Cationic polybenzoxazines. a novel polyelectrolyte class with adjustable solubility and unique hydrogen-bonding capabilities

Article abstract of DOI:10.1021/ma201414t

This article fuses the world of benzoxazine chemistry with the field of polyelectrolyte chemistry and introduces a new approach toward water-soluble polybenzoxazines. The synthesis of a novel class of polybenzoxazine-based polyelectrolytes carrying cation

Full text of DOI:10.1021/ma201414t

ARTICLE
pubs.acs.org/Macromolecules
Cationic Polybenzoxazines. A Novel Polyelectrolyte Class with
Adjustable Solubility and Unique Hydrogen-Bonding Capabilities
Christian Sawaryn,†,‡ Katharina Landfester,† and Andreas Taden†,‡,*
†Max-Planck-Institute for Polymer Research, Mainz, Germany
‡Adhesive Research, Henkel AG & Co. KGaA, D€usseldorf, Germany
ABSTRACT: This article fuses the world of benzoxazine
chemistry with the field of polyelectrolyte chemistry and
introduces a new approach toward water-soluble polybenzox-
azines. The synthesis of a novel class of polybenzoxazine-based
polyelectrolytes carrying cationic charges in the side chain and/
or the backbone is described. Tertiary nitrogen atoms of the
backbone or side chain have been methylated in a polymer
analogous reaction leading to cationic polybenzoxazines with
structure dependent, adjustable solubility in various solvents
including water providing unique hydrogen-bonding capabilities. The polymers have been investigated by 13C NMR, SEC, TGA,
and DSC.
This publication introduces a new species of linear cationic
water-soluble polyelectrolytes of pendant and integral type
holding quaternary ammonium groups based on 1,3-benzoxazine
chemistry. In contrast to a previous contribution by Yagci et al. in
which the quaternization of pyridine-substituted benzoxazine
monomer has been used to create intercalated benzoxazine clay,5
our approach aims at water-solubility by creating linear poly-
benzoxazine polyelectrolytes derived by thermal ring-opening
polymerization followed by a polymer-analogous quaternization
reaction.
Benzoxazines have gained increasing interest over the past decade
due to their convenient synthesis and their ability to thermally poly-
merize without the addition of catalysts or initiators.6 Depending
on the molecule design, highly cross-linked thermoset materials
with excellent material properties can be obtained that open a wide
range of possible applications from composite materials over brake
adhesives to printed circuit boards.7 By blocking the para-position of
the phenolic ring, network formation can be prevented and linear
polybenzoxazines8,9 can be obtained offering new options for this
vivid field of research (Scheme 1).
Recently our group investigated water-soluble benzoxazine
systems that are able of stabilizing oil-in-water (o/w) emulsions.
The first publication in this regard introduced nonionic surfac-
tants containing thermally polymerizable benzoxazine units.10
Another article picked up this idea and combined it with the
concept of advanced chemical induced phase separation (advanced
CIPS) to obtain main-chain type benzoxazine prepolymers that
function as protective colloids in emulsion systems and are further-
more able to improve fracture toughness of a rigid matrix.11,12
1. INTRODUCTION
Polymeric compounds bearing quaternary ammonium moi-
eties are a class of polyelectrolytes with unique properties
resulting from the distribution and density of the positive charges
along the macromolecule. Like other polyelectrolytes, these
polymers find a wide range of different applications in daily life
and industrial processes as they can function as stabilizers,
flocculants, thickeners, antibacterial agents, dye transfer inhibi-
tors, or drag reducers for example and are a valuable additive in
water treatment, oil recovery, soaps, shampoos, and cosmetics.1
Different synthetic pathways have been described in literature
as chain growth and step growth polymerization of suitable
cationic monomers or species that turn cationic during the
polymerization process as well as chemical transformations of
