Cyclic polyphosphoesters synthesized by acyclic diene metathesis polymerization and ring closing metathesis

Article abstract of DOI:10.1016/j.reactfunctpolym.2013.06.011

This article describes the synthesis of cyclic polyphosphoester (PPE) by the ring-closing metathesis (RCM) of different difunctional linear PPEs. Linear PPE precursors were prepared through a selective head-to-tail acyclic diene metathesis polymerization of phenyl dienephosphate monomer using 2-hydroxyethyl acrylate as a selective chain terminator, followed by the transformation of the terminal acrylate functional group into a hydroxyl group utilizing a thiol-Michael addition click reaction. These products were then reacted with the corresponding acyl chloride containing a vinyl end group. The subsequent end-to-end intramolecular coupling reaction was performed under highly dilute conditions. The successful transformation of the linear PPE precursors to cyclic PPE was confirmed by NMR spectroscopy and gel permeation chromatography. The thermal and flame retardant properties of linear and cyclic PPEs were investigated, and their thermal degradation and flame retardance were evaluated, as these are important features for future applications.

Full text of DOI:10.1016/j.reactfunctpolym.2013.06.011

Reactive & Functional Polymers 73 (2013) 1242–1248
Contents lists available at SciVerse ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Cyclic polyphosphoesters synthesized by acyclic diene metathesis
polymerization and ring closing metathesis
⇑
Liang Ding , Rong Lu, Jing An, Xueqin Zheng, Jun Qiu
School of Materials Engineering, Yancheng Institute of Technology, Yancheng 224051, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 May 2013
Received in revised form 22 June 2013
Accepted 24 June 2013
Available online 1 July 2013
This article describes the synthesis of cyclic polyphosphoester (PPE) by the ring-closing metathesis (RCM)
of different difunctional linear PPEs. Linear PPE precursors were prepared through a selective head-to-tail
acyclic diene metathesis polymerization of phenyl dienephosphate monomer using 2-hydroxyethyl acry-
late as a selective chain terminator, followed by the transformation of the terminal acrylate functional
group into a hydroxyl group utilizing a thiol-Michael addition click reaction. These products were then
reacted with the corresponding acyl chloride containing a vinyl end group. The subsequent end-to-end
intramolecular coupling reaction was performed under highly dilute conditions. The successful transfor-
mation of the linear PPE precursors to cyclic PPE was confirmed by NMR spectroscopy and gel permeation
chromatography. The thermal and flame retardant properties of linear and cyclic PPEs were investigated,
and their thermal degradation and flame retardance were evaluated, as these are important features for
future applications.
Keywords:
Macrocycles
Polyphosphoester
Selective head-to-tail acyclic diene
metathesis polymerization
Ring-closing metathesis
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
of advances in synthetic technology [14]. However, few reports are
available on cyclic PPEs due to the challenges associated with these
Phosphorus-containing polymers are frequently used as flame-
retardant materials because a protective char is formed during
combustion [1]. Flame retardance has become a highly desirable
polymer property [2]. Poly(phosphoester)s (PPEs), potentially
degradable and biocompatible polymers with repeating phospho-
ester bonds along the backbone, are prominent flame retardant
materials [3,4]. The chemical versatility of the monomeric phos-
phate allows for the design of functional materials with tunable
and complex architectures and a wide range of properties. Over
the past several decades, research efforts to explore new phospho-
rus-containing polymers have predominantly focused on linear
phosphorus polymers with side-chain or main-chain architectures
[5–9].
more demanding synthetic procedures.
Cyclic polymers can generally be prepared by a variety of strat-
egies. Intramolecular ring closure from linear precursors to cyclic
polymers is a popular method for widely available monomers
[15–17]. Among the diverse approaches to intramolecular ring clo-
sure, ring-closing metathesis (RCM) has been developed as a pow-
erful tool for the synthesis of various carbocyclic and heterocyclic
ring systems of different sizes over the last few years, but RCM is
infrequently used to synthesize cyclic polymers and has not often
been combined with the other synthetic strategies [17–20]. Xie
prepared cyclic poly(
enyne metathesis, and click reactions of different difunctional lin-
ear PCLs prepared by ring-opening polymerization of -caprolac-
e-caprolactone) (PCL) via RCM, ring closing
e
In contrast to linear polymers, cyclic polymers with various
chemical structures, compositions, molecular characteristics, and
architectures have attracted extensive attention in macromolecu-
lar science due to the absence of chain ends [10]. Cyclic polymers
usually have a smaller hydrodynamic volume, higher refractive in-
dex, reduced viscosity, and higher glass transition temperature.
