A ladder polymer is a double-strand macromolecule
consisting of an uninterrupted linear sequence of rings,
with adjacent rings having two or more atoms in common .
The ladder repeat unit has two pairs of open bonds. Each pair
is formally linking two contiguous units together and
thereby makes a strand. In the following example the upper
open imino bond is linked with the upper open aromatic bond
of the next unit, defining the upper strand, and the lower open
imino bond is connected with the lower open aromatic bond,
defining the lower strand:
A dictionary entry with key i is required
in each +n annotation to assure
correct connectivity. The entry with key ilu,
which is required in one of the two +n
annotations, identifies all nodes belonging to the ladder
unit—in this case all nodes of the notation.