Polymer Basics - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 98

can be formed by the reaction of boron trifluoride catalyst with adventitious
water. Isobutylene is protonated to give the tertiary carbocation. This isobutyl
carbocation can then react with another molecule of isobutylene to form a
new carbocation. This continues and the molecular weight increases. This
will continue until either there is no more isobutylene present or until the
polymerization is terminated. Termination occurs by loss of a proton to form
an alkene. The counterion of the cation can act as a base and promote the
termination reaction.

BF 3 + H 2 O H+BF 3 OH

H

x

BF 3 OH

H

H
H

C Termination

x

x

In a truly living polymerization, there is no termination. However, if
the termination step is reversible and if the rate of the reversible reaction
is greater than the rate of the polymerization propagation, then we have a
quasi-living polymerization [20]. An ideal living polymerization is kinet-
ically indistinguishable from a quasi-living polymerization. In the case of
cationic polymerization, it is actually quasi-living. When the original living
polymerization of isobutylene was quenched with a nucleophile such as an
amine or alcohol, the polymer did not exhibit alkene ends but rather tertiary
alkyl chlorides.

O O

CH 3

CH 3 OH

Cl

BCl 3 x x

This can be explained if the termination is reversible by the following
equilibrium.