Page 186 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology en012i-947 July 26, 2001 11:11

694 Polymers, Inorganic and Organometallic

been synthesized with the metallocene group pendant to a
hydrocarbon chain (Fig. 40C).
Although metallocenes have been made with many dif-
ferent metals, current work and commercial applications
have focused mainly on M = Fe (ferrocene). Therefore,
this section will explore the synthesis and properties of
polyferrocenes. The great interest in polyferrocenes stems
from the variety of their useful properties (e.g., high
thermal stability, radiation resistance, diversified chem-
FIGURE 39 Iron-containing coordination polymer with herbicidal istry, high energy content, and, recently, catalytic activity).
properties.
There are numerous examples of ferrocenyl polymers as
constituent parts of elastomers, adhesives, electron trans-
example, the herbicide 2,4-dichlorophenoxyacetic acid ferresins,semiconductors,UVabsorbers,solid-propellant
forms a coordination polymer with iron (Fig. 39) that additives, combustion activators, heat transfer fluids, lu-
slowly and effectively releases the bioactive agent in soil. bricants, and antioxidants.
The metallic by-product is nontoxic. Polyferrocenes were first obtained by heating ferrocene
in the presence of a free radical catalyst [Eq. (31)]. Al-
◦
though the product was stable to 400 C, the yields were
B. Metallocene Polymers low (∼10%) and the polymer had a relatively low MW
Metallocenes are organometallic compounds that contain (<3000). Grignard and Wurtz-type coupling reactions on

a metal atom (e.g., Fe, Cr, Ti, Mn, etc.) “sandwiched” 1,1 -diiodoferrocene have improved yields considerably
between two aromatic cyclopentadienyl (Cp) rings. Met- (>80%); however, the degree of polymerization remained
low.
allocene polymers are constructed by extending a chain
either through one (homoannular) or both (heteroannu-
lar) Cp rings (Fig. 40A, B). In addition, polymers have
n Fe 200 C
Peroxide
Fe (31)
I Cu n
n Fe
I

Since the Cp functionality has aromatic properties, it
undergoes electrophilic substitution reactions in much
the same manner as benzene. Therefore, many ring-
FIGURE 40 Polyferrocenes. (A) Homoannular. (B) Heteroannu- containing derivatives can be synthesized for use in step-
lar. (C) Pendant. growth, chain-growth, and ring-opening polymerization

FIGURE 41 Polyferrocenes with various bridging groups.