Page 245 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GNH/GRI P2: GTV Final pages
Encyclopedia of Physical Science and Technology EN012B-596 July 27, 2001 18:18

754 Polymers, Synthesis

Thus, one can imagine only a small percentage of the
monomer being activated per unit time, and perhaps only
about 10 −8 mole/liter of growing chains may be active
at any one time. In this case, the monomer concentra-
tion will decrease steadily throughout the reaction. Living
polymerizations continue to receive intense academic and
industrial attention. In a living polymerization, in which
the chain-end activity is maintained throughout the over-
all polymerization (the rate of any transfer or termina-
tion steps is essentially zero), one may observe molecular
weight increasing linearly with conversion. Such polymer-
izations are of great interest since they allow predictable
molecular weights and narrow molecular weight distribu-
tions to be obtained, as well as the possible generation of
block copolymers by the use of the active chain end of one
FIGURE 1 Molecular weight versus percent conversion for
species to initiate a new monomer.
(a) step-growth polymerization and (b) chain growth polymeriza-
tion and living polymerization processes.
III. LINEAR STEP-GROWTH
POLYMERIZATIONS
the fractional extent of conversion, usually designated P.
For example, if one started a reaction with 1 mole of car- In Scheme 2, a direct esterification process illustrates lin-
boxylic acid, it would be necessary to consume ∼99% of ear step-growth polymerization. The implication is that
that mole; in other words, reduce the concentration to 0.01 all of the oligomeric species are capable of reacting with
mole, before one could anticipate reaching high molecu- suitably terminated species. Therefore, as the molecular
lar weight. The degree of polymerization is proportional weight slowly builds through the early portion of the reac-
to 1/(1 − P), where P is the fractional extent of reaction. tion, a variety of oligomeric sizes exist that must react with
This means that there are a number of requirements one another to ultimately generate high molecular weight.
for obtaining high molecular weight. First, one must work These reactions are often protic or Lewis acid-catalyzed,
with very pure monomers. Typical purities of commercial- as illustrated, to allow the protonated species to be more
grade monomers are 99.99%. If they were less than 99% readily attacked by the nucleophile. This is consistent with
pure, it would be impossible to make more than 100 units the usual mechanics in small-molecule esterification.
in a chain, which would generally be insufficient to obtain Since the esterification is an equilibrium process, water
adequate mechanical properties. Second, if coreactants are typically has to be removed in order to drive the reaction
utilized,aperfectstoichiometricbalanceofthetwodifunc- to the right, that is, toward higher molecular weight. The
tional monomers must be introduced. Third, there must be polymerization rate in such a case is proportional to the
no side reactions. Finally, conditions must exist that will group collision frequency and not dependent on the dif-
allow the reaction to be pushed to very high conversions. fusion rate of the entire chain. The statistical segment is
This implies the absence of any side reactions. In essence, more mobile than the whole molecule. Many workers have
step-growth polymerization can be viewed as a form of demonstrated that the second functional group on a grow-
quantitative organic chemistry. Most organic reactions ing chain is not influenced by the first if several carbon
are not suitable for obtaining such high yields. Surpris- atoms separate it. Essentially, this leads to the principle
ingly, however, a large number of systems have been pro- of equal reactivity. This implies essentially that the reac-
duced in the laboratory, not only from simple polyesters tivity of the growing molecules will not change signifi-
and polyamides, but also from high-temperature materi- cantly during most of the polymerization process. There
als, such as polyimides and liquid crystalline polymers. are some exceptions to this, based on highly sterically re-
The only necessity is that the four requirements discussed stricted groups, and, of course, heterogeneous processes
above be met for any of the step-growth polymerization would also not fall into this category. Nevertheless, the
systems. principle of equal reactivity has made it possible to treat
Chain-growth reactions, by contrast, often show molec- these reactions statistically. The fact that the molecular
ular weight versus conversion curves, as illustrated in the weights and molecular distributions can be predicted this
lower portion of Fig. 1. Free-radical reactions that in- way is an indication that the principle is typically correct.
volve initiation, propagation, and termination often yield Another way of looking at this process is summarized in
high-molecular weight chains at very low conversions. Scheme 3. Any two species in the reaction mixture must