7530 Communications to the Editor
Macromolecules, Vol. 38, No. 18, 2005
Table 1. Molar Extinction Coefficients of Initiating
Monomers (L/(mol cm))
1
2
3
4
254 nm
313 nm
1262
3529
83
6813
379
7913
448
been synthesized and examined as photoinitiators and
comonomers. (see Supporting Information for detailed
synthesis and characterization). We previously reported
that 1 exhibits a red-shifted UV absorption spectra with
an absorption tail approaching 300 nm while monofunc-
tional analogues of 1 show UV absorption peaks below
230 nm.4 The acrylate group appears to be the core
chromophore while the vinyl group contributes to a
bathochromic (red-shifted) UV absorption. As antici-
pated on the basis of the chemical structure, 2, 3, and
4 also exhibited red-shifted UV absorption compared to
the monofunctional analogues, diethyl fumarate and
diethyl maleate. In particular, the species with two vinyl
substituents (3 and 4) are characterized by strong UV
absorption at wavelengths longer than 300 nm, a critical
property for efficient photoinitiators. Quantitative in-
formation regarding the promising aspects of the UV
absorption of these monomers can be provided by the
molar extinction coefficients (ꢀ) of 1-4 at 254 and 313
nm (two wavelengths emitted by mercury lamp sources)
as shown in Table 1. Compounds 2-4 exhibit high ꢀ
values at 254 nm, comparable to ꢀ values of conventional
photoinitiators, which range from approximately 5 × 103
to 1.6 × 104 L/(mol cm) at 254 nm.1 As shown in Table
1, ꢀ values of compounds 3 and 4 are certainly in this
range, while compound 2 absorbs significantly less light
at this wavelength. All of the synthesized monomers,
2-4, have much larger ꢀ values at 254 nm than 1, for
which ꢀ is approximately 1 × 103 L/(mol cm). Interest-
ingly, 3 and 4 have molar extinction coefficients ap-
proximately 2 times greater than that of 2 at 254 nm
due to the presence of two vinyl substituents. Moreover,
their absorption extends to wavelengths greater than
350 nm. As shown in Table 1, ꢀ values at 313 nm for
monomers 3 and 4 are approximately 5 times higher
that the ꢀ value for compound 2 at 313 nm.
The true test of the viability of these materials is their
ability to initiate polymerization. To examine their
relative initiation efficiency, 1,6-hexanediol diacrylate
(HDDA) was polymerized in the presence of 2-4 while
conversion rates were monitored using real-time FTIR,
as shown in Figure 2A. All polymerization experiments
were performed in oxygen-free conditions between two
salt plates. HDDA does not polymerize in the absence
of a photoinitiator while samples with 2-4 polymerize
rapidly. These samples polymerize quickly, attaining
50% conversion within 30 s, demonstrating their ef-
fectiveness as photoinitiators. The conventional initia-
tor, dimethoxyphenyl acetophenone (DMPA), has a
higher initiation efficiency than the initiating monomers
at equal molar concentration. Additionally, the DMPA
system reaches much higher conversion than the pho-
toinitiating monomers. While the conversion is higher
with the conventional photoinitiator, much higher HDDA
conversion can be achieved at higher light intensity and
different photoinitiating monomer concentrations. It is
also important to note that the vast majority of DMPA
remains unreacted after polymerization is complete. On
the other hand, the initiating monomers can copolymer-
ize as well as photoinitiate and therefore be completely
consumed and be incorporated into the cross-linked
network during polymerization. To demonstrate the self-
Figure 2. (A) Real-time IR conversion vs time plots of HDDA
polymerizations in the presence of 3.9 × 10-2 mol/L of (a)
DMPA, (b) 3, (c) 4, (d) 2, (e) 1, and (f) no photoinitiator in
HDDA. Light intensity (full spectrum UV light) is 20 mW/cm2,
and sample is 15 µm in thickness. (B) UV absorption spectra
before and after polymerization of HDDA with 2.9 × 10-1 mol/L
of 3 (DiVF). Sample is cured at 75 mW/cm2 for 3 min with a
medium-pressure mercury lamp.
inititating monomer consumption, Figure 2B shows the
UV absorption of 3 during polymerization of HDDA, for
which 3 was the only added photoinitiator. The shoulder
peak observed from 240 to 330 nm before polymerization
disappears after polymerization, indicating that the
absorbing chromophore is disrupted as the initiating
monomer is consumed by polymerization with the
acrylate and becomes part of the cross-linked polymer
film. The photolysis products have also been examined,
indicating that two initiating radicals are produced by
an R-cleavage reaction as observed for â,γ-unsaturated
ketones.7 The details will be published elsewhere.
Having shown that these vinyl ester monomers serve
as initiating monomers for acrylate polymerization,
their use as a comonomer with other monomers is
considered. Since fumarate ester monomers exhibit a
strong alternating copolymerization tendency (r1 , 1,
r2 , 1, r1r2 = 0) with electron-rich ene monomers due
to a large electron density difference between the two
double bonds,7 it might be expected that they would
copolymerize readily with the electron-rich vinyl mono-
mers and produce cross-linked networks without an
external photoinitiator. To illustrate this point, 2 was
copolymerized with triethylene glycol divinyl ether
(DVE) without addition of an external photoinitiator.
As is evident from the results in Figure 3, the polym-
erization of both the vinyl ether and vinyl ester groups
proceeds at a substantial rate. Even without external
photoinitiator, fumarate group conversion reaches ∼60%,
with ∼30% conversions of both the vinyl ether and vinyl
ester groups, after 60 s of exposure. Interestingly, the
sum of vinyl ester and vinyl ether conversion is ap-
proximately the same as the fumarate conversion
throughout the polymerization, indicating that the
fumarate group of 2 polymerizes in an alternating
fashion with both vinyl ester of 2 and vinyl ether double