Amphiphilic Maleic Acid-Containing Alternating
Copolymers—1. Dissociation Behavior and Compositions
E. SAUVAGE,1 D. A. AMOS,1 B. ANTALEK,1 K. M. SCHROEDER,1 J. S. TAN,1 N. PLUCKTAVEESAK,2
R. H. COLBY2
1
Research & Development Laboratories, Eastman Kodak Company, Rochester, New York 14650-2116
2
Department of Materials Science and Engineering, The Pennsylvania State University,
University Park, Pennsylvania 16802
Received 13 November 2003; revised 7 April 2004; accepted 5 May 2004
DOI: 10.1002/polb.20202
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The dissociation of the two adjacent carboxylic acids in maleic acid-containing copolymers is expected to differ from those of poly(acrylic acid) and poly(methacrylic acid) where the acids are separated by two carbons on the backbone. In
this work, we have employed potentiometric titration and NMR spectroscopy to characterize the dissociation behavior and chemical compositions of several water-soluble
maleic acid-containing copolymers. A distinct two-step process corresponding to the
dissociation of the two adjacent carboxylic acids is observed in aqueous CaCl2 (0.02 N)
solution for copolymers of maleic acid and isobutylene, diisobutylene, and styrene. Such
a two-step ionization process is less recognizable, however, for the copolymers of maleic
acid and linear alkenes ranging from n-hexene to n-octadecene. Nevertheless, the
compositions of all copolymers, including the extent of neutralization and the ratio of
the comonomer moieties, are estimated from the titration curves. The chemical composition derived from potentiometry and NMR spectroscopy for all copolymers are
approximately 1 : 1 (maleic acid : comonomer). With the exception of the hydrophobically modified copolymer of isobutylene-maleic acid, no obvious conformational transition was observed over the whole range of ionization for these hydrophobic maleic
acid-containing copolymers. This is related to the aggregated state of these copolymers
in aqueous media. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3571–3583,
2004
Keywords: maleic-anhydride alternating copolymers; pH titration; 13C NMR; extent
of neutralization; chemical composition
INTRODUCTION
Amphiphilic alternating copolymers of maleic anhydride and hydrophobic monomers form an important class of materials with many industrial
applications. For example, they have been pat-
Correspondence to: R. H. Colby (E-mail: rhc@plmsc.psu.edu)
or J. S. Tan (E-mail: juliatan@rochester.rr.com)
Journal of Polymer Science: Part B: Polymer Physics, Vol. 42, 3571–3583 (2004)
© 2004 Wiley Periodicals, Inc.
ented as good dispersants/emulsifiers for pigments and solid particle suspension1 and oil-soluble photographically useful compounds,2 adhesives, binders, coating and paper additives,3
cleaning and floor polishing agents,4,5 scale inhibitors for water,6 and hydraulic drilling fluid additives.7 The most distinct chemical features of
these copolymers are (1) the adjacent neighboring
position of the two carboxyl groups in the maleic
acid comonomer, and (2) the alternating sequencing of the hydrophobic and the maleic monomers.
3571
3572
SAUVAGE ET AL.
The former provides chelating sites for complexation with cations or positively charged dyes. The
alternating sequencing of the hydrophobic and
hydrophilic comonomers provides the flexibility
for forming a comb-like structure, which may facilitate aggregation of the hydrophobic segments
along the chain. We expect that chemical structures have profound influence on the properties of
these polymers, and hence their various applications. They can be made as water-soluble surfaceactive polymeric surfactants as long as the hydrophobic and hydrophilic comonomers are properly
balanced. The low molecular weight copolymers
are useful as colloidal-dispersing agents to stabilize water-insoluble organic molecules.2 The parent maleic anhydride copolymers can also be
made as oil-phase additives or modifiers with specific chemical moieties incorporated in the maleic
anhydride site. The high molecular weight maleic
acid copolymers usually display high viscosity
values, and can be used as associative thickeners
or as rheology modifiers if the hydrophobic
comonomers contain long alkyl side chains.
Therefore, understanding the solution behavior of
these copolymers is necessary for applying them
to the formulation and characterization of oil-inwater emulsion systems.
Our main focus for the present work is to characterize the structure and dissociation process of
the hydrolyzed copolymers as a function of the
comonomer chemical nature and hydrophobicity.
We expect that such molecular information will
be intimately related to their solubility and aggregation behavior, surface, and interfacial activities, and rheology in aqueous solutions, to be
discussed in forthcoming publications.
The dissociation behavior for the alternating
copolymers of maleic acid and alkyl vinyl ethers
was studied in detail by Dubin and Strauss.8,9
They found that the dissociation involves two
steps corresponding to the dissociation of the two
adjacent neighboring carboxylic acids in the maleic acid comonomer unit. This process was discussed in terms of chain expansion from a compact coil at low pH, resulting from intrachain
aggregation of the hydrophobic monomers, to an
extended coil at high pH. When the alkyl group in
the vinyl ether comonomer is longer than n-butyl,
a conformational transition region was observed
around ␣ ⫽ 0.1– 0.2. Here, the extent of neutralization, ␣, is defined as zero when the polymer is
in its unionized form and is unity when both the
carboxylic acids are fully ionized. The two-step
dissociation behavior was also shown for the co-
polymers of maleic acid and styrene;10,11 maleic
acid and propene;12 and maleic acid and isobutylene.13 The value of the second dissociation constant pK2 for the copolymer of maleic acid and
propene12 was found to be greatly decreased in
the presence of the divalent cation Ca2⫹. The
second-step dissociation was identified in the titration curve only in the presence of CaCl2
(⬍0.02) for this copolymer. This is attributed to
the chelating ability of the two maleic carboxylates to Ca2⫹ ion.
In the present article, we report the effect of
the chemical structure of the hydrophobic
comonomer on the dissociation behavior of several maleic acid-containing copolymers. They include the alternating copolymers of maleic acid
and isobutylene, diisobutylene, styrene, and nalkenes ranging from n-hexene to n-octadecene.
As we are interested in only the amphiphilic surface active materials,2 the copolymer of ethylene
and maleic acid is excluded from this study because of the extreme solubility of the copolymer in
water. The copolymer compositions (molar ratios
of carboxylate, carboxylic acid, and the hydrophobic comonomers) were determined by 13C NMR
and compared with potentiometric titration results. Finally, the titration data for all maleic acid
copolymers were analyzed to assess whether any
conformational transition occurs during neutralization. The results will be discussed in light of
chain characteristics of these copolymers in aqueous solution.
EXPERIMENTAL
Materials
All polymers employed in this work were in their
hydrolyzed form, containing maleic acid in its
fully neutralized or partially neutralized sodium
salt form. Chemical structures are shown in
Figure 1. The materials were exhaustively dialyzed and concentrated using an Amicon diafiltration unit with a 3000-MW cutoff membrane to
remove excess small ions and low molecular
weight impurities. The pure acid form of the polymer solid was obtained by acidifying the aqueous
polymer solutions with a mixed-bed ion exchange
resin, followed by freeze drying.
Kuraray Co., Ltd. (Kuraray America, Inc., NY)
supplied the alternating copolymer of maleic anhydride and isobutylene. Its trade name is Isobam (600, 04, 06, 10, and 18, corresponding to
AMPHIPHILIC MALEIC ACID-CONTAINING ALTERNATING COPOLYMERS—1
Figure 1. Chemical structures of the maleic acidcontaining copolymers.
nominal molecular weights of 6, 65, 80, 160, and
350 kg/mol, respectively, as specified by the manufacturer) and is designated as IBMA in this report. Solid maleic anhydride copolymers (in the
form of white powder) were first dissolved in water and hydrolyzed overnight at 50 °C with addition of an excess (2.2 ⫻ [maleic anhydride]) of
NaOH before dialysis, freeze drying, and vacuumoven drying.
The hydrophobically modified IBMA copolymers (IBMA-NHR) were obtained by dissolving
3573
the parent anhydride copolymer in a dried organic solvent (DMSO, approximately 3–5%) with
a molar excess (1.2 ⫻ [maleic anhydride]) of alkyl
amines, such as n-butyl amine and n-hexyl amine
to initiate the anhydride ring-opening reaction.
The solution was refluxed at 60 °C overnight with
constant stirring under nitrogen. The clear organic solution containing the resultant carboxylic
acid/amide copolymer was added to a slight excess
of aqueous NaOH solution (1.2 ⫻ [maleic anhydride]) to ensure complete neutralization. This
mixture, containing partially precipitated polymer solid (a mixture of carboxylic acid, sodium
carboxylate, and amide), was subjected to exhaustive dialysis and diafiltration to remove DMSO
and excess ions. The hydrolyzed polymer was collected from the clear dialyzed polymer solution by
freeze drying and vacuum-oven drying.
The hydrolyzed copolymer of maleic acid and
diisobutylene (referred to as DIBMA in this report) was obtained in aqueous solution from two
commercial sources: (1) Aldrich, supplied in a
25% clear aqueous solution with excess NaOH;
and (2) Rohm & Haas, supplied as the hydrolyzed
polymer solid containing excess NaOH, having a
trade name of Acusol 460N and a nominal molecular weight of 12–15 kg/mol. These polymers were
dissolved in water and exhaustively dialyzed and
diafiltered to remove the excess NaOH and freeze
dried to a white fluffy solid.
The unhydrolyzed copolymer of maleic anhydride and diisobutylene was synthesized using a
known solution polymerization procedure14 using
the monomers, maleic anhydride and 2,4,4-trimethyl-1-pentene (Aldrich). The following is an
example of the polymerization procedure. The
monomers (maleic anhydride, 50 g and olefin,
70 g) were dissolved in MEK (methyl ethyl ketone, 1200 g) and polymerized at 65–70 °C overnight using AIBN (2 2⬘ azobisisobutyronitrile, 5 g)
as a free radical initiator. The reacted solution
was reduced to 200 mL by rotary evaporation, and
the polymer was precipitated with ether and
dried in an aspirated filter funnel. The hydrolyzed
polymer was obtained by the same procedure as
that described above for IBMA. Variation in the
concentrations of the monomers and initiator was
employed to control the molecular weight of the
product copolymer.
The unhydrolyzed copolymers of maleic anhydride and n-alkenes (referred to as n-CmMA in
this report, where m ⫽ 6, 8, 10, 12, 14, 18) were
obtained from S. C. Johnson & Sons, Inc. Based
on an earlier patent,15 several of the copolymers
3574
SAUVAGE ET AL.
of maleic anhydride and higher n-alkenes (from
C10 to C18, obtained from Monomer-Polymer &
Dajac Labs, Inc.) were synthesized in 1,2-dichloropropane using benzoyl peroxide as the initiator.
These materials have higher molecular weights
(from 20 to 60 kg/mol) and lower polydispersity
index. For example, n-dodecene (80.8 g) was
added to a mixture of maleic anhydride (23.6 g)
and benzoyl peroxide (0.85 g) in 1,2-dichloropropane (80 g) under nitrogen. The solution was
heated and refluxed for 4 h under nitrogen atmosphere. The reaction solution was poured slowly
into 3.5 liters of isopropanol at room temperature
to precipitate the polymer. The white polymer
solid was isolated and hydrolyzed prior to drying
as described above for IBMA.
The unhydrolyzed copolymer of maleic anhydride and styrene (referred to as SMA in this
report) was obtained from Elf Atochem North
America, Inc., having the trade name, SMA 1000,
and molecular weight of 5 kg/mol. A higher molecular weight (280 kg/mol) version of the same
polymer was obtained from Scientific Polymer
Products, Inc. This compound with a molar ratio
of 1 : 1 for maleic anhydride:styrene is soluble in
aqueous NaOH solution. The hydrolyzed and dialyzed copolymer was obtained by the same procedure as that described for the other copolymers.
