Vol. 3

INORGANIC POLYMERS

29

INORGANIC POLYMERS

Introduction

Inorganic polymer science represents an area that has been held back by the syn-
thetic problem of constructing macromolecular chains. However, advances in the
last two decades or so of the twentieth century have led to the preparation of
a variety of new polymers that contain main group elements, transition-metals,
and even lanthanides. It is plausible that some of these new materials, with prop-
erties that are difficult or impossible to achieve with existing organic materi-
als, may fulfill the requirements of specialized markets; such developments re-
main an interesting future challenge. Polysiloxanes (silicones) represent the most
well-developed class of inorganic polymers and these materials are discussed in
a separate article (see S

ILICONES

). In addition, although surveyed briefly in this

article, more detailed information on P

OLYPHOSPHAZENES

and P

OLYSILANES

and

P

OLYCARBOSILANES

can be found elsewhere in the Encyclopedia. Here we focus on

other main classes of inorganic polymers.

Inorganic Polymers Based on Main Group Elements

Polyphosphazenes.

Polyphosphazenes (1) have a polymeric backbone

composed of alternating phosphorus and nitrogen atoms. The side groups, R, can
be alkoxy, aryloxy, amino, alkyl, aryl, inorganic, or organometallic groups. This
large range of accessible structural variations is accompanied by a wide range of
polymer properties that are highly dependent upon the nature of the side groups
(see Table 1).

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

30

INORGANIC POLYMERS

Vol. 3

Table 1. Properties of Selected Polyphosphazenes

a

Formula

T

g

,

◦

C

T

m

,

◦

C

Properties

[NP(O

n

C

4

H

9

)

2

]

n

−105

Elastomer

[NP(O

n

C

3

H

7

)

2

]

n

−100

Elastomer

(NPF

2

)

n

−96

−68, −40 Hydrolytically unstable

elastomer

[NP(OC

2

H

5

)

2

]

n

−84

Elastomer

[NP(OCH

2

CH

2

OCH

2

CH

2

OCH

3

)

2

]

n

−84

Water-soluble elastomer

[NP(OCH

3

)

2

]

n

−76

Elastomer

(NPCl

2

)

n

−66

−7.2 (39)

b

Hydrolytically unstable

elastomer

[NP(OCH

2

CF

3

)

2

]

n

−66

242

Microcrystalline thermoplastic

(films)

[N

3

P

3

(OCH

2

CF

3

)

5

(CH

3

)]

n

−63

Elastomer

[N

3

P

3

(OCH

2

CF

3

)

x

(CH

2

Si(CH

3

)

3

)]

n

−61

Elastomer

[NP(OCH

2

CF

3

)(OCH

2

(CF

2

)

x

CF

2

H)]

n

−60

c

Elastomer

[NP(OC

9

H

19

)

2

]

n

−56

Elastomer

[NP(CH

3

)(alkyl)]

n

−50

d

Amorphous gums or waxes

[NP(CH

3

)

2

]

n

−46

143

Microcrystalline powder

[NP(

n

C

6

H

13

)

2

]

n

−29

129

Wax-like solid

(NPBr

2

)

n

−15

Hydrolytically unstable,

leathery material

[NP(OC

6

H

5

)(OC

6

H

4

C

2

H

5

)]

n

−10

e

f

Elastomer

[NP(OC

6

H

5

)

2

]

n

−8

390

Microcrystalline thermoplastic

(films, fibers)

[NP(OC

6

H

4

COOH)

2

]

n

−5

Glass, soluble in aqueous base

[NP(OC

6

H

4

COOC

2

H

5

)

2

]

n

8

127

Microcrystalline thermoplastic

(films)

[NP(OC

6

H

4

CH

3

)(OC

6

H

4

CHO)]

n

11

Thermoplastic

[NP(NHCH

3

)

2

]

n

14

Water-soluble glass and

film former

[NP(OC

6

H

5

)(OC

6

H

4

C

6

H

5

-o)]

n

24

Glass

[NP(NHC

2

H

5

)

2

]

n

30

Glass, soluble in aqueous acid

[NP(CH

3

)(C

6

H

5

)]

n

37

[NP(OC

6

H

5

)(OC

6

H

4

C

6

H

5

-p)]

n

43

Glass

[NP(CH

3

)(alkyl)]

n

∼50

g

Amorphous gums or waxes

[N

3

P

3

(OCH

2

CF

3

)

4

(C

5

H

4

FeC

5

H

4

)]

n

g

61

Amber-colored glass

[NP(NHC

6

H

5

)

2

]

n

91

Glassy thermoplastic

[NP(OC

6

H

4

C

6

H

5

-p)

2

]

n

93

>350

Microcrystalline

thermoplastic (high

η)

a

Refs. 1,2–3.

b

For the stretched polymer.

c

Varies with values x and ratio of side groups.

d

Broad, poorly defined transitions.

e

Varies with ratio of side groups.

f

Complex melting phenomena.

g

Ferrocenyl polymer.

Vol. 3

INORGANIC POLYMERS

31

In addition, the phosphorus–nitrogen backbone inherently possesses a

unique range of unusual properties. For example, it is extremely flexible which in
turn can give rise to low glass-transition temperatures, particularly in the case
of poly(alkoxyphosphazenes) such as the n-butoxy derivative (T

g

= −105

◦

C) (1,4).

Furthermore, the backbone is thermally and oxidatively stable, as well as optically
transparent from 220 nm to the near infrared region, which makes it resistant
to breakdown in many harsh environments, as evidenced by the flame-retardant
properties of many polyphosphazenes.

Ring-Opening Polymerization (ROP).

The first polyphosphazene synthe-

sized, poly(dichlorophosphazene) (2), was prepared in cross-linked form by Stokes
at the end of the nineteenth century by the thermal ROP of the cyclic trimer
[Cl

2

PN]

3

(3) (1). This material, referred to as “inorganic rubber,” remained a

chemical curiosity because of its intractability and hydrolytic instability until
the mid-1960s when it was shown that if the ROP of pure [Cl

2

PN]

3

is carried

out carefully, uncross-linked poly(dichlorophosphazene) (2), which is soluble in or-
ganic solvents, is formed (5). Subsequent reaction of this highly reactive polymeric
species with nucleophiles has been shown to yield a wide range of hydrolytically
stable poly(organophosphazenes) (eq. 1) (1,6–8).

(1)

This macromolecular substitution route, used primarily with alkoxides, ary-

loxides, or primary amines, is largely responsible for the immense structural
diversity of poly(organophosphazenes) and allows the tuning of certain prop-
erties and the introduction of others through the choice of nucleophile. High-
lights concerning the use of the macromolecular substitution route with 2 and
related materials involve the introduction of side groups, which lead to liquid
crystallinity (see polymer 4) (9), photochromism (10), photocross-linkability (11–
13), and the preparation of novel polymers such as (5) with short-chain branching
(14,15).

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INORGANIC POLYMERS

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The thermal ROP route requires the synthesis and careful purification of

the cyclic trimer [Cl

2

PN]

3

and the use of elevated temperatures where control of

molecular weight is very difficult and cross-linking can take place at high conver-
sion, which can limit the yield.

Condensation Polymerization.

Although several polyphosphazenes have

been commercialized, much work has focused on the development of cheaper
and more convenient methods for making these materials. Also, the reaction
of poly(dichlorophosphazene) with organometallic reagents such as Grignard or
organolithium reagents generally leads to chain cleavage as well as substitution,
and thus macromolecular substitution does not provide a satisfactory route to
polymers with only alkyl and aryl side groups bound by direct P C bonds. To
these ends several promising condensation routes have been developed.

In the early 1980s, a condensation route to polyphosphazenes from phospho-

ranimines was discovered (eq. 2) (16).

