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Polymer International 47 (1998) 340È344
N -Vinylimidazole-N -vinyl-2-pyrrolidone
Copolymers.
Part I. Reactivity Ratios, Solubility and
Viscosimetric Study¤
Francisco Mart• nez-Pin8 a, Ligia Gargallo* & Deodato Radic
Depto. Qu• mica F• sica, Facultad de Qu• mica, PontiÐcia Universidad Catolica de Chile, Casilla 306, Correo 22, Santiago, Chile
(Received 19 January 1998 ; revised version received 2 April 1998 ; accepted 8 May 1998)
Abstract : Copolymers of N-vinylimidazole (VI) with N-vinyl-2-pyrrolidone (VP),
(VI-co-VP) were obtained by radical polymerization using a,a@-azobisisobutyronitrile as initiator. The copolymers were characterized by 1H NMR and viscosimetric measurements. The monomer reactivity ratios were estimated to be r \
1
1É00 for VP and r \ 0É07 for VI by the FinemanÈRoss and KelenÈTuŽdos linear
2
procedures. These parameters were also estimated using a computer program
based on a non-linear minimization algorithm (NLMA) starting from the values
of r and r obtained by the linear procedures. All copolymers could be con1
sidered
as 2random. The solubility in chloroform, n-butanol and n-propanol
depends on the copolymer composition. Water, methanol and ethanol are solvents, irrespective of the copolymer compositions. The viscosimetric behaviour of
unfractionated copolymers of one composition was studied in dilute solution in
water (pH 2É0 and 6É0) at 0É01 M (NaCl) ionic strength and in methanol. ( 1998
Society of Chemical Industry
Polym. Int. 47, 340È344 (1998)
Key words : random copolymers ; reactivity ratios ; N-vinyl-2-pyrrolidone ;
N-vinyl imidazole ; non-linear minimization algorithm ; reactivity ratios error-invariable model
counterion binding and dye binding, have been extensively studied.3,4 Furthermore, the conformational
behaviour of poly(N-vinylimidazole) (PVI) has been
described in terms of nature of the solvent, quaternizing group, ionic strength and counterion type.4,5 PVI
can be considered as a good polymeric model for investigations about interactions with neutral salts, complexing metal ions and dye molecules.
Since 1978, copolymers containing N-vinyl-2-pyrrolidone (VP) have been the focus of many studies because
of its extensive properties.6h12 Poly(N-vinyl-2-pyrrolidone) (PVP) contains a highly polar amide group which
confers hydrophobic and polar-attracting properties,
and apolar methylene and methine groups in the backbone and the ring which confer hydrophobic properties.13 Because of these characteristics, this polymer
INTRODUCTION
It is known that macromolecules containing imidazole
groups play an important role in a-chymotrypsin
catalyses.1 Investigations concerning synthetic polymeric enzyme models containing pendent imidazole
groups have been reported.2 The roles of imidazole
groups in polymers, such as metalÈion complexation,
¤ Dedicated to the memory of Prof. Irmina HernandezFuentes.
* To whom all correspondence should be addressed : fax ]56
2 5525692 ; e-mail lgargall=puc.cl
Contract/grant sponsor : Fondecyt.
Contract/grant numbers : Projects 8970011 and 2970007.
Contract/grant sponsor : Catedra Presidencial en Ciencias Ï95.
340
( 1998 Society of Chemical Industry. Polymer International 0959È8103/98/$17.50
Printed in Great Britain
Reactivity ratios, solubility and viscometry of V I-co-V P
341
Copolymer preparation
Scheme 1
shows a high capability to interact with di†erent kinds of
molecules, such as detergents,14 drugs,15 dyes, aromatic
compounds16h18 and other complementary polymers.19h25 The amphiphilic character of PVP means
that the nature of the interactions should vary depending
on the hydrophobic/hydrophilic properties of the cosolute.
Binding characteristics and correlation between conformation, counterion binding, and dye binding of PVP
and PVI have been previously reported.3,26 The results
suggest that the binding capacity for PVI is dictated
primarily by its charge. The binding strength between
methyl orange (anionic dye) and polycation
(quaternized poly(N-vinylimidazole) homopolymers) is
inÑuenced by coulombic interactions. The results of the
binding constants and the thermodynamic parameters
of the binding of methyl orange by PVP could be interpreted satisfactorily by hydrophobic interactions
between hydrocarbon portions of the dye and nonpolar parts of the polymer.16,26 Copolymers containing
N-vinylimidazole (VI) and VP as comonomers should
be very interesting because they could have the capacity
to interact by di†erent mechanisms. The combination of
these two monomers could be a way to obtain highly
functionalized polymers with speciÐc properties (a type
of bifunctional polymeric system). In this work, we
report the preparation of copolymers of VI and VP (see
Scheme 1), their monomer reactivity ratios, and the
solubility and preliminary viscosimetric behaviour in
dilute solution.
The reactivity ratios r and r of (VI-co-VP) were
1
2
determined in order to estimate the copolymer
sequence. The r and r values are discussed in relation
1
2
to other copolymers reported in the literature.
