Chemistry Reference
In-Depth Information
The new free-radical site on the polymer backbone starts chain growth that results in formation of
a branch. The same reaction can take place between a polymer-radical and a location on another
polymer chain. In either case, fresh chain growth results in formation of a branch.
Whether chain transferring can take place to an initiator depends upon the initiator's chemical
structure. It was believed in the past that chain transferring to
a
0
-azobisisobutyronitrile does not
occur. Later it was shown that chain transferring to this initiator does occur as well, at least in the
polymerizations of methyl methacrylate [
118
,
119
].
The amount of chain transferring that takes place to monomers is usually low because the reaction
requires breaking strong carbon-hydrogen bonds. Monomers, however, such as vinyl chloride and
vinyl acetate have fairly large chain transferring constants. In the case of vinyl acetate, this is
attributed to the presence of an acetoxy methyl group. This explanation, however, cannot be used
for vinyl chloride.
The chain transferring constants, are usually defined as:
a
,
C
M
¼ k
tr
;
M
=k
p
for monomers
C
S
¼ k
tr
;
S
=k
p
for solvents
C
I
¼ k
tr
;
I
=k
P
for initiators,
The values can be found in handbooks and other places in the literature. Presence of chain
transferring agents in a polymerization reaction requires redefining the degree of polymerization to
include the chain termination terms. The number average degree of polymerization has to be written
as follows:
R
p
DP
¼
ðR
t
=
2
Þþk
tr
;
M
½
M
½
M
þk
tr
;
s
½
M
½
S
þk
tr
;
I
½
M
½
I
It can also be expressed in terms of the chain transferring constants as follows:
1
=
DP
¼
2
R
P
=R
I
þC
M
þC
S
½
S
=½
M
þC
I
½
I
=½
M
This can also be written in still another form:
¼ k
t
R
P
=k
P
2
2
þC
I
k
t
R
P
2
=k
P
2
3
1
=
DP
½
M
þC
M
þC
S
½
S
=½
M
fk
d
½
M
When a polymerization reaction is conducted in a concentrated solution, or in complete absence of a
solvent, the viscosity of the medium increases with time, (unless the polymer precipitates out). This
impedes all steps in the polymerization process, particularly the diffusions of large polymer-radicals
[
54
]. The decreased mobility of the polymer-radicals affects the termination process. It appears that
this is common to many, though not all, free-radical polymerizations. All molecular processes in the
termination reactions are not fully understood, particularly at high conversions [
119
] This is a complex
process that consists of three definable steps. These can be pictured as follows. First, two polymer
radicals migrate together by means of translational diffusion. Second, the radical sites reorient toward
each other by segmental diffusion. Third, the radicals overcome the small chemical activation barriers
and react. The termination reaction is, therefore, diffusion controlled. At low concentrations, this will
be segmental diffusion while at medium or high concentrations it will be translational diffusion.
Present theories of terminations suggest that at intermediate conversions, terminations are domi-
nated by interactions between short chains formed by transfer and entangled long chains [
121
].
