Anionic ring-opening polymerization of a strained phosphirene: A route to polyvinylenephosphines

Article abstract of DOI:10.1039/b606311j

A strained 1-phenyl-2,3-dimethylphosphirene undergoes anionic ring-opening polymerization upon initiation with n-butyl lithium at ambient temperature to yield polyvinylenephosphine, an unsaturated organophosphorus polymer. The Royal Society of Chemistry 2

Full text of DOI:10.1039/b606311j

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Anionic ring-opening polymerization of a strained phosphirene: A route
to polyvinylenephosphines{
Lawrence A. Vanderark,a Timothy J. Clark,a Eric Rivard,a Ian Manners,*b J. Chris Slootwegc and
Koop Lammertsma*c
Received (in Cambridge, UK) 3rd May 2006, Accepted 7th June 2006
First published as an Advance Article on the web 29th June 2006
DOI: 10.1039/b606311j
A strained 1-phenyl-2,3-dimethylphosphirene undergoes anio-
nic ring-opening polymerization upon initiation with n-butyl
lithium at ambient temperature to yield polyvinylenephosphine,
an unsaturated organophosphorus polymer.
Functional polymers that incorporate inorganic elements in their
main chain are of current interest due to their unique properties
and potential applications.1 Phosphorus-containing polymers are
attracting particular current attention as self-assembled materials,2
p-conjugated materials3 and catalyst supports.4 Until recently, they
have been mainly accessible only by traditional condensation
polymerization and thermal ring-opening polymerization (ROP)
routes.1 Over the past decade, a variety of new and versatile
synthetic routes to these materials have been described, including
chain growth and metal-catalyzed polycondensation, Lewis acid-
catalyzed and anionic ROP at ambient temperature, and addition
polymerization.5 In this communication, we report the successful
anionic polymerization of a strained three-membered organopho-
Scheme 1
sphorus heterocycle, which provides
a convenient ambient
temperature route to a novel class of unsaturated vinylenepho-
sphine polymers.
In order to induce the ROP of 1a, we treated this species with
sub-stoichiometric quantities of an anionic initiator. The reaction
of 1a with 0.1 equiv. of nBuLi followed by quenching with MeOH
resulted in ca. 10% yield of a ring-opened product, as detected by
31P NMR spectroscopy. Signals at d 22.1 and 226.7 closely
matched values reported for the corresponding cis- and trans-
protonated alkenyl phosphines (2a and 2b).7 Unfortunately, the
ring-opening reaction did not propagate beyond the ring-opened
vinylphosphine. We attribute this to the steric bulk of the phenyl
groups (and possibly electronic effects), which renders chain
propagation kinetically, and perhaps also thermodynamically,
unfavourable. We anticipated that phosphirenes which possess
smaller ring substituents might function as more suitable
monomers. The less sterically encumbered 1-phenyl-2,3-dimethyl-
phosphirene monomer (1b) was therefore reacted with nBuLi
(0.1 equiv.) at 278 uC and allowed to gradually warm to room
temperature. New signals, assigned to anionic ring-opened
products, were observed by 31P NMR at d 24.9 and 214.4, in
addition to those of the starting material. As a general rule,
Duncan and Gallager have noted that more sterically compressed
Phosphirenes are interesting organophosphorus three-mem-
bered rings containing a CLC double bond.6 These species contain
considerable ring-strain, as demonstrated by their ability to
undergo stoichiometric ring-opening reactions.7 This suggests that
they are potentially useful as monomers in the synthesis of
vinylenephosphine polymers via ROP, but no successful polymer-
ization has been reported to date.8 Treatment of triphenylpho-
sphirene (1a) (Scheme 1) with a stoichiometric amount of n-butyl
lithium (nBuLi) has previously been shown to result in attack at the
phosphorus center, with concurrent ring-opening, to afford a
mixture of phosphine isomers (cis and trans).7 The resulting
products could be readily sulfurized and oxidized, and were
isolated as their air-stable sulfide and oxide derivatives, respec-
tively. Ring-opening of 1a was also reported in the presence of
hydrogen peroxide, however the products were difficult to isolate
due to oligomerization.7 Apart from these studies, the polymeriza-
tion of phosphirenes remains, to our knowledge, unexplored.
aDepartment of Chemistry, University of Toronto, 80 St. George Street,
Toronto, Ontario, Canada M5S 3H6
bSchool of Chemistry, University of Bristol, Bristol, UK BS8 1TS.
E-mail: Ian.Manners@bristol.ac.uk; Fax: +44 (0)117 9290509;
Tel: +44 (0)117 9287650
cDepartment of Chemistry, Faculty of Science, Vrije Universiteit, De
Boelelaan 1084, 1081 HV Amsterdam, The Netherlands.
E-mail: k.lammertsma@few.vu.nl
{ Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b606311j
Scheme 2
3332 | Chem. Commun., 2006, 3332–3333
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Table 1 Summary of results for the polymerization of 1b with nBuLi initiator
Initiator Monomer Calculated MW Experimental
Polydispersity
index
Degree of
polymerizationc
Percentage yield by 31P NMR
(Percentage isolated yield)
Trial (I)/mmol (M)/mmol [M]/[I]0 (Mn)/g mol21a MW (Mn)/g mol21a
1
2
3
4
5
a
0.06
0.09
0.09
0.09
0.12
0.6
2.2
2.2
2.2
6.2
10/1
24/1
24/1
24/1
51/1
2000
4700
4700
4700
9900
1700
8000
8200
8600
18 000
1.42
1.35
1.45
1.58
1.23
11
50
51
53
63 (17)b
87 (84)
84 (81)
87 (83)
85 (84)
c
111
b
Of sulfurized polymer 3. The low isolated yield is a result of difficulties in precipitating this low molecular weight polymer. DPn
