Synthesis and properties of hyperbranched polyamide-esters derived from 1,3,5-tris(4′-hydroxyphenylcarbamoyl)benzene

Article abstract of DOI:10.1016/j.tet.2009.11.108

A novel aromatic triol was synthesized and polycondensed with various diacid chlorides resulting in the preparation of a series of hydroxy-terminated hyperbranched polyamide-esters without gelation. Structure and degree of branching of the ensuing polymers were confirmed by FTIR, ¹H and ¹³C NMR analyses. These thermally stable polymers were found to be soluble in aprotic solvents. Inherent viscosities and T_g values lie in the range of 0.15-0.21 dL/g and 74-112 °C, respectively.

Full text of DOI:10.1016/j.tet.2009.11.108

Tetrahedron 66 (2010) 1389–1398
Contents lists available at ScienceDirect
Tetrahedron
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Synthesis and properties of hyperbranched polyamide–esters derived from
1,3,5-tris(40-hydroxyphenylcarbamoyl)benzene
,
Saima Shabbir a, Sonia Zulfiqar a, Zahoor Ahmad b, Muhammad Ilyas Sarwar a
*
a Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan
b Department of Chemistry, Faculty of Science, Kuwait University, PO Box 5969, Safat 13060, Kuwait
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel aromatic triol was synthesized and polycondensed with various diacid chlorides resulting in the
preparation of a series of hydroxy-terminated hyperbranched polyamide–esters without gelation.
Structure and degree of branching of the ensuing polymers were confirmed by FTIR, 1H and 13C NMR
analyses. These thermally stable polymers were found to be soluble in aprotic solvents. Inherent vis-
cosities and Tg values lie in the range of 0.15–0.21 dL/g and 74–112 ꢀC, respectively.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 24 April 2009
Received in revised form
20 October 2009
Accepted 26 November 2009
Available online 11 December 2009
Keywords:
Hyperbranched polyamide–ester
Synthesis
Characterization
1. Introduction
trimesic acid (B3). Furthermore, Jikei and co-workers,20 as well as
Voit and co-workers21 have explored these HBPs in more detail.
The preparation of hyperbranched polymers (HBPs) from AB2
monomers emerged with the initial report by Flory in 1952.1 Massive
research has recently commenced in an endeavor to develop poly-
meric products with similar structures and performance to perfect
dendrimers via a more facile approach.2 Many strategies have been
developed to prepare polymers with highly branched topologies and
one-step polycondensation of ABx monomers has been of increasing
interest.3–10 HBPs have received great attention owing to their
unique combination of low viscosity, excellent solubility, and facile
synthesis. HBPs prepared via one-step polycondensation of ABx
monomers typically revealed highly irregular structures and wider
molecular weight distributions.11 Consequently, it was recom-
mended that HBPs resembled conventional networks immediately
prior to the gel point.12–14 The correspondence between in-
termediates in an A2þB3 process and intermediates in the de-
velopment of polymeric networks, directed chemists to consider the
polycondensation of A2 with B3 monomers as a substitute synthetic
route. This comparable method is remarkable since many A2 and B3
monomers are readily accessible and have gained considerable focus
in the synthesis of conventionally branched polymers.15–24 Jikei and
co-workers15 described the synthesis of aromatic hyperbranched
polyamides (HBPAs), derived from aromatic diamines (A2) and
Kinetic calculations helped to envision that the first condensation
reaction of A2 with B3 was faster than the following propagation,
thereby leading to an accumulation of A–ab–(B)2 intermediates. For
this reason, the rest of the process resembled the more familiar AB2
polycondensation, and the final products demonstrated analogous
structures. There are numerous reports regarding this more facile
approach to HBPs and several novel families of HBPs were reported
using this methodology.16–24 Many fresh efforts have focused on the
synthesis of hyperbranched analogs of engineering polymers to
obtain the useful properties of HBPs such as low melt viscosity,
multifunctionality, the possibility of acting as molecular encapsu-
lants and good thermal stability.8 These novel HBPs are valuable as
modifiers in blends with linear polymers, thin coatings, gas sepa-
ration membranes and microelectronic materials.25,26 Turner and
co-workers reported the synthesis of hyperbranched poly(aryl es-
ter)s using commercially available AB2 monomers, such as 5-ace-
toxyisophthalic acid and 5-(2-hydroxyethoxy)-isophthalic acid.27,28
In addition, several bisphenols containing different linking groups,
such as biphenol-A, 4,40-(hexafluoroisopropylidene)diphenol,
4,40-sulfonyldiphenol, or 4,40-bis(hydroxyphenyl)phenylphosphine
oxide, were investigated to improve the performance of linear poly-
(aryl ester)s.29 These monomers led to HBPs with unique properties,
such as liquid crystalline morphologies, high thermal stabilities, and
low dielectric constants. The monomers involved in the approaches
mentioned earlier are limited, especially for synthesizing HBPAEs
containing both ester groups and peptide links, which make them
* Corresponding author. Tel.: þ92 51 90642132; fax: þ92 51 90642241.
E-mail address: ilyassarwar@hotmail.com (M. Ilyas Sarwar).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.11.108

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S. Shabbir et al. / Tetrahedron 66 (2010) 1389–1398
more potentially useful. There is little research on the synthesis of
HBPAEs except from carboxylic anhydrides and secondary amines
with two hydroxyls by van Benthem and co-workers.30,31 In addi-
tion, a series of polyaromatic dendrimers have been synthesized
possessing a repetitive amide–ester coupling sequence.32 However,
the direct condensation of bisphenols and diacids cannot yield high
molecular weight polyesters because of the strong leaving tendency
of the phenylate moiety, so high molecular weight linear aromatic
polyesters are prepared either using an activated acid (acid chloride)
or by acetylating or silylating the phenolic function. However, fam-
ilies of HBPs with moderately high molecular weight can be obtained
via the appropriate selection of monomers and the optimization of
reaction conditions. On account of the aforementioned consider-
ation, herein, we tried synthesizing aromatic and semi-aromatic
HBPAEs from a triol and different diacid chlorides. Novel aromatic
triol, 1,3,5-tris(40-hydroxyphenylcarbamoyl)benzene (THPCB), was
synthesized and its polymerization with terephthaloyl chloride,
isophthaloyl chloride, sebacoyl chloride and adipoyl chloride resul-
ted in the preparation of four hyperbranched polyamide–esters, i.e.,
HBPAE 1, HBPAE 2, HBPAE 3, and HBPAE 4, respectively. The use of A2
and B3 monomers represents a facile route to novel HBPAEs. Here we
have reported the synthesis of HBPAEs via solution polymerization of
different diacid chlorides (A2) and THPCB (B3) using a 1:1 M ratio of
monomers (2 equiv of A groups and 3 equiv of B groups). These re-
actions were stopped immediately prior to the gel points to form
highly branched molecules by controlled monomer ratio and con-
centration. Contrary to previous reports of HBPAs using less reactive
temperature, which is generally known to be fast and quantitative
(Scheme 1). Aromatic triol, THPCB, was synthesized via single step
nucleophilic substitution reaction of 1,3,5-benzenetricarbonyl tri-
chloride (BTC) and 3-fold molar excess of 4-aminophenol in the
presence of propylene oxide (PO). The structure of THPCB was
characterized by elemental analysis, FTIR, 1H and 13C NMR analyses.
