Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 524-530
524
cross-linking,and/or other reactions appear to occur. The improved thermal stability (TGA data) for acid-treated PAN may be attributed to the cyclized structures. Good char yields have been reported in the literature in the case of polymers having fused structures (van Krevelen, 1975). The mechanism presented agrees with this result. (b) Another potential mechanism to explain some of these results is a radical initiated oxidation. The PAN may contain hydroperoxides which decompose and initiate the chain reaction (Reich et al., 1969; Brandrup and Peebles, 1968). In this mechanism the benzoic acid would act as a catalyst to decompose the hydroperoxidesto free radicals and thus accelerate the chain reaction (Reich and Stivala, 1969). The following free radical mechanisms could ensue and also result in the ketonic groups found by IR (1705-1725 cm-’). Initiation:
-
-CHz-CH-CHZ-CH-CH~-CH
I
CN
I -
I
I
CN
CN
Brandrup, J.; Peebles, L. H., Jr. Macromo4cu4s 1968, 7, 64. Burlant, W. J.; Parsons, J. L. J . folym. Scl. 1958, 22, 349. Cagilostro, D. E. Texfl4 Res. J . 1980, 50. 632. Dwight, D. W.; McGrath, J. E.; Wghtman, J. P. J . Appl. folym. Scl., Appl. folym. Symp. 1970, No. 34, 35-47. Grassie, N. “Developments in Polymer Degradation”,Applied Science Publishers: London, 1977; pp 137-169. Grassie, N.; Hay, J. N. SCI Monogr. 1961, 73, 184-199. Grassie, N.; McGuchan, R. M. Eur. folym. J. 1971, 7, 1357. Grassie, N.; McNeili, I. C. J. folym. Scl. 1059, 39, 211. Jenkins, G. M.; Kawamura, K. “Poiymerlc Carbons-Carbon Fiber, Glass and Char”, Cambridge Unhwslty Press, 1976; pp 86-106. OvemOff, D.; Wlnkler, E.; Muelier, D. “Process for Manufacture of Carbon or Graphite Fibers”, German Patent 2220614. Nov 1973. Peebles, L. H., Jr. “Degradation of Acrylonitrile Polymers”, “Encyclopediaof Polymer Science and Technology”, Mark, H.; Gaylord, N., Ed.; Supplement -. .. v. i., 1976, pp 1-25. Reich, L.; Stivala, S. S. “Autooxidation of Hydrocarbon and Polyoiefinics”, Marcel Dekker: New York. 1969; pp 53-59. Schurz. J. J . folym. Scl. 1958, 28, 438. Skoda, W.; Schurz, J.; Bayzer, H. 2. phys. Chem. 1959, 270, 35. van Krevelen, D. W. Polymer 1975, 76, 615.
OZ
OOH
I
CHz
- CH -CH I
CN
-CH-
I
CN
Decomposition of hydroperoxides:
-
CH2
H 0 0
- CH
I -CH
- CH I
I
CN
CN
II
I
I
CN
Received for review October 17, 1980 Accepted April 28, 1981
0
OH -CH2-CH-CH-CH-
*CCHz-CH
I
CN
-
Termination: Radicals inert products (may be cross-linked products not formed in the initial stages). Thus it is difficult to pinpoint one type of mechanism during PAN degradation in presence of benzoic acid. Observed increase in oxygen content and loss of hydrogen of degraded fibers, as well as appearance of C=O peaks (1705-1725 cm-’) give support to oxidation reaction (mechanism b). Improvement in the char yield, discoloration and appearance of new absorption bands in IR spectra lend support to mechanism a. Both of these mechanisms might be contributing towards the benzoic acid initiated degradation of PAN fibers. Acknowledgment We wish to express our appreciation to Dr. A. E. Pavlath of the Department of Agriculture at the Western Regional Research Laboratory, Albany, Calif., and to Pat Zajicek of Surface Science, Inc., Palo Alto, Calif., for running and discussing the ESCA spectra. Literature Cited
I
CN
-C--CH-
I
CN
Presented a t the Second Chemical Congress of the North American Continent, Las Vegas, NV, Aug 25-29, 1980.
