Supramolecular Polymerizations

518 A. Ciferri
Figure 8. (a) Subunits used for supramolecular networks. [10] (b)
PEO/PPO block copolymer networks based on supramolecular
(upper) and covalent crosslinks. [52] (c) Hydrogen-bond-induced
compatibilization in a polymer blend. [53]
5.1.2 H-Bonded Random Networks
In the scheme of Figure 7 all unimers exhibit a complete
match of the donor/acceptor components of either single
or multiple H-bond units. If this match does not occur, or
tetra and bifunctional unimers are mixed, planar or threedimensional
networks are possible. [2] Networks based on
triacids and bipyridine derivatives, or on tetrafunctionalized
pyridine and difunctional benzoic acid compounds
(cf. Figure 8a), have been reported. [10, 51] Single H-bonds
connect chain segments emanating from tetrafunctional
crosslinkages. Meijer et al. [52] have reported functionalized
copolymers of propylene oxide and ethylene oxide
exhibiting a strong four H-bond scheme (Figure 8b).
These networks exhibit peculiar rheological features to
be described below. H-bonding (Figure 8c) between
polymers, such as poly(4-vinylpyridine) and poly(4hydroxystyrene),
[53] was described as a factor promoting
[55, 90]
compatibility in polymer blends.
5.1.3 Coordination Polymers
The scheme of linear coordination polymerization was
discussed by Lehn. [2] The unimers are ditopic ligands
with two binding groups forming main-chain bonds
through metal-ion coordination (Figure 9a). Several metal
binding groups (bidentate, tridentate) and metal ions with
tetra-, penta- and hexa-coordination are available. Among
Figure 9. (a) Schematization of a linear coordination SP showing
bidentate and tridentate metal binding group and metal ions
with tetra-, penta-, and hexa-coordination. [2] (b) Degree of polymerization
of Be(Bu2PO2)2 vs concentration in CHCl3 at room
temperature. The chain backbone is schematized on the right. [18]
(c) Self-assembly of a cobalt porphyrin polymer by coordination
of two covalently attached pyridine ligands. [20]
the earliest reports of soluble, reversible coordination
polymers we find systems based on three-atom-bridging
phosphinate groups connected by tetrahedral metal atoms
reported by Ripamonti and coworkers [18] in 1968. Figure
9b illustrates the variation of DP (by means of vapor
pressure osmometry) with the concentration of beryllium
dibutylphosphinate (Be(Bu2PO2)2) dissolved in CHCl3, a
non-coordinating solvent. Fiber-forming properties, suggesting
larger DPs, were exhibited by anisotropic gels
occurring in more concentrated solution. Substantial evidence
of depolymerization with dilution was observed,
confirming the dynamic reversibility typical of supramolecular
polymers. [4] The chain structure, deduced from Xray
diffraction, is based on the alternate singly and triply
bridged structure shown in Figure 9b.
Among the most recent reports, [20] the functionalized
porphyrin polymer in Figure 9c was shown to attain DP
L 100 in a 7610 –3 m solution in CHCl3 (by means of
size-exclusion chromatography (SEC)). Here, coordination
occurs between the Co atom (hexa-coordination) and
the two pyridine ligands.