Phosphinoyl radicals: Structure and reactivity. A laser flash photolysis and time-resolved ESR investigation

Article abstract of DOI:10.1021/ja982463z

The photochemistry of a series of bis(acyl)phosphine oxides and the rate constants of the reactions of their phosphorus radicals with n-butylacrylate, thiophenol, bromotrichloromethane, oxygen, and methyl viologen have been investigated by laser flash pho

Full text of DOI:10.1021/ja982463z

J. Am. Chem. Soc. 1998, 120, 11773-11777
11773
Phosphinoyl Radicals: Structure and Reactivity. A Laser Flash
Photolysis and Time-Resolved ESR Investigation
Steffen Jockusch and Nicholas J. Turro*
Contribution from the Department of Chemistry, Columbia UniVersity, New York, New York 10027
ReceiVed July 13, 1998
Abstract: The photochemistry of a series of bis(acyl)phosphine oxides and the rate constants of the reactions
of their phosphorus radicals with n-butylacrylate, thiophenol, bromotrichloromethane, oxygen, and methyl
viologen have been investigated by laser flash photolysis. The results were compared to a mono(acyl)phosphine
oxide ((2,4,6-trimethylbenzoyl)diphenylphosphine oxide). The variation in reactivity of the different phosphorus
radicals was correlated with the degree of radical localization and s-character on the phosphorus atom, as
reflected by the 31P hyperfine coupling constant, which was measured by time-resolved ESR. High P hyperfine
splitting is observed for high degree of spin localization in a σ-orbital. It was shown that typical radical
31
3
1
reactions (addition to monomers and oxygen, and atom abstractions) correlate well with the P hyperfine
coupling. An inverse correlation was observed for electron-transfer reactions (reduction of methyl viologen),
were a higher p-character of a localized orbital enhances the reaction.
Introduction
phinoyl radical (3) to a double bond, a mono(acyl)phosphine
oxide unit is formed, located at one end of the growing polymer.
Bis(acyl)phosphine oxides have attracted attention in recent
years because of their superior characteristics as initiators in
photochemical curing processes involving free radical polymeriza-
This moiety can undergo a second R-cleavage reaction and form
another benzoyl and phosphinoyl radical (see eq 2).7
1-5
tion.
In particular, these photoinitiators are of interest, since
their optical absorption extends well into the visible region
allowing for the curing of pigmented formulations.3 Moreover,
the long-wavelength absorption, which is due to the aroyl-
phosphinoyl chromophore, is destroyed during the photoreaction.
The resulting bleaching allows a progressively deeper penetra-
tion of the incident light into the coating layer, which enables
the curing of thick layers.4
,4
Mono(acyl)phosphine oxides also undergo rapid R-cleavage
6
,8-10
from the triplet state to produce radicals (eq 3).
For more
At the molecular level upon irradiation, bis(acyl)phosphine
6
oxides undergo R-cleavage with high efficiency (ΦR ≈ 0.6) to
1
-4
produce a benzoyl-phosphinoyl radical pair (see eq 1). In a
than a decade mono(acyl)phosphine oxides have been widely
used as photoinitiators in free radical polymerization.1
1-14
The
photophysics and photochemistry of mono(acyl)phosphine
oxides, which are similar to those of the bis(acyl)phosphine
oxides, have been the subject of a number of studies and are
6
,8-11,15-17
now fairly well understood (see eq 3).
previous paper we showed that the R-cleavage typically occurs
from a short-lived triplet state.6 As a result of the very fast
In this paper we describe a systematic study of the photo-
chemistry of a series of bis(acyl)phosphine oxides (1b-e) and
cleavage, competing bimolecular triplet quenching (e.g., reaction
with oxygen), which would lower the yield of radicals, is
generally not significant.
