Pyrimidine based carboxylic acid terminated aromatic and semiaromatic hyperbranched polyamide-esters: Synthesis and characterization

Article abstract of DOI:10.1016/j.tet.2010.06.065

Carboxylic acid terminated aromatic and semiaromatic hyperbranched polyamide-esters (HBPAEs) containing pyrimidine moieties were prepared by polycondensation of 4-hydroxy-2,6-diaminopyrimidine (CBB′) to a double molar ratio of various diacid chlorides (A₂) without any catalyst. The products were soluble in organic solvents, such as N,N-dimethylformamide, N-methyl-2-pyrrolidone and displayed glass transition temperature (T _g) between 180 and 244 °C. The polymerization products have been investigated with FTIR, ¹H and ¹³C NMR analyses and the degree of branching was higher than 60%. Amorphous polymers had inherent viscosity (η_inh) ranging between 0.21-0.28 dL/g and had excellent thermal stability with 10% weight loss at 346-508 °C.

Full text of DOI:10.1016/j.tet.2010.06.065

Tetrahedron 66 (2010) 7204e7212
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Pyrimidine based carboxylic acid terminated aromatic and semiaromatic
hyperbranched polyamide-esters: synthesis and characterization
,
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:
Carboxylic acid terminated aromatic and semiaromatic hyperbranched polyamide-esters (HBPAEs)
containing pyrimidine moieties were prepared by polycondensation of 4-hydroxy-2,6-diaminopyr-
imidine (CBB0) to a double molar ratio of various diacid chlorides (A2) without any catalyst. The products
were soluble in organic solvents, such as N,N-dimethylformamide, N-methyl-2-pyrrolidone and dis-
played glass transition temperature (Tg) between 180 and 244 ꢀC. The polymerization products have been
investigated with FTIR, 1H and 13C NMR analyses and the degree of branching was higher than 60%.
Amorphous polymers had inherent viscosity (hinh) ranging between 0.21e0.28 dL/g and had excellent
thermal stability with 10% weight loss at 346e508 ꢀC.
Received 20 November 2009
Received in revised form 4 May 2010
Accepted 24 June 2010
Available online 19 July 2010
Keywords:
Hyperbranched polyamide-ester
Pyrimidine
Ó 2010 Elsevier Ltd. All rights reserved.
Synthesis
Characterization
1. Introduction
permit to prepare HBPs on the basis of readily available commercial
monomers. In this perspective, our recent effort has focused on the
The interest in hyperbranched polymers (HBPs) increased rap-
idly over the past decade due to their unique physical and chemical
properties as compared to their linear analogues.1e13 HBPs were
prepared mainly by polycondensation of an ABx type monomer,
which has one ‘A’ functional group and x ‘B’ functional groups.14
This approach was widely used to synthesize a variety of HBPs,
development of a facile synthetic approach to more families of
HBPs. We found that the reactivity of three types of functionalities
in a trifunctional monomer, 4-hydroxy-2,6-diaminopyrimidine
(HDAP), is different in polymerization with diacid chlorides.
Therefore, the intermediate with diacid chloride groups and a -
hydroxyl group may form at the initial stage of the reaction. Further
polycondensations of such intermediates would lead to HBPAEs
without gelation, as shown in Scheme 1. So, HBPAEs with terminal
carboxylic acid group were formed via A2C type intermediate
(Scheme 1), which is produced through the reaction of the
more reactive B and B0 groups with the double molar concentration
of A2 monomer. The approach reported here would facilitate fur-
ther optimization of HBPAEs for wide applications. Thus,
copolymerization reaction of HDAP with terephthaloyl chloride
(TPC), isophthaloyl chloride (IPC), resulted in two fully aromatic
HBPAE 21 and HBPAE 22, whereas two semi-aromatic HBPAE 23
and HBPAE 24 originated from sebacoyl chloride (SC), and adipoyl
chloride (AC). Conversely to classical A2þCBx approach, in this case
the molar ratio of A2 to CBB0 equals 2:1 and no gelation occurs in
solution polymerization where A2C dimer is formed initially which
can be considered as a new A2B type of monomer. Further reaction
among A2C molecules resulted in the formation of HBPAEs. Con-
trary to the classical methods, the strategy reported here capitu-
lated HBPAEs without gelation by unequal reactivity of CBB0
monomer. In this context, A2 monomer was added in molar excess
to CBB0 monomer and during the progress of synthesis, excess
18
such as polyethers,15 polyesters,16 polyamides,17 polyurethanes,
and so forth. Since, most of ABx type monomers are unavailable
commercially and synthesis of a new ABx monomer is cumber-
some. Therefore, the approaches via the polymerization of multi-
functional monomers with equal reactivity were developed,
however, A2þBx, must be performed under stringent conditions to
avoid gelation, such as very low monomer concentrations, strictly
controlled feed molar ratios, slow monomer addition rates and
low monomer conversions.19e21 Nevertheless, A2þBx approaches
adopting monomers of suitable unequal reactivity have been
demonstrated to be practical to prepare HBPs from commercially
available monomers in one-pot processes.22e26 These monomers
with unequal reactivity can be Bx type monomers with B of dif-
22e24
ferent reactivity
or CBx type monomers.25,26 Despite the
numerous developments in the field of HBPs, the main challenge,
however, lies in the development of facile synthetic strategies that
* Corresponding author. Tel.: þ92 51 90642132; fax: þ92 51 90642241; e-mail
address: ilyassarwar@hotmail.com (M.I. Sarwar).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.06.065

S. Shabbir et al. / Tetrahedron 66 (2010) 7204e7212
7205
Scheme 1. Scheme for the synthesis of HBPAEs.
