Synthetic Strategy and Performances of a UV-Curable Poly Acryloyl Phosphinate Flame Retardant by Carbene Polymerization

Article abstract of DOI:10.1080/10426507.2015.1049741

A route to synthesize poly ethyl (4-acrylamidebenzyl) phosphinate (PPAC) was discussed and the optimal route was determined. Diethyl benzylphosphonate was synthesized by Arbuzov reaction, then, a nitryl was introduced onto the aromatic ring by the nitration reaction of ethyl benzylphosphonate. Subsequently the carbene polymerization of the product was carried out. Finally, the nitryls were reduced and amidated to introduce the acryloyl group into the prepolymer to obtain the target product (PPAC). PPAC can rapidly photopolymerize under UV light irradiation. The addition of PPAC decreased the smoke production rate (SPR), CO₂ production (CO₂P), and CO production (COP) of the UV-cured resin.

Full text of DOI:10.1080/10426507.2015.1049741

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Synthetic Strategy and Performances of a
UV-Curable Poly Acryloyl Phosphinate Flame
Retardant by Carbene Polymerization
Jia Yu, Minglei Li, Yong Yu, Yanjing Gao, Jiancheng Liu & Fang Sun
To cite this article: Jia Yu, Minglei Li, Yong Yu, Yanjing Gao, Jiancheng Liu & Fang Sun (2015)
Synthetic Strategy and Performances of a UV-Curable Poly Acryloyl Phosphinate Flame
Retardant by Carbene Polymerization, Phosphorus, Sulfur, and Silicon and the Related
Elements, 190:11, 1958-1970, DOI: 10.1080/10426507.2015.1049741
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Date: 09 February 2016, At: 03:05

Phosphorus, Sulfur, and Silicon, 190:1958–1970, 2015
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2015.1049741
SYNTHETIC STRATEGY AND PERFORMANCES OF A
UV-CURABLE POLY ACRYLOYL PHOSPHINATE FLAME
RETARDANT BY CARBENE POLYMERIZATION
Jia Yu,1,2 Minglei Li,2 Yong Yu,1,2 Yanjing Gao,2 Jiancheng Liu,3
and Fang Sun1,2
1State Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, Beijing 100029, PR China
2College of Science, Beijing University of Chemical Technology, Beijing 100029,
PR China
3Department of Chemical and Biological Engineering, University of Colorado
Boulder, Boulder, USA
GRAPHICAL ABSTRACT
A route to synthesize poly ethyl (4-acrylamidebenzyl) phosphinate (PPAC) was discussed and
the optimal route was determined. Diethyl benzylphosphonate was synthesized by Arbuzov
reaction, then, a nitryl was introduced onto the aromatic ring by the nitration reaction of
ethyl benzylphosphonate. Subsequently the carbene polymerization of the product was carried
out. Finally, the nitryls were reduced and amidated to introduce the acryloyl group into the
prepolymer to obtain the target product (PPAC). PPAC can rapidly photopolymerize under
UV light irradiation. The addition of PPAC decreased the smoke production rate (SPR), CO2
production (CO2P), and CO production (COP) of the UV-cured resin.
Keywords Carbene polymerization; photopolymerization; flame retardant
INTRODUCTION
In recent years, UV polymerization technology has received more and more attention
because of its advantages of energy-saving and environmentally protection. Accordingly,
Received 18 January 2015; accepted 25 April 2015.
Address correspondence to Fang Sun, Science College, State Key Laboratory of Chemical Resource Engi-
neering, Beijing University of Chemical Technology (PO Box 144), 15 Bei San Huan East Road, Beijing, 100029,
China. E-mail: sunfang60@yeah.net
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gpss.
1958

SYNTHETIC STRATEGY OF POLY ACRYLOYL PHOSPHINATE
1959
UV-curable materials have been developed rapidly and become a significant part of the green
industry movement. They are widely used in various fields, such as paints, coating, ink and
films.1 Although UV-curable materials are known as “environmental-friendly materials,”
UV-curable resins are usually flammable, which limits the application of the resins in fields
of fiber paints, wood paints and plastic coatings. Therefore, it is of great importance to de-
velop flame retardants for UV-curable materials. In the 1980s, UV-curable flame retardants
drew researchers attention. Specially, phosphine-containing flame retardants gained pop-
ularity because their flame-retardant mechanism2 involves condensed phase or condensed
phase and gaseous phase, and less gas and smoke are generated during their combustion. A
phosphine-containing polyfunctional monomer (TGMAP) was synthesized by Chen3 et al.,
which was introduced into polymer chains by photopolymerization technology to obtain
a UV-cured film with flame-retardant effect. The flame retardant significantly improved
the formation of carbon residue (carbon residue > 36.3%) by catalyzing the formation of
polyphosphoric acid structure in the condensed phase. A phosphorous polyester acrylate
4
˙
oligomer (BEPPPO) containing aromatic ring was synthesized by Inan et al., and some
related performance tests on the UV-cured film were carried out. The results indicated
that the char yield of the UV-cured film containing the oligomer at 600◦C reached 24.5%
and the average self-extinguishing time was 32 s. Wang5 et al. synthesized a difunctional
phosphate methacrylate (DMPE), and discussed the thermal degradation process of systems
with different additive amounts in detail. The temperature of weight loss 10% of all the
systems was above 300◦C and the char yield was above 12.5%.
Compared with traditional polymerization methods, carbene polymerization has the
advantage of high stereoregularity. The stereoregularity can offer polymers some special
structures and performances that are impossible to obtain by using other polymerizations.
For example, Hetterscheid6 synthesized poly (ethyl 2-ylidene-acetate) with a high syndio-
tactic structure by carbene polymerization. The polymer had a high crystallinity due to
its high syndiotactic structure. Doulut7 studied the effect of isotacticity on the α delay
process of poly methyl methacrylate (PMMA). Nevertheless, the synthesis of UV-curable
poly acryloyl phosphinate flame retardant by carbene polymerization is seldom reported.
The carbene polymerization would endow the UV-curable poly acryloyl phosphinate flame
unique structures, resulting in the improvement of performances of the UV-curable mate-
rials.
We have already reported the synthetic method and some performances of poly ethyl
(4-acrylamidebenzyl) phosphinate (PPAC)8.
However, the design and synthetic route of PPAC have not been discussed in detail.
In addition, during the combustion process, the effect of PPAC on the smoke release of the
resin, an important parameter in evaluating the flame resistance, has not been investigated.
In this paper, the route to synthesize PPAC by carbene polymerization was discussed
in detail. The optimal route and conditions of the synthesis were determined, and the
structures of reaction intermediates and the target product were characterized. The effect of
PPAC on the flame resistance of UV-cured resins was further investigated by measuring CO
and CO2 productions as well as the smoke production rate (SPR) during the combustion
process.
RESULTS AND DISCUSSION
The synthesis of the target prepolymer involved two main reaction steps, namely
carbene photopolymerization reaction and introduction of photopolymerizable groups.

1960
J. YU ET AL.
Therefore, we designed two synthetic strategies: first, introducing the photopolymeriz-
able groups to the target compound before carbene polymerization; second, introducing
the photopolymerizable groups to the target compound after carbene polymerization. Two
synthetic strategies were discussed in detail.
Introducing Photopolymerizable Groups before Carbene
Polymerization
Route 1. This design ideal is that the propylene acyl group, which is a photopoly-
merizable group, is first introduced into benzyl phosphoryl chloride, then the carbene
polymerization of the benzyl phosphoryl chloride with propylene acyl group is carried out
to synthesize the target product, propylene acyl poly benzyl phosphinate.
Introducing acryloyl into monoethyl benzylphosphonate directly by Friedel-Crafts
acylation9,10 may induce side reactions. Therefore, Route 1 was designed to introduce
the acryloyl by amidation reaction. First, the amino group was introduced into diethyl
benzylphosphonate, then, acryloyl ethyl benzylphosphonate was obtained by amidation
reaction and hydrolysis reaction. Finally, the target product was synthesized by carbene
polymerization. Scheme 1 shows the particular synthetic route.
The 1H NMR spectra of intermediate products (1)–(4) were shown in Figures S1–S4
(Supplemental Materials). The hydrolysis reactions of diethyl (4-acrylamidobenzyl) phos-
1
phonate under different conditions were investigated by H NMR. The results indicated
that the phosphonate group did not hydrolyze in 2 mol/L NaOH aqueous solution at room
temperature or under reflux condition, or even in the NaOH alcoholic solution under reflux
condition.
As reported11,12 before, the phosphonate group can be hydrolyzed in 2 mol/L aqueous
solution at room temperature, while the acylamido group cannot, because acylamido group
Scheme 1 Scheme of synthetic route 1.

SYNTHETIC STRATEGY OF POLY ACRYLOYL PHOSPHINATE
1961
Scheme 2 Scheme of synthetic route 2.
is more stable than the phosphonate group in theory. However, in the experiment, it was
found that the acryloyl group was hydrolyzed, whereas the phosphonate group did not.
We considered the reason causing the result to be the carbon–carbon double bond in the
acryloyl group, which makes acryloyl group less stable, resulting in the hydrolyzation of
acryloyl.
Introducing Photopolymerizable Groups after Carbene Polymerization
Because the aforementioned route was unfeasible, we designed a new strategy
whereby poly phosphinate was obtained by the carbene polymerization of benzylphos-
phoryl chloride, then, photopolymerizable group was introduced into the target product.
Two synthetic routes were investigated in detail.
Route 2. As shown in Scheme 2, the main idea of route 2 is that the carbene polymer-
ization of benzylphosphoryl chloride was carried out first, and then the photopolymerizable
groups, acryloyl groups, were introduced by transesterification of oxethyl in branched
chains of poly ethyl hydrogen (4-acryloylbenzyl) phosphinate and 2-hydroxyethyl acrylate
(HEA).
The 1H NMR spectra of intermediate products (5)–(7) were shown in Figures S5–S7
(Supplemental Materials).
Two products were obtained by the carbene polymerization of benzylphosphoryl
chloride, one was solid; the other was liquid. The solid product was considered to be
polymer with extremely high molecular weight and could not further react with HEA
because of its poor solubility. Therefore, the liquid product, poly phenylethyl phosphinate
(8) that further reacted with HEA, was characterized. Its IR and 13C NMR spectra are shown
in Figures S8 and S9. Figure S10 and Table 1 show the GPC result of the compound (8).
Table 1 Molecular weight and molecular weight distribution of poly phenylethyl phosphinate (8)
Mn
Mw
Polydispersity
1.33
1070
1425

1962
J. YU ET AL.
Scheme 3 Mechanism of phosphoryl carbene insertion.
The number-average molecular weight (Mn) and weight-average molecular weight (Mw)
of the prepolymer were 1070 and 1425, respectively. The degree of polymerization of the
prepolymer was about 5 according to its structure. Meanwhile, there were a small number
of polymers with a higher molecular weight and small molecules which did not polymerize.
As shown in Scheme 3, the phosphoryl carbene inserted at α-C because of the
steric hindrance, thus, compound (8) presented a linear structure.13 Figure 1 is 31P NMR
spectrum of the compound (8). There are two peaks at 28.0 and 29.0 ppm in the 31P NMR
spectrum because the compound (8) has two chiral centers and a pair of epimers. It is
further demonstrated that compound (8) possesses a syndiotactic structure.13
Figure S11 shows the 1H NMR result of the product purified by column chromatog-
raphy. The 1H NMR result indicated that the molecule already contained aryl and acryloyl
units.
Because intermediate product (8) and the product P-HEA were both prepolymers,
they are very difficult to purify, thereby resulting in a very low yield of the transesterification
(about 10%), According to route 2, the total yield of the target product was less than 1%
Figure 1 31P NMR spectrum of compound (8).

SYNTHETIC STRATEGY OF POLY ACRYLOYL PHOSPHINATE
1963
Scheme 4 Scheme of synthetic route 3.
due to the low yield of the transesterification and too many reaction steps. Hence, the route
needs to be further improved to increase the yield.
Route 3. Route 2 is feasible, but the yield is very low. Therefore, route 3 was
designed based on the above mentioned synthetic methods. Diethyl benzylphosphonate
was synthesized by Arbuzov reaction, then, diethyl benzylphosphonate was partially hy-
drolyzed to obtain monoethyl benzylphosphonate. Nitryl was introduced into the aromatic
ring of monoethyl benzylphosphonate by a nitration reaction, subsequently the carbene
polymerization of the product was carried out. Finally, nitryls were reduced and amidated
to introduce acryloyl groups into the prepolymer to obtain the target product (PPAC). The
reaction process is shown in Scheme 4.
1
The H NMR and GPC spectra of intermediate products (10)–(13) are shown in
Figures S12–17. The final product PPAC was characterized by IR, 1H NMR, 13C NMR, 31
NMR and GPC. The results are shown in Figures 2–5, Figure S18 and Table 2.
P
Table 2 Molecular weight and molecular weight distribution of PPAC
Mn
Mw
Polydispersity
2.12
1012
2149

