Chemistry Reference
In-Depth Information
Structure 11-9 is that of an allyl carbanion which can react at carbon 1 or carbon 3.
When the metal is lithium, which is unsolvatedinhydrocarbonmedia,itisattachedto
the chain end at carbon 1, as shown in 11-10.
Cis
-1,4 chain units can result from a con-
certed attack of isoprene on the bond between carbon 1 and Li while the incoming
monomer is held in a
cis
conformationbycoordinationtothesmallLi
"
ion.
CH
3
CH
3
CH
3
CH
3
L
i
L
i
(11-25)
L
i
CH
3
CH
3
11-10
If isomerization can occur, then the configuration of end units is finally fixed
only when a new monomer is added because the only units that can isomerize are
terminal ones.
Trans
isoprenyl forms are more stable than
cis
terminal isomers in
hydrocarbon media, but the rate of isomerization from the initial
cis
form shown
above is not very fast and the
cis
configuration can be fixed in the polymer struc-
ture if the new monomers are added sufficiently quickly to the terminal unit. The
cis
-1,4 content in the polymer decreases as the number of active centers is
increased or the monomer concentration is lowered.
Butadiene reacts generally slower than isoprene, and the difference in propa-
gation rates between
cis
and
trans
terminal units is less. Thus, the
cis
configura-
tion is not as favored in 1,4-polybutadiene as it is in polyisoprene.
A variation of the sequential monomer addition technique described in
Section 11.2.6.1
is used to make styrene
styrene triblock thermoplastic
rubbers. Styrene is polymerized first, using butyl lithium initiator in a nonpolar
solvent. Then, a mixture of styrene and the diene is added to the living polystyryl
macroanion. The diene will polymerize first, because styrene anions initiate diene
polymerization much faster than the reverse process. After the diene monomer is
consumed, polystyrene forms the third block. The combination of Li
"
initiation
and a nonpolar solvent produces a high
cis
-1,4 content in the central polydiene
block, as required for thermoplastic elastomer behavior.
diene
11.3
Group Transfer Polymerization
Although group transfer polymerization does not involve ionic reactions, it is
reviewed in this chapter because it bears many practical similarities to anionic
polymerizations and is an alternative route to (meth-)acrylic polymers and block