Cation Complexation Properties of Epoxy Polymers and Exchangers Containing Diazacrown Ethers and a Cryptate Phlllppe Gramain’ and Yves Frere Centre de Recherches sur les Mecromol6cules (CNRS),6, rue Rousslngault, 67083 Strasbourg-Cedex, France
The synthesis and complexation properties of various poly(diazacrown ethers) and one poly(diazacryptate) are presented. They are obtained by condensation polymerization of various diepoxides with the diamines [22], [21], and the cryptate [222,]. According to the starting depoxide, they are water soluble or not. Their binding properties toward alkali, alkaline earth, and some transition cations are studied and compared with those of their monomeric analogues by using data obtained from potentiometric, UV, and NMR spectroscopic, and water-chloroform extraction measurements. In addition, complexing exchangers are prepared by grafting on a Merrifield polymer two selected ligands, and their efficiency to separate cations by liquid chromatography is tested. We show that the properties of the polymers are, in a general rule, comparable to those of the analogous monomers. In some cases new properties are observed due to various structural factors such as the nature of the neighboring groups and of the connecting chains.
Introduction The preparation and properties of crown compounds have received considerable attention in recent years, and, 0196-4321/81/1220-0524$01.25/0
in an exhaustive review Bradshaw and Stott (1980) enumerate more than 655 macrocycles synthesized since the pioneer’s works of Pedersen in 1967 and Lehn in 1969 0 1981 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 3, 1981 525
(Dietrich et al., 1969). Due to their ability to complex selectively metal cations, various applications have been studied, including the following: the solubilization of salts in organic solvents of low polarity (Christensen et al., 1971; Pedersen and Frensdorff, 1972);the enhancement of the cation reactivity through efficient shielding of the cation (for a review see Weber and Vogtle, 1977,and for applications in polymerization see Boileau et al., 1974;Lacoste et al., 1976);the separation of cations (Izatt et al., 1977; Tadeka et al., 1979) including Lanthanides (King and Heckley, 1974;Izatt et al., 1977);the selective transport of cations through membranes (McLaughlin et al., 1972; Reusch and Cussler, 1973;Wong et al., 1974;Kobuke et al., 1976; Christensen et al., 1978); the use in chelation therapy (Mtiller, 1970,1974;Baudot et al., 1977;Kulstad and Malmsten, 1979);the isotopic enrichment (Jepson and Dewitt, 1976;Hermann and Schiefer, 1980); the stabilization of organic compounds (Bartsch and Juri, 1980; Ahern and Gokel, 1979);the complexation of molecules (Simon, 1976;Vogtle, 1980)and of anions (Graff and Lehn, 1976;Schmidtchen, 1977);the separation of optically active molecules (Sousa et al., 1974;Timko et al., 1978). Considering those possibilities, the preparation of polymeric macrocyclic ligands is an attracting and promising field since their polymeric nature provides additional practical properties and offers real possibilities for industrial applications. A polymeric ligand is easier to use, purify, recover, and can e made in various forms. Its solubility or swelling properties can easily be regulated by choosing the chemical nature of the polymeric chain. Attractive materials can be prepared leading to various practical uses: resins, membranes, or coatings can be elaborated for purification, separation, extraction, reaction, protection, or detection processes. However, relatively little has been published concerning the synthesis and the study of such polymers. A first contribution in this field was made by Kopolow et al. in 1971. They synthesized and studied various poly(crown ethers) obtained by polymerization of vinyl crown ether derivatives. Later, Varma et al. (1979),Kimura et al. (1979),and Yabi et al. (1980)studied polyacrylamide or polymethacrylic derivatives. Other types of polymeric ligands have been obtained by polycondensation of difunctional derivatives of crown ethers (Feigenbaum and Michel, 1971;Bromann et al., 1975;Shchori and Jagur-Grodzinski, 1976). For analytical or preparative purposes resins have also been prepared (Cinquini et al., 1976;Dotsevi et al., 1976,1979;Blasius et al., 1978;Warshawsky et al., 1979;Kutchukov et al., 1980). Although the results obtained are promising, the development of a commercial application still requires many studies. We have prepared and studied a variety of condensation polymers containing diazacrown ethers (Gramain and FrGre, 1979, 1980; Gramain et al., 1980) and cryptates, together with exchangers and, in this review, we describe the synthesis and binding properties of polymers obtained by polycondensation of azacrown ethers with diepoxides. Synthesis The polymers are readily prepared by reacting the commercially available cyclic diamines [22] or [21], or the cryptate [222N]prepared according Lehn and Montavon (1976),with epichlorohydrin (Epic), diepoxyoctane (EpiO), or the diglycidic ether of 2,2-bis(I-hydroxyphenyl)propane (EpiDPP).
