Isomerization polymerization of the phosphaalkene MesP=CPh2: An alternative microstructure for poly(methylenephosphine)s

Article abstract of DOI:10.1002/anie.201301881

Unique pathway: The radical-initiated addition polymerization of MesP=CPh₂ propagates through the ortho-bound CH₃ group of the Mes moiety after C-H bond activation (see scheme, Mes=2,4,6-trimethylphenyl, tht=tetrahydrothiophene, TEMPO=2,2,6,6-tetramethyl-l-piperidinoxyl). This unique isomerization polymerization mechanism contrasts the previously suggested head-to-tail enchainment typically observed for olefins. Copyright

Full text of DOI:10.1002/anie.201301881

Angewandte
Chemie
DOI: 10.1002/anie.201301881
Phosphorus Polymers
=
Isomerization Polymerization of the Phosphaalkene MesP CPh2: An
Alternative Microstructure for Poly(methylenephosphine)s**
Paul W. Siu, Spencer C. Serin, Ivo Krummenacher, Thomas W. Hey, and Derek P. Gates*
The synthesis and study of main-group-element analogues of
radical alkoxyamine initiators. These striking results led to
a revision of the proposed microstructure for poly(methyl-
enephosphine) that was produced by a radical reaction.
In an effort to understand the initiation step in the radical
polymerization of 1, we investigated its reaction with TEMPO
(1–2 equiv). 31P NMR spectroscopic analysis of the reaction
mixtures suggested the formation of multiple products,
including radical species, which were detected by EPR
spectroscopy. To date, none of these products have been
successfully isolated or unambiguously identified. In contrast,
employing the complex 1·AuCl[10,11] instead of 1 affords
a single product with TEMPO. Specifically, treatment of
a solution of 1·AuCl in toluene with TEMPO (1 equiv)
resulted in 50% conversion of 1·AuCl (d = 167.1) to a new
species, as determined by 31P NMR spectroscopy. Addition of
more TEMPO (1 equiv, that is, 2 equiv total) resulted in
complete conversion of 1·AuCl to two products in a ratio of
approximately 1:3, which displayed 31P signals at 135.6 ppm
À
alkenes and alkynes containing genuine (p p)p bonds
involving p-block elements is a central theme of inorganic
chemistry.[1,2] The prospect to “copy” the predictable and
=
ꢀ
sophisticated reaction chemistry of C C and C C bonds
utilizing functional inorganic systems is particularly enticing.
However, in many instances, the investigation of multiple
bonds of heavy elements leads to fascinating, albeit unex-
pected, outcomes that reinforce the fundamental differences
between the first and subsequent periods.
=
=
Inspired by the intriguing analogy between P C and C C
bonds in molecular chemistry,[3] we developed the addition
polymerization of phosphaalkenes as a route to new func-
tional phosphorus-containing polymers (Scheme 1).[4]
(d,
JPH = 18 Hz) and 130.5 ppm (d, JPH = 18 Hz). The
Scheme 1. The isolobal analogy between olefins and phosphaalkenes
as applied to addition polymerization to afford polyolefins and
poly(methylenephosphine).
magnitudes of the 31P–1H coupling constants are not consis-
tent with the expected product Mes(TEMPO)P(AuCl)-C-
(TEMPO)Ph2.
Colorless crystals were obtained by slow diffusion of
hexanes into the reaction mixture at À308C. Analysis of the
crystals by X-ray crystallography showed that the product was
the intriguing di-TEMPO species 2·AuCl (Figure 1).[11] As
anticipated, one of the TEMPO moieties was bound to
phosphorus. Surprisingly, the second TEMPO moiety was not
Although the synthesis of phosphorus-containing macromo-
lecules is of widespread interest because of their attractive
properties and potential applications,[5] the study of the
=
addition polymerization of P C bonds remains in its infancy.
=
Our studies showed that MesP CPh2 (1) and related mono-
=
mers polymerize in the presence of radical or anionic
initiators to afford poly(methylenephosphine) (Scheme 1).[6]
The living anionic polymerization of 1 permits the formation
of functional phosphine-containing block copolymers,[7,8] and
the radical-initiated copolymerization of 1 with styrene
affords random copolymers.[9]
In order to explore the mechanism of radical addition to
P = C bonds during polymerization, we investigated the
reactions of monomer 1 with TEMPO-derived radical sour-
ces. Herein, we report the discovery of a fascinating isomer-
ization polymerization of phosphaalkene 1 in the presence of
bound to the methylene carbon atom of the former P C bond.
Instead, this moiety was transformed to a CHPh2 group and
the former ortho-bound CH3 moiety of the mesityl group was
now a CH2(TEMPO) group. Presumably, TEMPO adds to
1·AuCl to afford a carbon-centered radical intermediate (i.e.,
{Mes(TEMPO)P(AuCl)}Ph2CC) to which HC from the ortho-
bound CH3 of the Mes group migrates. Although the chemical
or electrochemical one-electron oxidation and reduction of
phosphaalkenes has been studied extensively,[12] to our
knowledge, the nature of the addition of neutral radicals to
[13]
1
P C bonds is not well understood. The 31P and H NMR
spectra and elemental analysis were consistent with the
formulation of the product 2·AuCl. Interestingly, there are
two isomers of 2·AuCl, which we speculate are diastereomers
resulting from the stereogenic phosphorus center and atrop-
isomerism of the substituents.
=
[*] P. W. Siu, S. C. Serin, Dr. I. Krummenacher, Dr. T. W. Hey,
Prof. Dr. D. P. Gates
Department of Chemistry, University of British Columbia
2036 Main Mall, Vancouver, BC, V6T 1Z1 (Canada)
E-mail: dgates@chem.ubc.ca
In light of the observed formation of 2·AuCl from 1·AuCl,
we hypothesized that a similar C H activation might occur in
the polymerization of 1 using radical initiators. Thermolysis of
phosphaalkene 1 in the presence of substoichiometric quanti-
ties of TEMPO did not affect polymerization. In contrast,
heating the neat monomer 1 in the presence of the alkoxy-
À
[**] We thank the Natural Sciences and Engineering Research Council
(NSERC) of Canada for funding this work. P.W.S. thanks NSERC for
a PGS D fellowship. Mes=2,4,6-trimethylphenyl.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301881.
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Communications
Figure 2. 13C{1H} NMR spectra (151 MHz, CDCl3, 298 K) of: a) poly-
(methylenephosphine) (4) prepared from 1 and PhMeCH·TEMPO
(2 mol%); b) PhCH2-P(Mes)-CHPh2; c) poly(methylenephosphine)
([13C]-4; Mn =10000 gmolÀ1, PDI=1.3) prepared from MesP=13CPh2
and PhMeCH·TEMPO (5 mol%).
Figure 1. Molecular structure of 2·AuCl (50% probability ellipsoids).
With the exception of H1, all hydrogen atoms are omitted for clarity.
amine PhMeCH·TEMPO[14] under conditions typical for
nitroxide-mediated polymerization (NMP; neat, [M]:[I] =
50, T= 1258C, t = 3 h; M = monomer, I = initiator),[15]
afforded poly(methylenephosphine). Subsequent analysis of
the solution of the crude reaction mixture in THF by
31P NMR spectroscopy showed signals that can be assigned
to monomer (d = 233) and polymer (d = À10) in ratios that
suggested approximately 41% conversion[16] to poly(methy-
lenephosphine). After precipitation from the THF solution
with hexanes (ꢀ 3), polymer free of monomer 1 was isolated in
10% yield [gel-permeation chromatography (GPC) with light
scattering: Mn = 10000 gmolÀ1, polydispersity index (PDI) =
1.3]. We speculate that the low yield of isolated polymer likely
results from fractionation of the low-molecular-weight poly-
mer during precipitation. Importantly, heating of monomer
1 under identical conditions in the absence of initiator did not
afford polymer. Therefore, the alkoxyamine is necessary to
affect the polymerization of 1. However, the present data
does not confirm whether or not an NMP mechanism is
followed and further studies are underway to investigate this
interesting possibility.
more pronounced than that in our previously reported spectra
(measured at 75 MHz). In our earlier studies, the broadening
of the signals assigned to the ortho-bound CH3 groups in 3 was
rationalized as arising from restricted rotation of the Mes
group. Similar broadening had been noted in the
13C{1H} NMR spectra of the phosphines MesRP-CPh2R’
(R = Bu, R’ = H; R = Me, R’ = Me, P(NEt2)2, SiHMe2,
SiMe3), the structures of which were confirmed by X-ray
crystallography.[17] In light of the C H activated 2·AuCl,
À
a
13C APT (attached-proton test) NMR experiment was
conducted on the polymer. Remarkably, this experiment
suggested that the signal at 33.0 ppm could not result from
a CH3 moiety and must result from either a quaternary carbon
atom or a CH2 group. Therefore, we speculated that this signal
might be indicative of an Ar-CH2-P moiety within a polymer
with microstructure 4. For comparison, we prepared and fully
characterized
benzylphosphine
PhCH2-P(Mes)-CHPh2,
which displayed a similar 13C NMR chemical shift for the
1
Ph-CH2-P moiety (d = 32.5, d, JCP = 20 Hz; Figure 2b).
The broad resonance at 52.4 ppm (Figure 2a), formerly
assigned to the backbone carbon atom (e.g. P-CPh2-P),
provided additional evidence for microstructure 4. The
13C APT NMR experiment suggested that this resonance
could not be from a quaternary P-CPh2-P moiety and must
be either from a CHR2 or a CH3 moiety. To obtain further
The microstructure of the polymer obtained under NMP
conditions was elucidated using NMR spectroscopy. The
13C{1H} NMR spectrum (151 MHz; Figure 2a) was recorded
using a high sensitivity cryoprobe. At first glance, the
spectrum looks similar to that previously described for
VAZO-polymerized 1 (VAZO = 1,1’-azobis(cyclohexanecar-
bonitrile) and assigned to microstructure 3.[6] Closer exami-
nation shows a very broad signal at 33.0 ppm, which is much
1
support for the presence of a CHPh2 moiety, the H NMR
spectrum (600 MHz) was recorded (Figure 3a). Alone, the
1H NMR spectrum is fairly uninformative, showing very
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(2 equiv), these data are consistent with microstructure 4 for
poly(methylenephosphine).
