study of novel pu-type polymeric photoinitiators comprising of side-chain benzophenone and...

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Study of Novel PU-Type PolymericPhotoinitiators Comprising of Side-ChainBenzophenone and Coinitiator Amine:Effect of Macromolecular Structure onPhotopolymerization

Jun Wei, Hongyu Wang, Xuesong Jiang, Jie Yin*

Three PU-type polymeric photoinitiators containing side-chain benzophenone and coinitiatoramine, PUSBA-h, PUSBA-t and PUSBA-i, were synthesized via polycondensation of 3,5-diamino-40-thiophenylbenzophenone, different diisocyanates and N-methyldiethanolamine.FT-IR, 1H NMR and GPCanalyses confirm thestructures of all poly-mers. The UV-vis spectraof polymeric photo-initiators are similarand all exhibit themaxi-mal absorption near320 nm. ESR spectra show PUSBA-h and PUSBA-t can efficiently generate free radicals. Thephotopolymerization of trimethylolpropane triacrylate and the PUprepolymer, initiated by thesepolymeric photoinitiators, was studied by photo-DSC. The results indicate that the macromol-ecular structure has an important effect on photopolymerization, and different photoinitiatorsexhibit different behavior towards different monomers: PUSBA-t is the most efficient for TMPTAand PUSBA-h is the most efficient for PU prepolymer. The final conversion for the photopoly-merization of PU prepolymer initiated by PUSBA-h is greater than 97%.

J. Wei, H. Wang, X. Jiang, J. YinResearch Institute of Polymer Materials, School of Chemistry &Chemical Technology, State Key Laboratory for CompositeMaterials, Shanghai Jiao Tong University, Shanghai 200240,People’s Republic of ChinaFax: þ86 21 5474 7445; E-mail: [email protected]. WeiDepartment of Polymer Materials and Engineering, School ofMaterial Engineering, Yancheng Institute of Technology, Yan-cheng 224051, People’s Republic of China

Macromol. Chem. Phys. 2007, 208, 287–294

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Introduction

Photopolymerization has been the subject of increasing

interest due to its widespread applications.[1–3] Compared

with low molecular weight analogues, polymeric photo-

initiators have drawn much attention recently due to

several advantages, such as low odor, non-toxicity and

compatibility improvement with formulation compo-

nents.[4–18] In particular, due to the efficient energy

DOI: 10.1002/macp.200600520 287

J. Wei, H. Wang, X. Jiang, J. Yin

288

migration between the excited and ground state of the

photosensitive moieties along the polymer chain, the

photoefficiency of polymeric photoinitiators bearing

side-chain photosensitive moieties can be greatly

improved.[14,19] Among the polymeric photoinitiators,

benzophenone derivatives are widely used in vinyl

polymerization, and their photoefficiency can be promoted

in the presence of hydrogen donors, such as tertiary

amines.[20–27] The incorporation of both benzophenone

and coinitiator amine into the same polymer chain has

obvious advantages such as intramolecular reactions

responsible for the formation of more active species, pro-

tecting the active species by macromolecular chain, and

avoiding the migration of low molecular weight coin-

itiator amine in the post-cured materials.[9,23] As for aro-

matic ketone systems containing thio functional-

ities,[17,28–31] aside from the bimolecular hydrogen-

abstraction reaction, these photoinitiators may also

undergo photolysis reactions at the C–S bond, resulting

in the promotion of their photoefficiency.[30]

In our previous work,[32,33] we synthesized a thio-

containing PU-type polymeric photoinitiator with high

photoactivity for the polymerization of PU prepolymer.

After further research of introducing different amine

coinitiators into the PU-type polymeric photoinitiators, we

found the structure of amine coinitiators has an important

effect on photopolymerization. However, few studies have

ever been reported about the effect of the macromolecular

structure on the photoefficiency of polymeric photoinitia-

tors.

In this context, in order to investigate the influences of

macromolecular structure on photopolymerization, we

synthesized three PU-type polymeric photoinitiators

via polycondensation of 3,5-diamino-40-thiophenylbenzo-

-thiophenylbenzophenone (DATBP), different diisocya-

nates and N-methyldiethanolamine (MDEA). Three typical

diisocyanates of isophorone diisocyanate (IPDI), hexam-

ethylene-1,6-diisocyanate (HDI) and toluene-2,4-diisocya-

nate (TDI) were chosen as monomers for polycondensation

to create the polymers PUSBA-i, PUSBA-h and PUSBA-t,

respectively. UV-vis and ESR spectra were studied to

investigate their photochemical behavior. The photopoly-

merization of a trifunctional trimethylolpropane triacry-

late (TMPTA) and a difunctional PU prepolymer, initiated

by these polymeric photoinitiators, was studied by photo-

DSC.

Experimental Part

Materials

TDI, N,N-dimethylformamide (DMF), dibutyltin dilaurate (T12)

(Medicine Group of China), IPDI (Acros), HDI (Fluka), MDEA

(Kewang Chemical Reagent Company), PU prepolymer (UA-4200,



CAS No. 199875-93-9, Shin-Nakamura Chemical Co. Ltd), and

TMPTA (Nantong Litian Chemical Company) were used as

received. DATBP was synthesized according to the previously

described procedure.[32] Other chemicals used were of analytical

grade except if noted otherwise.

Synthesis of Polymeric Photoinitiators

To a three-necked flask containing 6.00 mmol TDI, IPDI or HDI and

10 mL DMF, which was magnetically stirred under a nitrogen

atmosphere, a solution of 3.00 mmol (0.961 g) DATBP in 10 mL

DMF was added dropwise within 10 min and stirred at room

temperature for 1 h. The mixture then was heated at 50 8C for a

further 1 h. During this process, the molar ratio of amino group

(NH2) to isocyanate group (NCO) was maintained at 1:2. After

cooling the mixture down to ambient temperature, a solution of

4.5 mmol (0.536 g) MDEA in 5 mL DMF and a drop of T12 were

added, and the molar ratio of NCO:hydroxyl group (OH) was kept

at 1:1.5. The mixture was heated at 60 8C for 6 h. The resultant

solution was poured into 10-fold diluted aqueous solution of

ammonia water and filtered to yield a yellow product, which was

dried in vacuo to obtain polymeric benzophenone photoinitiator

containing coinitiator amine.

PUSBA-h: Mn ¼5.1� 103, Mw=Mn ¼ 1.35 (determined by GPC

using DMF as eluent). 1H NMR (DMSO-d6, 400 MHz):d¼8.53 (4H,

NH), 7.67–7.63 (2H, aromatic), 7.52 (2H, aromatic), 7.45 (2H,

aromatic), 7.33 (1H, aromatic), 7.25–7.24 (2H, aromatic), 7.08–7.07

(2H, aromatic), 6.10–6.08 (1H, aromatic), 3.95 (4H, CH2), 3.02–2.84

(8H, CH2), 2.52–2.48 (3H, NCH3), 2.41–2.39 (4H, CH2), 1.35–1.19

(12H, CH2). FT-IR (KBr, cm�1): 3 344 (NH), 2 928, 2 854 (CH2), 1 700

(C––O of –NH–CO–), 1 652 (C––O of Ar–CO–Ar), 1 078 (C–S).

PUSBA-t: Mn ¼ 5.8�103, Mw=Mn ¼1.36 (determined by GPC

using DMF as eluent). 1H NMR (DMSO-d6, 400 MHz): d¼ 8.60 (4H,

NH), 7.93–7.90 (2H, aromatic), 7.84 (1H, aromatic), 7.70–7.69 (2H,

aromatic), 7.53–7.51 (2H, aromatic), 7.49–7.46 (2H, aromatic),

7.39–7.37 (2H, aromatic), 7.18 (1H, aromatic), 7.12–7.10 (2H,

aromatic), 6.91 (1H, aromatic), 4.13–4.11 (2H, CH2), 3.47 (2H, CH2),

2.71 (2H, CH2), 2.67–2.66 (3H, NCH3), 2.26 (2H, CH2), 2.08 (3H, CH3).

FT-IR (KBr, cm�1): 3 346 (NH), 2 930, 2 858 (CH2), 1 696 (C––O

of –NH–CO–), 1 660 (C––O of Ar–CO–Ar), 1 080 (C–S).

PUSBA-i: Mn ¼ 5.7� 103, Mw=Mn ¼1.29 (determined by GPC

using DMF as eluent). 1H NMR (DMSO-d6, 400 MHz): d¼ 8.32 (4H,

NH), 7.65–7.63 (2H, aromatic), 7.53–7.49 (2H, aromatic), 7.47–7.46

(2H, aromatic), 7.31–7.29 (2H, aromatic), 7.10–7.09 (1H, aromatic),

6.98–6.97 (2H, aromatic), 6.10 (1H, aromatic), 5.35 (1H, CH),

3.97–3.96 (4H, CH2), 2.72–2.69 (2H, CH2), 2.53 (3H, NCH3), 2.20–2.18

(6H, CH2), 1.48–1.45 (4H, CH2), 0.93–0.84 (9H, CH3). FT-IR

(KBr, cm�1): 3 356 (NH), 2 924, 2 860 (CH2), 1 690 (C––O of –NH–

CO–), 1 648 (C––O of Ar–CO–Ar), 1 078 (C–S).

Measurements

Molecular weights were determined by gel permeation chroma-

tography (GPC) on a Perkin Elmer Series 200 apparatus on the basis

of linear polystyrene (PS) standards. DMF was used as the eluent.1H NMR spectra were recorded on a Mercury Plus 400 MHz

spectrometer with DMSO-d6 as the solvent. FT-IR spectra were

DOI: 10.1002/macp.200600520

Study of Novel PU-Type Polymeric Photoinitiators Comprising of Side-Chain Benzophenone and . . .

recorded on a Perkin-Elmer Paragon1000 FT-IR spectrometer. The

samples were prepared as KBr disc. UV-Vis spectra were recorded

in chloroform solution using a Perkin-Elmer Lambda 20 UV-vis

spectrophotometer. Electron spin resonance (ESR) experiments

were carried out with a Bruker EMX EPR spectrometer at 9.77 GHz

with a modulation frequency of 100 kHz with 5,5-dimethyl-

1-pyrroline-N-oxide (DMPO) as radical capturing agent. A high-

pressure mercury lamp was used for irradiation in the ESR

spectrometer cavity. The concentration of polymeric photoinitia-

tors dissolved in dichloromethane were 1� 10�3M. 0.5 mL of each

sample was placed into a quartz ESR tube and then purged with

nitrogen to remove oxygen.

Photocalorimetry (Photo-DSC)

The photopolymerization of TMPTA and PU prepolymer was

carried out on a DSC 6200 (Seiko Instrument Inc) photo-DSC

according to the literature.[23] Approximately 2 mg of the sample

mixture was placed in the aluminum DSC pans.

Heat flow versus time (DSC thermogram) curves were recorded

in the isothermal mode under a nitrogen flow of 50 mL �min�1.

The reaction heat liberated in the polymerization was directly

proportional to the number of vinyl groups reacted in the system.

By integrating the area under the exothermic peak, the conversion

of the vinyl groups (C) or the extent of reaction could be deter-

mined according to:

Sch

Macrom

� 2007

C ¼ DHt=DHtheor0 (1)

Where DHt is the reaction heat evolved at time t, and DHtheor0 is the

theoretical heat for complete conversion, DHtheor0 ¼86 kJ �mol�1

for an acrylic double bond.[34] The rate of polymerization (Rp) is

eme 1. Synthesis routes for polymeric photoinitiators.

ol. Chem. Phys. 2007, 208, 287–294

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

directly related to the heat flow (dH/dt) by the following

equation:

Rp ¼ dC=dt ¼ ðdH=dtÞ=DHtheor0 (2)

Results and Discussion

Synthesis of Polymeric Photoinitiators

The main purpose of this paper is to investigate the

influence of the macromolecular structure of polymeric

photoinitiators on photopolymerization. Three polymeric

photoinitiators containing side-chain benzophenone and

coinitiator amine were synthesized according to Scheme 1.

By keeping the molar ratio of amino group (NH2)/

isocyanate group (NCO) as 1:2, NCO-terminated polymer

containing side-chain benzophenone were synthesized via

the polycondensation of DATBP and different diisocya-

nates. Then via a chain-extending reaction of NCO-

terminated polymer with MDEA (NCO/OH¼ 1:1.5), coin-

itiator amine was successfully introduced into the

macromolecular backbone. These OH-terminated poly-

mers can work both as polymeric photoinitiators and

prepolymers. As polymeric photoinitiators, they may

greatly reduce the migration of small molecules in the

post-cured materials. Meanwhile, as prepolymers, they

can further react with other monomers. For example, if

these prepolymers were used to synthesize even higher

molecular weight polyurethanes applied in UV-curable

www.mcp-journal.de 289


290

systems, they may undergo self-initiation process upon

exposure to UV light without addition of any photo-

initiators. FT-IR, 1H NMR and GPC spectra confirmed the

structures of all polymers. The appearance of signals

related to the urethane and urea group in the FT-IR and1H NMR spectra of polymers is considered as evidence for

the completion of the reaction. This is further confirmed by

the molecular weight of polymers as determined by GPC.

UV-Vis Spectra

UV absorption spectra of the three polymeric photoinitia-

tors in chloroform are shown in Figure 1. Their maximum

absorption (lmax) and the values of molar extinction

coefficient at lmax (e) are summarized in Table 1, which

are important in terms of their photochemical activity. As

for benzophenone derivatives, the main benzenoid p-p�

type transition is usually in the region of 250–300 nm.[35]

These polymeric photoinitiators all exhibit significantly

red-shifted maximal absorption near 320 nm, which may be

Figure 1. UV-Vis absorption spectra of polymeric photoinitiatorsin chloroform solution (0.05 � 10�3 M in term of benzophenonemoieties).

Table 1. Absorption properties of PUSBA-t, PUSBA-i and PUSBA-hin chloroform solution.

Photoinitiatora) llmax ee� 10�4

nm L �mol�1 � cm�1

PUSBA-t 318 1.836

PUSBA-i 317 1.808

PUSBA-h 320 1.827

a)The photoinitiator concentration is 0.05 � 10�3M in terms

of benzophenone moieties.



ascribed to the strong electron donation effect via the

thiophenyl group. Therefore, This red-shifted maximum

makes these polymers attractive as photoinitiators, suitable

for UV-curing near to the wavelength of visible light. Mean-

while, these polymeric photoinitiators have similar max-

imal absorption, which shows that the macromolecular

structure has no significant influence on the UV-vis absorp-

tion of benzophenone moieties in polymeric photoinitiators.

ESR Spectroscopy

In order to investigate the mechanism of photoinitiation,

ESR studies of PUSBA-h, PUSBA-t and PUSBA-i were carried

out in dichloromethane, and the results are shown in

Figure 2. Because the life of amine radicals is very short, we

choose DMPO as a radical capturing agent in the ESR

measurements. The amine radicals released from photo-

initiator systems will react with DMPO, and generate

relatively stable nitrogen-oxygen radicals.[33] As shown in

Figure 2(c), the six-line hyperfine splitting is usually

explained by a triplet with a-nitrogen and a further split

into a doublet with a b-proton.[5] Because the electro-

n-donating ability of the methylene groups is greater than

that of the methyl groups in the coinitiator moieties, the

excited triplet benzophenone moieties may abstract

hydrogen mainly from the methylene groups instead of

methyl groups.[36] Therefore, the methenyl radical marked

in Figure 2(c) is the main radical produced.

Compared with PUSBA-i, the signal intensity of PUSBA-h

and PUSBA-t is much higher, which indicates that lots of

free radicals were generated and its concentration is

relatively higher. As for PUSBA-h, the mobility of polymer

chain would be enhanced due to the flexible aliphatic

Figure 2. ESR spectra of (a) PUSBA-i, (b) PUSBA-t and (c) PUSBA-hin dichloromethane, irradiated for 5 min.

DOI: 10.1002/macp.200600520


chain of HDI moieties, which certainly favors the bimole-

cular hydrogen-abstraction reaction between the excited

benzophenone moieties and MDEA moieties. As for PUSBA-

t, the introduction of a rigid phenyl group may decrease

the mobility of the polymer chain. Thus, the electron

transfer in PUSBA-t is not as efficient as that in PUSBA-h.