uncharged reactive precursor polymers.1
The most significant polymer-analogous reactions to intro-
duce quaternary nitrogen in the polymer are the quaternization
of a halogen-containing polymer with a tertiary amine, the
arylation or alkylation of a polymeric tertiary amine, the addition
of agents like 2,3-epoxypropyltrimethylammonium chloride to
polymers containing OH-groups or the base-catalyzed Mannich
reaction of a polymeric amide with formaldehyde and dimethy-
lamine followed by quaternization.1 Depending on the location
of the cationic charge, one differs between pendant (charges
located in the side chain) and integral type (charges in the
backbone) systems. Typical examples for the integral type are
ionenes, polycations that carry quaternary nitrogen as the
charged species in the polymer backbone.2 They can be synthe-
sized by either the Menshutkin reaction of bis-tertiary amines
and dihalides,3,4 the self-condensation of aminoalkyl halides or by
the polyaddition of dimethylamine or the reaction of bis-tertiary
amines3,4 or secondary amines with epichlorohydrin in the
presence of HCl.4
Received: June 22, 2011
Revised:
August 15, 2011
Published: September 14, 2011
r
2011 American Chemical Society
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Scheme 1. Principle of the Polymerization of Benzoxazines
Leading to Linear or Cross-Linked Polybenzoxazines Using
the Example of N-methyl-3,4-dihydro-2H-1,3-benzoxazine
(P-m) Derivates
Poly(3-[3-(1H-imidazole-1-yl)propyl]-6-methyl-3,4-dihydro-2H-
1,3-benzoxazine) (PpC-napi). The polymer PpC-napi was synthesized
through the thermal curing of pC-napi. The monomer pC-napi was
transferred into aluminum pans for curing. Curing was done in an
autoclave at 160 °C for 5 h under 6 bar. The resulting polymer was
purified by dissolving it in ethanol and precipitating it three times in
chilled methanol. The supernatant was decanted and discarded, while
the precipitate was put under vacuum at room temperature for 1 day to
give a yellow solid (yield: 74%).
Poly[(3,4-dihydro-6-methyl-3-phenyl-2H-1,3-benzoxazine)-co-(3-
[3-(1H-imidazole-1-yl)propyl]-6-methyl-3,4-dihydro-2H-1,3-benzox-
azine)] (PpC-a/napi). The copolymer PpC-a/napi was synthesized
through the thermal curing of pC-a/pC-napi resin mixture in an
autoclave at 160 °C for 5 h under 6 bar of pressure. The resulting
polymer was purified by dissolving it in THF and precipitating it three
times in chilled methanol. The supernatant was decanted and disposed,
while the precipitate was put under vacuum at room temperature for 1
day to give a yellow solid (yield: 75%).
Methylated Poly(3,4-dihydro-6-methyl-3-phenyl-2H-1,3-benzoxaz-
ine) (MePpC-a). PpC-a was dissolved in DMSO and reacted with 2 mol
of dimethyl sulfate for every 1 mol of monomer units. The solution was
heated to 80 °C, and dimethyl sulfate was added dropwise. The solution
was stirred at 80 °C for 4 h. The solvent was evaporated to give a red
brown solid. The crude product was dissolved in methanol and
precipitated three times in diethyl ether. After precipitation, the super-
natant was decanted and the precipitate was dried under vacuum
overnight to give a red brown solid (yield: 70%).
Following the pathway of transferring benzoxazines to aqu-
eous media, this article fuses the world of benzoxazine chemistry
with the field of polyelectrolyte chemistry and introduces a new
approach toward water-soluble polybenzoxazines.
Methylated Poly(3-[3-(1H-imidazole-1-yl)propyl]-6-methyl-3,4-di-
hydro-2H-1,3-benzoxazine) (MePpC-napi). PpC-napi) was dissolved in
ethanol and reacted with 2 mol of dimethyl sulfate for every 1 mol of
monomer units. The solution was heated to reflux, and dimethyl sulfate
was added dropwise. The solution was stirred under reflux for 4 h. The
solvent was evaporated to give a yellow solid. The crude product was
dissolved in ethanol and precipitated three times in diethyl ether. After
precipitation, the supernatant was decanted and the precipitate was
dried under vacuum overnight to give a yellow solid (yield: 64%).