1964
J. YU ET AL.
Figure 2 IR spectrum of PPAC.
Mn and Mw of PPAC were 1012 and 2149, respectively. The chemical shift of
1
aromatic hydrogen changed again in the H NMR spectrum, indicating that the p-amino
group became an amide group. Similarly, there are two peaks at 21.3 and 21.4 ppm in the
31P NMR spectrum, demonstrating that PPAC is a syndiotactic prepolymer. The yield of
PPAC was greatly enhanced through optimizing synthetic route and reached 48%.
Photopolymerization Kinetics of PPAC
The photopolymerization ability of PPAC was investigated by real-time infrared
(RTIR) spectroscopy. As shown in Figure 6, the photopolymerization systems containing
PPAC and different monomers can rapidly polymerize under the irradiation of 365 nm UV
light, and the final double bond conversions of the systems were more than 95% after the
irradiation for 3 min, indicating that PPAC had great photopolymerization performance.
Figure 3 1H NMR spectrum of PPAC.

SYNTHETIC STRATEGY OF POLY ACRYLOYL PHOSPHINATE
1965
Figure 4 13C NMR spectrum of PPAC.
In addition, the photopolymerization rate of the system with TMPTA was higher than that
with HEA due to the high functionality of TMPTA. Furthermore,Figure 7 exhibits that the
polymerization rate of the systems slightly increased with the increase of the photoinitiator
content, while the final double bond conversion did not obviously change.
Smoke Release
The cone calorimeter has been available for the bench-scale assessment of several
reactions of fire and fire parameters, and many research groups have taken interest in
quantitative material flammability analysis.14–18 In this work, the cone calorimeter was
employed to evaluate the flame resistance of PPAC during the combustion of UV-cured
resin samples.
The SPR is the principal cause of death during fire. Therefore, the measure of SPR
for a variety of polymer components will be useful in indicating the relative risk under
well-ventilated conditions presented by these materials.19 Incomplete combustion of the
Figure 5 31P NMR spectrum of PPAC.

1966
J. YU ET AL.
Figure 6 Photopolymerization kinetics of PPAC. System composition: PPAC/monomer = 2/97 (wt%); photoini-
tiator 1173, 1 wt%; HEA/TMPTA = 5/1 (wt%).
pyrolyzed polymer volatile fuels causes the formation of CO, which occurs at low temper-
atures in early stages of fire development. When the fire develops, the high temperature
avails the formation of CO2, which is especially dependent on oxygen availability to the
fire.20
The productions for CO2 and CO are presented in Figures 8 and 9, respectively. The
CO and CO2 productions of the resin samples without PPAC were relatively high compared
with those with PPAC. With the increase of PPAC content from 1 to 5 wt%, the production
for CO decreased from 0.024 to 0.023 g/s, while the production for CO2 decreased from
0.92 to 0.85 g/s. SPR curves of the resin samples are shown in Figure 10. SPR of the resin
without PPAC was 0.31 m2/s, while those with 1 and 5 wt% PPAC decreased to 0.27 and
0.24 m2/s, respectively. These results proved that PPAC provided the smoke suppression
and exhibited a flame retardant effect.
EXPERIMENTAL
Materials
Triethyl phosphite (P(OEt)3) was purchased from Sinopharm Group Chemical
Reagent Co. (Beijing, China). Acryloyl chloride was obtained from Ouhechem Co. (Bei-
jing, China). Benzyl chloride, hydrochloric acid (HCl), concentrated sulfuric acid (H2SO4),
nitric acid (HNO3), sodium hydroxide (NaOH), potassium carbonate (K2CO3), ammo-
nium chloride (NH4Cl) and zinc powder (Zn) were all supplied by Beijing Chemical
Works. Trimethylolpropane triacrylate (TMPTA), isobornyl acrylate (IBOA), and HEA
were purchased from Eternal Specialty Chemical Co. (Zhuhai, China). The photoinitia-
tor 2-hydroxyl-2-methyl-1-phenylpropane-1-one (Darocur 1173) was purchased from Ciba
Geigy Co. Diazomethane (CH2N2) was synthesized by following previously reported pro-
cesses.21 The water in tetrahydrofuran (THF) and trichloromethane (CHCl3) was removed
by Na and P2O5, respectively, before use.

