Synthesis of pyrene end-capped A6 dendrimer and star polymer with phosphazene core via "click chemistry"

Article abstract of DOI:10.1002/pola.24756

Novel hexa-armed and pyrene (Pyr) end-capped phosphazene dendrimer [N ₃P₃-(Pyr)₆] and star polymer with poly(ε-caprolactone) (PCL) arms [N₃P₃-(PCL-Pyr) ₆] were prepared via two series of reactions. In these series, core-first approach was used starting from a hexa-hydroxy functional phosphazene derivative (N₃P₃-(OH)₆). It was used as an initiator in the ring-opening polymerization of ε-caprolactone to prepare a hexa-armed PCL star polymer (N₃P₃-(PCL-OH)₆). Hydroxyl functionalities of N₃P₃-(OH)₆ and N₃P₃-(PCL-OH)₆ were then successfully converted into bromide and azide, in turn. Further end-group modifications of azide functional dendrimer precursor (N₃P₃-(N₃) ₆) and star polymer (N₃P₃-(PCL-N ₃)₆) were achieved quantitatively via the Cu(I) catalyzed click reaction between azide functional groups and 1-ethynyl pyrene in the final step. Moreover, the pyrene end-capped phosphazene dendrimer and star polymer were used in noncovalent functionalization of multiwalled carbon nanotubes.

Full text of DOI:10.1002/pola.24756

Synthesis of Pyrene End-Capped A6 Dendrimer and Star Polymer with
Phosphazene Core via ‘‘Click Chemistry’’
MESUT GORUR, FARUK YILMAZ, ADEM KILIC, ZEYNEP M. SAHIN, ALI DEMIRCI
Department of Chemistry, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey
Received 6 January 2011; accepted 27 April 2011
DOI: 10.1002/pola.24756
Published online 18 May 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Novel hexa-armed and pyrene (Pyr) end-capped
phosphazene dendrimer [N3P3-(Pyr)6] and star polymer with
poly(e-caprolactone) (PCL) arms [N3P3-(PCL-Pyr)6] were pre-
pared via two series of reactions. In these series, core-first
approach was used starting from a hexa-hydroxy functional
phosphazene derivative (N3P3-(OH)6). It was used as an initiator
in the ring-opening polymerization of e-caprolactone to prepare
a hexa-armed PCL star polymer (N3P3-(PCL-OH)6). Hydroxyl
functionalities of N3P3-(OH)6 and N3P3-(PCL-OH)6 were then
successfully converted into bromide and azide, in turn. Further
end-group modifications of azide functional dendrimer precur-
sor (N3P3-(N3)6) and star polymer (N3P3-(PCL-N3)6) were
achieved quantitatively via the Cu(I) catalyzed click reaction
between azide functional groups and 1-ethynyl pyrene in the
final step. Moreover, the pyrene end-capped phosphazene
dendrimer and star polymer were used in noncovalent func-
C
V
tionalization of multiwalled carbon nanotubes.
2011 Wiley
Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 3193–
3206, 2011
KEYWORDS: click chemistry; dendrimers; noncovalent functional-
ization; polyphosphazenes; pyrene; star polymers
INTRODUCTION Phosphazenes, having the repeating unit
A[N¼¼PR2]A with trivalent nitrogen and pentavalent phos-
phorus atoms, are in the form of linear short-chain, cyclic, or
high-molecular weight polymers and constitute one of the
most important compound families in inorganic chemistry.
When ‘‘R’’ is the halogen moiety, it can be substituted swim-
mingly by alkoxy, aryloxy, or amino groups via nucleophilic
substitution reactions.1 Linear high polymers are the most
widely studied phosphazene macromolecules2; besides cyclo-
linear,3 star,4 dendrimeric,5 and cyclomatrix6 phosphonitrilic
macromolecules have also been synthesized. Moreover, cyclic
phosphazenes substituted with organic compounds were
attached as pendant groups to organic polymers.7 Depending
on the nature of the substituents, small-molecule phospha-
zene derivatives and macromolecules display distinctive
properties suitable for wide range of applications; such as,
light emitting diodes,3d,8 advanced elastomers,9 flame retard-
ant materials,10 biodegradable polymers,11 biologically inert
polymers,12 anticancer agents,13 tissue engineering,14 ion
conductive layers,4b,15 fuel cell membranes,16 nonlinear
optics,17 liquid crystalline materials,18 hydraulic fluids,19 and
lubricants.20 Moreover, phosphazene derivatives show high
chemical and thermal stabilities. Thus, phosphonitrilic chlo-
ride trimer can function as a useful platform for the synthe-
sis of variety of molecules and star polymers.
unique shape and possible processing advantages resulting
from their compact structure.21 The branched structure of
the star polymers resulted in lower solution and melt viscos-
ities compared with those of their linear counterparts of the
same molecular weight.22 Moreover, their higher degree of
end-group functionalization makes them convenient for
many specialized applications.23 There are two major
approaches for the synthesis of star-shaped polymers: Core-
first (divergent) and arm-first (convergent). The core-first
method requires a multifunctional initiator molecule, and the
number of arms of the star polymer is defined by the num-
ber of initiating functional groups on the initiator.24 On the
contrary, the arm-first method involves the use of previously
synthesized polymer arms and a multifunctional coupling or
terminating agent.25
The functional group modification of the polymers with a
high conversion rate subsequent to the accomplishment of
polymerization is a very important task in macromolecular
science. In this connection, ‘‘click chemistry’’ technique is a
versatile tool to carry out reactions in quantitative yields
without side reactions and time-consuming purification proc-
esses.26 The method invented by Huisgen, which includes
thermally induced 1,3-dipolar cycloaddition reaction between
alkynes and azides, is accepted as the premier example of
‘‘click’’ chemistry.27 However, Huisgen-type cycloadditon
reactions generally give mixtures of 1,4- and 1,5-triazole ste-
reoisomers (nearly 1:1). Later, Sharpless28 developed this
method and used Cu(I) salt catalysts to promote the reaction
Materials with controlled molecular architecture and compo-
sition have been the focus of polymer research for many
years. Star-shaped polymers intrigued scientists due to their
Additional Supporting Information may be found in the online version of this article. Correspondence to: F. Yilmaz (E-mail: fyilmaz@gyte.edu.tr)
C
V
Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 3193–3206 (2011) 2011 Wiley Periodicals, Inc.
PHOSPHAZENE DENDRIMER AND STAR POLYMER WITH PYRENE END-GROUPS, GORUR ET AL.
3193

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
between a nonactivated alkyne and an alkyl or aryl azide
giving substantially high yields at moderate temperature
with superior regioselectivity (1,4-triazole formation). More-
over, Sharpless-type click reactions are highly tolerant to the
functional groups and can be conducted in a variety of
solvents and solvent mixtures without compromising the
efficiency.4a,29
stored over dry 4-Å molecular sieves. Hexachlorocyclotri-
phosphazene (N3P3Cl6; Alfa Aesar; 98%) was purified by
fractional crystallization from dry hexane. Tetrahydrofuran
(THF; Merck; 99.8%) was dried by refluxing over Na-K alloy
and then distilled in a dry argon atmosphere. Sodium
hydride (Merck, 60% in mineral oil) was washed with dry
hexane just before use. Triethylamine (TEA, ꢁ99.5%) was
obtained from Fluka, dried over CaH2, and stored over 3-Å
molecular sieves. Methanol (Merck, 99.9%) was dried over
activated 4-Å molecular sieves and distilled before using.
Dichloromethane (DCM, Merck, 99.8%) was dried over phos-
phorus pentoxide. Dimethylformamide (DMF; Merck; 99.8%)
was both dried and stored over activated 4-Å sieves. 4-
Hydroxybenzaldehyde (Alfa Aesar; 98%), tetraethylammo-
nium bromide (Aldrich; 99%), sodium borohydride (NaBH4;
Alfa Aesar; 98%), tin(II) 2-ethylhexanoate (Sn(Oct)2; Aldrich;
95%), 4-bromobutyryl chloride (Aldrich; 98%), sodium
azide (Sigma Aldrich; 99.5%), 1-ethynyl pyrene (Alfa Aesar;
96%), N,N,N0,N00,N00-pentamethyldiethylenetriamine (PMDETA;
Aldrich; 99%), copper(I) bromide (Sigma-Aldrich; 98%), and
multiwalled CNT (Aldrich, 95%, diameter: 20–50 nm, length:
5–20 lm) were used as purchased without further purifica-
tion. Azido functional Merrifield resin was prepared accord-
ing to the literature.41
In the last two decades, pyrene containing polymers have
gained great deal of attention for their potential use in
fluorescent probes,30 because pyrene has long fluorescence
lifetime, ability to form excimers, and a well-defined fluores-
cence spectrum, which is sensitive to the polarity of the
environment.31 Moreover, pyrene functional materials have
been used in light-emitting diodes8b,32 and organic field
effect transistors.33
Carbon nanotubes (CNTs) are produced in the form of bundles
of intimately entangled long tubes, interacting by van der
Waals attraction. These characteristics are responsible for
their insolubility and weak dispersibility in common solvents
and matrices.34 On the other hand, dispersion of CNTs is nec-
essary to use them in a technical application. A variety of
methods have been used to disperse CNTs, including chemical
modification,35 covalent attachment of solubilizing compo-
nents (monomers, oligomers or polymers),36 adsorption of
charged surfactants,37 wrapping with polymers,38 and com-
plexation with p–p interactions.34,39 The main shortcoming of
the CNTs, which are chemically functionalized or covalently
modified is the generation of sp3 centers and the change in
their electronic structure.39b Pyrene is electronically analo-
gous to graphite and can be regarded as cut piece from a
graphite sheet.40 Besides, CNT-CNT and pyrene-CNT interac-
tions are very similar in their nature.39b Thus, large aromatic
compounds such as pyrene have been utilized in the noncova-
lent functionalization of single-walled or multiwalled CNTs.
