Synthesis and characterization of side-chain oxazoline-methyl methacrylate copolymers bearing azo-dye

Article abstract of DOI:10.1016/j.reactfunctpolym.2010.07.018

We synthesized new polymeric structures by attaching a side-chain azo-moiety on poly(oxazoline) and poly(oxazoline-co-methyl methacrylate)s. For the polymer analogous transformation, we took advantage of the highly effective ring-opening addition of carbo

Full text of DOI:10.1016/j.reactfunctpolym.2010.07.018

Reactive & Functional Polymers 70 (2010) 827–835
Contents lists available at ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Synthesis and characterization of side-chain oxazoline–methyl methacrylate
copolymers bearing azo-dye
a
b
b
Valentin Victor Jerca a, , Florica Adriana Nicolescu , Adriana Baran , Dan Florin Anghel ,
*
Dan Sorin Vasilescu c, Dumitru Mircea Vuluga a
a Centre for Organic Chemistry ‘‘Costin D. Nenitescu”, Romanian Academy, 202B Spl. Independentei CP 35-108, Bucharest 060023, Romania
b ‘‘Ilie Murgulescu” Institute of Physical Chemistry, Colloid Department, 202 Spl. Independentei CP 12-194, Bucharest 060021, Romania
c University ‘‘POLITEHNICA” of Bucharest, Department of Polymer Science, 149 Calea Victoriei, Bucharest 010072, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 May 2010
Received in revised form 19 July 2010
Accepted 20 July 2010
Available online 25 July 2010
We synthesized new polymeric structures by attaching a side-chain azo-moiety on poly(oxazoline) and
poly(oxazoline-co-methyl methacrylate)s. For the polymer analogous transformation, we took advantage
of the highly effective ring-opening addition of carboxyl group to the oxazoline cycle. The comonomers
feed ratio allowed us to control the composition of the products while the kinetic treatment, employing
an integral method, revealed a statistical copolymerization tendency of 2-isopropenyl-2-oxazoline with
methyl methacrylate in acetonitrile at 70 °C. The elemental analysis and 1H NMR spectroscopy provided
almost identical composition data for both the substrates and the side-chain copolymers. The UV
spectroscopy sustained the quantitative addition of 4-(4-hydroxy-3,5-dimethylphenylazo)benzoic acid
to the oxazoline rings. Both the unmodified copolymers and the coloured ones exhibited good thermal
stabilities, up to 371 °C and 302 °C, respectively. The glass transition temperatures ranged from 141.5
to 177.5 °C and from 153.8 to 200.9 °C for the substrates and for the modified copolymers, respectively.
Preliminary investigations showed fluorescence activity for all copolymers bearing azo-moieties.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
2-Isopropenyl-2-oxazoline
Azobenzene
Methyl methacrylate
Polymer analogous reaction
Ester–amide
1. Introduction
of N-alkylation of the 2-vinyl-2-oxazoline with a strong alkylating
agent leads to poly[1-(3-alkyl-2-oxazolinium-2-yl)ethylene]s [15–
Cyclic imino ethers, also known as 2-oxazolines generated a
considerable amount of research since their discovery by Kagiya
in 1960s [1–6]. The 2-oxazolines can be polymerized only by
cationic ring-opening mechanism, leading to well-defined polymers
of narrow average molecular weight distribution, through a living
or quasi-living process. From the organic chemistry point of view
the oxazoline ring can give hydrolysis, oxidation and addition reac-
tions with organic acids, phenols, thiols or aniline [7,8]. Due to this
variety of reactions, 2-oxazolines with dual functionality, which
could undergo both radical and cationic polymerization, were early
introduced by Kagiya with his original cyclic imino ether bearing
an isopropenyl substituent attached in the 2-position of the oxaz-
oline ring [9]. Depending on the reaction conditions, a variety of
macromolecular architectures is available. For example, 2-vinyl-
2-oxazoline can be polymerized by four mechanisms. The cationic
ring-opening produces alkenyl substituted poly(ethyleneimine),
while radical, anionic [9–12] or cationic polymerization [13,14]
with retention of the oxazoline ring, give poly[1-(2-oxazoline-2-
yl)alkene]s. The spontaneous polymerization during the process
19]. A series of reactive polymers were developed from these
monomers, with aim to producing thermosets, adhesives, coatings,
hydrogels and composites [8,20,21].
Polymers of 2-isopropenyl-2-oxazoline and its copolymers with
methyl methacrylate or styrene, obtained through radical mecha-
nism [9], were reported in the 1970s but their physical properties
were not thoroughly investigated. Specific applications such as
powder water-borne coatings and humidity sensors, rely on their
ability to undergo polymer analogous reactions (cross-linking of
oxazoline with dicarboxylic acids or ammonium salts) [22,23].
Compatibilization of immiscible polymers [24,25] and synthesis
of glass-ionomer cements [26] emerged as another important area
of application. Addition reactions of poly(2-isopropenyl-2-oxazo-
line) with carboxylic acids or thiols yielded poly(ester–amide)s
and poly(thio-ester–amide)s, respectively [27,28]. These reactions
provide highly effective modification, when working at elevated
temperatures. These features underline the usefulness of the 2-iso-
propenyl-2-oxazoline (co)polymers in the synthesis of various
polymeric structures if the modifying agent has a suitable acid
group.
The design of molecular systems that change their physical and
chemical properties in response to light is of interest because of a
* Corresponding author.
E-mail address: victor_jerca@yahoo.com (V.V. Jerca).
1381-5148/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.reactfunctpolym.2010.07.018

828
V.V. Jerca et al. / Reactive & Functional Polymers 70 (2010) 827–835
wide range of applications. Investigation of azobenzene derivatives
as promising nonlinear optical materials relies on their advantages
like easy preparation, increased solubility and high hyperpolariz-
ability [29,30].