In the FTIR spectrum of THPCB, two bands at 3205 and 3328 cmꢁ1
are attributed to the stretching vibration of amide N–H and primary
alcohol O–H groups. Aromatic C–H stretching and C]O amide
stretching vibrations appear at 3082 cmꢁ1 and 1661 cmꢁ1, re-
spectively. The band characteristic of 1,3,5-trisubstituted benzene is
found at 675 cmꢁ1 and the band at 823 cmꢁ1 is assigned to 1,4-
disubstituted benzene ring vibration (Fig. 1). Primary alcohol C–C–
O vibration appears at 511 cmꢁ1. The 1H NMR peak assignment
ascribed to THPCB is given in Figure 2. Amide proton peak appeared
at 10.374 ppm, while the 8.644 ppm proton corresponded to 1,3,5-
trisubstituted benzene, which provided basis for the estimation of
degree of branching. The terminal hydroxyl protons emerged as
a broad peak at 9.393 ppm. All carbon nuclei furnished well sepa-
rated peaks in the 13C NMR spectrum (Fig. 3). Carbonyl carbons of
amide groups resonate in the downfield region 164.50 ppm. FTIR,
1H and 13C NMR spectra supported that amino groups in 4-ami-
nophenol were completely converted to amide linkages and, data
from elemental analysis were in good agreement with the calcu-
lated values. These characterizations demonstrated that the
chemical composition of THPCB is consistent with the proposed
structure.
OH
COCl
DMAc
PO
NH
C
+
HO
NH2
3
N2 atm
O
ClOC
COCl
C
O
C
O
HN
NH
HO
Scheme 1. Scheme for the synthesis of THPCB monomer.
OH
THPCB
diacid monomers,15 a much more reactive functional monomer, di-
acid chloride, was used, along with a synthetic methodology, i.e.,
slow addition of a reactive monomer, to avoid gelation. For that
reason, the design and development of this facile, versatile and cost
efficient route to HBPAEs are significant. Furthermore, character-
ization of the derived HBPAEs via FTIR, 1H, 13C NMR, hinh, TGA, DSC,
XRD, and solubility are also reported. In most cases rigid polyamides
are infusible and insoluble, which restrict the synthesis and appli-
cations especially of high molecular weight materials. Modification
of high performance materials by increasing the solubility through
copolymerization while maintaining thermal stability is of particular
interest. Consequently, the present investigation is aimed to enhance
the solubility of rigid polyaromatic amide without deteriorating the
thermal properties.
2. Results and discussion
2.1. Monomer synthesis
Figure 1. FTIR spectrum of THPCB monomer.
2.2. Synthesis of HBPAEs
The key step for the preparation of rigid amide type monomer
(THPCB) used in this study is the synthesis of amide linkage via
A facile synthetic approach to aromatic and semi-aromatic hy-
droxy-terminated HBPAEs via direct polymerization of THPCB (B3)
Schotten–Baumann-type
polycondensation
below
room

S. Shabbir et al. / Tetrahedron 66 (2010) 1389–1398
1391
concentration. Structure elucidation of the synthesized HBPAEs was
carried out by FTIR and NMR techniques and degree of branching
was determined through 1H and 13C NMR. The solubility of these
HBPAEs in different solvents was assessed. Thermal stability, Tg,
hinh, and crystallinity were scrutinized using the corresponding
techniques as described below. The representative FTIR spectra of
the HBPAEs are displayed in Figure 4. The strong and broad band
between 3331 and 3325 cmꢁ1 is chiefly attributed to O–H stretch-
ing of the H-bonded OH groups, which is the distinguishing feature
of the phenol group. This broad band in the range of 3331–
3325 cmꢁ1 is also typical for the stretching of H-bonded O–H and
N–H groups. The spectra of HBPAE 1, 2, 3, and 4 showed bands
around 3160–3082 cmꢁ1 for aromatic C–H stretching. Amide C]O
Figure 2. 1H NMR spectrum of THPCB monomer.
Figure 3. 13C NMR spectrum of THPCB monomer.
with different diacid chlorides (A2) was explored. The poly-
condensation reaction of the prepared triol with terephthaloyl
chloride (TPC), isophthaloyl chloride (IPC), sebacoyl chloride (SC),
and adipoyl chloride (AC) resulted in the preparation of four
HBPAEs, i.e., HBPAE 1, HBPAE 2, HBPAE 3, and HBPAE 4, respectively.
The general method adopted in the present assignment is to syn-
thesize HBPAEs via solution polycondensation reaction of an aro-
matic triol with various carboxylic acid dichlorides in an amide
solvent using triethylamine as an acid scavenger. Appropriate
monomer addition order to yield hydroxy-terminated HBPAEs was
followed, i.e., the addition of A2 to B3. Therefore, the proper molar
ratio between A2 and B3 is 1:1, and correspondingly, the molar ratio
between acid chloride and amine groups of the monomers is 2/3. As
a result, no free acid chloride groups existed in the polymers, and
these polymers were hydroxy-terminated, which is similar to the
case for conventional HBPs prepared from AB2 type monomers.
Additionally, the monomer was added so slowly that the concen-
tration of diacid chloride approached zero to avoid any high local
Figure 4. FTIR spectra of HBPAEs.

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S. Shabbir et al. / Tetrahedron 66 (2010) 1389–1398
stretch was observed between 1661 and 1656 cmꢁ1. The ester C]O
band appeared in the region of 1730–1705 cmꢁ1 confirming the
structure. Moreover, IR spectra for HBPAE 3 and 4 displayed bands
within the range of 2928–2969 cmꢁ1 for aliphatic C–H stretching
vibration (Table 1). The bands at 1598 and 1575 cmꢁ1 are due to
aromatic ring C]C stretching and bands at 1050 and 1290 cmꢁ1 are
due to the C–O stretching vibrations. All the spectra are in com-
pliance with the proposed structures.
hydroxyl groups. Conventional HBPs that are derived from a one-
step polymerization of ABx monomers only possess B functional
groups as terminal units. However, Voit and co-workers have
reported that composition of the terminal groups of HBPAs derived
from A2 and B3 monomers depended on the molar ratio of two
monomers.21 In the present work, the final molar ratio of A2:B3 was
1:1, and the terminal groups were expected to be identical to HBPs
derived from ABx monomers. Two methodologies of 1H NMR and
Table 1
FTIR data of different HBPAEs
Polymer
Observed frequencies (cmꢁ1) of possible group
N–H
O–H
C]O (amide)
C]O (ester)
C–H
Ar
C–H
Ali
1,3,5-Ar
1,4-Ar
1,3-Ar
C–O
HBPAE 1
HBPAE 2
HBPAE 3
HBPAE 4
3325
3331
3327
3330
1661
1661
1656
1656
1730
1729
1707
1705
3082
3094
3156
3160
d
675
676
677
677
823
826
824
823
d
1194
1194
1170
1172
d
750
d
2969
2928
d
2.3. Degree of branching in HBPAEs
The structural perfection of HBPs is generally characterized by
FTIR spectroscopy were used to confirm the structure of terminal
groups. A detectable resonance related to the OH group (B func-
tionality) was observed in the 1H NMR spectra of HBPAEs. In ad-
dition, no peak corresponding to COCl group was observed,
indicating that no free COCl groups existed in the polymer; this fact
was documented by the absence of COCl peak in the FTIR spectra of
the corresponding HBPAEs. These results indicated that when the
ratio of two monomers was 1:1, HBPAEs did not contain residual A
groups and resembled products derived from AB2 monomers. It
was also found that hydroxy-terminated HBPAEs gave a clear dif-
ference in chemical environment among D, T, and L units. The ar-
omatic protons of the central aromatic ring in a given unit can be
clearly distinguished with the support of 1H NMR measurement.
Therefore, the same method was used to study the distribution of D,
T, and L units in as-prepared HBPAEs. Figure 6 shows the 1H NMR
spectra of the hydroxy-terminated HBPAE 1, 2, 3, and 4 in DMSO-d6
along with the spectra of model compounds 1 and 2 for compari-
son. 1H NMR spectra of the polymers showed signal broadening due
to polymerization, but the chemical shifts are consistent with the
proposed polymer structures. There is prominent intermolecular
H-bonding in the terminal hydroxyl groups because of the abun-
dant OH groups. Since H-bonding diminishes shielding, OH and NH
signals move downfield when intermolecular H-bonding is en-
hanced. The H-bonding is accompanied by exchange of protons
from one molecule to another, resulting in OH signal broadening.
Although H-bonding exists between the amide groups, it is much
weaker than the hydroxyl group because the amide groups lie in
the interior of polymer molecule and the group’s density is much
3
´
degree of branching (DB), which is defined by Frechet as follows :
DB ¼ ðD þ TÞ=ðD þ T þ LÞ (1)
where D, T, and L refer to the numbers of dendritic, terminal, and
linear units in the polymer, respectively. Experimentally, DB is
usually determined from 1H or 13C NMR spectroscopy by comparing
the integration of peaks for respective units in the HBPs.3 Figure 5
shows the structural units of HBPAE 2 as an example. 1H NMR
spectra of the HBPAEs showed aromatic hydroxyl proton peaks in
the range of 9.45–9.89 ppm to confirm the presence of terminal
H
N
T2
OH
Tc
O
O
T1
O
NH
O
O
O
O
O
Dc
HN
OH
NH
D
O
NH
O
HN
O
O
O
O
O
O
O
O
Lc
NH
NH
L1
O
L2
O
HN
O
O
O
NH
O
H
N
OH
O
O
O
Figure 5. Structural repeat units of HBPAE 2 and their notations for 1H and 13C NMR
Figure 6. 1H NMR spectra of A: model 1; B: model 2; C: HBPAE 1; D: HBPAE 2; E:
spectra.
HBPAE 3; F: HBPAE 4.

S. Shabbir et al. / Tetrahedron 66 (2010) 1389–1398
1393
smaller than the hydroxyl group; amide protons of the polymer do
not have so many signal peaks in the 1H NMR spectra as the hy-
droxyl protons. In dendritic units of the polymer, there is no hy-
droxyl group. 1H NMR spectroscopy also indicated the presence of
low levels of residual TEA salts (1.56 and 3.44 ppm) after purifica-
tion (Fig. 6). The peaks in the range 7.07–8.45 ppm are assigned to
the protons of p-phenylene residues, while the peaks between 8.78
and 9.15 ppm correspond to the protons in the central aromatic
rings of THPCB residues, which enclose the information on degree
of branching. To make clear the detailed assignment of the peaks in
this region, two kinds of model compounds were synthesized from
p-nitrobenzoyl chloride and THPCB as shown in Scheme 3. Model
compounds 1 and 2 were prepared by exploiting 1:1 and 1:2 M
ratios of p-nitrobenzoyl chloride to THPCB. THPCB residues in
model compounds 1 and 2 are similar to the counterparts in the T
and L units of the HBPAEs, respectively. For comparison purposes,
their 1H NMR spectra are also presented in Figure 6. Thus, the as-
signment of peaks for T, L, and D units of the OH-terminated
HBPAEs can be achieved by comparison to 1H NMR spectra of the
model compounds. It is obvious that for HBPAE 1, the peaks at 8.83
and 8.78 ppm should be attributed to T1 and T2 units, respectively.
The peaks at 8.94 and 9.03 ppm should be related to L1 and L2 units,
respectively. Note that these two peaks also appeared in the 1H
NMR spectra of the hydroxy-terminated HBPAE 2, 3, and 4 with
a slight up field shift. However, the peak around 9.15 ppm is related
to neither T units nor L ones. Thus, for OH-terminated HBPAE 1, the
peak at 9.15 ppm should be ascribed to the D part after excluding
the peaks for T and L units. On the basis of integration of the
deconvoluted peaks assigned to different units, the molecular
fractions of dendritic (Xd), linear (XL1þXL2) and terminal units
(Xt1þXt2) have been calculated, as shown in Table 2. According to
Eq. (1), DB is thus estimated to be 0.56 for HBPAE 1. Using the same
analyzing approach discussed above for the HBPAE 1, DB values
were evaluated to be 0.55, 0.53, and 0.51 for HBPAE 2, 3, and 4,
respectively (Table 2). The structural facet of high T and lower D
content supports the absence of intramolecular cross-linking, thus
hampering the development of a microgel. This may be the ratio-
nale why these HBPs show low solution viscosity and good solu-
bility. The DB value of HBPAE 2 is lower than that of HBPAE 1, which
may result from the fact that meta-oriented polymers have more
steric constraints in the proximity of unreacted hydroxyl group
than para-oriented ones, lowering the reactivity. In addition, the DB
of the polymer was also determined by 13C NMR spectra. The res-
onance of the amide carbonyl carbon atoms of HBPAE 1 should be
assigned to 164.95, 164.89, 164.78, 164.65, 164.58 ppm. While, sig-
nals of the ester carbon atoms of HBPAE 1 display three peaks,
166.45, 165.92, 165.64 ppm. It is very obvious that the peak of
165.64 ppm should be the resonance signal of the ester carbon
atoms in terminal units (Tc) of HBPAE 1 because the resonance of
ester carbon atom of model compound 1 (Fig. 7) was at 165.53 ppm,
and the terminal units possess a similar chemical environment to
the model compound 1. The signal at 165.92 ppm should be
Figure 7. 13C NMR spectra of A: model 1; B: model 2; C: HBPAE 1.