Measurements
1H and 31P NMR spectra were recorded in CDCl3 and DMSO-
d6 solutions on a Varian Unity INOVA 500-MHz (202 MHz for
31P) spectrometer using tetramethylsilane (TMS) as an inter-
1
nal reference for H NMR and 85% H3PO4 as an external ref-
erence for 31P at 25 ꢀC. Mass spectra were acquired on
Bruker MicroTOF LC–MS (electron spray ionization) and
Bruker microflex LT MALDI-TOF MS (nitrogen UV-laser ioni-
zation at 337 nm) spectrometers. ATR FTIR spectra were
taken on Perkin-Elmer Paragon 1000 spectrometer equipped
with PIKE MIRacleTM Diamond ATR, and the results were
uncorrected. UV–vis and fluorescence spectra were recorded
using 1-cm path length cuvettes on Shimadzu 2001UV and
Varian Eclipse spectrophotometers, respectively. Raman spec-
troscopic investigations were carried out using a Renishaw
inVia Reflex Raman microscopy system using the 514-nm
line of an argon ion laser for excitation. Average molecular
weights and molecular weight distributions of the polymers
were determined on an Agilent GPC Instrument (Model
1100) consisting of a pump, a refractive index detector and
two Waters Styragel columns (HR 5E and HR 4E) and using
THF as eluent at a flow rate of 0.5 mL minꢂ1 at 23ꢀC and
toluene as an internal standard. Melting points (Tm) and
crystallization temperatures (Tc) of the polymers were meas-
ured on DSC 822 (Mettler Toledo). Differential scanning calo-
rimetry analysis (DSC) was performed on a DSC 822 (Mettler
Toledo) thermal analysis system under nitrogen flow (10 mL
minꢂ1). All the samples were firstꢂh1eated from 0 to 100 ꢀC
with a heating rate of 10 ꢀC min and held for 5 min to
erase the thermal history, then cooled to 0 ꢀC at 10 ꢀC
minꢂ1, and finally heated to 100 ꢀC at 10 ꢀC minꢂ1. Ther-
mogravimetric analysis (TGA) was performed on a TGA/
SDTA 851 (Mettler Toledo) thermogravimetric analyzer with
In this study, novel hexa-armed pyrene end-capped phospha-
zene-cored dendrimer and star polymer were prepared in a
two series of reactions using core-first approach starting from
a hexa-hydroxy functional phosphazene derivative (N3P3-
(OH)6). In the first step, N3P3-(OH)6 was used as an initiator in
the ROP of e-caprolactone (e-CL) using Sn(Oct)2 as a catalyst
and a hexa hydroxy-functional star-shaped poly(e-caprolac-
tone) (PCL) having a phosphazene core (N3P3-(PCL-OH)6) was
prepared. The end groups of the obtained star polymer and
N3P3-(OH)6 were chemically modified in two different
synthetic routes yielding N3P3-(PCL-N3)6 and N3P3-(N3)6,
respectively. Finally, their azide end groups were quantita-
tively functionalized with pyrene via Cu(I) catalyzed 1,3-dipo-
lar cycloaddition reaction. Subsequently, the pyrene functional
phosphazene dendrimer and star polymer were used in non-
covalent grafting of multiwalled CNT (MWCNT) by taking the
advantage of electronic similarities.
EXPERIMENTAL
Materials
e-CL (Aldrich; 97%) was dried over calcium hydride (CaH2;
Sigma-Aldrich; 95%) distilled at 97 ꢀC under 10 mmHg and
3194
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
a heating rate of 10 ꢀC minꢂ1 from room temperature to
700 C under nitrogen atmosphere.
Synthesis of Pyrene-Functionalized A6 Dendrimer
(N3P3-(Pyr)6) (4)
ꢀ
1,1,3,3,5,5-Hexakis(4-(azidomethyl)phenoxy)cyclotriphosphos-
phazene (3; 0.25 g, 0.244 mmol) and 1-ethynyl pyrene
(0.365 g, 1.612 mmol) was dissolved in degassed DMF (5 mL)
under argon atmosphere. PMDETA (0.762 g, 4.395 mmol) was
added, and the solution was gently purged with argon for
5 min. Then, copper(I) bromide (0.631 g, 4.395 mmol) was
added to the reaction mixture, and it was again degassed by
purging with argon for 5 min. After the reaction mixture was
stirred at room temperature for 48 h, azido functional Merri-
field resin (0.2 g) was added; the suspension was purged
with argon for 5 min, and it was left stirring for additional
72 h at ambient temperature. The insoluble resin was
removed by gravity filtration through a paper filter (Schleicher
and Schuell, blue ribbon, 2 lm) and washed with DCM. Then,
the solution was diluted to 100 mL with DCM and washed
with brine (50 mL) and water (50 mL). After the collected
organic phases were combined and dried with MgSO4, the
solvent was removed by rotary evaporator, and dark yellow
product was obtained.
Synthesis of N3P3-(OH)6 (1)
1,1,3,3,5,5-Hexakis(4-(hydroxymethyl)phenoxy)cyclotriphos-
phosphazene (1) was synthesized according to the literature
method.4a
Yield: 90%. mp: 218–219 ꢀC. MS (m/z): calcd for
C
42H42N3O12P3, 873.72; found, 874.37 [MþH]þ. FTIR (ATR,
cmꢂ1): 3350 (broad, OH); 2908 and 2876 (CAH); 1256
and 1160 (P¼¼N); 952 (PAOAC); 1606 (CCPh). 1H NMR
´
(500 MHz, DMSO-d6, d, ppm): 7.20, 7.19 and 6.81, 6.80 (AA
´
BB pattern, 24H, C6H4); 5.22 (t, 6H, OH); 4.47 (d, 12H, CH2).
31P NMR (202 MHz, DMSO-d6, d, ppm): 9.93 (s).
Synthesis of N3P3-(Br)6 (2)
1,1,3,3,5,5-Hexakis(4-(hydroxymethyl)phenoxy)cyclotriphos-
phosphazene (1; 2 g, 2.289 mmol) was dissolved in anhy-
drous THF (50 mL) in a two-necked flask equipped with a
magnetic stirrer and a rubber septum under inert argon
atmosphere. The reaction vessel was cooled to ꢂ10 ꢀC by
using ice-salt mixture. Phosphorus tribromide (PBr3; 4.957 g,
18.313 mmol) was added dropwise via syringe through the
septum to the mixture over a 15-min period, and the mix-
ture was stirred at room temperature for 24 h. The solution
was concentrated to 10 mL by rotary evaporation and pre-
cipitated into 400 mL of deionized water. A beige precipitate
was collected and washed with water. After it was dried
under reduced pressure at room temperature, the obtained
material was dissolved in DCM and eluted from a silica gel
column with the same solvent.
ꢀ
Yield: 0.523 g (90%). Tg: 140 C (4 showed only Tg with no
observable melting point, which confirms amorphous nature
of 48b). FTIR (ATR, cmꢂ1): 2920 and 2851 (CH); 1605
1
(CCPh); 1262 and 1162 (P¼¼N); 960 (PAOAC). H NMR (500
MHz, CDCl3, d, ppm): 8.42 (d, 6H, C2HN3 (triazole ring));
7.95–7.77 (m, 54 H, C16H9 (Pyr)); 7.10, 7.08 and 6.85, 6.83
(AA BB pattern, 24H, C6H4); 5.40 (s, 12H, C6H4CH2). 31P
NMR (202 MHz, CDCl3, d, ppm) ¼ 8.80 (s).
´
´
Synthesis of Hydroxy End-Functional Star-Shaped
Hexa-Armed PCL (N3P3-(PCL-OH)6) (P1)
N3P3-(PCL-OH)6 (P1) was synthesized according to the
literature method.4a
Yield: 1.776 g (62%). mp: 147–149 ꢀC. MS (m/z): calcd for
C
42H36Br6N3O6P3, 1251.10; found, 1290.2 [MþK]þ. FTIR (ATR,
cmꢂ1): 2908 and 2876 (CH); 1603 (CCPh); 1264 and
1160 (P¼¼N); 952 (PAOAC). 1H NMR (500 MHz, CDCl3,
Yield: 3.961 g (95%). FTIR (ATR, cmꢂ1): 3350 (OAH, broad);
2944 and 2866 (CAH); 1722 (C¼¼O); 1468 (CAH); 1240
((C¼¼O)AO); 1292 and 1172 (P¼¼N); 1046 (CAOAC);
956 (PAOAC). 1H NMR (CDCl3, d, ppm): 7.18, 7.16 and
´
´
d, ppm): 7.25, 7.23 and 6.91, 6.89 (AA BB pattern, 24H, C6H4);
4.48 (s, 12H, CH2). 31P NMR (202 MHz, CDCl3, d, ppm): 9.67 (s).
