preparation and characterization of a water soluble methylated β-cyclodextrin/camphorquinone...

Research Article

Received: 9 July 2008, Revised: 14 August 2008, Accepted: 18 August 2008, Published online in Wiley InterScience: 28 January 2009

(www.interscience.wiley.com) DOI: 10.1002/pat.1290

Preparation and characterization of a watersoluble methylated b-cyclodextrin/camphorquinone complexy

Jing Zhanga, Pu Xiaoa, Suqing Shib and Jun Niea*

Awater soluble methylated b-cyclodextrin/camphorq

Polym. Adv

uinone (MCD/CQ) complex, based on methylated b-cyclodextrin(MCD) and camphorquinone (CQ), was prepared and its structure was characterized by FTIR, 1H-NMR, and UV–visspectra. The photopolymerization kinetics of MCD/CQ in the water soluble monomer systemwas studied by Real-timeInfrared spectroscopy (RT-IR). Compared to the photopolymerization carried out under nearly identical conditions butwithout MCD, the polymerization rate and final conversion initiated by a CQ-triethanolamine photoinitiator systemwere slightly lower. The effects of different MCD/CQ concentration, triethanolamine concentration, and light intensitywere also studied. Copyright � 2009 John Wiley & Sons, Ltd.

Keywords: camphorquinone; cyclodextrin; water soluble photoinitiator; RT-IR; kinetics

* Correspondence to: J. Nie, State Key Laboratory of Chemical Resource Engin-

eering and Key Laboratory of Beijing City on Preparation and Processing of

Novel Polymer Materials, Beijing University of Chemical Technology, Beijing

100029, P. R. China.

E-mail: [email protected]

a J. Zhang, P. Xiao, J. Nie

State Key Laboratory of Chemical Resource Engineering and Key Laboratory

of Beijing City on Preparation and Processing of Novel Polymer Materials,

Beijing University of Chemical Technology, Beijing 100029, P. R. China

b S. Shi

Department of Chemistry, Northwest University, Xi’an 710069, P. R. China

y Jing Zhang and Pu Xiao contributed equally to this work. 7

INTRODUCTION

Photopolymerization technology has various industrial appli-cations such as coatings, adhesives, printing inks, photoresists,and biomaterials.[1–4] This technology is based on the use ofphotoinitiator systems suited to absorb a light radiation of theappropriate wavelength and to produce primary radical speciesto convert a multifunctional monomer into a cross-linkednetwork.[3,5] Because of a legislation-led drive away from organicsolvents and toward water-borne formulations in the paints andcoatings industry, water soluble photoinitiators have receivedincreasing attention.[6] The introduction of water soluble groupsonto a conventional oil soluble photoinitiator is a relatively simpleprocedure to develop water soluble photoinitiators.[6–8] Forexample, Liska combined photoinitiators with carbohydrates toobtain water soluble photoinitiators.[8] Yagci and coworkers[9]

investigated the photoactivity of thioxanthone chromophoricgroup chemically attached to b-cyclodextrin and itsefficiency inthe photopolymerization of methyl methacrylate in an aqueousmedium.Cyclodextrins (CD) are cyclic oligosaccharides consisting of

glucopyranose units with a hydrophobic cavity and hydrophilicexterior.[4,9–11] They could enclose various hydrophilic moleculesinto their cavity due to the special molecular structure.[12,13] Theformation of an inclusion complex leads to a significant change inthe solution properties and reactivity of the guest mol-ecule.[4,10,11] Recently, Ritter and coworkers[10] have reportedthat the complexation of a nearly water insoluble type Iphotoinitiator 2-hydroxy-2-methyl-1-phenylpropan-1-one withmethylated b-cyclodextrin results in the formation of a watersoluble host/guest complex. Other studies,[4,11] have demon-strated a water soluble complex of the methylated b-cyclodextrinand 2,2-dimethoxy-2-phenyl acetophenone. The methylatedb-cyclodextrin/photoinitiator complexes mentioned above were

