Radical copolymerization of N-isopropylacrylamide with anhydrides of maleic and citraconic acids

European Polymer Journal 38 (2002) 2143–2152 www.elsevier.com/locate/europolj

Radical copolymerization of N-isopropylacrylamide with anhydrides of maleic and citraconic acids €seli, Hande Kesim, Zakir M.O. Rzaev *, Erhan Pisßkin Sevil Dincßer, Volkan Ko Department of Chemical Engineering and Bioengineering Division, Faculty of Engineering, Hacettepe University, Beytepe, 06532 Ankara, Turkey Received 19 February 2002; received in revised form 22 April 2002; accepted 23 April 2002

Abstract Radical-initiated copolymerization of N-isopropylacrylamide (NIPA) with maleic (MA) and citraconic (CA) anhydrides was carried out in the presence of 2,20 -azobisisobutyronitrile (AIBN) as an initiator in 1,4-dioxane at 65 °C under nitrogen atmosphere. Structure and monomer unit compositon of the copolymers obtained from a wide range of monomer feed were determined by elemental analysis (content of N for NIPA units), Fourier transform infrared and 1 H NMR spectroscopy. Monomer reactivity ratios for NIPA (M1 )–MA (M2 ) and NIPA (M1 )–CA (M2 ) pairs were determined by Kelen–T€ ud~ os (KT) and non-linear regression (NLR) methods using elemental and 1 H NMR spectroscopy analyses data. They are r1 ¼ 0:45 and r2 ¼ 0:08 (KT, N analysis), r1 ¼ 0:44 and r2 ¼ 0:10 (KT, 1 H NMR), r1 ¼ 0:45 and r2 ¼ 0:078 (NLR) for NIPA–MA monomer pair and r1 ¼ 0:52 and r2 ¼ 0:02, r1 ¼ 0:44 and r2 ¼ 0:04, r1 ¼ 0:51 and r2 ¼ 0:014 for NIPA–CA monomer pair, respectively. Observed tendency towards alternating copolymerization at 650 mol% NIPA concentration in monomer feed and relatively high activity of NIPA growing radical was explained by Hbond formation between C@O (anhydride) and NH (amide) fragments during chain growth reactions. Intrinsic viscosity, molecular weight and thermal behaviour of the synthesized copolymers were found to depend on the type of comonomer and the amount of NIPA units in the copolymers. These functional amphiphilic copolymers containing anion- and cation-active groups show both temperature and pH sensitivity and can be used for biological purposes as physiologically active macromolecular systems. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: N-isopropylacrylamide; Maleic anhydride; Citraconic anhydride; Radical copolymerization; Reactivity ratios; Molecular weight; Thermal behaviours

1. Introduction Alternating copolymers of maleic anhydride can be regarded as preactivated polymers due to the presence of anhydride moieties susceptible to the reaction with a primary amine of a biomolecule [1]. Poly[(maleic anhydride)-alt-(methyl vinyl ether)], poly[(maleic anhydride)alt-(divinyl ether)] and poly[(citraconic anhydride)alt-(divinyl ether)] were used in various applications in

*

Corresponding author. Tel.: +90-312-297-6439; fax: +90312-297-6439/299-2124. E-mail address: [email protected] (Z.M.O. Rzaev).

diagnostics [2,3] and in chemotherapy as effective antitumor agents [4]. Synthesis of pH-sensitive phase separating maleic anhydride–methyl methacrylate– acrylic acid and acrylamide–methacrylic acid–N-acryloxysuccinimide terpolymers and their application in immunoasssay was reported by Zhou et al. [5]. A commercially available pectiase was chemically modified with poly[(maleic anhydride)-co-(polyalkyleneoxide)]. It was shown that enzymatic charateristics of prepared biosystem were changed singnificantly, depending on the hydrophilicity of the copolymer-modifier and the degree of modification [6]. Acrylamide and its derivatives can undergo alternating copolymerization with maleic anhydride under the

