Supramolecular Polymerizations
Supramolecular Polymerizations
Supramolecular Polymerizations
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<strong>Supramolecular</strong> <strong>Polymerizations</strong> 517<br />
Figure 6. Mean-field phase diagram for amorphous diblock<br />
copolymers (A)n-(B)m. Main phases are: lamellar (L), hexagonal<br />
cylindrical (H), closed packed spheres (CPS), disordered (DIS).<br />
f is the volume fraction of A segments. (i Am. Chem. Soc.<br />
1996 [87] ).<br />
flexibility of the A and B blocks. On this basis, cubic,<br />
hexagonal, lamellar, and other phases are predicted in<br />
discrete regions of v N N versus m/n phase diagrams as<br />
that illustrated in Figure 6 (N: total number of A and B<br />
units). A review of the most recent elaboration that unifies<br />
the weak [85] and strong [86] segregation regimes is<br />
given in the literature. [87] Noolandi et al. [88] extended the<br />
(self-consistent) mean-field approach to the calculation of<br />
the relative stability of liquid-crystalline phases occurring<br />
in solutions of copolymers composed of three flexible<br />
blocks.<br />
5 Polymers from <strong>Supramolecular</strong><br />
Polymerization<br />
5.1 Linear Chains and Applications<br />
5.1.1 H-Bonded Polymers<br />
This class of systems has attracted considerable interest<br />
due to the intrinsic characteristics of H-bonds, such as<br />
directionality and the possibility of increasing bond<br />
strength by multiple pairwise interactions. Figure 7 illustrates<br />
typical cases in which flexible or rigid segments<br />
are functionalized with groups able to form single or multiple<br />
H-bonds. The binding constant K is the primary<br />
parameter controlling DP in terms of the simple MSOA<br />
mechanism schematized in Figure 4a, predicting, according<br />
to Equation (1), K A 106 –107 m –1 needed for DP above<br />
the oligomeric range. If one takes K = 500 m –1 as an average<br />
value for a single H-bond (reported for the pyridine/<br />
benzoic acid dimerization), [7] it appears that at least 4 Hbonds<br />
are needed to produce DPs of interest. In fact, the<br />
polymer in Figure 7c based on the dimerization of ureidopyrimidone<br />
residues (K =66107 m –1 in CDCl3), [4] was<br />
Figure 7. <strong>Supramolecular</strong> polymers stabilized by main-chain<br />
links based on (a) one, [11] (b) three, [2, 15] and (c) four [4, 52] Hbonds.<br />
reported to attain DP in the order of 1000 in dilute, iso-<br />
[4, 52]<br />
tropic solution.<br />
In the case of polymer (b), based on the 3 H-bond<br />
scheme of rigid anthracene segments terminated by uracil<br />
(A-A) or pyridine residues (B-B), one therefore would<br />
not expect a significant DP to occur in virtue of the<br />
MSOA mechanism. In fact, no evidence for appreciable<br />
DP was reported for isotropic solutions. However, the<br />
development of liquid crystallinity evidenced the formation<br />
of large DPs in moderately concentrated solutions in<br />
organic solvents, [14] likely triggered by the mechanism of<br />
growth-coupled-to orientation.<br />
When the rigid segments in (b) were replaced by flexible,<br />
tartaric acid spacers, the occurrence of liquid crystallinity<br />
was observed only in undiluted (thermotropic) systems.<br />
[13] Polymer (c), based on flexible segments terminated<br />
by diacid (A-A) and dipyridil (B-B) residues forming<br />
single H-bonds, [8–12] displayed analogously only thermotropic<br />
behavior. The occurrence of liquid crystallinity<br />
in the melt does not provide evidence of significant polymerization.<br />
[1] In fact, the role of the growth-coupled-toorientation<br />
mechanism for worm-like chains displaying<br />
soft interaction was shown to be extremely small. [10, 11] On<br />
the other hand, Equation (1) offers compelling evidence<br />
that no large DPs are produced in the case of a single Hbond.<br />
Other linear H-bonded systems of interest include the<br />
polycaps [89] based on the polymerization of capsular host<br />
complexes (calixarenes) functionalized with urea. The<br />
formation of main-chain bonds occurred only when a<br />
guest molecule was hosted in the capsule (CHCl3 acted<br />
both as a capsule host and as a solvent). Whitesides and<br />
coworkers [16] who had discussed the formation of closed<br />
structures by the self-assembly of melanine and cyanuric<br />
acid earlier, reported the formation of linear nanorods by<br />
introducing a mismatch in the spacers of the AA and BB<br />
groups of these monomers. Ladder-type supramolecular<br />
polymers, [10] and ribbon-type polymers have also been<br />
reported. [17]