Bis(acyl)phosphine oxides are highly effective photoinitiators
since they can produce four radicals, each of which is an effi-
cient initiator. For example, after addition of the (acyl)phos-
(7) Dietliker, K.; Leppard, D.; Jung, T.; Koehler, M.; Valet, A.; Kolczak,
U.; Rzadek, P.; Rist, G. RadTech Asia 97 Conf. Proc. 1997, 292.
(8) Sumiyoshi, T.; Schnabel, W.; Henne, A.; Lechtken, P. Polymer 1985,
2
6, 141.
(9) Baxter, J. E.; Davidson, R. S.; Hageman, H. J.; McLauchlan, K. A.;
Stevens, D. G. J. Chem. Soc., Chem. Commun. 1987, 73.
10) Koptyug, I. V.; Sluggett, G. W.; Landis, M. S.; Turro, N. J.;
(
(
1) Kolczak, U.; Rist, G.; Dietliker, K.; Wirz, J. J. Am. Chem. Soc. 1996,
18, 6477.
2) Sluggett, G. W.; McGarry, P. F.; Koptyug, I. V.; Turro, N. J. J. Am.
Chem. Soc. 1996, 118, 7367.
3) Leppard, D.; Dietliker, K.; Hug, G.; Kaeser, R.; Koehler, M.; Kolczak,
U.; Misev, L.; Rist, G.; Rutsch, W. Radtech ‘94 North Am. 1994, 693.
4) Rutsch, W.; Dietliker, K.; Leppard, D.; Koehler, M.; Misev, L.;
Bentrude, W. G. J. Phys. Chem. 1996, 100, 14581.
1
(11) Schnabel, W. In Laser in Polymer Science and Technology:
Applications; Fouassier, J. P., Rabek, J. F., Eds.; CRC: 1991; Vol. 2, p 95.
(12) Jacobi, M.; Henne, A. J. Radiat. Curing 1983, 10, 16.
(13) Jacobi, M.; Henne, A. Polym. Paint Colour J. 1985, 175, 636.
(14) Dietliker, K. In Radiation Curing in Polymer Science and Technol-
ogy; Fouassier, J.-P., Rabek, J. F., Eds.; Elsevier Applied Science: New
York, 1993; Vol. II, p 155.
(15) Sluggett, G. W.; Turro, C.; George, M. W.; Koptyug, I. V.; Turro,
N. J. J. Am. Chem. Soc. 1995, 117, 5148.
(16) Turro, N. J.; Khudyakov, I. V. Chem. Phys. Lett. 1992, 193, 546.
(17) Kajiwara, A.; Konishi, Y.; Morishima, Y.; Schnabel, W.; Kuwata,
K.; Kamanchi, M. Macromolecules 1993, 26, 1656.
(
(
(
Kolczak, U. In XXth International Conference in Organic Coatings Science
and Technology; Athens, 1994; p 467.
(
5) Rutsch, W.; Dietliker, K.; Leppard, D.; Koehler, M.; Misev, L.;
Kolczak, U.; Rist, G. Prog. Org. Coat. 1996, 27, 227.
6) Jockusch, S.; Koptyug, I. V.; McGarry, P. F.; Sluggett, G. W.; Turro,
N. J.; Watkins, D. M. J. Am. Chem. Soc. 1997, 119, 11495.
(
1
0.1021/ja982463z CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/03/1998

11774 J. Am. Chem. Soc., Vol. 120, No. 45, 1998
Jockusch and Turro
Scheme 1. Structure of the Bis(acyl)phosphine Oxides (1b-e) and Associated Phosphinoyl Radicals (3b-e)
reactions of their phosphinoyl radicals (3b-e) with a series of
different substrates, including unsaturated monomers, oxygen,
atom donators, and an electron acceptor. The results were
compared with the diphenylphosphinoyl radical (3a) generated
from a mono(acyl)phosphine oxide (1a). The structure of the
bis(acyl)phosphine oxides and phosphinoyl radicals investigated
are shown in Scheme 1.
I Nd:YAG laser, a Bruker ER 100D X-band ESR spectrometer, and a
PAR boxcar averager and signal processor (models 4420 and 4402).
