A reappraisal of the photopolymerization kinetics of triethyleneglycol dimethacrylate initiated by...

A reappraisalof the photopolymerization kinetics of triethyleneglycoldimethacrylate initia ted by camphorquinone-N,N-dimethyl-p-toluid ine fordental purposes

J.Nie+, L.A. Linden+, J.F. Rabek*++ , J.P. Fouassier++ , F. Morlet-Savary++ , F. Scigalski#, A.Wrzyszczynski# andE. Andrzejewska##

+ PolymerResearchGroup,Department of Dental BiomaterialsScience,KarolinskaInstitute(RoyalAcademyof Medicine),Box 4064,S-14104Huddinge(Stockholm),Sweden

+ + LaboratoiredePhotochimieGeneraleCNRSno 431,ENSCMU,3 rueAlfred Werner, F-68200Mulhouse,France# Facultyof ChemistryandChemical Engineering,Universityof TechnologyandAgriculture,Seminaryjna3, PL-85-326Bygoszcz,

Poland## Instituteof ChemicalTechnologyandEngineering,PoznanUniversityof Technology, Pl.Sklodowskiej-Curie2, PL-60-965Poznan,

Poland

This work describesthecharacteristics of triethyleneglycoldimethacrylate(TEGDM) polymerizationwhenphotoinitiatedby camphorquinone(CQ) aloneandin the presence of N,N-dimethyl-p-toluidine (DMT) in air and/orin N2. The ratesofpolymerization(RP), doublebondconversion(p), monomerconversion (pm), temperature,anddifferent concentrationsofCQ andDMT weremeasuredandanalyzed.ThesecondRp maximumwasfoundfor thepolymerizationof TEGDM in thepresenceof CQ (without DMT) in N2 andin thepresenceof CQ + DMT in air. Formation of thesecondRp maximumhasbeenexplainedby thedifferentmobility of the initiating monomerradicalsin the polymermatrix asa polymerization pro-ceeds.The2,2,6,6-tetramethylpiperidineand4-hydroxy-2,2,6,6-tetramethylpiperidinoxyradicalswereusedin orderto eva-luatetheroleof amineoxy andamineperoxyradicalsin thepolymerizationof TEGDM in air. Finally, photophysicalstudiesallowed amoredetailedevaluationof theroleof excitedstatesof CQandDMT in thephotoinitiationprocess.

1. Intr oduction

Clinical photocuring of polymeric restorative resins indentistry occurs under special conditions, which differfrom anytypeof industrially appliedcuring. Thesespecialconditionsarethefollowing:

1. The whole procedure is performed in vivo, and isrestricted by biophysiological demands such as the oraltemperature,which should not exceed508C (maximum708C); necessity of application of visible light over400nm, which avoids photocancerogenicand photoaller-gic effects (causedby UV radiation) andtherisk of tissueburning[1]; andthepresenceof air, water andsaliva (thelasttwo canbetechnically limi tedto someextent).

2. The monomers for dental restorative resinsare notpurified and are usedas delivered. They contain about0.01%of inhibitors(suchashydroquinone,methyl etherofhydroquinone, p-methoxyphenol, resorcinol, pyrogallol,benzoic acid, and thymol, however, the most common is2,4-dimethyl-6-tert-butyl phenol (Topanol A, I.C.I.)) topreventprematurepolymerizationduring storage. In addi-tion, somemonomers can contain other impurities orig-inatingfrom their synthesisandstorage.

3.Thephotopolymerization reaction occursin acompli-cateddentalformulation systemthat, besidesa mixtureofdifferent mono-, di- and tri-functional monomers andphotoinitiator (and coinitiator), contains different addi-tives, such as stabilizers, pigments, reinforcing fill ers(evenup to 70 wt.-%), and coupling agents[2–6]. Theconcentration of the photoinitiator in the resin must besuchthat it will react at the proper wavelength of visiblelight and be present in sufficient quantities. Excessiveinitiator concentrationhasa detrimental effect on storage

of thecomposite resinandmayberesponsiblefor biologi-calhazards.

4. There arevery strict toxic, neurotoxic, cancerogenic,mutagenic, andallergenicrestrictionsfor theuseof dentalformulation components [7, 8]. Meth(acrylic) monomersreleasedinto saliva may causecertainreactions, suchasredness,swelling andpain of the oral mucosa[9, 10]. Inaddition, thesemonomers can, after penetration of theskin, producepersistentparasthesia of thefingersin surgi-calanddentalpersonneldueto damageof peripheralmye-linized nerve fibres and the Ranvier nodes [11]. Theaminespresent belong to hepatotoxins, which can causeactivation of toxic substancesin the liver, andwhich canact as hepatocarcinogens [8]. An amine such as N,N-dimethyl-p-toluidine(DMT) usedascoinitiator is cancero-genicandmutagenic [12], but in spite of this it is widelyusedin dental compositions to accelerate thepolymeriza-tion process.

5. The intra-oral photocuring must be carried out forhighestpossible monomer conversion sincetheunreactedmonomerwil l diffuseout from thepolymermatrix into thesaliva in the oral cavity and wil l be swallowed by thepatient.

6. The amount of the photoinitiating system should belimi tedto aconcentrationthatis just sufficient to obtainanoptimumof photocuringreactionwith thehighestpossiblemonomerconversion. Unreactedphotoinitiators and restproducts of their photolysis (if they are not permanentlybound to apolymernetwork) wil l alsodiffuseoff from thepolymermatrix into thesaliva.

7. Unreactedmonomersaswell asphotoinitiators (andtheir photolysisproducts)canbeextractedby water(fromsaliva),whichpenetratesinto aphotocuredresin in a toothduring use.

Acta Polymer., 49, 145–161 i WILEY-VCH VerlagGmbH,D-69451Weinheim1998 0323-7648/98/0404-0145$17.50+.50/0 145

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146 Nie et al. Acta Polymer., 49, 145–161(1998)

Themostcommonphotoinitiator usedfor thephotocur-ing of dentalrestorativeresinsis camphoroquinone(CQ)(bornanedione, 1,7,7-trimethylbicyclo(2,2,1)heptane-2,3-dione), which belongs to the a-diketones.It is usedwithcoinitiatorssuch as amines,which accelerate the rate ofpolymerization [3–6, 13]. Thekineticsof thephotocuringprocessby aCQ/aminecouplehavebeenstudiedmainly inanitrogenatmosphere[14,15],butnot in anoxygenatmo-sphere(air).

Fordentalpurposes,thefollowing informationwaslack-ing: 1. How doesthe photopolymerization kineticsoccurunderconditionscloseto thosein theoralcavity (presenceof air, fille rs, andsaliva)? 2. How cancamphoroquinoneitself initiatephotocuring reactionsin air andneutralatmo-sphere(nitrogen),and how can the role of amine underacceleratedpolymerizationbereexamined?3.How high isthe realmonomer conversion andwhat is thedependenceon thetime of irradiation in dimensions of curedlayer?4.How much of the unreacted monomer can be extractedfrom acuredsampleby thesaliva?In orderto answerthesequestionswedecidedto concentrateourstudyof photocur-ing using only one of the most common monomersemployedin thecommercially available dentalrestorativeformulations, i. e. triethyleneglycol dimethacrylate(TEGDM), with a camphorquinone(CQ) asa photoinitia-tor andN,N-dimethyl-p-toluidine(DMT) asacoinitiator.

