light ispromoted to the excited singlet state (1PIlowast) followed byintersystem crossing (isc) to the excited triplet state (3PIlowast)3PIlowast is quenched by the amine (Am) by electron transfer toform a semireduced 3PI∙minus and a semioxidized 3Am∙+ radicalpair [3(PI∙minus + Am∙+)] This is followed by proton transferfrom the Am∙+ radical to the PI∙minus radical to produce neutral


PI and Am radicals The free radicals thus formed in thereaction would add to the double bonds of HEMAmonomerand initiate the polymerization process The rate and extentof polymerization would depend on the solvent polarity andviscosity

4 Conclusion

The polymerization of HEMA in the presence of RF CQand ST as photoinitiators and TEOHA as a coinitiator showsthat RF is more efficient than CQ and ST The rate ofpolymerization is a linear function of the solvent dielectricconstant indicating the involvement of polar intermediatesin the photoinitiated reaction The effect of decrease influorescence intensity of RF on the rate of the reaction is dueto a decrease in solvent polarity causing the quenching ofthe excited singlet state The rate of the reaction is inverselyproportional to the solvent viscosity as a result of the diffusioncontrolled process The study highlights the role of solventcharacteristics in the efficiency of the polymerization ofHEMA in the presence of the photoinitiators used

Conflict of Interests

There is no conflict of interests

References

[1] E S Amis and J F Hinton Solvent Effects on ChemicalPhenomena Academic Press New York NY USA 1973

[2] C Reichardt Solvents and Solvent Effect in Organic ChemistryWiley-VCH New York NY USA 2nd edition 1988

[3] E Buncel R A Stairs and H Wilson The Role of the Solventin Chemical Reactions Oxford University Press New York NYUSA 2003

[4] P J Sinko Martinrsquos Physical Pharmacy and PharmaceuticalSciences LippincottWilliamsampWilkins Philadelphia Pa USA5th edition 2006

[5] N J Turro V Ramamurthy and J S ScaianoModernMolecularPhotochemistry of Organic Molecules University Science BooksSausalito Calif USA 1st edition 2010

[6] JW NicholsonTheChemistry of Medical and Dental MaterialsThe Royal Society Cambridge UK 2002

[7] M V Encinas A M Rufs M G Neumann and C MPrevitali ldquoPhotoinitiated vinyl polymerization by safranineTtriethanolamine in aqueous solutionrdquo Polymer vol 37 no 8pp 1395ndash1398 1996

[8] 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

[9] 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

[10] Y Wang P Spencer X Yao and Q J 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

[11] 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

[12] 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

[13] P F Heelis ldquoThe photophysical and photochemical propertiesof flavins (isoalloxazines)rdquoChemical Society Reviews vol 11 no1 pp 15ndash39 1982

[14] 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

[15] C M Previtali S G Bertolotti M G Neumann I A PastreA M Rufs and M V Encinas ldquoLaser flash photolysis study ofthe photoinitiator system safranine T-aliphatic amines for vinylpolymerizationrdquoMacromolecules vol 27 no 25 pp 7454ndash74581994

[16] M V Encinas A M Rufs S G Bertolotti and C M PrevitalildquoXanthene dyesamine as photoinitiators of radical polymer-ization a comparative and photochemical study in aqueousmediumrdquo Polymer vol 50 no 13 pp 2762ndash2767 2009

[17] 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

[18] P S Song and D E Metzler ldquoPhotochemical degradation offlavins IV Studies of the anaerobic photolysis of riboflavinrdquoPhotochemistry and Photobiology vol 6 no 10 pp 691ndash7091967

[19] C G Hatchard and C A Parker ldquoA new sensitive chemicalactinometer II Potassium ferrioxalate as a standard chemicalactinometerrdquo Proceedings of Royal Society London A vol 235no 1203 pp 518ndash536 1956

[20] I Ahmad K Iqbal M A Sheraz et al ldquoPhotoinitiatedpolymerization of 2-hydroxyethyl methacrylate byRiboflavinTriehanolamine in aqueous solution a kineticstudyrdquo ISRN Pharmaceutics vol 2013 Article ID 958712 7pages 2013

[21] I Ahmad and G Tollin ldquoSolvent effects of flavin electrontransfer reactionsrdquo Biochemistry vol 20 no 20 pp 5925ndash59281981

[22] I Ahmad Q Fasihullah and F H M Vaid ldquoPhotolysis offormylmethylflavin in aqueous and organic solventsrdquo Photo-chemical and Photobiological Sciences vol 5 no 7 pp 680ndash6852006

