Biomedical Engineering Reference
In-Depth Information
CpCo(CO)
2
under UV irradiation and from the polymerizations of metallolyl
(
57
and
58
) and silyldiynes (
66
-
70
) catalyzed by tantalum halides [86-91].
The homopolycyclotrimerizations of other aromatic diynes (
38
-
56
)allpro-
ceeded very rapidly, giving polymeric products that were only partially or to-
tally insoluble in common organic solvents due to the involved cross-linking
reactions. The large free volumes and irregular molecular structures gener-
ated by the nonlinear carbazolyl, diphenylamine, metallolyl and silyl groups
may have helped endow the homopolymers (
hb
-P
57
to
hb
-P
70
)withtheex-
cellent solubility.
Structural analysis of the homopolymers by spectroscopic methods con-
firmedthatthediyneshadundergone[2+2+2]polycyclotrimerizationsby
forming new benzene rings from their acetylenic triple bonds. The ratio of
the 1,2,4- to 1,3,5-isomers of the trisubstituted benzene rings was estimated to
be
2.2 : 1. Careful evaluation of the
1
H NMR spectra unveiled that the num-
ber of terminal triple bonds in the final
hb
-PAs was much smaller than that in
an “ideal” hyperbranched structure produced by the diyne polycyclotrimer-
ization. This result suggests that intra-sphere ring formation might have been
involved in the cyclotrimerization polymerization.
To account for the “missing” acetylenic protons, three possible pathways
for the consumption of the triple bonds were proposed and are depicted in
Scheme 25. The first pathway is via a normal P
1
M
2
growth mode, where P and
M stand for polymer and monomer, respectively. The second is via an intracy-
clotrimerization mode of P
2
M
1
type, with two triple bonds from two polymer
branches and one from a monomer involved. The third is via another “pure”
intracyclotrimerization mode of P
3
type, with the three triple bonds all from
the branches of a polymer. The experimental results indicate that one or both
of the intracyclotrimerization modes must have been at play in the polycy-
clotrimerization of silylenediynes. However, the newly formed benzene rings
by these intracyclotrimerizations are undistinguishable from each other and
are also indistinct from those formed by the normal polycyclotrimerization
reactions in the NMR spectra, making it difficult to experimentally determine
the probabilities of the intracyclotrimerization reactions.
To solve this problem, computational simulation was exercised. The
models of the polymers were built, and the probabilities of the growth modes
were estimated according to the minimized energy of the structures, using
the Materials Studio program
1
. An example of the outputs of the com-
puter simulations is shown in Fig. 2. The overall structure of
hb
-P
66
looks
like a star-shaped macromolecule containing a number of small cyclic units
(Fig. 2, lower right panel). The total number of the triple bonds left inside
the hyperbranched structure and the total number of the aromatic protons
were in agreement with the numbers estimated from the
1
H NMR analysis.
∼
1
Materials Studio is a software environment that brings together the world's most advanced mate-
rials simulation and informatics technology. It is a product of Accelrys Inc.