2. Experimental

Camphorquinone(CQ,Aldrich) wasusedasa photoini-tiator in this work. It was usedalone and/or with N,N-dimethyl-p-toluidine (DMT, Fluka) as a coinitiator. Themonomer, triethyleneglycol dimethacrylate (CH22C-(CH3)COO[CH2CH2O]3COC(CH3)2CH2) (TEGDM,Fluka) was usedas received. Chromatographic purifica-tion of TEGDM wasdoneusingacolumnfilled with threetypes of aluminum oxide: acidic type 504C Brackmann(Aldrich), basictype 5016A Brackmann (Fluka)and neu-tral type II Brackmann (Reenal). Purification of theTEGDM had no influence on polymerizationkineticsandabsorption measurements. The presence of 100ppmhydroquinone methyl ether in the TEGDM had no influ-enceon the polymerization kineticsaccording to anotherreport [16]. As free radical scavengers, 2,2,6,6-tetra-methylpiperidine (TMP-H, Janssen) and 4-hydroxy-2,2,6,6-tetramethylpiperidinoxy radical (TMPO9, EGAChemie)wereused.

Commercially producedinorganicfill er (Ketac-Fil) wasdeliveredby ESPE.

Synthetic saliva (similar to humansaliva), consistingof1.09mM CaCl2, 0.68mM KH2PO4, 3.0 mM KCl and2.6mM NaF, wasbuffered at pH 7.0with 5.0mM N-2-hydro-xyethylpiperazine-N-2-ethanesulfonic acid[17,18].

Thepolymerizationkineticswasmonitoredby a differ-entialscanningcalorimeter(Perkin-Elmer DSC-4)adaptedfor photochemical measurements. A Perkin-Elmer 3600Data Station was employed to read and store data on

floppy disks.Thepolymerizationwascarriedoutata tem-peratureof 40l 0.018C (closeto thehumanbodytempera-tureof 378C),unlessotherwisestated.Accuratelyweighedsamples (l20 mg) of the photocurablecomposition werepolymerized in 6.5mm diameter open aluminum DSCpans.When polymerization was carried out in an inert(nitrogen) atmosphere, the sample was first allowed toreachequilibrium in theapparatusundera stream of oxy-gen-free nitrogen for 5 min (time sufficient to completereplacementof oxygen), and then a very slow nitrogenflow was allowed during irradiation.The Philips 500 Wlamp (type PF 318 E/49) emitting visible light from400nmwasusedfor initiation of thepolymerization.Thistype of lamp is equippedwith a spherical reflectorwhichgivesemitted light at the sameintensity over a large sur-facearea.The light intensity measuredat the level of thesurfaceof curedsampleswas60mW cm–2.

The reaction ratesvs. time werecalculatedby dividingthepeak height dH/dt (expressedin kJ/mol s),ateachpoly-merization point, by the theoretical heat of the reaction,DH0 = 56 kJ/mol per one double bond (calculated formethacrylates [19]). The dependences of fractional con-versionson the polymerization time were found by inte-gration of the areaunderthe curveof the polymerizationrate. Conversion at the time at which the polymerizationratedecreasedto 0 wastaken asthefinal conversion.TheDSCdataobtained werecorrectedfor changesin thebaseline.

Extraction of non-reacted monomer and initi ator frompolymerizedsampleswascarriedout using saliva for 12 hat378C (temperatureof thehuman body).

The UV andFTIR measurements werecarriedout on aBeckmanDU 7500andaPerkin Elmer 1600spectrophoto-meter, respectively.

Correctedemissionspectrawererecordedusing a Hita-chi F-4500 spectrofluorimeter with a time-resolved unitusedto isolatephosphorescencefrom the total lumines-cencespectrum.

The rateconstantsfor the quenching wereobtainedbyplotting thepeak-height ratiosof CQ(DMT) luminescencein the absenceandpresence of the quencherDMT (CQ)concentration,asaStern–Volmerplot [21]:

U0/Uq = 1 + k0 s0 [quencher] (1a)

where: U0 and Uq are luminescence intensities in theabsence and presenceof the quencher (Q), respectively.Theslopeof theplot, kq s0, is theproductof thebimolecu-lar quenching constant and the lifetime of excitedCQ(DMT) state in the absence of quencher (DMT)(CQ).The precision of the Stern–Volmer plots was l20%. AStern–Volmerplot of

k = k0 + kq [quencher] (1b)

yields kq, wherek0 andkq arethe decay rateconstantsofthetriplet of DMT in theabsenceandpresenceof CQ.

Transient decay profiles and absorption spectra wererecorded using a nanosecondflash photolysis apparatusutili zing the third harmonic of theNd-YAG Quantel laser

Acta Polymer., 49, 145–161(1998) A reappraisalof thephotopolymerizationkineticsof triethyleneglycoldimethacrylate... 147

astheexcitationsource(kexc= 355nm),aphotomultiplier/monochromator detection system, and a 250 MHz fastmeasuring scopeconnectedto acomputerizeddatastation.Therisetimeof thesystem waslessthan3 ls[20].

3. Resultsand comments

3.1.Absorption spectra

Camphorquinone (CQ) absorbs light in the region of200–300 nm dueto the p,p* transition and400–550 nm(responsible for its yellow color) dueto then,p* transitionof thedicarbonylgroup(Fig. 1) [22–24].Thereis asignif-icantdifferencebetween emax for the two transitions,withemax about 10 000 for the p,p* transition andonly 40 forthe n,p* transition (maximum at 468 nm, measured inethanol). This reflectsthe fact that the n,p* transition issymmetry forbidden,whilst thep,p* transition is allowed.N,N-Dimethyl-p-toluidine(DMT) (colorless)absorbslightin the region of 200–350 nm due to the p,p* transition(Fig. 1).Triethyleneglycol dimethacrylate(TEGDM) (col-orless) doesnotabsorbabove308nm(Fig. 1).

The low quantumyield (b = 0.07l 0.01) for CQ disap-pearance (bleaching) uponirradiation at 436nm (in care-fully degassedsolution)suggeststhe absence of a radicalchainprocess,and showsvery low CQ* photoactivity. Thequantumyield for the photooxidation of CQ at the samewavelength is b = 0.16l 0.01, showing that oxygen doesnotplayarole in thephotooxidation of CQ[25].

3.2.Isothermal DSCmeasurements

IsothermalDSCmeasurementsyield information on therateof polymerization (Rp, s–1), the highestrate of poly-merization (Rmax

p , s–1), thedoublebondconversion(p, %),the highestdegree of doublebondconversion (pmax, %),the time in which Rmax

p appears (tmax, s), and inhibitiontime (tinh, s) [26]. Rp andp areusually versustime of irra-diation (t, s), andRp versusp (Figs.2, 4, 6, 8, 10, 12 and13). Reproducibility of results wasl5%. Several distinc-tive features of the photopolymerization canbe observed

in theserate profiles,which arepartially interpreted in alaterpart of thispaper.

