Synthesis and cyclopolymerization of diallylammoniomethanesulfonate.
In addition to the synthesis of a variety of polyelectrolytes, Butler's cyclopolymerization (1-5) involving diallylammonium salts has also been a convenient protocol to generate polyzwitterions (PZs) (6-11) and polyampholytes (PAs) (12) (Scheme 1). Cyclocopolymerization of diallyammonium salts with sulfur dioxide have also been reported to lead to the synthesis of alternate copolymers (1), (13), (14). While the PZs have charges of both algebraic signs on the same repeating unit, the PAs have the presence of cationic and anionic motifs on different repeating units in the same polymer chain with or without charge symmetry (12). The cyclopolymers having five-membered ring embedded into the polymer architecture represent the eighth most important structural feature for synthetic polymers (15), (16).
Butler's cyclopolymers are of tremendous scientific and technological interest; the cationic polyelectrolyte poly(diallyldimethylammonium chloride) alone accounts for over 1000 patents and publications (1). The PZs and PAs, whose structure and behavior seem to mimic biopolymers like proteins or DNA that mediate life processes, have offered many new applications in medicine, biotechnology, hydrometallurgy, and oil industry (2), (17), (18). The permanent dipole of amphoteric PZs or PAs, unlike polyelectrolytes, can exhibit antipolyelectrolyte behavior, i.e., enhancements of viscosity and solubility in the presence of added electrolytes (e.g., NaCl) owing to the neutralization of the ionically crosslinked network in a collapsed coil conformation (19-25). The high dipole moment of PZs renders the properties of excellent polar host matrix in which only target ions can migrate (26), (27). The stoichiometric blends of some PZs with alkali metal salts have produced excellent matrices having high ionic conductivity (28). The unique "antipolyelectrolyte" behavior makes PZs attractive candidates for application in enhanced oil recovery, drag reduction, personal care products, cosmetics, and pharmaceuticals (29-31). The PZs have been utilized for efficient separations of biomolecules (3) and to develop procedures for DNA assay (32). They have also drawn attention in the field of ion exchange; their abilities to chelate toxic trace metal ions (H[g.sup.2+], C[d.sup.2+], [Cu.sup.2+], and [Ni.sup.2+]) have been exploited in wastewater treatment (2), (29).
For the synthesis of PZs 3 or the zwitterions/sulfur dioxide copolymer 4, monomer may either be cationic (+) 1 or zwitterionic ([+ or -]) 2; the pendent substituent 'X (e.g., C[O.sub.2]R) in the former could then be converted into [Y.sup.-] (e.g., C[O.sub.2.sup.-]) so as to provide entry to polycarbo ([Y.sup.-] = C[O.sub.2.sup.-]) (33), (34), phosphono ([Y.sup.-] = P[OR][=0][0.sup.-]) (35), (36), and sulfobetaines ([Y.sup.-] = S[0.sub.3.sup.-]) (37), (38) (Scheme 1). A polysulfobetaine (PSB) remains zwitterionic over a wide range of pH owing to the strong acid nature of the sulfonic acid [p[K.sub.a]: (-) 2.11, while a polycarbobetaine (PCB) or polyphosphonobetaine (PPB), because of the weak acid nature of the carboxylic or phosphonic acid group, can be rendered cationic by lowering the pH of the aqueous medium (3). The pH-responsive PSBs and PCBs having unquenched valency of nitrogens can also be changed to polyanions having trivalent nitrogens by neutralization of the ammonia proton in 3 or 4 (R=H) (6-9). The synthesis of copolyzwitterions incorporating 25 mol% carboxybetaine and 75 mol% sulfobetaine (SB) and its interesting pH-dependent solution properties have been reported (39). Nearly monodisperse PZs have been synthesized by group transfer polymerization (GTP) of 2-(diniethylamino)ethyl methacrylate (DMAEMA) followed by quantitative betainization using 1,3-propanesultone (40).
While the PZs having carboxylate and phosphonate moieties in the [alpha]-position with respect to the positive nitrogens as in 3 (m = 1) are reported (34), (41), the corresponding PSB (3, m = 1, [Y.sup.-] = S[O.sub.3.sup.-]) is not known to date. Even outside the domain of cyclopolymers, the literature lacks, to our knowledge, any report that describes a PSB having a single methylene spacer separating the zwitterionic charges. We report herein the synthesis and cyclopolymerization of a new SB monomer 7 (Scheme 2) having a single methylene spacer to produce homo-(11) (Scheme 3) and copolymer (16) (Scheme 4) with an intent to examine and compare their solution properties with the corresponding PSBs 19 and 20 (Scheme 5) having [(C[H.sub.2]).sub.3] spacer separating the charges in the zwitterionic motifs (37), (42). The newly synthesized polymers are expected to demonstrate interesting solution properties as a result of the closer proximity of the charges in the zwitterionic unit.
