_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

The first three periodic groups
Here the driving force is provided by the formation of a carbanion adjacent to a
stabilizing group (R 3 Si- or dithioacetal). Organolithium reagents, however, add
to conjugated dienes and hence initiate their polymerization. In hydrocarbon
solvents n-butyl lithium and butadiene result in mostly 1, 4-addition. The
polymer contains a mixture of cis and trans stereochemistry about the double
bonds.
Buli + H2C=CHCH=CH2
initiat ion
C4H6 ~
-p-r-op_a_g-at-io_n_ Bu(CH2CH=CHCH2)n Li - """"-CH2 CH2 0H
35% cis 1 14-,54% trons 1,4- ond
11% 1,2- polybutodiene lntroduction of terminal functional groups
In the presence of polar ligands such as TMEDA 1, 2-addition is favoured, giving
the following type of polymer.
Bu{CH 2 CH(CH=CH 2 ) }.CH 2 CH(CH=CH 2 )Li+
The structures of the intermediates have been studied by n.m.r. but no clear
detailed mechanism has yet emerged. These intermediates have 'living' characteristics.
This means that they are capable offurther polymerization on addition of
more monomer at any time, provided of course that they are protected from
oxygen and moisture. If the rate of initiation is fast relative to the rate of
propagation, the resulting polymer has a narrow molecular weight range which
is controlled by the mole ratio of initiator to monomer. The biggest use of
alkyllithium catalysts is in the production of styrene-butadiene rubber for tyre
treads. While simple olefins like ethene are not normally polymerized
(BuLi · TMEDA in hydrocarbons does polymerize ethene ), reactive o nes like
styrene (phenylethene) are. The 'living' character of the intermediates is
particularly suitable for the formation of block copolymers by alternate separate
Table 3.4. World production figures for rubber in 1977
Emulsion polymer (styrene butadiene rubber)
-free radical initiation.
Polyisoprene (natural rubber)
Polybutadiene (transition metal catalyst)
Polyisoprene (transition metal catalyst)
Polybutadiene and styrene butadiene rubber
-alkyl-lithium initiation
Tonnes( x 10")
4.8
3.6
1.1
0.6
0.8
48