Synthesis, photopolymerization, and adhesive properties of hydrolytically stable phosphonic acid-containing (meth)acrylamides

Article abstract of DOI:10.1002/pola.27025

Three novel dental monomers containing phosphonic acid groups (1a and 2a, based on diethyl amino(phenyl)methylphosphonate and 3a based on diethyl 1-aminoheptylphosphonate) were synthesized in two steps: the reaction of α-aminophosphonates with acryloyl ch

Full text of DOI:10.1002/pola.27025

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Synthesis, Photopolymerization, and Adhesive Properties of
Hydrolytically Stable Phosphonic Acid-Containing (Meth)acrylamides
Ayse Altin,1 Burcin Akgun,1 Zeynep Sarayli Bilgici,1 Sebnem Begum Turker,2 Duygu Avci1
1Department of Chemistry, Bogazici University, 34342 Bebek, Istanbul, Turkey
2Department of Prosthodontics, Marmara University, 34365 Nisantasi, Istanbul, Turkey
Correspondence to: D. Avci (E-mail: avcid@boun.edu.tr)
Received 21 August 2013; accepted 8 November 2013; published online 28 November 2013
DOI: 10.1002/pola.27025
ABSTRACT: Three novel dental monomers containing phosphonic
acid groups (1a and 2a, based on diethyl amino(phenyl)methyl-
phosphonate and 3a based on diethyl 1-aminoheptylphospho-
nate) were synthesized in two steps: the reaction of
a-aminophosphonates with acryloyl chloride (for monomers 1a
and 3a) or methacryloyl chloride (for 2a) to give monomers
with phosphonate groups, and the hydrolysis of phosphonate
groups by using trimethyl silylbromide. Their (and the inter-
mediates’) structures were confirmed by FTIR, 1H, 13C, and 31P
NMR spectroscopy. All the monomers dissolve well in water
(1<pH<2) and are hydrolytically stable. Their homo- and
copolymerizations with 2-hydroxyethyl methacrylate (HEMA)
and HEMA/glycerol dimethacrylate were investigated with
photo-DSC. Thermal polymerization of the new monomers in
water or in ethanol/water solution was investigated, giving
polymers in good yields. X-ray diffraction results showed only
dicalcium phosphate dehydrate formation upon interaction of
1a-3a with hydroxyapatite indicating its strong decalcification
and that monomer-Ca salts are highly soluble. Some results
were also compared to those with
a bisphosphonic acid-
containing methacrylamide (4a) previously reported; and the
influence of monomer structure on polymerization/adhesive
properties is discussed. These properties, especially hydrolytic
stability and good rates of polymerization, make these new
monomers suitable candidates as components of dental adhe-
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sive mixtures. 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part
A: Polym. Chem. 2014, 52, 511–522
KEYWORDS: adhesives; dental polymers; hydrolytic stability;
hydroxyapatite; photopolymerization; phosphonic acid
in the mouth. In order to realize the first requirement,
(meth)acrylate- and (meth)acrylamide-based acidic mono-
mers are extensively used in dental applications due to their
high reactivities.
INTRODUCTION In recent years, self-etching adhesives have
become popular in restorative dentistry in order to achieve a
strong bond between the dental hard tissues (dentin and
enamel) and the restorative material.1–3 Such adhesives are
water based and have several components: an acidic mono-
mer [e.g., 4-methacryloxyethyl trimellitic anhydride (4-
META) or 10-methacryloyloxydecyl dihydrogen phosphate
(MDP)], crosslinking monomers {e.g., 2,2-bis[4-(2-hydroxy-3-
methacryloyloxypropoxy) phenyl] propane (bis-GMA), and
Extensive research has been conducted to satisfy the second
requirement, that is, to develop monomers with acidic func-
tional groups, which may strongly bond to dental tissue, in
particular, HAP. For example, bisphosphonates, structural
analogs of naturally existing pyrophosphate with increased
chemical and enzymatic stability and strong affinity for HAP,
can be incorporated into the monomers.4–7 This interaction
can occur by chemical bonds, such as covalent or ionic
bonds, stemming from the reaction of acidic and chelating
groups with HAP; and physical bonds due to van der Waals
forces, London dispersion forces, hydrogen bonding, or
charge-transfer complexes.3 According to the “Adhesion-
Decalcification” concept of Yoshida et al., the bonding per-
formance of the adhesives depends on the chemical stability
of the monomer-Ca salts formed with the interaction of the
acid monomer and HAP.8–11 Therefore, the structure of the
acid monomer is very important and small differences such
urethane dimethacrylate},
a monofunctional comonomer
(e.g., HEMA), an initiator, and fillers.
Acidic monomers, usually with dihydrogen phosphates, car-
boxylic or phophonic acid groups, are the most important
component in a self-etching adhesive. They are used in the
adhesive in order to partially demineralize the tooth tissues
as well as to form a strong chemical bond with hydroxyapa-
tite (HAP). Therefore, it is desired that self-etching adhesive
monomers have the following properties: (i) high rate of
homopolymerizability or copolymerizability with the other
monomers in the adhesive, (ii) ability to form strong bonds
with tooth tissue, (iii) sufficient stability both in storage and
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as polarity in the monomer structures cause significant dif-
ferences in their adhesive performances. For example, while
4-META and methacryloxyethyl hydrogen phenylphosphate
(Phenyl-P) deposit unstable calcium salts due to dissolution,
MDP forms hydrolysis resistant calcium salts due to its
hydrophobic decyl group.
