1243
Macromol. Rapid Commun. 2001, 22, 1243–1248
Communication: Living potassium poly(N,N-dimethylacrylamide) initiates the polymerization of styrene and
butadiene, and adds 1,1-diphenylethylene in THF solution. The model compound a-potassio-N,N-dimethylpropionamide also polymerizes styrene and butadiene in contrast to esterenolates, which are known to be incapable of
such reactions. The IR spectra and SEC traces of the polymers obtained unequivocally prove that styrene and butadiene initiation proceeds directly via the amidoenolate
anion. Apparently, this is the first case observed where the
polymerization of a nonpolar monomer can be initiated by
the growing chain end of a polar polymer.
Copolymerization of N,N-Dimethylacrylamide with
Styrene and Butadiene: The First Example of Polar
Growing Chain End/Nonpolar Monomer CrossInitiation
Boris I. Nakhmanovich,1 Tatyana N. Prudskova,1 Alexander A. Arest-Yakubovich,* 1 Axel H. E. Müller 2
1
Karpov Institute of Physical Chemistry, Vorontsovo pole 10, Moscow 103064, Russia
Fax: +7-095-975-2450; E-mail: [email protected]
2
Makromolekulare Chemie II and Bayreuther Institut für Makromolekülforschung, Universität Bayreuth,
D-95440 Bayreuth, Germany
Introduction
As long as 50 years ago Mayo et al. showed that the anionic copolymerization of an equimolar mixture of methyl
methacrylate (MMA) and styrene resulted in almost pure
PMMA.[1] Since then, the inability of MMA to copolymerize with styrene, isoprene, and other nonpolar monomers has been well documented.[2 – 5] Graham et al. suggested that this phenomenon is due to the low basicity of
the PMMA anion.[3] The pKa values for the conjugated
acids, corresponding to anions of acrylic esters and typical nonpolar monomers, differ by more than 10 units
pKa = 30–31;
PhCH3,
pKa = 43;
(CH3COOC2H5 ,
CH22CHCH3 , pKa = 44[6]). The inability of nonpolar
monomers to add to esterenolate anions and to polar living chains in general is assumed, explicitly or implicitly,
in all text books on the theory of anionic polymerization,[6 – 9] and there has been a lack of new experimentation
in this direction.
Recently, the anionic polymerization of another acrylic
monomer, N,N-dimethylacrylamide (DMAA) has drawn
Macromol. Rapid Commun. 2001, 22, No. 15
attention.[10 – 12] The basicity of its anion is several units
higher than that of anions of acrylic esters (for the conjugated acid CH3CONMe2 , pKa = 34–35[6]), which reduces
the difference between DMAA and nonpolar monomers.
Besides, in a recent publication Xie and Hogen-Esch
reported that fluorenyl caesium cannot polymerize
DMAA.[10] In contrast, the salts of the fluorenyl anion
(fluorene, pKa = 22.6[6]) are known to initiate the polymerization of MMA,[3, 6] but not that of nonpolar monomers.[6] These considerations prompted us to examine the
possibility of DMAA copolymerization with styrene and
butadiene and to perform test experiments that have led
to quite unexpected results.
Experimental Part
Reagents
Tetrahydrofuran (THF) and monomers were purified by standard procedures[8] finally being treated with liquid K-Na
alloy (THF) and BuLi (styrene, butadiene and 1,1-diphenyl-
i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001
1022-1336/2001/1510–1243$17.50+.50/0
1244
B. I. Nakhmanovich, T. N. Prudskova, A. A. Arest-Yakubovich, A. H. E. Müller
Table 1.
Run
No.
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
k)
Summary of the experiments on DMAA (M1) copolymerization with nonpolar monomers (M2) in THF.
