Structure-reactivity relationships of alkyl α-hydroxymethacrylate derivatives

Article abstract of DOI:10.1002/pola.24743

A series of alkyl α-hydroxymethacrylate derivatives with various secondary functionalities (ether, ester, carbonate, and carbamate) and terminal groups (alkyl, cyano, oxetane, cyclic carbonate, phenyl and morpholine) were synthesized to investigate the ef

Full text of DOI:10.1002/pola.24743

Structure–Reactivity Relationships of Alkyl a-Hydroxymethacrylate
Derivatives
_
_
OZLEM KARAHAN, DUYGU AVCI, VIKTORYA AVIYENTE
Department of Chemistry, Bogazici University, 34342 Bebek, Istanbul, Turkey
Received 18 February 2011; accepted 22 April 2011
DOI: 10.1002/pola.24743
Published online 16 May 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A series of alkyl a-hydroxymethacrylate derivatives
with various secondary functionalities (ether, ester, carbonate,
and carbamate) and terminal groups (alkyl, cyano, oxetane, cyclic
carbonate, phenyl and morpholine) were synthesized to investi-
gate the effect of intermolecular interactions, H-bonding, p–p
interactions, and dipole moment on monomer reactivity. All of
the monomers except one ester and one ether derivative are
novel. The polymerization rates, determined by using photo-
DSC, showed the average trend (aromatic carbamate > hydroxyl
> ester > carbonate ꢀ aliphatic carbamate ꢀ ether), with several
exceptions due to the differences in terminal groups. There is a
correlation between the chemical shift differences of the double
bond carbons, the calculated dipole moments, and the reactiv-
C
V
ities only for nonhydrogen bonded monomers. 2011 Wiley Peri-
odicals, Inc. J Polym Sci Part A: Polym Chem 49: 3058–3068, 2011
KEYWORDS: dipole moment; methacrylates; photopolymeriza-
tion; radical polymerization; structure–property relations
INTRODUCTION Acrylates and methacrylates are the most
commonly used monomers in photoinitiated polymerizations
due to their high reactivities and excellent polymer proper-
ties. They are used in dental materials, biomaterials, coat-
ings, adhesives, and photolithography.1–3 Because of the wide
variety of application areas, extensive research has been con-
ducted to understand the relationship between the monomer
structure and reactivity and to develop monomers with
enhanced reactivity.
metry experiments also proved a correlation between reduc-
tion potential of the monomers and Michael addition and
photopolymerization reactivities.5
Andrzejewska reported that the presence of the heteroatom in
the ester group led to enhanced reactivity due to the CH2 group
attached to the heteroatom donating hydrogen to give a cross-
linked network. This reaction also leads to a reduction in oxy-
gen inhibition and faster rates and higher conversions in air.12
Jansen et al. investigated the rate of polymerization of differ-
ent acrylates as a function 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-bonded analogs possess-
ing ester and carbonate groups.13 The high reactivities were
suggested to be due to preorganization via hydrogen bond-
ing to bring the double bonds close to each other, enhancing
propagation, although reduction in termination rate may also
be involved or be the cause. Hoyle et al. showed that the
degree of hydrogen bonding and the rate of polymerization
of hydroxyalkyl acrylates are directly related and both
decrease with increasing temperature. Although they could
not find a quantitative relationship between hydrogen bond-
ing and termination rate constants, they claimed that highly
hydrogen-bonded systems behave as multifunctional mono-
mers and have low termination constants.14
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,
secondary functionalities).
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.4–11
It was observed that the presence of secondary functional-
ities (carbamates, carbonates, cyclic carbonates, cyclic ace-
tals, morpholine, oxazolidones, hydroxyl, and aromatic rings)
enhances the reactivity by reducing the activation energies
in both Michael addition and photopolymerizations as indi-
cated by a monotonic correlation between reactivities of
monomers with respect to both processes. The cyclic voltam-
Jansen et al. also investigated the effect of monomer polarity
on the rate of polymerization and found a direct correlation
Additional Supporting Information may be found in the online version of this article. Correspondence to: D. Avci (E-mail: avcid@boun.edu.tr)
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between the maximum rate of polymerization and the dipole
moment for the monomers having dipole moments higher
than 3.5 Debye. However, Kilambi et al. found no monotonic
correlation between monomer reactivity and molecular
dipole moment during bulk polymerization of various acry-
late monomers.7 They suggested that a low dipole moment
conformation of some monomers may be more reactive due
to intermolecular hydrogen bonding than a conformation
with a higher dipole moment.
mL). The organic phase phase was separated, dried with an-
hydrous sodium sulfate, and evaporated under reduced pres-
sure. The crude products were purified by column chroma-
tography (silica gel 0.063–0.200 mm) using hexane initially
and gradually changing to CH2Cl2 as eluent.
Ethyl 2-((2-morpholinoethoxy)methyl)acrylate (Monomer
1). Monomer 1 was obtained aꢃs2a colorless liquid in 62%
yield (bp ¼ 76–78 ꢂC/1.2 ꢁ 10 mbar). Viscosity ¼ 0.014
Pa s.
