Chemistry Reference
In-Depth Information
H
H
H
C
CH
2
C
CH
2
C
+
CH
2
CH
2
C
CH
2
CH
2
C
X
X
X
X
X
X
(8-82)
Reaction (8-80), on the other hand, does not result in branched polymers
because the end C atoms in the unsaturated polymer end are both substituted and
unreactive toward vinyl polymerization (cf.
Section 8.2
).
Chain transfer reactions of monomers with
methyl groups are likely to pro-
ceed predominantly by reaction (8-79) because the new radicals produced are rel-
atively stable:
α
CH
3
CH
3
CH
3
CH
2
C
(8-83)
CH
2
+
CH
2
C
CH
2
C
H +
CH
2
C
X
X
X
X
Chain transfer can also occur with side groups of some monomers. With vinyl
acetate, for example,
H
H
H
C
CH
2
+
CH
2
C
CH
2
CH
2
+
CH
2
C
O
O
O
O
(8-84)
CO
CO
CO
CO
CH
2
CH
3
CH
3
CH
3
This reaction also eventually produces branched macromolecules.
Monomer chain transfer constants are generally less than 10
2
4
. Reaction
(8-79) involves breaking the strong vinyl C
H bond and the products of reaction
(8-80) are not appreciably more stable than the reactants.
Transfer to monomer sets an upper limit to the molecular weight of the polymer
that can be produced. This is not normally a problem, however, except with allylic
mono
mer
s like propylene (
Section 8.8.5
).
C
M
is about 10
2
4
for styrene. The maxi-
mum DP
n
of polystyrene that can be achieved is then 10
4
[from
E
q.
(8-77)
assum-
ing very low [I] and vanishingly small
R
p
]. This corresponds to an
M
n
of
about 10
6
.
Commercial molding and extrusion grades of polystyrenes have
M
n
of about
1
a
10
5
, which are considerably lower than this limiting value.
C
M
can be measured from experiments based on
Eq. (8-77)
which reduces to
1
DP
n
5
10
5
3
2
2.5
3
k
tc
R
p
k
p
½
2
k
td
R
p
k
p
½
C
I
½
I
C
M
1
2
1
2
1
(8-85)
½
M
M
M
in the absence of solvent or other chain transfer agents. However,