Argon-saturated solutions were flown through a quartz flow cell (∼0.3
mm thick) in the rectangular cavity of the ESR spectrometer. Further
details are described elsewhere.1
9,20
Results
Laser Flash Photolysis. Laser flash photolysis (355 nm
excitation) of the phosphorus photoinitiators 1 affords readily
detectable transient absorption spectra (see Figure 1) which
Experimental Section
Materials and Solvents. (2,4,6-Trimethylbenzoyl)diphenylphos-
phine oxide (1a) (BASF) was recrystallized from diethyl ether. Bis-
decay on the microsecond time scale with mixed kinetics and
(2,6-dimethoxybenzoyl)-phenylphosphine oxide (1b), bis(2,6-dimeth-
6,8,15
have been assigned to the phosphinoyl radicals 3.
The
oxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (1c), bis(2,4,6-tri-
methylbenzoyl)-phenylphosphine oxide (1d), and bis(2,4,6-trimethyl-
benzoyl)-2,4,4-trimethylpentylphosphine oxide (1e) (Ciba Specialty
Chemicals) were recrystallized from ethanol. Thiophenol (Aldrich Gold
Label), 1-octanethiol (Aldrich), methyl viologen (Aldrich), bromotri-
chloromethane (Aldrich Gold Label), n-butylacrylate (Polysciences,
Inc.), and acetonitrile (Aldrich spectrophotometric grade) were used
as received.
8,15
6
transient absorption spectra of 3a and 3c have been published
earlier. All (acyl)phosphinoyl radicals (3b-e) show strong
absorptions centered at about 450 nm. The radicals 3b and 3d,
also show a transient absorption at about 330 nm. This
additional absorption band is caused by the phenyl substituents
on the phosphorus. In contrast, the radicals 3c and 3e, which
possess alkyl substituents on the phosphorus, do not show the
strong absorption at 330 nm. The benzoyl radicals 2a-e do
not interfere with the kinetics since they possess only a very
weak absorption in the area where the phosphorus radicals
Laser Flash Photolysis Experiments. Laser flash photolysis experi-
ments employed the pulses (355 nm, ca. 8 mJ/pulse, 8 ns) from a
Continuum Surelite I Nd:YAG laser and a computer-controlled system
1
8
which has been described elsewhere. Solutions of the phosphorus
photoinitiators were prepared at concentrations such that the absorbance
was ∼0.3 at the excitation wavelength employed. Transient absorption
spectra were recorded employing a Suprasil quartz flow cell (1 × 1
cm) to ensure that a fresh volume of sample was irradiated by each
laser pulse. Quenching rate constants were measured using argon-satu-
rated static samples contained in 1 × 1 cm Suprasil quartz cells. Fresh
solutions were prepared at each quencher concentration. Quenching
rate constants with oxygen were measured as follows: Mixtures of
oxygen and nitrogen of precisely known composition were prepared
using a mass flow gas controller-mixer system in connection with an
Oxygen Analyzer 2566 (Illinois Instruments). A gas splitter system
diverted a small fraction of this flow through the sample in a fine stream
of bubbles. Particular care was taken to keep the temperature of the
sample cell constant at 24 °C during the bubbling by using a water
circulator bath, to ensure a constant oxygen concentration in the sample
solutions.
2
1,22
absorb.
It was possible, however, to detect selectively by
1
5
time-resolved IR the benzoyl radicals 2a, which were gener-
ated by laser flash photolysis of 1a.
Time-Resolved ESR (TR-ESR). Laser flash photolysis (355
nm excitation) of argon-saturated acetonitrile solutions of the
bis(acyl)phosphine oxides 1b-e affords TR-ESR spectra shown
in Figure 2. The spectra display absorptive doublets at low
and high field relative to g ) 2 due to the (acyl)phosphinoyl
radicals 3 and broad absorptive lines near g ) 2, assigned to
benzoyl radicals 2, whose small couplings are not resolved. In
the case of the radicals 3c and 3e, the doublet of the phosphorus
radical is split further into a doublet of triplets, caused by the
(
19) Koptyug, I. V.; Ghatlia, N. D.; Sluggett, G. W.; Turro, N. J.;
Ganapathy, S.; Bentrude, W. G. J. Am. Chem. Soc. 1995, 117, 9486.