These unique properties have thus led to novel properties or im-
proved performance in many fields such as drug delivery, liquid
crystal behavior, and self-assembly [11–13]. Recently, the synthe-
sis of cyclic polymers has experienced rapid development because
tone and subsequent reaction with acyl chloride containing a
vinyl or azido end group [21].
Acyclic diene metathesis (ADMET) polymerization is a quantita-
tive reaction that can tolerate many functional groups, yields only
the desired linear polymer, and releases ethylene as a byproduct
[22]. Usually, symmetric a,x-dienes are used to obtain polymers
with a defined repeat unit structure. Meier recently introduced
the concept of a selective head-to-tail ADMET polymerization
using a monomer containing both a terminal double bond and an
acrylate. Such monomers polymerize with high cross metathesis
selectivity, enabling access to different polymer architectures if a
selective and irreversible chain transfer agent (mono- or multi-
functional acrylate) is added [23,24]. The chain transfer agent
⇑
Corresponding author. Tel.: +86 515 88298872.
E-mail address: dl1984911@ycit.edu.cn (L. Ding).
1381-5148/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.reactfunctpolym.2013.06.011

L. Ding et al. / Reactive & Functional Polymers 73 (2013) 1242–1248
1243
Scheme 1. Schematic representation of the synthesis of cyclic PPEs.
allows control over the molecular weight and direct functionaliza-
tion of the ADMET polymer by selective reaction with one of the
end groups (terminal double bond). Based on this principle, diverse
homopolymers and (amphiphilic) diblock copolymers were syn-
thesized, and they can be regarded as the linear precursors for
the preparation of cyclic polymers.
Consequently, building on our experience with ADMET poly-
merization and RCM, we wanted to exploit the cross metathesis
selectivity between terminal olefins and acrylates to synthesize de-
fined PPEs architectures. This synthesis was accomplished via AD-
MET polymerization to obtain the linear precursor in the presence
of Grubbs catalyst, followed by RCM to obtain the cyclic PPEs
(Scheme 1).
Bruker DPX spectrometer. Relative molecular weights and molecu-
lar weight distributions were measured by gel permeation chroma-
tography (GPC) in an instrument equipped with an Isocratic HPLC
pump, a refractive index detector, and a set of Waters Styragel col-
umns (7.8 Â 300 mm, 5
l
m bead size; 103, 104, and 105 Å pore
size). GPC measurements were carried out at 35 °C using THF as
the eluent with a flow rate of 1.0 mL/min. The system was cali-
brated with polystyrene standards. Matrix-assisted laser desorp-
tion ionization time-of-flight mass measurement (MALDI-TOF
MS) was performed using a Bruker Biflex III MALDI-TOF mass spec-
trometer equipped with a 337 nm nitrogen laser producing 3 ns
pulses. Mass spectra were recorded in the linear or reflector de-
layed extraction mode with an accelerating voltage of 19 kV and
a delay time of 200 ns. All data were reprocessed using the Bruker
XTOF software. The 1,8,9-trihydroxyanthracene (dithranol) matrix
was dissolved in CHCl3 (10 mg/mL), and the solution was mixed
2. Experimental
with the polymer solution (8.0 mg/mL in CHCl3). Then, 1.0
the analyte solution was spotted directly onto the thin layer
formed by depositing 1.0 L of saturated NaI cationizing agent in
lL of
2.1. Materials
l
methanol. Thermogravimetric analysis (TGA) was performed using
an SDTA851e/SF/1100 TGA Instrument under N2 flow at a heating
rate of 10 °C/min from room temperature to 800 °C. Differential
scanning calorimetry (DSC) was performed on a Perkin–Elmer
Pyris 1 instrument in a nitrogen atmosphere. All the samples were
first heated from room temperature to 200 °C and then held at this
temperature for 3 min to eliminate the thermal history, and then
the samples were quenched to À80 °C and heated again from
À80 to 200 °C at 10 °C/min. GC/MS measurements were performed
with a Varian Saturn 2100 GC/MS system with GC-3900 using a VF-
Phenyl dichlorophosphate (99%), 2-hydroxyethyl acrylate
(>96%),
[1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylid-
(Hov-
ene]dichloro(o-isopropoxyphenylmethylene)ruthenium
eyda–Grubbs second generation catalyst) (98%), acrylic acid
(99%), benzylidene-bis(tricyclohexylphosphine)dichlororuthenium
(first generation Grubbs catalyst) (98%), 2-mercaptoethanol (98%),
undecylenic acid (99%), allyl alcohol (98%), 1-[3-(dimethyl-
amino)propyl]-3-ethylcarbodiimide
hydrochloride
(EDCIÁHCl,
99%), and 4-dimethylaminopyridine (DMAP) (98%) were purchased
from Energy Chemical and used as received without purification.
