Ring-Opening Polymerization of Cyclic Phosphonates: Access to Inorganic Polymers with a PV-O Main Chain

Article abstract of DOI:10.1021/jacs.8b13435

We describe a new class of inorganic polymeric materials featuring a main chain consisting of P^V-O bonds and aryl side groups, which was obtained with >70 repeat units by ring-opening polymerization of cyclic phosphonates. This monomer-polymer system was found to be dynamic in solution enabling selective depolymerization under dilute conditions, which can be tuned by varying the substituents. The polymers show high thermal stability to weight loss and can be easily fabricated into self-standing thin films. Structural characterizations of the cyclic 6-and 12-membered ring precursors are also described.

Full text of DOI:10.1021/jacs.8b13435

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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Ring-Opening Polymerization of Cyclic Phosphonates: Access to
Inorganic Polymers with a PV−O Main Chain
Marius I. Arz,† Vincent T. Annibale,† Nicole L. Kelly,‡,§ John V. Hanna,‡ and Ian Manners*,†,∥
†School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom
‡Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, United Kingdom
§MAS CDT, Senate House, University of Warwick, Coventry CV4 7AL, United Kingdom
∥Department of Chemistry, University of Victoria, Victoria, British Columbia V8W 3V6, Canada
S
* Supporting Information
An early report from 1963 described the insoluble material
ABSTRACT: We describe a new class of inorganic
polymeric materials featuring a main chain consisting of
PV−O bonds and aryl side groups, which was obtained
with >70 repeat units by ring-opening polymerization of
cyclic phosphonates. This monomer−polymer system was
found to be dynamic in solution enabling selective
depolymerization under dilute conditions, which can be
tuned by varying the substituents. The polymers show
high thermal stability to weight loss and can be easily
fabricated into self-standing thin films. Structural charac-
terizations of the cyclic 6- and 12-membered ring
precursors are also described.
obtained from dehydrochlorination of MePOCl2 and MePO-
(OH)2 as polymeric [−P(O)Me−O−]n, but no character-
ization was provided.4 The formation of poorly characterized
polymers of type B as decomposition products of transiently
generated dioxophosphoranes (RPO2) under pyrolytic con-
ditions was also mentioned.5 Polyphosphonates B can be
regarded as hybrids between polyphosphoesters [−P(O)R−
O−Lc−O−] (C, Chart 1; Lc = carbon-based linker), an
emerging class of flame-retardant polymers recently recognized
for their promising biomedical applications,6,7 and ionic
n−
polyphosphates [PO3]n (D, Chart 1), which are applied in
diverse areas ranging from cation sequestering, fertilizers, flame
retardants and in biomedicine.8 Analogies with polynucleo-
tides, which also possess P−O units in the main chain, are also
intriguing. These considerations all suggest that polyphosph-
onates B are interesting synthetic targets with potentially useful
properties.
he development of new polymers based on p-block
T
elements is of continued interest due to the possibility to
find materials with unique properties which may lead to new
applications in polymer and material science.1,2 For example,
polysiloxanes [SiRR′−O]n (A, Chart 1) evolved from their
The polymers of type B are polymeric derivatives of cyclic
phosphonates, which remained surprisingly unexplored since
the early discovery of “PhPO2” by Michaelis and Rothe in
1892.9 Only two cyclic phosphonates, specifically the six-
Chart 1. Different Types of Inorganic Polymers Relevant to
a
10
membered rings (tBuPO2)3 and [(2-MeO-C6H4)PO2]3,11
This Work
have been previously structurally characterized by single-crystal
X-ray diffraction analysis (XRD). Notably, (nPrPO2)3 (T3P) is
used as a particularly mild dehydration reagent for various
synthetic applications,12 but little is known about the structure
of this reagent.9h The related polyphosphates [−P(OR′)(O)−
O−]n (so-called “Langheld esters” with R′ = alkyl or silyl
group) also find use as mild dehydration reagents, but consist
of complex mixtures of short-chain cycles and oligomers.13
Herein, we demonstrate that the ring-opening polymerization
(ROP) of cyclic phosphonates leads to high molar mass
polyphosphonates of type B with >70 repeat units.
a
Lc = carbon-based linker.
