Figure 1. Structural difference between a linear and a two-dimensional
(2D) polymer. In the former, linearly connecting monomers result in a
thread-like linear polymer, while in the latter laterally connecting
monomers result in a sheet-like 2DP with regularly tessellated repeat
units (here of square geometry). The repeat units are marked in red,
whereby the number n describes the degree of polymerization. While a
linear polymer has two end groups, a 2DP has an infinite number of end
groups that are positioned all along the sheet edges (green arrows).
A
two-dimensional polymer (
2DP) is a sheet-like monomolecular macromolecule consisting of laterally connected repeat units with end groups along all edges.
[1] [2] This recent definition of 2DP is based on
Hermann Staudinger’s
polymer concept from the 1920s.
[3][4]
According to this, covalent long chain molecules (“Makromoleküle”) do
exist and are composed of a sequence of linearly connected repeat units
and end groups at both termini.
Topologically, 2DPs may thus be understood as covalent structures
made up from regularly tessellated regular polygons (the repeat units).
Figure 1 displays the key features of a linear and a 2DP according to
this definition. For usage of the term “2D polymer” in a wider sense,
see “History”.
There are several examples of 2DPs which include the individual layers or sheets of graphite (called
graphenes),
MoS2,
(BN)x and layered
silicates.
As required by the above definition, these sheets have a periodic
internal structure. A potential repeat units of, e.g., graphene is a
sp2-hybridized
carbon atom. Individual sheets can in principle be obtained by
exfoliation procedures though in reality this is a non-trivial
enterprise.
Synthesis
Some of the known 2DPs can be obtained by pyrolytic or related
procedures which require forcing conditions. However, until recently
there was no rational synthesis available that would operate under
conditions mild enough to allow for implementing the repertoire of
organic chemistry in terms of precise structure control including the
exact positioning of functional groups on the sheets’ faces and edges.
The first rational organic synthesis of a molecular sheet meeting the
above 2DP definition was achieved in 2012 and based on monomer
structure with photoreactive anthracene and acetylene moieties.
[5]
It rests upon monomers with three reactive sites that crystallize in
layered single crystals, such that these sites of neighbouring monomers
are directly opposing each other. Light irradiation allows connecting
them through these sites, and subsequent exfoliation affords monolayer
sheets. The internal periodicity is supported by
electron microscopy imaging,
electron diffraction and
Raman-spectroscopic analysis.
2DPs should in principle also be obtainable by, e.g., an interfacial
approach whereby proving the internal structure, however, is more
challenging and has not yet been achieved. For a recent approach, see
Ref.
[6]
2DPs as two dimensional sheet macromolecules have a crystal lattice,
that is they consist of monomer units that repeat in two dimensions.
Therefore, a clear diffraction pattern from their crystal lattice should
be observed as a proof of crystallinity. A successful synthesis of such
a polymer, that is graphene oxide was achieved in 2012.
[7]
Applications
2DPs are expected to be superb membrane materials because of their
defined pore sizes. Further they can serve as ultrasensitive pressure
sensors, as precisely defined catalyst supports, for surface coatings
and patterning, as ultrathin support for
cryo-TEM,
and many other applications. 2DPs are also important to polymer
physics. It needs to be explored inasmuch the concepts which were
developed for linear polymers can be applied to 2DPs.
History
First attempts to synthesize 2DPs date back to the 1930s when Gee
reported interfacial polymerizations at the air/water interface in which
a
monolayer of an unsaturated fatty acid derivative was laterally polymerized to give a 2D cross-linked material.
[8]
Since then a number of important attempts were reported in terms of
cross-linking polymerization of monomers confined to layered templates
or various interfaces.
[1][9]
These approaches provide easy accesses to sheet-like polymers. However,
the sheets’ internal network structures are intrinsically irregular and
the term “repeat unit” is not applicable (See for example:
[10][11]). In organic chemistry, creation of 2D periodic network structures has been a dream for decades.
[12] Another noteworthy approach is "on-surface polymerization"
[13][14] whereby 2DPs with lateral dimensions not exceeding some tens of nanometers were reported.
[15][16]
Laminar crystals are readily available, each layer of which can ideally
be regarded as latent 2DP. There have been a number of attempts to
isolate the individual layers by exfoliation techniques (see for
example:
[17][18][19]).
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