Two homopolymers, poly(acrylic acid) (PAA,
MW ⬃300 and 60 kg/mol) and poly(methacrylic
acid) (PMA, MW ⬃30 kg/mol) were obtained from
Polysciences, Inc. Both of the aqueous polymer
solutions were added to excess NaOH, extensively
diafiltered and freeze dried.
The low molecular weight model dicarboxylic
acids, that is, maleic acid, succinic acid, and fumaric acid, were obtained from Eastman Laboratory Chemicals. L-Aspartic acid was obtained
from Fluka and L-ascorbic acid from Fisher
Scientific.
Potentiometric Titration
All pH titrations were carried out with an Orion
semimicro combination pH glass electrode in conjunction with a Corning pH meter at room temperature, ⬃22 °C. The concentration of the polymer for each titration in water or in 0.02 N CaCl2
was 3– 6 mg/mL. The aqueous 0.1 N HCl or NaOH
titrant was delivered into the stirred polymer solution in a 20-mL vial from a calibrated Rainin/
Gilson Pipetman pipettor P-20 or P-200 in aliquots of 5 to 20 ␮L. The pH readings for the
solution were stable and accurate to 0.01 pH unit.
The end-point determinations were unaffected by
corrections employing blank titrations. Dissociation behavior of all hydrolyzed copolymers was
studied by both “forward” and “backward” titration procedures. The “forward” pH titration was
performed with 0.1 N NaOH as the titrant for the
acid form of the polymer (either acidified with the
mixed-bed ion exchange resin (MB) or in the presence of excess HCl). Conversely, the “backward”
titration was conducted with 0.1 N HCl as the
titrant for the hydrolyzed/dialyzed sample, which
may contain a mixture of carboxylic acid and
sodium carboxylate.
Size-Exclusion Chromatography
The size-exclusion chromatography was performed for the parent maleic anhydride copolymers at 30 °C using three Polymer Laboratories
mixed-gel columns. The mobile phase was THF or
DMF, depending on the solubility of the parent
maleic anhydride copolymers. The values for the
poly(ethylene oxide) equivalent MW and polydispersity index for all the copolymers studied are
summarized in Table 1.
For the water-soluble hydrolyzed copolymers,
aqueous size-exclusion chromatography was also
employed. A mixture of MeOH/aqueous 0.1 M
Na2SO4 (1 : 1) was used as the mobile phase. Two
Synchropak GPC columns, 500/100, or 1000/300,
were used in conjunction with a refractive index
detector. The molecular weight was calibrated
based on the weight-average molecular weights of
several sodium polystyrenesulfonate standards
purchased from Phenomenex. These results are
also included in Table 1.
Infrared Spectroscopy
Infrared (IR) spectroscopy was used to verify the
complete hydrolysis of the maleic anhydride moiety. IR spectra was collected using a Diamond
ATR accessory in a Bio-Rad FTS-60S spectrometer. In Figure 2, the parent IBMA copolymers
show the presence of the distinct anhydride bands
around 1760 –1850 cm⫺1. For the hydrolyzed copolymers, these bands are completely displaced to
the stretching vibration of the carboxylate at
1560 cm⫺1 along with the appearance of the OH
band near 3400 cm⫺1.
NMR Spectroscopy
NMR was used to determine the chemical compositions of the copolymers. Both 1H and 13C NMR
3575
AMPHIPHILIC MALEIC ACID-CONTAINING ALTERNATING COPOLYMERS—1
Table 1. Molecular Weights and Molecular-Weight Distribution of Various Maleic Anhydride Copolymers
(determined in THF and in DMF) and Their Maleic-Acid Copolymers (Determined in Aqueous
MeOH/0.1 M Na2SO4)
MW(kg/mol)
(PEO eq in THF or DMF)
(a) IBMA (Isobutylene-alt-Maleic
Isobam 600
Isobam 04
Isobam 06
Isobam 10
Isobam 18
Mw/Mn
Anhydride Copolymersa)
(6.5) 3.8, 4.5
(65) 65
(85) 98, 76
(165) 156
(350) 327
1.75
2.60
2.1, 2.8
3.04
2.40
MW(kg/mol)
(PEO eq in DMF)
Mw/Mn
(b) DIBMA (Di-isobutylene-alt-Maleic Anhydride Copolymers)
Sampleb-1
9.0
Sampleb-2
10.5
Sampleb-3
13.6
Sampleb-4
45.4
Sampleb-5
75
Sampleb-5
66
Sampleb-7
113
Sampleb-8
11.0
Sampleb-9
9.7
DIBMAc
Low MW(kg/mol)
(PEO eq in THF)
(c) n-CmMA (n-Alkene-alt-Maleic Anhydrided)
n-C6MA
10.6
n-C8MA
3.5
n-C10MA
n-C12MA
11.8
n-C14MA
7.8
n-C18MA
7.8
n-C20MA
(d) SMA (Styrene-alt-Maleic Anhydridee)
SMA-1000
4.25
Scientific Polymer Products, Inc.
MW(kg/mol)
(NaPSS eq in MeOH/
0.1 MNa2SO4)
6.5
MW(kg/mol)
(NaPSS eq in MeOH/
0.1 M Na2SO4)
1.33
3.3
1.5
1.33
1.44
1.52
1.6
1.45
1.2
4.0, 3.5
6.5
6.8, 7.5
28, 37
35, 41
53, 65
90, 100
15, 21
Mw/Mn
2.6
2.3
3.1
3.7
4.5
High MW (kg/mol)
(PEO eq in THF)
Mw/Mn
56.5
55.7
20.1
23.2
27.6
35.8
23.9
4.2
3.7
2.0
2.1
2.5
3.0
7.3
280f
4.7
2.7
a
These anhydride copolymers were supplied from Kuraray. The nominal MW listed in parentheses is from the manufacturer.