0014-3057/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 2 ) 0 0 1 2 7 - 1

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given conditions [7–11]. These copolymers are potentially useful as flocculants, for purification of industrial waste water, as coatings for microcapsule production and for paper dry-strength agents [11]. Homo- and copolymers of N-isopropylacrylamide (NIPA) exhibited pH and thermal sensitivity and were used in biologically active systems (in protein conjugation) as cation-active polymers solube in water and physiological medium [12– 16], as well as carrier system for DNA delivery [17], for affinity separation of genotoxins [18] and as reversible bioconjugates [19]. The copolymer hydrogels were synthesized by radical crosslinking copolymerization of NIPA with acrylic, itaconic and maleic acids [20], and NIPA with acrylamidolactamine [21] in the presence of N; N 0 -methylene-bis-acrylamide. Recently Tuncel et al. [22] and Bokias et al. [23,24] have reported the synthesis and characterizaiton of new water soluble at low temperature and shear-responsive NIPA–N; N -[(dimethylamino)propyl] methacryl amide copolymers and their alkyl bromide derivatives having a delicate balance between thermosensitive, hydrophobic and ionic groups. Similar behaviour was observed in amphiphilic NIPA– acrylic acid random and graft copolymers [25], and also in maleimide-terminated oligo(NIPA) which can be used as a temperature-sensitive polymer to a genetically engineered protein [26]. Recently, synthesis and characterization of cationic stimuli-responsive acrylic acid-terminated poly(NIPA) potentially useful as carrier for gene delivery, conjugates of poly(NIPA) with amino acids as prodings, antitumor active binary and ternary copolymers of maleic anhydride, vinyl acetate and acrylic acid (or 2,3-dihydropyran) have been reported [27–30]. In the present work, radical-initiated copolymerization of NIPA with maleic (MA) and citraconic (CA) anhydrides has been studied as a way to obtain new reactive amphiphilic water-soluble polymers potentially useful as carriers for gene delivery. More specifically, monomer reactivity ratios have been determined using different methods, and the effect of H-bond formation of NIPA units on the copolymer composition-properties relationship has been described and discussed.

20 1 213.5 °C, n20 D ¼ 1:4712, d4 ¼ 1:2468. H NMR spectra: CH@, 1H quarter with 6.93–6.92 ppm and CH3 , 3H doublet with 2.19 and 2.18 ppm. a; a0 -Azoisobutyronitrile (AIBN) (Fluka) was twice recrystallized from methanol: m.p. 102.5 °C.

2.2. Copolymerization Copolymerizations of NIPA with MA and CA using various monomer feed ratios were carried out in 1,4dioxane at 65 °C with AIBN radical initiator at constant total concentration of monomers under the nitrogen atmosphere. Reaction conditions for the both systems: ½Mtotal ¼ 2:78 mol/l, ½AIBN ¼ 6:5  103 mol/l and monomer ratios of ½NIPA=½anhydride ¼ 0:25–4.0, conversion 6 10%. Appropriate quantities of monomers, 1,4-dioxane and AIBN were placed in a standard pyrexglass tube, and the reaction mixture was cooled by liquid nitrogen and flushed with dried nitrogen gas for at least 2 min, then soldered and placed in a thermostated silicon oil bath at 65  0:1 °C. The NIPA–MA and NIPA– CA copolymers were isolated from reacted mixture by precipitation with diethyl ether, then washed with several portions of benzene and dried under vacuum at 30 °C. The copolymer compositions were found by elemental (N content for NIPA units) and chemical (acid number for anhydride units), and 1 H NMR spectroscopy using integral area of chemical shifts of monomer functional groups for quantitative analysis. Copolymers prepared from equimolar ratio of initial monomers in similar conditions have the following average characterictics: NIPA–MA copolymer:

2. Experimental section 2.1. Materials NIPA monomer (Aldrich) was purified before use by distillation under vacuum and recrystallization from diethyl ether solution: b.p. 91.5 °C/2 mm, m.p. 61.6 °C. MA monomer (Fluka) was purified by recryctallization from anhydridous benzene and sublimation in vacuum: m.p. 52.8 °C. 1 H NMR spectra: CH@, 2H singlet with 7.34 ppm. CA monomer (Aldrich) was purified by distillation under vacuum before use: b.p.