assigned to the resonance of ester carbons in linear units (Lc) of the
HBPAE 1 because it is closer to ester carbonyls, 165.84 ppm, of
model compound 2, and the chemical environment of linear units is
analogous to that of the model compound 2. Lastly, the peak at
166.45 ppm should be the resonance of ester carbons in the den-
dritic units (Dc), which is identified after excluding the peaks for
´
terminal and linear units. DB (Frechet) of HBPAE 1 was calculated to
be 0.56 from the integration ratio of peaks (Tc:Lc:Dc¼
0.31:0.44:0.25), which is in great agreement with the result de-
termined by 1H NMR spectra. Similarly, the DB of HBPAE 2, 3, and 4
was determined as 0.55, 0.53, and 0.51, respectively, thereby, con-
firming the reliance of the determination of DB using 1H NMR
spectroscopy.
2.4. Viscometry and molar mass
The hinh values of the polymers were calculated at a concentra-
tion of 0.5 g/dL in DMSO at 30 ꢀC. The viscosity of HBPAEs was in
the range of 0.15–0.21 dL/g (Table 2). Comparison is made among
HBPAE 1, 2, 3, and 4 to illustrate the effect of A2 monomer structure.
Their hinh increased with the increase of para-phenylene unit. This
indicates that polymer chain rigidity increases as a function of their
para-linkages. Thus, HBPAE 2 with meta-linkages has the lower hinh
,
whereas HBPAE 1 with para-linkages has the highest one. The
higher hinh value of para-oriented type of polymer can also be at-
tributed to the higher rigidity and much interchain hydrogen
bonding, associated with higher chain symmetry and packing ef-
ficiency, which is similar to the closely related dendrimers pos-
sessing repetitive amide–ester sequence.32 Conversely, hinh
decreases to a lesser extent, with the incorporation of a flexible
alkyl linkage in HBPAE 3 and 4, when compared with HBPAE 1 and 2
(Table 2). Additionally, GPC measurements were carried out for
molar mass analysis of HBPAE 3 and 4, which are soluble in THF and
their molar mass values are 5790 and 6927 g/mol, respectively.
However, GPC measurement of HBPAE 1 and 2 cannot be performed
with THF because they were insoluble in THF. In general, the molar
mass was not as high as those from the AB2 system,18 due to
competing gel formation before a high molar mass can be achieved.
The polydispersity was broad, as expected for systems close to gel
point.
Table 2
DB and hinh of the HBPAEs in DMSO
Polymer
hinh
DB (Frechet)b¼
Unit fraction
a
(dL/g) (XdþXt1þXt2)/
b
b
b
b
b
Xd
XL1
XL2
Xt1
Xt2
(XdþXt1þXt2þXt1þXt2
0.56
0.55
0.53
0.51
)
HBPAE 1 0.21
HBPAE 2 0.18
HBPAE 3 0.16
HBPAE 4 0.15
0.26 0.14 0.30 0.10 0.20
0.23 0.15 0.30 0.11 0.21
0.22 0.15 0.32 0.11 0.20
0.21 0.16 0.33 0.10 0.20
2.5. Organosolubility
Qualitative solubility behavior of the polymers was studied in
different solvents and the results are summarized in Table 3. The
polymers demonstrated good solubility in polar aprotic solvents
including DMF, DMAc, NMP, and DMSO and to some extent in THF.
However, this showed a relative decrease of solubility with respect
a
Measured in DMSO at a concentration of 0.5 g/dL at 30 ꢀC.
Xd, dendritic unit fraction; XL1 and XL2, linear unit fractions; Xt1 and Xt2, terminal
b
unit fractions. The five unit fractions are calculated from the integration of the
deconvoluted peaks assigned to the central aromatic ring of the B3 monomer in the
1H NMR spectra.

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S. Shabbir et al. / Tetrahedron 66 (2010) 1389–1398
Table 3
and 4 exhibited the lowest Tg due to the presence of alkyl chain in
Solubilitya behavior of different HBPAE samples
the polymer architecture. The higher Tg of HBPAE 1 (112 ꢀC) in
comparison with HBPAE 2 (105 ꢀC) can be attributed to hindered
segmental rotation as a result of para-oriented benzoate units,
which have denser polymer chain packing than the corresponding
meta-oriented ones due to the larger conformational entropy of
meta-oriented chain.34 Consequently, the former (para-oriented)
has higher Tg than the latter (meta-oriented). Besides, Morikawa
previously demonstrated that the Tg value of HBP could be in-
creased by the incremental insertion of para-phenylene units be-
tween the polymerizable groups and 1,3,5-substituted phenyl
moiety, this results in an elevation of 20 ꢀC per para-phenylene
unit.35 The thermal stabilities of HBPAEs were evaluated by TGA
and the initial decomposition temperatures (T0) were 197–336 ꢀC
(Fig. 9). T10 values that are significant for estimation of thermal
stability were between 246 and 408 ꢀC and also the char yields of
polymers at 600 ꢀC were about 20–37%, which is indicative of good
thermal stability (Table 4). The obtained results for HBPAEs ren-
dered high thermal stability and the fully aromatic ones were more
stable than the semi-aromatic HBPAEs. High thermal stability and
improved solubility could be assigned to the incorporation of
branching units. Hence, all the resulting HBPAEs are thermally
stable owing to their aromatic amide structure. Accordingly, TPC-
derived HBPAE 1 revealed the highest thermal stability among the
prepared polymers. The higher thermal stability could be accredi-
ted to the chain stiffness and intermolecular hydrogen bonding
between amide and OH groups. So the thermal stability in all cases
is good as the T10 values of the HBPAEs are higher than 246 ꢀC. On
the other hand, 1H NMR spectroscopy revealed a large amount of
residual TEA-based ammonium salt even after the polymer was
extensively washed with basic, acidic and deionized water. Since
HBPs are not expected to be extensively engaged in chain entan-
glement, therefore, their Tg originates from a relaxation phenom-
enon different from that of linear polymers. Particularly, the end
Polymer
Solventb
NMP
DMAc
DMF
DMSO
THF
GHCl3
HBPAE 1
HBPAE 2
HBPAE 3
HBPAE 4
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þꢁ
þꢁ
þþ
þþ
ꢁ
ꢁ
þꢁ
þꢁ
a
Solubility:þþ soluble at room temperature; þꢁ partially soluble at room tem-
perature; ꢁ insoluble.
