Chemistry Reference
In-Depth Information
Copolymerization of a bifunctional monomer with a polyfunctional comono-
mer produces branches that can continue to grow by addition of more monomer.
An example is the use of divinylbenzene (1-62) in the butyl lithium
initiated
polymerization of butadiene (
Section 12.2
). The diene has a functionality of 2
under these conditions whereas the functionality of 1-62 is 4. The resulting
HCCH
2
HCCH
2
1-62
elastomeric macromolecule contains segments with structure 1-63. Long branches
such as these can interconnect and form cross-linked network structures depend-
ing on the concentration of polyfunctional comonomer and the fractions of total
monomers which have been polymerized. The reaction conditions under which
this undesirable occurrence can be prevented are outlined in
Section 9.9
.
H
H
H
H
H
CH
2
C
CCH
2
CH
2
C
CH
2
CC
CH
2
H
H
CH
2
C
CH
2
CC
CH
2
H
1-63
Another type of branching occurs in some free-radical polymerizations of
monomers like ethylene, vinyl chloride, and vinyl acetate in which the macro-
radicals are very reactive. So-called self-branching can occur in such polymeriza-
tions because of atom transfer reactions between such radicals and polymer
molecules. These reactions, which are inherent in the particular polymerization
process, are described in Chapter 8.
Although the occurrence of constitutive isomerism can have a profound effect
on polymer properties, the quantitative characterization of such structural varia-
tions has been difficult. Recent research has shown that the
13
C chemical shifts of
polymers are sensitive to the type, length, and distribution of branches as well as
to positional isomerism and stereochemical isomerism (
Section 1.12.2
). This tech-
nique has great potential when the bands in the polymer spectra can be assigned
unequivocally.
