Key Terms 097scientists, and biopolymers, which are synthesized by or-
ganisms. Synthetic polymers can be divided into two
classes, depending on their method of preparation:chain-
growth polymers(also called addition polymers) and
step-growth polymers (also known as condensation
polymers).
Chain-growth polymers are made by chain reactions—
by the addition of monomers to the end of a growing chain.
These reactions take place by one of three mechanisms:
radical polymerization,cationic polymerization, or an-
ionic polymerization. Each mechanism has an initiation
step that starts the polymerization, propagation steps that
allow the chain to grow at the propagating site, and termi-
nation steps that stop the growth of the chain. The choice of
mechanism depends on the structure of the monomerand
the initiator used to activate the monomer. In radical poly-
merization, the initiator is a radical; in cationic polymeriza-
tion, it is an electrophile; and in cationic polymerization, it
is a nucleophile. Nonterminated polymer chains are called
living polymers.
Chain-growth polymerization exhibits a preference for
head-to-tail addition. Branching affects the physical prop-
erties of the polymer because linear unbranched chains can
pack together more closely than branched chains can. The
substituents are on the same side of the carbon chain in an
isotactic polymer, alternate on both sides of the chain in a
syndiotactic polymer, and are randomly oriented in an
atactic polymer. The structure of a polymer can be con-
trolled with Ziegler–Natta catalysts. Natural rubber is a
polymer of 2-methyl-1,3-butadiene. Synthetic rubbers have
been made by polymerizing dienes other than isoprene.
Heating rubber with sulfur to cross-link the chains is called
vulcanization.
Homopolymers are made of one kind of monomer,
whereas copolymersare made of more than one kind. In an
alternating copolymer, the two monomers alternate. In a
block copolymer, there are blocks of each kind of monomer.
In a random copolymer, the distribution of monomers is
random. A graft copolymercontains branches derived from
one monomer grafted onto a backbone derived from another.
Step-growth polymers are made by combining two
molecules with reactive functional groups at each end.
There are two types of step-growth polymers. One type is
formed by using a single monomer with two different func-
tional groups, A and B. The other is formed by using two
different bifunctional monomers, one containing two A
functional groups and the other containing two B function-
al groups. The formation of step-growth polymers does not
involve chain reactions.
Nylon is a polyamide. Aromatic polyamides are called
aramides. Dacron®is a polyester. Polycarbonatesare
polyesters with two ester groups bonded to the same car-
bon. A urethaneis a compound that has an ester and an
amide group bonded to the same carbon.
Crystallitesare highly ordered regions of a polymer.
The more crystalline the polymer is, the denser, harder,
and more resistant to heat it is. Thermoplastic polymers
have crystalline and noncrystalline regions. Thermoset-
ting polymers have cross-linked polymer chains. The
greater the degree of cross-linking, the more rigid is the
polymer. An elastomeris a plastic that stretches and then
reverts to its original shape. A plasticizeris an organic
compound that dissolves in the polymer and allows the
chains to slide past one another. Biodegradable polymers
can be broken into small segments by enzyme-catalyzed
reactions.Key Terms
addition polymer (p. 073)
alternating copolymer (p. 088)
anionic polymerization (p. 074)
aramide (p. 090)
atactic polymer (p. 085)
biodegradable polymer (p. 096)
biopolymer (p. 072)
block copolymer (p. 088)
cationic polymerization (p. 074)
chain-growth polymer (p. 073)
chain reaction (p. 073)
chain transfer (p. 076)
condensation polymer (p. 074)
conducting polymer (p. 085)
copolymer (p. 088)
cross-linking (p. 087)
crystallites (p. 093)
elastomer (p. 095)
epoxy resin (p. 092)
graft copolymer (p. 088)
head-to-tail addition (p. 077)
homopolymer (p. 088)
isotactic polymer (p. 085)
living polymer (p. 082)
materials science (p. 073)
monomer (p. 072)
oriented polymer (p. 095)
plastic (p. 073)
plasticizer (p. 096)
polyamide (p. 089)
polycarbonate (p. 091)
polyester (p. 074)
polymer (p. 072)
polymer chemistry (p. 073)polymerization (p. 072)
polyurethane (p. 093)
propagating site (p. 074)
radical polymerization (p. 074)
random copolymer (p. 088)
ring-opening polymerization (p. 084)
step-growth polymer (p. 074)
syndiotactic polymer (p. 085)
synthetic polymer (p. 072)
thermoplastic polymer (p. 094)
thermosetting polymer (p. 094)
urethane (p. 093)
vinyl polymer (p. 074)
vulcanization (p. 087)
Ziegler–Natta catalyst (p. 085)alpha olefins (p. 074)
AU: Want to add term to listing? If not, make lightface on p. 074