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An Overview of Inorganic
and Organometallic Polymers
Martel Zeldin
*
INTRODUCTION
The chemistry and technology of polymeric materials have had a major impact on all
facets of our life and the economy of the world. Competition among nations for leadership
positions in the development of materials science and technology is economically driven. In fact,
it can be predicted that the counties with the most advanced and sophisticated materials
technology will offer their citizens the highest standards of living and the greatest hope for
national security in the decades to come.
In spite of the importance and widespread use of organic polymers, increasing attention
has been given to macromolecules that contain metals and metalloids, that is, inorganic and
organometallic polymers. The reasons for the growth in these "hybnd" species are linked to
need. For example:
Organic polymers are often susceptible to reaction with oxygen and ozone. The
presence of metals or metalloids reduces that propensity.
Organic polymers thermally decompose to volatile materials that burn; the presence
of inorganics increases thermal stability, reduces inflammability of the volatiles, or
increases nonvolatile residues.
Organic oolvmers degrade when exposed to ultraviolet and higher energy radiation;
norgan~cs are less susceptible to radiative degradation.
Organic polymers are somewhat more soluble or swellable in solvents.
Inorganics generally form stronger bonds and offer greater resistance to free radical
cleavage reactions than carbon compounds.
Longer bond lengths and higher valencies give rise to greater torsional mobility and
thus greater flexibility and other dynamic-mechanical properties.
~ A- ~
· · .
MAJOR CLASSIFICATIONS OF INORGANIC
AND ORGANOMETALLIC POLYMERS
Although many elements (metals, semimetals, and nonmetals) have been incorporated in
one manner or another into an inorganic and/or organometallic polymer unit, certain elements
lend themselves better to products with commercial potential. Thus, for example, silicon-
containing polymers are used extensively for industrial and consumer products, whereas boron-
containing polymers (i.e., as prepolymer for boron nitride (BN) ceramics or as carborane
*Science and Technology, The College of Staten Island-City University of New York.
i51
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Improved Fire- aM Smoke-Resistant Materials
siloxane ultra-high-temperature elastomers) are produced in limited quantity for very special
applications. The following is a brief discussion of some of the more important and interesting
inorganic and organometallic polymers. Although commercialization has been attained only in
a few examples, the potential for design of materials with unusual and significant properties is
unlimited.
Silicon-Based Polymeric Materials
Polysilaxanes
R
ski - -
R
H. hydrocarbon
Polysiloxanes (PSs), compounds containing Si-O bonds in the polymer backbone, are the
oldest and largest class of inorganic-organometallic polymers of commercial significance
(Rochow, 1987~. Worldwide production amounts to about 0.5 million metric tons, with
commercial value exceeding $3.5 billion. Their physical properties (see Table I), which are
sufficiency different from those of organic polymers, continue to catch the attention of academic
scientists and industrial research and development laboratories. Thus, the growth and
development of this class of polymer have remained unabated. In fact, a review of the current
literature reveals an increase in the number of publications about and patents for PSs throughout
the world. Some of the reasons for the remarkable and sustained growth in this area are:
The raw materials, principally silicon dioxide, methy~chioride and other organic
compounds, for the production of PSs are readily available.
Consumer and industrial applications of PSs are found in commodity and other
industries, for example, the plastics, rubber, paper, coatings, automotive,
construction, food, biomedical, cosmetics, and specialty chemicals inclustries.
PSs' copolymers can be prepared for tailoring physical, dynamic-mechanical, and
chemical properties of materials.
What is on the horizon for PSs? Some of the significant developments in the field are
summarized below:
.
Recent progress in understanding the mechanism(s) of PS polycondensation and
ring-opening polymerization will permit the synthesis of linear polymers with regio-
and stereoselectivity. Synthetic control of these materials will inevitably lead to
dominion over polymer microstructure and hence macroscopic properties
(Chojnowski, 1993~.
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Marte! cretin
Polysilazar~es
153
Advances in methods for copolymer synthesis, both siloxane-siloxane (Kennan,
1993) and siloxane-organic (Smith et al., 1992) polymers, offer greater flexibility
in modifying and improving properties for specific applications.
New functionalized polysiloxanes enable the formation of miscible polymer
mixtures and polymer blends for improved properties (Lu et al., 1993~.
Controlled synthesis of cross-linked network PSs, for example, silsesquioxanes and
related copolymers (Lichtenhan et al., 1993), interpenetrating networks (U.K.
