Makromol. Chem. 194,3329-3339 (I993) 3329

Amido-thioether polymers containing pendant carboxylato groups: thermodynamics of protonation and copper(I1) complex formation in dilute aqueous solution

Mario Casolaro*

Dipartimento di Chimica, Universita di Siena, Pian dei Mantellini 44, 53100 Siena, Italy

Elisabetta Ranucci, Fabio Bignotti, Paolo Ferruti

Laboratorio di Chimica Macromolecolare, Dipartimento di Ingegneria Meccanica, UniversitA di Brescia, Via Branze, 25 123 Brescia, Italy

(Received: December 23, 1992; revised manuscript of March 10, 1993)

SUMMARY: Calorimetric data are reported for the enthalpy of protonation of pendant carboxylato groups

in new polymers la -d containing amido and thioether moieties in their main chain. These data, together with potentiometric and viscometric data, afford a complete thermodynamic description of the protonation process of the new polyacids, which are water-soluble in a narrow range of a (degree of protonation) values. Comparison with the non-macromolecular model 2 revealed that the thermodynamic features are attributable to polyelectrolyte effects. In contrast with model 2, all polymers studied form stable hydroxocomplex species with copper(I1) ion. The hypothesized [CU(OH),L,]~- (where L is the monomeric unit of the polymer) stoichiometry fits the potentiometric data well when processed with the'SUPERFIT program. The trend of the corresponding stability constant (log /3) values is to decrease with increasing pH over a wide range.

Introduction

Many polyamides carrying pendant carboxyl groups have been studied in aqueous solution for their thermodynamics of protonation and copper(I1) complexing ability's*). They include polymers with a flexible vinyl chain, such as polyacrylic or polymethacrylic derivatives containing amino/carboxy groups 2-4), or the more rigid bis(amidic) unit present in poly(amido-amino acids) 's6). Evaluation of the stability constants (as log /I) at each pH from potentiometric titration data was performed by the computer SUPERFIT program6) which takes into account polyelectrolyte behaviour relative to protonation, i. e. the dependence of the logarithm of the basicity constant log K on the degree of protonation a.

New poly(carboxy1ic acid)s carrying thioether moieties have recently been studied with a view to pharmaceutical use as promoieties for polymeric and oligomeric prodrugs ').

0 1993, Hiithig & Wepf Verlag, Basel CCC 0025-1 16)</93/$05.00

3330 M. Casolaro, E. Ranucci, F. Bignotti, P. Ferruti

a b c d

1 1 I -R-

-CHzCH2- -CH2CH2CH2- -CHzCH2CH2CHz- -CHzCH-CHCHz-

I I OH OH

Polymers 1 a- d are water-insoluble in their neutral form. The carboxylato groups impart polymer-solubility only if they are in the ionized state to a large extent. The degree of protonation at which the polymer precipitates out depends on the number of the methylene groups between the sulfur atoms, any increase of the hydrocarbon chain narrowing the a-solubilitya) range of the polymer. This is closely related to the hydrophobic character of the aliphatic chain that superimposes on the hydrophilic quality imparted by the amido and carboxy groups.

In this paper, we present enthalpy data on the protonation of the carboxylato group of the above polymers, which are assumed to gradually collapse with decreasing charge density along the chains. This data provides an insight into the energetics of protona- tion. To better ascertain the specific macromolecular effects, we also synthesized and studied a non-macromolecular model compound 2, whose structure closely corre- sponds to that of the monomeric unit of the polymer:

0 0

HO-CH~CH2-S-CH2CHz-C-NH-CH-NH-C-CHzCH2-S-CH2CHz-OH II II

I COOH

2

We also decided to examine in greater detail the thermodynamics of the interaction of the new polyelectrolytes with copper(I1) ions. This study was carried out by means of thermodynamic (potentiometry, calorimetry, viscometry) and spectroscopic (ultra- violet-visible (UV/VIS), electron paramagnetic resonance (EPR)) techniques in aqueous media. The potentiometric data was analyzed with the SUPERFIT program as a further test of our proposed stability constant calculation method for polymers with “apparent” basicity constants2*6B8*9).

Experimental part

Synthesis

2,2’-bis(acrylamido)acetic acid, has been reported elsewhere ’). The synthesis of the polymers, prepared by hydrogen-transfer polyaddition of bis(thio1s) to

a) The maximum degree of protonation that keeps the macromolecule in solution (clear solution) is called a-solubility.

