Page 217 - Carrahers_Polymer_Chemistry,_Eighth_Edition

180 Carraher’s Polymer Chemistry

monomer that is nucleophilic and a second that is electrophilic in nature. The interaction of these
two comonomers gives a zwitterion that is responsible for both the initiation and the propagation.

+ −
Initiation MN + ME → MN–ME (5.61)
−
+
−
+
−
Propagation + MN–ME + MN–ME → MN–ME–MN–ME → → → →
+ MN−(−ME–MN−)−ME − (5.62)
If growth involves only addition and condensation reactions then an alternating copolymer is
formed. Sometimes a lateral reaction occurs where the zwitterion interacts with one of the mono-
mers giving a product that is a statistical copolymer.
5.13 ISOMERIZATION POLYMERIZATION
Isomerization polymerizations are polyaddition reactions where the propagating species rearranges
to energetically preferred structures before subsequent chain growth.

nA → –(–B–) – (5.63)
n

In 1962, Kennedy reported the first isomerization polymerization using 3-methyl-1-butene to
give a 1,1-dimethyl PP as below.
CH 3 H
+ C + R CH 3 R 1
R
CH 3 C + CH 3
R
CH 2 H CH 3 R (5.64)
H C 1
3
CH 3
H C
3
3-Methyl-1-butene Poly-1,1-dimethylpropylene
Isomerization polymerizations can be associated with coordination catalysts systems, ionic cata-
lysts systems, and free radical systems. The cationic isomerization polymerization of 4-methyl-1-
pentene is of interest because the product can be viewed as an alternating copolymer of ethylene and
isobutylene. This structure cannot be obtained by conventional approaches.
In the presence of certain ZNCs, an equilibrium exists between cis- and trans-1,3-pentadiene.
Here, the cis-1,4-polypentadiene is formed from trans-1,3-pentadiene or from a mixture of the cis
and trans isomers.
Such isomerizations are sometimes desired and sometimes are the cause of or explanation for
unwanted structures. In the cationic polymerization forming poly(1-butene), nine different struc-
tural units have been found. Classical 1,2-hydride and 1,2-methide shifts, hydride transfer, and
proton elimination account for these structures.
Unwanted branching of many polymers probably occurs through such isomerizations. PP, formed
using cationic polymerization, has methyl, ethyl, n-propyl, n-butyl, isopropyl, gem-dimethyl, isobu-
tyl, and t-butyl groups connected to the main chain.

5.14 PRECIPITATION POLYMERIZATION
Precipitation polymerization, also called slurry polymerization, is a variety of solution polymer-

ization where the monomer is soluble but the polymer precipitates as a fi ne flock. The formation
of olefin polymers via coordination polymerization occurs by a slurry process. Here the catalyst is

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