Clays and Clay Minerals, 1968, Vol. 16, pp. 3 1 - 3 9 .
Pergamon Press.
Printed in G rear Britain
C L A Y M I N E R A L S AS E L E C T R O N A C C E P T O R S
A N D / O R E L E C T R O N D O N O R S IN O R G A N I C
REACTIONS
D. H. SOLOMON
Diyision of Applied Mineralogy,
C.S.I.R.O., G.P.O. Box 4331, Melbourne, Australia
(Received 28August 1967)
Abstract-Certain clay minerals have the ability to catalyze the polymerization of some unsaturated
organic compounds (styrene, hydroxyethyl methacrylate) and yet to inhibit polymer formation from
other closely related monomers (e.g. methyl methacrylate). This apparently contradictory behaviour of
the clay minerals can be rationalized in terms of electron accepting and electron donating sites in the
silicate layers. The electron acceptor sites are aluminium at crystal edges and transition metals in the
higher valency state in the silicate layers; the electron donor sites are transition metals in the lower
valency state.
The catalyzed polymerizations involve the conversion of the organic molecule to a reactive intermediate; thus where the clay mineral accepts an electron from the vinyl monomer a radical-cation is
formed, where the organic compound gains an electron it forms a radical-anion. Examples of these
reactions are discussed.
The inhibition of polymerization processes involves the conversion of reactive organic intermediates, such as free radicals, which have been formed by heat or radical initiators, to non-reactive
entities. For example, loss of an electron from the free radical gives a carbonium ion; in some cases
this will not undergo polymerization. An example of this type is the thermal polymerization of methyl
methacrylate.
The color reactions on clay minerals are useful in predicting the electron accepting or electron
donating behaviour of the clay minerals because they proceed by similar mechanisms to the polymerization reactions. For example, the benzidine blue reaction is a one electron transfer from the organic
molecule to the electron accepting sites in the mineral (aluminium edges, transition metals in higher
valency state).
Masking of the crystal edge with a polyphosphate destroys the electron accepting properties of the
crystal edge; this technique can be used to control the reactivity of the mineral and to distinguish
between the crystal edge and transition metal sites as electron-acceptor sites in the clay minerals.
INTRODUCTION
Table 1. The influence of kaolinite on the polymerization
of methyl methacrylate and styrene
(Conditions of reaction: Monomer/benzene 50/50 heated
for 30 min in presence of kaolinite (4 pts.))
WHEN a vinyl or acrylic m o n o m e r , that is a compound of the general formula
X
I
System
CHIC,
I
Styrene/benzene
Styrene/benzene/kaolinite
Methyl methacrylate/benzene
Methyl methacrylate/benzene/
kaolinite
Y
is heated with a clay mineral, two apparently
c o n t r a d i c t o r y effects have b e e n noted. S o m e
m o n o m e r s undergo a vigorous catalyzed polymerization w h e r e a s with other m o n o m e r s , the clay
mineral inhibits the normal thermal polymerization.
F o r e x a m p l e T a b l e 1 s h o w s the results obtained
w h e n kaolin is heated with methyl m e t h a c r y l a t e or
styrene; the styrene p o l y m e r i z a t i o n is c a t a l y z e d
w h e r e a s that of methyl methacrylate is inhibited.
Yield of polymer
(%)
0.9
100
0.9
0" 1
T h e organic p o l y m e r scientist is well aware that
various m e c h a n i s m s can o p e r a t e during the polymerization o f vinyl o r acrylic m o n o m e r s . F u r t h e r more, s o m e m o n o m e r s are more likely to polymerize by, for example, a cationic m e c h a n i s m
31
32
D. H. SOLOMON
whereas others favor anionic polymerization; the
substituents X and Y govern to some extent the
susceptibility of the double bond to attack by
cationic, free radical, or anionic reagents.
