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Monomer reactivity ratios of N-isopropylacrylamide–itaconic acid copolymers at low and high conversions
European Polymer Journal 45 (2009) 1728–1737
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Monomer reactivity ratios of N-isopropylacrylamide–itaconic acid copolymers at low and high conversions Candan Erbil *, Bürgehan Terlan, Özgür Akdemir, Argun T. Gökçeören _ Istanbul Technical University, Science and Letters Faculty, Chemistry Department, 34469 Maslak, Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 4 March 2008 Received in revised form 24 February 2009 Accepted 26 February 2009 Available online 6 March 2009
Keywords: Poly (N-isopropylacrylamide) N-isopropylacrylamide/itaconic acid copolymer Monomer reactivity ratios Conductometric and potentiometric titrations FTIR spectroscopy
a b s t r a c t Copolymers of N-isopropylacrylamide (NIPAAm) and itaconic acid (IA) having various compositions were synthesized using free radical solution polymerization in 1,4-dioxane at 50 °C with a,a0 -azobisisobutyronitrile (AIBN) as initiator. The structures of the copolymers were confirmed by Fourier transform infrared (FTIR) spectroscopic technique. The copolymer compositions were determined by conductometric and potentiometric methods from the inflection points in the acid–base titration curves and by FTIR spectroscopy through recorded analytical absorption bands for NIPAAm (1620 cm1 for C@O stretching of secondary amides) and for IA (1704 cm1 for C@O stretching) units, respectively. Monomer reactivity ratios of IA (F1)–NIPAAm (F2) pair were estimated using the Finemann–Ross, the inverted Finemann–Ross, the Kelen–Tüdós and the extended Kelen–Tüdós graphical methods. The values ranged from 0.40 to 0.60 for r1 and from 1.20 to 1.90 for r2, depending on the conversion percentage, calculation methods of monomer reactivity ratios and determination methods of copolymer compositions. In all cases, r1r2 < 1 and r1 < r2 indicate the random distribution of the monomers in the final copolymers and the presence of higher amount of NIPAAm units in the copolymer than that in the feed, respectively. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Acrylamide (AAm), acrylic acid (AA), itaconic acid (IA) and N-isopropylacrylamide (NIPAAm) can easily polymerize by free radical polymerization technique [1–4]. Among these water-soluble polymers indicated above, PNIPAAm has a great interest because of its temperaturesensitive character in aqueous solutions. It exhibits a sharp phase transition close to body temperature (32–34 °C) and this phase transition temperature, which is known as lower critical solution temperature (LCST) can be modified by copolymerizing a more or less hydrophilic monomer [5–9]. Further, the temperature-sensitive PNIPAAm copolymerized with pH-sensitive monomers is the most intensively studied member of N-alkylacrylamide polymers and * Corresponding author. Tel.: +90 (212) 285 3234; fax: +90 (212) 285 6386. E-mail address: [email protected] (C. Erbil). 0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2009.02.023
copolymers because those factors are variables that change in physiological, biological and chemical systems [10–18]. Itaconic acid (IA) is an unsaturated dicarbonic organic acid with one carboxyl group conjugated to the methylene group. It can be regarded as an a-substituted acrylic or methacrylic acid. The potential for substitution of petrochemical-based acrylic or methacrylic acid in polymers is high, because rather low comonomer contents lead to copolymers with effective acidity due to the two carboxylic groups of IA. In addition, the outstanding properties in polymer chemistry, pharmacy and agriculture open up new applications [19]. As a result of increasing studies of NIPAAm and IA polymers, various methods for the synthesis of these polymers and their copolymers with the monomers such as acrylonitrile, styrene and acrylic esters have been reported, in most cases radical polymerizations [20–26]. These polymerizations are usually performed in organic solvents using peroxide-type initiators or in aqueous solutions using redox initiation systems.
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The compositional heterogenity of copolymers is of great importance in considering copolymer properties. For example, in the case of NIPAAm based copolymers, the structural differences, resulting from the distribution of comonomers along the polymer chains may be effected on the thermal behaviours of the product polymers. The copolymer composition in the free radical polymerization systems are dependent on the monomer reactivity ratios. Direct and indirect methods are applied to determine the compositions of copolymers [27–30]. For determination of monomer reactivity ratios, linear leastsquares methods such as Finemann–Ross, Kelen–Tüdós and extended Kelen–Tüdós and, nonlinear least-squares analysis can be applied to a wide range feed and corresponding copolymer compositions [31–37]. In the present study, NIPAAm/IA copolymers with different monomer ratio were prepared through free radical copolymerization in 1,4-dioxane using a,a0 -azobisisobutyronitrile (AIBN). Copolymerizations were carried out in both low and high conversion conditions. The reaction times were selected in such a way that the conversions were less than 15% and 70% of weight, for low and high conversions, respectively. The compositions of copolymers were determined by conductometric and potentiometric acid–base titrations, and FTIR spectroscopic technique. The reactivity ratios of NIPAAm/IA copolymers were calculated from the initial monomer feed composition, the low and high conversions and the copolymer composition, by means of Finemann–Ross (FR), Inverted Finemann–Ross (IFR) Kelen–Tüdós (KT) and extended Kelen–Tüdós (EKT) methods. 2. Experimental 2.1. Reagents N-isopropylacrylamide (NIPAAm) and itaconic acid (IA) were products of Aldrich and Fluka, respectively. Both monomers were used without further purification. The initiator, a,a0 -azobisisobutyronitrile (AIBN, Merck), was recrystallized from methanol. 1,4-Dioxane were used as purchased from Merck A. G. 2.2. Copolymerization The feed mixtures, containing a total monomer concentration of 1 M and 0.01 M AIBN in 1,4-dioxane were introduced into large glass tubes with 30 mm diameter, closed with rubber septa. Each of the tubes was flushed with dry nitrogen for 20 min using a syringe needles to remove oxygen dissolved in the reaction mixtures. During the reactions, copolymerization solutions were separated into a cloudy viscous layer (lower phase, LP) and a clear layer (upper phase, UP). The amount of LP increased with increase in IA content of copolymerization mixtures. For low conversions, the tubes were placed in the constant temperature bath at 50 °C to observe the phase separation and decide the reaction time. The time required to obtain approximately 10 % conversion (between 20 and 40 min) was found to vary slightly depending on the ratio of NIPAAm and IA, due to the difference in reactivity of these
two monomers. Low % conversion is required to minimize any potential composition drift which may result from depletion of any one comonomer in the feed. For high % conversions (to observe the effect of conversion percentage on the copolymer composition), polymerizations were carried out in an air oven regulated at 50 °C and the reactions were allowed to proceed for 24 h. After the reaction time is over, the tubes were removed from the bath and/or air oven and each phase of the reaction mixtures was precipitated by pouring into n-hexane. The product polymers were left to dry at ambient temperature for 24 h prior to drying to constant weight in a vacuum oven at 25 °C. Conversions were obtained gravimetrically. 2.3. Characterization FTIR spectra of PNIPAAm and NIPAAm/IA copolymers with different composition were recorded on Perkin-Elmer Spectrum One (FTIR-reflectance, Universal ATR with diamond and ZnSe) spectrophotometer using the samples in powder form. Monomer reactivity ratios of NIPAAm/IA copolymers were determined by conductometric and potentiometric titration methods. WTW-LF 2000 model conductometer and WTW 523 model pH meter were used for conductivity and pH measurements, respectively. Acid–base titrations were carried out in a doublewalled glass cell at 25 °C. For each titration experiment, the cell was filled with 20 ml 0.1 N NaCl solution in which 0.07 g of solid polymer was dispersed by magnetic stirring. After the polymer completely dissolved, the solution was titrated with 0.1 N NaOH, which was added from a microburette. Conductivity and pH values were plotted against the volume of titrant. Compositions of copolymers and monomer reactivity ratios were determined from the inflection points in these titration curves. 3. Results and discussion Copolymer composition in this system involving IA which contains two carboxylic acid groups was ascertained by conductometric and potentiometric titrations of the acidic comonomer. For this purpose, physical mixtures of PNIPAAm and PIA homopolymers were prepared with different compositions. The end points obtained from the
Table 1 Conductometric and potentiometric titration results in the FIA range of 5– 95 (in mol %) of physical mixtures of PNIPAAm and PIA. FIA(mol %)
Potentiometric titration Inflection points V (ml of 0.1 N NaOH)
Conductometric titration Inflection points V (ml of 0.1 N NaOH)
5.72 10.06 26.70 35.33 52.59 56.04 75.59 91.24
0.93 1.59 3.85 4.60 6.37 6.50 8.13 9.01
1.00 1.60 4.00 4.80 6.60 6.80 8.40 9.40
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conductometric and potentiometric titration curves of the aqueous solutions of these mixtures were plotted against the content of PIA to make the calibration curves (Table 1, Figs. 1 and 2). Then the inflection points of the conductometric and potentiometric curves of the copolymers were placed on the calibration curves to estimate the acidic comonomer content in the copolymers and the copolymer compositions were found directly from these curves (Figs. 3 and 4). For both low and high conversions, the compositions of the feeds and the copolymers as well as the conversions of LPs and UPs for the NIPAAm/IA copolymers are given in Tables 2–5. For the LPs of the low conversion samples, FTIR technique was also employed to calculate the reactivity ratios of monomer pair, based on the determination of the composition of NIPAAm/IA copolymers obtained at four different monomer feed ratios [27,29]. C@O stretching (1704 cm1) and –OH dimerization (3500–2500 cm1) bands in the FTIR spectra of these copolymers indicate the presence of IA units in the chains while the bands due to –C@O stretching and NH– bending for secondary amides at 1620 and 1540 cm1 and, a double band for iso-
Fig. 3. Conductometric titration curves of copolymers containing IA. () 40.0, (e) 50.0, (D) 60.0 and (h) 70.0 mol % in the feed (fIA) (at low conversion for LPs).
propyl group at 1385 and 1370 cm1 are characteristic absorptions of PNIPAAm (Fig. 5).
