Chemistry Reference
In-Depth Information
where,
k
tr
is the chain transferring constant in a reaction:
K
tr
!
M
þ
X
A
M
X
þ
A
Examples of molecules that have particularly labile atoms and contribute readily to chain-
transferring are mercaptans and halogen compounds, such as chloroform, carbon tetrachloride, etc.
A polymer that was prematurely terminated in its growth by chain transferring may be a
telomer
.
In most cases of telomer formation, the newly formed radical and the monomer radical are active
enough to initiate new chain growth. Thus, the life of the kinetic chain is maintained.
An illustration of a
telomerization reaction
can be free-radical polymerization of ethylene in the
presence of chloroacetyl chloride:
Initiation ROOR
2RO
O
O
2RO
+
ROCl
+
Cl
CH
2
Cl
Cl
CH
2
O
O
Propagation
+
CH
2
Cl
Cl
O
n
Cl
n
O
O
O
O
+
Cl
Termination
CH
2
Cl
Cl
n
Cl
Cl
+
Cl
CH
2
n
Chain transferring is affected by temperature but not by changes in the viscosity of the reaction
medium [
115
]. When a transfer takes place to a monomer, it is independent of the polymerization rate
[
116
,
117
]. When, however, transfer takes place to the initiator, the rate increases rapidly [
118
].
A chain transferring reaction to a monomer can be illustrated as follows:
X
X
X
hydrogen
X
+
1.
+
abstraction
X
X
X
X
hydrogen
+
+
2.
donation
A transfer reaction can also occur from the terminal group of the polymer-radical to a location on
the polymeric backbone. This is known as
backbiting
:
