Multicyclic Carbonates as Environmentally Benign
Starting Materials for Methacrylate-Urethane Monomers
of Low Oxygen Inhibition and Polymerization Shrinkage
G. Rokicki, M. Biernat, K. Słowik and P. G. Parzuchowski,
Warsaw University of Technology, Faculty of Chemistry,
Noakowskiego 3, 00-664 Warsaw, Poland
Email:[email protected]
Introduction. UV-cured materials technology exhibits several
advantages, such as solvent free formulations, fast curing times,
low-temperature processing, and energy efficiency. However, the
main disadvantage of methacrylate monomer photopolymerization is
susceptibility to oxygen inhibition. The upper layer of the coatings
remains uncured and tacky [1].
Recently, a new class of monomethacrylates have been developed
which exhibit significant enhanced photopolymerization kinetics.
These monomers contain urethane, “inverted urethane”, cyclic
carbonate functionalities, sometimes in conjugation with phenyl rings
[2]. An understanding of the exact causes to the enhanced reactivity of
the novel methacrylates has not been established, though multiple
theories have been examined. Various factors have been proposed to
impact the monomer reactivity including molecular dipole interactions,
hydrogen bonding, hydrogen abstraction or autoacceleration effects
generated due to cross-linking. It has been proposed that the enhanced
reactivity occurs due to a very efficient and rapid chain-transfer
process which generates active centers more prone to propagation than
termination [3,4].
In this work we present our recent results concerning synthesis
and photopolymerization of multimethacrylic monomers containing
urethane linkages, which were obtained via a non-isocyanate route
from six- and five-membered cyclic carbonates (1, 2 and 3) using
different aminoalcohols, and methacryloyl chloride as starting
materials. These multimethacrylic monomers exhibit good
polymerization reactivity and a low polymerization shrinkage as well
as low oxygen inhibition.
Experimental. Materials. 2,2’-Oxybis(methylene)bis(2-ethyl1,3-propane-1,3-diol) [di(trimetylolpropane)], 2,3-epoxypropane-1-ol
(glycidol) and dimethyl carbonate were purchased from Aldrich.
Diglycidyl ether of bisphenol A (DGBA) was isolated from epoxy
resin Epidian 6 (Z.Ch. “Sarzyna” S.A., Poland) by distillation under
reduced pressure. Other reagents (Aldrich) were used without
additional purification.
Syntheses of methacrylate monomers with urethane groups.
Monomer 1b. To the solution of di(trimethylolpropane) (22.5 g, 0.09
mol) in 500 mL of THF cooled to below 0 °C 40,2 g (0,36 mol) of
ethyl chloroformate was added dropwise maintaining reaction
temperature at –3 to –10 °C. Then, triethylamine (36.4 g, 0.36 mol)
was added under the same conditions. Reaction was carried out for 30
min at 0 °C and 20 min at r.t. The reaction product was recrystallized
from THF leading to white crystals of six-membered biscyclic
carbonate 1 (18 g, 66%). The mixture of biscyclic carbonate 1 (2.0 g,
6.6 mmol) and ethanolamine (1.43 g, 23.2 mmol) in 25 mL of THF,
was stirred at room temperature for 72 h up to disappearance of the
absorption band corresponding to the carbonyl group of cyclic
carbonate. The liquid viscous product (tetrahydroxyurethane
derivative) was reacted with methacryloyl chloride according to
procedure reported in [5]. The viscous liquid (0.6 Pa⋅s was obtained
(yield 60%).
Monomer 2b. Tetramethacrylate diurethane monomer 2b based on
2,2-bis[4-(2,3-dihydroxypropoxy)phenyl]propane dicarbonate was
prepared according to the method reported by us in [5].
Hyperbranched methacrylate 3b. Hyperbranched polymer (HBP)
terminated with five-membered cyclic carbonate groups was obtained
according to the method reported by us in [6]. The procedure of
obtaining methacrylate-urethane derivative was analogous to the
above described.
Photopolymerization. The layers (ca. 30 μm) of the monomers
containing
3%
of
camphorquinone/2-(dimethylamino)ethyl
methacrylate as a photoinitiator system were irradiated using a lamp
Megalux (75 W) in an air atmosphere.
