Chemistry Reference
In-Depth Information
transfer agents), or by using a divinyl monomer in which the second vinyl group
is less reactive than those in the monovinyl comonomer.
Note that the foregoing analysis assumes that no cross-links are wasted. The
calculations thus function best at low diene concentrations. In practice, gelation
will be observed at higher values of conversion than predicted, and the error will
increase with increasing diene concentration and hence with increasing wastage
by cross-linking different parts of the same molecule.
9.10
Reactivities of Radicals and Monomers
The reactivities of various chemical species are usually assessed by comparing
rate constants for selected reactions. This is not a convenient procedure in free-
radical polymerizations, however, because absolute rate constant measurements
are rare. More convenient and plentiful parameters in free-radical systems are
functions of more than one rate constant as in the
k
1
=
2
t
ð
k
p
=
Þ
factor, reactivity
ratios, and chain transfer constants.
The relative reactivities of various monomers toward a given radical can be
computed from the reciprocals of the reactivity ratios. Thus, if the reference
monomer is M
1
its reactivity ratio with each comonomer is
r
1
5
k
11
/k
12
, with
k
11
constant for the particular series. Relative reactivities of test monomers toward
radical M
:
1
are given by comparing 1/
r
1
5
k
12
/k
11
. Similar considerations apply to
transfer constants, such as
C
S
5
k
tr
,
S
/k
p
, for different transfer agents in the homo-
polymerization of a particular monomer.
The relative reactivities of different radicals toward the same monomer in
copolymerization or toward the same chain transfer agent in homopolymerization
can be assessed by comparing values of
k
12
5
k
11
/r
1
and
k
tr,S
5
k
p
C
S
5
k
11
C
S
,
respectively.
When such comparisons are made it becomes clear that the reactivities of
radicals, monomers, or transfer agents depend on the particular reaction being
considered. It is not possible to conclude, for example, that poly(vinyl acetate)
radical will always react
x
times more rapidly than polystyrene radical in addition
reactions or
y
times as rapidly in the atom abstraction reactions involved in chain
transfer. Similarly the relative order of efficiency of chain transfer agents will not
be the same for all radical polymerizations. This is because resonance, steric, and
polar influences all come into play and their effects can depend on the particular
species involved in a reaction.
9.10.1
Resonance Effects
As a general rule, the controlling factor in reactivity of a vinyl monomer toward
radical homopolymerization appears to be the stability of the radical formed by
addition of the monomer to the initial radical.