Synthesis of N3P3-(N3)6 (3)
´
´
6.92, 6.90 (AA BB pattern, C6H4); 5.05 (s, C6H4CH2O);
4.06 (m, CH2O(C¼¼O) in PCL); 3.65 (t, terminal CH2OH
in PCL); 2.30 (m, O(C¼¼O)CH2 in PCL), 1.63 (m,
O(C¼¼O)CH2CH2CH2CH2CH2O(C¼¼O) in PCL), 1.39 (m,
O(C¼¼O)CH2CH2CH2CH2CH2O(C¼¼O) in PCL).
1,1,3,3,5,5-Hexakis(4-(bromomethyl)phenoxy)cyclotriphos-
phosphazene (2; 1.5 g, 1.199 mmol) and sodium azide
(0.779 g, 11.989 mmol) was dissolved in anhydrous DMF
(10 mꢀL) under inert argon atmosphere and stirred for 24 h
at 90 C. After it was cooled to room temperature, the reac-
tion mixture was transferred into water (300 mL) and
extracted with diethylether (2 ꢃ 50 mL), the organic phases
are collected and dried with magnesium sulphate (MgSO4).
Consequently, the solvent was removed by rotary evaporator,
and the compound was isolated as a white solid by column
chromatography on silica gel using ethyl acetate/hexane
(3:7) as eluent.
Synthesis of Bromide End-Functional Star-Shaped
Hexa-Armed PCL (N3P3-(PCL-Br)6) (P2)
N3P3-(PCL-OH)6 (P1; 3 g, 0.216 mmol) and triethylamine
(0.262 g, 2.593 mmol) were dissolved in DCM (20 mL) in a
two-necked flask equipped with magnetic stirring bar and
pressure-equalized dropping funnel under argon atmosphere.
Then, the reaction mixture was cooled to ꢂ10 ꢀC using
ice-NaCl mixture. 4-Bromobutyryl chloride (0.481 g,
2.593 mmol) was dissolved in DCM (5 mL) and added drop-
wise to the miꢀxture in 20 min. The reaction mixture was
stirred at ꢂ10 C for 2 h; after it was allowed to attain room
temperature, stirring was continued for additional 46 h at
that temperature. The reaction mixture was diluted with
DCM (100 mL) and washed with saturated NaHCO3 solution
Yield: 1.105 g (90%). mp: 125–126 ꢀC. MS (m/z): calcd for
C
42H36N21O6P3, 1023.79; found, 1024.37 [Mþ1]þ. FTIR (ATR,
cmꢂ1): 2923 and 2859 (CH); 2091 (N3); 1606 (CCPh); 1263
and 1157 (P¼¼N); 960 (PAOAC). 1H NMR (500 MHz, CDCl3,
´
´
d, ppm): 7.16, 7.14 and 6.97, 6.96 (AA BB pattern, 24H,
C6H4); 4.31 (s, 12H, CH2). 31P NMR (202 MHz, CDCl3,
d, ppm): 8.41 (s).
PHOSPHAZENE DENDRIMER AND STAR POLYMER WITH PYRENE END-GROUPS, GORUR ET AL.
3195

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
(2 ꢃ 25 mL) and deionized water (2 ꢃ 25 mL). After the
organic layers were combined and dried with MgSO4, the sol-
vent was concentrated to about 5 mL by rotary evaporator,
and the bromide end-functional star polymer was isolated by
precipitating in cold methanol. The recovered polymer was
dried under reduced pressure at ambient temperature until
a constant weight.
Subsequent to collecting and drying of organic phases with
MgSO4, the solvent was concentrated to 5 mL by rotary evapo-
rator, and the pyrene end-functional star polymer was isolated
as a yellow product by precipitating from cold hexane.
Yield: 1.017 g (93%). IR (ATR, cmꢂ1): 2945 and 2866
(CAH); 1721 (C¼¼O); 1470 (CAH); 1239 ((C¼¼O)AO);
1294 and 1175 (P¼¼N); 1046 (CAOAC); 958 (PAOAC). 1H
NMR (500 MHz, CDCl3, d, ppm): 8.72 (d, C2HN3 (triazole
ring); 8.25–8.01 (m, C16H9 (Pyr)); 7.17, 7.16 and 6.92,
Yield: 3.033 g (95%). FTIR (ATR, cmꢂ1): 2944 and 2866
(CAH); 1722 (C¼¼O); 1470 (CAH); 1240 ((C¼¼O)AO); 1294
and 1172 (P¼¼N); 1046 (CAOAC); 958 (PAOAC). 1H
´
´
6.90 (AA BB pattern, C6H4); 5.04 (s, C6H4CH2O); 4.63
(m, terminal O(C¼¼O)CH2CH2CH2-triazole); 4.11 (m, terminal
CH2O(C¼¼O)(CH2)3-triazole); 4.06 (m, CH2O(C¼¼O) in PCL);
2.50 (m, terminal O(C¼¼O)CH2CH2CH2-triazole); 2.40 (m, ter-
minal O(C¼¼O)CH2CH2CH2-triazole); 2.30 (m, O(C¼¼O)CH2 in
PCL); 1.63 (m, O(C¼¼O)CH2CH2CH2CH2CH2O(C¼¼O) in PCL);
1.38 (m, O(C¼¼O)CH2CH2CH2CH2CH2O(C¼¼O) in PCL).
´
´
NMR (CDCl3, d, ppm): 7.18, 7.16 and 6.92, 6.90 (AA BB
pattern, C6H4); 5.05 (s, C6H4CH2O); 4.06 (m, CH2O(CAO)
and terminal CH2O(CAO)(CH2)3Br in PCL); 3.46 (m, terminal
O(C¼¼O)CH2CH2CH2Br);
2.49
(m,
terminal
O(C¼¼O)
CH2CH2CH2Br); 2.30 (m, O(C¼¼O)CH2 in PCL); 2.17 (m, ter-
minal O(C¼¼O)CH2CH2CH2Br); 1.65 (m, O(C¼¼O)CH2CH2CH2
CH2CH2O(C¼¼O) in PCL); 1.38 (m, O(C¼¼O)CH2CH2
CH2CH2CH2O(C¼¼O) in PCL).
Noncovalent Grafting of MWCNT with Pyrene
Groups of 4 and P4
Synthesis of Azide End-Functional Star-Shaped
Functionalization of MWCNT was accomplished by mixing
3.00 mg of MWCNT with 60.00 mg of pyrene-ended phos-
phazene dendrimer and star polymer in separate vials in
15 mL of THF at room temperature for 24 h. The functional-
ized MWCNT were filtered through a filter paper (Schleicher
and Schuell, blue ribbon, 2 lm) and washed with THF. The
collected functionalized MWCNT were ultrasonicated in THF
for 10 min, filtered, and washed with THF again to remove
any unbound polymers. The obtained MWCNT complexes
were dried under reduced pressure at room temperature.
Pyrene functional macromolecule contents of the MWCNT
complexes were evaluated via TGA analysis.
Hexa-Armed PCL (N3P3-(PCL-N3)6) (P3)
N3P3-(PCL-Br)6 (P2; 2 g, 0.135 mmol) was dissolved in anhy-
drous DMF (15 mL) in a single-necked flask under argon.
Sodium azide (0.158 g, 2.436 mmol) was added; the reactiꢀon
mixture was purged with argon and stirred for 24 h at 45 C.
After the reaction was completed, the content of the flask was
transferred to a separating funnel with DCM (100 mL), and
the organic layer was washed successively with brine (2 ꢃ
50 mL) and deionized water (50 mL). The organic layers were
combined and dried with anhydrous MgSO4. The solvent was
concentrated to about
5 mL and azide end-functional
star polymer was isolated by precipitating in cold methanol.
The obtained polymer was dried under reduced pressure at
ambient temperature until a constant weight.
