Decisive reaction steps at initial stages of photoinitiated radical polymerizations

Article abstract of DOI:10.1002/anie.200904473

The first addition of photoinitiator radicals formed by the photolysis of a bisacylphosphine oxide photoinitiator to an acrylate double bond proceeds reversibly (see scheme; R¹ =tBu, H; R² = MeOC(=O), BuOC(=O)). Moreover, hydrogen tr

Full text of DOI:10.1002/anie.200904473

Angewandte
Chemie
DOI: 10.1002/anie.200904473
Radical Polymerization
Decisive Reaction Steps at Initial Stages of Photoinitiated Radical
Polymerizations**
Markus Griesser, Dmytro Neshchadin, Kurt Dietliker, Norbert Moszner, Robert Liska, and
Georg Gescheidt*
Radical polymerization has been the focus of interest in terms
of synthetic, technological, and mechanistic aspects.[1–3] Many
photoinitiating systems have been investigated, in which
magnetic resonance methodologies were used as an exceed-
ingly convenient tool to analyze the primary products.[4–6]
More recent developments in radical polymerization involve
“living procedures”,[7–9] which result in a rather uniform
molecular-weight distribution. Reversible addition/elimina-
tion steps are decisive for living radical polymerization
procedures.[9–13] Reversible steps in “classical” radical poly-
merizations have been established in the case of depolyme-
rizations (decompositions)[14–17] and during chain growth (also
characterized by the ceiling temperature).[4,18–23] Another
important feature of photoinitiated radical polymerizations is
the formation of benzaldehyde derivatives even at early
stages of the polymerization.[24]
Scheme 1. Radical products (BC and PC) formed after photocleavage of
bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide 1 and the “cage” (1
and 2) and “escape” products (3–5) formed by recombination.
Mes=mesityl=2,4,6-trimethylphenyl.
Here we address mechanistic aspects of the reversibility
and formation of aldehydes by using H CIDNP (chemically
The first question is whether the first addition to an
acrylate and the following growth of the polymer chain is the
only subsequent reaction of the initial radical pair BC/PC. To
begin with, a mixture of 1 and the sterically congested acrylate
t-BAM (3,3-dimethyl-2-methylenebutanoate, molar ratio 1/t-
BAM = 1:1–1:20) or butyl acrylate (BA, molar ratio 1/BA =
1:1–1:20) was irradiated in [D6]benzene and [D3]acetonitrile.
The 1H CIDNP technique afforded NMR spectra of products
formed exclusively from radical pairs on a short time scale
(ns–ms).[25,29] Figure 1 compares the 1H NMR spectrum of the
parent reaction mixture of 1 and t-BAM (top) with the
corresponding 1H CIDNP spectrum obtained after photolysis
with a Hg-Xe lamp (300 ms). Bearing in mind that only signals
of products formed via paramagnetic intermediates are visible
1
induced dynamic nuclear polarization) spectroscopy[25] to
follow the very early steps of the radical polymerization.
CIDNP is a particularly suitable tool for these investigations
since the subsequent products formed from chemical reac-
tions involving radical pairs are observed exclusively.
Here, bisacylphosphine oxide 1 serves as an ideal model
compound because it is a well investigated[3,6,26] and fre-
quently utilized[3] commercially available photoinitiator. It
has been shown that photolysis of 1 leads to benzoyl radical BC
and phosphinoyl radical PC (Scheme 1).[27] In the absence of a
quencher, BC and PC either recombine to yield parent 1 or form
products 2–5 through either “cage” or “escape” reactions.[27]
In the presence of, for example, acrylates, radicals BC and PC
initiate polymerization, and the rate constants for the addition
of BC- and PC-type radicals to double bonds were determined
by time-resolved optical, IR, and EPR spectroscopy.[6,26,28]
[*] M. Griesser, Dr. D. Neshchadin, Prof. Dr. G. Gescheidt
Institute of Physical and Theoretical Chemistry
Graz University of Technology, Technikerstrasse
4/I, 8010 Graz (Austria)
E-mail: g.gescheidt-demner@TUGraz.at
Dr. K. Dietliker
Ciba (BASF) AG, 4002 Basel (Switzerland)
Dr. N. Moszner
Ivoclar AG, Fl-1234 Schaan (Liechtenstein)
Prof. Dr. R. Liska
Institute of Applied Synthetic Chemistry
Vienna University of Technology, Getreidemarkt 9/163
1060 Vienna (Austria)
Figure 1. Top : 1H NMR spectrum of 1/t-BAM (1:10) in C6D6. Bottom:
The corresponding H CIDNP spectrum recorded immediately after
1
[**] Financial support from the Austrian Science Fund FWF and the