Methylated Poly[(3,4-dihydro-6-methyl-3-phenyl-2H-1,3-benzox-
azine)-co-(3-[3-(1H-imidazole-1-yl)propyl]-6-methyl-3,4-dihydro-2H-
1,3-benzoxazine)] (MePpC-a/napi) (Molar Ratio 1:1, 3:2, 7:3). PpC-a/
napi was dissolved in ethanol and reacted with 2 mol of dimethyl sulfate
for every 1 mol of monomer units. The solution was heated to reflux, and
dimethyl sulfate was added dropwise. The solution was stirred under
reflux for 4 h. The solvent was evaporated to give a yellow solid. The
crude product was dissolved in ethanol and precipitated three times in
diethyl ether. After precipitation, the supernatant was decanted and the
precipitate was dried under vacuum overnight to give a yellow solid
(yield: 67%).
Methylated Poly[(3,4-dihydro-6-methyl-3-phenyl-2H-1,3-benzox-
azine)-co-(3-[3-(1H-imidazole-1-yl)propyl]-6-methyl-3,4-dihydro-2H-
1,3-benzoxazine)] (MePpC-a/napi) (Molar Ratio 4:1, 9:1). PpC-a/napi
was dissolved in DMSO and reacted with 2 mol of dimethyl sulfate for
every 1 mol of monomer units. The solution was heated to 80 °C, and
dimethyl sulfate was added dropwise. The solution was stirred at 80 °C
for 4 h. The solvent was evaporated to give a red brown solid. The crude
product was dissolved in methanol and precipitated three times in
diethyl ether. After precipitation, the supernatant was decanted and the
precipitate was dried under vacuum overnight to give an orange solid
(yield: 67%).
2. EXPERIMENTAL SECTION
2.1. Materials. Paraformaldehyde, bisphenol A, N-(3-aminopropyl)-
imidazole and p-cresol were purchased from Acros Organics, aniline was
purchased from Merck, and dimethyl sulfate was purchased from Fluka.
All chemicals were used as received.
2.2. Synthesis. 3,4-Dihydro-6-methyl-3-phenyl-2H-1,3-benzoxa-
zine (pC-a). pC-a was synthesized from para-cresol, aniline, and
paraformaldehyde. The reactants were mixed in a 1:1:2.2 molar ratio
of p-cresol, aniline, and paraformaldehyde resulting in a 10% excess
of paraformaldehyde. The reaction was carried out in toluene with a
50 wt % solid content and refluxed for 5 h. Upon completion of the 5 h,
the product was allowed to cool, and it was washed three times with a
1.0 M NaOH solution followed by three times with deionized water.
After evaporation of the solvent the product was degassed for 2 h at
80 °C to remove residual solvent. A white crystalline solid was obtained
(yield: 87%).
3-[3-(1H-imidazole-1-yl)propyl]-6-methyl-3,4-dihydro-2H-1,3-ben-
zoxazine (pC-napi). pC-napi was synthesized from p-cresol, N-(3-
aminopropyl)imidazole, and paraformaldehyde. Synthesis was carried
out in analogy to the synthesis of pC-a. A highly viscous yellow product
was obtained (yield: 79%).
pC-a/pC-napi Resin Mixtures. Mixtures of different molar ratios
(1:1, 3:2, 7:3, 4:1, 9:1) of pC-a to pC-napi were prepared by mixing
the required amounts of the previously synthesized monomers
at 80 °C.
Poly(3,4-dihydro-6-methyl-3-phenyl-2H-1,3-benzoxazine) (PpC-a).
The polymer PpC-a was synthesized through the thermal curing of pC-a.
The monomer pC-a was cured in an autoclave at 160 °C for 5 h under 6
bar to prevent vaporization of the pC-a. Afterward the product was
postcured for 2 h at 200 °C in an oven. The resulting polymer was
purified three times by dissolving it in THF and precipitation in chilled
methanol. The supernatant was decanted and disposed while the
precipitate was put under vacuum at room temperature for 1 day to
give a red brown solid (yield: 72%).
6,60-(Propane-2,2-diyl)bis(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]
oxazine) (B-a). To 10 mL of toluene, paraformaldehyde (44 mmol,
1321.32 mg, 10% excess), bisphenol A (20 mmol, 2282.90 mg), and
aniline (20 mmol, 1862.60 mg) were added and refluxed for 5 h under
stirring. The reaction mixture was washed three times with 1 N NaOH
aqueous solution and three times with deionized water. After removal of
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Scheme 2. Synthesis of 3,4-Dihydro-6-methyl-3-phenyl-2H-
1,3-benzoxazine (pC-a) and 3-[3-(1H-Imidazole-1-yl)propyl]-
6-methyl-3,4-dihydro-2H-1,3-benzoxazine (pC-napi) and
Their Polymerization to Linear Polybenzoxazines
Scheme 4. Schematic Illustration of the Synthesis of Poly-
electrolyte MePpC-napi by Methylation of the Tertiary Ni-
trogen in the Backbone of the Linear Polybenzoxazine
Precursor and the Nitrogen of the Imidazole Group in the
Side Chain
Scheme 3. Schematic Illustration of the Synthesis of Poly-
electrolyte MePpC-a by Methylation of the Tertiary Nitrogen
in the Backbone of the Linear Polybenzoxazine Precursor
Table 1. Apparent Molecular Weights and Polydispersity
Indices (PDIs) of the Linear Polybenzoxazines Determined
by Size Exclusion Chromatography (SEC) (Eluent: DMSO)
Mw (g molꢀ1
)
Mn (g molꢀ1
)
PDI (Mw/Mn)
3
3
PpC-a
6300
5400
2400
2200
2000
3400
2.6
2.4
2.5
3.2
PpC-napi
PpC-a/napi(1:1)
PpC-a/napi(1:1)a
a With 2 wt % B-a.
5200
10 800
solvent via rotary evaporator, the product was heated to 80 °C for
another 2 h while high vacuum was applied to remove residual solvent.
The procedure afforded a white solid (yield: 84%).
on the other hand offers a second methylation side at the
nitrogen in the imidazole ring (Scheme 4). Therefore, linear
polybenzoxazine polyelectrolytes holding cationic charges in the
backbone and the side chain have been obtained.
Besides the homopolymers of PpC-a and PpC-napi, ran-
dom copolymers of pC-a and pC-napi have been synthesized.
The copolymer, PpC-a/napi, has been methylated according
to the principles for the synthesis of the homopolymers
(Scheme 3 and 4).
In general polybenzoxazines are hydrophobic materials of low
water uptake.13 This property is highly desired for most applica-
tions and a standard measure in material science. Because of the
introduction of cationic charges and the transformation of
polybenzoxazines in polyelectrolytes, water solubility can be
achieved which is quite unusual for this class of materials.
A detailed investigation can be found in chapter 3.2.
2.3. Instrumentation. Quantifiable 13C NMR spectra were ob-
tained on a Bruker 500 MHz Avance NMR spectrometer using inverse-
gated decoupling. Size exclusion chromatography (SEC) was measured
with Waters Alliance instrument with UV detector 2487 (254 nm) or
refractive index detector 2410. THF was used as a carrier solvent at a
flow rate of 0.9 mL minꢀ1 at 35 °C. Three polystyrene gel columns
(Waters Styragel HR1, 3 and 5) were used with measurable molecular
weights from 1 ꢁ 102 to 2 ꢁ 107. Polystyrene standards were used for
calibration to determine Mn and Mw/Mn. Differential scanning calorim-
etry (DSC) was measured on a TA Instruments Q1000 analyzer.
Samples (6ꢀ7 mg) were heated from ꢀ50 to 250 °C at a heating rate
of 5 °C/min under nitrogen. Thermogravimetric analysis (TGA) was
carried out on a TA Instruments Q5000 analyzer. Samples (6ꢀ7 mg)
were heated from room temperature to 500 °C at a heating rate of
5 °C minꢀ1 under nitrogen.