SYNTHETIC STRATEGY OF POLY ACRYLOYL PHOSPHINATE
1967
Figure 7 Photopolymerization kinetics of PPAC with different contents of photoinitiator 1173. (a) HEA; (b)
HEA/TMPTA = 5/1 (wt%). System composition: PPAC/monomer = 2/97 (wt%).
Characterization
The Fourier transform infrared (FTIR) spectra were obtained on a Nicolet 50XC
spectrometer (Nicolet, USA) and scanned between 400 and 4000 cm−1
.
1
The H NMR, 13C NMR and 31P NMR spectra were recorded on an AV400 unity
spectrometer (Bruker, Germany).
The molecular weight of the prepolymer was determined by Water 515–2410 gel
permeation chromatography (GPC, Water, USA). Tetrahydrofuran, 1.0 ml/L, was used as
the mobile phase. Calibration was carried out by linear polystyrenes of known molecular
weight and dispersity.
Photopolymerization
The systems of TMPTA and HEA were employed as a range of concentration of poly
acryloyl ethyl phosphinate (PPAC). All samples were investigated by real-time infrared

1968
J. YU ET AL.
Figure 8 CO2 production (CO2P) as a function of time. System composition: IBOA/photoinitiator 1173 = 100:1
(wt%).
(RTIR) spectroscopy (RT-IR; Nicolet 5700, Thermo Electron, USA, equipped with an
extended range KBr beam splitter and an MCT/A detector) according to the procedures
reported before.22
Smoke Release
The cone calorimeter tests were conducted to research the fire performance on a FTT
cone calorimeter according to ISO 5660–1. The samples with the dimension of 100 mm ×
100 mm × 3 mm were exposed to a radiant cone at a heat flux of 40 kW/m2.
Figure 9 CO production (COP) as a function of time. System composition: IBOA/photoinitiator 1173 = 100:1
(wt%).

SYNTHETIC STRATEGY OF POLY ACRYLOYL PHOSPHINATE
1969
Figure 10 SPR as a function of time. System composition: IBOA/photoinitiator 1173 = 100:1 (wt%).
FT-IR and NMR Spectra of PPAC
νmax (KBr)/cm−1: 1603, 1546 and 848 (Ar), 1043 and 1067 (P-O-C), 1209 and 1289
(P O), 3064 (C CH), 2989 and 2848 (CH2CH3), 1664 (C O), 1416 (CN).
1H NMR (400 MHz, CD3OD, δ): 7.51–7.56 (m, 8H, C6H4), 7.26–7.32, (m, 8H,
C6H4), 6.30–6.48 (m, 8H, CH2 CH), 5.70–5.80 (m, 4H, CH2 CH), 3.79–3.85(m, 8H,
POCH2CH3), 3.48–3.54 (d, JP, H = 10.4 Hz, 3H, POCH2OCH3), 2.92–3.00 (d, JP, H
=
20.4 Hz, 10H, POCH2), 2.80–2.90 (m, 3H, PCHCH2), 1.15–1.22 (t, JH, H = 7.0Hz, 12H,
POCH2CH3).
13C NMR (100 MHz, CD3OD, δ): 162.9 (C = O), 125.9–136.4 (C6H4, CH2 = CH),
59.1 (d, JP,C = 5.5 Hz, POCH2), 50.8 (OCH2CH3), 33.5–35.4 (Ph-CH2, PCH(CH2)Ph),
16.7 (OCH3).
31P NMR (162 MHz, CD3OD, δ): 21.3, 21.4.
CONCLUSIONS
The optimal route to synthesize poly ethyl (4-acrylamidebenzyl) phosphinate (PPAC)
was determined as route 3. The structures of all products were characterized by NMR, FT-
IR and GPC. PPAC had good photopolymerization performance and the final double bond
conversion of the system reached above 95% after the irradiation for 3 min. The results
of the cone calorimeter demonstrated that the addition of PPAC decreased the SPR, CO2P
and COP of the UV-cured resins, indicating that PPAC has flame retardant capability for
UV-curable resins.
FUNDING
Financial support from the National Natural Science Foundation of China (Grant
51273014) and the Fundamental Research Funds for the Central Universities (YS1406) are
gratefully acknowledged.

1970
J. YU ET AL.
SUPPLEMENTARY MATERIAL
Supplemental data for this article can be accessed on the publisher’s website at
http://dx.doi.org/10.1080/10426507.2015.1049741.
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