Thermal decomposition of 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane

Article abstract of DOI:10.1007/s10973-010-0945-z

Certain five-membered dioxaheterocyclic compounds (hetero atoms may be P, Si, S, etc.) contain a strained carbon-carbon bond which may undergo homolytic thermolysis at modest temperatures to generate a diradical capable of initiating vinyl polymerization. If substituents contain flame-retarding moieties this represents a convenient method for imparting flame retrdancy to a polymeric material. Of particular interest has been 2,4,4,5,5-pentaphenyl-1,3,2- dioxaphospholane. The thermal degradation of this compound has been studied using ¹³C NMR spectroscopy. This may conveniently be done by monitoring the intensity of the signal for the benzylic carbon atom as a function of time and temperature. A simple transformation is the conversion of the cyclic compound to the linear polymer.

Full text of DOI:10.1007/s10973-010-0945-z

J Therm Anal Calorim (2010) 102:517–521
DOI 10.1007/s10973-010-0945-z
Thermal decomposition of 2,4,4,5,5-pentaphenyl-1,3,2-
dioxaphospholane
•
B. A. Howell Y.-J. Cho
NATAS2009 Special Issue
´
´
Ó Akademiai Kiado, Budapest, Hungary 2010
Abstract Certain five-membered dioxaheterocyclic com-
pounds (hetero atoms may be P, Si, S, etc.) contain a strained
carbon–carbon bond which may undergo homolytic ther-
molysis at modest temperatures to generate a diradical
capable of initiating vinyl polymerization. If substituents
contain flame-retarding moieties this represents a conve-
nient method for imparting flame retrdancy to a polymeric
material. Of particular interest has been 2,4,4,5,5-penta-
phenyl-1,3,2-dioxaphospholane. The thermal degradation of
this compound has been studied using 13C NMR spectros-
copy. This may conveniently be done by monitoring the
intensity of the signal for the benzylic carbon atom as a
function of time and temperature. A simple transformation is
the conversion of the cyclic compound to the linear polymer.
these materials in the environment and potential negative
health impacts has spurred increasing effort directed toward
finding suitable alternatives. Organophosphorus compounds
may act as effective flame retardants and offer the greatest
potential to serve as replacements for organohalogen com-
pounds in many applications [3–5]. In particular, new
organophosphorus compounds have been developed to
replace tetrabromobisphenol A for flame-retarding epoxy
resins [6, 7]. In this instance, strained five-membered di-
oxaheterocyclic compounds have been utilized as vehicles
by which flame-retarding moieties may be incorporated into
vinyl polymers. One of these, 2,4,4,5,5-pentaphenyl-1,3,2-
dioxaphospholane, undergoes smooth thermal rearrange-
ment to a new compound, presumably, the linear polymer.
Keywords Phospholane stability Á Radical initiators Á
Phosphorus flame retardants Á Five-membered heterocycles
Experimental
Materials
Introduction
Common solvents and reagents were obtained from Ther-
moFisher Scientific or the Aldrich Chemical Company.
Tetrahydrofuran (THF) was distilled from lithium aluminum
hydride prior to use; methylene chloride from calcium
hydride. Chlorodiphenylphosphine (diphenylphosphinous
chloride) was obtained from the Aldrich Chemical Company
and used as-received. 1,1,2,2-Tetraphenyl-1,2-ethanediol
(benzopinacol) was obtained by reductive coupling of ben-
zophenone as previously described [8]. This is a white
crystalline solid melting at 171 °C (Fig. 1). Spectral data for
this compound are presented below (Figs. 2, 3, and 4).
The most useful spectral information for monitoring the
formation of this compound and its transformation to
phosphorus heterocycles is the absorption at d 82.3 (ben-
zylic carbon atom) in the carbon-13 NMR spectrum.
For most applications, polymeric materials must be flame
retarded. This may be accomplished in a number of ways
depending on the intended end use of the polymer [1]. In the
US, organohalogen compounds, principally bromi-
nated aromatics, are widely used [2]. These compounds,
particularly, decabromodiphenyl ether, are readily available
at modest cost and highly effective as flame retardants.
However, increasing concern about bioaccumulation of
B. A. Howell (&) Á Y.-J. Cho
Center for Applications in Polymer Science and Department of
Chemistry, Central Michigan University, Mount Pleasant, MI
48859-0001, USA
e-mail: bob.a.howell@cmich.edu
123