A2 monomer limited the molecular weight to increase so the risk of
gel formation would be avoided. Compared to core-dilution
method, the current synthetic stratagem was much simpler and it
was hard to produce outgrowth. Moreover, compared with slow
addition method, the present synthetic approach was more con-
venient and facilitated attaining HBPAEs without gelation. To the
best of our knowledge there was no report regarding the synthesis
of HBPAEs containing pyrimidine units through 2A2þCBB0
approach. Hence, the purpose of this article is to present our
findings on synthesis of HBPAEs incorporating pyrimidine entities
and its property profile in conjunction with the effect of in-
troducing pyrimidine rings, as well as their structure property
relationship. Therefore, HBPAEs were characterized in order to
acquire a clear understanding of the influence exerted by structural
variations in these polymers upon some of important properties,
such as solubility, molar mass, Tg, hinh, crystallinity, and thermal
stability. The interest in these HBPAEs stems from the fact that
heterocyclic pyrimidine units provide not only thermal stability,
but also high electron affinity for materials useful in electronic
applications.27 Hence, this promising potential led to the synthesis
of HBPAEs with the heterocyclic pyrimidine moiety. Additionally,
intermolecular hydrogen bonding between pyrimidine nitrogens
and the amide NHs of adjacent molecules provides the basis for
material uniqueness. Quintessentially, pyrimidine moieties play
a vital role in producing outstanding thermal facade of HBPAEs;
therefore, pyrimidine rings influenced the structural and material
characteristics of these HBPAEs.
strong leaving tendency of the phenylate moiety, so high molec-
ular weight aromatic polyesters are prepared either by activated
acid (acid chloride) or by acetylating or silylating the phenolic
function. This means that HBPAEs with moderately high molecular
weight can be obtained via the elaborate selection of monomers
and optimization of reaction conditions. Accordingly, in current
approach, HBPAEs were prepared based on CBB0 type monomer
(HDAP) and series of A2 type diacid chloride monomers (TPC, IPC,
AC, and SC) as starting materials. The lower reactivity of amine at 2
position as compared with amine at 6 position was already
demonstrated by 1H NMR.24 Due to electronegative nitrogen
atoms in pyrimidine ring amine group at 2 position is less reactive
than amine group at 6 position due to electron withdrawing
inductive effect of heteroatom (N). Consequently, this method
benefits from the fact that 2-amine group had a lower reactivity
than the 6-amine group in HDAP, so, no gelation was observed
during the polymerization probably due to the different
reactivities of amine groups at 2 and 6 positions. Besides, it is also
known that nucleophilic reactivity of amine group is much greater
than that of hydroxyl group.28 Hence, for this trifunctional
monomer with less steric hindrance for amine at 6 position,24 the
reactivity sequence of different groups is as follows; 6 amine (B0)>
2 amine (B)>4 OH (C). It is reasonable to speculate that the po-
lymerization of 2A2þCBB0 can be described by Scheme 1 on ac-
count of the reactivity sequence of CBB0 monomer. Initially, amine
at 6 position would react with A2 at 0 ꢀC, forming ABC type pre-
cursor, then the residual A2 would react with amine on 2 position
at the aforementioned temperature, producing A2C type in-
termediate and finally HBPAEs were obtained from polymerization
of in situ formed A2C type intermediate at 30 ꢀC. No gelation oc-
curred through this polymerization process. The formation of A2C
type intermediate was verified by the synthesis and isolation of
A2C intermediate, which was subsequently characterized by CHN,
FTIR, 1H, and 13C NMR techniques. These characterizations dem-
onstrated that chemical composition of A2C is consistent with the
proposed structure. Furthermore, data from elemental analysis
was in good agreement with the calculated values. According to
the mechanism described in Scheme 1, HBPAEs obtained may
2. Results and discussion
2.1. Polymer synthesis
Structure of the pyrimidine ring with both rigidity and polar-
izability was of interest. Thus, heteroaromatic monomer, con-
taining the pyrimidine unit was exploited to prepare novel
heteroaromatic HBPAEs with good thermostability and process-
ability. Typically, the direct condensation of bisphenols and diacids
cannot yield high molecular weight polyesters because of the

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S. Shabbir et al. / Tetrahedron 66 (2010) 7204e7212
contain focal units as those from normal AB2 type monomers.
Therefore, HBPAEs with carboxylic acid terminal groups were
prepared by performing polymerization of 2A2þCBB0 in DMAc.
The first reaction was amidation (formation of amide linkage)
producing an A2C unit (with two acid chloride (A) groups and one
alcohol (C) group). The following reaction was esterification giving
rise to hyperbranched structures, which indicated that carboxyl
chloride had mostly reacted with alcohol groups, and A2C units
had esterified to ensuing hyperbranched structures. So tempera-
ture of reaction mixture was gradually raised to 30 ꢀC and kept
there for 6 h to finish the esterification. Owing to the existence of
an amount of reactive terminal acid chloride groups, cross-linking
occurred easily in the purification processes of acid chloride
terminated HBPAEs, so solid pure HBPAEs could not be obtained.
Therefore, the terminal acid chloride groups were changed into
stable carboxylic acid terminal groups by quenching with water.
Structure elucidation of the synthesized HBPAEs was carried out
by FTIR and NMR techniques. Figure 1 shows representative FTIR
spectra of the HBPAEs. The broad band between 2500 and
3500 cmꢁ1 were attributed to H-bonded amide NeH and acid
OeH stretches. The strong and broad band between
3331e3325 cmꢁ1 is chiefly attributed to OeH stretching of the H-
bonded OH groups, which is the distinguishing feature of the
COOH group. This broad band in the range of 3331e3325 cmꢁ1 is
also typical for the stretching of H-bonded OeH and NeH groups.