Chemical and used as recieved. MDP was a gift from Ivoclar
Vivadent AG.
Characterization
Monomer characterization involved 1H, 13C, and 31P NMR
spectroscopy (Varian Gemini 400 MHz) and FTIR spectros-
copy (T 380). Elemental analyses were obtained on a
Thermo Electron SpA FlashEA 1112 elemental analyser
(CHNS separation column, PTFE; 2 m; 6 3 5 mm). Photopo-
lymerizations were performed using a TA Instruments Q100
differential photocalorimeter. The interactions of monomers
with HAP were studied by X-ray diffraction (Rigaku D/max-
2200/PC). Gel permeation chromatography (GPC) was per-
formed on an Agilent 1100 GPC Instrument equipped with a
refractive index detector, using water as a solvent and poly(-
acrylic acid sodium salt) standards. Combi Flash Companion
Teledyne ISCO Flash Chromatography with octadecyl bonded
silica gel (C 18 reverse phase silica gel) as a stationary phase
was used for purification of monomers.
Although many commercial dental adhesives accomplish
good binding to HAP, the main problem is hydrolysis of the
ester groups in (meth)acrylates forming the basis of these
monomers, which relates to the third requirement. Adhesive
mixtures contain water, and the mouth is an aqueous envi-
ronment, so ester-based adhesives generally can hydrolyze
both in storage and in the mouth, which results in both inad-
equate shelf life of adhesives and undesirably short useful
lifetime of the dental material. To overcome this disadvant-
age, hydrolytically stable ether- and/or amide-linked mono-
mers were synthesized.12–22
Recently we have focused our interest on the design and
synthesis of the new acidic monomers, which are expected
to meet the requirements mentioned above. In this study, we
report on three such monomers. We describe the synthesis,
characterization, photopolymerization behavior, hydrolytic
stability, adhesive properties, and HAP interactions of the
monomers 1a–3a (Fig. 1). Some of their properties were
compared with a bisphosphonic acid-containing methacryla-
mide (4a) previously reported5 by our group to investigate
structure–property relationships. Monomers 1a and 3a are
novel and 2a is mentioned in a patent.23
Synthesis of Monomers
Diethyl Acrylamido(phenyl)methylphosphonate
(Monomer 1)
A solution of acryloyl chloride (0.80 mL, 10 mmol) in anhy-
drous toluene (2 mL) was added dropwise, under N2, to a
solution of diethyl amino(phenyl)methylphosphonate (1.95 g,
8.02 mmol) and TEA (1.30 mL, 9.00 mmol) in anhydrous tol-
uene (7 mL) in an ice bath. Thereafter, more toluene (5 mL)
was added and the mixture was stirred at room temperature
for 2 h. The reaction was terminated by addition of distilled
water (10 mL) and the mixture filtered. The filtered solid
was dissolved in chloroform (113 mL) and extracted with
distilled water (3 3 40 mL), 2M HCl (3 3 40 mL), saturated
NaHCO3 (3 3 40 mL), and distilled water (3 x 40 mL) again.
The combined organic layers were dried over anhydrous
sodium sulfate and evaporated under vacuum to give mono-
mer 1 as a white solid with a melting point of 154 ꢀC in
30% yield.
EXPERIMENTAL
Materials
Diethyl amino(phenyl)methylphosphonate and diethyl 1-
aminoheptylphosphonate were prepared according to literature
procedures.24 Trimethylsilyl bromide (TMSBr) (Aldrich, Tauf-
kirchen, Germany) was distilled before use. Dichloromethane
was dried over activated molecular sieves (4 A0). Methacryloyl
chloride, acryloyl chloride, triethyl amine (TEA), 2-hydroxyethyl
methacrylate (HEMA), glycerol dimethacrylate (GDMA), 2,2-
bis[4-(2-hydroxy-3-methacryloyloxy propoxy) phenyl] propane
(bis-GMA), HAP, 2,20-dimethoxy-2-phenyl acetophenone (DMPA),
2,20-azobis(2-methylpropionamidine)dihydrochloride (V-50), bis
(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO), and all
other reagents and solvents were obtained from Aldrich
1H NMR (400 MHz, CDCl3, d): 1.02, 1.24 (t, 3JHH 5 7.1 Hz,
3JHH 5 7.2 Hz, 6H, OCH2CH3), 3.64, 3.83, 4.11 (m, 4H,
OCH2CH3), 5.50 (dd, 3JHH 5 11.6 Hz, 3JHH 5 9.7 Hz, 1H,
CH2@CH), 5.61 (dd, 2JHH 5 4.7 Hz, 2JHP 5 2.6 Hz, 1H, CHAP),
3
3
6.23 (d, JHH 5 11.6 Hz, 1H, CH2@CH), 6.25 (d, JHH 5 9.7 Hz,
1H, CH2@CH), 7.21–7.46 (m, 5H, Ar-H), 8.22 (br.s, 1H, NAH)
ppm. 13C NMR (400 MHz, CDCl3, d): 16.31 (OCH2CH3), 48.92,
51.04 (CHAP), 63.24 (OCH2CH3), 126.85, 128.46, 128.52,
FIGURE 1 Structures of the (bis)phosphonic acid monomers (4a) 1a–3a.