Patent 804,199; U.S. Patent 3,971,705; Lestel et al., 1990), and hyperbranched
hybrid PSs (Rubinsztajn, 1994; Rubinsztajn et al., in press), has led to materials
with significantly improved thermal, mechanical, and optical properties.
Development of the colloid chemistry of silica (sol-gel processes and technology)
has had a significant impact on the preceramic, ceramic, and surface coatings
industry (Blinker et al., 1988; Huang et al., 1988; Saklca et al., 1988; Schmidt,
1988; Bergna, 1994~.
, . .
TABLE 1 Some Properties and Major Uses of Polysiloxanes
Properties
Uses
Low glass transition temperature
Low thermal coefficient of viscosity
High thermal stability
Oxidative stability
Hydrophobicity-lipophilicity
Elasticity
Low surface energy
Biological inertness
Photochemical stability
Low dielectric constant
Heat exchange fluids
Dielectric fluids
Antifoams
Lubricants
Polishes
RTV and HTV rubbers
Chromatographic resins
Refractory materials
Damping fluids
Biomedical applications
R
Isis
1 1
~ nil
n n
R
H. hydrocarbon
Polysilazanes, compounds containing Si-N bonds in the polymer backbone are an example
of organometallic precursors to advanced non-oxide ceramic materials, for example, for
reinforcement of ceramic, plastic, and metal matrix composites (Zeigler and Fearon, 19871. Such
ceramic materials offer a wide variety of unique physical properties that include extreme
hardness, high structural and thermal stability under extremes of environment conditions, and
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Improved Fire- arm Smoke-Resistant Materials
potential for electronic and optical properties. The major challenges facing further
commercialization of polysilazanes and ceramic progeny are the following:
synthesis of tractable prepolymers, that is, precursors with solubility in common
organic solvents;
precursors with sufficiently high molecular weight or optimum degree of cross-
linking for shape retention in the pyrolytic process of ceramic formation;
ability to attain high ceramic conversion;
control of the pyrolysis process to minimize ceramic porosity; and
reduction of carbon content in the ceramic product.
Polycarbosilanes
R
~S'-Y ~ Y = CH2, unsaturated hydrocarbon
Like polysilazanes, polycarbosilanes, compounds containing Si-C bonds in the polymer
backbone are an example of organome~lic precursors for advanced silicon carbide (SIC)
ceramic materials. Their route through polysilanes has been commercialized by the Nippon
Carbon Co. into silicon carbide fibers ~CALON_) (see scheme below).
(CH 3) 2sia2
[(CH3~2~ ]
/ Na 1 1. air, 280 °C
ogle ~ ii CH2)D. i N29~300 0C SiC
\<~/ CHe
t(CH3~2Sit
SiC-based ceramic materials offer a wide variety of physical properties that include
extreme hardness, high structural and thermal stability, and future potential as coating materials
for electronic and optical devices. Recently, novel carbosilane dendritic macromolecules have
been prepared (Zhou and Roovers, 1993~. These starburst-hyperbranched polymers are three-
dimensional symmetric species with well-defined 32- and 64-arm structures of intermediate
molecular weight. These materials may have interesting physical properties. Additionally, stable
comb-like polymers with carbon un saturation in the polymer backbone and pendant
OCR for page 155
Marte! Ze~in
155
oligofoxyethylene3 units have been synthesized (Wang and Weber, 1993; Chen et al., 1993~.
Significantly, ceramic materials from their pyrolysis have high char yield. The combination of
solubility and hydrolytic and thermal stability of these polycarbosilanes offers opportunities to
tailor properties by using appropriate preceramic polymer blends. Similarly, new branched
polyhydridocarbosilanes are precursors to high-residue ~ ~ 70 percent) SiC ceramics (Froehling,
1993~. Patents for these materials are in progress.
Polysilar~es
R
HI ~ R
Organic
Although polysilanes, compounds with Si-Si bonds in the polymer backbone, have been
known for decades, only recently has synthetic methodology been sufficiently perfected to afford
soluble oligomers and high molecular weight polymers that can be fully characterized. These
substances are stable to 300 °C, mildly susceptible to hydrolysis, and, contrary to early
predictions, inert to oxygen at ordinary temperatures. Perhaps the most significant and surprising
discovery is that these materials possess interesting and commercially important photochemical
and electronic properties. For example, since poly~diorganosilane~s absorb light in the visible
and ultraviolet region of the spectrum as a function of substituent and chain length, and
photodecay to volatile products, they have great potential as positive photoresists in the
microelectronics industry and photoconductors in the electrophotography industry. In addition,
they serve as free radical photoinitiators in organic reactions and display nonlinear optical
properties for potential use in lasers and other optical devices (Mark et al., 1992~.
Phosphorus-Based Polymeric Materials
R
MAP
N- - R = Just about any type
of substituent :
Polyphosphazenes, compounds with P-N bonds in the polymer backbone is rivaled only
by polysiloxanes with respect to diversity of species and potential for commercialization. The
potential for growth and development of this unusual class of materials is reflected in the
synthetic ease with which framing substituents, R. can be changed and modified, and in the
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Improved Fire- arm Smoke-Resistant Materials
consequential variations in physical and chemical properties that occur from the compositional
versatility (AlIcock, 1988; AlIcock et al., 1992~. Thus, different pendant groups lead to
dramatically different materials, that is, elastomers or glasses, hydrophobic or hydrophilic
-I-' inert or highs reactive materials i ' - - -
po~ymers, inert or highly reactive materials, insulators or conductors, photoactive or photoinert
materials, and bioerodible or bioinert material. Table 2 summarizes some of the current and
projected applications of polyphosphazenes. Some unusual polyphosphazenes are noted below.
~-R' ~ Pe En
IN = CAN = I my l' = N -IF = N I P = No
OR OR ~oR oR
TABLE 2 Applications of Polyphosph~enes
Elastomers
Ionic electrical conductors
Membranes
Water repellents
Nonburning textile fibers
Foam heat and sound insulators
Hydrocarbon solvent-resistant O-r~gs and gaskets
Immobilized enzymes
Controlled drug-release agents
Hydrogels for prostheses and soft-tissue applications
Liquid crystalline polymers for nonlinear optical properties
Ceramics
Although other inorganic and semi-inorganic polymers containing phosphorus (e.g.,
phory} resins, poly~alkylene phosphates), polycarbophosphazenes) are known and, in some
instances, possess interesting properties (transparency, hardness, flame retardancy, adhesion,
biocompatibility, and elasticity), they appear to suffer from three major disadvantages high
cost, oxidative instability, and hydrolytic sensitivity. Perhaps further research on related
compositions will produce new classes of materials without the problems.
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Representative terms from entire chapter:
organic polymers
Marte! Ze~in
OTHER TYPES OF INORGANIC AND ORGANOMETALLIC POLYMERS
Boron- and Almnmu~n-Contammg Polymers
Polyborylamines and Polyborazines
R R
kB``N'B``N'
R' R'
R = Hydrocarbon
1
157
Hi H
`,,N B\,N:
N B x
H H
2a
/
158
Improved Fire- and Smoke-Resistant Materials
A new class of soluble boron-nitrogen-carbon (BNC)-containing polymers, prepared by
ally~boration polymerization, has recently been reporter! (Chujo et al., 1992~. Relatively low-
molecular-weight Ad,, 5,000-14,000), organic soluble oligomers in pure form are obtained and
have been fully characterized. Although pyrolysis of the polymer at 900 °C gives a BNC
ceramic-like material, the properties of the product have not been published.
Carborane Cage Polymers
.
o
Me- Si- Me
Me FIR
l l
~7 I~ si-o-s'~ X
Me Me
Synthesis of m-carborane cage polymers based on the decaborane cage structure,
CB~oH~oC-, was achieved in the 1960s (see Alicock and Lampe, 19901. Incorporation of
poly(dimethylsiloxyI) units between the cage functions results in elastomeric copolymers that
retain their thermal stability ~` > 500 °C), low Tg (-30 °C), and chemical-oxidative stability.
These properties lead to high-performance applications as specially (Dexil) resins for
chromatography. Related materials, which contain diamine groups, have resulted in BN-like
fibers. Although these sub spaces were studied extensively in the 1960s and 1970s, further
development of them has essentially ceased due to the difficulty and cost of producing the parent
carboranes.
Oligo (alurninoxane)s
MAIM
SAC OR
R = alkyl, Neal cy'
b
R' /~/
,0-~'
R', R" = alky', ary'
Marte! Zeldir~
159
Oligotaluminoxane~s, prepared by condensation polymerization of AlCI3 in the presence
of aqueous organic acids or amides, are low molecular weight, highly branched, soluble
materials with the Al-O bond as the basic repeat structural unit. These compounds have found
application as ~drag-reducingU agents for fluid toil or water) flow, drying and gelling agents,
coatings, fuel additives, catalysts, and lubricants. These oligomers and the amide analogs can
be melt-spun into fibers that, upon pyrolysis, yield Al203 (alumina) and AIN (aluminum nitrides.
