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Vinyl polymerization by metal complexes. IX. On the mechanism of photopolymerization of vinyl monomers initiated by Fe(III)salt-saccharide systems

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Die Angewandte Makromolekulare Chemie 36 (1974) 27-40 ( N r . 5 1 4 )
From the Faculty of Engineering, Osaka University, Suita, Japan
Vinyl Polymerization by Metal Complexes
IX. On the Mechanism of Photopolymerization of Vinyl Monomers
Initiated by Fe(I1I)saltSaccharide Systems
By Tomoyuki Okimoto, Yoshiaki Inaki, and Kiichi Takemoto
(Received 21 July 1973)
Photoinduced polymerization of acrylonitrile and acrylamide in the presence of Fe(lI1)
salt and saccharide was studied in aqueous solution. The effect of various Fe(lI1) salts
on the rate of polymerization was found to be in the following order: Fe(l1I) nitrate,
perchlorate and chloride > Fe(II1) ammonium sulfate > potassium ferricyanide. On
the other hand, the effect of neutral salts such as sodium nitrate, chloride and perchlorate
on the rate of polymerization was negligibly small. The maximal rate of the photopolymerization attained at the pH region of about 2 to 3, and the rate of Fe(II1) ion consumption
determined spectrophotometrically revealed also a maximum in the same pH region.
From these results, initiating species of the photopolymerization was discussed.
Die photoinduzierte Polymerisation von Acrylnitril und Acrylamid wurde in wPI3riger
Losung in Gegenwart vom System Fe(1II)salz und Saccharid untersucht. Es ergab sich,
daI3 der Effekt der verschiedenen Fe(IIl)salzc2 auf die Polymerisationsgeschwindigkeit
folgende Reihe hat: Fe(II1)-nitrat, -perchlorat und -chlorid > Fe(II1)-ammoniumsulfat
> Kaliumferricyanid. Dagegen war der Effekt von neutralen Salzen wie Natriumnitrat,
-chlorid und -perchlorat auf die Polymerisationsgeschwindigkeit vernachlassigbar. Die
rnaximaleGeschwindigkeit wurde im pH-Bereich von 2 bis 3 erreicht, und die spektrophotometrisch bestimmte Geschwindigkeit des Fe(I1I)ionenverbrauchs zeigte ein Maximum
im gleichen pH-Bereich. Aus den Ergebnissen wurde die Auslosungsspezies der in Frage
kommenden Photopolymerisation diskutiert.
In a series of our preceding papers, polymerization of vinyl monomers
initiated by copper complexes has been reported '. Photoinduced vinyl
polymerization in t h e presence of iron salt or its complex has been recently
T. Okimoto, Y. Inaki, and K. Takemoto
received considerable interest8- l . In connection with these studies, photopolymerization of vinyl monomers initiated by metal ion-saccharide systems has
been investigated, and particularly, Fe(I1I)ion-saccharide system was found
to be capable of initiating vinyl polymerizations efficiently under rather milder
conditions 1 2 . In the preceding paper, photopolymerization of vinyl monomers
with Fe(II1) salt-saccharide systems was studied kinetically in aqueous solution13, and it was found that the accelerating effect caused by the addition
of a saccharide to the reaction system containing Fe(II1) salt was mainly
concerned with the initiation step, and the photochemical reaction scheme
of Fe(II1) ion with saccharide was quite similar to that of the redox reaction
of Ce(1V) with saccharide 1 4 .
Interaction between Fe(111) ion and saccharide seemes to be influenced
by the kind of counteranions of Fe(II1) ion, its concentration, and particularly
pH of the reaction system, so that it is of interest to study the effect of
the factors above mentioned on the photopolymerization of vinyl monomers
initiated by Fe(II1) salt-saccharide systems.
1 . Reagents
Acrylonitrile was purified by repeated distillation under reduced pressure. Acrylamide
was dissolved in acetone and recrystallized using a freezing mixture of dry ice-methanol;
mp 843°C. Metal salts, saccharide, and polyvinyl alcohol used were of commercial
origin (analytical grade).
2. Polymerization
Photopolymerization was carried out in sealed tubes of hard glass with 1.8 cm diameter.