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135.07 (Ar-C), 128.02 (CH@CH2), 130.41 (CH@CH2), 165.19,
165.27 (C@O) ppm. FTIR (ATR): 3262 (NAH), 3052, 3027
(Ar-H), 2985, 2935 (CAH), 1672 (C@O), 1630 (C@C), 1540
(NH), 1216 (P@O), 1015 and 954 (PAO) cm21. ELEM. ANAL.,
Calcd. for C14H20NO4P: C, 56.56%; H, 6.78%; N, 4.71%; O,
21.53%; P, 10.42%. Found: C, 57.62%; H, 7.20%; N, 4.97%.
1234 (P@O), 1021 and 960 (PAO) cm21. ELEM. ANAL., Calcd.
for C15H22NO4P: C, 57.87%; H, 7.12%; N, 4.50%; O, 20.56%;
P, 9.95%. Found: C, 57.82%; H, 7.55%; N, 4.43%.
Methacrylamido(phenyl)methylphosphonic Acid
(Monomer 2a)
TMSBr (0.295 g, 1.927 mmol) was added dropwise to a solu-
tion of monomer 2 (0.2 g, 0.642 mmol) in dry dichlorome-
thane (2.80 mL) in an ice bath and under N2. After stirring
at 40 ꢀC for 2 h, the volatile components were removed
under vacuum. Methanol (8.5 mL) was added and the mix-
ture was stirred at room temperature overnight. The solvent
was evaporated and the crude product was purified by flash
chromatography on C18 reverse phase silica gel, eluting with
H2O/MeOH (35/65, v/v) to give monomer 2a as a white
Acrylamido(phenyl)methylphosphonic acid (Monomer 1a)
TMSBr (0.309 g, 2.018 mmol) was added dropwise to a solu-
tion of monomer 1 (0.2 g, 0.673 mmol) in dry dichlorome-
thane (2.90 mL) in an ice bath and under N2. After stirring
for 3 h at room temperature, the volatile components were
removed under vacuum. Methanol (9 mL) was added and
the mixture was stirred at room temperature overnight. The
solvent was evaporated and the crude product was purified
by flash chromatography on C18 reverse phase silica gel,
eluting with H2O/MeOH (70/30, v/v) to give monomer 1a as
ꢀ
solid with a melting point of 63–64 C in 56% yield.
1H NMR (400 MHz, D2O, d): 1.99 (s, 3H, CH3), 5.28 (d,
2JHP 5 18.1 Hz, 1H, CHAP), 5.54, 5.78 (s, 2H, CH2@C), 7.36–7.57
(m, 5H, Ar-H) ppm. 13C NMR (400 MHz, MeOD, d): 14.48 (CH3),
27.23, 27.36 (CHCH2), 23.71, 29.92, 30.50, 32.89
(CH2CH2CH2CH2), 47.44 (CHAP), 127.21(C@CH2), 131.85
(C@CH2), 168.10, 168.14 (C@O) ppm. FTIR (ATR): 3245 (NAH),
3000–2600 (OH), 2926, 2858 (CAH), 1655 (C@O), 1627 (C@C),
1547 (NH), 1097 (P@O), 998 and 937 (PAO) cm21
ꢀ
a white solid with a melting point of 70–71 C in 54% yield.
1H NMR (400 MHz, D2O:MeOD, d): 5.36 (d, 2JHP 5 21.1 Hz,
1H, CHAP), 5.74 (d, 3JHH 5 10.5 Hz, 1H, CH2@CH), 6.22 (d,
3JHH 5 17.6 Hz, 1H, CH2@CH), 6.33–6.44 (dd, 3JHH 5 17.6 Hz,
3JHH 5 10.5 Hz, 1H, CH2@CH), 7.24–7.46 (m, 5H, Ar-H) ppm.
13C NMR (400 MHz, D2O:MeOD, d): 52.72, 54.20 (CHAP),
128.96, 129.11, 129.16, 137.35 (Ar-C), 129.78 (CH@CH2),
131.15 (CH@CH2), 168.42, 168.50 (C@O) ppm. 31P NMR
(400 MHz, CDCl3, d): 15.26 ppm. FTIR (ATR): 3264 (NAH),
3063, 3028 (Ar-H), 3000–2600 (OH), 2958 (CAH), 1653
(C@O), 1620, 1606 (C@C), 1529 (NH), 1178 (P@O), 983 and
932 (PAO) cm21
Diethyl 1-Acrylamidoheptylphosphonate (Monomer 3)
A solution of acryloyl chloride (0.33 mL, 4.06 mmol) diluted
in anhydrous dichloromethane (3.3 mL) was added dropwise,
under N2, to a mixture of diethyl 1-aminoheptylphosphonate
(0.64 g, 2.53 mmol), and triethylamine (0.5 mL, 3.43 mmol)
in anhydrous dichloromethane (7.4 mL) in an ice bath. The
mixture was stirred at room temperature for 2 h. The reaction
was terminated by addition of distilled water (3.3 mL). After
addition of chloroform (35 mL), the organic layer was
extracted several times with distilled water (3 3 12 mL), 2M
HCl (3 3 12 mL), saturated NaHCO3 (3 3 12 mL), and dis-
tilled water (3 3 12 mL) and dried over anhydrous Na2SO4,
filtered, and evaporated under vacuum to give monomer 3 as
a yellow viscous liquid in 49% yield.