3.3.Effect of concentrationsof camphorquinone-aminecomponentson the kinetics of photopolymerization

1. TheTEGDM photopolymerization wascarriedout atdifferent CQ concentrations (without DMT) in air(Fig. 2a, b, c) andN2 (Fig. 2d, e, f). The obtainedresultsindicatethatCQalonecanphotoinitiatepolymerization ofTEGDM both in air and in N2. Polymerization in air(Fig. 2b) andN2 (Fig. 2e)occursafteraninhibition periodthat is caused by thepresenceof oxygenand/or inhibitor.The tinh decreases nonlinearly with increasing [CQ](Fig. 3b).Rp is verysensitiveto [CQ] overthewholerangestudied. Rmax

p increasesand tmax decreases(both nonli-nearly) with increasing[CQ] (Fig. 3a). Thekineticsof Rpvs. t in N2 shows theexistenceof two maxima, which arewell observedat low [CQ] (0.5 wt-%) (Fig. 2d). Thepmaxaremuchhigherin N2 (60l 5%) thanin air (40 l 3%), andare almost independent of [CQ] for [CQ] A 1 wt-%(Fig. 3b). At low [CQ] (0.5 wt-%), the doublebondcon-version(p) in air doesnot exceed15%(Fig. 2b), whereasin N2 it canreach a valueof 60% (however, only after along time of irradiation,A1400s) (Fig. 2e).Rp vs.p showsthat the double bond conversion in N2 occurs with thesameRp for agiven[CQ] concentration,after reachingp A15% (Fig. 2f). Considering theseresultswe decidedtochoose [CQ] = 1 wt-% (0.06M) for all otherexperiments,unlessotherwisestated.

2. TheTEGDM photopolymerization wascarriedout atthesameCQconcentration(1 wt-%, 0.06M) andwith dif-ferent DMT concentrations in air (Fig. 4a–c) and N2(Fig. 4d–f): The obtainedresultsindicate that increasing[DMT] causesRp (Fig. 4a, d) and p (Fig. 4b, e) todecrease,both in air and in N2. The kinetics of Rp vs. t(Fig. 4a) and Rp vs. p (Fig. 4c) show the existence ofdouble Rp maxima in air (but not in N2 Fig. 4d, f). Rmax

pdecreasesandtmaxincrases(bothnonlinearly)with increas-ing [DMT] (Fig. 5a). The double bond conversion (p)occurs with different rate constantsin air (Fig. 4b), anddecreaseswith increasing[DMT] both in air (Fig. 4b) andin N2 (Fig. 4e). The pmax rapidly decreasesin air withincreasing[DMT] in therangeof 0.5–1.0wt-% (Fig. 5b).The tinh rapidly increasesin air when[DMT] A0.5 wt-%(Fig. 5b). Thepmax andtinh in TEGDM polymerizedin N2changealmostlinearlywith increasing[DMT] (Fig. 5b).

3. TheTEGDM photopolymerization wascarriedoutatthesameDMT concentration,0.5wt-% (0.037 M), anddif-ferent CQ concentrations in air (Fig. 6a–c) and N2(Fig. 6d–f). The results indicate that increasing [CQ]causes Rp to increaseboth in air (Fig. 6a) and in N2(Fig. 6d). The kinetics of Rp vs. t (Fig. 6a) and Rp vs. p(Fig. 6c) showthe existenceof doubleRp maxima in air(but not in N2, Fig. 6d, f), asis alsoshown in Fig. 4. TheRmax

p increasesnonlinearlywith increasing[CQ] in both airandN2 (Fig.7a), asin Fig.3a. Thetmaxdecreasesrapidly inair with increasing[CQ], but slowly in N2 (Fig. 7a). The

Fig. 1. Absorptionspectraof (—) CQ(6.026 10-3 M); (- - -) DMT(3.76 10–4 M); (.....)TEGDM (3.56 10–2M) in ethanol.


tinh decreasesnonlinearlywith increasing[CQ] in both airandN2 (Fig. 7b), however, lessthanin Fig. 3b. The pmaxvaluesarealmost thesamein air (pmax = 48 l 3%) and inN2 (pmax= 61l 3%) atdifferent [CQ] (Fig. 7b).

3.4.Effect of antioxidant and fr eeradical scavengeron thekinetics of photopolymerization

1. TheTEGDM photopolymerizationwascarriedout atthe sameCQ andDMT concentrations:1 wt-% (0.06M)and0.5 wt-% (0.037 M), respectively, but in thepresenceof an antioxidant (TMP-H) which was added in different

concentrations. The kinetics were studied both in air(Fig. 8a–c) andin N2 (Fig. 8d–f). TheRp decreaseswithincreasing[TMP-H] (Fig. 8a,d and Fig. 9). ThesecondRpmaximumdisappearscompletely in thepresence of TMP-H (Fig. 8a, c). The p decreaseswith increasing[TMP-H]in air (Fig. 8b) andN2 (Fig. 8e),however, in thelattercasethisdecreaseis small.

2. TheTEGDM photopolymerization wascarriedout atthe same CQ andDMT concentrations: 1 wt-% (0.06M)and0.5 wt-% (0.037M), respectively, but in thepresenceof thefreeradical scavenger(TMPO9), whichwasaddedindifferentconcentrations.Thekineticswerestudiedbothinair ( Fig. 10a–c) and in N2 (Fig. 10d–f). TheRp decreases

Fig. 2. Rateof polymerization(Rp) anddoublebondconversion (p) of TEGDM,photopolymerizedin thepresenceof differentCQconcen-trations(0,0.5,1.0,1.5,2.0and3.0wt-%); in air (a,b andc) andin N2 (d,eandf).

Fig. 3. Maximum rateof polymerization (Rmaxp ) (a) (—), time at which Rmax

p appears(a) (- - -), maximumdoublebond conversion(pmax)(b) (—) andinhibition time (tinh) (b) (- - -), of TEGDM photopolymerizedin thepresenceof differentCQconcentrations;in air (0, h) andin N2 (9,H).


with increasing [TMPO9] (Fig. 10a, d and Fig. 9). With[TMPO9] F 0.5wt-% thephotopolymerizationof TEGDMdoesnot occur (Fig. 9). The p decreases with increasing[TMPO9] in the range0.05–0.3 wt-% much fasterin air(Fig. 10b) thanin N2 (Fig. 10e).

3.5.Effect of fillers on the kinetics of photopolymerization

The TEGDM photopolymerization was carried out atthe sameCQ andDMT concentrations:1 wt-% (0.06M)

and0.5 wt-% (0.037M), respectively, but in thepresenceof the inorganic fille r, which wasadded in differentcon-centrations. The kinetics were studied in a 1 mm thicklayerin thepresenceof air (Fig. 11).Therestorativedentalpolymer resins usedin clinical dentistry contain up to 70wt-% of inorganic filler, which improves reinforcementand reduces polymerization shrinkage. The addition ofinorganicfille r to thepolymerizing resin is very complexandcausesmanyproblems[27]. Theamountof addedfil-ler (10–50 wt-%) has no significant effect on Rp, but

Fig. 4. Rateof polymerization (Rp) anddouble bondconversion(p) of TEGDM, photopolymerizedat constant CQ concentration(1 wt-%),anddifferentDMT concentrations(0,0.2,0.3,0.5,0.6,0.8,1.0,1.2,1.5,2.0and3.0wt-%); in air (a,b andc) andin N2 (d,eandf).