. Technol. 2009, 20 723–728 Copyright �

all type I photoinitiators used in UV cured formulations. Yet, therehave been no literature reports of any type II visible-lightphotoinitiators enclosed into the hydrophobic cavities ofcyclodextrins to obtain a water soluble b-cyclodextrin/photo-initiator complex. For in vivo applications, systems that can becured under visible light are favorable, as UV light can damageliving tissues. It is, therefore, of interest to develop visible lightphotoinitiators for biomedical applications. Hussain and cow-orkers[14] have synthesized methacrylated b-cyclodextrin andinvestigated the complexation of methacrylated b-cyclodextrinand camphorquinone, but the water solubility was notconcerned. Camphorquinone (CQ) is the most popular photo-sensitizer in commercial visible-light cured resins[15] but cannotbe used in water-borne formulations.In this study, a water soluble methylated b-cyclodextrin/

camphorquinone (MCD/CQ) complex was prepared and itsstructure was characterized by FTIR, 1H-NMR, and UV–vis spectra.The photopolymerization kinetics of MCD/CQ in thewater solublemonomer systemwas studied by real-time infrared spectroscopy.

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J. ZHANG ET AL.

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The results indicated that the MCD/CQ complex is an effectiveinitiator in water-borne formulations.

EXPERIMENTAL

Materials

Camphorquinone (CQ) was purchased from Sigma–Aldrich (USA).Methylated b-cyclodextrin (MCD) was purchased from ShandongXinda Fine Chemical Co. (China). Triethanolamine (TEOHA) waspurchased from Sinopharm Group Chemical Reagent Co. (China).Polyethylene glycol (200) diacrylate (PEG200DA) was obtained bySartomer Company (Warrington, PA, USA). Hydroxyethyl acrylate(HEA) was obtained from JiangSu Sanmu Group (China).Methanol and cyclohexane were purified according to standardlaboratory methods.

Instrumentation

FTIR spectra were recorded on a Nicolet 5700 instrument(Thermo Electron Corporation, Waltham, MA, USA).The 1H-NMR spectra were recorded on a Bruker AV600 unity

spectrometer operated at 600MHz using D2O as a deuteratedsolvent.UV–vis absorption spectra were recorded on a Hitachi U-3010

UV–vis spectrophotometer (Hitachi High-Technologies Corpor-ation, Tokyo, Japan). A cell pathlength of 1 cm was employed.The visible light source was Spectrum 800 Curing Light

(Dentsply, Milford, DE, USA).Light intensity was recorded by the FZ-A Radiometer

(Photoelectric Instrument Factory, Beijing Normal University,Beijing, China).

Preparation of methylated b-cyclodextrin/camphorquinonecomplex (MCD/CQ)

MCD (6.29 g, 4.80mmol) was dissolved in methanol (36.0mL) andCQ (0.80 g, 4.80mmol) was added. The mixture solution wasstirred and then sonicated for 20min, followed by standingovernight at room temperature. After solvent evaporation thesolid complex was dried under vacuum at 408C for 12 hr.

Photoreduction

Photoreduction of MCD/CQ and TEOHA photoinitiator systemwas studied in an aqueous solution, and the concentrationsof MCD/CQ and TEOHA were 1.5 and 1.0� 10�2mol L�1,respectively. The photoreduction process was monitored withUV–vis absorption spectroscopy. The light intensity on thesurface of the cell was 30mWcm�2.

Photopolymerization kinetics in water solution

The Photoinitiation efficiency of water soluble MCD/CQ complexwas studied in by real-time Infrared spectroscopy (RTIR). Thewater soluble monomer system was composed of PEG200DA/HEA/H2O¼ 51:29:20 (wt%), with a different design concentrationof MCD/CQ (or CQ) and TEOHA as the photoinitiator system.The basic principle of RTIR spectroscopy consists in exposing thesample simultaneously to the UV light which induces thepolymerization, and to the infrared beam which serves tomeasure the monomer concentration at any given time. Theresulting decrease in the IR absorption band characterizing the

www.interscience.wiley.com/journal/pat Copyright � 2009

monomer is monitored continuously on a transient memoryrecorder. Since the increase in the absorbance (A0–At) is alwaysproportional to the amount of monomer which has polymerizedafter a given exposure, and thus to the degree of conversion, therecorded RTIR trace actually corresponds to a conversion versustime curve.[16,17] Conversion data were obtained by monitoringthe decay of the acrylate double bond ––C–H peak at about6165 cm�1. Upon irradiation, the decrease in the ––C–Habsorption peak area from 6125.50 to 6211.79 cm�1 accuratelyreflects the extent of the polymerization. After baselinecorrection, conversion of the functional groups could becalculated by measuring the peak area at each time of thereaction and is determined as follows:

DCð%Þ ¼ ðA0 � AtÞA0

� 100

where DC is the degree of acrylate double bond conversion attime t, A0 the initial peak area before irradiation, and At is the peakarea of the double bonds at time t.All samples were photocured in 1.5mm thick plastic molds

with an 8.0mm diameter central. The molds were clampedbetween two glass slides with spring loaded binder clips.[18] Thesamples were irradiated with a visible light source. Each spectrumwas the signal of one scan with the resolution of 4 cm�1 at roomtemperature. For each sample, the series RTIR runs were repeatedthree times.

RESULTS AND DISCUSSION

Preparation of MCD/CQ complex

CQ is a kind of water insoluble photoinitiator. Equimolar amountsof MCD and CQ were dissolved in methanol, and the solution wassonicated for 20min to form the MCD/CQ complex (Scheme 1).The water soluble solid complex was obtained after evaporationof the solvent.Several methods were applied to the characterization of MCD/

CQ complex. The most important feature is the water solubility ofthe complex, meaning that the water insoluble CQ was enclosedinto the hydrophobic cavity of MCD.The IR spectra (not shown here) were used to verify the

formation of the MCD/CQ complex. Comparing the spectra ofMCD/CQ with uncomplexed CQ, the band at about 1749 cm�1,which corresponds to the carbonyl vibrations of CQ, shifted to1753 cm�1 in MCD/CQ due to the influence of the MCD hostcomponent. This phenomenon is probably because of theformation of hydrogen bonds between the hydroxyl groups ofMCD and the carbonyl groups of CQ.

1H-NMR spectroscopy of MCD/CQ and MCD in D2O indicatedthe presence of the hydrophobic CQ. Figure 1 illustrates thecharacteristic 1H-NMR shifts (D2O) of MCD/CQ in comparison tounmodified MCD. The spectrum of MCD/CQ showed highlyresolved and sharp signals for the proton shifts of CQ in additionto typical magnetic shifts of some MCD protons. The signals ofMCD protons changed slightly due to the formation of thecomplex.Figure 2 shows the UV–vis absorption spectra of MCD/CQ and

CQ in methanol. It showed that they had a similar spectral shapewith the maximum absorbance at 467 nm, and the molarextinction coefficients were 30 L mol�1 cm�1 (MCD/CQ) and 32 Lmol�1 cm�1 (CQ), respectively. This showed that the complexa-

John Wiley & Sons, Ltd. Polym. Adv. Technol. 2009, 20 723–728

Scheme 1. Preparation of MCD/CQ complex in methanol.

METHYLATED �-CYCLODEXTRIN/CAMPHORQUINONE COMPLEX

tion of MCD did not significantly influence the UV–vis absorptionof CQ.UV–vis absorption was also used to prove the formation of the

MCD/CQ complex. The transmissions at 700 nm for the differentconcentrations of CQ and MCD/CQ are presented in Fig. 3. Onincrease in the concentration of CQ in water, the mixture becamemore and more turbid and a dramatic decrease in transmission at700 nm was registered as a consequence of scattering light.However, with increase in the concentration of MCD/CQ in water,the solution was still clear and no change of transmission wasobserved.