CIBA Jubilꢀumsstiftung is gratefully acknowledged.
irradiation.
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Communications
in the CIDNP experi-
ment, it is, at first sight,
surprising that the signals
of t-BAM (d = 5.31 and
5.97 ppm) appear as being
“polarized”
in
the
1H CIDNP
spectrum.
This observation reveals
that t-BAM must be
regenerated through
a
pathway involving a radi-
cal pair, which can be
rationalized by assuming
that the initial addition of
Scheme 2. Reversible reaction of radicals BC and PC with acrylates, as well as follow-up reactions.
t-BAM to BC and PC is reversible (Scheme 2). The reversibility
may be due to steric congestion of the tert-butyl group on t-
BAM. This hypothesis, however, was ruled out by performing
the analogous experiment with BA: this led to an equivalent
observation (polarizations at d = 5.15, 5.99, and 6.32 ppm for
parent BA, Figure 2). Hence, the addition of either BC or PC to
an acrylate double bond has to proceed (at least partly) in a
reversible way, independent of steric factors.
signal at d = 10.4 ppm is again observed. Thus, the H atom
which adds to the benzoyl-type (mesityloyl) radical has to
originate from an initially formed radical pair.
An alternative source of the extra H atom could be either
of the radicals BMC or PMC. Both species are generated at the
very beginning of the polymerization and, very likely still
carry the polarization of the BC/PC radical pair.[30] Hydrogen-
atom transfer between BC and BMC/PMC would lead to the
formation of the observed aldehyde BH and alkenes BM/PM
(Scheme 2). Indeed, the 1H resonances around d = 7 ppm are
in accord with these latter molecules (Figures 1 and 2). In all
the CIDNP spectra, and even in the presence of a 200-fold
excess of alkenes (t-BAM, BA, or S), the photoinitiator is
regenerated as an in-cage recombination product (see the
resonances indicated in Figures 1 and 2).
In summary, we have shown that a first (or very early)
addition of an initiator radical to an acrylate double bond may
occur in a reversible way. Moreover, disproportionation
(hydrogen transfer) takes place, which leads to the formation
of aldehyde BH and phosphine PH together with the “new”
substituted alkenes BM and PM. The latter species likely
contribute to the polymerization process, even though they
are present at a low concentration. It is remarkable that these
reactions already arise at the very early stages of the
photoinitiated radical polymerizations at room temperature
(248C), which should be well below the ceiling temperatures
of BA and S, even at low concentrations of these mono-
mers.[31–32] Preliminary experiments indicate that the reversi-
bility of the initial addition step can also be observed upon use
of benzoyl peroxide and azobisisobutyronitrile (AIBN).
Furthermore, results obtained for S (and other monomers)
indicate the reversibility of this reaction step.
Figure 2. Top : 1H NMR spectrum of 1/BA (1:10) in C6D6. Bottom: The
corresponding 1H CIDNP spectrum recorded immediately after irradi-
ation.
The second remarkable feature in the 1H CIDNP spectra
of 1/t-BAM and 1/BA (Figures 1 and 2, bottom) is the
occurrence of a singlet signal at d = 10.4 ppm, which is in the
characteristic region for an aromatic aldehyde.[27] Clearly, the
aldehyde can only originate from benzoyl radical BC, but the
source of the additional hydrogen atom has yet to be
identified. It has been shown that a benzoyl-type radical is
able to abstract hydrogen atoms from efficient H donors;[26]
however, neither t-BAM nor BA possesses readily abstract-
able H atoms, and the aldehyde is formed irrespective of the
solvent used. To check whether the aldehyde hydrogen atom
stems from the reaction of BC with a parent alkene or (even
more unlikely) an alternative species, we treated 1 with
styrene (S), which possesses no abstractable H atoms, under
It is also notable that the parent initiator is regenerated
even in the presence of an excess of polymerizable double
bonds. This finding indicates that the photoinitiator poten-
tially has activity even at later stages of the polymerization
process.
Received: August 10, 2009
Published online: November 4, 2009
Keywords: CIDNP spectroscopy · photoinitiators ·
.
1
the given reaction conditions. Remarkably, the H CIDNP
radical polymerization · reaction mechanisms
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