7 11 16 18 19 20 (p[K.sub.a] (~6.05) (~8.0) (~6.05) (10.8) (8.5) CSC KC1 - 2.26 M 3.99 M 0.638 M 1.56 M K1 - 3.47 M 3.48 M 0.135 M 0.201 M SCHEME 5. Comparative p[K.sub.a] and critical salt concentration (CSC) values for several zwitterions and their corresponding cyclopolymers.
Melting points were recorded in a calibrated Electro-thermal-IA9100-Digital Melting Point Apparatus. Elemental analysis was carried out on a Perkin Elmer Elemental Analyzer Series II Model 2400. IR spectra were recorded on a Perkin Elmer 16F PC FTIR spectrometer. [.sup.1]H and [.sup.13]C NMR spectra were measured in CD[C1.sub.3] (using TMS as internal standard) or [D.sub.2]0 at +25[degrees]C (using HOD signal at 4.65 and dioxane [.sup.13]C peak at [delta]67.4 as internal standards) on a JEOL LA 500 MHz spectrometer.
Viscosity measurements were madewith an Ubbelohde viscometer (viscometer constant = 0.005718 cSt/s at all temperatures) with C[O.sub.2]-free water under [N.sub.2] to avoid C[O.sub.2] absorption, which could affect the viscosity data.
t-Butylhydroperoxide (TBHP) (80% in ditertiarybutylperoxide), ammonium persulfate (APS), paraformaldehyde, diallyl amine (5) from Fluka Chemie AG (Buchs, Switzerland) were used as received. Azoisobutyronitrile (AIBN) from Fluka AG was purified by crystallization from a chloroform-ethanol mixture. Sodium bisulfite was purchased from Fisher Scientific Company. Dimethylsulfoxide (DMSO) was dried over calcium hydride overnight and then distilled under reduced pressure at a bp of 64-65[degrees]C (4 mm Hg). For dialysis, a Spectra/Por membrane with a MW cut-off value of 6000-8000 was purchased from Spectrum Laboratories.
Sodium Diallylaminomethanesulfonate (6). To a stirring heterogeneous mixture of NaHS[0.sub.3] (31.2 g, 0.3 mol) and paraformaldehyde (9.0 g, 0.3 mol) in water (50 [cm.sup.3]) was added diallylamine (5) (29 g, 0.3 mol) at 20[degrees]C. The exothermic reaction ensued, and the resultant homogeneous reaction mixture was stirred for 1 Ii, after which cooled to room temperature. The solution was then freeze-dried to obtain sodium diallylaminomethanesulfonate (6) (61 g, 95%) which was used without further purification for the subsequent step. The [.sup.H] NMR spectrum revealed the product as very pure. However an analytical sample was prepared by washing the white salt with acetone/ether. (Found: C, 39.2; H, 5.8; N, 6.4; S. 14.9. [C.sub.7][H.sub.12]NNa[0.sub.3]S requires C, 39.43; H, 5.67; N, 6.57; S. 15.04%); mp 135[degrees]C--started to melt; did not melt completely;--220[degrees]C expands as yellowish solid; 320[degrees]C turned to black powder; [v.sub.max] (KBr): 3438, 3076, 2978, 2843, 1645, 1423, 1191 (very strong), 1048 (strong), 995, 910, 848, and 788 [cm.sup.-1]. [[delta].sub.H] ([D.sub.2]0): 3.43 (6H, d, J = 6.8 Hz), 3.82 (2 H, s), 5.28 (4 H, m), 5.88 (2 H, m), (HOD: 4.65). [[delta].sub.c] ([D.sub.2]0): 56.5, 68.8, 120.5 (=C[H.sub.2]), 134.7 (-CH=), (dioxane, 67.40 ppm). [.sup.13]C spectral assignments of the alkene carbons were supported by DEPT 135 NMR analysis. The [.sup.1]H and [.sup.13]C NMR spectra are displayed in Figs. La and 2a, respectively.
Diallylammoniomethanesulfonate (7). To a solution of above salt 6 (0.25 mmol diallylamine) in water (75 [cm.sup.3]) was added dropwise concentrated HCl until the pH became 1.5 (it required about 21 [cm.sup.3] concentrated HCl). The resulting solution was then freeze-dried. The residual thick liquid was triturated with ethanol (100 [cm.sup.3]) and filtered to remove NaCl. After removal of ethanol, the residue was dried under vacuum at 40[degrees]C to a constant weight to obtain 7 (44.5 g, 85%) as a thick liquid which solidified upon cooling inside a freezer. The spectral data revealed that the above product contained a mixture of 7 and 8 in a 1:1 ratio. The average molar mass of 7 and 8 was used in the percent yield calculation. Since it is a mixture, its elemental analysis was not carried out.