Table 2 shows a classification of the monomers
into classes representing the most favored mechanism for polymer formation; the influence of clay
minerals on the polymer forming reactions is also
shown. It can be readily concluded that the influence of a clay mineral on these polymerization
reactions is not simply the favoring of one particular mechanism. In the particular case of hydroxyethyl methacrylate it is even found that some
minerals catalyze polymer formation while others
act as inhibitors (Solomon and Swift, 1967). This
situation has led to a number of reports in the
literature that the surface of a clay is not conveniently classified in terms of its effect on polymer
forming reactions. In this paper, it will be suggested
that these polymerization reactions do fit into a
pattern provided that two factors are considered,
namely (i) the mechanism of the polymerization
reaction in the presence of the clay mineral, and
(ii) the nature and location of the active sites on the
mineral surface. To illustrate these principles, the
influence of clay minerals on the polymerization of
styrene, methyl methacrylate, and hydroxyethyl
methacrylate, will be considered in detail. It is
hoped to show that:
(I) The catalyzed polymerization of styrene takes
place as a result of interaction of the sytrene
molecule with the edge of the silicate structure.
(2) The inhibition of the polymerization of methyl
methacrylate results from the interaction of the
propagating free radical (formed by the action of
heat or by radical initiators) with the crystal edge.
(3) The catalyzed polymerization of hydroxyethyl
methacrylate (HEMA) is a monomer/transition
metal interaction that takes place between the
silicate layers. Inhibition of the polymerization of
Table 2. General relationship between monomer structure and favored
polymerization mechanism
Cationic
polymerization
Free radical or
anionic polymerization
Butene-2
(catalyzed)*"
Hydroxyethyl methacrylate
(catalyzed or inhibited)c
Alkyl vinyl ethers
(inhibited)~
Methyl melhacrylate
(inhibited)c
Acrylamide
(catalyzedy'
Vinyl pyridines
~catalyzed)c
Cationic or free radical
polymerization
Methyl vinyl ketone
(inhibited)d
Free radical
polymerization
Vinyl esters
(inhibited)d
Cationic, anionic or free radical
polymerization
Styrene
(catalyzed) b
N-vinyl carbazole
(catalyzed) a
*Action of mineral on polymerization.
"Friedlander, H. Z. (1963) Organized polymerization, Part I: J. Polymer
Sci., C. 4, 1291.
Friedlander, H. Z., and Fink, C. R. (1964) Organized polymerization
Part III: J. PolymerSci., B 2,475.
bSolomon, D. H., and Rosser, M. J. (1965) Reactions catalyzed by
minerals, Part 1: J. Appl. Polymer Sci. 9, 1261.
r
D. H., and Loft, B. C. (1967) Reactions catalyzed by
minerals, Part III: J. Appl. Polymer Sci. 11, In Press.
Solomon, D. H., and Swift, J. D. (1967) Reactions catalyzed by minerals,
Part II: J. Appl. PolymerSci. ll, In press.
aSolomon, D. H., and Swift, Jean D. (1967) Unpublished observations.
CLAY MINERALS IN ORGANIC REACTIONS
H E M A occurs when interlayer complexes are not
formed; this effect is similar to that found with
methyl methacrylate.
CATALYZED POLYMERIZATION OF STYRENE
When styrene is heated with aluminium silicates
a vigorous exothermic polymerization takes place
as soon as the styrene-water azeotrope distills from
the reaction mixture. This reaction occurs also at
room temperature if a pre-dried aluminium silicate
is added to styrene. Quantitative yields of polystyrene are formed rapidly when the mineral used
is kaolinite or attapulgite; montmorillonite is
slightly less active as a catalyst followed by pyrophyllite. Talc does not initiate the polymerization.
The results suggest that the important factors in
this type of catalytic activity include:
(1) The presence of aluminium in octahedral coordination, i.e. pyrophyllite is active, talc is not.
(2) The acidity of the mineral is significant; aluminium oxides and hydroxides are not active and
these are known to be weakly acidic compared with
aluminium silicates.
(3) The crystal edges are involved in the reaction
since attapulgite is much more active than montmorillonite. Both minerals have a 2:1 type structure but attapulgite has a higher edge/surface area
ratio. The importance of crystal edges in initiating
the polymerization of styrene is clearly established
by the loss of activity which results from polyphosphate treatment of the minerals; this treatment
is known to coat specifically the mineral edge.
It has been suggested (Solomon and Rosser,
1965) that the catalytic activity is related to aluminium in octahedral co-ordination at crystal edges
and that the mineral is acting as a Lewis acid. Thus
the drying operation removes co-ordinated water
molecules which are acting as a Lewis base and so
makes the acid site available for other molecules
(Fig. 1). This concept also explains why polar
solvents, monomers or polymers inhibit the catalyzed polymerization of styrene by clay minerals;
these polar compounds are stronger Lewis bases
than styrene and compete successfully for the
Lewis acid sites.
