Chemistry Reference
In-Depth Information
The main distinction between the dehydrohalogenation approach and
polycondensation processes is that in the former case the carbon backbone
is formed first by polymerizing relevant monomers, and the task for synthe-
sizing polyyne and/or polycumulene is reduced to the complete elimination
of hydrogen halide with the retention of one-dimensional structure.
The vast majority of studies published in this field through 1997 have
been extensively reviewed in 1999 [3]. Since then several studies exploring
new possibilities have been published, and in the present review I will con-
centrate on those new accomplishments, although some issues of general
importance from that review will be mentioned or briefly discussed as well.
In addition, some relevant studies, which, for whatever reason, were not
included in the 1999 review, will also be covered here.
12.2 ORIGINAL POLYMERS
The formation of carbyne by means of dehydrohalogenation is possible only
if equal numbers of the halogen and hydrogen atoms are attached to neigh-
boring carbon atoms in the original polymer chain. Theoretically, this
allows complete elimination of side substituents in two steps to leave a
'naked' carbon chain, i.e. chemical carbonization of the original polymer.
This requirement is satisfied in poly(vinylidene halides) (PVDHs) [4-8],
poly(1,2-dihaloethylenes) [9-11], as well as in poly(1,1,2-trichlorobutadiene)
and poly(1,2,3-trichlorobutadiene) [12,13]. All these types of polymers were
used as original materials in numerous attempts to synthesize carbyne by
the dehydrohalogenation reaction [3].
The dehydrohalogenation of halogen-containing polymers is generally
held to proceed in two stages. In the first stage, the one molecule of hydro-
gen halide (HX) is eliminated from each repeating link of the polymer
backbone to afford a halogen-substituted polyene structure 1 (
Scheme 12.1
,
X
F, Cl, Br).
As it is seen from Scheme 12.1, the dehydrohalogenation of both
poly(vinylidene halide)s and poly(1,2-dihaloethylene)s at the first stage
results in the formation of the same (or very similar) polyhalovinylene, 1.
However, comparing these two types of polymers as starting materials,
one should keep in mind that poly(vinylidene halide)s are produced by
polymerization of corresponding 1,1-dihaloethylenes, whereas poly(1,2-
dihaloethylenes), (-CHX-CHX-)
n
, where X
¼
Cl or Br, cannot be synthe-
sized directly by radical polymerization of corresponding 1,2-dichloro-(or
dibromo)ethylene because of steric hindrances [14]. Therefore, such poly-
mers were prepared by halogenation of polyacetylene (polyvinylene), which
can be synthesized either by polymerization of acetylene [9,10] or by dehy-
drohalogenation of poly(vinyl chloride) [11]. It should be noted here that
each polymer-analogous transformation introduces additional chemical
¼
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