Monomer unit ratio, m1:m2 ¼ 56:9:43.1; Content of N 7.62% (by elemental analysis); Acid number 440 mg KOH/g; ½gin 0.075 dl/g in THF at 25  0:1 °C; M n 7500 (by GPC); Tg 56.8 °C [by differential scanning calorimetry (DSC)] and Tm 208.5 °C [by DSC and differential thermal analyses (DTA)]. Fourier transform infrared (FTIR) spectra (KBr pellet), cm1 : 3300 (w) and 3100 (w) broad bands for NH secondary amide, 2971 (s), 2915 (m) and 2861 (s) CH

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stretching in CH, CH2 and CH3 groups, 1990–1885 (w) C@O overtones, 1870–1790 (m) antisym. C@O stretching, 1845–1785 (m) sym. C@O stretching of anhydride group, 1825–1770 (m) H-bonded C@O stretching, 1742 (s) C@O of free anhydride group, 1710 (m), 1695 (m) and 1670 (m) broad triplet band for complexed anhydride C@O group, 1650 (s) C@O stretching of amide I band, 1630 (m) NH deformation of secondary Hbonded amide, 1545–1516 (vs) broad NH amide II band, 1460 (m.-s) CH2 scissor vibration and CH3 antisym. deformation, 1425 (m) CH2 deformation, 1385 (m) and 1365 (m) doublet band for CH3 deformation in isopropyl group, 1340 (w) and 1315 (w) CH2 bending, 1260 (m) trans-amide III band, 1190 (w.-m) broad C–O–C anhydride or C–N stretching, 1130 (vs) C–O, C–O–C stretching and CH3 rocking, 1030 (w) NH bending in –NH  O@C–, 882 (vs) C–C stretching of main chain, 750 (w) NH deformation, 590–675 (w.-m) CH bending for anhydride unit. 1 H NMR spectra (in DMSO-d6 at 50 °C), ppm: (1) 2H, CH2 1.39–1.78, (2) 1H, CH 1.78–2.38, (3) 1H, NH 6.92– 8.04, (4) 1H, CH 3.88, (5) and (6) 6H, CH3 0.78–1.39 for NIPA unit; (7) and (8) 2H, CH 4.17 for maleic unit. NIPA–CA copolymer:

a JEOL 6X-400 (400 MHz) spectrometer with DMSO-d6 as a solvent at 50 °C. The compositions of the copolymers synthesized using various monomer feed ratios were determined by known NMR method [31] and were achieved by comparing the integrals of the isopropyl, methyne and methyl group regions in the spectra of NIPA, MA and CA units, respectively. Molar fractions of the comonomer units (m1 and m2 ) in NIPA–MA and NIPA–CA copolymers using 1 H NMR analysis data were calculated according to the following equations: Am1 ðCH3 Þ=Atotal ¼ n1 m1 =ða1 m1 þ b2 m2 Þ

ð1Þ

Am2 ðCH or CH3 Þ=Atotal ¼ n2 m2 =ða1 m1 þ b2 m2 Þ

ð2Þ

where Am1 and Am2 are the normalized areas per H from the corresponding functional groups of the monomer unit regions in 1 H NMR spectra; Atotal is the total area of protons in the copolymer; n1 and n2 are the integers of proton(s) in the functional group of the monomers; a and b are the integers of protons in the monomer units (m1 and m2 ); in the case of ðm1 þ m2 Þ ¼ 1, monomer unit ratios can be calculated from Eqs. (1) and (2) using the following simplified form: m1 =m2 ¼ f ¼ n2 Am1 ðCH3 Þ=n1 Am2 ðCH or CH3 Þ

