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Surfactant effects on heterogeneous polymer reaction Nucleophilic substitution of poly(vinyl chloride) in water.

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Die Angewandte Makromolekulare Chemie 28 (1973) 121-119 ( N r . 422)
From the Research Laboratory of Resources Utilization, Tokyo Institute of
Technology, Ookayama, Meguro-ku, Tokyo (Japan)
Surfactant Effects on Heterogeneous Polymer
Reaction: Nucleophilic Substitution
of Poly(viny1 chloride) in Water
By M. TAKEISHI,
Y. NAITO,and M. OKAWARA
(Eingegangen am 4. Juli 1972)
SUMMARY:
The reactions of poly(viny1 chloride) (PVC) with some nucleophiles such as
azide, thiophenoxide and dithiocarbamate ions do not take place in water. These
heterogeneous reactions are remarkably catalyzed by cationic surfactants. Urea and
methanol which destroy hydrophobic bondings inhibit the reaction. On the basis of
the experimental data it was assumed that the cationic surfactants are adsorbed
on the surface of the suspended PVC powder and that the positive charges attract
the nucleophiles.
ZUSAMMENFASSUNG :
Die Reaktionen von Polyvinylchlorid (PVC) mit einigen nucleophilen Agentien
wie Azid-, Thiophenoxid-, und Dithiocarbamat-Ionen kommen in Wasser nicht
zustande. Die heterogenen Reaktionen werden von kationenoberfllichenaktiven
Substanzen wirksam katalysiert. Harnstoff und Methylalkohol, die hydrophobe
Bindungen zerbrechen, inhibieren die Reaktion. Auf Grund experimenteller Daten
wird angenommen, da13 die kationenoberflachenaktiven Substanzen von dem in
Wasser suspendierten pulvrigen PVC adsorbiert werden und da13 die positive
Ladung die nucleophilen Agentien heranbringt .
1. Introduction
Most earlier studies of surfactant effects on organic reactions in water are
concerned with micellar catalysis, and they have been reviewedl-3. Recently,
remarkable enhancements in reaction rates were observed in surfactant media4Ji.
These reactions in micellar systems are of interest in relation with enzymatic
reactions because the incorporation of a substrate into t h e organic phase is a
prerequisite for both micellar a n d enzymatic catalysis.
Another surfactant effect is the"phase-transfer" catalysis, in which an
ionic surfactant carries a n ionic reagent from the aqueous phase t o the organic
phase containing a substrate61 7.
111
M. TAKEISRI,
Y.NAITO,and M. OKAWARA
I n this paper we wish to report on the kinetic effect of a surfactant on a
heterogeneous polymer reaction. Poly(vinyl chloride) (PVC) powder suspended
in water does not react with sodium azide, which easily reacts with PVC in
polar organic solventss. However, the reaction was observed to proceed in
water in the presence of some cationic surfactants.
2. Results and Discussion
2.1 Kinetics
Cationic surfactants catalyze the nucleophilic substitution of PVC powder
with sodium azide in water (Table 1). '
Table 1. Catalysis of cationic surfactants in the reaction of PVC with sodium
azide.
Surfactant
None
(CzH5)4N+C1(n-C4Hg)4N+CI(n-C4H9)4N+Br(n-C4Hg)dN+I(CH3)3(PhCHz)N+Cl(CiaH37)(CH3)z(PhCHz)NfC1(CiaH37)z(CH3)zN+Cllaurylpyridinium chloride
sodium laurylbenzenesulfonate
I Temp. ("C)
I Conversion ( o / ~ )
80
80
80
80
60
80
80
80
80
80
0
0
10.0
11.0
4.3
0.4
0.9
0.5
0.6
0
Water: 10 ml; PVC: 0.5 g ; NaN3: 0.5 g ; surfactant: 0.5 g ; 16 hrs.
Tetra-n-butylammonium salts are strong catalysts, and the catalytic effect
of other cationic surfactants is week. The initial rates are calculated from the
amounts of reacted sites in a unit volume, similarly to a homogeneous reaction;
they are plotted against the concentrations of tetra-n-butylammonium bromide
(TBAB)(Fig. 1).