The spectra of HBPAE 21, 22, 23, and 24 showed bands around
3185e3012 cmꢁ1 for aromatic CeH stretching. Amide C]O stretch
was observed between 1642e1648 cmꢁ1. While carboxylic acid
C]O and ester C]O bands appeared in the region of 1700e1687
and 1735e1725 cmꢁ1
, respectively, confirming the structure.
Moreover, IR spectra for HBPAE 23 and 24 displayed bands within
the range of 2856e2948 cmꢁ1 for aliphatic CeH stretching
vibration. The bands at 1515, 1500 and, 1450 cmꢁ1 are due to ar-
omatic ring C]C stretching and bands at 1048e1056 and
1200e1242 cmꢁ1 are due to the ester CeO stretching vibrations.
All spectra are in compliance with the proposed structures. Based
on the preceding results, analytical data indicated that HBPAEs
contained carboxyl acid end groups, ester bonds and amide link-
ages, which are in conformity with objective polymers. Degree of
branching was determined through 1H NMR analysis as proton
nuclei gave well separated peaks. The solubility of these HBPAEs in
different solvents was evaluated. Thermal stability, Tg, hinh, and
crystallinity were examined using the corresponding techniques
as described below.
2.2. Degree of branching in HBPAEs
The degree of branching (DB) of HBPAEs can be evaluated
by NMR spectroscopy, which was defined in the following
29
equation:
DB ¼ ðnumber of dendritic units
þ number of terminal units=number of total unitsÞ
Figure 1. FTIR spectra of HBPAEs.
Here DBs of HBPAEs should be assessed by taking the A2C type
intermediate as a starting monomer. Accordingly, A2C type in-
termediate was synthesized, which, through a series of reactions,
led to the formation of terminal, linear, and dendritic model com-
pounds 1, 2, and 3, respectively. In this way, the terminal, linear, and
dendritic units were defined in Figure 2. Yet, the structural differ-
ence among the terminal, linear, and dendritic unit could be dis-
tinguished in 1H NMR, so the DBs according to Fréchet’s definition
observed. Compared with the 1H NMR spectra of the model
compounds, the peaks of 1H NMR spectra of the polymers were
broader, including the carboxyl acid proton, due to the repeat units
and the branched architecture of the polymers. The inbuilt property
of HBPs is their broad molecular size distribution, which also led to
significant peak broadening in 1H NMR spectrum. The peak of
carboxyl proton became broad and shifted downfield slightly
(13.48,13.40, 13.31, 13.29 ppm) because of intermolecular hydrogen
bonding. There is prominent intermolecular H-bonding in amide
29
were easy to be measured. In the 1H NMR spectrum of HBPAEs,
the resonance of the proton of the amide group appears in the
range of 10.01e10.95 ppm, and a broad peak at 13.29e13.48 ppm
from the proton of the terminal carboxylic acid group is also

S. Shabbir et al. / Tetrahedron 66 (2010) 7204e7212
7207
unit as the criterion for the estimation of DB. These include one
dendritic unit (D), which has no COOH group; two linear units
(L1 and L2) with one COOH group and one terminal unit (T) with
two COOH groups (Fig. 2). In some cases, it is also possible to
identify the single focal unit A per molecule. But, as expected, the
content of structural units with A function (one focal unit per
molecule) is so small that it cannot be detected.30 In addition,
electro-deficient characteristic of pyrimidine ring causes broad
COOH resonance peak in more downfield region (13.48 ppm). The
amide proton region (10.01e10.95 ppm) is inappropriate for
quantification due to concentration effects on chemical shifts and
line widths. Since the chemical environment of proton in pyrimi-
dine ring of D, L1, L2, and T units will be substantially different; the
relative percentage of D, L1, L2, and T can be evaluated by 1H NMR
spectroscopy. To discriminate four subunits of the polymer back-
bone, model compounds 1, 2, and 3 were synthesized (Scheme 3)
and used as the model compound for terminal, linear, and dendritic
subunits, respectively. Comparison of the 1H NMR spectra of these
model compounds with 1H NMR spectrum of HBPAE 1 allowed the
resonance of D, L1, L2, and T subunits to be identified. Figure 3 shows
the 1H NMR spectra of model compounds 1e3, and HBPAEs. Owing
to structural uniqueness of pyrimidine moiety two types of linear
subunits were identified in 1H NMR spectrum of HBPAE 21 along
with model compound 2. Two distinct resonances for linear model
compound 2, were observed at 6.52 and 6.68 ppm while the cor-
responding proton for the dendritic model compound 3, and ter-
minal model compound 1, appeared at 6.90 and 6.43 ppm,
respectively. Multiplets in the region of 7e8 ppm belong to aro-
matic ring protons.
Whereas, in 1H NMR spectrum of HBPAE 21 related protons
were observed at 6.51e6.90 ppm. As compared with the 1H NMR
spectra of the model compounds, resonances at 6.62 and 6.80 ppm
could be assigned to L2 and L1 subunits, while the resonances at
6.51 and 6.90 ppm could be assigned to the T and D subunits,
respectively. Peak integration of resonances allowed the relative
percentage of three subunits, dendritic (Xd), linear (XL1, XL2), and
terminal unit (Xt) in HBPAE 21 and DB was evaluated. Therefore, on
the basis of the 1H NMR measurements, HBPAE 21 is a hyper-
branched macromolecule with numerous terminal units and DB
calculated by Fréchet’s equation29 was 0.67. Using the same ana-
lyzing strategy discussed above for the HBPAE 21, DB values were
evaluated to be 0.64, 0.62, and 0.61 for HBPAE 22, 23, and 24, re-
spectively (Table 1). Furthermore, since functional groups of
monomers exhibit different reactivity, the degree of branching for
HBPAEs is higher than 60%.