ROP of inorganic heterocycles provides an important route
to high molar mass polymers based on p-block elements.1−3,14
As a prerequisite to an exploration of a ROP route to
polyphosphonates using sterically demanding substituents, we
synthesized the cyclic monomers (RPO2)3 [1R3; R = Ph
(1Ph3), 4-tBu-C6H4 (1tBuAr3), Mes (1Mes3), 4-tBu-2,6-Me2-
C6H2 (1tBuAr′3)] via the dehydrochlorination of phosphonic
early discovery as a laboratory curiosity to widely used versatile
materials, mainly resulting from the fruitful interplay of the
unique features imposed by the Si−O main chain and the
tunable carbon-based (organic) side chains.3 In this context,
polymers consisting of a main chain of alternating P−O bonds
with modifiable organic side chains such as polyphosphonates
of type [−P(O)R−O−]n (B, Chart 1) may as well exhibit
interesting properties, but have only been briefly noted in the
literature.
Received: December 17, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.8b13435
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
dichlorides RPOCl2 and phosphonic acids RPO(OH)2 in
refluxing toluene (Scheme 1). After workup, the compounds
first example of a neutral (PO)n ring with n > 3 [ionic
n−
cyclophosphates (PO3)n (“metaphosphates”) were structur-
ally characterized for ring sizes up to n = 12].16
1H, 13C and 31P NMR spectroscopy in CDCl3 and toluene-
d8 revealed that all cyclotriphosphonates 1R3 consist of six-
membered rings with time-averaged Cs symmetry in solution,
leading to diagnostic AB2 patterns in the 31P{1H} NMR
spectra in the range of 0.8−4.1 ppm. Minor signals of low
intensity (<10%) were also visible in the 31P{1H} NMR spectra
in several instances (see SI, section 3), which could be
attributed to oligomeric 1Rn species. We speculate that one of
these species could be the cyclohexaphosphonate 1R6, which
was also found in the single crystal structure of 1Ph6. This is
corroborated by the ESI-MS spectra obtained from CH2Cl2
solutions of 1R3, which showed the expected ion peaks for the
cyclotriphosphonates 1R3 and also intense ion peaks for the
cyclohexaphosphonates 1R6.
a
Scheme 1. Synthesis of the Cyclic Phosphonates 1R3
a
Yields are given in brackets.
The dynamic covalent chemistry of the cyclotriphospho-
nates 1R3 in solution was further investigated by mixing 1Ph3
and 1Mes3 in CDCl3, which led to highly complex 31P{1H}
NMR spectra after ca. 24 h at ambient temperature, suggesting
that ring-opening/ring-closure equilibria occur in solution
leading to substituent scrambling, as further confirmed by ESI-
MS (see SI, section 3.5). In comparison, variable temperature
1H and 31P NMR spectroscopy of 0.5 M solutions of 1R3 in
CDCl3 revealed no significant changes in the temperature
range of −60 to +52 °C.
Studies of the cyclic precursors 1R3 by differential scanning
calorimetry (DSC) suggested that melting of 1R3 is
accompanied by thermally induced ROP leading to the
polymers 1Rn. In each case, the first heating cycles of 1R3
displayed an intense endothermic signal characteristic of a
melting of the material, and a small ROP exotherm was
discernible (except for 1tBuAr′3). The melting transition was
absent in the subsequent heating cycles and instead a new
deflection associated with a change in gradient emerged, which
is diagnostic of a glass transition, providing an indication for
the formation of polymeric 1Rn. The glass transition
temperatures (Tg) of the materials differed significantly
depending on the aryl group [Tg = 66 (1Phn), 103 (1tBuArn),
133 (1Mesn), 140 °C (1tBuAr′n)], whereby higher degrees of
substitution lead to larger Tg values, probably due to enhanced
rigidity of the polymer chains.17
1R3 were isolated as hydrolytically sensitive, colorless solids
and fully characterized by solution NMR spectroscopy, single-
crystal XRD, ESI-MS, FT-IR spectroscopy and elemental
analysis (see SI, section 3).15
Single crystal XRD analyses of 1Mes3, 1tBuAr3 and 1tBuAr′3
confirmed that these cyclic phosphonates consist of six-
membered (PO)3 rings in the solid state, which adopt boat
conformations with one aryl substituent in trans orientation
(Figure 1a). In contrast, crystallization of 1Ph3 yielded
reproducibly minor quantities of single crystals consisting of
the 12-membered ring 1Ph6 (Figure 1b), which represents the
The preparative synthesis of the polymers 1Rn was achieved
by thermal ROP of the cyclic precursors 1R3 (Scheme 2). The
Scheme 2. Synthesis of the Polymers 1Rn via Thermal ROP
of 1R3
monomers were heated slightly above the temperature of
melting of 1R3 as previously determined by DSC, which led to
a considerable increase of the viscosity of the melt. After
keeping the materials under thermal ROP conditions for 1 h
and cooling to ambient temperature, glassy solids were
obtained. These solids showed only the previously observed
Figure 1. Molecular structures of 1Mes3 (a) and 1Ph6 (b) by single-
crystal XRD; H atoms omitted for clarity.