(2)

The polymerization is in fact a chain-growth reaction and allows access to

high molecular weight polyphosphazenes such as poly(dimethylphosphazene) and
poly(methylphenylphosphazene) (2). Methyl deprotonation/substitution of these
polymers as well as electrophilic aromatic substitution of the phenyl substituents
in poly(methylphenylphosphazene) have been developed as versatile strategies
for the derivatization of both of these polymers (eq. 3) (3).

(3)

Vol. 3

INORGANIC POLYMERS

33

An alternative, direct route to fluoroalkoxy phosphazene polymers and aryl deriva-
tives which also permits access to block copolymers has been developed (eqs. 4
and 5) (17,18).

(4)

(5)

The development of condensation routes to poly(dichlorophosphazene) have

also been reported. One promising route operates at 200

◦

C (eq. 6) (19).

(6)

In 1995, details of a synthesis of poly(dichlorophosphazene) which operates

at room temperature and allows for good molecular weight control were reported
(eq. 7) (20). It involves the condensation of phosphoranimines in the presence of
cationic initiators such as a PCl

5

. The polymerization has been shown to pro-

ceed via a cationic chain-growth mechanism that shows “living” characteristics
(21). Two equivalents of the initiator react with one equivalent of a chlorinated
phosphoranimine to form a reactive ion pair (eg, [Cl

3

P N PCl

3

]

+

PCl

6

−

), which

further reacts with monomer to propagate chain growth.

(7)

This synthetic method has been extended to the direct synthesis of

poly(organophosphazenes) as well as the development of star and block copoly-
mers. For example, triarmed star-branched polyphosphazenes (eg, 6) can be syn-
thesized through the initiation of trifunctional phosphoranimines (22). It has
also been shown that the presence of “living” active sites at the termini of the
polymer chains allows for addition of a second monomer and the formation of
block copolymers (23), such as (7) which is formed through the initiation of a
difunctional linear phosphoranimine and the subsequent introduction of two dif-
ferent monomers (24). These developments offer the prospect of improved routes

34

INORGANIC POLYMERS

Vol. 3

to phosphazene polymers, which may well facilitate more rapid and extensive
commercialization.

Monomer

Synthesis.

Both

the

ROP

and

condensation

routes

to

poly(dichlorophosphazene) require high purity monomers. For the traditional ROP
process, ultrahigh purity [NPCl

2

]

3

is required. The most common method for its

preparation is through the reaction of PCl

5

with NH

4

Cl in a high boiling halo-

genated solvent, such as chlorobenzene or tetrachloroethane, at 150

◦

C. The yields

can approach 70–80% under specialized conditions, but 50% is more typical (25).

The phosphoranimine most commonly used in the aforementioned conden-

sation route to poly(dichlorophosphazene) is Cl

3

P NSi(CH

3

)

3

. The synthesis of

this compound has been described (21) and is based on a modification of an ear-
lier procedure (26) involving the reaction of PCl

5

with LiN(Si(CH

3

)

3

)

2

at

−78

◦

C

in hexane. However, the purification required by this method is a challenge and
leads to an overall yield of

<40%.

It has also been reported that the reaction of N(Si(CH

3

)

3

)

3

with PCl

5

can

be manipulated to maximize yields of either the cyclic trimer (76%) or the pure
phosphoranimine monomer (40%) through variations in reaction conditions (25).
These reactions use milder conditions than previously reported methods, with
higher yields. In addition, the major side product, (CH

3

)

3

SiCl, can be recycled to

form one of the starting materials, N(Si(CH

3

)

3

)

3

.

N-Silylphosphoranimines with alkyl and aryl substituents can be synthe-

sized by reaction of commercial starting materials such as PCl

3

, ((CH

3

)

3

Si)

2

NH,

and Grignard reagents by way of a three-step process, with typical yields of
60–70%. These precursors are also accessible via a “one-pot” synthesis, which
requires only isolation of the final product with yields of about 75% (3).

Uses.

Some of the most useful polyphosphazene derivatives are

fluoroalkoxy derivatives, and amorphous copolymers are very useful as
flame-retardant, hydrocarbon-solvent, and oil-resistant elastomers and have
found aerospace and automotive applications. Polymers such as the amorphous
comb polymer poly[bis(methoxyethoxyethoxy)phosphazene] (8) are of consider-
able interest as components of polymeric electrolytes in battery technology (1).

Vol. 3

INORGANIC POLYMERS

35

Polyphosphazenes are also of interest as biomedical materials and bioinert,
bioactive, membrane-forming, and bioerodable materials (1).

Polysilanes

Properties.

Since the first reports of soluble and processible polysilanes

(9) in the late 1970s, these macromolecules have attracted widespread interest
from both a fundamental and an applied perspective (1,27–29). The use of Wurtz
coupling has allowed the preparation of a range of polymers with alkyl or aryl sub-
stituents at silicon that are high in molecular weight with M

n

> 10

5

. Since 1980,

there has been a remarkable growth in interest concerning these polymers and
they have been found to possess a variety of fascinating properties. The backbone
of silicon atoms gives rise to unique electronic and optical properties.

One of the most remarkable features of the all-silicon backbone is that it

leads to the delocalization of

σ electrons, a phenomenon which is virtually un-

known in carbon chemistry (30). This can be understood in terms of the nature
of the molecular orbitals associated with the Si Si

σ bonds. These are more dif-

fuse than those associated with C C

σ bonds as they are constructed from higher

energy 3s and 3p atomic orbitals. This leads to significant interactions between
the adjacent Si Si

σ bonds along a polysilane chain, a situation analogous to

that for the

π bonds in π-delocalized polymers such as polyacetylene. Thus, a

band model is more appropriate than a localized model (1,28). As a consequence
of the delocalization of

σ electrons, the σ σ ∗ transition, which occurs at 220 nm

in (CH

3

)

3

Si Si(CH

3

)

3

, moves to lower energy as the number of silicon atoms in

the chain increases. In the high polymers, the

σ σ ∗ band-gap transitions occur

in the near uv region at ca 300–400 nm (Table 2). The lowest absorption bands of
polysilanes have also recently been assigned to excitonic rather than interband
transitions (32). The electron delocalization also leads to appreciable electrical
conductivity following doping. For example, conductivities of up to 0.5 S/cm have
been reported for polysilastyrene (10), after doping with AsF

5

(1). In addition,

many of the polymers are thermochromic as the conformations adopted by the
polymer change with temperature, which alters the degree of

σ-delocalization

along the main chain. Because of their low energy

σ σ∗ transitions, polysilanes

36

INORGANIC POLYMERS

Vol. 3

Table 2. Ultraviolet–visible Absorption Data for Typical Polysilanes

a

,b

R

1

R

2

λ

max

, nm

ε/SiSi

CH

3

n

C

3

H

7

306

5600

CH

3

n

C

4

H

9

304

CH

3

n

C

6

H

13

306

5100

CH

3

n

C

12

H

25

309

5000

CH

3

C

2

H

4

-C

6

H

5

303

9950

CH

3

c

C

6

H

13

326/320

7390

n

C

3

H

7

n

C

3

H

7

310

n

C

4

H

9

n

C

4

H

9

314

8400

n

C

6

H

13

n

C

6

H

13

316/317

9700

n

C

14

H

29

n

C

14

H

29

318

8400

n

C

3

H

7

n

C

6

H

13

322

10600

n

C

6

H

13

n

C

8

H

17

318

8785

CH

3

C

6

H

5

341

9300

CH

3

p-C

6

H

4

CH

3

337/338

8600

CH

3

p-C

6

H

4

OCH

3

344

8180

CH

3

p-C

6

H

4

O

n

C

6

H

13

354

5400

CH

3

p-C

6

H

4

C

6

H

5

352

4000

CH

3

β-naphthyl

350

2800

p-C

6

H

4

C

2

H

5

p-C

6

H

4

C

2

H

5

390

10200

p-C

6

H

4

n

C

4

H

9

p-C

6

H

4

n

C

4

H

9

395

26600

p-C

6

H

4

s

C

5

H

11

p-C

6

H

4

s

C

5

H

11

390

16200

p-C

6

H

4

t

C

4

H

9

p-C

6

H

4

t

C

4

H

9

376

3400

p-C

6

H

4

n

C

6

H

13

p-C

6

H

4

n

C

6

H

13

397

23300

m-C

6

H

4

n

C

6

H

13

m-C

6

H

4

n

C

6

H

13

400

21300

p-C

6

H

4

n

C

6

H

13

p-C

6

H

4

n

C

6

H

13

394

18600

a

In solution.

b

Refs. 28 and 31.

are photosensitive and have attracted considerable attention as photoresist ma-
terials in microlithography (1,27,28).

