Clay Minerals (1988) 23, 35--43
A CONTRIBUTION
TO T H E S T U D Y OF T H E
C A T A L Y T I C A C T I O N OF C L A Y S O N T H E
POLYMERIZATION
OF S T Y R E N E :
II. R E A C T I O N
MECHANISM
D. N J O P W O U O ,
G. R O Q U E S * AND R. W A N D J I
Laboratoire de Chimie Min~rale Appliqu~e, Facultd des Sciences, B.P. 812 Yaoundd, Cameroun, and
*Laboratoire de Chimie Physique Macromoldculaire, ENSIC, 1 rue Grandville. 54042 Nancy, France
(Received 27 April 1987; revised 16 November 1987)
ABSTRACT : The study of 13C NMR spectra of polystyrene thermally obtained on clays
shows that thermal polymerizationof styreneon these minerals proceeds by two parallel reaction
mechanisms: the radical mechanism due to the thermal effect, and the cationic mechanism in
which the initiatory cation H+ is provided by the clay. The latter mechanism, accelerated by
increase in temperature, becomes more important as the clay content increases in the reaction
medium. Otherwise, the thermal polymerizationon clays is accompanied by the hydrogenation
of some g carbons of the polystyrene molecule and the oxidation of certain aromatic carbons.
In a previous study, it was shown that mildly acid-treated clays catalyse the thermal
polymerization of styrene. The gel permeation chromatograms of the polystyrenes formed
revealed the intervention of two polymerization mechanisms (Njopwouo et al., 1987). Heat
acts on polystyrene by producing free radicals to start polymerization, and in this case, the
radical mechanism is evident. A supplementary polymerization test was carried out at
ambient temperature to demonstrate the intervention of the other mechanism; the control
tests (without clay), and those with the three clays used in the preceding study showed that at
the same clay catalyst concentration P (P = 0.8%), polymerization of styrene started after 3
days on Bo4 (KPIM* mixture), 4 days on Bal (halloysite) and 5 days on Bol (kaolinite) with
conversion rates z of 1-9%, 1-5% and 1.0%, respectively. In the control test, polymerization
began after 7 days. These results showed that clays could initiate the polymerization of
styrene by a non-radical mechanism. A cationic mechanism can be envisaged, and according
to Fripiat et al. (1965, 1971), silico-alumina surfaces possess Lewis and BrSnsted acid sites,
the latter being predominant in hydrated silico-alumina (as in clays). In other respects Conley
& Althoff (1971) showed that acid-treated kaolinite possesses BrSnsted acid sites (H3 O+) on
its surface at low temperature due to silanol groups, and Lewis acid sites (A13+) at high
temperature, from the octahedral gibbsitic sheet. In the trials reported, the acid sites,
especially Brfnsted style, could be responsible for the cationic polymerization mechanism
occurring in parallel with the radical mechanism. The present study is aimed at confirming
the presence of these two mechanisms from observations on ~3C nuclear magnetic resonance
(NMR).
* KPIM
-- kaolinite-palygorskite-illite-montmorillonite.
9 1988 The Mineralogical Society
D. Njopwouo et al.
36
MATERIALS AND EXPERIMENTAL
METHOD
Polystyrenes were obtained thermally, with or without clay catalysts, from styrene monomer
with a refluxing time of 30 min. The clay catalysts were Bol (kaolinite), Ba~ (halloysite) and
B04 (KPIM mixture) (Njopwouo et al., 1987).
A literature survey produced no information regarding ~3C NMR studies of polystyrenes
synthesised by a purely cationic mechanism. Consequently, the NMR spectrum of
polystyrene produced by a radical mechanism is used as reference in the current work. The
spectrum is characterized by three groups of peaks (Randall, 1975, 1976), symbolized by the
letters A, B and C where: (i) A corresponds to the resonance of aromatic carbon C1 with
chemical shift between 145 and 146.5 p.p,m. ; (ii) B comprises the resonance peak of aromatic
carbons C2, C3, C s, C6 and that of C4 appearing respectively from 127 to 130 p.p.m, and from
125.2 to 127 p.p.m.; (iii) C represents resonance peaks of aliphatic carbons -CH2- (42 to 47.5
p.p.m.) and - C H - (39 to 32 p.p.m.).
I
The NMR studies were carried out on a Jeol JNMS 100 NMR spectrometer at room
temperature. The solvent used was deuterium chloroform (CDC13) and the internal reference
was tetramethylsilane (TMS).
RESULTS AND DISCUSSION
The spectrum of polystyrene obtained without the catalyst (Fig. la) is similar to that of
polymer synthesised by the radical mechanism. However, in addition to the three groups of
peaks A, B, C and peaks of solvent CDCIa between 75 and 80 p.p.m., and of TMS at 0 p.p.m.,
a small peak designated B" around 115 p.p.m, and a group of peaks with very low intensities
between 135 and 143 p.p.m., named A", are observed.