Although the NMR spectroscopic data discussed thus far
are consistent with the proposed microstructure 4 for the
polymer produced by radical reaction, the aforementioned
experiments could not rule out the presence of small amounts
of microstructure 3. In order to investigate this possibility, the
13C-labeled polymer (Mn = 10000 gmolÀ1, PDI = 1.3) was
prepared from MesP = 13CPh2 ([13C]-1)[18] and PhMeCH·
TEMPO (5 mol%) as a sensitive probe for the presence of
traces of P-13CPh2-P enchained polymer (i.e., microstructure
[13C]-4/[13C]-3 where x > y). The corresponding 13C{1H} NMR
spectrum is shown in Figure 2c. Importantly, the dominant
signal is assigned to 13CHPh2 (d = 52.4). To confirm 31P–13C
coupling, we measured the 31P–13C HMQC NMR spectrum of
the labeled polymer, which showed the expected cross
correlation. It must be noted that the 13C{1H} NMR spectrum
exhibits smaller signals slightly downfield from the main
broad signal (d = 55–60). The 13C APT NMR spectrum indi-
cates that these moieties are not quaternary carbon atoms
and, therefore, cannot be attributed to P-13CPh2-P enchained
[13C]-3. It is plausible that these small signals may be
attributable to tacticity, end groups, or trace impurities. In
conclusion, the head-to-tail microstructure [13C]-3 does not
appear to be present in the polymer prepared by radical
initiation, but rather the polymer consists entirely of structure
[13C]-4.
Figure 3. 1H NMR spectra (600 MHz, CDCl3, 298 K) of: a) poly(methy-
lenephosphine) 4 prepared from 1 and PhMeCH·TEMPO (2 mol%);
b) PhCH2-P(Mes)-CHPh2.
To place these striking observations for the radical-
initiated polymerization of 1 into context, the reader is
reminded that rearrangements and isomerizations may occur
in the addition polymerization of many olefins. The simplest
example is perhaps the highly irregular branched structure
that results from H migrations in the radical polymerization
of ethylene. In contrast, the intramolecular H migration that
occurs in the present work appears to be highly selective and
affords regioregular 4. Moreover, the radical polymerization
mechanism for 1 appears to be unique for systems with double
Figure 4. 1H–13C HSQC NMR spectrum (600 MHz for 1H, CDCl3,
298 K) of poly(methylenephosphine) 4. The ordinate shows the
1
13C APT NMR spectrum and the abscissa shows the H NMR spec-
trum.
À
bonds. Activation of the C H bond of an ortho-bound CH3
group has been observed in the molecular chemistry of
compounds with P-Mes moieties, however it typically involves
broad resonances in the aromatic and the aliphatic regions.
However, the 1H–13C HSQC NMR spectrum (Figure 4)
showed two important cross correlations. Namely, signals
were detected and assigned to the CHPh2 moiety (d(13C) =
52.4, d(1H) = 4.8) and the CH2 moiety (d(13C) = 33.0, d(1H) =
À
atom insertion into the C H bond rather than H migration, as
observed herein.[19] It is worth noting that an early report on
=
the anionic polymerization of trimethylstyrene (MesCH
CH2) considered the possibility of C H rearrangements,
1
À
3.6) in 4. For comparison, these H NMR chemical shifts are
similar to those assigned to the methine and diastereotopic
involving the ortho- or para-bound CH3 moieties during
propagation, but these possibilities were not considered
further in subsequent publications.[20]
In conclusion, we have discovered that the radical-
initiated addition polymerization of phosphaalkene 1 affords
poly(methylenephosphine) 4 with an unexpected, but highly
regioregular, microstructure. The unusual isomerization poly-
methylene protons in the model PhCH2-P(Mes)-CHPh2 (Fig-
2
ure 3b; d = 5.06 (d, CHPh2, JPH = 4 Hz), 3.40 (dd, CHaHb,
2JHH = 13 Hz, 2JPH = 3 Hz), 2.93 (dd, CHaHb, 2JHH = 13 Hz,
2JPH = 3 Hz)). Together with the aforementioned structure of
2·AuCl, which results from treating 1·AuCl with TEMPO
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acterization data, including X-ray crystallography data, are given
in the Supporting Information.
merization described herein has not been reported for olefin
monomers and may be applicable to other Mes-containing
monomers. We are currently investigating, whether analogous
isomerization processes may be occurring in the anionic
polymerization for 1 and the radical-initiated copolymeriza-
tion of 1 and olefins.
[11] CCDC 927353 (1·AuCl) and 927352 (2·AuCl) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Received: March 6, 2013
Published online: June 5, 2013
À
Keywords: C H activation · inorganic polymers ·
phosphaalkenes · phosphorus · radical polymerization
.
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=
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=
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=
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À
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