RESULTS AND DISCUSSION
Yield: 1.871 g (95%). FTIR (ATR, cmꢂ1): 2944 and 2866
(CAH); 2098 (N3); 1720 (C¼¼O); 1470 (C-H); 1240
((C¼¼O)AO); 1294 and 1176 (P¼¼N); 1046 (CAOAC); 958
The Synthesis of Pyrene End-Capped A6 Phosphazene
Dendrimer
The novel hexa-armed phosphazene dendrimer with pyrene
end-groups were prepared via a series of reactions starting
1
(PAOAC). H NMR (CDCl3, d, ppm): 7.18, 7.16 and 6.92, 6.90
´
´
(AA BB pattern, C6H4); 5.05 (s, C6H4CH2O); 4.06 (m,
CH2O(C¼¼O) and terminal CH2O(C¼¼O)(CH2)3N3 in PCL); 3.35
(m, terminal O(C¼¼O)CH2CH2CH2N3); 2.40 (m, terminal
O(C¼¼O)CH2CH2CH2N3); 2.30 (m, O(C¼¼O)CH2 in PCL);
1.90 (m, terminal O(C¼¼O)CH2CH2CH2N3); 1.65 (m,
O(C¼¼O)CH2CH2CH2CH2CH2O(C¼¼O) in PCL); 1.38 (m,
O(C¼¼O)CH2CH2CH2CH2CH2O(C¼¼O) in PCL).
from
hexakis(p-(hydroxymethyl)phenoxy)cyclotriphospha-
zene (1) utilizing core-first approach in a three-step proce-
dure (Scheme 1). In the first step, the synthesis of 2 with
bromide functionality is accomplished by means of the reac-
tion of 1 with PBr3. Azidification of 2 provided a phospha-
zene derivative with azide end-functional groups (3). In the
final step, the attachment of pyrene units to the ends of each
arm of 3 was succeeded by taking the advantage of the
Cu(I)-catalyzed ‘‘click’’ reaction between ACBC (alkyne) and
AN¼¼N¼¼N (azide) functional groups of 1-ethynyl pyrene and
phosphazene derivative, respectively. Following the comple-
tion of the reaction, the excess of the alkyne functional py-
rene was successfully scavenged by a click reaction with az-
ide-functional partially (2%) crosslinked polystyrene resin
(Merrifield resin). The synthetic attraction of the ‘‘click’’
chemistry arises from its versatility, reliability and specificity.
Because, this synthetic technique requires no protecting
groups, proceeds with high conversion rates, and has exclu-
sive selectivity for the 1,4-triazole formation.
Synthesis of Pyrene End-Functional Star-Shaped
Hexa-Armed PCL (N3P3-(PCL-Pyr)6) (P4)
P3 (1 g, 0.069 mmol) and 1-ethynyl pyrene (0.112 g,
0.495 mmol) were dissolved in deaerated DMF (5 mL) under
inert argon atmosphere. PMDETA (0.214 g, 1.237 mmol) was
added, and the solution was gently purged with argon for
5 min. Then, copper(I) bromide (0.177 g, 1.237 mmol) was
added to the reaction mixture, and it was again deaerated by
purging with argon for additional 5 min. After the reaction
mixture was stirred at room temperature for 48 h, the
solution was diluted to 150 mL with DCM and washed
successively with brine (2 ꢃ 50 mL) and water (50 mL).
3196
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
SCHEME 1 The synthesis of pyrene functionalized phosphazene derivative (A6 dendrimer).
The structural characterizations of the phosphazene deriva-
tives were investigated by FTIR and 1H NMR spectroscopic
methods. In the FTIR spectra [Fig. 1(a)], the hydroxyl peak
of 1 observed at 3350 cmꢂ1 disappeared on bromination
with PBr3 [Fig. 1(b)]. After azidification, the peak indicating
the presence of azide functional group was observed at
2091 cmꢂ1 [Fig. 1(c)]. The absence of azide peak at
2091 cmꢂ1 in the FTIR spectrum of 4 [Fig. 1(d)] obviously
confirmed the completeness of the click reaction.
In the 1H NMR characterization of 1 and 2, dimethyl sulfox-
ide and chloroform were used as the deuterated solvents,
respectively. The chemical shift values for a specific proton
in different deuterated NMR solvents may not be the
same.42 Therefore, the expected chemical shift difference
could not be observed for the methylene protons (Hc) of
2 [Fig. 2(b)]. On the other hand, the signal of Hc protons
(d ¼ 4.48 ppm) was observed as doublet due to coupling
with the hydroxyl proton (Hd; J ¼ 5 Hz) in Figure 2(a) and
the doublet methylene signal changed into singlet on
FIGURE 1 FTIR spectra of (a) N3P3-(OH)6 (1), (b) N3P3-(Br)6 (2),
(c) N3P3-(N3)6 (3), and (d) N3P3-(Pyr)6 (4). [Color figure can be
viewed in the online issue, which is available at wileyonline
library.com.]
PHOSPHAZENE DENDRIMER AND STAR POLYMER WITH PYRENE END-GROUPS, GORUR ET AL.
3197

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 2 1H NMR spectra of (a) N3P3-(OH)6 (1), (b) N3P3-(Br)6 (2), (c) N3P3-(N3)6 (3), and (d) N3P3-(Pyr)6 (4).
bromination [Fig. 2(b)]. Moreover, the triplet signal observed
at 5.22 ppm in Figure 2(a), which is attributed to the
hydroxyl protons of 1, disappeared completely in
Figure 2(b) as a result of bromination. The consistency
between measured and theoretical integral ratio of the
aromatic Ha protons to that of methylene protons (Hc) of
2 (Ha/Hc, measured ¼ 0.96, the theoretical value is 1.00) is a
further proof for complete functionalization.
Cu(I)-catalyzed 1,3-dipolar cycloaddition reaction between
azide functional groups of 3 and 1-ethynyl pyrene in deoxy-
genated DMF by using Cu(I)Br/PMDETA catalyst/ligand sys-
tem provided pyrene end-capped phosphazene dendrimer
(4) with a substantial yield at room temperature. The com-
1
parison of the H NMR spectrum of 4 [Fig. 2(d)] to that of 3
[Fig. 2(c)] clearly supports this conclusion. The methylene
peak depicted at 4.31 ppm in the 1H NMR spectrum of
the precursor compound (3) drifted to low magnetic field by
As for the 1H NMR spectrum of 3, the singlet peak at 4.48
ppm in Figure 2(b), which is attributed to the methylene
protons (Hc) of 2, shifted to upfield by 0.17 ppm after azidi-
fication [Fig. 2(c)]. This result explicitly confirmed that the
bromide functional groups were quantitatively converted
into azide functional groups.
1.09 ppm upon Cu(I)-catalyzed [3
þ
2] cycloaddition
reaction. The new peaks belonging to aromatic pyrene and
triazole methyne protons of 4 are presented at 7.95–7.77
and 8.42, respectively [Fig 2(d)]. The integral ratio between
aromatic pyrene protons and the methylene proton (Hc)
signals is 4.53, which is very close to the theoretical value
3198
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
SCHEME 2 The synthesis of pyrene end-capped e-caprolactone star polymer.
(HPyr/Hc ¼ 4.50). All the obtained data indicate that azide
groups of 3 were substantially converted into pyrene as a
result of successful ‘‘click’’ reaction.
1H NMR spectroscopic techniques and GPC studies. In the
FTIR spectrum of P1 [Fig. 3(a)], the intensity of hydroxyl
groups at 3350 cmꢂ1 are considerably weakened after ROP
with respect to that of the initiator (1) in Figure 1(a). The
reaction of P1 with 4-bromobutyryl chloride generated the
bromide end-capped star polymer P2. In the FTIR spectrum
of P2 [Fig. 3(b)], no hydroxyl peak was detected between
3300 and 3600 cmꢂ1 indicating that hydroxyl functional
groups of P1 were converted into bromide. The detection of
the peak at 2098 cmꢂ1 in the FTIR spectrum of 3 [Fig. 3(c)]
designates the presence azide functional groups. The disap-
pearance of the azide signal at 2098 cmꢂ1 was a clear
indication of the successful click reaction [Fig. 3(d)]. Also,
the strong and sharp peak at 1722 cmꢂ1 in FTIR spectra of
the star polymers was attributed to the carbonyl groups in
the repeating units of the PCL chains.
The Synthesis of Hexa-Armed and Pyrene End-Capped
PCL Star Polymer with Phosphazene Core
The novel hexa-armed pyrene end-capped PCL star polymer
emanating from a phosphazene core was synthesized in a
four-step procedure (Scheme 2). Ring-opening polymeriza-
tion (ROP) and Cu(I)-catalyzed ‘‘click’’ reaction was used in
the first and final steps, respectively. Initially, starting from 1
as a multifunctional initiator, hexa-armed PCL star polymer
with hydroxyl end-groups P1 was synthesized via ROP of
e-CL catalyzed by tin(II) 2-ethylhexanoate at 115 ꢀC in bulk.
The further end-group modification of P1 was accomplished
via derivatization of the hydroxyl-functional end-groups. The
incorporation of bromide functional groups at each chain
ends of P1 was successfully achieved by the esterification
reaction with 4-bromobutyryl chloride giving P2. The com-
plete azidification of P2 and then the Cu(I)-catalyzed ‘‘click’’
reaction between the resultant polymer (P3) and 1-ethynyl
pyrene provided pyrene end-functional star polymer with a
phosphazene core (P4). The results of the star polymers are
given in Table 1.
1
The comparison of H NMR spectra of 1 [Fig. 2(a)] to that of
P1 [Fig. 4(a)] proves the successful ROP of e-CL using 1 as
an initiator. The triplet signal at 5.22 ppm ascribed to the
hydroxyl protons of the initiator (1) disappeared and the
peaks of the methylene protons next to the benzene ring
(Hc) shifted to low magnetic field and detected as a singlet
at 5.05 ppm after polymerization. The methylene protons
(Hd, He, Hf, and Hg) in the repeating units of PCL arms gave
resonances at 2.30, 1.63, 1.39, and 4.06 ppm, respectively.