EXPERIMENTAL
Materials
Commercial N-vinylimidazole (VI) was distilled under
vacuum before use. N-Vinyl-2-pyrrolidone (VP) was
dried over CaCl , Ðltered and distilled under reduced
2
pressure before copolymerization. All the solvents used
for measurements were puriÐed by appropriate procedures. Water was distilled twice.
POLYMER INTERNATIONAL VOL. 47, NO. 3, 1998
Radical copolymerization of VI with VP was carried
out in bulk under vacuum, using a,a@-azobisisobutyronitrile (AIBN) as initiator at 323 K for a period of 8 h. The
conversion of most of the copolymers was rather low
(\12%). PuriÐcation of the copolymers was achieved
by reprecipitation procedures using methanol as solvent
and acetone as precipitant. The purity was checked by
1H NMR spectroscopy ; the spectra were registered on a
Bruker AC 200 spectrometer using TMS as an internal
standard and deuterium oxide as solvent.
RESULTS AND DISCUSSION
The copolymer compositions (mol%) were determined
by 1H NMR analysis. A calibration curve was used for
this purpose, using the integral peak ratios for VI
(5É0 ppm and 5É5 ppm (four peaks)) and for VP (3É5 ppm
and 2É5 ppm (six peaks)).
The composition of the copolymers (VI-co-VP)
ranges from 49 mol% to 19 mol% of VI. (Vi-co-VP)
copolymers are soluble in water, methanol and ethanol
at all compositions studied. The solubility in chloroform, n-butanol and n-propanol depends on the copolymer composition. The solubility results are summarized
in Table 1 together with the copolymer compositions.
(VI-co-VP) copolymers of di†erent compositions were
characterized by viscosity measurements using a
DesreuxÈBischo† dilution viscosimeter27 with negligible
kinetic energy corrections. Intrinsic viscosity [g] was
determined according to the SolomonÈGotessman
relationship.28 Values in water at 298 K for copolymers
of di†erent compositions are also summarized in Table
1 ; these [g] values are generally very useful for estimating the degree of polymerization.
Reactivity ratios were evaluated by the least squares
method according to Fineman and Ross29 considering
VI as monomer 1 or monomer 2 (FR and FR ,
1
2
respectively). Nevertheless, VI is considered as
monomer 1 in the following analysis.
r and r were also determined by the KelenÈTuŽdoŽs30
1
2
(KT) treatment. Table 2 compiles the experimental
values of the copolymerization data for copolymers
containing VI with VP, together with the FinemanÈ
Ross and KelenÈTuŽdos parameters G and F, g, m and
a.30 No good agreement is observed for the values of r
1
and r obtained by the FR and KT methods. The
2
results shown in Table 3 would indicate that both linear
methods present a qualitative discrepancy. However,
irrespective of the method used to determine r and r
1
2
the copolymers could be considered as approximately
random. These copolymers containing VI units follow
the classical behaviour for radical copolymerizations in
the sense that r ¹ 1 and r \ 1. Di†erences in the
1
2
values of the reactivity ratios obtained by both methods
F. Mart• nez-Pin8 a, L . Gargallo, D. Radic
342
TABLE 1. Solubilitya at 298 K of (VI-co -VP) at different compositions
Copolymer
VI-co -VPb
Water
Methanol
n -Propanol
Chloroform
ÍgËc
(dl gÉ1)
1
2
3
4
5
48·8/51·2
39·8/60·2
37·0/63·0
27·0/73·0
19·0/81·0
½
½
½
½
½
½
½
½
½
½
É
É
½
½
½
É
É
É
½
½
0·35
0·54
0·73
0·74
0·85
a ½, Soluble ; É, insoluble.
b Copolymer compositions (mol%), determined by 1H NMR analysis.
c Determined in water at 298 K by the Solomon–Gotessman relationship.
TABLE 2. Copolymerization data for copolymer containing VI with VP
M a
1
88·88
66·65
57·87
34·98
24·98
dM b
1
xc
yd
Ge
Fe
gf
mf
48·76
39·79
36·96
27·00
19·03
0·106
0·424
0·617
1·574
2·543
0·890
1·281
1·444
2·290
3·603
É0·013
0·093
0·190
0·887
1·837
0·013
0·140
0·263
1·082
1·795
É0·079
0·318
0·456
0·718
0·943
0·076
0·478
0·632
0·876
0·921
af ¼ 0·1530
a Concentration of VI (mol%) in feed.
b Concentration of VI (mol%) in the copolymer.
c Ratio of monomer concentrations.
d Concentration ratio of copolymer components.
e Fineman–Ross parameter.
f Kelen–TuŽ dos parameter.
are expected according to data already reported.8,30,31
This is very common in the use of linear estimation
methods where gross inaccuracies have been documented.32,33 The main disadvantages of these linear
methods is the use of statistically invalid assumptions.33
Reactivity ratios are now usually estimated using procedures based on a statistically valid error-in-variables
model (EVM).32,33 Then, reactivity ratios of (VI-co-VP)
were also estimated using a computer program based
on a non-linear minimization algorithm (Reactivity
ratios error-in-variables method, RREVM), starting
TABLE 3. Monomer reactivity ratios for copolymers containing (VI-co -VP)
Method
r (VP)
2
r (VI)
1
r r
1 2
FR a
1
FR a
2
KTb
RREVMc
1·007
0·884
0·922
1·009
0·075
0·033
0·037
0·010
0·075
0·029
0·034
0·010
a Values obtained by the Fineman–Ross method considering VI as monomer 1 or monomer 2 (FR or FR ,
1
2
respectively).
b Kelen–TuŽ dos method.
c Reactivity ratios error-in variables method.
from the values of r and r obtained by the KT
1
2
method. Figure 1 shows the corresponding plot of the
95% posterior probability contour for (VI-co-VP).
