Hyperbranched polyphosphoesters with reactive end groups synthesized via acyclic diene metathesis polymerization and their transformation to crosslinked nanoparticles

Article abstract of DOI:10.1002/pola.26845

A novel hyperbranched polyphosphoester (HBPPE) was prepared by acyclic diene metathesis (ADMET) polymerization of an AB₂ monomer using the second-generation Hoveyda-Grubbs catalyst. IR analysis, gel permeation chromatography with multiangle laser light scattering, and NMR spectroscopy confirmed their controlled synthesis. The effect of the ADMET polymerization time in the thermal properties of HBPPEs was studied and their thermal degradation and flame-retardant properties were evaluated. These reactive HBPPEs were used to synthesize polyphosphoesters nanoparticles by intramolecular crosslinking of the end acrylate groups in dilute solution. The successful preparation of this new type of polymer nanoparticles was proven by NMR spectroscopy and gel permeation chromatography. The morphologies of nanostructures were investigated via dynamic light scattering, atom force microscopy, and transmission electron microscopy. The results revealed that the crosslinked nanoparticles, which have a comparatively uniform size, were stable and reproducible in organic solvents.

Full text of DOI:10.1002/pola.26845

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Hyperbranched Polyphosphoesters with Reactive End Groups
Synthesized via Acyclic Diene Metathesis Polymerization and
their Transformation to Crosslinked Nanoparticles
Liang Ding, Jun Qiu, Rong Lu, Xueqin Zheng, Jing An
Department of Polymer Materials and Engineering, School of Materials Engineering, Yancheng Institute of Technology,
Yancheng 224051, People’s Republic of China
Correspondence to: L. Ding (E-mail: dl1984911@ycit.edu.cn)
Received 29 April 2013; accepted 19 June 2013; published online 29 July 2013
DOI: 10.1002/pola.26845
ABSTRACT: A novel hyperbranched polyphosphoester (HBPPE)
was prepared by acyclic diene metathesis (ADMET) polymerization
of an AB2 monomer using the second-generation Hoveyda–
Grubbs catalyst. IR analysis, gel permeation chromatography with
multiangle laser light scattering, and NMR spectroscopy confirmed
their controlled synthesis. The effect of the ADMET polymerization
time in the thermal properties of HBPPEs was studied and their
thermal degradation and flame-retardant properties were eval-
uated. These reactive HBPPEs were used to synthesize polyphos-
phoesters nanoparticles by intramolecular crosslinking of the end
acrylate groups in dilute solution. The successful preparation of
this new type of polymer nanoparticles was proven by NMR
spectroscopy and gel permeation chromatography. The morpholo-
gies of nanostructures were investigated via dynamic light scatter-
ing, atom force microscopy, and transmission electron
microscopy. The results revealed that the crosslinked nanopar-
ticles, which have a comparatively uniform size, were stable and
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Polym. Sci., Part A: Polym. Chem. 2013, 51, 4331–4340
KEYWORDS:
acyclic
diene
metathesis
polymerization;
hyperbranched polyphosphoesters; intramolecular crosslinking;
polymer nanoparticles
enormous impact on polymer chemistry, especially on the
field of the living polymerization to prepare highly functional
polymers.10–12 However, as far as we are concerned, there
are comparatively few pathways toward hyperbranched
structures, which have been described to date. Grubbs set an
exciting starting point with the development of HBPs via acy-
clic diene metathesis (ADMET) polymerization of ABn-type
monomers under mild conditions in conjunction with
ruthenium-based catalysts.13 Xie lately originated a facile
approach named tandem chain transfer ring-opening metath-
esis polymerization (ROMP) and ADMET polymerization, on
the basis of the great differences in reactivity between
ROMP, crossmetathesis, and secondary metathesis, for syn-
thesizing long-chain highly branched polymers in one-pot
procedure.14
INTRODUCTION Hyperbranched polymers (HBPs), as a new
category of hyper-structured materials, have attracted con-
siderable attention owing to their unique highly branched
architectures.1–3 In contrast to their linear systems, HBPs
have a large number of terminal functional groups, which
show remarkable properties. Unlike the high structural regu-
larity and monodispersity in dendrimers, HBPs generally
have less well-defined structures and broad molecular
weight distributions.1–5 In many cases, HBPs are discussed
as a more rapidly prepared and more economical replace-
ment of perfect dendrimers in various applications. Tradi-
tionally, one of the common methods of preparation of HBPs
is polycondensation of ABn-type monomers (n ꢀ 2),6 self-
condensing vinyl polymerization,7 ring-opening multibranch-
ing polymerization,8 and self-condensing ring-opening poly-
merization (SCROP)9 are also used for preparing HBPs.
However, these methods also exhibit some drawbacks,
including the need for complex monomers and harsh reac-
tion conditions, very broad molecular weight distribution,
and the gelation caused by side reactions.
The topology architecture of the polymer chains and the
types of monomers used to synthesize polymers provide dif-
ferent properties and permit further modifications.15 Up to
now, a great number of functional groups have been intro-
duced to polymer or nanoparticle backbones from which
several advantages were obviously displayed, such as
improved material’s solubility, the melt and thermal
Olefin metathesis polymerization, as one of the most conven-
ient synthetic tools for the synthesis of polyolefins, had an
Additional Supporting Information may be found in the online version of this article.