The triplet peak at 3.65 ppm, corresponding to the terminal
The structures of the star-shaped polymers PCL polymers
with different end groups were characterized by FTIR and
PHOSPHAZENE DENDRIMER AND STAR POLYMER WITH PYRENE END-GROUPS, GORUR ET AL.
3199

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Results of the Phosphazene-Cored Star Polymers with
[Fig. 4(c)] provides an evidence for quantitative ‘‘Click reac-
tion.’’ The methylene protons of the terminal butyryl group
was deshielded by the triazole and pyrene groups and there-
fore shifted to lower magnetic fields. The highest chemical
shift difference was observed for Hl protons by 1.11 ppm,
the other chemical shift differences were 0.10 and 0.50 ppm
for Hi and Hk protons, respectively. Moreover, the signals of
methyne protons in the triazole rings (8.72 ppm) and aro-
matic pyrene protons (8.01–8.25 ppm) were also observed
Different Chain End Functionalities
End-Functional
a
b
b
Polymers Mn,NMR
Mn,GPC
Mw/Mn
Groups
P1c
P2
13,886
14,780
11,400
12,300
1.34
1.36
AOH
P3
P4
14,553
15,911
12,000
13,200
1.35
1.37
1
in the H NMR spectrum of P4 [Fig. 4(d)].
31P NMR spectra of 1–4 and P4 contain only one resonance
peak of the phosphorus atoms, which serves as an additional
evidence for the successful synthesis and complete function-
alization (see Figure S1 and S2 in Supporting Information).
a
Mn,NMR was determined by 1H NMR spectroscopy. Degree of polymer-
ization was calculated by comparing the integral ratio of methylene pro-
tons next to the benzene ring (AC6H4CH2OA, 5.05 ppm (Hc in Fig. 4) to
that of methylene protons of main PCL (ACH2CH2CH2CH2CH2O(C¼O)A)
at 4.06 ppm (Hg in Fig. 4). Average number of e-CL per PCL arm was
found to be 19.
The number- and weight-average molecular weights of the
synthesized star polymers along with their polydisperity
index values were determined by gel permeation chromatog-
raphy and related data are given in Table 1. The symmetrical
and unimodal elution peaks and low polydispersity values of
the star polymers indicated that the purified products con-
tained the targeted star polymers only (Supporting Informa-
tion Fig. S3). The 1H NMR spectra of the star polymers per-
mitted us to calculate the composition and Mn, NMR values as
well. The degree of polymerization (DPn) was found as 19
for each PCL arms of the star polymers, which are equal to
the integral ratios between methylene protons of PCL main
chain (ACH2CH2CH2CH2CH2O(C¼¼O)A) (Hg) at 4.06 ppm and
methylene protons next to the benzene ring (AC6H4CH2OA)
(Hc) at 5.05 ppm (Fig. 4). Thus, number-average molecular
b
Mn,GPC and Mw/Mn were determined by GPC analysis with polystyrene
standards. THF was used as eluent (RI detector).
c
T ¼ 115 ꢀC; bulk; [CL]/[SnOct2] ¼ 300; polymerization time ¼ 24 h.
methylene protons in the PCL arms, indicates that the arms
of P1 were capped with hydroxyl end groups. Moreover, the
experimental and theoretical integral ratios between the ter-
minal methylene protons (Hh) and methylene protons next
to the benzene ring (Hc) are nearly the same (Hh/Hc, experi-
¼ 1.01, the theoretical value should be 1.00). These
mental
results suggest that e-CL monomers were inserted into the
‘‘AC6H4CH2OAH’’ groups of the phosphazene derivative 1 via
selective acyl-oxygen cleavage of e-CL and that a phospha-
zene-cored star polymer with hydroxyl functional groups
was successfully synthesized.
1
weights by H NMR were calculated with the following equa-
tion: Mn,NMR ¼ 6 ꢃ [19 (DPn of each PCL arm) ꢃ 114.14] þ
MW of phosphazene core þ 6 ꢃ MW of chain end group. As
expected, the discrepancies between Mn,GPC and Mn,NMR val-
ues are due to the star-shaped structures of the synthesized
polymers. Because hydrodynamic volumes of the star-shaped
polymers are smaller than those of linear polystyrene stand-
ards with the same molecular weight, GPC analysis, which
utilizes refractive index (RI) or ultraviolet (UV) detection,
The chemical shift of the terminal methylene protons (d ¼
1
3.65 ppm) in the H NMR spectrum of P1 [Fig. 4(a)] drifted
to 4.06 ppm and was overlapped by the signal of Hg protons
in that of P2 [Fig. 4(b)] as a result of esterification reaction
with 4-bromobutyryl chloride. Some new peaks related to the
methylene protons of the terminal 4-bromobutyryl moiety
(Hi, Hk, and Hl) appeared at 2.49, 2.17, and 3.46 ppm, respec-
tively. Additionally, the integral ratios between the proton sig-
nal of the methylene group adjacent to the benzene ring and
those of the methylene groups in the terminal 4-bromobu-
tyryl moiety are 1.09 (Hc/Hi), 0.98 (Hc/Hk), and 1.08 (Hc/Hl).
These are very close to the theoretical values 1.00. These
results obviously show that all the hydroxyl end groups of the
P1 were quantitatively functionalized with bromide. After
azidification reaction, the signals of the methylene protons
(Hi, Hk, and Hl) in the terminal 4-azidobutyryl group shifted
to higher magnetic fields at 2.40, 1.90, and 3.35 ppm, respec-
tively, as seen in 1H NMR spectrum of P3 [Fig. 4(c)].
Cu(I)-catalyzed 1,3-cycloaddition reaction between azide
chain ends and 1-ethynyl pyrene in DMF using CuBr/
PMDETA catalyst/ligand system afforded pyrene end-capped
star polymer (P4) in a very high yield. The comparison of
the 1H NMR spectrum of P4 [Fig. 4(d)] with that of P3
FIGURE 3 FTIR spectra of (a) N3P3-(PCL-OH)6 (P1), (b) N3P3-(PCL-
Br)6 (P2), (c) N3P3-(PCL-N3)6 (P3), and (d) N3P3-(PCL-Pyr)6 (P4).
[Color figure can be viewed in the online issue, which is available
at wileyonlinelibrary.com.]
3200
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
FIGURE 4 1H NMR spectra of (a) N3P3-(PCL-OH)6 (P1), (b) N3P3-(PCL-Br)6 (P2), (c) N3P3-(PCL-N3)6 (P3), and (d) N3P3-(PCL-Pyr)6 (P4).
always underestimates the molecular weight star-shaped
polymers.43 Hence, the molecular weights of the star-shaped
polymers determined by 1H NMR are more reliable than
those obtained from GPC analysis.
Thermal Properties
DSC technique was used to determine the melting and crys-
tallization behaviors of the star polymers with different end-
functional groups. Figure 5 depicts the DSC curves of the
phosphazene-cored star polymers in the first heating run,
the cooling run and the second heating run. The DSC curve
of the linear PCL (LPCL), also included in Figure 5, provides
a comparison for characteristic transitions of semicrystalline
PCL polymers such as melting and crystallization. The data
FIGURE 5 DSC thermograms of the star-shaped polymers.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
in Table
2 demonstrate that the melting points (Tm),
PHOSPHAZENE DENDRIMER AND STAR POLYMER WITH PYRENE END-GROUPS, GORUR ET AL.
3201

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 Thermal Properties of the Star Polymers
Char
Tm,onset DHm1 Xc1
Tonset Tmax
(ꢀC)
yield
(%)f
c
d
Polymers
LPCLg
(ꢀC)a
(J/g)b (%)
(ꢀC)e
56.83
103.53 74.16 326.11 364.14 2.47
99.03 70.94 287.39 330.32 8.52
98.23 70.37 351.90 358.75 5.64
91.82 65.77 384.11 388.32 13.90
81.49 58.37 396.00 402.62 9.18
N3P3-(PCL-OH)6 52.07
N3P3-(PCL-Br)6 54.84
N3P3-(PCL-N3)6 55.77
N3P3-(PCL-Pyr)6 54.68
a
Tm,onset denotes the onset melting point of the polymers in the first
heating run of the DSC experiments.
b
DHm1 denotes the fusion enthalpy of the polymers in the first heating
run of the DSC experiments.
Xc1 ¼ DHm1/DHm0, DHm is the enthalpy of fusion for perfectly crystal-
c
0
line PCL. DHm ¼ 139.6 J/g.43(a)
0
d
FIGURE 6 TGA thermograms of the star-shaped polymers.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
Tonset is the onset decomposition temperature of the polymers in TGA
experiments.
e
Tmax is the temperature corresponding to the maximum rate of weight
loss in TGA experiments.
f
The percent of char yield at 700 ꢀC in TGA experiments.
g
LPCL was synthesized by using ethanol as an initiator (Mn,NMR ¼ 8200;
Br, N3, and Pyr groups induced a decrease in Xc1 and DHm1
values in the following order: P1 > P2 > P3 > P4.
