Synthesis and optical properties of sulfur-containing monomers and cyclomatrix polyphosphazenes

Article abstract of DOI:10.1039/b926069b

Novel cyclotriphosphazenes with sulfur-containing spirocyclic side groups were synthesized and polymerized to cyclomatrix materials for potential optical applications. The cyclotriphosphazenes were designed to give a high content of the phosphazene unit and sulfur atoms, as well as the capability for polymerization by ring- opening of the side groups. The chemical structures of the monomers were confirmed by NMR spectrometry, mass spectrometry, and single-crystal X-ray diffraction. Transparent solids were obtained by thermal bulk polymerization, and these were analyzed by the use of DSC, infrared spectroscopy, and mass spectrometry. One of the resultant cyclomatrix polyphosphazenes had a refractive index at 589 nm of 1.6465 and an Abbe number of 39. The contribution of the phosphazene unit to the refractive index is discussed.

Full text of DOI:10.1039/b926069b

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis and optical properties of sulfur-containing monomers and
cyclomatrix polyphosphazenes†
Toshiki Fushimia and Harry R. Allcock*b
Received 4th January 2010, Accepted 5th April 2010
First published as an Advance Article on the web 4th May 2010
DOI: 10.1039/b926069b
Novel cyclotriphosphazenes with sulfur-containing spirocyclic side groups were synthesized and
polymerized to cyclomatrix materials for potential optical applications. The cyclotriphosphazenes were
designed to give a high content of the phosphazene unit and sulfur atoms, as well as the capability for
polymerization by ring- opening of the side groups. The chemical structures of the monomers were
confirmed by NMR spectrometry, mass spectrometry, and single-crystal X-ray diffraction. Transparent
solids were obtained by thermal bulk polymerization, and these were analyzed by the use of DSC,
infrared spectroscopy, and mass spectrometry. One of the resultant cyclomatrix polyphosphazenes had
a refractive index at 589 nm of 1.6465 and an Abbe number of 39. The contribution of the
phosphazene unit to the refractive index is discussed.
electronic transitions in the UV region.3,5 Generally, the closer
Introduction
the ultraviolet absorption wavelength is to the visible region, the
higher is the optical dispersion. Thus, transparency both in the
visible region and in the UV region is required for low optical
dispersion.
One of the chemical groups, which gives high refractive
index, transparency, and low optical dispersion is the alkyl
sulfide unit.5,11,12 There have been many studies based on the intro-
duction of alkyl sulfide units into polymeric systems. Monomers
with refractive indices higher than 1.6 have been synthesized, and
transparent polymers with refractive indices as high as, and optical
dispersions as low as those of inorganic glasses have been prepared
by bulk ring-opening or bulk addition polymerization, with the
avoidance of by-products or solvent inclusion.5,12
Hybrid organic–inorganic cyclophosphazenes are candidates
for the preparation of polymeric optical materials. Cyclophosp-
hazenes form a broad class of compounds with an inorganic ring
of alternating nitrogen and phosphorus atoms, and with a wide
range of side groups linked to the phosphorus atoms. Organic side
groups are usually introduced by reactions of chlorocyclophosp-
hazenes, (NPCl2)n (n = 3, 4, and higher), with nucleophiles.13–22
The phosphazene ring has the following outstanding advantages
Organic polymeric materials have the potential to replace inor-
ganic glasses in the field of optics because they have the advantage
of lightness of weight, flexibility, and ease of processing. Examples
are lenses in cameras or eyeglasses,1 optical filters, and optical
fibers.2 Three important optical properties for lenses are the
refractive index, transparency, and optical dispersion. Optical
dispersion is the dependence of refractive index on the wavelength
of light, and is a factor that causes undesirable color fringing of
an image after passing through a lens. A combination of high
refractive index and low optical dispersion (high Abbe number) is
desirable.
These optical properties are determined by the following mate-
rial’s properties: polarizability,3,4 absorption of light by electronic
transitions,3,5 and homogeneity. A high refractive index requires
a high electronic polarizability. The polarization is caused by
displacement of electrons in the oscillating electromagnetic field
of light. Organic materials generally show lower refractive index
values than inorganic materials because of their lower electron
density.6 The introduction of electron-rich groups such as organic
conjugated units7–9 and heavy elements4,5,8–12 enables the synthesis
of polymeric materials with refractive indices higher than 1.6.
