Scope of sulfur dioxide incorporation into alkyldiallylamine-maleic acid-SO2 tercyclopolymer

Article abstract of DOI:10.1039/c8ra08723g

Alternate copolymerization of diallylamine derivatives [(CH₂CH-CH₂)₂NR; R = Me, (CH₂)₃PO(OEt)₂, and CH₂PO(OEt)₂] (I)-maleic acid (MA) and (I·HCl)-SO₂ pairs have been carried out thermally using ammonium persulfate initiator as well as UV radiation at a λ of 365 nm. The reactivity ratios of ≈0 for the monomers in each pair I-MA and I·HCl-SO₂ ensured their alternation in each copolymer. However, numerous attempted terpolymerizations of I-MA-SO₂ failed to entice MA to participate to any meaningful extent. In contrast to reported literature, only 1-2 mol% of MA was incorporated into the polymer chain mainly consisting of poly(I-alt-SO₂). Quaternary diallyldialkylammonium chloride [(CH₂CH-CH₂)₂N⁺R₂Cl^-; R = Me, Et] (II) also, did not participate in II-MA-SO₂ terpolymerizations. Poly((I, R = Me)-alt-SO₂) III is a stimuli-responsive polyampholyte; its transformation under pH-induced changes to cationic, polyampholyte-anionic, and dianionic polyelectrolytes has been examined by viscosity measurements. The pK_a of two carboxylic acid groups and NH⁺ in III has been determined to be 2.62, 5.59, and 10.1. PA III, evaluated as a potential antiscalant in reverse osmosis plants, at the concentrations of 5 and 20 ppm, imparted ≈100% efficiency for CaSO₄ scale inhibition from its supersaturated solution for over 50 and 500 min, respectively, at 40 °C. The synthesis of PA III in excellent yields from cheap starting materials and its very impressive performance may grant PA III a prestigious place as an environment-friendly phosphate-free antiscalant.

Full text of DOI:10.1039/c8ra08723g

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Scope of sulfur dioxide incorporation into
alkyldiallylamine–maleic acid–SO2
tercyclopolymer†
Cite this: RSC Adv., 2018, 8, 38891
*
*
Ibrahim Y. Yaagoob, Shaikh A. Ali,
and Mohammad A. J. Mazumder
Hasan A. Al-Muallem
Alternate copolymerization of diallylamine derivatives [(CH2]CH–CH2)2NR; R ¼ Me, (CH2)3PO(OEt)2, and
CH2PO(OEt)2] (I)–maleic acid (MA) and (I$HCl)–SO2 pairs have been carried out thermally using ammonium
persulfate initiator as well as UV radiation at a l of 365 nm. The reactivity ratios of z0 for the monomers in
each pair I–MA and I$HCl–SO2 ensured their alternation in each copolymer. However, numerous attempted
terpolymerizations of I–MA–SO2 failed to entice MA to participate to any meaningful extent. In contrast to
reported literature, only 1–2 mol% of MA was incorporated into the polymer chain mainly consisting of poly(I-
alt-SO2). Quaternary diallyldialkylammonium chloride [(CH2]CH–CH2)2N+R2Clꢀ; R ¼ Me, Et] (II) also, did not
participate in II–MA–SO2 terpolymerizations. Poly((I,
R
¼
Me)-alt-SO2) III is a stimuli-responsive
polyampholyte; its transformation under pH-induced changes to cationic, polyampholyte-anionic, and
dianionic polyelectrolytes has been examined by viscosity measurements. The pKa of two carboxylic acid
groups and NH+ in III has been determined to be 2.62, 5.59, and 10.1. PA III, evaluated as a potential
antiscalant in reverse osmosis plants, at the concentrations of 5 and 20 ppm, imparted z100% efficiency for
CaSO4 scale inhibition from its supersaturated solution for over 50 and 500 min, respectively, at 40 ꢁC. The
synthesis of PA III in excellent yields from cheap starting materials and its very impressive performance may
grant PA III a prestigious place as an environment-friendly phosphate-free antiscalant.
Received 21st October 2018
Accepted 13th November 2018
DOI: 10.1039/c8ra08723g
rsc.li/rsc-advances
several patents12–14 to give terpolymers in respective ratios of
50 : 25 : 25 (ref. 12) and 70 : 25 : 5.14 However, the patents do not
describe any micro-structural analysis. To our knowledge, no
journal article has appeared to date to describe such terpolymeri-
zation involving A–maleic acid–SO2 to give E (Scheme 1).
Amino-carboxylate and -phosphonate are important anti-
scalant and metal chelation motifs; as such, the decoration of
repeating units with a high density of these potent functional
groups is expected to impart, for instance, an antiscalant
behavior. Looking at the zero reactivity ratios for both the
1. Introduction
Butler's cyclopolymerization protocol1 has etched a place of
distinction in the synthesis of polymers of tremendous techno-
logical importance.2–4 Free radical-initiated polymerization of dia-
llylammonium salts A led to B having pyrrolidine rings embedded
into the macromolecules (Scheme 1). The macromolecule B
represents the eighth most important architecture among
synthetic high polymers.5,6 Polydiallyldimethylammonium chlo-
ride B (R1]R2]Me), alone accounting for over 1000 patents and
publications, has been produced to the tune of millions of pounds
for a variety of industrial applications including water treatment.1
Sulfur dioxide has been extensively used to synthesize alternate
cyclocopolymers C having numerous applications.7–9 The reactivity
monomer pairs: rA z rSO z 0 and rA z rMA z 0 (Scheme 1), we
2
do fancy the success of the terpolymerization to pave the way to
a great many interesting tripolymers E. Herein, we report
a systematic study involving the cyclopolymerization of A, A–
SO2, A–MA, and A–MA–SO2 to investigate the course of action of
an amine terminal radical –Ac whether it adds exclusively or
preferentially onto SO2 or MA or both.