M
w/Mn ¼ 1.28).
TGA experiments of the star polymers were peꢂrf1ormed
under nitrogen atmosphere at a rate of 10 ꢀC min from
room temperature to 700 ꢀC. Percent weight loss against
temperature curves of the star polymers are plotted in Fig-
ure 6, and data related to Tonset, Tmax, and percent char yield
are given in Table 2. Although the incorporation of phospha-
zene core remarkably increased char yield of the star-shaped
polymers, the increase in the number of PCL arms with
respect to LPCL, particularly in the number of OH end func-
tional groups, diminished the onset decomposition tempera-
ture (Tonset) and Tmax of N3P3-(PCL-OH)6. By comparing Tonset
and Tmax values of the star polymers, it can be obviously
concluded that modification of the thermally labile hydroxyl
crystallinities (Xc), and enthalpies of fusion (DHm) of all star
polymers are lower than that of LPCL. The lower crystallinity
of the star-shaped polymers with any functional groups can
be attributed to the disturbed crystallization rearrangement
of PCL arms mainly due to the highly branched framework.44
The arms of the star polymers were bound to a phosphazene
core, and therefore, chain movements were limited and crys-
tallizability of the star polymers declined. On the other hand,
such a constraint did not exist in LPCL. The temperature at
which the star polymers start to melt in the first heating
runs of the DSC experiments (Tm,onset) were close to each
other, whereas the conversion of OH-functional chain ends to
FIGURE 7 (a) Absorption spectra
of 1-ethynyl pyrene (1.473 ꢃ 10ꢂ5
mol Lꢂ1), N3P3-(Pyr)6 (4; 1.512 ꢃ
10ꢂ5 mol
L
ꢂ1), and N3P3-(PCL-
Pyr)6 (P4; 1.508 ꢃ 10ꢂ5 mol Lꢂ1);
(b) excitation spectra of 1-ethynyl
pyrene (1.473 ꢃ 10ꢂ7 mol Lꢂ1),
N3P3-(Pyr)6 (4; 1.512 ꢃ 10ꢂ7 mol
L
ꢂ1), and N3P3-(PCL-Pyr)6 (P4;
1.508 ꢃ 10ꢂ7 mol Lꢂ1) in DMF at
room temperature. [Color figure
can be viewed in the online
issue, which is available at
wileyonlinelibrary.com.]
3202
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
of the pyrene groups of P4 due to the higher mobility of the
pyrene end-capped polymer arms.
Figure 8 demonstrates the fluorescence emission spectra of
4, P4, and the precursor 1-ethynyl pyrene in DMF at room
temperature. 1-ethynyl pyrene exhibits characteristic pyrene
monomer emissions46; whereas, for 4 and P4 the excimer
emission peaks were observed around 490 nm due to the
interaction of the excited pyrene moieties with those in their
ground states (dynamic excimer) as well as the excitation of
ground state dimeric species (static excimer formation). The
excimer emissions were much more pronounced in the case
of 4 than in P4. Besides, although the peak broadenings in
the excitation spectra of P4 was broader than 4, 4 gave
much stronger excimer emission than P4, indicating the con-
tribution of dynamic excimers.
FIGURE 8 Emission spectra of 1-ethynyl pyrene (1.473 ꢃ 10ꢂ7
Figure 9 demonstrates a visual investigation of the fluores-
cence properties of dilute and concentrated pristine pyrene
as well as pyrene functional phosphazene dendrimer (4) and
star polymer (P4) in DMF recorded under irradiation of 365
nm UV lamp. In the dilute pyrene concentration (10ꢂ5 M),
monomer emission was prevalent. On the contrary, in the
concentrated pyrene solution, excimer formation due to the
interaction of excited pyrene molecules with the others in
the ground state resulted in blue emission. The same fluores-
cence emission observed with phosphazene dendrimer (4)
and star polymer (P4) indicates the incorporation of pyrene
into 4 and P4 serves to effectively increase the local pyrene
concentration.46
mol Lꢂ1, kExc ¼ 359 nm), N3P3-(Pyr)6 (4; 1.512 ꢃ 10ꢂ7 mol Lꢂ1
kExc ¼ 350 nm), and N3P3-(PCL-Pyr)6 (P4; 1.508 ꢃ 10ꢂ7 mol Lꢂ1
,
,
kExc ¼ 350 nm) in DMF at room temperature. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.
com.]
end groups with bromide groups enhanced thermal stabil-
ities of the star polymers. Additionally, the substitution of
bromide end groups by first azide and then inclusion of
pyrene moieties by click chemistry technique improved the
resistance to thermal decomposition of the corresponding
star polymers.
UV–Vis and Fluorescence Experiments
Noncovalent Grafting of MWCNT
The presence of pyrene groups in the phosphazene den-
drimer (4) and star polymer (P4) were further supported by
the absorption and fluorescence spectroscopies. The pyrene
end-functional phosphazene macromolecules gave the typical
pyrene absorption bands above 300 nm with strong peak
broadening, which is a clear indication of ground state py-
rene-pyrene association [Fig. 7(a)].45 The peak broadening in
the excitation spectra of 4 and P4 [Fig. 7(b)] with respect to
those of 1-ethynyl pyrene provided additional support for
this conclusion.45b This effect was more extensive for P4
than for 4, suggesting the stronger ground state association
MWCNT grafting capability of the pyrene end-capped macro-
molecules were investigated by TGA analysis of the dried
MWCNT complexes, which were free of any unbound pyrene
functional dendrimers or star polymers (Fig. 10). The sam-
ples of MWCNT grafted with N3P3-(Pyr)6 (4) and N3P3-(PCL-
Pyr)6 (P4), denoted as MWCNT-4 and MWCNT-P4, respec-
ꢀ
tively, were heated from room temperature to 700 C with a
heating rate of 10 ꢀC minꢂ1 under nitrogen atmosphere.
MWCNT-P4 demonstrated greater weight loss (20.71%) than
MWCNT-4 (10.55%), where the pristine MWCNT lost only
7.54% of its initial weight. Therefore, according to the
FIGURE 9 A visual investigation
of (a) dilute pyrene (10ꢂ5 mol
L
ꢂ1), (b) concentrated pyrene, (c)
dilute N3P3-(PCL-Pyr)6 (P4; 10ꢂ5
mol
L
ꢂ1), and (d) N3P3-(Pyr)6
(4; 10ꢂ5 mol Lꢂ1) in DMF under
365 nm UV lamp.
PHOSPHAZENE DENDRIMER AND STAR POLYMER WITH PYRENE END-GROUPS, GORUR ET AL.
3203

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 12 Raman spectra of (a) pristine MWCNT and (b)
MWCNT-P4 taken using laser excitation at 514 nm. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Better dispersion stability of P4 might be attributed to high
affinity of PCL arms for THF solvent.
FIGURE 10 TGA thermographs of (a) pristine MWCNT, (b)
MWCNT-4, and (c) MWCNT-P4 complexes. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.
com.]
Raman spectroscopy of the MWCNT-P4 dispersion (Fig. 12)
taken using laser excitation at 514 nm clearly proved that
the observed stable dispersion contained MWCNT. For the
comparison, Raman spectrum of the pristine MWCNT was
also included in Figure 12. Raman spectrum of drop-cast
sample of MWCNT-P4 dispersion shows three characteristic
signals of MWCNT at 1341 cmꢂ1 (D band), 1567 cmꢂ1 (G
band), and 2689 cmꢂ1 (G0 band), demonstrating the pres-
ence of MWCNT within the sample.39a,47 The Raman spectra
of pristine MWCNT and MWCNT-P4 did not have any observ-
able differences. Thus, it can be stated that the grafting of
MWCNT did not induce any structural change in the nano-
tube framework.
calculations, the grafted MWCNT-P4 and MWCNT-4 contained
14.30% star polymer (P4) and 3.26% dendrimer (4) by
weight, respectively. These data indicate that 4 was more
effectively grafted onto the surface of MWCNT than P4 by
mole. On the other hand, the visual investigation of the
grafted samples demonstrates that P4 was better MWCNT
dispersion stabilizer compared with 4 as seen in Figure 11.
A total of 2.00 mg samples of MWCNT-4 and MWCNT-P4
were ultrasonicated in THF and dispersion stabilities were
recorded after standing for 10 days. MWCNT-P4 did not
show any observable precipitation, whereas MWCNT-4 did.
CONCLUSIONS
We synthesized pyrene end-capped hexa-armed phosphazene
dendrimer and star polymer using core-first approach. A
hexa-hydroxy functional phosphazene derivative (N3P3-
(OH)6) was used as a starting compound for both dendrimer
and star polymer. Hexa-armed hydroxy end-functional PCL
star polymer (N3P3-(PCL-OH)6) was prepared by ROP of
e-CL. Hydroxy functionalities of the both compounds, N3P3-
(OH)6 and N3P3-(PCL-OH)6, were consecutively converted
into bromide and azide via proper functional group transfor-
mations. In the last step, pyrene end-groups were introduced
into azide functional dendrimer precursor (N3P3-(N3)6) and
the star polymer (N3P3-(PCL-N3)6) via Cu(I)-catalyzed ‘‘click’’
reaction. 1H NMR analysis clearly indicated that functionali-
zation of the arm ends of the dendrimer and the star
polymer were quantitative. Absorption and fluorescence
properties of pyrene end-functional phosphazene dendrimer
(N3P3-(Pyr)6) and star polymer (N3P3-(PCL-Pyr)6) were
investigated. Both of them showed ground state association
as proved by absorption and excitation fluorescence spectra.