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properties, hydrophilicity and biodegradability, facilitated
drug conjugation, and further derivatization.16–21 Conse-
quently, for HBPs bearing a number of end groups per single
chain, modification of these end groups is an important
strategy for improving the properties of branched polymers
and finding new applications.22
One very desirable property for polymers is the flame
retardancy.23 Many kinds of flame retardants have been
devoted to improving the flame resistance of polymers and
have been applied to commercial products. Among them,
polymers containing phosphorus are the most common
owing to the formation of a protecting char during combus-
tion.24 Within the different available polymers, poly(phos-
phoester)s (PPEs), which were earlier used as flame-
retardant materials, are the most prominent examples owing
to their potential degradability and biocompatiblility.25,26
The chemical versatility of the monomeric phosphate allows
the design of functional materials with tunable and complex
architectures, and many different properties. In spite of this
obvious benefit, PPEs are only scarcely found in recent stud-
ies, even if they are easily accessible and allow the feasible
synthesis of a great variety of functional materials.27 Yan28
synthesized a series of water-soluble hyperbranched poly-
phosphoesters (HBPPEs) with a great number of terminal
hydroxyl group through SCROP, and used as a carrier for
intracellular drug delivery, which exhibited excellent biocom-
patibility and biodegradability. Very recently, Wurm29 pre-
sented an expansion of the metathesis polymerization
toward PPEs from a step-growth ADMET polymerization to
the chain-growth ROMP,30 which could broaden the spectrum
of PPEs synthesized by metathesis polymerization to more
sophisticated polymer architectures. However, HBPPEs con-
taining a lot of terminal electron-deficient and electron-
sufficient alkenes groups have not been synthesized and fur-
ther exploited via ADMET polymerization to date.
SCHEME 1 Schematic representation for the synthesis of
reactive HBPPEs and the subsequent transformation to
nanoparticles.
ne]dichloro(o-isopropoxyphenylmethylene)ruthenium (Hov-
eyda–Grubbs second-generation catalyst) (98%), acrylic acid
(99%),
1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide
hydrochloride (EDCI-HCl, 99%), benzoin (99%), 3-mercapto-
1,2-propanediol (901%), allyl alcohol (98%), 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.
Characterization
1H (500 MHz) and 13C (125 MHz) NMR spectra were
recorded using tetramethylsilane as an internal standard in
CDCl3 on a Bruker DPX spectrometer. Phosphoric acid (85%)
was used as the external reference for 31P NMR analysis.
Relative molecular weights and molecular weight distribu-
tions were measured by gel permeation chromatography
(GPC) equipped with a Isocratic HPLC pump, a refractive
index detector, and a set of Waters Styragel columns (7.8 3
300 mm, 5 lm bead size; 103, 104, and 105 Å pore size).
GPC measurements were carried out at 35 ꢁC using tetrahy-
drofuran (THF) as the eluent with a flow rate of 1.0 mL/
min. The system was calibrated with polystyrene standards.
Absolute molecular weights were determined on a DAWN
EOS multiangle laser light scattering–gel permeation chroma-
tography (MALLS–GPC) instrument equipped with a viscome-
ter (Wyatt Technology); HPLC-grade THF (at 30 ꢁC) was
used as eluent at a flow rate of 1.0 mL/min. The chromato-
graphic system consisted of a Waters 515 pump, differential
refractometer (Optilab rEX), and one column, MZ 103 Å 300
3 8.0 mm. A MALLS detector (DAWN EOS), quasi-elastic
light scattering (QELS), and a differential viscosity meter
(ViscoStar) were placed between the GPC and the refractive
index detector. The molecular weight and molecular weight
distribution were determined by a GPC/DAWN EOS/Optilab
rEX/QELS model. The refractive index increment (dn/dc)
value of sample in THF solution was determined by an
Toward this attractive goal, herein, we aim to develop a gen-
eral strategy for convenient preparation of HBPPEs through
ADMET polymerization of a newly designed AB2 monomer.
This type of AB2 monomer contains phosphate moiety, which
functionalized with one electron-rich olefin—a terminal allyl
group, and two electron-poor olefins—acrylates. When
treated with ruthenium-based catalysts, electron-poor olefins
do not homodimerize (or do so very slowly) but do partici-
pate in a secondary metathesis reaction with homodimers of
more reactive olefins,31–34 which can be polymerized into a
hyperbranched structure successfully. Bearing a variety of
peripheral acrylate groups, the reactive HBPPEs could be
effectively further transformed to unimolecular polymer
nanoparticles by an intramolecular crosslinking process
(Scheme 1).35–37
EXPERIMENTAL
Materials
Phenyl dichlorophosphate (99%), 2-hydroxyethyl acrylate
(>96%), [1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylide-
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ꢁ
Optilab rEX detector at 30 C through batch model. Elemen-
tal analysis (EA) was conducted with an Elementar vario EL.
Thermogravimetric analyzer (TGA) was performed using an
SDTA851e/SF/1100 TGA Instrument under N2 flow atꢁ a
observed. EI/HRMS: Calcd. for C14H17O6P: 312.2549; found:
312.2512. Anal. calcd for C: 53.85, H: 5.49, O: 30.74; Found
C: 53.84, H: 5.45, O: 30.77.