Monomers based on alkyl a-hydroxymethacrylates (RHMA)
and their halide derivatives offer great versatility for func-
tionalization. Conversion of the alcohol group to various
ester, ether, and other derivatives has been demonstrated by
us and others.15–22 Although these derivatives have smaller
kp values than methyl methacrylate due to the steric effect of
the a-substituent, they have good polymerizability due to
their low kt values, and this type of polymerization has been
called ‘‘steric hindrance assisted polymerization’’ by Yamada
et al.23 Studies on the reactivities of several ester derivatives
of ethyl a-hydroxymethacrylate indicated that aromatic
esters are more reactive than nonaromatic ones, ether deriv-
atives, and methyl methacrylate.16,23
1H NMR (400 MHz, CDCl3, d, ppm): 1.3 (t, 3H, CH3), 2.5 (t,
4H, CH2AN), 2.6 (t, 2H, CH2AN), 3.6 (t, 2H, CH2AO), 3.7 (t,
4H, CH2AO), 4.1 (m, 4H, CH2ACH3, CH2AO), 5.8 (s, 1H,
CH2¼¼C), 6.3 (s, 1H, CH2¼¼C). 13C NMR (400 MHz, CDCl3, d,
ppm): 14.0 (CH3), 53.9, 58.1 (CH2AN), 60.5 (CH2ACH3),
66.8, 68.3, 69.1 (CH2AO), 125.3 (CH2¼¼C), 137.3 (C¼¼CH2),
165.7 (C¼¼O). FTIR (cmꢃ1): 2941 (CAH), 1713 (C¼¼O), 1638
(C¼¼C), 1108 (CAO). ELEM. ANAL., Calcd. for C12H21NO4: C,
59.26%; H, 8.64%; N, 5.76%; O 26.34%. Found: C, 58.98%;
H, 9.03%; N, 5.67%; O, 26.32%.
Characterization results of monomers 2–7 are given as Sup-
porting Information.
In this work, we report the synthesis and photopolymeriza-
tions of ester, ether, carbonate, and carbamate derivatives of
RHMA to evaluate the role of hydrogen bonding, dipole
moment, and p–p interactions on the reactivity.
General Procedure for the Synthesis of Monomers 8 and 9
Ethyl chloroformate (5.43 g, 50 mmol) in 10-mL CCl4 was
added dropwise to the mixture of RHMA (38 mmol) and pyr-
idine (3.95 g, 50 mmol) in 20-mL CCl4 under nitrogen. The
mixture was stirred at room temperature for 0.5 h. After re-
moval of the solvent, the mixture was poured into ice water
and neutralized with NaHCO3 (5%). The mixture was
extracted with CH2Cl2, dried with anhydrous sodium sulfate
and evaporated under reduced pressure. The crude product
was purified by column chromatography (silica gel 0.063–
0.200 mm) using hexane initially and gradually changing to
CH2Cl2 as eluent.
EXPERIMENTAL
Reagents
Ethyl a-hydroxymethacrylate (EHMA), t-butyl a-hydroxymetha-
crylate (TBHMA), t-butyl a-bromomethacrylate (TBBr), and
ethyl a-bromomethacrylate (EBBr) were synthesized according
to literature procedures.24,25 2-[2-(2-Methoxyethoxy)ethoxy]-
acetic acid, 3-hydroxypropionitrile, 3-hydroxymethyl-3-methyl-
oxetane, ethyl chloroformate, 4-(2-hydroxyethyl)morpholine,
4-(hydroxymethyl)-1,3-dioxolan-2-one, 2,6-di-tert-butyl-4-methyl
phenol (BHT), trifluoroacetic acid, triethyl amine (TEA), 2-hydrox-
yethyl methacrylate (HEMA), butyl isocyanate, and phenyl
isocyanate were obtained from Aldrich and Fluka and used as
obtained.
Tert-butyl 2-((ethoxycarbonyloxy)methyl)acrylate (Mono-
mer 9). Monomer 9 was obtained as a light yellow liquid in
35% yield (bp ¼ 67–68 ꢂC/1.2 ꢁ 10ꢃ2 mbar). Viscosity ¼
0.006 Pa s.
1H NMR (400 MHz, CDCl3, d, ppm): 1.3 (t, 3H, CH3ACH2), 1.5
(s, 9H, CH3AC), 4.2 (q, 2H, CH2ACH3), 4.8 (s, 2H, CH2AO),
5.8 (s, 1H, CH2¼¼C), 6.3 (s, 1H, CH2¼¼C). 13C NMR (400 MHz,
CDCl3, d, ppm): 14.2 (CH3), 27.9 (CH3AC), 65.6 (CH2ACH3),
68.9 (CH2AO), 81.4 (CACH3), 126.3 (CH2¼¼C), 136.4
(C¼¼CH2), 154.7 (C¼¼O), 164.1 (C¼¼O). FTIR (cmꢃ1): 1706
(C¼¼O), 1751 (C¼¼O), 1640 (C¼¼C).
Characterization
The monomer characterization involved 1H and 13C nuclear
magnetic resonance (NMR) spectroscopy (Varian Gemini 400
MHz) and Fourier transform infrared (FTIR) spectroscopy (T
380). The photopolymerizations were carried out on a TA
Instruments (Q100) photodifferential scanning calorimeter
(photo-DSC). Elemental analyses were obtained from Thermo
Electron SpA FlashEA 1112 elemental analyser (CHNS sepa-
ration column, PTFE; 2 m; 6 ꢁ 5 mm2).
Characterization results of monomer 8 are given as Support-
ing Information.