Figure 2. Structural repeat units of HBPAE 21.
NH and nitrogen of pyrimidine (Scheme 2) group because of the
abundant amide NH in the interior of the polymer molecule. Since
H-bonding diminishes shielding, the NH signals move downfield
while intermolecular H-bonding is enhanced. Although H-bonding
KT
KL
Terminal / Linear / Dendritic
When the rate of the formation of linear units is greater than
rate of formation of terminal units (kL>kT), the amount of dendritic
units would be increased during the polymerization of ABx type
monomers.30 It was observed (Table 1) that XL became larger than
Xt thus leading to enhanced DB of HBPAEs.
2.3. Organosolubility
Normally, high solubility is a preferred requirement for pro-
cessing polymers. All the carboxylic acid terminated HBPAEs had
good solubility in DMSO and NMP. Effect of introducing rigid amide
structure and heteroaromatic pyrimidine units in the backbone was
strongly depicted in solubility besides thermal properties (Table 2).
Enhanced solubility of HBPAEs could be endorsed to both nature of
terminal functional groups and highly branched architecture.
Nevertheless, inferior solubility relative to earlier HBPAEs 26 is due
to the presence of pyrimidine structural units that aggravate mac-
romolecule hydrogen bonding between amide NHs and pyrimidine
Scheme 2. Schematic representation of hydrogen bonding in HBPAEs.
exists between the COOH groups, it is much weaker than the amide
group because amide groups lie in the interior of polymer along
with pyrimidine units. The H-bonding is accompanied by exchange
of H’s from one molecule to another, resulting in signal broadening.
For HBPAE 21, the overall structure reveals four different types of
subunits based on pyrimidine proton, thus, rendering pyrimidine

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S. Shabbir et al. / Tetrahedron 66 (2010) 7204e7212
Scheme 3. Scheme for the synthesis of model compounds.
Table 1
hinh, Mw, and DB of HBPAEs in DMSO
a
b
Polymer
hinh
(dL/g)
Mw
PDI¼
b
DB (Fréchet)c¼
Unit fraction
Mw/Mn (XdþXt)/
cXd cXL1 XL2 cXt
c
(XdþXtþXL1þXL2
)
HBPAE 21 0.28
HBPAE 22 0.25
HBPAE 23 0.23
HBPAE 24 0.21
__
__
__
__
0.67
0.37 0.12 0.21 0.30
0.35 0.13 0.23 0.29
0.31 0.13 0.25 0.31
0.30 0.14 0.25 0.31
0.64
0.62
0.61
58,858 2.5
41,954 2.2
a
Measured in DMSO at a concentration of 0.5 g/dL at 30 ꢀC.
Determined by GPC based on polystyrene standards.
Xd, dendritic unit fraction; XL1 and XL2, linear unit fractions; Xt, terminal unit
b
c
Figure 3. 1H NMR spectra of A: model compound 1; B: model compound 2; C: model
compound 3; D: HBPAE 21; E: HBPAE 22; F: HBPAE 23; G: HBPAE 24.
fraction. The four unit fractions are calculated from the integration of the decon-
voluted peaks assigned to the central pyrimidine unit in the 1H NMR spectra.

S. Shabbir et al. / Tetrahedron 66 (2010) 7204e7212
7209
Table 2
-2
Solubilitya behavior of different HBPAEs
Polymer
Solventb
NMP
DMAc
DMF
DMSO
THF
CHCl3
HBPAE 21
HBPAE 22
HBPAE 23
HBPAE 24
þ þ
þ þ
þ þ
þ þ
þ ꢁ
þ ꢁ
þ þ
þ þ
þ ꢁ
þ ꢁ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
-1
0
a
Solubility: þþ soluble at room temperature, þꢁ partially soluble at room
temperature, ꢁ insoluble.
HBPAE 21
HBPAE 22
HBPAE 23
HBPAE 24
b
Scale: 15 mg/1 mL.
nitrogens. Figure 3 clearly demonstrates the existence of hydrogen
bonding as deshielding via intermolecular hydrogen bonding
caused COOH and NH signals to move downfield. Prominently the
presence of alkyl groups in the semi-aromatic HBPAEs derived from
aliphatic acid chlorides caused improvement of solubility in con-
trast to fully aromatic HBPAEs. In view of that, HBPAE 21 and 22,
derived from aromatic diacid chlorides with more rigid structure,
exhibited lower solubility in various solvents.
0
50
100
150
200
250
300
Temperature (°C)
Figure 4. DSC thermograms of HBPAEs at heating rate of 10 ꢀC/min in N2.
Table 3
Thermal analysis data of different HBPAEs
2.4. Viscometry and molar mass
Polymer
Tg (ꢀC)a
T0 (ꢀC)b
T10 (ꢀC)c
Tmax (ꢀC)d
Yc at 600 ꢀ
C
(%)e
Low viscosity of HBPAEs relative to linear analogues was
suggested to originate from their compact molecular nature, i.e.,
globular shape with less physical entanglement based on highly
branched structure. However, hinh of HBPAEs, 0.21e0.28 dL/g
(Table 1), is comparable to hyperbranched polyester-amides
reported by Liu et al.31 indicating a moderate molecular weight.
Their hinh amplified with the increase of p-phenylene unit. This
indicated that the polymer chain rigidity increased as a function
of their para linkages (Table 1). On the contrary, hinh decreased
only slightly with the incorporation of flexible alkyl linkage in
HBPAE 23 and 24, when judged against HBPAE 21 and 22. Table 1
also lists the Mw and polydispersity index (PDI) of the resultant
HBPAE 23 and 24 by GPC analysis. It is previously established
that this method has limited suitability for HBPs and usually
undervalues the exact molar mass of HBPs due to their smaller
hydrodynamic radii compared to linear analogues.29 On the
other hand, the interactions of the polar end group with solvent
are stronger, which overrates the actual value of molecular
weight. But for a HBP containing a long linear aliphatic chain,
these two effects may decline and the determined mass may be
close to the true value. Besides, the HBPAEs exhibited moderate
molecular weights with broad molecular weight distributions
due to the highly branched structure. The PDI were high, which
was a typical result for ABx polycondensation predicted by Flory.1
HBPAE 21
HBPAE 22
HBPAE 23
HBPAE 24
244
227
207
180
435
425
318
281
508
498
364
346
583
564
543
532
71
68
55
54
a
Tg: Glass transition temperature.
b
c
T0: Initial decomposition temperature.
T10: Temperature for 10% weight loss.
Tmax: Maximum decomposition temperature.
Yc: Char yield; weight of polymer remained.
d
e
2.5. Thermal analyses
Figure 5. TGA curves of HBPAEs at heating rate of 10 ꢀC/min in N2.
The thermal profile of HBPAEs was evaluated in N2 at a heating
rate of 10 ꢀC/min using DSC and TGA techniques. Polymers showed
Tgs ranging from 180 to 244 ꢀC (Fig. 4) according to their DSC
analysis (Tg was taken as the midpoint of the change in slope of the
baseline). In the present investigation, COOH-terminated HBPAE 21
has a Tg of 244 ꢀC and HBPAE 22, 23, and 24 was lower, 227, 207, and
180 ꢀC, respectively (Table 3). HBPAE 23 and 24 exhibited much
lower Tgs than HBPAE 21 and 22 due to the plasticization of the
more flexible alkyl groups.
evaluation of thermal stability, were in the range of 346e508 ꢀC
and also char yields at 600 ꢀC were about 54e71%, which is in-
dicative of good thermal stability. It was found that T10 of HBPAE 21
was slightly higher than that of HBPAE 22, which may be attributed
to the fact that the meta-oriented architecture have less dense
polymer chain packing than p-oriented ones due to the smaller
conformational entropy of p-oriented chain. Consequently, the
former has better thermal stability than the latter.
In addition, the higher Tg of HBPAEs compared to that of former
26
HBPAEs
might be ascribed to the existence of heteroaromatic
pyrimidine rings. Besides, thermal stabilities of HBPAEs were
evaluated by thermogravimetric analysis (TGA) and initial de-
composition temperature (T0) ranged between 281e435 ꢀC (Fig. 5).
Temperatures for 10% gravimetric loss (T10), significant for the
According to the acquired results, these HBPAEs established
high thermal stability that could be ascribed to the incorporation
of heterocyclic pyrimidine moieties and pre-formed amide groups
as well as the existence of intermolecular hydrogen bonding

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S. Shabbir et al. / Tetrahedron 66 (2010) 7204e7212
between pyrimidine nitrogen and amide NH. Hence, all resulting
HBPAEs are thermally stable owing to their aromatic heterocyclic
structure.
No. FTSW 300 MX manufactured by BIO-RAD. NMR spectrum was
recorded at room temperature using BRUKER Spectrometer oper-
ating at 300.13 MHz for 1H NMR spectra. Solvent used was deu-
terated dimethyl sulfoxide (DMSO-d6). Elemental analysis was
performed using a PerkineElmer 2400 CHN elemental analyzer.
Inherent viscosity (hinh¼lnhr/c) was measured in DMSO at 30 ꢀC
with an Ubbelohde viscometer on polymer solutions with a con-
centration of 0.5 g/dL. Qualitative solubility was determined with
15 mg of polymer in 1 mL of various solvents at room temperature.
Gel permeation chromatography (GPC) was performed in DMF
containing 0.01 mol/L of lithium bromide as an eluent. The absolute
molecular weight was calculated by GPC with a light scattering
detector. Thermal stability of the polymers was determined by
METTLER TOLEDO TGA/SDTA 851e thermogravimetric analyzer
using 1e5 mg of the sample in an 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
temperature, differential scanning calorimetry (DSC) was per-
formed by a METTLER TOLEDO DSC 822e differential scanning cal-
orimeter, using 5e10 mg of samples encapsulated in aluminum
pans and heated at a rate of 10 ꢀC/min under nitrogen atmosphere.
X-ray diffraction pattern was performed at room temperature on an
2.6. X-ray diffraction
According to Figure 6, HBPAE 21 and 22 possess a slight degree
of ordering with hydrogen bonding derived from amide and
pyrimidine components exhibiting a minute crystalline pattern.
However, there were no crystalline melting transitions (Tm)
detected by DSC (Fig. 4). This probably indicated that either HBPAEs
were predominately amorphous with only a low degree of molec-
ular ordering, or that the Tm value of ensuing HBPAEs was higher
than the measurement range used in DSC experiment. By and large,
HBPs are identified not to crystallize and amorphous behavior
recommended that the branched structure hindered crystalliza-
tion. Finally, the amorphous character is also supported by the
reasonable solubility of HBPAEs.
X-ray diffractometer (3040/60 X’pert PRO) using Ni-filtered Cu K
a
radiation (40 kV, 30 mA) with scanning rate of 0.04ꢀ/s.