ꢁ
Synthesis of Phenyl Allyl Dihydroxylphosphate
Compound (2)
heating rate of 10 C/min from room temperature to 800 C.
Differential scanning calorimeter (DSC) was performed on a
Perkin-Elmer Pyris 1 in nitrogen atmosphere. All the samples
1 (5.62 g, 18 mmol), 3-mercapto-1,2-propanediol (2.92 g, 27
mmol), triethylamine (3.64 g, 36 mmol), and CH2Cl2 (80 mL)
were charged into a 250-mL round-bottom flask equipped
with a magnetic stirrer under nitrogen atmosphere, and the
mixture was stirred overnight at room temperature. The
resulting solution was washed with 1 M of HCl, saturated
NaHCO3 aq., and water. The organic layer was separated and
dried over anhydrous Na2SO4, and concentrated to afford a
clear liquid 2 (6.29 g, 83.2% yield).
ꢁ
were first heated from room temperature to 200 C and held
at this temperature for 3 min to eliminate the thermal his-
tory, and then they were quenched to 280 ꢁC and heated
again from 280 to 200 ꢁC at 10 ꢁC/min. Gas chromatogra-
phy (GC) was measured by Agilent 6890 series GC system
instrument equipped with a flame ionization detector and a
capillary column (HP-5, 0.25 mm 3 30 m), using decane as
an internal standard. Tinj 280 ꢁC, Tdetec 280 ꢁC, Tinit 50 ꢁC
ꢁ
(10 C/min), carrier gas: N2. High-resolution mass spectrom-
1H NMR (CDCl3), d (ppm): 7.36–7.32 (m, 2H, m-ArH), 7.23–
7.19 (d, 2H, o-ArH), 7.14–7.10 (m, 1H, p-ArH), 5.95–5.87 (m,
1H, CH2@CHCH2O), 5.30–5.16 (d, 2H, CH2@CHCH2O), 4.33–
4.27 (m, 4H, CH2CH2OCOCH2 1 CH2@CHCH2O), 3.88–3.76
(m, 5H, CH2CH2OCOCH2 1 CH2OH 1 CHOH), 2.84–2.79 (m,
2H, CH2SCH2CH2OCO), 2.57–2.43 (m, 4H, CH2SCH2CH2OCO 1
etry (HRMS) data were recorded on a Waters GCT Premier
mass spectrometer with electron ionization mode. EA was
conducted with an Elementar vario EL. The hydrodynamic
diameter was determined by means of dynamic light scatter-
ing (DLS) analysis using a Malvern Zetasizer Nano-ZS light
scattering apparatus (Malvern Instruments, United Kingdom)
with a He–Ne laser (633 nm, 4 mW). Atom force microscopy
(AFM) observations were performed on SPM AJ-III atomic
force microscope at a measure rate of 1.0005 Hz in the tap-
ping mode. Transmission electron microscopy (TEM) was
performed on a JEM-200CX microscope operating at an
acceleration voltage of 200 kV.
CH2SCH2CH2OCO). 13C NMR (CDCl3),
d (ppm): 172.13,
153.32, 136.57, 131.33, 129.79, 120.47, 117.66, 116.26,
77.31, 69.41, 68.56, 68.21, 67.02, 36.64, 35.10, 28.28. GC:
single peak was observed. EI/HRMS: Calcd. for C17H25SO8P:
420.4144; found: 420.4109. Anal. calcd for C: 48.57, H: 5.99,
O: 30.45; Found C: 48.57, H: 5.95, O: 30.48.
Synthesis of Phenyl Phosphate AB2 Monomer (3)
Acrylic acid (2.59 g, 36 mmol), 2 (5.04 g, 12 mmol), and
DMAP (0.36 g, 2.9 mmol) were dissolved in CH2Cl2 (60 mL),
and the mixture was stirred at 0 ꢁC for 15 min. EDCI (6.90
g, 36 mmol) was then added to the former solution, and
stirred for 3 days under nitrogen flow after the solution
warmed to room temperature. The resulting solution was
washed three times with deionized water (3 3 80 mL), and
the organic layer was dried over anhydrous Na2SO4. Solvent
was then evaporated and the crude product was purified by
silica gel chromatography eluted with methylene chloride/
petroleum ether (10/1) to give a viscous colorless liquid 3
(4.93 g, 77.8% yield).
Polymerizations were carried out in Schlenk tubes using of a
nitrogen flow to drive off the ethylene condensate for
ADMET.
Synthesis of Phenyl Dienephosphate Compound (1)
To a round-bottom flask, phenyl dichlorophosphate (6.33 g,
30 mmol) was added and dissolved in 100 mL of dry CH2Cl2
ꢁ
and cooled to 0 C; allyl alcohol (1.57 g, 27 mmol) and pyri-
dine (2.61 g, 33 mmol) were added to this solution via
syringe, and subsequently stirred for 8 h under nitrogen
flow, then the solution was cooled to 0 ꢁC, 2-hydroxyethyl
acrylate (3.14 g, 27 mol) and pyridine (2.61 g, 33 mmol)
were added via syringe, the reaction was stirred overnight at
room temperature. After repeatedly washing with 1 M of HCl
and deionized water, the organic layer was dried over anhy-
drous Na2SO4, filtered, concentrated under reduced pressure,
and the crude product was purified by silica gel chromatog-
raphy eluted with methylene chloride to give a clear color-
less liquid 1 (6.12 g, 72.6% yield).
