Engineering Chemistry S Datta

442 ENGINEERING CHEMISTRY
and halides of V, Zr, Cr, Mo etc.) along with organometallic compounds (triethyl/trimethyl
aluminium) polymerization of olefins leads to stereospecific polymerization.
Mechanism of Ziegler-Natta polymerization:
Initiation:
Cat R′ + CH 2
= CHR ⎯⎯→ Cat CH 2
CH(R)R′
Propagation:
Cat CH CHR + CH = CHR Cat CH CHCH CHR
2 n
2 2 2
R R R n
Termination:
Cat CH CHCH CHR +HX Cat X+
CH CHCH CHR
2
2 3 2
R
R n R R n
polymer
By earlier free radical polymerization of ethylene, propylene yielded polymers of low
density having highly branched chain polymer of low crystallinity and mechanically weaker
structure like LDPE (low density polyethylene). Ziegler-Natta polymerization yielded
stereospecific structure of high mechanical strength, unbranched, high melting and highly
crystalline polymer like HDPE (high density polyethylene).
Stereochemistry of polymers
(i) Isotactic polymers have all the groups in one side of the polymeric backbone and the
monomers are joined in a regular head to tail arrangement.
H
C
Y
C
H
C
Y
C
H
C
Y
C
H
H
H
H
H
H
(ii) Syndiotactic polymers have similar head to tail arrangements but here Y groups
appear on opposite sides of polymer backbone alternately.
H
C
H
C
H
C
Y
C
H
C
H
C
H
C
H
Y
H
H
H
Y
H
(iii) Atactic polymers have Y groups arranged randomly along the polymeric backbone
and the material is soft, elastic, rubbery.
H
C
H
C
H
C
Y
C
H
C
Y
C
H
C
H
Y
H
H
H
H
H