Self-Assemblies, Folding, and Properties of Perylene Monomers, Oligomers, and Polymers - Molecular Self-Assembly: Advances and Applications - page 137

incorporated, the foldable polymer can be cleaved from the support to
yield a monodispersed polymer. The advantage of this method is that
the lengths of all the macromolecules are identical. The disadvantage
is that the quantity of the final polymer is small, even when the yield
of each coupling step is very high. Nonetheless, oligonucleotides and
peptides are routinely synthesized using this method.
The third approach is a stepwise solution-phase synthesis [18].
Again, this method requires an asymmetric building block with one
end activated and the other end protected. Instead of coupling to
a solid support, such properly activated building block can react
to a molecule that serves as a polymer chain anchor. This newly
developed method is called stepwise solution approach to construct
foldable polymers, in which the molecular weight can be controlled
as the solid-phase synthesis. Unique properties of such polymers are
their single molecular weight and precisely controlled sequences,
orientations, and folding.
Although the aforementioned three strategies provide general
guidance, the tactics to implement them in the construction of
alternating rigid and flexible sequences still remain critical. Coupling
the rigid sequence to the flexible sequence in order to construct
foldable polymers represents a conceptually simple approach.
However, the problem may arise that the rigid sequence becomes so
insoluble to render the coupling reaction low yield and ill defined.
Another approach is to sandwich the rigid sequence between two
half flexible sequences and initiate the coupling between the two
flexible ends. The opposite of this tactic is to sandwich the flexible
sequence with two half rigid sequences and to carry out coupling
reactions between the two rigid ends. There are known reactions
such as Wittig or Suzuki coupling, which will yield conjugated rigid
sequences. The foldamers presented here uses the second tactic,
employing phosphoramidite chemistry to connect flexible OEG. The
main reason is that phosphoramidite chemistry produces high yields
and the coupling of soft chains is relatively easier than that of rigid
chromophores, especially for macromolecules.
Accordingly, two building blocks are synthesized: one as the
chain anchor and the other to grow the chain. The chain anchor
has a perylene core with one end blocked from reactions by mono-
benzoylation and the other end functionalized with a hydroxyl
group ( 1A ). The chain grower is protected with a removable

′

DMTr (4,4 -dimethoxyl trityl) group on one end and activated