Effect of aliphatic spacer substitution on the reactivity of phenyl carbamate acrylate monomers

Article abstract of DOI:10.1021/ma048359l

Novel monoacrylate monomers with a carbamate secondary functionality and a terminal aryl group were synthesized to study the mechanisms that allow for increased reactivity of these monomers compared to typical monoacrylate monomers. To test for the possibility of hydrogen abstraction from the two-carbon aliphatic spacer group, varying methyl substituents were added to one or both of the spacer carbons. Single methylation at the α and β carbons showed no drastic effect on the polymerization rate. However, dual methylation of the a carbon reduced the overall polymerization rate by approximately 5-fold. Conversely, dual methylation at the β carbon reduced the polymerization rate by only 2-fold. Unsteady state analysis of the α,α-methylated monomer showed that the apparent kinetic constants approach that of traditional monoacrylate monomers. Thus, hydrogen abstraction appears to be a likely explanation for these unique substitution effects on monoacrylate reactivity where the substitution number and location have a profound impact on the polymerization rate of these novel monomers. Disubstitution at the α-position has the most profound influence on the rate.

Full text of DOI:10.1021/ma048359l

Macromolecules 2005, 38, 3093-3098
3093
Effect of Aliphatic Spacer Substitution on the Reactivity of Phenyl
Carbamate Acrylate Monomers
Eric R. Beckel,† Jeffrey W. Stansbury,‡ and Christopher N. Bowman*,†,‡
Department of Chemical and Biological Engineering, University of Colorado,
Boulder, Colorado 80309-0424, and Department of Restorative Dentistry,
University of Colorado Health Sciences Center, Denver, Colorado 80045-0508
Received August 9, 2004; Revised Manuscript Received November 30, 2004
ABSTRACT: Novel monoacrylate monomers with a carbamate secondary functionality and a terminal
aryl group were synthesized to study the mechanisms that allow for increased reactivity of these monomers
compared to typical monoacrylate monomers. To test for the possibility of hydrogen abstraction from the
two-carbon aliphatic spacer group, varying methyl substituents were added to one or both of the spacer
carbons. Single methylation at the R and â carbons showed no drastic effect on the polymerization rate.
However, dual methylation of the R carbon reduced the overall polymerization rate by approximately
5-fold. Conversely, dual methylation at the â carbon reduced the polymerization rate by only 2-fold.
Unsteady state analysis of the R,R-methylated monomer showed that the apparent kinetic constants
approach that of traditional monoacrylate monomers. Thus, hydrogen abstraction appears to be a likely
explanation for these unique substitution effects on monoacrylate reactivity where the substitution number
and location have a profound impact on the polymerization rate of these novel monomers. Disubstitution
at the R-position has the most profound influence on the rate.
Introduction
Decker et al. have developed a novel class of monoacry-
late monomers that react extremely rapidly despite
having only one vinyl group and still form cross-linked,
insoluble polymer early in the polymerization.3,14-18
These polymers are characterized by a unique combina-
tion of excellent hardness and flexibility in the same
material. These novel acrylates were designed to include
secondary functionalities such as carbonate, cyclic car-
bonate, carbamate, and oxazolidone moieties.14,15 The
polymerization rate was most rapid for a monoacrylate
containing both cyclic carbonate and linear carbonate
secondary functionalities.14 Additional novel monoacry-
late monomers that exhibit similar enhanced polymer-
ization characteristics were developed and analyzed in
our laboratory.19-21 Unfortunately, the exact mechanism
or mechanisms by which these unique polymer proper-
ties develop are yet to be fully understood.
Three distinct mechanistic theories to account for the
enhanced reactivity of these monoacrylates have been
advanced: (1) hydrogen abstraction/chain transfer, (2)
hydrogen bonding, and (3) electronic and resonance
effects. Decker et al. first proposed a highly efficient
hydrogen abstraction mechanism to account for the
increased reactivity and cross-linked polymer formation,
implicating the labile hydrogens associated with the
secondary functionality as the abstractable species.2,22
In addition to hydrogen abstraction, hydrogen bonding
may also contribute to the increased reactivity by
increasing monomer ordering and subsequently increas-
ing the efficiency of radical propagation through the
prealigned double bonds.23 Furthermore, hydrogen bond-
ing increases the overall viscosity of the bulk monomer
solution, thus hindering radical termination and causing
an increase in radical concentration during polymeri-
zation. This increase in radical concentration will
directly increase the polymerization rate. Recently, in
a systematic study of monomethacrylate monomers with
varying secondary functionalities and end groups, Ber-
chtold et al. determined that hydrogen bonding appears
The photopolymerization process is widely used in
today’s industrial market to convert liquid monomer
rapidly to solid polymer for a variety of applications.
