Step-Growth Polymerizations - Polymer Science and Engineering - page 307

H H

(7-3)

HOROH + OCN-R ′ -NCO OROC

O

N R ′ N C

x

O

7. Step-growth reactions of the type of Eq. (7-3) are essentially irreversible.
Interchange reactions are not significant. The reactions are usually very fast,
and high degrees of polymerization can be realized, depending on the
stoichiometric balance of the reactants. The polymers may be capable of
further growth in size if additional reactants are added after the nominal end
of the polymerization reaction. (This is illustrated in Section 1.5.4 and
Problem 3-15.)

Chain-growth polymerizations have the following distinguishing features:

1. Each polymer molecule increases in size at a rapid rate once its growth has
been started. When the macromolecule stops growing it cannot generally react
with more monomers (barring side reactions).
2. Growth of polymer molecules is caused by a kinetic chain of reactions. (The
name chain-growth reflects the existence of a chain reaction. Unfortunately,
macromolecules are often also called chains because they are composed of
linked identical entities. There is no necessary connection between the two
usages. Some polymer chains are made by chain-growth polymerizations, and
some are made by step-growth reactions.)
3. Chain-growth polymerization involves the reaction of monomers with active
centers that may be free radicals, ions, or polymer-catalyst bonds.
4. In chain-growth polymerizations the mechanisms and rates of the reactions
that initiate, continue, and terminate polymer growth are different.
5. Chain-growth polymerization is usually initiated by some external source
(energy, highly reactive compound, or catalyst), and the reaction is allowed to
proceed under conditions in which monomers cannot react with each other
without the intervention of an active center.
6. Polymers made by chain-growth reactions are often addition polymers by
Carothers's definition. The most common polymers made by these processes
have only carbon

carbon links in their backbones.

An example of step- and chain-growth reaction mechanisms is provided by
alternative pathways to poly(tetramethylene oxide) (1-31), which can be made by
the self-condensation of tetramethylene glycol:

H+

(7-4)

n HO - CH 2 CH 2 -CH 2 CH 2 -OH OCH 2 CH 2 CH 2 CH 2

n + n H 2 O

and by the acid-catalyzed ring opening polymerization of tetrahydrofuran:

HClO 4

n

OCH 2 CH 2 CH 2 CH 2

(7-5)

n

O