Monomer unit ratio, m1:m2 ¼ 56:93:43.07; Content of N 7.62% (by elemental analysis); Acid number 400 mg KOH/g; ½gin 0.055 dl/g in THF at 25  0:1 °C; M n 7300 (by GPC); Tg 53.6 °C (by DSC) and Tm 185.3 °C (by DSC and DTA). FTIR spectra of this copolymer is similar to spectra of NIPA–MA copolymer. 1 H NMR spectra (in DMSO-d6 at 50 °C), ppm: (1) 2H, CH2 1.34–1.76, (2) 1H, CH 1.76–2.28, (3) 1H, NH 6.82–7.75, (4) 1H, CH 3.85, (5) and (6) 6H, CH3 0.78– 1.34 for NIPA unit; (7) 1H, CH 4.18 and (8) 3H, CH3 1.18 for citraconic unit. 2.3. Measurements FTIR spectra of the copolymers (KBr pellet) were recorded with FTIR Nicolet 510 spectrometer in the 4000–400 cm1 range, where 30 scans were taken at 4 cm1 resolution. Proton NMR spectra were recorded on

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ð3Þ

DSC and DTA of copolymers were performed on a DuPont TA 2000 calorimeter and Setaram Labsys TGDTA 12 Termal Analyzer, respectively, under nitrogen atmosphere at a heating rate of 10 °C/min. The CHNS-932 Model LECO Elemental Analyzer was used for the determination of C, H and N contents in the copolymers synthesized. Molar fractions (mol%) of comonomer units (m1 and m2 ) in NIPA–MA and NIPA–CA copolymers using elemental analysis data (content of N) were calculated according to the following equations: m1 ¼ M2 =½ðAN =BÞ  DM  102 

ð4Þ

where M2 is the molecular weight of MA or CA units; AN is the atom weight of N; B is the content of N in the copolymers (%); DM ¼ M1  M2 (M1 is the molecular weight of the NIPA unit). Acid numbers (AN) of the anhydride-containing copolymers and terpolymers were determined by standard titration method. Intrinsic viscosities of the copolymers with different compositions were determined in THF at 25  0:1 °C in the concentration range of 0.1–1.0 g/dl using an Ubbelohde viscometer. Molecular weights of copolymers were determined by Gel-Permeation Chromatography using GPC with Shimpack 804 colunm and THF as a mobile phase-eluent.

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3. Results and discussion 3.1. H-bonding effect in radical copolymerization Judging by the nature of the conjugation between double bonds and functional groups (C@O of amide and anhydride), all three monomers (NIPA, MA and CA) can be considered as electron acceptors. However, this fact does not prevent the monomers from having sufficient activity in free-radical copolymerization of NIPA–MA and NIPA–CA monomer pairs thanks to the interaction between functional groups of the comonomers or macroradicals through H-bonding. This effect can be illustrated as following:

Copolymerizations were carried out to low conversions ( 6 10%) in order to determine monomer reactivity ratios in the steady-state kinetics by using known terminal model of the Kelen–T€ ud~ os (KT) equation [32]: g ¼ ðr1 þ r2 =aÞn  r2 =a