20) Lipson, M.; McGarry, P. F.; Koptyug, I. V.; Staab, H. A.; Turro,
(
Time-Resolved ESR Experiments. TR-ESR experiments employed
the pulses (355 nm, ca. 8 mJ/pulse, 8 ns) from a Continuum Surelite
N. J.; Doetschman, D. C. J. Phys. Chem. 1994, 98, 7504.
(21) Huggenberger, C.; Lipscher, J.; Fischer, H. J. Phys. Chem. 1980,
8
4, 3467.
(
18) McGarry, P. F.; Cheh, J.; Ruiz-Silva, B.; Hu, S.; Wang, J.;
(22) Fischer, H.; Baer, R.; Hany, R.; Verhoolen, I.; Walbiner, M. J. Chem.
Soc. Perkin Trans. 2 1990, 787.
Nakanishi, K.; Turro, N. J. J. Phys. Chem. 1996, 100, 646.

Phosphinoyl Radicals
J. Am. Chem. Soc., Vol. 120, No. 45, 1998 11775
This is expected to be observed due to the large phosphorus
2
3
hyperfine coupling. Conversely, the spectra of 1b and 1c,
shown in Figure 2 are more dominated by a radical pair
mechanism. The spectra in Figure 2 were recorded 300 to 400
ns after the laser flash. At an earlier time scale, the polarization
by the triplet mechanism was clearly observed, which demon-
strates that the radicals were formed from the triplet state of 1
by R-cleavage. Interestingly, the polarization by the triplet
mechanism is much weaker for the methoxy substituted bis-
(acyl)phosphine oxides 1b and 1c compared to that for the
methyl substituted compounds 1d and 1e. This effect was also
observed by Baxter et al. for (2,6-dimethoxybenzoyl)- and
9
(2,4,6-trimethylbenzoyl)diphenylphosphine oxide.
Reactions of Phosphinoyl Radicals. The high reactivity (as
measured by the second-order rate constant) of phosphinoyl rad-
icals toward unsaturated compounds is one of the most desirable
properties of phosphine oxide photoinitiators for radical polym-
2,4,5,11,17
erization.
Table 1 summarizes the bimolecular rate con-
stants (kacrylate) for the reaction of phosphinoyl radicals 3a-e
Figure 1. Transient optical absorption spectra recorded 200-700 ns
following laser excitation (355 nm, 8 ns) of 1b, d, e in argon-saturated
acetonitrile solutions at 23 °C.
coupling of the two R-hydrogens of the alkyl chain (aH ) 4.6
G (3c) and aH ) 5.3 G (3e)).
There are some notable differences between the polarized
spectra shown in Figure 2. In the case of 1d and 1e, all of the
lines are clearly absorptive, due to the triplet mechanism
polarization. The small distortion is due to the emission/
absorption radical pair mechanism polarization from the gemi-
nate radical pair, which dominates at later observation times.
with n-butylacrylate (eq 4), obtained by laser flash photoly-
sis.
Figure 2. TR-ESR spectra recorded 300-400 ns following laser excitation (355 nm, 8 ns) of 1b-e in argon-saturated acetonitrile solutions at 23 °C.

