Development of reactive methacrylates based on glycidyl methacrylate

Article abstract of DOI:10.1002/pola.24163

Six methacrylate monomers have been synthesized for use as reactive diluents in dental composites and evaluated to investigate the relationship between molecular structure and monomer reactivity. Four were synthesized by reactions of glycidyl methacrylate (GMA) with various acids, 2-(2-methoxyethoxy)acetic acid (1), 2-(2-(2-methoxyethoxy)ethoxy)acetic acid (2), cyanoacetic acid (3), and benzoic acid (4); others were synthesized by reactions of GMA with diethyl hydrogen phosphate (5) or methanol (6). Monomers 1 and 2 are novel, 3 seems to be novel, 4 and 6 were synthesized via a novel method, and the synthesis of 5 was described in the literature. The monomers showed high crosslinking tendencies during thermal bulk polymerizations. The photo-, homo-, and copolymerization behavior of the monomers with 2,2-bis[4-(2-hydroxy-3-methacryloyloxy)phenyl]propane (Bis-GMA) were investigated. The maximum rate of polymerizations of monomers 2-6 was found to be greater than triethyleneglycol dimethacrylate, Bis-GMA, 2-hydroxyethyl methacrylate, and glycerol dimethacrylate. For the more reactive monomers (2, 3, and 4), the oxygen sensitivity of polymerization was found to be low due to a hydrogen abstraction/chain transfer reaction. The computationally calculated dipole moment and lowest unoccupied molecular orbital energies indicated that there seems to be a correlation between these quantities and reactivity for ester linked monomers (1-5), which was also supported by ¹³C NMR data.

Full text of DOI:10.1002/pola.24163

Development of Reactive Methacrylates Based on Glycidyl Methacrylate
OZLEM KARAHAN, KINYAS AYDIN, SEDA EDIZER, NIHAN ODABASI, DUYGU AVCI
Department of Chemistry, Bogazici University, 34342 Bebek, Istanbul, Turkey
Received 5 March 2010; accepted 28 May 2010
DOI: 10.1002/pola.24163
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Six methacrylate monomers have been synthesized
for use as reactive diluents in dental composites and evaluated
to investigate the relationship between molecular structure and
monomer reactivity. Four were synthesized by reactions of gly-
cidyl methacrylate (GMA) with various acids, 2-(2-methoxy-
ethoxy)acetic acid (1), 2-(2-(2-methoxyethoxy)ethoxy)acetic acid
(2), cyanoacetic acid (3), and benzoic acid (4); others were syn-
thesized by reactions of GMA with diethyl hydrogen phosphate
(5) or methanol (6). Monomers 1 and 2 are novel, 3 seems to
be novel, 4 and 6 were synthesized via a novel method, and
the synthesis of 5 was described in the literature. The mono-
mers showed high crosslinking tendencies during thermal bulk
polymerizations. The photo-, homo-, and copolymerization
behavior of the monomers with 2,2-bis[4-(2-hydroxy-3-metha-
cryloyloxy)phenyl]propane (Bis-GMA) were investigated. The
maximum rate of polymerizations of monomers 2–6 was found
to be greater than triethyleneglycol dimethacrylate, Bis-GMA,
2-hydroxyethyl methacrylate, and glycerol dimethacrylate. For
the more reactive monomers (2, 3, and 4), the oxygen sensitiv-
ity of polymerization was found to be low due to a hydrogen
abstraction/chain transfer reaction. The computationally calcu-
lated dipole moment and lowest unoccupied molecular orbital
energies indicated that there seems to be
a correlation
between these quantities and reactivity for ester linked mono-
13
C
V
mers (1–5), which was also supported by C NMR data. 2010
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48:
3787–3796, 2010
KEYWORDS: crosslinking; dental polymers; glycidyl methacry-
late; photopolymerization; radical polymerization
INTRODUCTION Photopolymerization processes have found a
large variety of industrial applications, mainly in dental
materials, biomaterials, coatings, and photolithography.1–3
Acrylate and methacrylate monomers are used in these pro-
cesses because of their high polymerization rates and excel-
lent final polymer properties.
Decker and Bowman formulated new monoacrylate mono-
mers with carbonate, cyclic carbonate, carbamate, and oxazo-
lidone groups that react extremely rapidly despite one vinyl
group and form crosslinked polymers. They mentioned that
crosslinking due to hydrogen abstraction reactions causes an
increase in viscosity, earlier gelation, and autoacceleration,
which lead to high rate of polymerization.4–10
2,2-Bis[4-(2-hydroxy-3-methacryloyloxy)phenyl]propane (Bis-
GMA) is the most common methacrylate used in dental com-
posites due to its high mechanical strength, low volatility,
and low polymerization shrinkage. However, it has very high
viscosity and low polymerization conversion. Although reac-
tive diluent monomers such as triethyleneglycol dimethacry-
late (TEGDMA) are added to decrease viscosity and increase
conversion, some drawbacks arise such as increased poly-
merization shrinkage, cytotoxicity, and water absorption.
Therefore, development of highly reactive novel monomers
as reactive diluents alternative to TEGDMA with lower water
absorption, polymerization shrinkage, and cytotoxicity is one
of the most important challenges in this area.
Jansen et al.11 investigated the rate of polymerization of dif-
ferent acrylates in terms of hydrogen bonding capability for
systems containing amide, urethane, and urea groups and
found that the monomers capable of forming hydrogen
bonds show three to six times higher polymerization rates
compared with their nonhydrogen bonding analogues pos-
sessing ester and carbonate groups. The high reactivities
were suggested to be due to preorganization via hydrogen
bonding to bring the double bonds close to each other,
enhancing the rate of polymerization, although reduction in
termination rate may also be involved or be the cause. They
also investigated the effect of monomer polarity on rate of
polymerization and found a direct correlation between the
maximum rate of polymerization and the dipole moment of
the monomer above 3.5 Debye. However, Kilambi et al.7
found no monotonic correlation between monomer reactivity
and molecular dipole moments during bulk polymerization
of various acrylate monomers. They suggested that a low
In recent years, several factors leading to the enhanced reac-
tivity of (meth)acrylates were hypothesized. These are
hydrogen abstraction from labile hydrogens in monomers,
hydrogen bonding, and electronic effects (dipole moment
and secondary functionalities).
Correspondence to: D. Avci (E-mail: avcid@boun.edu.tr)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 3787–3796 (2010) 2010 Wiley Periodicals, Inc.
DEVELOPMENT OF REACTIVE METHACRYLATES, KARAHAN ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
were carried out using Gemini 150 Rheometer system (Boh-
lin Instruments).
Synthesis of Monomers
Monomer 1
GMA (1.01 g, 7.1 mmol), 2-(2-methoxyethoxy)acetic acid
(0.99 g, 7.40 mmol), BHT (2.6 mg, 0.012 mmol), and triethyl-
amine (TEA) (102.2 mg, 1.0 mmol) were added to a round-
bottom flask with a waterꢀcondenser and nitrogen inlet. The
mixture was stirred at 60 C for 18 h. The purification of the
crude product by column chromatography (silica gel 0.063–
0.200 mm) using dichloromethane initially and gradually
changing to ethyl acetate (EAc) as elutant resulted in a light
yellow oil in 46.6% yield.
FIGURE 1 Structures of phosphonated methacrylates synthe-
sized from GMA in our previous works.
dipole moment conformation of a monomer may be more re-
active due to intermolecular hydrogen bonding than a con-
formation with a higher dipole moment.
1H NMR (400 MHz, CDCl3, d, ppm): 1.87 (s, 3H, CH3AC),
3.31 (s, 3H, CH3AO), 3.51 (t, 2H, CH2AO), 3.63 (t, 2H,
CH2AO), 4.04–4.30 (m, 7H, CH2ACH, CH2AC¼¼O), 5.53, 6.06
(s, 2H, C¼¼CH2). 13C NMR (400 MHz, CDCl3, d, ppm): 18.14
(CH3AC), 58.89 (CH3AO), 64.97, 65.56 (CH2AO), 67.64
(CHAOH), 68.38, 70.72 (CH2AO), 71.82 (CH2AOACH3),
125.89 (CH2¼¼C), 135.65 (C¼¼CH2), 167.13 (CH2¼¼CAC¼¼O),
170.33 (C¼¼O). FTIR (cmꢁ1): 3430 (OAH), 2888 (CAH),
1754 (C¼¼O), 1716 (C¼¼O), 1635 (C¼¼C).
Monomer 2 was obtained with the same procedure using 2-
(2-(2-methoxyethoxy)ethoxy)acetic acid. The purification of
the crude product by column chromatography (silica gel
0.063–0.200 mm) using hexane initially and gradually chang-
ing to ethyl acetate as elutant resulted in a light yellow oil in
49.7% yield. Characterization data are given next.
It was observed that the presence of secondary functional-
ities (carbamates, carbonates, cyclic carbonates, cyclic ace-
tals, morpholine, oxazolidones, hydroxyl, and aromatic rings)
enhances reactivity by reducing activation energies in both
Michael addition and photopolymerizations indicated by a
monotonic correlation between them. The cyclic voltammetry
experiments also proved a correlation between reduction
potential of the monomers and Michael addition and photo-
polymerization reactivities.5
In our previous works, we synthesized phosphonated metha-
crylates based on glycidyl methacrylate (GMA) for dental
applications (Fig. 1). These monomers showed high polymer-
ization rates and crosslinking tendencies.12,13 Unexpectedly,
monomers A and C that are monomethacrylates were found
to be more reactive than dimethacrylate monomers such as
glycerol dimethacrylate (GDMA) and Bis-GMA.
Monomer 2
1H NMR (400 MHz, CDCl3, d, ppm): 1.92 (s, 3H, CH3AC),
3.35 (s, 3H, CH3AO), 3.53–3.80 (m, 8H, CH2AO), 4.14–4.60
(m, 7H, CH2ACH, CH2AC¼¼O), 5.60, 6.13 (s, 2H, CH2¼¼C). 13C
NMR (400 MHz, CDCl3,d, ppm): 18.18 (CH3AC), 58.93
(CH3AO), 64.98, 65.6 (CH2AO), 67.58 (CHAOH), 68.39, 69.4,
70.72 (CH2AO), 71.83 (CH2AOACH3), 126.25 (CH2¼¼C),
135.65 (C¼¼CH2), 167.15 (CH2¼¼CAC¼¼O) , 170.34 (C¼¼O).
FTIR (cmꢁ1): 3432 (OAH), 2883 (CAH), 1753 (C¼¼O), 1716
(C¼¼O), 1635 (C¼¼C).
The factors for enhancement of monomer reactivity are not
very clear yet, nor is their relative importance. To examine
the relationship between monomer structure and reactivity,
here, a series of monomers based on GMA and whose struc-
tures were changed in the third functionality were designed,
synthesized, characterized, and evaluated for their rate of
polymerization.
Monomer 3
EXPERIMENTAL
GMA (0.99 g, 6.99 mmol), cyano acetic acid (0.63 g, 7.41
mmol), BHT (2.9 mg, 0.013 mmol), and TEA (99.1 mg 0.98
mmol) were added to a round-bottom flask with a water
condenser and nitrogen inlet and stirred at 40 ꢀC for 5 h.
The residue was diluted with CH2Cl2 and extracted with
NaHCO3 (5%) solution. After the drying of the organic phase
with anhydrous Na2SO4 and evaporation of the solvent, the
crude product was purified by column chromatography
(silica gel 0.063–0.200 mm) using hexane initially and chang-
ing to ethyl acetate as elutant. The pure product was
obtained as colorless oil in 24.3% yield.
Materials
GMA, 2-(2-methoxyethoxy)acetic acid, 2-(2-(2-methoxyeth-
oxy)ethoxy)acetic acid, diethylchlorophosphate, 2,6-di-tert-
butyl-4-methylphenol (BHT), cyanoacetic acid, benzoic acid,
hexyl acrylate (HA), 2-hydroxyethyl methacrylate (HEMA),
Bis-GMA, TEGDMA, GDMA, and 2,2-dimethoxy-2-phenylaceto-
phenone (DMPA) were used as received from Aldrich.
Measurements
The monomer characterization involved 1H and 13C NMR
spectroscopy (Varian Gemini 400 MHz) and Fourier-trans-
form infrared (FTIR) spectroscopy (T 380). Photopolymeriza-
tions were carried out on a TA Instruments Q100 differential
photocalorimeter (DPC). Thermogravimetric analysis was
done with a TA Instrument (Q50). Viscosity measurements
1H NMR (400 MHz, CDCl3, d, ppm): 1.93 (s, 3H, CH3AC),
3.55 (s, 2H, CH2ACN), 4.15–4.6 (m, 5H, CH2ACH, CH), 5.61,
6.12 (s, 2H, CH2¼¼C). 13C NMR (400 MHz, CDCl3, d, ppm):
18.14 (CH3AC), 24.59 (CH2ACN), 64.89, 67.09 (CH2AO),
69.42 (CHAOH), 113.06 (CNACH2), 126.54 (CH2¼¼C), 135.54
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(C¼¼CH2), 163.05 (CH2¼¼CAC¼¼O), 167.27 (C¼¼O). FTIR
(cmꢁ1): 3494 (OAH), 2963 (CAH), 1750 (C¼¼O), 1712
(C¼¼O), 1635 (C¼¼C).
Monomer 4 was obtained with the same procedure using
benzoic acid. The purification of the crude product by col-
umn chromatography (silica gel 0.063–0.200 mm) using hex-
ane initially and gradually changing to ethyl acetate as elu-
CHAOH, CH2AO), 5.59, 6,13 (s, 2H, CH2¼¼C). 13C NMR (400
MHz, CDCl3, d, ppm): d¼¼18.25 (CH3AC), 59.20 (CH3AO),
65.69, 68.73 (CH2AO), 73.43 (CHAOH), 126.01 (CH2¼¼C),
135.92 (C¼¼CH2), 167.40 (C¼¼O), 170.34 (C¼¼O). FTIR
(cmꢁ1): 3434 (OAH), 2929 (CAH), 1715 (C¼¼O), 1635
(C¼¼C).
Polymerization Procedure
tant resulted in
a
colorless oil in 24.3% yield.
Approximately 3.0 or 4.0 mg of sample was placed in an alu-
minum differential scanning calorimeter (DSC) pan. The pho-
toinitiator (DMPA), which was dissolved in CH2CI2 was
added with a microsyringe to give a final concentration in
the monomer of 2.0 mol percent after evaporation of the sol-
vent. The sample and the reference pans were placed in the
DSC chamber, the system was purged with nitrogen flow to
remove air and CH2Cl2 for 10 min before polymerization and
purging was continued during polymerization. Heats of pho-
toreactions were measured using a DPC equipped with a
mercury arc lamp. The samples were irradiated for 10 min
Characterization data are given next.
Monomer 4
1H NMR (400 MHz, CDCl3, d, ppm): 1.96 (s, 3H, CH3), 4.20–
4.50 (m, 5H, CH2ACH), 5.62, 6.16 (s, 2H, CH2¼¼C), 7.40–7.50
(m, 2H, Ar-CH), 7.50–7.65 (m, 1H, Ar-CH), 8.00–8.15 (m, 2H,
Ar-CH). 13C NMR (400 MHz, CDCl3,d, ppm): 17.97 (CH3AC),
65.25, 65.47 (CH2AOH), 67.81 (CHAOH), 126.17 (CH2¼¼C),
128.17, 129.45, 133.01 (Ar-CH), 135.52 (C¼¼CH2), 166.41
(CH2¼¼CAC¼¼O), 167.21 (C¼¼O). FTIR (cmꢁ1): 3432 (OAH),
2958 (CAH), 1714 (C¼¼O), 1636 (C¼¼C).
at 40 C with an incident light intensity of 20 mW cmꢁ2. The
ꢀ
heat flux as a function of reaction time was monitored using
DSC under isothermal conditions and both the rate of poly-
merization (Rp) and conversion were calculated as a function
of time. The theoretical values used for the heats of reaction
(DHp) were 13.1 and 20.6 kcal molꢁ1 and for methacrylate
and acrylate double bonds.14,15 Rates of polymerization were
calculated according to the following formula:
Monomer 5
Diethylchlorophosphate (5.00 g, 28.9 mmol) and water (3.10
g, 172.2 mmol) were added to a round-bottom flask, and the
mixture was stirred at room temperature for 1 h. The solu-
tion was extracted with dichloromethane. After the drying of
the organic phase and evaporation of the solvent, diethyl
hydrogen phosphate was obtained.