The spectra of polystyrenes formed on clays (Fig. lb, c) comprise peaks or groups of peaks
observed on the previous spectrum, and in addition, upfield to downfield shifts: (i) a group of
peaks D at about 20 p.p.m., (ii) the multiple group of peaks C with the appearance of other
groups of peaks C" (36.5-38.5 p.p.m.) and C' (47.5-51 p.p.m.) on either side of C, (iii) a group
of peaks B' close to B, between 120 and 125-2 p.p.m. (iv) a group of peaks A' close to A,
between 146.5 and 151 p.p.m.
To examine the evolution of the intensity of each peak or group of peaks relative to the clay
catalyst concentration P, the height hi between the base line and the plateau of the integral
curve corresponding to a peak or group of peaks i were considered. The equipment and
conditions of obtaining the spectra were such that the nuclear Overhauser effect was
eliminated, and so it is considered that hi is proportional to the number of active sites
responsible for the peak or group of peaks i. The values of hi were measured for all spectra at
the same enlargement (X16).
With hAo, hBo and hco as values of hA, hB and hc, respectively, for polystyrene obtained
without the catalyst, the evolution of the base peak A, B, and C, characteristic of normal
polystyrene (obtained by the radical mechanism), can be followed using the variation of the
ratios hA/hAo, he/heo and hc/hco. Figs. 2a,c show that irrespective of the clay catalyst, the
number of active sites responsible for peaks A and C decreases significantly when the clay
concentration P increases. On the contrary, the number of active sites responsible for peaks B
(Fig. 2b) increases notably with P (hs/hBco > 1) for polymers formed on kaolinite-based clays
BOl and Bo4; this increase is maintained up to P = 1.8~ofor Bol (kaolinite) and to P = 2% for
Bo4 (KPIM mixture). In the polymer formed on Bal (halloysite), the intensities of peaks A, B
Polymerization of styrene on clays
37
C2
C3
CH-CH
6
C~
2
3
~,
4
/ n
--CH--
J
CI
(a) Without catalyst
l
(B")
(C)
CDCI
TMS
(b) On clay Bo~ at P = 0-4%
Integral curve
(c) On clay Bo~ atP = 2%
(A')
(A")
(c")
/
(c')
(B')
/
%
t.
160
140
120
100
80
4~0
0
~ (ppm)
FIG. 1. 13C N M R spectra of thermally obtained polystyrenes. (a) without catalyst; (b) on clay
Bol at P = 0.4~; (c) or~ ctay Bo I at P = 2%,
D. Njopwouo et al.
38
hA/hA,
1'3
(a)
1.0
o ~.,.,,.,...,~.t.
~
0.5
0~4
~
0!8
1~2
1!6
2:'0
P(%)
!
12
I
1-6
-=
2.0
P(%)
1.2
1.6
2.0
P(%)
hB/hs0
15t
t~
1.0
0.5
§ 2 4 7
~~
I
0.4
0
(b)
I
0.8
hc/hc~
11.3
(c)
0.5
0
0-4
0-8
FIG. 2. Variation of the ratios (a) hA/hAo; (b) hB/hao; (c) hc/hcoas a function of the content P of
the clay. + = c l a y B o t ; 9 = clayBat; 0 = clayBo4.
and C vary similarly with an irregular decrease for P < 0.4%; the decrease in the intensity of
B is less (hB/hso >i 0.8) than those of A (hA/hAo > 0"6) and C (hc/hco > 0-5).
The presence of clay probably results in: (i) a decrease of the number of aromatic carbons
C1 (peak A) and of the number of aliphatic carbons (peaks C); this decrease became
relatively more significant when the clay concentration P increased, (ii) an increase in the
number of phenyl carbons C2, C3, C4, C5 and C6 for polymers formed on kaolinite-based
clays (Bol and Bo~) and a decrease in the number of these carbons for polymers formed on
halloysite (Ba~).
39
Polymerization of styrene on clays
To attempt to explain these observations, it was logical to consider the evolution of the
intensities of new peaks or groups of peaks D, C", C', B", B' A" and A'. According to
Abraham & Loftus (1979) and Wehrli & Wirthlin (1980), the carbons responsible for these
peaks or groups of peaks are :
for D (15-25 p.p.m.)
- C H 2 - for C" (36.5-38.5 p.p.m.)
- C H - for C' (47.5-51 p.p.m.)
I
=CH2 for B" (110-120 p.p.m.)
= C H - for B' (120-125.2 p.p.m.)