518
B. A. Howell, Y.-J. Cho
–30
–20
–10
0
10
20
30
40
50
60
70
80
90
100
60
80
100
120
140
160
180
200
Temperature/°C
Fig. 1 DSC thermogram for 1,1,2,2-tetraphenyl-1,2-ethanediol
(benzopinacol)
140
130
120
110
100
90
80 ppm
Fig. 4 13C NMR spectrum of 1,1,2,2-tetraphenyl-1,2-ethanediol
106
104
102
100
98
scanning calorimetry (DSC) using a TA Instruments 2910
MDSC at a heating rate of 5 °C min-1 in a constant
nitrogen purge of 50 cm3 min-1. Thermal decomposition
of the phospholane rearrangement product was examined
by thermogravimetry using a TA Instruments 2950 Hi-Res
TGA instrument interfaced with the Thermal Analyst 2100
control unit at a heating rate of 5 °C min-1. Samples
(5–10 mg) were contained in a platinum pan. The sample
compartment was purged with dry nitrogen at 50 cm3
min-1 during analysis. Infrared (IR) spectra were obtained
using thin films between sodium chloride plates or solid
solutions (1%) in anhydrous potassium bromide (as disks)
and a Nicolet MAGNA-IR 560 spectrophotometer.
96
94
92
90
88
86
84
82
80
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers/cm–1
Fig. 2 IR spectrum for 1,1,2,2-tetraphenyl-1,2-ethanediol
Synthesis
2,4,4,5,5-Pentaphenyl-1,3,2-dioxaphospholane may be read-
ily prepared by treating 1,1,2,2-tetraphenyl-1,2-ethanediol
with (phenyl)dichlorophosphine in the presence of triethyl-
amine. While it is difficult to isolate it in pure form, it has been
characterized spectroscopically and it (or the linear polymer
analog) has been utilized to initiate styrene polymerization to
produce a polymer-containing phosphorus in the main chain
[9, 10]. The conversion of the diol to the pospholane may be
conveniently monitored using 13C NMR spectroscopy. In a
typical experiment, a solution of equimolar amounts of
benzopinacol and dichloro(phenyl)phosphine (20 vol.%) in
deuterochloroform was cooled to -20 °C. Triethylamine
(two moles/mole of phosphine) was added, the mixture was
mixed well, and a portion transferred to an NMR tube which
was placed in the instrument cavity cooled to -20 °C. The
sample was allowed to stand for 10 min for temperature
equilibration and a spectrum was recorded. The temperature
was increased to -10 °C, the sample allowed to stand for
temperature equilibration, and a second spectrum was recor-
ded. This procedure was repeated at 10° intervals until a
temperature of 20 °C was reached. The spectra are displayed
8
7
6
5
4
3
2
1
ppm
Fig. 3 Proton NMR spectrum for 1,1,2,2-tetraphenyl-1,2-ethanediol
Instrumentation
Nuclear magnetic resonance (NMR) spectra were obtained
using a 10–25% solution in deuterochloroform and a
Varian Plus 300 MHz or Varian 500 MHz spectrometer.
Proton and carbon chemical shifts are reported in parts per
million (d) with respect to tetramethylsilane as internal
reference (d = 0). Phosphorus chemical shifts are in d with
respect to triphenylphosphate as internal reference (d =
-18.0). Melting points were determined by differential
123

Thermal decomposition of 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane
519
in Fig. 5. As may be seen, the initial mixture contains ben-
zopinacol as noted by the absorption for the benzylic carbon
atom at d 82.3.
As the temperature is allowed to increase the absorption
for the benzylic carbon atom of the diol (d 82.3) diminishes
in intensity while that for the phospholane (d 92.7) increases
in intensity as a function of time. The transformation to
dioxaphospholane is also apparent in the corresponding
phosphorus-31 spectra. As a function of time, the absorp-
tion for dichloro(phenyl)phosphine (d -57.2) decreases in
intensity and that for the dioxaphospholane (d -40.6)
increases. The 13C NMR spectrum of the dioxaphospholane
formed is shown in Fig. 6. The most noteworthy peak in the
spectrum is the doublet (JC–P = 5–6 Hz) at d 92.7 corre-
sponding to the benzylic carbon atoms of the compound.
The phospholane is relatively stable in chloroform at low
temperature.
P
O
O
92.7
ppm
80
140
130
120
110
100
90
Fig. 6 13C NMR spectrum of 2,4,4,5,5-pentaphenyl-1,3,2-
dioxaphospholane
preparation may be conveniently monitored using 13C
NMR spectroscopy.
The dioxaphospholane is relatively stable at low tem-
perature but at room temperature and above it undergoes
thermal degradation. This process may be conveniently
monitored by either 13C or 31P NMR spectroscopy. As may
be seen from the spectra collected in Fig. 7, the absorption
for the benzylic carbon atoms of the dioxaphospholane
(d 92.7) decreases with time and increasing temperature
while the intensity of a product peak at d 97.8 increases.
Ultimately, the transformation to the thermally rearranged
Results and discussion
The preparation of a five-membered heterocycle containing
flame retarding properties was based on a suitable 1,2-diol
as a precursor. In initial work phosphorus reagents were
utilized in an attempt to form phospholanes. This was done
for several reasons. First, the necessary diol is generally
available from reductive coupling of ketones. Further there
was precedent for the generation of cyclic phosphorus
compounds from fully functionalized 1,2-diols [11, 12].
This is illustrated in Scheme 1. Similar compounds have
been prepared from less highly functionalized 1,2-diols
[13, 14].
OH
HO
The synthesis of 2,4,4,5,5-pentaphenyl-1,3,2-dioxa-
phospholane is depicted in Scheme 2. As noted above, the
H3PO3
DCCI
PCl2F
20°C
F
P
O
H
O
P
O
O
O
10°C
0°C
Scheme 1 Formation of phosphorus heterocycles from fully-func-
tionalized 1,2-ethanediols
–10°C
92.7
–20°C
P
Et3N
O
O
+
82.3
HO
OH
P
CH2Cl2
Cl
Cl
ppm
80
140
130
120
110
100
90
Fig. 5 13C NMR spectral changes which accompany the conversion
of benzopinacol to the corresponding dioxaphospholane
Scheme 2 Synthesis of 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane
123

520
B. A. Howell, Y.-J. Cho
product, presumably the linear polymer, is completed at
20 °C.
This is also apparent in the corresponding phosphorus-
31 spectra. Absorption for the phosphorus atom of the
dioxaphospholane (d -40.6) decreases in intensity with
time and temperature while that for the phosphorus atom of
the product (d 31.4) increases in intensity. The 13C NMR
spectrum for the thermal decomposition (rearrangement) of
the dioxaphospholane is shown in Fig. 8. This spectrum
contains a single absorption for benzylic carbon atoms at d
97.8. Thus, the thermal degradation of the dioxaphospho-
lane would seem to correspond to a clean rearrangement to
a single new material which is depicted as the linear
polymer.
C
O
P
O
C
δ97.8
97.8
ppm
140
130
120
110
100
90
80
Fig. 8 13C NMR spectrum of the product of the thermal rearrange-
ment of 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane
The IR spectrum of this material (Fig. 9) contains P–O–C
absorptions at 1159 and 1000 cm-1
.
This material is stable at room temperature and may be
stored indefinitely. At higher temperature, it undergoes
degradation (onset at 227 °C) in three steps. This may be
readily observed using thermogravimetry. The thermogram
is displayed in Fig. 10.
64
62
60
58
56
54
52
50
48
46
44
42
40
38
36
34
32
Conclusions
30
28
26
The cyclic heterocyclic compound, 2,4,4,5,5-pentaphenyl-
1,3,2-dioxaphopholane, has the potential to function as a
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers/cm–1
Fig. 9 IR spectrum of the product from the thermal rearrangement of
2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane
20°C
10°C
100
80
60
40
20
0
0°C
97.8
–10°C
50
250
450
650
Temperature/°C
Fig. 10 Thermal decomposition of the 2,4,4,5,5-pentaphenyl-1,3,2-
dioxaphospholane rearrangement product
–20°C
92.7
radical initiator for vinyl polymerization. It may be pre-
pared from a readily available diol precursor, 1,1,2,2-tet-
raphenyl-1,2-ethanediol. Its thermal stability may be
conveniently monitored using 13C or 31P NMR
140
130
120
110
100
90
80
70 ppm
Fig. 7 13C NMR spectral changes as 2,4,4,5,5-pentaphenyl-1,3,2-
dioxaphospholane undergoes thermal decomposition
123

Thermal decomposition of 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane
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