Page 186 - Carrahers_Polymer_Chemistry,_Eighth_Edition

5 Ionic Chain-Reaction and

Complex Coordination

Polymerization

In contrast to the relatively slow step-reaction polymerizations discussed in Chapter 4, chain polym-
erizations are usually rapid, and the initiation species continues to propagate until termination. Thus,
in the extreme case, a single initiation species could produce one high molecular chain, leaving all of
the other monomer molecules unchanged. In any case, the concentration of monomer, which is often
a substituted vinyl compound, decreases continuously throughout the reaction. In contrast to stepwise
polymerization, the first species produced in chain polymerizations is a high molecular polymer.

A kinetic chain reaction usually consists of at least three steps, namely (1) initiation, (2) propa-
gation, and (3) termination. The initiator may be an anion, a cation, a free radical, or a coordination
catalyst. While coordination catalysts are the most important commercially, the ionic initiators will

be discussed first in an attempt to simplify the discussion of chain-reaction polymerization.
In almost all of the polymerizations described in this chapter, there is a sensitive and critical balance
between the activity of the catalyst and polymerization. For instance, if the catalyst is too active, it may
bind at unwanted sites, including the solvent. If the catalyst complex is not sufficiently active, then ready

initiation does not occur. The choice of the solvent is also important. Some solvents will react with the cat-
alysts binding it rather than allowing the catalyst to initiate the desired polymerization. Others may “hold”
the catalyst complex together rather than allowing the catalyst to initiate polymerization. Still others may
not allow the catalyst complex to form. As in much of science, the precise ingredients and conditions were
developed through a combination of intuition, science, art, and research. This process continues.
While many vinyl monomers undergo free radical polymerization (Chapter 6), a smaller num-
ber undergo ionic polymerization. Cationic polymerizations require monomers that have electron
releasing groups such as an alkoxy, phenyl, or vinyl group. Anionic polymerization occurs with
monomers containing electron-withdrawing groups such as carboxyl, nitrile, or halide. This selec-
tivity is due to the strict requirements for stabilization of anionic and cationic species.
Compared to free radical polymerizations, the kinetics of ionic polymerizations are not as well

defined. Reactions can use heterogeneous initiators, and they are usually quite sensitive to the pres-
ence of impurities. Thus, kinetic studies are difficult and the results sensitive to the particular reac-

tion conditions. Further, the rates of polymer formation are more rapid.
Cationic and anionic polymerizations are similar. Both involve the formation and propagation
of ionic species. While high energy, low stability, ions would be expected to react with most double
bonds, ionic species that are stable enough to propagate are difficult to form and are easily destroyed.

The “energenic-window” that allows the formation of such charged species that promote polymer
formation is narrow. While polar solvents might be desirable to solvate the ions, and hence help sta-
bilize them, they often cannot be used. Some polar solvents, such as water and alcohols, react with
and destroy most ionic initiators. Other polar solvents, such as ketones, prevent initiation because of
the formation of stable complexes with the initiators. Ionic polymerizations are therefore conducted
in low or moderately polar solvents, such as hexane and ethylene dichloride.
By bulk, almost all vinyl polymers are made by four processes (Table 5.1)—free radical (about
50%), complex coordinate (about 20%), anionic (10%–15%), and cationic (8%–12%). Three of these
techniques are covered in this chapter.

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