These anhydride copolymers were synthesized by Dr. I. Ponticello.
c
This copolymer was supplied by Rohm & Haas in the hydrolyzed form only.
d
These copolymers were either from S.C. Johnson or synthesized by Dr. I. Ponticello.
e
The low MW SMA was supplied by Elf Atochem, Inc. The high MW SMA was purchased from Scientific Polymer Products, Inc.
f
The intrinsic viscosity of the high molecular weight SMA in THF was measured to be 0.79 dL/g, which suggests a higher
molecular weight of M ⫽ 340,000 based on the Mark-Houwink relation for SMA in THF.16 The SEC equivalent molecular weight
in THF is generally erroneous for SMA.17
b
measurements were made on a Varian Inova
spectrometer operating at 400 MHz for 1H and
101 MHz for 13C. The solid samples were dissolved in D2O at a concentration of 0.25 g/5 mL.
An internal chemical shift reference, 3-(trimethylsilyl) propionic-2, 2, 3, 3-d4 acid, sodium salt
(TSP), was used. For the 1H NMR, the acquisition
parameters are as follows: acquisition ⫹ delay
3576
SAUVAGE ET AL.
Figure 2. IR spectra of the unhydrolyzed and hydrolyzed IBMA. (a) IBMA (Isobam 18), (b) IBMA hydrolyzed (Isobam 18). (c) IBMA (Isobam 600), (d) IBMA
hydrolyzed (Isobam 600).
time ⫽ 7 s; pulse width ⫽ 45°; sweep width
⫽ 4600 Hz; points ⫽ 28,700. For the 13C NMR, the
acquisition parameters are as follows: acquisition
⫹ delay time ⫽ 15 s; pulse width ⫽ 60°; sweep
width ⫽ 24,750 Hz; points ⫽ 74,300; decoupling
on during acquisition only. An inversion-recovery
experiment was performed to measure the T1 values of each carbon to ensure quantitative conditions. The acquisition ⫹ delay time was set to
greater than five times the longest T1. For all the
copolymers, the longest T1 observed, 2.4 s, belongs to the end methyl group of the copolymers
containing a linear alkyl chain.
CaCl2, curve (2). This result is similar to that
reported for another copolymer of maleic acid and
propene.12
Alternatively, the mixture of the acid and its
salt form in the polymer sample can be converted
to the pure acid form by adding an excess of HCl.
In the forward titration shown as curve (3) in
Figure 3, the two dissociation steps remain sharp
and identifiable in 0.02 N CaCl2, although the
whole titration curve is shifted to higher titrant
volume region because of the presence of excess
HCl.
However, the end point for the excess (unused)
HCl cannot be detected in curve (3). This may be
related to the value of pK1 of the polymer sample,
as discussed below.
Before we analyze the pH titration data shown
in Figure 3 and for all other copolymers, titration
results for several low molecular weight model
dicarboxylic acids were necessary. In the absence
of possible interference of the comonomers in
these model compounds, simple titration results
are expected, and they can provide aids in characterizing the dissociation behavior of the copolymers. Five dicarboxylic acid compounds including maleic acid (1.83, 6.07), fumaric acid (3.03,
4.42), L-aspartic acid (3.86, 9.82), succinic acid
(4.16, 5.61), and L-ascorbic acid (4.16, 11.79) were
studied using the same forward and backward
titration methods. Shown in the parentheses for
each acid are the pK1 and pK2 values taken from
the CRC Handbook of Chemistry and Physics.
With the exception of fumaric acid, pK1 and pK2
RESULTS AND DISCUSSION
Dissociation Behavior
IBMA [Copoly(isobutylene-alt-maleic acid)]
The forward pH titration curves for IBMA (Isobam 600) (60 mg) from the acid to the dissociated
form are shown in Figure 3. To convert the copolymer to its acid form, the polymer solution was
treated with a mixed-bed ion exchange resin
(MB). The forward titration in water for the MBtreated sample is shown as curve (1) with only
one inflection point. In the presence of 0.02 N
CaCl2, however, two distinct inflection points for
this sample can be identified in curve (2), corresponding to the dissociation of the two carboxylic
acid groups in the maleic acid comonomer. The
point of inflection in water, curve (1), coincides
with that of the first carboxylic acid in 0.02 N
Figure 3. pH titrations for IBMA (Isobam 600, 60
mg): curve (1) mixed-bed resin-treated sample in water;
curve (2) mixed-bed resin-treated sample in 0.02 N
CaCl2; curve (3) hydrolyzed sample ⫹ excess HCl (7 mL
0.1 N) in 0.02 N CaCl2.
AMPHIPHILIC MALEIC ACID-CONTAINING ALTERNATING COPOLYMERS—1
are far enough apart to yield two end points in
titration.
The forward titration of maleic acid (cis-configuration) exhibited two steps for the dissociation,
but any excess unused HCl present cannot be
detected because of the low pK1 value of maleic
acid (1.83). Fumaric acid (trans-configuration)
only shows a single dissociation in forward titration and the unused HCl again cannot be detected. The absence of the two-step titration curve
is attributed to the closeness of its two pKs (3.03,
4.42), and is indicative of the absence of interference of the two acids in their trans-configuration
around the double bond. L-aspartic acid shows a
two-step dissociation, corresponding to the two
acids involved with very different pKs. However,
the end point for the excess unused HCl cannot be
recognized even with a pK1 value as high as 3.86
for L-aspartic acid. The end point for the excess
unused HCl is clearly seen in the forward titration of succinic acid, consistent with its higher
pK1 value of 4.16. The total amount of dicarboxylate in the forward titration of succunic acid is
equal to that of the dicarboxylic acid in the backward titration. The end point for the unused HCl
is also observed in the forward titration of Lascorbic acid with a similar pK1. Summarizing
the titration data for the model dicarboxylic acids,
we have obtained two important findings. (a) The
end point for the excess unused HCl is recognizable only if the pK1 value for the dicarboxylic acid
is above 4.0. (b) The two-step dissociation behavior can be clearly displayed for the dicarboxylic
acid only when the two acid groups are in close
proximity to exert influence on each other. In
other words, the ionization of the first carboxylic
acid can impose steric and electrostatic hindrance
on the ionization of the second carboxylic acid
3577
group when the two acids are in close proximity.
With these findings, we then analyze the pH titration curves of all maleic acid-containing copolymers.
Returning to Figure 3, curve (3), for the hydrolyzed sample of IBMA, the absence of the end
point for the unused HCl suggests that the pK1
value for IBMA is lower than 4.0. This is in good
agreement with a value of 3.2–3.3 for IBMA and a
value of 3.7 for copoly (propene-alt-maleic acid)
reported previously.12 The equivalent base between the two end points for the first and second
carboxylic acids is equal to that required to neutralize the second half of the maleic acid present.
Therefore, the total equivalent amount of carboxylic groups is equal to two times this quantity.
This quantity establishes the end point shown by
arrow A [see curve (3)] for the excess unused HCl.
Thus, from the amount of consumed HCl, one can
calculate the amounts of carboxylate and carboxylic acid present in the copolymer sample. The
extent of neutralization is defined as,
␣ ⫽ [COO⫺]/[COOH ⫹ COO⫺]
(1)
and ␣ can be calculated using the consumed HCL
as [COO⫺] and two times the equivalent between
the two end points as the total [COOH ⫹ COO⫺].
This value (⬃0.7) is shown for all hydrolyzed and
dialyzed IBMA samples with different molecular
weights (see Table 2). Additionally, the IBMA
solution is completely soluble throughout the
whole range of pH in either forward or backward
titration in the presence of 0.02 N CaCl2.
IBMA-NHR (Hydrophobically Modified
IBMA Copolymers)
The titration curves for the n-butylamine-modified
IBMA and n-hexylamine–modified IBMA are
Table 2. pH Readings of the Hydrolyzed Polymer Solutions (30 mg/5 mL) in Water or in 0.02N CaCl2, Extent of
neutralizationa (␣), and Comonomer Compositionb (␤) by pH Titration and 13C NMR
C6MA
C8MA
C12MA
C14MA
C18MA
SMA
IBMA
DIBMA
pH in water
pH in 0.02N CaCl2
8.3
6.8
8.2
7.3
8.9
7.9
8.3
7.8
8.8
8.2
8.0
7.8
9.2
7.1
6.2
5.6
␣
0.91
0.87
0.94
0.75
0.87
0.54
0.72
0.39
0.7
0.83
1.2
0.81
0.6
0.84
0.7
0.98
0.9
0.87
1.0
—
1.1
1.0
1.3
0.98
␤ (pH Titration)
␤ (13C NMR)
a
b
The extent of neutralization is defined as ␣ ⫽ [COO⫺]/[COOH ⫹ COO⫺].
Comonomer composition is defined as ␤ ⫽ [hydrophobic monomer]/[maleic anhydride].
3578
SAUVAGE ET AL.
shown in Figures 4(a) and 4(b), respectively. The
chemical structure of the n-butylamine and n-hexylamine-modified IBMA is described as follows,
where R ⫽ n-butyl or n-hexyl, and b ⫽ (x⫹y)/2.
There are two distinct sharp end points for the
backward titration of n-butylamine–modified
IBMA sample in excess NaOH, marked by arrows
A and B in Figure 4(a). The solution is clear in the
basic region above pH ⬃6.5, and becomes hazy in
the acid region. The first end point indicated by
arrow A is related to the amount of excess unused
NaOH and the onset of protonation of the carboxylate, and the second end point indicated by arrow B denotes the complete protonation of all
carboxylate present. For the forward titration of
n-hexylamine–modified IBMA with an addition of
excess HCl, there are two distinct end points as
marked by two arrows A and B in Figure 4(b). The
solution in the acid region is hazy, and becomes
clear above pH 8.0. The end point shown by arrow
A corresponds to the end point for the unused
HCl, and the second shown by arrow B corresponds to the dissociation of all carboxylic acid
present. The absence of any additional point of
inflection in Figure 4(a) and (b) suggests there is
no significant amount of maleic acid remaining in
the NHR-modified copolymers.
The total amount of carboxylate (x) can be calculated from the end points. Assuming equal molar amounts of isobutylene and maleic anhydride
in the parent polymer, this information can be
used to estimate the compositions of the modified
polymers. Identical results of the composition of
1 : 1 : 1 for isobutylene (a ⫽ 1) : carboxylate (x
⫽ 1) : amide (y ⫽ 1) were obtained for both samples, suggesting nearly complete modification on
the anhydride by the amines. This finding also
reveals that the amide in these copolymers is not
hydrolyzed during the modification treatment.
DIBMA [Copoly(di-isobutylene-alt-maleic acid)]
Similar to the case for IBMA (see Fig. 3), a twostep dissociation is clearly identified for DIBMA
in the presence of 0.02 N CaCl2 (data not shown).
Figure 4. pH titrations of (a) n-butylamine-modified
IBMA (Isobam 600, 60 mg) plus excess NaOH (3 mL 0.1
N); (b) n-hexylamine-modified IBMA (Isobam 600, 30
mg) plus excess HCl (2 mL 0.1 N) in water.
Whereas the end point for the unused HCl in
IBMA [see curve (3), Fig. 3] is not obvious, the end
point for the unused HCl in forward titration of
DIBMA is apparent. This suggests that the pK1
for DIBMA is slightly higher than that for IBMA
and likely exceeding 4.0. With the known amount
of polymer used and the end points obtained, the
value for ␣ can be estimated to be approximately
0.4 for DIBMA (see Table 2).
There is one difference in solubility behavior,
however, between DIBMA and IBMA. The
DIBMA solution becomes hazy below pH ⬍ 4.5,
whereas IBMA remains soluble over the whole
range of pH. Nevertheless, this does not seem to
affect the end point determination in pH titration.