Sulfur-Containing, Polythiazy' (S-N)X, Polymers
Sulfur-containing polymers have been studied for decades. Elemental sulfur itself is a
polyatomic, polymorphic material, and exists as chains and rings (i.e., S6, Ski. Although the
element is low in cost and plentiful, there was only intermittent research and development
activity until the 1970s.
,N So
Nit
x
In 1975, the spotlight was turned onto S-N polymers because of their remarkable metal-
like appearance and their properties, for example, malleability, electrical conductivity, oriented
epitaxial structure, optical polar~zability, and superconductivity (at 0.3 °C) (Mikulski et al.,
1975~. The polymer can be fabricated as thin films on glass or synthetic polymeric surface by
epitaxial polymerization. They have a gold color by reflected light and blue color by transmitted
light. They are "relatively" inert to air and moisture; however, they detonate on compression
and depolymerize under ambient conditions on long standing. Polythiazyls are the forerunners
of polyacetylenes.
Tin-Containing Polymers
R
R
Drum
~ ~ FIR
R
_ 914
R 0~1
R
ladder
/
60
Improved Fire- and Smoke-Resistant Materials
Most notable of the tin-containing macromolecules are the ladder-like polymers. Interest
in these materials is relatively recent and a result of their high temperature stability. Moreover,
it has been found that these stannsequioxanes exist in several structural types ("ladder," "drum,"
and "butterfly") and contain hypervalent tin (Holmes et al., 1988~. The biological activity of tin
species has discouraged their continued investigation as commercially useful materials.
Ferroceny' Polymers
Homoannular
Heteroannutar
it"""' (~CH-CH2=
~C~ -
M (CO)3
Pendant
Metallocenyl polymers, which contain a wide variety of metallic elements, have been
known for a long time. Most of the studies on these materials occurred in the 1960s, principally
with iron as the metal center (Neu se, 19821. The structural units depicted above (homoannular,
heteroannular, and pendant) represent only a few of the types that have been synthesized. The
major limitations in further development of home- and heteroannular materials results from low
solubility, low-to-moderate molecular weights, and lack of mechanical properties for
applications. Recently, pendant-type water-soluble polymers, such as polyaspartamide-bound
ferrocene compounds, have been prepared and characterized. These materials were found to have
salient biomedical applications (Neuse et al., 1990~. A strained-ring ferrocenyl-silane (below),
which can be thermally polymerized to an amorphous magnetic iron-containing SiC ceramic, has
been recently reported (Manners, 1993).
Hi
R
-R' heat
R
Mn, 30000-800000
500°C, Fe-Si~
Marte! Ze~in
~0
it>
014
gN,$~
:~-I N - N
161
Phthalocyan~ne (shiskabab) Polymers
Phthalocyanines are planar macrocyclic rings composer'
of aromatic groups and nitrogen atoms capable of tetra-
coordina~cion with a meW or Adenoid. Polycondensation of a
monomeric Riot derivative, where M = Si, Ge, or Sn, leads to
one-dimensional stacked me~Ioxy chain macromolecules with
unusual physical and dynamic-mechanical properties. These
polymers dissolve in strong acids and are spinnable into fibers.
Since they are highly oriented, they can be co-spun with such
materials as polyaramids (e.g., Kevia~) to impart strength and
thermal stability. Moreover, upon doping with iodine, before
or after spinning, the phthalocyanine polymers possess
interesting magnetic and electronic (conductor and semi-
conductor), as well as optical, properties (Marks et al., 1988).
MetalloporphyFin Polymers
`1 `~
I N ~
, \ 1 ,
I ,N-M N
/
Hi\ INS
FAX
M = Fe(III), Co(II), Ni(II), Zn(II)
X = Organic function
Metalloporphyrin polymers have been popular recently due to potential catalytic,
electrochemical, and electrical-photonic properties. For example, work by Lindsey and
colleagues has demonstrated that, with M = Zn and the appropriate in-chain polymer spacer
groups, an oligomer can act as a 9-nary macromolecular photonic wire with 76 percent energy
transfer efficiency (Wagner and Lindsey, 1994~.
162
Improved Fire- aM Smoke-Resistant Materials
SUMMARY
Research on inorganic and organometallic polymers has grown enormously over the past
30 years. The above overview represents a relatively small perspective of the breadth and scope
of the field that has taken numerous, interesting diversions based, on the one hand, on
Intellectual curiosity and creativity, and on the other, on economic objectives. As long as special
properties and function are desired, new organic, inorganic, organometallic, hybrid materials
will be discovered.
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