740

OLEFIN FIBERS

Vol. 10

OXIDATIVE
POLYMERIZATION

Introduction

Oxidative polymerization is, formally, abstraction of two hydrogen atoms from a
monomer to give a polymer, and thus may be classified as polycondensation. The
applicable monomers are mainly aromatic compounds. Electrical and chemical
oxidation methods are often used, in which catalysis with dioxygen or hydro-
gen peroxide is the most favorable; the reaction temperature is moderate and
the by-product is water only. Oxidative polymerization is one of the cleanest and
lowest loading methods in polycondensation. However, the reaction mechanism
is unclear in many cases and the coupling selectivity is not generally easy to
control.

This article deals with oxidative polymerization of phenols, anilines, thio-

phenol derivatives, aromatic hydrocarbons, heterocyclic aromatics, and other
monomers. The reaction mechanism, the coupling selectivity, and the character-
istics of the resulting polymers are discussed.

2,6-Disubstituted Phenols

Until the 1950s, oxidation of 2,6-dimethylphenol (2,6-Me

2

P) by an oxidant

like benzoyl peroxide (1) or alkaline ferricyanide (2) mainly gave 3,3



,5,5



-

tetramethyldiphenoquinone (DPQ) (Fig. 1). In 1959, oxidative polymerization of
2,6-Me

2

P catalyzed by CuCl/pyridine (Py) under dioxygen leading to poly(2,6-

dimethyl-1,4-phenylene oxide) (Poly-2,6-Me

2

P) was discovered (3). Various cat-

alysts such as copper/substituted-ethylenediamine complexes were developed
and Poly-2,6-Me

2

P was found completely miscible with polystyrene (4) (see

P

OLYETHERS

, A

ROMATIC

).

Oxidative Coupling Mechanism.

Poly-2,6-Me

2

P is a C O coupling prod-

uct and DPQ is a product via C C coupling. Control of the C O coupling is a most
important question (5). Three possible reaction mechanisms for the C O coupling

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

Vol. 10

OXIDATIVE POLYMERIZATION

741

Fig. 1.

Oxidation of 2,6-dimethylphenol.

selectivity have been proposed as follows: (1) coupling of free phenoxy radicals re-
sulting from one-electron-oxidation of 2,6-Me

2

P, (2) coupling of phenoxy radicals

coordinated to a catalyst complex, and (3) coupling through phenoxonium cation
formed by two-electron-oxidation of 2,6-Me

2

P (Fig. 2).

Coupling of Phenoxy Radicals Coordinated to Catalyst Complex.

Be-

cause the oxidative coupling of 2,6-Me

2

P with benzoyl peroxide or alkaline fer-

ricyanide affords DPQ as the main product (1,2), it seems that the free phenoxy
radical leads to C C coupling. C O coupling results from the phenoxy radical

Fig. 2.

Three possible mechanisms for C O coupling in the oxidative polymerization of

2,6-dimethylphenol.

742

OXIDATIVE POLYMERIZATION

Vol. 10

coordinated to the copper complex [(ii) in Fig. 2]. In CuCl/Py catalysis, increasing
the amount of Py to copper (6,7) favors the C O coupling. Substituents at the
2,6-positions of Py (6) or high reaction temperature (8) makes the C C coupling
favorable.

The kinetic study of Cu/Py catalysis showed that the oxidative polymeriza-

tion proceeded by a Michaelis–Menten-type reaction mechanism and the C O
coupling is preferred when at least one of the radicals is coordinated to copper (9).
ESR measurements using a copper(II) acetate/Py complex (10), where Py/Cu

= 20,

showed two phenoxo-copper(II) complexes, and the oxidative polymerization gave
mainly Poly-2,6-Me

2

P. For Py/Cu

= 2, none of phenoxo-copper(II) complexes was

detected and the major product was DPQ. Thus, these data suggest that coupling
via the coordinated phenoxy radicals mainly leads to the C O coupling.

Coupling of Phenoxonium Cation with 2,6-Dimethylphenol.

No ESR sig-

nals of free phenoxy radicals in the oxidation of 2,6-Me

2

P with a [CuCl(OCH

3

)Py]

complex were detected (11). It was assumed that the phenoxy radical was oxidized
by the Cu(II) complex (totally two-electron-oxidation from the phenol) to give the
phenoxonium cation, which can couple with the phenol leading to C O coupling
[(iii) in Fig. 2]. For a Cu/N-methylimidazole (NMI) catalyst, the reaction order in
copper changed from 1.22 at NMI/Cu=10 to 1.70 at NMI/Cu=75, and the selectivity
for the C O coupling reached a maximum at the NMI/Cu ratio of at least 30 (12).
From the data, the key intermediate may be a

µ-phenoxo dicopper(II) complex, in

which two-electron-transfer from phenoxo moiety to two copper atoms can give a
phenoxonium cation.

