Novel phosphorus-silicon synergistic flame retardants: Synthesis and characterization

Article abstract of DOI:10.1016/j.cclet.2012.04.024

Three novel flame retardants containing both phosphorus and silicon elements in their structures, including [(1,1,3,3-tetramethyl-1,3- disiloxanediyl)di-2, 1-ethanediyl]bis(diphenylphosphine oxide) (FR-1), [(2,4,6,8-tetramethylcyclotetra-siloxane-2,4,6,8-tetrayl)tetra-2,1-ethanediyl] tetrakis[diphenylphosphine oxide] (FR-2) and 1,3,5,7,9,11,13,15-octakis(di- phenylphosphine oxide-2,1-ethanediyl)pentacyclo[9.5.1.1^3,9.1 ^5,15.1^7,13]octasiloxane (FR-3) were synthesized by a convenient pathway from the reaction of diphenylphosphine oxide (DPPO) and vinyl-terminated siloxanes under the catalysis of triethylborane. The chemical structures of the target compounds were confirmed by nuclear magnetic resonance (¹H NMR, ¹³C NMR, ²⁹Si NMR and ³¹P NMR), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and Fourier transform infrared (FT IR) measurements. Thermogravimetric analysis (TGA) results indicated that the new flame retardants possessed good thermal stability both in nitrogen and in air. FR-3 containing polyhedral oligosilsesquioxanes (POSS) moiety exhibited the best thermal properties with a 10% weight loss temperature >400°C and a residual weight ratio >39% at 700°C.

Full text of DOI:10.1016/j.cclet.2012.04.024

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 793–796
www.elsevier.com/locate/cclet
Novel phosphorus–silicon synergistic flame retardants:
Synthesis and characterization
*
Tao Song, Zhi Sheng Li, Jin Gang Liu , Shi Yong Yang
*
Laboratory of Advanced Polymer Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Received 13 March 2012
Available online 9 June 2012
Abstract
Three novel flame retardants containing both phosphorus and silicon elements in their structures, including [(1,1,3,3-
tetramethyl-1,3-disiloxanediyl)di-2, 1-ethanediyl]bis(diphenylphosphine oxide) (FR-1), [(2,4,6,8-tetramethylcyclotetra-silox-
ane-2,4,6,8-tetrayl)tetra-2,1-ethanediyl]tetrakis[diphenylphosphine oxide] (FR-2) and 1,3,5,7,9,11,13,15-octakis(di-phenylpho-
sphine oxide-2,1-ethanediyl)pentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane (FR-3) were synthesized by a convenient pathway
from the reaction of diphenylphosphine oxide (DPPO) and vinyl-terminated siloxanes under the catalysis of triethylborane.
The chemical structures of the target compounds were confirmed by nuclear magnetic resonance (1H NMR, 13C NMR, 29Si NMR
and 31P NMR), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and Fourier
transform infrared (FT IR) measurements. Thermogravimetric analysis (TGA) results indicated that the new flame retardants
possessed good thermal stability both in nitrogen and in air. FR-3 containing polyhedral oligosilsesquioxanes (POSS) moiety
exhibited the best thermal properties with a 10% weight loss temperature >400 8C and a residual weight ratio >39% at 700 8C.
# 2012 Jin Gang Liu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Phosphorous-containing siloxanes; POSS; Flame retardant; Synergistic effect
Flame retardants (FRs) are one of the most important components for common engineering polymers, such as
engineering plastics, textiles and molding compounds for electronic packaging [1]. In recent rears, various efforts have
been performed in order to improve the flame retardant efficiency of the FRs. Among various methodologies, the
attempts taking advantage of the synergistic effects of specific elements with flame retardant characteristics, such as
boron, silicon, phosphorus, nitrogen, halogen (chlorine, bromine and iodine), antimony and bismuth have gained much
attention in recent years [2–4]. Various synergistic couples, for instance, P–N, P–Si, Sb–Cl, Bi–Cl and many other
combinations have been incorporated into the structures of high performance FRs [5–7]. In the synergistic systems
mentioned above, the P–Si system has been proved to be of great value due to its inherent high flame retardant
efficiency, good environmental compatibility and good thermal durability at elevated temperatures [8,9]. Various P–Si
synergistic FRs, for instance, those containing 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and
POSS components can usually increases the ignition resistance of common polymers significantly while maintaining
the thermal stability of polymer matrixes [10,11].
* Corresponding authors.
E-mail addresses: liujg@iccas.ac.cn (J.G. Liu), shiyang@iccas.ac.cn (S.Y. Yang).
1001-8417/$ – see front matter # 2012 Jin Gang Liu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2012.04.024