b
Scale:15 mg/1 mL.
to the analogous dendrimer structures reported previously.32 The
presence of alkyl groups in the semi-aromatic HBPAEs derived
from aliphatic acid chlorides caused enhancement of solubility in
comparison to fully aromatic HBPAEs. Consequently HBPAE 1 and
2, derived from aromatic diacid chlorides with more rigid struc-
tures, exhibited lower solubility in the solvents. To the converse,
presence of alkyl group caused improved solubility and accord-
ingly AC and SC derived HBPAE 3 and 4 showed the highest sol-
ubility among the prepared HBPs. It is a known fact that the special
features of HBPs are better solubility and lower solution viscosity
compared with linear analogs due to the branched and denser
molecular structure.1 In agreement with this, it was found that all
the synthesized HBPAEs were soluble in organic solvents due to
the consecutive branching structure. Water-soluble HBPs ensuing
from 5-acetoxyisophthalic acid with carboxylic ammonium or so-
dium salts as terminal groups were reported earlier.22,33 However,
the HBPAE 1, 2, 3, and 4 salts were not water soluble. Compared
with the earlier water-soluble HBPs, these HBPAEs have a repeat
unit consisting of phenyl rings. Thus, the charge density of the
acquired HBPAEs was lower than the preceding water-soluble
polyesters and not sufficiently high to end in a fully water-soluble
polymer.
2.6. Thermal analyses
100
HBPAE 1
HBPAE 2
The thermal behavior of HBPAEs was evaluated in N2 at a heat-
ing rate of 10 ꢀC/min using DSC and TGA techniques. Polymers
showed Tgs in the region of 74–112 ꢀC (Fig. 8) according to their DSC
analysis (Tg was taken as midpoint of change in slope of the base-
line). As expected, Tg was dependent on the structure of the diacid
chloride component and decreased with increasing flexibility of the
polymer backbones. Therefore, HBPAE 1 derived from TPC exhibi-
ted the highest Tg due to the rigid p-benzoate units and HBPAE 3
80
60
40
20
0
HBPAE 3
HBPAE 4
2
HBPAE 1
HBPAE 2
100
200
300
400
500
600
700
800
HBPAE 3
HBPAE 4
0
-2
-4
-6
Temperature (°C)
Figure 9. TGA curves of HBPAEs at a heating rate of 10 ꢀC/min in N2.
Table 4
Thermal analysis data of different HBPAE samples
a
b
c
d
e
Polymer
Tg (ꢀC)
T0 (ꢀC)
T10 (ꢀC)
Tmax (ꢀC)
Yc at 600 ꢀ
C
HBPAE 1
HBPAE 2
HBPAE 3
HBPAE 4
112
105
86
335
336
218
197
408
401
264
246
540
565
528
524
20
37
33
30
74
a
Tg: glass transition temperature.
T0: initial decomposition temperature.
T10: temperature for 10% weight loss.
Tmax: maximum decomposition temperature.
Yc: char yield; weight of polymer remained.
50
100
150
200
250
300
b
c
Temperature (°C)
d
e
Figure 8. DSC thermograms of HBPAEs at heating rate of 10 ꢀC/min in N2.

S. Shabbir et al. / Tetrahedron 66 (2010) 1389–1398
1395
group contribution becomes significant36 and an alkyl group end-
capped HBP causes Tg to be decreased.15 Therefore, it was presumed
that residual TEA salts plasticized HBPAEs, thus, HBPAEs exhibited
lower Tgs relative to their linear analogs. HBPs made via this
methodology exhibited excellent thermal stability and no weight
loss was observed before the onset of polymer degradation (300 ꢀC,
Fig. 9). In view of good thermal stability in conjunction with the low
Tg these novel HBPAEs may potentially be used as additives in
a melt process. In general, higher Tg values of the HBPAE 1 in
comparison with other polymers can be attributed to hindered
segmental rotation and greater rigidity due to para units, while
HBPAE 3 and 4 contain more flexible alkyl backbone, so their Tg
values were lower. The building blocks of HBPAE 3 and 4 with ex-
actly the same chain ends (hydroxyl) are almost similar, but their
Tgs have a discrepancy of about 12 ꢀC. The Tg of HBPAE 3 is higher
than that of HBPAE 4, which can be credited to the difference of
their end group densities as a result of the difference of their DBs.
The DB of HBPAE 3 is higher than that of HBPAE 4, which produces
the greater terminal-group density and results in the stronger in-
termolecular interaction of the H-bond. The case of HBPAE 1 and 2
is similar to that of HBPAE 3 and 4. As a result, Tg values increase
with the increase of DB for the HBPAEs with hydroxyl end groups.
and processable HBPAEs without too much sacrificing thermal
stability. Initially, an aromatic triol, 1,3,5-tris(40-hydroxyphenyl-
carbamoyl)benzene (THPCB), was synthesized from BTC and three
equivalents of 4-aminophenol, subsequently, a series of HBPAEs
were successfully prepared by THPCB and various diacid chlorides.
The resulting HBPAEs exhibited good solubility in polar solvents
such as DMAc, DMSO, and NMP. T10 are above 246 ꢀC, and Tgs are in
the range of 74–112 ꢀC. 1H NMR and FTIR spectroscopy indicated
that the acid chloride functionalities were quantitatively consumed
during the polycondensation. 1H NMR was used to identify L, D, and
T units, and the final DB was determined to be >50% due to pre-
formed dendritic (amide) linkage in the monomer THPCB. All the
polymers were amorphous and their hinh values ranged between
0.15and 0.21 dL/g.
4. Experimental
4.1. Materials
4-Aminophenol, 1,3,5-benzenetricarbonyl trichloride (BTC)
(99%), propylene oxide (PO), anhydrous triethylamine (TEA) (99%),
terephthaloyl chloride (TPC), isophthaloyl chloride (IPC), sebacoyl
chloride (SC), and adipoyl chloride (AC) were procured from Aldrich
and used as received. N,N-Dimethylformamide (DMF), N-methyl-
pyrrolidinone (NMP), and dimethyl sulfoxide (DMSO) (Merck, high
purity) were used as received. Methanol purchased from Fluka was
used as received. Anhydrous N,N-dimethylacetamide (DMAc) was
obtained by the courtesy of Aldrich and purified by distilling over
calcium hydride under reduced pressure.