Solvents were distilled over drying agents under nitrogen prior to
use. Triethylamine (Et3N) and pyridine were freshly distilled and
dried.
5 MS, 30 m  0.25 mm  0.25
lm diffused silica capillary column.
Elemental analysis (EA) was conducted with an Elementar Vario
EL. Thin layer chromatography (TLC) was performed on silica gel
TLC cards (0.20 mm layer thickness). Limiting oxygen index (LOI)
values were measured on a Stanton Redcroft instrument provided
with an oxygen analyzer in vertical tests. The samples were
impregnated on glass fiber plaques using concentrated solutions
of the polymers in THF, and LOI values were taken as the average
of three measurements.
2.2. Characterization
1H (500 MHz) and 13C (125 MHz) NMR spectra were recorded
using tetramethylsilane as an internal standard in CDCl3 on a

1244
L. Ding et al. / Reactive & Functional Polymers 73 (2013) 1242–1248
Polymerizations were carried out in Schlenk tubes using of a
nitrogen flow to drive off the ethylene condensate for ADMET.
CHCH2 + CH2CH2OCO + HOCH2CH2OCO), 3.88–3.73 (m, CH2CH2OCO +
HOCH2CH2OCO + HOCH2CH2SCH2), 2.73 (m, HOCH2CH2SCH2), 2.67–
2.52 (m, HOCH2CH2SCH2 + HOCH2CH2SCH2CH2). GPC: Mn = 4500,
Mw/Mn = 1.35 for 3a; Mn = 9600, Mw/Mn = 1.56 for 3c; NMR:
Mn = 3200 for 3a; Mn = 6500 for 3c.
2.3. Synthesis of phenyl diene phosphate (1)
In a round bottom flask, phenyl dichlorophosphate (6.33 g,
30 mmol) was dissolved in 100 mL of dry CH2Cl2 and cooled to
0 °C. Allyl alcohol (1.57 g, 27 mmol) and pyridine (2.61 g, 33 mmol)
were added to this solution by a syringe, and then stirred for 8 h
under nitrogen flow. The solution was cooled to 0 °C; 2-hydroxy-
ethyl acrylate (3.14 g, 27 mmol) and pyridine (2.61 g, 33 mmol)
were added by a syringe; and the reaction was stirred overnight
at room temperature. The progress of the reaction was monitored
by TLC using methylene chloride/petroleum ether (5:1) as the elu-
ent (an Rf value of 0.5 was obtained from TLC). After repeatedly
washing with 1 M HCl and deionized water, the organic layer
was dried over anhydrous Na2SO4, filtered, and concentrated under
reduced pressure. Then, the crude product was purified by silica
gel chromatography and eluted with methylene chloride/petro-
leum ether (1:1) to give the clear colorless liquid 1 (6.12 g, 72.6%
yield). 1H NMR (CDCl3), d (ppm): 7.36–7.31 (m, 2H, m-ArH),
7.23–7.19 (d, 2H, o-ArH), 7.13–7.09 (m, 1H, p-ArH), 6.38–6.33 (d,
1H, OCOCH@CH), 6.21–6.19 (m, 1H, OCOCH@CH), 6.00–5.97 (m,
1H, CH2@CHCH2O), 5.83–5.74 (d, 1H, OCOCH@CH), 5.29–5.18 (d,
2H, CH2@CHCH2O), 4.32–4.28 (m, 2H, CH2CH2OCOCH@CH2),
4.21–4.17 (d, 2H, CH2@CHCH2O), 3.88–3.84 (m, 2H, CH2CH2-
OCOCH@CH2). 13C NMR (CDCl3), d (ppm): 165.45, 152.26, 136.90,
130.65, 129.13, 128.08, 120.29, 117.11, 116.17, 68.86, 68.12,
67.05. 31P NMR, d (ppm): À6.11. GC: single peak was observed.
EI/MS: Calcd. for C14H17O6P: 312.3; found: 312.2. Anal. calcd for
C: 53.85, H: 5.49, O: 30.74; Found C: 53.84, H: 5.45, O: 30.77.