[v.sub.max] (neat): 3417, 3003, 2784, 1645, 1452, 1203 (very strong), 1037 (strong), 996, 949, 782, and 603 [cm.sup.3]. The [.sup.1]H NMR spectrum revealed the following signals for 7 and 8:
7:[[delta].sub.H] ([D.sub.2]0): 3.63 (6H, d, J = 6.6 Hz), 4.23 (2 H, s), 5.46 (4 H, m), 5.90 (2 H, m), (HOD: 4.65). [[delta].sub.c] ([D.sub.2]0): 49.5, 75.0, 124.6 (=C[H.sub.2]), 128.2 (--CH=) (dioxane, 67.4 ppm). [.sup.13]C spectral assignments of the alkene carbons were supported by DEPT 135 NMR analysis.
8:[[delta].sub.H] ([D.sub.2]0): 3.97 (6H, d, J = 7.3 Hz), 4.34 (2 H, s), 5.64 (4 H, m), 5.90 (2 H, m), (HOD: 4.65). [[delta].sub.c]: ([D.sub.2]0): 57.0, 64.5, 125.9 (--CH=), 128.6 (=C[H.sub.2]) (dioxane, 67.4 ppm). [.sup.13]C spectral assignments of the alkene carbons were supported by DEPT 135 NMR analysis. The [.sup.1]H and [.sup.13]C NMR spectra are displayed in Figs. 1 and 2, respectively.
General Procedure for the Homopolymerization of a Mixture of 7 and 8. All the polymerizations were carried out using conditions as described in Table 1. In a typical experiment, a solution of the monomer 7/8 (1:1) in a suitable solvent in a small round bottomed flask was purged with [N.sub.2], and after adding the required amount of the initiator, the mixture was stirred in the closed flask at the specified temperature for 48 h. For instance in entry 10 (Table 1), when monomer 7/8 (15 mmol) in DMSO (3.0 g) and APS (375 mg) was heated at 65[degrees]C for 48 h, followed by precipitation in acetone, gave a yellow polymeric powder (50%). The polymer was dissolved in water in the presence of NaCl and dialyzed against deionized water for 24 h. During dialysis, the polymer 11 precipitated which was dried under vacuum at 55[degrees]C to a constant weight (10%). The water-soluble material after freeze-drying also gave intractable mixture of polymeric materials (34%). When the water-soluble polymer was again dialyzed against water for longer duration (72 h), only a small amount of polymer (6%) was recovered.
TABLE 1. Homopolymerization of the monomer 7 (a). Entry Initiator (mg) Solvent (g) Temperature Yield ([degrees]C) (b) (%) 1 AIBN (45) DMSO (1.5 g) 70 trace 2 AIBN (75) DMSO (3.8 g) 60 4 3 AIBN (75) DMSO (1.5 g) 70 4 4 AIBN (150) DMSO (2.3 g) 65 5 5 AIBN (225) DMSO (2.3 g) 60 8 (c) 6 AIBN (270) DMSO (2.4 g) 70 9 (d) 7 MEN (300) DMSO (2.7 g) 60 5 8 APS (75) DMSO (3.8 g) 60 6 9 APS (150) DMSO (3.8 g) 60 8 10 APS (375) DMSO (3.0 g) 65 10 (e) 11 APS (400) DMSO (3.0 g) 65 11 (f) 12 APS (450) DMSO (2.7 g) 40 Trace 13 APS (80) Water (1.2 g) 65 Trace 14 TBHP (25) 6 M HCl (1.2 g) 65 Trace (g) 15 TBHP (135) 1 N NaCl (1.2 g) 65 Trace (g) (a) Polymerization reactions were carried out in a solution of monomer 7 (15 mmol) in solvent DMSO or water (in the presence or absence of NaC1 or HCl) using initiator APS or TBHP or AIBN for 48 h. (b) Yield for the water-insoluble polymer. (c) Total yield for water soluble + water insoluble polymer = 33%. (d) Total yield for water soluble + water insoluble polymer = 48%. (e) Total yield for water soluble + water insoluble polymer = 50%. (f) Total yield for water soluble + water insoluble polymer = 50%. (g) Exothermic reaction set in immediately after addition of the initiator at 20[degrees]C with evolution of gas.
When the polymerization of 1:1 mixture of monomers 7/8 was carried out in IN NaCl or 6M HC1 with the monomer/solvent wt ratio of 70:30 using initiator TBHP, an immediate exothermic reaction set in at 20[degrees]C, and the solution turned brown. After heating at 65[degrees]C for 48 h it gave a trace amount of polymeric material which was insoluble in methanol but soluble in water.