[23] P Maurel ldquoRelevance of dielectric constant and solventhydrophobicity to the organic solvent effect in enzymologyrdquoTheJournal of Biological Chemistry vol 253 no 5 pp 1677ndash16831978

[24] J D Biasutti G E Roberts F P Lucien and J P A Heuts ldquoSub-stituent effects in the catalytic chain transfer polymerization of2-hydroxyethyl methacrylaterdquo European Polymer Journal vol39 no 3 pp 429ndash435 2003

[25] I Ahmad Q Fasihullah and F H M Vaid ldquoA study of simul-taneous photolysis and photoaddition reactions of riboflavin inaqueous solutionrdquo Journal of Photochemistry and PhotobiologyB vol 75 no 1-2 pp 13ndash20 2004


[26] B Orellana A M Rufs M V Encinas C M Previtali andS Bertolotti ldquoThe photoinitiation mechanism of vinyl poly-merization by riboflavintriethanolamine in aqueous mediumrdquoMacromolecules vol 32 no 20 pp 6570ndash6573 1999

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Applied ChemistryJournal of



Theoretical ChemistryJournal of


Journal of

Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International

ElectrochemistryInternational Journal of



CatalystsJournal of


2-Hydroxyethyl methacrylate Riboflavin

Camphorquinone Safranin T

Triethanolamine

O

OOH

CH3

H2C

H3C N

OH

HO

NH

N

HO

OH

ClminusN+H2N

O

O

O

O

N

H3C N CH3

NH2

OH

OHN

HO

H3C

H3C CH3

H3C

Figure 1 Chemical structures of HEMA and photoinitiators

highlights the effect of solvent on the kinetics and mode ofpolymerization reactions The chemical structures of HEMAand the photoinitiators used in this study are shown inFigure 1

2 Experimental

21 Materials Riboflavin (RF) camphorquinone (CQ) andsafranin T (ST) were obtained from Sigma Triethanolamine(TEOHA Sigma) and 2-hydroxyethyl methacrylate (HEMAAldrich) were distilled under reduced pressure before useWater was purified using a Millipore Milli-Q system

22 Method of Polymerization Polymerization of HEMA(monomersolvent ratio 121 10 1M) was carried out in thepresence of photoinitiators RF CQ and ST (absorbance ofeach photoinitiator at the 120582max was not more than 0125to avoid inhomogenous free radical distribution) [17] and001M TEOHA as a coinitiator in aqueous and organicsolvents under anaerobic conditions at 25∘C The solutionswere irradiated with a low intensity General Electric 15W

fluorescent lamp (emission in the visible region) fixed hori-zontally at a distance of 25 cm from the center of the vessel

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

24 Fluorescence Measurements Fluorescence measure-ments of RF in various HEMA solutions were carried out atroom temperature (sim25∘C) using a SpectraMax 5 fluorimeter(Molecular Devices USA) in the end point mode using120582ex = 374 nm and 120582em = 520 nm [18] The fluorescence wasrecorded in relative fluorescence units using a pure 005mMRF solution as standard

25 Measurement of the Light Intensity Themeasurement ofthe intensity of General Electric 15W fluorescent lamp wascarried out by potassium ferrioxalate actinometry [19] and avalue of 285 plusmn 026 times 1016 quanta sminus1 was obtained




(sminus1)b





ln 119896 = ln 119896119863=infinminus 119870(

1

119863

) (1)




CQ

RF

ST

00

10

20

30

40

50


kob

stimes104

(sminus1)




) (2)











RF gt CQ gt ST (3)





CQRF

ST

00

10

20

30

40

50

kob

stimes104

(sminus1)








119886



Am

PI

H+ transfer


h isc




119886







4 Conclusion




References





































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




(sminus1)b





ln 119896 = ln 119896119863=infinminus 119870(

1

119863

) (1)




CQ

RF

ST

00

10

20

30

40

50


kob

stimes104

(sminus1)




) (2)











RF gt CQ gt ST (3)





CQRF

ST

00

10

20

30

40

50

kob

stimes104

(sminus1)








119886



Am

PI

H+ transfer


h isc




119886







4 Conclusion




References





































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of










RF gt CQ gt ST (3)





CQRF

ST

00

10

20

30

40

50

kob

stimes104

(sminus1)








119886



Am

PI

H+ transfer


h isc




119886







4 Conclusion




References





































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


CQRF

ST

00

10

20

30

40

50

kob

stimes104

(sminus1)








119886



Am

PI

H+ transfer


h isc




119886







4 Conclusion




References





































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of



4 Conclusion




References





































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of












Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

research article solvent effect on photoinitiator reactivity in the...

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