Initiator
M2
½M2 Š0
mol N Lÿ1
Conversion of M2
%
100
63
1a)
2a)
3a)
4g)
SDMAb)
K-a-MStd)
K-a-MStd)
K-a-MStd)
styrenec)
styrenec)
DPEe)
styrene
1.0
1.0
0.2
2.4
60–70
5g)
6g)
7j)
8j)
K-a-MStd)
K-a-MStd)
K-DMPAk)
K-DMPAk)
styrene
butadiene
styrene
butadiene
2.6
2.3
1.9
2.2
100
L30
100
L50
f)
Molar mass averages
—
—
—
—
103 N M n,theor
103 N M n,exp
M w /M n
50
80
12
30h)
85i)
480
36
L1
1.4
93
240
18
43h)
162i)
1.44 6 103
–
3.6
–
3.1
1.9
1.9
2h)
2i)
–
–
2
–
Addition of initiator to the monomer mixture at –30 8C.
Sodium dihydrobis(methoxyethoxy)aluminate, NaAlH2(OC2H4OCH3)2 , partly hydrolyzed (see Experimental Part).
DMAA/styrene equimolar mixture.
Living dipotassium-oligo(a-methylstyrene).
DMAA/DPE mixture.
DPE adds to PDMAA chain ends.
Polymerization of DMAA at –30 8C, then the nonpolar monomer was added at room temperature.
DMAA prepolymer.
Total copolymer.
Homopolymerization of nonpolar monomer at room temperature.
a-Potassio-N,N-dimethylpropionamide.
ethylene). DMAA (Aldrich) was dried over calcium hydride
and distilled under vacuum into ampoules equipped with
break-seals.[10, 12]
Living dipotassium-oligo(a-methylstyrene) (K-a-MSt) was
synthesized according to common procedure by the reaction
of a-methylstyrene monomer with potassium mirror in THF
at room temperature.[8] Sodium dihydrobis(2-methoxyethoxy)aluminate (NaAlH2(OC2H4OCH3)2 ; SDMA; Institute of
Inorganic Chemistry, Prague) was obtained as a 70% toluene
solution, diluted with toluene to 1 mol N L–1 and partly hydrolyzed (the extent of hydrolysis was 50%) in order to increase
its reactivity towards styrene polymerization.[13] The model
amidoenolate initiator, a-potassio-N,N-dimethylpropionamide (CH3CH(K)CON(CH3)2 ; K-DMPA) was synthesized
similar to the procedure described previously.[14] The solution of DMPA (6 mmol) in 10 mL of dry hexane was added
dropwise, with stirring, at –5 8C to the solution of potassium
hexamethyldisilazanate (6 mmol) in 20 mL toluene. After
the addition was finished, the reaction mixture was warmed
to room temperature and the solvents were distilled off under
vacuum. The solid product was washed with dry hexane and
dried under vacuum up to constant weight. 0.8 g (96%) of KDMPA were obtained as a white powder.
C5H10NOK (139.244): Calcd. C 43.13, H 7.24, N 10.06;
Found C 43.02, H 7.18, N 9.95.
Polymerizations
All operations were carried out under high vacuum conditions
in an all-glass apparatus using break-seal techniques as
described earlier.[12, 15] The polymerization of DMAA was performed at –30 8C, and the polymerization of nonpolar monomers at ambient temperature. After completion, polymeriza-
tion was quenched with a drop of methanol, the polymer was
precipitated by pouring the reaction mixture into a large
excess of hexane and then dried in vacuo to constant weight.
Typical monomer concentrations in polymerization experiments were ca. 1–2 mol N L–1, and initiator concentrations
were (5–10) 6 10–3 mol N L–1. Monomer conversion was
determined by gravimetry and was taken into account in cal—
culation of theoretically expected molecular weight (M n,theor).
Molecular weights and molecular weight distributions of
the polymers were measured by means of size-exclusion
chromatography (SEC) with a Waters-510 instrument
equipped with RI and UV (264 nm) detectors as described
earlier;[16] CHCl3 with 3% methanol was used as the eluent.
Calibration was performed using polystyrene (PSt) standards. IR spectra were obtained with a Perkin-Elmer FTIR1710 spectrometer, polymers were deposited on KBr surface
by evaporation of the solvent from CHCl3 solution.
