A cooperative role for the counteranion in the PCl5-initiated living, cationic chain growth polycondensation of the phosphoranimine Cl 3P=NSiMe3

Article abstract of DOI:10.1021/ja307703h

The counteranion associated with the cationic initiator [Cl ₃P=N=PCl₃]⁺ ([4]⁺) generated during the PCl₅-initiated living, cationic chain growth polycondensation of the N-silylphosphoranimine Cl₃P=NSiMe₃ (3) to give poly(dichlorophosphazene), [N=PCl₂]_n (2), has been found to have a dramatic effect on the polymerization. When the counteranion of [4]⁺ was changed from PCl₆^- or Cl^- to the weakly coordinating anions [Bar*^F₄] ^- and [BAr^F₄]^- (Ar* ^F = 3,5-{CF₃}₂C₆H₃, Ar^F = C₆F₅) instead of the polymerization of 3 being complete in 4-6 h, no reaction was observed after 24 h. Remarkably, the polymerization of 3 may be initiated by Cl^- anions even in the absence of an active cation such as [4]⁺. However, in the presence of [4]⁺, the reaction proceeded significantly faster and allowed for molecular weight control. These results reveal that the currently accepted mechanism for the PCl₅-initiated living polymerization of 3 needs to be revised to reflect the key role of the counteranion present.

Full text of DOI:10.1021/ja307703h

Communication
pubs.acs.org/JACS
A Cooperative Role for the Counteranion in the PCl5‑Initiated Living,
Cationic Chain Growth Polycondensation of the Phosphoranimine
Cl3PNSiMe3
Vivienne Blackstone,† Stefan Pfirrmann,† Holger Helten,‡ Anne Staubitz,§ Alejandro Presa Soto,∥
George R. Whittell, and Ian Manners*
School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.
S
* Supporting Information
route to polyphosphazenes.7,8 This process, a rare example of a
living chain growth polycondensation,9 involves the treatment
ABSTRACT: The counteranion associated with the
cationic initiator [Cl3PNPCl3]+ ([4]+) generated
during the PCl5-initiated living, cationic chain growth
polycondensation of the N-silylphosphoranimine Cl3P
NSiMe3 (3) to give poly(dichlorophosphazene), [N
PCl2]n (2), has been found to have a dramatic effect on the
polymerization. When the counteranion of [4]+ was
of trichloro(N-trimethylsilyl)phosphoranimine, Cl3PNSiMe3
(3),10 with PCl5. In contrast to other routes, this method allows
the molecular weight of the polymer to be controlled by
altering the monomer to initiator ratio to give 2 in high yield
with a relatively narrow molecular weight distribution.
Furthermore, block copolymers may be prepared via sequential
monomer addition to the living chain ends or the use of
macroinitiation methods.11,12 The improvement in the control
of the molecular weight and polydispersity enabled access to
well-defined materials suitable for self-assembly studies12 as well
as to biodegradable macromolecular anticancer drug carriers.13
Although the discovery of the PCl5-initiated living polymer-
ization route to 2 has permitted a range of synthetic advances,
the polymerization mechanism has not been fully elucidated.
The proposed mechanism involves initiation by the reaction of
monomer 3 with PCl5 to form the cationic species [Cl3PN
PCl3]+ ([4]+) (Scheme 1). The latter reacts with further
−
changed from PCl6 or Cl− to the weakly coordinating
anions [BAr*F ]− and [BArF ]− (Ar*F = 3,5-{CF3}2C6H3,
4
4
ArF = C6F5) instead of the polymerization of 3 being
complete in 4−6 h, no reaction was observed after 24 h.
Remarkably, the polymerization of 3 may be initiated by
Cl− anions even in the absence of an active cation such as
[4]+. However, in the presence of [4]+, the reaction
proceeded significantly faster and allowed for molecular
weight control. These results reveal that the currently
accepted mechanism for the PCl5-initiated living polymer-
ization of 3 needs to be revised to reflect the key role of
the counteranion present.