Results and Discussion. Multifunctional methacrylic resins
containing urethane linkages were obtained from different six- and
five-membered biscyclic carbonates and hyperbranched polyglycerol
containing five-membered cyclic carbonate as terminal groups.
Six-membered biscyclic carbonate 1 (Scheme 1) was obtained from
commercially
available
di(trimethylolpropane)
and
ethyl
chloroformate. Five-membered biscyclic carbonate - 2,2-bis[4-(2,3-dihydroxypropoxy)phenyl]propane dicarbonate (2), was obtained from
epoxy resin based on bisphenol A and CO2. Such crystalline products
can be easily purified by recrystallization. For obtaining
hyperbranched polymer with cyclic carbonate groups glycidol or
glycerol carbonate were used as starting materials (Scheme 3) [7].
Six-membered biscyclic carbonate 1 was first converted to a
diurethane tetrol 1a by reaction with ethanoloamine. The urethane
tetrol product was reacted with methacryloyl chloride to give the
corresponding tetramethacrylate diurethane 1b (Scheme 1).
O
O
OH
OH
OH
O
Et3N
+ 4 Cl
O
O
OH
O
O
O
O
O
OH
1
2 NH2CH2CH2OH
HN
OH
O
O
O
O
O
O
NH
HO
OH
4
O
Cl
O
1a
Et3N
HN
O
O
O
O
O
O
O
NH
O
O
1b
O
O
Scheme 1. Synthesis of tetramethacrylate diurethane monomer 1b.
In the tetramethacrylate diurethane (1b) based on trimethylolpropane dimer there are two types of methacrylic groups, one
connected with the core via an urethane linkage and the second one
via an oxygen atom. Additionally, between methacrylic groups there
is longer spacer in comparison to that in multimethacrylic resin based
on five-membered carbonates. The characterization of the synthesized
monomers was carried out using 1H , 13C NMR and IR spectroscopies.
The spectral data are in agreement with the expected structures.
Figure 1. 1H NMR spectrum (400 MHz DMSO-d6) of 1b.
In a similar manner five-membered biscyclic carbonate 2 was
transformed into tetramethacrylate diurethane 2b (Scheme 2). In
contrast to monomer 1b, in the reaction of 2 with ethanolamine three
regioisomers are formed. From the comparison of the signals intensity
in the 1H NMR spectrum it has been found that the isomer with
primary hydroxyl group is formed with 50% higher yield than that
with secondary OH group.
Hyperbranched
multimethacrylates
were
prepared
by
polymerization of glycidol using partially ionized trimethylolpropane
as initiator (molar ratio 10:1) [8]. The selective introduction of cyclic
carbonate groups was achieved by the reaction terminal vicinal OH
groups with dimethyl carbonate carried out according to the method
reported earlier by us (Scheme 3) [6].
In contrast to bis-GMA, the lacquer prepared from 1b and 2b after
photocuring in an air atmosphere was not tacky and indicated
relatively high hardness (König hardness > 0.5).
CH3
O
O
C
O
O
O
2
O
+ 2 H2N CH2CH2 OH
O
CH3
O
HO
O
O
HO
O
O
N
H
CH3
C
CH3
OH
OH
N
H
O
O
0.25
3b
2a
+ 2 isomers
O CH3
Cl C C CH2
1b
0.2
Et3N
0.15
O
O
N
H
CH3
C
CH3
O
O
O
O
4
0.1
O
O
O
2b
p
O
O
N
H
O
O
0.05
O
2b
+ 2 isomers
0
Scheme 2. Synthesis of tetramethacrylate diurethane monomer
containing bisphenol A core 2b.
HO
K
O
HO
OH
OH
HO
OH
OH
OH
O
OH
THF
O
+
H3C
95-100
O
oC
O
O
H3C
OH
O
O
HBPG
HO
O
O
K2CO3
O
O
O
O
O
O
O
O
O
O
O
H3C
O
O
O
OH
HO
O
O
O
O
H3C
ZnCl2
OH
O
O
O
O
O
+
O
O
O
OH
O
O
O
O
O
O
O
3a
O
O
O
O
O
O
O
O
O
O
HO
HO
O
O
O
HO
1. NH2CH2CH2OH
O
O
2.