SMA [Copoly (styrene-alt-maleic anhydride)]
In the forward titration of SMA (data not shown),
the acid form of the copolymer is not soluble in
0.02 N CaCl2, so the titration was conducted in
water. The solution is hazy in the acid region up
to pH ⬃3.0. Above this pH, the solution becomes
clear throughout the high pH region. The backward titration for SMA in excess NaOH was conducted in 0.02 N CaCl2. The solution is hazy in
AMPHIPHILIC MALEIC ACID-CONTAINING ALTERNATING COPOLYMERS—1
the basic region from pH 12 to 7, although a sharp
inflection point is observed and attributed to the
presence of CaCl2. In the range of pH from 2.5 to
7.0, the solution is clear but becomes hazy again
below pH ⬍ 2.5. The backward titration shows
three end points that are approximately equally
separated. The first end point is related to the
amount of the unused NaOH and the onset of
protonation of one of the carboxylate ions. The
second end point is the end of protonation of the
first carboxylate and the onset of protonation of
the second carboxylate. The total carboxylate in
the copolymer sample is about the same as the
equivalent base added between the first and third
end points. The value for ␣ of the hydrolyzed SMA
sample is calculated to be 0.54 (see Table 2). From
the appearance of the end point of the excess
unused HCl in the forward titration of SMA, we
expect that the pK1 value for SMA is also higher
than 4.0.
3579
or DIBMA (␣ ⫽ 0.54, 0.72, and 0.39 respectively). The absence of the two-step ionization
process for the copolymers of n-alkenes and maleic acid indicates that the neighboring effect
for the two carboxylic acids in the maleic acid
comonomer is hampered by the n-alkene
comonomer. This is also consistent with the
higher extents of neutralization upon hydrolysis found for these copolymers.
Chemical Compositions of the Copolymers
The extent of neutralization of the hydrolyzed
copolymers have been determined by using potentiometric titration data. The ratio of the comonomers can also be obtained from the same titration
data. Based on the titration curves, we have obtained the numbers of moles of carboxylic acid x
and sodium carboxylate y for each polymer sample as described by the following formula,
n-CmMA [Copoly(n-alkene-alt-maleic anhyride)]
The n-CmMA copolymers are not soluble in 0.02 N
CaCl2, even in their hydrolyzed form. Therefore,
titrations of these samples were conducted in water (data not shown). The forward titrations for
n-C6MA with excess HCl are similar to that for
SMA, with two obvious points of inflection. The
backward titration clearly showed the onset of the
second carboxylate protonation and the end of its
total protonation, but the first protonation at high
pH is not observable. Addition of 0.01 N CaCl2 did
not sharpen this end point either. Therefore, to
estimate the total carboxylate moiety in the hydrolyzed polymer sample, we have used two times
the equivalent acid or base between the two lower
pH end points to locate the high pH end point.
The value of ␣ can be estimated as 0.91 (see
Table 2).
For copolymers with higher n-alkenes, the
characteristics of the titration curves are very
similar to those for n-C6MA, and the calculated
extents of neutralization are also listed in
Table 2. Unlike the cases for IBMA, DIBMA, or
SMA, the uncertainties in the determination of
the values for ␣ are much larger for the n-CmMA
series (⫾10 –15%) because of the lack of clear end
points for the second carboxylic acid.
Based on the ␣ values listed in Table 2 for all
hydrolyzed copolymers, it appears that the hydrophobic n-alkene-maleic acid series have
higher extents of neutralization upon hydrolysis (0.75 ⬍ ␣ ⬍ 0.91) than those of SMA, IBMA,
where b ⫽ (x ⫹ y)/2. By using the known amount
of polymer sample in each titration, one can calculate the number of mol of the hydrophobic
monomer a.
Here, we define the composition of the copolymer as the molar ratio of the hydrophobic
comonomer and the maleic acid or the original
anhydride as
␤ ⫽ a/b.
(2)
The results are listed in Table 2. The uncertainties in ␤ are somewhat large (⫾10 –15%) because of the uncertainties involved in the titration end point determinations.
Additionally, we have obtained the compositions by NMR spectroscopy. The 1H and 13C spectra for IBMA are shown in Figure 5 and pertinent
data summarized in Table 3. The assignments are
consistent with the work of Komber.18 The chemical composition of the polymers was determined
from the integrals of all baseline-resolved resonances in the quantitative 13C spectrum. In all
cases, integrals of the carbonyl resonances were
obtained. For the copolymers with alkyl side
chains, the resonances of the three carbons in the
chain furthest from the backbone were well resolved and integrals obtained. ␤ was calculated
3580
SAUVAGE ET AL.
ure 5(b), is narrow and has no shoulder or complicated structure to indicate any sequence heterogeneity. The case of IBMA is simpler, however, than the other polymers because the
hydrophobic moiety has no asymmetric center.
This is not true for the other polymers. For
example, the backbone methine carbon in the
hydrophobic moiety of n-C14MA was very broad,
and no specific sequencing information was obtained. Although we expect that the copolymers
investigated here are alternating, besides the
case of IBMA, there is little NMR data to substantiate such an assumption. We can only use
the chemical composition results to support the
alternating nature of this class of copolymers.
Figure 5. (a) 1H and (b)
(Isobam 18).
13
C spectra of IBMA
Conformational Transition
for each polymer and included in Table 2 for comparison. The uncertainty in ␤ is considered to be
⬍10%. From the results obtained, we conclude
that the composition of all the parent maleic
anhydride– hydrophobic monomer is approximately 1 : 1.
Sequencing information can generally be obtained from 13C NMR. For these polymers, an
alternating sequence is expected. The details
are examined for IBMA. The fine structure
found for the carbon resonances a and c [Fig.
5(b)] is attributed to tacticity brought about by
the asymmetric carbon centers of the acid
groups. No features are observed that would
indicate anything other than an alternating sequence. For example, the resonance for the
backbone quaternary carbon, labeled “b” in Fig-
Table 3.
