Chemistry Reference
In-Depth Information
r
1
5
ðQ
2
=Q
1
Þ
exp
½
2
e
2
ðe
2
2
e
1
Þ
(9-70)
and
2
r
1
r
2
5
exp
½
2
ð
e
1
2
e
2
Þ
(9-71)
e
1
) for a
given comonomer pair. In order to obtain numerical values of the four
Q
and
e
parameters from two reactivity ratios, two of the former are assigned arbitrary
values. Originally, the scheme was anchored by selecting styrene as the reference
monomer with
Q
Experimental reactivity ratios provide values for (
Q
1
/Q
2
) and (
e
2
2
1.0 and
e
0.8. Later modifications have broadened the
calculation base to include styrene copolymerization data of other well-researched
monomers
[26]
.
Extensive lists of
Qe
values are given in handbooks and texts on copolymer-
ization.
Table 9.2
contains a brief list. As a general rule monomers with electron-
rich double bonds have more negative
e
values and those which are extensively
resonance stabilized have higher
Q
numbers.
The
Q
5
52
e
numbers are obtained by fitting to experimental reactivity ratios,
many of which are not very accurate. It is not surprising, then, that this prediction
scheme is not quantitatively reliable. It is nevertheless very simple and convenient
to use and it is at least qualitatively reasonable. Monomers with very different
e
values will evidently react with higher absolute values of (
e
1
2
e
2
)
2
values (
Eq. 9-
71
). They will also be more likely to be influenced in their behavior by complex-
ing agents (
Section 9.12.4
). An alternative approach is the
patterns of reactivity
method
[27]
in which the rate constant for the reaction of a radical and any given
substrate is expressed in a three-parameter form. This approach has been used to
Table 9.2
Qe Values
[26]
Monomer
e
Q
Butadiene
0.50
1.70
2
Styrene
0.80
1.00
2
Vinyl acetate
0.88
0.03
2
Ethylene
0.05
0.01
Vinyl chloride
0.16
0.06
Vinylidene chloride
0.34
0.31
Methyl methacrylate
0.40
0.78
Methyl acrylate
0.64
0.45
Acrylic acid
0.88
0.83
Methacrylonitrile
0.68
0.86
Acrylonitrile
1.23
0.48
Methacrylamide
0.05
0.40
2
Maleic anhydride
3.69
0.86