Studies on the mechanism of the polymerization
of styrene on clay minerals have shown that the
reaction has the characteristics of both a free
radical and an ionic mechanism (Table 3). F o r
example, the reaction is inhibited by benzoquinone,
diphenylpicryl hydrazyl is decolourized, and cobalt
ions increase the rate of reaction; all of these observations are consistent with a free radical or one
electron transfer type mechanism. On the other
hand, no chain transfer takes place in carbon tetrachloride and so free radicals are not involved in the
propagation step. Also the molecular weight of the
CCM-D
33
INFLUENCE OF DRYING ON
ACTIVITY
MINERAL
I
HEAT
|
_ _ ~-q
-I- HzO
+ ~]/CH = CHz
Styrene
__
/
CH2
6
Fig. 1. Influence of drying on mineral activity.
Table 3. Characteristics of styrene polymerization on
montmorillonite and attapulgite
Observation
Decolorizes DPPH, inhibited
by benzoquinone
Rate proportional to
temperature
Co ++ montmofillonite much
more active than Na §
montmorillonite
Carbon tetrachloride and
benzene as solvents give
comparable mol. wts. No
chlorine in polymer.
Mol. wt. of polymer low
(1000-4000) and high yield
of dimers, etc.
Reaction proceeds in presence
of CC14, CHCIz,
C 6 H t , CHa. CnH~ inhibited
by C2HsOH, CH3COOC2H~,
(C2H5)20,
CH3COCH3, dioxane
Inference
Radical mechanism
Radical mechanism
Radical mechanism
Not radical
Cationic
Adsorption process
involved in initiation
of reaction
polymer is relatively low and under some conditions
high yields of dimers and trimers are formed. This
suggests an ionic mechanism. Of the possible
reactive intermediates that can be formed from a
vinyl monomer (Fig. 2) a mechanism involving
radical-cations has been preferred since it satisfies
both the free radical and cationic characteristics
of the polymerization. This mechanism is shown in
Fig. 3. In support of this mechanism it should be
34
D . H . SOLOMON
~)H2-- )H
R
CH~'--iH
R
* ~
CAT&ON
R A D I C A L ANION
Benjidine
HzN~
N
H
Colorless
CH~~H
R
FREE RADICAL
ANION
Possible reactive intermediates from vinyl
monomers.
Inilia|ion
Clo~
4+
CH m
CH|
IL
CIOI J -
'(-
9
~
-e
C=CH z
6 C-
C --
~/
Silicate
Anthracene
CH - - C ~ |
+
CHz
-
Aluminium
Silicate
-- CHz -- C~
Fig. 4. One electron-transfer reactions on aluminium
silicates.
Propogot,or)
tion of a reactive intermediate that brings about
polymer formation.
CH -- CHz
+A +
Terminabon
+A-f-~-c.~c.,-~:.
,-
Lea,.,<>
%rc.,-c.~c.-c. +.*
L
CH - - CH~--CH. - - CH
o
2
~ o
Aluminium=
c5
Z
o
_1,I - DiphemjI. ethcjlene
R
RADICAL CATION
Fig.
+
z~
HzN= = ~ = = ~ N H
Aluminium
$ilicale
Blue
r.r
"
"9
Fig. 3. Proposed mechanism for polymerization of styrene
on clays.
noted that non-polymerizable hydro-carbons such
as anthracene and 1-1 diphenylethylene form
radical-ions on aluminium silicates (Rooney and
Pink, 1962). The oxidation ofbenzidine to benzidine
blue also involves a one electron transfer, and this
reaction takes place readily on many clay minerals,
particularly montmorillonite (Fig. 4). The benzidine
oxidation proceeds at crystal edges but additional
sites within the silicate layers are able to oxidize
benzidine to benzidine blue. Consequently the
activity of the minerals is not the same as found with
styrene.
Thus the polymerization of styrene by clay
minerals is an example of interaction between the
mineral and the organic molecule and the genera-
INHIBITION OF FREE RADICAL
POLYMERIZATIONS
When methyl methacrylate or similar monomers
are heated with clay minerals no catalyzed polymerization similar to that noted for styrene is
observed (Solomon and Swift, 1967). This can be
explained by the unfavorable orientation of the
adsorbed monomer for electron-transfer reactions
(Fig. 5), and/or the unlikely polymerization even if
electron transfer did occur, because methyl methacrylate is not susceptible to cationic polymerization. However, if methyl methacrylate is heated for
comparatively long periods (2 hr) in the presence of
a clay mineral, the normal thermal polymerization
which is a free radical reaction is inhibited (Table 4).
