Page 565 - Carrahers_Polymer_Chemistry,_Eighth_Edition

528 Carraher’s Polymer Chemistry

Block copolymers are also produced using such macromers. Again, the various units of the polymer
can be designed to act as needed with hard/soft or hydrophilic/hydrophobic or other combinations. Also,
these macromers generally have two active functional groups that allow polymerization to occur.
Telechelic polymers can also be used as cross-linkers between already formed polymers because
both ends are active. These functional groups could be either two vinyl ends or two Lewis acids or
bases, such as two hydroxyl or amine groups. Interpenetrating polymer networks (IPNs) and related
structures such as dendrites and stars can also be formed using macromers.
The use of placing specific end groups onto polymers and oligomers is widespread and will be

illustrated here for dimethylsiloxanes, the most widely used of the siloxanes. These end groups are
given various names depending on their intended use. Some names for these end groups are cap-

ping agents, blocking agents, chain-length modifiers, and coupling agents. Typical siloxane end
groups include trimethylsiloxyls, acyloxyls, amines, oximes, alcohols, and alkoxyls. These reactive
end groups are reacted forming a wide variety of useful materials. Most silicone room tempera-
ture vulcanizing (RTV) adhesives, sealants, and caulks, are moisture curing, that is, they contain
a hydroxyl-capped siloxyls that are reacted with acyloxyls, amines, oximes, or alkoxyls moisture-
sensitive compounds.

16.6 SUPRAMOLECULES AND SELF-ASSEMBLY
The terms self-assembly, self-organization, and self-synthesis are closely related and sometimes used
interchangeably. Self-assembly involves the aggregation of molecules, including macromolecules,
into thermodynamically stable structures that are held together often using secondary bonding,
including hydrogen bonding, Van der Waal’s and electrostatic forces, pi–pi interactions and hydro-
phobic and hydrophilic interactions. The term self-organization is used for situations where the sec-

ondary bonding interactions are more specific and directional, giving a higher degree of order to the
self-assembled molecules. Finally, the term self-synthesis includes self-assembly and self-organization,
but also includes situations where self-replication and template-type synthesis occurs.
Self-assembly is the spontaneous organization of molecules into stable, well-defi ned structures
with the driving forces being noncovalent associations. The final structure is normally near or at the

thermodynamic equilibrium arrangement allowing it to readily spontaneously form. Such formations
can be done under conditions where defects are either minimized or eliminated. In nature, self-
assembly is common such as the folding of proteins, formation of the DNA double helix, and so on.
Self-assembled monolayers (SAMs) are the most widely studied nonnatural self-assembly sys-
tems. They are generally spontaneously formed from chemisorption and self-organization of organic
molecules onto appropriate surfaces.
Natural polymers utilize a combination of primary and secondary forces and bond angles and dis-
tances to form polymers with both long-range (multimacromolecular) and short-range structures with
both structures essential for the “proper” functioning of the macromolecular structure. While most
synthetic polymer chemists have focused on what is referred to as primary and secondary structures
(short-order structure control), work is just beginning on developing the appropriate structure control
to allow tertiary and quaternary structural control (long-range control). While the “backbones” of these
structures are held together with primary bonds, the secondary, tertiary, and quaternary structures are
generally “driven” by secondary forces with the resulting tertiary and quaternary structures fixed in place

through a combination of these secondary forces and small amounts of ionic and covalent cross-linking.
As noted above, self-assembled structures normally are the result of what are often referred to as
weaker force interactions that include Van der Waal’s forces, pi–pi interactions, capillary interac-
tions, and so on. Structures that successfully form ordered structures often take advantage of these
short-range attractive forces but also of longer-range repulsive forces. Almost all of these structures
have some thermodynamic stability with the overall structure more stable than the unassembled
parts. These somewhat overall weak-energy driving forces make the structures especially suscepti-
ble to small variations in the conditions leading to ready reversibility.

9/14/2010 3:43:06 PM
K10478.indb 528
K10478.indb 528 9/14/2010 3:43:06 PM