However, the conjugated structure of TDI moieties would

enhance the electrophilic ability of the two urea bonds

that directly link to benzophenone moieties. As a result,

the proton transfer in PUSBA-t is very efficient. Therefore,

the bimolecular hydrogen-abstraction reaction rate of

PUSBA-t is also very high. In contrast to PUSBA-t, due to the

strong electron donating ability of the saturated hexacyclic

structure in PUSBA-i, the nucleophilic reaction between

the excited state of benzophenone moieties and MDEA

moieties does not easily occur, resulting in a low concen-

tration of active species.

Photopolymerization of TMPTA

The photo-DSC profiles of TMPTA for PUSBA-t, PUSBA-i and

PUSBA-h are shown in Figure 3. Their polymerization

behavior appears similar to other multifunctional mono-

mers.[37–41] Figure 4(a) shows that the conversion corre-

sponding to the maximal of polymerization rate (Rp, max) is

dependent on the photoinitiator used. The data for Rp,max

and final conversion of TMPTA is shown in Table 2. From

Figure 3, 4 and Table 2, PUSBA-h exhibits similar photo-

polymerization behavior to PUSBA-i, and PUSBA-t is the

most efficient photoinitiator for the polymerization of

TMPTA. This result may be ascribed to the more efficient

proton transfer in PUSBA-t than that in PUSBA-h and

Figure 3. Photo-DSC profiles for polymerization of TMPTAinitiated by PUSBA-h, PUSBA-t and PUSBA-i systems, cured at25 8C by UV light with an intensity of 50 mW � cm�2 (photoini-tiator concentration¼0.02 M in terms of benzophenone moi-eties).

Figure 4. (a) Rate vs. conversion; (b) Conversion vs. time forpolymerization of TMPTA for PUSBA-h, PUSBA-t and PUSBA-isystems, cured at 25 8C by UV light with an intensity of50 mW � cm�2 (photoinitiator concentration 0.02 M in terms ofbenzophenone moieties).

Table 2. Photopolymerization of TMPTA initiated by PUSBA-t,PUSBA-i and PUSBA-h, cured at 25 8C by UV light with an intensityof 50 mW � cm�2.

Photoinitiatora) Rp, max Final

conversion

Hmax Tmax

10S2 sS1 % mW �mgS1 s

PUSBA-t 2.930 51.36 20.95 9.8

PUSBA-h 2.098 48.27 15.10 11.2

PUSBA-i 1.773 47.57 12.75 13.4

a)The photoinitiator concentration is 0.02 M in term of

benzophenone moieties, where Rp, max ¼maximal

polymerization rate, Hmax¼maximal heat flow, and

Tmax ¼ time to reach maximal heat flow.


� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mcp-journal.de 291


Figure 5. Photo-DSC profiles for polymerization of PU prepolymerinitiated by PUSBA-h, PUSBA-t and PUSBA-i systems, cured at 25 8Cby UV light with an intensity of 50 mW � cm�2 (photoinitiatorconcentration is 0.02 M in terms of benzophenone moieties).

Figure 6. (a) Rate vs. conversion; (b) Conversion vs. time forpolymerization of PU prepolymer for PUSBA-h, PUSBA-t andPUSBA-i systems, cured at 25 8C by UV light with an intensityof 50 mW � cm�2 (photoinitiator concentration is 0.02 M in termsof benzophenone moieties).

292

PUSBA-i, as observed from the ESR analysis. Moreover, the

relatively large steric hindrance of the macromolecular coil

could hinder the recombination reaction between the

propagating radicals and the macroradicals, thus strongly

limiting the extent of termination and producing a relati-

vely high concentration of the active species.

Compared with the linear hexamethylene structure in

PUSBA-h, the electron donating ability of the saturated

hexacyclic structure in PUSBA-i is much higher. Therefore,

the nucleophilic reaction between the excited state of

benzophenone moieties and MDEA moieties in PUSBA-h is

more efficient than that of PUSBA-i, which is also con-

firmed by the Tmax values from Table 2. As a result,

PUSBA-h possesses slightly higher polymerization rates

and final conversion of TMPTA than PUSBA-i.

Photopolymerization of PU Prepolymer

Photopolymerization of PU prepolymer initiated by these

polymeric photoinitiators behaves similarly to that of

Table 3. Photopolymerization of PU prepolymer initiated by PUSBA-t,50 mW � cm�2.