11776 J. Am. Chem. Soc., Vol. 120, No. 45, 1998
Jockusch and Turro
Table 1. 31P Hyperfine Coupling and Bimolecular Rate Constants for Quenching of Phosphinoyl Radicals 3 by n-Butylacrylate, Thiophenol,
Bromotrichloromethane, Oxygen, and Methyl Viologen in Acetonitrile Solution
3
a
3b
294 G
2.4 × 10
6.3 × 10
2.2 × 10
3.0 × 10
1.2 × 10
3c
286 G
1.5 × 10
3d
270 G
1.1 × 10
2.3 × 10
0.9 × 10
2.7 × 10
2.8 × 10
3e
255 G
0.4 × 10
3
1
P hyperfine coupling
369 G
2.8 × 10
-
1
-1
7
7
5
8
9
9
7
7
5
8
9
9
7
k
k
k
k
k
n-butylacrylate (M
PhSH (M
BrCCl3 (M
s
)
-
1
-1
5
s
)
42 × 10
-
1
-1
8
9
8
9
9
8
9
9
s
)
6.9 × 10
2.4 × 10
2.6 × 10
1.5 × 10
0.8 × 10
2.5 × 10
2.6 × 10
-
1
-1)
oxygen (M
s
4.2 × 10
methyl viologen (M s-1
-
1
)
<10
6
Figure 3. Pseudo-first-order decay rate constant (kobs) of the radicals
a-e versus n-butylacrylate concentration; laser flash photolysis of
argon-saturated acetonitrile solutions of 1a-e (λex ) 355 nm) in the
presence of different n-butylacrylate concentrations at 23 °C.
Figure 4. Pseudo-first-order decay rate constant (kobs) of the radicals
3a-e versus oxygen concentration; laser flash photolysis of acetonitrile
solutions of 1a-e (λex ) 355 nm) in the presence of different oxygen
concentrations at 24 °C.
3
The pseudo-first-order treatment of the decay of the absorp-
tion of 3a-e according to eq 9 yields the second-order rate
Synchronously with the decay of 3, the growth of the thiophe-
noxyl radical at 450 nm was observed. Since the absorption of
thiophenoxyl radicals interferes with the decay kinetic of 3b-e
at 400 to 480 nm, rate constants were only determined for the
radicals 3a, b, and d, which possess strong transient absorptions
at 335 nm where the thiophenoxyl radical does not interfere.
The results are summarized in Table 1. The radical of
1-octanethiol does not show an optical absorption in the visible
range. Therefore, we attempted to employ 1-octanethiol as
hydrogen donor to study the differences in reactivity of the
phosphinoyl radicals 3a-e. In the case of the diphenylphos-
phinoyl radical (3a), the rate constant for hydrogen abstraction
kobs ) k + kacrylate[acrylate]
(9)
0
constants kacrylate, where kobs represents the observed pseudo-
first-order rate constant at various concentrations of n-butyl-
acrylate, and k0 the first-order (estimated) rate constant for the
decay of the radical in the absence of added quencher (see Figure
3
).
An important common side reaction, which reduces the
initiation efficiency of polymerization, is the addition of oxygen
to the radicals 3a-e (eq 5). The rate constants koxygen were
obtained by pseudo-first-order treatment (see above, and Figure
5
-1 -1
was 7.1 × 10 M s , nearly 1 order of magnitude smaller
than for the analogous reaction using thiophenol. For the other
phosphorus radicals 3b-e, no change in the decay kinetics of
the radicals was observed with increasing 1-octanethiol con-
centrations. From the quencher concentrations utilized and
considering the accuracy of decay kinetic measurements, we
conclude that if hydrogen abstraction were to occur, the rate
4
) and variation of the oxygen concentration by saturation of
the acetonitrile solutions with oxygen nitrogen mixtures using
.1 mM as saturation concentration at 1 atm partial pressure
9
24
oxygen at 24 °C. As expected for such organic radicals, all
9
-1 -1
rate constants koxygen range from 2 to 3 × 10 M
s
(see
5
-1 -1
Table 1).
constant should be <10 M s .
