Synthesis of functionalized phenylphosphinic acid resins through Michael reaction and their ion-exchange properties

Reactive & Functional Polymers 44 (2000) 9–19 www.elsevier.com / locate / react

Synthesis of functionalized phenylphosphinic acid resins through Michael reaction and their ion-exchange properties Andrzej W. Trochimczuk* Institute of Organic and Polymer Technology, Wrocł aw University of Technology, 50 -370 Wrocł aw, Poland Received 4 January 1999; received in revised form 5 July 1999; accepted 13 July 1999

Abstract Novel, functional resins, containing variously substituted phenylphosphinic acid ligands, have been obtained via the Michael reaction. Reaction has been carried out on phenylphosphinic acid resin crosslinked with 2 wt.% of divinylbenzene using the following electrophiles: methyl chloroformate, ethyl bromoacetate, ethyl 2-bromopropionate, ethyl acrylate, ethyl methacrylate, acrylonitrile and methacrylonitrile giving the desired products with 60–97% yield. Resulting resins, having carboxylic acid function in a, b and g position in respect to the phosphinic group have been used in ion-exchange / coordination of Cu(II), Cd(II), Ni(II), Zn(II) and Eu(III) from nitric acid solutions. It has been found that resins with carboxyl groups in a and b positions display higher divalent metal uptake, when the pH of the solution is above 1.5 and ion exchange is a prevailing process. Resins with either carboxyl or nitrile group in the g position are less effective in metal ion uptake than the parent, phenylphosphinic polymer. In experiments with Eu(III) uptake from 0.1–1.0 M nitric acid solutions, where resins are supposed to operate mostly through coordination, none of synthesized resins performs better than the phenylphosphinic one. This means that introduction of carboxyl group to the phenylphosphinic acid ligand does not give a synergistic effect in coordination of metal ions.  2000 Elsevier Science B.V. All rights reserved. Keywords: Functional resins; Chemical modification; Metal uptake; Phosphinic acid

1. Introduction Insoluble, mostly suspension type polymers with ion-exchange or chelating groups attached to them are useful ion-exchangers capable of removing various metal ions from water, industrial waste and from leaching solutions in many hydrometallurgical processes. Efficiency of the removal process depends on the process *Tel.: 148-71-320-3273; fax: 148-71-320-3678. E-mail address: [email protected] (A.W. Trochimczuk)

parameters (such as type of targeted and ballast ions, pH of the solution, ion concentration) and on the properties of ion-exchanger (such as crosslinking degree, swelling and to a large extent on type and structure of immobilized ligand). Ion-exchange and chelating resins were the subject of many reviews [1–3]. Among various types of ion-exchangers with acidic ligands those having phosphonate functionality are of particular interest since they are selective towards heavy metal cations [4–6]. Development of this type of ion-exchangers started in the late 1940s [7] with phosphorylation of

1381-5148 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S1381-5148( 99 )00072-3

10

A.W. Trochimczuk / Reactive & Functional Polymers 44 (2000) 9 – 19

poly(vinyl alcohol) using various phosphorylating agents. Next, were reports on introducing phosphonic [8] and phosphinic [9] functionality to styrene / divinylbenzene and to its chloromethyl derivative. This type of modification was also presented in Ref. [10], where more attention was paid to the type of polymeric matrix and in Refs. [11,12], where ionexchange / complexing properties of phosphinic and phosphonic resins in the form of acid, monoester and diester were presented. Other types of phosphonic functionality immobilized on insoluble polymeric matrices include methylenediphosphonate [13], ethylenediphosphonate and carboxyethyl phosphonate [14] immobilized on vinylbenzyl chloride / divinylbenzene copolymers. The latter one contained also a carboxylate group as part of the ligand and the same situation pertained in the case of 1,1-dicarboxylate-2-ethanephosphonate and 1,1-diphosphonate-2-ethanecarboxylate immobilized on vinylbenzyl chloride / styrene / divinylbenzene copolymer [15]. In all cases resins displayed good selectivity towards multifunctional cations. The resin with phosphonate groups in the geminal position retained its ion-exchange / coordinating properties even at very low pH [13], whereas ethylenediphosphonate and carboxyl containing resins, being less acidic, were more selective at pH 1–2. In some cases immobilization of the entire ligand is not a viable option and instead multistep modification of polymeric precursor is necessary. In the field of phosphonate resins, the latest example of such an approach is presented in Ref. [16], where by Arbusov and subsequent Perkow reactions on styrene / divinylbenzene beads ion-exchange / coordination resins bearing various ketophosphonate ligands are obtained. Recently, facile synthesis of low molecular functionalized phenylphosphinic acid has been published [17]. Possibilities offered by the reaction presented in that work and the fact that the yield of obtained derivatives has been very high prompted this investigation. The aim of this work is to synthesize series of

new ion-exchange resins with phosphinic and carboxylic groups by addition of appropriate electrophiles to a phenylphosphinic resin and to determine their ion-exchange properties.

2. Experimental Starting copolymer of styrene (St) and technical divinylbenzene (DVB) is prepared with 0.5 wt.% of benzoyl peroxide as initiator using the suspension polymerization technique. The nominal crosslinking degree is 2 wt.% and polymerization is carried out in the presence of toluene (50 wt.% in respect to monomers) in order to obtain an expanded gel structure of the resultant polymer. After polymerization, beads are washed with hot water, water, acetone and dried. The polymer is preswollen in toluene and extracted with this solvent for 12 h in a Soxhlet apparatus. Phosphorylation of St / DVB polymer follows the procedure published in Ref. [18]. Thus, 17 g of polymer beads are placed in a round bottom flask together with 128 ml of PCl 3 and allowed to swell for 30 min. Fresh, powdered aluminum chloride is added (20.2 g) and the entire mixture is shaken at RT for 24 h. After that time beads are filtered off, washed briefly with dioxane and dropped into a 1-l beaker containing ice and sodium chloride solution. After washing with distilled water, beads are left overnight in ca. 3 M NaOH solution followed by standard conditioning. One gram of thus obtained St / DVB polymer, containing 4.05 mmol of phosphinic acid groups per gram is placed in a 100-ml Erlenmeyer flask together with 20 g of dry 1,2-dichloroethane. After 2 h of preswelling, 2.20 g (17 mmol; 4.2 3 excess in respect to phosphinic groups) of diisopropylethylamine is added followed by an equimolar amount of trimethylchlorosilane. The entire mixture is shaken under Ar for 24 h. After that time 9.0 mmol of appropriate electrophile are added and shaking under Ar is continued for 48 h. Work up consists of rinsing

A.W. Trochimczuk / Reactive & Functional Polymers 44 (2000) 9 – 19

the polymer with 1,2-dichloroethane, dioxane and 1 M HCl in which polymer is kept overnight followed by passing of distilled water through the polymer. Hydrolysis of the resulting resins containing methyl and ethyl ester functionality is done by refluxing the polymer in 3 M sodium hydroxide solution for 4 h. After the reaction, polymer is washed with distilled water and conditioned with 1 M HCl, water, 1 M NaOH, water, 1 M HCl and finally with water. Water regain of the resins is measured using a centrifugation method and calculated as:

11

of the appropriate metal 10 24 N ion solution in nitric acid for 24 h and then metal concentration is determined using the atomic absorption method on a Perkin-Elmer AAnalyst 100 with wavelength set at 213.9, 228.8, 232.0 and 324.8 nm for Zn(II), Cd(II), Ni(II) and Cu(II), respectively. Affinity of the resins towards Eu(III) is determined by contacting of a resin equivalent to 0.1 mmol of phosphorus with 10 ml of 3310 24 N europium nitrate solutions in nitric acid. Final metal concentration is measured using emission atomic spectroscopy with wavelength 459.4 nm.

W 5 (m w 2 m d ) /m d where m w is the weight of polymer after centrifugation in a small column with fritted-glass bottom and m d is the weight of polymer after drying at 1008C overnight. Acid capacity is measured by immersing a known amount of centrifuged polymer in the protonated form in 100 ml of 0.1 M sodium hydroxide solution for 24 h and titrating the resultant solution with 0.1 M hydrochloric acid. pKa of the obtained resins is measured using ca. 0.1 g (in respect to dry weight) of polymer equilibrated with 45 ml of 1 M KCl. To this stirred (200 rev. / min) suspension 0.15 ml of 0.100 M KOH is injected every 90 min using a ¨ Kuhn&Bayer liquid processor. pH is recorded every 15 min. Phosphorus content is measured after mineralization of ca. 20 mg polymer sample in a perchloric acid solution using molybdate method. Readings are taken at 700 nm. Nitrogen capacity is measured after mineralization of the resin sample in a concentrated sulfuric acid containing copper sulfate and potassium sulfate and follows the Kiejdahl procedure. IR spectra of the resins are taken in KBr pellets in the range 4000 to 650 cm 21 using a Carl Zeiss M-80 spectrophotometer. In order to determine performance of the resins as cation exchangers a resin equivalent to 0.1 meq of acid capacity is shaken with 20 ml