PZ 11: The onset of thermal decomposition (closed capillary): mp 25.5[degrees]C (brown)-265[degrees]C (black); [v.sub.max] (KBr): 3433 (very broad), 2970, 2927, 2749 (br), 1635, 1467, 1421, 1307, 1214, 1126, 1038, 853, 779, 593, and 518 [cm.sup.-1]. (Found: C, 44.5; H, 7.3; N. 7.7; S. 16.1. [C.sub.7][H.sub.3]N[O.sub.3]S requires C. 43.96; H, 6.85; N, 7.32; S. 16.76 %). The [.sup.1]H and [.sup.13]C NMR spectra are displayed in Fig. 3.
General Procedure for the Copolymerization of 1:1 Mixture of Monomers 7/8 with S[O.sub.2]. All the polymerizations were carried out using conditions as described in Table 2. In a typical experiment, S[O.sub.2] was absorbed in a solution of the monomers 7/8 in DMSO. The required amount of the initiator (AIBN) was then added under [N.sub.2] and the closed flask was stirred using magnetic stir-bar at 60[degrees]C for 24 h. Within hours, the magnetic bar stopped stirring, and initial reaction mixture became a solid mass of white polymer. At the end of the elapsed time, the polymerization mixture was dropped into water. The polymer was crushed to powder and filtered. White copolymer 16 was dried under vacuum at 60[degrees]C to a constant weight. The onset of thermal decomposition (Closed capillary): mp 235[degrees]C (brown)-260[degrees]C (black), effervescence of S[O.sub.2], [V.sub.max] (KBr): 3700-2500 (very broad), 1639, 1462, 1418, 1304, 1214, 1125, 1041, 775, and 618 [cm.sup.-1]. (Found: C. 32.7; H, 5.3; N, 5.3; S, 24.8. [C.sub.7][H.sub.13]N[0.sub.5][S.sub.2] requires C, 32.93; H, 5.13; N, 5.49; S. 25.12%). The [.sup.1]H and [.sup.13]C NMR spectra are displayed in Figs. 4 and 5, respectively.
TABLE 2. Cyclocopolymerization (a) of the monomer 7 with S[O.sub.2]. Entry 7 Initiator DMSO Yield [[eta]] (b) [bar.Mn] (mmol) (mg) (g) (%) (dL [g.sup.-1]) 1 6.0 AIBN (32) 1.5 60 0.0567 67,500 2 6.0 AIBN (32) 2.1 87 0.0553 65,000 3 6.0 AIBN (32) 3.1 83 0.0512 62,500 4 6.0 AIBN (18) 1.5 82 0.0572 67,000 5 52 AIBN (156) 13 80 0.0545 64,000 (a) Polymerization reactions were carried out in a solution of monomer 7 in DMSO using initiator AIBN at 60 [degrees]0 for 24 h. (b) Viscosity of 1.5-0.5% polymer solution of 5 in the presence of 1 equivalent NaOH in 0.1 N NaCl at 30 C was measured with Ubbelohde Viscometer (K = 0.005718).
Conversion of PSB 16 to Anionic Polyelectrolyte (APE) 17. To a stirred heterogeneous mixture of PSB 16 (1.53 g, 6.0 mmol) in water (5 [cm.sup.3]) at 5-10[degrees]C was added powdered NaOH (240 mg, 6.0 mmol) in three portions (ca. 5 min). After the mixture became homogeneous, another part of solid NaOH (240 mg, 6.0 mmol) was added, briefly stirred (30 sec), and precipitated into methanol (30 [cm.sup.3]). APE 17 thus generated was filtered, washed with methanol, and dried under vacuum at 55[degrees]C to a constant weight (1.38 g, 83%). The onset of thermal decomposition (Closed capillary); 225[degrees]C (brown)-245[degrees]C effervescence of SO2; (Found: C, 30.0; H, 4.5; N, 4.9; S. 22.8. [C.sub.7][H.sub.2]NNa[0.sub.5][S.sub.2] requires C, 30.32; H, 4.36; N, 5.05; S, 23.12%).
Reconversion of (APE) 17 to PSB 16. To a stirred mixture of APE 17 (0.278 g, 1.0 mmol) in water (1 [cm.sup.3]) at 20[degrees]C was added IN HCl (1.2 mL, 1.2 mmol). The precipitate thus formed was kept under stirring for 2 h after which it was filtered and dried under vacuum at 55[degrees]C to a constant weight of PSB 16 (0.237 g, 93%).
Solubility Measurements and Cloud Point Titrations in Aqueous Salt Solutions. The critical (minimum) salt concentration (CSC) required to promote water solubility of PSB 11 or 16 at 23[degrees]C was measured by titration of 1% w/w polymer solution at sufficiently high salt concentration with deionized water. The accuracy of the CSC values, obtained by visual determination of the first cloud point, was approximately -[+ or -]2-4%. The results of the solubility and CSC values are given in Tables 3 and 4.
