Chemistry Reference
In-Depth Information
hydrogen atom abstraction from propylene which gives an allyl radical and a
terminated oligomer.
CH
3
CH
3
CH
3
CH
3
.
RCH
2
C
.
C
R
H
2
C
CH
2
CH
2
C
H
H
H
H
CH
3
H
.
CH
3
CH
2
H
C
H
RCH
2
.
+
C
RCH
2
C
H
CH
2
C
H
H
H
H
2
C
C
Allyl radical
H
Polypropylene has excellent fatigue resistance. Fatigue is the process by
which materials fail after cyclic loading. Consider a piece of bare copper
wire. It is strong and cannot be separated by hand. However, if you bend it
back and forth several times, eventually it will fail and break apart. This is
fatigue failure. Polypropylene is very resistant to fatigue. Therefore it is used
in applications, such as plastic hinges, which require repeated flexing, bend-
ing, or opening and closing. Other polymers develop cracks and break upon
repeated bending.
Polypropylene makes good fibers and is used for clothing, ropes, and car-
pets. Unlike wool or nylon, polypropylene does not soak up water so it is used
for outdoor carpeting and athletic clothing. Other applications for polypropy-
lene include yogurt cups, straws, bottle caps, and automobile bumpers.
8.3 POLYVINYL CHLORIDE
Although vinyl chloride has been polymerized with organometallics [2],
commercial poly(vinyl chloride) (PVC) is made by a free radical polymeriza-
tion. The dominant process is the suspension process. Vinyl chloride (most
commonly referred to as vinyl chloride monomer or VCM) is suspended as
droplets in water and an initiator that is soluble in the VCM is added. VCM
has a boiling point of
13
∘
C. It is reacted under pressure as a liquid. The
resultant PVC powder is separated from the water and dried. PVC can also be
prepared by a mass or bulk polymerization where the initiator is added to the
liquid VCM. A third polymerization technique, an emulsion polymerization,
employs liquid VCM, water, and an emulsifier. A water soluble initiator is
added to the VCM/water emulsion.
The PVC which is formed in the reactor has the tendency to adhere to the
walls of the reactor, the agitator, and reactor baffles. If it later flakes off, it
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