Fig. 5. Maximumrateof polymerization (Rmaxp ) (a) (—), time at which Rmax

p appears (a) (- - -), maximumdoublebondconversion (pmax) (b) (—) andinhibition time (tinh) (b) (- - -), of TEGDM photopolymerizedatconstantCQconcentra-tion (1 wt-%) andatdifferentDMT concentrations; in air (0,h) andin N2 (9,H).


clearly increasesp (Fig. 11). This leadsto the conclusionthat photoinitiated polymerization in the layer accessibleto light penetration (closeto thetopsamplesurface)propa-gatedthroughawholesamplefill edwithaninorganicfiller(this conclusion is propably valid only for the1 mm thicklayer usedfor this polymerization). In dentalclinical cur-ing, it is recommended that the polymerization is carriedout in layer not exceeding 1–1.5 mm thickness.Theoccurrence of a specific depthof cure (below which nopolymerization occurs)[28, 29] for samplesthicker than

2–3 mm canbe explained in termsof the attenuation ofradiation with depth by filler particles [4] and the con-sumption of free radicals by oxygen (and perhaps inhibi-tor) presentin thedeeperlayers[30–32]. In astartingsam-ple the concentration of oxygen is uniformly distributedthroughout the material. Under irradiation,initiator radi-calsareformed, leading to theproductionof polymerchainradicals. However, oxygen competes with polymer netgrowth and inhibits polymerization until virtually all theoxygen molecules are consumed.Thus, there is a depth

Fig. 6. Rateof polymerization (Rp) anddoublebondconversion(p) of TEGDM, photopolymerizedat constantDMT concentration (0.5wt-%),andatdifferentCQconcentrations(0.5,1.0,1.5,2.0,3.0and5.0wt-%); in air (a,b andc) andin N2 (d,eandf).

Fig. 7. Maximumrateof polymerization (Rmaxp ) (a) (—), time at which Rmax

p appears (a) (- - -), maximumdoublebondconversion (pmax) (b) (—) andinhibition time(tinh) (b) (- - -), of TEGDM photopolymerizedatconstantDMT concentra-tion (0.5wt-%) anddifferent atCQconcentrations;in air (0,h) andin N2 (9,H).


abovewhich the oxygen is depleted and where polymeri-zationhasoccurred,and adepthbelowwhichthepolymer-izationis totally inhibited.

3.6.Effect of temperature on the kinetics ofphotopolymerization

1. TheTEGDM photopolymerizationwascarriedout atthe sameCQ concentration (1 wt-%, 0.06 M) (withoutDMT), but at different temperatures.The kinetics werestudiedboth in air (Fig. 12a, b) and in N2 (Fig. 12c, d).The rangeof temperaturesbetween408C and 1008C was

chosen. However, exceeding 708 is not clinically accepta-ble, because of pulp damage. Temperatures exceeding808C are usedfor resin inlay and onlay restorations. Rp(Fig. 12a, c) and p (Fig. 12b, d) increaseboth in air(Fig. 12a, b) andin N2 (Fig. 12c, d) with increasingtem-perature.At temperaturesA1008C thepolymerization ratedecreases, which is partly dueto the effect of depropaga-tion of the kinetic chain. During the photopolymerizationcarriedout in N2, two distinct Rp maximawere formedathigher temperatures(Fig. 12c). Thesamedouble maximawere observed during the polymerization of TEGDM atdifferent[CQ] (Fig. 2d).

2. TheTEGDM photopolymerization wascarriedout atthe same CQ andDMT concentrations: 1 wt-% (0.06M)and0.5wt-% (0.037 M), respectively, butatdifferenttem-peratures.Kinetics werestudiedin both air (Fig. 13a, b)and N2 (Fig. 13c, d). The increasing temperature in aircausesadecreaseof thefirst Rp maximum andanincreaseof the secondRp maximum (Fig. 13a), and ananomalousformationin thekineticsof thep (Fig. 13b).Theseanoma-lies were not detected when polymerization was carriedoutin N2 atelevatedtemperatures(Fig. 13c,d).

3.7.Determination of monomerconversion

On the surface of the photopolymerizedTEGDM sam-plesin air, therealwaysremainsaliquid layerof unreactedmonomer(which was confirmed by FTIR spectroscopy).In this thin layer, photopolymerization does not occurbecauseit is inhibited by air. The monomer conversion

Fig. 8. Rateof polymerization (Rp) anddoublebondconversion(p) of TEGDM, photopolymerized at constantCQ (1 wt-%) andDMT(0.5wt-%),andatdifferent(TMP-H) concentrations(0,0.05,0.1,0.3,0.5and1.0wt-%); in air (a,b andc) andin N2 (d,e,andf).

Fig. 9. Maximum rate of polymerization (Rmaxp ) of TEGDM,

photopolymerized at constantCQ (1 wt-%) andDMT (0.5 wt-%),andat differentTMP-H (- - -) or TMPO9 (—) concentrations; in air(0,h) andin N2 (9,H).


(Pm, %) in air in agivensample dependsontheratioof sur-facewhich is in contactwith air to the weight of sampleandits layer thickness. For samples having the samesur-face area(0.355 cm2) the Pm increaseswith increasinglayer thickness and weight of a polymerized sample(Fig. 14a). In a layerof 0.1mm thickness,no polymeriza-tion occurs. The highest degreeof Pm was obtained forsampleswith thicknessexceeding 1 mm, Pm L 70–90%.ThePm alsodependson time of irradiation. In thecaseofCQ-initiated polymerization of TEGDM in air or N2 (1mmthick layer), thePm was32%after 6 min of irradiation,whereas in the presence of CQ + DMT, Pm = 85% wasobtainedafter 60 s of irradiation(Fig. 14b). The time ofphotocuring is very important in clinical dentistry. The 1mm composite layer should be photopolymerizedwithin20–40 s,andthepolymerization timeshouldnever exceed

oneminute.Thesameamountof unreactedmonomer wasextracted by acetone,which wasusedinstead of saliva.This indicatesthatsalivacancompletely extract unreactedmonomerfrom the cured sample(1 mm thick) in a shorttime (12 h). The UV/vis spectra show that unreactedmonomer, non-reactedinitiators(CQ or CQ + DMT) andtheir photoreactionproductsarealsoextractedby saliva.Thisincompletemonomerconversionis aprimaryconcernin theapplicationof polymerrestorative resinsin dentistry.Thisproblem hasalsobeenstudiedby othersunderdiffer-entcuringconversionsandresincompositions[33,34].

3.8Effect of salivaon kinetics of photopolymerization

In theclinical procedure,dentistsalwayshaveproblemswith contamination by saliva present in the oral cavity.Our measurements show that a thin layer of saliva (0.1mm) on thesurfaceof polymerizedTEGDM sample in airdoesnotsignificantlyinfluenceRp andp values. However,thesurfaceof thetooth should beabsolutely dry in ordertoobtaingoodadhesionto apolymer.

3.9.Spectroscopicalstudy of the camphorquinone-aminephotoinitiating system

The UV/vis absorption spectra of light-irradiated CQ[(1 wt-%, 0.06M) (Fig. 15a) andCQ (1 wt-%, 0.06M) +DMT (0.5 wt-%, 0.037 M) (molar ratio 2:1) (Fig. 15b) inethanol (in air andin N2), and thendissolved to the mea-surementsconcentration for CQ = 6.02 6 10–2 M and

Fig. 10. Rateof polymerization(Rp) anddoublebondconversion(p) of TEGDM, photopolymerizedat constantCQ (1 wt-%) andDMT(0.5wt-%),andatdifferent(TMPO9) concentrations(0,0.05,0.1and0.3wt-%); in air (a,b andc) andin N2 (d,eandf).