Amido-thioether polymers containing pendant carboxylato groups . . . 3331

The non-macromolecular model compound 2 was synthesized from 2-mercaptoethanol and 2,2'-bis(acry1amido)acetic acid, as follows: 2,2'-bis(acry1amido)acetic acid (1,982 g; 10 mmol) was dissolved in water (10 mL) and sodium bicarbonate was added to regulate the pH of the solution up to 8,0-8,5. After that, 2-mercaptoethanol(1,5 mL) was added to the solution under an inert atmosphere. The reaction mixture was left with stirring for three days and was then acidified with a concentrated aqueous solution of HCl. The solvent was removed by evaporation under reduced pressure. The crude solid so obtained was washed with diethyl ether (2 x 50 mL) to remove unreacted 2-mercaptoethanol, and then with 2-propanol (5 x 50 mL). The 2-propanol solution was then evaporated under reduced pressure. Yield 1,249 g (35%). Thin-layer chromatography (silica gel, chloroform/2-propano1): R , = 0,O.

The proton nuclear magnetic resonance ('H NMR) spectrum was recorded on a 60 MHz 360 A Varian spectrometer, using CDCl, as internal reference. 'H NMR (ppm): 2,l-2,8 (m, 12H, CH2S, CH2CON-), 3,l-3,6(m,4H,CH20-),4,1-5,0(broadm,2H,OH), 5,1-5,3(t, lH,

The infrared (IR) spectrum was obtained on an FT-IR 5300 Jasco spectromer. IR (cm-'): 3 100 CH-), 8,3-8,4 (d, 2H, NH).

(V (0-H)), 2960 (V (CH,)), 1 738 (V (-C=O)), 1658 (V (-C-NH)), 1 541 (S (N-H)), 1419 II 0

I 0

(CHz)).

C,zH22N20&. (1/2)H,O (363,46) Calc. C 39,66 H 6,38 N 7,71 Found C 39,71 H 6,18 N 7,64

Model compound 2 is readily soluble in water and the potentiometric titration assay of the carboxy group with standard sodium hydroxide revealed a purity of 87%.

Potentiometric measurements

Potentiometric titrations were carried out according to a previously described procedure lo). A digital PHM-84 Radiometer potentiometer, equipped with a glass electrode (Ross, mod 81-01) and a reference electrode (Ross, mod 80-05), and a Metrohm Multidosimat piston burette were connected to an Olivetti M20 computer that automatically controlled the titration experiments. All titrations were carried out in a thermostated glass Fell kept at 25 "C. For each experiment the glass cell was filled with ca. 100 mL of 0,l M NaCl in which a weighed solid polymer and a known amount of standardized NaOH solution was dispersed by magnetic stirring. To avoid C 0 2 contamination, a presaturated nitrogen stream was maintained over the surface of the solution. The titration with standardized HC1 solution started only when the initial electromotive force (e. m. f.) value reached a steady-state condition. This was observed within a few hours. On the contrary, the model compound, titrated with standard NaOH solution, dissolved in a few minutes. The experimental details for the protonation study are summarized in Tab. 1. The copper(I1)- complex formation study was performed in a similar manner by stepwise addition of HCI or NaOH titrant solutions to a polymer/Cu(II) (mole ratios 3/1 or 2/1) solution in 0,l M NaCl (ca. 100 mL) that initially contained a known large excess of sodium hydroxide or hydrochloric acid, respectively ('lhb. 2). The basicity and the stability constants of the polymers were computed by the APPARK and SUPERFIT programs '* ' I ) , running on an Olivetti M20 and a Rainbow PCl00 (Digital Eq.) computer, respectively. The basicity constant for the non-macromolecular model 2 was evaluated using the SUPERQUAD program on an Olivetti M28 computer ',).

Calorimetric measurements

Calorimetric titrations were performed according to a previously described method with a Tronac calorimeter (mod. 1250) equipped with a 50 mL dewar or a 25 mL stainless-steel reaction vessel for isoperibol and isothermal experiments, respectively "1. For all the polymers, continuous titrations were performed in the isothermal mode at 25 "C and 0,l M NaC1. The solution, containing a weighed amount of polymer and a measured volume of standardized NaOH, was titrated with HCl standard solution at a constant buret delivery rate (0,0999 mL/min).