The preparation of a transparent material requires not only
the absence of an inherent absorption of light from electronic
transitions in the visible region, but also the absence of scattering
caused by a heterogeneous morphology. Bulk polymerization to
an amorphous material without the formation of by-products is
also preferred for the production of polymers free from inclusions,
which would cause scattering. Optical dispersion correlates with
=
for incorporation into optical materials. Firstly, the–P N–unit
in the cyclotriphosphazene skeleton has a molar refraction of
6.88 cm3 mol-1 and a molar volume of 14.5 cm3 mol-1, which
corresponds to a refractive index as high as 1.9.15 Secondly,
the inorganic skeleton is transparent down to wavelengths near
230 nm.13 Thirdly, the ease of chemical modification of the side
groups allows an enhancement of the refractive index and the
introduction of functional groups for polymerization.
Novel cyclotriphosphazenes to be used as monomers for optical
materials should have a high electron density and a chemical
structure appropriate for polymerization. The polymerization
process should not produce by-products and preferably can be
carried out without the use of solvents. The chlorine atoms in
chlorocyclophosphazene starting compounds must be replaced by
organic substituents because the P–Cl bonds are susceptible to
hydrolysis in the atmosphere. Ideally, the substituents should have
aResearch and Development Department, Fushimi Pharmaceutical Co., Ltd.,
1676 Nakazu-cho, Marugame, 763-8605, Japan
bDepartment of Chemistry, The Pennsylvania State University, University
Park, Pennsylvania, 16802, USA. E-mail: hra@chem.psu.edu
† Electronic supplementary information (ESI) available: Additional data.
CCDC reference numbers for 1, 2, 3 and 4 are 760670, 760671, 760672 and
760673, respectively. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b926069b
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a minimal contribution to the molecular weight so that the weight
percent of the phosphazene skeleton remains high. They should
also provide a mechanism for polymerization.
hydrochloride (20.00 g, 17.60 mmol) in absolute ethanol (250 cm3).
The suspension was stirred at ca. 20 ◦C overnight. The precipitate
and the solvent were removed by filtration and evaporation. The
product was distilled from the resultant oil at 34 ◦C and 2.0 ¥
101 mmHg. The purified product was obtained as a transparent,
colourless oil in a yield of 39.3%; dH(360.13 MHz; CDCl3;
(CH3)4Si) 1.38 (2 H, s, NH2), 2.09 (3 H, s, SCH3), 2.59 (2 H, t,
CH2S) and 2.88 (2 H, t, CH2N).
Cyclotriphosphazenes with spiro rings are particularly inter-
esting systems.16–21 These are formed by the replacement of two
geminal chlorine atoms in N3P3Cl6 with use of a difunctional
substituent. A cyclotriphosphazene with spiro- or transannular-
side groups would have a higher content of the phosphazene com-
ponent than counterparts with non-cyclized side groups because
the former requires only three substituents per phosphazene ring
while the latter requires six. In addition, spiro rings provide a
mechanism for polymerization. Some cyclotriphosphazenes with
five-membered spiro rings, such as N3P3(O2C6H4)3, are known to
form transparent crosslinked cyclomatrix polymers by opening
of the spiro ring.17,18 This polymerization takes place without a
solvent and produces no by-products.
In this article, we report the synthesis, characterization, and
polymerization of novel cyclotriphosphazenes with spiro rings,
and describe the optical properties of a polymer derived from
one of these species. 2-Mercaptoethylamine was chosen as the
functional substituent because of its low mass, its capacity to form
a strained five-membered spiro ring, and because the presence of
a sulfur atom should enhance the refractive index. The results
show that this phosphazene is a promising candidate for optical
materials with a high refractive index and a low optical dispersion.
Synthesis of 3,3,5,5-tetrachloro-1,1-(epithioethanoimino)cyclo-
triphosphazene (N3P3Cl4(NHCH2CH2S)) (1). Triethylamine
(13.36 g, 132.0 mmol) was added to a suspension of finely ground
2-mercaptoethylamine hydrochloride (5.00 g, 44.0 mmol) and
N3P3Cl6 (15.30 g, 44.01 mmol) in tetrahydrofuran (300 cm3). The
suspension was stirred at ca. 20 ◦C overnight. The precipitate
and the solvent were removed by filtration and evaporation,
and the crude product was recrystallized from chloroform.
Compound 1 was obtained as colourless transparent crystals in
a yield of 57% with respect to N3P3Cl6; dH(360.13 MHz; CDCl3;
(CH3)4Si) 3.27 (1 H, s, NH) and 3.40–3.55 (4 H, m, CH2CH2);
dP(145.78 MHz; CDCl3; H3PO4) 21.7 (2 P, d, 2 ¥ PCl2) and 47.1
(1 P, t, P(NHCH2CH2S)); m/z 351 (MH+).