TABLE 3. Critical salt concentration for aqueous solutions of polysulfobetaines 11 and 16 at 23[degrees]C. Salt CSC (M) for 11 CSC (M) for 16 NaCl 3.06 Insoluble KCl 2.26 3.99 KBr 3.28 3.78 KI 3.47 3.48
RESULTS AND DISCUSSION
Zwitterionic diallylammoniomethanesulfonate (7), a SB, was prepared using procedure described for the synthesis of dimethylammoniomethanesulfonate (DAMS; ([Me.sub.2]N[H.sup.+]C[H.sub.2]S[O.sub.3.sup.-]) (43). Diallyl amine (5), on treatment with paraformaldehyde and sodium bisulfite, afforded sodium diallylaminomethanesulfonate (6), which on treatment with HCl gave a mixture of diallylammoniomethanesulfonate (7) and its corresponding sulfonic acid 8 in excellent yield (Scheme 2). The IR spectra of 6 and a mixture of 7 and 8 indicate the presence of the sulfonate group by its characteristic bands at ~1200 and ~1040 c[m.sup.-1]. [.sup.1]H and [.sup.13]C NMR spectrum of 6-8 are displayed in Figs. 1 and 2, respectively. Figs. la and 2a confirm the structure of monomer precursor 6 having four types of hydrogens and carbons. Sodium salt 6 upon addition of about 0.4 equivalent of HCl generated a mixture of 6 and 7 as indicated by the NMR spectra presented in Figs. lb and 2b. New sets of four signals attributed to 7 appeared in the spectra. The synthesis of SB 7 in the current study led to a mixture of 7 and 8 in a 1:1 as described in the experimental section. The analysis of the NMR spectra (Fig. 3c and d) led to this conclusion. It is interesting to note that the MIR signals for the apparently equilibrating mixture of 6P (Figs. lb and 2b) and 7/8 (Figs. 1c and 2c) were distinct for each species. For instance, the signal of carbon or protons marked d and d' should appear at an average chemical shifts as expected for a fast equilibrating mixture. The observation that they appear at two different chemical shifts led us to rationalize that the specie 6 and 7 are either frozen or equilibrating slowly with respect to NMR time scale. Slow equilibration makes sense since for the species 7, equilibration must involve breaking of the H-bond followed by deprotonation to 6. The equilibration for the pair involving 7 and 8 also seems to be frozen.
A 1:1 mixture of monomers 7/8 was subjected to cyclopolymerization (Scheme 3)under various conditions using a variety of initiators to give water-insoluble PSB 11 in very low yields as given in Table 1. In entry 10, for instance, a polymeric powder was obtained in 50% yield of which the water-insoluble polymer 11 only accounted for 10%, while the remainder being water-soluble. The water-soluble portion constituted polymer of low molecular mass as it escaped the dialysis bag (of MWCO 6000-8000) upon prolonged dialysis. PSB 11, upon neutralization with sodium hydroxide, was converted into the corresponding unstable APE 12 which could not be reconverted to PSB 11 upon acidification. Generally, the bisulfite compounds are unstable; the reactions which lead to their formation are readily reversible and markedly influenced by acids and alkalis.
The [.sup.1]H and [.sup.13]C NMR spectra of water-insoluble 11 are displayed in Fig. 3. The spectra are approximately consistent with the polymer structure. The water-soluble fraction of the polymer as described before did not show the NMR signal attributed to the C[H.sub.2] (marked "d") group (not shown using a figure). In some samples, it was completely absent whereas in other cases incomplete hydrolysis led to the presence of C[H.sub.2]S[O.sub.3]- pendent to variable extents as indicated by the lower intensities of the C[H.sub.2] (marked "d") signals. Even in the polymer spectra of water-insoluble 11 given in Fig. 3, the intensity of the C[H.sub.2] (d) carbon seems to be less than the other carbons. The elemental analysis was not that satisfactory. The results are rationalized in terms of structural instability of the bisulfite/formaldehyde addition product. As outlined in Scheme 3, the breakdown of 12 to immonium salt 13 is the consequence of electron push from the trivalent nitrogen to the bisulfite leaving group. The formation of [H.sub.2]O addition product 14 followed conversion to 15 and formaldehyde. The appearance of a minor signal around ([delta]81 ppm could be attributed to the NC[H.sub.2]OH carbon of 14 (Fig. 3b).