3
All structures were analyzed in detail. The apparent molecular
weights and the according polydispersity indices have been
determined by size exclusion chromatography (SEC) (Table 1).
Since the molecular weights of the nonmethylated are approxi-
mately identical to the values of the methylated species, SEC
experiments were carried out with the uncharged polymers to
avoid strong interactions with the stationary phase of the SEC
3. RESULTS AND DISCUSSION
Linear cationic polyelectrolytes of pendant and integral type
holding quaternary ammonium groups based on 1,3-benzoxazine
chemistry have been successfully synthesized. Two different
monomers based on p-cresol have been created according to
Scheme 2. The methyl group of p-cresol serves as a blocking
group to inhibit para-substitution and guide the thermal ring-
opening polymerization process to linear polybenzoxazines
(Scheme 2).
These linear polybenzoxazines served as precursors for the
synthesis of cationic polyelectrolytes. Because of the different
chemical nature of the side groups either integral type or a com-
bination of integral and pendant type have been received. The
aniline-based polybenzoxazine has been methylated at the tertiary
nitrogen in the polymer backbone using dimethyl sulfate as the
alkylation agent (Scheme 3). This reaction leads to cationic
integral type polyelectrolytes. The imidazole-based polybenzoxazine
column. Weight-averaged molecular weights of 5000ꢀ6000 g
3
molꢀ1 have been achieved with polydispersities of around 2.5.
From previous work, it is known that, in benzoxazine polymeriza-
tions, the chain growth is strongly dependent on the chemical
structureand reaction conditions, e.g., the use of catalyst or varying
reaction temperatures.14,15 However, in case of monobenzoxazine
polymerization usually low molecular weight species of similar or
even lower Mw values are obtained.16
The molecular weight can be increased by addition of
2 wt % of a difunctional benzoxazine, 6,60-(propane-2,2-diyl)bis(3-
phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine) (B-a), to the
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Figure 1. Quantifiable 13C NMR of the methylated polybenzoxazine
structure of MePpC-a in DMF-d7. The degree of methylation of the
backbone nitrogen (B) was determined to be 54.1%.
Figure 3. DSC analysis (endo down) of pC-a (solid line) and pC-napi
(dashed line). The heating rates in both cases were 5 K minꢀ1
.
3
The peaks at 29, 34, and 162 ppm are solvent peaks.
The spectrum of MePcC-napi is shown in Figure 2. Because of
the second methylation side two values for the degree of
methylation can be calculated. A value of 96.3% can be calculated
for the methylation of the nitrogen atom in the backbone from
the signals at 20 and 52 ppm. For the nitrogen atom in the
imidazole ring a value of 85.9% was achieved according to the
ratios of the integrals of the peaks at 20 and 35 ppm. Therefore,
the polymer can be regarded as a copolymer of four different
repeat units where either both, one or none of the tertiary amines
are methylated. These different structures lead to slightly shifted
signals that let the spectrum appear crowded. The signals at
around 150 and 137 ppm can be assigned to the aromatic carbons
in the backbone next to the hydroxyl group and the imidazole
carbon between the nitrogen atoms. The peaks in the range of
115 to 125 ppm can be assigned to the rest of the aromatic
carbons. The signals at around 25 ppm can be assigned to the
second methylene group in the side chain. The methylene groups
of the backbone and the methylene group of the side chain next
to a charged nitrogen overlap with the signal for the methyl group
of the quaternary nitrogen. The signals for the methylene groups
of the side chain next to uncharged nitrogen can be found in the
region of 46 ppm. The solvent peak appears at 39 ppm.