Fig. 11. Rateof polymerization(Rp) of TEGDM photopolymerizedatconstantCQ(1 wt-%) andDMT (0.5wt-%) andatdifferentfillerconcentrations(0,10,30and50wt-%) in air.


DMT 3.76 10–4 M (Fig. 15aand b)] differ from thatmea-suredwhen the CQ and CQ + DMT solutionswere irra-diatedat measurementsconcentration. Irradiation of theCQ + DMT at higher concentration causesa decreaseofDMT absorption spectrum, whereasirradiation of the CQ+ DMT at lower concentrationcausesanincreaseof DMTabsorption spectrum with someshifting of maximum ofthe absorption (Fig. 15b). These results indicate thatphotolysisof CQ + DMT dependson theconcentrationofCQ and DMT in a solution. Chemical yields of DMTphotolysisafter1600sof irradiationin air andN2 were40l 3%and 30l 5%,respectively.

The CQ* can abstractthe H-atom from DMT (abbre-viatedhereasAM-H):

CQ* + AM-H e CQH9 + AM 9 (chemicalquenching) (2)

and/orfrom amonomer(RH):

CQ* + RHe CQH9 + R9 (chemicalquenching) (3)

Reactions(2) and (3) may be competitive, depending ontheconcentration of CQ + DMT (andtheir molar ratio) insolution(monomer) andon the rateconstantsof the pro-

cess.Formation of CQH9 in monomershasbeenconfirmedby ESR spectroscopy [13]. The CQH9 radical gives astrong ESRsignal consistingof four main multiplets[13,35–38].

Emission spectroscopy shows that the air does notquench the CQ* fluorescence.However, it quenchestheCQ* phosphorescencein liquid TEGDM (at room tem-perature), but not in frozen monomer (–808C) and rigidTETGDM polymer (at roomtemperature) (Fig. 16).Fromtheseresults it is evident that oxygen quenchesonly thetriplet state,notsingletstates(S1 and S2) of CQ*:

CQ* (T1) + O2e CQ+ O2 (+D) (physical quenching) (4)

The lifetime of the CQ* (S1) in TEGDM is s = 9.398610–8 s. Oxygendeactivatesketone triplet states, with sec-ond-order rateconstantskq around(1–3)6 109 M–1 s–1 insolution. In mostorganicmedia[O2] L 2 6 10–3 M, andthe term kq [O2] is typically (2–6) 6 106 s–1 [38]. Thisterm cannotbe neglected in dentalcuring, where [mono-mer]L 10M and[AMH] F 0.1M.

Theenergy of CQ* (T1) in TEGDM is E(T1) = 51.5kcalmol–1 (at556nm),andis similar to thatreportedelsewhere[39]. The low CQ* E(T1) shows thatCQ reactionscanbe

Fig. 12. Rateof polymerization (Rp) anddoublebondconversion(p) of TEGDM, photopolymerizedatconstant CQ(1wt-%) atdifferenttemperatures(408C,608C,808C and1008C); in air (aandb) andN2 (c andd).


Fig. 13. Rateof polymerization(Rp) anddoublebondconversion(p) of TEGDM, photopolymerizedatconstantCQ(1wt-%) andDMT (0.5wt-%) atdifferenttemperatures(408C,608C,808C and1008C); in air (aandb) andN2 (c andd).

Fig. 14. Monomer conversion(Pm) of TEGDM photopolymerizedat: (a)CQ(1 wt-%) in air (0) andin N2 (h), and(b): (—) atCQ(1 wt-%)in air (0) andN2 (9); (- - -) atCQ(1 wt-%) andDMT (0.5wt-%) in air (H) andN2 (h); vs.weightor thicknessof sample(a),andvs.timeofirradiation(b).


sensitizedby avariety of triplet sensitizers.Theefficiencyof CQ* phosphorescenceexcitationis greateruponexcita-tion into CQ* S2 than into CQ* S1, and the intersystemcrossing S2 e T1 (T2) is more efficient thanS1 e T1 [40].The excitation efficiency in the S2 E S0 region is higherthan that in the S1 E S0 absorption region by a factor ofL10 [41]. Thequantum yield for the intersystem crossing(ISC) of CQ* is UISC = 1, regardlessof the solvent used[25]. The singlet 0–0 bandlies near 495 nm in the closeoverlapof absorption (Fig. 1) andfluorescence (Fig. 16).Thisgivesasinglet–triplet splitting of 2100cm–1 [39].

Emission spectroscopy shows that DMT (physically)quenches fluorescence and phosphorescenceof CQ* infrozen(–808C) TEGDM in thepresenceof air (Fig. 17):

CQ* (T1) + DMT e CQ+ DMT* (5)

The deactivation of CQ* fluorescence and phosphores-cencein the presenceof DMT follows the Stern–VolmerEq. (1) (Fig. 18). The quenching rate constant of CQ*fluorescenceby DMT in TEGDM is kq = 4.436 108 M–1

s–1. The quenching mechanism of CQ* by DMT is prob-

ably similar to thequenchingof biacetyl (aliphatica-dike-tone) by amines,which occursat high rateconstants[42].Aminesquenchketoneexcitedsingletstates.However, thesinglet state lifetime is soshort(in thenanosecondrange)that an efficient quenching, capableof competing withISC, maybeexpectedonly at very high amineconcentra-tions(A0.1M). Themechanismsuggestedfor the interac-tion betweenaminesandketone singlet andtriplet excitedstatesis a chargetransferinteractionfrom thelonepair of

Fig. 15. Change of absorptionspectra of DMT (3.76 10-4 M) oftwo samplesof DMT (0.5wt-%) andCQ(1 wt-%) whichwerelightirradiated(800and1600s) at differentconcentrations:3.76 10–4

M (.....)and3.7 6 10–2 M (- - -); andbeforeirradiation(—), in air(a)andN2 (b).

Fig. 16. Emissionspectraof CQ* (1.56 10–2 M): fluorescence(F)and phosphorescence (P) in solid polymerizedTEGDM at roomtemperature(rT) (—); F in liquid TEGDM at rT (- - -), andP insolidfrozen(–808C) TEGDM in air (.....).

Fig. 17. Quenchingof the fluorescence(a) and phosphorescence(b) of CQ* (1.56 10–2 M) by DMT atdifferentconcentrations: (a)CQ without DMT; (b) CQ + DMT (1:0.5); (c) CQ + DMT (1:1);(d) CQ+ DMT (1:1.5)in solidfrozen(–808C) TEGDM in air.


theamineand theelectrophilic half vacantn orbital of thecarbonylic oxygen [43]. Theproposedmechanismis sup-portedby the increasingof the quenching rate constantswith the increasingenergy of thequenchedexcitedstated(thesinglet stateis moreefficiently quenchedthanthetri-plet state) andwith the decreasingionisationpotential oftheamines.In fact, suchacombination of H-atomabstrac-tion andelectron-transfermechanismmaywell beoperat-ing in thequenching of luminescenceof CQ* by DMT.

The laser excitation (at 266nm) of CQ in cyclohexanecausesthe excitation of CQ to the CQ* (S2) state(p,p*

transition). The observed CQ* transient(Fig. 19) decaysaccording to asecond-orderlaw with k = 66 106 M–1 s–1.The end of the pulseabsorption spectragiven in Fig. 19(insert) shows that themaximumlies above350nm (withoptical densityaround0.3).This transientdecayshowsnosensitivit y to oxygen. The observed spectrum in Fig. 19cannot be attributed to the CQ* (T1) or triplet–tripletabsorption,andis notassignedyet.