R-h
R=H
n = 0:[21] n = 1:[22]
R = EtOH
[21]EtOH or [22]EtOH
R = EtOH
[222~]EtOH
WA0?
W
The repeat unit structures of the obtained polymers are depicted below
~ o + + - @ o ? OH
p 121 I E ~ ~ D P P
and
p -22‘ EpDDP
p - 2 2 1 EplO
and
p r 2 2 2 ? Epio
OH
N
p
2 1 j EPIC
and
p ‘22’ EplC
The best results have been obtained when the polymerization is carried out in THF/MeOH (1/3)under reflux for 72 h with 80-90% yield. Tonometric or GPC analysis give M , = 1500 (p[22]EpiC), M, = 4300 (p[22]EpiO), M , = 5000 (p[22]EpiDPP) and M, = 3500 (p[222,]EpiO) indicating respectively an average of 5, 10,9,and 7 ligand units in the polymeric chains. If the polymerizations are carried out in mass or in nonstoichiometric conditions, insoluble gels are obtained. In order to compare the binding properties of the polymers to those of the analogous molecules, the dihydroxyethyl derivatives of [21], [22], and [222N] have been also prepared according the process described by Gramain et al. (1980). Complexation Properties The cation complexation properties of these polymeric ligands have been studied by potentiometric titrations, liquid-liquid extractions, and NMR and UV spectroscopies, and compared to the properties of the monomeric analogues. Water-Soluble Polymers Polymers prepared from Epic and EpiO are water soluble and using the fact that the nitrogen atoms of the ring, participate in the complexation of cations and become more difficult to protonate, it is possible to determine the stability constants of complexes in water by potentiometric titration (Dietrich et al., 1973). With nonpolymeric ligands a first titration without cations leads to the determination of the basicity constants K1,K,, K,,corresponding to the equilibria Ki
L+H+SLH+
(1)
526
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20,No.
K2
LH+ + H+ eLH22+ LH,-l(n-l)+
K" + H+ S LHnn+
with [LH22+1 K1 = [LH+I [L][H+]' K2 = ILH+I[H+l -e
and more generally K, =
LHnn+ [LH,-,(n-l)+][H+]
A second titration in the presence of cations, allows determination of the stability constants of the complexes according to the equilibria
+ Mn+ Kd LMn+ + L L
LMn+
(4)
LMn+L
( 5)
with [LM"+] Ksl =
*
[LI W"+l'
Ks2 =
[LMn+L] [LM"+I[LI
and K's2 = Ks, x Ks2 With cryptates the formation of complexes between the protonated ligands and a metal is possible (Lehn and Montavon, 1978) but this case is not considered here. (All the considered constants are "concentration" constants assuming the activity coefficients of the species equal to 1.) However, the case of a polymeric ligand must be considered in more detail. A polyelectrolyte effect due to the interactions between the ionizable groups can affect the determination of pK and only apparent pK can be calculated. For a nonmacromolecular electrolyte with two ionizable groups, it can be demonstrated (Souchay and Lefebvre, 1969) that, at any moment of the titration, the concentration into free H+ is given by pH=
PK1+ PK2
+
n
L
log
[
2(2 - n) [R2(1- lt)2 4n(2 - fi)]''* - R ( l - rt)
+
]
(6)
with
Ho - H
fi=----. log R = PK1- PK2 . 2 ' LO where Lo is the initial concentration of ligand and Ho the total concentration in H+. For a polyelectrolyte, relation 6 is not generally valid and an additional term varying with the ionization degree must be added. The use of the relation 6 leads to apparent pK1 and pK2 whose values vary with rt. When the quantity pH - log [XIcalculated from the experimental titration curves of the studied polymers is plotted over rt, no variation is observed, showing that the polymers do not exhibit typical polyelectrolyte behavior. Although similar behavior has been observed with polyamines containing diacylpiperazine groups (Barbucci et al., 1978), this independence of the repeating units is possibly due to the presence of the monocycles sheltering the positive charges on the pro-