Synthesis of trans-5,5-dichloro-1,1:3,3-bis(epithioethanoimino)-
cyclotriphosphazene (trans-N3P3Cl2(NHCH2CH2S)2) (2). Tri-
ethylamine (13.36 g, 132.0 mmol) was added dropwise over
30 min to a suspension of finely ground 2-mercaptoethylamine
hydrochloride (10.00 g, 88.02 mmol) and N3P3Cl6 (15.30 g,
44.01 mmol) in acetonitrile (200 cm3). The suspension was stirred
at ca. 20 ◦C overnight. Triethylamine (13.36 g, 132.0 mmol) in
acetonitrile (100 cm3) was added to the suspension over 4 h, and
the suspension was stirred at ca. 20 ◦C overnight. The precipitate
and the solvent were removed by filtration and evaporation, and
the crude product was dried under vacuum overnight. The crude
product (20.44 g) was stirred in chloroform (40 g) at ca. 20 ◦C,
and then the finely divided solid in the suspension was collected
by filtration, and was rinsed with cold chloroform. Compound 2
was obtained as a white powder in a yield of 53% with respect to
N3P3Cl6; dH(360.13 MHz; CDCl3; (CH3)4Si) 3.15 (2 H, s, 2 ¥ NH)
and 3.35–3.50 (8 H, m, 2 ¥ CH2CH2); dP(145.78 MHz; CDCl3;
H3PO4) 24.0 (1 P, t, PCl2) and 49.5 (2 P, d, 2 ¥ P(NHCH2CH2S));
m/z 356 (MH+). Decomposition to a powder, which was insoluble
in tetrahydrofuran or dichloromethane but soluble in water, took
place when the solution was exposed to air.
Experimental
Scheme 1 shows the synthetic scheme for the series of the
cyclotriphosphazenes in this study.
Scheme 1 Structures and synthetic route of the cyclotriphosphazenes
used in this study.
Synthesis of cis-5,5-dichloro-1,1:3,3-bis(epithioethanoimino)-
cyclotriphosphazene (cis-N3P3Cl2(NHCH2CH2S)2) (3). Triethy-
lamine (13.36 g, 132.0 mmol), finely ground 2-mercaptoethylamine
hydrochloride (5.00 g, 44.0 mmol), and N3P3Cl6 (7.65 g,
22.0 mmol) were added simultaneously to dioxane (300 cm3). The
Materials
Iodomethane and 2-mercaptoethylamine hydrochloride were
purchased from Alfa Aesar. Hexachlorocyclotriphosphazene
[N3P3Cl6 or (NPCl2)3] was supplied by Fushimi Pharmaceutical
Co., Ltd. (Japan) and was purified by recrystallization from
heptane followed by sublimation. All reactions were carried out
under a nitrogen atmosphere and in dried solvents.
◦
suspension was stirred at ca. 20 C for 3 d. The precipitate and
the solvent were removed by filtration and evaporation. The crude
product was then dried under vacuum overnight. The dried crude
product was recrystallized from chloroform (70 wt% solution) at
the boiling point. The crystals were collected by filtration, and were
rinsed with cold chloroform. Compound 3 was obtained as a white
powder in a 34% yield with respect to N3P3Cl6; dH(360.13 MHz;
CDCl3; (CH3)4Si) 3.11 (2 H, s, 2 ¥ NH) and 3.35–3.50 (8 H, m, 2 ¥
CH2CH2); dP(145.78 MHz; CDCl3; H3PO4) 24.1 (1 P, t, PCl2)
Synthesis of 2-(methylthio)ethylamine (NH2CH2CH2SCH3).
A
solution of potassium hydroxide (19.76 g, 35.21 mmol) in ab-
solute ethanol (150 cm3) was added dropwise to a solution of
methyl iodide (24.99 g, 17.60 mmol) and 2-mercaptoethylamine
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and 49.6 (2 P, d, 2 ¥ P(NHCH2CH2S)); m/z 356 (MH+). A
decomposition pattern similar to that of 2 was detected.
exposure time of 5 or 10 s flame-1. The flames were integrated
with a software package (Bruker SAINT) using a narrow-frame
integration algorithm. Structures were solved and refined by
the use of a software package (Bruker SHELXTL version 6.1).
