Download Redox polymerization of vinyl monomers, initiated by transition

CHAPTER I
1.1.
GENERAL ASPECTS
A
large
macromolecule
number
or
of
smaller
polymer.
This
molecules
unite
chemical
process
to
form
is
a
called
polymerization reaction. A “monomer” is defined as a chemical
compound containing one or more polymerizable structural units1-5.
The macromolecules consisting of identical monomers are called
homopolymers. When different types of monomers are used, the
resulting polymer is called copolymer or mixed polymer.
The
polymerization reactions are broadly classified into two types, namely
condensation
polymerization
and
addition
polymerization.
In
condensation polymerization, the polymer formation is in general
accompanied by the elimination of smaller molecules such as water and
methanol.
But in addition polymerization, which is confined to a
particular type of unsaturated compounds namely olefins and their
derivatives, the polymer is formed without the elimination of simple
molecule.
Addition polymerization is further classified into two categories
namely ionic polymerization and free-radical polymerization. The ionic
polymerization proceeds through the formation of carbonium ion,
whereas the free-radical polymerization proceeds through the formation
of active free-radicals.
1
The
free-radical
polymerization
involves
a
chain
reaction,
proceeding in three steps namely (1) activation or initiation (2)
propagation and (3) termination.
It is observed that the addition of
small amount of foreign substances like oxygen, iodine and quinone
suppresses the rate of polymerization due to quenching of free radicals.
The active radicals generated in the initiation step successively
combine with monomer molecules to give a long chain active radical.
R–M1
R–M2
R–M3
kp
+
M
kp
+
M
kp
+
M
R–M2
R–M3
R–M4 …
The rate of propagation in each step is assumed to be the same.
In the final step the growing polymer radicals lose their activity and
become an inactive polymer. The termination may proceed through the
following reactions:
(i)
By recombination


R-CH2-CHX + XCH- CH2-R
(ii)
kt
R- CH2-CHX-XHC-CH2R
By disproportionation


R-CH2-CHX + XCH-CH2-R
kt
R-CH2-CH2X + R-CH=CH-X
2
(iii)
By metal

R-CH2-CHX + Metal ion
(iv)
kt
Polymer + Reduced metal ion
By primary radical


R-CH2-CHX + R
kt
R- CH2-CHXR
The retarders like nitrobenzene reduce the rate of polymerization
while the inhibitors like benzoquinone, almost completely inhibit the
reaction. A well known and an effective inhibitor for vinyl polymerization
is molecular oxygen. The stable free radical diphenylpicrylhydrazyl
(DPPH) is also an effective inhibitor for polymerization.
The polymerization reaction can be initiated by various systems
such as organic substances, inorganic salts, metal ions and metal ion
complexes of organic ligands and metal ion complexes of inorganic
ligands in aqueous medium.
1.2
METALLIC REDOX SYSTEMS AS INITIATORS
Redox polymerization of vinyl monomers, initiated by transition
metal ions in their higher oxidation states in aqueous medium, can
provide valuable information. Among the metals, the transition metal
ions are most widely used to initiate polymerization in aqueous
solutions. Transition metal ions are easily reducible to the lower valency
state resulting in the formation of free-radicals of the reducing agent.
The metal ions can be used to initiate polymerization as its simple salt
3
or in combination with complexing ligands. A survey of the available
literature on the polymerization of vinyl monomers using the metal ion
redox initiating systems has been given.
Viswanathan and Santappa6 have shown that in the presence of
reducing agents such as n-butanol, ethylene glycol, cyclohexanone and
acetaldehyde, Cr(VI) is capable of initiating vinyl polymerization. It was
found that the rate of polymerization decreased with increasing [Cr(VI)]
and was proportional to [H+] indicating the involvement of Cr(VI) in the
termination step and also the importance of protonated species in the
reaction. Similar cases of metal ion termination were reported in the
polymerization of AN initiated by Cr(VI) thiourea7, and Cr(VI)-glycerol
redox systems8.
Wang9 reported the polymerization of N-vinyl carbazole and
4-vinyl pyridine initiated by copper(II) nitrate. Tazuke and Gkamura10
reported that the polymerization of 4-vinylpyridine was initiated by an
one electron transfer from monomer to the copper(II) acetate in a
complex.