The numerous backbones used as scaffolds for azo-moieties,
include methacrylates [31], maleimides [32], amide–imides [33],
imides [34], thiophenes [35], urethanes [36], styrenes [37], and
others [38,39]. A thorough survey of the literature background on
polymers containing azobenzene derivatives reveal that the poly
(oxazoline)s or related copolymers were never used as backbones
for azo-moieties. The lack of data concerning the physical proper-
ties of such azo-polymers led us to investigate them.
However, it is known that azobenzene molecules do not fluo-
resce in solution with appreciable quantum yield due to their abil-
ity to undergo trans–cis isomerization [40]. Reports on tuning
fluorescence properties of azobenzene derivatives mostly dealt
with small organic molecules, rather than polymers. For instance,
self-assembled membranes [41,42] built from azobenzene-con-
taining amphiphiles, boron-substituted azobenzene [43,44] and
ortho-metalated azobenzene complexes [45,46] were reported to
exhibit fluorescence.
There are few papers in the literature dealing with azobenzene
based polymers (main-chain or side-chain) exhibiting intense
fluorescence at room temperature. Fluorescence emission from
an azobenzene diblock copolymer-containing a hydrophilic quat-
ernized poly(4-vinyl pyridine) and a hydrophobic liquid crystalline
polymethacrylate bearing azobenzene side groups in solution was
reported [47]. The micellization of the polymer in water induce the
fluorescence emission at 450 nm upon excitation of azobenzene
groups at 360 nm. Fluorescence in the 600 nm region was also
noticed from some azobenzenes dendrimers due to the formation
of J-aggregates [48].
2.2. Physico-chemical characterization
2.2.1. General measurements
FT-IR spectra were recorded on a Bruker Vertex 70 spectrometer
fitted with a Harrick MVP2 diamond ATR device. 1H NMR spectra
were recorded on a Varian Unity Inova 400 spectrometer in CDCl3
and in DMSO-d6, at 30 °C and at 60 °C, respectively. The number
average molecular weights (Mn) have been evaluated by SEC with
Agilent 1200 Series Refractive Index Detector, (G1310A)-ISO HPLC
Pump; using dimethylformamide as eluent (flow rate 1 mL/min),
against polystyrene standards (102–106 Da) at 23 °C. The thermal
analysis (simultaneous TGA–DSC, MS hyphenated) was performed
on a NETZSCH STA 449C Jupiter system, coupled to an Aeolos II MS
detector. TGA–DSC were typically carried out, for all the samples,
from ambient temperature up to 700 °C at a heating rate of 5 °C/
min under helium gas flow. The elemental analysis was carried
out on a Costech ECS 4010 CHNS analyzer. The UV–Vis spectra
were recorded on a Varian Cary 100 Bio UV–Vis spectrophotome-
ter. Steady-state fluorescence measurements were carried out on
a Horiba-Jobin-Yvon Fluoromax 4P spectrofluorimeter. Excitation
was set at 262 nm recording the emission in the 280–580 nm
range, and at 380 nm monitoring the emission from 400 to 550 nm.
2.2.2. Theory section
The composition dependence of the copolymer Tg was analyzed
using Fox (Eq. (1)) [51] and Gordon–Taylor (Eq. (2)) models [52].
1
w1 w2
¼
þ
ð1Þ
Tg Tg1 Tg2
w1Tg1 þ kw2Tg2
w1 þ kw2
Tg
¼
ð2Þ
One of the issues of successful use of azobenzene derivatives
implies the availability of suitable matrix materials. It is
believed that the structure of the chromophore can be tailored
for this particular application, if suitable substituents are
introduced into azobenzenes rings. Rau [49] proved that hydrox-
yazobenzene has fluorescence at room temperature, while
Smitha and coworkers [50] recently reported that 4-(4-hydroxy-
phenylazo)benzoic acid exhibits fluorescence activity. How-
ever, attempts to synthesize a fluorescent polymer starting from
4-(4-hydroxy-phenylazo)benzoic acid failed due to the synthetic
approach.
The main objective of the current work is to obtain
binary copolymers with controlled content of oxazoline units as
scaffolds for tailored fluorescent dye, the 4-(4-hydroxy-3,5-dim-
ethylphenylazo)benzoic acid, attached at the carboxyl function.
Advanced physico-chemical characterization of the newly
synthesized side-chain polymers as well as preliminary identifica-
tion of the fluorescence activity are the subsidiary goals of the
study.
Tg is the glass transition temperature of copolymer, wi and Tgi are
the weight fractions and the Tg values of the homopolymers, respec-
tively; the subscript 2 refers to the component with the higher Tg.
The constant k takes account of the specific coefficients of expan-
sion of the two homopolymers 1 and 2 in the melt (liquid or rub-
bery state) and in the glassy state as follows:
D
D
a2q1
a1q2
ð3Þ
where
qi, is the density of the polymer i and
D
ai is the change in
cubic expansion coefficient of the ith component at its glass
transition temperature. In most cases k is used as an adjustable
parameter.
2.3. Synthesis
2.3.1. Synthesis of 4-(4-hydroxy-3,5-dimethylphenylazo)benzoic acid
(C0) [53]
ABN (50.8 mmole) was mixed with 10 mL H2O and NaNO2
(51 mmole) to form a paste, which was subsequently poured into
a mixture of crushed ice (15–20 g) and 18 mL HCl (36%). The
reaction was carried out for 0.5 h in an ice bath. The diazonium
solution was allowed to warm up to room temperature and then
slowly added over the phenoxide solution prepared as follows:
DMP (50.9 mmole) was dissolved in sodium hydroxide solution
(100 mL warm water with NaOH (51 mmole) and Na2CO3
(51 mmole). The reaction was left to continue overnight. Then,
the crude precipitate was filtered off and washed several times
with saline water (saturated), and finally, dried under vacuum
at 60 °C for 24 h. The obtained orange crystals, recrystallised
from chloroform, have mp: 226–228 °C, yield: 80%; kmax (DMSO):
2. Experimental
2.1. Materials
4-Aminobenzoic acid (ABN, Merck, 98%), 2,6-dimethyl phenol
(DMP, Aldrich, 99.5%), tetrahydrofuran (THF, Merck, 99.5%) and
diethyl ether (Merck, 99.5%), were used as such. Acetonitrile
(Merck) was distilled from CaCl2 prior to use. 2-Isopropenyl-2-
oxazoline (IPRO, Aldrich), 2-ethyl-2-oxazoline (EtOZO, Aldrich),
methyl methacrylate (MMA, Merck) and N,N-dimethylformamide
(DMF, Merck) were purified by low pressure distillation. 2,20-azo-
bis(2-methylpropionitrile) (AIBN, Aldrich) was recrystallized from
ethanol prior to use.