3. Results and discussion

3.1. Resin synthesis In order to investigate the Michael reaction on a polymeric resin it has been necessary to obtain material having primary phosphinic acid ligands only. Phosphorylation is achieved by reacting St / DVB resin with PCl 3 in the presence of Friedel-Crafts catalyst. Although the reaction is well known and relatively simple, special care should be taken in order to avoid unwanted side reactions. Therefore synthesis of the phosphinic resin followed the procedure described in Ref. [18], in which the influence of such variables as temperature, amount of catalyst and crosslinking level of the St / DVB resin on the purity of phosphinic resins is described in detail. It has been found that using the reaction conditions described in Section 2, a resin having phosphorus content 4.05 mmol / g and acid capacity 4.02 mmol / g is obtained. To determine the number of primary phosphinic acid groups, a small sample of phosphinic resin has been oxidized by refluxing with 6 M nitric acid for 5 h. The product of oxidation has 7.27 mmol / g of acid capacity (expected value if only primary phosphinic sites were present before oxidation was 7.60 mmol / g), which proves that within experimental error monofunctional primary phosphinic resin was obtained in the Friedel-Crafts reaction.

12

A.W. Trochimczuk / Reactive & Functional Polymers 44 (2000) 9 – 19

It should be also pointed out that the yield of phosphinic resin synthesis ([4.05 / 5.77]31005 70%, where 5.77 mmol / g is theoretical phosphorus content in fully functionalized resin, that is when each styrene mer has one phosphinic group) is higher than reported in the case of polymer crosslinked with 2% of DVB [18] and the probable reason for that is the expanded gel structure of polymer used in this work, which makes the entire structure more accessible due to the better swelling of such St / DVB material in PCl 3 during phosphorylation. Fig. 1 shows a three-step reaction leading from the phenylphosphinic resin, through bis(trimethylsilyl)phosphonite, addition of electrophile with the creation of new C-P bond and subsequent hydrolysis, to a series of resins having variously substituted secondary phosphinic ligands. All of the above reactions have been carried out using general conditions described in Ref. [17] with slight modifications arising from the fact that polymer bound products are insoluble in water and organic solvents and because due to the diffusion barrier addition of electrophile is expected to be slower than in the case of low molecular compounds. Thus, addition of electrophile has been carried out at RT and the mixture has been shaken for 48 h. Polymer has been filtered off and washed with reaction solvent, then dioxane in order to remove dichloroethane and make the polymer compatible with water based solutions of hydro-

chloric acid. Table 1 summarizes data obtained for the resulting resins. As can be seen, phosphorus content is lower than 4.05 mmol / g, determined for starting phenylphosphinic resins, as the result of the mass gain during modification. Phosphorus capacity is used to calculate yield of the reaction, which is presented in the last column. It can be noticed that for the majority of the electrophiles used in this work yield is very high and in some cases higher than reported in Ref. [17]. Probably it is due to insolubility of the polymer bound product and thus lack of product losses during work up of the post reaction mixture. Such losses were for example reported during modification of low molecular phosphinic acids in Ref. [19]. The only exception is modification with methyl methacrylate where the yield is 60%. Yield, calculated from the phosphorus content, is in all cases in good agreement with the acid capacity, which should be twice higher than P content. Resins have similar water uptake to that displayed by the starting phenylphosphinic one (resin R in Table 1). Significantly lower hydrophilicity is presented by resin 5, modified with acrylonitrile, where the non-ionizable nitrile group is introduced, whereas ca. 30% higher water uptake is displayed by resin 1, where addition of a carboxylic group proceeded without simultaneous addition of one or two methylene groups as in the case of resins 2–4. The structure of the ligands attached to the resins is

Fig. 1. Scheme of resin synthesis.

A.W. Trochimczuk / Reactive & Functional Polymers 44 (2000) 9 – 19

13

Table 1 Characteristics of resins obtained Resin no.

Electrophile

Phosphorus content mmol / g

Acid capacity mmol / g

Percentage of solid %

Water regain g/g

%

R 1 2

None Methyl chloroformate Ethyl bromoacetate Ethyl 3-bromopropionate Ethyl acrylate Acrylonitrile Ethyl methacrylate

4.05 3.69 3.47

4.02 6.04 6.01

49.20 43.07 49.6

1.03 1.32 1.02

N /A 91 93

3.15

5.98

51.40

0.95

89

3.07

5.84

53.71

0.86

97

3.25 / 3.03 a 3.39

3.76 5.30

62.84 51.54

0.59 0.94

93 60

3 4 5 6 a

Yield

Nitrogen content.

further supported by the IR spectra. In the spectra of resin R the P–OH peak appears at 975 cm 21 , P=O is seen at 1130 cm 21 and P–H is present at 2350 cm 21 . Peaks at 700 and 760 cm 21 corresponding to a substituted phenyl ring are present in the spectra of all resins. In the spectra of resin 1 an additional peak at 1630 cm 21 appears, corresponding to C=O bond. In the spectra of resin 2 the P–OH peak position is shifted to 960 cm 21 , P=O is at 1130 cm 21 and carbonyl at 1715 cm 21 . The spectra of resins 3 and 4 are very similar to the spectra of resin 2. In the spectra of resin 5, P–OH is at 960 cm 21 , P=O is present at 1130 cm 21 with a shoulder at 1165 cm 21 . Additionally, a peak at 2245 cm 21 corresponding to the nitrile group can be seen. Proposed structures of the resins are presented in Fig. 2.

3.2. Ion uptake In order to investigate the effect of ligand structure on the mechanism and extent of metal ions, binding resins have been contacted with Me(II) solutions in nitric acid of various concentration. Since resin 6, modified with ethyl methacrylate, is functionalized only in 60% its use in a metal contact study is pointless. For the rest of the resins under applied conditions, where there is a large excess of ligand number

over available metal cations, distribution coefficients are determined and plotted as the function of pH of the solution. Because the most important advantage of phosphorus-containing resins is their good performance under acidic conditions combined with much better selectivity than selectivity displayed by strongly acidic resins, contact studies have been limited to the 0.01, 0.02 and 0.05 M nitric acid solutions. Results are displayed in Fig. 3. As can be seen in each case curves of log Kd vs. pH of the solution fall into two groups. The first one, with higher log Kd values consists of resins R, 1 and 2. These are resins with the most strongly acidic ligands. It can be supposed that introduction of a carboxylic group in either a position (as in resin 1) or b position (as in resin 2) to phosphinic group would result in decrease of the pKa of the resultant acid compared to the resin R phenylphosphinic acid ligand. Fig. 4 shows pH titration curves of the investigated polymers. It can be noted that curves of resins R and 5 are typical of a monobasic acid and those of resins 1, 2 and 3 are characteristic of dibasic acids. This result supports the proposed model of the functional groups in resins. It should be also noted that dibasic character shows more clearly (inflections on curves) in the case of resins 1 and 2. It has been demonstrated by Helfferich in Ref. [20], that the pKa values of the acidic

14

A.W. Trochimczuk / Reactive & Functional Polymers 44 (2000) 9 – 19

Fig. 2. Structure of ligands in the resins obtained.

groups of resins can be calculated knowing the external pH at half conversion, concentration of cation (which can be regarded as constant since titration is performed in the presence of a 1 M salt solution) and swelling of the resins at half conversion. Swelling has been determined in separate experiments and after calculations the