Results and Discussion
Several series of experiments were performed using various initiators and ways of introducing the reactants into
the system. The experimental conditions and results are
given in Table 1.
In the first series (runs 1, 2) a thin-walled glass bulb
containing the initiator solution was broken at –30 8C
inside an ampoule containing an equimolar mixture of
DMAA and styrene in THF. Both experiments showed
the following qualitative pattern: instantaneous polymerization of DMAA on contact with the initiator, accompanied by heat evolution and a sharp increase in viscosity as
described in our preceding paper,[12] then a short quiet
1245
Copolymerization of N,N-Dimethylacrylamide with Styrene and Butadiene ...
Figure 1. SEC traces of DMAA-styrene copolymer obtained
by simultaneous addition of monomers (run 2).
period (10–15 min at room temperature), and after that a
second exothermic polymerization accompanied by
foaming of the reaction mixture. A high total yield of
polymer (Table 1) confirms the participation of styrene at
this stage. The instantaneous disappearance of the characteristic UV absorption band of a-methylstyryl anions at
330 nm on contact with DMAA (run 2) excludes the possibility of a participation of residual initiator in the initiation of the second stage of the process.
The polymers obtained in runs 1 and 2 are soluble in
CHCl3 . The SEC traces are multimodal (Figure 1).
Because the absorption coefficient of PDMAA at 264 nm
is very low (see Figure 2 a and Table 2) one can conclude
that the low-molecular weight part of the copolymer is
composed mainly of PDMAA, whereas the high-molecular weight fraction undoubtedly contains a marked
amount of styrene. This conclusion agrees well with the
low nucleophilicity of the DMAA anion leading to a low
initiation efficiency in the second stage and, consequently, to the presence of unreacted DMAA homopolymer. Other reasons for multimodal MWD of the copolymer will be discussed in more detail below.
The polymerization of a mixture of DMAA with 1,1diphenylethylene (DPE; run 3) proceeds similarly to runs
1 and 2. After the initial stage of violent DMAA polymerization the solution gradually acquired the red color characteristic of DPE anions. According to the optical density
of the UV band at 480 nm, approximately 20% of growing PDMAA chains added DPE. The SEC curve obtained
Figure 2. SEC traces of PDMAA prepolymer (a) and the total
copolymer (b) resulting from the sequential copolymerization of
DMAA and styrene (run 4).
Table 2. Analysis of copolymer from run 4 using the ratio of
integral intensities of signals from UV (k = 264 nm) and RI
detectors.
Polymer
polystyrene
(reference sample)
prepolymer (PDMAA)
total copolymer
fractions:
methanol-soluble
methanol-insoluble
ðUV=RIÞ ratio DMAA weight fraction
arbitrary units
%
38
0
0.46
27
100
30
3.5
35
92
8
with the use of the UV detector (not shown) confirms the
presence of DPE in macromolecules.
The behavior of the system resembles the well-known
styrene-butadiene copolymerization initiated by alkyllithium compounds in hydrocarbon solvents where butadiene polymerizes first, and only after its consumption
does styrene start to polymerize, resulting in the formation of a tapered copolymer.[8, 9] The rate constant of the
addition of styrene to the living poly(butadienyl lithium)
is about 4 orders of magnitude lower than the rate constant of the reverse process.[17] For the DMAA-styrene
system in THF this pattern seems to be similar and consistent with the large difference in the nucleophilicity of
the corresponding anions.
The next series (runs 4–6) was performed as follows.
First, a living PDMAA prepolymer was obtained by the
reaction of DMAA with the dipotassium salt of oligo(a-
1246
B. I. Nakhmanovich, T. N. Prudskova, A. A. Arest-Yakubovich, A. H. E. Müller
Figure 3. IR spectrum of DMAA-styrene copolymer (run 4):
(a) methanol-soluble, (b) methanol-insoluble fraction.
Figure 4.