ð5Þ

where g ¼ ½F ðf  1Þ=f =ðF 2 =f þp aÞ;ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n ¼ ðF 2 =f Þ=ðF 2 =ffi f þ aÞ; a ðarbitrary constantÞ ¼ ðF 2 =f Þmin ðF 2 =f Þmax ; F ¼ ½NIPA=½MA (or [CA]) and f ¼ m1 =m2 . For comparison, non-linear regression (NLR) procedure using a microcomputer program [33] has also been applied to recalculate copolymerization constants. Results of 1 H NMR and elemental analyses for various initial monomer ratios of both copolymers are illustrated in Figs. 1 and 2, and summarized in Table 1. Copolymer compositions calculated using elemental

analysis data (content of N were in a very good agreement with those obtained from 1 H NMR analysis using Eqs. (3) and (4). As evidenced from data of Table 1, copolymerization of monomers in both NIPA–MA and NIPA–CA systems has a tendency towards alternation, especially at ½NIPA 6 50 mol%. Monomer reactivity ratios (r1 and r2 ) were evaluated using experimental data, presented in Tables 1 and 2, from KT plots of n vs g (Fig. 3) and NLR analysis. As evidenced from these values, which are summarized in Table 3, alternating copolymerizations are realized in both monomer systems with visibly high degree of alternation of monomer units in NIPA–CA system, which can be explained by relatively low electron-acceptor

properties of citraconic double bond due to a CH3 group in the a-position. Copolymerization constants determined by KT method and calculated by NLR method have similar values indicating good agreement between the two methods. As an additional conformation of the alternating tendency of the monomers in the both studied systems, the monomer sequence lengths (l1 and l2 ) are calculated from the well known equations [34] l1 ¼ 1 þ r1 ðm1 =m2 Þ

ð6Þ

l2 ¼ 1 þ r2 ðm2 =m1 Þ

ð7Þ

The values of l1 and l2 are presented in Table 2. As seen from these values for different monomer-copolymer compositions, the value of l1 (NIPA unit sequence length) visible changes from 1.48 to 2.01 and from 1.50

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Fig. 1. 1 H NMR (400 MHz) spectra of poly(NIPA-co-MA) in DMSO-d6 at 27 °C.

Fig. 2. 1 H NMR (400 MHz) spectra of poly(NIPA-co-CA) in DMSO-d6 at 27 °C.

to 1.88 in the both NIPA–MA and NIPA–CA systems, respectively, in increasing NIPA feed concentration. While, the mean unit sequence lengths for MA and CA anhydride units (l2 ) have relatively low and almost non-changed values. This fact observed is correlated with the low values of r2 and confirmed both the alternating tendency of the two copolymers and relatively high tendency to alternate of the NIPA–CA monomer pair.

In general, these results allow to assume that the chain growth reactions proceed predominantly by the addition of anhydride comonomers to NIPA macroradical through intermediate formation of H-bond between secondary amide and anhydride carbonyl groups according to the first above mentioned scheme. The fact that radical copolymerization of isostructural analogies of NIPA unable to form H-bonding such as alkyl acrylates and N ; N -dialkylacrylamides with MA

Table 1 1 H NMR and element analysis data for determining the composition of poiy (NIPA-co-MA) and poly (NIPA-co-CA) synthesized from various initial monomer mixtures Monomer feed (mol%)

Am1 (NIPA unit)a

Am2 (M2 unit)a

N (%)

Copolymer composition (mol%) 1

H NMR analysis

Nitrogen analysis

m1

m2

m1

m2

[NIPA] 70 60 50 40 30

[MA] 30 40 50 60 70

0.104 0.035 0.065 0.016 0.021

0.050 0.025 0.049 0.014 0.022

8.63 7.99 7.62 6.96 6.65

67.5 58.3 57.0 53.3 48.7

32.3 41.7 43.0 46.7 51.3

66.6 61.2 58.2 53.6 50.2

33.4 38.8 41.8 46.4 49.8

[NIPA] 70 60 50 40 30

[CA] 30 40 50 60 70

0.040 0.790 0.030 0.030 0.030

0.020 0.460 0.023 0.024 0.026

8.45 8.01 7.42 7.05 6.71

66.7 63.2 56.6 55.6 52.6

33.3 36.8 43.4 44.4 47.4

68.1 64.5 59.7 56.7 54.0

31.9 35.5 40.3 43.3 46.0

a

Integral area for CH chemical shift of NIPA (isopropyl group) and MA (CA) anhydride (methyne group) units.