General Procedure for the Synthesis of Monomers 10–12
The acid (10 mmol), EBBr or TBBr (10 mmol), K2CO3 (1.38
g, 10 mmol), and methyl ethyl ketone (MEK; 25 mL) were
added to a round-bottomed flask with a nitrogen inlet. The
mixture was stirred at 60 ꢂC for 24 h. After removal of the
solvent, the mixture was diluted with CH2Cl2 and extracted
with water (3 ꢁ 5 mL). The organic phase was separated,
dried with anhydrous sodium sulfate, and evaporated under
Synthesis of the Monomers
General Procedure for the Synthesis of Monomers 1–7
To a mixture of alcohol (10 mmol) and TEA (3.86 g, 38
mmol) in 5 mL THF in an ice bath, EBBr or TBBr (10 mmol)
in 5-mL THF was added dropwise. The mixture was stirred
at 60 ꢂC for 24 h. After removal of the solvent, the mixture
was diluted with CH2Cl2 and extracted with water (3 ꢁ 5
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
reduced pressure. The crude products (10 and 11) were
purified by column chromatography (silica gel 0.063–0.200
mm) using hexane initially and gradually changing to CH2Cl2
as eluent. Monomer 12 was purified by distillation.
mW/cm2. The heat flux as a function of reaction time was
monitored using DSC under isothermal conditions and both
the rate of polymerization (Rp) and conversion were calcu-
lated as a function of time. The theoretical values used for
the heats of reaction (DHp) were 13.1 kcal/mol for methac-
rylate double bonds.26,27 Rates of polymerization were calcu-
lated according to the following formula:
Ethyl 2-((2-(2-(2-methoxyethoxy)ethoxy)acetoxy)methyl)
acrylate (Monomer 10). Monomer 10 was obtained as a
colorless liquid in 41% yield. Viscosity ¼ 0.010 Pa s.
1H NMR (400 MHz, CDCl3, d, ppm): 1.3 (t, 3H, CH3), 3.4 (s,
3H, CH3AO), 3.55 (t, 2H, CH2AO), 3.65 (t, 2H, CH2AO), 3.69
(t, 2H, CH2AO), 3.75 (t, 2H, CH2AO), 4.2 (m, 4H, CH2AO,
CH2ACH3), 4.9 (s, 2H, CH2AO), 5.9 (s, 1H, CH2¼¼C), 6.4 (s,
1H, CH2¼¼C). 13C NMR (400 MHz, CDCl3, d, ppm): 14.1 (CH3),
58.9 (CH3AO), 60.9 (CH2ACH3), 62.5, 68.5, 70.4, 70.5, 70.8
(CH2AO), 127.6 (CH2¼¼C), 135.0 (C¼¼CH2), 164.9 (C¼¼O),
169.8 (C¼¼O). FTIR (cmꢃ1): 2878 (CAH), 1757 (C¼¼O), 1717
(C¼¼O), 1640 (C¼¼C), 1106 (CAO). ELEM. ANAL., Calcd. for
ðQ=sÞM
Rate ¼
(1)
nDHp
m
where Q/s is the 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
Calculation of Dipole Moments
Boltzman-averaged dipole moments were calculated with
PM3 for all the monomers. For this purpose, all posible rota-
tions around single bonds were considered for a given acry-
late to generate all the conformations corresponding to sta-
tionary points. Minimization, followed by the calculation of
the Boltzmann-averaged dipole moments for all the confor-
mations was carried out with PM3 by using Spartan ’06.28
The unique structures were sorted in the order of increasing
energy. The dipole moments of the first 100 conformers are
Boltzmann averaged at 298.15 K according to the following
formula:
C13H22O7: C, 53.79%; H, 7.59%; O, 38.62%. Found: C,
53.61%; H, 7.79%; O, 38.60%.
Characterization results of monomers 11 and 12 are given
as Supporting Information.
General Procedure for the Synthesis of Monomers 13–16
The isocyanate (7.69 mmol) was added dropwise to a mix-
ture of RHMA (7.69 mmol) and BHT (4 mg, 0.018 mmol) in
an ice bath. After stirring (1.5 h for monomers 13 and 14
and 10 h for monomers 15 and 16) at 60 ꢂC, the mixture
was washed with hexane.
eDH =RT
X
j
X
Ethyl 2-((phenylcarbamoyloxy)methyl)acrylate (Mono-
mer 13). Monomer 13 was obtained as a white solidꢂ after
recrystallization from methanol in 79% yield (mp ¼58 C).
P
hlcalci ¼
Dj
¼
Djpj
(2)
eDH =RT
i
j
j
i
1H NMR (400 MHz, CDCl3, d, ppm): 1.3 (t, 3H, CH3), 4.3 (q,
2H, CH2ACH3), 4.9 (s, 2H, CH2AO), 5.9 (s, 1H, CH2¼¼C), 6.4
(s, 1H, CH2¼¼C), 6.7 (s, 1H, NHAC), 7.1 (t, 1H, Ar-CH), 7.3–
7.4 (m, 4H, Ar-CH). 13C NMR (400 MHz, CDCl3, d, ppm): 14.2
(CH3), 61.0 (CH2ACH3), 63.2 (CH2AO), 118.7, 123.6 (Ar-CH),
127.6 (CH2¼¼C), 129.1 (Ar-CH), 135.7 (C¼¼CH2), 137.6 (Ar-C),
152.8 (C¼¼O), 165.3 (C¼¼O). FTIR (cmꢃ1): 3349 (NAH), 2982
(CAH), 1728 (C¼¼O), 1691 (C¼¼O), 1637 (C¼¼C), 1601 (C¼¼C),
1540 (NAH). ELEM. ANAL., Calcd. for C13H15NO4: C, 62.65%; H,
6.03%; N, 5.63%; O, 25.69%. Found: C, 62.35%; H, 6.26%; N,
5.81%; O, 25.58%.
where Dj is the dipole moment of the conformation j, DHj is
the heat of formation of conformation j, T is the absolute
temperature, R is the Boltzmann constant, and pj is the prob-
ability of finding the monomer in conformation j at the tem-
perature T.13
RESULTS AND DISCUSSION
Synthesis of the Monomers
The general structure of the synthesized monomers can be
given as
Characterization results of monomers 14–16 are given as
Supporting Information.