3332 M. Casolaro, E. Ranucci, F. Bignotti, P. Ferruti

lhb. 1. Experimental details from potentiometric and calorimetric measurements for the protonation') of polymers l a - d and the non-macromolecular model 2 in 0,l M NaCl at 25 "C

Substance pH-Range lo3 TL/molb) lo3. ToH-/molC) CT/(mol * dm-3)d) Pointse)

Potentiometry: l a 4,61 - 3,44 l b 4,84 - 3,77

4,89 - 3,82 l c 4,91 - 4,16 I d 4,15 - 2,61

2,79 - 4,41 2 3,11 - 4,46

Calorimetry: l a 439 - 3.39

4,52 - 3,41 l b 4,55 - 3,85 l c 4,40 - 4,29 I d 3,80 - 2,58

4,03 - 2,61 2 2,69 - 3,80

0,2103 0,2487 0,4357 0,2269 0,1538 0,1538 0,1074

0,1679 0,3770 0,4540 0,7118 0,1399 0,1940 0,0989

0,2981 0,2473 0,0603 0,3931 0,2183

-0,2095 -0,1074

0,0543 0,0543 0,0688 0,0724 0,0616 0,0543

-0,2122

0,1127 0,1127 0,1127 0,1127 0,1127

- 0,1240 - 0,1472

0,1087 0,1087 0,1087 0,1087 0,1087 0,1087

- 0,1472

24 16 42 7

75 50 16

24 44 16 16 20 45 15

a) Reaction L - + H + * LH (L- is monomeric unit of polymer (ionized form)). b, TL = Initial amount of monomeric unit as ligand. c) T ~ ~ - = Initial quantity of hydroxide ions (negative values refer to hydrogen ions). d, C, = HCl titrant concentration (negative values refer to NaOH titrant). e, Number of points in the potentiometric and calorimetric titrations.

lhb. 2. Experimental details from potentiometric measurements for copper(I1)-complex forma- tion of polymers l a - c in 0,l M NaCl at 25 "C

Polymer 1 0 3 - ~ L a ) 103-7 - ,b ) 103-rH+ c) C, d) Points mol mol mol mol . dm-3

l a 0,1529 0,0495 0,0744 -0,1459 18 0,1898 0,0944 - 0,1020 0,1207 14

l b 0,1622 0,0562 - 0,1296 0,1207 10 l c 0,1776 0,0527 -0,1142 0,1207 8

a) TL = Initial amount of monomeric unit as ligand. b, T~ = Initial amount of copper(I1) ions.

TH+ = Initial quantity of hydrogen ions (negative values refer to hydroxyl ions). d, C, = HCl titrant concentration (negative values refer to NaOH titrant).

In case of model 2 the calorimetric titration was performed in the isoperibol mode by titrating, with standard NaOH solution, 50 mL of 0,l M NaCl containing known amounts of dissolved 2 and HCl. The corrections for the heats of dilution were made by adding the standard titrant solution (HCl or NaOH) to a 0,l M NaCl solution. Experimental details are summarized in 'Itib. 1. All the experiments were automatically controlled by a North Star CCP 930 computer and the titration data stored on a floppy disk. The enthalpy values were computed on the Olivetti M24 computer with the FITH program described elsewhere I ) .

Amido-thioether polymers containing pendant carboxylato groups . . . 3333

Viscometric measurements

Viscometric titration data was obtained with an AVS 3 10 automatic Schott-Gerate viscometer at 25 "C. Solutions were prepared by dissolving a weighed amount of polymer (50-80 mg) in 25 mL of 0,l M NaCl containing a known excess of standard sodium hydroxide solution. Pure C0,-free water was used throughout and the polymer solutions were titrated immediately with 0,l M HCl delivered by a Metrohm Multidosimat piston buret.

Spectroscopic measurements

Electron paramagnetic resonance (EPR) spectra were recorded at low temperature (110 K) on a Bruker ER-200 spectrometer (X-band frequencies) using 2,2-diphenyl-l-picrylhydrazyl as field marker 13). Solutions were prepared under alkaline conditions (pH > 10) in 111 (volume ratio) water/ethylene glycol with 211 and 3/1 mole ratios of polymer/Cu(II).

The ultraviolet/visible electronic spectra (UV/VIS) were recorded at room temperature on a Perkin-Elmer 320 spectrophotometer using 1 cm silica cells. The 0,l M NaCl solutions, containing polymer (ca. 50 mg) and Cu(I1) ions in a 3/1 mole ratio with an excess of sodium hydroxide (100 mL total volume), were titrated with 0,l M HCI in a stepwise manner.

Results and discussion

Protonation

The behaviour towards protonation of the carboxylato groups was studied in aqueous media by means of viscometry, potentiometry and calorimetry. Since the polymers are soluble in the fully ionized form, the solutions were prepared under alkaline conditions and titrated with 0,l M HCI. Viscometric data (Fig. 1) showed that the highest reduced viscosity gi/c was obtained when the polymer was completely ionized. In this form polymer 1 d, being more solvated for the presence of hydroxyl groups, also revealed the highest gi/c values. Lengthening of the methylene chain from 1 a to 1 b decreased the reduced viscosity values. This trend can be ascribed to different chain extensions of the ionized macromolecule in water. Since the polymers showed almost the same intrinsic viscosity values [q] in dimethyl sulfoxide as solvent'), the different pattern in water is probably due to a different solvation effect6). This is confirmed by polymer behaviour on protonation of the carboxylato groups, which

Fig. 1. Reduced viscosity qi/c of polymers l a (l), l b (2) and Id (3) in 0,l M NaCl at 25 "C plotted against volume of 0,l M HCl titrant (arrows indicate excess of titrated sodium hydroxide) 0,OL

0 1 2 - 3 Volume of HCI titrant in mL

3334 M. Casolaro, E. Ranucci, F. Bignotti, P. Ferruti

leads to precipitation in spite of the fact that many COO- groups remain unprotonated. The behaviour of the polymer solutions was similar except for their hydrophobic character. The a-solubility limit of the polymers decreased from 0,4 to 0,2 from 1 a to 1 c. In all cases, however, the viscometric titration data showed a decrease in coil dimension in this narrow range of a values, after which the polymers precipitated. In the a-solubility range of the polymers, the thermodynamic functions (free energy of protonation -AGO, enthalpy of protonation - AHo and entropy of protonation ASo) were evaluated and compared with those of the non- macromolecular model 2 (Fig. 2). The polyelectrolyte behaviour of the polymers, i. e. the dependence of the basicity constant (log K ) values on a, was evaluated using the Henderson-Hasselbalch equation’), and the free energy of protonation of the COO - group in polymers for each given a value was obtained.

Combined use of the calorimetric and potentiometric data, yields the entropy of protonation values ASo = (AHo - AGo)/T which are reported in Fig. 2.

The - AGO of polymer Id, which is soluble over the widest range of a, is the lowest of all the polymers but is still higher than that of the model. As seen previously’) its behaviour is described well by the Henderson-Hasselbalch equation modified by Katchalsky and Spitnik for polyelectrolytes 14). The presence of charges on the polymers resulted in higher free energies of protonation. Low electrostaticity, reached at almost complete protonation (a close to one), brought the - AGO of the polymer close to that of the corresponding model (-AGO = 3,52 kcal/mol (= 14,7 kJ/mol). This usual behaviour is found in many polyacids and the corresponding low- molecular-weight a n a l o g u e ~ ~ * ~ # ’’). To better understand the protonation process, calorimetric data were obtained by titrating dilute solutions of the ionized polymer with hydrochloric acid. The calorimetric titration data were analyzed by taking into account the variation of log Kvalues on a. The calculated enthalpy changes, however, showed a distinctly different pattern. Unlike the large exothermic - AHo of model 2 (- AHo = 2,54 kcal/mol (= 10,6 kJ/mol)), the protonation reaction of the COO- groups on polymer 1 d was found to be endothermic (- AHo = - 0,SO kcal/mol (- 2,l k J/mol)) and ‘real’, i. e. independent of a. The anomalously larger enthalpy change may be due to the highly symmetrical and energetically more favoured structure reached upon protonation of the carboxylate anion in the model c o m p o ~ n d ~ ~ ~ ~ ) . The presence of hydroxyl groups at the extremes could give rise to a new and more stable orientation of water molecules around the symmetrical neutralized species 15).