Summary of 1H and 13C NMR Spectral Characteristics for IBMA (Isobam 18)
Constituent
1
H␦
(ppm)a
CH2 (a)
C (b)
CH3 (c)
CH (d)
CH (e)
CAO (f)
CAO (g)
1.58, 1.95
—
1.05
2.35
2.70, 2.78
—
—
a
Amphiphilic copolymers can be made surface active
if the hydrophobic and the hydrophilic moieties are
properly balanced to render reduction of their surface free energies. The thermodynamic driving force
for this phenomenon is the tendency for aggregation
of the hydrophobic segments of the copolymers at
the oil/water or air/water interface. In the bulk
aqueous phase, these amphiphilic copolymers also
have a tendency to self-aggregate, forming intramolecular or intermolecular microdomains. The formation of such hydrophobic microdomains in the bulk
aqueous phase has been a subject of interest for
many studies on polymers containing hydrophobic
groups as a side chain in a homopolyelectrolyte,19 –21 or as a comonomer in random22–29 and
alternating8 –10,30 –33 copolymers.
One important aspect of the solution properties
of water-soluble hydrophobic polyelectrolytes is
their conformational behavior in various solution
13
C ␦ (ppm)a
48.4,
41.3,
27.8,
68.9,
46.9
185.0
188.7
49.1
41.4
28.9, 29.5
69.7
13
C
integralb
1.00
1.05
2.00
0.94
0.97
0.99
1.01
13
C T1 (s)
0.36
1.3
0.15–0.22
0.20
0.16
0.94
0.72
3-(trimethylsilyl)propionic-3,3,2,2-d4 acid, sodium salt (TSP) was used as an internal chemical shift reference; 1H ⫽ 0 ppm; 13C ⫽ 1.7 ppm.
b
The 13C CH3 integral was referenced to 2.00.
AMPHIPHILIC MALEIC ACID-CONTAINING ALTERNATING COPOLYMERS—1
3581
conditions. For example, chain dimensions of a
linear polycarboxylic acid are expected to increase
with increasing pH, resulting from electrostatic
repulsion of the charged carboxylate groups along
the chain. The electrostatic free energy required
for chain expansion can be related to the apparent
dissociation constant as
pKapp ⫽ pH ⫺ log[␣/(1 ⫺ ␣)]
⫽ pK° ⫹ (0.434/RT)共␦G/␦␣兲 (3)
where pK° is the intrinsic dissociation constant
and G is the electrostatic free energy per mol of
the maleic acid on the chain. Therefore, the shape
of the titration curve of pKapp versus ␣ can reveal
information about conformational changes of a
polycarboxylic acid with pH or ␣. For example,
pKapp for PAA increases smoothly with ␣. This
suggests that the electrostatic free energy for dissociation is related to the charging of each carboxylic acid on the chain in a monotonic manner. The
result is reproduced here in Figure 6(a) for a PAA
sample with a molecular weight of 60 kg/mol.
A change in slope of pKapp versus ␣ has been
reported for copolymers of maleic acid and n-alkylvinylether8 –10 and PMA19,20. The slope of
pKapp versus ␣ is larger in the lower ␣ region than
that in the high ␣ region, suggesting that a higher
free energy is required to dissociate the polyacid
in this region. This result is reproduced also in
Figure 6(a) for a PMA sample with a molecular
weight of 35 kg/mol. The anomaly around the low
pH region (␣ ⬃0.2) is attributed to an extra free
energy for dissociation, resulting from a conformational transition of the chain from a very compact coil at low pH to an extended state at
higher pH.
We have examined the titration curves of our
current maleic acid-containing copolymers in
light of the above argument. The forward pH titration data for IBMA, DIBMA, the n-hexylamine–modified IBMA, SMA, and n-C8MA were
used to calculate pKapp using eq 3 and plot pKapp
versus ␣, as shown in Figure 6(a)–(c). All copolymer samples were converted to the acid forms for
the titrations. The PAA curve is included in all
three figures for visual comparison. With the exception of the n-hexylamine–modified IBMA
[Fig. 6(a)], which shows the anomaly of pKapp increase at low ␣, all maleic acid copolymers studied
here do not seem to yield any initial increase in
pKapp at low ␣. The conformational change in the
n-hexylamine–modified IBMA is consistent with
Figure 6. pKapp versus ␣ for (a) PAA, PMA, and
IBMA-NHR; (b) PAA, IBMA, and DIBMA; (c) PAA,
n-C8MA, and SMA.
our later light-scattering finding34 of an intramolecular aggregation caused by the n-hexyl side
chains of the modified IBMA copolymer. Such intramolecular aggregation is weakened upon ionization, leading to expansion of the chain.
The higher pKapp beyond ␣ ⬎ 0.5 for IBMA,
DIBMA [Fig. 6(b)], and SMA [Fig. 6(c)] is consistent with the expectation that ionization of the
second carboxylic acid in these copolymers requires much higher free energy than the first. The
gradual increase with ␣ of the curve for n-C8MA
in Figure 6(c) suggests that the conformation of
the chain remains very much the same, although
we suspect that n-C8MA is in an aggregated state
3582
SAUVAGE ET AL.
throughout the whole range of ␣.32 It should be
noted that the shape of the curve for SMA obtained here is very different from those reported
by Ohno and coworkers,10 although a similar result to ours was also reported by Iida.33
The lack of anomaly in the pKapp versus ␣ at
low ␣ for all the copolymers, IBMA, DIBMA,
SMA, and n-C8MA, suggests the absence of a conformational change in this region. The copolymer
IBMA is extremely soluble in water over the
whole range of pH ⬎ 4, and is not expected to
render any significant conformation change.