ðQsꢁ1ÞM
The formed product, diethyl hydrogen phosphate (1.75 g,
11.36 mmol) and GMA (1.34 g, 9.43 mmol) were added to a
round-bottom flask with a nitrogen inlet. The mixture was
Rate ¼
;
nDHpm
ꢀ
where Q sꢁ1 is heat flow per second, M the molar mass of
the monomer, n the number of double bonds per monomer
molecule, DHp is the heat released per mole of double bonds
reacted, and m the mass of monomer in the sample.
stirred at 50 C for 5 h. The crude product was washed with
water, cyclohexane, and petroleum ether and then further
purified by column chromatography (silica gel 0.063–0.200
mm) using CH2Cl2 initially and gradually changing to 1%
methanol in CH2Cl2 as elutant. The pure product was
obtained as a viscous yellow oil in a 31.5% yield.
Computational Simulation
All monomers were fully optimized by density functional
theory (DFT) using Gaussian 03 program at the B3LYP/6-
31þG(d) to obtain some quantum chemical descriptor to
find experimental and computational correlation.16 Four
descriptors were calculated, which are dipole moment, the
energy of the lowest unoccupied molecular orbital (ELUMO),
natural bond orbital (NBO) charges of the carbon alpha to
the carbonyl group, and terminal vinylic carbon.
1H NMR (400 MHz, CDCl3, d, ppm): 1.34 (s, 3H, CH3ACH2),
1.94 (t, 3H, CH3AC), 2.6 (bs, 1H, OH), 4.07–4.24 (m, 9H,
CH2ACH), 5.59, 6.13 (s, 2H, CH2¼¼C). 13C NMR (400 MHz,
CDCl3, d, ppm): 16.35 (CH3ACH2), 18.59 (CH3AC), 64.64,
68.92 (CH2AO), 64.97 (CH2ACH3), 69.09 (CHAOH), 126.60
(CH2¼¼C), 136.06 (C¼¼CH2), 167.55 (C¼¼O). FTIR (cmꢁ1):
3379 (OAH), 2983 (CAH), 1717 (C¼¼O), 1637 (C¼¼C), 1249
(P¼¼O), 1017 cmꢁ1 (PAOAEt).
Calculation of Dipole Moments
Monomer 6
Boltzman-averaged dipole moments (lcalc) were calculated using
the following method. First, all free rotations around single
bonds were considered for a given monomer. Spartan ’04 pro-
gram was used to calculate the Boltzmann-averaged dipole
moments.17 The number of conformations generated is depend-
ent on both the number of bonds and their types. All these con-
formations were minimized at the PM3 level of theory. The con-
vergence criterion for the maximum gradient was 0.0001 a.u.,
and the maximum number of geometry optimization cycles was
taken to be 20 þ the number of independent geometrical param-
eters for geometry optimization. The unique structures were
sorted in the order of increasing energy. The dipole moments of
GMA (0.84 g, 5.92 mmol), 30 mL methanol, and Amberlyst-
15 (600.4 mg) were added to a round-bottom flask and
placed in an ultrasonic bath. After 2 h at 22–30 C, the cata-
lyst was filtered, methanol was evaporated, and the residue
was washed with hexane to remove unreacted GMA. The
pure product was obtained as a colorless liquid after column
chromatography (silica gel 0.063–0.200 mm), starting CH2Cl2
elutant and changing to ethyl acetate: CH2Cl2 (20:80)
gradually.
ꢀ
1H NMR (400 MHz, CDCl3, d, ppm): 1.95 (s, 3H, CH3AC),
3.4–3.6 (m, 5H, CH3AO, CH2AOACH3), 4.00–4.40 (m, 3H,
DEVELOPMENT OF REACTIVE METHACRYLATES, KARAHAN ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 2 Synthesis of monomers.
the first 100 conformers are Boltzmann averaged at 298.15 K
according to the following formula:
functionality. The hydrogen bonding ability of the monomers
will be similar due to OH group. The structure of the third
functionality is varied to obtain highly polar monomers.
eDH =RT
X
j
X
The general procedure for the synthesis of monomers 1–4
involved a simple one-step ring-opening reaction of GMA
with different acids in the presence of TEA as catalyst (Fig.
2). The following acids were used to provide monomers with
high polarities: 2-(2-methoxyethoxy)acetic acid, 2-(2-(2-
methoxyethoxy)ethoxy)acetic acid, cyanoacetic acid, and ben-
zoic acid. Monomers 1 and 2 are novel, 3 seems to be novel
(we could not find its synthesis procedure18). In the litera-
ture, monomer 4 was synthesized using tetrabutylammo-
nium bromide as catalyst in acetonitrile.19 The reactions
were conducted at 60 ꢀC, and BHT was used to prevent
homopolymerization of GMA during reactions. Monomers
were obtained as viscous oils after purification with column
chromatography. The solubilities and viscosities of the mono-
mers are reported in Table 1. All of the synthesized mono-
mers were found to be more viscous than TEGDMA due to
hydrogen bonding.