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Table 2 Radical-intiated copolymerization of NIPA (M1 ) with MA and CA (M2 ) Monomer ratio

By 1 H NMR

Parameters of KT equation

F

f

F 2 =f þ ab

g

NIPA=MA 2.33 1.50 1.00 0.67 0.43

2.08 1.40 1.33 1.14 0.95

3.31 2.31 1.45 1.09 0.89

NIPA=CA 2.33 1.50 1.00 0.67 0.43

2.00 1.72 1.30 1.25 1.11

3.39 1.99 1.45 1.04 0.85

By N analysis

Parameters of KT equation

Mean sequence lengtha

n

f

F 2 =f þ ab

g

n

l1

l2

0.37 0.19 0.17 0.08 )0.02

0.79 0.70 0.52 0.36 0.21

1.99 1.58 1.39 1.15 1.01

3.43 2.12 1.42 1.09 0.88

0.34 0.26 0.20 0.08 0.01

0.80 0.67 0.51 0.36 0.21

1.89 1.71 1.63 1.52 1.45

1.04 1.05 1.06 1.07 1.08

0.34 0.31 0.16 0.13 0.05

0.80 0.66 0.53 0.35 0.20

2.13 1.82 1.48 1.31 1.17

3.19 1.88 1.32 0.98 0.80

0.39 0.36 0.25 0.16 0.08

0.80 0.66 0.51 0.35 0.20

2.11 1.95 1.77 1.16 1.09

1.01 1.01 1.01 1.02 1.02

Reaction conditions: Solvent: 1,4-dioxane, ½Mtotal ¼ 2:78 mol/1, ½AIBN ¼ 6:41  103 mol/l, 65  0:1 °C, conversion 6 10%. a These values were calculated by using the following r1 and r2 values: 0.45 and 0.08 for NIPA–MA pair and 0.52 and 0.02 for NIPA–CA pair. pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi b a ðarbitrary constantÞ ¼ ðF 2 =f Þmin ðF 2 =f Þmax ¼ 0:70 for NIPA–MA, and 0.68 and 0.64 for NIPA–CA system, respectively.

predominantly leads to the formation of random copolymers enriched with acrylic monomer units (r1 ¼ 3:5 and r2 ¼ 0:03 for methyl methacrylate–MA pair [35]) also confirms that a H-bond effect is responsible for an increased tendency towards alternation in the radical copolymerization of NIPA with MA or CA. The parameters of specific activity ðQ1 Þ and polarity ðe1 Þ for NIPA monomer were calculated using the Q–e scheme of Alfrey and Price [36] in the form of the following equations: Fig. 3. KT plots for the copolymerization of NIPA with MA (- - and -N-) and CA (-- and -D-) using (-N- and -M-) 1 H NMR and (- - and --) elemental analysis data. Slope ¼ r1 þ r2 =a and intercept r2 =a.

Table 3 The values of copolymerization constants (r1 and r2 ) for NIPA (M1 )–MA (M2 ) and NIPA–CA (M2 ) monomer pairs determining by KT and NLR methods using 1 H NMR and elemental analysis techniques Monomer pair