According to the fluorescence emission spectra, the excimer
formation was more extensive in N3P3-(Pyr)6 than in N3P3-
(PCL-Pyr)6 due to the increased proximity of the pyrene moi-
eties. Moreover, N3P3-(Pyr)6 and N3P3-(PCL-Pyr)6 were used
in the noncovalent functionalization of MWCNT and the later
showed very much better results as confirmed by TGA
experiments and visual investigations.
FIGURE 11 The visual investigation for the dispersion stabil-
ities of MWCNT-P4 and MWCNT-4 after standing for 10 days.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
3204
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
rials 1997, 18, 1147–1152; (b) de Wolf, H. K.; Luten, J.; Snel, C.
J.; Oussoren, C.; Hennink, W. E.; Storm, G. J Control Release
2005, 109, 275–287; (c) Lakshmi, S.; Katti, D. S.; Laurencin, C. T.
Adv Drug Deliv Rev 2003, 55, 467–482.
Further studies for the synthesis of phosphazene cored-star
polymers with different arms and funtional chain ends are
now in progress.
The authors thank ‘‘The Gebze Institute of Technology Research
Foundation’’ for financial support (Project No: 2009-A-5) and
Mustafa Çulha from Yeditepe University for the Raman spectra
measurements.
12 Andrianov, A. J Inorg Organomet Polym Mater 2006, 16,
397–406.
13 Sassus, J. L.; Graffeuil, M.; Castera, P.; Labarre, J. F. Inorg
Chim Acta 1985, 108, 23–27.
14 (a) Deng, M.; Nair, L. S.; Nukavarapu, S. R.; Jiang, T.; Kan-
ner, W. A.; Li, X. D.; Kumbar, S. G.; Weikel, A. L.; Krogman, N.
R.; Allcock, H. R.; Laurencin, C. T. Biomaterials 2010, 31,
4898–4908; (b) Deng, M.; Nair, L. S.; Nukavarapu, S. P.; Kum-
bar, S. G.; Jiang, T.; Krogman, N. R.; Singh, A.; Allcock, H. R.;
Laurencin, C. T. Biomaterials 2008, 29, 337–349.
REFERENCES AND NOTES
1 (a) Allcock, H. R. Acc Chem Res 1979, 12, 351–358; (b) Kilic,
A.; Kilic, Z.; Shaw, R. A. Phosphorus Sulfur Silicon Relat Elem
1991, 56, 157–164; (c) Krogman, N. R.; Singh, A.; Nair, L. S.;
Laurencin, C. T.; Allcock, H. R. Biomacromolecules 2007, 8,
1306–1312.
15 Allcock, H. R.; Bender, J. D.; Morford, R. V.; Berda, E. B.
2 (a) Allcock, H. J Inorg Organomet Polym Mater 2006, 16,
277–294; (b) Chang, Y.; Cho, S. Y.; Steely, L. B.; Allcock, H. R.
J Polym Sci Part A: Polym Chem 2009, 47, 692–699; (c) Chang,
Y.; Powell, E. S.; Allcock, H. R. J Polym Sci Part A: Polym
Chem 2005, 43, 2912–2920.
Macromolecules 2003, 36, 3563–3569.
16 (a) Carter, R.; Wycisk, R.; Yoo, H.; Pintauro, P. N. Electro-
chem Solid-State Lett 2002, 5, A195–A197; (b) Wycisk, R.; Lee,
J. K.; Pintauro, P. N. J Electrochem Soc 2005, 152, A892–A898;
(c) Hofmann, M. A.; Ambler, C. M.; Maher, A. E.; Chalkova, E.;
Zhou, X. Y.; Lvov, S. N.; Allcock, H. R. Macromolecules 2002,
35, 6490–6493.
3 (a) Allcock, H. J Inorg Organomet Polym Mater 2007, 17,
349–359; (b) Tunca, U.; Hizal, G. J Polym Sci Part A: Polym
Chem 1998, 36, 1227–1232; (c) Allcock, H. R.; Kellam, E. C.; Hof-
mann, M. A. Macromolecules 2001, 34, 5140–5146; (d) Stone,
D. A.; Chang, Y.; Allcock, H. R. J Polym Sci Part A: Polym
Chem 2006, 44, 69–76.
17 Dembek, A. A.; Allcock, H. R.; Kim, C.; Devine, R. L. S.;
Steier, W. H.; Shi, Y.; Spangler, C. W. In Materials for Nonlinear
Optics; Marder, S. R.; Sohn, J. E.; Stucky, G. D., Eds.; ACS
Symposium Series 455; American Chemical Society: Washing-
ton, DC, 1991; pp 258–266.
4 (a) Gorur, M.; Yilmaz, F.; Kilic, A.; Demirci, A.; Ozdemir, Y.;
Kosemen, A.; San, S. E. J Polym Sci Part A: Polym Chem 2010,
48, 3668–3682; (b) Inoue, K.; Miyamoto, H.; Itaya, T. J Polym
Sci Part A: Polym Chem 1997, 35, 1839–1847; (c) Chang, J. Y.;
Ji, H. J.; Han, M. J.; Rhee, S. B.; Cheong, S.; Yoon, M. Macro-
molecules 1994, 27, 1376–1380.
18 (a) Jaglowski, A. J.; Singler, R. E.; Atkins, E. D. T. Macromo-
lecules 1995, 28, 1668–1672; (b) Tanaka, H.; Gomez, M. A.;
Tonelli, A. E.; Chichester-Hicks, S. V.; Haddon, R. C. Macromo-
lecules 1989, 22, 1031–1036.
19 (a) Singler, R. E.; Deome, A. J.; Dunn, D. A.; Bieberich, M. J.
Ind Eng Chem Prod Res Dev 1986, 25, 46–57; (b) Singler, R.
J Inorg Organomet Polym Mater 2006, 16, 307–309.
5 Rele, S. M.; Cui, W.; Wang, L.; Hou, S.; Barr-Zarse, G.; Tatton,
D.; Gnanou, Y.; Esko, J. D.; Chaikof, E. L. J Am Chem Soc 2005,
127, 10132–10133.
20 Liu, W.; Ye, C.; Zhang, Z.; Yu, L. Wear 2002, 252, 394–400.
6 (a) Kaskhedikar, N.; Burjanadze, M.; Karatas, Y.; Wiemhofer,
¨
H. D. Solid State Ionics 2006, 177, 3129–3134; (b) Cho, S. Y.;
Allcock, H. R. Chem Mat 2007, 19, 6338–6344.
21 Clouet, E.; Fillaut, J. L.; Gnanou, Y.; Astruc, D. In Macromo-
lecular Engineering: Recent Advances; Mishra, M. K.; Nuyken,
O.; Kobayashi, S.; Yagci, Y.; Sar, B., Eds. Plenum Press: New
York, 1995; pp 47–66.
7 (a) Fei, S. T.; Wood, R. M.; Lee, D. K.; Stone, D. A.; Chang, H.
L.; Allcock, H. R. J Membr Sci 2008, 320, 206–214; (b) Devapal,
D.; Packirisamy, S.; Reghunadhan Nair, C.; Ninan, K. J Mater
Sci 2006, 41, 5764–5766.
22 (a) Fre´chet, J. M. J.; Hawker, C. J. React Funct Polym 1995,
26, 127–136; (b) Wnek, G. E. In Encyclopedia of Biomaterials
and Biomedical Engineering; Wnek, G. E.; Bowlin, G. L., Eds.
Informa Healthcare USA, Inc.: New York, 2008; Vol. 3, pp
2275–2281.
8 (a) Bolink, H. J.; Santamaria, S. G.; Sudhakar, S.; Zhen, C.;
Sellinger, A. Chem Commun 2008, 2008, 618–620; (b) Bolink, H.
J.; Barea, E.; Costa, R. D.; Coronado, E.; Sudhakar, S.; Zhen, C.;
Sellinger, A. Org Electron 2008, 9, 155–163; (c) Sudhakar, S.;
Sellinger, A. Macromol Rapid Commun 2006, 27, 247–254.
23 Deng, G.; Chen, Y. Macromolecules 2003, 37, 18–26.
24 (a) Suzuki, A.; Nagai, D.; Ochiai, B.; Endo, T. J Polym Sci
Part A: Polym Chem 2005, 43, 5498–5505; (b) Abraham, S.;
Choi, J. H.; Ha, C.-S.; Kim, I. J Polym Sci Part A: Polym Chem
2007, 45, 5559–5572; (c) Taton, D.; Saule, M.; Logan, J.; Duran,
R.; Hou, S.; Chaikof, E. L.; Gnanou, Y. J Polym Sci Part A:
Polym Chem 2003, 41, 1669–1676; (d) Tasdelen, M. A.; Demirel,
A. L.; Yagci, Y. Eur Polym J 2007, 43, 4423–4430.
9 (a) Allcock, H. R. Chem Mat 1994, 6, 1476–1491; (b) Allcock,
H. R.; Visscher, K. B.; Kim, Y.-B. Macromolecules 1996, 29,
2721–2728.