2.6. Synthesis of linear PPE bearing two vinyl end groups (4)
Under a nitrogen atmosphere, (COCl)2 (2.6 mL, 30 mmol) was
added by syringe to undecylenic acid (1.1 g, 6.0 mmol) at room
temperature with rapid stirring. After 6 h, the excess (COCl)2 was
removed in vacuo to yield 10-undecenoyl chloride, which was then
added by syringe to the solution of 3 (0.6 mmol) in 10 mL of CH2Cl2
and 1.0 mL (7.5 mmol) dry triethylamine at 0 °C. The reaction mix-
ture was then allowed to warm to room temperature and stirred
overnight. The precipitate was filtered off and the filtrate was
washed with water; then the organic layers were dried over anhy-
drous Na2SO4, and the concentrated residue was precipitated twice
from methanol and dried for 24 h in a vacuum oven to afford the
diene end-functionalized polymer 4 at high yield. 1H NMR (CDCl3),
d (ppm): 7.47–7.32 (m, m-ArH), 7.25–7.05 (m, o-ArH + p-ArH),
7.00–6.86 (m, OCOCH@CH), 5.93–5.71 (m, OCOCH@CH + CH2-
@CHCH2), 4.99–4.85 (m, CH2@CHCH2CH2), 4.43–3.96 (m, OCOCH@
CHCH2 + CH2CH2OCO + OCOCH2CH2OCO + OCOCH2CH2OCO + CH2-
CH2SCH2CH2OCO), 3.91–3.68 (m, CH2CH2OCO), 2.77–2.56 (m,
CH2CH2SCH2CH2OCO + CH2CH2SCH2CH2OCO + CH2CH2SCH2CH2OCO),
2.36–2.25 (m, OCOCH2CH2CH2), 2.09–2.01 (m, CH2@CHCH2), 1.75–
1.68 (m, OCOCH2CH2CH2), 1.46–1.25 (CH2CH2CH2CH2CH2). 31P
NMR, d (ppm): À6.39. GPC: Mn = 4800, Mw/Mn = 1.30 for 4a;
Mn = 9500, Mw/Mn = 1.52 for 4c; NMR: Mn = 3400 for 4a;
Mn = 6600 for 4c.
2.4. General procedure for ADMET polymerizations in the presence of a
selective chain terminator
2.7. Synthesis of cyclic PPEs (5) by RCM
A solution of 4 (0.025 mmol) in CH2Cl2 was degassed with three
freeze–vacuum–thaw cycles to obtain an initial l-PPEs precursor 4
concentration of 5 Â 10À5 mol/L, and in the presence of N2 sparg-
ing, first generation Grubbs catalyst was added (8.3 mg,
0.01 mmol). The reaction mixture was stirred for 48 h at 40 °C.
After cooling, the solvent was removed under reduced pressure
and the residue was precipitated in methanol, filtered and dried
under vacuum to obtain cyclic PPE (c-PPE) at a high yield. 1H
NMR (CDCl3), d (ppm): 7.52–7.29 (m, m-ArH), 7.25–7.07 (m, o-
ArH + p-ArH), 7.03–6.88 (m, OCOCH@CH), 5.92–5.76 (m, OCOCH@
CH), 5.47–5.36 (m, CH2CH@CHCH2), 4.52–4.16 (m, OCOCH@CHCH2 +
CH2CH2OCO + OCOCH2CH2OCO + OCOCH2CH2OCO + CH2CH2SCH2CH2
OCO), 3.87–3.65 (m, CH2CH2OCO), 2.78–2.53 (m, CH2CH2SCH2CH2
OCO + CH2CH2SCH2CH2OCO + CH2CH2SCH2CH2OCO), 2.27–1.71 (m,
OCOCH2CH2CH2 + CH2CH@CHCH2 + OCOCH2CH2CH2), 1.49–1.12 (CH2CH2
CH2CH2CH2). 31P NMR, d (ppm): À6.31. GPC: Mn = 3900, Mw/Mn =
1.38 for 5a; Mn = 7300, Mw/Mn = 1.45 for 5c; NMR: Mn = 3300 for
5a; Mn = 6300 for 5c.
In a nitrogen-filled Schlenk tube, monomer 1, the desired
amount of the selective chain terminator (2-hydroxyethyl acry-
late), and CH2Cl2 were degassed by three freeze–vacuum–thaw cy-
cles. The mixture was heated to 40 °C while stirring and then a
solution of Hoveyda–Grubbs second generation catalyst (0.5 mol%
with respect to monomer 1) in 0.5 mL of CH2Cl2 was degassed fol-
lowing the same procedure. After the reaction mixture was stirred
for 24 h, the polymerization was quenched by adding THF (2 mL)
and ethyl vinyl ether with stirring for 30 min. The solution was
precipitated into an excess of methanol, and the precipitate was
isolated by filtration and dried under vacuum for 24 h to give the
ADMET polymers 2. 1H NMR (CDCl3), d (ppm): 7.49–7.35 (m, m-
ArH), 7.21–7.01 (m, o-ArH + p-ArH), 6.99–6.86 (m, OCOCH@CH),
6.47–6.35 (d, OCOCH@CH), 6.18–6.07 (m, OCOCH@CH), 5.95–5.78
(m, OCOCH@CH + OCOCH@CH), 4.49–4.31 (m, OCOCH@CHCH2 +
CH2CH2OCO + HOCH2CH2OCO), 3.92–3.83 (m, CH2CH2OCO + HOCH2-
CH2OCO). GPC: Mn = 4600, Mw/Mn = 1.34 for 2a; Mn = 6800, Mw/
Mn = 1.42 for 2b; Mn = 9200, Mw/Mn = 1.51 for 2c; NMR: Mn = 3200
for 2a; Mn = 6500 for 2c.
3. Results and discussion
2.5. End group functionalization of 2 via a thiol-Michael addition click
reaction
3.1. Head-to-tail ADMET polymerization of diene monomer
The ADMET polymers 2 (1.0 mmol), 2-mercaptoethanol (0.32 g,
2 mmol) and triethylamine (0.2 g, 2 mmol) were dissolved in
10 mL of THF in a Schlenk tube and stirred overnight at room tem-
perature. The product was then precipitated quantitatively from
methanol and dried under vacuum for 24 h to give the linear PPE
(l-PPE) 3 with two hydroxyl end groups. 1H NMR (CDCl3), d (ppm):
7.53–7.39 (m, m-ArH), 7.29–7.06 (m, o-ArH + p-ArH), 6.98–6.81 (m,
OCOCH@CH), 5.92–5.70 (m, OCOCH@CH), 4.61–4.26 (m, OCOCH@
ADMET polymerization monomer 1 was synthesized by an
esterification reaction of phenyl dichlorophosphate with allyl alco-
hol and 2-hydroxyethyl acrylate in the presence of a pyridine base.
The crude product was purified by column chromatography to pro-
vide pure colorless liquid 1 with a good yield of 72.6%. The 1H NMR
spectrum (Fig. 1a) showed the resonance signals of CH2@CHOCOA
protons (g, h) at 6.34 ppm, 5.78 ppm, CH2@CHOCOA protons (f) at
6.19 ppm, CH2@CHCH2OA protons (a) at 5.28–5.26 ppm, and

L. Ding et al. / Reactive & Functional Polymers 73 (2013) 1242–1248
1245
Fig. 1. 1H NMR spectra for (a) phenyl diene phosphate compound 1, (b) the ADMET polymer 2a, (c) the linear PPE 3a bearing two hydroxyl end groups 3, (d) diene end-
functionalized PPE 4a, and (e) cyclic PPE 5a.
CH2 = CHCH2OA protons (b) at 5.98 ppm. Furthermore, the molec-
ular weight (Mn = 312.2) of 1 was identified by MS to be in good
accordance with the calculated value, and the product is also
highly pure as estimated from the single peak of the GC chromato-
gram. All of these points confirmed the successful preparation of
monomer 1 with the expected structure.
Based on this principle, we chose the commercially available 2-
hydroxyethyl acrylate to act as a selective chain terminator in the
ADMET polymerization of monomer 1. Varying the [Monomer]/
[chain terminator] ratio should allow for the synthesis of PPEs with
different molecular weights (Fig. 2 and Table 1). The reaction of 1
and the chain terminator 2-hydroxyethyl acrylate at
a
Diene monomers containing both a terminal olefin and an acry-
late functional group will only polymerize by head-to-tail addition,
and a high terminal olefin-to-acrylate metathesis selectivity of 97%
can be achieved when using the Hoveyda–Grubbs second genera-
tion catalyst [23]. The allyloxy functionality has been proven not
to be self-reactive in ADMET polymerization, perhaps due to the lack
of sufficiently strong metathesis reactivity, but it can participate in
cross metathesis with acrylate in an equilibrium step propagation
condensation fashion, based on the electron-rich feature of the allyl-
oxy functionality itself [25–27]. Therefore, a monomer bearing both
an allyloxy functionality and an acrylate functional group will have
higher metathesis selectivity. If mono-functional acrylate is added,
diverse homopolymers and diblock copolymers are synthesized.
As the mono-functional acrylate can only react with one end of
the polymer chain (acrylates have a very low tendency to dimerize
during olefin metathesis due to the negative neighboring group), it
can be considered a selective chain terminator [23,24].