The big difference in the degree of methylation at the back-
bone nitrogen of MePpC-napi and MePpC-a can be explained by
the difference in basicity. In case of MePpC-a the nitrogen atom is
attached to an aromatic group in contrast to PpC-napi where an
alkyl chain is present. Comparing the pKb values for N,N-
dimethylaniline (pKb = 8.9) and triethylamine (pKb = 3.2) as a
reference, the basicity of the aromatic amine is significantly
weaker. Therefore, the backbone methylation can be pushed to
much higher percentage in the case of PcP-napi. The pKb of
the unsaturated nitrogen in the imidazole ring can be estimated
to be roughly 7. This estimation is based on the pKb of imidazole
(pKb = 7), since its basicity is originated from the free electron
pair of the unsaturated nitrogen located in the sp2-orbital
orthogonal to the aromatic ring electrons. The degree of
methylation is therefore expected to reach a value somewhere
between the values for the aromatic and alkyl-substituted backbone
nitrogen. The experimental value of around 86% follows this
theoretically derived trend.
Figure 2. Quantifiable 13C NMR of the methylated polybenzoxazine
MePcP-napi in DMSO-d6. The degrees of methylation of the polymer
backbone (B) and the imidazole in the side chain were determined to be
96.3% and 85.9%, respectively.
monomer before curing leading to molecular weights of around
11,000 g molꢀ1
.
3
3.1. Characterization of Polyelectrolytes. 13C NMR Analysis.
The chemical structures of the polyelectrolytes have been con-
firmed by quantifiable 13C NMR. The spectrum of MePpC-a is
shown in Figure 1. The spectrum allows the calculation of the
degree of methylation by comparing the integrals of the signals of
the methyl group in para-position to the hydroxyl group at 20
ppm and the methyl group at the quaternary nitrogen at 40 ppm.
1
An analysis via H NMR could not be achieved due to over-
lapping signals. 54.1% of the tertiary nitrogen atoms in the
backbone of the precursor were successfully transferred into
charged quaternary species. The resulting polymer can be
regarded as a copolymer of charged and uncharged repeat units.
These different structures lead to slightly shifted signals that let
the spectrum appear crowded. The aromatic carbons of the side
group and the backbone can be found in the region of 120 to 140
ppm. The peaks overlapping at 150 ppm can be assigned to the
aromatic carbons attached to the oxygen and nitrogen. The
methylene groups between the nitrogen and the aromatic ring of
the backbone can be assigned to the signal at 49 ppm, in case of
the nonmethylated, and 53 ppm, in case of the methylated
repeat unit.
3.2. Thermal Analysis. DSC and TGA analysis has been
carried out to investigate thermal behavior of the polyelectro-
lytes, unmethylated polybenzoxazines as well as the starting
monomers. Figure 3 shows the DSC thermograms of pC-a and
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Table 2. Glass Transition Temperatures of Unmethylated
Linear Polybenzoxazines Derived by DSC Analysis
sample
Tg/°C
PpC-a
72.5
47.1
73.2
PpC-napi
PpC-a/napi (1:1)
Figure 5. TGA thermograms of the polybenzoxazine polyelectrolytes.
The heating rates were 5 K minꢀ1
.
3
Figure 5 shows the degradation behavior of the polycationic
polybenzoxazines. It can be observed that the T5% values drop
significantly by around 60 °C in case of the imidazole containing
polymers and 90 °C for PpC-a. Therefore, the thermal stability is
drastically decreased.
In contrast to the thermograms of the umethylated species, the
graphs of the polyelectrolytes show an irregular decrease starting
at roughly 50 °C. This effect has its origin in the hygroscopicity of
the samples. The samples absorb water from the atmosphere
which is than evaporated during the heating process in the TGA
causing unwanted deflections.
Figure 4. TGA analysis of the unmethylated linear polybenzoxazines.
The heating rates were 5 K minꢀ1
.
3
pC-napi. The aniline based benzoxazine pC-a shows a pro-
nounced endothermic melting peak at 48 °C due to the high
crystallinity of the compound. The curing starts at 170 °C and
reaches its maximum at 213 °C. The exothermic curing of pC-
napi can be observed at lower temperatures with Tonset = 140 °C
and Tmax = 190 °C. On the basis of these results we chose the
matching curing cycles.
