Supramolecular Polymerizations

Supramolecular Polymerizations 527
of properties of different polymers, while preventing the
undesirable (incompatibility-driven) macrophase separation
of unconnected components. [47–49, 106] Pre-assembly
followed by the growth of specifically designed unimers
could represent a novel strategy toward the fabrication of
composite structures with a prescribed distribution of
components.
6 Topics for Further Investigation
Several aspects of fundamental and applied character
appear to need more extensive investigation. Here we
restrict attention to the polymer-like properties of SPs
and the basic polymerization mechanisms.
A main problem is the assessment of DP. A characteristic
feature of these systems is that DP is a function of
concentration. However, the determination of DP at a
given concentration becomes complicated when using
conventional molecular-weight determinations, requiring
extrapolations to infinite dilution. Measurements under
theta conditions should be preferred. The assessment of
DP by means of SEC is also questionable for kinetically
unstable SPs that display weak contact forces. In the case
of actin, a successful determination of the DP distribution
was reported using electron microscopy. [28] A reliable
determination of the complete DP/concentration dependence
may require the evaluation of binding constants and
the application of theoretical expressions valid for a specific
growth mechanism. For instance, Equation (1) can
be used in the low concentration regime when MSOA is
expected to prevail. The analysis of data at higher concentration
could allow the detection of cooperative contributions
arising from the HG or SLC mechanisms.
There are several other parameters that could be determined
by an extension of conventional polymer physical
chemistry. One is the characterization of the rigidity of
the assembly. The usual determination of the persistence
length from solution studies may not be a viable one in
the present context. An alternative approach, suitable for
large values of q, is based on flexural rigidity data
extracted from the thermally driven fluctuation of shape
or end-to-end distance. [114, 115] Calculation approaches are
also possible. [116] Rheological characterization of processing
parameters under shear and elongational flow should
aim to the coupling of polymer viscoelasticity and bond
lability. Analogies should be considered with the behavior
of living polymers, [91] covalent networks exhibiting
labile crosslinkages, including those formed in the
oriented state. The assessment of the mechanical strength
of supramolecular bonds, particularly for the linear Hbond
materials discussed in Section 5.1.1, is another open
topic of great importance, e.g., for the performance of
supramolecularly extended covalent polymers. An evaluation
of the most suitable method [117] for calculating the
elastic constants of SPs is needed. The characterization of
the difference in the properties of reversible and irreversible
polymers and copolymers could be better assessed by
comparative studies on a given SP and on its analog
obtained by transforming the supramolecular main-chain
bonds into covalent ones.
Analysis and more detailed investigations of the
growth mechanisms are also needed. The sudden occurrence
of growth when the nematic phase appears ought to
be documented for other SPs, the rigidity of which needs
to be independently assessed. The theoretical reasons preventing
the observation of a nematic phase for block
copolymers and some other amphiphiles need to be clarified
and experimentally verified. Systems for which the
contact energy, or equilibrium constant, can be systematically
altered (e.g. by altering the number of pairwise
interactions at given functionality) should be considered
for a more stringent test of the relationship between K
and DP. In this context, recent work has shown that
bonds based on DNA base pairing produce well-behaved
SPs. [118] An analysis of model systems in which only the
site distribution is altered could allow an assessment of
the parameters controlling linear versus helical growth.
The detailed analysis of the steps contributing to the formation
of host/guest composites (Section 5.4), and other
hierarchical processes (Section 5.3), should help to elucidate
the scaling up from nanometric to mesoscopic
assemblies.
Acknowledgement: The author expresses his appreciation to
Prof. Paul van der Schoot for clarifying discussions and constructive
criticism.
Received: March 23, 2002
Revised: May 13, 2002
Accepted: May 13, 2002
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