Chemistry Reference
In-Depth Information
In the above process, the formation of crystalline domains involves consecutive insertions from
one of the lateral coordination sites of the catalyst so as to give rise to isotactic sequences, whereas
consecutive insertions at the other site (2) give rise to atactic amorphous sequences. Interconversion
between these two states must occur within the lifetime of a given polymer chain in order to generate
a physically cross-linked network and is believed to occur via occasional isomerizations of the
polymer chains (i.e., interconversion at the metal center). Preparation of an oscillating catalyst that
yields an elastomeric polypropylene was also reported by others [
441
].
9.7.7 Simultaneous Use of Free Radical and Ionic Chain-Growth
Polymerizations
This technique allows formation of many different types of block copolymers [
437
]. Lithium metal
can be used to initiate polymerizations in solvents of varying polarity. Monomers, like styrene,
-
methylstyrene, methyl methacrylate, butyl methacrylate, 2-vinylpyridine, 4-vinyl pyridine, acryloni-
trile, and methyl acrylate, can be used. The mechanism of initiation depends upon formation of ion-
radicals through reactions of lithium with the double bonds:
a
CH
2
Li
Li
+
Propagation reactions proceed from both active sites, the radical and the carbanion. When two
different monomers are present, free-radical propagation favors formation of copolymers, while
propagation at the other end favors formation of homopolymers. There is a tendency, therefore, to
form AB—B type block copolymers.
Several publications appeared recently that describe use of controlled/“living” radical
polymerizations to form block copolymers. Thus, Jerome et al. [
435
] described formation of block
copolymers by using an initiator capable of initiating simultaneously dual living polymerizations:
N
n
O
HO
O
O
m
O
N
H
O
O
O
m
n
