Lectures presented at the 39th IUPAC Congress and 86th Conference of
the Canadian Society for Chemistry: Chemistry at the Interfaces, Ottawa,
Canada, 10-15 August 2003
Part I - Controlling the self assemb y in macromolecular systems:
From nature to chemistry to functional properties
Preface
Molecular self-assembly is the spontaneous organization of molecules
under thermodynamic equilibrium conditions into structurally well-defined
and rather stable arrangements through a number of non-covalent interactions
such as hydrogen bonds, ionic bonds, and van der Waals interactions
[G.M. Whitesides, J.P. Mathias,C.T. Seto. Science 254
,1312 (1991); J.M. Lehn. Science 260 ,1762 (1993)].
The collective interactions of these weak chemical bonds result in well-defined
and stable hierarchical macroscopic structures. The key elements in
molecular self-assembly are chemical complementarity and structural
compatibility.
Molecular self-assembly is ubiquitous in Nature, and silk is a well-known
example [J. Feltwell. The Story of Silk, Alan Sutton Publishing,
Phoenix Mill, UK (1990)]. The monomeric silk fibroin protein is approximately
1 mm, but a single silkworm can spin fibroins
into silk materials over 2 km in length, two billion times longer. These
building blocks are often at the nanometer scale. Similarly, individual
phospholipid molecules are approximately 2.5 nm in length, but they
can self-assemble into millimeter-sized lipid tubules with defined helical
twist, many million times larger.
While it is obvious that the noncovalent interactions mentioned above
play a role in self-assembly of molecular motifs, it is equally important
to understand the mechanism of self-assembly, in terms of atomic and
molecular sequences. In other words, how the primary structure of a
molecule or a sequence of molecules controls the secondary and tertiary
structures, which are the ultimate entities that dictate the functional
properties of a molecular system. Silk is an example, in which the structural
integrity and strength are consistent throughout the length of the fiber.
Chain folding is commonly observed during the crystallization of polymers.
Controlled, reversible self-assembly via chain folding and unfolding,
has also been demonstrated, using an appropriate sequence of foldable
and rigid moieties [A.P. Nowak, V. Breedveld, L. Pakstis, B. Ozbas,
D.J. Pine, D. Pochan, T.J. Deming. Nature 417 , 424 (2002);
A.D.Q. Li, W. Wang, L.-Q. Wang. Chem. Eur. J. 9, 4594
(2003); J.P. Schneider and D.J. Pochan. Polym. Mater. Sci. Eng.
90, 243 (2004)]. Comprehension of such factors is key to the
sophisticated use of liquid crystals, block copolymers, hydrogen-and
pi-bonded complexes, and the natural polymers.
The title of the symposium, "Controlling the self assembly
in macromolecular systems: From nature to chemistry to functional properties",
intentionally emphasizes control. While self-assembly is enabled by
the choice of motifs, controlling the extent of such self-assembly to
precisely tailor the functional properties is a challenge. For example,
can we command the same polymer system to self-assemble using, say,
only 60 % of the hydrogen-bondable groups for one application, and 100
% for another?
Thus, discussion related to controlling the nature and the extent of
self-assembly was the intent of this symposium. A broad range of topics
were covered, including block copolymer vesicles, micelles for drug
delivery, hydrogen-bonded supramolecular structures, and dendritic polymers,
with specific functional properties.
Symposia and special issues of this nature serve to judge the current
state of the science and enable charting the course for future directions.
It was my pleasure to organize this symposium, and I thank all the authors
for their contributions and cooperation in completing this project.
P.R. Sundararajan
Chair, Symposium on "Controlling the self assembly in macromolecular
systems:
From nature to chemistry to functional properties"