370 nm. 1H NMR (DMSO-d6, 30 °C)
J = 8.4 Hz, 2 aromatic H ortho to COOH), 7.75–8 (d, J = 8.0 Hz, 2
d (ppm): 8.02–8.20 (d,

V.V. Jerca et al. / Reactive & Functional Polymers 70 (2010) 827–835
829
aromatic H meta to COOH), 7.59 (s, 2 aromatic H meta to OH), 2.26
(s, 6H); 13C NMR (DMSO-d6, 30 °C) d (ppm): 166.78, 157.66, 154.64,
144.98, 131.68, 130.50, 124.99, 123.73, 121.94, 16.62. Elem. Anal.
Calcd. C, 66.66%, H, 5.22%, N, 10.36%; Found C, 66.65%, H, 5.22%,
(DMSO-d6, 30 °C) d (ppm): 3.9–4.2 (2 aliphatic protons vicinal to
oxygen atom in oxazoline ring), 3.6–3.85 (2 aliphatic protons
vicinal to nitrogen atom in oxazoline ring), 3.37–3.6 (OCH3 protons
form MMA), 1.4–2 (methylene protons from the backbone both
IPRO and MMA), 0.5–1.3 (methyl protons from the backbone both
IPRO and MMA). When calculating the reactivity ratios, we used
only samples having the conversions of the comonomers less than
20%.
N, 10.38%. FT-IR (ATR)
m
(cmꢀ1): 3384 br (OH), 1681 (C@O).
2.3.2. Synthesis of 2-isopropenyl-2-oxazoline–methyl methacrylate
copolymers (Px)
IPRO was copolymerized with MMA using the following molar
feed ratios between comonomers: IPRO:MMA = 4:1, IPRO:MMA =
2.33:1, IPRO:MMA = 1:1, IPRO:MMA = 1:2.33 and IPRO:MMA = 1:4,
respectively (see Scheme 1a).
2.3.3. Synthesis of azo-dye functionalized copolymers (Px-C0)
The modified copolymers were obtained by oxazoline ring-
opening addition in the presence of C0 (see Scheme 1b). In a typical
run, 15 mL of dried DMF were added to 1.33 mmole of C0 and
2.37 mmole of P3 (IPRO units: 1.21 mmole) in a round bottom flask.
The solution was heated at 160 °C and left under stirring for 10 h.
The reaction mixture was precipitated in diethyl ether after cooling
and the resulting coloured polymer was filtered and dried. The
product was purified twice by reprecipitation from THF in diethyl
ether and dried at 60 °C for 48 h under vacuum. The polymer anal-
ogous reaction for P0 homopolymer was carried out under the
same conditions, leading to the modified homopolymer P0-C0.
1H NMR for P3-C0 (DMSO-d6, 30 °C) d (ppm): 7.96–8.34 (2 aro-
matic H ortho to COOH), 7.69–7.94 (2 aromatic H meta to COOH),
7.36–7.67 (2 aromatic H meta to OH), 4.18–4.5 (2 aliphatic protons
vicinal to ester group), 2.7–3.7 (2 aliphatic protons vicinal to amide
group overlap with OCH3 protons form MMA), 2.1–2.36 (6H), 1.4–2
A representative copolymerization was as follows: 4.5 mmole of
IPRO and 4.5 mmole of MMA were taken in a vial. Then, AIBN
(5 ꢁ 10ꢀ3 M) and 2 mL of freshly distilled acetonitrile were added
to prepare the final solutions. The resulting mixture was well de-
gassed and sealed off under argon cushion. The polymerizations
were carried out at 70 °C for 24 h. After cooling at room tempera-
ture, the solutions were precipitated in diethyl ether. The copoly-
mers were purified from unreacted monomers by reprecipitation
from THF in diethyl ether. Subsequently, the products were dried
under vacuum at 70 °C for 48 h. A homopolymer of IPRO (P0) was
synthesized under the same conditions and purified in the same
manner as the MMA–IPRO copolymers. All copolymers were
obtained with good purities and with the following yields: P0
60%; P1 65%; P2 69%; P3 71%; P4 81%; P5 83%. 1H NMR for P3
Scheme 1. Synthetic route for Px, the related Px-C0s copolymers, and the C0-Oxa structure.

830
V.V. Jerca et al. / Reactive & Functional Polymers 70 (2010) 827–835
(methylene protons from the backbone both IPRO and MMA), 0.5–
1.3 (methyl protons from the backbone both IPRO and MMA).
3.1. Solubility, kinetics and molecular weights
The Px polymers showed excellent solubility in polar solvents,
such as: CHCl3, CH2Cl2, dichlorobenzene, dioxane, THF, acetone,
DMF, dimethylsulfoxide, and even in alcohols. The solubility in
alcohols and water increases with the IPRO content. The chemi-
cally modified structures (Px-C0s) became insoluble in chlorinated
solvents, while P3–5-C0 remained partially soluble in THF. Dimeth-
ylsulfoxide and DMF proved to be the best solvents for all
copolymers.
The Px copolymer molar compositions were determined by two
different methods, namely: elemental analysis and 1H NMR spec-
troscopy (see Table 1). The compositions given by elemental anal-
ysis were calculated based on nitrogen content, while the
compositions given by 1H NMR spectroscopy were calculated
based on the OACH2 protons integral at 3.9–4.2 ppm from the
oxazoline ring against the methoxy protons integral at 3.37–
3.6 ppm from MMA. The values provided by these two methods
of characterization were almost identical, consequently we con-
sider the copolymer compositions reliable ones.
The MMA–IPRO system was investigated for the first time by
Kagya et al. in benzene in 1970 [9]. They reported only monomer
reactivity ratios (at low conversions), calculated by differential
methods such as Fineman–Ross (FR) or Kelen–Tüdös (KT), and
the corresponding ‘‘Q–e values”. No investigations regarding the
physical properties were given. We used an integral method and
software, namely the OptPex2 program provided by Hagiopol and
his coworkers [54], to calculate reactivity ratios and the corre-
sponding ‘‘Q–e” values. Using the molar compositions given by
1H NMR, the reactivity ratios were calculated and the values are
listed in Table 2 together with the ‘‘Q–e” values. The reactivity
ratios obtained by us are slightly different from those reported
by Kagya and coworkers, probably due to the better accuracy of
the integral method. The r1 and r2 values are very close to one
another, pointing out that both radicals are more reactive towards
the other monomer (cross-propagation). The diagram of composi-
tion was plotted for MMA–IPRO system (see Fig. 1), to underline its
particularities. In our case, the behaviour is not ideal; though we
can notice an azeotrope point at exactly 0.5 M fraction. This type
of diagram and the value of r1r2 < 1 reveals a statistic distribution
of monomer units in the copolymers backbone.
2.3.4. Synthesis of 2-propionamidoethyl 4-(4-hydroxy-3,5-
dimethylphenylazo) benzoate (C0-Oxa)
1.85 mmole of C0 was dissolved in 10 mL DMF, then 1.85 mmole
of EtOZO was added under continuous stirring (see Scheme 1c).
The mixture was heated at 130 °C and the reaction was allowed
to continue for 10 h. After that, the solvent was removed under
reduced pressure. A carrot-orange compound was obtained after
recrystallization from benzene. Yield: 80%; mp: 138 °C, UV–Vis
kmax (DMSO): 380 nm; kmax (THF): 373 nm; kmax (Dioxane):
368 nm; kmax (CH2Cl2): 364 nm. The molar extinction coefficient
(e = 23,552 L/mol cm) was calculated in DMSO, due to the fact that
the final polymers (Px-C0) were investigated in the same solvent.
1H NMR (CDCl3, 30 °C) d (ppm): 8.09–8.18 (d, J = 8.7 Hz, 2 aromatic
H ortho to COOH), 7.82–7.9 (d, J = 8.8 Hz, 2 aromatic H meta to
COOH), 7.64 (s,
2 aromatic H meta to OH), 4.39–4.51 (t,
J = 5.3 Hz, 2 aliphatic protons vicinal to ester group), 3.6–3.77
(dd, J = 5.5 Hz, J = 10.8 Hz, 2 aliphatic protons vicinal to amide
group), 1.96 (s, 6H), 1.7–1.98 (q, J = 7.6 Hz, 2H), 0.72–0.9 (t,
J = 7.6 Hz, 3H); 13C NMR (CDCl3, 30 °C) d (in ppm): 174.46,
166.47, 156.46, 155.79, 146.38, 130.76, 130.51, 124.31, 124.20,
122.42, 64.09, 39.08, 29.80, 16.24, 9.92. Elem. Anal. Calcd. C,
65.03%, H, 6.28%, N, 11.37%; Found C, 65.05%, H, 6.22%, N, 11.38%.
FT-IR (ATR)
m
(cmꢀ1): 3277 (br OH, NH), 1721 (C@O), 1620 (amide
I), 1556 (amide II).
3. Results and discussion
The present study addresses a very interesting issue which is
the synthesis of azo-dye fluorescent polymers. Firstly a fluorescent
azobenzene dye with designed functions (hydroxyl for fluores-
cence properties and carboxyl for chemical linkage) was synthe-
sized and its fluorescent properties were investigated. Secondly a
synthetic strategy was developed in order to preserve this property
onto polymeric structures.
The first step consisted in the preparation of several IPRO–MMA
copolymers by radical copolymerization with controllable oxazo-
line content. The second step was based on the quantitative addi-
tion reactions of oxazoline (co)polymers with carboxylic acid.
Thus, we were able to obtain structures with desired azobenzene
content and tune up their fluorescent properties.
High number average molecular weights (Mn) were attained by
copolymerization and the values are given for all Px copolymers in
Table 1
Physical features for Px and the related Px-C0 copolymers.
Polymer code
IPRO feed fraction
aCop. comp.
bCop. comp.
Mn (Da)
PDI
Tg (°C)
cTd (°C)
P0
P1
P2
P3
P4
P5
1
1
1
25,500
26,300
27,700
28,900
29,600
30,500
1.97
1.88
1.85
1.79
1.61
1.49
177.5
166.1
162.3
152.5
146
371.0
352.8
348.4
328.8
329.4
317.0
0.8
0.7
0.5
0.3
0.2
0.74
0.66
0.50
0.35
0.25
0.74
0.66
0.52
0.34
0.26
141.5
Polymer code
Yield (%)
aCop. comp.
dDeg. (%)
eDeg. (%)
fDeg. (%)
Tg (°C)
cTd (°C)
P0-C0
P1-C0
P2-C0
P3-C0
P4-C0
P5-C0
44
59
65
70
84
91
1
95
92
90
98
90
89
95
94
94
96
93
90
96
95
93
97
94
92
200.9
188
186.2
175.6
168.9
153.8
255.4
261.4
271.0
276.4
286.8
302.6
0.74
0.66
0.50
0.35
0.25
IPRO molar fraction in copolymer.
a
Given by 1H NMR.