P
hlcalci ¼
Dj
¼
Djpj;
i eDH =RT
i
j
where Dj is the dipole moment of the conformation j, DHj is
the difference between the heat of formation of conformation
j and the heat of formation of the global minimum conforma-
tion, T is the absolute temperature, R is the ideal gas con-
stant, and pj is the probability of finding the monomer in
conformation j at the temperature T.11
Calculation of ELUMO and NBO Charges
ELUMO and NBO charges of vinylic carbon atoms have been
performed with the Gaussian03 package at the B3LYP/6-
31þG(d) level.
RESULTS AND DISCUSSION
Monomer Synthesis and Characterization
The ring-opening reaction of GMA with four carboxylic acids,
a phosphoric acid derivative, and an alcohol was used for
the preparation of new functional monomers with the fol-
lowing structure:
Monomer 5 was synthesized with a procedure similar to
that given in the literature.20 GMA was reacted with diethyl
hydrogen phosphate, which was prepared from the reaction
of diethylchlorophosphate and water (Fig. 2). The pure prod-
uct was obtained after column chromatography as a colorless
oil. It was soluble almost in all organic solvents except hex-
ane, and it was also soluble in water (Table 1).
The ring-opening reaction of the epoxides is not regiospe-
cific. There are two possible sites for attack of alcohols,
acids, and anhydrides.21,22 If the attack occurs from the less-
where R1 is methyl due to methacrylates, R2 is a secondary
functionality that is an OH group, and R3 is a third
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ARTICLE
Acetone
TABLE 1 Viscosities and Solubilities of the Synthesized Monomers and TEGDMA
Monomer
Viscosity (Pa s)
H2O
Ether
Methanol
CH2Cl2
THF
Hexane
TEGDMA
0.009
0.047
0.072
0.960
0.258
0.059
0.136
ꢁ
þ
þ
ꢁ
ꢁ
þ
6
þ
þ
þ
þ
þ
þ
ꢁ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
þ
þ
þ
þ
þ
þ
þ
1
2
3
4
5
6
hindered side, the linear isomer is obtained otherwise the
branched isomer or both isomers are produced. The crude
product yields of acid reactions were very high, containing
mostly the linear isomer with different amounts of the
branched isomer.
The FTIR spectra of the monomers 1–4 showed the presence
of alcoholic OH bond at ꢂ3400 cmꢁ1, the double bonds at
1634 cmꢁ1, two different ester C¼¼O bonds at ꢂ1750 and
1716 cmꢁ1 (Fig. 6) except monomer 4 where two C¼¼O
peaks overlap at 1713 cmꢁ1. Monomer 5 showed one C¼¼O
peak at 1717 cmꢁ1 and also showed P¼¼O and PAOEt group
The 1H NMR of monomer 1 showed methyl protons at 1.87
ppm, methoxy protons at 3.31 ppm, two methylene protons
at 3.51 and 3.63 ppm, other methylene and methine protons
between 4.04 and 4.30 ppm, and double bond protons at
5.53 and 6.06 ppm (Fig. 3). The small peaks ꢂ4.4, 5.2, and
6.0 ppm are due to the branched isomer. The ratios of linear
to branched isomers for monomers 1, 2, and 5 were found
to be greater than 7.5:1.
peaks at 1249 and 1017 cmꢁ1
.
The alcoholysis of epoxides is conducted under acidic and
basic conditions and requires long reaction times and high
temperature. Liu et al.23 have demonstrated that Amberlyst-
15 is an efficient heterogeneous catalyst for the regioselec-
tive ring-opening reactions of epoxides (such as phenyl gly-
cidyl ether, styrene oxide, and cyclohexene oxide) by alcohols
in the formation of b-alkoxy alcohols under ultrasonication
at room temperature. We used the same method for the syn-
thesis of our last monomer (monomer 6; Fig. 2). GMA was
reacted with methanol inꢀthe presence of Amberlyst-15 in an
ultrasonic bath at 25–30 C. However, the major product was
found to be a a-alkoxy alcohol (linear isomer: 78%) with a
small amount of b-alkoxy alcohol (branched isomer: 22%)
The 13C NMR spectrum of monomers 3 and 4 also showed
two isomers (Figs. 4 and 5). For example, the spectrum of
monomer 4 showed characteristic peaks for methyl carbon
at 17.97 ppm, a tertiary carbon at 67.81 ppm, methylene
carbons at 65.25 and 65.47 ppm, double bond carbons at
126.20 and 135.52 ppm, aromatic carbons at 128.17, 129.45,
and 133.01 ppm, and carbonyl carbons at 166.41 and
167.21 ppm (Fig. 5). The small peaks at 61.02 (CH2) and
62.75 (CH2) ppm are due to the branched isomer, small CH
peak is difficult to see. The ratio of CH2 peaks for linear iso-
mer to that of branched isomer indicates relative ratios of
the isomers, which is about 4:1. This ratio was also con-
1
determined from H NMR of the crude product. In the litera-
ture, the reaction of GMA with methanol at room tempera-
ture using BF3 as catalyst gave a mixture of products with
similar regioselectivity (branched:linear 20/80%).24,25
When the crude product was subjected to a chromatographic
separation on silica gel, fractions containing mixture of iso-
mers were obtained. The 13H NMR spectra of three isomer
mixtures were shown at Figure 7. The ratio of double bond
peaks or double bond to CH peak of branched isomer (ꢂ5.0
ppm) indicates relative ratios of the isomers. Mixture 1 con-
tains mainly the linear isomer with a very small amount of
branched isomer. Mixtures 2 and mixture 3 contain linear to
1
firmed by H NMR. Another fraction of column chromatogra-
phy indicated a ratio of 14:1. Similarly, the ratio of linear to
branched isomers for monomer 3 was also found to be high
(4:1). These results indicated the amount of branched iso-
mers formed is greater for the acids with more rigid
structures.