10:1 ꢀ 20:1 M ratio yielded the PPEs 2. Increasing the molar ratio
led to a moderate increase in the polymer molecular weight (Mn)
from 4600 to 9200, with a reasonably low polydispersity index
(PDI) ranging from 1.34 to 1.51.
Moreover, the head-to-tail metathesis selectivity of PPEs 2
could be validated by the 1H NMR analysis. Fig. 1b showed the
1H NMR spectrum of 2a, in which a new peak (x) appeared at
6.99–6.86 ppm due to the formation of internal a, b-unsaturated
ester functions. Importantly, the terminal allyloxy (a, b) disap-
peared after ADMET polymerization indicating a head-to-tail
metathesis selectivity of >99%. By comparing the peak integration
areas of internal a, b-unsaturated ester protons on the PPE back-
bone at 6.99–6.86 ppm (Hx) with that of olefinic protons at the
end of the PPE backbone at 6.21–6.19 ppm (Hf), the average degree
of polymerization (n) was calculated using the following formula:
n = Sx/Sf = 10. The integrated ratio was used to determine the num-
ber-average molecular weight of PPE 2, Mn,NMR = (Sx/Sf) Â M(1)

1246
L. Ding et al. / Reactive & Functional Polymers 73 (2013) 1242–1248
Fig. 2. GPC traces of the linear PPEs 2, 3, and 4.
Table 1
Analytical data of PPEs prepared via head-to-tail ADMET polymerization with
selective chain stopper.
Yielda (%)
Mn,GPC
PDIb
Mn,NMR
b
c
Polymer
[M]/[CS]
t (h)
2a
2b
2c
10:1
15:1
20:1
24
24
24
92
88
91
4600
6800
9200
1.34
1.42
1.51
3200
ndd
6500
Fig. 3. FTIR spectra for (a) the ADMET polymer 2a, (b) the linear PPE 3a bearing two
hydroxyl end groups, (c) diene end-functionalized PPE 4a, and (c) cyclic PPE 5a.
a
Obtained gravimetrically from the dried polymer.
Number average molecular weight (Mn) and polydispersed index (PDI) were
b
Table 2
determined by gel permeation chromatography in THF relative to monodispersed
polystyrene standards.
Characteristics of linear and cyclic PPEs via RCMa.
c
Mn,NMR = (Sx/Sf) Â M(1) + M(CS) À M(ethylene) was calculated by 1H NMR spectros-
c
Polymer
Yieldb (%)
Mn,GPC
PDIc
Mn,NMR
LOIg
copy, where M(1) = 312, M(CS) = 116, and M(ethylene) = 28 are the molar masses of
monomer 1, chain stopper, and ethylene, respectively.
3a
4a
5a
3c
4c
5c
98
96
81
83
90
92
4500
4800
3900
9600
9500
7300
1.35
1.30
1.38
1.56
1.52
1.45
3200d
3400e
3300f
6500d
6600e
6300f
23.2
23.6
24.5
24.2
24.6
25.3
d
Not determined.
(312) + M(CS) (116) À M(ethylene) (28) = 3200. However, the value of
Mn,GPC was higher than that of Mn,NMR, which could be due to dif-
ferences in the hydrodynamic volume of PPE or differences in the
polystyrene standards used for calibration. In the IR spectrum of
the ADMET polymer 2a in Fig. 3a, the absorption bands’ character-
istic peaks at 3420, 2921, 1725, 1597, 1512, 1295, 1126, and
1065 cmÀ1 were attributed to the stretching vibrations of hydroxyl
groups, unsaturated C@CAH and saturated CAH, ester (C@O), P@O,
PAO, and AOAC (PAOAC), respectively. All of these results demon-
strated the success of ADMET polymerization.
a
Reaction conditions for preparation of 5: reaction temperature = 40 °C, reaction
time = 48 h, [M]0 = 5 Â 10À5 mol/L.
b
Obtained gravimetrically from the dried polymer.
Number average molecular weight (Mn) and polydispersed index (PDI) were
c
determined by gel permeation chromatography in THF relative to monodispersed
polystyrene standards.
d
Mn,NMR = (2St/Sv) Â M(1) + M(CS) + M(mercaptoethanol) À M(ethylene) was calculated by
1H NMR spectroscopy.
e
Mn,NMR = (4Sx/Sa) Â M(1) + M(CS) + M(mercaptoethanol) + 2 Â M(undecylenic acid) À M(eth-
was calculated by 1H NMR spectroscopy.
ylene)
f
Mn,NMR = (2St/SA) Â M(1) + M(CS) + M(mercaptoethanol) + 2 Â M(undecylenic
acid) À 2 Â
M(ethylene) was calculated by 1H NMR spectroscopy.
3.2. End group functionalization of linear PPEs
g
LOI values were taken as the average of three measurements.
The generated ADMET PPEs 2 contain acrylate end groups,
allowing for the direct functionalization of these macromolecules.
The thiol-Michael addition click reaction is an advantageous tool
because it is a rapid and quantitative reaction that achieves high
conversion with little to no photoinitiator under atmospheric con-
ditions, and it is insensitive to water and oxygen [23,28–30]. Acry-
late-functionalized polymers are known to react rapidly with thiols
by the thiol-Michael addition click reaction [23,30]. Therefore, we
reacted PPEs 2 with mercaptoethanol in the presence of triethyl-
amine catalyst. The reaction proceeded at room temperature until
complete and selective functionalization of the acrylate end groups
3 was achieved, and the products were then subjected to 1H NMR
spectroscopy. The 1H NMR spectrum and the corresponding peak
assignments of 3a are shown in Fig. 1C. After 2 reacted with
mercaptoethanol, two groups of new resonance peaks appeared
at v (2.73 ppm) and u, w (2.67–2.52 ppm), which can be ascribed
to the protons of methylene. Importantly, the resonance signals
of terminal acrylate protons (g,h,f) at 6.47–6.35 ppm and 6.18–
6.07 ppm completely disappeared, indicating the complete trans-
formation of the terminal acrylates into hydroxyls. The IR spec-
trum for 3a is shown in Fig. 3b, and the absorption bands were
almost identical to those in 2a. Furthermore, the GPC elution
curves of 3 displayed nearly the same Mns and PDIs as those of 2
(Table 2 and Fig. 2), suggesting that ADMET polymer 2 has a similar
backbone structure to that of l-PPEs 3.
The esterification of 3 with an excess of 10-undecenoyl chloride,
which was synthesized by reacting undecylenic acid with oxalyl
chloride, was carried out at room temperature in the presence of
triethylamine to produce the l-PPE precursors 4. The 1H NMR spec-
trum of 4a (Fig. 1d) showed the resonance signals of terminal ole-
finic protons at 5.78 ppm (b) and 4.99 ppm (a). Moreover, the
resonance signals of methylene protons z (2.36–2.25 ppm), i
(2.09–2.01 ppm), m (1.75–1.68), and l (1.46–1.25 ppm) can also
be clearly observed, corresponding to the chemical structure of
undecylenic acid. Comparing the IR spectrum of the l-PPEs precur-

L. Ding et al. / Reactive & Functional Polymers 73 (2013) 1242–1248
1247
observe the clean shift of the elution peak of c-PPEs 5 to the lower
molecular weight position from Fig. 4, compared to those of l-PPE
precursors 4, which can be ascribed to the lower hydrodynamic
volume of c-PPEs. These findings confirmed that the intramolecular
ring closure was complete. The cyclic PPE 5a and its linear precur-
sor 4a were also examined by MALDIÀTOF MS spectra (Fig. 5). The
peaks were separated by 284 mass units, corresponding to the
molecular weight of the monomer unit (284.20). We were able to
calculate the average degree of polymerization (DPn) by analyzing
these peaks [16a]. For example, the highest peak at 3364.8 in
Fig. 5b corresponded to 5a with a DPn of 10, which was derived
from the following formula: M(each monomer unit) (284) Â DPn + M(end
(512) À M(ethylene) (28) = 3364.8. Similarly, the DPn of 4a
groups of4a)
was calculated by the formula: M(each
M(end
(284) Â DPn +
monomer unit)
(512) = 3392.6, and DPn was again 10. Moreover,
groups of4a)
the Mns of 4a and 5a calculated from the MALDIÀTOF spectra are
3406 and 3378, which are in good agreement with the values
determined by GPC and calculated via 1H NMR. As the cyclic PPE
product is produced from a linear precursor, elimination of an eth-
ylene molecule should result in a molecular weight difference of 28
mass units, as confirmed from the Mns of the two polymers. More-
over, the absorption bands of c-PPE 5a in the IR spectrum (Fig. 3d)
were almost identical to those of the l-PPE precursor 4a. Therefore,
we concluded that the backbone composition of the l-PPE precur-
sor is similar to that of the resulting c-PPE.