These applications include inks, coatings, adhesives,
stereo-lithography,1-3 dental restoratives,4-6 and con-
tact lenses.7,8 The photopolymerization curing mode is
utilized for numerous reasons, which include solvent-
free polymerizations, energy efficiency, and spatial and
temporal control. However, there are several limitations
to the photopolymerization process that restrict the
industrial viability of these materials. These limitations
include residual unstaturation,9,10 oxygen inhibition,11-13
low polymerization rate, and reduced polymer proper-
ties. A typical method to increase the polymerization
rate of acrylic systems is to utilize monomers with more
than one vinyl group. By increasing the monomer
functionality, diffusional limitations are encountered
earlier in the polymerization, and thus, termination is
more restricted. The reduction in termination leads to
more profound autoacceleration and increased polym-
erization rates. Additionally, in this case, cross-linked
materials are formed, which leads to increased modulus
and hardness. Unfortunately, increasing the cross-
linking density additionally leads to brittleness and
residual unsaturation, leading to polymer property
variations with age and possible biocompatibility issues.
These tradeoffs between polymerization rate, polymer
properties, and residual unsaturation are a key con-
stituent in the development and selection of monomers
for use in polymerization applications. There has been
a long-standing desire to counteract these limitations
and develop monomers that polymerize to a higher
extent of reaction with greater polymerization rates.
† University of Colorado.
‡ University of Colorado Health Sciences Center.
* To whom correspondence should be addressed: Fax 303 492
4341; e-mail christopher.bowman@colorado.edu.
10.1021/ma048359l CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/11/2005

3094 Beckel et al.
Macromolecules, Vol. 38, No. 8, 2005
Table 1. Molecular Structures of All Monomers Studied
to have an impact on the polymerization characteristics
of these monovinyl monomers.24 Nonetheless, by sys-
tematically varying the polymerization temperature, it
was determined that the polymerization rate was
relatively unaffected by temperature. This study dem-
onstrated that hydrogen bonding may be important but
is not the central factor affecting reactivity.
Finally, electronic and resonance effects may also
impact the reactivity of these monoacrylate monomers.
Specifically, resonance may allow for such mechanisms
as an increase in the amount of labile hydrogens or (de)-
stabilization of radicals. Recently, Jansen et al. have
developed the theory of increasing polymerization rate
with increasing dipole moment of the monomer.25-27 In
this dipole moment theory the polymerization rate
increases linearly with dipole moment when the overall
dipole moment of the solution exceeds a threshold value
of 3.5 D. Recently, experimental data have shown that
the resonance and electronic effects do not appear to be
the primary source of the enhanced reactivity for a
specific class of novel monomers. Novel acrylic mono-
mers with a carbamate functionality and aryl end group
were synthesized with differing electron-withdrawing
and electron-donating substituents attached to the aryl
end group.28 These monomers have varying overall
dipole moments, ranging from 1.8 to 3.7 D. Additionally,
a high dipole moment solvent was added to the higher
dipole moment monomers to create a greater overall
dipole moment medium (∼4.3 D). In all cases, there did
not appear to be a clear correlation between electronic
and resonance effects and overall polymerization rate.
These three mechanistic theories are interrelated and
are extremely difficult to decouple in experimentation,
but specific experiments can be developed to emphasize
each of the above mechanisms individually to test their
validity.
Each of the three mechanistic theories has been
shown to affect polymerization rates in certain monomer
systems. However, no clear explanation for the dramati-
cally enhanced reactivity of these monomers relative to
standard monoacrylate momomers has been identified.
Hydrogen-bonding and electronic and resonance effects
appear to contribute to the reactivity of the monomers
but do not appear to be the primary contributors.
This work develops and analyzes the possibility of
hydrogen abstraction from a two-carbon aliphatic spacer
group between the acrylic moiety and the secondary
functionality. This research expands on previous work
where the base monomer is a monoacrylate with an aryl
carbamate secondary functionality. However, in this
research, the two-carbon aliphatic spacer group is
methyl-substituted at the R and/or â carbon positions.
This systematic aliphatic spacer group methylation is
designed to test for the possibility of hydrogen abstrac-
tion from the spacer group and to provide an indication
of the primary abstraction site, if present.
FTIR Analysis. Conversion vs time profiles for all mono-
mers were determined via steady-state analysis using real
time Fourier transform infrared (RT-FTIR) spectroscopy.29-31
Conversion data were obtained by monitoring the decay of the
acrylate double bond peak, either ∼1630 cm-1 (CdC stretching
vibration) or ∼810 cm-1 (CdC twisting vibration), using a
Nicolet Magna 760 FTIR spectrometer (Madison, WI) with an
XT-KBr beam splitter and MCT/B detector. The temporal
resolution for this instrument is approximately 200 ms. For
ease of sample handling, a custom horizontal transmission
accessory (HTA) was utilized to allow for the samples to
remain in a horizontal configuration during analysis.4,32 All
samples were placed between NaCl crystals to form a laminate
and were irradiated with 5 mW/cm2 ultraviolet (UV) light
(filtered and centered at a wavelength of 365 nm) from an
EXFO Ultracure 100ss (Mississauga, Ontario, Canada) light
source; steady-state irradiation duration was 5 min. Addition-
ally, all samples were preheated to a temperature of 67 °C for
analysis using a custom temperature cell,19,24 since most of the
monomer samples are solids or highly viscous liquids at room
temperature.
Kinetic parameter data were determined via unsteady state
analysis, utilizing similar RT-FTIR methodology as described
above. Unsteady state analysis is equivalent to the steady state
analysis, except the UV irradiation is extinguished prior to
complete conversion and the dark polymerization is monitored.
To calculate the kinetic parameters, the reaction diffusion
coefficient is first determined from the following equation:4,29,33
1
2R
∆[M] )
ln(2RRp0t + 1)
(1)
Experimental Section
where ∆[M] is the change in double-bond concentration in the
dark, R is the reaction diffusion coefficient, defined as the ratio
of the termination kinetic constant over the propagation
termination constant times the double-bond concentration (kt/
kp[M]), Rp0 is the polymerization rate at time zero (shutter
closed) in the dark, and t is the time that the double-bond
concentration is monitored in the dark. To decouple the kinetic
constants, the reaction diffusion coefficient is combined with
Materials. All monomers evaluated in this work were
synthesized in our laboratory. Table 1 gives the molecular
structures of all monomers studied. Synthesis and purification
information can be found in the Supporting Information. In
all cases no inhibitors were added to the monomers. All
samples were initiated with 0.1 wt % 2,2-dimethoxy-2-phenyl-
acetophenone (DMPA), purchased and used as received from
Ciba Geigy (Hawthorne, NY).

Macromolecules, Vol. 38, No. 8, 2005
Phenyl Carbamate Acrylate Monomers 3095
the steady state rate expression (eq 2) just prior to the
extinction of light.
1/2
kp
kt1/2
Ri
Rp )
[M]
(2)
(2 )
In eq 2, Ri is defined as the initiation rate.34 It is very
important to note that the kinetic constants for propagation
and termination calculated by this method are apparent
kinetic parameters that lump in many different reactions and
radical types for both propagation and termination. For these
systems, it is not possible to perform PLP/SEC experiments
because of the extremely high chain transfer rates and the
tendency to form gel at low conversions.
Molecular Modeling. Globally minimized molecular mod-
els of specific monomers were evaluated to determine possible
conformational effects on the reactivity of these monomers.
To perform these molecular models, CAChe 4.5 (Fujistu
America, Inc., Sunnyvale, CA) was utilized to determine the
lowest energy conformations of the monomers. In these
simulations each molecule was first globally minimized twice
via an augmented molecular mechanics MM2 simulation,
followed by a semiempirical AM1 simulation to determine the
lowest energy conformation of the molecule.
Figure 1. Acrylate conversion vs time for R,â-substituted
monomer. Phenyl carbamate acrylate (1) and phenyl carba-
mate R,â-methyl acrylate (2) are shown. Only a slight initial
decrease in polymerization rate is demonstrated by the
substituted monomer; otherwise, polymerization rate profiles
are similar. All polymerizations were initiated with 0.1 wt %
DMPA at 5 mW/cm2 and 67 °C.
group between the carbamate secondary functionality
and the acrylic moiety were exchanged with deuteriums.