Table 3 also summarizes r and r values obtained
1
2
using these non linear methods. A good agreement is
found when the reactivity ratios are determined by the
RREVM. However, r and r values determined by the
1
2
TidwellÈMortimer34 method were 0É0085 and 1É009 for
monomers 1 and 2, respectively. The corresponding
joint conÐdence contour shows a worse accuracy for
monomer 1 than the RREVM method.
The relative monomer reactivity in radical copolymerization can be conditioned by steric hindrance, but
Fig. 1. 95% posterior probability contour of r and r for
1
2
VI-co-VP.
POLYMER INTERNATIONAL VOL. 47, NO. 3, 1998
Reactivity ratios, solubility and viscometry of V I-co-V P
mainly by stabilization through the resonance of the
radicals generated during the copolymerization process.
The stabilization increases as the electronic delocalization increases, as was found in the case of 2- and
4-vinylpyridine.12
In Table 4 the values of the reactivity ratios for
copolymers containing a hydrophilic monomer and VP
are compiled. These values were obtained under conditions analogous to those used in this work.35,36 The
results presented in Table 4 show that the reactivity of
some monomers is difficult to explain. The formation of
a monomerÈVP complex, which could decrease the
activity of the radicals, cannot be disregarded during
the polymerization process.
The viscosimetric behaviour of unfractionated
copolymers of one composition was studied in dilute
solution in water (pH 2É0 and 6É0) at 0É01 M (NaCl)
ionic strength, and in methanol. The results are shown
in Fig. 2. A concave-upward curvature was observed in
the viscosity plots, i.e. g
versus c, for the aqueous
sp@c
solutions at pH 2É0 and 6É0, indicating the e†ect of the
ionization of the copolymer upon dilution. However,
linear viscosity plots were found in methanol.
Reduced viscosity (g ) increases (apparently without
sp@c
limit) as the concentration decreases along the curve
(concave upward). This phenomenon can be attributed
to polyions expanding rapidly under the inÑuence of
increasing ionization at higher dilution. According to
Strauss et al.37 concerning the behaviour of poly-
343
Fig. 2. Plot of g
versus c for copolymer 2 (39É8 mol% VI) :
sp@c
…, in water at pH 6É0 ; L, in water at pH 2 ; K, in methanol.
electrolytes, a moderate ionic strength would eliminate
this e†ect.
Attempts to eliminate the electrostatic repulsion with
added salt or variation of pH lead to the results summarized in Table 5. The [g] values reÑect the great
inÑuence of electrostatic interactions on the hydrodynamic behaviour of this copolymer. If the electrostatic
contribution is suppressed by the ionic strength, the
TABLE 4. Reactivity ratios of some monomers with N -vinyl-2-pyrrolidone
where r corresponds to VP and r to the other monomer
2
1
r
Monomer
2
r
1
r r
1 2
Ref.
Monomethyl itaconate
1·86
0·51
0·949
11
N -vinylimidazole
1·00
0·07
0·010
This work
2-Vinylpyridine
0·54
6·34
3·42
12
4-Vinylpyridine
0·47
3·60
1·69
12
Vinylacetate
0·26
2·3
0·60
34
Acrylamide
0·001 4
4·84
0·006 8
35
POLYMER INTERNATIONAL VOL. 47, NO. 3, 1998
F. Mart• nez-Pin8 a, L . Gargallo, D. Radic
344
TABLE 5. Intrinsic viscosity values for copolymer 2
in water at pH 2·0 and 6·0 at four different ionic
strengths
Ia
0·010
0·150
0·303
0·553
ÍgË (dl gÉ1)b
pH 2·0
pH 6·0
18·23
5·32
1·74
1·50
1·32
1·08
1·04
1·05
a Ionic strength in terms of molarity (M) of NaCl
b Estimated from the Solomon–Gotessman equation.28
macromolecules assume the random-coil conformation
of nonionic polymers. The compact form in the uncharged
state, in aqueous salt solutions, reaches the [g] value
found in methanol for this polymer. Further investigations are in progress to clarify these preliminary Ðndings.
ACKNOWLEDGEMENTS
We express our thanks to Fondecyt Project 8970011 for
Ðnancial help. F.M.-P. thanks Conicyt for a doctoral
fellowship and Fondecyt Project 2970007 for partial
Ðnancial help. D.R. acknowledges Ðnancial support of
Catedra Presidencial en Ciencias Ï95.
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POLYMER INTERNATIONAL VOL. 47, NO. 3, 1998
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