Given by elemental analysis.
b
c
Temperature represents 10% weight loss in TGA measurements at heating rate of 5 °C/min degree of substitution at IPRO units.
d
e
f
Given by 1H NMR.
Given by Elem. Anal.
Given by UV–Vis.

V.V. Jerca et al. / Reactive & Functional Polymers 70 (2010) 827–835
831
Table 2
feed and chains termination by combination at low IPRO feed
ratios.
Reactivity ratios and ‘‘Q–e” values for MMA–IPRO system.
Polymer
code
Conv
(%)
IPRO feed
fraction
aCop.
comp.
br1
r2
Q1
e1
3.2. Thermal analysis
P1
P2
P3
P4
P5
15.1
16.2
14.9
14
0.8
0.7
0.5
0.3
0.2
0.74
0.66
0.50
0.35
0.25
The copolymer Tg values ranged between 106–177.5 °C, and
106–200.9 °C for the Px and Px-C0, respectively. Also, it can be no-
ticed that the glass transition temperatures increased with the
content of the oxazoline or C0 units in copolymers (see Table 1).
The variation of Tg with the composition of the copolymers is
shown in Fig. 2. The glass transition temperatures of Px copolymers
increase rapidly and nonlinearly in the 0–0.3 range of weight frac-
tion of IPRO units. In this interval, the polymer analogous reaction
induces a steep increase of Tg. For molar fractions of IPRO units in
copolymer higher than 0.3, the chemical modification regularly
rises the Tg by 20 °C.
0.71 0.63 1.77 1.29
16.5
Given by 1H NMR.
IPRO was considered as the first monomer.
a
b
Fox (Eq. (1)) and Gordon–Taylor (Eq. (2)) equations were used
to fit the experimental Tg data of the Px and Px-C0 copolymers.
One may see that the experimental Tg values for both modified
and unmodified copolymers do not agree with the theoretical val-
ues predicted by Fox equation (Fig. 2), which assumes that the Tg of
a copolymer depends only on the relative amounts of each mono-
mer and Tg of the respective pure homopolymers. Therefore, we
used the Gordon–Taylor method which takes into account the den-
sities and the change in expansivity at the glass transition of the
homopolymers. Fig. 2 shows the best fit curve based on the Gor-
don–Taylor equation, where k is 0.48 for Px and 0.36 for Px-C0 (k
was taken as adjustable parameter). The reasonable prediction of
the Gordon–Taylor equation for Tg composition curve of Px and
Px-C0 sustained the fact that the copolymers have a random
distribution.
Fig. 1. The diagram of composition for Px copolymers.
The thermal stability was investigated for all copolymers by
TGA analysis. Again, in Px series the thermal stability (Td, tempera-
ture at 10% weight loss) was influenced by the molar content of
IPRO units. The best Td was attained by the P0 homopolymer, due
to a better stability of the oxazoline heterocycle towards the ester
bonds from MMA units (see Table 1). The situation is quite
reversed in the Px-C0 series; consequently, the thermal stability
decreased with the increase of molar content of C0 moieties. This
reversed situation is directly linked to the thermal degradation of
the azo-moiety (C0). Previous studies were carried out by Smitha
regarding the thermal degradation of this particular azo-dye [57].
The reported Td of C0 was in the range of 237 °C. According to
our TGA–MS analysis, the first step of weight loss is attributed to
Table 1 together with their polydispersity indices (PDI). The theo-
retical values of Mw/Mn for polymers produced via radical combi-
nation and disproportionation are 1.5 and 2.0, respectively [55].
The polydispersity index of P0 suggested that chain termination
by disproportionation outweighs coupling, consistent with in-
creased rigidity of the chains provided by the high IPRO contents.
The values of Mw/Mn in copolymerization are also known to de-
pend on chain termination in the same way as in homopolymeriza-
tion [56]. The values of Mw/Mn for the copolymers suggested a
tendency for chain termination by disproportionation at high IPRO
Fig. 2. The dependence of Tg-s as a function of weight fractions for Px copolymers (a); the dependence of Tg-s as a function of weight fractions for Px-C0 copolymers (b).

832
V.V. Jerca et al. / Reactive & Functional Polymers 70 (2010) 827–835
several fragments that came out from the scission of N@N double
bonds, which are known as most unstable. Mass fragments corre-
sponding to nitrogen (m/z = 28) and aromatic ring (m/z = 76) could
be detected in the first degradation step. Therefore, the more
azo-moieties are present in the copolymers, the lower the thermal
results from overlapping of two different carbonyl stretching
vibrations. This peak is useful for identifying newly formed ester
group in IPRO homopolymer only.
3.4. 1H NMR characterization
stability is. The elimination of CO2 due to
aromatic and aliphatic) ester linkages took place at higher
temperature.
a-quaternary (both
The related Px-C0s were also characterized by 1H NMR spectros-
copy. After the ring-opening, the distinctive signals of ACH2AOA
group at 4.12 ppm are shifted downfield at 4.37 ppm, while the
signals from ACH2AN@ group that appear like a shoulder at
3.7 ppm in the P4 spectrum are shifted upfield under the signal
of methoxy groups (AOCH3) at 3.5 ppm in the P4-C0 spectrum. Fur-
thermore, the aromatic protons from the azo-moiety show up at
7.5 to 8.2 ppm in the P4-C0 spectrum due to the chemical linkage,
while the o,o0-methyl signals can be found at 2.25 ppm. The back-
bone protons lie in the range of 0.5–2.0 ppm in both P4 and P4-C0
spectra. The degree of chemical modification (Deg.%) was appreci-
ated from the Px-C0s spectra based on the areas of aromatic signals
relatively to those of the backbone. The values are summarized in
Table 1. According to these values, an almost quantitative modifi-
cation was obtained on Px copolymers (regardless of their initial
compositions). Typical examples of 1H NMR spectra are given in
Fig. 4 for P4-C0 in contrast with its initial copolymer P4. These
features were displayed by all Px-C0s.