FIGURE 3 1H NMR of monomer 1.
FIGURE 4 13C NMR of monomer 3.
DEVELOPMENT OF REACTIVE METHACRYLATES, KARAHAN ET AL.
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FIGURE 5 13C NMR of monomer 4.
FIGURE 6 FTIR spectrum of monomer 3.
branched isomers with the ratio of 3.4:1 and 1:1. The FTIR
spectrum of this monomer showed the characteristic peaks
of hydroxyl, carbonyl, and C¼¼C groups at 3434, 1715, and
group. The thermal gravimetric analysis of this polymer (re-
sidual monomer was removed by several washings with
methylene chloride) showed a major weight loss ꢂ270 ꢀC
and a char yield of 33.3%. This char yield was higher than
those of GMA-based crosslinked phosphonated polymers
(poly-A, poly-B, and poly-C) with the char yields of 25–30%.
The enhanced char production of phosphates compared with
phosphonates is due to formed phosphoric acid, which is
more effective at promoting further crosslinking of polymers
instead of the weaker phosphonic acid.26
1635 cmꢁ1
.
Thermal Polymerizations
Bulk polymerizations of monomers 1–6 were carried out
with azobisisobutyronitrile (AIBN) (0.5 wt %) at 60 ꢀC.
These monomers polymerized very fast to give crosslinked
polymers, as indicated by swelling in various solvents. This
behavior can be explained by a hydrogen abstraction chain
transfer mechanism from the labile hydrogens on the car-
bons attached to oxygen, which can be followed by reinitia-
tion. The crosslinking will result high mechanical properties,
which is an important property for dental materials.
Photopolymerization
Photopolymerization of the monomers were followed by
DPC. First, the homopolymerization behavior of the synthe-
sized monomers was investigated and compared with those
of commercial monomers such as HEMA, Bis-GMA, GDMA,
The crosslinked polymer obtained from monomer 5 may
also have potential as flame retardant due to its phosphate
FIGURE
7
1H NMR of isomer
mixtures of monomer 6 (mixture
1-bottom, mixture 2-middle, and
mixture 3-top).
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ARTICLE
TABLE 2 Rate of Polymerizations and Conversions of
Monomers 1–6, Bis-GMA, TEGDMA, HEMA, and GDMA
The synthesized monomers were also evaluated as reactive
diluents to Bis-GMA. The maximum rate and conversion val-
ues of Bis-GMA:monomer (50:50 mol %) mixtures were
measured and compared with a control Bis-GMA:TEGDMA
(50:50 mol %) system. It was observed that on addition of
monomers (except monomer 1), both maximum rate of poly-
merizations and conversions were improved, and the values
are comparable with the control (Table 3). We could not
explain why addition of a very flexible monomer (monomer
1) to Bis-GMA did not increase the conversion. The maxi-
mum rate of polymerization and conversion of the mixtures
(except Bis-GMA:1) were between the rates of polymeriza-
tions and conversions of the two monomers. For example,
the maximum rate of polymerization of Bis-GMA and mono-
mer 2 were 0.037 and 0.060 sꢁ1, whereas the mixtures of
Bis-GMA with monomer 2 gave maximum rate of polymeriza-
Monomer (Linear:Branched)
Rp (sꢁ1
)
Conversion (%)
1 (>7.5:1)
2 (>7.5:1)
3 (5:1)
0.034 6 0.003
0.060 6 0.001
0.074 6 0.002
0.070 6 0.002
0.061 6 0.002
0.066 6 0.003
0.065 6 0.001
0.058 6 0.001
0.056 6 0.002
0.037 6 0.003
0.037 6 0.002
0.040 6 0.00
91 6 3
95 6 2
88 6 2
72 6 2
74 6 2
64 6 3
81 6 3
68 6 1
79 6 2
47 6 2
92 6 1
58 6 1
4 (14:1)
4 (4:1)
5 (>7.5:1)
6 (>7.5:1)
6 (1:1)
TEGDMA
Bis-GMA
HEMA
tion of 0.056 sꢁ1
.
GDMA
Effect of Monomer Structure on Oxygen Inhibition
Although free radical polymerizations are inhibited by oxy-
gen, most of the curing processes are conducted in air. The
presence of oxygen results in long induction times, low con-
versions, slow polymerization rates, short chains, and poor
polymer properties.28 In general, oxygen inhibition depends
on viscosity of the medium that is effected by monomer
functionality, molecular weight, and functional groups pres-
ent in the monomer structure (such as OH). When the vis-
cosity is high or increases during polymerization, diffusion
rate of oxygen to the monomer will be low. Other than vis-
cosity, monomers with abstractable hydrogens such as ether
groups were showed to reduce oxygen inhibition by forming
peroxy radical. For example, Hoyle and coworkers found that
TEGDMA shows oxygen inhibition in air with less than 15%
decrease in polymerization rate, while 1,12-dodecane dime-
thacrylate shows ꢂ35% reduction in rate. They also con-
cluded that methacrylates were less sensitive to oxygen than
acrylates. The reduction of rate of polymerization of HA was
found to be ꢂ80%.28
and TEGDMA. Table 2 shows the results in terms of rate of
polymerizations and conversions.
