Chemistry Reference
In-Depth Information
Grafting reactions to polybutadiene, however, proceed via chain transferring from the growing chain
radical to the backbone [
299
]. Nevertheless, strong evidence also shows that the initiator radicals can
interact directly with polymeric backbones [
299
,
300
].
When graft copolymers of polystyrene to natural rubber form, the chain length of the attached
branches equals to the chain lengths of the unattached polystyrene homopolymer that forms simulta-
neously [
301
]. This led to the following conclusions [
303
]:
1. As the concentration of rubber increases, the length of the grafted branches diminishes, while their
number remains the same.
2. When the concentration of the initiator increases, the length of the branches diminishes, but the
number of branches increases.
3. When the concentration of monomers increases, the length of the branches also increases, but their
number diminishes.
4. When the polymerization time increases, the length of the branches remains the same, but their
number increases.
9.6.3 Preparation of Graft Copolymers with the Aid of Macromonomers
The chain-transferring methods for preparing graft copolymers suffer from the disadvantages of low
efficiency and contamination by homopolymers. The efficiency in forming graft copolymers, how-
ever, increases with the use of
A macromonomer is a macromolecular monomer, an
oligomer, or a polymer with a polymerizable end group. When macromonomers are copolymerized
with other monomers, comb-shaped polymers form [
302
,
303
]. The copolymerizations can be free
radical or ionic in mechanism. Some examples of macromonomers are presented in Table
9.1
.
A preparation of graft copolymers with the aid of macromonomers can be illustrated as follows [
304
]:
macromonomers.
O
O
R'
R'
m
S
n
+
O
O
R
H
o
Y
OH
X
R'
initiator
n
m
Y
O
O
HO
O
O
R
R'
S
H
o
X
Many variations on the above technique are possible.
