Novel multifunctional hyperbranched polymeric

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www.rsc.org/materials | Journal of Materials Chemistry

Novel multifunctional hyperbranched polymeric photoinitiators with built-in amine coinitiators for UV curing{ Yu Chen,*ab Johan Loccufier,c Luc Vanmaelec and Holger Frey*b Received 13th June 2007, Accepted 12th July 2007 First published as an Advance Article on the web 17th July 2007 DOI: 10.1039/b708986d

A new class of hyperbranched polymeric photoinitiators with built-in amine coinitiators has been developed, showing high functionality, low viscosity, good compatibility with the usual radiation curable formulations, high photoactivity and low extractability from the cured sample. Crosslinked polymers derived from photopolymerization are important for a broad variety of applications, such as photocurable coatings, varnishes, lacquers, optical discs, electronic circuits, printing inks and adhesives.1,2 For all applications, a photoinitiator system with high photoactivity, good solubility in the curable medium, low odor and toxicity, no darkening deriving from the presence of migratory residues in the network and good storage stability is desired. To satisfy most of the aforementioned requirements, one possible strategy is the synthesis of polymeric photoinitiators by incorporation of the low molecular weight photoinitiator into the main or side chain of polymers.3–10 In radiation-curable formulations, polymeric photoinitiating systems with high functionality and low viscosity are preferred. To date, nearly all known polymeric photoinitiating systems are based on conventional linear polymer structures. For such topologies, an enhancement of the functionality is usually accompanied by an increase of the molecular weight. This, however, enhances the viscosity of the radiation-curable formulations significantly.3 Hyperbranched polymers and dendrimers combine high functionality with low viscosity,11–14 in contrast to linear polymers. To date, reports on polymeric photoinitiating systems derived from dendrimers15,16 or hyperbranched polymers17,18 are very scarce. Herein, we report new, conveniently manufactured hyperbranched polymeric photoinitiators (PPIC) with built-in amine coinitiators which possess the following advantages: (1) high functionality; (2) good compatibility with the usual radiation-curable formulations; (3) low viscosity; (4) high photoactivity; (5) low extractability from the cured sample. Transparent, yellowish hyperbranched polyglycerols (PG) with an average of 17, 33, 83 and 179 hydroxyl end-groups, were used as scaffolds for the syntheses of multifunctional hyperbranched PPICs. Benzophenone (BP) has been widely used as a Norrish type II photoinitiator due to its low cost, good solubility, good activity and low yellowing on cure. Therefore, it was selected for a

Department of Chemistry, School of Sciences, Tianjin University, 300072, Tianjin, People’s Republic of China. E-mail: [email protected] Institut fu¨r Organische Chemie, Johannes Gutenberg-Universita¨t, Duesbergweg 10-14, 55099, Mainz, Germany c Agfa Graphics NV, Septestraat 27, B-2640, Mortsel, Belgium { Electronic supplementary information (ESI) available: Details of experiment, UV curing process, table of radiation-curable formulations, NMR. See DOI: 10.1039/b708986d b