Other common reactions which lower the initiation efficiency
of polymerization are atom abstractions from materials that are
required to be present in a photocuring formulation. To
distinguish the reactivity between the different phosphinoyl
radicals 3a-e toward atom abstraction, we chose model
compounds with high atom donor tendency. Thiophenol is often
employed as standard hydrogen donor in laser flash photolysis,
since the thiophenoxyl radical resulting from hydrogen atom
For halogen atom abstraction reactions, bromotrichloro-
methane was selected as substrate. Earlier studies showed that
bromotrichloromethane is an efficient bromine donor compared
to other donors. The rate constants kBrCCl3 of reaction 7 are
listed in Table 1.
2
Besides addition reactions (eqs 4 and 5) and atom abstractions
(eqs 6 and 7), electron-transfer reactions of phosphorus radicals
are also possible. As a model for electron-transfer, methyl
abstraction can be directly detected by its absorption at around
viologen, which possesses a high reduction potential (E1/2 ) -
5
4
50 nm (eq 6).2 Laser flash photolysis of 1 yielded the radicals
26
0
.46 eV in acetonitrile, SCE) and is often used as a standard
3
whose lifetime was reduced upon addition of thiophenol.
electron acceptor, was selected. The reduced form of methyl
viologen, the paraquat radical cation (eq 8), possesses a strong
(
(
23) Buckley, C. D.; McLauchlan, K. A. Mol. Phys. 1985, 54, 1.
24) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochem-
2
7
optical absorption with a maximum at 608 nm. Laser flash
istry, 2nd ed.; Marcel Dekker: New York, 1993.
25) Bonifacic, M.; Weiss, J.; Chaudhri, S. A.; Asmus, K.-D. J. Phys.
Chem. 1985, 89, 3910.
(
(26) Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Mever, T. J.; Whitten,
D. G. J. Am. Chem. Soc. 1979, 101, 4815.

Phosphinoyl Radicals
J. Am. Chem. Soc., Vol. 120, No. 45, 1998 11777
photolysis of 1b-e yielded the radicals 3b-e, whose lifetime
was reduced upon addition of methyl viologen. Simultaneous
with the decay of 3b-e, the growth of the paraquat cation
radical was observed at 600 nm. The rate constants for reaction
8
, listed in Table 1, were obtained by pseudo-first-order
treatment of the decay of the radicals 3b-e (see above) by
variation of the methyl viologen concentration. In the case of
laser photolysis of 1a in the presence of high methyl viologen
concentration (0.6 mM), no change in the lifetime of 3a and no
production of paraquat cation radicals were observed. Estimat-
ing from the quencher concentrations used in these studies and
the accuracy of decay kinetic measurements, if reaction 8 were
6
-1 -1
to occur, the rate constant should be < 10 M s .
Discussion
Figure 5. Reaction rate constants of phosphorus radicals with
n-butylacrylate and methyl viologen versus P hyperfine coupling.
Phosphinoyl radicals (3) possess a high reactivity toward
unsaturated substrates. The radical 3a, generated from the
mono(acyl)phosphine oxide 1a, shows the highest rate constant
to n-butylacrylate, compared to the radicals 3b-e, generated
from the bis(acyl)phosphine oxides 1b-e (see Table 1). But
this does not necessarily imply or guarantee, that mono(acyl)-
phosphine oxides will be better photoinitiators than bis(acyl)-
phosphine oxides. In fact, the bis(acyl)phosphine oxides 1c and
31
delocalization) causes a decrease of the 31P hyperfine coupling
and reactivity. Thus, for typical radical reactions, i.e., addition
to olefinic monomers (eq 4), atom abstractions (eqs 6 and 7),
and addition of oxygen (eq 5), a high degree of spin localization
on the phosphorus would result in both high rate constants and
3
1
a high value of P hyperfine coupling. This is the case for the
d perform significantly better than 1a in most industrially
radicals 3a and 3b. For all reactions (eqs 4-7) the rate constants
relevant applications.7
,28,29
This better performance of 1c and
31
correlate well with the P hyperfine coupling. Minor ir-
d could be caused by the bathochromic shifted optical absorption
of the bis(acyl)phosphine oxides compared to the mono(acyl)-
phosphine oxide among other possible factors. In addition, the
role of the radicals produced by initiator photolysis in the
polymerization process is complex and involves also chain
termination.
regularities can be caused by small errors in the determination
of the rate constants by laser flash photolysis.