The presence of the mineral causes an increase in
the molecular weight of the polymer formed, and a
reduction in the yield of polymer. Similar results
are obtained with a benzoyl peroxide initiated
reaction. These observations (lower yield, higher
molecular weight) suggest that the mineral reduces
Orientation when adsorbed at mineral edcje
-•.{0o•
C-- CI = CHz
CHS CHs
c t Styrene where the
vintll double bond
is the electron donor
Fig. 5. Unfavorable orientation of methyl methacrylate
for catalyzed polymerization by minerals.
CLAY MINERALS IN ORGANIC REACTIONS
Table 4. Thermal polymerization of methyl methacrylate
Polymerization
conditions
Yield of
polymer (%)
(1) Heat under reflux in
benzene with stirring
for 2 hr
(2) As above but with 7
per cent attapulgite
(3) As above but with 7
per cent kaolinite
(4) As in (1) but with
7 per cent talc
Mol.wt.
3.68
1-364 • 106
none
-
0.512
2-882 • l06
2-20
2.543 • 106
the number of free radicals available for chain
initiation or propagation. A number of observations
suggest that a site of prime importance in this reaction is aluminium located at the crystal edge of the
silicate structure. The evidence for this conclusion
includes:
(1) A significant reduction in the inhibiting power
of the mineral after the crystal edge has been coated
with a polyphosphate (Table 5).
(2) The greater inhibition shown by attapulgite
compared with that by montmorillonite.
(3) The greater inhibiting power of aluminium
silicates compared with magnesium silicates.
Therefore, once again the Lewis acidity of the
clay minerals is involved and additionally the
manner in which this Lewis acidity inhibits a free
radical polymerization. Two likely mechanisms by
which weak Lewis acids such as the clay minerals
could inhibit free radical reactions involve preferential adsorption by the Lewis acid of the initiating
or propagating free radical and then either termination by combination or disproportionation, or by
electron transfer from the radical to the Lewis site
and the formation of a carbonium ion. These possibilities are shown in Fig. 6.
In the particular case of methyl methacrylate the
cation is not a propagating species and adds preferentially to the carbonyl double bond of the
monomer.
35
Support for the electron transfer mechanism
comes from a study of the action of the clay minerals
on stable organic free radicals and on simple
compounds. The triphenylmethyl radical is converted to a triphenylmethyl carbonium ion by
aluminosilicates (attapulgite, kaolinite, montmorillonite) but not by talc. This reaction is readily
followed by E S R and by visual spectroscopy. The
triphenyl-methyl carbonium ion eventually forms
triphenylcarbinol by abstracting a hydroxyl ion
from the mineral surface (Fig. 7).
Although the evidence tends to favor electron
transfer as the mechanism responsible for the
inhibition of free radical polymerizations the alternative of enhanced chain termination cannot be
eliminated. The possibility of enhancing the interaction of two radicals is supported by the work of
Hall (1965), who has shown that the thermal
decomposition of benzoyl peroxide in the presence
of an electron acceptor gives increased yields of
diphenyl as a result of combination of phenyl
radicals.
Thus this reaction is one in which an active
intermediate in the polymerization process is
de-activated by the mineral. It should be noted that
in both the catalyzed polymerization of styrene and
the inhibition of the free radical polymerization of
methyl methacrylate the mineral edge is acting as
an electron acceptor.