Diethyl Methacrylamido(phenyl)methylphosphonate
(Monomer 2)
A solution of methacryloyl chloride (0.60 mL, 6 mmol)
diluted in anhydrous toluene (1.2 mL) was added dropwise,
under N2, to a mixture of diethyl amino(phenyl)methyl-
phosphonate (1.20 g, 5. mmol) and triethylamine (0.8 mL,
5.5 mmol) in anhydrous toluene (4.2 mL) in an ice bath.
Thereafter, more toluene (3 mL) was added and the mixture
was stirred at room temperature for 2 h. The reaction was
terminated by addition of distilled water (6 mL) and the
mixture was filtered. The filtered solid was dissolved in chlo-
roform (57 mL) and extracted with distilled water (3 3 20
mL), 2M HCl (3 3 20 mL), saturated NaHCO3 (3 3 20 mL),
and distilled water (3 3 20 mL). After drying the organic
phase with anhydrous Na2SO4, the solvent was evaporated
under vacuum to give monomer 2 as a white solid with a
1H NMR (400 MHz, CDCl3, d): 0.79 (t, 3JHH 5 6.9 Hz, 3H,
CH2CH3), 1.13–1.30 (m, 14H, OCH2CH3, (CH2)4), 1.58, 1.73
(m, 2H, CHCH2), 4.06 (m, 4H, OCH2CH3), 4.47 (m, 1H,
3
3
CHAP), 5.58 (dd, JHH 5 10.6 Hz, JHH 5 9.2 Hz, 1H, CH2@CH),
3
3
6.25 (d, JHH 5 10.6 Hz, 1H, CH2@CH), 6.27 (d, JHH 5 9.2 Hz,
1H, CH2@CH), 7.45, 7.48 (d, 3JHH 5 9.9 Hz, 1H, NAH) ppm.
13C NMR (400 MHz, CDCl3, d): 12.78 (CH3), 15.58
(OCH2CH3), 21.66 (CHCH2), 24.87, 27.84, 28.51, 30.54
(CH2CH2CH2CH2), 43.31, 44.79 (CHAP), 61.19, 62.08
(OCH2CH3), 125.47 (CH@CH2), 129.59 (CH@CH2), 164.59,
164.64 (C@O) ppm. FTIR (ATR): 3253 (NAH), 2980, 2928
(CAH), 1664 (C@O), 1630 (C@C), 1539 (NH), 1222 (P@O),
ꢀ
melting point of 114 C in 40% yield.
1H NMR (400 MHz, CDCl3, d): 1.05, 1.27 (t, 3JHH 5 7.0 Hz,
3JHH 5 7.1 Hz, 6H, OCH2CH3), 1.93 (s, 3H, CH3), 3.67, 3.89,
4.11 (m, 4H, OCH2CH3), 5.32, 5.71 (s, 2H, CH2@C), 5.53 (dd,
2JHH 5 11.6 Hz, 2JHP 5 9.5 Hz, 1H, CHAP), 7.06 (br s, 1H,
NAH), 7.27–7.46 (m, 5H, Ar-H) ppm. 13C NMR (400 MHz,
CDCl3, d): 16.35 (OCH2CH3), 18.97 (CH3), 49.84, 51.26
(CHAP), 63.36 (OCH2CH3), 120.59 (CH2@C), 128.30, 128.41,
128.80, 135.45 (Ar-C), 140.43 (CH2@C), 167.89, 167.97
(C@O) ppm. FTIR (ATR): 3280 (NAH), 3048, 3029 (Ar-H),
2982, 2908 (CAH), 1659 (C@O), 1621 (C@C), 1524 (NH),
1022 and 959 (PAO) cm21
.
1-Acrylamidoheptylphosphonic Acid (Monomer 3a)
TMSBr (0.490 g, 3.203mmol) was added dropwise to a solu-
tion of monomer 3 (0.326 g, 1.068 mmol) in dry dichlorome-
thane (4.60 mL) in an ice bath and under N2. After stirring
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for 3 h at room temperature, the volatile components were
removed under vacuum. Methanol (14.4 mL) was added and
the mixture was stirred at room temperature overnight. The
solvent was evaporated and the crude product was purified
by flash chromatography on C18 reverse phase silica gel,
eluting with H2O/MeOH (45/55, v/v) to give monomer 3a as
tin with a microbrush for 10 s. The adhesive layer was air-
dried for 15 s and light cured for 10 s with a LED curing
light. A 3-mm thick Teflon mold (Gapi, Bergamo, Italy) with
a central 2-mm diameter circular hole was fixed on the sur-
face. The nanohybrid composite was placed into the mold.
The samples were polymerized for 40 s with LED curing
light. The samples were finally stored in water at 37 ꢀC for
24 h before being tested. The shear bond strength (SBS) was
measured using a universal testing machine (Zwick) at a
crosshead speed of 0.8 mm min21. Ten samples were tested
for each adhesive.
ꢀ
a white solid with a melting point of 50–51 C in 80% yield.