IR spectrum of DMAA-butadiene copolymer (run 6).
methylstyrene) (K-a-MSt) in THF at –30 8C at a ratio
[DMAA]/[K-a-MSt] of L150 mol/g-equiv. A part of the
prepolymer was taken for analysis, and the rest was added
at room temperature to the second monomer (styrene or
butadiene). The prepolymer and copolymers obtained in
run 4 were dissolved in CHCl3 and examined by means of
SEC (Figure 2). As one can see from Table 1, the average
molar mass of the prepolymer (based on PSt standards) is
close to the theoretical value whereas the molar mass of
the copolymer is higher than the theoretical value. Apparently, this is due to the occurrence of some secondary
reactions resulting in the formation of graft copolymers
as will be discussed further.
Next, the copolymer was fractionated by precipitation
from CHCl3 into methanol, which is a precipitant for PSt,
but a good solvent for PDMAA. The large difference
between the UV absorption coefficients of DMAA and
styrene units enabled the estimation of the composition of
products from the ratio of integral intensities of the signals of the UV and RI detectors. The results of the measurements are given in Table 2. These data strongly suggest that the methanol-soluble fraction contains mostly
unreacted PDMAA prepolymer, with some copolymers
of styrene and DMAA, whereas the methanol-insoluble
fraction mostly contains copolymer and, possibly, some
styrene homopolymer. The IR spectra definitely indicate
the presence of PSt (characteristic bands of the monosubstituted aromatic ring at 697 and 745 cm–1) in the methanol-soluble fraction and the presence of PDMAA (characteristic band of the carbonyl group at 1 640 cm–1) in the
insoluble one (Figure 3). In a similar experiment (run 5)
the precipitate obtained after the precipitation of the total
copolymer from CHCl3 into methanol, after thorough
washing with hot methanol, contains 3% DMAA (according to NMR data). As methanol is a poor solvent for PSt
and a good one for PDMAA, these results prove the presence of styrene units in the methanol solution, and
DMAA units in the precipitate in the form of blocks or
grafted branches, but in no case as homopolymers.
A similar experiment with butadiene (run 6) gave principally the same result. In spite of a relatively low butadiene conversion, the reasons for which will be discussed
below, butadiene also adds to PDMAA anions. This is
shown not only by the increase in the total weight of the
product of polymerization, but also by properties of the
copolymer obtained. The copolymer was rubber-like,
well soluble in THF and THF/CHCl3 mixture (solvents
for both components) but insoluble in heptane (solvent
for polybutadiene), or in water and water-methanol mixture (solvents for PDMAA). The IR spectrum of the
copolymer shows strong bands of the carbonyl group
(1 640 cm–1) and the 1,2-butadiene unit (910 cm–1) and a
smaller one of the trans-1,4-butadiene unit (967 cm–1)
(Figure 4).
For an additional confirmation of the ability of the
PDMAA anion to initiate the polymerization of nonpolar
monomers, K-DMPA was synthesized as its model.
When this compound was added to a styrene solution in
THF at room temperature (run 7), the polymerization
started after 5 min, as was indicated by a warming and
foaming of the reaction mixture and the appearance of a
red color characteristic of PSt anions. The polymer yield
was close to quantitative (see Table 1). The low initiator
efficiency again indicates slow initiation. Butadiene polymerizes more slowly than styrene, the polymer yield after
3 h of polymerization at room temperature was ca. 50%
1247
Copolymerization of N,N-Dimethylacrylamide with Styrene and Butadiene ...
(run 8). It is essential that the IR spectra of both polymers, purified from possible traces of unreacted initiator
by the reprecipitation from THF into methanol, contain
the characteristic band of the carbonyl group, which confirms the initiation by amidoenolate anions.
Therefore, the results described above prove the ability
of living PDMAA chain ends to add nonpolar monomers.