1H NMR (CDCl3), d (ppm): 7.37–7.31 (m, 2H, m-ArH), 7.23–
7.19 (d, 2H, o-ArH), 7.14–7.09 (m, 1H, p-ArH), 6.38–6.32 (d,
2H, OCOCH@CH), 6.23–6.16 (m, 2H, OCOCH@CH), 6.01–5.90
(m, 3H, CH2@CHCH2O 1 OCOCH@CH), 5.29–5.23 (d, 2H,
CH2@CHCH2O), 4.87–4.85 (s, 1H, CHOCO), 4.32–4.27 (m, 2H,
CHCH2OCOCH@CH2), 4.24–4.18 (m, 4H, CH2CH2OCOCH2
1
CH2@CHCH2O), 3.89–3.83 (m, 2H, CH2CH2OCOCH2), 2.88–
2.83 (m, 2H, CH2SCH2CH2OCO), 2.74–2.60 (m, 2H,
CH2SCH2CH2OCO), 2.49–2.41 (m, 2H, CH2SCH2CH2OCO). 13C
NMR (CDCl3), d (ppm): 171.26, 165.38, 152.72, 136.77,
131.65, 129.54, 127.88, 122.05, 117.39, 116.82, 77.46, 69.02,
68.76, 68.35, 67.30, 35.94, 32.67, 28.23. 31P NMR, d (ppm):
26.12. GC: single peak was observed. EI/HRMS: Calcd.
for C23H29SO10P: 528.5091; found: 528.5065. Anal. calcd
for C: 52.27, H: 5.53, O: 30.27; Found C: 52.25, H: 5.51, O:
30.30.
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, CH2CH2O-
COCH@CH2), 4.21–4.17 (d, 2H, CH2@CHCH2O), 3.88–3.84 (m,
2H, CH2CH2OCOCH@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, and 67.05. GC: single peak was
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Representative Procedure for ADMET Polymerization
In a nitrogen-filled Schlenk tube, a solution of Hoveyda–
Grubbs second-generation catalyst (0.5 mol % to AB2 mono-
mer 3) in 0.5 mL of degassed toluene with three freeze–vac-
uum–thaw cycles was added to a solution of monomer 3
(280 mg, 0.5 mmol) in 0.5 mL of toluene degassed with the
same procedure. After the reaction mixture was stirred at 60
ꢁC for 1–7 days, the polymerization was quenched by adding
ethyl vinyl ether with stirring for 30 min. The solution was
precipitated into an excess of methanol, and the precipitate
was isolated by filtration, dried under vacuum for 24 h to
give the polymer 4 as a semisolid.
1H NMR (CDCl3), d (ppm): 7.49–7.35 (m, m-ArH), 7.21–7.01
(m, o-ArH 1 p-ArH), 6.99–6.89 (m, OCOCH@CH), 6.47–6.31
(d, OCOCH@CH), 6.18–6.03 (m, OCOCH@CH), 5.90–5.71 (m,
CH2@CHCH2O 1 OCOCH@CH 1 OCOCH@CH), 5.43–5.32 (m,
CH2@CHCH2O), 4.88–4.82 (s, CHOCO), 4.49–4.10 (m,
CHCH2OCOCH@CH2 1 CH2@CHCH2O 1 CH2CH2OCOCH2),
3.87–3.74
(m,
CH2CH2OCOCH2),
2.67–2.59
(m,
CH2SCH2CH2OCO
1
CH2SCH2CH2OCO), 2.48–2.34 (m,
CH2SCH2CH2OCO). 31P NMR, d (ppm): 26.10. MALLS–GPC:
Mn 5 4.6 kDa, Mw/Mn 5 2.76 for 1 day; Mn 5 10.7 kDa,
Mw/Mn 5 2.68 for 3 days; Mn 5 16.3 kDa, Mw/Mn 5 2.49
for 5 days; Mn 5 28.2 kDa, Mw/Mn 5 2.33 for 7 days. GPC:
Mn 5 3.3 kDa, Mw/Mn 5 2.98 for 1 day; Mn 5 6.9 kDa, Mw/
Mn 5 2.93 for 3 days; Mn 5 10.8 kDa, Mw/Mn 5 2.76 for 5
days; Mn 5 15.7 kDa, Mw/Mn 5 2.62 for 7 days.
Transformation of Reactive HBPs to Nanoparticles
The intramolecular polymerization of the reactive HBPs was
typically performed as follows: 50 mg of HBPPEs was dis-
solved in 300 mL of toluene (C 5 6 3 1026 mol/L) in a dry
round-bottom flask under a nitrogen atmosphere. The reac-
tion mixture was degassed under vacuum, then benzoin (10
mg, 20 wt %) was added. The flask was placed under a UV
lamp (900 W Hg lamp, 365 nm) for 4 h under stirring. The
solution was concentrated and precipitated into an excess of
methanol, the precipitate was isolated, dried under vacuum
for 24 h to give a solid polymer 5 (yield, >95%).