Photopolymerization
Approximately 3.0 or 4.0 mg of sample was placed in an alu-
minum DSC pan. The photoinitiator, which was dissolved in
CH2Cl2, was added with a microsyringe to give a final con-
centration in the monomer of 2.0 mol % after the evapora-
tion of the solvent. 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 polymer-
ization. Heats of photoreactions were measured using a DSC
equipped with a mercury arc lamp. The samples were irradi-
where R1 is ethyl or t-butyl group due to methacrylate, R2 is
the secondary functionality, which displays the ester, ether,
carbonate, and carbamate groups, and R3 is the terminal
group. The terminal groups were varied to incorporate highly
polar or cyclic groups.
The synthesis of all the monomers is illustrated in Figure 1.
The ether derivatives (1–7) were synthesized by the reaction
of alcohols with EBBr and/or TBBr using TEA as catalyst in
THF at 60 ꢂC for 24 h: 4-(2-hydroxyethyl)morpholine with
EBBr and TBBr gave 1 and 2; 3-hydroxypropionitrile with
ꢂ
ated for 10 min at 40 C with an incident light density of 20
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ARTICLE
monomer 8 may be explained by one of the two following
mechanisms: (i) sorption of carbon dioxide released during
storage of the monomer to residual pyridine and formation
of protonated pyridine bicarbonates in the presence of
water30 and (ii) attack of residual pyridine to double bond
of the monomer with the release of carbonate ion and form-
ing a carbonate salt. Because we were not able to solve colo-
ration problem, we did not further investigate this monomer.
The ester derivatives were synthesized from the reactions of
EBBr and/or TBBr and 2-[2-(2-methoxyethoxy)ethoxy]acetic
acid and benzoic acid to increase the dipole moment in an
effort to investigate the electronic effect of high dipole
moment (l) and also p–p interactions. Also, the monomers
obtained from 2-[2-(2-methoxyethoxy)ethoxy]acetic acid will
contain two ether linkages with abstractable hydrogens,
which may reduce the oxygen inhibition.31 The reaction was
carried out at 60 ꢂC in methyl ethyl ketone in the presence
of the catalyst potassium carbonate and gave the ester deriv-
atives 10, 11, and 12.
To investigate the importance of hydrogen bonding and pꢃp
interactions in the polymerization rate, the carbamate deriv-
atives 13–16 were prepared via a reaction of the isocyanates
(phenyl and butyl isocyanates) with EHMA and TBHMA with
catalyst (dibutyltin dilaurate) at room temperature or with-
out catalyst at 60 ꢂC. The reactions were monitored by fol-
lowing the disappearance of the isocyanate peak in the FTIR
spectrum.
The synthesis of monomers gave the crude products in high
yields (80–90%). All of the monomers were liquids except
monomer 13, which was a white solid (mp 58 ꢂC) at room
temperature. The monomers were purified by column chro-
matography, distillation, or recrystallization and character-
ized by using FTIR, 13C NMR, and 1H NMR spectra (Figs. 2
and 3). The 1H NMR of monomer 1 showed methyl protons
at 1.3 ppm, two methylene protons adjacent to nitrogen at
2.5 and 2.6 ppm, four methylene protons adjacent to oxygen
at 3.6, 3.7, and 4.1 ppm and double bond protons at 5.8 and
6.3 ppm (Fig. 2). The 13C NMR spectrum of monomer 6
showed characteristic peaks for methyl carbon at 14.1 ppm,
methylene carbons at 60.8, 66.1, 69.6, and 74.9 ppm, a
methine carbon at 69.8 ppm, double bond carbons at 126.3
and 136.4 ppm, and carbonyl carbons at 154.8 and 165.5
ppm (Fig. 3). The FTIR spectra of monomers 13 and 14 are
shown in Figure 4.
FIGURE 1 Synthesis of monomers.
EBBr and TBBr gave 3 and 4; 3-hydroxymethyl-3-methyl-
oxetane with EBBr gave 5, 4-(hydroxymethyl)-1,3-dioxolan-2-
one with EBBr gave 6 and phenol with EBBr gave 7. Four of
these (1, 2, 5, 6) contain cyclic terminal groups, which were
found to enhance the reactivities of acrylates and methacry-
lates.5 The other two (3 and 4) contain a cyano group and
are designed to assess the electron withdrawing effect of the
substituent. Monomer 7 was synthesized to evaluate the im-
portance of pꢃp interactions. The reaction time for mono-
mer 3 was decreased to 7 h due to unexpected polymeriza-
tion in 24 h. This monomer polymerizes without an initiatꢂor
to give a polymer with Mn and Tg values of 3200 and 45 C
during the synthesis; other indications and possible reasons
for the high reactivity of this monomer are discussed later in
this work.
Photopolymerizations
The reactivities of the synthesized monomers in photopoly-
merization were investigated with photodifferential scanning
calorimetry and compared with those of EHMA, TBHMA, and
HEMA. Photopolymerizations were carried out using 2,2-
The carbonate derivatives (8 and 9) were prepared from the
reactions of EHMA and TBHMA with ethyl chloroformate
using pyridine as the catalyst; the aim being to evaluate the
enhancement of reactivity due to carbonates, as discussed
above. Although the crude products were purified by column
chromatography, monomer 8 quickly turned to dark red
color after storage, even refrigerated. In the literature, the
use of pyridine and dimethylaniline during the synthesis of
(2-oxo-1,3-dioxolan-4-yl)methyl acrylate produced yellow
and dark purple colors.29 The color change during storage of
ꢂ
dimethoxy-2-phenylacetophenone (DMPA; 2 mol %) at 40 C.
EHMA, TBHMA, and HEMA polymerizations behave in the
same way in which a distinct shoulder at the onset of
autoacceleration was observed (Fig. 5 and Table 1). This
shoulder was observed earlier for EHMA and HEMA when
compared with TBHMA. It is known that intermolecular
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FIGURE
2
1H NMR spectra of
monomers 1, 10, and 13.