isomers show more shielded 31P nuclei.9 Consequently, the more
shielded signal at d 214.4 was assigned to the trans isomer, while
that at d 24.9 was attributed to the cis isomer. Upon further
reaction for 14 h, the 31P NMR signal of monomer 1b was no
longer observed, while two broad signals were detected at d 25.1
and 29.6, consistent with successful chain propagation and
polymer formation.{ The polymer product 2c was isolated by
concentration of the THF solution and subsequent precipitation
into hexanes as a beige solid in 17% yield. The 1H and 13C NMR
spectra of 2c were also consistent with the assigned structure. The
material was rendered air-stable through sulfurization with
elemental sulfur in THF to afford the corresponding sulfide 3.
This resulted in the downfield shift of the 31P NMR resonances, as
well as an overlap of the signals from each isomer, as the chemical
shift difference was reduced. The sulfurization step also permitted
analysis of the molecular weight by gel permeation chromato-
graphy (GPC) in THF, as polymers containing uncoordinated
phosphorus(III) centers often adsorb to size exclusion columns.5e
The weight-average molecular weight (Mw) was found to be
2000 g mol21 and the number-average molecular weight (Mn) was
1400 g mol21 vs. polystyrene standards. These values correspond to
a number-average degree of polymerization (DPn) of ca. 10 repeat
units, as might be expected for a 1 : 10 initiator to monomer ratio.
To generate higher molecular weight materials, polymerizations
were performed with decreased amounts of initiator. Reactions
We anticipate that ROP should be generally possible for strained
rings of this type, providing that the steric encumbrance is not too
severe. Experiments aimed at testing this assertion, and exploring
properties and applications are underway.
L. A. V. would like to thank the University of Toronto for a
Fellowship. T. J. C. thanks the Ontario Government for an
Ontario Graduate Scholarship in Science and Technology as well
as CIBA Speciality Chemicals for a graduate student award. E. R.
thanks NSERC for a Graduate Fellowship. I. M. thanks the E. U.
for a Marie Curie Chair and the Royal Society for a Wolfson
Research Merit Award.
Notes and references
{ We tentatively assign the peak at d 25.1 to P atoms flanked by cis and
trans CLC bonds, and that at d 29.6 to those in a trans–trans environment.
Tacticity effects, which provide an alternative explanation for the
environment of two peaks, cannot be disproved at this stage.
1 D. P. Gates, Annu. Rep. Prog. Chem., Sect. A: Inorg. Chem., 2004, 100,
489; R. D. Archer, Inorganic and Organometallic Polymers, Wiley-VCH,
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2 L. Cao, I. Manners and M. A. Winnik, Macromolecules, 2001, 34, 3353;
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1998, 120, 1466.
5 (a) J. A. Gruneich and P. Wisian-Neilson, Macromolecules, 1996, 29,
5511; (b) C. H. Honeymann, I. Manners, C. T. Morrissey and
H. R. Allcock, J. Am. Chem. Soc., 1995, 117, 7035; (c) V. Chunechom,
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M. Yam and D. P. Gates, J. Am. Chem. Soc., 2003, 125, 1480; (h)
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n
using 0.04 equiv. of BuLi relative to 1b were performed. After
14 h, the 31P NMR spectrum showed the same broad peaks at
d 25.1 and 29.6 associated with the polymer (87%), but also
showed some unreacted 1b (13%). Further reaction resulted in no
change in these signals, and can possibly be attributed to side
reactions quenching the propagating anionic species. One likely
possibility is a ‘‘chain transfer to monomer’’ reaction, where the
propagating anion abstracts a proton from one of the methyl
substituents on the phosphirene ring. Rearrangement of the
resulting compound may afford
a stable allenylphosphine
structure, which would be unreactive towards 1b (Scheme 2).
Such a mechanism has been reported for the nucleophilic attack of
1-mesityl-2,3-dimethylphosphirene, where the bulky mesityl group
prevents nucleophilic attack at the phosphorus.7b Despite the
possible occurrence of this chain transfer reaction, this procedure
has been used to repeatedly and controllably prepare polymeric 3
with molecular weights of Mw = ca. 8000–8600 g mol21. This
corresponds to a DPn of 24. In preliminary experiments, polymers
with molecular weights as high as Mw = 18 000 g mol21 have been
obtained by using smaller quantities of initiator (Table 1).
6 F. Mathey, Chem. Rev., 1990, 90, 997; A. Marinetti, F. Mathey, J. Fischer
and A. Mitschler, J. Am. Chem. Soc., 1982, 104, 4484; H. Heydt, Sci.
Synth., 2002, 9, 125; T. P. Goumans, A. W. Ehlers, K. Lammertsma and
E. U. Wu¨rtherwein, Eur. J. Org. Chem., 2003, 2941; R. E. Bulo,
H. Jansen, A. W. Ehlers, F. J. J. de Kanter, M. Schakel, M. Lutz,
A. L. Spek and K. Lammertsma, Angew. Chem., Int. Ed., 2004, 43, 714.
7 (a) A. Marinetti and F. Mathey, J. Am. Chem. Soc., 1985, 107, 4700; (b)
F. Nief and F. Mathey, Tetrahedron, 1991, 47, 6673.
8 For a brief report of the cationic polymerization of saturated cyclic
phosphine (tBu)3C6H2PCH2CH2, see: S. Kobayashi and J. I. Kadokawa,
Macromol. Rapid Commun., 1994, 15, 567.
In summary, we have demonstrated a facile route to high
molecular weight polyvinylenephosphines through the anionic
ring-opening polymerization of 1-phenyl-2,3-dimethylphosphirene.
9 M. Duncan and M. J. Gallager, Org. Magn. Reson., 1981, 15, 37.
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