150
200
2b
120
O
O
100
t [s]
O
HO
HO
50
Figure 2. Conversion versus time plots for different
multimethacrylate-urethanes: 1b, 2b, 3b and a typical dental resin:
bis-GMA-TEGDMA (7:3) (4), cured in an air atmosphere.
3
Cl
Bending strength [MPa]
+
0
M10
100
1b
80
M3
M1
3b
HBPG2b
Bis-GMA/
TEGDMA
60
40
20
0
Figure 3. Bending strength of cured multimethacrylates: 1b, 2b, 3b
and commercial dental methacrylic resins: bis-GMA and M1.
O
O
NH
O
O
O
O
NH
O
O
O
O
O
O
O
O
O
O
O
O
O
O
H3C
O
O
O
O
O
O
O
O
O
O
It was found that multimethacrylates with urethane linkages
exhibited lower polymerization volume shrinkage than that obtained
for bis-GMA - TEGDMA compositions.
Table 1. Volume polymerization shrinkages of multifunctional
methacrylate-urethane monomers.
O
O
3b
Monomer
O
Scheme 3. Synthesis of hyperbranched multimethacrylate 3b.
To reduce hydrophilic properties of HBPG the remaining hydroxyl
groups in the inner sphere were blocked in the reaction with acetic
anhydride. The viscosity of this branched product (2.5 Pa⋅s, 25 °C)
was much lower than that of bis-GMA (500 Pa⋅s, 25 °C)
Photopolymerization of the multimethacrylate monomers and
theirs
compositions
with
typical
dental
components
(bis-GMA-TEGDMA) was investigated with respect to oxygen
inhibition. Kinetic investigations of photopolymerization were carried
out by measuring of double bonds conversion using FTIR
spectroscopy as well as confocal Raman microscopy.
It was found that the conversion of multifunctional monomers
containing urethane linkages is greater than that of composition with
bis-GMA for the same irradiation time (Fig. 2). The presence of four
methacrylic groups in a monomer molecule leads to fast increase in
viscosity which decrease the oxygen solubility and suppresses
inhibition in an upper layer of the resin. Additionally, formation of
hydrogen bonds between urethane groups leads to faster crosslinking
of the resin.
Fig. 2 shows the effect of monomer structure on the conversion.
The highest conversion was observed for hyperbranched methacrylate
3b, followed in order by tetramethacrylate based on
di(trimethylolpropane) 1b and bisphenol A 2b. Despite of lower
conversion of double bonds the cured 2b exhibited the highest
bending strength, 100 MPa (Fig.3) and volumetric polymerization
shrinkage (Table 1).
2b
3b
Bis-GMA - TEGDMA
Volumetric shrinkage,
(%)
3.9 ± 0.2
5.0 ± 0.2
10.4 ± 0.4
Developed method of synthesis of multimethacrylates containing
urethane linkages via cyclic carbonates is safer and more
environmentally friendly than those involving isocyanates and
phosgene. Such monomers after UV curing in air are characterized by
higher conversions and reaction rates, lower volume shrinkage and
good mechanical properties.
Acknowledgements. This paper is based upon work supported by
the Polish Ministry of Science and Higher Education (3 T09B 072 29;
2005-2008).
References:
[1] Andrzejewska, E. Prog. Polym. Sci. 2001, 26, 605.
[2] Jansen, J. F. G.; Dias, A.A.; Dorschu, M.; Coussens, B.
Macromolecules 2003, 36, 3861.,
[3] Podszun, W.; Schäpers, K.; Finger, W.; Heiliger, L.; Cesser, C. US
Pat. 5 621 119, 1997.
[4] Jansen, J. F. G. A.; Dias, A. A., Dorschu, M.; Coussens, B.
Macromolecules 2002, 35, 7529.
[5] Biernat, M.; Rokicki, G. e-Polymers 2005, P32.
[6] Parzuchowski, P. G.; Kiźlińska, M.; Rokicki, G.; Polymer 2007,
48, 1857.
[7] Rokicki, G.; Rakoczy, P.; Parzuchowski, P. G.; Sobiecki, M.
Green Chem. 2005, 7, 529.
[8] Sunder, A.; Hanselmann, R. Frey, H. Mülhaupt, R.
Macromolecules, 1999, 32, 4240.
[9] Ge, M.; Trujillo, J.; Stansbury, J. Dental Materials 2005, 21, 1163.