Chemistry Reference
In-Depth Information
The products are cationic ionomers. Anionic ionomers form very similarly through coupling of
chains with bifunctional anionic “chain lengtheners” [
435
]:
H
2
N
NH
2
NO
ON
+
O
O
O
O
O
O
NNN
N
n
H
H
H
H
Because of interactions between the chains, the polyurethane ionomers are similar in properties to
cross-linked elastomers. In solution, they are strongly associated.
9.7.6 Block Copolymers of Poly(
a
-Olefin)s
These block copolymers form readily when appropriate Ziegler-Natta catalysts are used [
436
]. This is
discussed in Chaps.
4
and
6
.
In addition, there is a special technique for preparations of such block
copolymers. At the outset of the reaction, only one monomer is used in the feed. A typical catalyst
might be
-TiCl
3
/(C
2
H
5
)
3
Al. After the first monomer has been bubbled in for a short period, perhaps
5 min, the addition is stopped and the unreacted monomer removed by evacuation or by flushing. The
second monomer is then introduced and may also be bubbled in for the same period. The addition of
the second monomer may then be stopped, the second monomer evacuated, and the whole thing might
be repeated. If equal length of each block is desired, the addition times of each monomer may be
varied to adjust for different rates of polymerization [
436
].
Kuhlman and Klosin [
436
] reported forming multiblock copolymers of polyethylene and tuning
block composition by catalyst selection. Chain shuttling, the fast exchange of growing polymer
chains among catalyst centers mediated by a chain shuttling agent (CSA), has enabled the production
of ethylene-based olefin block copolymers. Diethyl zinc was used as a chain shuffling agent. Thus,
copolymerization of ethylene and
a
-olefin by certain catalyst pairs in the pretense of a CSA produces
blocks that are alternately highly or lightly branched. Chain shutling polymerizations are bet
conducted in continuous reactors.
A catalyst system derived from titanocene complex and methyl-aluminoxane (see Chap.
5
) has
been used to polymerize propylene and, depending on polymerization conditions, produce a block
copolymer of crystalline and amorphous, elastomeric polypropylene [
440
]:
a
Site 2
Ti
Ti
Site 1
↓
↓
isotactic
sequences
atactic
sequences
