Polyphenylene Nanostructures - Cluster for Molecular Chemistry

1772 Chemical Reviews, 1999, Vol. 99, No. 7 Berresheim et al.
Scheme 40 Chart 9
processable PPPs with high molecular weight. 52-54,63
In view of the expected large persistence length (for
definition see below) of the main chain and of the
flexibility of the side chains, such molecules have
been termed hairy-rod polymers. 32 Unlike other rigidrod
polymers, PPPs show significant optical polarizability
and anisotropy, which is particularly important
when trying to obtain information on the
orientational dynamics in solution. 236-238 Useful methods
for studying the dynamic behavior of rigid-rod
chains are static and dynamic light scattering as they
are very sensitive probes for collective orientation
fluctuations. Measurements obtained from dilute
solutions lead to the conclusion that poly(2,5-di-ndodecyl-para-terphenyl-4,4′′-diyl)
(167) forms small
aggregates typically consisting of trimers with a
parallel arrangement of the molecules. 239 In semidilute
solution, additional relaxation processes (slow
and ultra-slow) can be detected in addition to the
faster cooperative diffusion and reorientation of the
trimers. X-ray scattering experiments suggest that
these can be ascribed to the formation of large
anisotropic clusters of about 570 nm size with inherent
crystallization. 239 The formation of these large
clusters at room temperature requires approximately
1 week after preparation from the solution, and the
clusters disappear upon heating to 65 °C. After
standing, the clusters precipitate under gravity and
form an opaque sediment. 239
In the 1920s, Vorländer reported that quinquephenyl
(168) and sexiphenyl (169) formed liquid
crystalline melts (Chart 11). 240,241 The liquid crystalline
character of methyl-substituted oligophenylenes
was also established. 242 It is generally assumed for
the melts of monodisperse polymers that an axial
ratio of 6.2 is required for the occurrence of liquid
crystallinity. 243-245 Unfortunately, since the melt
contains molecular weight distributions, induced
fractionation could occur. 246 Lower molecular weight
poly(2,5-di-n-dodecyl-1,4-phenylene)s (171a) give only
isotropic phases upon melting while the higher molecular
analogues exhibit anisotropic phases as well
as isotropic ones. 247 Later investigations of the high
molecular weight fraction of polymers 171a with Mw
approximately 137 000 g mol -1 showed that only one
anisotropic phase is obtained. 243 It is important, in
this context, that rigid-rod polymers with a stiff
backbone and shape anisotropy are predicted to have
outstanding mechanical properties. While the presence
of alkyl chains is important for solution and
melt-processing often under the formation of order,
the presence of side chains could compromise the
mechanical properties of PPPs due to the chemical
instability of the side groups and to their liquidlike
character at relatively low temperatures. 243
Conduction processes in linear conjugated polymers
such as PPP cannot be discussed without referring
to information on 3-dimensional structures. The
difficulty of obtaining crystal structures from conju-