FIGURE 1 1H NMR spectra for (a) phenyl phosphate AB2 mono-
mer 3, (b) reactive HBPPE 4d for 7 days, and (c) crosslinked
nanoparticle 5d.
colorless liquid 1 with good yield of 72.6%. The 1H NMR spec-
trum (Supporting Information Fig. S1a) showed the resonance
signals of CH2@CHOCOA protons (g, h) at 6.34 and 5.78 ppm,
CH2@CHOCOA protons (f) at 6.19 ppm, and CH2@CHCH2OA
protons (a) at 5.28–5.26 ppm, CH2@CHCH2OA protons (b) at
1H NMR (CDCl3), d (ppm): 7.46–7.31 (m, m-ArH), 7.23–7.02
(m, o-ArH 1 p-ArH), 6.99–6.86 (m, OCOCH@CH), 5.90–5.73
(m, CH2@CHCH2O
1
OCOCH@CH), 5.44–5.37 (m,
CH2@CHCH2O), 4.90–4.82 (s, CHOCO), 4.49–4.08 (m, CHCH2O-
COCH@CH2 1 CH2@CHCH2O 1 CH2CH2OCOCH2), 3.85–3.66
5.98 ppm. Furthermore, the molecular weight (Mn
5
(m, CH2CH2OCOCH2), 2.78–2.49 (m, CH2SCH2CH2OCO
1
CH2SCH2CH2OCO), 2.48–2.33 (m, CH2SCH2CH2OCO). 31P NMR,
d (ppm): 26.36. GPC: Mn 5 13.5 kDa, Mw/Mn 5 2.58.
312.2512) of 1 from MS analysis was in good accordance with
the calculated value, and the product also has a high purity
estimated from the single peak of GC chromatogram. All of
these points affirmed the successful preparation of 1 with the
expected structures by esterification.
RESULTS AND DISCUSSION
Phenyl Phosphate AB2 Monomer Synthesis
The AB2-type monomer in this study was successfully pre-
pared by a three-step technique. As the first step, commer-
cially available phenyl dichlorophosphate was esterified with
allyl alcohol and 2-hydroxyethyl acrylate, respectively, in the
presence of a base, pyridine (Scheme 1). The crude product
was purified by column chromatography to provide pure
Phenyl dihydroxylphosphate compound, 2, was synthesized
by thiol–Michael addition click reaction.33,37–39 The com-
pound 1 contained one acrylate group, which could allow for
the direct reaction with 3-mercapto-1,2-propanediol in the
presence of triethylamine. As Supporting Information Figure
S2a shows the 1H NMR spectrum of compound 2, we can
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TABLE 1 Characteristics of Reactive HBPPEs by ADMET Polymerization using Hoveyda–Grubbs Second-Generation Catalysta
GPC Analysisc MALLS Analysisd
f
Polymer
t (day)
Yield (%)b
Mn (kDa)
PDI
Mn (kDa)
Mw (kDa)
PDI
[g]B (dL/g)e
[g]L (dL/g)e
g0
4a
4b
4c
4d
1
3
5
7
89
86
91
93
3.3
2.98
2.93
2.76
2.62
4.6
12.8
28.7
40.6
65.7
2.76
2.68
2.49
2.33
0.128
0.149
0.185
0.223
0.230
0.292
0.431
0.587
0.56
0.51
0.43
0.38
6.9
10.7
16.3
28.2
10.8
15.7
a
d
e
Polymerization conditions: polymerization temperature 5 60 ꢁC, [M]0
5 0.5 mol/L, and (Hoveyda–Grubbs second-generation catalyst) 5 2.5 3
1024 mol/L.
Obtained from multiangle laser light scattering analysis.
Measured by Ubbelohde viscometer. The B indicates hyperbranched
polymers, and the L implies linear polymers with the same molecular
weights of hyperbranched polymers.
b
Obtained gravimetrically from the dried polymer.
c
f
Number average molecular weight (Mn) and PDI were determined by
g0 5 [g]B/[g]L.
GPC in THF relative to monodispersed polystyrene standards.
observe the appearance of three groups of new resonance
peaks at u, v (3.81–3.76 ppm), s (2.80 ppm), and t, q (2.56–
2.43 ppm), which can be ascribed to the protons of methyl-
ene and methylidyne. Furthermore, peaks g, h, and f initially
at 6.34, 5.78, and 6.19 ppm in Supporting Information Figure
S1a disappeared in Supporting Information Figure S2a,
which meant acrylate group was virtually transformed.
between the olefin and the ether oxygen or the ester from
the oxygen side. To avoid suffering a negative neighboring
group effect, the number of methylene spacers between
these functionalities is at least two for successful metathesis
to occur.29,40 Consequently, the acrylate functionality is
always failed to (homo)polymerize.13,41,42 On the other hand,
the allyloxy functionality has been proved to be “unreactive”
to ADMET polymerization with itself (maybe stemming from
the lack of strong enough metathesis reactivity), but could
take part in crossmetathesis with acrylate in an equilibrium
step propagation condensation fashion, on the basis of some
extent electron-rich feature of the allyloxy functionality
itself.14,29,36
AB2 monomer 3 was prepared from a similar esterification
reaction of compound 2 with acrylic acid catalyzed by DMAP
in CH2Cl2 at room temperature. 1H NMR spectrum and the
corresponding peak assignments of
3 are shown in
Figure 1(a). After 2 reacted with the acid, it was a clearly
observed that a clean chemical shift of the peaks of both
protons u and v was initially at 3.81–3.76 ppm, neighboring
to two hydroxyl groups in Supporting Information Figure
S2a to the higher field at 4.30 and 4.86 ppm for protons of
methylene and methylidyne after forming the acrylate as
shown in Figure 1(a). Moreover, the resonance signals of g, h
(6.36 and 5.92 ppm), and f (6.22 ppm) were newly formed,
which strongly suggested complete transformation of termi-
nal hydroxyl into terminal acrylate. GC–MS analysis also
affirmed that AB2 monomer 3 had an exact molecular weight
(Mn 5 528.5065) and a high purity.
