Engineering Chemistry S Datta

POLYMERS 441
Propagation:
Termination:
+
XCH2 CH CH2 CH X CH2 CH CH2CH Z
Y Y
Y Y
n
Z –
Anion
Polymer
If the medium is of dielectric strength, the overall effect leads to an increased
polymerization rate and also increased degree of polymerization, since better charge separation
of ion and counter ion occurs.
Anionic polymerization mechanism:
Initiation:
Propagation:
Termination:
(–)
Z (–) + CH 2
= CH
Y ⏐
(–)
⎯⎯→ ZCH 2
– CH
⏐
Y
Carbanion
ZCH2 CH + CH 2 = CH z CH2CHCH2CH
Y Y Y Y
(–)
CH CHCH CH
2 2
Y
n
(–)
Y
n
(–)
nCH = CHY
2
H +
CH2CHCH2CH:
Cation
ZCH CHCH CH
Y n Y
Y n Y
polymer
2 2 2
Polymerization of olefins with metallic sodium, sodium ethoxide takes place via this
mechanism.
(c) Co-ordination polymerization or Ziegler-Natta polymerization. It was observed by
Ziegler and Natta that in presence of a combination of transition metal halides (TiCl 4
, ZrBr 3
,