Fig. 1. Conductometric calibration curve which is used to obtain copolymer compositions.
Fig. 2. Potentiometric calibration curve which is used to obtain copolymer compositions.
Fig. 4. Potentiometric titration curves of copolymers containing IA. (a) 40.0, (b) 50.0, (c) 60.0 and (d) 80.0 mol % in the feed (fIA) (at high conversion for LPs).
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C. Erbil et al. / European Polymer Journal 45 (2009) 1728–1737 Table 2 Itaconic acid composition in the feed (f) and copolymer chain (F) at low and high conversions for lower phases (LPs) of the samples. Sample No. (LC)b
fIA
V (ml)
FIAa (conversion %)
Sample No. (HC)c
fIA
V (ml)
FIAa (conversion %)
1 2 3 4 5
40 40 50 60 70
3.99 4.20 4.95 5.55 6.59
27.48 29.27 42.32 46.44 60.04
– 6 7 8 9
– 40 50 60 80
– 4.35 5.00 5.48 6.40
– 30.57 36.48 41.12 50.76
a b c
(14.34) (8.62) (8.43) (9.56) (2.28)
(74.12) (67.50) (50.74) (36.95)
The results obtained by means of conductometric titration method. Low conversion. high conversion.
Table 3 Itaconic acid composition in the feed (f) and copolymer chain (F) at low and high conversions for lower phases (LPs) of the samples. Sample No. (LC)b
fIA
V (ml)
FIAa (conversion %)
Sample No. (HC)c
fIA
V (ml)
FIAa (conversion %)
1 2 3 4 5
40 40 50 60 70
3.45 3.80 4.95 5.55 6.59
24.34 27.37 38.07 44.18 55.85
– 6 7 8 9
– 40 50 60 80
– 3.85 4.48 4.90 5.95
– 27.81 33.55 37.58 48.49
a b c
(14.34) (8.62) (8.43) (9.56) (2.28)
(74.12) (67.50) (50.74) (36.95)
The results obtained by means of potentiometric titration method. Low conversion. high conversion.
Table 4 Itaconic acid composition in the feed (f) and copolymer chain (F) at high conversion for upper phases (UPs) of the samples. Sample No. (LC)a
fIA
Va (ml)
FIA (conversion %)
Sample No. (HC)b
fIA
Vb (ml)
FIA (conversion %)
6 7 8 9
40 50 60 80
4.87 6.14 6.57 8.40
35.26 47.93 52.66 75.76
6 7 8 9
40 50 60 80
4.07 5.36 5.61 7.90
29.77 42.20 44.81 73.16
(17.20) (7.50) (24.20) (62.70)
(17.20) (7.50) (24.20) (62.70)
Inflection points obtained from (a) conductometric and (b) potentiometric acid–base titrations of copolymers.
Table 5 Itaconic acid composition in the feed (f) and copolymer chain (F) at low conversion for upper phases (UPs) of the samples. Sample No.
fIA
Va (ml)
FIA (conversion %)
Sample No.
fIA
Vb (ml)
FIA (conversion %)
1 2 3 4 5
40 40 50 60 70
10.2 10.2 10.2 10.2 10.2
1.00 1.00 1.00 1.00 1.00
1 2 3 4 5
40 40 50 60 70
10.0 10.0 10.0 10.0 10.0
1.00 1.00 1.00 1.00 1.00
(48.40) (39.16) (45.69) (53.02) (69.72)
(48.40) (39.16) (45.69) (53.02) (69.72)
Inflection points obtained from (a) conductometric and (b) potentiometric acid–base titrations of copolymers.
For the compositional analysis of the NIPAAm/IA copolymers, characteristic absorption bands of 1704 cm1 (for IA units) and 1620 cm1 (for NIPAAm units) were chosen as analytical bands. The least changing absorption band of 725 cm1 were used as a standard band (A = log Io/I, DAC = AC/A725) to calculate the copolymer composition. The ratios of the mole fractions of comonomer units (F1 and F2) of IA (1)/NIPAAm (2) copolymers were calculated using the following relation:
F 1 =F 2 ¼ ½DA1704 =M 1 =½DA1620 =M2
ð1Þ
The mole fractions (in mol %) of NIPAAm (2) and IA (1) in the copolymers of four different compositions, calculated by using FTIR analysis data and the inflection points on the conductometric/potentiometric titration curves are given in Table 6.
Reactivity ratios of NIPAAm and IA were determined by the application of conventional linearization methods such as Finemann–Ross (FR), Inverted Finemann–Ross (IFR), Kelen–Tüdós (KT) and extended Kelen–Tüdós (EKT) methods. Finemann–Ross (FR) method is the earliest way for determining reactivity ratios [31]. G and H have linear relationship with each other according to the following equation:
G ¼ r IA H r NIPAAm
ð2Þ
The inverted FR equation (IFR) gives rIA as the intercept and rNIPAAm as the slope:
G=H ¼ rIA r NIPAAm ð1=HÞ
ð3Þ
The FR and IFR plots obtained by linear regression analysis for NIPAAm/IA copolymers are given in Figs. 6 and 7.
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Fig. 6. FR method for determining monomer reactivity ratios in the copolymerization of IA and NIPAAm by using potentiometric titration data (at low conversion for LPs).
η 0.2 y = 1.359 x - 0.743 2 R = 0.8610
0.1 0 -0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
ζ
-0.2 -0.3
Fig. 5. FTIR spectra of the samples (a) 40.0, (b) 50.0, (c) 60.0 and (d) 70.0 given in Table 6.
-0.4
Kelen and Tüdós (KT) apply these two parameters, G and H in the linearized copolymerization equation, along with new parameters such as a, g and f [32]:
g ¼ ½rIA þ rNIPAAm =af rNIPAAm =a
ð4Þ
The intercepts at f = 0 and f = 1 of the g versus f plots yield – rNIPAAm/a and rIA, respectively. The KT plots for low conversion data of this system are given in Figs. 8 and 9. The effect of conversion is considered in the extended Kelen–Tüdós equation (EKT) [33]:
g ¼ rIA f rNIPAAm =að1 fÞ
ð5Þ
The effect of conversion is given by partial molar conversion
nNIPAAm ¼ wðl þ XÞ=ðl þ YÞ
ð6Þ
Fig. 7. IFR method for determining monomer reactivity ratios in the copolymerization of IA and NIPAAm by using potentiometric titration data (at low conversion for LPs).
where w is the weight conversion of polymerization and l is the ratio of molecular weight of NIPAAm (2) to that of IA (1). The partial molar conversion of IA is
nIA ¼ nNIPAAm Y=X
ð7Þ
Then,
z ¼ log ð1 nIA Þ=log ð1 nNIPAAm Þ
ð8Þ
EKT parameters were calculated from the above equations using experimental data in Tables 2–4. EKT plots for high conversion samples of NIPAAm/IA copolymers are shown in Figs. 10 and 11.
Table 6 Itaconic acid composition in the feed (f) and copolymer chain (F) at low conversion for lower phases (LPs) of the samples. Sample No. (LC)
fIA
V (ml)
Conversion (%)
FIAa
FIAb
Sample No. (LC)
fIA
FIAc
1 2 3 4 5
40 40 50 60 70
3.99 4.20 4.95 5.55 6.59
14.34 8.62 8.43 9.56 2.28
27.48 29.27 42.32 46.44 60.04
24.34 27.37 38.07 44.18 55.85
– 2 3 4 5
– 40 50 60 70
– 25.15 45.33 48.90 56.65
a b c
The results obtained by means of conductometric titration method. Potentiometric titration method. FTIR spectroscopy.
C. Erbil et al. / European Polymer Journal 45 (2009) 1728–1737
Fig. 8. KT method for determining monomer reactivity ratios in the copolymerization of IA and NIPAAm by using conductometric titration data (at low conversion for LPs).
Fig. 9. KT method for determining monomer reactivity ratios in the copolymerization of IA and NIPAAm by using potentiometric titration data (at low conversion for LPs).
For all graphical methods, the plots were linear. This indicates that the reactivity of a polymer radical is determined only by the terminal monomer unit. The reactivity ratios are summarized in Tables 7 and 8. For both the LPs at low % conversion and each of the phases (LPs and UPs) at high % conversion, the higher reactivity ratio value of NIPAAm confirms its reactivity compared with that of IA. In all cases, the value of r1 is less than 1 and that of r2 is greater than 1, which indicates the presence of higher amount of NIPAAm units in the copolymer than that in the feed. This means that the polymer radical with a NIPAAm unit at the chain end is considerably more reactive than with an IA unit, and the probability of NIPAAm entry into the chain is greater than that of IA entry.
Fig. 10. EKT method for determining monomer reactivity ratios in the copolymerization of IA and NIPAAm by using conductometric titration data (at high conversion for LPs).
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Fig. 11. EKT method for determining monomer reactivity ratios in the copolymerization of IA and NIPAAm by using conductometric titration data (at high conversion for UPs).
In the determination of reactivity ratios in any copolymerization reaction, the experimental errors involved in the analysis method which is used to determine copolymer compositions together with systematic errors arising from assumptions made in the derivation of various equations cannot be avoided. In this work, when different calculation methods were applied to a certain set of experimental results and/or a single calculation method was used to the results of different experimental techniques r values observed to vary considerably. The r1 values were found to change between 0.36 (the lowest value for high conversion is 0.003) and 0.64 and the r2 values range between 1.20 (0.89 for high conversion) and 1.89 (Tables 7 and 8). The results show that for the same set of experimental data different equations lead to the different standard deviations of r values. The precision of experimentally determined monomer reactivity ratios depends on the experimental design and the technique used to analysis the data. Tables 7 and 8 show discrepancies between the values obtained by the different methods. For the low conversion set of experimental data and four different conventional linearization methods, when FTIR technique was used as one of the characterization methods r1 and r2 values differed from the ones obtained with the conductometric and potentiometric titration methods This may obviously indicate the unadequency of the FTIR technique for the determination of the reactivity ratios of NIPAAm–IA pair. In addition, in the case of conductometric method, conductivity values in the vicinity of the equivalent point had no important effect on the construction of the graph, which consist of two straight lines intersecting at the inflection point, whereas for FTIR spectroscopic technique and potentiometric titration method , there might be an indefiniteness resulting form the areas of the absorption bands and the end points of the acid–base titration curves on the transmittance vs wave number spectra and the pH vs titrant volume plots because of the overlaps between the peaks selected for specific groups and the alkali metal poisoning encountered with glass electrode, respectively. The effect of reaction medium on free radical copolymerization has been well documented in previous reported studies [38], suggesting factors like electrostatic interactions, hydrogen bonding monomers with solvent, polar–
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Table 7 Comparison of reactivity ratios by various methods for NIPAAm/IA copolymers. Low conversion
LP-FR LP-IFR LP-KT LP-EKT
Conductometric method
Potentiometric method
FTIR Spectroscopy
r1
r2
r1
r2
r1
r2
0.62 ± 0.05 0.64 ± 0.17 0.62 ± 0.03 0.61 ± 0.03
I.59 ± 0.20 1.61 ± 0.12 I.58 ± 0.24 0.63 ± 0.21
0.53 ± 0.04 0.54 ± 0.17 0.53 ± 0.03 0.52 ± 0.02
1.80 ± 0.21 l.82 ± 0.12 I.81 ± 0.36 I.88 ± 0.24
0.44 ± 0.08 0.41 ± 0.18 0.44 ± 0.06 0.40 ± 0.06
1.26 ± 0.21 1.20 ± 0.27 1.26 ± 0.34 1.23 ± 0.26
Table 8 Comparison of reactivity ratios by various methods for NIPAAm/IA copolymers. High conversion
LP-EKT LP-EKT
Conductometric method
Potentiometric method
r1
r2
r1
r2
0.016 ± 0.004 0.53 ± 0.02
1.29 ± 0.29 0.89 ± 0.47
0.003 ± 0.001 0.36 ± 0.025
1.89 ± 0.46 1.19 ± 0.77
polar interactions, solvent dielectric effects etc as responsible for monomer and radical reactivities in copolymerization. Klumperman et al. have reported that for acrylic (AA) and methacrylic acid (MAA), methyl acrylate, acrylonitrile and acrylamide copolymerized with styrene the solvent can have an apparent influence on the values of r1 and r2 [39,40]. The influence of the solvent may be associated with the tendency of the monomers to partition themselves between the bulk medium and the solvent sheath surrounding the growing copolymer chains, and different solvents may have different partition coefficients. The free radical solution copolymerizations of olefinically-unsaturated carboxylic acid containing one activated carbon-to-carbon olefinic double bond and two carboxyl group, i.e., IA (polar monomer) with monoolefinically unsaturated amide having one hydrogen and one isopropyl group on the amide nitrogen, i.e., NIPAAm (hydrogen bond forming monomer) in dioxane, being moderately hydrogen bonded (d = 9.9) and electron-donor solvent that have zero dipole moment (e = 2.2) at 50 °C and under nitrogen atmosphere have been investigated. It was assumed that the phase separations during the polymerizations arised from both viscosity and solubility differences of low conversion and high conversion NIPAAm-rich and IA-rich growing chains under our experimental conditions. It is known that the polymerizations of NIPAAm produce long polymer chains while in the case of IA, chain transfer to the monomer because of the presence of allylic hydrogens in the molecular structure results in the production of PIA chains with low molecular weight. Therefore, the viscosities of the LPs containing NIPAAm-rich chains were higher than the UPs. For both low and high conversion LPs, growing chains formed slightly turbid viscous solutions. The hydrogen bondings between NIPAAm and dioxane, IA and dioxane molecules and their competitions with hydrophobic interactions between the isopropyl groups prevent the aggregations of the NIPAAm-rich growing chains at 50 °C although PNIPAAm and its copolymers exhibit inverse temperaturesensitive behaviour at and above LCST (T P 32–34 °C, by depending on the copolymer composition) in aqueous solutions. In addition, the presence of the large number
of NIPAAm units in the NIPAAm/IA copolymer chains increases the solubility of growing chains during the polymerizations in this work whereas the PIA and its IA-rich copolymers in dioxane are obtained by heterogeneous solution polymerizations. The results reported by Huglin et al. also support our observations [41]. They synthesized the NIPAAm/AA, NIPAAm/MAA and NIPAAm/AMPS (2-methyl-2-acrylamidopropane sulfonic acid) copolymers in dioxane, ethanol and water. The compositions of these linear copolymers were determined by conductometric titration method. As to their results, NIPAAm has a strong propensity to enter the growing chain and the general form of the copolymerizations indicates a compositional heterogenity up to high conversions for the copolymers that exhibit thermo-sensitive solution behaviour [41]. The product of the monomer reactivity ratios (r1r2) for a given binary copolymerization system is often used to indicate the sequencing in the resultant copolymer composition. When the value of the product r1r2 is lesser than 1, the system deviates from straight line corresponding to ideal copolymerization. Copolymer compositional plots in the present work were obtained from the experimental data. The correlation between the feed monomer molar ratio, f, and the copolymer composition, F, is provided in Fig. 12. It is seen that all the compositional curves generated for NIPAAm with IA lie below the ideal compositional line. None of these experimental curves has an intercept with the fIA = FIA curve. This case is referred as nonideal nonazeotropic copolymerization. The positions of NIPAAm/IA plots clearly indicate that IA is less reactive than NIPAAm and that it is incorporated into the copolymer at levels much lower than those present in the monomer feed ratios. Further, the result that rIA < rNIPAAm, indicates that the copolymers are richer in NIPAAm units as confirmed in Fig. 12. Monomer reactivity ratios are generally determined at low conversion. The change in reaction medium with conversion affects the monomer reactivity ratio values. In this study, the effect of conversion on the monomer reactivity ratio values has also been investigated.
C. Erbil et al. / European Polymer Journal 45 (2009) 1728–1737
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Fig. 12. Composition diagram for NIPAAm–IA system. (j) Conductometric titration, (N) potentiometric titration and (d) FTIR Spectroscopy results.
From the phase separations during the polymerizations, the values of monomer reactivity ratios, which are given in Tables 7 and 8, and the compositional curves in Fig. 12, it can be suggested that the structures and solubilities of NIPAAm/IA copolymers are dependent on the distribution of monomeric units within both the single chains and LPs/UPs. According to the FTIR spectra and the conductometric/ potentiometric titration results of UPs and LPs for both low and high conversion NIPAAm/IA copolymers, copolymer composition is mainly determined by the conversion percentages of the total amount of monomers but not the molar ratio of the monomers in the feeds. For example, in the case of the low conversions the LPs of samples, i.e., cloudy viscous layers contain random copolymers of NIPAAm/IA (0 < r1r2 < 1) while in the clear phases, oligomeric IA chains are formed (Figs. 13–15 and Table 5). Both the appearance of the characteristic absorptions relating to only IA units in the FTIR spectra and the inflection points on the conductometric/potentiometric titration curves (and the mole fractions of copolymers, calculated from these end points) for the clear UPs of the low conversion copolymers indicates the presence of the oligomers of IA
Fig. 13. FTIR spectra of the samples 2 (a) and 5 (b) given in Table 5.
Fig. 14. Conductometric (a) and potentiometric (b) titration curves of sample 2 in Table 5.
but not NIPAAm/IA copoymers. As polymerization conversion increasing, NIPAAm units are easily adsorbed by NIPAAm-rich polymeric radicals in the LP and therefore, on the contrary of the low conversion samples, the value of the reactivity ratio for IA in the LPs of the high conversion samples is nearly zero (Tables 7 and 8) while the UPs contain NIPAAm/IA copolymer chains. The drift of the composi-
Fig. 15. Conductometric (a) and potentiometric (b) titration curves of sample 5 in Table 5.
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tions of the LPs and UPs with increasing conversion is associated with the enhancement of the viscosity of the reaction medium leading to a lower diffusion rate of IA to the growing macroradicals of NIPAAm in the LPs. 4. Conclusion In this work, copolymers of the IA (1)/NIPAAm (2) system were prepared with AIBN as initiator at 50 °C for both low conversions and high conversions. During the reactions, free radical copolymerization solutions were separated into a cloudy viscous layer (lower phase, LP) and a clear layer (upper phase, UP). The reactivity ratios of NIPAAm with IA were determined using linear graphical methods. Despite the rather large standard deviations (i.e., uncertainty limits) in the reactivity ratios for both the LPs at low conversion and each of the phases (LPs and UPs) at high conversion, they indicate that there are greater content of NIPAAm units in the copolymer than in the feed. Further, the value of the product of r1r2 was found to be lesser than 1. This means that copolymerization nature is nonideal and it is confirmed by the feed vs. copolymer composition curve. References [1] Larez CV, Canelon F, Millan E, Perdemo G, Katime I. New results on the polymerisation of the itaconic acid in aqueous medium. Polym Bull 2002;49:119–26. [2] Bokias G, Durand A, Hourdet D. Molar mass control of poly(Nisopropylacrylamide) and poly(acrylic acid) in aqueous polymerizations initiated by redox initiators based on persulfates. Macromol Chem Phys 1998;199:1387–92. [3] Erbil C, Akpınar FD, Uyanık N. Investigation of the thermal aggregation in aqueous poly(N-isopropylacrylamide-co-itaconic acid) solutions. Macromol Chem Phys 1999;200:2448–53. [4] Erbil C, Gökçeören AT, Polat YO. N-isopropylacrylamideacrylamide copolymers initiated by ceric ammonium nitrate in water. Polym Int 2007;56:547–56. [5] Schild HG. Progress in polymer science. Pergamon Press Ltd.; 1992. [6] Yin X, Hoffman AS, Stayton PS. Poly(N-isopropylacrylamide-copropylacrylic acid) copolymers that respond sharply to temperature and pH. Biomacromolecules 2006;7(5):1381–5. [7] Yoo MK, Sung YK, Cho CS, Lee YM. Effect of polymer complex formation on the cloud-point of poly(N-isopropyl acrylamide) (PNIPAAm) in the poly(NIPAAm-co-acrylic acid): Polyelectrolyte complex between poly(acrylic acid) and poly(allylamine). Polymer 1997;38(11):2759–65. [8] Yu H, Grainger DW. Modified release of hydrophilic, hydrophobic and peptide agents from ionized amphiphilic gel networks. J Contr Rel 1995;34:117–27. [9] Ramkissoon-Ganorkar C, Gutowska A, Liu F, Baudys M, Kim SW. Polymer molecular weight alters properties of pH-/temperaturesensitive polymeric beads. Pharm Res 1999;16(6):819–27. [10] Fujishige S, Kubota K, Ando I. Phase transition of aqueous solutions of poly(N-isopropylacrylamide) and poly(N-isopropylmethacrylamide). J Phys Chem 1989;93:3311–3. [11] Ito S. Phase transition of aqueous solution of poly(N-Alkylacrylamide) derivatives – effects of side chain structure. Kubonshi Ronbunshu 1989;46:437–43. [12] Feil H, Bae YH, Feijen J, Kim SW. Effect of comonomer hydrophilicity and ionization on the lower critical solution temperature of Nisopropylacrylamide copolymers. Macromolecules 1993;26: 2496–500. [13] Takai Y, Aoki T, Sanui K, Ogata N, Sakurai N, Okano T. Dynamic contact angle measurement of temperature-responsive surface properties for poly(N-isopropylacrylamide) grafted surfaces. Macromolecules 1994;27:6163–6. [14] Chen G, Hoffman AS. Graft copolymers that exhibit temperatureinduced phase transitions over a wide range of pH. Nature 1995;373:49–52.
[15] Casolaro M. Solution behaviour of poly(N-acryloyl-l-leucine) and its copolymers with N-isopropylacrylamide. Polymer 1997;38(16): 4215–22. [16] Xue W, Hamley IW, Castelletto V, Olmsted PD. Synthesis and characterization of hydrophobically modified polyacrylamides and some observations on rheological properties. Eur Polym J 2004;40: 47–56. [17] Zhu XX, Avoce D, Liu HY, Benrebouh A. Copolymers of Nalkylacrylamides as thermosensitive hydrogels. Macromol Symp 2004;207:187–92. [18] Erbil C, Kazancıog˘lu E, Uyanık N. Synthesis, characterization and thermoreversible behaviours of poly(dimethyl siloxane)/poly(Nisopropyl acrylamide) semi-interpenetrating networks. Eur Polym J 2004;40:1145–54. [19] Willke T, Vorlop KD. Biotechnological production of itaconic acid. Appl Microbiol Biotechnol 2001;56:289–95. [20] Bajaj P, Paliwal DK, Gupta AK. Acrylonitrile – Acrylic acids copolymers I synthesis and characterization. J Appl Polym Sci 1993;49: 823–33. [21] Xue W, Champ S, Huglin MB. Observations on some copolymerisations involving N-isopropylacrylamide. Polymer 2000;41: 7575–81. [22] Cowie JMG, McEwen IJ, Yule DJ. The influence of solvent on the apparent reactivity ratios in free radical copolymerisation reactions between itaconic acid and 2-hydroxyethyl acrylate. Eur Polym J 2000;36:1795–803. [23] Uyanık N, Erbil C. Monomer reactivity ratios of itaconic acid and acrylamide copolymers determined by using potentiometric titration method. Eur Polym J 2000;36:2651–4. [24] Erbil C, Özdemir S, Uyanık N. Determination of the monomer reactivity ratios for copolymerization of itaconic acid and acrylamide by conductometric titration method. Polymer 2000;41:1391–4. [25] Virtanen J, Tenhu H. Studies on copolymerization of Nisopropylacrylamide and glycidyl methacrylate. J Polym Sci Part A Polym Chem 2001;39:3716–25. [26] Devasia R, Nair CPR, Ninan KN. Copolymerization of acrylonitrile with itaconic acid in dimethylformamide: effect of triethylamine. Eur Polym J 2003;39:537–44. [27] S ß olpan D, Güven O. The copolymerization of allyl glycidyl ether with acrylonitrile initiated by gamma-rays. J Polym Sci Part A Polym Chem 1996;34:833–8338. [28] Patel MV, Dolia MB, Patel JN, Patel RM. Synthesis and characterization of novel acrylic copolymers: determination of monomer reactivity ratios and biological activity. React Funct Polym 2005;65:195–204. [29] Pekel N, Sßahiner N, Güven O, Rızaev ZMO. Synthesis and characterization of N-vinylimidazole–ethyl methacrylate copolymers and determination of monomer reactivity ratios. Eur Polym J 2001;37:2443–51. [30] Save NS, Jassal M, Agrawal AK. Stimuli sensitive copolymer poly (N-tert-butylacrylamide-ran-acrylamide): synthesis and characterization. J Appl Polym Sci 2005;95:672–80. [31] Finemann M, Ross S. Linear method for determining monomer reactivity ratios in copolymerization. J Polym Sci 1950;5:259–62. [32] Berger M, Kuntz I. The distinction between terminal and penultimate copolymerization models. J Polym Sci A 1964;2(4):1687. [33] Kelen T, Tüdós F. Analysis of the linear methods for determining copolymerization reactivity ratios I. A new improved linear graphic method. J Macromol Sci A 1975;9:1–27. [34] Kelen T, Tüdós F, Foldes-Berezsnich T, Turcsanyi B. analysis of linear methods for determining copolymerization reactivity ratios III. Linear graphic method for evaluating data obtained at high conversion levels. J Macromol Sci A 1976;10:1513–40. [35] Polic AL, Duever TA, Penlidis A. Case studies and literature review on the estimation of copolymerization reactivity ratios. J Polym Sci Polym Chem 1998;36:813–22. [36] Tidwell PW, Mortimer GA. An improved method of calculating copolymerization reactivity ratios. J Polym Sci A 1965;3:369–87. [37] Dube M, Amin Sanayei R, Penlidis A, O’Driscoli KF, Reilly PM. A microcomputer program for estimation of copolymerization reactivity ratios. J Polym Sci Polym Chem 1991;29:703–8. [38] Klumperman B, O’Driscoll KF. Interpreting the copolymerization of styrene with maleic anhydride and with methyl methacrylate in terms of the bootstrap modell. Polymer 1993;34(5):1032–7. [39] Klumperman B, Kraeger IR. The effect of solvent on the copolymerization of Styrene and Acrylonitrile – Application of the Bootstrap effect to the penultimate unit model. Macromolecules 1994;27(6):1529–34.
C. Erbil et al. / European Polymer Journal 45 (2009) 1728–1737 [40] Plochocka K. Effect of the reaction medium on radical copolymerization. J Macromol Sci Rev Macromol Chem and Phys 1981;C20(1):67–148.
[41] Xue W, Champ copolymerizations 2000;41:7575–81.
