  The development of synthetic strategies to engineer \pi-conjugated polymers
is of paramount importance in modern chemistry and materials science. Here we
introduce a theoretical and experimental synthetic paradigm based on the search
for specific vibrational modes through an appropriate tailoring of the
\pi-conjugation of the precursors, in order to increase the attempt frequency
of a chemical reaction. First, we on-surface design a 1D \pi-conjugated polymer
with specific \pi-topology, which is based on bisanthene monomers linked by
cumulene bridges that tune specific vibrational modes. In a second step, upon
further annealing, such vibrational modes steer the two-fold cyclization
reaction between adjacent bisanthene moieties, which gives rise to a long and
free-defect pentalene-bridged conjugated ladder polymer featuring a low band
gap. In addition, high resolution atomic force microscopy allows us to identify
by atomistic insights the resonant form of the polymer, thus confirming the
validity of the Glidewell and Lloyd's rules for aromaticity. This on-surface
synthetic strategy may stimulate exploiting previously precluded reactions
towards novel pi-conjugated polymers with specific structures and properties.