The rate increases with increasing ammonium salt concentration, but a
saturation phenomenon was observed a t high TBAB concentrations. Azide ion
seems unable to attack solid PVC powder suspended in water, because the
polymer does not wet sufficiently and the azide ion is strongly solvated by
112
Substitution of Poly(viny1 chloride)
C T BABlo
M
Fig. 1. Catalysis of tetra-n-butylammonium bromide (TBAB). PVC: 1 mole/l;
NaN3: 1 mole/l; 80°C.
water. Accordingly, the catalysis could be interpreted as follows : the cationic
surfactant is adsorbed on the surface of solid PVC, by hydrophobic bonding,
and then the positive charge attracts azide ions to let them react with PVC.
Generally, the adsorption of an ionic surfactant on hydrophobic surface is of
the LANcMmR-type and its adsorption on hydrophilic surface is of the BETtype of two layers. A colloidal carbon, Graphon, which is free of superficial
oxigenated sites, hence, carries essentially no electrical charge on its surface;
however, it adsorbs sodium lauryl sulfate in a process of the two-layer typeg.
Even though the adsorption of an alkyl-ammonium salt on the surface of PVC
is of the BET-type, only the first layer will participate in the catalysis, and
consequently, the LANaMnm-type adsorption must be taken into account.
Since the amount of adsorbed TBAB is too small in comparison with the
added TBAB to be determined by a method of ordinary potentiometric titration with silver nitrate, the equilibrium concentration of TBAB, [TBAB],,, in
the aqueous phase is approximately equal to its initial concentration, [TBAB],.
According to the LANGMUIRadsorption the amount of adsorbed TBAB,
(TBAB),,is
(TBAB), =
K a [TBAB] eq - K a [TBAB],,
1
K [TBAB]. ’
1
K [TBABIeq
+
+
where K is the adsorption constant and a is the maximum adsorbed amount.
The initial rate is assumed to be directly proportional t o the amount of adsorbed
TBAB, and taking equation (1) into account, one gets:
VO
= c(TBAB)B =
c K a [TBAB].
1
K
+ [TBAK
’
(2)
€13
M. TAKEISHI,
Y. NAITO,and M. OKAWARA
where c is a constant which includes a PVC concentration term, a sodium azide
term and the rate constant itself. Eq. (2) predicts a saturation phenomenon in
the relationship between the rates and TBAB concentrations. I n order to
examine the validity of eq. (2) the reciprocals were plotted according to eq. (3)
(Fig. 2):
_1 _-- .
vo
1
1
cKa
[TBAB].
+ -c1.a
(3)
The good linear relationship suggests that the reaction proceeds by the assumed mechanism.
5
10
C TBABfJ
15
M-'
Fig. 2. Plots of reciprocals, corresponding t o Fig. 1.
Next, the rates were plotted against reaction temperature (Fig. 3). The
plots exhibit a sharp increase around 80 "C, and this finding cannot be ascribed
only to the increase of the rate constant. One reasonable explanation is that
the hydrophobic interaction increases a t higher temperature within the
examined range, in such a way that the amount of adsorbed surfactant increases with increasing temperature. This effect will be stronger for chlorides
than iodides, because the formers are more ionic and more soluble than the
latters in water; accordingly, the adsorption of the former will be relatively
hindered a t lower temperature. Consequently, the inversion of catalytic effect
between chloride and iodide around 60 " to 80 "C may be ascribable to the above
assumption.
If the adsorption is prerequisite, the reaction rate must be controlled by the
surface area of the PVC powder and may thus dependonparticle size. However, the reaction rates did not depend on particle size of PVC powder over a
range 48 mesh to 325 mesh. On the basis of these results and of the unexpected
114
Substitution of Poly(vinyl chloride)
Fig. 3.
Catalysis of tetra-n-butylammonium halide: 0:Cl-;
:
Br-; 0:I-.PVC: 0.80mole/l;
NaN3: 0.77 mole/l; ammonium halide: 0.22 mole/].
Temperture
"C
high conversion ratio attained in the reaction with thiophenoxide, as discussed
in 2.4, the surface area must be large enough, regardless of the particle size.