Photoreduction

Photoreduction of CQ occurs by an intermolecular hydrogenabstraction from hydrogen-donor molecules such as alcohols,ethers, and amines.[2] CQ absorbs visible light in the blue regiondue to the n-p� transition of the dicarbonyl group. The excitedn-p� transition CQ abstracts a hydrogen atom from ahydrogen-donor molecule and produces a primary radical, whichattacks the carbon double bonds of monomers. These processesresult in new radicals, called primary propagating radicals, whichattack other monomers and initiate polymerization.[15]

The photoreduction study of the MCD/CQ-TEOHA photoini-tiator system was conducted by monitoring the changes of

Figure 1. 1H-NMR spectra of MCD/CQ and MCD in D2O.

Polym. Adv. Technol. 2009, 20 723–728 Copyright � 2009 John

UV–vis spectra with visible light irradiation in the region of350–600 nm. During the visible light irradiation, the spectra ofwater solution containing TEOHA (Fig. 4) clearly showedbleaching of MCD/CQ in the range of 400–500 nm. The decrease(absorbance at 0min minus absorbance at t) in the maximumabsorbance of MCD/CQ at 460 nm in the presence of TEOHA andthe absence of amine are presented in Fig. 5. It could be clearlyseen that the observed absorption peak of MCD/CQ alonedecreased slightly with irradiation, but the addition of TEOHAmade the maximum absorbance peak of MCD/CQ decreasedramatically. It means that TEOHA is an efficient hydrogen donorto MCD/CQ in water solution. MCD can also act as ahydrogen donor because of the existence of alcohol and ethergroups in it, but it is not as efficient as TEOHA.

Photopolymerization kinetics

Photopolymerization kinetics of MCD/CQ in the water solublemonomer system, composed of PEG200DA/HEA/H2O¼ 51:29:20(wt%) was studied by real-time infrared spectroscopy.

Comparison of MCD/CQ and CQ

Conversion versus time plots for the polymerization ofPEG200DA/HEA/H2O induced by MCD/CQ-TEOHA and CQ-TEOHA

Figure 2. UV–vis spectra of MCD/CQ and CQ in methanol solution

([MCD/CQ]¼ 1.5� 10�2mol L�1, [CQ]¼ 1.5� 10�2mol L�1).

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Figure 3. Transmission at 700 nm of the aqueous dispersions of CQ and

the solution of MCD/CQ at different photoinitiator concentrations.

Figure 5. Decrease in the MCD/CQ absorption band at 460 nm with/

without the present of TEOHA in water during visible light irradiation ([MCD/

CQ]¼ 1.5� 10�2mol L�1, [TEOHA]¼ 1.0� 10�2mol L�1, I¼ 30mWcm�2).

J. ZHANG ET AL.

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photoinitiator systems are shown in Fig. 6. Although the CQconcentrations are the same, the polymerization rate and finalconversion initiated by MCD/CQ-TEOHAwere slightly higher thanthat of CQ-TEOHA. This is probably because MCD/CQ had betterwater solubility than CQ. On the other hand, MCD can act as acoinitiator except for TEOHA because of the existence of alcoholand ether groups in it. It may also implicate that the complexationof MCD had a certain influence on the photoinitiation process ofCQ.

Effect of MCD/CQ concentration

The plots of conversion versus irradiation time for PEG200DA/HEA/H2O incorporating different MCD/CQ concentrations in thepresence of 27.4mmol g�1 TEOHA as the initiating system arepresented in Fig. 7. As expected, the polymerization rate and finalconversion increased with increase in MCD/CQ concentration,but they tended to level off at the concentration of23.9mmol g�1. Further increase in the concentration of MCD/CQ led to a decrease in the polymerization rate and final

Figure 4. Change in the UV–vis absorption spectra of MCD/CQ in thepresence of TEOHA in water during visible light irradiation ([MCD/

CQ]¼ 1.5� 10�2mol L�1, [TEOHA]¼ 1.0� 10�2mol L�1, I¼ 30mWcm�2).