ratios of amine (salt) (rA) and rSO being zero led to the alternation
2
in C. Likewise, the almost zero reactivity ratios of rA and maleic
acid (MA) (rMA) led to the exclusive formation of alternate copol-
ymer D from alkyldiallylamine/MA ion pair A.10,11 Interestingly,
diallyldimethylammonium chloride (A)–MA–SO2 is reported in
2. Experimental section
2.1 Materials
Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran
31261, Saudi Arabia. E-mail: shaikh@kfupm.edu.sa; Web: http://faculty.kfupm.edu.
sa/CHEM/shaikh/; Fax: +966 13 860 4277; Tel: +966 13 860 3830
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra08723g
Paraformaldehyde was purchased from Fluka Chemie AG. Dia-
llylamine from Merck Schuchardt OHG, ammonium persulfate
(APS), azobisisobutyronitrile (AIBN) and maleic acid from Sigma-
Aldrich were used as received. Diallyldimethylammonium
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Scheme 1 Synthesis of homocyclopolymer and alternate copolymers from monomeric pairs A/maleic acid and A/SO2 and terpolymerization of
1 or 5/MA/SO2.
chloride (15a) from Sigma-Aldrich, obtained as 65 wt% solution, measured in D2O on a JEOL LA 500 MHz spectrometer. The
was freeze-dried and crystallized from methanol/acetone. residual HOD signal of D2O at d 4.65 ppm and dioxane 13C
Phopsphonate ester 1a,15 1b,16 and 5 (ref. 17) were prepared as resonance at d 67.4 were used as internal and external references,
described.15–17 For dialysis, Spectra/Por membrane (MWCO: respectively. Viscosities were measured using an Ubbelohde
6000–8000) from Spectrum Laboratories Inc. was utilized.
viscometer (viscometer constant ¼ 0.005718 cSt sꢀ1). An SDT
analyzer (Q600: TA instruments, USA) was utilized for Thermog-
ravimetric analysis (TGA) under N2 owing at a rate of 50
cm3 minꢀ1. For photoinitiated polymerization, the samples were
irradiated at 365 nm using UV Lamp 365 nm/254 nm SPEC-
TROLINE (Model: ENF-240C) from Spectronic Corp., Westbury,
NY, USA, with LONGLIFE Filter to ensure highest UV intensity
and Fluorescence Analysis Cabinet SPECTROLINE CM-10A.
2.2 Physical methods
Elemental analysis was carried out in a Perkin Elmer Elemental
Analyzer Series 11 Model 2400 (Waltham, Massachusetts, USA).
IR spectra were recorded on a Thermo Scientic FTIR spec-
trometer (Nicolet 6700, Thermo Electron Corporation, Madison,
WI, USA). 1H and 13C NMR spectra of the polymers were
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distilled water for 6 h. The dialyzed solution was acidied with
concentrated HCl (1.0 mL) and freeze-dried to obtain 4a. Note
that the acidication was intended to keep the nitrogens and
carboxyls protonated. The FTIR and NMR spectra of the polymers
resembled with the spectra of 1a$HCl/SO2 and 1b HCl/SO2
copolymers 4a20 and 4b21 (y z 0), respectively.
For the polymerization involving monomer 5,17 the crude
reaction mixture was dialyzed against 2 M HCl followed by
deionized water. Polymer 6 (y z 0), precipitated out during
dialysis, was dried under vacuum. The spectral analysis revealed
the identical nature of the material 6 (y ¼ 0) described
elsewhere.20
2.4.2 Free radical polymerization of monomer 8. A solution
of salt 8 (8.86 g, 60 mmol) in water (3.5 g) and concentrated HCl
(36 wt%) (0.5 g) in an RB ask (50 mL) tted with a condenser
was degassed with N2 and then heated to 100 ꢁC. APS was added
in three portions (3 ꢂ 250 mg) with intervals of 3 min (Scheme
2). The temperature of the exothermic polymerization rose to
110 ꢁC. The reaction mixture was continued to stir at the oil
bath temperature of 100 ꢁC for an additional 30 min. The
product mixture was then dialyzed against distilled water and
freeze-dried to obtain polymer 9 (7.7 g, 87%) as a white powder.
(Found: C 56.7; H 9.58; N 9.4%. C7H14ClN requires C 56.94; H
9.56; N 9.49%); vmax (KBr): 3500 (broad), 2946, 2724, 1649 (H2O
bending), 1461, 110ꢁ7, 1053, 973, 909, 628, and 483 cmꢀ1. [h]
0.0805 dL gꢀ1 at 30 C in 0.1 M NaCl.
2.3 Synthesis of methyldiallylamine (7) and its hydrochloride
salt 8
The procedure for the methylation of amines is adapted from
the literature.18 Formic acid (35 g, 85%, 0.646 mol) was added
slowly (30 min) onto an ice-cold diallylamine (1) (49 g, 0.506
mol) in a 1 L RB ask under stirring. Paraformaldehyde (18.3 g,
0.613 mol) and water (33 mL) were added to the ask. The
mixture in the ask, tted with a long condenser, was stirred at
75–77 ꢁC during which CO2 evolution happened (30 min).
ꢁ
Thereaer, the reaction mixture was heated to 90 C; aer the
gas evolution stopped (15 min), the reaction mixture was cooled
and saturated with the addition of a solution of NaOH (30 g,
0.75 mol) in water (30 mL). The separated organic layer of amine
7 was dried over NaOH and distilled at 107 ꢁC (lit. 19 bp 109 ꢁC)
to obtain 7 as a colorless liquid (49 g, 87%).
A dry HCl gas was passed to a solution of methyldiallylamine
(7) (25 g, 0.225 mol) in ether (150 mL) at 0 ꢁC. Immediate
precipitation of white salt 8 occurred. The passage of HCl was
continued until the supernatant liquid becomes clear instead of
cloudy. Aer ltration, the white salt was dried under vacuum to
obtain 8 (30 g, 90%). dH (D2O) 2.70 (3H, s), 3.65 (4H, d, J 7.3 Hz),
5.50 (4H, m), 5.82 (2H, m); dC (D2O) 39.57 (1C, Me), 58.30 (2C,
NCH2), 126.60 (2C, ]CH), 127.33 (2C, ]CH2). The 13C spectral
assignments was supported by DEPT 135 NMR analysis.
2.4 Polymer syntheses
2.4.3 Photochemically initiated polymerization of mono-
mer 8. A sample of monomer 8 (200 mg, 1.4 mmol) in a vial was
exposed to UV radiation at room temperature for 4 days. No
polymerization occurred as indicated by 1H NMR spectrum. Like-
wise, a solution of monomer 8 (295 mg, 2.0 mmol) in water (120
mg) was exposed to UV radiation at room temperature for 4 days
2.4.1 Procedure for 1 or 5/2/SO2 terpolymerization. Table 1
describes the polymerizations conditions (Scheme 1). For exper-
iment under entry 1, SO2 (705 mg, 11 mmol) was absorbed in
a solution of monomer 1a (1.38 g, 5.0 mmol) and MA 2 (0.70 g, 6.0
mmol) in DMSO (2.6 g) in an RB ask (10 mL). Aer purging the
mixture with N2, initiator APS (0.260 g) was added to the stirring
mixture in the closed ask at 21 ꢁC for 24 h. The polymer mixture
was dialyzed against 0.2 M HCl for 24 h with frequent change of
0.2 M HCl. Finally, the polymer solution was dialyzed against
1
giving 9 only in 5% yield as revealed by H NMR spectrum. The
yield was improved to 7.6% aer further irradiation for 2 days.
2.4.4 Copolymerization of 8 with SO2. To a solution of
monomer 8 (5.02 g, 34 mmol) in DMSO (8.4 g) was absorbed SO2
Table 1 Attempted cycloterpolymerizationa of monomers 1a, 1b and 5 with maleic acid 2 and SO2
Entry
Amine 1 (mmol)
MA 2 (mmol)
SO2 mmol
Initiatorc (mg)
Temp.
Yieldb
[h]c (dL gꢀ1
)
1
2
3
4
5
6
7
8
1a: 5
1a: 5
1a: 5
1a: 5
1a: 5
1a: 5
1a: 5
1a: 5
1a: 5
1b: 5
5: 5
6
6
6
11
11
11
16
11
11
11
11
11
11
11
11
5
APS (260)
APS (70)
APS (260)
APS (260)
APS (96)
APS (210)
APS (250)
AIBN (46)
AIBN (100)
APS (140)
APS (200)
AIBN (50)
APS (140)
TBHP (100)
21
45
45
45
65
65
65
65
65
65
65
65
4a: 39 (67)
4a: 34 (68)
4a: 48 (86)
4a: 62 (90)
4a: 11 (72)
4a: 53 (60)
4a: 48 (81)
4a: trace (28)
4a: 27 (60)
4b: 49 (72)
6: 61 (67)
0.0721
0.0562
0.0571
0.0462
0.0401
0.0454
0.0228
—
0.0211
0.0355
0.532
0.519
—
10
6
6
6
6
6
6
6
6
6
6
9
10
11
12
13
14
5: 5
—
—
6: 67 (71)
No polymer
No polymer
45
5
Temp.d
—
a
Copolymerization reactions were carried out in DMSO (2.6 g) in the presence of ammonium persulfate (APS) or azoisobutyronitrile (AIBN) or tert-
butylhydroperoxide for 48 h. Polymer obtained is written in bold followed by isolated yields; the percent conversion determined by 1H NMR
b
c
analyses of the crude reaction mixture are written in parentheses. Viscosity of 1–0.25% polymer solution in 0.1 M NaCl at 30 ꢁC was measured
with Ubbelohde viscometer (K ¼ 0.005317 mm2 sꢀ2). At ꢀ10 ꢁC for 2 days, then at 0 and 25 ꢁC for 2 days each.
d
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Scheme 2 Synthesis of homopolymer from 8, alternate copolymers from 7/MA, 15/MA, 8/SO2, 15/SO2, and terpolymers from 7 or 8/MA/SO2.and
15/MA/SO2.