Methods

r1

r2

NIPA–MA

1

H NMR analysis (KT) NLR Elemental analysis (KT) NLR

0.44 0.47 0.45 0.45

0.10 0.12 0.08 0.078

NIPA–CA

1

0.44 0.45 0.52 0.51

0.04 0.038 0.02 0.014

H NMR analysis (KT) NLR Elemental analysis (KT) NLR

e2 ¼ e1  ð ln r1 r2 Þ

ð8Þ

Q1 ¼ ðQ2 =r2 Þ exp½e1 ðe1  e2 Þ

ð9Þ

Using known values of Q2 ¼ 0:23 and e2 ¼ 2:25 for the MA comonomer [35], the parameters of Q1 ¼ 0:26 and e1 ¼ 0:71 have been calculated for NIPA, which describe the energy of localization, order and p-electron density of NIPA double bond. 3.2. Copolymer structure-composition and property relationships Two characteristic peaks (3.88 ppm CH in isopropyl group of NIPA unit, 4.17 ppm CH in maleic unit and 4.18 ppm CH in citraconic unit) can be identified in the 1 H NMR spectra of the copolymers (Figs. 1 and 2) and used as analytical signals for quantitative analysis of copolymer composition. This structure-composition relationship for both copolymers is illustrated in Fig. 4. FTIR spectra of the copolymers are characterized by the typical absorption bands for NIPA units (3300–3100

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tween alternating NIPA–MA diads of macromolecules as follows:

Fig. 4. Fragments of 1 H NMR spectra of (a) poly(NIPA-coMA) and (b) poly(NIPA-co-CA) synthesized using various monomer mixtures: (1) 70:30, (2) 60:40, (3) 40:60 and (4) 30:70.

cm1 broad bands for secondary NH amide, 2971–2861 cm1 CH bands in isopropyl group, 1650 cm1 strong C@O amide band I, 1545–1516 cm1 strong NH amide II band and 1260 cm1 amide III band) and anhydride units (1845 and 1742 cm1 symmetrical and antisymmetrical C@O bands, 1190 and 1130 cm1 C–O and C– O–C anhydride bands). Spectra of these copolymers also contain characteristic bands for H-bonded C@O groups (1825–1770 cm1 for anhydride C@O and 1710–1670 broad band for amide C@O) and H-bonded secondary amide NH group (1630 and 1030 cm1 NH deformation of secondary H-bonded amide in –NH  O @ C–complex). It can be proposed that intermolecular H-bonded fragments are most probably formed be-

The results of DSC and DTA studies of copolymer composition–thermal behavior relationship for the synthesized poly(NIPA-co-MA) and poly(NIPA-co-CA) can also serve as an additional confirmation for the formation of intermolecular H-bonded structure in these systems. Fig. 5 shows the DSC thermograms of both copolymers prepared from the different monomer feed compositions. These results indicate that the intensity and position of higher temperature endo-peaks, which are associated with the melting point (Tm ), significantly depend on the monomer unit ratios in the copolymers, and especially on the degree of their alternation. It is known that the high melting points of polymers are associated with many factors including inter- and intramolecular structural regularity and rigidity of macromolecules [37]. The lower temperature endo-effects on the DSC curves, associated with the glass transition temperature (Tg ), change insignificantly with appreciable increasing NIPA unit concentration for both copolymers. This observed phenomenon indicates that the mechanism of glasstransition is similar in these copolymers. The values of Tg , Tm and DH (enthalpy) for the copolymers are presented in Table 4. It is shown that the decrease of NIPA-unit content in the copolymers increases the value of Tm from 195.3 to 216.9 °C for poly(NIPA-co-MA) and from 173.3 to 194.8 °C for poly(NIPA-co-CA). This indicates that the copolymers with MA units form relatively more rigid structure than those containing CA units. The observed difference between Tm values for the two copolymers is related to the effects of a-CH3 group of CA unit and relatively lower intrinsic viscosity, i.e., lower molecular weight of poly(NIPA-co-CA) (Table 4). The higher values of Tm are observed for the copolymers with composition close to 1:1 (curves 3 and 4 for both copolymers). Therefore, rigid H-bonded structure provides high Tm in the studied copolymers. The similar effect was observed for the semicrystalline polyamide (Nylon 11) with Mn 16.000,

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Fig. 5. DSC thermograms of synthesized copolymers with different compositions. (a) poly(NIPA-co-MA) with monomer unit ratios of f ðm1 =m2 Þ: (1) 1.99, (2) 1.58, (3) 1.15 and (4) 1.01; (b) poly(NIPA-co-CA) with f : (1) 2.13, (2) 1.82, (3) 1.31 and (4) 1.17.