ꢃ1
ꢂ
hydrogen bonding and/or chain transfer reactions leading to
an increase in viscosity and a decrease in termination rate
cause autoacceleration. The extent of autoacceleration was
observed in the following order: HEMA > EHMA > TBHMA,
this was also the order of their maximum rates of polymer-
ization. Increasing the size of the ester group from ethyl to
t-butyl decreased the polymerization rate due to steric
effects. We observed this behavior in all of the synthesized
monomers here.
50 C as 944 and 2563 L mol
s
ꢃ1. The activation energy of
HEMA (21.9 kJ molꢃ1) is not significantly different from the
one of EHMA (20.4 kJ molꢃ1) but the frequency factor (which
reflects the steric effect) for HEMA (8.88 ꢁ 106 L molꢃ1 sꢃ1
)
is higher than that of EHMA (1.87 ꢁ 106 L molꢃ1
s
ꢃ1). Davis
et al. also reported the activation energy and the frequency
factor for EHMA as 14–17 kJ molꢃ1 and (3–8) ꢁ 106 L molꢃ1
s
ꢃ1, respectively.33,34 They also found an important solvent
effect on kp of EHMA that changes between 580 L molꢃ1 sꢃ1
(THF) and 1860 L molꢃ1 sꢃ1 (xylene) at 15 ꢂC.35
The high reactivity of HEMA compared with EHMA was also
observed by Buback et al.32 They reported propagation rate
coefficients for bulk polymerizations of EHMA and HEMA at
In general, overall monomer conversion increases for systems
that are polymerized at faster rates. As the polymerization
FIGURE
3
13C NMR spectra of
monomers 4 and 6.
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ARTICLE
FIGURE 4 FTIR spectra of monomers 13
and 14.
rate increases, the volume relaxation is unable to keep pace
with conversion, leading to increased free volume formation.
The free volume in excess of equilibrium values causes higher
mobility, which results in increased conversion. Also, conver-
sion depends on the polymerization temperature and the Tg
of the system, which indicates its mobility. The Tg of a poly-
merization system depends on flexibility of monomers. For
example, polar groups are responsible for intramolecular and
intermolecular interactions and decrease flexibility of the sys-
tem, increase Tg and decrease conversion.
rate, whereas chain transfer increases with increasing tem-
perature which tends to increase the polymerization rate. To
investigate the effect of temperature change on polymeriza-
tion rate, the monomers were polymerized at 25, 40, and 70
ꢂC. We found that rate of polymerizations of these monomers
increase roughly with temperature (Table 1). This can be
explained by competing effects of activation, which occurs by
increasing temperature and deactivation, which is due to
decreased hydrogen bonding. The FTIR spectrum of HEMA
clearly shows two carbonyl peak maxima at 1713 and 1699
cmꢃ1 due to a hydrogen-bonded carbonyl and free carbonyl
stretching bands. However, FTIR spectrum of EHMA and
TBHMA shows one peak at 1704 and 1707 cmꢃ1 corre-
sponding more to a hydrogen-bonded carbonyl stretching
band (Fig. 6). Additionally, thermal bulk polymerization of
HEMA and EHMA gave crosslinked polymers, indicating the
importance of chain transfer reactions.
The overall conversions were 90, 69, and 51% for HEMA,
EHMA, and TBHMA. It was observed that the greater the
extent of autoacceleration, the higher the conversion. The Tg
values for poly-HEMA, poly-EHMA, and poly-TBHMA are 83–
25
ꢂ
95, 64, and 120 C. Low conversion of TBHMA is due to a
combination of high Tg of its polymer and low rate of
polymerization.
The rates of polymerization of the phenyl carbamate mono-
mers (monomers 13 and 14) were found to be significantly
higher than those of the monomers with other functional
groups (Fig. 7, Table 1); even higher than EHMA and
TBHMA. The intermolecular interactions due to hydrogen
bonding and/or p–p stacking are probably the reason for the
high reactivity of monomers 13 and 14.
It is known that hydrogen bonding decreases with increasing
temperature, which tends to decrease the polymerization
The different states of monomer 13 and 14 (solid vs. liquid)
indicate their different extent of hydrogen bonding. The
extent of hydrogen bonding was evaluated by NH and C¼¼O
stretching modes observed by FTIR spectra of monomers
(Fig. 4). The locations of NH peaks were 3349 and 3331
cmꢃ1 (monomers 13 and 14), both correspond to hydrogen-
bonded NH peaks.36 The locations of C¼¼O peaks were 1690
(carbamate) and 1728 cmꢃ1 (ester) for monomer 13.
Because monomer 13 is crystalline and its C¼¼O groups are
completely bonded to NH, the peak at 1690 cmꢃ1 is due to
ordered carbamate C¼¼O. The other C¼¼O peak at 1728 cmꢃ1
is due to the free ester C¼¼O. However, monomer 14 is liquid
and both C¼¼O groups (carbamate and ester) can make
FIGURE 5 Rate of polymerization versus time for HEMA,
EHMA, and TBHMA.