following pKa values are obtained: 3.12 for resin R, 3.06 for resin 1, 3.00 for resin 2, 3.59 for resin 3 and 3.86 for resin 5. Lower pKa and hence the possibility of operating also by ionexchange mechanism at low pH is demonstrated in the values of metal sorption displayed by resins R, 1 and 2. In the studied pH range sorption of either divalent metal is in the case of resins belonging to the first group ca. two times higher at pH 2 and ca. 5–6 times higher at pH 1.36 than sorption observed in the case of resins 3–5. Similar dependence was observed earlier [21] for other difunctional ligands containing phosphonic and carboxylic moieties. When both were in the acid form, sorption of Cu(II) from 0.01 M nitric acid was 1.45 mg / g of polymer and log Kd was 3.28. But when a carboxylic group in the same ligand was present the form of ester sorption dropped by 30% to 1.00 mg / g and distribution coefficient almost three times to 679, proving that the presence of carboxylic acid influenced metal sorption. In this work the observed slope of log Kd vs. pH is close to 2 and supports the ion-exchange mechanism of Cu(II) sorption. Resin 5 is an exception and for this resin the slope is equal to 1.90, which indicates participation of coordination in the removal of copper ions by this polymer. For other metal ions the slope of log Kd vs. pH is lower than 2, indicating that other than ionexchange interaction with resins, ligands must be also operating. Lowest slopes are observed for Ni(II). The second set of log Kd vs. pH curves includes resins 3–5. As can be seen in Fig. 3 distribution coefficients are ca. 10 times lower than in the first group of resins and are accompanied by lower sorption of metal ions. However, at the upper end of the pH used in this work, sorption increases. It can be assumed that at higher pH more groups are deprotonated and able to participate in ion-exchange. Deprotonation of acidic phosphinic groups proceeds to a small extent also at pH lower than pKa . The mechanism of the metal uptake remains unaffected (slope of log Kd vs. pH does not change)

A.W. Trochimczuk / Reactive & Functional Polymers 44 (2000) 9 – 19

Fig. 3. Log of distribution coefficient and sorption of Me(II) as a function of pH.

15

16

A.W. Trochimczuk / Reactive & Functional Polymers 44 (2000) 9 – 19

Fig. 4. pH titration of investigated resins.

only the amount of metal on the resin gets higher. In order to check the capacity of the resins towards Cu(II) as a function of the ligand to metal ions ratio, sorption experiments have been carried out using 20 ml of Cu(II) in 0.02 M nitric acid solution and various amounts of swollen resins. The results are depicted in Fig. 4, where copper sorption is plotted as the function of R i (meq of copper ions / meq of acid capacity). As can be seen sorption is much higher in the case of resin R, 1 and 2 in the entire range of R i . Moreover, for these resins, sorption reaches a plateau when copper ions to acid capacity is ca. 0.5 (in the case of resin 1) or above this value (for resins R and 2). This means that in the above resins ligands can be used effectively and that they are able to interact with an increased number of metal ions. The above effect can be ascribed to the fact that their groups are in close proximity and in the ion-exchange process can satisfy requirements

of divalent copper ions. Contrary to that, resins 3–5 have their groups further apart and certain geometrical requirements must be met before ion exchange of divalent ion is possible. So, the efficiency of ligand use is low and only a fraction of them is operating, which results in much lower metal uptake than in the case of resins R, 1 and 2. In order to investigate the difference between ligand performance in each type of the studied resins it was necessary to carry out ion uptake experiments under moderately acidic conditions using Eu(III) as the model ion. Under such conditions the predominant mode of metal ion uptake is coordination [13,16]. Ion exchange by carboxyl group is supposed to be largely eliminated and this fact is supported by data presented in Ref. [22], where the fraction of Eu(III) complexed was plotted as a function of pH for selected chelating agents. None of the carboxylate ligands presented there (with the exception of oxalate) was able to complex Eu(III) when

A.W. Trochimczuk / Reactive & Functional Polymers 44 (2000) 9 – 19

the pH was lower than 1. In case of the functionalized phenylphosphinic resins presented here, Eu(III) is complexed by all polymers as can be seen in Fig. 5. The situation is similar to that observed for less acidic solutions and Me(II). The first three resins perform much better than resins 3 and 5. From the slopes, which are 2.25, 2.57, 2.53, 1.68 and 1.73 it can be concluded that low metal uptake by resins 3 and 5 is caused by a smaller contribution of ion exchange to the metal sorption. Resin 5 has no other potentially ionizable group but phosphinic and this one under applied conditions apparently operates mostly by coordination. It is substituted by a 2-cyanoethyl group, which makes it less acidic compared to the phenylphosphinic acid ligand. Ion exchange plays a more important role in the case of resins R, 1 and 2. It is obvious that when the carboxyl group is in the a and b position, acidity of phosphinic acid increases due to the induction effect and ionexchange influences metal uptake. Comparing the values of log Kd and sorption of Eu(III) one can see that there is no advantage in introducing a functionalized substituent instead of hydrogen in the phenylphosphinic ligand (Fig. 6). Both sorption and log Kd for