All non-hydrogen atoms were refined anisotropically, and then
hydrogen atoms were rode on their parental atoms. Disorder in the
structure was identified with the help of difference Fourier map.
CCDC reference numbers for 1, 2, 3, and 4 are 760670, 760671,
760672, and 760673, respectively. The details of data collection and
refinement, a cif file for the crystallographic data of 1, 2, 3, and 4,
as well as their molecular structures are available in supplementary
information.†
The density of the cyclomatrix polymer formed from 5 was
determined by measurements of the density of an aqueous calcium
chloride solution that caused suspension of the samples at a
mid point in the solution. The estimated error was 0.2%. The
refractive indices of the polymer at 25 ◦C and at 486, 589, and
656 nm were measured at OHARA Inc (Japan) by the use of a
V-block refractometer (PR-2, Carl Zeiss Jena).
Synthesis of trans-1,1:3,3:5,5-tris(epithioethanoimino)cyclotri-
phosphazene (trans-N3P3(NHCH2CH2S)3) (4). Triethylamine
(1.71 g, 16.9 mmol), finely ground 2-mercaptoethylamine hy-
drochloride (0.64 g, 5.6 mmol), and 2 (2.00 g, 5.62 mmol)
were stirred in tetrahydrofuran (300 cm3) at ca. 20 ◦C for 6
d. The resulting suspension was concentrated to a slurry, and
the precipitate was collected, and dissolved in dichloromethane.
The triethylamine hydrochloride in the precipitate was partly
removed by precipitation into acetone. The product was then
recrystallized from methanol. Compound 4 was obtained as
transparent colourless rhombic crystals in a yield of 2.5% with
respect to 2; dH(360.13 MHz; CDCl3; (CH3)4Si) 3.03 (3 H, s, 3 ¥
NH) and 3.30–3.45 (12 H, m, 3 ¥ CH2CH2); dP(145.78 MHz;
CDCl3; H3PO4) 52.2 (3 P, s, 3 ¥ P(NHCH2CH2S)).
Synthesis of trans-5,5-bis[2-(methylthio)ethylanimo)]-1,1:3,3-
bis(epithioethano imino)cyclotriphosphazene (trans-N3P3(NHCH2-
CH2S)2(NHCH2CH2SCH3)2) (5). Triethylamine (5.00 g,
49.4 mmol), 2-(methylthio)ethylamine (4.51 g, 49.4 mmol), and 2
(8.00 g, 22.5 mmol) were dissolved in tetrahydrofuran (50 cm3).
Results and discussion
◦
Synthesis and characterization of the cyclotriphosphazenes
The suspension was stirred at ca. 20 C for 1 d. The precipitate
and the solvent were removed by filtration and evaporation.
The crude product was dissolved in dichloromethane (100 cm3),
washed with water (100 cm3), and then recrystallized twice from
methanol (33 wt% solution). The crystals were dried under
vacuum at ca. 20 ◦C overnight. Compound 5 was obtained as
colourl◦ess transparent crystals in a yield of 54% with respect to 2;
N3P3Cl4(NHCH2CH2S) (1). When triethylamine was added
stepwise to
a solution of N3P3Cl6 with suspended 2-
mercaptoethylamine hydrochloride, the 31P NMR spectrum of the
solution revealed two stages of substitution. In the first stage, as
three molar equivalents of triethylamine with respect to N3P3Cl6
were added, a conversion occurred from a 31P singlet, ascribed to
N3P3Cl6, to a doublet and a triplet. In the second stage, a change
occurred from the preceding spectrum to partially overlapped
two pairs of a doublet and a triplet, until three additional
molar equivalents of triethylamine had been added. Assuming
the consumption of one molar equivalent of triethylamine for the
liberation of one molar equivalent of 2-mercaptoethylamine from
its hydrochloride salt, two molar equivalents of triethylamine were
consumed in one stage of the substitution. This indicates that two
molar equivalence of chlorine atoms were replaced in each stage.
The product formed during the first stage of the substitu-
tion was purified and characterized to determine its chemi-
cal structure. The mass spectrum showed that the molecular
weight did not correspond to N3P3Cl4(NHCH2CH2SH)2 or to
N3P3Cl4(SCH2CH2NH2)2 but was correct for 1. The fine structure
originating from the presence of the isotopes of the chlorine atoms
showed that the molecule had four remaining chlorine atoms.
No protons ascribable to the thiol group were detected in the
1H NMR spectrum in CDCl3. These results showed that both
functional groups of 2-mercaptoethylamine had reacted with the
same phosphorus atom, and thus formed a spiro ring structure.
This molecular structure was further confirmed by single-crystal
X-ray crystallography.
mp 78 C; nmax(KBr pellet)/cm-1 3145 (NH) and 1172 (–N P–);
=
dH(360.13 MHz; CDCl3; (CH3)4Si) 2.10 (6 H, s, 2 ¥ SCH3), 2.66 (4
H, t, 2 ¥ CH3SCH2), 2.84 (2 H, s, 2 ¥ NHCH2CH2SCH3), 2.92 (2
H, s, 2 ¥ (NHCH2CH2S)P), 3.13 (4 H, dt, 2 ¥ NHCH2CH2SCH3)
and 3.30–3.45 (8 H, m, 2 ¥ (NHCH2CH2S)P); dP(145.78 MHz;
CDCl3; H3PO4) 16.0 (1 P, t, P(NHCH2CH2SCH3)2) and 53.4 (2 P,
d, 2 ¥ P(NHCH2CH2S)); m/z 466 (MH+).
Thermal bulk polymerization of trans-5,5-bis[2-(methylthio)-
ethylanimo]]-1,1:3,3-bis(epithioethanoimino)cyclotriphosphazene
(5). This phosphazene (1.5 g) was placed in a Teflon well (12 ¥
12 ¥ 7 mm), and was dried at 125 ◦C under a vacuum of 8 ¥ 10-1 mm
Hg for 48 h. It was heated at 165 ◦C in a nitrogen atmosphere for
5.5 h. The density of the polymer was 1.447 g cm-3 at 25 ◦C.
Measurements
1H and 31P NMR spectra were obtained by the use of a multi-probe
NMR spectrometer (Bruker AMX-360). The chemical shifts of the
protons in the amino groups in 1–5 moved toward a lower field as
the concentration in the CDCl3 solution increased after exceeding
a certain value. The chemical shifts reported here were obtained
from CDCl3 solutions diluted sufficiently for the 1H NMR spectra
to be independent of concentration. The IR spectra of 5 and the
cyclomatrix polymer formed from 5 were obtained by the use of
an IR spectromer (HORIBA FT-720).
N3P3Cl2(NHCH2CH2S)2 (2 and 3). The product formed dur-
ing the second stage of the substitution was purified and char-
acterized to determine its chemical structure. The two pairs
of a doublet and a triplet detected in the 31P NMR spectrum
indicate the presence of two phosphazenes with AB2 structures.
The difference in chemical shift was small enough for the pairs to
overlap. These two products, 2 and 3, were isolated as described
X-Ray intensity data were collected by the use of a diffrac-
tometer (SMART APEX CCD area detector system) equipped
with a graphite monochromator and a MoKa fine-focus sealed
˚
tube for X-ray radiation at a wavelength of 0.71073 A. A total
of 1850 flames were collected with a scan width of 0.3◦ in w and
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in the Experimental. The 1H NMR spectra of both of the purified
products in CDCl3 were similar to each other. These spectra were
also similar to that of 1. The similarity in the 31P NMR spectra
indicates that the two phosphazenes have similar structures. The
similarity in 1H NMR spectra between 1, 2, and 3 indicates that the
substituents in them are the same spiro rings. These results show
that the two phosphazenes are trans and cis isomers originating
from different mutual configurations of the two asymmetric spiro
rings. The trans isomer has the two sulfur atoms on opposite sides
of the phosphazene ring, while the cis isomer has both sulfur
atoms on the same side. The presence of trans and cis isomers has
been reported for a similar phosphazene with two spiro rings with
oxygen atoms in place of the sulfur atoms.20 The diastereomeric
configurations of 2 and 3 were confirmed to be trans and cis
isomers by single-crystal X-ray crystallography.
as lenses. The weight percent of the phosphazene ring and the
total weight percent of phosphorus and sulfur are 29.0 and 47.5%,
respectively.
Single-crystal X-ray crystallography of N3P3(NHCH2CH2S)3
(4). Fig. 1 shows the molecular structure of 4.
The reactions up to this stage gave the mono- or the di-spiro
phosphazenes selectively. The 31P NMR spectrum of the reaction
solution for 1 showed that 90 mol% of the products was 1.
Only ca. 5 mol% of the phosphazenes progressed to 2 and 3.
This was similar to the reaction for the di-spiro phosphazenes.