It is known that, as the monomer functionality increases, the
rate of polymerization increases while the conversion
decreases. However, the synthesized monofunctional mono-
mers except one (monomer 1) showed higher rates of poly-
merization (0.058–0.074 sꢁ1) than Bis-GMA (0.037 sꢁ1) and
GDMA (0.040 sꢁ1) and similar or higher rates of polymeriza-
tion than TEGDMA (0.056 sꢁ1). In addition to hydrogen
abstraction mechanism, the high reactivity of the synthesized
monomers can be explained by their hydrogen bonding capa-
bilities leading to high viscosity and earlier Trommsdorf
effect. Monomer 1 was found to be the least reactive mono-
mer, which also correlates with its low viscosity, as expected.
These properties may be understood to be due to a combi-
nation of low dipole moment (in dipole moment section)
and low hydrogen bonding capability. The conversions of all
the synthesized monomers (64–95%) were higher than Bis-
GMA (47%) and GDMA (58%). Monomers 1 and 2 with very
flexible structure gave very high conversions (91 and 95%).
Since we propose a hydrogen abstraction mechanism for the
formation of crosslinked structures from our monomers, we
investigated their photopolymerization behavior in the pres-
ence and absence of oxygen. Figures 8 and 9 show the poly-
merization exotherms and conversions of monomer 2 to-
gether with HA in the presence and absence of oxygen. The
It was observed that the rate and conversion data of a given
monomer is different for different fractions of column chro-
matography, which indicated different reactivities of the two
isomers. To see the effect of isomer structure on the rate of
polymerization, two fractions obtained from column chroma-
tography of monomer 4 and 6 were polymerized. It was
observed that as the branched isomer content in the isomer
mixture is increased, the maximum rate of polymerization is
decreased. For example, the maximum rate of polymerization
of monomer 4 decreased from 0.070 (14:1, linear:branched)
to 0.061 sꢁ1 (4:1, linear:branched). Similarly, rates were
found to be 0.065 (mixture 1) and 0.058 sꢁ1 (mixture 3) for
two different isomer mixtures of monomer 6. We observed
similar behavior during polymerizations of ethyl a-chlorome-
thacrylate-GDMA isomer mixtures.27 The reason for the
higher reactivity of the linear isomers is not obvious. We
suggest an explanation by the polarity differences of isomers
in the next section.
TABLE 3 Rate of Copolymerizations and Conversions of
Monomers 1-6 and TEGDMA with Bis-GMA
Monomers (50:50 mol %)
Rp (sꢁ1
)
Conversion (%)
Bis-GMA:1
0.035 6 0.002
0.056 6 0.003
0.040 6 0.03
0.050 6 0.003
0.043 6 0.001
0.049 6 0.002
0.052 6 0.003
43 6 2
70 6 2
50 6 2
57 6 2
57 6 2
52 6 3
56 6 3
Bis-GMA:2
Bis-GMA:3
Bis-GMA:4
Bis-GMA:5
Bis-GMA:6
Bis-GMA:TEGDMA
DEVELOPMENT OF REACTIVE METHACRYLATES, KARAHAN ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 4 Photopolymerization Rates and Conversions
of Monomers 2, 3, 4, and HA in the Presence and
Absence of Oxygen
Monomer
Rp (sꢁ1
)
Conversion (%)
2
2a
0.060 6 0.001
0.050 6 0.002
0.074 6 0.002
0.069 6 0.002
0.070 6 0.002
0.061 6 0.001
0.043 6 0.003
0.010 6 0.002
95 6 2
89 6 2
88 6 2
80 6 2
72 6 2
75 6 2
82 6 4
60 6 2
3
3a
4
4a
HA
HAa
a
In air.
FIGURE 8 Photopolymerization rate versus time for monomer
2 and HA in nitrogen and air.
ues of 3.49, 3.22, and 2.82 (Table 5). The difference for
monomer 3 was very significant, which may explain the
photopolymerization results of monomers 2, 3, and 4 were
listed in Table 4. The synthesized monomers showed little
oxygen inhibition in air (10–13% decrease in rate), whereas
HA (76% decrease in air) was found to be very sensitive to
air. These findings support our proposed hydrogen abstrac-
tion chain transfer mechanism.
lower reactivity of monomer
expected.
3 (isomer mixture) than
Calculation of NBO Charges and ELUMO
Computer simulations were used to observe the electron
density surfaces and partial charges on double bond carbons
of the synthesized monomers. These values were compared
with the chemical shift differences of these carbons deter-
mined by 13C NMR spectra.
Dipole Moment
The synthesized monomers were evaluated in terms of
dipole moment to find a relation between monomer struc-
ture and reactivity. The Boltzmann-averaged dipole moments
of monomers were calculated for minimum energy conform-
ers (Table 5). When we consider ester linked monomers (1–
5), there seems to be a correlation: the most reactive mono-
mer has the highest dipole moment, the least reactive one,
lowest; with the other three in between (the intermediate
ones are too close together for their differences to have
meaning).