The DMT haslow absorption at 355 nm (Fig. 20), andwhen irradiatedin air with light at 355nm at –808C exhi-bits fluorescenceandweakposphorescence(Fig. 21).Thelaserexcitation(at355nm) of DMT givesatransientspec-trum with the maximum situated at 450 nm (Fig. 22). Asthe signal is sensitive to air, the spectrum is attributedtotriplet absorption. The deactivation of DMT* (T1) in thepresenceof CQoccursby

DMT* + CQe DMT + CQ* (physical quenching) (6)

and follows the Stern–Volmer Eq. (1b). The experimen-tally measuredrateconstants(k) areplottedasa functionof theCQconcentrationin Fig. 23.According to Eq.(1b),thedeactivation rateconstant kq wasfoundto be1.46 109

M–1 s–1.The two major primary processesof triplet states of

mosta-diketonesarehydrogenabstraction and addition to

Fig. 18. Stern–Volmer plots of the quenching of fluorescence(a)andphosphorescence(b) of CQ* (1.56 10–2 M) by DMT atdiffer-entconcentrationsin solidfrozen(–808C) TEGDM in air.

Fig. 19. Transientabsorptionspectrum of CQ* (1.56 10–2 M) indeaeratedcyclohexanesolution resultingfrom laserexcitation at266nm.Theinsetshowsthetransientsignalat360nm.

Fig. 20. Absorptionspectraof DMT in ethanol atdifferentconcen-trations:(a)3.76 10–2 M; (b) 3.76 10–3 M and(c) 3.76 10–4 M.

Fig. 21. Emissionspectra of DMT* (1.56 10–2 M): fluorescence(—) andphosphorescence(- - -) in solidfrozen(–808C) TEGDM inair.


multiple bonds in a process analogous to the Paterno-Buchi reaction[44–47]. In thepresenceof oxygen,photo-excited diketonesare converted to intermediate peroxyradicals. Whenalkenes arepresent,the intermediateper-oxy radicals reactwith the double bonds [47]. The quan-tumyieldsfor intermolecular reactionsof dionesarein therange0.01to 0.5.This is partly dueto competing deactiva-tion of theexcitedstate and maybetheresult of chemicalreactionsthat regenerate starting materials from reactionintermediates.Unlike other a-diketones,the CQ photo-chemistry appears to be much more complicated. In oxy-gen-free atmosphere, CQ is reduced to ketols [25, 48],whereasin the presenceof oxygen it is oxidized to cam-phoricanhydrideand reducedto mixturesof acyloins[35].

4. Discussionof results

4.1.Primary photoreactionsof CQ with monomer(TEGDM)

Wecanconsidertwo cases:1. Monomer moleculesform a cage free of oxygen in

which CQ* (T1) canbe formed.The products of the pri-mary photoreaction of CQ* (T1) with the surrounding

H-atomdonor monomer molecules(RH) areCQH9 andR9radicals (reaction (3)). In suchcases,separation into freeradicals may competewith intersystem crossingto an in-cagepair thatshouldundergovery rapidcoupling. Consid-eringtheTEGDM structure, theH-atomabstractionoccursfrom themethylenegroup(1CH21), ratherthanfrom themethyl group (1CH3). Hence the initiating R9 has thestructure1CH91.

2. OxgenandCQarepart of acageformedby monomermolecules. TheCQ* (T1) is quenchedby oxygen(reaction(4)), andtheH-atomabstraction doesnot occur. Theche-mically induced dynamic electron polarization (CIDEP)hasprovedthat the CQH9 radicals canbe formeddirectlyfrom theCQ* excitedsingletstate(S2) [36]. In suchacase,oxygen doesnot quenchCQ* (S2) even in the monomercage.TheCQH9 andR9 radicals thathaveleft thecagemayparticipate in the secondarynon-photochemical reactionssuchas:

i. Oxygen present in the monomer can react with pri-mary CQH9 radicals that are trappedin the polymer net-work,giving peroxyradicals:

CQH9 + O2e CQHOO9 (7)

R9 + O2e ROO9 (8)

ii. They may initiate propagation and/orparticipate intermination reactions. Residual, trapped radicals havebeen experimentally detected using ESR spectroscopy[49–51].

It is generally accepted that the CQH9 radical cannotdirectly initiate the propagation reaction [52]. In theabsenceof acoinitiator(amine),only R9 canberesponsiblefor initiationof polymerizationin N2:

R9 +1CH2CH2e1CH91CH2R (9)

Peroxy radicalscanabstracttheH-atomfrom themono-mer moleculesand form hydroperoxides (reactions (10)and(11)), and/or by thecoupling eachother canform per-oxides(reactions(12)and(13)):

CQHOO9 + RHe CQHOOH+ R9 (10)

ROO9 + RHe ROOH + R9 (11)

CQHOO9 + CQHOO9e CQHO1OCQH + O2 (12)

ROO9 + ROO9e R1OO1R + O2 (13)

There is littl e information available on whetherCQHOO9 canparticipatein thesereactions.ReactionsofCQH9 andR9 radicalswith oxygen((7) and (8)) will occuras long as oxygen is presentin the monomer(inhibitionperiod, seee.g. Fig. 3b, Fig. 5b andFig. 7b). Otherminorreactionsare:coupling of two R9 radicals, disproportiona-tion of CQH9 radical to giveareduceddiketoneCQH2 andstartingCQ(theinitial product of disproportionationis theenediol whichketonizesto givehydroxyketone),or dimer-ization of CQH9 to thermally labile diketopinacol CQH2[47].

Fig. 22. Transientabsorptionspectrum of DMT* (1.56 10–2 M)in deareatedtoluenesolutionresultingfrom laserexcitationat 355nm.

Fig. 23. Stern–Volmer plot of the quenching of the triplet state(T1) of DMT* (1.56 10–2 M) by CQatdifferent concentrations.


4.2.Primary photoreactionsof CQ-aminesand monomer(TEGDM)

The mechanism of initi ation proposedfor camphorqui-none(CQ)-amine(AMH) systemswasbasedontheresultsobtainedfor the monoketone-aminesystem[52–57]. Atthe present, the primary stepis consideredasthe genera-tion of a short-lived charge-transfer complex (CTC)betweenketoneandaminefollowed by formation of acon-tact ion pair (CIP) anda solvent (monomer)-separatedionpair (SSIP), bothin equilibrium [58]. While H-atomtrans-fer definitely occursin CQ–aminereactions(reaction(2)),clear-cut evidence for an electron transfer step will begiven in a separatepaper. It is generally acceptedthat theAM 9, but not CQH9, radical initiates thepropagation reac-tion [52]. A low chemical yield of primary (chemical)photoreactionof CQ with DMT canbe dueto competing(physical) deactivationof CQ* by amines(reaction (5)).

