Environmental Engineering Reference
In-Depth Information
that can be easily extruded or molded. The modulus of the glassy polymer
decreases by several orders of magnitude during the heating process.
3.3 SYNTHESIS OF POLYMERS
Two broad classes of polymers are based on the two types of polymerization
reactions: addition polymerization and condensation polymerization of
monomers. Most high-volume thermoplastics such as PP, PE, PS, and PVC
are addition polymers. Those such as nylons or PC are examples of
condensation polymers.
3.3.1 Addition or Chain Growth Reaction
Most monomers are either stable gases or liquids; but when mixed with an
initiator compound, they yield active radicals or an ion. Organic peroxides
[R-O-O-R′], hydroperoxides [R-O-O-H], and azo compounds
[R-N=N-R′] are generally efficient initiators. The radicals react readily
with unsaturated monomers to yield a monomer radical. Addition or chain
growth polymerization of unsaturated monomers (with a double or a triple
bond in the structure) progresses by adding on repeat units sequentially at
the end of the growing radical. The initiation reaction is the formation of
free radical species
R
• and it propagates via sequential addition of more
monomer to create a growing chain (hence the name). The DP of the chain
increases with each addition of an unsaturated monomer M:
There are no by-products of this reaction as addition is a mere
rearrangement of atoms. Eventually, the macroradials terminate by a
combination of a pair of these long-chain radicals. The free radicals in
reaction mixture can react not only with a monomer but also with the
polymer chain. In this case, it abstracts a hydrogen from the chain creating
a chain radical. Polymerization initiates at this point too and gives rise to
branches, either short or long branches depending on how far removed
the branch point is from the growing chain end (Mishra and Yagc, 2009).