3.3. FT-IR characterization
The FT-IR spectra were recorded for all copolymers (Pxs and
Px-C0s) and, homopolymers (P0 and the related P0-C0). After the
polymer analogous reaction took place, a significant spectral
change was noticed in all Px-C0s. When the oxazoline ring opens,
an ester–amide group is generated due to the linkage of C0 onto
the Px backbone (see Scheme 1b). To underline this, the IR spectrum
of P0 is given in Fig. 3a together with that of the modified homopoly-
mer (P0-C0). The P0 spectrum showed characteristic absorptions of
oxazoline ring at 1651 and 1118 cmꢀ1 attributable to C@N and
CAO stretching, while other ring skeletal vibrations can be found
at 986 and 951 cmꢀ1. The sharp band at 1651 cmꢀ1 from the former
P0 spectrum appeared as a broad band in the current P0-C0 spec-
trum due to the absorption of amide I (
mC@O) band. The amide II
band (
m
CAN) can be found at 1520 cmꢀ1 in the P0-C0 spectrum along
3.5. UV–Vis characterization
with the characteristic ester bands (mC@O, mCAOAC and mCAO) at 1710,
1265 and 1115 cmꢀ1, respectively. The band at 1596 cmꢀ1 from the
P0-C0 spectrum can be assigned to the substituted benzene rings
vibrations from the azo-moiety. Also, the trans N@N absorption
To determine with accuracy the C0 content in the modified
copolymers we synthesized the C0-Oxa structure. This saturated
band from the azo-dye can be found at 1478 cmꢀ1
.
The FT-IR spectra for the P1–5 and related Px-C0 copolymers are
more complicated since the MMA units contribute to the abun-
dance of esters groups. Nevertheless, the same ester–amide group
is formed in the case Px-C0s too and its typical absorptions could be
identified at 1712 (
m
C@O), 1262 and 1115 (mCAOAC and
mCAO), 1648
(amide I, C@O) and 1521 (amide II,
m
m
CAN), all in cmꢀ1. The Px-C0s
spectra are given together in Fig. 3b to underline some of their
particularities. One may notice that the intensity of the amide II
peak increases with the molar ratio of IPRO in the Px copolymers
(see arrow). The same tendency has the peak at 1595 cmꢀ1 from
the azo-moiety of the benzene rings. We can conclude that the
azo-dye content is proportional with the oxazoline units and
increases in the following order: P5-C0 < P4-C0 < P3-C0 < P2-C0 <
P1-C0. The amide I peak cannot be discussed as the former one
since it may overlap with the peak from the residual oxazoline
Fig. 4. The 1H NMR spectra for P4 and P4-C0 copolymers.
ring. The peak at 1712 cmꢀ1
(mC@O) is affected by errors since it
Fig. 3. The FT-IR spectra for P0 and P0-C0 polymers (a), and for the Px-C0 copolymers (b).

V.V. Jerca et al. / Reactive & Functional Polymers 70 (2010) 827–835
833
Fig. 5. The UV–Vis spectra for C0-Oxa in different solvents at 3 ꢁ 10ꢀ5 M (a); the UV–Vis spectra for P3-C0 copolymers in different solvents at 3 ꢁ 10ꢀ5 M (b).
can be attributed to the
The molar extinction coefficient,
p
–
pꢂ transitions of the aromatic rings.
, was determined for the first
e
absorption band (380 nm, red shifted towards C0 kmax = 370 nm)
and used to calculate the degree of C0 addition to IPRO rings for
all polymers; the values are given together in Table 1. Solvatochro-
mic effects of C0-Oxa were also investigated. The extent of solvato-
chromic response is related to the molecular polarizability. The
solvatochromic shifts in solvents of varying polarities like dioxane,
THF, dichloromethane and DMSO are shown in Fig. 5a. The C0-Oxa
showed a positive solvatochromism with the maximum shift in
DMSO suggesting higher polarizability in this solvent. Such
behaviour is characteristic for charge-transfer transitions with an
increase of dipole moment upon excitation. This behaviour was
preserved upon modification of the Px copolymers with C0 (see
Fig. 5b). P3-C0 was chosen for exemplification due to its similar
solubility as the C0-Oxa. The solvatochromic response is almost
identical with the C0-Oxa in dioxane, THF and DMSO. Dichlorme-
thane became out of question due to the fact that all the modified
copolymers, regardless their C0 ratio, are insoluble. This proves
that our synthesis strategy was a successful one in preserving
intact the optical properties of the C0-Oxa structure when it was
embedded in oxazoline polymeric structures.
Fig. 6. The UV–Vis spectra for Px-C0 copolymers at 3 ꢁ 10ꢀ5 M in DMSO.
structure is similar with the units found in all Px-C0 (co)polymers
after ring-opening. The UV–Vis spectra of C0-Oxa in DMSO had
an absorption maxima at k = 380 nm which corresponds to a
p–
The absorption spectra of all Px-C0 polymers showed two in-
tense bands at 262 and 380 nm in the UV spectral region (Fig. 6).
pꢂ first single electron transition, while the maximum at 262 nm
Fig. 7. Emission spectra for the C0 and C0-Oxa (a), and Px-C0 copolymers (b) at 3 ꢁ 10ꢀ5 M; kexcitation = 262 nm (1) and kexcitation = 380 nm (2).

834
V.V. Jerca et al. / Reactive & Functional Polymers 70 (2010) 827–835
We can notice that the absorbance decreases in intensity in the
same order that the C0 content decreases, when the molar concen-
tration of modified copolymers is kept constant at 3 ꢁ 10ꢀ5 M.
Acknowledgements
This work was partially achieved by means of PN2 CNMP Par-
teneriate 71-029 ‘‘Nabieco” and 31-056 ‘‘Biopolact” Projects (fi-
nanced by the Romanian Ministry of Education & Research), and
PN-II-ID-2008-2 ‘‘Macromolecular materials containing chro-
mophores” Project (financed by Romanian National Council for Sci-
entific Research in Higher Education).
3.6. Fluorescence characterization
The fluorescence of hydroxy azobenzene can be explained in
terms of azo-hydrazone tautomerism [49]. Smitha and Asha [50]
synthesized an azo-dye polymer with a similar structure starting
from 4-(4-hydroxy-phenylazo)benzoic acid methyl ester. They
mentioned that the fluorescence was lost upon coupling the azo-
dye with methacryloyl chloride and subsequent polymerization.
This fact sustained the idea that the emission properties are due
to the hydrazone form. For that reason, we used a different syn-
thetic strategy that preserved the hydroxyl group. At the same
time, supplementary methyl groups were introduced in the 30
and 50 positions of the benzene rings, because they are known to
shifting the tautomeric equilibrium towards the hydrazone species
[58].
References
[1] K. Aoi, M. Okada, Prog. Polym. Sci. 21 (1996) 151–208.
[2] R. Hoogenboom, Angew. Chem. Int. Ed. 48 (2009) 7978–7994.
[3] S. Kobayashi, H. Uyama, J. Polym. Sci. Part A: Polym. Chem. 40 (2002) 192–209.
[4] A. Levy, J. Polym. Sci. Part A: Polym. Chem. 6 (1968) 57–62.
[5] A. Levy, M. Litt, J. Polym. Sci. Part A: Polym. Chem. 6 (1968) 1883–1894.
[6] R. Hoogenboom, Macromol. Chem. Phys. 208 (2007) 18–25.
[7] J.A. Frump, Chem. Rev. 71 (1971) 483–505.
[8] B.M. Culbertson, Prog. Polym. Sci. 27 (2002) 579–626.
[9] T. Kagiya, T. Matsuda, M. Nakato, R. Hirata, J. Macromol. Sci. Chem. 6 (1972)
1631.
[10] J.M. Havard, M. Yoshida, D. Pasini, N. Vladimirov, J.M.J. Frechet, D.R. Medeiros,
K. Patterson, S. Yamada, C.G. Willson, J.D. Byers, J. Polym. Sci. Part A: Polym.
Chem. 37 (1999) 1225–1236.
[11] V.V. Jerca, A.F. Nicolescu, A.-M. Albu, D.M. Vuluga, Mol. Cryst. Liq. Cryst. 483
(2008) 78–88.
Solutions of C0, C0-Oxa and Px-C0 (3 ꢁ 10ꢀ5 M) in DMSO were
excited at 262 nm corresponding to the p–
pꢂ transition of the fluo-
rophore. The C0 had the highest fluorescence emission intensity,
while the emission intensity of Px-C0 copolymers was slightly
reduced as compared to C0-Oxa (see Fig. 7). The emission intensity
of the polymers is directly related to the content of azo-dye, and
increases with the molar ratio of the chromophore. The emission
[12] Z. Ning, H. Stephan, S. Anita, L. Robert, J. Rainer, Macromolecules 42 (2009)
2215–2221.
[13] M. Masatoshi, L. Peter, K. Shin-ya, S. Takeo, Polym. Bull. 34 (1995) 249–256.
[14] D.A. Tomalia, B.P. Thill, M.J. Fazio, Polym. J. 12 (1980) 661–675.
[15] M. Miyamoto, Y. Sano, Y. Kimura, T. Saegusa, Macromolecules 18 (1985) 1641–
1648.
spectra at the
p
–
pꢂ transition of the azobenzene units (kexcitation
=
[16] M. Miyamoto, Y. Sano, Y. Kimura, T. Saegusa, Makromol. Chem. 187 (1986)
1807–1817.
380 nm) were recorded for all the above-mentioned solutions.
The C0 and C0-Oxa had identical spectra (the profiles are given in
Fig. 7a); suggesting that the newly formed ester–amide structure
does not interfere with the emissive properties of C0.
[17] M. Miyamoto, Y. Sano, T. Saegusa, Makromol. Chem. 187 (1986) 2747–2752.
[18] M. Miyamoto, Y. Sano, T. Saegusa, Polym. J. 19 (1987) 557–566.
[19] M. Miyamoto, Y. Sano, T. Saegusa, J. Macromol. Sci. A 25 (1988) 627–641.
[20] S. Nivedita, M. Amol, S. G, F. Sushmita, G. Hua, J. Appl. Polym. Sci. 117 (2010)
1718–1730.
We ascribe the lower intensity of P0-C0 as compared to the
C0-Oxa to the azo-hydrazone tautomerism. The tautomerism in
polymers is shifted toward the azo form due to the interactions
between chains. The same dependence of fluorescence intensity
on the molar ratio of azo-dye was noticed in all our polymers
(Fig. 7b). However, we have to stress that here we reported only
preliminary results; further investigations on this matter are now
in progress and will be reported later.
[21] J.T. Yoona, W.H. Joa, M.S. Leeb, M.B. Koc, Polymer 42 (2001) 329–336.
[22] C.-W. Lee, H.-S. Park, M.-S. Gong, Sens. Actuators B 109 (2005) 256–263.
[23] D.L. Schmidt, R.F. Brady Jr., K. Lam, D.C. Schmidt, M.K. Chaudhury, Langmuir 20
(2004) 2830–2836.
[24] W.E. Baker, M. Saleem, Polymer 28 (1987) 2057–2062.
[25] H. Matthias, B. Matthias, L. Holger, G. Wolfram, M. Rolf, J. Polym. Sci. Part A:
Polym. Chem. 36 (1998) 1821–1827.
[26] B.M. Culbertson, J. Dent. 34 (2006) 556–565.
[27] T. Nishikubo, A. Kameyama, H. Tokai, Polym. J. 28 (1996) 134–138.
[28] T. Kagiya, T. Matsuda, Polym. J. 3 (1972) 307–314.