conversion. It might be attributed to the light absorption processwith higher MCD/CQ concentrations, which reduced thepenetration of light into the sample due to the internal filteringeffect of MCD/CQ.[17]

Effect of TEOHA concentration

Figure 8 shows the conversion versus time plots for PEG200DA/HEA/H2O initiated by 24.0mmol g�1 MCD/CQ with differentTEOHA concentrations. Photopolymerization proceeded slowly inthe condition of MCD/CQ alone as the photoinitiator. Addition ofTEOHA accelerated the photopolymerization dramatically due tothe formation of an amine-derived radical.[3] The polymerizationrate and final conversion increased with increase in TEOHAconcentration. This was because a very high coinitiatorconcentration yielded many radicals by the incident light, whichled to the high polymerization rate and final conversion. At53.4mmol g�1 of TEOHA, the polymerization rate and finalconversion were almost the same as that measured for

Figure 6. Conversion versus irradiation time plot of PEG200DA/HEA/

H2O initiated by MCD/CQ-TEOHA and CQ-TEOHA (Concentration: MCD/CQ 23.9mmol g�1, CQ 23.9mmol g�1 and TEOHA 27.4mmol g�1; light

intensity: 45mWcm�2).


Figure 7. Effect of MCD/CQ concentrations on photopolymerization

of PEG200DA/HEA/H2O (TEOHA concentration: 27.4mmol g�1; light inten-sity: 45mWcm�2).

Figure 9. Effect of light intensity on photopolymerization of PEG200DA/

HEA/H2O (Concentration: MCD/CQ 4.8mmol g�1, TEOHA 27.4mmol g�1;

light intensity: 45mWcm�2).

METHYLATED �-CYCLODEXTRIN/CAMPHORQUINONE COMPLEX

35.8mmol g�1 of TEOHA, which meant 35.8mmol g�1 was anoptimum concentration for TEOHA.

Effect of light intensity

Figure 9 shows the conversion versus time plots for PEG200DA/HEA/H2O initiated by 4.8mmol g�1 MCD/CQ in the presence of27.4mmol g�1 TEOHA at different intensities (I). The polymeri-zation rate and final conversion increased with increase in I. Thiswas because the higher light intensity could yield more radicalswhich led to an increase in the polymerization rate and finalconversion. At the same time, the induction period wasshortened with increase in I; it happened because whenphotopolymerization was carried out in the presence of air,oxygen molecules dissolved in the formulation scavenged theinitiating radicals. As the oxygen was consumed, the monomermolecules became capable of successfully reacting with theinitiator radicals. Increase in intensity could rapidly produce largeamounts of free radicals to overcome oxygen inhibition.[17]

Figure 8. Effect of TEOHA concentrations on photopolymerization ofPEG200DA/HEA/H2O (MCD/CQ concentration: 24.0mmol g�1; light inten-

sity: 45mWcm�2).

Polym. Adv. Technol. 2009, 20 723–728 Copyright � 2009 John

CONCLUSIONS

The complexation of MCD with hydrophobic photoinitiator CQwas successfully achieved, resulting in the formation of an MCD/CQ complex, which had stable water solubility. Compared to thephotopolymerization kinetics in the water soluble monomersystem, the polymerization rate and final conversion initiated byMCD/CQ-TEOHA were slightly higher than that of CQ-TEOHAwhich was also carried out under nearly identical conditions. Thepolymerization rate and final conversion reached the maximumat the optimum MCD/CQ concentration of 23.9mmol g�1 with aconstant TEOHA concentration of 27.4mmol g�1. Further increasein the concentration of MCD/CQ led to a decrease in thepolymerization rate and final conversion. The polymerization rateand final conversion increased with increase in TEOHAconcentration and light intensity.

Acknowledgements

The authors appreciate the support from the Program for Chang-jiang Scholars and Innovative Research Team in University.

7

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preparation and characterization of a water soluble methylated β-cyclodextrin/camphorquinone...

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