(2.18 g, 34 mmol). Aer adding initiator AIBN (200 mg), the
reaction mixture was heated at 65 C inside a closed vessel for
2.4.5 Methyldiallylamine (7) and maleic acid (2) ion pair
12. Maleic acid 2 (1.16 g, 10.0 mmol) was added to a biphasic
mixture of amine 7 (1.11 g, 10.0 mmol) in water (8 mL) and
stirred at 25 ꢁC for 30 min. Exothermic neutralization led to
a homogeneous solution which was freeze-dried to obtain ion
pair 12 as a colorless liquid in quantitative yield. vmax (neat):
3496 (strong), 3375 (strong and broad), 3031, 2962, 2656
(R3NH+), 2516 (R3NH+), 2356 (R3NH+), 1704, 1623, 1581, 1477,
1363, 1202, 1082, 988, 869, 710, 653, and 568 cmꢀ1.dH (D2O) 2.55
(3H, s), 3.43 (2H, ABX, J 7.5, 13.4 Hz), 3.57 (2H, ABX, J 7.0, 13.4
ꢁ
24 h. The resultant polymer mixture was dialyzed against
distilled water. The dialyzed solution was treated with 2.0 g
concentrated HCl (37%, 20 mmol) and then freeze-dried to
obtain a white powder of copolymer 10 (6.7 g, 93%). [h] 0.276 dL
gꢀ1 at 30 C in 0.1 M NaCl. (Found: C 39.4; H 6.8; N 6.5%. C7-
ꢁ
H14ClNO2S requires C 39.71; H, 6.67; N 6.62%); vmax (KBr): 3452
(broad), 2955, 2719, 1636 (H2O bending), 1466, 1413, 1304,
1128, 1036, 963, 856, 773, and 517 cmꢀ1
.
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Hz), 5.35 (4H, m), 5.67 (2H, m), 6.08 (2H, s); dC (D2O) 39.37 (1C, by dialysis against distilled water for 6 h. The resulting solution
Me), 58.07 (2C, NCH2), 126.37 (2C, ]CH), 127.19 (2C, ]CH2), freeze-dried to obtain polymer 16. However, the dialyzed solution
135.09 (CH]CH), 171.14 (C]O). The 13C spectral assignments of 18, aer acidication with concentrated HCl (1.0 mL), was
were supported by DEPT 135 NMR.
freeze-dried. (Acidication was carried out to keep the carboxyl
2.4.6 Copolymerization of 7 and MA 2. In an RB ask tted groups protonated). In the case where MA was not involved, the
with a condenser, were taken amine 7 (2.22 g, 20.0 mmol), MA 2 polymer 17 was simply puried by dialysis against distilled water.
(2.54 g, 22.0 mmol) and water (2.04 g). The homogeneous The FTIR and NMR spectra of the polymer 18 resembled with the
ꢁ
mixture was stirred at 100 C under N2, and initiator APS was copolymer 17, thereby implying the value of y z 0.
added (2 ꢂ 200 mg) onto the mixture over 4 min. The
exothermic polymerization started, and the mixture was stirred
at 100 ꢁC (10 min), cooled, dialyzed against deionized water,
and freeze-dried to obtain a white powder of alternate poly-
ampholyte (PA) 13 (3.68 g, 81%). The crude mixture revealed the
monomer conversion of 86% as indicated by 1H NMR spectrum.
(Found: C 57.8; H 7.7; N 6.0%. C11H17NO4 requires C 58.14; H
7.54; N 6.16%). vmax (KBr): 3455 (broad), 2949, 2729, 2357, 1714,
2.5 Determination of basicity constants by titrations
The basicity constant (K) of the three basic centers in PA 13 were
determined using reported procedures11,22 and conditions
described in Table S1.† log Kis were calculated using the Hen-
derson–Hasselbalch equation (eqn (2); Scheme 3).
1635, 1574, 1464, 1391, 1207, 1053, 810, 661 and 566 cmꢀ1
.
2.6 Use of synthesized PA 13 as an antiscalant
2.4.7 Procedure for 7 or 8/2/SO2 terpolymerization. Table 2
describes the polymerizations conditions. The polymerizations
were carried out as described under Section 2.4.1. Aer
completion of the polymerization, the crude mixture was dia-
lyzed against 0.2 M HCl with frequent change of the 0.2 M HCl
for a duration of 24 h, and nally, dialyzed against distilled
water for 6 h. The resulting solution was acidied with
concentrated HCl (1.0 mL) and freeze-dried to obtain polymer
14. Note that the acidication was carried out to keep the
nitrogens and carboxyls protonated. The FTIR and NMR spectra
of the polymer 14 resembled with the spectra of 7 HCl/SO2
copolymer 10, thereby implying the value of y z 0.
2.4.8 Procedure for 15/2/SO2 co- and terpolymerization.
Table 3 describes the polymerizations conditions (Scheme 2). The
polymerizations of monomers 15 were carried out as described
under Section 2.4.1. Aer the elapsed time, the reaction mixture
(involving MA) was dialyzed against 0.5 M HCl for 24 h with
frequent change of the 0.5 M HCl to remove excess MA, followed
The inhibition of CaSO4 scale formation from its supersaturated
solution was examined in the presence of various concentrations
of PA 13 in an aqueous solution containing Ca2+ (2600 mg Lꢀ1
)
and SO4 (6300 mg Lꢀ1) at 40 ꢃ 1 ꢁC as described.22 The
conductivity decreased suddenly aer an induction time, owing
to the formation of scale as conrmed by visual inspections.
2ꢀ
3. Results and discussion
3.1 Polymer synthesis
The results of APS-initiated terpolymerization of monomers 1a
or 1b or 5 with SO2 and MA 2 are given in Table 1 (Scheme 1).
The attempted copolymerization of MA–SO2 was unsuccessful
(entries 13 and 14). Terpolymerization of 1a–MA–SO2 was con-
ducted under various conditions using APS or AIBN initiator.
