Chemistry Reference
In-Depth Information
in the average molecular weight and the breadth of the distribution of molecular
weights. If chain transfer to polymer can occur (
Section 8.8.4
), this will be most
significant at higher conversions when the polymer concentration is high. This
results in a further skewing of the molecular weight distribution to higher molecu-
lar weights, because the larger polymer molecules are the most likely to suffer
transfer reactions and then grow even larger.
M
ol
ecular weight distributions in commercial polymers are characterized by
M
w
=
M
n
ratios of about 3 for substances like polystyrene in which transfer to
polymer does not appear to be important. Where long branches can be formed by
chai
n t
ra
nsf
er to polymer, the molecular weight distribution will be even broader
and
M
w
=
M
n
ratios of 50 and more are observed in some polyethylenes made by
free-radical syntheses.
Note that the molecular weight distributions of high-conversion polymers
made under conditions where the growth of macromolecules is limited primarily
by chain transfer will
b
e
ran
dom, as described in
Section 8.14.1
for low-
conversion cases. Then
M
w
=
M
n
will be 2. An exception to this rule occurs when
the chain transfer reactions that determine the polymer molecular weight are to
monomer and can result in branching [as in reactions (8-79) or (8-84)]. The
molecular weight distributions of the branched polymers that are produced will
be broader
than the random one, and bimodal distributions may also be
observed.
Poly(vinyl chloride) made by suspension polymerization (Chapter 10) is a
polymer in which molecul
ar
w
eig
ht control is effectively by chain transfer—to
monomer in this case. The
M
w
=
M
n
ratio is slightly higher than the expected value
of 2 because the polymerization is performed at rising temperatures, rather than
isothermally, as assumed in the ideal kinetics discussed above, and possibly
because of autoacceleration effects.
When chain transfer agents are used to control polymer molecular weight the
molecular weight distribution will tend to narrow toward the random case from
the characteristics it would have in the absence of transfer agents.
8.15
Free-Radical Techniques for Polymers with Narrower
Molecular Weight Distributions
Free-radical polymerization is the most economical process for use with vinyl
monomers because the reaction mixture does not require the high-purity reactants
and rigorous exclusion of moisture, air, and other impurities that is needed for
successful operation of the alternative ionic or organometallic catalyses described
in Chapter 11. The molecular weight distributions that are characteristic of free-
radical reactions are, however, not optimum for some polymer applications, such
as toners for reprographic processes
[20]
. Process variations have been developed
that yield narrow molecular weight polymers from free-radical polymerizations.