This journal is ß The Royal Society of Chemistry 2007

incorporation into the PPICs as the photoactive moiety. Aromatic and aliphatic tertiary amines, 4-dimethylaminobenzoate (DMB) and 1-piperidinepropionate (PP), were incorporated as coinitiator structures. Since the PPICs with solely BP and DMB or PP functional groups exhibited poor solubility in the usual monomers dipropylene glycol diacrylate (DPGDA) and trimethylolpropane triacrylate (TMPTA), compatibilizing groups, such as 2-[2-(2-methoxyethoxy)ethoxy] acetate (MEEA), were introduced to enhance their solubility in the radiation-curable formulations. For the synthesis of hyperbranched PPICs with aromatic tertiary amine DMB as coinitiator, the inexpensive raw material 4-dimethylaminobenzoic acid was used. The acid-catalyzed direct condensation between PG and 4-dimethylaminobenzoic acid in toluene failed. Subsequently, the 1,19-carbonyldiimidazole (CDI)activated carboxylic acid method was employed, as shown in Scheme 1. First PG was partially modified with MEEA under acid-catalyzed condensation conditions. The material obtained was soluble in low boiling point solvents, such as THF and chloroform. Subsequently the CDI-activated 4-dimethylaminobenzoic acid reacted with partially MEEA-modified PG in THF, and the residual hydroxyl groups were modified by CDI-activated (4-benzoylphenoxy)acetic acid. Compared to the PPICs with an aromatic tertiary amine coinitiator, the synthesis of the PPIC with aliphatic tertiary amine PP coinitiator is more simple (Scheme 2) and can be conducted in one pot, using the acid-catalyzed direct condensation between PG and the other raw materials with carboxylic acid groups. All resulting PPICs were characterized by 1H NMR (ESI,{ Fig. S1 and S2) and FTIR spectroscopy, confirming the successful incorporation of the functional groups into PG. Subsequently, the average degree of substitution with BP, DMB, PP and MEEA moieties can be calculated and the results are compiled in Table 1. The molar ratio of initiator and coinitiator moieties in the resulting PPICs is around 1 : 1, just as expected. The total functionality of initiator plus coinitiator moieties is around 60%, slightly less than the targeted 66%. The average functionality of initiator plus coinitiator moieties in the subject PPICs is in the range of 8 to 106, showing that the obtained PPICs have high functionality. The resulting PPICs have good solubility in the usual radiationcurable monomers DPGDA and TMPTA. Thus, with the mixture of DPGDA and TMPTA as the monomers six radiation-curable formulations based on the PPICs have been prepared (ESI,{ Table S1). For comparison, two formulations with the low molecular weight photoinitiator methyl (4-benzoylphenoxy)acetate (MBPA) and photo-coinitiator 2-ethylhexyl 4-dimethylaminobenzoate (EHA) were also prepared (ESI,{ Table S1). The content of J. Mater. Chem., 2007, 17, 3389–3392 | 3389

Scheme 1 Synthesis of hyperbranched polymeric photoinitiator with built-in aromatic tertiary amine coinitiator.

photoinitiator moiety and the photo-coinitiator moiety in all formulations was adjusted to be similar, as shown in Table 2. The photoactivity of the resulting PPICs with aromatic tertiary amine coinitiator moieties was compared with the corresponding

Scheme 2

low molecular weight MBPA photoinitiator and EHA coinitiator mixture. All curing experiments were performed under a nitrogen blanket. The exposure time of all reactive mixtures under the UV lamp was kept constant. The output power of the UV lamp was

Synthesis of hyperbranched polymeric photoinitiator with built-in aliphatic tertiary amine coinitiator.

3390 | J. Mater. Chem., 2007, 17, 3389–3392


Table 1 Compositions of all multifunctional hyperbranched polymeric photoinitiators with built-in amine coinitiators Average functionality of the functional moieties in PPICb Polymera

BP

DMB

PP

MEEA

Xc

Mn (61023)

General formula

PPIC-1 PPIC-2 PPIC-3 PPIC-4 PPIC-5 PPIC-6

4.8 11.4 4.3 8.2 22.7 49.0

4.8 9.3 0 0 0 0

0 0 4.6 8.9 24.8 56.7

7.4 12.3 8.1 15.9 35.5 73.3

17 33 17 33 83 179

4.3 8.4 4.2 7.9 20.5 44.4

PG17(BP)4.8(DMB)4.8(MEEA)7.4 PG33(BP)11.4(DMB)9.3(MEEA)12.3 PG17(BP)4.3(PP)4.6 (MEEA)8.1 PG33(BP)8.2(PP)8.9(MEEA)15.9 PG83(BP)22.7(PP)24.8(MEEA)35.5 PG179(BP)49(PP)56.7(MEEA)73.3

a PPIC: polymeric photoinitiators with built-in amine coinitiators. b BP represents the benzophenone moiety, DMB represents the 4-dimethylaminobenzoate moiety, PP represents 1-piperidinepropionate, MEEA represents the 2-[2-(2-methoxyethoxy)ethoxy] acetate moiety. c X gives the total functionality of the PPIC.

Table 2 Comparison of curing speed and viscosity of the radiation-curable formulations based on monomeric and polymeric photoinitiating systems No.a

Photoinitiating systemb

BP (%)

DMB (%)

PP (%)

MEEA (%)

PMOc (%)

Viscosity/mPa s

1 2 3 4 5 6 7 8

MBPA + EHA MBPA + EHA PPIC-1 PPIC-2 PPIC-3 PPIC-4 PPIC-5 PPIC-6

3.7 3.7 3.7 4.4 3.3 3.4 3.6 3.6

3.3 4.1 3.3 3.3 0 0 0 0

0 0 0 0 3.1 3.2 3.4 3.6

0 0 5.5 4.7 6.2 6.5 5.6 5.3

50 50 50 50 50 50 50 50

26.6 26.9 55.5 69.4 48.9 53.4 54.4 75.4

a Radiation-curable standard formulation with dipropylene glycol diacrylate and trimethylolpropane triacrylate as monomers. b MBPA is the low molecular photoinitiator methyl (4-benzoylphenoxy)acetate; EHA is the monomeric photo-coinitiator 2-ethylhexyl 4-dimethylaminobenzoate. c PMO: Percentage of maximum output of the UV lamp.

adjusted to assure that the formulations were fully cured within the same time. A sample was considered as fully cured when scratching with a Q-tip caused no visual damage, which is a simple standard test. The percentage of maximum output (PMO) of the UV lamp was taken as a measure for the curing speed. The lower this number was, the higher the curing speed. Comparing the PMO data listed in Table 2, it is obvious that the curing speed of the formulations derived from the PPICs (No. 3 and 4 in Table 2) was the same as those of the formulations derived from their corresponding low molecular weight analogues (No. 1 and 2 in Table 2). From Table 2 it is also obvious that the type of coinitiator moiety and the molecular weight of the respective PPICs does not affect the photoactivity. Low viscosity is preferred in nearly all radiation-curable formulations. Especially for ink-jet application, a significant increase in the solution viscosity has to be avoided to keep the ink jettable. Use of linear polymers usually enhances the viscosity of the radiation-curable formulation significantly.3 In order to keep viscosity low, oligomers with molecular weight most preferably lower than 800 g mol21 are preferred.19 In our case, the molecular weight of the PPICs used is much higher than 800 g mol21, being in the range of 4200 to 44,400 g mol21. However, compared with the formulations based on the low molecular weight photoinitiator and coinitiator, the viscosity of the formulations based on the subject PPICs did not increase dramatically, being still jettable as inkjet ink. Thus, it can be concluded that the hyperbranched polymers with complex functionalities showed remarkably low viscosity that did not increase significantly with molecular weight. This phenomenon is characteristic of compact spheroidal structures, present in both dendritic polymers20–22 and star polymers.23,24 This journal is ß The Royal Society of Chemistry 2007

Low extractability in the cured samples is preferred, since the extractable low molecular weight residues remain mobile and can deteriorate the physical properties of the packaging materials. Moreover, in food packaging printed with such radiation-curable compositions the low molecular weight residues might be extracted into the packaged food. Thus, we also measured the low molecular weight residue extractability of the PPIC samples and found that the extracted amount was lower than the detection limit of the methods used to accurately determine the extracted amounts, which means that the extracted amount of residues is clearly below 50 mg m22. This illustrates that almost no photoreactive residues can be extracted from the cured samples, in contrast to the completely extracted photoinitiator residues for the low molecular weight photoinitiator MBPA, and around 30% for the hyperbranched polymeric photoinitiator without tertiary amine coinitiator moieties.25 Thus, it is obvious that the tertiary amine coinitiator moieties improve the permanent fixation of the PPICs in the final polymer network. In conclusion, new, conveniently manufactured hyperbranched polymeric photoinitiators with built-in amine coinitiators have been developed. The obtained PPICs possessed high functionality, low viscosity, and good compatibility with the usual radiation curable formulations. Furthermore, high photoactivity and almost no extractable low molecular weight residues in the cured sample were observed. Hyperbranched polymeric photoinitiators, such as the ones studied, appear to be promising for further exploration in food packaging applications.

Notes and references 1 J. P. Fouassier, Photoinitiation, photopolymerization, and photocuring: fundamentals and applications, Hanser, Munich, 1995.

J. Mater. Chem., 2007, 17, 3389–3392 | 3391

2 C. Roffy, Photogeneration of reactive species for UV-curing, Wiley, New York, 1997. 3 T. Corrales, F. Catalina, C. Peinado and N. S. Allen, J. Photochem. Photobiol., A, 2003, 159, 103. 4 A. M. Sarker, A. Lungu and D. C. Neckers, Macromolecules, 1996, 29, 8047. 5 L. Angiolini, D. Caretti and E. Salatelli, Macromol. Chem. Phys., 2000, 201, 2646. 6 X. Jiang and J. Yin, Polymer, 2004, 45, 5057. 7 V. Castelvetro, M. Molesti and P. Rolla, Macromol. Chem. Phys., 2002, 203, 1486. 8 L. Angiolini, D. Caretti, S. Rossetti, E. Salatelli and M. Scoponi, J. Polym. Sci., Part A: Polym. Chem., 2005, 43, 5879. 9 J. Wei, H. Wang, X. Jiang and J. Yin, Macromolecules, 2007, 40, 2344. 10 X. Jiang and J. Yin, Macromol. Rapid Commun., 2004, 25, 748. 11 G. R. Newkome, C. N. Moorefield and F. Vo¨gtle, Dendritic Molecules: Concepts, Synthesis, Perspectives, VCH, Weinheim, 2001. 12 J. M. J. Fre´chet, J. Polym. Sci., Part A: Polym. Chem., 2003, 41, 3713. 13 B. Voit, J. Polym. Sci., Part A: Polym. Chem., 2005, 43, 2679.

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14 15 16 17 18 19 20 21 22 23 24 25

A. Sunder, J. Heinemann and H. Frey, Chem.–Eur. J., 2000, 6, 2499. A. Dias and J. Jansen, WO Pat., 9907746, 1999. X. Jiang and J. Yin, Macromolecules, 2004, 37, 7850. B. Pettersson, WO Pat., 02/22700, 2002. S. R. Davidson, Exploring the Science Technology and Applications of UV and EB-curing, SITA Technology Ltd, London, 1999. R. E. Burrows, R. S. Davidson and S. L. Herlihy, WO Pat., 03/033452, 2003. S.-E. Stiriba, H. Kautz and H. Frey, J. Am. Chem. Soc., 2002, 124, 9698. Y. Chen, Z. Shen, L. Pastor-Pe´rez, H. Frey and S.-E. Stiriba, Macromolecules, 2005, 38, 227. H. Liu, Y. Chen, D. Zhu, Z. Shen and S.-E. Stiriba, React. Funct. Polym., 2007, 67, 383. Y. Chen, Z. Shen, E. Barriau, H. Kautz and H. Frey, Biomacromolecules, 2006, 7, 919. H. Liu, Y. Chen, Z. Shen and H. Frey, React. Funct. Polym., 2007, 67, 156. Y. Chen, J. Loccufier, L. Vanmaele, E. Barriau and H. Frey, Macromol. Chem. Phys., 2007, DOI: 10.1002/macp.200700086.


Supplementary material (ESI) for Journal of Materials Chemistry This journal is © The Royal Society of Chemistry 2007 Journal of Materials Chemistry

ESI

Novel multifunctional hyperbranched polymeric photoinitiators with built-in amine coinitiators for UV curing Yu Chen,* a,b Johan Loccufier,c Luc Vanmaelec and Holger Frey*b

Experimental Part Materials 2-[2-(2-Methoxyethoxy)-ethoxy]

acetic

acid

(MEEAA,

tech)

and

1-piperidinepropionic acid (PPA, 99%) were purchased from Aldrich and used as received. 4-Dimethylaminobenzoic acid (98%), 1,1’-carbonyldiimidazole (CDI, 97%) and p-toluenesulfonic acid monohydrate (99%) were purchased from Acros. Dipropylene

glycol

diacrylate

(DPGDA)

was

purchased

from

UCB.

Trimethylolpropane triacrylate (TMPTA) was purchased from BASF. 2-Ethylhexyl 4-dimethylamino benzoate (EHA) was purchased from Aceto. Hyperbranched polyglycerol samples with trimethanolpropane (TMP) core: PG17 (number-average molecular weight M n =1.21 × 103, M w M n = 1.6), PG33 ( M n =2.32 × 103,

M w M n =2.6), PG83 ( M n =5.99×103, M w M n =2.6), and PG179 ( M n =1.31×104, M w M n =2.9) were prepared as reported previously.[1] The syntheses of (4-benzoylphenoxy)acetic acid (BPAA) and methyl (4-benzoylphenoxy)acetate

Supplementary material (ESI) for Journal of Materials Chemistry This journal is © The Royal Society of Chemistry 2007

(MBPA) were described elsewhere. Poly(ethylene terephthalate) (PET) substrate with an anti-blocking layer having anti-static properties on the backside was available from Agfa Graphics as P125C PLAIN/ABAS.

Abbreviation of the resulting PPIC samples The resulting PPICs were designated (X)y: t represents the average end group number of PG; X designates the functional groups attached to PG, which include benzophenone

(BP),

2-[2-(2-methoxyethoxy)-ethoxy]acetate

(MEEA),

4-dimethylaminobenzoate (DMB) and 1-piperidinepropionate (PP) groups. y represents the average functionality of X moieties of PG.

Synthesis of multifunctional hyperbranched PPICs with BP, DMB, and MEEA moieties The synthesis is exemplified for the polymer PG17(BP)4.8(DMB)4.8(MEEA)7.4: The solution of 1.67g (10.0mmol) of 4-dimethylaminobenzoic acid and 1.63 (10.0mmol) of CDI in 20ml of THF was refluxed for 3h. Then it was added to the flask containing 1.44g (1.19mmol) of PG and the mixture was refluxed overnight under vigorous stirring. The solution of 2.07g (8.0mmol) of BPAA, 1.23ml (8.0mmol) of MEEAA and 2.61g (16.0 mmol) of CDI in 20ml of THF was stirred at ambient temperature for 1h, and subsequently added to the solution containing PG partially modified with DMB. The mixture was stirred at ambient temperature overnight. Water was added to destroy the residual CDI and CDI activated acid. After removing most of the volatile


components under vacuum, the residue was dissolved in chloroform. The mixture was washed twice with 2N of HCl aq, three times with deionized water, twice with 10% of NaOH aq and several times with NaCl aq until pH=7. After removing the volatile components under vacuum, the residual water was removed by forming an azeotrope with toluene. After filtration, most of the toluene was removed and then the residue was kept at 40ºC in vacuum oven overnight. Yield=50%. 1H NMR (CDCl3): δ = 0.77, 1.32 (TMP core of PG); 2.99 ((CH3)2N-); 3.07-5.52 (protons of PG and MEEA moieties, -OCH2COO-); 6.56, 6.92, 7.34-8.02 (protons of aromatic ring of BP and DMB moieties).

Synthesis of multifunctional hyperbranched PPICs with BP, PP and MEEA moieties The synthesis is exemplified for the polymer, PG17(BP)4.3(PP)4.6(MEEA)8.1: 2.05g (1.69mmol) of PG17, 2.21g (8.57mmol) of BPAA, 1.35g (8.57mmol) of PPA, 1.77ml (11.4mmol) of MEEAA and 2.18g (11.4mmol) of p-toluenesulfonic acid monohydrate were added into a 100ml one-neck flask equipped with Dean-Stark and condenser. Then 40ml of toluene was added. The mixture was heated to 136oC and stirred for around 2h. Then 0.88ml (5.7mmol) of MEEAA was introduced. The mixture was stirred under the same conditions for an additional 4h. After removing most of the volatile components under vacuum, the residue was dissolved in chloroform. The mixture was washed twice with 10% of NaOH aq and several times with NaCl aq until pH=7. After removing the volatile components under vacuum, the residual water was removed by forming an azeotrope with toluene. After filtration, most of the


toluene was removed and then the residue was kept at 40ºC in vacuum oven overnight. Yield=86%. 1H NMR (CDCl3): δ = 0.77, 1.32 (TMP core of PG); 1.14-1.64 (β and γ CH2 in piperidine ring); 1.98 (CH3COO-); 2.13-2.66 (α CH2 in piperidine ring,

-NCH2CH2COO-);

2.98-5.42

(protons

of PG

and

MEEA

moieties,

-OCH2COO-); 6.62-8.05 (protons of aromatic ring of BP moieties).