17

three resins are comparable and no synergistic effect of phosphinic and carboxyl groups is observed. The most likely explanation would be that in the phenylphosphinic ligand contact with metal ions results in formation of two equal P–O bonds in the place of P–O and P=O and thus such a ligand acts as a bidentate. Introduction of a functionalized fragment through the Michael reaction removes the labile acidic hydrogen and at the same time introduces carboxylate functionality (in resins 1–4). However, only when carboxylate is close to the phosphinate, that is in either a or b position, is it able to act together with phosphinate. Such ligands (in resin 1 and 2) are potentially bidentate and perform similarly to phenylphosphinate.

4. Conclusions A new group of functionalized resins was obtained through Michael reaction on phenylphosphinic resin crosslinked with divinylbenzene. It has been proved that for most of the electrophiles used, the yield of this chemical modification is high, allowing to intro-

Fig. 5. Sorption of Cu(II) as a function of ion to ligand ratio.

18

A.W. Trochimczuk / Reactive & Functional Polymers 44 (2000) 9 – 19

Fig. 6. Log of distribution coefficient and sorption of Eu(III).

duce new functional groups without unwanted side reactions. This, in turn, makes possible the use of newly introduced groups in further

modification. Metal ion uptake studies showed that the affinity of resins with a carboxyl group in the a and b position towards divalent metal

A.W. Trochimczuk / Reactive & Functional Polymers 44 (2000) 9 – 19

ions such as Cu(II), Cd(II), Ni(II) and Zn(II) is slightly better than the affinity displayed by parent phenylphosphinic resin, which should be ascribed to the increased acidity of the obtained ligands. References [1] S.K. Sahni, J. Reedijk, Coord. Chem. Rev. 59 (1984) 1. [2] S.D. Alexandratos, D.W. Crick, Ind. Eng. Chem. Res. 35 (1996) 635. [3] R.A. Beauvais, S.D. Alexandratos, React. Funct. Polym. 36 (1998) 113. [4] M. Marhol, J. Chmielnicek, A.B. Alovidtinov, C.W. Kockarova, J. Chromatogr. 295 (1983) 4. [5] H. Egawa, K. Yamabe, A. Jyo, J. Appl. Polym. Sci. 52 (1994) 1153. [6] J. Kaiser, B. Schmied, N. Trautmann, W. Vogt, Makromol. Chem. 193 (1992) 799. [7] R.E. Ferrel, H.S. Olcott, H. Fraenkel-Conrat, J. Am. Chem. Soc. 70 (1948) 2101.

19

[8] J.I. Bregman, Y. Murata, J. Am. Chem. Soc. 74 (1952) 1867. [9] E.L. McMaster, W.K. Glesner, US Patent 2980,721 (1961). [10] R. Bogoczek, J. Surowiec, J. Appl. Polym. Sci. 26 (1981) 4161. [11] S.D. Alexandratos, D.R. Quillen, M.E. Bates, Macromolecules 20 (1987) 1191. [12] S.D. Alexandratos, D.L. Wilson, Macromolecules 19 (1986) 280. [13] S.D. Alexandratos, A.W. Trochimczuk, E.P. Horwitz, R.C. Gatrone, J. Appl. Polym. Sci. 61 (1996) 273. [14] A.W. Trochimczuk, Eur. Polym. J. 34 (1998) 1047. [15] A.W. Trochimczuk, Eur. Polym. J., in press. [16] S.D. Alexandratos, L.A. Hussain, Macromolecules 31 (1998) 3235. [17] F.A. Boyd, M.E.K. Boyd, V.M. Loh Jr., Tetrahedron Lett. 37 (1996) 1651. [18] S.D. Alexandratos, M.A. Strand, D.R. Quillen, A.J. Walder, Macromolecules 18 (1985) 829. [19] J.K. Thottathil, C.A. Przybyla, J.L. Moniot, Tetrahedron Lett. 25 (1984) 4737. [20] F. Helffrich, Ion Exchange, McGraw-Hill, New York, 1962. [21] A.W. Trochimczuk, J. Jezierska, Polymer 38 (1997) 2431. [22] K.L. Nash, J. Alloys Comp. 249 (1997) 33.