3.3. Solubility. The solubility of the different linear methy-
lated and unmethylated polybenzoxazines in a variety of solvents
has been investigated. Table 3 gives an overview of the results on
a scale from insoluble to very good solubility. DMF and DMSO
are able to solubilize all species. In toluene, acetone and ethyl
acetate the polymers proved to be insoluble. Methanol and
ethanol is able to solubilize the polyelectrolytes, whereas THF
can only be used as a solvent for the nonmethylated species.
As expected, all uncharged polymers do not show water
solubility. Furthermore, even MePpC-a is insoluble in water.
Only the pC-napi containing methylated polymers show very
good water solubility. These results show that the partial
methylation of the backbone nitrogen of ca. 54% in MePpC-a
is not sufficient to transform the polybenzoxazine into its water-
soluble form. The introduction of methylated imidazole groups
in the side chain in conjunction with an almost complete back-
bone nitrogen methylation is the key toward water solubility due
to the increase in the number of charges in the macromolecule. It
can be speculated that if a higher degree of methylation could be
achieved for MePpC-a, water-solubility is obtained.
Glass transition temperatures of the unmethylated linear
polybenzoxazines have been obtained by DSC measurements
(Table 2). The Tg of PpC-a of around 73 °C exceeds the value of
PpC-napi by over 25 °C. The difference can be explained by the
higher stiffness of the polymer chain of PpC-a caused by the
aromatic benzene ring linked to the nitrogen of the backbone. In
case of PpC-napi an alkyl spacer in the length of 3 methylene
groups is present which links the aromatic imidazole to the
backbone and results in higher chain flexibility and lowers the
glass transition temperature. Interestingly, the Tg for the copo-
lymer slightly exceeds the value of PpC-a. This is surprising, since
the more flexible imidazole based monomer unit is incorporated.
It can be speculated that increased hydrogen bonding due to the
unsaturated nitrogen in the imidazole ring and π,π-interactions
between the aromatic rings are the origin of this phenomena. The
formation of exceptionally strong hydrogen bonds is a well-
known characteristic of polybenzoxazines and has been already
described in literature.9
Further solubility experiments have been carried out to detect
a critical pC-a to pC-napi ratio in the copolymer MePpC-a/napi
up to which water-solubility is maintained (Table 4). Above the
ratio of 4:1 the polyelectrolytes are insoluble in water.
The degradation behavior has been studied by TGA. Thermal
stability is one of the key features of benzoxazines and an
important property for a wide range of applications. Figure 4
shows the thermograms and the resulting T5% values (weight loss
reaches 5%) of the unmethylated polybenzoxazines. PpC-a can
stand temperatures up to 271 °C compared to 258 and 259 °C
for PpC-napi and the copolymer, respectively. The comparable
values of the pC-napi based species lead to the thesis that pC-napi
is the limiting factor in terms of stability.
On the basis of the resulting molar ratios and the estimated
degrees of methylation obtained by 13C NMR for the homo-
polymers an overall degree of methylation for the backbone and
the side chain has been calculated (Table 4). For the weight ratio
of 4:1 a critical degree of methylation for the backbone of 62%
and 15% for the side chain is calculated. Of course, this value can
only be regarded as a rough estimation since it assumes that
water-solubility has its only origin in the number of charges and
neglects other important interactions. On the basis of these
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Table 3. Solubility of the Methylated and Unmethylated Polybenzoxazines in Different Solvents Rated on a Scale from “Insoluble”
(--), “Poor” (-), and “Good” (+) to “Very Good” (++) Solubility
PpC-a
MePpC-a
PpC-napi
MePpC-napi
PpC-a/napi (1:1)
MePpC-a/napi (1:1)
H2O
--
--
--
--
--
+
--
-
--
--
--
+
-
--
--
--
--
+
-
++
--
--
--
--
--
-
++
--
acetone
EtAc
--
--
MeOH
EtOH
THF
++
+
++
+
--
--
+
--
+
--
+
--
+
++
+
--
toluene
DMF
--
+
++
-
--
--
++
-
++
-
DCM
DMSO
-
+
+
+
Table 4. Solubility of MePpC-a/napi Copolymers of Differ-
ent pC-a/pC-napi Ratios in Water on a Scale from “Insoluble”
(--), “Poor” (-), and “Good” (+) to “Very Good” (++)
Solubility and the According Overall Degrees of Methylation
different linear homopolymers and copolymers of the monomers
3,4-dihydro-6-methyl-3-phenyl-2H-1,3-benzoxazine (pC-a) and
3-[3-(1H-imidazole-1-yl)propyl]-6-methyl-3,4-dihydro-2H-1,3-
benzoxazine (pC-napi) have been synthesized by thermal ring-
opening polymerization. In a second step, cationic polyelectro-
lytes have been obtained by methylation of the tertiary backbone
nitrogen and, in case of the pC-a based species, the nitrogen of
the imidazole ring.