The polymer 4a was isolated aer extensive dialysis against
aqueous HCl to remove unreacted MA and replace the maleate
anion with Clꢀ ions. The spectral (NMR and IR) analysis
Table 2 Attempted cycloterpolymerization of monomer 7 or 8 with SO2 and maleic acid 2
Entry
7 or 8 (mmol)
MA 2 (mmol)
SO2 (mmol)
DMSO (g)
Initiator (mg)
Temp (ꢁC)
Time (h)
Yielda
[h]b (dL gꢀ1
)
1c
7: (5)
7: (5)
7: (5)
7: (5)
7: (5)
7: (5)
7: (5)
8: (1)
8: (5)
7: (5)
8: (5)
8: (5)
8: (5)
8: (5)
8: (5)
8: (5)
6
6
11
11
16
16
16
31
11
25
5
2.3
2.3
2.3
2.3
2.3
2.8
2.3
4.6
0
0
0
0
APS: 85
APS: 150
APS: 85
APS: 175
APS: 260
APS: 260
AIBN: 75
APS: 360
Dark
UV
UV
UV
UV
45
45
45
45
65
65
65
45
23
23
23
72
72
72
48
24
24
36
48
24
168
24
4
24
6
24
144
14: 21 (63)
14: 31 (57)
14: 23 (63)
14: 47 (60)
14: 32 (83)
14: 62 (85)
14: 0 (10)
14: 51 (67)
14: 0 (0)
14: 26 (43)
14: 45 (62)
14: 43 (67)
14: 43 (83)
14: 15 (35)
10: 54 (72)
9: — (7.6)
0.141
0.125
0.121
0.182
0.129
0.172
—
2c
3
4
5
11
11
11
22
6
10
5
5
5
5
5
5
6
7c
8d
9d
10d
11e
12e
13e
14
15
16
—
—
6
0.278
0.329
0.273
0.471
0.192
0.836
—
10
10
10
10
4.5
0
ꢀ15
1.0
MeOH: 1.0
0
23
ꢀ10
23
UV
UV
UV
0
0
H2O: 0.3
23
a
Isolated yields is followed by the percent conversion written in parentheses. As determined by 1H NMR analyses of the crude reaction mixture.
b
c
Viscosity of 1–0.25% polymer solution in 0.1 M NaCl at 30 ꢁC was measured with Ubbelohde viscometer (K ¼ 0.005317 mm2 sꢀ2). Cloudy
reaction mixture aer 4 h. MA remained partially soluble. e Solidied.
d
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Table 3 Attempted cycloterpolymerization of monomers 15 with SO2 and maleic acid 2
Entry
15 (mmol)
MA 2 (mmol)
SO2 (mmol)
DMSO (g)
Initiator (mg)
Temp (ꢁC)
Time (h)
Yield (%)
[h]a (dL gꢀ1
)
1
2c
3
4
5
6
7
15a: 5
15a: 5
15a: 5
15a: 5
15b: 5
15b: 5
15b: 5
6
—
6
6
—
—
6
—
6
11
6d,e
6
H2O (1.0)
0
APS: 250
UV
APS: 215
UV
APS: 250
AIBN: 29
APS: 125
100
0.25
48
24
48
2
16a: 27b
17a: 80
18a: 87
18a: 71
17b: 96
17b: 87
18b: 88
0.102
1.86
0.317
1.67
0.552
0.253
0.401
23
55
23
55
55
55
3.0
0
2.6
2.6
2.8
6
10
72
48
a
b
Viscosity of 1–0.0625% polymer solution in 0.1 N NaCl at 30 ꢁC was measured with Ubbelohde viscometer (K ¼ 0.005317 mm2 sꢀ2). 98%
conversion as revealed by 1H NMR of crude mixture. White solid mixture in the beginning and no phase change throughout. Not been able
c
d
to add more SO2. e Slightly turbid in the beginning but hardened towards the end.
revealed that terpolymer 4a resembled the 1a–SO2 copolymer 4a
Likewise, 1b–MA–SO2 (entry 10) and 5–MA–SO2 (Table 1,
(y ¼ 0),20 except a minor 13C C]O peak at zd 180 ppm (vide entries 11 and 12) terpolymerizations afforded 4b and 6,
infra) and a minor vibration at z1728 cmꢀ1 attributed to C]O respectively (Scheme 1), whose spectra are identical to those of
of CO2H in the IR spectrum. The absence of alkene carbons at 4b (y ¼ 0)21 and 6 (y ¼ 0),20 except that a very minor peak due to
d 135 ppm excluded the presence of MA either as a free acid or C]O in the 13C NMR and IR spectra indicate the very minor
the maleate ions; however, the presence of minor C]O signals incorporation of MA.
suggests the incorporation of maleic acid into the polymer
Monomer 8 was homo- and co-polymerized with SO2 to
backbone. The MA does undergo propagation, but the addition obtain cationic polyelectrolytes 9 and 10 in excellent yields of 87
is reversible as indicated by the presence of fumaric acid (FA) and 93%, respectively (see Experimental) (Scheme 2). Homo-
(trans-HO2CC]CHCO2H) in the unreacted reaction mixture, in polymerization of 8 in an aqueous solution under UV light (l ¼
addition to its original cis-counterpart MA in a z 1 : 4 ratio. The 365 nm) was very slow; aer 6 days, it gave CPE 9 in a very low
radical having MA terminal: R–CH(CO2H)–CH(CO2H)$ may yield of 7.6% (entry 16, Table 2). The bulk photo-
undergo quick reversion to give R$ and MA as well as FA.
copolymerization 8–SO2 (entry 15, Table 2) in the absence of
Scheme 3 pH Induced changes in the charge types in the backbone of 13, 19–21.
