Chemistry Reference
In-Depth Information
The first reaction can be either one of combination or of disproportionation. In a combination
reaction, two unpaired spin electrons, each on the terminal end of a different polymer-radical, unite to
form a covalent bond and a large polymer molecule. In disproportionation, on the other hand, two
polymer-radicals react and one abstracts an atom from other one. This results in formation of two
inactive polymer molecules. The two differ from each other in that one has a terminal saturated
structure and the other one has a terminal double bond. Usually, the atom that is transferred is hydrogen.
It was suggested [
111
] that a basic rule of thumb can be applied to determine which termination reaction
predominates in a typical homopolymerization. Thus, polymerizations of 1,1-disubstituted olefins are
likely to terminate by disproportionation because of steric effects. Polymerizations of other vinyl mono-
mer, however, favor terminations by combination unless they contain particularly labile atoms for
transferring. Higher activation energies are usually required for termination reactions by disproportion-
ation. This means that terminations by combination should predominate at lower temperatures.
For a polymer radical that simply grows by adding monomeric units and still possesses an active
center after the growth, the number of monomeric units (
) added to a radical center during the time
interval t, according to Tobita [
112
], conforms to the following Poisson distribution:
r
e
y
y
r
=r
pðrÞ¼
where
y
is the expected number of monomeric units added to a radical center, given by
y ¼ k
p
½
t
M
where
k
p
is the propagation rate coefficient and [M] is the monomer concentration. If the number of the
added monomeric units,
. For bimolecular termina-
tion reactions that are independent of chain length, the required time for bimolecular termination
between a particular radical pair is also given by the following most probable distribution [
112
]:
r
, is large enough, one can approximate that
r
y
p
t
ðy
t
Þ¼x
exp
ðxy
t
Þ
where
k
t
is the bimolecular termination rate coefficient. The imaginary time
for chain stoppage by bimolecular termination must be considered for all radical pairs that exist in the
reaction medium [
112
].
The third type of a termination reaction is chain transferring. Premature termination through
transferring results in a lower molecular weight polymer than can be expected from other termination
reactions. The product of chain transferring is an inert polymer molecule and, often, a new free radical
capable of new initiation. If, however, the new radical is not capable of starting the growth of a new
chain, then this is
degenerative chain transferring
. It is also referred to as a
first-order termination
reaction
. The molecules that accept the new radical sites (participate in chain transferring) can be any
of those present in the reaction medium. This includes solvents, monomer molecules, inactive
polymeric chains, and initiators.
The ease with which chain transferring takes place depends upon the bond strength between the labile
atoms that are abstracted and the rest of the molecule to which they are attached. For instance, chain
transferring in methyl methacrylate polymerization to the solvent occurs in the following order [
115
]:
x ¼ k
t
/(
k
p
[M]
nN
A
), and
benzene
<
toluene
<
ethyl benzene
<
cumene
The rate of a chain transferring reaction is,
R
tr
¼ k
tr
½
M
½
XA
