Chemistry Reference
In-Depth Information
The most widely used chain transfer agents are compounds with one relatively weak
bond like thiols, disulfides, CCl
4
,orCBr
4
, but even hydrogen and propane can be used
with the very reactive polyethylene radical. If
C
, as defined in
Eq. (8-76)
, is relatively
large, it can be expected that a chain transfer agent will be used up relatively early in a
free-radical polymerization. For example, if C
1, the transfer agent will be entirely
consumed when not all of the monomer has been polymerized in a batch (all ingredients
present in the reaction vessel before the process is started) reaction. In such cases it is
advisable to meter the transfer agent into the vessel during the polymerization.
5
8.8.4
Chain Transfer to Polymer
Transfer to polymer yields a radical on the polymer chain. Polymerization of
monomer from this site produces a polymer with a long branch:
H
H
X
C
CH
2
+
CH
2
C
CH
2
CH
2
X
+
CH
C
X
X
X
H
X
X
CH
2
C
+
CH
2
C
CH
2
C
CH
2
C
X
CH
2
CH
2
H
X
H
C
X
(8-88)
Transfer Constants to Chain Transfer Agents
a,b
(C
3
10
4
at 60
C)
Monomer
Table 8.2
Transfer agent
Styrene
Methyl methacrylate
Vinyl acetate
Benzene
0.03
0.04
1.5
Toluene
0.2
0.2
20
Isopropylbenzene
1
2
10
Isopropanol
3
1
45
Chloroform
0.5
1
15
Carbon tetrachloride
100
200
8,000
1-Butanethiol
200,000
480,000
a
These values are approximate averages of the data listed in the third edition of the Polymer
Handbook
[8]
. There is a considerable spread in reported results, probably because of effects
described in Section 8.13.
b
By convention, most free-radical kinetic data are measured at 60
C, but commercial polymerizations
are performed at higher temperatures, where feasible, in order to complete the reaction faster.