Cocyclopolymerization with S[O.sub.2]
While we were dismayed by the synthetic results to obtain homopolymer 11 which represents the first example of a PSB with a single methylene spacer, we were delighted to synthesize effectively the sulfur dioxide copolymer. The 1:1 monomer mixture of 7/8 in the presence of 1 molar equivalent of S[O.sub.2] was subjected to polymerization in DMSO using AIBN as the initiator to give water-insoluble PSB 16 in excellent yields (Scheme 4). The results of the polymerization under various conditions molar masses of PSB 16 and intrinsic viscosities in the presence of 1 equivalent NaOH are given in Table 2. The polymer precipitated during the polymerization process. The neutralization of PSB 16 with an excess of NaOH followed by precipitation into methanol afforded water-soluble APE 17 which, on addition of 1 equivalent of HC1, reverted to PSB 16 in 93% yield. The IR spectrum of PSB 16 indicates the presence of the sulfonate group by its characteristic bands at ~1200 and ~1040 c[m.sup.-1]. The two strong bands at ~1300 and ~1120 c[m.sup.-1] were assigned to the asymmetric and symmetric vibrations of S[O.sub.2] unit.
ULU Thermal degradation of PSB 16 appeared to happen at around 230-250[degrees]C and is attributed to the loss of sulfur dioxide. The 1H and [.sup.13]C spectra are displayed in Figs. 4 and 5, respectively. The absence of any residual alkene proton or carbon signal in the spectra indicated the chain transfer process for the termination reaction involving the macroradical abstracting the labile allylic hydrogen of monomer 7 (44-46). Note that the NMR signals of PSB 16 are shifted down-field than that of APE 17 as a result of the former having the presence of electron-withdrawing positive nitrogens (Fig. 4). The [.sup.13]C NMR spectrum of PSB 16 (Fig. 5a) revealed the presence of four types of signals as expected. However, for APE 17, the carbon "b" appeared as two signals in a respective ratio of 80:20, attributed to the symmetrical major cis (b,b) and unsymmetrical minor trans (b,b') disposed substituents in the five-membered ring structure (Fig. 5b, Scheme 4) (7). The integration of the relevant [.sup.13]C peaks yielded the cis/trans ratio which is similar to that observed for the related cyclopolymers (7), (47), (48). Note that while the [.sup.13]C spectrum of the structurally frozen PSB 16 is much simpler than that of mobile APE 17 as a result of the equilibration:
--NC[H.sub.O]S[O.sub.3.sup.-](17) + [H.sub.2]O [??] --N[H.sup.+]C[H.sub.2]S[O.sub.3.sup.-](16) + O[H.sup.-]
Molar masses of the copolymers were determined using end group analysis. In cyclopolymerization reaction, the termination involves degradative chain transfer reaction involving the abstraction of the labile allylic hydrogens; the resultant stable allylic radical can not reinitiate polymerization. In that scenario, it would be a good approximation to assume that each polymer chain will have one initiator fragment [i.e., the primary radical [Me.sub.2]C(CN)] as an end group (34). The end group hydrogens of the initiator fragment in PSB 16 or APE 17 appeared as a broad singlet at [delta]1.3 ppm (Fig. 4b). The area (A) at [delta]1.3 thus integrates for six protons of the initiator fragment, while the area (B) for rest of the signals at [delta]2-4 ppm belongs to all the 12 protons of the repeating units. The degree of polymerization (DP) is then calculated as the ratio of the area of a single proton of the repeating unit and initiator fragment [i.e., DP = (B/12)/(A/6)]. The number average M of PSB 16 was then obtained by multiplying the DP with the molar mass (255.30) of each repeating unit. The molar masses are reported in Table 2.
Solubility Behaviors of the Homo- and Copolymers
Like the overwhelming majority of reported PZs, PSB 11 was found to be insoluble in water (11), (23), (27). However, as anticipated for zwitterionic polymers, it was found to be soluble in the presence of added salts. The low molecular weight anions and cations of the added electrolyte enter and partially neutralize a portion of the intrachain interactions in PSB, thus allowing the collapsed coil in pure water to expand in the presence of small ions. For various salts, the CSCs required to promote water solubility of PSB 11 at 23[degrees]C are shown in Table 3. For a common cation, [K.sup.+], the sequence of increasing solubility power (i.e., decreasing order of CSC values) was found to be:
[I.sup.-] < [Br.sup.-] < [Cl.sup.-]
It is interesting to note that the order is the reverse of solubility power known for various PSBs (42), (23). The iodide anion is the most polarizable (soft), hence it is anticipated to be the most effective in neutralizing ionic crosslinks thus increasing the solubility of the PSB. We are at this stage unable to rationalize the apparent anomaly in the solubility behavior for the current polymer 11. There is a considerable difference in the concentration of common electrolytes required to promote solubility of PSB 11 having a C[H.sub.2]S[O.sub.3.sup.-]pendent. For instance, a much lower concentration of 0.638M KC1 is required to promote solubility of PSB 19 containing longer [(C[H.sub.2]).sub.3]S[0.sub.3.sup.-] pendent (42), whereas the CSC of KCI required for the solubility of 11 was found to be 2.26M (Scheme 5, Table 3). Increasing spacer length is thus found to decrease the CSC values. It is interesting to note that the corresponding PCB having a C[H.sub.2]C[O.sub.2.sup.-] pendent is readily soluble in salt-free water (34). The p[K.sub.a] of polyelec-trolytes having--N[H.sup.+][C[H.sub.2]C[O.sub.2]H motifs is reported to be 2.5 (34) and that of sulfonic acid group in--N[H.sup.+]C[H.sub.2]S[O.sub.3]H is expected to be or even lower than--2.1, the normal p[K.sub.a] of a sulfonic acid. The SB moiety (NI-1+C[H.sub.2]S[0.sub.3]-), having more dispersed charges and thus being less hydrated, is able to exert stronger zwitterionic interactions. The negative charges on the carbobetaines (N[H.sup.+]+C[H.sub.2]C[O.sub.2.sup.-]), having higher carboxyl p[K.sub.a] values are expected to be less dispersed, hence more hydrated (34), (41), (47) and as such tend to exhibit weaker Coulombic interactions with the cationic charges on the nitrogens (49) thus imparting solubility in salt-free water.