pC-a/pC-napi ratioa
0:1 1:1 3:2 7:3 4:1 9:1 1:0
++ ++ ++ -- --
backbone 96 74 70 66 62 58 54
H2O
+
-
overall degree of
methylationb/%
13C NMR experiments allowed the calculation of a degree of
methylation for the individual methylation sides. Depending on
the basicity of the according nitrogens, values of around 54% for
the backbone nitrogen of the pC-a-based polybenzoxazine, 96%
for the backbone nitrogen of the pC-napi-based polybenzoxa-
zine, and 86% for the nitrogen of the imidazole ring have been
achieved.
side chain 86 40 32 23 15
8
0
a Molar ratio based on the amount of used monomer. b Estimated overall
degree of methylation based on the experimentally calculated degree of
methylation of the homopolymers (0:1 and 1:0).
results, a backbone alkylation of over 60% is necessary to obtain
water-solubility. The other 40% of the backbone groups is therefore
still available for the formation of hydrogen bonds that have proven
to be of major importance in polybenzoxazine chemistry.17ꢀ20
Please note that the already mentioned unique hydrogen bonding
capabilities of 1,3-benzoxazine-based polymers are a structure
inherent property and are fundamentally maintained even in the
solubilized state. To our knowledge our contribution in addition
with a recently published patent21 describes for the first time that
a partial polymer analogous alkylation reaction can be used for
adjusting the solubility of N-alkyl and N-aryl type polybenzoxazine
derivatives in various solvents including water. Especially the non-
modified repeat units due to the partial nitrogen quarternization
degree throughout the polymer structure are expected to provide
specific interactions in environments, which are typically not
accessible to polybenzoxazines. For example novel dye-transfer
inhibition properties, as desired for anticolor transfer detergents,
were already observed in aqueous media.21 It shall be emphasized that
this property relies fundamentally on intermolecular dyeꢀpolymer
interactions.22 Especially for this application the polycationic
nature of our novel polybenzoxazine derivatives supports in a
synergistic manner the adhesion to a variety of important substrates,
including polyesters, polyamides or wool. In conjunction with the
great design flexibility of polybenzoxazines the (partial) alkylation
therefore offers a versatile pathway to a novel polyelectrolyte class of
adjustable solubility and unique hydrogen bonding capabilities.
TGA experiments revealed that methylation decreases the
temperature stability by 60 to 90 °C leading to T5%-values of
181 °C for the pC-a-based homopolymer and 198 °C for pC-
napi-based polymers.
Special focus was lead on the solubility behavior of the
different polymers in various solvents especially water. The
partial methylation of 54% did not lead to water-soluble poly-
electrolytes. A series of PpC-a/PpC-napi copolymers of different
repeating unit ratios has been investigated to derive a critical
degree of methylation necessary to obtain water-soluble poly-
mers. A backbone methylation of over 60% in combination with a
side chain methylation of about 15% turned out to be the key
toward water-solubility. The unique capability for the formation
of hydrogen bonding, an important characteristic of polybenzox-
azines, is maintained due to the presence of unmethylated
nitrogen atoms in the polymer.
’ ACKNOWLEDGMENT
The authors thank Dr. Manfred Wagner for his NMR expertise
as well as our intern Benjamin Bangasser for the lab support.
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Different polyelectrolytes have been obtained by a polymer
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