Chain-Reaction A Polymerization (Addition)
The monomers normally employed in this type of polymerization
contain a carhon-carbon double bond that can
oatticinate in a chain reaction. As in the chain reactions
studied in organic chemistry, e.g., the free-radical halogenation
of alkanes. the mechanism of the ~olvmerizationco nsists
of three distinct steps. In the hitiition Step an initiator
molecule(s) is thermally decomposed or allowed to undergo
a chemical reaction to generate an "active species." This
"active species," which can he a free radical, a cation, an anion,
or a coor~clinationc omplex, then'initiatri the polymerization
by adding to the monumrr's carhn-carbon duuble bond. The
reaction occurs in such a manner that a new free radical, cation,
anion, or complex is generated. The initial monomer hecomes
the first repeat unit in the incipient polymer chain. In
the Propagation Step, the newly generated "active species"
adds to another monomer in the same manner as in the Initiation
Step. 'rhis procedure is repentrd ovw and over again
until thr find step of the vrw:rss, Termination, occurs. In this
step, the growing chain'terminates through reaction with
another arowina chain, bv reaction with another species in the
po~ymerEationkixtureb, r by the spontaneous decomposition
of the active site. Under certain conditions, anionic and
coordination polymerizations can be carried out without the
Termination Step to generate so-called "living" polymers.
The following are several general characteristics of this type
of polymerization: