Biomedical Engineering Reference
In-Depth Information
Scheme 26
Synthesis of
hb
-PP through copolycyclotrimerization of aryldiyne with aryl-
monoyne
Spectral characterization confirmed the proposed hyperbranched archi-
tecture of the copolyarylenes. Estimations of the ratio of diyne to monoyne
incorporated into the
hb
-PAs revealed that the monoynes functioned as the
growth-controlling agents, impeding intracyclotrimerization reactions [86].
As an optimized ratio of diyne to monoyne, 1 : 1.5 has often been found
to work well to yield completely soluble, high molecular weight
hb
-PAs.
Copolycyclotrimerization of aromatic triynes such as
20
is inherently much
more difficult to control. While for aliphatic monoynes like 1-octyne (VI)
a monoyne to triyne ratio of 3 : 1 was sufficient to obtain a soluble
hb
-PA,
a larger ratio (4 : 1) was necessary for aromatic monoynes such as phenyl-
acetylene (IX) [86, 100].
Detailed structural investigations revealed that even when such a large ex-
cess of growth-controlling monoyne was used, the resultant polymers still
contained internal cyclic structures. According to the ratio of monoyne to
triyne found in
hb
-P(
20
-VI), different propagation modes accounting for the
intracyclotrimerization reactions were proposed as depicted in Scheme 27.
Taking the 1,2,4- and 1,3,5-isomeric structures of the trisubstituted ben-
zenes into account, the formation of “small”- and “medium”-sized cycles
is highly possible. Similar to the silyldiyne homopolycyclotrimerization, the
close intramolecular proximity of the two triple bonds originating from an
ortho-connection are likely to be terminated by the triple bond of 1-octyne,