Table 4 Effect of NIPA unit on the thermal behaviours of poly(NIPA-co-MA) and poly(NIPA-co-CA) Content of NIPA units (mol %)

DSC and DTA analysis

Acid number (mgKOH/g)

½gin in THF at 25  0:1 °C

16.7 37.3 35.5 21.0 33.9

345 – 440 – 530

0.062 – 0.075 – 0.088

30.3 31.8 40.2 41.8 98.4

320 – 400 – 460

0.026 – 0.055 – 0.058

Tg (°C)

DH (lV)

Tm (°C)

DH (mJ)

NIPA-co-MA 66.6 61.2 58.2 53.6 50.2

59.6 58.6 56.8 58.1 58.9

0.80 1.01 0.63 0.33 1.12

195.3 205.1 208.5 216.0 216.9

NIPA-co-CA 68.1 64.5 59.7 56.7 54.0

55.9 58.6 53.6 61.7 60.8

0.52 0.56 0.45 0.41 0.60

173.3 180.4 185.3 193.6 194.8

having lower Tg (42 °C) and higher Tm (184 °C) values [37,38]. For the poly(NIPA) synthesized under similar conditions, the relatively low value of Tm (143.3 °C) with small enthalpy is obtained (Table 4). Fig. 6 illustrates DTA curves for poly(NIPA-co-MA) with different compositions. The first broad exo-effects around 95–125 °C on the DTA curves might be related to a ‘‘physical aging’’ of H-bonded copolymers which is accompanied by the breaking of inter- and intramolecular –NH  O@C–bonds of the macromolecules. It is necessary to note that the character and position of these

peaks, as well as Tg endo-effects on the DSC curves, are unaltered for the copolymer samples which are subjected to thermotreatment at 150 °C for 30 min before DTA and DSC analyses. The lower endo-effects on the curves are related to Tm as on the DSC curves. The flowing (Tf ) and decomposition (Td ) temperature regions for the both copolymers are around 220–250 and 275–380 °C. The results of DSC and DTA studies of copolymes with different compositions indicate the formation of semicrystalline structure on account of H-bonded intermolecular alternating fragments of macromolecules.

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process as carriers for gene delivery. The results of this investigation in detail will be the subject of our further publication.

Acknowledgements This study was carried out in accordance with Polymer Science and Technology Program of Chemical Engineering Department and Bioengineering Division, Hacettepe University. Support by TBTAK (Turkish National Scientific and Technical Research Council) through Project MISAG–146 is acknowledged.

References

Fig. 6. DTA curves of poly(NIPA-co-MA) with monomer unit ratios of f ðm1 =m2 Þ (values of f as in Fig. 5).

4. Conclusion The NIPA–MA and NIPA–CA monomer pairs have been used to determine monomer reactivity ratios (r1 and r2 ) in the radical-initiated copolymerization, and the properties, including thermal behaviour, of the resulting copolymers have been investigated. By using KT and NLR methods, and 1 H NMR spectroscopy, elemental and chemical analysis data of copolymer compositions, the values of r1 and r2 for both monomer pairs were found, and Q1 and e1 parameters for NIPA were calculated. Relatively high activity of studied pair monomers, having tendency towards alternating copolymerization, was explained by the effect of H-bond formation between C@O (anhydride) and NH (amide) groups during chain growth reactions. The formation of alternating copolymers, predominantly at 6 50 mol% of NIPA in the monomer feed, was also confirmed by the results of copolymer composition-property relationship studies. The results of DSC and DTA studies on the copolymer composition–thermal behaviour relationship for the synthesized poly(NIPA-co-MA) poly(NIPA-coCA) can also serve as an additional confirmation for the formation of intermolecular H-bonded structure in these systems. Synthesized and characterized new amphiphilic water-soluble copolymers can be used in bioengineering

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