B
DOI: 10.1021/jacs.8b13435
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Journal of the American Chemical Society
Communication
glass transitions for 1Rn in the DSC analysis and distinct
changes were observed for the ν(P−O) absorption bands in
the ATR FT-IR spectra, which can be explained by the changes
in the P−O bond energies due to ROP (see SI, section 4).
Slow decomposition to the corresponding phosphonic acids
occurred upon exposure of the polymers 1Rn to atmospheric
moisture, but the materials were found to be indefinitely stable
under a dry nitrogen atmosphere at ambient temperature.
The polymer 1Phn was found to be insoluble in common
organic solvents, whereas 1tBuArn readily dissolved in CDCl3,
toluene or hexane, but rapidly depolymerised in solution to the
cyclic precursor 1tBuAr3 according to 1H and 31P NMR
spectroscopy, preventing further analysis of these materials by
1
solution-based techniques. By contrast, the H and 31P NMR
spectra of the polymers 1Mesn and 1tBuAr′n (data discussed
below) containing sterically more congested side chains with
methyl groups in the ortho positions showed only slow
depolymerization to the cyclic precursors 1Mes3 and 1tBuAr′3
in solution (t1/2 > 15 days), suggesting that these polymers are
kinetically trapped in solution by the presence of the bulkier
side groups. The identity of the polymers 1Phn and 1tBuArn
was further corroborated by their solid state 31P CP-MAS
NMR spectra (Figure 2a), showing broad isotropic 31P
Figure 3. (a) Reaction of 2Ph with B(C6F5)3 to give 2Ph·B(C6F5)3.
(b) Molecular structures by single crystal X-ray diffraction of (b) 2Ph
and (c) 2Ph·B(C6F5)3 (H atoms omitted for clarity).
thesized from 1R3 and 4-dmap and fully characterized by XRD,
NMR and elemental analysis (for details see SI, section 5).
Remarkably, the adducts 2R (R = Ph, Mes) exist in a
dissociative equilibrium with 1R3 and free 4-dmap in CDCl3
solution. The estimated equilibrium constants derived by
NMR spectroscopy amount to K = 1.11 · 10−7 (2Ph) and 3.49
· 10−4 mol L−1 (2Mes). Attempts to abstract the 4-dmap
substituent from 2R with the Lewis-acidic boranes BPh3 or
B(C6F5)3 resulted in either mixtures of mainly the six-
membered rings 1R3 and (4-dmap)·BPh3 or the “push-pull”
adduct (4-dmap)PhPO2[B(C6F5)3] [2Ph·B(C6F5)3, Figure 3],
which was also isolated and fully characterized, but no
formation of polymeric 1Rn was detected during these
reactions by NMR spectroscopy.
Figure 2. (a) Representative 31P CP-MAS NMR spectrum of 1tBuArn;
the inset shows the isotropic 31P NMR chemical shifts of all materials.
(b) 31P{1H} NMR spectrum of 1Mesn in CDCl3.
Analysis of dilute solutions of 1Mesn and 1tBuAr′n in CH2Cl2
by dynamic light scattering (DLS) revealed the presence of
particles with average hydrodynamic radii of 3.1 and 3.3 nm,
respectively. These values corroborate the presence of
polymeric material and can be roughly compared to
polystyrene particles of similar size, which possess weight-
average molecular weights (Mw) of 14 400 or 16 100 g mol−1,
corresponding to degrees of polymerization (DPw) for 1Mesn
and 1tBuAr′n of ca. 79 and 72 repeat units. These molecular
weights are reasonably consistent with the MALDI-ToF mass
spectra of 1Mesn and 1tBuAr′n (Figure 4a), which suggest an
upper limit to the degree of polymerization of >70,
corresponding to molecular weights of ca. 12 900 and 17 300
g mol−1, respectively. A significant decrease of intensity toward
high molar masses was observed in the MALDI-ToF mass
spectra, which indicates that short-chain molecules are
preferably ionized and the measured intensity distribution
does not reflect the molecular weight distribution determined
by DLS.
resonances at 1.3 and 0.2 ppm, respectively. These signals
are similar to the 31P NMR signals of 1Mesn and 1tBuAr′n in
the solid state [δiso = 0.3 ppm (1Mesn), −1.5 ppm (1tBuAr′n)].