In contrast to above situation, electron delocalization and
enhanced p-character should improve the reactivity in electron-
transfer reactions. Indeed, oxidation of phosphorus radicals
3b-e by methyl viologen (eq 8) correlates inversely with the
3
1
The rate constants of the different phosphorus radicals vary
almost 1 order of magnitude toward a given substrate (see Table
P hyperfine coupling (see Table 1 and Figure 5). High rate
3
1
constants were observed for radicals with low P hyperfine
coupling (higher degree of spin delocalization, more p-charac-
ter). Under our experimental conditions, no reaction of radical
3a with methyl viologen was observed, suggesting that the
carbonyl R to the phosphorus in radicals 3b-e is essential for
the reaction.
1
ity.
). Radical reactivity is often correlated with radical stabil-
3
0,31
The latter in turn correlates inversely with localization
of the radical or s-character of a localized orbital. However,
since s-character and orbitals are theoretical concepts, an
experimental parameter, which is expected to correlate with
31
s-character, is required. Such a parameter is the P hyperfine
coupling, which correlates well with s-character in localized
Summary and Conclusion
3
1
Phosphinoyl radicals (3) generated by photolysis of acyl and
bis(acyl)phosphine oxides, possess high rate constants of
addition to acrylates. This high reactivity is the basis of their
effectiveness as photoinitiators for free radical polymerization.
The differences in the rate constants can be explained with
different degrees of radical localization and s-character on the
phosphorus atom. A physical parameter, which reflects this
orbitals. The P hyperfine coupling decreases in the order of
a > 3b > 3c > 3d > 3e. Similarly, the rate constants for
acrylate addition (eq 4) decrease in the same order (see Table
and Figure 5). Thus, there is a strict correlation between
3
1
3
1
reactivity (k) and P hyperfine coupling. The high reactivity
of the phosphinoyl radical 3a has been attributed to a nonplanar
structure which results in a high degree of s-character and spin
3
1
localization on the phosphorus.2
,32-34
degree of radical localization, is the P hyperfine coupling. It
was shown that typical radical reactions (addition to monomers
A decrease of s-character
and increase of p-character (more planar structure and spin
and oxygen, and atom abstractions) correlate well with the 31
P
(27) Bockman, T. M.; Kochi, J. K. J. Org. Chem. 1990, 55, 4127.
(28) Jung, T.; Koehler, M. J. Oberflaechentech. 1998, 38, 26.
(29) Rees, M. T. L.; Russell, G. T.; Zammit, M. D.; Davis, T. P.
hyperfine coupling. An inverse correlation was observed for
electron-transfer reactions (reduction of methyl viologen).
Macromolecules 1998, 31, 1763.
30) Kochi, J. K., Ed. Free Radicals; John Wiley & Sons: New York,
973.
31) Hay, J. M. ReactiVe Free Radicals; Academic Press: London, New
York, 1974.
(
Acknowledgment. We thank the National Science Founda-
tion and Ciba Specialty Chemicals for their generous support
of this research and Dr. P. Lechtken (BASF, Germany) and
Dr. M. Kunz (Ciba Specialty Chemicals) for samples of 1a and
1b-e, respectively. The authors thank Dr. M. Kunz (Ciba
Specialty Chemicals) for comments on the manuscript.
1
(
(
(
32) Geoffroy, M.; Lucken, E. A. C. Mol. Phys. 1971, 22, 257.
33) Kerr, C. M. L.; Webster, K.; Williams, F. J. Phys. Chem. 1975, 79,
2
650.
34) Bentrude, W. G. In Free Radicals; Kochi, J. K., Ed.; John Wiley &
Sons: New York, 1973; Vol. 2; p 600.
(
JA982463Z