1H NMR (400 MHz, MeOD, d): 0.89 (t, 3JHH 5 6.9 Hz, 3H,
CH2CH3), 1.23–1.46 (m, 8H, (CH2)4), 1.65, 1.88 (m, 2H,
CHCH2), 4.29 (dt, 3JHH 5 12.8 Hz, 2JHP 5 4.2 Hz, 1H, CHAP),
5.66, 5.68 (dd, 3JHH 5 9.8 Hz, 3JHH 5 2.5 Hz, 1H, CH2@CH),
6.21, 6.25 (dd, 3JHH 5 15.1 Hz, 3JHH 5 2.5 Hz, 1H, CH2@CH),
6.31, 6.35 (dd, 3JHH 5 15.1 Hz, 3JHH 5 9.8 Hz, 1H, CH2@CH)
ppm. 13C NMR (400 MHz, MeOD, d): 14.48 (CH3), 27.23,
27.36 (CHCH2), 23.71, 29.92, 30.50, 32.89 (CH2CH2CH2CH2),
47.44 (CHAP), 127.21 (C@CH2), 131.85 (C@CH2), 168.10,
168.14 (C@O) ppm. 31P NMR (CDCl3): 22.76 ppm. FTIR
(ATR): 3245 (NAH), 3000–2600 (OH), 2926, 2858 (CAH),
1655 (C@O), 1627 (C@C), 1547 (NH), 1097 (P@O), 982 and
933 (PAO) cm21
Photopolymerizations
Photopolymerizations were conducted using a DSC equipped
with a mercury arc lamp. The samples (3–4 mg) containing
2.0 mol % initiator were irradiated for 10 min with an inci-
dent light intensity of 20 mW cm22 and a nitrogen flow of
20 mL min21. Photopolymerization rates were calculated
using the following formula:
ðQ=sÞM
Rate 5
nDHpm
Methacrylamidomethylenediphosphonic Acid
(Monomer 4a)
where Q/s is the heat flow per second, M is the molar mass
of the monomer, n is the number of double bonds per mono-
mer molecule, DHp is the heat released per mole of double
bonds reacted, and m is the mass of monomer in the sample.
The values used for the DHp of acrylamide and methacryla-
mide double bonds were 20.6 and 13.1 kcal mol21 [25].
Monomer 4a was synthesized in our previous study.5
Interactions of Monomers with Hydroxyapatite
FTIR Spectroscopy Technique
HAP particles (0.2 g) were dispersed in 1.00 g of monomer/
EtOH/H2O (15/45/40 wt %) solution under stirring as
described by Yoshihara et al.9 After 24 h, the monomer-
coated HAP particles were isolated by centrifugation and
washed with water (3 3) and then with ethanol and dried
at room temperature.
Thermal Polymerizations
Thermal homopolymerizations of monomers (1a, 2a, and
3a) were carried out in solutions of water or EtOH/water
(2.5/1, v/v) containing V-50 at 65 ꢀC using the standard
freeze-evacuate-thaw procedure. Large amounts of THF
(1a) or water (2a, 3a) were then added to precipitate the
polymers. After filtration, the polymer was dried under
vacuum.
XRD Technique
HAP samples were prepared as in the case of FTIR spectros-
copy technique, washed with ethanol (3 3) and water (3 3),
and dried at room temperature before analysis. The crystal
phases on the monomer-coated HAP particles were identified
by a powder XRD operated under 40 kV acceleration and 40
mA current and scanning rate of 2ꢀ min21 for 2h/h scan.
RESULTS AND DISCUSSION
Synthesis and Characterization of Monomers
Two a-primary aminophosphonates (diethyl amino(phenyl)-
methylphosphonate and diethyl 1-aminoheptylphosphonate)
were used as starting materials for the synthesis of the
monomers. They were prepared in a solvent-free one-pot
reaction of benzaldehyde or heptaldeyde, ammonium carbon-
ate, and diethyl phosphite in the presence of catalytic
Al(OTf)3 (Fig. 2).24
Hydrolytic Stability
The hydrolytic stabilities of monomers were studied by 1H
NMR measurements of 5 wt % solutions of the monomers in
methanol-d4/D2O (1/1, v/v) after storage at 37 ꢀC for 20
days.
Shear Bond Strength Measurements
Caries-free human molars were used in this study. The roots
of each tooth were embedded in autopolymerizing acrylic
resin (Meliodent, # 64713278; Heraeus Kulzer, Hanau, Ger-
many). A horizontal dentin bonding surface was prepared
with a high speed of diamond disc [Isomet 1000 Precision
saw (Buehler, Easton Ill)]. A phosphoric acid gel was applied
on the dentin surface for 15 s. The etchant gel was then
removed with a water spray, and the excess of moisture was
removed. The adhesive was then rubbed on the etched den-
The new phosphonic acid-containing (meth)acrylamides (1a–
3a) were prepared in two steps (Fig. 2). In the first step,
acryloyl or methacryloyl chloride was reacted with diethyl
amino(phenyl)methylphosphonate and diethyl 1-amino
heptylphosphonate in the presence of TEA to give
phosphonate-containing monomers (1–3). The monomers 1
and 2 were solids with melting points of 154 and 114 ꢀC,
respectively, and isolated in 30 and 40% yields, whereas
monomer 3 was a viscous liquid in 49% yield.
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FIGURE 2 Synthesis of a-aminophosphonates and phosphonic acid monomers 1a–3a.
In the second step, the silylation of phosphonate monomers
(1–3) using TMSBr, followed by methanolysis of the silyl
ester groups provided the new acid monomers 1a–3a in
54–80% yields after purification with C18 reversed-phase
flash chromatography. The monomers 1a–3a were obtained
as white solids with melting points of 70–71, 63–64, and
50–51 ꢀC, respectively. The solubility of phosphonic acid
monomers in water, EtOH, acetone, THF, diethyl ether, and
CH2CL2 (3 mg/5 mL), were investigated (Table 1). All mono-
mers are soluble in EtOH and water, which is very important
for dental adhesive applications; but insoluble in diethyl
ether and CH2CL2. Solubility of monomer 1a was also deter-
mined as 0.2, 0.6, and 1.2 mg mL21 at pH values of 4, 7,
and 10. Monomers 1a–3a are also soluble in THF and ace-
tone whereas monomer 4a is not, due to its more polar
nature.
and 3.83 ppm and protons of the other methylene were
observed at 4.11 ppm for monomer 1 (Fig. 3). The rigid
structure of the molecule provided by the benzyl group may
be the reason for this behavior. Moreover, monomers 1 and
2 also have a peak seen as doublet of doublet for methine
proton due to phosphorus and resonance form of amide link-
1
age. The broad NH peak at 8.22 ppm in the H NMR spectra
of monomers 1 indicates a monosubstituted carbamide
group. The 13C NMR spectrum of monomer 3 is shown in
Figure 4. The doublet seen at 43.31, 44.79 ppm is due to the
carbon attached to phosphorus. In the FTIR spectra of the
monomers, monomer 3 showed peaks at 3253 cm21 (amide
V region), 1630 cm21 (amide I region) and 1539 cm21
(amide II region). Monomer 3 also showed absorption peaks
of C@C, P@O, and PAO peaks at 1630, 1222, 1022, and
959 cm21
.
The monomers (1–3 and 1a–3a) were characterized using
FTIR, 1H, 13C, and 31P NMR spectroscopies, as well as ele-
mental analysis. In the 1H NMR spectra of monomers 1 and
2, we observed two different triplet peaks for methyl pro-
tons and three different multiplets for methylene protons,
indicating special conformations of these monomers. Diaster-
eotopic protons were observed for one methylene at 3.64
1H NMR spectrum of monomer 1a shows the complete dis-
appearance of phosphonic ester peaks at 1.02, 1.24, 3.64,
3.83, and 4.11 ppm (Fig. 3). The FTIR spectrum of monomer
1a shows broad peaks in the region of 3000–2600 and
2300–2100 cm21 due to OH stretching, 1670–1600 cm21
due to OH bending and strong peaks at 1653, 1620, and
1529 cm21 due to C@O, C@C, and NH stretching,
TABLE 1 Solubility of Monomers 1a–4a in Selected Solvents
Monomer
H2O
EtOH
THF
Acetone
Diethyl Ether
CH2Cl2
1a
2a
3a
4a
1
1
1
1
1
1
1
1
1
1
1
–
1
1
1
–
–
–
–
–
–
–
–
–
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FIGURE 3 1H NMR spectra of monomers 1 and 1a.
respectively. Also the strong bands at 983 and 932 cm21
correspond to the symmetric and asymmetric vibration of
PAO. In the 13C NMR spectrum of monomer 3a, the disap-
pearance of the characteristic peaks of the ethyl groups of
the phosphonates at 15.58, 61.19, and 62.08 ppm shows
that the deprotection is complete (Fig. 4).
FIGURE 4 13C NMR spectra of monomers 3 and 3a.
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Acidity and Interactions of Monomers with
Hydroxyapatite
The pH values of aqueous solutions of the monomers (2 wt
%) were found to be 1.38, 1.42, 1.54, and 1.21 for mono-
mers 1a–4a, respectively, in the range expected for mild self-
etching adhesives. These results show that bisphosphonic
acid monomer (4a) is more acidic in comparison with the
others, due to electron withdrawing effect of the second
phosphonic acid group in its structure. The substitution of
the phenyl group by the hexyl group results a decrease in
pH, due to electron donating effect of alkyl chain.
To investigate the interaction of the synthesized monomers
with HAP, we used FTIR spectroscopy and XRD techniques.
HAP particles were treated with a mixture of monomer/
EtOH/H2O for 24 h and washed with water and ethanol. The
solid and the liquid separated from the mixture were ana-
lyzed by FTIR (Fig. 5). The adhesive monomer’s characteris-
tic C@O band was observed on the solid part, indicating
very low adsorption of monomers on the HAP surface due to
hydrogen bonding or complex formation. FTIR analysis of
the liquid part showed characteristic stretching bands of the
monomers and some changes of the P@O and PAO peaks
when compared with the corresponding monomer. For exam-
ple, the peaks at 983 and 932 cm21 due to symmetric and
asymmetric vibration of PAO in monomer 1a decreased and
a new peak appeared around 1026 cm21. This peak overlaps
FIGURE 6 XRD patterns of HAP, MDP-HAP, 1a-HAP, 2a-HAP,
and 3a-HAP samples.