This selective formation is probably due to the reduction of
reactivity caused by the replacement of two chlorine atoms in
a step of substitution. Replacement of the electron-withdrawing
chlorine atoms by organic groups reduces the electrophilicity of
the remaining PCl groups.22 This allows the N3P3Cl6 to consume
the reactants before N3P3Cl4(NHCH2CH2S) undergoes the next
stage of the substitution.
Fig. 1 Molecular structure of 4 determined by single-crystal X-ray
crystallography.
The nitrogen and the sulfur atoms in the substituent connect
to a same phosphorus atom. Thus the side group ring is spiro
rather than transannular. The exocyclic S–P–N angle in the
spiro ring was 96◦, which is as small as the exocyclic angle
of 97◦ found in the strained spiro ring in N3P3(O2C6H4)3.16,17,19
This suggests the presence of ring strain imposed by the five-
membered rings. Two P–N–P angles in the phosphazene ring
were 123◦ and the other was 125◦. Two N–P–N angles in the
phosphazene ring were 115◦, and the other was 118◦. The
P–N–P angle of 125◦ is at the high end of the range for
P–N–P angles in cyclotriphosphazenes. The N–P–N angle of 115◦
is at the low end of the range for N–P–N angles in cyclotriphos-
phazenes except for rare cases where bulky organosilicon groups
or ferrocene units are present.13 These large P–N–P and small N–
P–N angles are probably due to the re-hybridization of atomic
orbitals caused by the strain at the phosphorus atoms. The bond
lengths between the nitrogen and the phosphorus atoms in the
N3P3(NHCH2CH2S)3 (4). Although 2-mercaptoethylamine
was introduced exclusively as the spiro ring in the synthesis
of 1, 2, and 3, this was not the case for this synthesis. The
31P NMR spectrum of the reaction solution showed that ca.
40 mol% of the phosphorus atoms were in a phosphazene
similar to 5 described in the next section. This is probably
trans-N3P3(NHCH2CH2S)2(NHCH2CH2SH)2 formed by a reac-
tion between N3P3Cl(NHCH2CH2S)2(NHCH2CH2SH) and 2-
mercaptoethylamine before the third spiro ring closes. The yield
of 5 was low because of the presence of a competitive pathway
as well as the experimental difficulty in its separation from other
compounds.
The purified product showed one singlet in its 31P NMR
spectrum and similar signals to those of 2 and 3 in its 1H
NMR spectrum. The 31P NMR spectrum shows that the three
phosphorus atoms are equivalent. Moreover, the 1H NMR
spectrum indicates that the substituents are identical spiro rings.
These results indicate that the structure of this product is trans-
N3P3(NHCH2CH2S)3. This structure was confirmed by single-
crystal X-ray crystallography. The weight percent of the phosp-
hazene ring in this compound is 37.4%. This value is even higher
than, for example, that in N3P3(OCH2CH3)6, which is one of the
smallest cyclotriphosphazenes stable in air at room temperature.
The total weight percent of phosphorus and sulfur atoms is 52.5%.
These numbers demonstrate that replacement of chlorine atoms in
N3P3Cl6 by spiro rings is an effective method for the preparation
of phosphazenes with a high phosphorus content.
˚
phosphazene ring ranged from 1.58 to 1.60 A, which are typical
values for cyclotriphosphazenes with organic substituents.13 The
˚
distances between atoms on the different spiro rings exceeded 4 A,
which indicates the absence of steric hindrance between them.
DSC measurements
N3P3(NHCH2CH2S)3 (4). Strained spiro rings in cyclo-
triphosphazenes such as N3P3(O2C6H4)3 and N3P3(S2C6H3CH3)3
open during heating and connect to neighbouring phosphazene
molecules to form cyclomatrix polymers.17,18 Because each phos-
phazene molecule has three spiro rings, the polymers become
crosslinked and thus insoluble.13 The smaller exocyclic angle in 4
compared to N3P3(O2C6H4)3 suggests the possibility of formation
of a cyclomatrix polymer in a similar manner during heating.
A single crystal of 4 became insoluble in dichloromethane after
N3P3(NHCH2CH2S)2(NHCH2CH2SCH3)2 (5). In this com-
pound a flexible chain was introduced to lower the melting point
so that the phosphazene can be used as a convenient monomer for
bulk polymerization in various shapes of molds for objects such
◦
heating at 200 C for 30 min. This change in solubility suggests
formation of a cyclomatrix polymer. Fig. 2 shows the DSC curve.
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Fig. 2 DSC curve for 4. The heating rate was 10 ◦C min-1.