Deuterated acrylate monomers had a very slow initial
polymerization rate that autoaccelerated into a maxi-
mum polymerization rate that was 4 times lower than
that of the undeuterated monomer.35 Therefore, hydro-
gen abstraction does appear to be a key and rate-
limiting reaction in the overall reactivity of these
monomers.
Here, we explore further the possible importance of
hydrogen abstraction from the aliphatic spacer group
and the impact of this abstraction on the polymerization
rate of the monomer. Systematic studies were performed
where the aliphatic spacer group was substituted with
methyl groups, and the effect on the steady state
polymerization rate was analyzed.
r,â-Dimethyl Acrylate. Initial studies have impli-
cated the aliphatic spacer group as the location for
hydrogen abstraction; however, it is uncertain which
carbon is the active site for this hydrogen abstraction.
Systematic elimination of hydrogen abstraction sites by
methylation allows for insight into the possible active
center for the abstraction. The first methylation experi-
ment utilized a single methyl at each of the R and â
carbons of the spacer group. The addition of the methyl
groups to the molecule has a 3-fold effect on hydrogen
abstraction. First, each methyl group will remove a
single hydrogen from the molecule that could have
otherwise been abstracted. Second, the addition of the
methyl group increases the stability of a radical at that
specific carbon, thus allowing for the possibility of
increased hydrogen abstraction from that site. Third,
the methyl substituents will act as a steric hindrance
to abstraction from the aliphatic spacer group, thus
possibly reducing the total hydrogen abstraction from
the spacer group. Because of the symmetry of the
molecule, the single methylation of the R and â carbons
will not reveal the major site of the hydrogen abstrac-
tion; however, it will give a greater indication into the
possible abstraction mechanism hypothesized for en-
hanced reactivity of this novel monomer. Figure 1 shows
the steady state polymerization rate of the unsubsti-
tuted and methylated molecules, and Table 2 gives the
Rp,max and time for the polymerization to proceed from
10 to 50% conversion for all monomers studied. Figure
1 and Table 2 show that there is not a significant
Results
The theory of hydrogen abstraction proposed is that
the labile hydrogens associated with the secondary
functionality lead to a highly efficient hydrogen abstrac-
tion/chain transfer reaction. Recent results that tested
the possibility of electronic and resonance effects on the
overall polymerization rate indicate a possible contra-
diction to this hydrogen abstraction theory from the
secondary functionality and end group. In these experi-
ments a monoacrylate with a carbamate secondary
functionality and a perfluorinated aryl end group was
synthesized, and subsequent steady state analysis of
this monomer showed only a 2-fold decrease in polym-
erization rate with respect to the hydrocarbon analogue,
with no major inhibition or slow initial polymerization
rate.28 On the basis of the assumption that abstraction
occurs from the aryl ring, a more profound decrease in
polymerization rate would be expected, since the fluo-
rines are not prone to abstraction. Additionally, it was
found that very little cross-linking occurred in these
systems, including the unsubstituted monomer. Thus,
the end group does not appear to be the location for
abstraction in these molecules. Additionally, experi-
ments by Berchtold with other monomer structures
indicate that hydrogen abstraction from the secondary
functionally may not be occurring.19 Berchtold synthe-
sized and analyzed a monoacrylate monomer with a
carbamate secondary functionality and benzyl end group
where the N-H of the carbamate was substituted with
a methyl group (N-CH3). Steady state polymerization
of this monomer revealed that the polymerization rate
of this monomer was slower than the unsubstituted
monomer. However, the reduction in polymerization
rate was solely attributed to the reduction in viscosity,
leading to an increase in mobility, due to the elimination
of the strong hydrogen bond donor. Thus, it appears that
hydrogen abstraction from the secondary functionality
and end group is not the primary mechanism for the
enhanced polymerization rates.