2.7. X-ray diffraction
The polymers showed low crystallinity (below 10%) and there-
fore, were almost amorphous. Figure 10 presents the X-ray dif-
fractograms for HBPAEs. XRD patterns offered an interesting
comparison and gave evidence for meager crystallization of the
copolymers as HBPAE 1 displayed small peaks around 25ꢀ. This
feature was attributed to the para substitution of the backbone
benzene rings, which can lead to a better packing of the chains. In
addition, these weak and broad reflections become detectable,
which are characteristic of the crystal lattice of p-benzamide units.
Due to hydrogen bonds among amide groups it is reasonable that
the amide groups of the p-benzamide units dominate the chain
packing. Hence XRD results of the copolymers correlated well with
DSC outcome as HBPAE 1 displayed the highest Tg at 112 ꢀC (Table 4)
owing to its stiffer structure by virtue of additional p-benzoate
units derived from TPC. Subsequently thermal traits are in good
compliance with X-ray results. XRD patterns play an important role
in polymer morphology, which suggests that the molecular order is
mainly based on interaction via hydrogen bonds.
4.2. Monomer synthesis
4.2.1. Synthesis of 1,3,5-tris(40-hydroxyphenylcarbamoyl)benzene
(THPCB). A 250 mL two-necked, round-bottomed flask equipped
with a magnetic stirrer, nitrogen gas inlet tube and calcium chloride
drying tube was charged with 4-aminophenol (15 mmol) and dry
DMAc (50 mL). The mixture was stirred at 0 ꢀC for 30 min. Then PO
(ca. 9 mL) was added, and after a few minutes BTC (5 mmol) was
added and the mixture was agitated at 0 ꢀC for 30 min (Scheme 1).
The temperature of the solution was raised to room temperature
and the stirring was continued for an additional 6 h. THPCB was
precipitated by pouring the reaction mixture into water. Then it
was filtered, re-dissolved in minimum amounts of DMAc and re-
precipitated in water. The precipitate was filtered again, washed
several times with hot water and dried overnight under vacuum at
70 ꢀC. Yield: 2.32 g gray brown powder, 96%; mp 347 ꢀC; IR (KBr)
ymax: 3328, 3205, 3082, 1661, 1607, 1557, 1516, 1457, 823, 675,
511 cmꢁ1 1H NMR (300 MHz, DMSO-d6, ppm):
; d 10.374 (s, 3H,
amide NH), 9.393 (br, 3H, Ar OH), 8.644 (s, 3H, 1,3,5-trisubstituted
benzene), 7.613 (d, 6H, J¼9.2 Hz, p-Ar), 6.755 (d, 6H, J¼9.1 Hz, p-Ar).
13C NMR (75 MHz, DMSO-d6, ppm):
d 164.50, 154.37, 136.08, 130.95,
129.73, 122.76, 115.54. Anal. Calcd for C27H21N3O6: C, 67.07; H, 4.38;
N, 8.69%. Found: C, 67.04; H, 4.21; N, 8.60%.
4.3. Hyperbranched polyamide–esters synthesis
THPCB (10 mmol) and TEA (1.52 g, 0.015 mol) were added to
NMP (100 mL) in a 250 mL, thoroughly dried, three-necked flask
equipped with a magnetic stir bar, reflux condenser and dropping
funnel. Slight heating was required to completely dissolve THPCB.
Corresponding diacid chloride (10 mmol) was dissolved in NMP
(50 mL) and the solution was slowly added to the reaction flask
with a dropping funnel in 1 h. The homogeneous reaction was
maintained at 30 ꢀC for 24 h. Water (10 mL) was subsequently
added to quench the reaction and the heterogeneous solution was
stirred for 30 min. The solution was washed twice with basic,
Figure 10. X-ray diffraction curves of the HBPAEs.
3. Conclusions
To extend the utility of thermally stable materials a novel aro-
matic triol (B3 monomer) was synthesized, which offered soluble

1396
S. Shabbir et al. / Tetrahedron 66 (2010) 1389–1398
acidic, and deionized water then precipitated into methanol. The
product was filtered and dried at 40 ꢀC in a vacuum oven for 24 h
(Scheme 2). Yield: 6.70 g clay pink powder, 83%; HBPAE 1; IR (KBr)
ymax: 3325, 3082, 1730, 1661, 1194, 823, 675. 1H NMR (300 MHz,
129.50, 122.54, 122.24, 121.72, 121.56, 62.45, 33.50, 29.48, 29.14,
25.25, 24.56, 14.86.
4.3.1. Synthesis of model compound 1. In a dried 100 mL three-neck
flask THPCB (1.0 mmol) and TEA (0.0015 mol) were dissolved in
NMP (10 mL) with slight heating to ensure complete dissolution. To
this solution 1.0 mmol of p-nitrobenzoyl chloride in NMP (5 mL)
was added over 1 h (Scheme 3). The reaction mixture was main-
tained at 30 ꢀC for 24 h. Water (2 mL) was subsequently added to
quench the reaction and stirred for 30 min. It was washed twice
with basic, acidic, and deionized water then precipitated into
methanol. The product was filtered and dried at 40 ꢀC in a vacuum
oven for 24 h. Yield: 0.581 g dirty white powder, 92%; mp 353 ꢀC; IR
(KBr) ymax: 3323, 3081, 1728, 1660, 1192, 821, 677; 1H NMR
DMSO-d6, ppm): d 10.352–10.785 (m, amide NH), 9.741 (br, Ar OH),
8.789–9.152 (1,3,5-trisubstituted benzene), 7.076–8.450 (p-Ar),
3.44 (br, –NCH2CH3), 1.56 (br, –NCH2CH3). 13C NMR (75 MHz,
DMSO-d6, ppm):
d 166.45, 165.92, 165.64 (ester C]O), 164.95,
164.89, 164.78, 164.65, 164.58 (amide C]O), 155.21, 154.52, 148.62,
137.46, 135.64, 132.47, 131.32, 129.84, 128.85, 124.50, 123.21, 122.38,
121.64, 116.48, 62.40, 14.84. Yield: 6.86 g dirty pink powder, 85%;
HBPAE 2; IR (KBr): 3331, 3094, 1729, 1661, 1194, 826, 750, 676. 1H
NMR (300 MHz, DMSO-d6, ppm):
d 10.282–10.758 (m, amide NH),
9.724 (br, Ar OH), 8.748–9.125 (1,3,5-trisubstituted benzene),
7.074–8.351 (Ar), 3.44 (br, –NCH2CH3), 1.56 (br, –NCH2CH3). 13C
(300 MHz, DMSO-d6, ppm):
d 10.234–10.524 (m, 3H, amide NH),
NMR (75 MHz, DMSO-d6, ppm):
d
166.42, 165.90, 165.66 (ester
9.445 (br, 2H, Ar OH), 8.834 (s, 1H, 1,3,5-trisubstituted benzene),
8.785 (s, 2H,1,3,5-trisubstituted benzene), 8.403 (d, 2H, J¼8.8 Hz, p-
Ar), 8.175 (d, 2H, J¼8.9 Hz, p-Ar), 7.817 (d, 2H, J¼9.1 Hz, p-Ar), 7.572
(4H, d, J¼8.6 Hz, p-Ar), 7.230 (d, 2H, J¼9.0 Hz, p-Ar), 6.913 (d, 4H,
C]O), 164.95, 164.86, 164.72, 164.763, 164.51 (amide C]O), 156.34,
154.26, 148.75, 145.81, 138.53, 135.71, 132.67, 132.13, 130.62, 129.90,
128.85, 125.63, 124.82, 123.87, 122.24, 122.65, 62.41, 14.82. Yield:
6.60 g gray powder, 86%; HBPAE 3; IR (KBr): 3327, 3156, 2969, 1707,
J¼8.7 Hz, p-Ar). 13C NMR (75 MHz, DMSO-d6, ppm):
d 165.53 (ester
1656, 1170, 824, 677. 1H NMR (300 MHz, DMSO-d6, ppm):
d
10.275–
C]O), 164.82 (amide C]O), 164.54 (amide C]O), 154.17, 153.45,
147.26, 136.42, 134.46, 132.74, 131.23, 129.48, 128.58, 123.05, 122.12,
121.83, 121.06, 116.21. Anal. Calcd for C34H24N4O9: C, 64.56; H, 3.82;
N, 8.86%. Found: C, 64.54; H, 3.80; N, 8.83%.