By ab initio calculation, the atomic charges for the phenol, phenolate anion,

phenoxy radical, phenoxonium cations of 2,6-Me

2

P were determined (13). Since

the oxygen atoms in all the species were negative, it was considered that the
species susceptible to the C O coupling has positive charge on the para-carbon,
and therefore, it should be the phenoxonium cation. The coupling of the phenoxy
radicals was excluded, because the treatment of 2,6-Me

2

P with benzoyl peroxide

yielded DPQ (1). The reaction between the phenoxy radical and phenol was ruled
out, because 2,6-dimethylanisole did not react in the oxidative polymerization of
2,6-Me

2

P (14).

If the oxidative coupling of 2,6-Me

2

P proceeds via the phenoxonium cation,

in the presence of a nucleophilic reagent in excess to the phenol, the phenoxo
cation should react with the nucleophile, and hence, Poly-2,6-Me

2

P cannot be

obtained. In fact, it was found that the oxidative polymerization of 2,6-Me

2

P

catalyzed by a Cu(tmed) (tmed: N,N,N



,N



-tetramethylethylenediamine) complex

with an excess amount of n-pentylamine produced Poly-2,6-Me

2

P in 75% yields (5).

The nucleophilicity of n-pentylamine toward benzyl chloride was confirmed to be
much greater than that of 2,6-Me

2

P. These facts strongly indicate that such elec-

trophilic intermediates as a phenoxonium cation are not involved in the oxidative
polymerization.

Coupling of Free Phenoxy Radicals.

The above two mechanisms [(ii) and

(iii) in Fig. 2] are based on the assumption that the coupling via free phenoxy rad-
icals of 2,6-Me

2

P leads to the C C coupling mainly. With alkaline ferricyanide (2),

DPQ was produced in 45–50% but the other half of the oxidized material was a yel-
low nonketonic polymer, probably Poly-2,6-Me

2

P. On the other hand, the oxidation

with benzoyl peroxide afforded the C C coupling products in total 60% yield (1).

Vol. 10

OXIDATIVE POLYMERIZATION

743

However, detailed analysis for this oxidation reaction showed that the reaction
mechanism did not involve radical intermediates but rather a benzoyl perester
intermediate (15). It was reported that the thermal decomposition of benzoyl 2,6-
dimethylphenyl carbonate, which should generate the phenoxy radical, produced
Poly-2,6-Me

2

P and DPQ in 35–38% and 10% yield, respectively (16). From these

studies, it seems that some complicated experimental results and different under-
standings were involved in the above assumption of the C C coupling selectivity
in the free-radical coupling, which was pointed out before (17).

In addition, many studies on the oxidative coupling of 2,6-Me

2

P with inor-

ganic oxidants were performed, and in some cases the free radical was observed.
The oxidation with MnO

2

, PbO

2

, and Ag

2

O produced Poly-2,6-Me

2

P in 60–95%

yields. For MnO

2

(18) and Ag

2

O (19), the free radicals of 2,6-Me

2

P and Poly-2,6-

Me

2

P were detected by ESR measurements. By the addition of increasing amount

of triethylamine in oxidative polymerization with PbO

2

, the formation of DPQ

was suppressed and the yield of Poly-2,6-Me

2

P was increased (20). On the other

hand, the oxidation by hexachloroiridate(IV) anion in an acidic aqueous solution
afforded DPQ in 55–65% (21).

It was considered from the above studies (20,21) that the selectivity for the

C O or C C coupling of 2,6-Me

2

P was much affected by a coexisting base or

acid. Recently, oxidative coupling of 2,6-Me

2

P with Ag

2

CO

3

was examined in the

presence of excess n-pentylamine (nPA) or acetic acid (AcOH) (5). The ratio of the
products Poly-2,6-Me

2

P and DPQ was 50/50 without these additives,

>99/∼0 with

nPA, and 0/100 with AcOH. These data indicate that generally in the oxidative
coupling of 2,6-Me

2

P the addition of a base would lead to the C O coupling and

that of an acid to the C C coupling.

The above observations may be explained by the following proposal (22): in

basic reaction media, a free phenoxy radical is formed leading to C O coupling [(i)
in Fig. 2], and in acidic reaction media, a phenoxonium cation could be generated to
give the C C coupling. In the electrochemical oxidation of 2,6-Me

2

P, the oxidation

above pH 5.2 takes place via two monoelectronic steps, the first of which forms the
free phenoxy radical; the oxidation below pH 5.2 undergoes one dielectronic step,
which might generate the phenoxonium cation (23). It is still unclear whether or
not the phenoxonium cation is really involved as the intermediate in the oxidative
coupling; however, at least the phenoxonium cation should be easier to form under
acidic conditions than under basic conditions.

To summarize on the above observations about the reaction mechanisms, the

C O/C C coupling selectivity in the oxidative polymerization of 2,6-Me

2

P appears

to be as follows (5).

(1) The coupling via coordinated phenoxy radicals and the coupling of free phe-

noxy radicals under basic conditions would mainly lead to the C O coupling.

(2) The coupling of free phenoxy radicals under acidic conditions or the coupling

of phenoxonium cations from the two electron oxidation with phenol would
favor the C C coupling.

Chain Extension Mechanism.

The chain extension mechanism in the

oxidative polymerization of 2,6-Me

2

P has been almost established (Fig. 3). It has

744

OXIDATIVE POLYMERIZATION

Vol. 10

Fig. 3.

Chain extension in the oxidative polymerization of 2,6-dimethylphenol.

been widely accepted that two dimeric phenoxy radicals couple with each other
to give a quinone-ketal intermediate, which has not been detected. No reactiv-
ity of the tail phenoxy group (marked B in Fig. 3) was observed, because the
π-conjugation is cut off by the ether bond (25); ie, the head-tail coupling mecha-
nism, in which the oxygen atom of head phenol unit couples at the para-carbon
atom of tail phenoxy group (24), is excluded. The oxidation of a 4-phenoxyphenol
marked with methyl group (let it be A-B as two distinguishable aromatic rings)
yielded none of the dimer through a head-tail coupling (A-B-A-B) but the dimer
through a quinone-ketal intermediate (B-A-A-B) (26).

Two reaction routes from a quinone-ketal intermediate to a tetramer were

proposed; one is a quinone-ketal redistribution to give a monomer radical and a
trimer radical (27), and the other is a quinone-ketal intramolecular rearrangement
(28). The oxidative polymerization from 2,6-Me

2

P dimer produced the oligomers of

even numbers such as 2,6-Me

2

P itself and the trimer (27,29). From these data, the

extension mechanism for the oxidative polymerization of 2,6-Me

2

P definitely in-

volves quinone-ketal redistribution and it is unclear whether or not quinone-ketal
rearrangement actually occurs. For 2-methyl substituted and 2,6-unsubstituted
4-phenoxyphenols, only quinone-ketal rearrangement took place at a low tem-
perature; however, at higher reaction temperatures, quinone-ketal redistribution
also occurred (26). Application of the redistribution mechanism resulted in the
modification (30–32) and depolymerization (33) of Poly-2,6-Me

2

P.

In general, the oxidative polymerization of phenols undergoes a stepwise

growth mechanism, although the electrochemical oxidative polymerization has
been suggested to be a chain reaction mechanism (34). However, phenolic dimers

Vol. 10

OXIDATIVE POLYMERIZATION

745

and higher oligomers have electron-donating phenoxy groups at p-positions and
become easier to oxidize (more reactive) than phenolic monomers. Therefore, until
the monomer is almost consumed, the reaction mixture consists mainly of the
monomer and polymer, and so it often seems to proceed formally via chain reaction
mechanism, but it is actually reactive intermediate polycondensation (35).

Other 2,6-Disubstituted Monomers.

Various poly(2,6-substituted-1,4-

phenylene oxide)s possessing alkyl, aryl, alkoxyl, and halogen groups have been
produced (see P

OLYETHERS

, A

ROMATIC

). Recently, some functional polymers (36–

39) were synthesized, one of which was converted to a heterocyclic ladder polymer
(39). Poly(2,6-difluoro-1,4-phenylene oxide)s with crystallinity (40) and no crys-
tallinity (41) were synthesized. By enzyme catalysis, oxidative polymerization of
3,5-disubstituted-4-hydroxybenzoic acids, with liberation of carbon dioxide, pro-
duced poly(2,6-disubstituted-1,4-phenylene oxide)s (42,43).

2- and/or 6-Unsubstituted Phenols

A phenoxy radical intermediate has four reactive positions: the oxygen and para-
carbon as well as two ortho-carbons. Therefore, for o-unsubstituted phenols, it is
difficult to regulate the coupling selectivity. Enzyme catalysts and enzyme model
catalysts have been studied for the control of the polymerization of 2- and/or 6-
unsubstituted phenols.

Enzyme Catalyst.

Oxidative coupling of phenols is involved in some bi-

ological reactions; for example, formation of lignin (qv) or melanin is catalyzed
by oxidoreductase enzymes such as peroxidase, oxidase, or oxygenase (44,45). In
1983, horse radish peroxidase (HRP) catalyst was used to remove phenols from
waste water, affording water-insoluble low molecular weight phenolic polymers
(46). Since HRP was employed as the polymerization catalyst (47–49), the poly-
merization of various phenols by using HRP with hydrogen peroxide or laccase
(an oxidase) with dioxygen has been extensively investigated (50,51). These en-
zymes show high catalytic activity for generating free radicals from phenols, but
are unable to control the coupling selectivity (52).

In oxidative polymerization of phenol by HRP catalyst (53,54), soluble

polyphenol was obtained for the first time by controlling the polymer structure
and molecular weight (55). Regio-controlled poly(hydroxyphenylene) was synthe-
sized by using poly(ethylene glycol) (PEG) as template (eq. 1) (56). The enzyme-
catalyzed oxidative polymerizations of alkylphenols (57–61), biphenols (62–64),
and various functional phenols (65–70) were also performed. For glucose-

β-

D

-

hydroquinone, only C C coupling at o-positions was claimed (eq. 2) (65). Chemos-
elective polymerization of a phenolic monomer having a methacryloyl group (eq. 3)
(71) and an ethynyl group (72) was achieved and gave the corresponding polyphe-
nol. “Artificial urushi” was prepared by laccase-catalyzed cross-linking reactions
of urushiol analogues (73,74). (see E

NZYMATIC

P

OLYMERIZATION

).

(1)

746

OXIDATIVE POLYMERIZATION

Vol. 10

(2)

(3)

Peroxidase Model Catalyst.

An N,N



-bis(salicylidene)ethylenediamino

iron [Fe(salen)] complex was found to be a cheap peroxidase model catalyst for
oxidative polymerization of phenols with hydrogen peroxide (75–79). Hematin
also polymerized ethylphenol in a reaction mechanism similar to that for HRP
(80).

Tyrosinase Model Catalyst (“Radical-Controlled” Oxidative Poly-

merization Catalyst).

Conventional copper catalysts were not able to give use-

ful polymers from phenols having at least one o-position unsubstituted (26,81,82).
Peroxidase (HRP), oxidase (laccase), and peroxidase-model [Fe(salen)] catalysts
also showed a limited ability for controlling the coupling selectivity of such phenols
(5,53). New copper catalysts were then studied. Copper(I)/diamine complexes, typ-
ical conventional catalysts, reacted with dioxygen to give bis(

µ-oxo) dicopper(III)

complexes (83). In the reaction of HRP with hydrogen peroxide, Fe(IV) O inter-
mediates were formed (84). These active oxygen complexes were subjected to the
reaction with phenols to afford “free” phenoxy radical species (83,85). These data
suggest that the regioselective coupling cannot be achieved by the catalysts gen-
erating “electrophilic” or “radical” active oxygen species.

Then, a working hypothesis was made (Fig. 4) (86,87): if a catalyst generates

only a “nucleophilic,” strictly speaking “basic,”

µ-η

2

:

η

2

-peroxo dicopper(II)complex

1 (88,89), it will abstract a proton (not a hydrogen atom) from phenol to give
phenoxo–copper(II) complex 2, equivalent to phenoxy radical–copper(I) complex
3. Intermediate species 2 and/or 3 are not “free” radicals but “controlled” rad-
icals, and therefore regioselectivity of the subsequent coupling will be entirely
regulated. The difficulty for conventional catalysts to control the regioselectivity
(90,91) is probably due to generation of free radicals. This new concept is charac-
terized by the exclusive formation of controlled phenoxy radicals, and hence the
new concept was termed a “radical-controlled” oxidative polymerization (87).

Vol. 10

OXIDATIVE POLYMERIZATION

747

Fig. 4.

Nucleophilic vs electrophilic active oxygen complexes.

Oxidative Polymerization of 4-Phenoxyphenol by Tyrosinase Model Cat-

alyst.

“Radical-controlled” oxidative polymerization of 4-phenoxyphenol (PPL)

catalyzed by tyrosinase model complexes has been developed (eq. 4)(86,87,92,93).

(4)

This was the first simple synthesis of crystalline poly(1,4-phenylene oxide)

(PPO) having a melting point by the catalytic oxidative polymerization method,
although other time-consuming synthesis procedures have been reported (94–
96). As the tyrosinase model, (hydrotris(3,5-diphenyl-1-pyrazolyl)borate) copper
[Cu(Tpzb)] complex (5) and (1,4,7-R

3

-1,4,7-triazacyclononane) copper [Cu(L

R

): R

=

isopropyl (iPr), cyclohexyl (cHex), and n-butyl (nBu)] complexes (6) were employed.

To examine the coupling selectivity, the ratio of oxidative coupling dimers

formed at the initial stage of polymerization of PPL was investigated (Table 1)
(86,87). CuCl/N,N,N



,N



-tetraethylethylenediamine (teed), which was the sole cat-

alyst reported for oxidative coupling of PPL (26), was also employed (entry 7). As
a model system of free phenoxy radical coupling, an equimolar amount of 2,2



-

azobisisobutyronitrile (AIBN) was used for the oxidation of PPL (entry 8). In the

748

OXIDATIVE POLYMERIZATION

Vol. 10

Table 1. Dimer Ratio at Initial Stage of Oxidative Polymerization of 4-Phenoxyphenol

Initial dimer ratio, %

Entry

Catalyst

Oxidant

Solvent

7

8

9

10

1

Cu(Tpzb)Cl

a

O

2

Toluene

91

9

0

0

2

Cu(Tpzb)Cl

a

O

2

THF

91

9

0

0

3

Cu(L

iPr

)Cl

2

a

O

2

Toluene

93

7

0

0

4

Cu(L

iPr

)Cl

2

a

O

2

THF

89

7

1

3

5

Cu(L

cHex

)Cl

2

a

O

2

Toluene

95

5

0

0

6

Cu(L

nBu

)Cl

2

a

O

2

Toluene

90

9

0

1

7

CuCl/teed

a

O

2

Toluene

79

6

2

13

8

b

AIBN

Toluene

82

4

2

12

a

Polymerization of PPL (0.60 mmol) with Cu complex (0.030 mmol) and 2,6-diphenylpyridine (0.30

mmol) in solvent (1.2 g) under dioxygen (101.3 kPa) at 40

◦

C. CuCl (0.030 mmol) and teed (0.015

mmol) was used as the Cu complex in entry 7.

b

Oxidized by AIBN (0.60 mmol) under nitrogen at 40

◦

C.

case of CuCl/teed, four dimers were detected and the structures of the dimers were
identified as 7, 8, 9, and 10. Products 7 and 8 are formed by C O coupling, and
formation of 9 and 10 is based on C C coupling.

For the CuCl/teed catalyst, considerable amounts of the two C C coupling

dimers of 9 and 10 were detected and 7 selectivity was low (79%). The dimer ratio
was very similar to that via free-radical coupling by AIBN oxidation, in which
considerable amounts of the C C coupling dimers were observed. However, for
Cu(Tpzb) (5) in toluene and in THF, and for the Cu(L

iPr

), Cu(L

cHex

), and Cu(L

nBu

)

(6) in toluene, none or very little of the C C coupling dimers were detected, show-
ing high regioselectivity of 7. The order of 7 selectivity was Cu(L

nBu

) (90%)

<

Cu(L

iPr

) (93%)

< Cu(L

cHex

) (95%), in good agreement with that of steric hindrance

of the substituents (87). These data show that the regioselectivity of phenoxy rad-
ical coupling can be controlled by these catalysts. The dimerization catalyzed by
the Cu(L

iPr

) in THF gave C C coupling dimers to some extent.

The resulting polymer was isolated as a methanol-insoluble part. In the cases

with little or no C C dimer formation (entries 1–3,5,6), white powdery polymers

Vol. 10

OXIDATIVE POLYMERIZATION

749

Fig. 5.

Reaction mechanism for copper complex catalysis of oxidative polymerization.

with M

w

of 700–4700 were obtained. The IR spectrum patterns of the polymers

were very similar to that of PPO synthesized by Ullmann condensation (95). From
the DSC analysis, the polymers showed melting points (T

m

) at 171–194

◦

C. In

polymerizations giving considerable amounts of the C C dimers (entries 4,7,8),
the brownish polymers showed no clear melting points in the DSC traces.

Reaction Mechanism of Catalytic Cycle.

On the basis of the above data,

the reaction mechanism for the copper complex catalysis is postulated as follows
(Fig. 5). First, the starting copper(II) chloride complex Cu(Tpzb)Cl (5) or Cu(L

R

)Cl

2

(6) reacts with PPL or oligomers of PPL to give phenoxo-copper(II) complex (2),
equivalent to phenoxy radical-copper(I) complex (3). Regioselective coupling takes
place between two molecules of 2 and/or 3 to produce copper(I) complexes (11) as
well as the phenylene oxide products having p-linkage selectively, because the
steric hindrance of the catalysts blocks the coupling at o-positions.

In case of the Cu(Tpzb) complex (5), formation of

µ-η

2

:

η

2

-peroxo dicopper(II)

complex (1) from 11 was confirmed under dioxygen (89) in both toluene and THF.
For the Cu(L

iPr

) complex as well as the Cu(L

cHex

) and Cu(L

nBu

) complexes (6), it

was reported that 11 afforded complex 1 in nonpolar solvents such as toluene (97).
1 reacts with phenols to regenerate 2 (98) and hydrogen peroxide (99). Hence, this
catalytic system would allow only the regioselective coupling process from 2 and/or
3 and completely exclude free-radical coupling reactions; the present reaction is
thus recognized as “radical-controlled” oxidative polymerization.

For the Cu(L

iPr

) complex under dioxygen in THF, 11 gave bis(

µ-oxo) di-

copper(III) complex (4) (97). 4 abstracts hydrogen atoms from phenols to give
bis(

µ-hydroxo) dicopper(II) complex (12) and free phenoxy radical (13). Therefore,

750

OXIDATIVE POLYMERIZATION

Vol. 10

Fig. 6.

Reaction mechanism for oxidative coupling and chain extension of 4-

phenoxyphenol.

this catalytic cycle involves the free-radical coupling with the formation of C C
linkages, although production of 2 from complex 12 also takes place (100). The
CuCl/teed complex also reacts with dioxygen to give 4 (83).

Two computational studies have been performed; one disagreed with the

above reaction mechanism (101), but the other was in good agreement (102).

Reaction Mechanism of Oxidative Coupling and Chain Extension.

Figure

6 shows a specrulative reaction mechanism for oxidative coupling of PPL to pro-
duce dimers (87,92). For simplicity, the mechanism is argued here by expressing
intermediate structures in the form of free radical rather than controlled rad-
icals. First, two phenoxy radicals are generated from PPL and couple to each
other (radical coupling); then only three reaction routes (a, b, and c) can take
place, giving rise to a quinone-ketal intermediate, 8 and 9, respectively. From
the quinone-ketal, the redistribution path (quinone-ketal redistribution, d) was

Vol. 10

OXIDATIVE POLYMERIZATION

751

ruled out, and therefore the rearrangement path (quinone-ketal rearrangement)
is proposed in the oxidative coupling of PPL. On cleavage of the ketal C O bond,
synchronous bond formation of route e to 7, that of f to 10, and that of h to 9 can
occur, but that of g regenerates the quinone-ketal.

In the “radical-controlled” oxidative coupling of PPL, the radical coupling

takes place from controlled phenoxy radical–copper(I) intermediate. Therefore,
the steric effect of the catalyst would suppress 8 formation (route b) and inhibit 9
formation (route c), mainly giving quinone-ketal intermediate (route a). Moreover,
almost no detection of 10 as well as 9 shows that the catalyst must be kept inter-
acting with the quinone-ketal intermediate during the rearrangement. Probably,
the carbonyl group of the quinone-ketal coordinates to the copper(I) atom of the
catalyst. Thereby, bond formation at the o-position to 10 and 9 via routes f and
h, respectively, would be protected and give 7 predominantly via route e. Further
chain extension follows in a similar way to that of dimerization.

Oxidative Polymerization of Other Phenols by Tyrosinase Model Catalyst.

The substituent effect of phenol monomers on the reaction rates has been inves-
tigated (103). For the Cu(tmed) catalyst, the reaction rates were governed by the
O H homolytic bond dissociation energies of the monomers, which are closely re-
lated to the electronic effect of substituents. On the other hand, for the Cu(L

iPr

)

catalyst, the steric effect of substituents at the o-positions depressed the reaction
rates.

In the oxidative polymerization of phenol, the Cu(L

iPr

) catalyst showed high

selectivity for C O coupling; however, it did not exclude the formation of C C
coupling (104). The resulting polymer consisted mainly of a 1,4-phenylene oxide
unit but contained a considerable amount of C C coupling structures, showing
no crystallinity. The oxidative polymerization of 2- and 3-methylphenol regiose-
lectively produced soluble poly(phenylene oxide)s, showing good thermal stability
(105,106).

The polymers obtained from 2,5-dialkylphenols [2,5-R

2

P: R

= methyl (Me),

ethyl (Et), n-propyl (nPr)] are noteworthy (107) (eq. 5). The oxidative polymeriza-
tion of 2,5-Me

2

P catalyzed by Cu(L

iPr

)Cl

2

in toluene under dioxygen produced a

white polymer. The M

w

was 19,300 and the structure was composed exclusively

of a 2,5-dimethyl-1,4-phenylene oxide unit. The melting temperatures in the first
and second scan (T

m1

and T

m2

) were detected at 308 and 303

◦

C, respectively. Poly-

2,5-Me

2

P (eq. 5) showing heat-reversible crystallinity was synthesized for the first

time. The isomeric polymer Poly-2,6-Me

2

P (Fig. 1) showed a melting point at 237

◦

C

(T

m1

), but once the crystaline part had been totally melted, recrystallization never

occurred (T

m2

not detected) by slow cooling or after annealing (108). Since ther-

moplastic polymers are mainly used as melt-moldings, Poly-2,6-Me

2

P is generally

accepted as an amorphous polymer; however, Poly-2,5-Me

2

P is considered as a

crystalline one.

(5)

752

OXIDATIVE POLYMERIZATION

Vol. 10

In the oxidative polymerization of 2,5-Et

2

P and 2,5-nPr

2

P (107), white poly-

mers with M

w

of 23,100 and 32,200, respectively, possessed only the 1,4-phenylene

oxide units. The latter showed heat-reversible crystallinity with T

m2

at 276

◦

C,

however, the former did not show a detectable T

m2

. The recrystallization for

poly(alkylated phenylene oxide)s after melting seems to be governed by both the
position and nature of alkyl substituents.

Oxidative Polymerization of Naphthol Derivatives.

Oxidative poly-

merization

of

2-naphtol

(109)

and

1,5-dihydroxynaphthalene

(110)

has

been done using enzyme catalysts. Solid-state polycondensation of 2,6-
dihydroxynaphthalene with FeCl

3

catalyst (111) has been accomplished. Asym-

metric oxidative coupling polymerization of 2,3-dihydroxynaphthalenes and their
derivatives was achieved by chiral copper catalysts (112–115).

Oxidative Polymerization of Anilines

Oxidative polymerization of aniline produces polyaniline (PAN) (eq. 6). This poly-
mer was obtained as aniline black about a century ago (116,117) and has been re-
vived as an electrically conducting polymer (see E

LECTRICALLY

A

CTIVE

P

OLYMERS

).

(6)

Polyaniline has been synthesized electrochemically (118,119) and with chem-

ical oxidizing agents (120–123); a typical oxidizing agent is ammonium persulfate
(120,121). Catalytic oxidative polymerization of aniline using iron salt catalyst
(124) or HRP enzyme catalyst (125–129) with hydrogen peroxide, iron salt cata-
lyst with ozone (130), and copper salt catalyst with dioxygen (131) has been done.
A photo-induced catalytic system (132,133) and a gas-phase plasma method (134)
have also been reported. Strongly acidic reaction conditions in the polymerization
are normally selected, because polymer structure and electrical conductivity of
PAN depend on the pH of the polymerization reaction (135,136).

The reaction mechanism of oxidative polymerization of aniline has been a

big controversy (Fig. 7). The dimerization step is generally proposed as (i), in
which aniline is one-electron-oxidized to a cation radical, followed by coupling
of two molecules of the cation radical to a dimer. The subsequent steps of chain
extension are under discussion; routes involving coupling of cation radicals such
as (ii)–(iv) (137–140) and routes via electrophilic attack of a two-electron-oxidized
quinodal diiminium ion (v) or nitrenium ion (vi) (141,142) have been proposed. The
addition of electron-rich arenes does not inhibit the polymerization, and therefore
the route through the nitrenium ion (vi) seems to be rejected (137).

Protonic doping is necessary to convert PAN from an insulator to a con-

ductor (136), and so various dopants such as acid-substituted polymers (143–146)
and chiral sulfonic acids (147–149) have been employed. Emulsion polymerization
of aniline-afforded PAN particles (150–156) and PAN–silica particles (157,158).
Polyaniline nanofibers have been synthesized by template-guided polymerization

Vol. 10

OXIDATIVE POLYMERIZATION

753

Fig. 7.

Reaction mechanism for oxidative polymerization of aniline.

(159,160) and aqueous/organic interfacial polymerization (161–163). Intercalated
PANs in V

2

O

5

(164), VOPO

4

(165), MoO

3

(166), and FeOCl (167) have been ob-

tained, and included PANs in zeolite molecular sieves (168) and mesoporous chan-
nel hosts (169).

Many substituted polyanilines were also synthesized (170). Typical examples

are self-doping polymers, such as sulfonic acid ring-substituted PANs (171–177),
mercaptopropanesulfonic acid substituted PANs (178), N-alkylsulfonic acid sub-
stituted PANs (179–181), and N-phenylsulfonic acid substituted PANs (182,183).
Polyanilines having boronic acid for detecting sugar and dopamine (184,185) and
for controlling self-doped states (186) have been produced. Long-chain-substituted
anilines were polymerized at an air–water interface by the Langmuir–Blodgett
technique (138,187,188). (see L

ANGMUIR

-B

LODGETT

F

ILMS

). Polyanilines with liquid

crystalline substituents (189,190) and redox-active disulfide unit (191) were syn-
thesized. Oxidative polymerization of 1-aminonaphthlenes gave PAN-like poly-
mers (192,193).

Oxidative Polymerization of Thiophenols and Their Derivatives

Thiophenol is oxidatively coupled to give diphenyldisulfide (eq. 7), because forma-
tion of an S S bond, resulting from the coupling of thiophenoxyl radicals, happens
more readily than formation of a C S bond (194). Oxidative polymerization of 1,3-
dimercaptobenzene afforded poly(1,3-phenylenedisulfide) (195–197).

(7)

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OXIDATIVE POLYMERIZATION

Vol. 10

Poly(phenylene sulfide) (PPS) has been synthesized by oxidative polymer-

ization of diphenyl disulfide, which was obtained via oxidation of thiophenol
(eq. 8) (198–208). PPS is an engineering plastic with a high melting point, which
is manufactured by condensation polymerization eliminating a salt.

(8)

The oxidative polymerization of diphenyl disulfide was carried out in the

presence of a strong acid by electrolysis (198) and by reaction with Lewis acids such
as SbCl

5

(199–201) or quinones such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone

(202–205). VO catalysts such as vanadyl acetylacetonate with O

2

have also been

used for the oxidative polymerization (206–208), and the catalytic reaction mech-
anism involving four-electron reduction of O

2

has been discussed (209–215).

The chain extension mechanism is shown in Figure 8 (201). Diphenyl disul-

fide is oxidized to a cation radical, which reacts with diphenyl disulfide to give
phenylbis(phenylthio)sulfonium cation, followed by electophilic attack of diphenyl
sulfide on the cation.

The use of various diaryl disulfides as the monomer produced PPSs substi-

tuted with methyl groups at the 2- or 3-, 2,3-, 2,5-, or 2,6-positions, methoxy at
the 2-position, etc. (204,207). Reactive functionalized oligomers (216) and block
polymers containing alkylene and perfluoroalkylene groups (217,218) have been
obtained. Cyclic hexakis(1,4-phenylene sulfide) was synthesized (219), and ring-
opening polymerization of such cyclic oligomers was also found (220).

The synthesis of PPS via poly(sulfonium cation) as a soluble precursor (221)

was developed through oxidative polymerization of methyl 4-(phenylthio)phenyl
sulfide (222), followed by dealkylation (eq. 9) (223–225).

Fig. 8.

Reaction mechanism for oxidative polymerization of diphenyl disulfide.

Vol. 10

OXIDATIVE POLYMERIZATION

755

(9)

Oxidative Polymerization of Aromatic Hydrocarbons

In the 1960s, oxidative polymerization of benzene by CuCl

2

-AlCl

3

was discovered

(eq. 10) (226). Other oxidants such as AlCl

3

-MnO

2

, FeCl

3

, and MoCl

5

were em-

ployed. These systems allowed the polymerization of various monomers such as
toluene, chlorobenzene, diphenyl, naphthalene, and phenanthrene (227).

(10)

Oxidative polymerization of bis(1-naphthoxy) monomers is known as the

Sholl Reaction (eq. 11) (228). 1,4-Dialkoxybenzenes have been polymerized using
FeCl

3

(229,230) and oxovanadium catalyst with dioxygen (231) to give poly(2,5-

dialkoxy-1,4-phenylene)s (eq. 12). Poly(4,6-di-n-butyl-1,3-phenylene) was also ob-
tained (232).

(11)

(12)

Three reaction mechanisms of C C coupling in oxidative polymerization of

benzenes have been reported (Fig. 9). The first step of each mechanism is one
electron oxidation of a monomer to give a cation radical. The next step may be (i)
coupling of two cation radicals (233,234), (ii) coupling of one cation radical with
one neutral molecule (226,228), or (iii) coupling of one cation radical with many
neutral molecules (stair-step mechanism) (235).

Oxidative Polymerization of Heterocyclic Aromatics

Oxidative polymerization of pyrrole (236), thiophene (237), furan (238), and se-
lenophene (239) was performed with electrical methods around two decades ago

756

OXIDATIVE POLYMERIZATION

Vol. 10

Fig. 9.

Reaction mechanism for oxidative polymerization of benzene.

(eq. 13). More recently, numerous studies on the resulting polymers, particularly
polypyrroles and polythiophenes, have been performed in terms of electrical con-
ductivity (see E

LECTRICALLY

A

CTIVE

P

OLYMERS

).

(13)

For the oxidative polymerization of heterocyclic aromatics, two reaction

mechanisms, similar to that of benzenes, have been proposed (Fig. 10). One is
coupling of two cation radicals [(i) in Fig. 10] (240,241), and the other is coupling
of one cation radical with one neutral molecule [(ii) in Fig. 10] (242,243).

Oxidative Polymerization of Pyrroles.

The synthesis of pyrrole blacks

was performed by chemical oxidative polymerization with a variety of oxidizing
agents such as hydrogen peroxide, lead dioxide, quinines, ferric chloride, and per-
sulfates (244) before the electrical method was employed (236). The catalytic ox-
idative polymerization under dioxygen to give polypyrrole (PPY) has also been
developed (245–247).

Fig. 10.

Reaction mechanism for oxidative polymerization of heterocyclic aromatics.

Vol. 10

OXIDATIVE POLYMERIZATION

757

Fig. 11.

Regioisomers of 3-substituted polythiophenes.

Various PPYs having 1-substituted (248), 3-substituted (249,250), and 3,4-

disubstituted (251–253) groups, and transition metal complex moieties (254–256)
have been obtained.

PPY particles were produced by colloidal dispersion method (257–259).

Monolayer and multilayer PPYs were obtained by thiol–Au interaction (260,261),
by deposition on YBa

2

Cu

3

O

7

−δ

(262,263), by intercalation in FeOCl (264), and by

monomer amphiphilicity (265). PPY wires (266–268), PPY tubes (269–271), and
PPY microcontainers (272,273) have also been synthesized.

Oxidative Polymerization of Thiophenes.

Thiophene is oxidatively

polymerized to give polythiophene (PTH) by electrochemical oxidation (237); by
chemical oxidation with AsF

5

(274), NO salts (275), and FeCl

3

(276); and by cat-

alytic oxidation with dioxygen (277).

PTH can be substituted at the 3- and/or 4-positions (278). 3-Substituted

PTHs have regioisomers (Fig. 11), and the regioregularity greatly affects conju-
gation length and electronic properties. For poly(3-alkylthiophene)s, high regios-
electivity of 91–98% HT has been obtained by organometalic reaction (279,280);
however, a maximum 89% HT (Fig. 11) has been obtained by oxidative polymer-
ization (281,282). In oxidative polymerization of 3-arylthiophenes (283–285) and
3-alkoxy-4-methylthiophenes (286), 94–96% HT contents were achieved. This re-
gioselectivity has been discussed as arising from the spin density of the radical
cations (287,288).

Poly(3,4-ethylenedioxythiophene) was developed around 1990, and its com-

bination with poly(styrene sulfonic acid) became practical because of its high film-
formability, conductivity, transparency, and stability (289). Many functional PTHs
were prepared possessing crown ether (290,291), tetrathiafluvalene-like struc-
tures (292,293), C

60

-attachment (294,295), probes for affinity chromism (296–298),

and various metal complexes (299–306).

PTH film obtained by electrochemical deposition was stronger than alu-

minum film (307). PTH fibers were produced by using capillary flow cell (308) and
with electrically independent connections (309). PTHs were included in zeolite
(310,311) and such PTH wires (312) were obtained. PTH catenanes and rotaxanes
have also been synthesized (313,314).

Oxidative Polymerization of Other Monomers

Treatment of m-diethynylbenzene with a copper catalyst and dioxygen gave a
poly(phenylene butadiynylene) (315). Use of Cu-incorporated mesoporous mate-
rials as the oxidative polymerization catalyst for 1,4-diethynylbenzene produced
highly conjugated poly(1,4-phenylene-1,4-butadiynylene) (316).

758

OXIDATIVE POLYMERIZATION

Vol. 10

Bifunctional p-tolylcyanoacetic esters (317) and monofunctional phenyl-

cyanoacetic esters (318) were oxidatively polymerized using a copper catalyst with
dioxygen.

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H

IDEYUKI

H

IGASHIMURA

Sumitomo Chemical Company Ltd.
S

HIRO

K

OBAYASHI

Kyoto University