Novel multifunctional hyperbranched polymeric photoinitiators with built-in amine coinitiators for UV curing

Article abstract of 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. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b708986d

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
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
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 trimethy-
lolpropane 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.
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 photocur-
able 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 func-
tionality 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.
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.
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
The resulting PPICs have good solubility in the usual radiation-
curable 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
aDepartment of Chemistry, School of Sciences, Tianjin University,
300072, Tianjin, People’s Republic of China. E-mail: chenyu@tju.edu.cn
bInstitut fu¨r Organische Chemie, Johannes Gutenberg-Universita¨t,
Duesbergweg 10-14, 55099, Mainz, Germany
cAgfa 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
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 3389–3392 | 3389

View Article Online
Scheme 1 Synthesis of hyperbranched polymeric photoinitiator with built-in aromatic tertiary amine coinitiator.
photoinitiator moiety and the photo-coinitiator moiety in all low molecular weight MBPA photoinitiator and EHA coinitiator
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
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
Scheme 2 Synthesis of hyperbranched polymeric photoinitiator with built-in aliphatic tertiary amine coinitiator.
3390 | J. Mater. Chem., 2007, 17, 3389–3392
This journal is ß The Royal Society of Chemistry 2007

View Article Online
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
17
33
17
33
83
179
Mn (61023
)
General formula
PPIC-1
PPIC-2
PPIC-3
PPIC-4
PPIC-5
PPIC-6
a
4.8
11.4
4.3
8.2
22.7
49.0
4.8
9.3
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
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
0
b
PPIC: polymeric photoinitiators with built-in amine coinitiators. 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.
X gives the total functionality of the PPIC.
c
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
a
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
50
50
50
50
50
50
50
50
26.6
26.9
55.5
69.4
48.9
53.4
54.4
75.4
b
5.5
4.7
6.2
6.5
5.6
5.3
Radiation-curable standard formulation with dipropylene glycol diacrylate and trimethylolpropane triacrylate as monomers. MBPA is the
low molecular photoinitiator methyl (4-benzoylphenoxy)acetate; EHA is the monomeric photo-coinitiator 2-ethylhexyl
4-dimethylaminobenzoate. PMO: Percentage of maximum output of the UV lamp.
c
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 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 hyper-
branched polymeric photoinitiator without tertiary amine coin-
itiator moieties.25 Thus, it is obvious that the tertiary amine
coinitiator moieties improve the permanent fixation of the PPICs
in the final polymer network.
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
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.
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
Notes and references
1 J. P. Fouassier, Photoinitiation, photopolymerization, and photocuring:
fundamentals and applications, Hanser, Munich, 1995.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 3389–3392 | 3391

View Article Online
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.
14 A. Sunder, J. Heinemann and H. Frey, Chem.–Eur. J., 2000, 6, 2499.
15 A. Dias and J. Jansen, WO Pat., 9907746, 1999.
16 X. Jiang and J. Yin, Macromolecules, 2004, 37, 7850.
17 B. Pettersson, WO Pat., 02/22700, 2002.
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.
18 S. R. Davidson, Exploring the Science Technology and Applications of
UV and EB-curing, SITA Technology Ltd, London, 1999.
19 R. E. Burrows, R. S. Davidson and S. L. Herlihy, WO Pat., 03/033452,
2003.
20 S.-E. Stiriba, H. Kautz and H. Frey, J. Am. Chem. Soc., 2002, 124, 9698.
21 Y. Chen, Z. Shen, L. Pastor-Pe´rez, H. Frey and S.-E. Stiriba,
Macromolecules, 2005, 38, 227.
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.
22 H. Liu, Y. Chen, D. Zhu, Z. Shen and S.-E. Stiriba, React. Funct.
Polym., 2007, 67, 383.
23 Y. Chen, Z. Shen, E. Barriau, H. Kautz and H. Frey,
Biomacromolecules, 2006, 7, 919.
24 H. Liu, Y. Chen, Z. Shen and H. Frey, React. Funct. Polym., 2007, 67,
156.
25 Y. Chen, J. Loccufier, L. Vanmaele, E. Barriau and H. Frey, Macromol.
Chem. Phys., 2007, DOI: 10.1002/macp.200700086.
3392 | J. Mater. Chem., 2007, 17, 3389–3392
This journal is ß The Royal Society of Chemistry 2007