10.657 (m, amide NH), 9.647 (br, Ar OH), 8.324–8.676 (1,3,5-
trisubstituted benzene), 6.954–7.455 (p-Ar), 1.29 (br, CH2),
1.761–2.012 (m, CH2), 2.247–2.461 (m, CH2), 3.44 (br, –NCH2CH3),
1.56 (br, –NCH2CH3). 13C NMR (75 MHz, DMSO-d6, ppm):
d 172.32,
168.64, 166.58 (ester C]O), 165.92, 165.76, 165.31, 164.83, 164.67
(amide C]O), 157.63, 145.28, 134.47, 128.54, 122.56, 122.03, 121.86,
121.50, 62.43, 33.28, 24.56, 14.85. Yield: 7.91 g light brown powder,
90%; HBPAE 4; IR (KBr): 3330, 3160, 2928,1705,1656,1172, 823, 677.
4.3.2. Synthesis of model compound 2. Using the same method,
synthesis of model compound 2 was pursued except that p-nitro-
benzoyl chloride (2.0 mmol) was used, finally the reaction solution
was poured into methanol, and the precipitate was dried at 40 ꢀC in
a vacuum oven for 24 h (Scheme 3). Yield: 0.711 g light yellow
powder, 91%; mp 361 ꢀC; IR (KBr) ymax: 3321, 3082, 1729, 1660, 1191,
1H NMR (300 MHz, DMSO-d6, ppm):
d 10.215–10.686 (m, amide
NH), 9.653 (br, Ar OH), 8.369–8.791 (1,3,5-trisubstituted benzene),
6.947–7.454 (p-Ar), 1.763–2.014 (m, CH2), 2.427–2.681 (m, CH2),
3.44 (br, –NCH2CH3), 1.56 (br, –NCH2CH3). 13C NMR (75 MHz,
823, 674. 1H NMR (300 MHz, DMSO-d6, ppm):
d 10.216–10.518 (m,
3H, amide NH), 9.514 (br, 1H, Ar OH), 8.987 (s, 2H, 1,3,5-tri-
substituted benzene), 8.868 (s, 1H, 1,3,5-trisubstituted benzene),
8.410 (d, 4H, J¼8.8 Hz, p-Ar), 8.182 (d, 4H, J¼8.9 Hz, p-Ar), 7.819 (d,
DMSO-d6, ppm):
d 172.41, 168.67, 166.60 (ester C]O), 165.93,
165.78, 165.32, 164.82, 164.69 (amide C]O), 157.87, 146.38, 135.71,
OH
OH
HN
C
O
O
NH
O
C
C
O
H
N
C
O
O
Y
C
O
O
O
O
C
HN
C
C
HN
NH
O
HO
OH
TEA
+
NH
C
NMP
ClOC
COCl
R
O
O
C
C
HN
NH
O
C
O
O
O
C
Y
Y
Hyperbranched polyamide-ester
(CH2)
(CH2)
R
4
8
HBPAE
3
1
2
4
Scheme 2. Scheme for the synthesis of HBPAE 1, 2, 3, and 4.

S. Shabbir et al. / Tetrahedron 66 (2010) 1389–1398
1397
C
O
NO2
O
OH
1
m/n = 1/1
NH
O C
NH
C
n
O
C
O
C
O
NH
HN
C
O
C
O
NH
HN
HO
OH
HO
OH
+
C
O
NO2
O
m
O2N
COCl
NH
O C
2
m/n = 2/1
C
O
C
O
NH
HN
O
OH
O C
NO2
Scheme 3. Scheme for the synthesis of model compounds 1 and 2.
4H, J¼9.1 Hz, p-Ar), 7.528 (2H, d, J¼8.6 Hz, p-Ar), 7.252 (d, 4H,
J¼9.0 Hz, p-Ar), 6.921 (d, 2H, J¼8.7 Hz, p-Ar). 13C NMR (75 MHz,
a rate of 10 ꢀC/min under nitrogen atmosphere. X-ray diffraction
pattern was obtained at room temperature on an X-ray diffrac-
DMSO-d6, ppm):
d
165.84 (ester C]O), 164.86 (amide C]O), 164.61
tometer (3040/60 X’pert PRO) using Ni-filtered Cu K
(40 kV, 30 mA) with scanning rate of 0.04ꢀ/s.
a radiation
(amide C]O), 154.18, 153.46, 147.26, 136.42, 134.46, 132.74, 131.25,
129.48, 128.60, 123.05, 122.12, 121.84, 121.06, 116.21. Anal. Calcd for
C41H27N5O12: C, 63.00; H, 3.48; N, 8.96%. Found: C, 62.97; H, 3.40; N,
8.95%.
Acknowledgements
S. Shabbir gratefully acknowledges the Higher Education Com-
mission of Pakistan (HEC) for fiscal assistance in this work under
Indigenous 5000 PhD Fellowship Scheme Batch-I.I. (042-111295-
PS2-175).