Scheme 1. Proposed Reaction Sequence in the PCl5-Initiated
Polymerization of 3
ell-defined polymers and block copolymers with
Winorganic elements as a key component are emerging
as an interesting and broad class of easily processed materials
with properties and functions that complement those of state-
of-the-art organic macromolecular materials.1 Polyphospha-
zenes, [NPR2]n, represent one of the most versatile classes of
main-group polymers, and the ability to tune their chemical and
physical properties through the choice of the substituents (R)
has enabled a broad range of promising applications.2 The most
well-developed route to these materials involves the thermal
ring-opening polymerization (ROP) of the cyclic phosphazene
trimer (NPCl2)3 (1) at 200−250 °C to yield poly-
(dichlorophosphazene), [NPCl2]n (2),3,4 which offers access
to various polyphosphazenes via macromolecular substitution
of the P−Cl bonds. Although a range of other synthetic
methods affording phosphazene polymers have been devel-
oped,5 including examples that operate at room temperature,6
all suffer from poor molecular weight control and the formation
of broad molecular weight distributions due to the likely
presence of a combination of chain transfer, chain termination,
and slow initiation events.
equivalents of 3 to form a growing polymer chain, [Cl3PN−
(PCl2N)n−PCl3]+, with concomitant elimination of the
condensation byproduct Me3SiCl.7,8 According to this mech-
anistic sequence, the PCl5-initiated polymerization of 3 may be
classified as a cationic chain growth polycondensation process.
We recently examined the reactivity of the species [4]+ and
[Cl3PN−PCl2N−PCl3]+ ([5]+) formed in the initiation
step and the first propagation step, respectively, using model
In 1995, our group and Allcock and co-workers collabo-
Received: August 3, 2012
ratively reported the first example of a living polymerization
Published: September 5, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
compound studies.14 Although the reactivity detected was
consistent with the propagation step in Scheme 1, we also
−
found that PCl6 , which might be formed as the counteranion
of [4]+ via the reaction of monomer 3 with two molecules of
PCl5,15 is not innocent as previously assumed but may actually
participate in the polymerization of 3. Thus, stoichiometric
reactions between 3 and the salt [Ph3PNPPh3]PCl6
([6]PCl6), which contains an inert countercation devoid of
P−Cl bonds, resulted in the formation of cationic oligomers
[Cl3PN−(PCl2N)x−PCl3]+ in which the phosphorus
−
center of the PCl6 anion was incorporated into the
phosphazene chain.14 To eliminate unwanted effects of
“noninnocent” counteranions on the molecular weight
distribution of the resulting polyphosphazenes, we explored
the potential of salts of [4]+ with inert, weakly coordinating
counteranions for use as polymerization initiators. With this
strategy, ion pairing effects should also be largely suppressed,
which was expected to lead to highly efficient initiation. In this
preliminary communication, we present the unexpected results
of these studies, which disclose additional fundamental insight
into the mechanism of the PCl5-initiated living polymerization
of 3.
The new potential polymerization initiators [4][B(3,5-
{CF3}2C6H3)4] (denoted as [4][BAr*F ]) and [4][B(C6F5)4]
4
(denoted as [4][BArF ]) were prepared via the salt metathesis
4
reactions of [4]Cl16 with Na[BAr*F ] and Na[BArF ],
4
4
respectively. Surprisingly, when the polymerization of 3 was
attempted using [4][BAr*F ] or [4][BArF ] in CH2Cl2 at 25 °C
Figure 1. (a) 31P{1H} NMR spectrum of a mixture of [4][BAr*F ] and
4
4
4
(Scheme 2), instead of highly efficient initiation, no reaction
10 equiv of 3 after 24 h in CH2Cl2. (b) Amount of 3 remaining (as
determined by 31P{1H} NMR signal integration) during the reaction
between 3 and [4][BAr*F ] (25:1) before and after the addition of [6]
Scheme 2. Attempted Polymerization of 3 Using
4
Cl (1 equiv with respect to [4][BAr*F ]).