-CH
3
CC-C
(Car-C) for A" (135-143 p.p.m.)
CC-O
(Car--O) for A' (146.5-151 p.p.m.)
Figs. 3a, b, c show the variations of hi relative to these peaks or groups of peaks as a function
of the clay catalyst concentration P. Amongst these curves, hA.,and hB,.evolve peculiarly: (i)
they have non-zero values for P = 0~ (thermal polymerization without catalyst) of 21.0 and
6.5cm respectively; (ii) for polymers formed on Box and Bo4, hA. increases greatly for P <
0.1~ and decreases similarly for P > 0-1~ (Fig. 3a, c); for polymers obtained on Bal, hAincreases up to P = 0"4~o before decreasing (Fig. 3b); (iii) up to P = 0.1~o, hB, increases
greatly for polymers formed on Bol and Bal, varies slightly for polymers obtained on Bo4, and
for P > 0.1~, it decreases and is practically zero at P = 2~.
For other peaks or groups of peaks (D, C", C', and A'), the values of hi are zero for P = 0~
and despite slight irregularities observed on hA, and ho for P < 0.4~, they globally increase as
P increases.
The clay catalyst was probably the cause of the formation of the methyl group ( - CH3)
responsible for the group of peaks D (15-25 p.p.m.); ho is zero for polymers formed without
the catalyst, and its values increase with increase in clay concentration, implying an increase
in the number of active sites responsible for peaks D. The formation of the methyl group is
easily explained by the intervention of a cationic mechanism in the polymerization of
styrene:
H
0r+ ',
~b-CH=CH2
H+
,
" "
~b-Ch--CH2
+
, ~b-CH-CH3
(I)
+
+
4~-CH + n (CH,=CH-q~)
l
CH 3
,
q~-eH
CH2-CH
CH2-CH
l /
I /
I
CH3 ~k~
~b / n - 1
~b
(2)
By elimination of a proton, the macromolecular cation would give rise to an ~methylpolystyrene possessing a C-C double bond according to the reaction:
~b-CH
~
cH3\
CH2-CH
~-
+-H+
CHz-CH
/._,
~ ~-CH
-~
CHz-CH
CH \
~-
CH=CH
/.-1
*
(3)
D. Njopwouo et al.
40
6'
i-_-
"
2,
0
t
e
- ~ ' ~ ' ~
P(%)
~
4
"=-
0
6~
211.~
2
O r.
I
O=hc"
(b)
e
r
.......
~
J
I=
h w.
.~
P(%)
0I;S:~
/r~
0.4
:---'-
0-8
1.2
1.6
2-0
P(%)
FIG. 3. Variation of h i as a function of the content P of the clay. (a) Bol ; (b) Ba~ ; (c) Bo4.
Polymerization of styrene on clays
41
This polymer might be responsible for peaks D (15-25 p.p.m.) due to the presence of
- C H 3 group and for peaks B' (120-125.2 p.p.m.) coming from = C H - carbon. The proton
that initiated this mechanism is provided by the clay as the increase in P is accompanied by
an increase in ho and hB,.
The group of peaks C" (36-5-38-5 p.p.m.) possibly resulted from the presence of clay which
acted as a hydrogenation agent. Indeed, after hydrogenation of ct carbons (see formula 1) on
an amorphous polystyrene, Randall (1975) observed a considerable upfield shift of the
- f l C H 2 - carbons in the hydrogenated polystyrene; the resonance of these carbons passed
from 42-47 p.p.m, to 33-39 p.p.m. This modification was accompanied by the decrease in the
height and splitting into a doublet of the peaks of ct carbons and by the multiplication and
large increase in the height of the peaks of fl carbons at 33-39 p.p.m. The result obtained in
the present Work is similar to that reported by Randall (1975). The clay probably entailed the
hydrogenation of ~ carbons and the appearance of peaks C" at 36.5-38.5 p.p.m.
The hydrogenation of 9 carbons was probably a substitution of the phenyl group by
hydrogen, resulting in an increase of - CHE - at the expense of the decreased ct carbons, and
in the appearance of more or less free aromatic rings in the medium. The decrease ofhc/hco as
P increases (Fig. 2c) is probably due to the decrease in the number of ct carbons which
contribute significantly to the peak C (Fig. la). On the other hand hc. increases with increase
in P as more ct carbons are hydrogenated.