The polymerization of AB2 monomer is easily monitored by
1H NMR spectroscopy. Figure 1(b) shows the 1H NMR spec-
trum of the resulting polymer 4 for 7 days. It can be seen
that the terminal allyloxy (a) almost disappeared after 7
days of polymerization. Moreover, as expected, a new peak
(x) appeared at 6.99 ppm owing to the formation of internal
acrylates (AB olefins), and importantly, the integration area
ratio of x and the remaining unreacted acrylate peak f char-
acteristic resonances of 1:1 was well agreed with the fact
that there are twice as many B groups as A groups in an
AB2 monomer, which indicated that polymerization proceeds
to completion, consuming almost all of the terminal alkenes,
and there are half of the free acrylate groups left in the final
polymer. However, the ratio of 1:4 and 1:2 for HBPPEs 4a
and 4b, respectively (Supporting Information Figure S5)
almost consuming half of the terminal aliphatic alkenes in
Supporting Information Figure S5b. All these results implied
that the conversion of monomer increased with longer poly-
merization time.
Reactive HBPPEs by ADMET Polymerization
Monomers containing both a terminal olefin and an acrylate
functional group would polymerize only by head-to-tail addi-
tions, and the high terminal olefin-to-acrylate metathesis
selectivity of 97% can be achieved when using Hoveyda–
Grubbs second-generation catalyst.33 Thus, HBPPEs were
synthesized via ADMET polymerization in conventional reac-
tion conditions utilizing Hoveyda–Grubbs second-generation
catalyst. It must be pointed out that the addition of a new
batch of catalyst (0.5 mol % to 3) at about 2-day interval
was found to be necessary to observe metathesis activity
owing to the length of the catalyst’s lifetime in the reaction
solution. The polymer was precipitated to an excess of meth-
anol to provide pure product.
To provide a better understanding of the polymerization sys-
tems described above, the GPC method is used to trace the
polymerization of AB2 monomer 3, and the results are sum-
marized in Table 1. It is well known that HBP is consisted of
multicomponent polymers, and hence the GPC elution curves
displayed multiple peak distributions. In Figure 2, GPC
curves of HBPPEs displayed multiple peaks. For HBPPE 4a
formed at the early polymerization stage for 1 day, it had a
Generally, in ADMET polymerization, the polymerizability of
a monomer is limited by the number of methylene spacers
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ing factor (g0) were used to describe the solution properties
of the branched polymer, which was distinctly different from
their linear analogues. However, in this study, DB of HBPPEs
4 cannot be obtained from the common technique owing to
the characteristic protons overlapped. Therefore, the [g] and
g0 were used to verify that the obtained polymer does, in
fact, have a branched architecture, and the results are sum-
marized in Table 1. The g0 can be expressed by the contract-
ing factor defined as [g]B/[g]L, where [g]B and [g]L were the
intrinsic viscosity of HBPPE and linear polymer with the
same molecular weight, respectively, measured by Ubbelohde
viscometer. To aid with comparisons, linear PPEs were syn-
thesized by ADMET polymerization of monomer 1 in the
same conditions. The [g]Bs were less than those of their lin-
ear reference as HBPPEs have smaller hydrodynamic vol-
umes and more compact structures than linear PPEs of
identical molecular weight. The g0 was reduced from 0.56 to
0.38 with increasing the molecular weight of HBPPEs. In a
word, the lower g0 value is, the higher DB value is.
FIGURE 2 GPC traces of reactive HBPPEs.
remarkable increase in molecular weight (Mn,GPC 5 3.3 kDa)
and a broad molecular weight distribution (polydispersity
index [PDI] 5 2.98). This observation clearly illuminated
that the monomer had been converted into HBP with rela-
tively low molecular weights via ADMET polymerization.
With evolution of ADMET polymerization, the peaks at low-
molecular-weight positions decreased, the elution curve of
HBPPE 4b (for 3 days) was gradually shifted to higher
molecular weight region (Mn,GPC 5 6.9 kDa), and the polydis-
persity was improved (PDI 5 2.93). After polymerization,
being conducted for 3 days, a fresh batch of 0.5 mol equiva-
lent of catalyst was then added to the reaction vessel. As a
result, a further molecular weight growth of HBPPE 4c was
detected by GPC as shown in Figure 2 after polymerization
for 5 days (Mn,GPC 5 10.8 kDa, PDI 5 2.76). At this time
point of 5 days, the addition of a second batch of catalyst to
the reaction vessel resulted in an obvious growth of HBPPE
4d for 7 days in the molecular weight (Mn,GPC 5 15.7 kDa,
PDI 5 2.62). Also, PDIs for all HBPPEs became narrower
with increasing molecular weight, indicating longer polymer-
ization time resulted in higher molecular weight and lower
molecular weight distribution, which is in accordance with
the mechanism of ADMET polymerization.