On the other hand, the other three polymers, being soluble in a narrower a-range, showed - AGO values sharply decreasing with a. They showed a higher polyelectrolyte effect that levelled off with increasing a. Moreover, the smaller coil of the fully ionized macromolecule revealed by viscometric data and due to a lower solvation of the polymers la , 1 b and l c enhanced the high -AGO value (see Fig. 2 at a = 0,l) since the COO- groups bring each other closer. Calorimetric data showed a different pattern in the series of homologous polymers 16). Unlike the “apparent”, i. e. dependent on a , behaviour of 1 b and 1 c, the more soluble 1 a polymer showed a “real”,

Amido-thioether polymers containing pendant carboxylato groups . . . 3335

Fig. 2. Free energy AGO (a), enthalpy AHo (b) and entropy of protonation AS' (c), plotted against degree of protonation (a) for polymers l a (l), l b (2), l c (3), I d (4) and the non-macromolecular model compound 2 (5); in SI units 1 cal = 4,184 J

5 m-m-m-m-¤

3 , /

/* 4 1

2 . P OLO-0-0-0-0 1

d-o--fo--o /*

0 0.1 0.2 0.3 0.L 0.5 0.6 0.7 Degree of protonation a

5 m-m-m-m-¤

0 0.1 0.2 0.3 0.L 0.5 0.6 0.7 Degree of protonation a

i. e. independent of a, - AHo value (- AHo = 0,36 kcal/mol (= 1 3 kJ/mol); Fig. 2) and a slightly exothermic effect comparable to that of the former polymers at higher a values. Polymer l b , in fact, showed an increasing trend of -AHo vs. a as

3336 M. Casolaro, E. Ranucci, F. Bignotti, P. Ferruti

protonation starts with negative - AHo values (Fig. 2). The extent of hydration of the macromolecular chain was reflected by the loss of entropy contribution on charge neutralization. Even if the ASo values decreased with increasing a, they always remained higher than those of the model compound 2. This is a general trend attributed to the larger desolvation effect in polymers. This effect generates endothermic contributions produced by the macromolecular chains during the process of charge neutralization Is). The hydration shell of a polyelectrolyte always embraces several repeating units, and the protonation process leads to a larger release of water molecules than that for the model compound. Moreover, the sharp drop of ASo with a for polymers with longer methylene chains reflects a ‘solvent exclusion’ effect, owing to their more hydrophobic nature lo).

Calorimetric data indicates that aliphatic chain length, charge density of polyions and extension of hydration are the main factors controlling the absolute value and sign, as well as the trend, of the - AHo of protonation of poly(carboxy1ic acids). The entity of such factors depends on a and on the hydration of the whole macromolecular chain. Similar effects have previously been observed in more soluble carboxylic acid copolymers with hydrophobic groups between the carboxylated units 16).

Copper(II)-complex formation

The copper(I1)-polymer mmplexes of this study displayed a characteristic reaction behaviour not observed in the reactions of low-molecular-weight analogues. This polymeric characteristic is due to the specific nature of the polymer chain, the polymer/metal ion interaction proceeding in a microhetesogeneous region occupied by the polymer chain (the so-called domain) having a physicochemical environment different from that of the bulk solution 17). Such interactions imply many functional groups on the macromolecular chain and the corresponding cooperative effects Is). In fact, we were surprised to find that the polymers, but not their non-macromolecular model, formed complex species with the copper(l1) ion in aqueous solution. We evaluated the spectroscopic (ultraviolet/visible (UV/VIS) and electron paramagnetic resonance (EPR)) and thermodynamic (stability constants) properties of the copper(I1) complexes. The spectroscopic data, recorded over a wide range of pH and with different Cu(II)/L- mole ratios (L is the monomeric unit of the polymer), did not change appreciably. The wavelength of maximum absorption Am= at 640 nm (15600 cm-’) was independent of changes in pH. This is compatible with a single complex species with high-energy donor atomsl3), and is usually found in complex species having a basic nitrogen in the monomeric unit 3). The EPR spectra confirmed the presence of a single complex with parameters independent of pH and close to those previously reported for a polymer containing amino acid residuesI3) (Bb. 3). As in the latter case, the EPR data in the present investigation are consistent with an octahedral tetragonally-distorted geometry whose complex species involves only oxygen donor atoms’”. For the present polymers we can reasonably hypothesize chemical bonds occurring only with the COO- groups belonging to different monomeric units. When these groups are bound to the Cu(1I) ion, they are always associated with electronic