_
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TABLE 1 Rates of Polymerization, Conversions, Calculated Boltzmann-Averaged Dipole Moments, and Differences
in Chemical Shift Values of Monomers 1–16, HEMA, EHMA, and TBHMA
Monomer
Monomer Structure
Rate (sꢃ1
)
Converison (%)
Dipole Moment (Debye)
2.99
Dd (ppm)
13
0.038b
0.045c
44
63
8.1
HEMA
0.031a
0.036b
0.042c
0.033b
81
90
83
89
3.55
10.0
12
6
3.18
3.99
8.5
0.023a
0.026b
0.030c
64
56
74
10.1
7
0.024b
80
3.66
2.67
9.6
14
0.022b
62
10.6
EHMA
0.016a
0.019b
0.023c
0.015b
61
69
71
94
3.19
14.0
10
3.60
2.76
7.4
TBHMA
0.012a
0.015b
0.016c
0.012b
49
51
61
90
16.4
3
3.35
10.4
1
0.009b
0.008b
0.007b
78
70
77
2.73
2.94
2.93
12.0
9.8
11
15
13.1
9
0.006b
0.005b
0.005b
85
82
72
2.26
2.54
2.35
10.1
11.8
15.5
5
16
(Continued)
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ARTICLE
TABLE 1 (Continued)
Monomer
Monomer Structure
Rate (sꢃ1
0.004b
)
Converison (%)
65
Dipole Moment (Debye)
2.01
Dd (ppm)
4
12.6
2
0.003b
66
2.21
14.3
a
At 25 ꢂC.
b
At 40 ꢂC.
c
At 70 ꢂC.
hydrogen bonding with NH. Therefore, carbamate C¼¼O shifts
to higher frequency and overlaps with hydrogen bonded
ester C¼¼O at 1710 cmꢃ1. No free ester C¼¼O exists. All these
results confirm a greater extent of hydrogen bonding in
monomer 13, which results in high rate of polymerization.
Therefore, photopolymerization of this monomer was investi-
gated in both crystalline (40 ꢂC) and noncrystalline states
(70 ꢂC; Table 1). The results showed that polymerization
occurs in both states and the rate increases with tempera-
ture. Also, higher conversions were obtained in noncrystal-
line state, as expected.
mate ethyl acrylate was 1.3 sꢃ1 whereas n-butyl carbamate
ethyl acrylate gave a much lower rate of polymerization of
0.38 sꢃ1. The conversions obtained for monomers 15 and 16
were higher than those of monomers 13 and 14 despite
their lower rates of polymerization, indicating the enhanced
mobility of these monomers.
Among the ester-linked monomers (10, 11, and 12), the aro-
matic ester monomer (12) capable of p–p interaction exhib-
ited about twice the rate of polymerization of the aliphatic
monomer (10), approaching even that of the most reactive
carbamate monomer (13; Fig. 8, Table 1). This result was
previously indicated with the evaluation of kp and kt of the
To investigate the effect of pꢃp stacking on monomer reac-
tivity, the aliphatic carbamate monomers (15 and 16) were
polymerized. These monomers were found to be about four
or five times less reactive than phenyl carbamate monomers
(13 and 14; Fig. 7, Table 1). They have rates of polymeriza-
tion comparable with ether derivatives. These monomers
were liquids and the locations of NH (3344 cmꢃ1) and over-
lapping ester and carbamate C¼¼O peaks (1710 cmꢃ1) in
their FTIR spectra were similar to those of monomer 14.
Thus, we can say that the strength of hydrogen bonding of
these monomers (14, 15, and 16) has no discernible effect
on the polymerization rate. The relatively low reactivity of
monomers 15 and 16 despite their ability to still make
hydrogen bonding brings out the importance of p–p stacking.
We observed a similar behavior in the literature.37 For exam-
ple, the maximum rate of polymerization of phenyl carba-
benzoate derivative (990 and 2.9 ꢁ 10ꢃ6 L molꢃ1
s
ꢃ1) of
EHMA and the acetate derivative (kp ¼ 350, kt¼ 2.1 ꢁ 10ꢃ6
L molꢃ1 ꢃ1) of MHMA.23,38 Also, Davis et al. indicated that
s
when going from EHMA to its acetate derivative both Eact
(12.4 kJ molꢃ1) and the frequency factor (8 ꢁ 104 L molꢃ1
s
ꢃ1) decrease.33 Monomer 10, due to its very flexible struc-
ture and high rate of polymerization exhibits the highest
conversion among the monomers studied in this work.
The carbonate derivative (9) was found to be less reactive
than ester and aromatic carbamate derivatives, EHMA,
TBHMA, and HEMA. It showed similar reactivity with
TBHMA-ether and TBHMA-aliphatic carbamate derivatives.
FIGURE 6 FTIR spectra of carbonyl stretching regions of
FIGURE 7 Rate of polymerization versus time for monomers 13
and 15.
HEMA, EHMA, and TBHMA.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
The rate differences of ether derivatives can be explained by
the differences in dipole moment as will discussed in the
next section.
Dipole Moment
The synthesized monomers were evaluated in terms of
dipole moment to find a relation between the monomer
structure and the reactivity. The Boltzmann-averaged dipole
moments of the monomers were calculated for their mini-
mum energy conformers (Table 1). When we consider non-
hydrogen bonding monomers (1–12), there seems to be a
correlation except for monomers 7 and 12 (Table 1, Fig. 9).
This exception may be due to an additional pꢃp interaction,
which increases the rate of polymerization. The high reactiv-
ity of monomer 6 can be explained by its high dipole
moment.
FIGURE 8 Rate of polymerization versus time for monomers 3,
6, 7, 10, and 12.
When we consider all hydrogen-bonding monomers (13–16,
HEMA, EHMA, and TBHMA), it was not possible to find a
correlation between monomer reactivity and dipole moment.