Fig. 4. GPC traces of the linear PPEs 4 and the cyclic PPEs 5.
sor 4a with 3a, we can also clearly observe in Fig. 3c that a strong
absorption peak at 3400 cmÀ1 disappeared, while the other absorp-
tion bands exhibited no significant change. All of these points con-
firmed that the esterification reaction was successful and that the
terminal hydroxyls were completely transformed into terminal
double bonds. The Mns and PDIs of 4 were similar to those of poly-
mers 2 and 3 because their structures were similar (Table 2 and
Fig. 2).
3.4. Thermal properties of linear and cyclic PPEs
3.3. Preparation of cyclic PPEs via RCM of linear PPEs
The DSC traces of typical PPEs are shown in Fig. 6a, and the ther-
mal stability was examined by TGA under a nitrogen atmosphere
(Fig. 6b). The flexible PAOAC groups in the backbone commonly
result in PPEs with low glass transitions (Tg = À40 °C) [32]. The
Fox equation for these polymeric systems estimated the Tg at
À30 °C for a PPE homopolymer, which is consistent with the role
of the internal plasticizer of the phosphoester monomer itself
[26]. The l-PPE 4a and c-PPE 5a exhibited different Tgs at À35.6
and À24.9 °C, without a melting peak (Fig. 6a). The olefinic double
bonds in the polymer microstructure resulted in the typical low Tg
of PPEs in this case.
The ADMET polymer 4 was telechelic, with two vinyl end
groups, and thus could be considered a functional linear precursor
for the preparation of cyclic polymers. Therefore, c-PPEs 5 were fi-
nally obtained when the intramolecular cyclization reaction was
achieved under highly dilute conditions by the end-to-end RCM
reaction between terminal alkene groups on the l-PPE precursor
4. The RCM reaction was performed at a low concentration of [4]
=5 Â 10À5 mol/L to ensure that most of the l-PPE precursor reacted
in the intramolecular cyclization reaction because the concentra-
tion of the linear polymer is usually the primary factor determining
whether an a,
condensation [31].
x
-functionalized precursor will prefer cyclization or
The TGA curves in Fig. 6b exhibit a one-step degradation pro-
cess, and the temperature range of 10% weight loss was deter-
mined at 396 and 405 °C, demonstrating the good thermal
stability of these polymers. Moreover, the thermal degradation of
PPEs leads to the formation of the phosphorus containing char,
which acts as a protective layer for the polymer surface. The char
yield is an important contributor to the flame retardance of poly-
mers, as high char yields always lead to high flame retardant activ-
ity of phosphorus flame retardants [1c]. The char yield of l-PPE 4a
and c-PPE 5a obtained at 800 °C reached a maximum of approxi-
mately 10% in both PPE series.
Fig. 1e shows the 1H NMR spectrum of the RCM product c-PPE
5a. New resonance peaks A at 5.36 ppm appeared after RCM cycli-
zation and can be ascribed to the protons of internal carbon–car-
bon double bonds. When compared with the spectrum of l-PPE
4a, peaks a and b at 4.99 and 5.78 ppm in Fig. 1d almost disap-
peared in Fig. 1e after RCM, indicating successful ring closure of
the linear PPEs via RCM. Monomodal peaks can be observed in
the GPC chromatogram (Fig. 4) for the as-obtained c-PPEs 5 with
relatively low PDIs (Table 2). Most importantly, we can also clearly
Fig. 5. MALDIÀTOF MS spectra of (a) the linear PPE 4a and (b) the cyclic PPE 5a.

1248
L. Ding et al. / Reactive & Functional Polymers 73 (2013) 1242–1248
addition click reaction and finally was reacted with corresponding
acyl chloride containing a vinyl end group to obtain linear PPE pre-
cursors. The linear and cyclic PPEs were characterized in detail by
NMR, FTIR, GPC, MALDIÀTOF MS, DSC, and TGA measurements. LOI
values up to 25.3 were obtained for PPEs, indicating the favorable
flame retardant properties of cyclic PPEs. These results may pro-
vide valuable guidance for the synthesis of linear and cyclic poly-
mers, especially functional polyolefins.
Acknowledgments
We thank the Initial Scientific Research Foundation of Yancheng
Institute of Technology for financial support of this research. We
also thank Dr. Zhenshu Zhu at China Pharmaceutical University
for his kind help with the MALDIÀTOF MS experiments.
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A novel cyclic PPE was successfully prepared via RCM of difunc-
tional linear PPE precursors under highly dilute conditions. The lin-
ear PPE bearing an end acrylate functional group was first
synthesized through a selective head-to-tail ADMET polymeriza-
tion of phenyl dienephosphate monomer using 2-hydroxyethyl
acrylate as a selective chain terminator. The end functional group
was then transformed into a hydroxyl group by a thiol-Michael