Chemistry Reference
In-Depth Information
Transfer to polymer is important with very reactive radicals like those in the
polymerizations of vinyl chloride, vinyl acetate, ethylene, and similar monomers
in which significant resonance stabilization is absent. It is also most significant in
high conversion reactions where the concentration of polymer in the system is rel-
atively high.
Chain transfer to polymer is not included in
Eq. (8-77)
. The occurrence of
reaction (8-88) does not change the number of monomers that have been polymer-
ized nor the
num
ber of polymer molecules over which they are distributed. It has
no effect on DP
n
for that r
easo
n. It will cause a change in the molecular weight
distribution, however, and DP
w
and higher averages are increased because the
polymers which are already large are more likely to suffer transfer reactions and
become yet bigger.
The existence of only a few long branches per molecule of radical-
polymerized polyethylene produces a more compact structure at given overall
molecular weight. Long branched macromolecules are less likely to become
mutually entangled and exhibit lower melt elasticities and better processing prop-
erties for some applications than equivalent molecular weight polyethylenes with-
out long branches, provided that the branches themselves are not long enough to
entangle.
Reactive radicals like that of polyethylene can also undergo self-branching by
a “backbiting” reaction in which a radical at the end of the polymer chain
obstructs a hydrogen from a methylene unit in the same chain:
H
H
X
C
CH
2
+
CH
2
C
CH
2
CH
2
X
+
CH
C
H
H
C
C
CH
2
CH
2
CH
2
CH
3
Transfer
C
H
CH
2
CH
2
CH
3
CH
2
CH
2
CH
2
CH
2
H
H
C
2
H
4
C
2
H
4
Transfer
CH
2
CH
2
C
CH
2
CH
3
CH
2
H
C
CH
2
3
3
CH
2
CH
2
CH
2
CH
2
H
Transfer
C
CH
2
CH
3
3
H
C
CH
2
C
CH
2
3
CH
2
CH
3
(8-89)