794
T. Song et al. / Chinese Chemical Letters 23 (2012) 793–796
In our previous study, a series of P–Si synergistic FRs have been developed from DOPO and vinyl-terminated linear
siloxanes [12]. However, the inferior hydrolysis resistance of the P–O–C bond in the DOPO ring limited their
applications in moisture-sensitive areas [13].
In this communication, a class of P–Si synergistic FRs containing P–C bonds instead of P–O–C bonds was
developed from diphenylphosphine oxide (DPPO) and vinyl-containing siloxanes. Compared with their DOPO
analogues, the P–C bond in the phosphonate moiety in DPPO is less sensitive to moisture and would not hydrolyze in
high temperature wet environments. The synthesis and characterization of the new FRs were investigated in detail.
1. Experimental
Triethylborane (as a 1 mol/L solution in hexane) and vinyl-terminated siloxanes, including 1,3-divinyl-1,1,3,3-
tetramethyldisiloxane, 1,3,5,7-tetramethyl-1,3,5, 7-tetravinylcyclotetrasiloxane and octavinyl POSS (OVPOSS) were
purchased from Sigma–Aldrich and used as received. FT IR spectra were recorded on a PE 2000 FT IR spectrometer.
1H NMR and 13C NMR were conducted on a Bruker AV-400 nuclear magnetic resonance spectrometer with
tetramethylsilane (TMS) as the internal standard. 29Si NMR and 31P NMR measurements were performed on a Bruker
DMX 300 spectrometer. MALDI-TOF-MS spectra were obtained on a Biflex III mass spectrometer. Differential
scanning calorimeter (DSC) measurements were performed with a TA Instruments Q100 thermal analysis system.
1,3,5,7,9,11,13,15-Octakis(diphenylphosphine oxide-2,1-ethanediyl)pentacyclo [9.5.1.13,9.15,15.17,13]octa-silox-
ane (FR-3) was synthesized as follows. Into a 250-mL three-necked bottle equipped with a mechanical stirrer, a
dropping funnel and a gas inlet was added diphenylphosphine oxide (DPPO) (16.16 g, 80 mmol), OVPOSS (6.31 g,
10 mmol) and tetrahydrofuran (60 mL). The mixture was stirred at room temperature in nitrogen until all of the
reactants dissolved. A stoichiometric triethylborane solution in hexane (16 mL, 16 mmol) was added dropwise. After
the addition, the progress of the reaction was monitored by thin layer chromatography (TLC). After the reaction was
judged complete (usually after 5–6 h) by TLC, the reaction mixture was evaporated to remove the tetrahydrofuran. The
resulting solid was successively washed by diethyl ether and distilled water for three times. Then, the resulting white
solid was further purified by recrystallization from tetrahydrofuran to afford FR-3 (20.46 g) with a yield of 91%.
Melting point: 259 8C (DSC peak temperature); 1H NMR (400 MHz, CDCl3): d 0.87 (m, 16H, –Si–CH2–), 2.28 (d,
16H, –CH2–P), 7.37 (m, 32H), 7.42 (m, 16H), 7.61 (m, 32H); 13C NMR (101 MHz, CDCl3): d 132.5, 131.8, 131.5,
130.7, 128.8, 22.7; 29Si NMR (60 MHz, CDCl3): d À66.72; 31P NMR (162 MHz, CDCl3): d 32.76; FT IR (KBr): n
3067, 3026, 2987, 2961, 1946, 1604, 1410, 1276, 1111, 1005, 970 cmÀ1; MALDI-TOF-MS: m/z calcd. for
C
112H112O20P8Si8 [M+Na]+: 2271.4, found 2271.2.
[(1,1,3,3-Tetramethyl-1,3-disiloxanediyl)di-2,1-ethanediyl]bis(diphenylphosphine oxide) (FR-1) and [(2,4,6,8-
tetramethylcyclotetrasiloxane-2,4,6,8-tetrayl)tetra-2,1-ethanediyl]tetrakis[diphenylphosphine oxide] (FR-2) were
synthesized with a similar procedure as described in the synthesis of FR-3 except that 1,3-divinyl-1,1,3,3-
tetramethyldisiloxane was used instead of OVPOSS for FR-1 and 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclo-
tetrasiloxane was used for FR-2.
FR-1: mp: 169 8C (DSC peak temperature); yield: 89%. 1H NMR (400 MHz, CDCl3): d À0.08 (s, 12H, –Si–CH3),
0.63 (d, 4H, –Si–CH2–), 2.05 (d, 4H, –CH2–P), 7.30 (d, 6H), 7.60 (t, 4H); 13C NMR (101 MHz, CDCl3): d 133.0,
131.7, 130.9, 128.7, 77.1, 23.4, 8.43, À0.05; 29Si NMR (60 MHz, CDCl3): d 8.63; 31P NMR (162 MHz, CDCl3): d
33.28; FT IR (KBr): n 3052, 2958, 2899, 1483, 1437, 1410, 1318, 1255, 1183, 1150, 1120, 1091, 999, 842 cmÀ1
MALDI-TOF-MS: m/z calcd. for C32H40O3P2Si2 [M+Na]+: 613.2, found 613.2.
;
FR-2: yield: 89%. 1H NMR (400 MHz, CDCl3): d À0.06 (s, 12H, –Si–CH3), 0.59 (d, 8H, –Si–CH2–), 2.01 (d, 8H, –
CH2–P), 7.22 (m, 24H), 7.61 (m, 16H); 13C NMR (101 MHz, CDCl3): d 134.9, 133.5, 132.7, 132.5, 131.7, 131.3,
131.2, 130.7, 128.7, 128.1, 23.0, 22.3, 7.1, À1.1; 29Si NMR (60 MHz, CDCl3): d À18.69; 31P NMR (162 MHz,
CDCl3): d 33.77; FT IR (KBr): n 3056, 2959, 2901, 1637, 1591, 1484, 1437, 1408, 1262, 1180, 1160, 1121, 1071,
918 cmÀ1; MALDI-TOF-MS: m/z calcd. for C60H68O8P4Si4 [M+Na]+: 1175.3, found 1175.3.
2. Results and discussion
Three P–Si synergistic FRs, FR-1, FR-2 and FR-3 were synthesized with a procedure shown in Scheme 1. This
procedure has been proven to be an efficient pathway for the formation of P–C bonds [14]. In our experiments, the
triethylborane-induced radical addition of DPPO to vinyl-siloxanes proceeded smoothly giving rise to the