Fig. 3 DSC curve for 5. The heating rate was 10 ◦C min-1.
evaluated from the amount of the soluble components extracted
by chloroform from the powdered product and from the 31P NMR
spectrum. Less than 1 wt% was non-polymerized phosphazene.
Thus, the conversion to the polymer was higher than 99%. A
polymer that was insoluble in methanol but soluble in chloroform
was obtained after heating at 165 ◦C for only 1 h. Direct evidence
for the polymerization was obtained from a mass spectrum of the
product in the chloroform solution as shown in Fig. 4.
An exothermic heat flow was detected in a temperature range
between 130 to 220 ◦C, and an endothermic heat flow was
detected at temperatures above 240 ◦C. The first transition is
ascribed to the polymerization because of the change in solubility
and the retention of the original weight, as shown by TGA
measurements at the corresponding temperature. The second
transition is ascribed to thermal decomposition, because of the
loss in weight and the appearance of yellow coloration at the
corresponding temperature. The polymerization of 4 occurred at
a much lower temperature than that of N3P3(O2C6H4)3,17 which
occurs above 240 ◦C. This difference may be due to the more
severe strain in 4 than in N3P3(O2C6H4)3, as indicated by the
narrower exocyclic angle, and/or weaker chemical bonding of P–S
compared to P–O.23
The single crystal maintained its transparency and colourless
appearance as well as its shape after polymerization at 200 ◦C.
The polymerization probably occurred at a temperature below the
melting point. This means that shaping by bulk-polymerization
in molds would be difficult, and thus may not be satisfactory
for optical materials fabrication. This led us to the synthesis and
polymerization of 5, where the melting point would be lower than
4 because of the presence of a flexible substituent.
Polymerization of N3P3(NHCH2CH2S)2(NHCH2CH2SCH3)2
(5). Fig. 3 shows the DSC curve for 5.
Fig. 4 Mass spectrum of the oligomers of 5 obtained after 1 h of
polymerization at 165 ◦C.
An endothermic flow and two exothermic flows were detected
at 78 ◦C, above 165 ◦C, and above 215 ◦C, respectively. The
endothermic flow at 78 ◦C is ascribed to the melting point based
on visual observation. The first exothermic flow is ascribed to
polymerization because of the loss of solubility and the detection
of oligomers, as described in the following paragraph. The second
exothermic flow is attributed to thermal decomposition evidenced
by the weight loss above 200 ◦C in TGA measurements. The
melting point is lower than the temperature of polymerization.
This allows shaping by bulk-polymerization in molds.
Successive major peaks with an interval corresponding to
the molecular weight of 5 were detected at m/z of 466, 931,
1396, and 1861. These peaks were ascribed to unreacted 5, the
dimer, the trimer, and the tetramer, respectively, with one proton
attached in the process of ionization. The same interval as the
molecular weight of the 5 also means that the polymerization is
not accompanied by any loss of the components. This is consistent
with the growth of the polymer by ring-opening of the spiro ring.
Evidence for the opening of the spiro ring was also obtained by
1H NMR spectroscopy, DSC measurements, and IR spectroscopy.
The 1H NMR spectrum of the soluble product showed a decrease
in intensity of the four CH2 protons of the spiro ring, which
indicates a change in chemical structure. The enthalpy of the
opening of the spiro ring was estimated from a DSC measurement
◦
Thus, heating of 5 under a nitrogen atmosphere at 165 C for
5.5 hours gave transparent, colourless solids that were insoluble
in solvents such as methanol or dichloromethane, which are good
solvents for 5. The solidity above the melting point of 5 and the in-
solubility indicate formation of a crosslinked cyclomatrix polymer.
The conversion of the cyclotriphosphazene to the polymer was
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Table 1 Refractive indices and Abbe number of the cyclomatrix polymer
formed from 5
at an early stage of the polymerization where the sample remained
soluble in CDCl3 and thus the conversion could be estimated from
1H NMR spectroscopy. The phosphazene was heated from 30 to
190 ◦C at a rate of 2 ◦C min-1 and then immediately quenched. The
change in enthalpy was estimated to be ca. -3 ¥ 101 kJ mol-1 from
the heat of the polymerization and the conversion. This change
is ascribable to the release of the strain in the spiro ring.24 This
supports the conclusion that the change in chemical structure
detected in the 1H NMR spectrum is due to opening of the spiro
rings accompanied by a release of the strain energy. Fig. 5 shows
the IR spectra of 5 before and after 12 h of heating at 165 ◦C.