The light source was a high-pressure mercury vapor lamp (Toshiba SHL-IOOUV-2)
in a hard glass envelope so that the light below about 300mp was screened off. The
tubes were rotated around the lamp keeping a constant distance of 1Ocm. Vinyl monomer,
metal salt and saccharide solution were charged successively in order into a tube in
the dark. The evacuated tubes were then irradiated in an ice-cold water bath at 0°C.
Polymer was obtained as a colorless precipitate by pouring the content into a large
excess of methanol, and purified by reprecipitation technique.
3. Molecular Weight of Polymers
Molecular weight of the polymers was determined by viscometry, using the following
equations :
for acrylonitrile: [1]=3.92.10-' Mu " (in DMF, at 25 C)",
for acrylamide: [ q ] =6.80. lo-' M" h6 (in H20, at 30°C)'".
Vinyl Polynwriznt ion by Metal Complexes
4. Appurutus
Fc(II1) ion concentration was determined spectrophotometrically ', and the rate of
Fc(II1) ion consumption was followed with 356 model Hitachi two-wavelength double
beam spectrophotometer equipped with a tungsten-iodine lamp for photoreactions.
U V and visible spectra were measured with 124 model Hitachi double beam spectrophotometer, and the pH measurement was done using the Hitachi Horiba pH-meter F-5.
Results and Discussion
1. Photopolymerization with various Fell11 j salt-Saccharide Systems
Photopolymerization ofacrylonitrile and acrylamide with a variety of Fe(II1)
salt-saccharide systems was studied in aqueous solution at 0°C. The results
obtained are shown in Table 1. In all cases, acrylamide polymerized more
facile than acrylonitrile. Thc ability of the Fe(II1) salt-saccharide systems
to initiate polymerizations was found to be in the following order: Fe(lI1)
nitrate, perchlorate and chloride > FdIII) ammonium sulfate > potassium
ferricyanide. This difference in the initiation ability might be attributed
Table I .
Photopolymerization of vinyl monomers initiated with iron salts (O'C, in
Conversion ( " h )
Iron salt
(15 min)
0.1 1 1
0.1 I I
0.1 I I
0.1 I I
0.1 I I
0.1 I I
[iron salt] = 5.0.10
.' mole/l,
[acrylonitrile] = 1.01 mole/l, [acrylamide]=0.94 mole,l.
I'. O L i n i o t o . Y . Inakt. and K Takcmoto
to the difference in the coordination strength of counteranions to the Fe(1lI)
ion. I t is to be noted that Fe(l1) perchlorate showed rather high activity
for the pliotopolymcrization of acrylamide, while the activity was not so
significant in the cast: of the polymericttion of acrylonitrile. The difference
might be related to the strength of interaction of Fc(l1) or Fc(l1l) ion with
the vinyl monomers.
S c r l r I</kcr
Photopolymerization of acrylonitrile initiated by Fe(l1I) salt-saccharide systems was carried out in 0.02 N-acidic solution in the presence of sodium
nitrate. perchlorate or chloride. Results are shown in Fig. 1. The ability
of the systems to initiate polymerizations was in the following order: Fe(l1l)
chloride > t-.e(III) perchlorate > Fe(1lI) nitrate. 'hc effect of the neutral
(C I
2 01
0 0.1 0.2 0.3 0.4 0.5
0 0.1 0.2 0.3 0.4 0.5
[NaCIOk] ( rnole.l-')
[NaNO,] ( moleel-')
0.1 0.2 0.3 0.1 0.5
[NaCI] (molel-')
salts on the photopolymerization was negligibly small in all cascs. Because
hydration of l~'e(I11)ion is assumed to be stronger than thosc o f three sorts
ofanions here mentioned. it might be explained that thc rate of polymeri7ation
was not affected in the presence of different amounts of the neutral salts.
3 . f j f i ~ c ~ofr A c i d or
The effect ofndding H N O , or N a O H on the polymerization of acrylonitrile
initiated by various systems was studicd in aqueous solution. Effect of f " 0 . 3
is shown in Fig. 2, and the relationship of conversion with [NuOH] [ Fe(lli)]
ratio ;ire shown in Fig. 3. I-rom these figures. i t is clear that the rate of
photopo1ymerii;ltion tends t o decrease by the addition o f nitric acid o r sodium
hydroxide. I t has already bccn confirmed that the effect of sodium and nitrate
ions on the rate of photopolymeri~..ationwas negligibly small. Therefore.
it is assumed that thc decrease of the rate of polymcriution is mainly duc
to the increase of the H or Of{ concentration of the reaction systems.
In relation to thc problem of the initiation mechanism, it is to be n o t d
that the rate of polymerization of acrylonitrile by ccric ion increased u i t h
increasing nitric acid concentration".
0.2 0.4 0.6 0.8
[HN03] ( mole4 1
Fig. 2. Concersion cs. nitric acid concentration (0 <'.in i!,O for 20 min): [ acrylonitrilcJ
= 1.01 mole 1. (0)
1 glucose1 -0.1 1 1 mote 1. (01
{glucose] =O.W) m o l e 1.
In Fig. 3 tht' cflect o f sodium hydroxide on the polymcri/ation is shown
in the absence o r presence of glucose. fructose or polyvinyl alcohol. In all
cast's. the decreasing tendency of the conversion curve was observed. and
T. Okimoto, Y. Inaki, and K. Takemoto
I NaOHl/[
Fe( III) 1
Fig. 3. Conversion vs. [NaOH]/[Fe(lll)] ratio (O'C, in H 2 0 for 20 min; [acrylonitrilc] = 1.01 mole/l).
( 0 )[Fe(N03)3.9H20]=5.0.10 mole/l.
( 0 )[Fc(N03)3.9H20] =5.0.10 mole/l, [glucosc]=O.I 1 I mole/l.
[Fe(N03)3.9H20]= 1.0.10-3molc/l, [glucose]=0.111 mole/l.
( A ) [Fe(N03)3.9H20]= 1.0.10- mole/l, [fructose]=O.I 1 1 mole/l.
( 0 )[Fe(NO,),.9H20] = 1.0.10-3 mole/l, polyvinyl alcohol: [OH] =0.227
mole/l, [NaCIO,] =0.1 mole/l.
[Fe(CIO4),.6H,O] =5.0.10-, mole/l, [NaCIO,]=O.l
mole/l, [glucose]
= 0.1 11 mole/l.
the conversion reaches almost zero at the point at which [NaOH]/[Fe(lII)]
ratio is in the range between 2.5 to 3.0, regardless of the initial Fe(II1) ion
concentration. The reason for the results will be discussed in a later chapter.
4. pH-Dependency of the Reaction Systems
Photopolymerization of acrylonitrile and acrylamide was carried out next
in aqueous solution in the pH region from 0 to 4. The pH of the solution
was adjusted with adding either sodium hydroxide or perchloric acid. The
results obtained are shown in Fig. 4 and 5. In the whole pH region, acrylamide
was polymerized more facile than acrylonitrile. In both cases of polymerization,
Vinyl Polymerization by M et a l Complexes
Fig. 4. Conversion vs. pH (0°C. in HzO for 20 rnin); [acrylonitrile]= 1.01 rnolefi,
[Fe(C104)3.6H20]=5.0.10.' rnole/l, [NaC101]=0.1 m o l d .
a) [glucose] =O.OOo rnolejl,
b) [glucose] = 0.1 1 I rnole/l.
conversion-pH relationship,
( 0 )molecular wcight-pH relationship.
Fig. 5. Conversion vs. pH (0'C, in HzO for 15 rnin); [acrylarnide] =0.94 rnoleil,
[t;e(C104).3.6H20] =5.0.
rnole/l, [NaCIO1]=O.l rnole/l.
a) [glucosc] =o.OOO rnole/l,
b) [glucose] = 0.1 1 1 rnoltyl.
conversion-pH relationship,
( 0 )rnolccular weight-pH rclationship.
T. Okimoto. Y. Inaki. and K. Takernoto
the maximal rate attained at the pH value of about 3. regardless to the
absence or presence of glucose in the reaction system. It is assumed here
that the acceleration of the photopolymerization depends on the rate of
initiation and/or that of propagation, and otherwise, depends reversely on
the rate of termination. In the polymerization of niethacrylamide in aqueous
solution. the rate constants of termination was rcported as in the following
order: Fe(II1)-monomer > Fe(llI).OH > Fe(ll1) ion'. In our experiments.
the molecular weight of the polymers obtained was the lowest under the
polymerization condition at pH about 3. Hencc, it can be concluded that
the accclcration was preferably caused by the increasc in thc rate of initiation.
From the fact that the maximal rate of initiation was observed in the rcaction
condition at pH about 3 , regardless to the kind of monomer and the absence
o r presence of glucose, it is expected that pH influenced mainly on the
hydrolysis of Fc(II1) ion. and/or interaction between Fe(I1l) ion and glucose.
5. U V utid Msihle S p r c w i in
p l i Rqioiis
U V and visible spectra of the Fc(ll1)ion-saccharidc systems were measured
in aqueous solution at different pH regions. Fig. 6 shows the absorption
spectra of the solution in various [NaOH]/[Fe(lll)] ratios in the presence
ofexcess amount ofglucose, fructoseand polyvinyl alcohol at higher concentration of Fc(II1) ion. being cqital to 5.10 -.'mole;l. The absorption was found
to increase by the addition of base, and the increase ceased above [NaOH]:'
[Fe(Ill)] =3, presumably due to the complction of the complex formation.
A definite isosbestic point was, however, not observed. I t can be expected
therefore that there are several absorbing chromophores, such as free Fe(ll1)
ions, hydrolyzed Fe(lII), polymer spccics of Fc(llI), Fe(1ll) ion-saccharide
or -polyvinyl alcohol. and so on.
For the case of FdIII) ion concentration as low as 5.10 ' mole,'l. the
absorption spectrum of Fc(l1l) ion-glucose system was also measured in
aqueous solution at difforcnt pH range. Results obtained arc shown in Fig.
7. Above pH about 2.9. any remarkable change in the absorption curves
was not observed with increasing pH. On the other hand. below pH about
2.0 the absorption in the region of 240mp incrcascd. &Me the absorption
in thc region of 3Wmp decroascd with decreasing pH. and there cxistcd
two isoshestic poipts at 223 mp and 27 I mp. This obscrwtion was similar
t o that seen in the absoncc of glucose. It has been reported that the absorption
band at about 300rnld was attributed to Fc(OI4)' ' and the pcak at 240mp
e 10
Wave length ( m t t )
Wave length ( mp)
Wave length ( rnp 1
Fig. 6. CJV and visible spectra of Fe(lll) ion at diffcrent [NaOH]'[Fc(IIll] ratios (in
H 2 0 a t room tempcraturc.with I mmcell): [Fe(C'I04)J.6H20]
=S.0.10- mole4,
[Na<'iO.,] = 0. I mole 'I.
;I) [glucose]=O.I I 1 mole,I, [NaOH];[l~c(lll)]ratios: a : 0.0: b: 0.5: c: 1.0: d :
2.6; 5 : 3.4 5.0.
b) [fructosc]=0.I I I molc,I. [NaOll]~[Fc~III)]
ratios: a : 0.0: b: 0.5: c : 1.0:
d : 2.0 2.6: e : 3.2: f : 3.X: g: 5.0.
c ) polyvinyl alcohol: [ OH]=0.??7 mole I. [ N a O H ] [I-e(Ill)] ratios: a : 0.0:
b: 0 . 5 : c : 1.0: d : 1.5: e : 2.6: f: 3.2 4.0.
T. Okimoto, Y. Inaki, and K . Takemoto
might be due to Fe3+ ion”. It seems to be sure that the increase of the
Fe(OH)+ concentration is partly related to the increase of the rate of polymerization below pH about 2.
Fig. 7.
Wave length (mp)
UVand visiblespectra of Fe(II1)ion at different pH (in H20at room temperature,
with lOmm cell); [Fe(CI01)3.6H20]= 5.0.10 - 4 molc/l, [NaCIO4] =O.l mole/l,
[glucose]=0.111 mole/l. pH: a: 0.24; b: 1.2; c: 1.5: d: 1.8; e : 2.0; f: 2.86;
8: 2.90; h : 3 . c - 3.5.
6. Fe( I l l ) Ion Consumption
It was confirmed throughout the experiments that the relationship between
the absorbance and the concentration of Fe(II1) ion was followed well to
the Beer’s law, regardless to the absence or presence of glucose and Fe(I1)
ion which might be produced during the reaction. The aqueous solution
of Fe(II1) salt and glucose was charged into a 1 : 1 cm quartz cell, and after
degassing the sealed cell was placed in the spectrophotometer cell holder.
After irradiating with a tungsten-iodine lamp at regular time intervals, Fe(II1)
ion concentration was determined spectrophotometrically. Relationship of
Time ( min 1
Fig. 8. Ratc of Fe(II1) ion consumption (in 0.01 N HCIO,, at room tcmperature,
[NaCIO,] = 0.1 mole/l).
(1) [glucose] =O.ooO molc/l,
(2) [glucose] =0.1 I I molc/l.
(1) light off,
(1) light on.
- 3
Fig. 9. Rate of Fe(II1) ion consumption vs. pH (in H20, at room temperature,
[NaCI04] = 0.1 mole;l).
[glucose] =O.ooO mole/l,
( 0 )[glucose] = 0.1 1 1 mole,il.
T'. Okimoto. Y . Inaki. and K . Takcmoto
logFc(I1I) concentration vs. time is shown in Fig. 8. Inhibition period was
hardly observed. and when the illumination was intcrruptcd, the rate of
reaction fell rapidly near to zero. The relationship between the ratc of Fc(1ll)
ion consumption and plI of the reaction system is shown in Fig. 9. In
this system, the hydrolyzed Fe(II1) precipitated gradually at pH above 2.
while in the presence of glucose it remained homogeneous at pH below
about 4. The maximal ratc of Fe(ll1) ion consumption was observed in the
pH region of about 2 to 3. It was found that the curvcs obtained here
were in accordance with the results of the photopolymerization irradiated
with a high-pressure mercury lamp (cf. Figs. 4 and 5).
1. Etrarioii
( ' i i r w s of
liiititition S j * s t m
Titration curves of the solution including Fe(IIIj ion and excess amount
of glucose separately. and both the compounds together are shown in Fig.
10. pH of the system was adjusted with sodium hydroxide. Each solution
was stored in a scaled tube overnight in the dark. in order to ensure the
accuracy for the pH measurement. As the hydrolyzed Fe(II1) ion precipitated
gradually above pH about 3 to 4 even in the presence of excess amount
of glucose, uncertainty might occur in the titration curves. As shown in
Fig. 10. pH of Fe(II1) ion solution decreased a little in the presence of glucose.
INaOH I /[ Fe(III) 1
Fig. 10. Titration curves ([KaC'104] = O . l moleil).
( I ) [glticosc] = 0.1 I 1 molc:l.
( 2 ) [Fcl('104),.6H20]=5.0.10 ' molc,'l.
( 3 ) [glucose]=0.1 I 1 moleil. [Fc(C10~).,.6H20]=5.0.
' mole/l.
k'rom the result, it is suggested that the hydrogen ions were released in
the complex formation between glucose and Fe(III) ion at pH as low as
3. The buffered pH region finishes by adding 3.0 base equivalent per mole
of Fe(II1)ion. This fact reveals that three hydrogen ions are directly generated
from glucose and/or H 2 0 for each Fe(II1)ion initially present, and the complex
formation is complete at pH 4.
8.1 F e ( l l 1 ) Ion System
The initiation mechanism for the photopolymerization in the presence of
Fe(lI1) ion in aqueous solution has been proposed by Dainton and Sisley
as follows'):
Fe(lll).f{,O -% F e ( l l ) + H ' +.OH.
For the initiation system including Fe(111) ion at different pH, following
equilibria are present:
From the results mentioned above, it is assumed that the Fe(III).OH
is more photosensitive here than both free Fc(II1) ion and Fe(III).[OH )3.
Thus the photopolymeri/;ltion is assumed to be initiated preferentially according to the equation ( I ) .
8.2 F e ( I I I ) I o n - S a c c h a r i d e System
The initiation mechanism for the photopolymerization in the presence of
k'e( I I I ) salt-saccharide system was assumed t o be as follows ? :
In the prcscnt paper. it was shown that three hydrogen ions were generated
from the saccharide and or H,O per mole of Fe(ll1) ion. In the relation.
T. Okimoto, Y. Inaki, and K. Takemoto
structures of ferric gluconate complexesl Yand sugar-iron complexes" have
been proposed. The following Fe(111) ion-saccharide complexes are therefore
assumed for our initiation system:
Fe( 111). SH,
Fcf111). SH, . O H
(or F e ( l I I ) ~ S H , - , ~ H , O )
r "I
dimer or polymeric form.
By considering equations (6) and (7), it seems to be likely that the rate
of photopolymerization is accelerated by the formation of the complex [I],
while the complex [I13 is less active to initiate photopolymerization.
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salt, photopolymerization, monomerl, metali, saccharina, mechanism, vinyl, system, complexes, iii, polymerization, initiate
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