Synthesis.

The first report of a soluble polysilane appeared in 1978 and

the material was prepared by the treatment of a mixture of organodichlorosilanes
with sodium metal (33). Instead of only the expected cyclic oligomers, a poly-
meric product, termed polysilastyrene (10), was formed. Poly(dimethylsilane) had
been previously prepared as a highly crystalline insoluble material (1,27,28). The
introduction of phenyl groups in the random copolymer reduces the crystallinity
and allows the material to be soluble and processible.

Vol. 3

INORGANIC POLYMERS

37

The

main

method

used

to

synthesize

polysilanes

involves

the

thermally-induced Wurtz coupling reaction of organodichlorosilanes with
alkali metals (eq. 8). Although improvements in this process have been reported
(eg, the use of ultrasound), the harsh conditions for this reaction tend to limit the
side groups that can be successfully introduced to nonfunctionalized alkyl and
aryl units and makes scale-up unattractive (1).

(8)

Because of these limitations, considerable effort has been focused on the

development of new synthetic routes to polysilanes. Transition-metal-catalyzed
dehydrogenative coupling, discovered in 1985 (eq. 9) (34), is potentially very at-
tractive; however, the molecular weights of the polysilanes formed to date are
generally quite low (M

n

< 8,000). The catalysts used for these coupling reactions

are usually titanocene or zirconocene derivatives (34,35).

(9)

The catalytic dehydrogenation route yields novel polysilanes with Si H func-

tionalities, which are of interest as ceramic precursors (36). In addition, it has been
shown that a variety of new side groups can be introduced using a derivatization
approach (eq. 10) (37).

(10)

In 1991, a novel ROP route to polysilanes was reported (eq. (11)) (38). The key

to this approach is to take readily accessible octaphenylcyclotetrasilane, which
is too sterically crowded to undergo ROP, and to replace some of the phenyl
groups by smaller methyl substituents (via a two-step process) to make the ring
polymerizable.

(11)

38

INORGANIC POLYMERS

Vol. 3

Another route to polysilanes that involves the anionic polymerization of dis-

ilabicyclooctadienes, which function as sources of masked disilenes (eq. 12), has
been described (39,40). Amphiphilic block copolymers formed by this anionic route,
such as poly(1,1-dimethyl-2,2-dihexyldisilene)-b-(2-hydroxyethyl methacrylate),
undergo self-assembly to form micelles (41).

(12)

Uses.

The delocalization of

σ electrons in polysilanes gives rise to unique

electronic and optical properties. Also, several polysilanes have been found to
function as useful thermal precursors to silicon carbide fibers and these materi-
als have attracted attention with respect to microlithographic applications and as
polymerization initiators (1,27,28). The use of these materials as hole transport
layers in electroluminescent devices has also been explored (42). Indeed, the pho-
toconductivity of poly(methylphenylsilane) doped with C

60

has been studied and

has been found to be comparable with the best materials available (43).

Polygermanes and Polystannanes

Properties.

The remarkable properties of polysilanes has led to significant

interest in the development of polymer chains based on the heavier Group 14
elements, germanium and tin.

Studies of polygermanes indicate that the

σ delocalization is even more ex-

tensive than for polysilanes and that the

σ σ ∗ band-gap transition for the high

polymers is significantly red-shifted by ca 20 nm in comparison to the silicon ana-
logues (44,45) (Table 3). Other studies have shown that these materials possess
semiconductive behavior upon oxidative doping (49) as well as significant nonlin-
ear optical behavior (50) and thermochromicity (45).

High molecular weight polystannanes possess

σ electrons that are exten-

sively delocalized as illustrated by the band-gap transition, which occurs at
384–388 nm for poly(dialkylstannanes) (in THF) and at even higher wavelengths
for some poly(diarylstannanes) (Table 3) (48,51). In addition, exposure of thin films
of the polymers to the oxidant AsF

5

leads to significant electronic conductivities of

ca 0.01–0.3 S/cm (52). Polystannanes are highly photosensitive and exhibit photo-
bleaching behavior, and on uv-irradiation depolymerize to yield cyclic oligomers.
The materials are thermally stable to 200–270

◦

C in air and, at more elevated

temperatures, function as interesting precursors to SnO

2

(52).

Synthesis.

Polygermanes (11) were prepared in the mid-1980s by Wurtz

coupling techniques similar to those used to prepare the silicon analogues (eq. 13)
(1). A variety of alkyl and aryl derivatives can be prepared by this method but the
harsh reaction conditions are not tolerant of many functional groups (53).

Vol. 3

INORGANIC POLYMERS

39

Table 3. Ultraviolet–visible Absorption Data for Some Typical Polygermanes
and Polystannanes [ER

1

R

2

]

n

a

E

R

1

R

2

λ

max

, nm

Ge

CH

3

C

6

H

5

332

b

/327

c

Ge

CH

3

p-C

6

H

4

F

336

b

Ge

CH

3

p-C

6

H

4

CF

3

332

b

Ge

CH

3

p-C

6

H

4

CH

3

326

b

Ge

CH

3

m-C

6

H

4

(CH

3

)

2

330

b

Ge

CH

3

p-C

6

H

4

OCH

3

338

b

Ge

C

2

H

5

C

2

H

5

293

c

Ge

n

C

3

H

7

n

C

3

H

7

312

c

Ge

n

C

4

H

9

n

C

4

H

9

325

c

Ge

n

C

5

H

11

n

C

5

H

11

327

c

Ge

n

C

6

H

13

n

C

6

H

13

325

c

Ge

C

6

H

5

n

C

4

H

9

337

c

Sn

p-C

6

H

4

t

C

4

H

9

p-C

6

H

4

t

C

4

H

9

432

d

Sn

p-C

6

H

4

n

C

6

H

13

p-C

6

H

4

n

C

6

H

13

436

d

Sn

o-C

6

H

4

C

2

H

5

o-C

6

H

4

C

2

H

5

468

d

,

e

Sn

p-C

6

H

4

O

n

C

4

H

9

p-C

6

H

4

O

n

C

4

H

9

448

d

Sn

o-C

2

H

5

-p-O

n

C

4

H

9

C

6

H

4

o-C

2

H

5

-p-

n

C

4

H

9

-C

6

H

4

506

d

Sn

p-C

6

H

4

N(Si(CH

3

)

3

)

2

p-C

6

H

4

N(Si(CH

3

)

3

)

2

450

d

a

All values were measured in THF unless otherwise noted.

b

Synthesized via demethanative coupling (46).

c

Synthesized via electrochemical polymerization (47).

d

Synthesized via catalytic polymerization (48).

e

Value obtained on a thin film of the polymer.

(13)

Dehydrocoupling has been investigated, but has proven relatively unsuccess-

ful (54). Electrochemical reduction of halogermanes has proven somewhat success-
ful and has provided a route to poly(germane–germane) and poly(germane–silane)
copolymers (47,55). The ruthenium catalyst, Ru(P(CH

3

)

3

)

4

(CH

3

)

2

, can be em-

ployed in the demethanative coupling of trimethylgermane, which gives relatively
high molecular weight polygermanes under mild conditions (25

◦

C) (eq. 14) (46,56).