4.2. Materials
4-Hydroxy-2,6-diaminopyrimidine (HDAP), benzoyl chloride,
TPC, IPC, SC, and AC were procured from Aldrich and used as
received. N-Methyl-2-pyrrolidone (NMP), dimethyl sulfoxide
(DMSO), and thionyl chloride (SOCl2) were obtained from Merck
and used as received. N,N-Dimethylacetamide (DMAc), methanol,
N,N-dimethylformamide (DMF), and tetrahydrofuran (THF) were
received from Aldrich and distilled prior to use.
Figure 6. X-ray diffraction curve of HBPAEs.
4.3. Hyperbranched polyamide-esters synthesis
3. Conclusions
The synthesis of HBPAE 21, 22, 23, and 24 was carried out in a dry
250 mL four-necked round bottom flask equipped with a magnetic
stirrer, a thermometer and a drying tube placed in a dry box. HDAP
(10 mmol) was charged into the reaction flask, followed by the ad-
ditionof50 mL dryDMAcwithcontinuousstirring. Thereaction flask
was placed in an ice-salt bath and cooled at 0 ꢀC for 15 min. After
complete dissolution, 20 mmol of the corresponding diacid chloride
was added at once (Scheme 1). Stirring was continued for an addi-
tional 2 h at the above-mentioned temperature. Then the tempera-
ture of the reaction mixture was allowed to rise gradually to 30 ꢀC
and further agitated for 6 h. The homogeneous reaction was main-
tained at 30 ꢀC for 24 h. Water (10 mL) was subsequently added to
quench the residual acid chloride and the heterogeneous solution
was stirred for 30 min. A stoichiometric amount of TEAwas added to
remove HCl produced during the polymerization reaction otherwise
it would act as an impurity that might yield a low molecular weight
polymer. The solution was centrifuged to isolate the precipitates
from the neat HBPAEs. Finally, the polymer solution was slowly
poured into methanol with uniform stirring uponwhich precipitates
of HBPAE immediately formed. The polymer was isolated by filtra-
tion, washed successively with methanol and water. Polymer sam-
ples were purified by repeated precipitation from their solutions in
DMAc using methanol as non-solvent. The precipitated polymers
were isolated, washed and dried under vacuum at 80 ꢀC for 24 h.
On the basis of the reactivity sequence for CBB0 type monomer
in the polymerization with A2 monomers, 2A2þCBB0 approach to
HBPAEs was set up. Pyrimidine containing HBPAEs with carboxylic
acid terminal groups were obtained by polymerization of CBB0 with
a double molar A2 via A2C type intermediate that originated from
its ABC type precursor when polymerization was performed in
DMAc from 0 to 30 ꢀC. HBPAEs obtained using a two-fold molar-
excess of A2 were characterized by FTIR and 1H NMR spectroscopy
and had hinh between 0.21e0.28 dL/g. Thermal behavior of these
soluble HBPAEs was studied by TGA exhibiting ample thermal
stability owing to amide linkages and pyrimidine groups.
The degree of branching determined by 1H NMR ranged between
0.67 and 0.61. DSC served to envisage Tg of amorphous polymers
sited between 180 and 244 ꢀC. Introduction of pyrimidine ring with
rigidity and aromaticity, as well as, the contribution of polariz-
ability resulting from nitrogen atom in pyrimidine ring has been
substantiated. Prospective utilizations of these HBPAEs include
reactive COOH chain ends that remain after polymerization, which
present sites that can be employed for surface or bulk modification.
4. Experimental
Yield: 4.52 g pale yellow powder, 85%; HBPAE 21; IR (KBr) nmax
:
4.1. Measurements
3300e2500 (NeH, OH), 3012, 3165 (Ar CeH),1735 (ester C]O),1700
(acid C]O), 1648 (amide C]O), 1515, 1450 (Ar C]C), 1321, 1242,
1202,1050 (CeO), 812 (p-Ar), 502, 451.1H NMR (300 MHz, DMSO-d6,
IR spectra of monomer and polymers were monitored at a res-
olution of 4 cmꢁ1, using Excalibur Series FTIR Spectrometer, Model
ppm):
d 13.49 (br, COOH), 10.01e10.69 (m, amide NH), 8.32 (d,

S. Shabbir et al. / Tetrahedron 66 (2010) 7204e7212
7211
J¼9.3 Hz, p-Ar), 8.05 (d, J¼9.1 Hz, p-Ar), 7.75 (d, J¼9.2 Hz, p-Ar), 7.51
(d, J¼9.0 Hz, p-Ar), 6.90 (s, pyrimidine), 6.81 (s, pyrimidine), 6.63 (s,
pyrimidine), 6.51 (s, pyrimidine).13C NMR (75 MHz, DMSO-d6, ppm):
The reaction mixture was maintained at 30 ꢀC for 24 h. Water (2 mL)
was subsequently added to quench the reaction and stirring was
continued for 30 min. A stoichiometric amount of TEA was used as
HCl scavenger. The solution obtained was poured into methanol and
precipitates were washed with water and methanol successively, to
furnish model compound 1 after drying in vacuum. Yield: 0.5054 g
d
176.82, 176.61, 176.36, 176.28 (acid C]O), 176.01, 175.84, 175.65,
175.43, 175.25, 175.04, 174.97 (ester C]O), 167.61, 167.35, 167.22,
167.08, 166.90, 166.74 (amide C]O), 163.51, 162.43, 160.51, 156.77,
152.68, 150.21, 142.76, 139.92, 135.26, 134.08, 130.75, 130.46, 129.53,
128.52, 128.86, 127.89, 127.63, 125.96, 121.18, 98.39, 97.65, 96.47,
91.28. Yield: 4.58 g pale yellow powder, 86%; HBPAE 22; IR (KBr):
3300e2500 (NeH, OH), 3046, 3185 (Ar CeH),1730 (ester C]O),1698
(acid C]O), 1648 (amide C]O), 1515, 1450 (Ar C]C), 1320, 1241,
1200, 1048 (CeO), 856, 798 (m-Ar), 502, 450. 1H NMR (300 MHz,
light yellow powder, 96%; mp 375e380 ꢀC; IR (KBr) nmax
:
3300e2500 (NeH, OH), 3040 (Ar CeH), 1698 (acid C]O), 1654
(amide C]O), 1602, 1515, 1450 (Ar C]C), 1232, 1204, 1036 (CeO),
835 (p-Ar), 740, 710 (monosubst Ar), 501.1H NMR (300 MHz, DMSO-
d6, ppm): d 13.65 (s, 2H, COOH),11.03 (s,1H, amide NH),10.87 (s,1H,
amide NH), 8.22 (d, 4H, J¼5.5 Hz, p-Ar), 8.34 (d, 4H, J¼5.6 Hz, p-Ar),
DMSO-d6, ppm): d 13.41 (br, COOH),10.95e10.69 (m, amide NH), 7e8
7e8 (m, 5H, Ar), 6.44 (s, 1H, pyrimidine). 13C NMR (75 MHz, DMSO-
(m, m-Ar), 6.69 (s, pyrimidine), 6.51 (s, pyrimidine), 6.40 (s, pyrimi-
d6, ppm): d 176.75, 176.59 (acid C]O), 175.93 (ester C]O), 167.56,
dine), 6.26 (s, pyrimidine). 13C NMR (75 MHz, DMSO-d6, ppm):
167.30 (amide C]O), 162.46, 160.51, 156.73, 148.65, 139.91, 134.07,
133.86,131.38,130.45,128.50,127.54, 93.38. Anal. Calcd for C27H18N4O8:
C, 61.60; H, 3.45; N, 10.64%. Found: C, 61.56; H, 3.40; N, 10.63%.
d
176.80, 176.58, 176.35, 176.29 (acid C]O), 175.97, 175.85, 175.67,
175.42, 175.21, 175.03, 174.96 (ester C]O), 167.60, 167.32, 167.21,
167.05, 166.89, 166.73 (amide C]O), 162.89, 161.25, 160.96, 158.08,
152.68, 151.36, 145.34, 136.50, 133.89, 130.54, 129.24, 128.75, 127.66,
127.57,121.32, 98.42, 97.67, 96.53, 91.36. Yield: 4.13 g yellow powder,
84%; HBPAE 23; IR (KBr): 3300e2500 (NeH, OH), 3125 (Ar CeH),
2925, 2856 (Ali CeH), 1728 (ester C]O), 1687 (acid C]O), 1645
(amide C]O), 1500, 1450 (Ar C]C), 1346, 1236, 1202, 1054 (CeO),
4.3.3. Synthesis of model compound 2. A sample of model com-
pound 1 (0.00316 g, 0.006 mmol) was dissolved in anhydrous DMF
and SOCl2 (0.95 mL, 0.01 mmol) was added. A clear solution was
obtained after about 10 min. The reaction mixture was refluxed for
6 h. The solvent and excess of SOCl2 were removed by rotary evap-
oration. Ensuing acid chloride was redissolved in DMAc (30 mL) and
used directly for further modification to model compound 2 using
0.006 mmol of phenol pursuing the above procedure (Scheme 3).
501, 452. 1H NMR (300 MHz, DMSO-d6, ppm):
d 13.31 (br, COOH),
10.85e10.44 (m, amide NH), 6.28 (s, pyrimidine), 6.11 (s, pyrimidine),
5.98 (s, pyrimidine), 5.84 (s, pyrimidine), 2.50e1.75 (m, CH2). 13C
NMR (75 MHz, DMSO-d6, ppm):
d
177.68, 177.53, 177.36, 177.23 (acid
Yield: 3.4706 g yellow powder, 96%; mp 362 ꢀC; IR (KBr) nmax
:
C]O),176.96, 176.78, 176.64, 176.48, 176.32,176.15,175.97 (ester C]
O), 168.54, 168.38, 168.23, 168.06, 1676.90, 167.72 (amide C]O),
160.47, 150.98, 145.79, 99.65, 98.29, 97.76, 92.42, 35.84, 33.42, 33.27,
32.96, 25.13, 24.50, 24.16. Yield: 5.20 g yellow powder, 86%; HBPAE
24; IR (KBr): 3300e2500 (NeH, OH), 3126 (Ar CeH), 2948, 2876 (Ali
CeH), 1725 (ester C]O), 1687 (acid C]O), 1642 (amide C]O), 1500,
1450 (Ar C]C), 1342, 1240, 1200, 1056 (CeO), 498, 446. 1H NMR
3300e2500 (NeH, OH), 3042 (Ar CeH), 1695 (acid C]O), 1652
(amideC]O),1602,1515,1450 (Ar C]C),1231,1206,1036 (CeO), 834
(p-Ar), 746, 711 (monosubst Ar), 502. 1H NMR (300 MHz, DMSO-d6,
ppm):
d 13.58 (s, 1H, COOH), 10.96 (s, 1H, amide NH), 10.81 (s, 1H,
amide NH), 8.11 (d, 2H, J¼5.3 Hz, p-Ar), 8.29 (d, 2H, J¼5.2 Hz, p-Ar),
8.32 (d, 2H, J¼5.4 Hz, p-Ar), 8.36 (d, 2H, J¼5.6 Hz, p-Ar), 7e8 (m,10H,
Ar), 6.52 (s, 1H, pyrimidine), 6.68 (s, 1H, pyrimidine). 13C NMR
(300 MHz, DMSO-d6, ppm):
d
13.29 (br, COOH), 10.60e10.31 (m,
(75 MHz, DMSO-d6, ppm): d 176.32, 176.14 (acid C]O), 175.79,
amide NH), 6.16 (s, pyrimidine), 6.01 (s, pyrimidine), 5.87 (s, py-
175.52, 175.36 (ester C]O), 167.14, 166.98 (amide C]O), 162.84,
160.90, 157.15, 151.32, 149.46, 142.78, 135.23, 133.85, 131.64, 130.50,
129.28, 128.79, 127.68, 125.62, 121.11, 96.52, 96.46. Anal. Calcd for
rimidine), 5.81 (s, pyrimidine), 2.51e2.13 (m, CH2), 1.89e1.47 (m,
CH2). 13C NMR (75 MHz, DMSO-d6, ppm):
d 177.69, 177.52, 177.38,
177.25 (acid C]O), 176.94, 176.81, 176.65, 176.50, 176.34, 176.14,
175.89 (ester C]O), 168.56, 168.37, 168.24, 168.05, 1676.92, 167.73
(amide C]O), 160.32, 150.92, 145.65, 99.72, 98.36, 97.84, 92.63,
36.22, 33.61, 29.43, 26.17, 29.06, 28.74, 25.78, 25.10.