Characterization 1

H and 13C NMR spectra were recorded on a Bruker ARX 300 spectrometer, operated

at 300 MHz and 75.4 MHz, respectively. FTIR spectra were recorded on a Nicolet 5DXC ATR-FTIR spectrometer. The viscosity of the radiation curable composition was measured with a Brookfield DV-II+ viscometer at 25 oC and shear rate 3RPM. Alltime C18 µm HPLC column (ALLtech, 150mm×3.2mm) was used to analyze the extractant. Fusion DRSE-120 conveyer equipped with a Fusion VPS/1600 lamp (D-bulb) was used for the UV curable process.

The process for radiation curing The UV curable compositions were coated on an unsubbed 100µm PET substrate using a bar coater and a 10µm wired bar. Each coated layer was cured using a Fusion DRSE-120 conveyer, equipped with a Fusion VPS/1600 lamp (D-bulb), which transported the samples under the UV lamp on a conveyer belt at a speed of 20m/min. A sample was considered as fully cured at the moment scratching with a Q-tip caused


no visual damage. The percentage of the maximum output of the lamp was taken as a measure for the curing speed. The lower the number was, the higher the curing speed.

Method of extraction for photoinitiators and coinitiators The cured sample (24x31 cm) and the PET overlay were extracted with THF by agitating the sample in a dipping tank filled with THF for 15 minutes. The samples were rinsed with THF and the combined THF-solutions were concentrated to 9 ml. The final volume of the THF-solutions was adjusted to 10 ml. 100µl of this solution was filtered over a 0.45µm PVDF-filter and injected on a 3x mixed B column set and eluted with THF/acetic acid 95/5. A refractive index detector was used. The concentration of extracted photoreactive polymers was estimated using standard solutions of the reference polymers.


Figure S1. Typical 1H NMR spectrum of multifunctional hyperbranched polymeric photoinitiators with built-in aromatic tertiary amine coinitiators [PG33(BP)11.4(DMB)9.3 (MEEA)12.3]


Figure S2. Typical 1H NMR spectrum of multifunctional hyperbranched polymeric photoinitiators with built-in aliphatic tertiary amine coinitiators [PG33(BP)8.2(PP)8.9 (MEEA)15.9] Table S1. Radiation-curable formulations based on monomeric and polymeric photoinitiating system Composition of radiation-curable formulation (wt%) a) No.

DPGDA

TMPTA

EHA

MBPA

PPIC-1

PPIC-2

PPIC-3

PPIC-4

PPIC-5

PPIC-6

DBP

1

47.0

40.0

5.5

5.5

—

—

—

—

—

—

2.0

2

45.5

40.0

7.0

5.5

—

—

—

—

—

—

2.0

3

40.0

40.0

—

—

18.0

—

—

—

—

—

2.0

4

40.0

40.0

—

—

—

18.0

—

—

—

—

2.0

5

40.0

40.0

—

—

—

—

18.0

—

—

—

2.0

6

40.0

40.0

—

—

—

—

—

18.0

—

—

2.0

7

40.0

40.0

—

—

—

—

—

—

18.0

—

2.0

8

40.0

40.0

—

—

—

—

—

—

—

18.0

2.0

Supplementary material (ESI) for Journal of Materials Chemistry This journal is © The Royal Society of Chemistry 2007 a)

DPGDA: dipropylene glycol diacrylate; TMPTA: trimethylolpropane triacrylate; EHA: 2-ethylhexyl 4-dimethylaminobenzoate; MBPA is the monomeric photoinitiator, methyl

(4-benzoylphenoxy)acetate; PPICs are polymeric photoinitiators shown in Table 1; DBP:

Dibutyl phthalate

[1] A. Sunder, R. Hanselmann, H. Frey, R. Mülhaupt, Macromolecules 1999, 32, 4240. [2] M. Aydin, N. Arsu, Y. Yagci, Macromol. Rapid Commun. 2003, 24, 718.