(14)

Early attempts to generate polystannanes by Wurtz coupling of organ-

odichlorostannanes have yielded only low molecular weight oligomers and
reduction products. The first high molecular weight materials were made using

40

INORGANIC POLYMERS

Vol. 3

transition-metal-catalyzed dehydrogenative coupling of secondary stannanes
R

2

SnH

2

(eq. 15) (51). Yellow polystannanes (12) (R

= n-butyl, n-hexyl, or n-octyl)

of substantial molecular weight (up to M

w

= ca 96,000, M

n

= ca 22,000)

were prepared using various zirconium catalysts. This method is applicable
to the preparation of poly(diarylstannanes) possessing solubilizing substituents
(48,57). Interestingly, a change in catalyst from zirconocene-based systems to
HRh(CO)(P(C

6

H

5

)

3

)

3

has been shown to lead to highly branched polystannane

structures (58).

(15)

Uses.

Both polygermanes and polystannanes may find applications be-

cause of their unique optical, electronic, and chemical properties. Some of these
potential uses include photoresist layers (44,59,60), third-order nonlinear optical
materials (50), charge transport polymers (61,62), photoconductors, microlitho-
graphic materials (63), and photoinitiators (59).

Boron-Containing Polymers

Boron-containing polymers are of considerable intrinsic interest, as possible reac-
tive intermediates and as precursors to boron-based ceramics (64–69). The synthe-
sis of polyborazines (13) (M

w

up to ca 7600, M

n

up to ca 3400) via thermally induced

dehydropolymerization of borazines (eq. 16) has been reported (68). The polymers
were isolated as white solids and characterization suggested the presence of a
significantly branched structure. Pyrolysis at 1200

◦

C yielded white turbostratic

boron nitride in 85–93% yield.

(16)

A wide range of novel polymers with boron in the backbone have been pre-

pared by means of boration polymerizations (65,70–82). Diynes can be polymer-
ized by hydroboration (70,71), phenylboration (65), and haloboration (72) to yield
polymers (14),(15), and (16) (eq. 17). When an appropriate aromatic or heteroaro-
matic diyne is used, the resulting polymers have been shown to have extended
π-conjugation through the vacant p-orbital of the boron atom (73,74). In fact, sev-
eral have been shown to exhibit blue fluorescence emission.

Vol. 3

INORGANIC POLYMERS

41

(17)

Diisocyanates have been shown to undergo haloboration–phenylboration

polymerization to give halosubstituted polymers (17) (75). They also undergo
alkoxyboration in the presence of mesityldimethoxyborane to produce poly(boronic
carbamates) (18) (eq. 18) (76).

(18)

Dicyano

compounds

undergo

hydroboration

to

produce

poly(cyclodiborazanes) (19), which have proven relatively stable towards air and
thermal oxidation (eq. 19) (77–79). Some examples have been prepared where
the dicyano compound allows for incorporation of a charge transferred structure
(80). These exhibit extended

π-conjugation through the cycloborazane unit.

(19)

42

INORGANIC POLYMERS

Vol. 3

Finally, the polycondensation reaction between bifunctional Grignard

reagents and aryldimethoxyboranes, carried out under a nitrogen atmosphere,
in THF, gives rise to poly(phenylene-boranes) (20) (eq. 20) (83). The conjugated
polymers may have potential applications in electronic devices such as LEDs.

(20)

Transition-metal-catalyzed dehydrocoupling of phosphine–borane adducts

(21) has been shown to give rise to high molecular weight polyphosphinoboranes
(22) (84). In the 1950s and 1960s, several claims of the synthesis of polyphos-
phinoboranes (22) were made (eq. 21). The main route studied was the thermal
dehydrocoupling of R

2

PH

·BH

3

adducts at 200

◦

C and above; however, structural

characterization of the polymers was minimal and reported yields and molecular
weights were very low (85,86).

(21)

The dehydrocoupling of various adducts (21) has now been studied in

the presence of various catalysts, such as RhCl

3

, [Rh(

µ Cl)(1,5-cod)]

2

, and

[Rh(1,5-cod)

2

]

+

(87). The bulky adduct (21) (R

1

= R

2

= C

6

H

5

) was shown to un-

dergo dehydrogenative coupling to form only a linear dimer or a mixture of the
cyclic trimer and tetramer, depending upon the temperature used. However, pri-
mary phosphine–borane adducts, such as C

6

H

5

PH

2

·BH

3

and iC

4

H

9

PH

2

·BH

3

, were

found to undergo catalytic dehydrogenative polymerization under similar condi-
tions to yield soluble polyphosphinoboranes (22) (eq. 22). When the polymerization
is carried out in solution, the resulting polymers are low in molecular weight (eg,
M

w

≈ 5600 for R = C

6

H

5

) whereas the neat polymerization affords high molecular

weight phosphorus–boron polymers (eg, M

w

≈ 31,000 for (22) R = C

6

H

5

). These

polymers are air and moisture stable in the solid state, and detailed studies of
the physical properties have yet to be conducted but the novel phosphorus–boron
backbone allows for interesting possibilities such as low temperature flexibility,
flame retardancy, and ceramic formation.

Vol. 3

INORGANIC POLYMERS

43

(22)

Polycarbophosphazenes

Polycarbophosphazenes possess a backbone of phosphorus, nitrogen, and carbon
atoms and can be regarded as derivatives of “classical polyphosphazenes” (1) in
which every third phosphorus atom is replaced by carbon. The first examples of
these materials were discovered in 1989 (88). Thermal ROP of a cyclic carbophos-
phazene was used to prepare the chlorinated polymeric species (23), which un-
dergoes halogen replacement reactions with nucleophiles such as aryloxides and
aniline to yield hydrolytically stable poly(aryloxycarbophosphazenes) (24) (M

w

=

ca 10

5

, M

n

= 10

4

) (eq. 23) (88–91). The polymer backbone in these materials was

found to be less flexible than in classical polyphosphazenes. For example, the halo-
genated polymer (23) possesses a T

g

of

−21

◦

C compared to a value of

−66

◦

C for

poly(dichlorophosphazene) (2).

(23)

The reaction of (23) with alkylamines has also been studied (91). The result-

ing poly(alkylaminocarbophosphazenes) are sensitive to hydrolysis. However, ary-
lamino derivatives are moisture stable and, in addition, a novel, regioselectively
substituted polymer (25) was successfully prepared via the sequential reaction
with NH(C

6

H

5

)

2

and trifluoroethoxide anions (eq. 24).

(24)

44

INORGANIC POLYMERS

Vol. 3

Sulfur–Nitrogen–Phosphorus Polymers

Sulfur–nitrogen–phosphorus polymers possess backbones that can be regarded
as compositional hybrids of those present in sulfur–nitrogen polymers, such as
the solid-state polymer poly(sulfur nitride) [SN]

x

or polyoxothiazenes [RS(O) N]

n

and classical polyphosphazenes, [R

2

P N]

n

(1) (92). Poly(sulfur nitride), [SN]

x

, pos-

sesses remarkable properties such as electrical conductivity at room temperature
and superconductivity below 0.3 K (93). [SN]

x

is insoluble and has a polymeric

structure in the solid state with interchain S

···S interactions. As these interac-

tions are crucial to the properties of the material, [SN]

x

is best regarded as a

solid-state polymer rather than a polymeric material with discrete macromolecu-
lar chains of the type discussed in this article.

The first well-characterized examples of sulfur–nitrogen–phosphorus ma-

terials, polythiophosphazenes, were reported in 1990 (94). These polymers
were prepared via the thermal ROP of a cyclothiophosphazene (eq. 25). This
yielded the hydrolytically sensitive polythiophosphazene (26) with a backbone of
three-coordinate sulfur(IV), nitrogen, and phosphorus atoms. Although reaction of
(26) with nucleophiles such as aryloxides yielded materials (27) with improved hy-
drolytic stability, degradation was still rapid except where very bulky substituents
such as o-phenylphenoxy were present.