tonated nitrogens and to the low molecular weight of our samples. The pH-metric titrations are perrformed according to a procedure described previously (Gramain and Frbre, 1979a, 1979b) and analyzed by a computer program. In Table I, the basicity Constants of the polymers are collected together with those for the molecule analogues. I t can be seen that the polymer structure influences the constants of first protonation very little, but decreases the basicity of the other amines of the ring, except for p[22]Epi0. This lowering is attributed to the interaction between the first proton with the other amines of the rings which are, in the polymers p[22]EpiC, p[21]EpiC, and ~ [ 2 2 2 ~ ] E p in i Oclose proximity. This type of interaction is favored by the polymeric nature of the ligands. Table I1 lista the pKsl values of the polymeric and analogue ligand complexes with various cations. Except for p[22]Epi0 with Ba2+,the observed stoichiometry was always of 1ligand for 1 cation in our concentration conditions ( L = 10-3M,cations in excess). For the alkali and alkaline earth cations, the stabilities of the complexes with cyclic ligands depend primarily on the relative size of the ring and of the cations, which can be considered as spherical. However, other factors affect the stabilities and selectivities. The replacement of 0 by N in the rings decreases the electrostatic interaction of the cation with the li and and somewhat the size of the cavity (N, 1.5 A; 0,1.4 ). In addition the chemical environment of the rings and in particular the presence of neighboring groups effect the stabilities of the complexes. The neighboring groups can perturb the electronic distribution of the donor atoms, change the hydration shell of the ligand, introduce a steric or conformational strain, or participate to the coordination of the cation. Such interactions are illustrated by our results. The evolution of the pK values gives a good indication of the disponibility of the electron pair of the nitrogens, and the higher the pK, the higher is the stability. This is clearly demonstrated: p[22]Epi0 and [22]EtOH have comparable pK and pK, but p[22]EpiC and p[21]EpiC, which have lower pK2 than [21]EtOH and [22]EtOH, have lower pK,. The neighboring group can participate to the formation of the complex as illustrated by the comparison between the ligands [22]CH3and [22]EtOH where the OH groups stabilize the complexes (Gramain and Frgre, 1979b) and inverse the selectivity Sn2+/Ba2+.With the ligands [222N]EtOH and ~ [ 2 2 2 ~ ] E p ithe O presence of the OH groups destabilizes the complexes-compared with pK, (Ba2+)= 6.7 obtained with [222N]CH,(Lehn and Montavon, 1976, 1978). Using a C.P.K. model, it is clearly seen that the hydroxyl groups shield the cavity from the external polar aqueous medium, thus decreasing the interaction between the cation and the solvent, and also they can easily interact with the oxygen atoms of the rings. Such effects are amplified by the polymeric structure (pK, = 4.1 for the polymer instead of 4.8 for the analogue). The case of the polymer p[22]Epi0 with Ba2+ is particularly interesting because it is an example where the polymeric nature leads to additional properties. It has been analyzed in detail previously (Gramain and Frbre, 1980): we observe a stoichiometry 2:l only with Ba2+and this polymer. NMR and viscosity studies demonstrate that an intramolecular "sandwich" structure is formed due to the insufficient size of the ring to accommodate the barium well and to the presence of flexible hydrophobic CH2 bridges between the rings. With the cations of the first series of transition (Co2+,Ni2+,Cu2+,Zn2+)and with Cd2+,
d
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 3, 1981 527 Table I. Basicity Constants of Polymeric Ligands and Models in Water at 20 "C in Presence of 0.1 M NMe,B, (The Values in Parentheses Are the Standard Deviations) ~
Pi:, Pk2 Pk, Pk,
[22]EtOHa
[21]EtOHa
8.44 (1) 6.88 (1)
8.64 ( 1 ) 7.35 (1)
[222~]EtOH p[22]Epi0 9.99 7.25 3.30 2.70
8.76 ( 2 ) 6.78 ( 2 )
p[22]EpiC
p[21]EpiC
p[222~]EpiO
8.33 ( 4 ) 4.75 ( 4 )
8.27 ( 4 ) 5.07 ( 4 )
9.13 5.24 2.00
According to Gramain and Frere (1979a). Table 11. Stability Constants of the Polymeric and Analogous Complexes in Water at 20 "C in Presence of 0.1 M NMe,B, cation Na K' Mg2 Ca2 SrZ BaZ Cd2 cu2 +
+
+
+
+
+
+
co2 Ni '+ +
Zn2
+
a
ionic radii, A
[22]EtOH
0.98 1.33 0.78 1.06 1.27 1.43 1.03 0.72 0.92 0.78 0.80
[21]EtOH
log Ks, [222~]EtOH p[22]EpiO p[22]EpiC
p[Zl]EpiC
1.0
0.9 1.4