4.8ꢀ and 7.1ꢀ appeared which are assigned to the formation
of hydrolytically stable MDP-Ca salts. These peaks are due to
assembling of MDP molecules into nanolayers caused by Ca
ions released upon partial dissolution of HAP. This nano-
layering forms a strong phase at the adhesive interface. XRD
spectra of HAP treated with acidic monomer (1a–3a) solu-
tions showed no peaks around 2h 5 2–8ꢀ which can be
assigned to monomer-Ca salts (Fig. 6). However, a new peak
was detected at around 2h 5 11.6ꢀ due to deposited CaH-
PO4.2H2O (DCPD), indicating presence of demineralization
which also contributes to chemical bonding. Similar results
were observed for the commercial adhesive Phenyl-P in
which no ionic bonding to HAP was detected.26 These results
can be explained by (i) decalcification of HAP by the synthe-
sized monomers and (ii) higher solubilities of monomer-Ca
salt than DCPD in water-EtOH solution. The intensity of the
peak corresponding to DCPD is lower for monomer 3a,
which can be explained by the less acidic property of this
monomer. Its decalcification ability is probably lower than
the other two monomers. All these results indicate that the
synthesized monomers will have bond strengths lower than
MDP. However, a comparison between the synthesized mono-
mers and MDP may not be totally appropriate, since the lat-
ter has a phosphoric acid group instead of a phosphonic acid
group.
with PO4 peak of HAP at 1022 cm21. These results indi-
32
cated that the synthesized monomers can decalcify HAP and
are also slightly adsorbed to HAP surfaces.
XRD patterns of monomer (1a–3a)-treated HAP particles
were explained according to the Adhesion-Decalcification
concept proposed by Yoshida et al.8 To compare the perform-
ance of the synthesized monomers, MDP, which has been
shown to have excellent binding ability, was used as a con-
trol monomer. Untreated HAP showed peaks at 2h5 26ꢀ and
32ꢀ (Fig. 6). Upon treatment with MDP, peaks around 2h5
Hydrolytic Stability
All the synthesized monomers have phosphonic acid groups
attached to the double bond through amide linkage so these
monomers are expected to be resistant to hydrolysis. In fact,
1H NMR spectra of the monomers in aqueous methanol at
37 ꢀC taken 20 days apart confirmed our expectation; no
hydrolysis was observed.
FIGURE 5 FTIR spectra of HAP, 1a-HAP-24 h-EtOH (HAP), 1a,
1a-HAP-24 h-EtOH (solution).
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TABLE 2 Compositions of the Evaluated Adhesives
rable methacrylates. Therefore, it is expected that monomers
1a and 3a will have higher rates of polymerization values
compared to 2a. This expectation was amply borne out by
the maximum rate of polymerization of 3a, but not by that
of 1a. The lower reactivity of monomer 1a is probably due
to its very rigid structure compared to monomer 3a. The
bisphosphonic acid-containing monomer (4a) was found to
be much less reactive than phosphonic acid-containing ones
(1a–3a). This can be also explained due to steric effect of
bulky bisphosphonic acid group compared to that of phos-
phonic acid.
Components
Proportions (wt %)
Acidic monomer
HEMA
11.1
20.0
11.0
33.1
24.0
0.80
GDMA
bis-GMA
EtOH
BAPO
Adhesive Properties
The overall conversions of the synthesized monomers were
found to be low: 41, 46, and 50% for monomers 1a, 2a, and
3a, respectively. These low conversions were expected due
to the high Tg of the polymerizing systems which depends
on flexibility of monomers.27 In fact, the order of conver-
sions, 3a > 2a > 1a > 4a, is correlated well with the flexibil-
ity of the monomers. The hydrogen bonded and aromatic
ring structures in monomers probably result in extremely
rigid systems and decreased conversion.
In order to evaluate the adhesive properties of the synthe-
sized acidic monomers in dental adhesives, experimental
adhesives were formulated. Each formulation contains a
phosphonic acid monomer, crosslinking monomers (GDMA
and bis-GMA),
a comonomer (HEMA), a photoinitiator
(BAPO), and EtOH in the same molar ratios (Table 2). A ref-
erence adhesive containing MDP was also prepared to com-
pare the efficiency of the synthesized monomers with a
commercial one. The adhesives were then used to induce
formation of a bond between the dental tissues and restora-
tive materials. SBS tests were carried out using a universal
testing machine (Instron) to evaluate the performance of
adhesives. The results of SBS tests reported in Table 3 show
the significant influence of the acidic monomer structure on
the adhesion performance. Each adhesive based on the syn-
thesized monomers has lower adhesion properties to dentin
than the formulations containing the reference, MDP. These
results can be explained by the higher acidity of MDP com-
pared to the synthesized monomers which allows deeper
penetration of MDP into dentin and also by lower solubility
of MDP’s salts which formed after interaction with HAP.
In order to test the potential of the synthesized monomers
to be used in self-etching dental adhesives, their copolymer-
ization kinetics with HEMA were also investigated. Various
formulations consisting of mixtures of HEMA and water (60/
40 wt %); HEMA, 1a–3a and water (at different ratios) were
photopolymerized (Fig. 8). The data indicated that replace-
ment of 15 wt % of HEMA with one of the synthesized
monomers in the formulation showed a remarkable effect on
both maximum polymerization rate and conversion values.