THE POLYMERIZATION OF HYDROXY
MONOMERS
When hydroxy monomers such as hydroxyethyl
methacrylate or hydroxypropyl methacrylate are
heated with a clay mineral two effects have been
noted. The polymerization is inhibited by aluminium
and magnesium silicates that do not form interlayer
complexes with the monomer (e.g. kaolinite) and
this aspect is similar to the influence of the minerals
on methyl methacrylate discussed above. On the
other hand, minerals that form interlayer complexes
and that have a transition metal in the lower valency
state in the silicate sheet catalyze the polymerization (Solomon and Loft, 1967). The reaction will
Table 5. Effect of a polyphosphate treatment on the ability of a mineral to inhibit the
thermal polymerization of methyl methacrylate
Polymerization conditions
Yield of Polymer
(%)
Mol.wt of Polymer
Monomer and solvent heated under
reflux with 7 per cent clay mineral
(a)
(b)
(c)
(d)
montmorillonite
phosphated montmorillonite
attapulgite
polyphosphated attapulgite
0'680
1"755
none
1.115
3-051 x 106
2"759 X 106
2.840 • 106
36
D . H . SOLOMON
Electron
transfer
with
formation
of
non propagating intermediate,
CH 3
Polymer
--
C H 2- -
~|
CH 3
Clay minerat
I
I|
---
Polymer --
C H 2 - - - CI
+
e
/
COOCH 3
COOCH 3
non
for
)ncreased
rate
propagating
cationic
X
of termination
I
Polymer - -
CH--C--
2
2
potymer-CH 2-
Ctalr mineral
species
polymerization
I
Y
X
I
C -- CH --
I
Y
Polymer
2
Comhinat ton
I
X
Y
X
I
Polymer
-- CH = C +
I
Polymer--CH2--CH
I
Y
Oispropor t=onation
I
Y
Fig. 6. Possible mechanisms for inhibition of free radical reactions by clay minerals.
l
9
Fig. 7. Mechanism of triphenyl methyl radical/kaolinite
reaction.
take place also at room temperature and, in contrast to the polymerization of styrene, in the
presence of water. In fact, limited amounts of water
actually increase the rate of polymerization.
Polymerizations of this type are not influenced to
any great extent by masking the crystal edges. Some
inhibition by the crystal edge would be expected
but under the conditions of the experiments, this
would be difficult to establish. Two likely mechanisms for these polymerizations which are initiated
between the silicate layers are electron transfer to
the monomer from the transition metal or peroxidation of the monomer followed by the catalyzed
breakdown of the hydroperoxides. The first of
these mechanisms is shown in Fig. 8.
The radical-anion is formed by electron transfer
from the mineral to the double bond; this mechanism
is similar to that proposed for transition metal
catalysts for olefin polymerization.
It is most likely that the anion would be destroyed
rapidly by reaction with proton donors associated
with the mineral surface (--OH of monomer, water
molecules). The free radical formed from the
radical-anion could then propagate between the
mineral layers and grow into the monomer external
to the mineral/monomer complex.
Indirect support for the feasibility of radicalanion formation on montmorillonites is given by a
study of the reaction of tetracyanoethylene
( T C N E ) with montmorillonite (Volclay). E S R
spectra show that the reduced form of the clay is
oxidized by T C N E (Fig. 9) which in the process is
converted to the radical-anion. Oxidized Volclay
CLAY MINERALS IN ORGANIC REACTIONS
I
Fe++
37
interlayer ~
+
complex
l MontmorittonitA:I
I
Montmorinonit:tI + CH2:
C-O
C=O
I
0
I
0
OH
OH
I
I
Fig. 8. Possible initiating mechanism for polymerization of hydroxyethyl
methacrylate by montmorillonite.
CN
CN
CN
I.on,o..,,on,7.I+Cl''
CN
CN
Fe+++ 1
Montrnaritton,tA:I -~- Oc~
,
CN
~C~
,
CN
Tetracyanaethylene
CN
Totrocyonoethylene radicaL
anion
(TCNE)
Fig. 9. Radical anion formation on montmorillonites.
did not react with T C N E . The accelerating effect
of water on the reaction is also in accord with an
oxidation type electron transfer mechanism
between the silicate layers since it has been noted
previously that water increases the rate of oxidation
of benzidine where the clay acts as an electronacceptor. The precise mechanism by which water
aids electron transfer has yet to be defined.
Evidence not in support of the hydroperoxide/
transition metal mechanism is that oxygenation of
the monomer prior to complex formation does not
accelerate the rate of polymerization. However, the
possibility that traces of oxygen adsorbed on the
mineral surface cause rapid hydroperoxidation, or
that the hydroperoxide content is not rate determining cannot be discounted.
[Montmori
Fe++ l onite All
~EMA
I ;.
i
Irrespective of the initiating reaction (electron
transfer or hydroperoxide breakdown) the suggested
propagation stage involves free r a d i c a l s - t h e most
likely intermediates for the chain growth of a
methacrylate under the conditions used.