4.3.Effects of oxygenon the CQ-AMH photoinitiatedpolymerization of TEGDM

Oxygenpresentin a monomercan:1. quenchthetripletstateof theCQ* (T1) (reaction (4)), and2. reactwith freeradicals formedfrom the initi atorsystem(CQH9 (reaction(7)), AM 9 (reaction (14)), monomer radical (R9) (reaction(8)), and/or growing polymer radicals (P9) (reaction (15)),giving peroxy radicals:

AM 9 + O2e AM1OO9 (14)

P9 + O2e P1OO9 (15)

Reactions(7), (8), (14) and(15) reducetherateof poly-merization and lead to an inhibiti on (induction) periodbetweenthe initi al polymerization and the beginning ofsignificant polymerization. Interaction between oxygenandaminesmayalsofavor theO2 consumption, thuslimit-ing its detrimentaleffect (reducing therateof polymeriza-tion, crosslinking and causingincompletefunctionalgroupconversion). The reaction of free radicals with oxygen isfastand therateconstantfor theformation of peroxy radi-calshasbeenfoundto beof theorder of 109 M–1 s–1 [59].On the otherhand, the peroxyradicals (ROO9) react veryslowly with monomers. (Rate constant for reaction ofROO9 with methyl methacrylate(MM) hasbeen found tobeof theorder0.24M–1 s–1 in comparisonto 515M–1 s–1

for P9 with MM. This impliesthatROO9 radicals arenearly2000 time lessreactive[60].) Peroxy radicals, however,caneasilyabstractthe H-atomfrom any type of H-donormolecule [52, 61, 62], e.g. from a monomer, in this wayinitiating a new reactiveradical. The ROO9 radicals canalsoparticipatein theterminationandothersidereactions.All thesereactionswill occuraslong asoxygenis presentin thephotocured sample.

4.4.Anomalousbehavior of TEGDM photopolymerization

Thepolymerizationrateof kinetics of mono-functionalmonomersis directly proportional to monomer concentra-

tion (is first order in monomerconcentration), but propor-tional to one-half the concentrationof initi ator (i. e., one-half order)([M][ I]1=2

0 ). Thepolymerization wil l slow downin an exponential fashion asthe initiator is usedup (finalterm).More initi ator would haveto be addedto keep thepolymerizationgoing. The rateof polymerization is pro-portional to kp/k

1=2t , wherekp andkt are rateconstantsof

propagationandtermination, respectively. In puremono-mers a marked acceleration of the polymerization isobservedatsomepoint.Thereason for this is thatthevisc-osity of themedium increasesaspolymer is formed. Thisdoesnot affect the rate of propagation, which dependsuponthebarelyaffecteddiffusion of smallmolecules. Ter-mination involves the much slower diffusion of largermacromolecular species, however, and this increaseinviscositycanresult in a largedecrease in therateof termi-nation. The “reactivit y” of the radicals is not altered,buttheability of the radicals to find eachotheris, so that theapparentvalueof kt is reduced.This has a dramaticeffectontherateof polymerizationbecauseof its dependenceonkp/k

1=2t . The rate of heatevolved from theseexothermic

reactions also increases.This phenomenonof autoacce-leration is oftencalledtheTrommsdorffeffect.

Theanomalousbehaviorof thepolymerization kineticsreactionsof multifunctional monomers in the high cross-linking regime is well known and described elsewhere[63–65]. In the caseof TEGDM this anomaly includesautoacceleration (increasein rate of polymerization) andautodeceleration (decreasein rate of polymerization).Both processesarewell observedin all kineticRp measure-ments(Figs.2,4, 6, 8, 10,12and 13). In thephotoinitiatedpolymerization of TEGDM, the propagation kinetic con-stants remains relatively constantat low to moderatecon-versionswherediffusionof initiatingandpropagation radi-cals is unhindered(termination reaction occursin thiscaseonly when two radicalsareableto diffusetogether). Asthepolymerization proceeds,radical diffusion continues todecrease(causing the terminationrateto be reduced)andthe radical concentration increases,which leads to higherratesof polymerization and henceautoacceleration. Whenpropagationreaction becomesdiffusion controlled (alongwith termination reaction),the mobility of free radicalsand reactive vinyl groups is reducedand polymerizationcan proceed for some time with the sameRp and thendecreases(in the latter the rate of propagation decreasesmuchmorerapidly thanrateof termination) [65]. Autode-celeration occurs because the propagation reactionbecomesdiffusion controlled along with the terminationreactions [66–68]. Reaction diffusion involves radicalsmoving by propagating through unreacted vinyl groups.As the mobility of the reactive vinyl group is furtherreduced,the functional groups become lessreactiveuntilthe reactionstopsfrom vitrification. Thenetwork derivedfrom thedivinyl monomer suchasTEGDM consistsof thesol component, containing residual monomer, andthegel,which consistsof divinyl units havingeitheronereactedvinyl group (attachedto a pendant vinyl group) or tworeacted vinyl groups (forming a tetrafunctional cross-link).


Newanomalousbehaviorsnot reportedearlier in thelit-erature aretheexistence of two polymerization ratemax-imain polymerizationcarried outin:

1. thepresenceof CQ(withoutDMT) in N2 (Fig. 2d andFig. 12c),and

2. thepresenceof CQ+ DMT in air (Fig. 4a,Fig. 6aandFig. 13a).

The formation of two Rp maxima alsohas an effect onthe kinetics of the p, which has in eachcasetwo slopes(Fig. 2b,4b and 6b).

In thefirst case, thesecondRp maximum (which is bestobservedat low [CQ] = 0.5wt-%,Fig. 2d) disappearswithincreasing[CQ], but it clearly formswith increasingpoly-merization temperature (Fig. 12c). The existenceof thedoubleRp maximacanbeexplained by thedifferentmobi-lity of the initiating monomer radical (R9) (formed fromthe TEGDM in reaction (3)) in the polymer matrix, aspolymerization proceeds.At thebeginningof thepolymer-ization, faster reaction ratescreate a larger excessfreevolume, which allows for the greater mobilit y of the R9radicals. As the level of conversion is reached where thesamereactionat equilibrium would stop, the excessfreevolume allows furtherreaction (thesecondRP) becauseofenhanced R9 mobility [69]. During the development of adensenetwork structure, further radicals R9 can becometrappedin thepolymer matrix, presumably asgel radicals[63, 70,71]. Increasing [CQ] increasesbothRmax

p (Fig. 7a)andpmax(Fig. 7b) in air aswell in N2, andthiscanbeeasilyexplainedby thefact thattherateof polymerizationis pro-portional to the squareroot of the initi ator concentration[14].

In thesecondcase,theformationof thesecondRp max-imum probably occurs due to polymerization causedbyamineperoxy (AM1OO9) and/or amineoxy (AM1O9)radicals that can propagate with unreacted vinyl groups.Theseradicalscanbe formedby the CQ* decompositionof aminohydroperoxides(AM1OOH) (reactions(16) and(17)) and amineperoxides (AM1OO1AM) (reaction(18)):

AM1OOH+ CQ* e AM1O9 + 9OH + CQ (16)

AM1OOH+ CQ* e AM1OO9 + CQH9 (17)

AM1OO1AM + CQ* e 2 AM1O9 + CQ (18)

It hasearlier been reportedthat CQ* sensitizesdecom-position of peroxides [72]. In separate studies [73], wehaveconfirmed that addition of peroxidesacceleratestheCQ photoinitiated polymerization of TEGDM, whereaswehavefoundthataddition of hydroperoxidesdeceleratespolymerization. Peroxy radicals exhibit low reactivitytowards monomers, whereasoxy radicals easily abstractH-atoms from any molecule.Peroxy radicals trappedin acrosslinked polymer matrix arevery long lived speciesatroomtemperature, and even athightemperatures[74]. ThesecondRp maximum is not formedwhen polymerizationwascarried out in N2 (Fig. 4d andFig. 6d), which elimi-natestheassumption thatAMH 9 mayberesponsible for its

formation. As [DMT] increasesboth Rmaxp (Fig. 5a) and

pmax (Fig. 5b) decrease.This can be the result of DMT(physical) quenchingof the excitedCQ* (T1) state (reac-tion (5)) (Fig. 17). However, the aminecanalsoact asaretarder andreduce the rateof polymerization by a chaintransfer reaction [14, 75]. Retardation of the polymeriza-tion by excess amine will result from chain transfer toamine if the rateof addition of amine radical (AMH9) tomonomer is much slower than the propagation reaction[14].