Refractive index at 25 ◦
C
486 nm
1.6583
589 nm
1.6465
656 nm
1.6417
Abbe number
39
Scheme 2 Proposed structure of the triad in the cyclomatrix polymer
formed from compound 5.
Optical properties of the cyclomatrix polymer formed from
N3P3(NHCH2CH2S)2(NHCH2CH2SCH3)2 (5). Table 1 shows
the refractive indices of the cyclomatrix polymer at different
wavelengths, together with the Abbe number (n), which is an
indicator of optical dispersion, calculated from the following
equation.
nD −1
n =
nF −nC
Fig. 5 IR spectra of 5 before (top) and after (bottom) polymerization at
165 ◦C for 12 h.
where nF , nD, and nC indicate refractive indices at 486, 589, and
656 nm, respectively. The larger the Abbe number, the smaller is
the optical dispersion.
The infrared spectrum measured before the heating showed a
strong peak and a broad peak at 1172 and 3145 cm-1, respectively.
Organic polymers and inorganic glasses with refractive indices
of 1.65 generally show Abbe numbers of 25 and higher than
30, respectively.6 Thus, the optical dispersion of this cyclomatrix
polymer is lower than that of organic polymers with similar
refractive indices, and is as low as many inorganic glasses. This
important result demonstrates that phosphazenes can be used for
optical materials that require high refractive indices and high Abbe
numbers, that are comparable to inorganic glasses.
=
The first is attributed to the stretching mode of the–P N–unit. The
second is ascribed to the N–H bond of the spiro ring, because the
wavenumber is similar to that of the absorption of the N–H bond
in N3P3[(NH)2C6H10]3.21 The spectrum measured after the heating
=
also showed peaks ascribable to the–P N–unit and a N–H bond
-1
=
at 1198 and 3278 cm , respectively. The peak from the–P N–unit
was broader than that before the heating, and the peak for the NH
group shifted to a longer wavenumber compared to the spectrum
before the heating. The broadening of the former may be due to
polymerization. The shift of the latter suggests an opening of the
spiro ring. Based on these results, we conclude that the opening of
the spiro ring is involved in the polymerization.
=
The contribution of the–P N–unit to the refractive index of the
cyclomatrix polymer was estimated on the basis of the difference
in refractive index with and without the contribution from the
cyclotriphosphazene skeleton. Refractive index n can be evaluated
from the following equation:25
n2 −1
n2 + 2
R
V
Information about the chemical structure of the cyclomatrix
polymer was obtained from mass spectrometry. Only the monomer
and the dimer were detected in the mass spectrum (not shown) of
N3P3(NHCH2CH2S)(NHCH2CH2SCH3)4, which is an analogue
of 5◦but has only one spiro ring, after 1 or 5 h of heating at
165 C. This implies that the opening of not one, but two spiro
rings is required to connect two cyclotriphosphazene molecules.
Scheme 2 shows the proposed chemical structure of the triad
in the cyclomatrix polymer. However, the insolubility of the
cyclomatrix polymer suggests that the actual structure is a more
complex crosslinked one. We conclude that the polymerization
could involve the formation of ladder-like linkages and possibly
other mechanisms causing crosslinking.
=
where R and V represent molar refraction and molar volume,
respectively. The latter is a ratio of molecular weight to density.
The molar refraction required to calculate the imaginary refractive
index excluding the contribution from the cyclotriphosphazene
skeleton can be obtained by subtraction of the molar refraction
of the–P N–unit from that of the cyclotriphosphazene repeat-
ing unit in the polymer.25 The molar volume can be obtained in
the same method. The molar refraction and the molar volume
=
=
of the –P N– unit were previously evaluated by the authors
to be 6.88 mol-1 and 14.5 cm3 mol-1, respectively, from other
5354 | Dalton Trans., 2010, 39, 5349–5355
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liquid cyclotriphosphazenes.15 Those of the cyclotriphosphazene
repeating unit were calculated from the refractive index and
the density of the polymer. The imaginary refractive index was
calculated to be 1.61, which is lower than that of the polymer by
ca. 0.04. The difference is ascribed to the contribution from the
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Acknowledgements
We thank OHARA, Inc (Japan) for the measurement of refractive
indices, Dr Hemant P. Yennawar and David K. Lee for the single-
crystal X-ray crystallography and helpful discussions, and NSF
chemistry grant CHE-0131112 for the X-ray instrumentation.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 5349–5355 | 5355
©