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Inorganic and Organometallic Polymers.
Ronald D. Archer
Copyright
2001 Wiley-VCH, Inc.
ISBNs: 0-471-24187-3 (Hardback); 0-471-22445-6 (Electronic)
CHAPTER 1
INORGANIC POLYMERS AND
CLASSIFICATION SCHEMES
1.1
INTRODUCTION
This is an exciting time to be involved in the field of inorganic polymers.
The advances being made in the core areas of inorganic polymer chemistry are
truly remarkable and outstanding, using any logical definition. Recent synthetic
breakthroughs are very impressive. Just a few years ago, no one envisioned
the synthesis of polyphosphazenes at room temperature or the ready synthesis of
organometallic polymers through ring-opening polymerizations. Both are realities
at the present time. These and other examples of both main group and metal-
containing polymers are discussed in Chapter 2.
Uses for inorganic polymers abound, with advances being made continually.
Polysiloxane and polyphosphazene elastomers, siloxane and metal-containing
coupling agents, inorganic dental polymers, inorganic biomedical polymers,
high temperature lubricants, and preceramic polymers are examples of major
applications for inorganic polymers. Conducting and superconducting inor-
ganic polymers have been investigated as have polymers for solar energy
conversion, nonlinear optics, and paramagnets. These uses are detailed in
Chapter 4. If we were to include inorganic coordination and organometallic
species anchored to organic polymers and zeolites, catalysis would also be a
major use.
Inorganic and Organometallic Polymers,
by Ronald D. Archer
ISBN 0-471-24187-3 Copyright
2001 Wiley-VCH, Inc.
1
2
INORGANIC POLYMERS AND CLASSIFICATION SCHEMES
1.1.1
What Is an Inorganic Polymer?
Inorganic by its name implies nonorganic or nonhydrocarbon, and polymer
implies many
mers,
monomers or repeating units. Organic polymers are char-
acteristically hydrocarbon chains that by their extreme length provide entangled
materials with unique properties. The most obvious definition for an inorganic
polymer is a polymer that has inorganic repeating units in the backbone. The inter-
mediate situation in which the backbone alternates between a metallic element
and organic linkages is an area where differences in opinion occur. We will
include them in our discussions of inorganic polymers, although, as noted below,
such polymers are sometimes separated out as inorganic/organic polymers or
organometallic polymers or are excluded altogether.
Various scientists have provided widely differing definitions of inorganic
polymers. For example, Currell and Frazer (1) define an inorganic polymer as a
macromolecule that does not have a backbone of carbon atoms. In fact, several
other reviews define inorganic polymers as polymers that have no carbon atoms
in the backbone (2–4). Such definitions leave out almost all coordination and
organometallic polymers, even though a sizable number of such polymers have
backbone metal atoms that are essential to the stability of the polymer chains.
Some edited books (3), annual reviews (5), and the present work include
metal-containing polymers in the definition by using titles like inorganic and
organometallic polymers. One text includes these polymers but only gives them
a few percent of the total polymer coverage (6). Research papers sometimes
use the term inorganic/organic polymers, inorganic/organic hybrid polymers,
organometallic polymers, or metal-containing polymers for polymers that have
both metal ions and organic groups in the backbone. MacCallum (7) restricts
inorganic polymers to linear polymers having at least two different elements in
the backbone of the repeat unit. This definition includes the coordination and
organometallic polymers noted above, but it classifies polyesters and polyamides
as inorganic polymers while leaving out polysilanes and elemental sulfur!
Holliday (8) is also very inclusive by including diamond, graphite, silica,
other inorganic glasses, and even concrete. Thus it seems that ceramics and
ionic salts would also fall under his definition. Anderson (9) apparently uses a
similar definition; however, Ray (10) suggests that the term inorganic polymers
should be restricted to species that retain their properties after a physical change
such as melting or dissolution. Although this would retain silica and other oxide
glasses, inorganic salts would definitely be ruled out. Whereas other definitions
could undoubtedly be found, the lack of agreement on the definition of inorganic
polymers allows for either inclusiveness or selectivity.
This book will explore the classifications of polymers that are included in
the more inclusive definitions and will then take a more restrictive point of
view in terms of developing the details of inorganic (including metal-containing
organometallic) polymer synthesis, characterization, and properties. The synthesis
and characterization chapters will emphasize linear polymers that have either
at least one metal or one metalloid element as a regular essential part of the
backbone and others that have mainly noncarbon main group atoms in the
CLASSIFICATIONS BY CONNECTIVITIES
3
backbone. Inorganic species that retain their polymeric nature on dissolution
will be emphasized rather than species that happen to be polymeric in the solid
state by lattice energy considerations alone.
For the main group elements, linear chain polymers containing boron, silicon,
phosphorus, and the elements below them in the periodic table will be emphasized
provided they have sufficient stability to exist on a change of state or dissolution.
For transition and inner transition elements, linear polymers in which the metal
atom is an essential part of the backbone will be emphasized, with the same
restriction noted for the main group elements.
To categorize inorganic polymers further, we must distinguish between
oligomers and polymers on the basis of degrees of polymerization. Too often
in the literature, a new species is claimed to be polymeric when only three or
four repeating units exist per polymer chain on dissolution. For our purposes,
we will use an arbitrary cut-off of at least 10 repeating units as a minimum for
consideration as a polymer. Anything shorter will be classed as an oligomer.
Note:
In step-growth and condensation polymers of the AA
C
BB type, where
the repeating unit is AABB, 10 repeating units, (AABB
10
, corresponds to a
degree of polymerization of 19. That is, 2n 1 reaction steps are necessary
to assemble the 20 reacting segments that make up the polymer. The reader
can verify this relationship with a simple paper-and-pencil exercise. One of the
greatest challenges in transition metal polymer chemistry has been to modify
synthetic procedures such that polymers rather than oligomers are formed before
precipitation (cf. Exercise 1.1).
1.2
CLASSIFICATIONS BY CONNECTIVITIES
N. H. Ray, in his book on inorganic polymers (10), uses
connectivity
as a method
of classifying inorganic polymers. Ray defines connectivity as the number of
atoms attached to a defined atom that are a part of the polymer chain or matrix.
This polymer connectivity can range from 1 for a side group atom or functional
group to at least 8 or 10 in some metal-coordination and metal-cyclopentadienyl
polymers, respectively. Multihapticity is designated with a superscript following
the
Á
for example, the cyclopentadienyl ligand in Figure 1.2b is
Á
5
.
An alternate designation of connectivity of the cyclopentadienyl ring is based
on the number of electron pairs donated to the metal ion. Thus a metal species
with a bis(cyclopentadienyl) bridge has a connectivity of 6 using this alternate
designation. This is more in keeping with its bonding.
Also note that double-ended bridging ligands in linear coordination polymers
are classed as bis(monodentate), bis(bidentate), bis(tridentate), bis(tetradentate),
etc. and provide connectivities of 2, 4, 6, or 8, respectively.
1.2.1
Connectivities of 1
Anchored metal-containing polymers used for catalysis can have connectivity
values as low as 1 with respect to the polymer chain as shown in Figure 1.1.
4
INORGANIC POLYMERS AND CLASSIFICATION SCHEMES
L
M L′′ L
L′
M
L′
L′′ L
M L′′ L
L′
M
L′
L′′
n/4
Figure 1.1
Schematic of anchored metal-containing polymer with a connectivity of 1,
where M might be palladium or platinum with three other ligands. For catalytic activity,
at least one of the three must be easily removed by a substrate.
P
Ni
OC CO
(a)
P
R
H
3
C
Mn
OC CO CO
R
CH
3
Mn
OC CO CO
(b)
CH
3
R
=
C
CO
2
C
8
H
17
Figure 1.2
Higher connectivities for metal-anchored polymers: (a). Schematic repre-
sentation of an anchored polymer that can convert dienes to cyclohexene aldehydes
under the right conditions. (b). Schematic representation of an anchored polymer that can
photolytically transport N
2
across membranes. The analogous manganese cyclopentadienyl
tricarbonyl monomer decomposes under comparable conditions.
TABLE 1.1 Dentate Number (Denticity) Designation of Metal Chelates.
Donor Atoms
on Metal
one
two
three
four
five
six
a
1990
b
1970
Designation in This Text
a
monodentate [Fig. 1.3d]
bidentate
b
[Fig. 1.8b,c]
tridentate [Fig. 1.10a]
tetradentate [Fig. 1.12]
pentadentate
hexadentate
Alternate Designation
unidentate
didentate
terdentate
quadridentate
quinquidentate
sexadentate
IUPAC nomenclature except when noted otherwise [text examples in brackets]
IUPAC nomenclature
CLASSIFICATIONS BY CONNECTIVITIES
5
Note that the metal can have other ligands (groups coordinated to the metal) as
well, but inasmuch as they do not affect the polymer connectivity, the metal is
defined as having a connectivity of 1. Important connectivities of 1 are fairly rare
because the inertness of a single metal connection to a polymer is appreciably less
than cases in which multidentate chelation (2 or more ligating atoms from a single
ligand are coordinated to the same metal atom; cf. Table 1.1) or multihapticity
(2 or more atoms from the same molecule interacting with the same metal atom
in an organometallic species; cf. Fig. 1.2) occurs.
1.2.2
Connectivities of 2
Sulfur and selenium in their chain polymer allotropes undoubtedly possess a
connectivity of 2. They also have a connectivity of 2 in their ring structures,
for example, the crown S
8
structure. Linear polyphosphates, polyphosphazenes,
poly(sulfur nitride), polycarboranes, pyroxenes (single-chain silicates), silicones
O
−
O
P
O
(a)
Cl
N
P
Cl
Cl
P
N
Cl
P
Cl
(c)
B
B
B
B
B
B
B
B
B
B
(e)
B
B
B
B
B
B
B
Cl
Cl
N
P
N
Cl
P
Cl
Cl
N
PR
3
Pt
PR
3
(d)
B
B
B
B
C
C
C
C
P
O
O
−
O
P
O
(b)
PR
3
Pt
PR
3
C
C
C
C
O
−
O
P
O
O
−
O
P
O
O
−
O
S
N
S
N
S
N
S
N
S
N
C
C
C
O
C
C
C
O
C
C
C
O
Figure 1.3
Examples of inorganic polymeric species with connectivity of 2: (a) poly-
(sulfur nitride); (b) linear polyphosphate; (c) poly(dichlorophosphazene); (d) poly[bis-
(R
3
phosphine)-
2
-diacetylenato-C
1
,
C
4
(2-)platinum(II)], where R is a large organic group;
(e) carborane oligomer with
meta-B
10
H
10
C
2
polyhedra linked by CO (although the
hydrogens on the boron atoms and the BH groups in the back of the B
10
H
10
C
2
polyhedra
are not shown). Carborane polymers with –SiR
2
OSiR
2
n
– linkages also exist and have
been shown to have practical applications (cf. Chapter 4).

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