[4][BAr*F ] or [4][BArF ] and Subsequent Addition of
4
4
4
[6]Cl
Scheme 3
tography (GPC) analysis (UV detection) of the latter showed it
to have high molecular weight [number-average molecular
weight (Mn) = 64 580 g mol−1; polydispersity index (PDI) =
1.36]. Moreover, after equimolar amounts of 3 and [6]Cl were
stirred for 24 h in CH2Cl2 at 25 °C, a 31P{1H} NMR spectrum
of the reaction mixture revealed full consumption of 3 and the
formation of the cyclic phosphazene trimer 1 (63%) and the
cyclic tetramer (NPCl2)4 (−5.9 ppm; 15%) as well as
polydichlorophosphazene 2 (22%).
was observed by 31P{1H} NMR spectroscopy after 24 h [Figure
1a and Figure S2 in the Supporting Information (SI)]. In
striking contrast, the polymerization of 3 initiated by [4]Cl
under otherwise identical conditions was complete within 5 h
(Figure S1).17 Moreover, upon subsequent addition of chloride
ions to the mixture of 3 and [4][BAr*F ] or [4][BArF ] in the
form of the salt [Ph3PNPPh3]Cl ([6]Cl; 1 equiv with
respect to [4]+), which has an inert countercation, rapid
polymerization of 3 occurred with quantitative conversion to 2
in less than 6 h (Figure 1b and Figure S3).
4
4
Although these experiments showed that the Cl− anion is
able to initiate the polymerization of 3, several features of this
reaction indicate that Cl− alone cannot account for the
formation of polymer 2 in the living, PCl5-initiated process.
First, the reaction is much slower than that involving [4]Cl,
wherein the cation contains reactive terminal P−Cl bonds
(reaction time of 3 weeks vs 4−6 h). Second, a significant
quantity of cyclic trimer 1 is formed, and broad polydispersities
result. To examine a potential cooperative role for Cl− and [4]+,
we investigated the effect of their concentrations on the
reaction time and the resulting molecular weight distribution in
more detail. Specifically, two sets of experiments were
conducted: First, to examine the influence of Cl− concen-
Clearly, the Cl− anion, which might be generated during the
PCl5-initiated polymerization of 3, is not an innocent spectator
but rather plays a crucial role in this process. This result
prompted us to investigate the possibility of initiating the
polymerization of 3 by Cl− in the absence of an active cation
such as [4]+. Indeed, when [6]Cl, in which the cation is devoid
of P−Cl bonds, was used as an initiator (25:1 ratio) in CH2Cl2
at 25 °C, 3 was completely consumed within 3 weeks (Scheme
3).18
A
31P{1H} NMR spectrum showed the presence of the
cyclic phosphazene 1 (20.6 ppm; ca. 3%) as well as a resonance
characteristic of 2 at −17.4 ppm (97%). Following derivatiza-
tion of 2 with NaOCH2CF3 to yield the air- and moisture-stable
polymer [NP(OCH2CF3)2]n (7), gel-permeation chroma-
tration, a solution of 3 and [4][BAr*F ] (4 mol %) in CH2Cl2
4
(Scheme 2) was treated with a CH2Cl2 solution of [6]Cl in
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Journal of the American Chemical Society
Communication
varying concentrations. Second, to examine the influence of the
cation, yielding polymers with high molecular weight and
moderate polydispersity. Although an active cation such as
[Cl3PNPCl3]+ ([4]+) is not required for the polymer-
ization of 3 to proceed, its presence dramatically increases the
rate of propagation and provides molecular weight control.
These preliminary results have led us to reassess the mechanism
of the PCl5-initiated living polymerization of 3. For example, it
is possible that the initiation of the polymerization of 3 requires
prior association of Cl− with the monomer. We are currently
working on further detailed mechanistic investigations in order
to provide an in-depth understanding of this interesting
polymerization reaction with the ultimative aim of broadening
the applications of polyphosphazenes in nanoscience.
active phosphazene cation [4]+, a solution of [4][BAr*F ] in a
4
range of concentrations in CH2Cl2 was added to a solution of 3
and [6]Cl (4 mol %) in CH2Cl2 (Scheme 2). The reactions
were monitored by 31P{1H} NMR spectroscopy and stirred at
25 °C until 3 was completely consumed to form 2.