The displaced aromatic rings can have two fates: (i) they can be fixed on the clay surface
and so are absent in the polymers; (ii) they can be fixed on the polystyrene and therefore
appear on the N M R spectrum. The latter possibly explains the presence and evolution of
peaks A' (146.5-151 p.p.m.) with P. These peaks correspond to the resonance of aryl carbons
bonded to oxygen C ~ r - O ; the clay acted in this case as an oxidant. The mechanisms of these
hydrogenation and oxidation reactions by clay are still uncertain but are possibly linked to its
acid pretreatment, chemical composition and structure. During the preparation of the
polymer, the precipitated polystyrene was not washed before drying, and compounds with
oxygenated aryl carbons have boiling points above the temperature at which the polymers
were dried (60-70 ~ It was then likely that the C , r - O compounds remained adsorbed on
the polystyrene and largely constituted molecules of low masses observed by gel permeation
chromatography (GPC) (Njopwouo et al., 1987). At P = 0.1% the intensities of peaks A' are
in the order:
hA,(Bo4) > hA,(Bol) > hA,(Bal)
similar to the GPC results on the amplitude of peaks M, corresponding to molecules of low
masses. The appearance of C~r - O was a result of the rupture of C1 - C of the polystyrene and
it is logical that the increase in P and concomitant increase in hA, entail a decrease in hA/hgo
(Fig. 2a).
The group of peaks C' (47.5-51 p.p.m.) probably represent the resonance of - C H I
carbons bonded to methyl - C H 3 (eqn. 3), a situation not found in a normal polystyrene (Fig.
4) as given by Roberts & Caserio (1968):
Flo. 4. Formula of a polystyrene molecule obtained by radical mechanism. (X = initiator
radical).
42
D. Njopwouo et al.
The groups of peaks B" (110-120 p.p.m.) and A" (135-143 p.p.m.), resonances for = CH2
and C a r - C carbons, respectively, probably resulted from the presence of styrene
~b- CH = CH2 molecules. Indeed, Wehrli & Wirthlin (1980) reported the resonance of phenyl
carbons at 128.5 p.p.m., but the presence of - C H = CH2 on Ct carbon entailed a downfield
shift of this carbon to 138 p.p.m, as in the case of styrene. Otherwise, in the first part of this
study (Njopwouo et al., 1987) it was shown that during the preparation of polystyrene, the
conversion rate was always < 100K for P < 1.6~ on Bol, P < 0.8~ on Bal and P < 0.4~ on
Bo4. As the polymerization was incomplete in these concentration ranges, it is possible that
styrene molecules remained adsorbed on the polymer, as the polymer was not washed after
preparation, and the boiling point of styrene (148 ~ is considerably higher than the drying
temperature. When P increased, the conversion rate increased and the polymerization
tended toward completion, resulting in a decrease of unpolymerized styrene that could be
adsorbed on the polymer. This decrease was directly related to the catalytic activity of the
clay, so that from the structure of the curves hA, and hB,, the following catalytic activity can be
deduced:
Bo4(KPIM mixture) > Bal(halloysite) > Bol(kaolinite).
This trend was observed on the effect of P on the conversion rate (Njopwouo et al., 1987). The
clay catalyst at low concentrations probably activated the adsorption of the unpolymerized
styrene on the polymer surface as peaks A" and B" are small in the polystyrene obtained
without catalyst. At P = 2~, hA- has values close to those of hc. and hc.. because of the
existence of C a r - C in the polystyrene, whilst the = CH2 carbons (peaks B") are absent (eqn.
3).
In addition to polystyrene, the presence of C a r - O compounds and adsorbed styrene
contributed to increasing the intensity of protonated phenyl carbons of resonance between
125.2 and 130 p.p.m. (peaks B), thus hB/hao > 1 ; the peaks B diminish as the adsorbed styrene
molecules decrease when P increases (Fig. 2b).
CONCLUSIONS
Although qualitative, the 13C N M R spectroscopic study of thermally-obtained polystyrenes
confirmed the hypothesis, made from GPC results, of two parallel mechanisms (Njopwouo et
al., 1987): (i) the radical mechanism due to the thermal effect; (ii) the cationic mechanism in
which the initiatory cation was H § provided by the clay. The latter mechanism, accelerated
by increase in temperature, became more important as the clay content increased in the
reaction medium. Otherwise, this study showed that the polymerization on clays was
accompanied by the hydrogenation of some ct carbons of the polystyrene molecule and the
oxidation of certain aromatic carbons.
ACKNOWLEDGMENTS
The authors thank Dr Cung Manh Thong of the Laboratoire de Chimie Physique Macromol6culaire de
I'ENSIC, Nancy, Dr Ngando Mpondo of the Laboratoire de Synth6seOrganique, Yaound6 and Dr U. Chinje
of the Laboratoire de Chiniie Min6rale Appliqu6e, Yaound6, for obtaining the NMR spectra, for help in
interpretation of the spectra, and for corrections to the manuscript, respectively.
Polymerization o f styrene on clays
43
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