Amido-thioether polymers containing pendant carboxylato groups . , . 3337

'kb. 3. Hyperfine splitting parameters g , , g , and A 11 of complex system with CuZf of poly- mers la -c from electron paramagnetic resonance and electronic spectral data (8 is molar absorption coefficient)

g, lo4 * A II/cm-' Electronic absorp- tion in cm - I (e in dm3 - mol-' . cm-I)

811 Polymer

l a 2,375 2,078 155 15 600 (E = 25) l b 2,366 2,070 155 15 600 l c 2,370 2,084 157 15 600

absorption spectra shifted to lower energy2'), and this is in contrast with our data. The differences observed can be interpreted in terms of increased equatorial bonding resulting from a substitution of water molecules with OH- as donor groups8). The hydrated Cuz+ ion showed a lower A ll value (138 cm-I) with respect to the complex species, and this is compatible with an increased field in the equatorial plane having larger A values (Tab. 3) 13). This was found to be consistent in the calculation of stability constants with Cu(OH),L:- stoichiometry. The potentiometric data, in fact, gave a good fit between the experimental pH and the corresponding pH calculated by the SUPERFIT program6P8). This calculation program allows the refinement of the log B value of a single complex species present in a given pH range. Each point of the potentiometric titration data is considered closer with respect to the previous one, and the equation

is iteratively solved by varying /l till the difference between the calculated pH (pH,,,) and the experimentally measured pH (pH,,,) is less than6) 0,Ol. Among the different stoichiometries tested, only the proposed one, Cu(OH),L,2-, permitted a fair agreement between observed and calculated pHs, enabling log B values to be evaluated. The trend of the stability constants for the polymers studied is plotted in Fig. 3 and

Fig. 3. Variations of Cu(OH),L;- stability constant log /3 (Cu2+ + 2L- + 2 0 H - L CU(OH)~L:-) in relation to pH for polymers l a (1, l'), 1 b (2) and 1 c (3) at different mole ratios Cu(II)/L-(l/J (1, 2, 3) and 112 (1'))

; 2L - 4 22 4

16

7 1 L 6 8 10 12

PH

3338 M. Casolaro, E. Ranucci, F. Bignotti, P. Ferruti

was found to decrease with increasing pH, in a similar manner to that reported for other classes of polyelectrolytes studied by US^^^,*). The formation of a negatively charged complex species gave rise to a sharp drop in log B values with pH. Previous results, in fact, showed that complex reaction with charge neutralization allows different trends in stability constant values, decreasing sharply as negative charges increase on complex species2T8). This was only due to entropy factors where no change in the structure or bonds occurs in the complex reacting species"). The stability constants (log p) were very sensitive to pH. In all cases, the log /3 values of the new polymers were close to the corresponding ones measured for a basic polymer containing a sulfone group in its main chain *I. A dihydroxycopper(I1) complex with N,N-bis(2-hydroxyethyl)ethylene- diamine also showed a loglo/3 value of 21,2 for a Cu(OH)*L species2'). The similarity of the log B values suggests that the new polymers have the same complexing behaviour towards copper(I1) ions. This is also supported by spectroscopic data, as the parameters were similar in all cases. The absorption coefficient E (Thb. 3) is relatively low, suggesting a less distorted and a more stable complex species 13). The methylene chain length does not play a significant role in the complexing properties, and the complex species are well solvated in a wide range of high pHs in all cases owing to the presence of ionic groups. The macromolecule was also more coiled when complexed, enabling greater solvation of monomeric units as a result of intramolecular cooperation. This hypothesis was confirmed for polymer l a by the viscometric data that showed a minimum value a t a Cu(II)/L - mole ratio close to 0,5.

This work was partially supported by the MURST (60% funds). Thanks are due to Dr. R Laschi (Department of Chemistry, Siena University) for recording EPR spectra.

M. Casolaro, R. Barbucci, in: 2nd International Symposium of Polymer Electrolytes, B. Scrosati, Ed., Elsevier Ltd., London and New York 1990, 31 1, and references therein

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