Most importantly, the compact branched structure could also
be confirmed by the empirical Mark–Houwink exponent a-
value ([g] 5 KMa). Ordinarily, an a parameter of 0.5–1.0 is
typical for randomly coiled linear polymers, polymers with a
rigid-rod shape have an a of 2.0, and spherically shaped
macromolecules are expected to have an a < 0.5.43 Support-
ing Information Figure S6 shows the corresponding loga-
rithm plots of [g] against molecular weight (Mw) for linear
and branched PPEs, which revealed a significantly depressed
(0.32) relative to that of linear reference (0.67). As expected,
the Mark–Houwink exponent a has characteristic value and
is typically below 0.5. Overall, the obtained a-value strongly
suggests a spherical shape in solution and, therefore, hyper-
branching for polymers 4.
Thermal Properties of Typical HBPPEs
The thermal behavior of HBPs is usually different from their
linear analogues owing to the branching effect. Generally,
glass transition temperature (Tg) of HBPs may be affected by
Although GPC is well known to determine the molecular
weight and molecular weight distribution for linear polymers
by using calibration with linear polystyrene, for HBP, its size
is smaller than that of linear ones with same molecular
weight because of the difference in hydrodynamic volume.
Therefore, we employed MALLS as an absolute molecular
weight characterization method to further characterize
HBPPEs. The typical polymerization results and MALLS chro-
matograms of HBPPEs obtained from different polymerization
time are shown in Table 1 and Figure 3, respectively. Indeed,
Mn,MALLS of HBPPEs 4 were significantly higher than their GPC
values based on polystyrene calibration, which is consistent
with the general trends for HBPs. It was shown that HBPPEs
4 had a moderate molecular weight from 4.6 to 28.2 kDa
with reasonable lower PDI ranging from 2.76 to 2.33.
Some important molecular parameters such as the degree of
FIGURE 3 Representative MALLS–GPC traces for reactive
HBPPEs.
branching (DB), the intrinsic viscosity ([g]), and the branch-
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TABLE 2 Thermal Property Data for HBPPEs with Different
value of 26.9 for HBPPEs 4d with a phosphorus content of
Molecular Weights
6.16%.
Polymer P Content (%)a Tg (ꢁC)b Td (ꢁC)c Char (%)d LOIe
Crosslinked Nanoparticles from Reactive HBPPEs
Next, the peripheral acrylate-functionalized HBPPEs have
been converted into nanoparticles by means of an intramo-
lecular polymerization. Owing to the lower reactivity of acry-
late groups bound to a polymer, UV initiation was chosen,
resulting in shorter reaction times. As the inter- and intra-
molecular reactions are chemically the same, the competition
and fraction of inter- and intramolecular reaction exclusively
depend on the concentration of the polymer, a series of dilu-
tions was examined.35 The resulting products were then ana-
lyzed by 1H NMR spectroscopy to determine the conversion
and by GPC curve to detect the possible presence of coupled
HBPPEs.
4a
4b
4c
4d
6.06
6.08
6.09
6.16
236.8
225.3
213.9
25.6
353
374
388
402
6.92
7.76
8.03
9.62
24.5
25.3
25.8
26.9
a
M
(Phosphorus)/M(HBPPEs) percentages.
b
c
Glass transition temperature measured by DSC.
Temperatures at 10% mass loss determined by TGA.
Residual quantity at 800 ꢁC.
d
e
LOI values were taken as the average of three measurements.
branch number, repeat units per branch, chemical composi-
tion, and polydispersity. Polymers with more branches have
augmentative free volume because of the abundant chain
ends, which will reduce Tg.44 On the other hand, the
branches also hinder the chain mobility, like bulky side
groups, which will elevate Tg.5 In this case, Tg of typical
HBPPEs was determined by DSC, whereas the thermal stabil-
ity was examined by TGA under nitrogen atmosphere, and
the results are summarized in Table 2. The flexible PAOAC
groups in the backbone commonly resulted in PPEs with low
glass transitions (240 ꢁC).45 The Fox equation for these
polymeric systems estimated the Tg at 230 ꢁC for a PPE
homopolymer, which is consistent with the role of an inter-
nal plasticizer of the phosphoester monomer itself.29 In DSC
curves of HBPPEs 4 [Fig. 4(a)], the typical low Tg of PPEs,
with a relevant contribution of the olefinic double bonds to
the polymer microstructure for HBPPEs, ranges between
236.8 and 25.6 ꢁC. No melting and crystallization peaks
were observed, indicating their amorphous state, which was
typical for branched polymers.