Recent results utilizing the primary isotope effect of
deuteriums have indicated the profound effect of hy-
drogen abstraction on the overall reactivity of several
novel acrylate monomers.19,35 Experiments were con-
ducted where the hydrogens on the aliphatic spacer

3096 Beckel et al.
Macromolecules, Vol. 38, No. 8, 2005
Table 2. Maximum Polymerization Rate, Polymerization
Time Required To React from 10 to 50% Conversion, and
Polyerization Time Required To React from 0 to 65%
Conversion for all Unsubstituted and Substituted
Monomers Studied
spacer substitution
Rp,max (1/s)
tx ) 10-50% (s) tx ) 0-65% (s)
unsubstituted
R,â-methyl
R-methyl
0.36 ( 0.03
0.35 ( 0.03
0.34 ( 0.02
0.28 ( 0.02
0.08 ( 0.02
0.16 ( 0.02
1.2
1.3
1.3
1.6
20
2.1
2.8
2.7
3.0
39
â-methyl
R,R-dimethyl
â,â-dimethyl
3.1
5.6
difference in reactivity between the methylated and
nonmethylated phenyl carbamate acrylate monomer.
There is a slight difference in initial reactivity, as the
nonmethylated monomer reacts more rapidly at the
initial stages of polymerization. Nevertheless, there is
no significant difference in the maximum polymeriza-
tion rates of the two monomers. Additionally, there is
not a significant difference in the time for each monomer
to polymerize from 10 to 50% conversion. The explana-
tion for the steady state results is theorized to be
dependent on the unique combination of reducing the
number of abstractable hydrogens while simultaneously
changing the resulting radical to a tertiary radical. In
general, tertiary radicals are more stable than second-
ary radicals, and this stable tertiary radical may stifle
secondary reactions at the onset of polymerization.
Thus, the initial polymerization rate would be slower
in the substituted monomer until the secondary reac-
tions that may facilitate increased reactivities occur,
allowing for approximately equivalent polymerization
rates in the substituted and unsubstituted monomers.
Figure 2. Acrylate conversion vs time for R- and â-substituted
monomers. Phenyl carbamate acrylate (1), phenyl carbamate
R-methyl acrylate (2), and phenyl carbamate â-methyl acrylate
(3) are shown. Again, only a slight initial decrease in polym-
erization rate is demonstrated by the substituted monomers;
otherwise, polymerization rate profiles are similar. All poly-
merizations were initiated with 0.1 wt % DMPA at 5 mW/cm2
and 67 °C.
Because of the symmetric nature of the aliphatic
spacer group, the R,â-disubstituted monomer does not
give much insight into the possible hydrogen abstraction
location on the aliphatic spacer group. To gain greater
insight into the possible hydrogen abstraction location,
methylation solely on either the R or â carbon is
required.
Figure 3. Acrylate conversion vs time for R,R- and â,â-
substituted monomers. Phenyl carbamate acrylate (1), phenyl
carbamate â,â-methyl acrylate (2), and phenyl carbamate R,R-
methyl acrylate (3) are shown. The R,R-substituted monomer
shows a greatly reduced polymerization rate profile as com-
pared to the unsubstituted monomer. Additionally, the â,â-
substituted shows a slightly decreased polymerization rate
profile as compared to the unsubstituted monomer. All poly-
merizations were initiated with 0.1 wt % DMPA at 5 mW/cm2
and 67 °C.
r(â)-Methyl Acrylate. To discern possible site-
specific abstraction effects, monomers with single
methylation at the R or â carbon were synthesized.
These monomers allow the individual carbons in the
spacer group to be analyzed for their effect on the
polymerization rate. Figure 2 shows the steady state
polymerization rate of the two singly methylated mono-
mers. The steady state results of the singly methylated
monomers greatly resemble that of the R,â monomer
results. Specifically, there is a slightly lower initial
polymerization rate, which autoaccelerates into a maxi-
mum polymerization rate that mimics that of the
unsubstituted monomer. More specifically, the â-sub-
stituted monomer attains a maximum polymerization
rate that is marginally slower than that of the unsub-
stituted monomer, and the R-substituted monomer
nearly matches that of the unsubstituted monomer.
Because of the initially slower polymerization rate, it
is again hypothesized that the addition of the methyl
groups promotes hydrogen abstraction and the forma-
tion of more stable tertiary radicals. Again, these stable
tertiary radicals may suppress secondary reactions that
could lead to increased polymerization rates at the onset
of polymerization. Once these secondary reactions occur,
the polymerization rates of the substituted and unsub-
stituted monomers would become approximately equiva-
lent.
As with the R,â-disubstituted monomer, the results
of the singly methylated monomer studies do not
significantly indicate which, if either, of the spacer
group carbons contribute to the enhanced reactivity.