The calculated charges on the terminal vinyl carbons and the
carbon alpha to the carbonyl group of the monomers were
similar to indicate a correlation with reactivities (Table 5).
Bowman and coworkers5 said that a molecule with smaller
ELUMO value shows higher susceptibility toward attack by
nucleophiles or radicals. Thus, during a polymerization of a
monomer with a lower LUMO energy, the activation energy
for the propagation is reduced, which results in an increase
in rate of polymerization. We also calculated the LUMO ener-
gies of the synthesized monomers using B3LYP DFT to
explain reactivity differences between them (Table 5). We
observed a correlation between the polymerization rate and
LUMO energies for the ester linked monomers (1–5); the
most reactive monomer (monomer 3) has the lowest LUMO
energy, whereas the least reactive one (monomer 1) has the
highest LUMO energy. The ether linked monomer with the
highest LUMO energy did not follow this trend.
We also calculated dipole moments of some of the branched
isomers. They were found to be lower than those of the lin-
ear isomers as expected. For example, linear isomers of
monomers 3, 4, and 6 were found to have dipole moments
of 5.30, 3.97, and 3.04, whereas branched isomers gave val-
13C NMR can be used to predict free radical polymerizability
of monomers. The chemical shift differences of the C¼¼C dou-
ble bonds (CbH2¼¼CaA) of the synthesized monomers are
given in Table 5. Vaidya et al. have reported that dCb and dCa
shift to lower and higher fields, respectively, with an increase
in electron withdrawing power of the substituents. There-
fore, Dd (dCbꢁdCa) shows the effects of substituents on poly-
merizability. The stronger the electron-withdrawing power of
the substituents, the smaller the difference and the higher
the radical polymerizability.29,30 Also, bulky substituents may
cause differences in chemical shifts and/or lead to lower
propagation enthalpy through steric hindrance. Comparison
FIGURE 9 Conversion versus time for monomer 2 and HA in
nitrogen and air.
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ARTICLE
TABLE 5 Dipole Moments, Chemical Shift Differences, LUMO Energies, and NBO Charges of the Monomers
Dipole
LUMO
Charges on the
Charges on the Carbon Alpha
to the Carbonyl Group
Monomer
Moment
Dd
Energies (eV)
Terminal Vinylic Carbon
2.81
9.76
ꢁ0.06817
ꢁ0.3749
ꢁ0.1238
3.94
5.30
3.49
9.40
9.00
–
ꢁ0.07487
ꢁ0.08463
ꢁ0.07391
ꢁ0.3659
ꢁ0.3733
ꢁ0.3894
ꢁ0.1302
ꢁ0.1301
ꢁ0.1247
3.97
3.22
9.35
ꢁ0.07198
ꢁ0.06757
ꢁ0.3877
ꢁ0.3713
ꢁ0.1235
ꢁ0.1206
ꢁ
3.34
3.04
9.50
9.91
ꢁ0.07361
ꢁ0.06295
ꢁ0.3872
ꢁ0.3879
ꢁ0.1243
ꢁ0.1268
2.82
–
ꢁ0.06295
ꢁ0.3719
ꢁ0.1243
DEVELOPMENT OF REACTIVE METHACRYLATES, KARAHAN ET AL.
3795

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
13 Sahin, G.; Avci, D.; Karahan, O.; Moszner, N. J Appl Polym
of the Dd values of the ester-linked monomers shows that
the most reactive monomer (monomer 3) has the lowest
(9.0 ppm) value and the least reactive one (monomer 1) has
the highest value (9.76 ppm). The monomers with similar
polymerization tendencies gave values in between (9.35–
9.50). The aforementioned correlation was not applicable to
the ether-linked monomer 6.
Sci 2009, 114, 97–106.
14 Anseth, K. S.; Wang, C. M.; Bowman, C. N. Macromolecules
1994, 27, 650–655.
15 Brandrup, J.; Immergut, E. H. Polymer Handbook; Wiley
Interscience: New York, 1975.
16 Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven,
T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.;
Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.;
Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.;
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.;
Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.;
Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo,
J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Moro-
kuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzew-
ski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.;
Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
Revision D. 01; Gaussian, Inc.: Wallingford, CT, 2004.
CONCLUSIONS
Six hydroxyl-containing methacrylate monomers with various
third functionalities were synthesized as mixtures of isomers.
Photopolymerization studies of these monomers indicated
that the monomers they can be used as alternative reactive
diluent to TEGDMA in Bis-GMA. Monomer 2 was found to be
the best monomer between all the monomers studied. Low
sensitivity of the monomers to oxygen is consistent with the
crosslinked polymer formation during thermal polymeriza-
tion due to a proposed hydrogen abstraction chain transfer
reaction. Although we could not see a monotonic correlation
between computationally calculated dipole moment and
LUMO energies, these values can be used to roughly estimate
relative reactivities of the monomers with similar structures.
The authors thank Viktorya Aviyente for valuable suggestions
during computational work and Oguz Okay and Deniz Ceylan
for viscosity measurements.
17 SPARTAN Version 4.0; Wavefunction, Inc.: Irvine, CA, 1995.
18 Naoki, Y.; Junji, Y.; Motoyoshi, Y. Jpn. Kokai Tokkyo Koho,
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