(25)

Although the backbone of polythiophosphazenes appears to be quite fragile,

a particularly interesting feature of the substitution reactions of (26) is that the
S Cl bond is much more reactive than the P Cl bonds. Regioselective substitu-
tion at the sulfur center is possible and yields macromolecules (29) with different
aryloxy substituents at sulfur and phosphorus (eq. 26) (94,95).

(26)

In 1991, another class of sulfur–nitrogen–phosphorus polymers, poly-

thionylphosphazenes, were reported (96,97). These materials, which possess
four-coordinate sulfur(VI) atoms in the backbone, possess improved stability and
were prepared by a thermal ROP of cyclic thionylphosphazenes (30), with ei-
ther chlorine or fluorine at the sulfur(VI) center, at 165–180

◦

C (eq. 27) (98). An

Vol. 3

INORGANIC POLYMERS

45

ambient temperature synthesis involving initiation by Lewis Acids such as GaCl

3

has been subsequently developed (99).

(27)

The halogenated polythionylphosphazenes (31) that are formed in these ROP

reactions (together with small quantities of macrocyclic byproducts) are quite sen-
sitive to hydrolysis but a variety of moisture-stable derivatives have been prepared
by reaction of this species with aryloxides or amines (100–102). Mixed substituent
aryloxy–alkoxy polymers have also been prepared (92). Interestingly, with arylox-
ides, regioselective substitution at phosphorus is observed and in the resulting
polymers (32) the sulfur(VI)-halogen bond remains intact. Remarkably, this re-
gioselectivity is the exact reverse of that detected for the polythiophosphazenes
described earlier where the sulfur(IV)-halogen bond is more reactive. In contrast
to the reactions with aryloxides, reaction with primary or secondary amine nucle-
ophiles leads to substitution at both the phosphorus and the sulfur(VI) centers to
give poly(aminothionylphosphazenes) (33) (102,103).

Ab initio calculations on isotactic polythionylphosphazene (31) (X

= Cl or F)

indicate a localized electronic structure for the polymer backbone and predict
that a cis–trans helical conformation is the most energetically favorable for
isolated macromolecules (104). This is in contrast to the analogous “classical”
polyphosphazene (1) where a trans-planar conformation is preferred. Studies of
the properties of the polythionylphosphazenes also reveal significant differences
in thermal transition behavior and polymer morphology compared to classical
polyphosphazenes. For example, the polymer [NSOF

{NP(OC

6

H

5

)

2

}

2

]

n

is an amor-

phous elastomer (T

g

= −15

◦

C), whereas the analogous classical polyphosphazene

[NP(OC

6

H

5

)

2

]

n

is a microcrystalline thermoplastic (T

m

= 390

◦

C, T

g

= −6

◦

C). The

T

g

s of the fluorinated polythionylphosphazenes are lower than those of the ana-

logues with chlorine at sulfur. For example, for (31) (X

= F) T

g

= −56

◦

C whereas

for (31) (X

= Cl) T

g

= −46

◦

C (101). Poly(aminothionylphosphazenes) possess high

gas permeability and have found utility as matrices for phosphorescent dyes for
oxygen sensing applications (105).

Also noteworthy is an interesting condensation route reported which leads

to polymers with backbones of alternating S(O) N and P N units (106).

46

INORGANIC POLYMERS

Vol. 3

Preliminary reports of the first polythiazylphosphazenes (34), which possess

three-coordinate sulfur(III) atoms, have appeared (eq. 28) (107). These materials
would represent true hybrids of poly(sulfur nitride) and polyphosphazenes and
further developments in this area should prove to be particularly interesting.

(28)

Polyoxothiazenes

Partially characterized polyoxothiazenes were briefly reported in the early 1960s
(108). However, in 1992 the first well-characterized examples (35) with alkyl or
aryl substituents at sulfur were described (109). These polymers, which possessed
estimated molecular weights of M

w

= ca 10

5

and M

n

= ca 10

4

, were synthe-

sized via the condensation polymerization of N-silylsulfonimidates at 120–170

◦

C

over 2–8 days (eq. 29). These reactions are catalyzed by added Lewis acids (eg,
BF

3

·O(C

2

H

5

)

2

) and bases (eg, fluoride). By using a mixture of different sulfonim-

idates, random copolymers such as [CH

3

S(O) N]

m

[C

6

H

5

S(O) N]

n

were also suc-

cessfully prepared. Free sulfonimidates were also found to thermally condense
to yield poly(organooxothiazenes) at lower temperatures than their N-silyl ana-
logues (eq. 29) (109,110).

(29)

Polyoxothiazenes appear to be highly polar. For example, [CH

3

S(O) N]

n

is soluble in DMF, DMSO, hot water, and concentrated H

2

SO

4

. Studies of

the thermal transition behavior of these materials have indicated that they
are amorphous, which is consistent with an atactic structure. Interestingly,
the T

g

of [CH

3

S(O) N]

n

is ca. 60

◦

C, which is dramatically higher than for

[(CH

3

)

2

P N]

n

(T

g

= −46

◦

C). This suggests a much less flexible backbone for

polyoxothiazenes compared to polyphosphazenes, as might be expected from

Vol. 3

INORGANIC POLYMERS

47

studies on polythionylphosphazenes (vide supra). Thermogravimetric analysis
(TGA) showed that the polymers are stable to weight loss up to ca 270

◦

C

(at a heating rate of 10

◦

C/min). Theoretical studies on [CH

3

S(O) N]

n

have

indicated that a cis–trans helical conformation is the most stable for this polymer
(110).

Inorganic Polymers Based on Transition Elements

Ferrocene-Based Polymers.

The excellent thermal stability and inter-

esting physical (eg, redox) properties associated with the ferrocene moiety have
led to extensive efforts aimed at the incorporation of this unit into polymer struc-
tures. The inclusion of this moiety in the side-group structure of polymers has
been very successful and requires only minor modifications of previously estab-
lished synthetic methodologies. For example, poly(vinylferrocene) (36) can be pre-
pared via the free radical addition polymerization of vinylferrocene (111). The
incorporation of ferrocene moieties into the main chains of polymers where the
metal atoms are separated by a considerable distance has also been achieved.
The extensive organic chemistry of the metallocene nucleus has allowed for the
preparation of well-defined difunctional ferrocenes that have been used in con-
trolled polycondensation reactions to yield well-defined products of appreciable
molecular weight. Examples of products derived from such reactions involve the
poly(arylene–siloxane–ferrocenes) (eg, 37) (112), ferrocene-containing polyesters
(113), and novel “accordian” type polymers (114,115).

The versatile chemistry of the ferrocenyl moiety has also allowed for the

preparation of a large number of dendrimeric structures with this group at the
periphery (116–118). A series of dendrimers based upon polypropylenimine cores
have been reported (119,120) and star-shaped macromolecules with ferrocene
units at the periphery have been produced (121), as well as amido-ferrocene den-
drimers (eg, 38) (122). A convergent approach has allowed for the synthesis of
dendrimers (39) for which the ferrocene units at the periphery display electronic
interaction (123).

48

INORGANIC POLYMERS

Vol. 3

Vol. 3

INORGANIC POLYMERS

49

In contrast, very few examples of polymers with ferrocene units in close prox-

imity, which could take advantage of interactions between the metal atoms, have
been prepared. The development of materials with interesting physical (eg, elec-
tronic and/or magnetic) properties might be anticipated based on the properties
of molecular species in which two ferrocene units are linked close together. Thus,
in such systems the iron atoms can interact and in some cases yield delocalized,
mixed valent species upon one-electron oxidation, even when the metal atoms are
up to 0.7 nm apart (124,125). Work in this field has largely yielded low molecular
weight (M

n

 10,000) and often poorly defined materials (126). For the synthesis

of well-characterized polymers (M

n

< ca 11,000) with main chains of ferrocene

groups and vinylene, divinylene, or oligovinylene units see Reference 127.

Polyferrocenylenes

A typical early route to polyferrocenylenes (40) with M

n

< 5000 involved poly-

condensation processes such as the recombination of ferrocene radicals gen-
erated via the thermolysis of ferrocene in the presence of peroxides. How-
ever, these materials have been found to possess other fragments such as
CH

2

and O in the main chain (128,129). More structurally well-defined poly-

ferrocenylenes (40) (M

n

< 4000) have been prepared (130) via the condensa-

tion reaction of 1,1-dilithioferrocene

·TMEDA (tetramethylethylenediamine) with

1,1-diiodoferrocene and, significantly, the reaction of 1,1



-dihaloferrocenes with

magnesium (eq. 30) has been shown to afford low molecular weight (M

n

= 4600

for soluble fractions) materials with appreciable crystallinity (131). In the latter
case, oxidation with 7,7,8,8-tetracyanoquinodimethane (TCNQ) afforded doped
polymers that were delocalized on the M¨ossbauer time scale (ca 10

− 7

s) at room

temperature and which possessed electrical conductivities of up to 10

− 2

S/cm.

(30)

Polyferrocenylsilanes

Properties.

Polyferrocenylsilanes possess a backbone of alternating fer-

rocene and organosilane units. Since the early 1990s when the first high molec-
ular weight, well-characterized examples were prepared by a ROP approach,
considerable effort has been directed towards understanding the properties of
polyferrocenylsilane materials, the vast majority of which are soluble in com-
mon organic solvents (132,133). It was noted early on that electrochemistry of
the high polymers such as (41) (R

= R



= CH

3

) possess two reversible oxidation

waves in a 1:1 ratio (132,134). This provided clear evidence for the existence of
interactions between the iron atoms, and led to the proposal that initial oxidation

50

INORGANIC POLYMERS

Vol. 3

occurred at alternating iron sites along the main chain. Work on model oligomers
with between two and nine ferrocene units has provided clear evidence in sup-
port of this postulate (135). Similar electrochemical behavior has subsequently
been detected for a range of other polyferrocenylsilanes (134). Oxidative doping
of poly(ferrocenyldimethylsilane) with I

2

has been shown to yield semiconduct-

ing materials (

σ = ca 10

− 4

S/cm) whereas the pristine materials are insulating

(

σ = ca 10

− 14

S/cm) (133). A report indicates that several tetracyanoethylene

(TCNE)-oxidized low molecular weight polyferrocenylsilanes (M

w

= ca 1500) show

electron delocalization on the M¨ossbauer time scale (ca 10

− 7

s) and also ferro-

magnetic ordering at low temperatures (136). Studies of high molecular weight
M

w

> 10

5

analogues have not reproduced this behavior (137). Thin films of the

homopolymers and block copolymers with organic or inorganic co-blocks are at-
tracting attention for numerous applications, such as chemomechanical sensors,
electrochromic materials, electrode mediators, variable refractive index materi-
als, hole transport layers, charge dissipation coatings for dielectrics, lithographic
resists, and photonic band-gap materials (133,138–144).

The polymers also exhibit interesting morphology and several of the symmet-

rically substituted derivatives will crystallize. For example, the dimethyl deriva-
tive (41) (R

= R



= CH

3

) is an amber, film-forming thermoplastic and possesses

a T

m

at 120–145

◦

C depending on crystallite size and a T

g

at 33

◦

C whereas, in

contrast, the n-hexyl analogue (41) (R

= R



= n-hexyl) is an amber, gummy amor-

phous material with a T

g

of

−26

◦

C (133). The packing in the crystalline regions of

polymer (41) (R

= R



= CH

3

) has been shown to be analogous to that of a linear pen-

tamer (135,141,142). In addition, as the iron atom in ferrocene acts as a “molecular
ball-bearing,” this gives these polymers a large degree of conformational flexibility
and consequently T

g

s are lower than might be expected for polymers with such a

bulky unit in the main chain (133) (Table 4).

Several polyferrocenylsilanes can be fabricated in the melt (eg, R

= R



=

CH

3

above 150

◦

C) (Fig. 1). Polyferrocenylsilanes have been found to exhibit ex-

cellent thermal stability to weight loss (up to 350–400

◦

C) and have been shown

to yield interesting composites containing Fe nanoparticles at 500–1000

◦

C (145–

147). Controlled cross-linking of the polyferrocenylsilanes can be used to make
magnetic ceramic films and monoliths with the same shape as the polymer pre-
cursor as well as solvent-swellable, redox-active gels (148,149). The solution prop-
erties of polyferrocenylsilanes such as the dimethyl derivative (41) (R

= R



= CH

3

)

have been well-characterized by light-scattering experiments and viscometry and
Mark–Houwink parameters have been established (150).

Vol. 3

INORGANIC POLYMERS

51

Table 4. Thermal Transition and gpc Molecular Weight Data for Selected
Polyferrocenylsilanes

a

R/OR

R



/OR



T

g

(T

m

)

b

,

◦

C

M

n

c

PDI

d

H

H

16 (165)

–

e

–

e

CH

3

CH

3

33 (122–143)

3.4

× 10

5

1.5

C

2

H

5

C

2

H

5

22 (108)

4.8

× 10

5

1.6

n

C

3

H

7

n

C

3

H

7

24 (98)

8.5

× 10

4

2.7

n

C

4

H

9

n

C

4

H

9

3 (116,129)

3.4

× 10

5

2.6

n

C

5

H

11

n

C

5

H

11

−11 (80–105)

3.0

× 10

5

1.6

n

C

6

H

13

n

C

6

H

13

−26

7.6

× 10

4

1.5

CH

3

H

9 (87, 102)

4.2

× 10

5

2.0

CH

3

CH

2

CH

2

CF

3

59

8.1

× 10

5

3.3

CH

3

CH CH

2

28

7.7

× 10

4

2.1

CH

3

n

C

18

H

37

1 (16)

5.6

× 10

5

2.5

CH

3

C

6

H

5

90

1.5

× 10

5

2.0

CH

3

Fc

f

99

7.1

× 10

4

2.3

CH

3

CH

2

CH

2

CH

2

Cl

27

2.7

× 10

5

1.5

OCH

3

OCH

3

19

1.5

× 10

5

1.9

OC

2

H

5

OC

2

H

5

0

3.8

× 10

5

2.1

OCH

2

CF

3

OCH

2

CF

3

16

2.2

× 10

5

1.2

O

n

C

4

H

9

O

n

C

4

H

9

−43

3.9

× 10

5

2.1

O

n

C

6

H

13

O

n

C

6

H

13

−51

0.9

× 10

5

2.4

O

n

C

18

H

37

O

n

C

18

H

37

(32)

2.3

× 10

5

2.2

OC

6

H

5

OC

6

H

5

54

2.3

× 10

5

2.0

(OCH

2

CH

2

)OCH

3

(OCH

2

CH

2

)OCH

3

−53

1.0

× 10

5

2.6

CH

3

(OCH

2

CH

2

)

x

OCH

3

g

−69, −72

h

5.6

× 10

4

, 1.9

× 10

5

2.3, 2.2

a

Refs. 133 and 143.

b

DSC data collected at a heating rate of 10

◦

C min

− 1

.

c

GPC data and molecular weight values are relative to polystyrene standards. Although in this

case gpc provides only molecular weight estimates, absolute determinations of M

w

by static light

scattering for several polymers have indicated that gpc underestimates the real values by a factor
of 2 (144).

d

PDI

= M

w

/M

n

.

e

Insoluble polymer.

f

Fc

= (η-C

5

H

4

)Fe(

η-C

5

H

5

).

g

x

∼ 8.

h

Two different molecular weight samples.

Water-soluble polyferrocenylsilanes have also been prepared and these pos-

sess, for example, oligoethoxy or ionic side chains (151,152). These materials can be
used in layer-by-layer assembly processes to form superlattices with a range of po-
tential applications (153). Novel random copolymers (42) with oligosilane spacers
have also been prepared by using a thermal copolymerization process (154,155).
These polymers possess interesting photophysical and charge transport proper-
ties. Indeed, the skeletons of the polysilane segments can be selectively cleaved
using uv light because of the photosensitive nature of the Si-Si bond. Block copoly-
mers containing polyferrocenylsilane blocks (vide infra) have demonstrated inter-
esting self-assembly behavior and are of interest for nanostructure applications
(156). For example, cylindrical worm-like micelles with a polyferrocenylsilane core

52

INORGANIC POLYMERS

Vol. 3

Fig. 1.

Samples of melt processed polyferrocenylsilanes.

and a polysiloxane corona can be fabricated and are of potential use as semicon-
ducting nanowires and as etching resists in nanolithography (157).

Synthesis.

Ring-Opening Polymerization.

Early attempts to prepare macromolecules

in which the ferrocene units are separated via an organosilane spacer group fo-
cused on the use of polycondensation reactions. Partially characterized, impure
polyferrocenylsilanes were prepared via the reaction of dilithioferrocene with
organodichlorosilanes. The molecular weights of 1400–7000 reported for these
materials are characteristic of polycondensation processes where exact reaction
stoichiometries are virtually impossible to achieve because one reactant, in this
case dilithioferrocene, cannot be readily prepared in pure form (116).

In 1992, the first synthesis of high molecular weight polyferrocenylsilanes

(41) (M

w

= 10

5

–10

6

, M

n

> 10

5

) via a thermal ROP route was reported (132,158).

This process involved heating silicon-bridged [1]ferrocenophanes (43) in the melt
at 130–220

◦

C (eq. 31). The presence of a single-atom bridging the ferrocene unit

in the monomer leads to a strained structure in which the planes of the cyclopen-
tadienyl rings are tilted with respect to one another by an angle of ca 21

◦

. In

contrast, in ferrocene the cyclopentadienyl rings are parallel. The presence of
strain in the ferrocenophane, which has been measured to be ca 80 kJ/mol for
(43) (R

= R



= CH

3

), is believed to provide the driving force for the ROP process

(132,158).

Vol. 3

INORGANIC POLYMERS

53

(31)

Since this initial discovery, a wide range of silicon-bridged [1]ferroceno-

phanes with either symmetrically or unsymmetrically substituted silicon atoms
have been prepared and similarly polymerized (Table 4). Polymerization of a mix-
ture of different silicon-bridged [1]ferrocenophanes has also been shown to yield
random copolymers (159).

Silicon-bridged [1]ferrocenophanes undergo living anionic ROP using ini-

tiators such as n-C

4

H

9

Li in THF (160,161). This has permitted the synthesis of

polyferrocenylsilanes with controlled molecular weights and narrow polydispersi-
ties and has also allowed the preparation of the first block copolymers containing
skeletal transition-metal atoms (160,161). Block copolymers such as (44) have
been prepared with other monomers that undergo anionic polymerization such as
cyclic siloxanes (see eq. 32) or organic monomers such as polystyrene and isoprene
(161,162). The resulting block copolymers undergo phase separation in the solid
state and, in solution, micellar aggregates are formed (156,163).

(32)

Also, the transition-metal-catalyzed ROP of silicon-bridged [1]ferroceno-

phanes in the presence of various transition-metal complexes (eg, Pt

II

, Pt

0

, Rh

I

,

Pd

II

) has been developed (164). This route, which takes place in solution at room

temperature, is much milder than, and doesn’t have the same stringent monomer
purity requirements as, anionic ROP. Furthermore, molecular weight control is
possible through the use of Si H containing capping agents such as (C

2

H

5

)

3

SiH,

and access to block and graft copolymers and star polymers is possible (164,165).

Monomer Synthesis.

Sila[1]ferrocenophane monomers such as (43) are

readily available on a substantial laboratory scale (

>100 g) from the reaction

of dilithioferrocene tetramethylethylenediamine (fcLi

2

·TMEDA) with the appro-

priate dichloroorganosilane (166). Spirocyclic sila[1]ferrocenophanes such as (45)
and (46) are also easily synthesized (167). Sila[1]ferrocenophane monomers with
alkoxy, aryloxy, and amino substituents at silicon are readily accessible through
reaction of dichlorosila[1]ferrocenophane with the appropriate alcohol, phenol or
amine in the presence of base (168).

54

INORGANIC POLYMERS

Vol. 3

Other Polyferrocenes

The ROP route has been extended to the synthesis of other polymers from [1]fer-
rocenophane precursors. Polyferrocenylgermanes (47) were first reported in 1993
and have been well-characterized and possess quite similar thermal transition
behavior, morphology, and electrochemical behavior to the analogous polyferro-
cenylsilanes (169). Poly(ferrocenylsilane-ferrocenylgermane) random copolymers
(48) have also been prepared via the thermal polymerization of mixtures of the
respective monomers (169).

Polyferrocenylphosphines (49) (and the corresponding phosphine sulfides)

are also accessible via the thermal ROP of phosphorus-bridged [1]ferrocenophanes
(170). Polymers of this type have been previously prepared by condensation routes
and the catalytic potential of some of their transition-metal derivatives has al-
ready been noted. In addition, the first sulfur-bridged [1]ferrocenophanes have
been prepared and polymerized to give polyferrocenylsulfides, (eg, 50) (171).

Hydrocarbon-bridged [2]-ferrocenophanes (51) possess strained ring-tilted

structures (tilt-angles

= ca 21

◦

) and these species have been found to yield poly-

ferrocenylethylenes (52) via ROP at 250–300

◦

C (eq. 33) (172). As a consequence

of the presence of a more insulating bridge, these polymers show much smaller
interactions between the iron atoms compared to polyferrocenylsilanes.

Vol. 3

INORGANIC POLYMERS

55

(33)

Analogous hydrocarbon-bridged [2]ruthenocenophanes (tilt angles

= ca

29–30

◦

) undergo thermal ROP to yield poly(ruthenocenylethylenes) (172).

These materials exhibit significantly different electrochemistry from their iron
analogues.

In early 1992, it was reported that [3]trithiaferrocenophanes, which are es-

sentially unstrained, function as precursors to poly(ferrocenylene persulfides) via
a novel atom abstraction polymerization route (eq. 34) (173). Thus, reaction of
[3]-trithiaferrocenophanes (53) with P(C

4

H

9

)

3

led to the formation of the phos-

phine sulfide S P(C

4

H

9

)

3

and the polymers (54).

(34)

The presence of a butyl substituent on the cyclopentadienyl ring is necessary

for the polymer to be soluble. The molecular weight (M

w

) of (54) (R

= n-C

4

H

9

) was

determined to be 40,000 by gel permeation chromatography (gpc). The [3]ferro-
cenophanes (53) (R

= H) and (53) (R = n-C

4

H

9

) can be copolymerized to give

soluble copolymers with M

w

= 25,000.

Poly(ferrocenylene persulfides) possess a range of novel properties (173–

175). They are photosensitive and the S S bonds can be reversibly reductively
cleaved with Li[B(C

2

H

5

)

3

H] and then regenerated upon oxidation with I

2

. Their

electrochemical behavior is similar to that detected for polyferrocenylsilanes ex-
cept that the interaction between the iron sites appears to be even greater. The
atom abstraction route using P(

t

C

4

H

9

)

3

as a desulfurization agent has also been

extended to the preparation of other poly(ferrocenylene persulfides) with t-butyl
substituents and also high molecular weight (M

w

= 50,000–1,000,000) network

polymers by the use of [3]ferrocenophanes with two trisulfido bridges as monomers
(174).

Face-to-Face Metallocene Polymers

The development of rigid-rod metallocene polymers with a multistacked structure
using condensation routes has been reported (176–180). This involved treatment
of the ferrocene monomers (55) with FeCl

2

and Na[N(Si(CH

3

)

3

)

2

)] and this yielded

purple polymers (56) with molecular weights up to M

n

= 18,000, although higher

56

INORGANIC POLYMERS

Vol. 3

molecular components were also present (eq. 35). Mixed metal copolymers contain-
ing Ni and Fe whose soluble fractions were of low molecular weight (M

n

< 3000)

were also reported by using Ni(acac)

2

instead of FeCl

2

(176). The electrical and

magnetic properties of these novel polymers and copolymers are clearly worthy of
investigation. Interestingly, structural work on well-defined oligomers suggests
that the stacked metallocene units in the polymer form a helical structure (180).

(35)

Coordination Polymers

In the past, attempts to prepare coordination polymers have been hindered by the
insolubility and consequent intractability of the products. These problems arise
from the inherent skeletal rigidity of these materials, and the introduction of solu-
bilizing or flexibilizing groups either in the polymer backbone or side-group struc-
ture is necessary for useful products to be obtained. Such modification has yielded
a range of interesting and well-characterized materials with intriguing proper-
ties. For example, novel liquid crystalline polymers (57) containing paramagnetic
Cu

II

centers have been prepared (181) and soluble, luminescent silver-containing

polymers (58) have been reported (182).

In addition, well-characterized lanthanide containing polymers (59), which

possess polyelectrolyte behavior and exhibit interesting photophysical properties,
have been reported (183). The tetradentate Schiff-Base ligands greatly stabi-
lize the lanthanide ions in solution and allow for efficient energy transfer to the

Vol. 3

INORGANIC POLYMERS

57

lanthanide centers. Several of the polymers are soluble in polar organic solvents
such as DMSO, and molecular weights (M

n

) up to 1.8

× 10

4

have been established.

The polymer (59) (Ln

= Eu or Eu/Y) is an interesting candidate for luminescent

and lasing applications (184). Similar polymers containing cerium and zirconium
in the main chain have also been prepared and these possess M

n

values up to

3

× 10

4

(185). Films of these polymers have conductivities of

∼ 10

− 7

S/cm which

increase to

∼10

− 3

S/cm upon I

2

doping (186).

Phthalocyanine-based polymers, such as the “shish-kebab” polymers (60),

are also of considerable interest and significant electrical conductivities of up to
ca 0.1 S/cm have been detected for chemically or electrochemically doped materi-
als (187,188). If flexible organic substituents are present on the periphery of the
phthalocyanine ring, these materials can also be soluble (at least low molecular
weight fractions).

Well-defined and readily soluble ruthenium coordination polymers (61) have

been synthesized through the reaction of a bisbidentate ligand and a metal center
that already possesses one bidentate ligand (189–191). In these complexes, the
random stereospecificity at the ruthenium centers results in a ribbon-like con-
formation of the polymers with the extension of the chain dependent upon the
substituent R. The Ru

II

centers appear to behave independently and the poly-

mers appear to be stable against heat and uv irradiation. They have molecular
weights on the order of M

w

= 40,000–50,000.

58

INORGANIC POLYMERS

Vol. 3

A similar strategy applied to the reaction of a bistridentate monomer with an

appropriate ruthenium compound gives rise to the rod-like polymers (62), which
are soluble due to the n-hexyl substituents (192,193). These polymers display a
pronounced polyelectrolytic effect in salt-free dimethylacetamide.

Polythiophene-metal complex hybrid polymers such as (63) and (64)

have been explored (194,195). The polythiophene-cobalt salen hybrid (63)
participates in the electrocatalytic reduction of oxygen and is highly conduct-
ing in nature. Polymers such as (64) have also been shown to be conduct-
ing. A variety of related structures have been prepared and similar strate-
gies have also resulted in the preparation of polymeric metallorotaxanes
(196).

Vol. 3

INORGANIC POLYMERS

59

The generation of dendrimeric coordination polymers has also been an area of

significant activity. Imaginative routes to novel Ru- or Pt-polypyridyl systems (eg,
65) (197) have been reported and many other ligand systems have been exploited
(117,118,198–201). These materials are of interest with respect to their photo-
physical and electrochemical properties and possibly their catalytic behavior.

60

INORGANIC POLYMERS

Vol. 3

Rigid-Rod Organometallic Polymers

Macromolecules with backbones that possess conjugated C C units and
transition-metal atoms, termed polymetallaynes, represent some of the
best-characterized examples of transition-metal-based polymers prepared to
date (202).

The first polymetallaynes contained nickel, palladium, or platinum atoms

in the main chain and were isolated in 1977 as yellow, film forming materials
(203–205). These were prepared by efficient copper halide-catalyzed coupling pro-
cesses (eg, eq. 36) and possessed estimated molecular weights (M

w

) from 13,000

to 120,000.

(36)

There has been an important expansion in this area which has yielded a

range of new rigid-rod materials via the creative use of a variety of new and
well-defined polycondensation strategies. For example, a new route to the poly-
platinynes (66) that involved the reaction of trans-PtCl

2

(PR

3

)

2

complexes with

bis(trimethylstannyl)diynes (eq. 37) has been reported (206). These rigid-rod poly-
mers possessed estimated weight-average molecular weights up to ca 100,000 ac-
cording to gpc measurements.

(37)

This synthetic procedure can be extended to allow the incorporation of other

transition elements into the polymer main chain such as iron (to give 67) by using
FeCl

2

[(C

2

H

5

)

2

PCH

2

CH

2

P(C

2

H

5

)

2

]

2

as the transition-metal-containing reactant

(207). In addition, condensation routes to organonickel polymers (68), have been
devised (208,209) and interesting organocobalt (210) and organozirconium (211)
polymers containing metallacyclopentadiene moieties in the main chain have been
reported.

Vol. 3

INORGANIC POLYMERS

61

In another interesting synthetic development, it was shown that a range

of rhodium-containing polymetallynes (69) are accessible via the reaction of the
unsubstituted diynes with Rh(PR

3

)

4

CH

3

in a reaction that involves reductive

elimination of methane and the loss of a phosphine ligand (eq. 38) (212). In
the case where trimethylphosphine ligands are attached to rhodium, the poly-
mers are insoluble but the tri(n-butyl)phosphine analogues are soluble and yield
free-standing solvent-cast films from THF.

(38)

Over the past 15 years the physical properties of polymetallaynes have re-

ceived continued attention because of their novel rigid-rod structures and their
conjugated backbones (213–222). Thus, polyplatinynes form ordered, liquid crys-
talline mesophases in solvents in which they are soluble such as trichloroethy-
lene (213), and these materials also possess novel, third-order nonlinear optical
properties (214) that are of interest for electrooptic device applications. Optical
absorption and photoluminescence spectroscopic studies and extended H ¨

uckel cal-

culations have shown that polymetallaynes possess a delocalized polymer back-
bone whose electronic structure is modified by the nature of the transition metal,
coligands, and the unsaturated hydrocarbon spacer (215–217). For example, op-
tical band gaps for a series of polyplatinynes with platinum centers joined by
σ-conjugated acetylide-arene linkages of varying length have been measured to
be in the range of 2.5–3.1 eV, which is lower than for model complexes and is
consistent with conjugation through the metal centers (216).

Other developments in the area of rigid-rod transition-metal-based polymers

include the synthesis of a range of thermotropic liquid crystalline organocobalt
polymers (eg, 70) in which the metal is bound to skeletal cyclobutadiene units (eq.
39) (223).

(39)

In addition, the preparation and characterization of novel lyotropic liquid

crystalline aramides (71) with complexed chromium(tricarbonyl) units have been

62

INORGANIC POLYMERS

Vol. 3

reported (224). These materials are soluble in organic solvents, which leads to po-
tential processing advantages for the uncomplexed organic polymer as the Cr(CO)

3

groups can be easily added and removed and poly(p-phenyleneterephthalamide)
is only soluble in concentrated sulfuric acid (224,225). Work on the coordination of
transition-metal fragments to

π-hydrocarbon units can be found in Reference 226.

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