Inove and Yamuchi11 reported the rapid polymerization of MMA
by a combination of organic acids and copper powder or cuprous
chloride. Guchi Tatsuro et al.12 observed that the high concentration of
Cu2+ inhibited the polymerization of MMA initiated by urea-CuCl2 redox
system. Yinghai et al.13 investigated the polymerization of AN initiated
by Cu(II) and urea in alkaline medium.
4
It is well known that vanadium(V) by itself does not initiate
polymerization but does so only in the presence of suitable reducing
agents. Santappa et al. were the first to make a systematic study of the
polymerization of vinyl monomers using V(V) in combination with
reducing agents. They used cyelohexanol14, lactic acid15 and pinacol16 in
the presence of V(V) for the polymerization of AN. In these systems, Rp
showed a second order dependence on (monomer), a first order
dependence on reductant and inverse first order dependence on V(V).
The termination of growing polymer chain was found to occur by the
reaction with V(V). Mutual terminations as well as termination by
primary radicals were considered to be unimportant in these systems.
Click and Senwar17 studied the polymerization of AN using
V(V) – malic acid system in sulphuric acid medium. The Rp was
proportional to [malic acid] and [monomer]2. Nayak et al. used
thiourea18, tartaric acid19, cyclohexanone20, propane-1,2-diol21 and
ethylene glycol22 as reducing agents in combination with V(V) for the
polymerization of AN. The V(V)–cyclohexanone23 redox system was also
used for the polymerization of MA and MMA. The results were
consistent with the formation of an intermediate complex between the
ketone and the metal ion, the decomposition of which leads to initiating
radicals.
Anuradha et al. studied the polymerization of AN by using
V(V) – glycerol24 redox system and proposed linear termination of
5
polymer chains. Das et al. reported the polymerization of AN and MMA
using V(V) ascorbic acid25 and V(V) –thioacetamide26 systems. The
kinetics and mechanisms of retardation of V(V) -cyclohexanone initiated
polymerization of AN by phenol were discussed by Jena et al27. The
polymerization of AN initiated by V(V) –thiourea redox systems was
reported by Jinynan et al28. Srinivas et al.29 reported the polymerization
of AN initiated by pervanadic acid (PVA). If the concentration of PVA is
greater than 5x10-3 M, there is
no induction period. The rate of AN
disappearance was proportional to [AN]2, [PVA]10 and decreased with
increase in [V5+] at high concentration.
Hussain and Dass30 reported the polymerization of MMA initiated
by a mixture of urea and CCl4 and accelerated by Fe(III)
complex
namely hexakis (dimethyl sulfoxide) iron(III) perchlorate. The maximum
rate of polymerization was reduced, when a 6:1 molar ratio of urea to
Fe(III) complex was achieved.
Dass and Baruah31 noticed the
accelerated rate of polymerization of MMA in the presence of hexahis
(dimethyl sulfoxide) iron(III) perchlorate in DMSO, even in the absence
of reducing agent at 60°C.
Arai Yoshio et al.32 investigated the
polymerization of MMA by iron(III) nitrate in aqueous solution
containing a non-ionic surfactant. The polymerization was hindered by
the addition of 2-phenyl-1,1-picryl-hydrozyl-1,4-benzoquinone. The
enhanced rate of polymerization of MMA, initiated by semicarbazide
complex of Fe(III) and CCl4 in DMF, was reported by Dass et al.33
6
The polymerization of MMA in CCl4 medium with ferric laurate, a
metal soap in combination with n-hexylamine as the initiator system
was studied by Saha and Chaudhuri34. The rate of polymerization was
found to be linear with the monomer concentration and proportional to
the square root of both ferric ion and amine concentration.
In the
absence of amines there was no polymerization even after 2 hours.
Jayasubramanian et al.35 reported the vinyl polymerization by
thallium(III) acetate. Like this thallium(III) sulphate36 was also found to
initiate polymerization of MMA.