T. Song et al. / Chinese Chemical Letters 23 (2012) 793–796
795
CH3
Si O Si
CH3
CH3
R
CH3 CH3
Si O Si
CH3 CH3
R
CH3
FR-1
R
CH3
H3C
Si
O Si
CH3
H3C
Si
O Si
O
R
(CH3CH2)3B in hexane
THF
O
O
R
O
Si
P
+
O
Si
Si
O
CH3
H
H3C
Si
O
CH3
H3C
O
P
DPPO
R
R=
FR-2
R
O
R
Si
O
O
Si
Si
O
O
R
Si
Si
O
O
Si
Si
R
O
O
Si
O
O
Si
O
O
R
Si
O
Si
Si
O
O
O
Si
O
Si
O
Si
O
O
Si
R
R
O
R
FR-3
Scheme 1. Synthesis of phosphorus–silicon FRs (FR-1–FR-3).
corresponding phosphorus-containing siloxanes FRs in a yield >85%. FR-1 and FR-3 were obtained as white crystals
with a melting point of 169 8C and 259 8C, respectively; while FR-2 was obtained as a white solid without clear
melting transition in the DSC measurement. The chemical structures of the FRs were confirmed by FT IR, NMR and
MALDI-TOF-MS measurements. For instance, in the 31P NMR spectra (not shown), the FRs exhibited clear
1
resonances at 33.28 ppm for FR-1, 33.77 ppm for FR-2 and 32.76 ppm for FR-3. In the H NMR spectra, the
characteristic chemical shifts of protons in the terminal vinyl (H2C CH–) at about 5.7–5.9 ppm disappeared in the
spectra of the final FRs, indicating the successful transition of the reactions. In addition, the measured molecular
weights for the three FRs agreed well with the theoretical values. Thus, all the characterizations indicate that we
successfully prepared the target FRs.
The effects of the structures of the FRs on their thermal properties were investigated by TGA techniques. Fig. 1
illustrates the TGA decomposition curves of the FRs measured both in air and nitrogen. Table 1 tabulates the thermal data
of the FRs. It can be obviously observed that the current FRs possessed good thermal stability up to 300 8C both in air and
nitrogen. The5%(T5%) and10%(T10%) weight loss temperaturesof the FRs increased inthe orderFR-1 < FR-2 < FR-3.
100
80
60
FR-1 in air
40
FR-2 in air
FR-3 in air
FR-1 in nitrogen
FR-2 in nitrogen
FR-3 in nitrogen
20
0
100 200 300 400 500 600 700 800
Temperature (ºC)
Fig. 1. Thermal stabilities of FRs in nitrogen and air.

796
T. Song et al. / Chinese Chemical Letters 23 (2012) 793–796
Table 1
Thermal data for FRs.
a
b
c
Rw
Sample
T5%
T10%
In air
In nitrogen
In air
In nitrogen
In air
In nitrogen
FR-1
FR-2
FR-3
312.9
329.4
395.5
317.8
331.6
383.5
331.0
366.5
411.1
336.5
361.9
401.2
1.2
31.3
42.4
2.8
30.3
39.0
a
Temperatures at 5% weight loss.
Temperatures at 10% weight loss.
Residual weight ratio at 700 8C.
b
c
For instance, FR-3 exhibited the highest T10% value of 411.1 8C in air, which is nearly 80 8C and 45 8C higher than those
of FR-1 and FR-2, respectively. The residual weight ratios (Rw) of FR-3 at 700 8C were 42.4% in air and 39.0% in
nitrogen, respectively. The best thermal stability of FR-3 might be ascribed to its highest phosphorous and silicon
contents in the series. In addition, the slightly higher Rw values of FR-3 in air than that in nitrogen might be due to the
formation of inert phosphorous or silicon oxide compounds at elevated temperatures in air. These residues increase the
weights ratio of the FRs.
The detailed thermal decomposition mechanism of the FRs in air and nitrogen and the effects of their structures on
the flame retardancy of epoxy systems would be discussed in our next work.
Acknowledgments
The authors thank the National Natural Science Foundation of China (No. 51173188) for financial support. The
authors thank Dr. Xuechao Zhang for his instructive discussion on this work.
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