  Polymers that extend covalently in two dimensions have attracted recent
attention as a means of combining the mechanical strength and in-plane energy
conduction of conventional two-dimensional (2D) materials with the low
densities, synthetic processability, and organic composition of their
one-dimensional counterparts. Efforts to date have proven successful in forms
that do not allow full realization of these properties, such as polymerization
at flat interfaces or fixation of monomers in immobilized lattices. A
frequently employed synthetic approach is to introduce microscopic
reversibility, at the cost of bond stability, to achieve 2D crystals after
extensive error correction. Herein we demonstrate a synthetic route to 2D
irreversible polycondensation directly in the solution phase, resulting in
covalently bonded 2D polymer platelets that are chemically stable and highly
processable. Further fabrication offers highly oriented, free-standing films
which exhibit exceptional 2D elastic modulus and yield strength at 50.9 +- 15.0
GPa and 0.976 +- 0.113 GPa, respectively. Platelet alignment is evidenced by
polarized photoluminescence centered at 580 and 680 nm from different dipole
transitions. This new synthetic route provides opportunities for 2D polymers in
applications ranging from composite structures to molecular sieving membranes.