Thus this reaction is one in which the monomer
forms an inter-layer complex with the mineral and
then forms a reactive intermediate as the result of
electron transfer from the mineral.
The reactions discussed in this paper contribute
to a more rational explanation of the apparently
contradictory behaviour shown by clay minerals on
polymer forming reactions. Thus the clay mineral
can act as an electron donor or an electron acceptot. The electron acceptor sites are aluminium at
the crystal edges and transition metals in the higher
Oz ~ l
Fe+'H"
E
ICHs
-CHZ-'i~,O
\
IF.:,,""
NC~,.... CN
NC/C- CxcN
All
s
CHz~
~ H2=.~=~'~ Hz
OCz H4OH
Fig. 10. Schematic representation of the influence of montmorillonite on
polymerization and related reactions.
38
D.H.
SOLOMON
v a l e n c y s t a t e in t h e silicate layers; t h e e l e c t r o n
d o n o r sites are t r a n s i t i o n m e t a l s in t h e l o w e r
v a l e n c y s t a t e in t h e silicate layers.
The catalyzed polymerizations involve the
c o n v e r s i o n o f t h e organic m o l e c u l e to a r e a c t i v e
i n t e r m e d i a t e ; t h u s , w h e r e t h e clay m i n e r a l a c c e p t s
a n e l e c t r o n f r o m t h e vinyl m o n o m e r , a r a d i c a l c a t i o n is f o r m e d , w h e r e the o r g a n i c c o m p o u n d gains
a n e l e c t r o n it f o r m s a r a d i c a l - a n i o n .
The inhibition of polymerization processes
involves the conversion of reactive intermediates,
s u c h as free r a d i c a l s f o r m e d t h e r m a l l y or b y
p e r o x i d e initiators, to n o n - r e a c t i v e entities, s u c h as
c a r b o n i u m ions w h i c h do n o t u n d e r g o c h a i n
polymerization.
The electron donating and electron attracting
r e a c t i o n s o f m o n t m o r i l l o n i t e are s h o w n s c h e m a t i cally in Fig. 10.
REFERENCES
Hall, C. D. (I965) Thermal decomposition of aroyl
peroxides in the presence of electron acceptors: Chem.
& Ind. (London), 384.
Rooney, J. J., and Pink, R. C. (1962) Formation and
stability of hydrocarbon radical-ions on a silica-alumina
surface: Trans. Faraday Soc. 58, 1632.
Solomon, D. H., and Loft, B. C. (1967) Reactions catalyzed by minerals, Part III: J. Appl. Polymer Sci. In press.
Solomon, D. H., and Rosser, M. J. (1965) Reactions
catalyzed by minerals, Part I: J. Appl. Polymer Sci. 9,
1261.
Solomon, D. H., and Swift, Jean D. (1967) Unpublished
observations.
Solomin, D. H., and Swift, Jean D. (1967) Reactions
catalyzed by minerals, Part II: J. Appl. Polymer Sci.
In press.
Rrsumr-Certains minrraux argileux ont le pouvoir de catalyser la polymrrisation de quelques composrs organiques non saturrs (styrene, hydroxyrthyl, mrthacrylate) et cependam, d'interdire la formation polym~re ~ partir d'autres monom~res 6troitement lids (par ex. le mrthacrylate de mrthyle). Ce
comportement apparamment contradictoire des minrraux argileux peut se rationaliser en termes de
zones d'acceptation d'rlectrons et de donation d'rlectrons dans les couches de silicate. Les zones
d'acceptation d'lectrons sont l'aluminium aux bords des cristaux et les mrtaux de transition dans
l'rtat de haute atomicit6 dans les feuillets de silicate; les zones de donation d'rlectrons sont les mrtaux
de transition dans l'rtat de plus basse atomicitr.
Les polymrrisations catalysres comprennent la conversion de la molrcule organique en un intermrdiaire de r~action; ainsi, quand le mineral argileux accepte un 61ectron du monom~re vinylique, il se
forme un radical-cation, quand le compos6 organique prend un 61ectron, il se forme un radical-anion.
Des exemples de ces rractions sont discutrs.