For various salts, the CSC required to promote water solubility of PSB 16 at 23[degrees]C are shown in Table 3. For a common cation, [K.sup.+], the sequence of increasing solubilizing power (i.e., decreasing CSC values) was determined to be:
[Cl.sup.-] < [Br.sup.-] < [I.sup.-]
To our knowledge, the zwitterionic interaction is the largest ever reported in literature for a PZ; it required a huge amount of KI (3.48M) to break the zwitterionic interaction and thus promote solubility. A much lower concentration of 0.201M KI was required to promote solubility of the corresponding PSB 20 containing longer [(C[H.sub.2]).sub.3]S[0.sub.3.sup.-] pendent (37). The intercharge distances of [Et.sub.3][N.sup.H][(C[H.sub.2]).sub.n]S[0.sub.3.sup.-] have been derived from their dipole moments and evaluated to be 3.90[angstrom] (for n = 2) and 4.80 [angstrom] (for n = 3) (50). Using calibration curve, the distance separating the charges of the unknown zwitterions (11 = 1) has been estimated to be 3.05[angstrom] which is lower than the minimum of 3.18[angstrom] required for the ion-pair conformation (50). The higher CSC values are consistent with the development of strong electrostatic attractive interactions between the charges of opposite algebraic signs as well as dipolar interactions promoting intragroup, intra-and interchain associations (Fig. 6) (2).
The linear increase of dipole moment [mu] with the increase in n values in [Et.sub.3.sup.N][(C[H.sub.2]).sub.n]S[0.sub.3] is more in favor of an extended conformation than a curled one. It has been reported that for n = 2 or 3, the zwitterion is expected to be practically in its fully extended conformation and has very little possibility of coiling especially in the case of n = 2. Increasing the number of methylene units between the charged centers increases the dipole moment and thus hydrophilicity and solubility (3), (50), (51). The higher CSC value for the polymer 16 (with a single methylene spacer between the charge centers separated by 3.05[angstrom]) than the corresponding copolymer 20 (37) having a spacer of three methylene units is attributed to the formation of intra group ion-pair (thus imparting more covalent character and lesser solubility in aqueous media). For the copolymer with n = 3, the intra--and interchain zwitterionic interactions seem to be the dominant interactions (Fig. 6).
Estimated p[K.sub.a] Values of the PSBs
We were unable to determine the p[K.sub.a] of SB 7 [i.e., Log (protonation constant K) of the conjugate base 61 by potentiometry as described elsewhere (34), (52). The gradual addition of titre HCl (0.1M) to a 2.5 X [10.sup.-3] M (molarity of the repeating units) solution (200 mL) of 6 led to a decrease in pH; however, considerable hydrolysis at around pH 6 led to higher pH values which cannot be justified by the amount of HCl added. As described in Scheme 2, hydrolysis of 6 led to the formation of basic diallylamine (5) which upon acid-base reaction with H20 generates O[H.sup.-] thereby increasing the pH. Similar hydrolysis was noted for DAMS, [Me.sub.2]N[H.sup.+]C[H.sub.2]S[O.sub.3.sup.-], which is reported (43) to have an approximate p[K.sub.a] value of 6.05 as determined using [.sup.1]H NMR technique. Monomer SB 7 is also expected to have a similar p[K.sub.a] value since both DAMS and 7 have single methylene spacer separating the zwitterions. The p[K.sub.a] value of 18 having a longer [(C[H.sub.2]).sub.3] spacer separating the zwitterions is reported to be 8.5 which is almost ~2.5 units higher than that of 7 (Scheme 5) (42).