10 (a) Lu, S. Y.; Hamerton, I. Prog Polym Sci 2002, 27,
1661–1712; (b) Mathew, D.; Nair, C. P. R.; Ninan, K. N. Polym Int
2000, 49, 48–56; (c) Reed, C. S.; Taylor, J. P.; Guigley, K. S.; Cole-
man, M. M.; Allcock, H. R. Polym Eng Sci 2000, 40, 465–472.
25 (a) Wu, Z.-M.; Liang, H.; Lu, J.; Deng, W.-L. J Polym Sci Part
A: Polym Chem 2010, 48, 3323–3330; (b) Cheng, G.; Fan, X.;
Tian, W.; Liu, Y.; Kong, J. Polym Int 2010, 59, 543–551.
11 (a) Vandorpe, J.; Schacht, E.; Dunn, S.; Hawley, A.; Stolnik,
S.; Davis, S. S.; Garnett, M. C.; Davies, M. C.; Illum, L. Biomate-
PHOSPHAZENE DENDRIMER AND STAR POLYMER WITH PYRENE END-GROUPS, GORUR ET AL.
3205

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
26 (a) Binder, W. H.; Sachsenhofer, R. Macromol Rapid Com-
mun 2007, 28, 15–54; (b) Uygun, M.; Tasdelen, M. A.; Yagci, Y.
Macromol Chem Phys 2010, 211, 103–110; (c) Akat, H.; Gacal,
B.; Balta, D. K.; Arsu, N.; Yagci, Y. J Polym Sci Part A: Polym
Chem 2010, 48, 2109–2114; (d) Durmaz, H.; Dag, A.; Gursoy, D.;
Demirel, A. L.; Hizal, G.; Tunca, U. J Polym Sci Part A: Polym
Chem 2010, 48, 1557–1564; (e) Dag, A.; Durmaz, H.; Tunca, U.;
Hizal, G. J Polym Sci Part A: Polym Chem 2009, 47, 178–187.
35 (a) Sun, Y.-P.; Fu, K.; Lin, Y.; Huang, W. Acc Chem Res 2002,
35, 1096–1104; (b) Dyke, C. A.; Tour, J. M. Chem Eur J 2004,
10, 812–817.
36 (a) Jung, D.-H.; Kim, B. H.; Ko, Y. K.; Jung, M. S.; Jung, S.;
Lee, S. Y.; Jung, H.-T. Langmuir 2004, 20, 8886–8891; (b) Qu, L.;
Veca, L. M.; Lin, Y.; Kitaygorodskiy, A.; Chen, B.; McCall, A. M.;
Connell, J. W.; Sun, Y.-P. Macromolecules 2005, 38,
10328–10331.
27 (a) Moses, J. E.; Moorhouse, A. D. Chem Soc Rev 2007, 36,
1249–1262; (b) Huisgen, R. Proc Chem Soc 1961, 1961, 357–369;
(c) Huisgen, R. Angew Chem Int Ed Engl 1968, 7, 321–328.
37 (a) Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.;
Smalley, R. E.; Schmidt, J.; Talmon, Y. Nano Lett 2003, 3,
1379–1382; (b) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson,
A. T.; Yodh, A. G. Nano Lett 2003, 3, 269–273; (c) Regev, O.;
ElKati, P. N. B.; Loos, J.; Koning, C. E. Adv Mater 2004, 16,
248–251.
28 (a) Demko, Z. P.; Sharpless, K. B. Angew Chem Int Ed 2002,
41, 2110–2113; (b) Demko, Z. P.; Sharpless, K. B. Angew Chem
Int Ed Engl 2002, 41, 2113–2116; (c) Rostovtsev, V. V.; Green, L.
G.; Fokin, V. V.; Sharpless, K. B. Angew Chem Int Ed Engl 2002,
41, 2596–2599.
38 (a) Rouse, J. H. Langmuir 2004, 21, 1055–1061; (b) Die-
ckmann, G. R.; Dalton, A. B.; Johnson, P. A.; Razal, J.; Chen, J.;
Giordano, G. M.; Mun˜ oz, E.; Musselman, I. H.; Baughman, R.
H.; Draper, R. K. J Am Chem Soc 2003, 125, 1770–1777; (c) Bas-
karan, D.; Mays, J. W.; Bratcher, M. S. Chem Mater 2005, 17,
3389–3397.
29 (a) Nagai, A.; Kamei, Y.; Wang, X.-S.; Omura, M.; Sudo, A.;
Nishida, H.; Kawamoto, E.; Endo, T. J Polym Sci Part A: Polym
Chem 2008, 46, 2316–2325; (b) Durmaz, Y. Y.; Sangermano, M.;
Yagci, Y. J Polym Sci Part A: Polym Chem 2010, 48, 2862–2868;
(c) Tasdelen, M. A.; Van Camp, W.; Goethals, E.; Dubois, P.; Du
Prez, F.; Yagci, Y. Macromolecules 2008, 41, 6035–6040; (d) Dur-
maz, H.; Dag, A.; Altintas, O.; Erdogan, T.; Hizal, G.; Tunca, U.
Macromolecules 2006, 40, 191–198.
39 (a) Bahun, G. J.; Adronov, A. J Polym Sci Part A: Polym
Chem 2010, 48, 1016–1028; (b) Meuer, S.; Braun, L.; Zentel, R.
Macromol Chem Phys 2009, 210, 1528–1535.
40 Meuer, S.; Braun, L.; Schilling, T.; Zentel, R. Polymer 2009,
50, 154–160.
30 (a) Segui, F.; Qiu, X.-P.; Winnik, F. M. J Polym Sci Part A:
Polym Chem 2008, 46, 314–326; (b) Danko, M.; Libiszowski, J.;
Biela, T.; Wolszczak, M.; Duda, A. J Polym Sci Part A: Polym
Chem 2005, 43, 4586–4599; (c) Yuan, W.; Yuan, J.; Zhou, M.;
Pan, C. J Polym Sci Part A: Polym Chem 2008, 46, 2788–2798.
41 Opsteen, J. A.; van Hest, J. C. M. Chem Commun 2005,
57–59.
42 Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. J Org Chem 1997,
62, 7512–7515.
31 Avnir, D.; Busse, R.; Ottolenghi, M.; Wellner, E.; Zachariasse,
43 (a) Celik, A.; Kemikli, N.; Ozturk, R.; Muftuoglu, A. E.; Yil-
K. A. J Phys Chem 1985, 89, 3521–3526.
maz, F. React Funct Polym 2009, 69, 705–713; (b) Xue, L.;
32 (a) Lo, M. Y.; Zhen, C.; Lauters, M.; Jabbour, G. E.; Sellinger,
A. J Am Chem Soc 2007, 129, 5808–5809; (b) Liu, F.; Zou, J.-H.;
He, Q.-Y.; Tang, C.; Xie, L.-H.; Peng, B.; Wei, W.; Cao, Y.; Huang,
W. J Polym Sci Part A: Polym Chem 2010, 48, 4943–4949.
Agarwal, U. S.; Zhang, M.; Staal, B. B. P.; Muller, A. H. E.;
¨
Bailly, C. M. E.; Lemstra, P. J. Macromolecules 2005, 38,
2093–2100.
44 Yuan, W. Z.; Tang, X. Z.; Huang, X. B.; Zheng, S. X. Polymer
33 (a) Zhang, H.; Wang, Y.; Shao, K.; Liu, Y.; Chen, S.; Qiu, W.;
Sun, X.; Qi, T.; Ma, Y.; Yu, G.; Su, Z.; Zhu, D. Chem Commun
2006, 755–757; (b) Wang, Y.; Wang, H.; Liu, Y.; Di, C.-A.; Sun,
Y.; Wu, W.; Yu, G.; Zhang, D.; Zhu, D. J Am Chem Soc 2006,
128, 13058–13059; (c) Shukla, D.; Welter, T. R.; Robello, D. R.;
Giesen, D. J.; Lenhard, J. R.; Ahearn, W. G.; Meyer, D. M.;
Rajeswaran, M. J Phys Chem C 2009, 113, 14482–14486.
2005, 46, 1701–1707.
45 (a) Winnik, F. M. Chem Rev 1993, 93, 587–614; (b) Tanhuan-
paa, K.; Cheng, K. H.; Anttonen, K.; Virtanen, J. A.; Somerharju,
¨ ¨
P. Biophys J 2001, 81, 1501–1510.
46 Toiserkani, H.; Yilmaz, G.; Yagci, Y.; Torun, L. Macromol
Chem Phys 2010, 211, 2389–2395.
47 DiLeo, R. A.; Landi, B. J.; Raffaelle, R. P. J Appl Phys 2007,
34 Petrov, P.; Stassin, F.; Pagnoulle, C.; Jerome, R. Chem Com-
101, 064307–064311.
mun 2003, 2904–2905.
3206
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA