Page 566 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Reactions on Polymers 529

Several strategies are being developed that allow this long-range control, including the use of
secondary forces to “hold” in place monomers that subsequently will be polymerized “in place.”
In another approach, already molecular “architectured “ templates are employed to hold the poly-
mer, prepolymer, or monomers in the desired shape with subsequent reactions and interactions
enacted producing a material with a somewhat “robust” tertiary and quaternary structure. Some
of these “molecular molds” are being produced using nanotechnology.
Self-assembly tendencies are apparent in the simple crystallization of inorganics and organics.
Structure, size, and chemical tendencies (such as “like-liking-like” and “unlikes” repelling, secondary
and primary bonding tendencies) are all involved. Proteins “self-assemble” in a much more diverse
manner than so-called “simple” crystallization of common organics and inorganics. As noted above,
we are just beginning to understand the nuances involved with the self-assembling, formation of giant
macromolecules, including organizations such as those present in the cells of our bodies. We are begin-
ning to understand the major factors involved in making the cell membranes and are starting to mimic
these features to form synthetic biological-like membranes. We are using self-assembling concepts and
approaches to develop a large number of interesting and potentially useful macromolecular materials.
One of the applications of molecular self-assembling is the formation of ultrathin fi lms using both
synthetic and natural surfaces as two-dimensional templates. As noted above, the same chemical and
physical factors that we recognize in other areas are at work here. We will begin considering the forma-
tion of a simple bilayer membrane such as that present in natural cell membranes. Using the concept of
like-liking-like and unlikes rejecting one another, the orientation of molecules with two different polar
environments will vary depending upon the particular environment in which they are placed. For a
common soap molecule with hydrophilic and hydrophobic ends, the like ends will congregate together
and will reside either internally together or externally together. This is exactly the same concept as
given in most general chemistry texts when considering the formation of micelles in commercial deter-
gents. In the presence of water the hydrophilic ends will face outward, and in a nonpolar organic solvent
the polar ends will face inward. Researchers have extended these simple concepts to include specially
designed molecules that contain not only the heads and tails, but also spacers, conductors, and to vary

the flexibility of the various parts of the molecule, spacing and number of heads and tails, and so on.
An important concept in the creation of some of these structures is that a primary driving force is
the solute–solvent immiscibilities (energy; enthalphic). Thus, the magnitude of the cohesive energy
of the solute may be a secondary factor in determining these supermolecular or supramolecular
structures for such systems.
The self-assembling character of bilayer membranes is demonstrated by the formation of free-

standing cast films from aqueous dispersions of synthetic bilayer membranes. The tendencies for

association are sufficiently strong as to allow the addition of “guest” molecules (nanoparticles, pro-

teins, and various small molecules) to these films where the connective forces are secondary in
nature and not primary. Synthetic polymer chemists have made use of these self-assembling tenden-
cies to synthesize monolayer fi lms. Essentially, a monomer that contains both reactive groups and
hydrophobic and hydrophilic areas is “cast” onto an appropriate template that “self-assemblies” the
monomer, holding it for subsequent polymerization. Thus, a bilayer structure is formed by
O O Me
– – – – –
15
2
2
10
H 3 C−(−CH −) −O−C−CH−NH−C−(−CH −) −N−Me, Br
– – (16.17)
H C−(−CH −) −O−C−CH 2 Me
15
3
2
– –
O
The bis-acrylate monomer (Equation 16.18)
O O
(16.18)
–
–
–
–
H C=CH−C−O−(−CH −CH −O−) −O−C−CH=CH 2
14
2
2
2
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