There is no evidence of aggregation for SMA34 to
suggest any conformational transition. For the
copolymer n-C8MA, intermolecular aggregation
in aqueous media is suspected over the whole
range of pH, and the absence of any conformation
transition is not surprising.34
CONCLUSIONS
We have examined the dissociation behavior of
several water-soluble alternating maleic acidcontaining copolymers. A distinct two-step process for the dissociation of the two adjacent
maleic acids is observed in the presence of 0.02
N CaCl2. From IR data and the titration curves,
the state of hydrolysis of the parent maleic anhydride and the state of neutralization can be
characterized. From the same potentiometric
titration data, the composition of the copolymer
can be calculated. The result for all copolymers
is approximately [comonomer]/[maleic anhydride] ⫽ 1 and is confirmed by 13C NMR. A
conformational transition region at low pH from
a compact coil to an expanded coil is observed
for the hydrophobically modified n-hexylamine–
modified IBMA but not for other copolymers
studied. The conformational change in this copolymer is consistent with our later lightscattering finding34 of an intramolecular aggregation caused by the n-hexyl side chains of the
modified IBMA copolymer. The lack of conformation transition in IBMA, DIBMA, SMA, and
n-C8MA is related to the conformational characteristics of these copolymers in aqueous solution. IBMA is extremely water-soluble, with no
aggregation at any pH. DIBMA and SMA are
considerably more hydrophobic than IBMA but
still show no signs of any conformational transition, although DIBMA may have some aggregation.34 The strongly hydrophobic n-C8MA is
most likely aggregated at all pH,34 and still
shows no conformational transition as a function of pH.
The authors thank Ms. A. Miller for obtaining the IR
spectra, Dr. T. Mourey, Mr. T. Bryan, and Ms. K. Le
for performing the size-exclusion chromatography,
and Mr. D. Linehan for conducting some initial titration work. We are also grateful to Dr. I. Ponticello for
synthesizing some of the n-alkene-alt-maleic anhydride copolymers and to Dr. C. J. Verbrugge for the
gift of some of the same polymer samples.
REFERENCES AND NOTES
1. Klingelhoefer, P.; Denzinger, W.; Reynolds, M.;
Hartmann, H. BASF Aktiengeselschaft, WO 94/
154706, July 21, 1994.
2. Tan, J. S.; Schroeder, K. M.; Amos, D. A. US
6,472,136 (October 29, 2002).
3. Kuraray Co. Ltd. JP 94025227 (Apr. 6, 1994).
Based on Masao, H.; Hirotoshi, M.; Toshimitsu, K.
Kuraray Co. Ltd., JP 62257913 (Nov. 10, 1987).
4. Sandvick, P.E. S. C. Johnson & Sons, Inc., US
4,613,646 (Sep. 23, 1986).
5. Verbrugge, C. J. S. C. Johnson & Sons, Inc., US
4,358,573 (Nov. 9, 1982).
6. Okazawa, T.; Tokumaru, S.; Ohira, H.; Kano, Y.
Tosoh Corporation, US 5,336,727 (Aug. 9, 1994).
7. Demartino, R. N. Celanese Corporation, US
4,172,055 (October 23, 1979).
8. Dubin, P. L.; Strauss, U. P. J Phys Chem 1967, 71,
2757.
9. Dubin, P. L.; Strauss, U. P. J Phys Chem 1970, 74,
2842.
10. Ohno, N.; Nitta, K.; Makino, S.; Sugai, S. J Polym
Sci, Polym Phys Ed 1973, 11, 413.
11. Brissova, M.; Staudner, E. Eur Polym J 1996, 32,
529.
12. Reinhardt, S.; Steinert, V.; Werner, K. Eur Polym J
1996, 32, 935.
13. Kitano, T.; Kawaguchi, S.; Anazawa, N.; Minakata,
A. Macromolecules 1987, 20, 2498.
14. Hazen, S. M.; Heilman, W. J. Gulf Research &
Development, US 3,553,177 (January 5, 1971).
15. Hazen, S. M.; Heilman, W. J. Gulf Research &
Development, US 3,560,457 (February 2, 1971).
16. Chow, C. D. J Appl Polym Sci 1976, 20, 1619.
17. Tacx, J. C. J. F.; Meijerink, N. L. J.; Suen, K.
Polymer 1996, 37, 4307.
18. Komber, H. Macromol Chem Phys 1996, 197, 343.
19. Barone, G.; Crescenzi, V.; Liquori, A. M.; Quadrifoglio, F. J Phys Chem 1967, 71, 2341.
20. Chu, D. Y.; Thomas, J. K. In Polymers in Aqueous
Media; Glass, J. E., Ed.; Advances in Chemistry
Series 223; American Chemical Society: Washington, DC, 1989, p 325.
AMPHIPHILIC MALEIC ACID-CONTAINING ALTERNATING COPOLYMERS—1
21. Strauss, U. P.; Jackson, E. G. J Polym Sci 1951, 6,
649.
22. Morishima, Y. Adv Polym Sci 1992, 104, 51.
23. Morishima, Y. Trends Polym Sci 1994, 2, 31.
24. Yamamoto, H.; Mizusaki, M.; Yoda, K.; Morishima,
Y. Macromolecules 1998, 31, 3588.
25. Strauss, U. P. In Polymers in Aqueous Media; Glass,
J. E., Ed.; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989, p 317.
26. Magny, B.; Iliopoulos, I.; Audebert, R. Polym Commun 1991, 32, 456.
27. Wang, T. K.; Iliopoulos, I.; Audebert, R. In WaterSoluble Polymers; Shalaby, S. W.; McCormick,
C. L.; Butler, G. B., Eds.; American Chemical Society: Washington, DC, 1991.
3583
28. Petit-Agnely, F.; Iliopoulos, I. J Phys Chem B 1999,
103, 4803.
29. Petit-Agnely, F.; Iliopoulos, I.; Zana, R. Langmuir
2000, 16, 9921.
30. Zdanowicz V. S.; Strauss, U. P. Macromolecules
1993, 26, 4770.
31. Anthony, O.; Zana, R. Macromolecules 1994, 27,
3885.
32. Hu, Y.; Smith, G. L.; Richardson, M. F.; McCormick, C. L. Macromolecules 1997, 30, 3526.
33. Iida, S. Biophys Chem 1995, 53, 219.
34. Sauvage, E.; Plucktaveesak, N.; Colby, R. H.;
Amos, D. A.; Antalek, B.; Schroeder, K. M.; Tan,
J. S. J. Polym Sci Part B: Polym Phys 2004, 42,
3584.