Chemistry Reference
In-Depth Information
Table 3.8
Some reactivity ratios [
132
,
136
]
Monomer 1
Monomer 2
r
1
r
2
Styrene
Butadiene
0.78
1.39
Styrene
Methyl methacrylate
0.52
0.46
Styrene
Vinyl acetate
55.0
0.01
Vinyl acetate
Vinyl chloride
0.23
1.68
Methyl acrylate
Vinyl chloride
9.00
0.083
considered, in terms of reactivity, as a poly(methyl methacrylate) radical. This assumption although
not always adequate [
52
] can be used to predict satisfactorily the behavior of many mixtures of
monomers. Based on this assumption, the copolymerization of a pair of monomers involves four
distinct growth reactions and two types of polymer radicals.
3.6.1 Reactivity Ratios
In a reaction of two monomers, designated as M
1
andM
2
, four distinct reactions can be written as follows:
.
k
11
1.
M
1
þ
M
1
!
M
1
M
1
.
k
12
2.
M
1
þ
M
2
!
M
1
M
2
.
k
21
3.
M
1
þ
M
2
!
M
1
M
2
.
k
22
4.
M
2
þ
M
2
!
M
2
M
2
The ratios of
k
11
/
k
12
and
k
22
/
k
21
are called
monomer reactivity ratios
. They can be written as follows:
r
1
¼ k
11
=k
12
r
2
¼ k
22
=k
21
The relationship can be express in terms of the ratio of the monomers, [M
1
]/[M
2
] that end up in the
formed polymer,
R
p
:
R
p
¼ R
m
ðr
12
R
m
þ
1
Þ=ðr
21
þR
m
Þ
where,
R
m
is equal to [M
1
]/[M
2
]. Table
3.8
illustrates a few typical reactivity ratios taken from the
literature. Many more can be found [
128
].
These reactivity ratios represent the relative rates of reactions of polymer radicals with their own
monomers vs. that with the comonomers. When
r
1
>
1, the radical ~M
1
is reacting with monomer
M
1
faster than it is with the comonomer
1, the opposite is true.
Based on the
r values
, the composition of the copolymers can be calculated from a
copolymerization
equation
[
52
] shown below:
M
2
. On the other hand, when
r
1
<
d
½
M
1
M
2
¼
½
M
1
r
1
½
M
1
þ½
M
2
d
½
½
M
2
r
2
½
M
2
þ½
M
1