4.4. Measurements
The FTIR spectra of monomer and polymers were recorded at
room temperature, at a resolution of 4 cmꢁ1, using Excalibur Series
FTIR Spectrometer, Model No. FTSW 300 MX manufactured by BIO-
RAD. NMR spectrum was recorded at room temperature using
BRUKER Spectrometer operating at 300.13 MHz for 1H and at
75.47 MHz for 13C NMR spectra. Solvent used was deutrated di-
methyl sulfoxide (DMSO-d6). Elemental analysis was performed
using a Perkin–Elmer 2400 CHN elemental analyzer. Inherent vis-
cosity (hinh) was measured in DMSO at 30 ꢀC with an Ubbelohde
viscometer on polymer solutions with a concentration of 0.5 g/dL.
Qualitative solubility was determined with 15 mg of polymer in
1 mL of solvent at room temperature. Gel permeation chromatog-
raphy (GPC) was performed in THF as an eluent. The molecular
weight was calculated by GPC with a refractive index (RI) detector.
Thermal stability of the polymers was determined through MET-
TLER TOLEDO TGA/SDTA 851e thermo gravimetric analyzer using 1–
5 mg of the sample in Al2O3 crucible heated from 25 to 900 ꢀC at
a heating rate of 10 ꢀC/min under nitrogen atmosphere with a gas
flow rate of 30 ml/min. In order to obtain glass transition temper-
ature (Tg) differential scanning calorimetry (DSC) was performed by
METTLER TOLEDO DSC 822e differential scanning calorimeter, using
5–10 mg of samples encapsulated in aluminum pans and heated at
References and notes
1. Flory, P. J. J. Am. Chem. Soc. 1952, 74, 2718–2723.
2. Kim, Y. H.; Webster, O. W. J. Am. Chem. Soc. 1990, 112, 4592–4593.
3. Hawker, C. J.; Lee, R.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1991, 113, 4583–4588.
´
4. Wooley, K. L.; Hawker, C. J.; Lee, R.; Frechet, J. M. J. Polym. J. 1994, 26, 187–197.
5. Fre´chet, J. M. J. Science 1999, 263, 1710–1715.
6. Kambouris, P.; Hawker, C. J. J. Chem. Soc. Perkin Trans. 1993, 1, 2717–2721.
7. Parker, D.; Feast, W. J. Macromolecules 2001, 34, 2048–2059.
8. Lin, Q.; Long, T. E. J. Polym. Sci. Part A: Polym. Chem. 2000, 38, 3736–3741.
9. Li, X. R.; Li, Y. S. Polymer 2003, 44, 3855–3863.
10. Jikei, M.; Kakimoto, M. Prog. Polym. Sci. 2001, 26, 1233–1285.
11. Mock, A.; Burgath, A.; Hanselmann, R.; Frey, H. J. Macromolecules 2001, 34,
7692–7698.
12. Fre´chet, J. M. J.; Hawker, C. J.; Gitsov, I.; Leon, J. W. J. Macromol. Sci. Pure Appl.
Chem. 1996, A33, 1399–1425.
13. Burchard, W. Adv. Polym. Sci. 1999, 143, 113–194.
14. Aharoni, S. M.; Murthy, N. S.; Zero, K.; Edward, S. F. Macromolecules 1990, 23,
2533–2549.
15. Jikei, M.; Chon, S. H.; Kakimoto, M.; Kawauchi, S.; Imase, T.; Watanabe, J.
Macromolecules 1999, 32, 2061–2064.
16. Emrick, T.; Chang, H. T.; Fre´chet, J. M. J. Macromolecules 1999, 32, 6380–6382.
´
17. Emrick, T.; Chang, H. T.; Frechet, J. M. J. J. Polym. Sci. Part A: Polym. Chem. 2000,
38, 4850–4869.

1398
S. Shabbir et al. / Tetrahedron 66 (2010) 1389–1398
18. Monticelli, O.; Mariani, A.; Voit, B. I.; Komber, H.; Mendichi, R.; Pitto, V.; Tab-
uani, D.; Russo, S. High Perform. Polym. 2001, 13, S45–S59.
19. Russo, S.; Boulares, A.; da Rin, A.; Mariani, A.; Cosulich, M. E. Macromol. Symp.
1999, 143, 309–313.
27. Turner, S. R.; Walter, F.; Voit, B. I.; Mourey, T. H. Macromolecules 1994, 27,1611–1616.
28. Turner, S. R.; Voit, B. I.; Mourey, T. H. Macromolecules 1993, 26, 4617–4623.
29. Knauss, D. M.; McGrath, J. E.; Kashiwagi, T. ACS Symp. Ser. (Fire and Polymers)
1995, 599, 41–55.
20. Hao, J. J.; Jikei, M.; Kakimoto, M. A. Macromolecules 2003, 36, 3519–3528.
21. Komber, H.; Voit, B. I.; Monticelli, O.; Russo, S. Macromolecules 2001, 34,
5487–5493.
22. Fang, J.; Kita, H.; Okamoto, K. Macromolecules 2000, 33, 4639–4646.
23. Kricheldorf, H. R.; Fritsch, D.; Vakhtangishvill, L.; Schwarz, G. Macromolecules
2003, 36, 4337–4344.
30. van Benthem, R. A. T. M. Prog. Org. Coat. 2000, 40, 203–214.
31. van Benthem, R. A. T. M.; Meijerink, N.; Gelade, E.; Koster, C. G.; Muscat, D.;
Froehling, P. E.; Hendriks, P. H. M.; Vermeulen, C. J. A. A.; Zwartkruis, T. J. G.
Macromolecules 2001, 34, 3559–3566.
32. Romagnoli, B.; Ashton, P. R.; Harwood, L. M.; Philp, D.; Price, D. W.; Smith, M.
H.; Hayesa, W. Tetrahedron 2003, 59, 3975–3988.
24. Yan, D.; Gao, C. Macromolecules 2000, 33, 7693–7699.
25. Munez, C. M.; Choiu, B.; Andrady, A. L.; Khan, S. A. Macromolecules 2000, 33,
1720–1726.
26. Viot, B. I.; Eigner, M.; Estel, K.; Wenzel, C.; Bartha, J. W. Macromol. Symp. 2002,
177, 147–154.
33. Kricheldorf, H. R.; Vakhtangishvili, D.; Fritsch, D. J. Polym. Sci. Part A: Polym.
Chem. 2002, 40, 2967–2978.
34. Mi, Y.; Stern, S. A.; Trohalaki, S. J. Membr. Sci. 1993, 77, 41–48.
35. Morikawa, A. Macromolecules 1998, 31, 5999–6009.
36. Kim, Y. H.; Beckenbauer, R. Macromolecules 1994, 27, 1968–1971.