In CDCl3 solution, the polymers 1Mesn and 1tBuAr′n give rise
to broad resonances in the 1H and 13C{1H} NMR spectra and
three broad signals are observed in the 31P{1H} NMR spectra
(δ = −1.3−1.5 ppm) with a rough intensity distribution of
2:1:1 (Figure 2b). These were assigned to rm/mr and either
mm or rr triads, respectively, suggesting the presence of
stereoirregular atactic polymer chains, as expected from the
forcing conditions of the thermal ROP.18 Notably, the much
broader 31P NMR signals of the polymers 1Rn appear in a
similar spectral region to those of the cyclic monomers 1R3 in
CDCl3 (0.8−4.1 ppm).
Further evidence for the assigned structure for 1Phn was
provided by treatment of a suspension of 1Phn in CDCl3 with
4-dimethylaminopyridine (4-dmap), leading to complete
dissolution of all insoluble material due to selective formation
of (4-dmap)PhPO2 (2Ph; Figure 3),19 which is the Lewis base
adduct of the transient dioxophosphorane PhPO2,5c,20,21 i.e.
the monomer unit of 1Phn. The adduct 2Ph as well as its
derivative (4-dmap)MesPO2(2Mes) were subsequently syn-
Thermal gravimetric analysis (TGA) revealed a high thermal
stability to weight loss for the polymers 1Rn with high T5% loss
values, which range from ca. 310 to 380 °C (see SI, section 4).
TGA also revealed low ceramic yields (ca. 10%), indicating an
almost complete thermal decomposition and vaporization of
C
DOI: 10.1021/jacs.8b13435
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Journal of the American Chemical Society
Communication
Notes
The authors declare no competing financial interest.
Crystallographic data have also deposited with Cambridge
Structural Database under the deposition numbers CCDC
1882836−1882844.
ACKNOWLEDGMENTS
■
M.I.A. acknowledges the Alexander von Humboldt foundation
for a Feodor Lynen research fellowship, V.T.A. thanks the
European Union for a Marie Curie Fellowship (H2020-MSCA-
IF-2016_748371) and N.L.K. thanks EPSRC for a PhD
studentship through the EPSRC Centre for Doctoral Training
in Molecular Analytical Science (EP/L015307/1). J.V.H.
thanks the EPSRC, the University of Warwick and the
Birmingham Science City Program for partial funding of the
solid state NMR infrastructure at Warwick. The latter program
accessed the Birmingham Science City Advanced Materials
Project 1: Creating and Characterizing Next Generation
Advanced Materials, which derived support from Advantage
West Midlands (AWM) and the European Regional Develop-
ment Fund (ERDF). I.M. thanks the Canadian government for
a Canada 150 Research Chair and the University of Bristol for
support. We also thank Dr. H. A. Sparkes for crystallographic
contributions and R. L. N. Hailes for help with the MALDI-
ToF MS measurements.
Figure 4. (a) Representative MALDI-ToF mass spectrum of 1Mesn
showing the presence of high molecular weight polymer; for details on
the MALDI-ToF mass spectra of 1Mesn and 1tBuAr′n see SI, sections
4.3 and 4.4. (b) Free-standing film (12 mm × 12 mm) of 1Mesn
prepared by drop-casting from 1,2-dichloroethane solution.
the materials at 575 °C. Initial studies aiming at the fabrication
of the polymers revealed that self-standing films of 1Mesn can
be prepared by drop-casting of a concentrated solution in 1,2-
dichloroethane onto a Teflon mold (Figure 4b), providing
evidence for the promising processability properties of these
materials.
In summary, we have established synthetic access to novel
polymers consisting of a P−O main chain and aryl groups at
phosphorus via ROP of cyclic phosphonates. The relatively
small ROP exotherms detected by DSC suggest that ΔH0ROP is
low for these materials, as expected for monomers with bulky
substituents, rationalizing that ROP only occurs at high
monomer concentration, i.e. in the melt phase. Careful
adjustment of the side groups leads to significant changes in
the properties of the polymers, including the solubility and the
stability in solution toward depolymerization. This tunable
reversible depolymerization behavior in solution, which can be
furthermore chemically triggered as demonstrated by the
depolymerization upon addition of a Lewis-base, suggests
potential for the development of recyclable materials.22 The
simple fabrication, the high thermal stability and the large
diversity of possible organic side groups opens up a large scope
for future explorations of these new inorganic soft materials.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.8b13435.
T., Eds.; John Wiley & Sons: Hoboken, NJ, 2018.
■
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AUTHOR INFORMATION
Corresponding Author
*ian.manners@bristol.ac.uk, imanners@uvic.ca
■
ORCID
Marius I. Arz: 0000-0002-1271-6343
Vincent T. Annibale: 0000-0002-0511-4504
John V. Hanna: 0000-0002-0644-3932
Ian Manners: 0000-0002-3794-967X
D
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