C33H22N4O8:
C, 65.75; H, 3.64; N, 9.27%.
C,
65.78;
H,
3.68;
N,
9.30%.
Found:
4.3.4. Synthesis of model compound 3. A similar method for the
synthesis of model compound 3 was followed, except that phenol
(0.0011 g, 0.012 mmol) was used (Scheme 3). Yield: 3.950 g yellow
powder, 97%; mp 358 ꢀC; IR (KBr) nmax: 3325 (amide NeH), 3046
(Ar CeH), 1652 (amide C]O),1601,1515,1450 (Ar C]C),1233,1204,
1037 (CeO), 837 (p-Ar), 741, 712 (monosubst Ar), 501. 1H NMR
4.3.1. Synthesis of A2C intermediate. HDAP (0.126 g, 1.0 mmol) was
dissolved in 5 mL of DMAc with continuous stirring under N2 flow.
To this solution TPC (0.406 g, 2.0 mmol) was added in DMAc (5 mL)
in 30 min (Scheme 3). The reaction mixture was maintained at 0 ꢀC
for 2 h. Water (2 mL) was subsequently added and stirring was
continued for 30 min. Finally, the precipitates of A2C were isolated
by filtration, washed with methanol and water in succession, and
dried under vacuum at 80 ꢀC for 24 h. Yield: 0.4012 g white powder,
95%; mp 346 ꢀC; IR (KBr) nmax: 3500e2500 (NeH, OH), 3028
(Ar CeH), 1700 (acid C]O), 1657 (amide C]O), 1606, 1515, 1450
(Ar C]C), 1236, 1202, 1038 (CeO), 832 (p-Ar), 502. 1H NMR
(300 MHz, DMSO-d6, ppm):
d 10.91 (s, 1H, amide NH), 10.79 (s, 1H,
amide NH), 8.32 (d, 4H, J¼5.7 Hz, p-Ar), 8.40 (d, 4H, J¼5.6 Hz, p-Ar),
7e8 (m, 15H, Ar), 6.90 (s, 1H, pyrimidine). 13C NMR (75 MHz,
DMSO-d6, ppm):
d 175.20, 175.03, 174.89 (ester C]O), 166.85,
166.69 (amide C]O), 163.44, 161.20, 158.05, 152.42, 150.26, 145.38,
136.53, 133.95, 131.84, 130.70, 129.58, 128.89, 127.88, 125.92, 121.91,
91.25. Anal. Calcd for C39H26N4O8: C, 69.02; H, 3.86; N, 8.26%.
Found: C, 68.97; H, 3.83; N, 8.22%.
(300 MHz, DMSO-d6, ppm):
d 13.65 (br, 2H, COOH), 10.86 (s, 1H,
amide NH), 10.77 (s, 1H, amide NH), 9.14 (br, 1H, Ar OH), 8.16 (d, 4H,
Acknowledgements
J¼5.3 Hz, p-Ar), 8.31 (d, 4H, J¼5.4 Hz, p-Ar), 6.45 (s, 1H, pyrimidine).
13C NMR (75 MHz, DMSO-d6, ppm):
d 173.68, 173.52 (acid C]O),
S.S. gratefully acknowledges the Higher Education Commission
of Pakistan (HEC) for fiscal assistance in this work under Indigenous
5000 PhD Fellowship Scheme Batch-II (042-111295-PS2-175).
165.13, 164.98 (amide C]O), 160.62, 156.47, 148.98, 139.51, 133.40,
130.23, 127.21, 91.04. Anal. Calcd for C20H14N4O7: C, 56.88; H, 3.34;
N, 13.27%. Found: C, 56.85; H, 3.31; N, 13.17%.
4.3.2. Synthesis of model compound 1. In a dried 100 mL three-neck
flask, A2C (0.422 g, 1.0 mmol) was dissolved in 5 mL of DMAc, the
mixture was stirred under N2 flow. To this solution was added ben-
zoylchloride(0.141 g,1.0 mmol)inDMAc(5 mL)in30 min(Scheme 3).
Supplementary data
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.tet.2010.06.065.

7212
S. Shabbir et al. / Tetrahedron 66 (2010) 7204e7212
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