Chemistry Reference
In-Depth Information
Termination can be by several different paths, all of which involve transfer reactions:
1.
Chain transfer to monomer:
H
H
M
C
CH
2
C
2
H
H
R
R
M
C
H+
CH
2
C
CH
2
C
2
n
+
CH
2
CH
R
R
R
R
(11-60)
where M is the transition metal;
2.
Chain transfer to organometallic compound:
H
H
H
M
Et + Et
2
Al
M
C
CH
2
C
CH
2
C
2
n
n+1
(11-61)
R
R
R
+ AlEt
3
3.
Chain transfer to active hydrogen transfer agent:
H
H
H
M
C
CH
2
C
+ TH
MT
+ CH
3
C
CH
2
C
2
n
R
R
R
(11-62)
where TH is the transfer agent containing a labile H.
4.
Internal hydrogen atom transfer:
H
H
H
(11-63)
M
C
CH
2
C
MH + CH
2
C
CH
2
C
2
n
n
R
R
R
R
Polymer molecular weight is often regulated in these reactions by using H
2
as a
transfer agent.
-hydrogen transfer (reaction 11-63) and transfer to monomer (reac-
tion 11-60) yield unsaturated polymer end-
gro
ups. In a process where either of
these mechanisms controls polymer growth,
M
n
of the product is readily measured
by infrared analysis of terminal C
β
C double bonds at a wave number of 908 cm
2
1
.
a
11.5.2
Kinetics of Ziegler
Natta Polymerizations
Generalizations are difficult to perceive in these systems because there is such
a wide variety of catalysts and reaction conditions. Industrial processes are usu-
ally heterogeneous and the rate of propagation is governed by the coordination
of
the monomer on the catalyst site. Active sites comprise only a small
