  The current state of the chemical functionalization of three types of single
sheet 2D materials, namely, graphene, molybdenum disulfide (MoS2), and black
phosphorus (BP) is summarized. Such 2D sheet polymers represent currently an
emerging field at the interface of synthetic chemistry, physics, and materials
science. Both covalent and non-covalent functionalization of sheet
architectures allows for a systematic modification of their properties, i.e. an
improvement of solubility and processability, the prevention of re-aggregation
or a band gap tuning. Next to successful functionalization concepts also
fundamental challenges are addressed. These include the insolubility and
polydispersity of most 2D sheet polymers, the development of suitable
characterization tools, the identification of effective binding strategies, the
chemical activation of the usually rather unreactive basal planes for covalent
addend binding, and the regioselectivity of plane addition reactions. Although
a number of these questions remain elusive in this review, the first promising
concepts to overcome such hurdles have been listed.