1737 S, Huglin MB. Observations on some involving N-isopropylacrylamide. Polymer
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Monomer reactivity ratios of N-isopropylacrylamide–itaconic acid copolymers at low and high conversions Candan Erbil *, Bürgehan Terlan, Özgür Akdemir, Argun T. Gökçeören _ Istanbul Technical University, Science and Letters Faculty, Chemistry Department, 34469 Maslak, Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 4 March 2008 Received in revised form 24 February 2009 Accepted 26 February 2009 Available online 6 March 2009
Keywords: Poly (N-isopropylacrylamide) N-isopropylacrylamide/itaconic acid copolymer Monomer reactivity ratios Conductometric and potentiometric titrations FTIR spectroscopy
a b s t r a c t Copolymers of N-isopropylacrylamide (NIPAAm) and itaconic acid (IA) having various compositions were synthesized using free radical solution polymerization in 1,4-dioxane at 50 °C with a,a0 -azobisisobutyronitrile (AIBN) as initiator. The structures of the copolymers were confirmed by Fourier transform infrared (FTIR) spectroscopic technique. The copolymer compositions were determined by conductometric and potentiometric methods from the inflection points in the acid–base titration curves and by FTIR spectroscopy through recorded analytical absorption bands for NIPAAm (1620 cm1 for C@O stretching of secondary amides) and for IA (1704 cm1 for C@O stretching) units, respectively. Monomer reactivity ratios of IA (F1)–NIPAAm (F2) pair were estimated using the Finemann–Ross, the inverted Finemann–Ross, the Kelen–Tüdós and the extended Kelen–Tüdós graphical methods. The values ranged from 0.40 to 0.60 for r1 and from 1.20 to 1.90 for r2, depending on the conversion percentage, calculation methods of monomer reactivity ratios and determination methods of copolymer compositions. In all cases, r1r2 < 1 and r1 < r2 indicate the random distribution of the monomers in the final copolymers and the presence of higher amount of NIPAAm units in the copolymer than that in the feed, respectively. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Acrylamide (AAm), acrylic acid (AA), itaconic acid (IA) and N-isopropylacrylamide (NIPAAm) can easily polymerize by free radical polymerization technique [1–4]. Among these water-soluble polymers indicated above, PNIPAAm has a great interest because of its temperaturesensitive character in aqueous solutions. It exhibits a sharp phase transition close to body temperature (32–34 °C) and this phase transition temperature, which is known as lower critical solution temperature (LCST) can be modified by copolymerizing a more or less hydrophilic monomer [5–9]. Further, the temperature-sensitive PNIPAAm copolymerized with pH-sensitive monomers is the most intensively studied member of N-alkylacrylamide polymers and * Corresponding author. Tel.: +90 (212) 285 3234; fax: +90 (212) 285 6386. E-mail address: [email protected] (C. Erbil). 0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2009.02.023
copolymers because those factors are variables that change in physiological, biological and chemical systems [10–18]. Itaconic acid (IA) is an unsaturated dicarbonic organic acid with one carboxyl group conjugated to the methylene group. It can be regarded as an a-substituted acrylic or methacrylic acid. The potential for substitution of petrochemical-based acrylic or methacrylic acid in polymers is high, because rather low comonomer contents lead to copolymers with effective acidity due to the two carboxylic groups of IA. In addition, the outstanding properties in polymer chemistry, pharmacy and agriculture open up new applications [19]. As a result of increasing studies of NIPAAm and IA polymers, various methods for the synthesis of these polymers and their copolymers with the monomers such as acrylonitrile, styrene and acrylic esters have been reported, in most cases radical polymerizations [20–26]. These polymerizations are usually performed in organic solvents using peroxide-type initiators or in aqueous solutions using redox initiation systems.
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The compositional heterogenity of copolymers is of great importance in considering copolymer properties. For example, in the case of NIPAAm based copolymers, the structural differences, resulting from the distribution of comonomers along the polymer chains may be effected on the thermal behaviours of the product polymers. The copolymer composition in the free radical polymerization systems are dependent on the monomer reactivity ratios. Direct and indirect methods are applied to determine the compositions of copolymers [27–30]. For determination of monomer reactivity ratios, linear leastsquares methods such as Finemann–Ross, Kelen–Tüdós and extended Kelen–Tüdós and, nonlinear least-squares analysis can be applied to a wide range feed and corresponding copolymer compositions [31–37]. In the present study, NIPAAm/IA copolymers with different monomer ratio were prepared through free radical copolymerization in 1,4-dioxane using a,a0 -azobisisobutyronitrile (AIBN). Copolymerizations were carried out in both low and high conversion conditions. The reaction times were selected in such a way that the conversions were less than 15% and 70% of weight, for low and high conversions, respectively. The compositions of copolymers were determined by conductometric and potentiometric acid–base titrations, and FTIR spectroscopic technique. The reactivity ratios of NIPAAm/IA copolymers were calculated from the initial monomer feed composition, the low and high conversions and the copolymer composition, by means of Finemann–Ross (FR), Inverted Finemann–Ross (IFR) Kelen–Tüdós (KT) and extended Kelen–Tüdós (EKT) methods. 2. Experimental 2.1. Reagents N-isopropylacrylamide (NIPAAm) and itaconic acid (IA) were products of Aldrich and Fluka, respectively. Both monomers were used without further purification. The initiator, a,a0 -azobisisobutyronitrile (AIBN, Merck), was recrystallized from methanol. 1,4-Dioxane were used as purchased from Merck A. G. 2.2. Copolymerization The feed mixtures, containing a total monomer concentration of 1 M and 0.01 M AIBN in 1,4-dioxane were introduced into large glass tubes with 30 mm diameter, closed with rubber septa. Each of the tubes was flushed with dry nitrogen for 20 min using a syringe needles to remove oxygen dissolved in the reaction mixtures. During the reactions, copolymerization solutions were separated into a cloudy viscous layer (lower phase, LP) and a clear layer (upper phase, UP). The amount of LP increased with increase in IA content of copolymerization mixtures. For low conversions, the tubes were placed in the constant temperature bath at 50 °C to observe the phase separation and decide the reaction time. The time required to obtain approximately 10 % conversion (between 20 and 40 min) was found to vary slightly depending on the ratio of NIPAAm and IA, due to the difference in reactivity of these
two monomers. Low % conversion is required to minimize any potential composition drift which may result from depletion of any one comonomer in the feed. For high % conversions (to observe the effect of conversion percentage on the copolymer composition), polymerizations were carried out in an air oven regulated at 50 °C and the reactions were allowed to proceed for 24 h. After the reaction time is over, the tubes were removed from the bath and/or air oven and each phase of the reaction mixtures was precipitated by pouring into n-hexane. The product polymers were left to dry at ambient temperature for 24 h prior to drying to constant weight in a vacuum oven at 25 °C. Conversions were obtained gravimetrically. 2.3. Characterization FTIR spectra of PNIPAAm and NIPAAm/IA copolymers with different composition were recorded on Perkin-Elmer Spectrum One (FTIR-reflectance, Universal ATR with diamond and ZnSe) spectrophotometer using the samples in powder form. Monomer reactivity ratios of NIPAAm/IA copolymers were determined by conductometric and potentiometric titration methods. WTW-LF 2000 model conductometer and WTW 523 model pH meter were used for conductivity and pH measurements, respectively. Acid–base titrations were carried out in a doublewalled glass cell at 25 °C. For each titration experiment, the cell was filled with 20 ml 0.1 N NaCl solution in which 0.07 g of solid polymer was dispersed by magnetic stirring. After the polymer completely dissolved, the solution was titrated with 0.1 N NaOH, which was added from a microburette. Conductivity and pH values were plotted against the volume of titrant. Compositions of copolymers and monomer reactivity ratios were determined from the inflection points in these titration curves. 3. Results and discussion Copolymer composition in this system involving IA which contains two carboxylic acid groups was ascertained by conductometric and potentiometric titrations of the acidic comonomer. For this purpose, physical mixtures of PNIPAAm and PIA homopolymers were prepared with different compositions. The end points obtained from the
Table 1 Conductometric and potentiometric titration results in the FIA range of 5– 95 (in mol %) of physical mixtures of PNIPAAm and PIA. FIA(mol %)
Potentiometric titration Inflection points V (ml of 0.1 N NaOH)
Conductometric titration Inflection points V (ml of 0.1 N NaOH)
5.72 10.06 26.70 35.33 52.59 56.04 75.59 91.24
0.93 1.59 3.85 4.60 6.37 6.50 8.13 9.01
1.00 1.60 4.00 4.80 6.60 6.80 8.40 9.40
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conductometric and potentiometric titration curves of the aqueous solutions of these mixtures were plotted against the content of PIA to make the calibration curves (Table 1, Figs. 1 and 2). Then the inflection points of the conductometric and potentiometric curves of the copolymers were placed on the calibration curves to estimate the acidic comonomer content in the copolymers and the copolymer compositions were found directly from these curves (Figs. 3 and 4). For both low and high conversions, the compositions of the feeds and the copolymers as well as the conversions of LPs and UPs for the NIPAAm/IA copolymers are given in Tables 2–5. For the LPs of the low conversion samples, FTIR technique was also employed to calculate the reactivity ratios of monomer pair, based on the determination of the composition of NIPAAm/IA copolymers obtained at four different monomer feed ratios [27,29]. C@O stretching (1704 cm1) and –OH dimerization (3500–2500 cm1) bands in the FTIR spectra of these copolymers indicate the presence of IA units in the chains while the bands due to –C@O stretching and NH– bending for secondary amides at 1620 and 1540 cm1 and, a double band for iso-
Fig. 3. Conductometric titration curves of copolymers containing IA. () 40.0, (e) 50.0, (D) 60.0 and (h) 70.0 mol % in the feed (fIA) (at low conversion for LPs).
propyl group at 1385 and 1370 cm1 are characteristic absorptions of PNIPAAm (Fig. 5).