Thus, studies with dual methylation on either the R or
â carbon are required to discern the possible effects of
this proposed hydrogen abstraction. The dual methyla-
tion eliminates any possible hydrogen abstraction from
one of the spacer group carbons.
r,r(â,â)-Dimethyl Acrylate. Figure 3 shows the
steady state results of the R,R- and â,â-dimethylated
monomers. From these steady state results it is evident
that the R,R-dimethylated monomer shows a slow initial
polymerization rate which autoaccelerates at about 20%
conversion. This autoacceleration leads to a maximum
polymerization rate that is approximately 4 times lower
than the maximum rate of the unsubstituted monomer.
Conversely, the â,â-dimethylated monomer does not
show this slow initial polymerization rate. Instead, early
in the polymerization, this monomer attains a maximum
rate that is only approximately half that of the unsub-

Macromolecules, Vol. 38, No. 8, 2005
Phenyl Carbamate Acrylate Monomers 3097
Figure 4. Globally minimized molecular structures of the unsubstituted (A), R,R-disubstituted (B), and â,â-disubstituted (C)
monomers. From these molecular models, it does not appear that the secondary functionality sterically hinders the radical chain
end to an extensive degree, except for the â,â-disubstituted monomer.
stituted monomer. From these steady state results, it
appears that the R carbon is the predominate site for
hydrogen abstraction, leading to enhanced reactivity of
these novel monomers. The evidence stems from the
extremely slow initial polymerization rate, delayed
autoacceleration, and a maximum polymerization rate
that is comparable to that of a typical monoacrylate.
disubstituted monomer are approaching that of tradi-
tional monoacrylate monomers. However, the â,â-sub-
stituted monomer still demonstrates the characteristics
of enhanced reactivity associated with the novel class
of monomers. More specifically, the apparent propaga-
tion kinetic constant for the â,â-disubstituted monomer
is approximately 1.5 times greater than that of the
unsubstituted monomer, but the apparent termination
kinetic constant is approximately 10 times greater than
the unsubstituted monomer. The large increase in the
termination reaction does not have a obvious explana-
tion at this point. Again, inspection of the steady state
polymerization rate equation reveals that the 2-fold
decrease in the â,â-substituted monomer rate is due to
the approximately 10-fold increase in the termination
kinetic constant. From the apparent kinetic constants
it is evident that elimination of the hydrogens at the
R-position decreases the reactivity of this monomer
drastically.
A potential explanation for the increased reactivity
of the unsubstituted monomer relies on steric blocking
of the radical chain end by the bulky secondary func-
tionality. This steric blocking of the radical chain end
would limit the two sterically blocked chain ends from
terminating, leading to a sharp decrease in the termina-
tion kinetic constant. This hypothesis is supported by
the unsubstituted monomer kinetic constant evaluation,
as the termination in this monomer is greatly decreased
over traditional monoacrylates. However, this hypoth-
esis is not supported by molecular modeling data of the
three monomers studied herein. Figure 4 shows the
globally minimized conformations of the unsubstituted
and R,R- and â,â-disubstituted monomers, with in-
tramolecular hydrogen bonding taken into consider-
ation. As seen from this figure, in the unsubstituted and
R,R-disubstituted monomers, the bulky secondary func-
tionality is distant from the radical chain end and does
not appear to cause significant steric blocking. However,
in the â,â-disubstituted monomer (Figure 4C), the
secondary functionality does appear to have a potential
steric blocking effect on the radical chain end despite
having the largest apparent termination kinetic con-
stant of the three monomers. Thus, steric blocking of
the radical chain end leading to decreased termination
does not appear to be the primary source of the
increased reactivities in these monomers.
Despite complex polymerization reactions that are not
completely known, it is important to ascertain the
kinetic relationships that lead to the variation in the
reactivity of the disubstituted monomers. To discern the
kinetic relationships of these monomers, unsteady state
analyses were performed to determine the apparent or
overall kinetic constants of each monomer at approxi-
mately 50% conversion. These unsteady state results
entail extinguishing the light at various extents of
conversion and monitoring the dark polymerization. It
is important to note that numerous reactions are
occurring during the polymerization process, such as
propagation, backbiting, and chain transfer to monomer
and polymer. The calculated kinetic constants reported
here are apparent or overall values for propagation and
termination. However, since a consistent method was
utilized to determine these values for each monomer,
qualitative analysis and comparison of the kinetic
relationships with respect to reactivity is feasible.