For example, monomers 13 and 15, both having approxi-
mately similar dipole moments (2.99 and 2.93 Debye),
showed completely different polymerization rates. Kilambi
et al. have also investigated hydrogen-bonding monomers
having high reactivity but low dipole moments. They said
that a particular conformation with a low dipole moment
may be energetically favored due to intermolecular hydrogen
bonding.7
On average, ether derivatives were found to be less reactive
than ester derivatives (Table 1). Yamada et al. previously
reported kp and kt values for the butyl ether derivative of
MHMA as 298 and 8.0 ꢁ 10ꢃ6 L mol sꢃ1, respectively.23,39
Although this kp value is similar to that of the acetate deriva-
tive, the higher kt value of the ether derivative is responsible
for the lower rate of polymerization. However, we observed
that rates of ether derivatives can be increased by modifica-
tions of the terminal groups. The maximum rate of polymer-
izations of EHMA ether derivatives (1–7) with different ter-
minal groups showed the following trend: cyclic carbonate >
phenyl > cyano > morpholine > oxetane, where Rp,max
(monomer 6) ¼ 1.08, Rp,max (monomer 7) ¼ 1.83, Rp,max
(monomer 3) ¼ 2.44, Rp,max (monomer 1) ¼ 4.4, and Rp,max
(monomer 5; Fig. 8, Table 1). This trend was similar to that
observed for acrylate monomers reported by Jansen et al.
where Rp,max (glycerol carbonate acrylate) ¼ 2.16, Rp,max
(cyano ethyl acrylate) ¼ 3.66 and Rp,max (oxethane acry-
late).13 The high reactivity of monomer 6 may be due to
hydrogen abstraction, ring stacking interactions, and rela-
tively high dipole moment. However, monomers that also
possess a heterocyclic ring (1, 2, and 5) did not exhibit such
rate enhancement. To check the possibility of hydrogen
abstraction, the photopolymerization of monomer 6 in the
presence of benzophenone was tried; no polymerization was
observed, indicating that there are no labile hydrogens under
our photopolymerization conditions. Additionally, the poly-
mers obtained from this monomer were totally soluble indi-
cating that chain transfer is not important. The effect of tem-
perature on photopolymerization of 6 was also investigated
(Table 1). Monomer 6 exhibited small changes in polymeriza-
Chemical Shift Values
13C NMR can be used to predict the free radical polymeriz-
ability of monomers. Vaidya et al. have reported that chemi-
cal shifts of the C¼¼C double bonds (CbH2¼¼Ca) depend on
the substituents. dCb and dCa shift to lower and higher fields,
respectively, with an increase in electron withdrawing power
of the substituents.40 Therefore, Dd (dCb–dCa) shows the
effects of substituents on polymerizability. The stronger the
electron-withdrawing power of the substituents the smaller
the chemical shift difference and the higher the radical poly-
merizability.40,41 Also, bulky substituents may cause differen-
ces in chemical shifts and/or lead to lower propagation en-
thalpy through steric hindrance.
To see a correlation between polymerizability and differen-
ces in chemical shift values (Dd), we evaluated the mono-
mers in three categories. In the first category (HEMA, EHMA,
and TBHMA), the reactivity trend was correlated to Dd
ꢂ
tion rate over the 25–70 C range, while the conversion was
increased by about 25%. Although monomer 6 has the high-
est rate of polymerization among the ether derivatives, its
conversion was lower, similar to cyclic carbonate-containing
(meth)acrylates reported in the literature,11 which may be
attributed to early autoacceleration and/or high Tg of its
polymer (85 ꢂC). Among the ether derivatives, monomer 3
with a high rate of polymerization and flexible structure
exhibited highest conversion.
FIGURE 9 Rate of polymerization versus calculated dipole
moment for monomers 1–11 (except for 7 and 12).
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ARTICLE
7 Kilambi, H.; Beckel, E. R.; Berchtold, K. A.; Stansbury, J. W.;
values (Table 1). As expected, TBHMA was found to be the
least reactive monomer due to steric effect. The second cate-
gory involved ester, ether, and carbonate derivatives (1–12)
of RHMA monomers. Here also, the reactivity of ester deriva-
tives, compared with carbonate and ether derivatives was
confirmed by low Dd values. For example, the maximum
rates of polymerization of TBHMA derivatives (11, 9, 4, and
2) with the chemical shift values of 9.8, 10.1, 12.6, and 14.3
Bowman, N. C. Polymer 2005, 46, 4735–4742.
8 Berchtold, K. A.; Nie, J.; Stansbury, J. W.; Hacioglu, B.; Beckel,
E. R.; Bowman, N. C. Macromolecules 2004, 37, 3165–3179.
9 Beckel, E. R.; Stansbury, J. W.; Bowman, N. C. Macromole-
cules 2005, 38, 9474–9481.
10 Beckel, E. R.; Nie, J.; Stansbury, J. W.; Bowman, N. C. Mac-
ppm were found to be 0.008, 0.006, 0.004, and 0.003 sꢃ1
.
romolecules 2004, 37, 4062–4069.
11 Berchtold, K. A.; Nie, J. N.; Stansbury, J. W.; Bowman, C. N.
Among the ether derivatives, monomers 3, 6, and 7 were
expected to have the highest reactivity, comparable with
those of the ester derivatives.
Macromolecules 2008, 41, 9035–9043.
12 Andrzejewska, E.; Andrzejesky, M.
J Polym Sci Part A:
Polym Chem 1998, 36, 665–673.
The last category involved carbamate derivatives (13–16) of
RHMA monomers. The Dd values were well correlated with
the reactivities, indicating the low reactivities of monomers
15 and 16.
13 Jansen, J. F. G. A.; Dias, A. A.; Dorschu, M.; Coussens, B.
Macromolecules 2003, 36, 3861–3873.
14 Lee, T. Y.; Roper, T. M.; Jonsson, S.; Guymon, C. A.; Hoyle,
C. E. Macromolecules 2004, 37, 3659–3665.
CONCLUSIONS
15 Jariwala, P. C.; Mathias, L. J. Macromolecules 1993, 26,
Novel RHMA derivatives were prepared and evaluated using
photopolymerization rates to understand the relation
between the monomer structure and the reactivity. It was
observed that the nature of both secondary functionalities
(ester, ether, carbonate, and carbamate) and the terminal
groups (phenyl, cyano, morpholine, butyl, etc.) have signifi-
cant effects on polymerization kinetics. Depending on the
type of the secondary functionality, the polymerization rate
may increase up to five times by changing terminal groups.