Fig. 1. Conductometric calibration curve which is used to obtain copolymer compositions.
Fig. 2. Potentiometric calibration curve which is used to obtain copolymer compositions.
Fig. 4. Potentiometric titration curves of copolymers containing IA. (a) 40.0, (b) 50.0, (c) 60.0 and (d) 80.0 mol % in the feed (fIA) (at high conversion for LPs).
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C. Erbil et al. / European Polymer Journal 45 (2009) 1728–1737 Table 2 Itaconic acid composition in the feed (f) and copolymer chain (F) at low and high conversions for lower phases (LPs) of the samples. Sample No. (LC)b
fIA
V (ml)
FIAa (conversion %)
Sample No. (HC)c
fIA
V (ml)
FIAa (conversion %)
1 2 3 4 5
40 40 50 60 70
3.99 4.20 4.95 5.55 6.59
27.48 29.27 42.32 46.44 60.04
– 6 7 8 9
– 40 50 60 80
– 4.35 5.00 5.48 6.40
– 30.57 36.48 41.12 50.76
a b c
(14.34) (8.62) (8.43) (9.56) (2.28)
(74.12) (67.50) (50.74) (36.95)
The results obtained by means of conductometric titration method. Low conversion. high conversion.
Table 3 Itaconic acid composition in the feed (f) and copolymer chain (F) at low and high conversions for lower phases (LPs) of the samples. Sample No. (LC)b
fIA
V (ml)
FIAa (conversion %)
Sample No. (HC)c
fIA
V (ml)
FIAa (conversion %)
1 2 3 4 5
40 40 50 60 70
3.45 3.80 4.95 5.55 6.59
24.34 27.37 38.07 44.18 55.85
– 6 7 8 9
– 40 50 60 80
– 3.85 4.48 4.90 5.95
– 27.81 33.55 37.58 48.49
a b c
(14.34) (8.62) (8.43) (9.56) (2.28)
(74.12) (67.50) (50.74) (36.95)
The results obtained by means of potentiometric titration method. Low conversion. high conversion.
Table 4 Itaconic acid composition in the feed (f) and copolymer chain (F) at high conversion for upper phases (UPs) of the samples. Sample No. (LC)a
fIA
Va (ml)
FIA (conversion %)
Sample No. (HC)b
fIA
Vb (ml)
FIA (conversion %)
6 7 8 9
40 50 60 80
4.87 6.14 6.57 8.40
35.26 47.93 52.66 75.76
6 7 8 9
40 50 60 80
4.07 5.36 5.61 7.90
29.77 42.20 44.81 73.16
(17.20) (7.50) (24.20) (62.70)
(17.20) (7.50) (24.20) (62.70)
Inflection points obtained from (a) conductometric and (b) potentiometric acid–base titrations of copolymers.
Table 5 Itaconic acid composition in the feed (f) and copolymer chain (F) at low conversion for upper phases (UPs) of the samples. Sample No.
fIA
Va (ml)
FIA (conversion %)
Sample No.
fIA
Vb (ml)
FIA (conversion %)
1 2 3 4 5
40 40 50 60 70
10.2 10.2 10.2 10.2 10.2
1.00 1.00 1.00 1.00 1.00
1 2 3 4 5
40 40 50 60 70
10.0 10.0 10.0 10.0 10.0
1.00 1.00 1.00 1.00 1.00
(48.40) (39.16) (45.69) (53.02) (69.72)
(48.40) (39.16) (45.69) (53.02) (69.72)
Inflection points obtained from (a) conductometric and (b) potentiometric acid–base titrations of copolymers.
For the compositional analysis of the NIPAAm/IA copolymers, characteristic absorption bands of 1704 cm1 (for IA units) and 1620 cm1 (for NIPAAm units) were chosen as analytical bands. The least changing absorption band of 725 cm1 were used as a standard band (A = log Io/I, DAC = AC/A725) to calculate the copolymer composition. The ratios of the mole fractions of comonomer units (F1 and F2) of IA (1)/NIPAAm (2) copolymers were calculated using the following relation:
F 1 =F 2 ¼ ½DA1704 =M 1 =½DA1620 =M2
ð1Þ
The mole fractions (in mol %) of NIPAAm (2) and IA (1) in the copolymers of four different compositions, calculated by using FTIR analysis data and the inflection points on the conductometric/potentiometric titration curves are given in Table 6.
Reactivity ratios of NIPAAm and IA were determined by the application of conventional linearization methods such as Finemann–Ross (FR), Inverted Finemann–Ross (IFR), Kelen–Tüdós (KT) and extended Kelen–Tüdós (EKT) methods. Finemann–Ross (FR) method is the earliest way for determining reactivity ratios [31]. G and H have linear relationship with each other according to the following equation:
G ¼ r IA H r NIPAAm
ð2Þ
The inverted FR equation (IFR) gives rIA as the intercept and rNIPAAm as the slope:
G=H ¼ rIA r NIPAAm ð1=HÞ
ð3Þ
The FR and IFR plots obtained by linear regression analysis for NIPAAm/IA copolymers are given in Figs. 6 and 7.
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Fig. 6. FR method for determining monomer reactivity ratios in the copolymerization of IA and NIPAAm by using potentiometric titration data (at low conversion for LPs).
η 0.2 y = 1.359 x - 0.743 2 R = 0.8610
0.1 0 -0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
ζ
-0.2 -0.3
Fig. 5. FTIR spectra of the samples (a) 40.0, (b) 50.0, (c) 60.0 and (d) 70.0 given in Table 6.
-0.4
Kelen and Tüdós (KT) apply these two parameters, G and H in the linearized copolymerization equation, along with new parameters such as a, g and f [32]:
g ¼ ½rIA þ rNIPAAm =af rNIPAAm =a
ð4Þ
The intercepts at f = 0 and f = 1 of the g versus f plots yield – rNIPAAm/a and rIA, respectively. The KT plots for low conversion data of this system are given in Figs. 8 and 9. The effect of conversion is considered in the extended Kelen–Tüdós equation (EKT) [33]:
g ¼ rIA f rNIPAAm =að1 fÞ
ð5Þ
The effect of conversion is given by partial molar conversion
nNIPAAm ¼ wðl þ XÞ=ðl þ YÞ
ð6Þ
Fig. 7. IFR method for determining monomer reactivity ratios in the copolymerization of IA and NIPAAm by using potentiometric titration data (at low conversion for LPs).
where w is the weight conversion of polymerization and l is the ratio of molecular weight of NIPAAm (2) to that of IA (1). The partial molar conversion of IA is
nIA ¼ nNIPAAm Y=X
ð7Þ
Then,
z ¼ log ð1 nIA Þ=log ð1 nNIPAAm Þ
ð8Þ
EKT parameters were calculated from the above equations using experimental data in Tables 2–4. EKT plots for high conversion samples of NIPAAm/IA copolymers are shown in Figs. 10 and 11.
Table 6 Itaconic acid composition in the feed (f) and copolymer chain (F) at low conversion for lower phases (LPs) of the samples. Sample No. (LC)
fIA
V (ml)
Conversion (%)
FIAa
FIAb
Sample No. (LC)
fIA
FIAc
1 2 3 4 5
40 40 50 60 70
3.99 4.20 4.95 5.55 6.59
14.34 8.62 8.43 9.56 2.28
27.48 29.27 42.32 46.44 60.04
24.34 27.37 38.07 44.18 55.85
– 2 3 4 5
– 40 50 60 70
– 25.15 45.33 48.90 56.65
a b c
The results obtained by means of conductometric titration method. Potentiometric titration method. FTIR spectroscopy.
C. Erbil et al. / European Polymer Journal 45 (2009) 1728–1737
Fig. 8. KT method for determining monomer reactivity ratios in the copolymerization of IA and NIPAAm by using conductometric titration data (at low conversion for LPs).
Fig. 9. KT method for determining monomer reactivity ratios in the copolymerization of IA and NIPAAm by using potentiometric titration data (at low conversion for LPs).
For all graphical methods, the plots were linear. This indicates that the reactivity of a polymer radical is determined only by the terminal monomer unit. The reactivity ratios are summarized in Tables 7 and 8. For both the LPs at low % conversion and each of the phases (LPs and UPs) at high % conversion, the higher reactivity ratio value of NIPAAm confirms its reactivity compared with that of IA. In all cases, the value of r1 is less than 1 and that of r2 is greater than 1, which indicates the presence of higher amount of NIPAAm units in the copolymer than that in the feed. This means that the polymer radical with a NIPAAm unit at the chain end is considerably more reactive than with an IA unit, and the probability of NIPAAm entry into the chain is greater than that of IA entry.