Although monomers 1a, 2a, and 4a in water have lower
maximum rates of polymerizations than HEMA in water,
their mixtures with HEMA gave values higher than both
HEMA in water and monomers in water. These results can
be explained by the flexibility increase of the polymerizing
systems. The more flexible monomer 3a with comparable
values of maximum polymerization rates is not affected. The
overall conversions were found to be 57, 67, 68, and 77%
for copolymers of 4a, 1a, 2a, and 3a with HEMA.
Photopolymerizations
Photopolymerizations of the synthesized monomers were
investigated with photodifferential scanning calorimetry.
First, the monomers were homopolymerized at 40 ꢀC in
water using BAPO as a photoinitiator. For comparison with
these monomers, commercial dental monomer HEMA was
also polymerized under the same conditions. Figure 7 shows
photopolymerization results such as the time to reach the
maximum polymerization rate (tmax), maximum rate of poly-
merization (Rpmax), and conversion. It was clearly seen that
all three synthesized monomers exhibit improved tmax com-
pared to HEMA, the acrylate monomers 1a and 3a having
shorter tmax than the methacrylate monomer 2a, consistent
with the trend that acrylates are more reactive than compa-
Bulk copolymerization of the acid monomers with a mixture of
HEMA and GDMA were also studied in the presence of DMPA
to compare reactivity of the synthesized monomers (Fig. 9).
The mixtures of HEMA/monomer/GDMA (5/2/3 mol/mol/
mol) and HEMA/GDMA (7/3 mol/mol) were photopolymer-
ꢀ
ized at 40 C. It was observed that the mixtures containing the
acid monomers showed improved tmax (4.8 s) values compared
to HEMA/GDMA. Maximum rates of polymerizations follow the
same order: HEMA/3a/GDMA > HEMA/2a/GDMA > HEMA/
1a/GDMA, as in monomer/water systems. The more flexible
monomer 3a has similar maximum rate compared to HEMA/
GDMA mixture.
TABLE 3 Shear Bond Strength Measurements
Acidic Monomer
Mean SBS (MPa)
SD (MPa)
Thermal Polymerizations
MDP
1a
27.0
13.7
18.0
15.9
8.3
2.3
9.0
3.1
Thermal homopolymerizations of monomers 1a, 2a, and 3a
were investigated in water at 65 ꢀC with V-50 as initiator.
The polymerization conditions and the characteristics of the
polymers are listed in Table 4. Although monomer 1a gave
2a
3a
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FIGURE 7 Rate–time and conversion–time curves for homopolymerizations of 1a, 2a, 3a and 4a using BAPO at 40 ꢀC.
soluble polymers in water, polymers of 2a and 3a precipi-
tated as insoluble solids during polymerization. The conver-
sions reached very high values (85–89%) within 24 h.
Soluble polymers of 3a were obtained during solution poly-
merization of this monomer in EtOH/water (2/1, v/v).
Although poly-1a is soluble in water, poly-2a and poly-3a
are not, due to their more hydrophobic character. All poly-
mers are soluble in ethanol.
FIGURE 8 Rate–time and conversion–time curves for copolymerizations of 1a, 2a, 3a, and 4a with HEMA using BAPO at 40 ꢀC.
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1H NMR spectrum of poly-1a indicated disappearance of
double bond peaks of the monomer at 5.74, 6.22, and 6.33–
6.44 ppm after polymerization (Fig. 10). The C@O stretching
(amide I) vibrations were observed at 1654 and 1642 cm21
in the FTIR spectra of monomer 3a and its polymer (Fig.
11). The NH bending (amide II) in the monomer and the
polymer appears at 1536 and 1528 cm21, respectively. These
differences are probably due to the differences of intermolec-
ular and intramolecular hydrogen bonding strengths. The
small peak observed at 1621 cm21, which corresponds to
C@C is absent in the polymer spectrum. The number average
molecular weight (Mn) for this polymer was found to be
200,000 g mol21, as estimated by GPC. The determination of
the molecular weight of poly-2a and poly-3a was not possi-
ble due to insolubility of these polymers in water or THF.
The glass transition temperatures (Tg) of the synthesized
polymers were measured by DSC to be 62 and 37 ꢀC for
poly-2a and 3a, respectively. The lower Tg of poly-3a is due
to the alkyl side chain in its structure. No Tg was observed
for poly-1a. In order to further investigate the reactivity of
the synthesized monomers, monomer 1a was copolymerized
with HEMA (1:9 mol ratio) in water (Table 4). FTIR spec-
trum of the crosslinked polymer obtained in 15 min showed
FIGURE 9 Rate–time and conversion–time curves for copoly-
merizations of 1a, 2a, and 3a with HEMA and GDMA using
DMPA at 40 ꢀC.
TABLE 4 Thermal Polymerization Resultsa
Monomer
[V-50]
Solvent
Time (h)
Conversion (%)
Mn
1a
0.025
0.025
0.025
0.05
H2O
24
85
200,000
2a
H2O
24
87
–
–
–
–
–
3a
H2O
24
89
3a
EtOH/H2O (2.5/1, v/v)
7 min
15 min
15 min
44
HEMA
0.025
0.025
H2O
H2O
Crosslinked
Crosslinked
HEMA/1a (9/1 mol %)
a
65 ꢀC [M]5 1 mol L21
.
FIGURE 10 1H NMR spectra of 3a and poly-3a.
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FIGURE 11 FTIR spectra of 3a and poly-3a.
REFERENCES AND NOTES
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