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solvent gave CPE 10 having much higher intrinsic viscosity [h] of
0.836 dL gꢀ1 than that of the thermally initiated copolymer with
a [h] of 0.276 dL gꢀ1 (Experimental section 2.4.4).
3.2 Solubility behavior
Cationic polymers 4a, b (y z 0), 9, 10 and 14 (y z 0) are water-
soluble, while the PZ 6 (y z 0) is insoluble in water but soluble
in 0.8 M HCl.20 The water-solubility of 13, unlike most electro-
neutral polymers,27–29 could be attributed to the less effective
Terpolymerization of 7 or 8–MA–SO2 was extensively studied
using free radical initiators APS, AIBN or UV radiation at l
365 nm (Scheme 2). The results are included in Table 2. Note
that the isolated yields aer dialysis are low as compared to the
NMR conversion since the low molecular weight polymer frac-
tions leave the dialysis bag. The 1H and 13C NMR spectra of the
samples from entries 1–14 dealing with the 7 or 8–MA–SO2
terpolymerizations afforded 14, which is almost identical to the
8–SO2 copolymer 10 except the IR has very minor C]O vibra-
tional peak at z1728 cmꢀ1 indicating the presence of MA
repeating units in 14. A series of IR spectra of copolymer 10
containing 0–5 mol% succinic acid (CH2)2(CO2H)2 helped us to
estimate the proportion of MA unit in 14 as z1–3 mol%. The
strong non-overlapping IR vibrations of SO2 unit in 10 at 1304
and 1128 cmꢀ1 and the C]O vibration of CO2H at z1728 cmꢀ1
were used to estimate the MA mol%.
Next, we focussed our attention to the polymerization of
quaternary ammonium salts 15 as outlined in Scheme 2; the
polymerization conditions and the results are included in
Table 3. The 15a–MA copolymerization afforded 16a in 27%
isolated yields, while the crude mixture before dialysis indi-
cated 98% conversion as calculated from 1H NMR spectrum
(entry 1). The spectral data matched with that reported for the
polymer.23 Bulk copolymerization of 15a–SO2 under UV light24
afforded copolymer 17a in excellent yield (entry 2). The very
high [h] value of 1.86 dL gꢀ1 indicated the high molar mass of
the polymer. The thermal 15b–SO2 (ref. 19) copolymerization
afforded copolymer 17b also in excellent yields (entries 5 and
6). The IR and NMR spectra of 17a, b matched with the re-
ported data.25,26 The terpolymerizations: 15a–MA–SO2 (entries
3 and 4) and 15b–MA–SO2 (entry 7) gave polymers 18a and 18b,
respectively; their NMR and IR spectra matched with those of
the respective copolymers 17a and 17b except the minor C]O
vibration at z1728 cmꢀ1 indicating the incorporation of z1–
3 mol% MA.
ꢀ
participation of the crowded CO2 to undergo electrostatic asso-
ciations with NH+ for manifestation of ampholytic interactions.2
Moreover, the hydrophilicity and solubility of the ampholytic
motifs increase as a result of the increased dipole moment (m)
ꢀ
because of the large separation between NH+ and CO2
.
30,31
3.3 Spectroscopic characterization
The strong IR bands at 1304 and 1128 cmꢀ1 were assigned to the
vibrations of SO2 unit in 10. The absorption band at z1714 and
1574 cmꢀ1 is attributed to the C]O stretch of CO2H and CO2
ꢀ
in (+c–) 13.32
The NMR spectra of several polymers are displayed in Fig. 1
and 2. For the monomer pair 12, alkene protons of the diallyl
amine part and maleate motif appeared at d 5.5–6 ppm and
6.08 ppm, respectively (Fig. 1a), while the corresponding
carbons are displayed at d 126.4 (]CH), 127.2 (2C, ]CH2), and
135.1 (CH]CH) (Fig. 2a). The low molar masses (as suggested
by low viscosity values) and the absence of spectral signal for
residual alkene protons or carbons of the polymers 9, 10 and 13
are indicative of degradative chain transfer33 involving allylic
hydrogens or a termination reaction by coupling.34 The compact
coil10 conformation of MA-copolymer (+c–) 13 is reected in the
broadened 1H as well as 13C NMR spectra (Fig. 1d and 2d),
whereas the spectral signals are found to be sharp for homo-
polymer 9 or SO2-copolymer 10 (Scheme 2) (Fig. 1b or 1c). The
incorporation of MA in (+c–) 13 is conrmed by the presence of
a broad signal around d 180 ppm (Fig. 2d). Elemental analysis
and the spectral data conrmed the formation of the alternate
copolymer because both the reactivity ratios ramine and rMA, are
expected to be close to zero.10,11 Integration of the relevant
carbon signals35,36 revealed a 3 : 1 cis/trans ratio of the ring
substituents at Cb,b (Scheme 2).