Fig. 10. EKT method for determining monomer reactivity ratios in the copolymerization of IA and NIPAAm by using conductometric titration data (at high conversion for LPs).
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Fig. 11. EKT method for determining monomer reactivity ratios in the copolymerization of IA and NIPAAm by using conductometric titration data (at high conversion for UPs).
In the determination of reactivity ratios in any copolymerization reaction, the experimental errors involved in the analysis method which is used to determine copolymer compositions together with systematic errors arising from assumptions made in the derivation of various equations cannot be avoided. In this work, when different calculation methods were applied to a certain set of experimental results and/or a single calculation method was used to the results of different experimental techniques r values observed to vary considerably. The r1 values were found to change between 0.36 (the lowest value for high conversion is 0.003) and 0.64 and the r2 values range between 1.20 (0.89 for high conversion) and 1.89 (Tables 7 and 8). The results show that for the same set of experimental data different equations lead to the different standard deviations of r values. The precision of experimentally determined monomer reactivity ratios depends on the experimental design and the technique used to analysis the data. Tables 7 and 8 show discrepancies between the values obtained by the different methods. For the low conversion set of experimental data and four different conventional linearization methods, when FTIR technique was used as one of the characterization methods r1 and r2 values differed from the ones obtained with the conductometric and potentiometric titration methods This may obviously indicate the unadequency of the FTIR technique for the determination of the reactivity ratios of NIPAAm–IA pair. In addition, in the case of conductometric method, conductivity values in the vicinity of the equivalent point had no important effect on the construction of the graph, which consist of two straight lines intersecting at the inflection point, whereas for FTIR spectroscopic technique and potentiometric titration method , there might be an indefiniteness resulting form the areas of the absorption bands and the end points of the acid–base titration curves on the transmittance vs wave number spectra and the pH vs titrant volume plots because of the overlaps between the peaks selected for specific groups and the alkali metal poisoning encountered with glass electrode, respectively. The effect of reaction medium on free radical copolymerization has been well documented in previous reported studies [38], suggesting factors like electrostatic interactions, hydrogen bonding monomers with solvent, polar–
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Table 7 Comparison of reactivity ratios by various methods for NIPAAm/IA copolymers. Low conversion
LP-FR LP-IFR LP-KT LP-EKT
Conductometric method
Potentiometric method
FTIR Spectroscopy
r1
r2
r1
r2
r1
r2
0.62 ± 0.05 0.64 ± 0.17 0.62 ± 0.03 0.61 ± 0.03
I.59 ± 0.20 1.61 ± 0.12 I.58 ± 0.24 0.63 ± 0.21
0.53 ± 0.04 0.54 ± 0.17 0.53 ± 0.03 0.52 ± 0.02
1.80 ± 0.21 l.82 ± 0.12 I.81 ± 0.36 I.88 ± 0.24
0.44 ± 0.08 0.41 ± 0.18 0.44 ± 0.06 0.40 ± 0.06
1.26 ± 0.21 1.20 ± 0.27 1.26 ± 0.34 1.23 ± 0.26
Table 8 Comparison of reactivity ratios by various methods for NIPAAm/IA copolymers. High conversion
LP-EKT LP-EKT
Conductometric method
Potentiometric method
r1
r2
r1
r2
0.016 ± 0.004 0.53 ± 0.02
1.29 ± 0.29 0.89 ± 0.47
0.003 ± 0.001 0.36 ± 0.025
1.89 ± 0.46 1.19 ± 0.77
polar interactions, solvent dielectric effects etc as responsible for monomer and radical reactivities in copolymerization. Klumperman et al. have reported that for acrylic (AA) and methacrylic acid (MAA), methyl acrylate, acrylonitrile and acrylamide copolymerized with styrene the solvent can have an apparent influence on the values of r1 and r2 [39,40]. The influence of the solvent may be associated with the tendency of the monomers to partition themselves between the bulk medium and the solvent sheath surrounding the growing copolymer chains, and different solvents may have different partition coefficients. The free radical solution copolymerizations of olefinically-unsaturated carboxylic acid containing one activated carbon-to-carbon olefinic double bond and two carboxyl group, i.e., IA (polar monomer) with monoolefinically unsaturated amide having one hydrogen and one isopropyl group on the amide nitrogen, i.e., NIPAAm (hydrogen bond forming monomer) in dioxane, being moderately hydrogen bonded (d = 9.9) and electron-donor solvent that have zero dipole moment (e = 2.2) at 50 °C and under nitrogen atmosphere have been investigated. It was assumed that the phase separations during the polymerizations arised from both viscosity and solubility differences of low conversion and high conversion NIPAAm-rich and IA-rich growing chains under our experimental conditions. It is known that the polymerizations of NIPAAm produce long polymer chains while in the case of IA, chain transfer to the monomer because of the presence of allylic hydrogens in the molecular structure results in the production of PIA chains with low molecular weight. Therefore, the viscosities of the LPs containing NIPAAm-rich chains were higher than the UPs. For both low and high conversion LPs, growing chains formed slightly turbid viscous solutions. The hydrogen bondings between NIPAAm and dioxane, IA and dioxane molecules and their competitions with hydrophobic interactions between the isopropyl groups prevent the aggregations of the NIPAAm-rich growing chains at 50 °C although PNIPAAm and its copolymers exhibit inverse temperaturesensitive behaviour at and above LCST (T P 32–34 °C, by depending on the copolymer composition) in aqueous solutions. In addition, the presence of the large number
of NIPAAm units in the NIPAAm/IA copolymer chains increases the solubility of growing chains during the polymerizations in this work whereas the PIA and its IA-rich copolymers in dioxane are obtained by heterogeneous solution polymerizations. The results reported by Huglin et al. also support our observations [41]. They synthesized the NIPAAm/AA, NIPAAm/MAA and NIPAAm/AMPS (2-methyl-2-acrylamidopropane sulfonic acid) copolymers in dioxane, ethanol and water. The compositions of these linear copolymers were determined by conductometric titration method. As to their results, NIPAAm has a strong propensity to enter the growing chain and the general form of the copolymerizations indicates a compositional heterogenity up to high conversions for the copolymers that exhibit thermo-sensitive solution behaviour [41]. The product of the monomer reactivity ratios (r1r2) for a given binary copolymerization system is often used to indicate the sequencing in the resultant copolymer composition. When the value of the product r1r2 is lesser than 1, the system deviates from straight line corresponding to ideal copolymerization. Copolymer compositional plots in the present work were obtained from the experimental data. The correlation between the feed monomer molar ratio, f, and the copolymer composition, F, is provided in Fig. 12. It is seen that all the compositional curves generated for NIPAAm with IA lie below the ideal compositional line. None of these experimental curves has an intercept with the fIA = FIA curve. This case is referred as nonideal nonazeotropic copolymerization. The positions of NIPAAm/IA plots clearly indicate that IA is less reactive than NIPAAm and that it is incorporated into the copolymer at levels much lower than those present in the monomer feed ratios. Further, the result that rIA < rNIPAAm, indicates that the copolymers are richer in NIPAAm units as confirmed in Fig. 12. Monomer reactivity ratios are generally determined at low conversion. The change in reaction medium with conversion affects the monomer reactivity ratio values. In this study, the effect of conversion on the monomer reactivity ratio values has also been investigated.
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1735
Fig. 12. Composition diagram for NIPAAm–IA system. (j) Conductometric titration, (N) potentiometric titration and (d) FTIR Spectroscopy results.
From the phase separations during the polymerizations, the values of monomer reactivity ratios, which are given in Tables 7 and 8, and the compositional curves in Fig. 12, it can be suggested that the structures and solubilities of NIPAAm/IA copolymers are dependent on the distribution of monomeric units within both the single chains and LPs/UPs. According to the FTIR spectra and the conductometric/ potentiometric titration results of UPs and LPs for both low and high conversion NIPAAm/IA copolymers, copolymer composition is mainly determined by the conversion percentages of the total amount of monomers but not the molar ratio of the monomers in the feeds. For example, in the case of the low conversions the LPs of samples, i.e., cloudy viscous layers contain random copolymers of NIPAAm/IA (0 < r1r2 < 1) while in the clear phases, oligomeric IA chains are formed (Figs. 13–15 and Table 5). Both the appearance of the characteristic absorptions relating to only IA units in the FTIR spectra and the inflection points on the conductometric/potentiometric titration curves (and the mole fractions of copolymers, calculated from these end points) for the clear UPs of the low conversion copolymers indicates the presence of the oligomers of IA
Fig. 13. FTIR spectra of the samples 2 (a) and 5 (b) given in Table 5.
Fig. 14. Conductometric (a) and potentiometric (b) titration curves of sample 2 in Table 5.
but not NIPAAm/IA copoymers. As polymerization conversion increasing, NIPAAm units are easily adsorbed by NIPAAm-rich polymeric radicals in the LP and therefore, on the contrary of the low conversion samples, the value of the reactivity ratio for IA in the LPs of the high conversion samples is nearly zero (Tables 7 and 8) while the UPs contain NIPAAm/IA copolymer chains. The drift of the composi-
Fig. 15. Conductometric (a) and potentiometric (b) titration curves of sample 5 in Table 5.