Bamford and Lind37,38 studied the polymerization of vinyl
monomers initiated by manganese(III) triacetonylacetonate. Nikolaev39
showed that the introduction of a carboxylate group into the above
ligand increased the rate of polymerization.
polymerization
of
AAM
manganese(III)
acetate,
and
methacrylamide
Santappa
et
al.40-42
In the study of the
(MAA)
initiated
proposed
that
by
the
termination was by mutual combination for the first two monomers
while termination was by Mn(III) ions for AA. The permanganate ion in
combination with organic acids such as oxalic acid,43-50 ascorbic acid,51
tartaric acid,52 malic acid,53 thioglycollic acid,54-55 and citric acid56 had
been used to initiate vinyl polymerization.
Ganga Devi and Mahadevan57 studied the polymerization of AN
using Mn(III) and reducing agents such as diacetone alcohol and
malonic acid.
No polymer was formed in the absence of a reducing
7
agent showing that there was no reaction between monomer and
Mn(III). Complex formation between Mn(III) and reducing agent and its
decomposition to produce the initiating free-radicals were proposed in
the mechanism.
During the polymerization of AN and MMA using
Mn(III)-malonic acid58 redox system, the polymerization was found to
get terminated by mutual combination of growing chains.
In the polymerization of AAM using Mn(III)-pinacol59 redox
system, the rate of polymerization was proportional to [AAM] and
[Pinacol] and was independent of [Mn(III)].
Jayakrishnan and
Mahadevan60 investigated the polymerization of AN initiated by Mn(III)cyanoacetic acid and Mn(III)-2-butanone redox systems and suggested
that the initiation was through the complex formed between Mn(III) and
reducing agent and termination by the mutual combination of growing
chains. Though the above authors suggested the complex formation,
the complex formation constant had not been evaluated.
Nayak et al.61 investigated the polymerization of AN with a series
of organic acids as reducing agents and found that the order of
reactivity varied as citric > tartaric > ascorbic > oxalic > succinic >
glutaric > adipic acid. Similarly in the polymerization of MMA, initiated
by Mn(III)-alcohol redox systems62, the reactivity of alcohols was found
to vary in the order : 1-propanol > glycerol > ethylene glycol > isobutyl
alcohol > 1-butanol > 1,2-propanediol > cycloheptanol > cyclohexanol >
cyclopentanol. The kinetics of polymerization of AN, initiated by Mn(III)
8
with a series of amides redox systems, was investigated by Samal et
al.63
It was reported that under identical conditions, the order of
reactivity of the amides was, thioacetamide > succinamide > acetamide
> formamide, the order of reactivity of the amides was explained by
considering the existence of resonance and hyperconjugation effects.
In the polymerization of AN, initiated by Mn(III)-glycerol and
Mn(III)-allyl alcohol64 systems, oxidation of both glycerol and allyl
alcohol proceeded through an intermediate metal complex.
Senapathy et al.65, who studied the polymerization of AN initiated
by Mn(III)-glycerol system in aqueous acetic acid, reported a decrease in
the rate of polymerization with an increase in the acetic acid
concentration.
Further, an increase in the concentration of H2SO4
caused an increase in the reaction rate initially but at higher acid
concentrations, the rate again decreased. Jeyakrishnan et al.66 studied
the polymerization of AN and MMA initiated by Mn(III)-propanedinitrile
redox system in aqueous H2SO4, DMF and glacial acetic acid. Mutual
termination was found to occur in all these systems.
Elayaperumal
et al.67 studied the kinetics of polymerization of AN and AAM initiated
by Mn(III)-diglycollic acid system. The rate coefficients were related to
reactivities of monomer and polymer radicals.
Balakrishnan and Subbu68 investigated the polymerization of
acrylamide initiated by manganese(III)-acetate-ethoxyacetic acid (EAA)
redox system in aqueous H2SO4.
The polymerization process was
9
initiated by the free-radical arising from the oxidation of EAA by Mn(III)
and terminated by mutual combination of growing polymer radicals.
They also found that the disappearance of Mn(III) followed a first order
reaction.
Kunyan et al.69 investigated the reactivity of a series of
substituted carboxylic acids in combination with Mn(III) during vinyl
polymerization and reported the following order of reactivity : citric acid
> lactic acid > glycine > n-butyric acid > methacrylic acid.
From
the
foregoing
survey,
it
becomes
evident
that
the
reactivity of the reducing agents depended on the resonance and
hyper conjugation effect.
In most of the above reactions, the rate of
disappearance of metal ions followed the first order reaction during
polymerization.
It was noticed that the formation of polymer was
terminated by mutual combination of growing chains. Further it was
reported that the rate of polymerization increased upto certain acid
concentrations but above which the rate of polymerization decreased.
1.3
CHEMISTRY OF CERIUM
Cerium belongs to the group of inner transition elements with the
incompletely filled 4f level. Ce(IV) (4f05s26p6) and Ce(III) (4f15s26p6) are
the two important oxidation states of cerium. Ce(IV) compounds are
more
acidic,
more
easily
hydrolysed
and
more
susceptible
to
complexation than Ce(III). The salts of quadrivalent cerium are noted for
10
their strong oxidizing power and are largely used in analysis and
synthesis. Oxidation of a substrate by Ce(IV) occurs through transfer of
a single electron from the substrate to the oxidant
Ce(IV) + e-
Ce(III)
For this reason, the mechanism of oxidation by Ce(IV) in many
cases is simpler than similar oxidations with chromate and manganate.
Moreover, the oxidation potential of Ce(IV)-Ce(III) couple is higher than
many of the common oxidizing agents. The oxidation potential of Ce(IV)
in an acid medium varies from 1.28-1.87 V depending on the nature
and concentration of the acid used. The ceric salts exist as different
ionic species in different acid media depending on the pH of the
solution. In perchloric acid medium, ceric salt exists as Ce(OH)3+ and
Ce(OH)22+ as observed by Sherill et al.
Ce(OH)3+ + H2O
70
Ce(OH)22+ + H+
The value of the above equilibrium constant K was proposed as
0.6. Heidt and Smith71 who studied the photochemical oxidation of
water by Ce(IV) in perchloric acid medium concluded that Ce(IV) forms a
dimeric species in 1 M perchloric acid.
In sulphuric acid medium, Moore and Anderson72 observed a
complex containing one Ce(IV) and one sulphate ion as the predominant
11
species
upto
a
concentration
of
0.01M
H2SO4.
Hardwick
and
Robertson73 have studied the association of ceric ions with sulphate ion
spectrophotometrically and proposed the existence of complexes of the
type Ce(SO4)2+, Ce(SO4)2 and (Ce(SO4)3)2- In nitric acid medium
Blaustein and Gryder74 identified the existence of the following dimers
and
2 Ce(IV)
(Ce(IV))2
Ce(IV) + Ce(III)
(Ce(IV) – Ce(III))
the association constants for these dimers were also reported.
Ceric salts were found to oxidise different kinds of organic
compounds. The oxidation of a number of aliphatic ketones and
aldehydes was studied by Shorter and Hinshelwood75 who correlated
their structures and enolisation constants with reactivity. The kinetic
study of the oxidation of acetone with ceric ammonium nitrate was
made by Shorter.76 Enol form of acetone and Ce(OH)3+ were proposed as
the reactive species. The oxidation was inhibited by bromine which
removed the enol form as it was formed. Venkatakrishnan and
Santappa77 also reported that the enol forms of aliphatic ketones were
reactive towards ceric ions. The studies on the kinetics of oxidation of
methanol,78 ethanol,79 2,3-butanediol,80 glycol,81 pinacol82 and benzyl
alcohol83 by Ce(IV) have been reported. The mechanism of oxidation of
12
the above organic substrates by Ce(IV) in perchloric or nitric acid
medium involved the complex formation between the substrate(s) and
Ce(IV) and then the decomposition of the complex to give a radical (R)
which subsequently reacted to yield the products.
K
S + Ce(IV)
kd
Complex
R + Ce(III) + H+
fast

R + Ce(IV)
products + Ce(III) + H+
Yadah and Bhagat84 studied the kinetics of oxidation of malonic
acid by ceric sulphate and observed a first order dependence with
respect to each reactant. Therefore, in the following section, the
literature on cerium ion as initator has been reviewed.
1.4.
CERIC ION AS AN INITIATOR OF VINYL POLYMERISATION
At first Bacon85 reported the initiation of vinyl polymerization by
Ce(IV). A qualitative study of the initiation capacities of ceric ions in
different
acid
media
was
made
by
Saldick86
in
1956.
Later
Venkatakrishnan and Santappa87 studied to some extent the kinetics of
ceric ion initiated polymerization. Ananthanarayanan and Santappa88
made a more comprehensive kinetic study on vinyl polymerization. The
rate of polymerizations was found to increase in the order HClO4 >
HNO3 > H2SO4 media. Ceric ions initiated as well as terminated the
polymerization. The rate of polymerization was found to bear a square
dependence on monomer concentration and be independent of Ce(IV)
13
concentration, confirming both the initiation and termination by ceric
ions.
Narita et al.89 proposed a mechanism for the polymerization of
AAM by ceric ammonium nitrate, wherein AAM formed a complex with
ceric ion which yielded the initiating species through the abstraction of
H from the amide group of the monomer. Karpenko et al.90 reported the
isothermal polymerization of AN in the presence of Ce(IV).
Pramanick and Sarkar91 discussed the mode of initiation by ceric
salt in the polymerization of MMA. It was found that the basic reaction
for the generation of initiating radicals was strongly dependent on the
acidity of the medium and independent of the nature of the anion. In a
moderately acidic medium, the primary reaction was the formation of
hydroxyl radicals from the oxidation of water by ceric ion. These
hydroxyl radicals initiated polymerization and appeared as end-groups
in the polymer. When ammonium ions were also present as in the case
of ceric ammonium sulphate, some of the hydroxyl radicals reacted with
ammonium ions producing ammonium radicals and hence polymers
with both hydroxyl and amine end groups were formed. In a strongly
acidic medium, no specific end groups could be identified in the
polymer.
Dainton92 pointed out the advantages of a redox system for the
initiation of vinyl polymerization. There was a decrease in activation
energy by about 80 kJ/mol for the radical production in a redox system
14
when
compared
to
a
non-redox
system.
This
would
permit
polymerization at a lower temperature, uncomplicated by side reactions.
Mino et al.93 investigated and found that the ceric salts such as ceric
ammonium nitrate and ceric ammonium sulphate were capable of
forming effective redox systems in the presence of organic reducing
agents such as alcohols, thiols, aldehydes and amines. They carried out
the polymerization of AAM initiated by ceric nitrate–3-chloro-1propanol redox system and found that at constant hydrogen ion
concentration, the rate of polymerization was independent of ceric ion
concentration.
Subramaniam and Santappa94 carried out the polymerization of
AN, MA and MMA in perchloric acid medium using ceric perchlorateformaldehyde [F] redox couple. The rate of polymerization was found to
be proportional to [M]2 [F] for all the systems. The rate was found to be
independent of [Ce(IV)] for the systems with MA or AN but inversely
proportional to [Ce(IV)] with MMA as the monomer. The rate of ceric ion
disappearance was directly proportional to [Ce(IV)], [F] and [M].
Termination of the polymer chain radical was reported to occur by
reaction with metal ions. The same authors also studied the
polymerization of MA and MMA initiated by ceric ammonium sulphatemalonic acid95 redox systems in sulphuric acid medium and reported
that the initiation was by the primary radical of malonic acid and
15
termination occurred by the mutual interaction between growing
radicals.
Saha and Chaudri96 reported the polymerization of AN by ceric
ion-triethylamine system. They97 also found out the effect of various
amines on the ceric ion initiated polymerization of vinyl monomers.
They explained that a redox reaction between ceric ions and the amines
was dependent on the electron donating ability of the substituents.
Narita et al.98 reported the polymerization of MMA using Ce(IV) –
pinacol redox system. They found that Ce(IV) was just an initiator at
lower concentrations and acted also as a terminator at higher
concentrations. Anuradha et al. studied the polymerization of AN using
Ce(IV) – glycerol99 and Ce(IV) – acetophenone100 redox systems. Both the
systems
followed
similar
kinetics
with
the
rate
of
monomer
disappearance being proportional to [AN]1.5, [Ce(IV)]0.5 and [reductant]0.5
and the rate of disappearance of Ce(IV) being proportional to [Ce(IV)]
and [reductant]. The results were explained by assuming the production
of initiating radicals by the oxidation of substrate by Ce[IV] and
termination by the mutual combination of growing polymer chains. The
above mentioned authors101 also reported in the polymerization of AN by
Ce(IV)-thiourea redox system that the rate of monomer disappearance
was directly proportional to [thiourea] and the square of [monomer] and
inversely proportional to [Ce(IV)]. The rate of Ce(IV) disappearance was
proportional to [Ce(IV)] and [thiourea] being independent of [monomer].
16
A kinetic scheme was given, suggesting the production of initiating
radicals from the oxidation of thiourea by Ce(IV) and termination by the
interaction of the chain radicals with Ce(IV).
The aqueous polymerization of AN by Ce(IV)- acetaldehyde redox
system at 25°C was reported by Riaz Ahamed et al.102 The results
obtained were found to be identical to those obtained for Ce(IV) –
glycerol and Ce(IV) –acetophenone systems. The activation energy for
the overall rate of polymerization was 67.0 kJ/mol and the value of the
rate constant was 2.48 mol dm-3s-1
Mohanty et al.103 investigated the polymerization of AN using
Ce(IV)-propane 1,2-diol (PDL) system in aqueous sulphuric acid
medium. The polymerization rate was proportional to [Ce(IV)], [PDL] and
to the square of [AN]. The polymerization of AN initiated by Ce(IV) –
mannitol104 redox system was also studied by the same authors. At low
ceric ion concentration, mutual termination was important whereas at
higher concentrations, linear termination was the exclusive step.
Misra et al.105 reported the homogenous polymerization of AAM
initiated by ceric ammonium sulphate citric acid system. The rate of
monomer disappearance was found to be proportional to [monomer] and
[Ce(IV)]0.58 and independent of [citric acid]. The independence of the rate
of polymerization on [citric acid] might be due to the balance that exists
between the generation and consumption of the free-radicals in the side
17
reactions. The rate of ceric ion disappearance was directly proportional
to [ceric ion] but independent of [monomer].
Polymerization of AN, initiated by Ce(IV)- thioacetamide redox
system, was investigated by Samal et al.106 The rate of polymerization
and the rate of Ce(IV) disappearance were measured. The effect of
certain compounds on the rate of polymerization was also studied.
Termination o the polymer chain was found to occur by the reaction
with metal ions.
The polymerization of AN, initiated by ceric ion organic sulphur
compound redox systems, was studied by Lenka and Nayak.107 The
reactivities of organic sulphur compounds used with ceric ammonium
sulphate to initiate the polymerization of AN decreased in the order :
cysteine > thiourea > 2- aminoethanethiol > thioglycollic acid. The rate
of polymerization and the chain length of polyacrylonitrile increased
with increasing monomer concentration while the latter alone decreased
with increasing ceric ion concentration.
Misra and Bhattacharya108 found out the polymerization of AAM
(M) initiated by ceric ammonium sulphate-thiourea (TU) system. The
polymerization rate depended on [M]1,2, [Ce(IV)]0.5 and [TU]0.5.
The
activation energy was reported to be 24 kJ/mol.
Misra and Khatib109 repoted the polymerization of AAM initiated
by Ce(IV)-lactic acid redox system. The rate of monomer disappearance
18
increased with increasing [lactic acid], [AAM] and reaction temperature.
It decreased with increasing [H2SO4] and [catalyst].
The termination
was, however, found to occur by the mutual combination of growing
chains.
The polymerization of AN by Ce(IV)-glucose system was studied by
Padhi and Singh110. The rate of polymerisation was proportional to
[monomer]2, [glucose] and [Ce(VI)]-l. A mechanism involving termination
by Ce(IV) was proposed. Nageswar Rao et al. pointed out the
polymerization of AN using Ce(IV)-diacetone alcohol111, Ce(IV)-malic
acid112 and Ce(IV)-ethyl methyl ketone113 redox systems. In all the
three
systems,
the
rate
of
polymerization
was
proportional
to
[monomer]1.5Ce(IV)]0.5 and [reductant]. A mechanism involving primary
radical initiation and mutual termination was proposed.
Pramanick and Chakraborthy114 studied the polymerization of
MMA, MA, AAM and AN using Ce(IV)-sulphamic acid redox system.
The rate of polymerization of MMA increased with [ceric ion] upto
0.7X10-3 M and then decreased. The kinetics of polymerization of AN by
ceric ion butane 1,4-diol system was studied by Mohanty et al115. There
was no evidence for the formation of a complex between the diol and the
oxidant. The results were consistent with a linear mode of termination.
The polymerization of AN initiated by Ce(IV)-dimethyl sulphoxide
(DMSO) system in perchloric acid was studied by Nageswar Rao et al.116
19
The rate of polymerization was proportional to [monomer]2 and [DMSO]
while being independent of [Ce(IV)]. Termination was found to occur
exclusively by the interaction of chain radicals with cerium (IV) ions.
Padhi and Singh117 investigated the polymerization of AN by
using Ce(IV) – sucrose redox system and reported that the rate of
polymerization was proportional to [AN]1.5 and the rate of Ce(IV)
disappearance was almost independent of [AN).
The kinetics of the aqueous polymerization of MMA, initiated by
Ce(IV)-thiourea(TU) system in 1 M H2SO4, was studied by Pramanick
and Chatterjee.118 It was found that the Ce(IV) and thiourea initially
formed 1:1 complex which then reacted with uncomplexed ceric ion. In
the presence of excess of Ce(IV)-TU complex, the polymerization of MMA
was initiated by TU radicals whereas the slight excess of TU was found
not to influence the rate of polymerization. The overall activation energy
for the polymerization reaction was determined, in the temperature
between 15°C and 25°C, as 42.65 kJ/mol.
Fernandez et al.119 reported the results of the polymerization of
MMA in aqueous nitric acid with ceric ammonium nitrate-2- butanol
(IBA) redox system. The rate of polymerization was proportional to
[monomer]1.5 and [IBA]0.5. The rate of ceric ion disappearance was Rce
directly proportional to [Ce(IV)]. The polymerization rate was found to
have increased with [Ce(IV)] from 1.0 x 10-3 to 3.0 x 10-3M. From [Ce(IV)]
=3.0 x 10-3 to 1.5 x 10-2 M, the rate became independent of [Ce(IV)] but
20
beyond this concentration, the rate was found to have decreased. The
half order on the [IBA] and 3/2 order on the [MMA] suggested the
mutual termination of growing polymeric radicals.
Fernanda et al.120 investigated the polymerization of MMA,
initiated by ceric ammonium nitrate-methanol redox system, in
aqueous nitric acid medium. The concentration of nitric acid in the
reaction medium was varied from 1.0 x 10-3 N to 1.0 N. An increase in
the acid concentration from 1 x 10-3 N to 9 x 10-2 N caused an
increase in the rate of polymerization, but for acid concentrations
above 9x10-2N the Rp values decreased progressively. At the high
concentration of nitric acid, the protonation of methanol did not favour
the complex formation between ceric ion and methanol.
Fernandez and Guzman121 reported the effects of nitric acid and
nitrate ion concentration on the polymerization of MMA initiated by
ceric ammonium nitate-2-propanol and 2-butanol redox systems. In
these systems, the Rp values initially increased rapidly on increasing the
nitric acid concentration upto 0.1 M and then decreased with further
increase of acid concentration. It was shown that ceric ions in acid
solution consist of different species such as Ce4+, Ce(OH)3+ and
(Ce-O-Ce)6+, whose relative amounts depend on the acid concentration,
according to the equilibria (1) and (2).
21
Ce4+ + H2O
(CeOH)3+ + H+
(1)
2(CeOH)3+
(Ce-O-Ce)6+ + H2O
(2)
Abdulkadir et al.122 studied the polymerization of acrylamide
initiated by Ce(IV)-tartaric acid redox system in sulphuric acid and
perchloric acid media. They found the first order dependence with
respect to [Ce(IV)]. Dong Jianhua et al.123 reported the polymerization
of AAM initiated by Ce(IV)-aceto acetanilide redox system. The rate of
polymerization was 170 times faster for the above system than with
Ce(IV) only. Pojman et al.124 studied the polymerization of AN by
Ce(IV)-bromoderivatives
of
malonic
acid
systems.
During
polymerization, the Ce(III) was reoxidised in an autocatalytic process
involving HBrO2 and the radical BrO2. The BrO2 radical was involved
in termination of polymer chains through disproportionation.
Wen-Cheng et al125 investigated the polymerization of acrylamide
by
Ce(IV)-EDTA
redox
spectrophotometric
system
process.
The
in
aqueous
radicals
medium
which
through
initiated
the
polymerization were found as
K
Ce(IV) + EDTA
Ce(IV) - EDTA
Complex C
kd
Ce(IV) – EDTA
Complex C
EDTA + Ce(III) + H+
R
22
The concentration of Ce(IV) decayed exponentially with reaction
time, following the first order kinetics. The complex formation constant
of the above system was larger than those obtained from other Ce(IV)alcohol complex systems. The enhanced stability of Ce(IV)-EDTA
complex indicated that a strong chelated complex occurred. The
complex formation constant (K) and disproportionation constant (kd)
of Ce(IV)-EDTA chelated complex were reported as 1.67 x 104 and
3.77 x 10-3 respectively. The rate dependence of polymerization on [M]
and [EDTA] both followed a second order reaction when the monomer
concentration was lower. The activation energy was found to be
62 kJ/mol.
The polymerization of acrylamide and acrylonitrile using ceric ion
as initiator can be promoted greatly by the tartaric acid, The kinetic
equation and overall activation energy of the polymerization of
acrylamide initiated by ceric ion/tartaric acid have been obtained126.
Aqueous polymerization of methyl methacrylate initiated by ceric
ion reducing agent systems in sulphuric acid medium was studied by
Venkataramana Reddy et a1.127 Polymerization of methacrylate (MMA)
was carried out in aqueous sulphuric acid medium at 30°C using
ammonium ceric sulphate (ACS) / methyl ethyl ketone (MEK) and
ammonium ceric sulphate / acetone as redox initiator systems. A short
induction period was observed with both the initiator systems, as well
as the attainment of limiting conversion for polymerization reactions.
23
The rate of ceric ion consumption RCe was first order with respect to
Ce(IV) concentration in the concentration range (0.5 - 5.5) x 10-3 M and
0.5 order with respect to reducing agent concentration in the
concentration ranges (0.0480 - 0.2967 M) and (0.0500 - 03912 M) for
Ce(IV) - MEK and Ce(lV)-acetone initiator systems respectively.
Nalla Mohamed128 has studied the polymerization of acrylamide
initiated by Ce(IV) -iminodiacetic acid (IDA) redox system in aqueous
solution. Polymerisation behaviour as a function of the concentration of
Ce(IV), IDA and acrylamide as well as temperature has been studied.
The rate of polymerization has been found to be first order with respect
to the concentration of monomer. The complex formation constant (K)
and disproportionation constant (kd) of Ce(lV) – IDA complex were
evaluated.
complex
The
effect
(initiator)
of
excess
concentrations
of
Ce(IV),
was
mineral
studied.
acid
Evaluation
and
of
complex formation during polymerization of acrylamide initiated by
Ce(IV)-aminopolycarboxylate ligands has been done by the same author.
Radical polymerization of acrylamide initiated by ceric ammonium
nitrate-methionine redox initiator system was studied by Betuldince
et al.129 The polymerization of acrylamide, initiated by [Ce(IV)]
ammonium nitrate-methionine redox initiator system was carried out in
an aqueous solution at different reaction conditions.
24
The kinetics of polymerization of acrylonitrite (AN), initiated by a
V(V)-cyclohexanone redox system in the presence of a surfactant, was
studied over a temperature range of 30°C-50°C in acidic medium by
Manabendra Patra et al.130
Nanda
and
Kishore131
have
studied
catalytic
oxidative
polymerization of vinyl monomer using cobalt phthalocyanine complex
and an exploratory investigation on the polymerization of vinyl acetate
has also been done.
A kinetic study has been done on the polymerization of
acrylonitrile initiated by the Cerium (IV) – Glutamine132 redox system.
Redox
polymerization
of
acrylonitrile,
initiated
by
Tristrinitratocerium(IV)paraperiodate-propane-l,2-diol133,
the
system
has
been
studied in aqueous sulphuric acid under nitrogen in the temperature
range 30°C - 40°C. The relation between the rate of polymerization and
[M]2 or [R] is linear.
The polymerization of acrylamide, acrylic acid and methyl
acrylate, initiated by Mn(III) -aspartic acid redox system in aqueous
sulphuric acid, has been studied by Shanmuga Sundari and Subbu134.
Rajendran and Subbu have studied the polymerization of
methylacrylate
and
acrylamide
initiated
by
the
redox
system
Mn(III)- tartaric acid in aqueous sulphuric acid medium at 15°C135.
25