Using the aforementioned unsteady state analysis,
the average propagation and termination kinetic con-
stants for the unsubstituted and R,R- and â,â-substi-
tuted monomers were determined. The average propa-
gation kinetic constant for the unsubstituted, R,R, and
â,â monomers are 2 × 104, 7 × 103, and 3 × 104 L/(mol
s), respectively, while the average termination kinetic
constant for the unsubstituted, R,R, and â,â monomers
are 7 × 104, 2 × 105, and 7 × 105 L/(mol s), respectively.
For comparison, the average propagation and termina-
tion kinetic constants for butyl acrylate at 20 °C
determined via PLP/SEC and subsequent simulations
are 7.6 × 103 and 3 × 107 L/(mol s), respectively.36 The
unsteady state results indicate that the R,R-disubsti-
tuted monomer exhibits a sharp decrease in the appar-
ent propagation kinetic constant with a slight increase
(2-fold) in the apparent termination kinetic constant.
Moreover, the apparent propagation kinetic constant
value for the R,R-substituted monomer is closer to that
of a typical monoacrylate, such as butyl acrylate.36 From
inspection of the steady state polymerization rate equa-
tion (eq 2), it is the 3-fold decrease in propagation and
3-fold increase in termination that is responsible for the
5-fold decrease in the maximum polymerization rate of
the R,R-disubstituted monomer as compared to the
unsubstituted monomer. Thus, the kinetics of the R,R-
From the results presented here, hydrogen abstrac-
tion from the R carbon remains as a likely primary
mechanism leading to the enhanced polymerization
kinetics of these novel monoacrylates. However, from
the steady state polymerization characteristics of the
R,R-substituted monomer, it appears that substitution

3098 Beckel et al.
Macromolecules, Vol. 38, No. 8, 2005
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at the â-position may also contribute to the reduced
reactivity. There are two distinct indications of the
substitution effect at the â-position. First, apparent
kinetic constants for the R,R-disubstituted monomer,
specifically the propagation kinetic constant, remain
elevated as compared to traditional monoacrylate mono-
mers. If abstraction were occurring only from the
R-position, removal of all hydrogens from this position
would render the kinetic constants much closer to
traditional monoacrylate monomers. The shape of the
steady state polymerization conversion of the R,R-
substituted monomer is indicative of the remaining
importance of the â-position. As shown, the conversion
curve initially shows a slow polymerization rate and
begins to autoaccelerate at approximately 20% conver-
sion, thereafter attaining the maximum polymerization
rate. If abstraction were occurring solely from the
R-position, the monomer would polymerize in a fashion
more characteristic of a monoacrylate forming linear
polymer and exhibit little to no autoacceleration.
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Conclusions
522.
Novel monoacrylate monomers were synthesized with
methyl substituents attached to the two-carbon spacer
group between the acrylic moiety and the secondary
functionality. These monomers were designed to test for
the effect of hydrogen abstraction on the polymerization
rate. Steady state results indicated that single meth-
ylation on one or both of the carbons had little impact
on the polymerization characteristics. However, steady
state results of the R,R-dimethylation showed a drastic
polymerization rate reduction. Unsteady state results
for the R,R-dimethylated monomer showed that the
apparent propagation kinetic constant is decreased
3-fold, while the apparent termination kinetic constant
is increased 2-fold; moreover, these kinetic constants
approach those of traditional monoacrylate monomers.
Conversely, â,â-dimethylation showed only a mild po-
lymerization rate reduction. These results are consistent
with the majority of the hydrogen abstraction occurring
at the R carbon, while hydrogen abstraction from the â
carbon contributes minimally to the polymerization rate
characteristics. Thus, the polymerization rate is most
drastically influenced by substitution at the R carbon.
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Acknowledgment. The authors thank Kathryn A.
Berchtold for the groundwork investigations on this
research project and Kelly Davis for synthetic chemistry
advice. The authors additionally thank the Department
of Education Graduate Assistantship in the Area of
National Needs fellowship program, the National Sci-
ence Foundation Industrial University Cooperative
Research Center for Fundamentals and Applications of
Photopolymerizations, and the National Institutes of
Health (Grant DE10959) for funding this project.
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Supporting Information Available: Synthesis and pu-
rification methods for all monomers analyzed herein. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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New York, 1991.
(35) Berchtold, K. A.; Beckel, E. R.; Nie, J.; Stansbury, J. W.;
Bowman, C. N., manuscript in preparation.
References and Notes
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