Background and Originality Content
Catenanes are compounds composed of two or more mechanical interlocked
macrocycles.[1-3] By introducing the catenanes
structures into polymers, polycatenanes with chain-like mechanically
interlocked structures can be obtained.[4] Since
catenanes are highly free and mobile, polycatenanes possess greater
flexibility and strength caused by different chain conformations
compared to conventional polymers.[5-8] The
categories of polycatenanes could be divided into a number of subclasses
as follow,[9-15] depending on the location and
connection of the catenane subunits: main-chain polycatenanes obtained
by introducing difunctionalized catenanes into the linear polymer main
chains or by interlocking a plurality of rings two by two; side-chain
polycatenanes obtained by introducing monofunctional catenanes into the
linear polymer side chains;polycatenanes based on cyclic polymers
formed by two large or one large ring hanging with small rings; Unlike
the above types of polycatenanes, polycatenane networks are composed of
multiple interlocked rings or of catenanes and polymers, showing
solid-like properties different from most of the liquid polycatenanes.
Liquid polycatenanes are mostly used for theoretical research and are
difficult to be widely used as macroscopic solid materials.
Through the linear crosslinking, polycatenanes could be converted from
liquid to gels and thus be widely used.[16] The
above-mentioned polycatenanes networks formed by multiple interlocked
rings are also a kind of crosslinked networks, while their structures
are complex making it difficult to recognize the exact
structure.[12] In recent years, many kinds of
poly[2]catenane networks have been developed. For example, As shown
in Scheme 1a, Huang, Wu and coworkers prepared a poly[2]catenane gel
by the “click” reaction between bisvinyl [2]catenanes and
poly(ethylene glycol) derivatives containing thiol
groups.[17] Rowan and coworkers designed a series
of poly[2]catenane gels with different numbers of interlocked units
by using doubly threaded pseudo[3]rotaxane crosslinkers and covalent
crosslinkers (Scheme 1b).[18] Besides, Wang, Chen
and coworkers designed a poly[2]catenane hydrogel prepared by
Tetra-PEG chains crosslinked by the recognition of metal ions, which was
a reversible coordination interlock structure (Scheme
1c).[19] It could be found from the above-reported
examples, the gel formation all relies on the polymer as part of the
backbone structure.
Herein, we prepared a new type of poly[2]catenane gel by entire
sequential assembly of small molecules without any polymer structures.
Poly[2]catenane gels were prepared based on the monomer M1 ,
monomer M2 (2,6-pyridinedicarbaldehyde) used for ring closing
reactions and the crosslinker monomer M3 , all of which were
small molecules. First, M1 of alkyl chain linking with two
isophthaloyl bis(diamine) groups, self-assembled to form linear
supramolecular polymers (SPs) through synergistic intramolecular
hydrogen bonding and π-π interactions at higher concentrations in
chloroform. When M2 and M3 were simultaneously added
to the M1 solution, the aldehyde groups of M2 could
spontaneously react with the amino groups of M1 to form dynamic
covalent bonds (imine bonds), and ring closing behaviors occurred. The
ring-closing behaviors was caused from: since the number of reactive
sites of M3 was twice as many as that of M2 , the amnio
groups in supramolecular polymer chains could react with M3 to
form the closed ring. Additionally, M3 acted as the crosslinker
to crosslink linear SPs. Therefore, the simultaneous addition ofM2 and M3 could transform linear SPs into
poly[2]catenane gels. The reaction process of forming imine bonds is
spontaneous and stable, and thus, it would not affect the existing
non-covalent bonds and disturb the SPs formed by self-assembly.Scheme 1 Schematic representation of the three types of
poly[2]catenane gels: (a) gels formed by cross-linking of catenane
modified in the side chains of polymers, (b) gels formed by polymers
acting as irregular catenane macrocycles, (c) gels formed by polymers
acting as regular catenane macrocycles, (d) gels formed by the
self-assembly of small molecules (M1 , M2 , M3 )
through non-covalent and dynamic covalent bonds