The pꢃp interactions were found to be an important rate
enhancing factor. Among the monomers studied here, aro-
matic carbamates capable of both pꢃp interactions and
hydrogen bonding were found to show highest rate of poly-
merization. Studies on the other aromatic carbamate deriva-
tives of RHMA monomers are continuing. The high reactivity
of cyclic carbonate-containing monomers was shown once
more with a new derivative. The relationship between the
polymerization and the dipole moment of monomers for
nonhydrogen-bonded monomers seems to hold.
5129–5136.
16 Avci, D.; Kusefoglu, S. H.; Thompson, R. D.; Mathias, L. J.
Macromolecules 1994, 27, 1981–1982.
17 Mathias, L. J.; Thigpen, K.; Avci, D. Macromolecules 1995,
28, 8872–8874.
18 Thompson, R. D.; Barclay, T. B.; Basu, K. R.; Mathias, L. J.
Polymer J 1995, 27, 325–338.
19 Avci, D.; Ziylan Albayrak, A. J Polym Sci Part A: Polym
Chem 2003, 41, 2207–2217.
20 Avci, D.; Mathias, L. J. J Polym Sci Part A: Polym Chem
2002, 40, 3221–3231.
21 Avci, D.; Mathias, L. J. Polymer 2004, 45, 1763–1769.
22 Avci, D.; Kusefoglu, S. H. J Polym Sci Part A: Polym Chem
1993, 31, 2941–2949.
23 Yamada, B.; Kobatake, S. Prog Polym Sci 1994, 19,
1089–1131.
24 Warren, S. C.; Mathias, L. J. J Polym Sci Part A: Polym
The computational resources used in this work were provided
by the TUBITAK ULAKBIM High Performance Computing Cen-
ter, and the National Center for High Performance Computing of
Turkey (UYBHM) under the grant number 20792009.
Chem 1990, 28, 1637–1648.
25 Mathias, L. J.; Warren, M. R.; Huang, S. Macromolecules
1991, 24, 2036–2042.
26 Anseth, K. S.; Wang, C. M.; Bowman, C. N. Macromolecules
1994, 27, 650–655.
REFERENCES AND NOTES
27 Brandrup, J.; Immergut, E. H. Polymer Handbook, 2nd Ed.;
Wiley-Interscience: New York, 1975; pp II-424.
1 Kloosterboer, J. G. Adv Polym Sci 1988, 84, 1–61.
2 Decker, C. Prog Polym Sci 1996, 21, 593–650.
28 Spartan version 4.0, 1995; Wavefunction, Inc.; Irvine, CA.
29 D’Alelio, G. F.; Huemmer, T. J Polym Sci Part A: Polym
3 Anseth, K. S.; Newman, S. M.; Bowman, C. N. Adv Polym Sci
Chem 1967, 5, 307–321.
1995, 122, 177–217.
30 Burton, B. L. In Proceedings of Epoxy Resin Formulators’
Meeting of the Society of the Plastics Industry, Baltimore, MD,
July, 2001.
4 Moussa, K.; Decker, C. J Polym Sci Part A: Polym Chem
1993, 31, 2197–2203.
5 Kilambi, H.; Reddy, S. K.; Schneidewind, L.; Stansbury, J. W.;
Bowman, N. C. J Polym Sci Part A: Polym Chem 2009, 47,
4859–4870.
31 Lee, T. Y.; Guymon, C. A.; Jonsson, E. S.; Hoyle, C. E. Poly-
mer 2004, 45, 6155–6162.
32 Beuermann, S.; Buback, M. Prog Polym Sci 2002, 27,
6 Lu, H.; Stansbury, J. W.; Nie, J.; Berchtold, K. A.; Bowman, C.
191–254.
N. Biomaterials 2005, 26, 1329–1336.
_
STRUCTURE–REACTIVITY RELATIONSHIPS OF RHMA, KARAHAN, AVCI, AND AVIYENTE
3067

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
33 Barner-Kowollik, C.; Vana, P.; Davis, T. P. In Handbook of
Radical Polymerization; Matyjaszewski, K.; Davis, T. P., Eds.;
Wiley: New York, 2002; Chapter 4, pp 197–198.
37 Kilambi, H.; Stansbury, J. W.; Bowman, C. H. Macromole-
cules 2007, 40, 47–54.
38 Sato, T.; Kamiya, I.; Tanaka, H.; Ota, T. Eur Polym J 1991,
34 Davis, T. P.; Heuts, J. P. A.; Barner-Kowollik, Harrisson, S.;
Morrison, D. A.; Yee, L. H.; Kapfenstein, H. M.; Coote, M. L.
Macromol Symp 2002, 182, 131–148.
27, 1087–1092.
39 Yamada, B.; Satake, M.; Otsu, T. Makromol Chem 1991, 192,
2713–2722.
35 Morrison, D. A.; Davis, T. P. Macromol Chem Phys 2000,
40 Vaidya, R. A.; Mathias, L. J. J Polym Sci Polym Symp 1986,
201, 2128–2137.
74, 243–251.
36 Coleman, M. M., Lee, K. H.; Skrovanek, D. J.; Painter, P. C.
41 Kodaira, T.; Fujisawa, T.; Liu, Q.-Q.; Urushisaki, M. Macro-
Macromolecules 1986, 19, 2149–2157.
molecules 1996, 29, 484–485.
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