Chemistry Reference
In-Depth Information
measurements of small angle neutron scattering, NMR spectroscopy, size exclusion chromatography,
and ultraviolet-visible spectroscopy [
383
]. They reported observing structural changes of living chains
during the polymerization process. They also found that copolymerization process is divided into two
time regions, defined by regions I and II. In region I, the copolymerization of styrene and isoprene
monomers occurred and all I monomers were consumed at the end of region I, while in region II, pure
polystyrene block chains were formed. In the beginning of region II, living polymers with terminal
isoprenyl anion started to add styrene monomer rapidly and changed into chains with terminal styryl
anions. This was followed by a slow change from polyisoprene to polystyrene in the latter part of region
II. As a consequence of the polymerization of styrene monomers in the presence of polyisoprene and
polystyrene, they found an increase in
M
n
. In addition. they also found that the conversion rate of
styrene monomers at these conditions is slow compared with one in homopolymerization.
Anionic copolymerizations are very useful in forming block copolymers (see Chap.
9
for discus-
sion). Ziegler-Natta catalysts also form block copolymers, similarly to anionic initiators. Much work
on copolymerization with coordinated anionic initiators was done to develop ethylene propylene
copolymers. Ethylene is considerably more reactive in these copolymerizations. To form random
copolymers, soluble Ziegler-Natta catalysts are used. This is aided further by carefully controlling
the monomer feed [
384
].
The 1,2-disubstituted olefinic monomers will usually not homopolymerize with the Ziegler-Natta
catalysts. They can, however, be copolymerized with ethylene and some
M
w
/
-olefins [
384
]. Due to
poorer reactivity, the monomer feed must consist of higher ratios of the 1,2-disubstituted olefins than
of the other comonomers. Copolymers of
a
-2 butene with ethylene, where portions of the
macromolecules are crystalline, form with vanadium based catalysts. The products have alternating
structures, with the pendant methyl groups in erythrodiisotactic arrangements [
197
]. Similarly,
vanadium based catalysts yield alternating copolymers of ethylene and butadiene, where the butadi-
ene placement is predominantly
cis
-1,4 [
197
].
Giuronett and Mecking carried out copolymerizations of ethylene with 1-olefins in supercritical
carbon dioxide by electron poor nickel complexes [
385
]. The catalyst can be illustrated as follows:
trans
F
3
C
CF
3
N
CF
3
CF
3
O
Ni
N
O
F
3
C
F
3
C
They reported that the catalyst showed high activity in supercritical CO
2
yielding high molecular
weight copolymers of ethylene with 1-hexene and with norbornene [
385
].
Copolymerizations of aldehydes take place by both anionic and cationic mechanisms. An elastic
copolymer of formaldehyde and acetaldehyde forms with triisobutylaluminum. The rate of copoly-
merization is very rapid at
78
C. The reaction is complete within 30 min [
364
]. The product,
however, is crosslinked. Aldehydes also copolymerize with some vinyl monomers [
386
]. An acetone
block copolymer forms [
387
] with propylene when Ziegler-Natta catalysts are used at
78
C.
Copolymers of acetone with other olefins and with formaldehyde were also prepared [
388
,
389
].
Many initiators are effective in copolymerizations of aldehydes, ketones, and epoxies [
387
,
390
].