Zwitterionic attraction is so prominent in PSB 16 that we were unable to determine the protonation constant, K, for the amine nitrogen in APE 17 using potentiometry (34), (52). Thus addition of even two drops of titre HCl (0.1M) to a 200 mL of 2.0 x [10.sup.-3] M solution (molarity of the repeating units) of 17 led to a cloudy solution followed by precipitation. Note that for the above solution containing 0.4 mmol of 17, a volume of 4.0 mL of 0.1M HCl (i.e., 0.4 mmol HCl) would be the acid equivalent to the basic nitrogens in it. The p[K.sub.a] values of monomer 18 and its S02-copolymer 20 are similar (Scheme 5) (37); as such the corresponding pair 7/16 is also assigned the same p[K.sub.a] value of 6.05. Based on the amine p[K.sub.a] value of 7.95 of 17 (i.e., 14--ammonium p[K.sub.a] value of 6.05 of 16), the following equilibration:
(17) + [H.sub.2]O [??] (16) + 0[H.sup.-]
is calculated to contain only 0.24 mol% of 16. A slight increase in its concentration led to precipitation thus demonstrating superior zwitterionic interactions operating in 16.
Comparative Stability of Homopolyrner 11 Versus
The question remains: why is homopolymer 11 unstable while copolymer 16 is stable? The reversible nature of the aldehyde/bisulfite addition reaction is well known. While the addition product 21 undergoes relatively faster desulfonation to 22, the acetyl protected 23 on the other hand is quite stable (Scheme 6). Since the electron push from the oxygen, as shown in the scheme, facilitates the removal of the sulfite group, the stability of 23 could be attributed to the lesser electron density on oxygen as a result of electron-withdrawing acetyl group. In a similar fashion, the stability of copolymer 17 (the conjugate base of 16) may thus be rationalized in terms of depleted electron charge density on the nitrogen as result of the presence of electron-withdrawing S[O.sub.2] units in the polymer backbone. Note that the presence of SO2 does indeed deplete electron density as indicated by the lower p[K.sub.a] value of copolymer 20 than that of homopolymer 19 by a significant 2.3 units (Scheme 5). The trivalent nitrogen in copolymer 17 is thus more electronegative hence less basic or nucleophilic than its counterpart in homopolymer 12, thereby imparting greater stability to the copolymer as it is unable to push the electrons required for decomposition to 26 as depicted in Scheme 6.
A convenient synthesis of the new zwitterionic monomer 7 has been achieved. The work described in this article represents the first example of a cyclopolymerization-derived PSBs having zwitterionic charges separated by a single methylene spacer. While the homopolymer 11 was unstable, the corresponding S[O.sub.2] copolymer 16 was found to be stable owing to the depleted electron charge density on the trivalent nitrogens in 17. The PSBs demonstrated very strong zwitterionic interactions as a result of the proximity of the charges of opposite algebraic signs leading to the formation of the ion-pairs.
The current challenge in the field of polymers is the construction of stimuli responsive advanced materials of PZs that have high added-value applications in drug delivery systems, separation materials, sensors, catalysts etc. Numerous applications of PZs also include their uses as fungicides, fire-resistant polymers, lubricating oil additives, emulsifying agents, bioadherent coatings (5), (51), (53), (54), drilling-mud additives (55), and chelator of toxic trace metals (Hg, Cd, Cu, and Ni) in wastewater treatment (2), (30). The current PSB's effectiveness in some of such applications has to be tested.
The authors acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST). Facilities provided by King Fahd University of Petroleum and Minerals are also gratefully acknowledged.
Correspondence to: Shaikh A. All; e-mail: firstname.lastname@example.org
Contract grant sponsor: Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM), the National Science, Technology and Innovation Plan; contract grant number: 11-ADV2132-04.
Published online in Wiley Online Library (wileyonlinelibrary.com).
[c] 2013 Society of Plastics Engineers
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NOMENCLATURE AIBN Azoisobutyronitrile APE Anionic polyelectrolyte APS Ammonium persulfate CSC Critical minimum salt concentration DAMS Dimethylammoniomethanesulfonate DMAEMA 2-(Dimethylamino)ethyl methacrylate DMSO Dimethylsulfoxide DP Degree of polymerization PA Polyampholyte PCB Polycarbobetaine PPB Polyphosphonobetaine PSB Polysulfobetaine PZ Polyzwitterion SB Sulfobetaine TBHP t-Butylhydroperoxide
Shaikh A. Ali, Othman Charles S.O. Al-Hamouz
Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
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|Author:||Ali, Shaikh A.; Al-Hamouz, Othman Charles S.O.|
|Publication:||Polymer Engineering and Science|
|Date:||Nov 1, 2013|
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