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Encyclopedia of Physical Science and Technology EN012B-596 July 27, 2001 18:18

Polymers, Synthesis 753

multiplied by the molecular weight of the structure within tion reactions. The latter may involve several distinct ki-
the repeating unit. The molecular weights of synthetic netic chain-growth steps such as initiation, propagation,
macromolecules usually exhibit a distribution in size. The and possibly termination. The reaction is most important
distributionsareoftenGaussian,butmayalsoapproachthe for the preparation of poly(ethylenes), poly(acrylates),
Poisson distributions in living and more controlled poly- poly(styrenes), poly(dienes), and related mono- or disub-
merization processes. An example is provided below for stituted analogs.
polystyrene. Thus, M n is the number-average molecular
weight, which is a product of N, the degree of polymer- II. BASIC CONSIDERATIONS

ization, times 104 g/mole. The end groups R and R are
negligible at high M n : A number of important requirements must be met if small
molecules(ontheleftsideofthearrowsinScheme1)areto
R (CH 2 CH) n R
be efficiently transformed into macromolecular structures.
One critical concept is the idea of functionality. In a step-
growth polymerization, in order to achieve high molec-
ular weight, it is essential that the reactions be perfectly
difunctional.Forexample,ifwereactedonlyoneofthehy-
droxyl or carboxyl groups, we would make simply a small
D. Living and Controlled Polymerization
molecule. Indeed, the idea of a step-growth reaction is that
Polymerization reactions wherein the terminal unit of the the reaction can proceed from both ends of the molecule. It
growing chain species has an indefinitely long lifetime must proceed until high values of n, the degree of polymer-
and will essentially react only with the monomer are ization, are achieved. Typical number-average molecular
called living or controlled polymerizations. Such behav- weights for step-growth polymers, such as poly(ethylene
ior permits molecular weight to increase linearly with terephthalate), are 20,000–30,000 g/mole (Da).
conversion and provides terminally active species, which By contrast, typical chain-growth reactions are required
may initiate distinctly different monomer structures, thus to proceed to significantly higher molecular weights, per-
producing blocklike copolymers. This is particularly haps 100,000–200,000 g/mole or higher in the case of
important for carbanion polymerizations, as illustrated polyacrylonitrile and related so-called vinyl polymers.
by the organolithium polymerizations of hydrocarbon The term vinyl polymer refers to the fact that the chain
monomers.Researchhasshownthatothertypesofreactive molecule is derived from a “vinyl-containing” starting ma-
intermediates, including coordination species with par- terial, even though the addition produces a saturated chain.
ticular attention to metallocene and single-site catalysts, The chain-growth reaction occurs in several distinct steps.
appropriate cationic or oxonium ion species and reversibly Initiation, propagation, and terminated processes are in-
stabilize free radical processes. Recent efforts in con- volved as the kinetic chain begins to react, propagates, and
trolled radical polymerization processes have focused on finally terminates. Thus, there are a number of differences
stable free radical polymerization (SFRP) using nitroxide between these two fundamental routes of polymerization.
mediation, atom transfer radical polymerization (ATRP), In the step-growth case, only one reaction is responsible
and radical addition fragmentation transfer (RAFT). The for polymer formation. In the example, this is esterifi-
use of “living” or “controlled” terminology depends on cation. By contrast, in the chain-growth case, initiation,
the mechanism of polymerization, and the “living” clas- propagation, and termination reactions proceed at differ-
sification is often reserved for processes where the rates ent rates and possibly different mechanisms. During the
of transfer or termination equal zero during propagation. polymer-growth step, any two molecular species present
can react during a step process, and one typically observes
E. Step-Growth (Polycondensation) a slow, random growth taking place. Again, by contrast,
Polymerization the growth step in the chain reaction usually occurs by
the rapid addition of one unit at a time to the active end
In these processes a macromolecule is built up via a series
of the polymer chain. As we shall demonstrate later, the
of reactions between functional groups, for example, the
active ends will be the typical intermediates of organic
hydroxyl–carboxyl reaction, to produce a macromolecule
chemistry, such as radicals, anions, cations, and coordi-
at very high conversions (>99.9%).
nation complexes. The point of the discussion here is that
the molecular weights vary quite differently as a function
F. Chain-Grown (Addition) Polymerization
of conversion for the two systems. This is illustrated in
In this process a reactive small molecule (often contain- Fig. 1. In the case of the step-growth reactions, molecular
ing an alkene group) is transformed via radical or ionic weight increases only slowly until one reaches very high
intermediates into a macromolecule by a series of addi- conversions. In fact, the overall process is governed by