Addition of 2,2,6,6-tetramethylpiperidine (TMP1H) totheCQ + DMT photoinitiatedpolymerizationof TEGDMcausesthesecondRP maximumnotto appear (Fig. 8a)andp has only one slope (Fig. 8b). Increasing [TMP1H]causesa decrease of both Rp (Fig. 8a) and p (Fig. 8b).TMP1H is a well known antioxidant(decomposeshydro-peroxy groups and/or reactswith peroxy radicals, givingstable oxy radical) [76]. We can assumethat TMP1Hreactswith AM1OO9 radicals butnotwith AM9 radicals:

TMP1H + AM1OO9e TMP9 + AM1OOH (19)

A similar effect (disappearanceof the secondRp maxi-mum) canbe observed whenthe 4-hydroxy-2,2,6,6-tetra-methylpiperidinoxy radical (TMPO9) is added.The latterdoesnot react with hydroperoxide radicals (AM1OO9),butonly scavengestheAM 9 radicals:

TMPO9 + AM 9e TMPO1AM (20)

There is a distinct difference in theRmaxp vs. [TMP1H]

and Rmaxp vs. [TMPO9] kinetics (Fig. 9), which indicates

that the TMPO9 retardsthe polymerization much moreeffectively thanTMP1H. TheAM 9 andAM1OO9 radicalshaveevidently differentreactivities with vinyl groups. Inthe early stage,mainly AM 9 radicals initiate polymeriza-tion, whereasthe AM1OO9 radicals areaccumulated.Asthe polymerization proceedsthe AM1OO9 radicals thatare trappedin the microgel domainslowly start to reactwith vinyl groups, andthe second Rp maximum appears.Addition of TMPO9 causes the AM 9 radicals to be sca-venged in theearlystageof polymerizationandpolymeri-zation becomesretarded(Fig. 9).

Increasingthe polymerization temperature, both in airandN2, causesanincreaseof Rp (Figs.12 and13, respec-tively). The secondRp maximum for polymerizationcar-ried out in air andin thepresenceof DMT doesnot disap-pear but evidently increaseswith increaseof temperature(Fig. 13a).As thetemperature is raised,theadditional freevolumeincreasesthesizeof thefreevolume packets, thusgiving someof thetrapped peroxy radicalsAM1OO9 suf-ficient mobility to react, and this probably causes anincreaseof the secondmaximum rateof polymerization.This explanation is based on the assumption that the freevolumeavailable to the radicals is not evenly spread, butexists in packetsof varying sizewhichcannotberedistrib-uted[70].

Aswenotedatthestartof ourdiscussionof thepolymer-ization of mono-functional monomers, termination reac-


tions play an important role in the kinetics. Because incross-linked systems bulk mobility of the radicals isseverlyhindered, gel-bound radicals tend to be trapped.Trapping occurs from the onset of polymerization [63,77–79].Thepresenceof trapped radicals indicatesthepre-senceof regions more crosslinked than the surroundingenvironment in the bulk [81]. Thus, in the polymerizingmediumof two-functional monomers thereare free radi-cals whosemobility is not directly restricted by spatialconstraint and trapped radicals [82]. This leads to twotypes of polymerization chains: the usual bimolecularinteractions of polymer radicals and a first-order processinvolving only onepolymer radical. The modeling of thetermination reactionsin thetermination of polymerizationof two-functional monomers haveshown that the mono-molecular termination is animportantreactionin thepoly-merization of multifunctional monomers, even at a lowdegreeof conversion[79].

5. Conclusions

It was shownthat photopolymerization of triethylene-glycol dimethacrylate(TEGDM) canbephotoinitiatedbycamphorquinone(CQ)both in air andin N2. TheRp is twotimeshigher in air than in N2; however, the double bondconversion is higher in N2 (L60%) than in air (L40%).Two Rp maxima appeared in the polymerization carriedout in N2. The photopolymerization is accelerated by theaddition of N,N-dimethyl-p-toluidine (DMT), which is anH-atom donor. Increasing [DMT], however, decreasesbothRp andp, becauseaminesquench both singletand tri-pletstatesof CQ.TwoRp maximaappearedin thephotopo-lymerizationcarriedout in thepresenceof CQandDMT inair. Formationof thedouble Rp maximacanbeexplainedby the differentmobility of the initiating radicals formedfrom CQandDMT in apolymermatrix, aspolymerizationproceeds.Theamount of monomer conversion(pm) in thesamplesat the samesurfacesdepends on their thickness,weight, andtime of irradiation. Thepm L 80–90%in theCQandCQ+ DMT photoinitiatedpolymerizationsboth inair andin N2 canbeobtainedafter30 min and60 sirradia-tion, respectively. This fact is a serious limitation for theapplicationof CQ only, instead of CQ + DMT, asa photo-initiator in dental restorative resins, in spite of the toxicand cancerogenic properties of the latter. Emission andlaserspectroscopyhaveprovedthat both the singlet (S2)andtriplet (T1) of theexcitedCQ* canbe involved in thereactionof CQ with DMT in TEGDM both in air and inN2. Thequenchingof theCQ* emissionincreases,whereasRp andp decreasewith increasing[DMT]. For agivenden-tal resinformulation, there is anoptimal amineconcentra-tion at which both Rp andp arethehighest.Thenon-poly-merizedmonomer, non-photoreactedphotoinitiator (CQ +DMT) (L60–70%)andits photoreaction productscanbeeasilyextractedby saliva during 12h extraction at 378C(human body temperature). These productsare swollenandmaycausecertainbiological reactionsthataredanger-ousfor humanbeings.

Theresultspresentedclearly demonstratethatusingCQ(without aminecoinitiators)effectively photocures dentalresins in N2. The technique will significantly reduce thedemaging bi-effects currently encountered when usingaminesin dentistry. We plan further research to minimizethe risk of toxic and cancerogenic reactionsassociatedwith usingphotocuring in clinical dentistry.

Acknowledgements

J.F. Rabekthanks the Foundation of ENSCMU for athree-monthstayin theLaboratoire dePhotochimie Gen-eraleCNRSas aninvited professor in 1991, duringwhichthe ideaof this joint projectwasformulated.The authorsalso gratefully acknowledgethe support of the SwedishInstitute,who generously provided post-docstipendia forE. Andrzejewska, J. Nie and A. Wrzyszczynski, thusenabling this projectto becontinued.This work wassup-portedby theSwedish Medical Research Council (ProjectNo.K97-24X-11650-02A).

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ReceivedJune10,1997

Final versionNovember24,1997

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