[29] S.K. Yesodha, C.K.S. Pillai, N. Tsutsumi, Prog. Polym. Sci. 29 (2004) 45–74.
[30] Y. Zhao, T. Ikeda, Smart Light Responsive Materials-Azobenzene-Containing
Polymers and Liquid Crystals, John Wiley & Sons, New Jersey, 2009.
[31] N.J. Li, J.M. Lu, X.W. Xia, Q.F. Xu, L.H. Wang, Polymer 50 (2009) 428–433.
[32] H.T. Pu, L. Liu, W.C. Jiang, X.W. Li, J.P. Chen, J. Appl. Polym. Sci. 108 (2008)
1378–1384.
4. Conclusions
The MMA–IPRO was the ideal system in our experiments due to
an excellent control upon the IPRO units in copolymer. The azo-
phores content in copolymers can be adjusted either from the reac-
tion time of the analogous substitution or from feed molar ratio.
Therefore, if a higher content of azo-dye is required then the P0
homopolymer would be an excellent choice, while other different
compositions can be provided by the Px-C0 copolymers. The ring-
opening method can be easily used to design other side-chain poly-
mers, without laborious work, keeping intact all features provided
by the first step of polymerization. The polymer analogous reaction
took place with a high degree of substitution, regardless of the Px
molar content in IPRO units. The molar compositions for Px-C0s
were appreciated from three distinct methods of characterization,
which were in a good agreement with one another. The modified
polymers (Px-C0) exhibited higher glass transition temperatures
compared to the Px copolymers, while the thermal stability de-
creased due to the azobenzene moieties. Our original synthetic ap-
proach favours, at least partially, the retaining of the fluorescent
activity in copolymers. These results suggest that a balanced choice
of chemical substituents and embedding environment can be valu-
able for exploitation of the azo-hydrazone tautomerism of C0 in
fluorescence applications.
[33] E. Schab-Balcerzak, B. Sapich, J. Stumpe, Polymer 46 (2005) 49–59.
[34] M. He, Y.M. Zhou, F. Dai, R. Liu, Y.P. Cui, T. Zhang, Polymer 50 (2009) 3924–
3931.
[35] L.H. Gan, X.L. Xia, Y.C. Chan, X. Hu, X.Y. Zhao, Polym. Adv. Technol. 14 (2003)
260–265.
[36] Y. Sui, D. Wang, J. Yin, Z.K. Zhu, Z.G. Wang, Chem. Phys. Lett. 339 (2001) 186–
190.
[37] A. Zerroukhi, A. Trouillet, D. Blanc, B. Boinon, A. Cachard, J.P. Montheard, J.
Appl. Polym. Sci. 51 (1994) 1165–1173.
[38] X.B. Chen, J.J. Zhang, H.B. Zhang, Z.H. Jiang, G. Shi, Y.B. Li, Y.L. Song, Dyes Pigm.
77 (2008) 223–228.
[39] S. Suresh, R.J. Gulotty, S.E. Bales, M.N. Inbasekaran, M.A. Chartier, C. Cummins,
D.W. Smith, Polymer 44 (2003) 5111–5117.
[40] H. Rau, Photoisomerization and photo-orientation of azobenzenes, in: Z.
Sekkat, W. Knoll (Eds.), Photoreactive Organic Thin Films, Academic Press,
Elsevier Science, 2002, pp. 3–38.
[41] M. Shimomura, T. Kunitake, J. Am. Chem. Soc. 109 (1987) 5175–5183.
[42] K. Tsuda, J. Am. Chem. Soc. 122 (2000) 3445–3452.
[43] J. Yoshino, N. Kano, T. Kawashima. Synthesis of the Most Intensely Fluorescent
Azobenzene by Utilizing the b–n Interaction, 2007, pp. 559–561.
[44] J. Yoshino, N. Kano, T. Kawashima, Chem. Lett. 37 (2008) 960–961.
[45] M. Ghedini, D. Pucci, G. Calogero, F. Barigelletti, Chem. Phys. Lett. 267 (1997)
341–344.
[46] Y. Wakatsuki, H. Yamazaki, P.A. Grutsch, M. Santhanam, C. Kutal, J. Am. Chem.
Soc. 107 (1985) 8153–8159.
[47] Q. Bo, Y. Zhao, Langmuir 23 (2007) 5746–5751.
[48] J. Nithyanandhan, N. Jayaraman, R. Davis, S. Das, Chem. Eur. J. 10 (2004) 689–
698.

V.V. Jerca et al. / Reactive & Functional Polymers 70 (2010) 827–835
835
[49] H. Rau, Angew. Chem. Int. Ed. 12 (1973) 224–235.
[50] P. Smitha, S.K. Asha, J. Phys. Chem. B 111 (2007) 6364–6373.
[51] T.G. Fox, Bull. Am. Phys. Soc. 1 (1956) 123.
[52] M. Gordon, J.S. Taylor, J. Appl. Chem. 2 (1952) 493–500.
[53] F.A. Nicolescu, V.V. Jerca, C. Draghici, D.M. Vuluga, D.S. Vasilescu, Des.
Monomers Polym. 12 (2009) 553–563.
[55] S. Teramachi, A. Hasegara, M. Atasuka, A. Yamashita, N. Takemoto,
Macromolecules 11 (1978) 1206–1210.
[56] H.W. Melville, B. Noble, W.F. Watson, J. Polym. Sci. 4 (1949) 629–637.
[57] P. Smitha, S.K. Asha, C.K.S. Pillai, J. Polym. Sci. Part A: Polym. Chem. 43 (2005)
4455–4468.
[58] J.-A. Farrera, I. Canal, P. Hidalgo-Fernandez, M.L. Perez-Garcia, O. Huertas, F.J.
Luque, Chem. Eur. J. 14 (2008) 2277–2285.
[54] C. Hagiopol, Copolymerization-toward
Publisher, New York, 1999.
a Systematic Approach, Plenum