Chemistry Reference
In-Depth Information
3. There is no termination step, and the end groups of the polymers are reactive throughout the
process of polymerization.
4. The same reaction mechanism functions throughout the process of polymerization.
The important features of chain-growth polymerizations are:
1. Chain-growth takes place by repeated additions of monomers to the growing chains at the reactive sites.
2. The monomer is consumed slowly and is present throughout the process of polymerization.
3. There are two distinct mechanisms during polymer formations. These are initiation and propagation.
4. In the majority of cases, there is also a termination step.
When the polymerization reaction takes place in three dimensions, after it has progressed to a
certain point, gelation occurs. This well-defined change during polymerization is known as the
gel
point
. At this point the reaction mixture changes from a viscous liquid to an elastic gel.
Before gelation, the polymer is soluble and fusible. After it, however, it is neither soluble nor
fusible. This is a result of restraining effects of three-dimensional space networks. Another classifi-
cation of polymers is also possible. It is based on whether the material can form crosslinked or gelled
networks. The polymers that eventually reach gelation are called
thermosetting
. Such polymers are
also called
crosslinkable
polymers. Once past gelation, raising the temperature will no longer attain
plasticity as the molecules can no longer move past each other. For the same reason they can no
longer be dissolved in any solvent.
Polymers that never gel or become crosslinked are called
thermoplastic
. Such polymers can
always be reflowed upon application of heat. They can also be dissolved again in appropriate
solvents.
The wiggly lines in the above illustration imply that the polymer extends further in their
directions. The above illustration is one of a thermoset polymer that is formed by the step-growth
mechanism. It is also possible to form crosslinked polymers by the chain-growth mechanism. This
requires presence in the polymerization mixture of a comonomer that possesses multiple function-
ality. Copolymerization of styrene with a comonomer like divinyl benzene can serve as an
example:
k
+
o
An
oligomer
is a very low molecular weight polymer. It consists of only a small number of mers.
The definition of a
telomer
is that of a chain-growth polymer that is composed of molecules with end
groups consisting of different species from the monomer units. Telomers can form by either free
radical or by ionic chain-growth polymerization mechanism.