Subsequently, 2 was subjected to halogen replacement to
yield the hydrolytically stable polymer 7, which was analyzed by
MALDI−TOF MS and GPC (Tables 1 and 2).19
Table 1. Effect of Changing the Amount of [6]Cl on the
Polymerization of 3 in the Presence of [4][BAr*F ] (Scheme
4
2) to Afford [N=P(OCH2CF3)2]n (7) after Derivatization of
[N=PCl2]n (2)
ASSOCIATED CONTENT
* Supporting Information
Experimental details, GPC traces, and MALDI−TOF and
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
a
b
b
Cl−:[4][BAr*F ]:3 molar ratio
time (min)
Mn (g mol−1
)
PDI
4
S
0.17:1:25
0.33:1:25
0.5:1:25
0.72:1:25
1:1:25
380
240
180
140
100
6350
7370
7670
7020
8220
1.04
1.04
1.04
1.02
1.02
AUTHOR INFORMATION
Corresponding Author
Ian.Manners@bristol.ac.uk
■
a
Time required for full consumption of 3 to form 2 as observed by
b
31P{1H} NMR spectroscopy (see the SI for details). Determined by
MALDI−TOF MS. At lower concentrations of Cl−, slightly larger
amounts of cyclic trimer and tetramer were formed.
Present Addresses
‡Institute of Inorganic Chemistry, RWTH Aachen University,
Landoltweg 1, 52056 Aachen, Germany.
Table 2. Effect of Changing the Amount of [4][BAr*F ] on
§Otto-Diels-Institute for Organic Chemistry, University of Kiel,
Otto-Hahn-Platz 3/4, 24118 Kiel, Germany.
∥Department of Organic and Inorganic Chemistry, IUQOEM,
4
the Polymerization of 3 in the Presence of [6]Cl (Scheme 2)
to afford [NP(OCH2CF3)2]n (7) after Derivatization of
[NPCl2]n (2)
́
University of Oviedo, Julian Claveria, 33006 Oviedo, Spain.
a
Cl−:[4][BAr*F ]:3 molar ratio
time (min)
Mn (g mol−1
)
PDI
Author Contributions
4
†V.B. and S.P. contributed equally.
b
b
1:0.17:25
1:0.33:25
1:0.5:25
1:0.72:25
1:1:25
<960
<960
450
37100
1.03
1.07
1.13
1.03
1.04
b
b
b
c
25200
Notes
b
20000
The authors declare no competing financial interest.
c
360
12970
c
c
100
8430
ACKNOWLEDGMENTS
■
a
Time required for full consumption of 3 to form 2 as observed by
V.B. thanks the States of Jersey for a Ph.D. scholarship. S.P. and
H.H. thank the Deutscher Akademischer Austauschdienst
(DAAD) and the Deutsche Forschungsgemeinschaft (DFG),
respectively, for postdoctoral fellowships. A.P.S. thanks the EU
for a Marie Curie Fellowship. I.M. thanks the EU for financial
support.
b
31P{1H} NMR spectroscopy (see the SI for details). Determined by
GPC (UV detection) vs polystyrene standards (a correction factor was
applied for Mn; see ref 19). Determined by MALDI−TOF MS.
c
This study revealed that an increase in the proportion of the
chloride ion led to a significant decrease in the time taken for
full consumption of 3 to form 2. The GPC data showed no
appreciable difference in the molecular weight of the prepared
polymer with different relative amounts of Cl− (Table 1). This
suggests that Cl− is not responsible for the molecular weight
control observed in the PCl5-initiated polymerization of 3. On
the other hand, an increase in the relative amount of the active
cation [4]+ clearly led to a decrease in the molecular weight of
the obtained polymer (Table 2). This indicates that the cation
may provide molecular weight control in the PCl5-initiated
polymerization of 3. No significant change in PDI was observed
with changes in the relative amount of either [4]+ or Cl−.
In summary, our studies have disclosed that the mechanism
of the PCl5-initiated living polymerization of 3 is much more
complex than originally proposed.7,8 The Cl− anion generated
at an early stage is not innocent, as has previously been implied,
but in fact plays a key cooperative role with the living cations in
the polymerization process in both the initiation and chain
propagation steps. Thus, we have shown that Cl− anions initiate
the slow polymerization of 3 even in the absence of an active
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