Macromolecules, Vol. 38, No. 23, 2005
Polymerization of (Meth)acrylate Monomers 9479
polymerization characteristics approximately equiva-
lent, but the steady-state polymerization rates are
additionally similar.
A potential explanation for the decrease in polymer-
ization rate with acid addition relies on hydrogen-
bonding reductions due to the presence of the acid. It is
theorized that the introduction of acid will block poten-
tial hydrogen-bonding sites at the carbonyl of the cyclic
carbonate moiety and/or the amine hydrogen of the
carbamate moiety, thus allowing for greater mobility
leading to lower polymerization rates. To check for
possible hydrogen-bonding reductions, the N-H stretch
vibration peak (∼3300 cm-1), N-H bend vibration peak
(∼1500 cm-1), and the carbonyl CdO stretch vibration
peak (∼1750 cm-1) were examined for frequency shifts
with and without acid addition. Decreased hydrogen
bonding can be identified by a frequency increase in the
N-H stretch vibration peak, a frequency increase in the
CdO stretch vibration peak, and/or a frequency reduc-
tion in the N-H bend vibration peak. Upon frequency
analysis of all the monomers with and without acid
addition, there did not appear to be any discernible
frequency shifts in any of the aforementioned peaks,
within the tolerance of the apparatus. Therefore, it is
believed that hydrogen bonding is not affected to any
significant extent by the addition of small amounts of
acid.
The results from these studies conclusively demon-
strate that the presence of a strong acid impurity has a
dramatic effect on the polymerization kinetics in both
acrylate and methacrylate monomers that have inher-
ently high reactivities. These studies do not provide
insight into the detailed mechanism, i.e., the chemical
identity and reactions of the particular reactive inter-
mediate that is responsible for the rate acceleration.
They do, however, indicate that it is highly probable that
an intermediate exists which has significant anionic
character either as a propagating active center or as a
transition compound from the traditional acrylic propa-
gating center to a more reactive active center. The
formation of this intermediate anionic species is re-
stricted by the presence of the methanesulfonic acid to
such an extent that reductions in polymerization rate
of up to 40 times are observed with as little as 400 ppm
acid. The control experiments involving multi(meth)-
acrylates and traditional mono(meth)acrylates demon-
strate that this mechanism is unique to this class of
monomers. Additionally, the control experiments il-
lustrate that the acid is not affecting the photodegra-
dation or the efficiency of the photoinitiator, and the
acid itself is not undergoing photodegradation or other
side reactions. Additional experiments are clearly re-
quired to ascertain the exact nature of both the reactive
intermediate and any mechanisms associated with its
formation. It is not at all obvious what the exact nature
of either the reactive intermediate or these mechanisms
are.
Figure 10. Ratio of polymerization rate averaged from 10 to
50% conversion for unaltered monomer divided by acid con-
taining monomer. Tetrahydrofurfuryl acrylate (b), 1,6-hex-
anediol diacrylate (9), cyclic carbonate acrylate (2), and phenyl
carbamate acrylate (1) are shown. The maximum rate reduc-
tion due to acid addition is ∼40-fold for 380 ppm methane-
sulfonic acid in phenyl carbamate acrylate.
the overall polymerization kinetics of the PhNCO Acr
with 250 ppm acid are slower than that of the hexyl
acrylate.
The results of the novel acrylate monomers with acid
addition showed a drastic increase in acid susceptibility
as compared to the methacrylate monomers. To dem-
onstrate further this increased susceptibility, Figure 10
shows the ratio of the average polymerization rates from
10 to 50% conversion for the pure monomer without acid
as compared to the monomer with acid for all of the
acrylate monomers studied in these experiments. From
Figure 10 it is apparent that the novel acrylate mono-
mers are influenced to a much greater extent by the acid
addition than their methacrylate counterparts.
Each of the steady-state methacrylic and acrylic
functionalized monomer studies indicates the possibility
of an anionic contribution to the overall polymerization.
A second analysis was performed to verify this hypoth-
esis. In this analysis an unsteady-state evaluation of
PhNCO Acr with 250 ppm methanesulfonic acid was
performed. Specifically, the light was extinguished at
less than 50% conversion, and the dark polymerization
was monitored. Along with this analysis, two control
studies were additionally performed. The first control
was the unsteady state of the pure PhNCO Acr, and the
second was the unsteady state of the traditional free-
radical monomer hexyl acrylate. Figure 9 show the
unsteady-state analysis of the PhNCO Acr with and
without acid and the unsteady-state analysis of the
PhNCO Acr with 250 ppm acid as compared to hexyl
acrylate, respectively. In this figure, the light was
extinguished at ∼38% for the unaltered PhNCO Acr,
and this monomer continued to react with ∼40% ad-
ditional conversion in the dark. On the contrary, for the
250 ppm solution, the light was extinguished at ∼44%
conversion, and this system achieved only ∼4% ad-
ditional conversion. This 4% dark conversion is indica-
tive of a more traditional free-radical polymerization
once the light source is extinguished. On the contrary,
the ∼40% dark conversion supports the anionic contri-
bution theory, as termination is clearly limited in this
system. The typical free-radical polymerization charac-
teristic is more evident in the figure, where the light is
extinguished at ∼49% conversion for hexyl acrylate, and
this monomer reacted only an additional ∼8% in the
dark. Figure 9 illustrates that not only are the dark
Monomer Degradation Analysis. To be certain
that the acid addition does not cause an acid-catalyzed
degradation of the monomer leading to the rate reduc-
tion, the NMR spectrum of phenyl carbamate acrylate
with 25 000 ppm (2.5 wt %) acid was evaluated. The
NMR spectrum of this solution is shown in Figure 11.
The NMR shows no apparent monomer degradation of
the monomer after 1 week at 25 °C. Additionally, FTIR
analysis of this sample does not indicate the presence
of monomer degradation, within the limits of the ap-