All the reactivity ratios in the pairs: amine salt–MA and
amine salt–SO2 are almost zero (vide supra) thereby implying
that under the reaction conditions, amine salt or MA or SO2
3.4 TGA and viscosity behaviors PA (+c–) 13
cannot undergo homopolymerization, instead they must give TGA plot for a PA 13 (dried under vacuum at 55 ꢁC for 6 h)
alternate copolymers as experimentally observed. MA–SO2 revealed the weight losses at various temperature ranges (Fig. 3a).
reaction has been shown to give neither homo- nor co-polymer The presence of adjacent CO2H groups would lead to the
(entries 13 and 14, Table 1). In the numerous amine salt–MA– formation of anhydride motifs with the loss of water as reected
SO2 terpolymerizations, the extent of MA incorporation is only by an 8.3% weight loss up to 200 ꢁC. In fact, a loss of 1 molecule
1–2 mol%. In every instance, amine salt–SO2 copolymer was of water per repeating unit is calculated to be 7.9%. In the 200–
ꢁ
obtained and no trace of amine salt–MA copolymer could be 380 C range, maleic anhydride is itself released from the poly-
identied. The claim in the patented literatures12–14 that the mer backbone thereby accounting for the loss of z41.2% as
formation of terpolymer: diallyldimethylammonium chloride– compared to a calculated value of 46.9%.10,37 A 48% loss in the
maleic acid–SO2 having repeating unit composition of 380–800 ꢁC range is attributed to the removal of the amine units.
50 : 25 : 25 and 70 : 25 : 5 are doubtful. A composition ratio of
The viscosities of PA (+c–) 13 are presented in Fig. 3b. The
50 : 25 : 25 may indeed be a result of a mixture of two alternate increase in viscosity with the increasing salt concentrations
copolymers of diallyldimethylammonium chloride–maleic acid demonstrates the antipolyelectrolytic38 nature of the poly-
and diallyldimethylammonium chloride–SO2. The current work ampholyte [cf. Fig. 3b(i–iv)]. The ampholytic dipole is not perfectly
conrms that the amine salt–MA–SO2 cannot be terpolymerized electroneutral, rather has a residual negative charge on CO2ꢀ since
to any meaningful extent.
Na+ cannot effectively neutralize it as compared to the greater
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Fig. 1 1H NMR spectrum of (a) 12, (b) 9, (c) 10, and (d) 13 in D2O.
shielding of N+ by Clꢀ.39–41 The magnitude of the excess negative
charge, hence the viscosity, increases with salt concentrations.
The intrinsic viscosities of the synthesized polymers 4a, 4b,
6, 9, 10, 13, 14, 16–18 are reported in the Tables 1–3. Since the
PA (+c–) 13 becomes a cationic polyelectrolyte 19 by proton- MA incorporation is very minimal, 4a, 4b and 6 with the value
ation of CO2ꢀ and as such its viscosity becomes higher in 0.1 M of y z 0 resembled the copolymers 4a, 4b and 6 (y ¼ 0) whose
HCl because of repulsion among positive charges (Fig. 3b-v) molar mass and [h] are amply described.20,21 Similarly, poly-
(Scheme 3). The neutralization of 13 with one or two equivalents mers 17a, b which are similar to 18a, b (y z 0)23,25,26 and 16a32
of NaOH leads to polyampholyte/anion (+c]) 20 and poly- have been extensively characterized in terms of viscosity and
dianion (]) 21, the greater charge imbalance in the latter molar mass.
makes it more viscous (Fig. 3b-vi and vii).
Strong interactions of materials of GPC columns with
functional groups like amine and CO2 prevented the deter-
ꢀ
3.5 Basicity constant
mination of molar masses of the polymers like 13. Similar
observation was reported earlier.10 Purication by extensive
dialysis with membrane of MWCO of 6000–8000 daltons
indicates the products as true polymers not oligomers. The
use of higher initiator dose along with degradative chain
transfer termination process33 led to PA 13, having low molar
mass as indicated by its lower [h] value.
As outlined in Scheme 3, the protonation constant K of three
basic centers in (]) 21 are calculated using eqn (3) where log Kꢁ
¼ pH at a (degree of protonation) ¼ 0.5. The variation from the
true basicity constant depends on the value of n which equals 1
for small molecules. The values of ‘n’ and log Kꢁ were extracted
from the slope and intercept of the pH vs. log[(1 ꢀ a)/a] straight-
line plot. The respective log K1ꢁ ; log K2ꢁ and log K3ꢁ were
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Fig. 2 13C NMR spectrum of (a) 12, (b) 9, (c) 10, (d) 13, and (e) 4a in D2O.
determined to be 10.1, 5.59 and 2.62, with the corresponding n1, log K2 and log K3, respectively, with increasing a, while the
n2 and n3 values of 1.15, 2.19 and 0.48 (Table S1†). log K1 remains almost constant since the associated n value of
All the n values, especially n2 and n3, are reective of the 1.15 is not far from 1.