L'interdiction des processus de polymrrisation comprend la conversion des intermrdiaires organiques de rraction, tels que les radicaux libres, qui ont 6t6 forms par des initiateurs de chaleur ou radicaux, 5. des entitrs de non-rraction. Par exemple, le perte d'un 61ectron du radical libre donne un ion de
carbonium; dans quelques cas ceci ne subira pas une polymrrisation. Un exemple de ce type est la
polymrrisation thermique du mrthacrylate de m&hyle.
Les rractions de couleurs sur les minrraux argileux sont utiles pour connMtre b. l'avance si les
minrraux argileux vont accepter ou donner des 61ectrons parce qu'ils proc6dent de mrcanismes similaires aux rractions de polymrrisation. Par exemple, la rraction bleue de la benzidine est un transfert
d'un 61ectron d'une molrcule organique aux zones d'acceptation d'61ectrons dans le minrral (bords en
aluminium, mrtaux de transition en 6tat de haute atomicitr).
Masquer le bord du cristal avec un polyphosphate d6truit les propri6t6s d'acceptation d'~lectrons
du bord du cristal; cette technique peut &re utilis6e pour contrfler la r6activit6 du min6ral et pour
distinguer entre le bord du cristat et les zones de m6tal de transition comme les zones d'acceptation
d'61ectron darts les min6raux argileux.
Kurzreferat- Gewisse Tonminerale haben die Eigenschaft die Polymerisation mancher unges~ittigter
organischer Verbindungen (Styrol, Hydroxy~ithyl-Methacrylat) zu katalysieren, w~ihrend sie die
Polymerbildung anderer, nahe verwandter Monomere (z.B. des Methylmethacrylates) inhibieren.
Dieses scheinbar widerspruchsvolle Verhalten der Tonminerale kann rational auf Grund des Vorkommens von elektronenaufnehmenden und elektronenabgebenden Stellen in den Silikatschichten erkl~irt
werden. Die elektronenaufnehmenden Stellen sind Aluminium an Kristallkanten und 0bergangsmetalle im hrheren Wertigkeitszustand in den Silikatschichten; die elektronenabgebenden Stellen sind
Obergangsmetalle im niedrigeren Wertigkeitszustand.
Die katalysierten Polymerisationen schliessen die Umwandlung des organischen Molekiils in ein
reaktives Zwischenprodukt mit ein. Wenn also das Tonmineral ein Elektron vom Vinylmonomer
aufnimmt, wird ein Radikal-Kation geformt und wenn die organische Verbindung ein Elektron
aufnimmt, so bildet sie ein Anion. Es werden Beispiele fiir diese Reaktion errrtert.
CLAY MINERALS IN O RGANIC
REACTIONS
Die Inhibierung von Polymerisationsprozessen schliesst die U m w a n d l u n g reakfiver organischer
Zwischenprodukte, beispielsweise freier Radikale, die sich durch W~irme oder RadikalauslSser gebildet
haben, in nicht-reaktive Verbindungen mit ein. So fiihrt der Verlust eines Elektrons aus dem freien
Radikal beispielsweise zu einem Carbonium-lon; in manchen F~illen wird dieses nicht polymerisationsfiihig sein. Ein Beispiel dafiir stellt die thermische Polymerisation von Methylmethacrylat dar.
Die Farbreaktionen an Tonmineralen sind fiir die Vorausbestimmung des elektronenaufnehmenden
oder elektronengebenden Verhaltens der Tonminerale geeignet, da dieselben einem Mechanismus
folgen, der ~ihnlich dem der Polymerisationsreaktionen ist. So ist beispielsweise die BenzidinblauReaktion eine Einzelelektroniibertragung aus dem organischen Molekiil auf die elektronenaufnehmenden Stellen in dem Mineral (Aluminiumkanten, Ubergangsmetalle im hSheren Wertigkeitszustand).
Eine Maskierung der Kristallkante durch ein Polyphosphat zerst6rt die elektronenaufnehmenden
Eigenschaften der Kristallkante; diese Technik kann dazu verwendet werden, um die Reaktionsf'fihigkeit des Minerals zu steuern und um zwischen den Kristallkanten und den Ubergangsmetallstellen
Ms Elektronenaufnahmestellen in den Tonmineralen zu unterscheiden.
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npnMepbl TaKnx
pea~uatl.
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cnoco6HblX opraHnqecgnx npoMe~KyTOqHb]X 3BeHbea, nanp. CBOSOaHblX pa~n~anoB, KOTOpble
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MeTH.qMeTaKpnJ]aTa.
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39