Although the reaction rates depend linearly on the concentration of sodium
azide within the examined range (Fig. 4),the detailed kinetics would be more
complicated, because both the equilibrium constant of ion-dissociation of the
quarternary salt and the salting out effect of sodium azide, discussed in 2.3,
seem to influence the reaction rate.
Fig. 4.
Rate dependenceon sodium azide concentration.PVC : 0.80 mole/l; TBAB :
0.22 mole/l; 80°C.
115
M. TAXEISHI,
Y .NAITO,and M. OKAWARA
2.2 Inhibition
If the adsorption by hydrophobic bonding plays an important role, the
reaction may be inhibited by destruction of the hydrophobic bondings. As
shown in Fig. 5, the reaction is retarded by urea which is a representative
reagent to destroy the hydrophobic bondings, and the reaction rates decrease
with increasing content of methanol in water.
C MeOHl v o l '10
10
20
30 ,
c
.0
!?
al
>
C
0
0
1.0
a5
C Urea3
1.5
9110ml
Fig. 5. hihibition by methanol and urea. PVC : 0.80 mole/l; TBAB : 0.22 mole/];
NaN3: 0.77 mole/l; 80°C; 10 hrs.
Laurylpyridinium chloride (LPC) does not appreciably catalyze the reaction,
but it seems to be adsorbed on the surface of PVC powder. Accordingly, it
will competitively inhibit the TBAB-catalyzed reaction by occupying the
binding sites. Fig. 6 shows that the competitive inhibition takes place by lauryl-
I . 0.2* . 0.4. . 0.6 . "0.6 '
C LPC I
Fig. 6.
116
g110ml
Competitive inhibition by laurylpyridinium chloride. PVC: 0.80 mole/] ;
TBAB: 0.22 mole/l; NaN3: 0.77 mole/l; 80°C; 10 hrs.
Substitution of Poly(viny1 chloride)
pyridinium chloride. However, that is weak. These results support the assumption that the catalysis is ascribable to hydrophobic adsorption of cationic
surfactants on the surface of the solid PVC powder.
2.3 Salting-out egect
The amount of adsorbed TBAB will be increased by the salting-out effect
of an inorganic salt; consequently, the reaction rate may be increased by the
addition of such a salt. On the other hand, an anion of the inorganic salt may
interact with the positive charge on the surface of PVC and may inhibit the
reaction by interrupting the interaction of the charge with an azide ion. This
inhibition is important in the micellar catalysislo, and the corresponding salt
effects were investigatedll. These two mutually competing effects were
examined by adding potassium chloride. Fig. 7 indicates that the salting-out
effect suppresses the other effect.
0.5
1.0
C KCl 1
Fig. 7.
1.5
S/lOml
SSelting-out effect of potassium chloride. PVC : 0.80 mole/l; TBAB :
0.22 mole/l; NaN3: 0.77 mole/l; 8 O O C ; 10 hrs.
2.4 Reaction with other nuckophih
The catalysis of the cationic surfactant was also investigated in the reactions of PVC with several nucleophiles other than the azide ion. The reaction
with thiophenoxide was remarkably catalyzed, and in one experiment about
30% of chlorine atoms converted into sulfide-structures (Fig. 8). This value is
rather large for a reaction which occurs only on the surface oftheparticles.
A PVC chain located on the surface of a particle can hardly be displaced by
another located in the interior, because the macro-BRomian movement of the
polymer chain is strongly inhibited below 80°C. These data suggest that solid
117
M. TAKEISHI,
Y . NAITO,and M. OKAWARA
PVC powder has an extremely large surface area. In the reaction with thiophenoxide the reciprocals also fitted eq. 3.
6 0"
J-
I"
b
-10
lime
Fig. 8.
15
hr
Catalysis of TBAB in the reaction of PVC with nucleophiles :
a ) CGH~S-:
0.80 mole/l; TBAB: 0.25 mole/l.
b) (CzH5)zNCSS-: 0.53 mole/l; TBAB: 0.05 mole/l.
PVC: 0.80 mole/l; 80°C.
The above catalysis is of much interest if it allows several reactions of
polymer suspended in water t o become feasible. Accordingly, t h e use of surfactants can make it possible for the water insoluble polymers to react in
checked aqueous media.