Photoinitiatora) Rp, max� 102 Fin

sS1

PUSBA-h 2.008

PUSBA-t 1.605

PUSBA-i 1.030

a)The photoinitiator concentration is 0.02 M in terms of benzop

rate, Hmax ¼maximal heat flow and,Tmax ¼ time to reach max



TMPTA, as shown in Figure 5, 6 and Table 3. It is well

accepted that the hydrogen-abstraction reaction is che-

mically controlled in the early stage of photopolymeri-

zation. Compared with trifunctional monomer TMPTA

PUSBA-i and PUSBA-h, cured at 25 8C by UV light with an intensity of

al conversion Hmax Tmax

% mW �mgS1 s

97.33 3.03 12.4

63.54 2.42 12.2

54.80 1.57 13.8

henone moieties, where Rp, max¼maximal polymerization

imal heat flow.

DOI: 10.1002/macp.200600520


(M¼ 296), the double bond content of difunctional PU

prepolymer is much lower (M¼ 1 300). Therefore, the PU

prepolymer possesses much lower maximal heat flows

(Hmax) than TMPTA, as shown in Figure 3 and 5. Mean-

while, gelation may occur at an even later stage of

photopolymerization, resulting in a relatively longer time

(Tmax) to reach the Rp, max and higher final conversion of PU

prepolymer than that of TMPTA, as shown in Table 2 and 3.

From Figure 5, 6 and Table 3, PUSBA-h is the most

efficient photoinitiator for the polymerization of PU

prepolymer, which is different from the polymerization

of TMPTA. Due to the very high viscosity (2 000 mPa �s � 8C�1) of the PU prepolymer, diffusion control may be the

most important factor for the photopolymerization.

Therefore, in order to attain efficient electron and proton

transfer, both benzophenone moieties and the coinitiator

moieties must get as close as possible. Therefore, consi-

dering this, the mobility of the macromolecules may

greatly affect the polymerization rate. The introduction of

different diisocyanates into the polymeric photoinitiators

may have a significant influence on the flexibility of the

polymer chains. Compared with IPDI of cyclic structure

and TDI of rigid phenyl, HDI possesses a flexible aliphatic

chain. Therefore, the intramolecular hydrogen-abstraction

reaction of PUSBA-h is more efficient than PUSBA-t

and PUSBA-i, leading to a higher polymerization rate

and final conversion of PU prepolymer. Compared with

PUSBA-i, though the macromolecular chain of PUSBA-t is

more rigid, it is more facile to generate free radicals as

observed in the ESR studies. Therefore, PUSBA-t possesses a

slightly higher final conversion of PU prepolymer than

PUSBA-i.

Conclusion

In this work, three PU-type polymeric photoinitiators con-

taining side-chain benzophenone and coinitiator amine

were synthesized via polycondensation of DATBP, differ-

ent diisocyanates and MDEA. UV-vis spectra showed that

the macromolecular structure has no significant influence

on the UV-vis maximal absorption. ESR spectra indicate

that PUSBA-h and PUSBA-t can generate free radicals more

efficiently than PUSBA-i. The photopolymerization of

TMPTA and PU prepolymer, initiated by these polymeric

photoinitiators, was studied by photo-DSC. The results

show that the photoefficiency of polymeric photoinitiators

is mainly depended on their structures: PUSBA-t is the

most efficient for TMPTA and PUSBA-h is the most efficient

for PU prepolymer. The final conversion for PU prepoly-

mers initiated by PUSBA-h is greater than 97%, which

indicates that this PU-type polymeric photoinitiator may

show potential applications in PU-based UV-curable

coatings.



Acknowledgements: The authors express their gratitude to theMinistry of Science and Technology of China (NO: 2004AA33H010),and the Ministry Education of China (Kuashiji Scholar Project) fortheir financial support.

Received: October 17, 2006; Revised: November 16, 2006;Accepted: November 22, 2006; DOI: 10.1002/macp.200600520

Keywords: benzophenone; polymeric photoinitiator; photopoly-merization; polyurethane; thio-containing initiator

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DOI: 10.1002/macp.200600520

study of novel pu-type polymeric photoinitiators comprising of side-chain benzophenone and...

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