The most convincing evidence is given in the following:
(i) a high total yield of polymer when both monomers
were simultaneously mixed with the initiator (runs 1, 2)
with spectrophotometric evidence for complete initiator
consumption well before the beginning of the second
stage of polymerization; (ii) the addition of DPE to
PDMAA growing chains (run 3) as confirmed by SEC
and UV spectroscopy; (iii) the ability of living PDMAA
prepolymer to initiate styrene and butadiene polymerization (runs 4–6), the simultaneous presence of polar (carbonyl group) and nonpolar (aromatic ring in the case of
styrene and vinyl bond in the case of butadiene) fragments in the thoroughly purified polymerization products
being proven by spectroscopy; (iv) finally, the ability of
the model potassium amidoenolate, K-DMAP, to initiate
the polymerization of styrene and butadiene, the presence
of the carbonyl group of the initiator in both polymers
being proven by IR spectroscopy (runs 7 and 8). Moreover, as we have shown in preliminary experiments,
potassium hexamethyldisilazanate, used in the synthesis
of the model amidoenolate, does not initiate styrene polymerization.
Apparently, this is the first observation of the polymerization of nonpolar monomers initiated by a polar chain
end. The exact chemistry of the process is not clear. In
the case when both monomers are introduced simultaneously, ideally one might expect the formation of
diblock or tapered copolymers. However, as can be predicted from the large difference in nucleophilicities of
polar and nonpolar living chain ends, and from the presence of highly nucleophilic nonpolar anions and reactive
polar functional groups, the copolymer composition can
be much more complex. Firstly, the former factor results
in a low initiation efficiency and therefore in the presence
of PDMAA homopolymer. Secondly, the attack of growing polystyrene (polybutadiene) chain ends on the
PDMAA side groups seems to be plausible; this would
result in the formation of graft copolymer under the
expulsion of N,N-dimethylamide anions (Scheme 1).
Such a reaction is well known for the interaction of living
polystyrene with PMMA;[18, 19] a similar reaction was
recently suggested by Hogen-Esch at al. for the anionic
polymerization of DMAA.[10] The ketone carbonyl group
formed according to Scheme 1 is more reactive than that
of the initial DMAA unit.[10] Therefore, it can be further
attacked by the next growing polystyrene (polybutadiene)
chain. Thirdly, Xie and Hogen-Esch suggested the possible deprotonation of the PDMAA methine protons by
Scheme 1.
Grafting of PSt anions onto PDMAA.
Scheme 2.
chains.
Proton transfer between PSt anions and PDMAA
strong bases.[10] This reaction, as well as the deprotonation of methine protons of newly formed ketone-containing monomer units, seem to be probable in our case due
to the high nucleophilicity of polystyrene (polybutadiene)
anions (Scheme 2).
Depending on the relative reactivities of the resulting
potassium N,N-dimethylamide (Scheme 1) or amidoenolate anions (Scheme 2) these reactions may have the character of chain transfer and/or chain termination. Chain
termination – explaining the incomplete conversion of
the nonpolar monomer – may proceed according to
Scheme 2 because the initiation ability of the sterically
hindered anion formed on a PDMAA chain should be
even lower than that of the living PDMAA anion. On the
other hand, as follows from several publications (see,
e. g., the paper by Lawson et al.[20] and references cited
therein), alkylamide anions are capable of initiating butadiene and styrene polymerization in hydrocarbon solvents
and in THF. Therefore, one cannot exclude chain transfer
according to Scheme 1. Butadiene seems to be more reactive in deprotonation but less reactive in reinitiation than
styrene, which explains its lower conversion.
The considerations given above enable us to qualitatively describe the results obtained. The exact mechanism
of the process and, especially, quantitative characteristics
of reactions need to be investigated in more detail. However, the reactions observed may be useful for the synthesis of new types of block and graft copolymers.
Acknowledgement: This work was supported by the Russian
Foundation of Basic Research (RFBR), project No. 00-0333209, and joint RFBR-INTAS Grant No. IR-97-278. The
authors would like to thank Prof. T. E. Hogen-Esch, Los
Angeles, for fruitful discussion and Dr. L. Lochmann, Prague,
for helpful advice on the K-DMPA synthesis.
Received: April 24, 2001
Revised: July 30, 2001
1248
B. I. Nakhmanovich, T. N. Prudskova, A. A. Arest-Yakubovich, A. H. E. Müller
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