3. Experimental
3.1 Materials
The PVC powder a was commercial product (Nippon Zeon Co. Ltd.) ; D P = 700,
48-100 mesh: 73.8y0, 100-200 mesh: 24.2y0, 200-325 mesh: 2.0%. The Surfactants
were of reagent pure grade (Wako Pure Chem. Ind. Co. Ltd.), their purities were
by halogen analysis (9G99.5yO).
3.2 Substitution reaction of PVC with azide ion catalyzed by cutionic surfactants
I n 10 ml water 0.5 g (TBAB: 1.80 mmole, distearyldimethylammonium chloride:
0.85 mmole) of cationic surfactant and 0.5 g (7.7 mmole) of sodium azide were
dissolved and then 0.5 g (8.0 mmole, based on monomer unit) of PVC powder was
suspended. The mixtures were sealed in ampoules, and they were shaken at 80°C.
The progress of the reaction was followed by I R spectroscopy. Conversions were
determined by elemental analysis.
3.3 Kinetics
Varying amounts of tetra-n-butylammonium bromide (TBAB) and 0.65 g
(10mmole) of sodium azide were dissolved in lOml water and then 0.6256 (IOmmole)
118
Substitution of Poly(viny1 chloride)
of PVC powder was suspended in it. The mixtures were sealed in ampoules, and
they were shaken a t 80°C. The reactions were stopped before conversions came to
5%, so that the kinetics was not disturbed. The reaction rates were calculated on
the basis of the data of elemental analysis of the obtained polymer.
3.4 Inhibition
Varying amounts of urea or laurylpyridinium chloride (LPC) were added to the
reaction mixtures prepared as described in 3.2 (TBAB: 0.7 g), and the reactions
were carried out similarly. The effect of methanol was investigated as follows: a
portion of water was displaced by methanol and the other procedures were similar
to 3.2 (TBAB : 0.7 g).
3.5 Salting-out egect
Varying amounts of potassium chloride were added to the reaction mixtures
prepared as described in 3.2 (TBAB 0.7 g), and the reactions were carried out similarly.
3.6 Substitution reactions of P VC with other nucleophiles
Sodium sulfite, sodium thiosulfite, sodium cyanide, potassium thioisocyanate,
sodium thiophenoxide and sodium diethyldithiocarbamate were examined. After
0.322 g ( 1 mmole) of tetra-n-butylammonium bromide (TBAB) and 3.2 mmole of
nucleophile were dissolved in 4 ml water, 0.2 g (3.2 mmole) of PVC powder was
suspended in it. The mixtures were sealed in ampoules, and they were shaken a t
80 "C. The structures of the obtained polymers were identified by I R spectroscopy.
The reaction yields were determined by elemental analysis.
I n all reactions the obtained PVC was purified by tetrahydrofuran-methanol
reprecipitation.
1
2
3
4
5
6
7
8
9
10
l1
E. J. FENDLER
and J. H. FENDLER,
Adv. Phys. Org. Chem. 8 (1970) 271.
E. H. CORDESand R. B. DUNLAP,Accounts Chem. Res. 2 (1969) 329.
H. MORAWETZ,Adv. Catal. Relat. Subj. 20 (1969) 341.
R. H. HAUTLA
and R. L. LETZINGER,
J. Org. Chem. 36 (1971) 3762.
JUNE-RUCHO and H. MORAWETZ,J. Am. Chem. SOC.94 (1972) 375.
A. BRANDSTORM
and L. JUNGGREN,
Acta Chem. Scand. 23 (1969) 2204.
C. M. STARKS,
J. Am. Chem. SOC.93 (1971) 195.
M. TAKEISKIand M. OEAWARA,
J. Polym. Sci. B 7 (1969) 201; B 8 (1970) 829.
R. E. DAY,F. G. GREENWOOD,
and G. D. PARFITT,
Proc. 4th Int. Cong. on Surface Activity, Vol. XI, 1967, p. 1005.
F. M. MENGERand E. PORTNOY
J. Am. Chem. SOC.89 (1967) 4698.
W. K. MATHEW,
J. W. LARSON,
and M. J. PIKAL,Tetrahedron Lett. 1972, 513.
119
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