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tions of the LPs and UPs with increasing conversion is associated with the enhancement of the viscosity of the reaction medium leading to a lower diffusion rate of IA to the growing macroradicals of NIPAAm in the LPs. 4. Conclusion In this work, copolymers of the IA (1)/NIPAAm (2) system were prepared with AIBN as initiator at 50 °C for both low conversions and high conversions. During the reactions, free radical copolymerization solutions were separated into a cloudy viscous layer (lower phase, LP) and a clear layer (upper phase, UP). The reactivity ratios of NIPAAm with IA were determined using linear graphical methods. Despite the rather large standard deviations (i.e., uncertainty limits) in the reactivity ratios for both the LPs at low conversion and each of the phases (LPs and UPs) at high conversion, they indicate that there are greater content of NIPAAm units in the copolymer than in the feed. Further, the value of the product of r1r2 was found to be lesser than 1. This means that copolymerization nature is nonideal and it is confirmed by the feed vs. copolymer composition curve. References [1] Larez CV, Canelon F, Millan E, Perdemo G, Katime I. New results on the polymerisation of the itaconic acid in aqueous medium. Polym Bull 2002;49:119–26. [2] Bokias G, Durand A, Hourdet D. Molar mass control of poly(Nisopropylacrylamide) and poly(acrylic acid) in aqueous polymerizations initiated by redox initiators based on persulfates. Macromol Chem Phys 1998;199:1387–92. [3] Erbil C, Akpınar FD, Uyanık N. Investigation of the thermal aggregation in aqueous poly(N-isopropylacrylamide-co-itaconic acid) solutions. Macromol Chem Phys 1999;200:2448–53. [4] Erbil C, Gökçeören AT, Polat YO. N-isopropylacrylamideacrylamide copolymers initiated by ceric ammonium nitrate in water. Polym Int 2007;56:547–56. [5] Schild HG. Progress in polymer science. Pergamon Press Ltd.; 1992. [6] Yin X, Hoffman AS, Stayton PS. Poly(N-isopropylacrylamide-copropylacrylic acid) copolymers that respond sharply to temperature and pH. Biomacromolecules 2006;7(5):1381–5. [7] Yoo MK, Sung YK, Cho CS, Lee YM. Effect of polymer complex formation on the cloud-point of poly(N-isopropyl acrylamide) (PNIPAAm) in the poly(NIPAAm-co-acrylic acid): Polyelectrolyte complex between poly(acrylic acid) and poly(allylamine). Polymer 1997;38(11):2759–65. [8] Yu H, Grainger DW. Modified release of hydrophilic, hydrophobic and peptide agents from ionized amphiphilic gel networks. J Contr Rel 1995;34:117–27. [9] Ramkissoon-Ganorkar C, Gutowska A, Liu F, Baudys M, Kim SW. Polymer molecular weight alters properties of pH-/temperaturesensitive polymeric beads. Pharm Res 1999;16(6):819–27. [10] Fujishige S, Kubota K, Ando I. Phase transition of aqueous solutions of poly(N-isopropylacrylamide) and poly(N-isopropylmethacrylamide). J Phys Chem 1989;93:3311–3. [11] Ito S. Phase transition of aqueous solution of poly(N-Alkylacrylamide) derivatives – effects of side chain structure. Kubonshi Ronbunshu 1989;46:437–43. [12] Feil H, Bae YH, Feijen J, Kim SW. Effect of comonomer hydrophilicity and ionization on the lower critical solution temperature of Nisopropylacrylamide copolymers. Macromolecules 1993;26: 2496–500. [13] Takai Y, Aoki T, Sanui K, Ogata N, Sakurai N, Okano T. Dynamic contact angle measurement of temperature-responsive surface properties for poly(N-isopropylacrylamide) grafted surfaces. Macromolecules 1994;27:6163–6. [14] Chen G, Hoffman AS. Graft copolymers that exhibit temperatureinduced phase transitions over a wide range of pH. Nature 1995;373:49–52.
[15] Casolaro M. Solution behaviour of poly(N-acryloyl-l-leucine) and its copolymers with N-isopropylacrylamide. Polymer 1997;38(16): 4215–22. [16] Xue W, Hamley IW, Castelletto V, Olmsted PD. Synthesis and characterization of hydrophobically modified polyacrylamides and some observations on rheological properties. Eur Polym J 2004;40: 47–56. [17] Zhu XX, Avoce D, Liu HY, Benrebouh A. Copolymers of Nalkylacrylamides as thermosensitive hydrogels. Macromol Symp 2004;207:187–92. [18] Erbil C, Kazancıog˘lu E, Uyanık N. Synthesis, characterization and thermoreversible behaviours of poly(dimethyl siloxane)/poly(Nisopropyl acrylamide) semi-interpenetrating networks. Eur Polym J 2004;40:1145–54. [19] Willke T, Vorlop KD. Biotechnological production of itaconic acid. Appl Microbiol Biotechnol 2001;56:289–95. [20] Bajaj P, Paliwal DK, Gupta AK. Acrylonitrile – Acrylic acids copolymers I synthesis and characterization. J Appl Polym Sci 1993;49: 823–33. [21] Xue W, Champ S, Huglin MB. Observations on some copolymerisations involving N-isopropylacrylamide. Polymer 2000;41: 7575–81. [22] Cowie JMG, McEwen IJ, Yule DJ. The influence of solvent on the apparent reactivity ratios in free radical copolymerisation reactions between itaconic acid and 2-hydroxyethyl acrylate. Eur Polym J 2000;36:1795–803. [23] Uyanık N, Erbil C. Monomer reactivity ratios of itaconic acid and acrylamide copolymers determined by using potentiometric titration method. Eur Polym J 2000;36:2651–4. [24] Erbil C, Özdemir S, Uyanık N. Determination of the monomer reactivity ratios for copolymerization of itaconic acid and acrylamide by conductometric titration method. Polymer 2000;41:1391–4. [25] Virtanen J, Tenhu H. Studies on copolymerization of Nisopropylacrylamide and glycidyl methacrylate. J Polym Sci Part A Polym Chem 2001;39:3716–25. [26] Devasia R, Nair CPR, Ninan KN. Copolymerization of acrylonitrile with itaconic acid in dimethylformamide: effect of triethylamine. Eur Polym J 2003;39:537–44. [27] S ß olpan D, Güven O. The copolymerization of allyl glycidyl ether with acrylonitrile initiated by gamma-rays. J Polym Sci Part A Polym Chem 1996;34:833–8338. [28] Patel MV, Dolia MB, Patel JN, Patel RM. Synthesis and characterization of novel acrylic copolymers: determination of monomer reactivity ratios and biological activity. React Funct Polym 2005;65:195–204. [29] Pekel N, Sßahiner N, Güven O, Rızaev ZMO. Synthesis and characterization of N-vinylimidazole–ethyl methacrylate copolymers and determination of monomer reactivity ratios. Eur Polym J 2001;37:2443–51. [30] Save NS, Jassal M, Agrawal AK. Stimuli sensitive copolymer poly (N-tert-butylacrylamide-ran-acrylamide): synthesis and characterization. J Appl Polym Sci 2005;95:672–80. [31] Finemann M, Ross S. Linear method for determining monomer reactivity ratios in copolymerization. J Polym Sci 1950;5:259–62. [32] Berger M, Kuntz I. The distinction between terminal and penultimate copolymerization models. J Polym Sci A 1964;2(4):1687. [33] Kelen T, Tüdós F. Analysis of the linear methods for determining copolymerization reactivity ratios I. A new improved linear graphic method. J Macromol Sci A 1975;9:1–27. [34] Kelen T, Tüdós F, Foldes-Berezsnich T, Turcsanyi B. analysis of linear methods for determining copolymerization reactivity ratios III. Linear graphic method for evaluating data obtained at high conversion levels. J Macromol Sci A 1976;10:1513–40. [35] Polic AL, Duever TA, Penlidis A. Case studies and literature review on the estimation of copolymerization reactivity ratios. J Polym Sci Polym Chem 1998;36:813–22. [36] Tidwell PW, Mortimer GA. An improved method of calculating copolymerization reactivity ratios. J Polym Sci A 1965;3:369–87. [37] Dube M, Amin Sanayei R, Penlidis A, O’Driscoli KF, Reilly PM. A microcomputer program for estimation of copolymerization reactivity ratios. J Polym Sci Polym Chem 1991;29:703–8. [38] Klumperman B, O’Driscoll KF. Interpreting the copolymerization of styrene with maleic anhydride and with methyl methacrylate in terms of the bootstrap modell. Polymer 1993;34(5):1032–7. [39] Klumperman B, Kraeger IR. The effect of solvent on the copolymerization of Styrene and Acrylonitrile – Application of the Bootstrap effect to the penultimate unit model. Macromolecules 1994;27(6):1529–34.
C. Erbil et al. / European Polymer Journal 45 (2009) 1728–1737 [40] Plochocka K. Effect of the reaction medium on radical copolymerization. J Macromol Sci Rev Macromol Chem and Phys 1981;C20(1):67–148.
[41] Xue W, Champ copolymerizations 2000;41:7575–81.
1737 S, Huglin MB. Observations on some involving N-isopropylacrylamide. Polymer