Page 267 - Carrahers_Polymer_Chemistry,_Eighth_Edition

230 Carraher’s Polymer Chemistry

Copolymers of VC and vinylidene chloride were introduced in the 1930s. The copolymer with
very high VC content is used as a plastic film (Pliovic), and the copolymer with high vinylidene

chloride content is used as a film and fi lament (Saran).
Polybutadiene, produced in emulsion polymerization, is not useful as an elastomer. However, the
copolymers with styrene (SBR) and acrylonitrile (Buna-N) are widely used as elastomers.
Ethylene–propylene copolymers (EPMs) show good resistance to ozone, heat, and oxygen and
are used in blends to make today’s external automotive panels. Two general types of ethylene–
propylene copolymers are commercially available. EPMs are saturated and require vulcanization if
used as a rubber. They are used in a variety of automotive applications, including as body and chas-
sis parts, bumpers, radiator and heater hoses, seals, mats, and weather strips. EPMs are produced
using Ziegler–Natta catalysts.
The second type of EPM is the ethylene–propylene–diene terpolymers (EPDMs). These are
made by polymerizing ethylene, propylene, and a small amount (3–10 mol %) of nonconjugated

diolefine employing Ziegler–Natta catalysts. The side chains allow vulcanization with sulfur. They
are employed in the production of appliance parts, wire and cable insulation, coated fabrics, gaskets,
hoses, seals, and high-impact polypropylene.

7.4 BLOCK COPOLYMERS
While block copolymers do not occur naturally, synthetic block copolymers have been prepared by
all known classical polymerization techniques. The first commercial block copolymer was a sur-

factant (Pluronics) prepared by the addition of propylene oxide to polycarbanions of ethylene oxide.
While neither water-soluble poly(ethylene oxide) nor water-insoluble poly(propylene oxide) exhibits
surface activity, the ABA block copolymer consisting of hydrophilic and lyophilic segments, is an
excellent surfactant. Each block has 20 plus repeat units of that variety (7.30).

CH 3
(7.30)
O O
n O R
R
m n
The ethylene oxide block is hydrophilic, while the propylene oxide block is (relatively) hydro-
phobic. The copolymer forms micelles in aqueous solutions with the hydrophilic portions pointing
outward, interacting with the water, while the hydrophobic portions form the inner core, shielded
from the water by the ethylene oxide derived block. A micelle is also formed in organic liquids, but
here the hydrophobic propylene oxide block “faces” outward while the ethylene oxide block acts as
the inner core.
Polymers that contain two reactive end groups are referred to as telechelic polymers. Joseph
Shivers, a DuPont chemist, invented Spandex in 1959 after about a 10-year search. It was fi rst
named Fiber K but DuPont chose the more appealing, smooth-sounding trade name of Lycra.
Elastomeric polyurethane fibers consisting of at least 85% segmented polyurethane are com-

mercially available under the name Spandex. They are block copolymers and are among the fi rst

products specifically designed using the concept of soft and hard segments. The soft or fl exible
segment is composed of poly(ethylene oxide) chains that contain two hydroxyl end groups (one at
each end of the poly(ethylene oxide) chain. The hard or stiff segment is derived from the reaction
of the diisocyanate with the hydroxyl end groups forming polar urethane linkages that connect
the poly(ethylene oxide) polymer segments through the urethane linkages. Such products are often
referred to as segmented-polyurethane fibers (Equation 7.31). These products have found wide use

in cloths including bras. Similar products have also been formed using hydroxyl-terminated poly-
ethylene in place of the poly(ethylene oxide).

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