“apparent”42,43 nature of the Kis which changes with the a as
With increasing a, the equilibrium: [(ZH+c]) 20 + H+
!
shown in Fig. 3c. The n value reects a measure of the poly- (ZH2+c–) 13] is shied to the right thereby decreasing log K2ꢁ as
electrolyte index. There are signicant decrease and increase of a consequence of decreasing negative charge density that
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Fig. 3 (a) TGA curves of 13; (b) the viscosity behavior at 30 ꢁC of polyampholyte 13 (i) in salt-free water, (ii) in 0.1 M NaCl, (iii) in 0.5 M NaCl, (iv)
in 1.0 M NaCl, (v) in 0.1 M HCl, and (vi) in the presence of 1 equiv. NaOH in 0.1 M NaCl, and (vii) in the presence of 2 equiv. NaOH in 0.1 M NaCl; (c)
plot for the apparent log Ki versus degree of protonation (a) in salt-free water (log K3, run 2, Table S1†), (log K2, run 2, Table S1†) and (log K3, run
2, Table S1†); (d) conductivity of a supersaturated solution of CaSO4 in the presence (5, 10, 15, 20 ppm) of 13 and in the absence (blank) of antiscalant.
encourages protonation. This is in contrast to the increase of the degree of hydration which leads to greater entropy changes
log K3ꢁ [involving (ZH2 c ) 13 + H+ ! (ZH3 c) 19], whereby because of release of a greater number of molecules during
protonation of a repeating unit also decreases the overall progressive protonation.
negative charges of the polymer chain. The n values > or < 1 are
+ –
+
caused by entropy effects.42,44 With each protonation,
3.6 Scale inhibition study
a repeating unit (RU) in ampholytic/anionic (ZH+c]) 20
+ –
In a Reverse Osmosis (RO) process, feed water is converted to
product water and rejects brine, in which supersaturation of
dissolved salts, like CaSO4 and CaCO3, may lead to precipitation
which reduces the efficacy of the membrane separation process.
The percent scaling inhibition (PSI) is calculated using eqn
(4):
collapses into an ampholytic RU of (ZH2 c ) 13. Ampholytic/
anionic (ZH+c]) 20 having excess negative charges is more
hydrated than PA (+c–) 13 as supported by the higher viscosity
value of the former (cf. Fig. 3b-vi vs. 3b-ii). The protonation of
a RU leads to an entropically favourable release of water of
hydration. For (ZH+c]) 20, as the a increases, the average
number of water molecules per RU in a polymer chain decreases
which leads to a decrease in the magnitude of the positive
entropy changes. Since the exothermic DHꢁ is independent of a,
the DGꢁ, hence K, decreases with the increasing a.44
½Ca2þꢄinhibitedðtÞ ꢀ ½Ca2þꢄ
blankðtÞ
PSI ¼
ꢂ 100
(4)
½Ca2þꢄinhibitedðt Þ ꢀ ½Ca2þꢄ
blankðtÞ
0
where [Ca2+] concentrations are used at time zero and t. The
inhibited and blank are used to describe the presence and
absence of the antiscalant.
For the protonation of (ZH2+c–) 13 to (ZH3+c) 19 (Scheme 3),
the latter is more hydrated thus having expanded polymer chain
as evinced by its greater viscosity value (cf. Fig. 3b-v vs. 3b-ii).
The more exposed RU makes the neutralization process easier
with the increasing a (Fig. 3c, i ¼ 3).45,46 With increasing a,
increasing imbalance in favour of positive charge also increases
The concentration of Ca2+ and SO42ꢀ, present in the reject
brine of a RO plant,47 was described as 1-CB (concentrated
brine). The scaling behavior in the presence of PA 13 was carried
out in a supersaturated solution of CaSO4 comprising 3-CB
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Table 4 Percent scale inhibition in the presence of PA 13 in 3 CBa
has been determined to be 2.62, 5.59, and 10.1, respectively. PA
13 was evaluated as an antiscalant for potential application in
reverse osmosis (RO) plants. At concentrations of 5 and 20 ppm,
it demonstrated remarkable efficiency of z100% for CaSO4
scale inhibition from its supersaturated solution for 50 and
500 min, respectively. Since an antiscalant should be effective
for the duration of brine's residence time (z30 min) in the
osmosis chamber, the synthesis of PA 13 in excellent yields from
cheap starting materials and its very impressive performance
may accord it a prestigious place among many an environment-
friendly phosphate-free antiscalant. Note that polyphosphate
additives used for controlling scale formation, when discharged
in the sea have deleterious inuence over the marine biota
picture.51
supersaturated CaSO4 solution at 40 ꢁ
C
Percent inhibition at times (min)
Entry Sample (ppm) 50 100 200 300 400 500 700
1
2
3
4
5
100
100 100
100 99
92
71
88
96
24
81
91
22
76
88
17 17
71 63
83 75
10
15
20
100 100 100 100 100 100 98
a
2ꢀ
Three times the concentration of Ca2+ and SO4
found in the
concentrated brine of an RO plant.
2ꢀ
concentration of Ca2+ (2600 ppm) and SO4 (6300 ppm) by
conductivity measurements. A sudden drop in the conductivity
indicates the onset of CaSO4 scaling (Fig. 3d).
Conflicts of interest
It is indeed satisfying that in the presence of 5 and 20 ppm
13, a 100% scale inhibition was registered for about 50 and
500 min, respectively (Table 4). Since the reject brine usually has
a residence time of z30 min in the osmosis chamber, the
current antiscalant may thus be very effective in inhibiting
CaSO4 scaling. As shown in Fig. 3d, the onset of precipitation
occurs aer an induction period; an accelerated growth of
CaSO4 crystals is indicated by a sudden drop in conductivity.
For a duration of 700 min, 20 ppm PA 13 did not show any
induction time. The crystal growth starts aer nucleation
process, which is interfered with the complexation of the metal
ions by the chelating ligands of the antiscalant.48,49 The gypsum
scale, i.e., CaSO4 in mineral form, occurs during several
processes involving production of water.50 PA 13 is found to be
remarkably efficient in preventing scale formation, and as such
it has the ability to mitigate the membrane fouling in RO plants.
There are no conicts to declare.
Acknowledgements
The facilities provided by KFUPM are gratefully acknowledged.
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