As shown in Figure 1(c), the almost complete disappearance
of the acrylate signals (g, f) demonstrated near quantitative
conversion. In Figure 5, the GPC analyses for different con-
centrations of HBPPE 4d are shown. Increasing the polymer
concentrations gradually resulted in the appearance of the
shoulders at high-molecular-weight fraction, caused by inter-
molecular reaction, whereas an insoluble gel was produced
TGA curves shown in Figure 4(b) exhibited a one-step degra-
dation process, and the temperature range of 10% weight
ꢁ
loss was determined to lie between 353 and 402 C, demon-
strating the good thermal stability of these polymers. More-
over, the thermal degradation of HBPPEs leads to the
formation of the phosphorus containing char, which acts as a
protective layer for the polymer surface. Therefore, the resi-
ꢁ
dues obtained at 800 C increase as the phosphorus content
does, reaching a maximum around 10% in both HBPPEs
series. Therefore, a conclusion could be made that the ther-
mal stability was largely dependent on the molecular weight
of HBPPEs, and higher molecular weight resulted in higher
Tg and decomposition temperature.
The flame retardancy properties of HBPPEs series were eval-
uated using the limiting oxygen index (LOI) test. The LOI is
the minimum concentration of oxygen determined in a flow-
ing mixture of oxygen and nitrogen that will just support the
flaming combustion of a certain material.24 The LOI values
obtained are listed in Table 2. A distinct and steady increase
of the LOI with increasing the molecular weight, that is
phosphorus content, was observed, and reaching a maximum
FIGURE 4 Thermal properties of HBPPEs 4 (a) DSC traces, and
(b) TGA curves.
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TABLE 3 Reaction Conditions and Results of the Formation of
Polymer Nanoparticles 5 by Means of an Intramolecular
Crosslinkinga
Yield
(%)b
Concentration
(mol/L)
Mn,GPC
(kDa)c
Nanoparticles
PDIc
5d1
5d2
5d3
5d4
5d5
96
95
93
97
99
5 3 1026
5 3 1025
5 3 1024
5 3 1023
5 3 1022
13.5
16.8
18.2
19.7
Gel
2.58
2.67
2.65
2.64
–
a
Crosslinking reactions were run at room temperature for 4 h with dif-
ferent concentrations.
b
Obtained gravimetrically from the dried polymer.
Number-average molecular weight (Mn) and PDI were determined by
c
FIGURE 5 GPC traces of HBPPE 4d and crosslinked nanopar-
ticles 5d with different reaction conditions.
GPC in THF relative to monodispersed polystyrene standards.
from reaction at concentrations above 1023 mol/L (Table 3).
A reduction of the concentration to an even lower value
would finally lead to a pure product. Therefore, a low con-
centration of 5 3 1026 mol/L was selected, and the cross-
linked unimolecular polymer nanoparticle 5d1 was
successfully fabricated and confirmed by its GPC curve (Fig.
5). We could observe a clear shift to smaller hydrodynamic
volume when compared to that of uncrosslinked HBPPE 4d
shown in Figure 5. To our knowledge, this is one of the very
few illustrations reported in the literature where the periph-
eral groups of HBPs are selectively reacted intramolecularly.
crosslinked nanoparticles were actually stable and reproduci-
ble in organic solvents.
The morphology of 5d1 was examined by AFM. One drop of
the dilute solution at concentration of 0.005 mg/mL in THF
was spin-coated on the freshly cleaved mica at spinning
speed of 1800 rpm for 30 s. The AFM image of the cross-
linked nanoparticle 5d1 is shown in Figure 6(b), which
exhibited a spherical morphology in comparatively uniform
size with an average diameter of 22 nm. Further evidence
for the formation of nanoparticles was obtained by TEM
analysis. The sample was prepared by dropping the cross-
linked nanoparticle solution with concentration of 0.005 mg/
mL in THF onto the carbon-coated copper grid, followed by
air-drying. As a typical TEM image, Figure 6(c) shows that
the crosslinked nanoparticle 5d1 takes an approximately
spherical shape, and good particle dispersion with no aggre-
gated structures. Most of the nanoparticles have an average
diameter of 24 nm, which is coincident with the DLS result,
considering the sample in dry state.
Size and Morphology of Crosslinked Polymer
Nanoparticles
To measure the hydrodynamic diameter and polydispersity
index of nanoparticles, a portion (1 mL) of the THF solution
was taken out from the original reaction mixture and ana-
lyzed by DLS. Figure 6(a) shows the size and distribution of
the crosslinked nanoparticle 5d1, giving the average hydro-
dynamic diameter of 26 nm and a PDI of 0.192 with a mono-
modal size distribution. When 5d1 in THF was precipitated
from methanol, dried under vacuum, and then redissolved in
THF or CHCl3, the LCHBP nanoparticles almost retained the
size and distribution. These results indicated that the
The obtained size of crosslinked nanoparticle 5d1 is reasona-
ble to consider these spherical objects as crosslinked nano-
particle from HBPPE molecules with a number-average
molecular weight of 28.2 kDa, leading to the formation of
FIGURE 6 Polymer nanoparticles 5d1 formed by crosslinking HBPPE 4d (a) size and distribution determined by means of DLS in
THF, (b) AFM image with concentration of 0.005 mg/mL, and (c) TEM image at 0.005 mg/mL.
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REFERENCES AND NOTES
larger sized corresponding nanoparticles than their linear
analogues, possibly owing to the significantly increased
hydrodynamic volume, the absence of chain entanglement,
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polymers, in which the coiling of branches or molecules
would not be restrained, resulting in a reduced size of
nanoparticles.35
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