Page 384 - Elements of Chemical Reaction Engineering Ebook

See. 7.3 Polymerization 355

Everyday Examples Polymerization is the process in which monomer units are linked
Polyethylene together by chemical reaction to form long chains. lt is these long chains that
Softdrinkcups set pohymers apart from other chemical species and gives them their unique
Sandwich bags characteristic properties.
Poly (vinyl chloride) The polymer chains can be linear, branched, or cross-linked (Figure 7-3).
Pipes
Shower curtains
Tygon tubing
Poly (vinyl acetate)
Chewing gum

Linear Branched Cross-linked
Figure 7-3 Types of polymer chains.

Homopolymers are polymers consisting of a single repeating unit, such as
[ -CH2-CH2- 1. Homopolymers can also be made from two different
monomers whose structural units form the repeating unit such as the formation
of a polyamide (e.g., Nylon) from a diamine and a diacid.
Polymerization reactions are divided into two groups known as step
reactions (also called condensation reactions) and chain reactions, aKso
known as addition reactions. Step reactions require bifunctional or polyfunc-
tional monomers, while chain reactions require the presence of an initiator.

Structural Structural
Monomer Monomer Unit 1 Unit 2
n‘NH,RNH,‘ fn kOOCR’COOfi --+ H[‘ NHkNH ‘ ‘ 0CR’C6ln OH + (2n - 1)H,O
Y
Repeating Unit
Copolymers are polymers made up of two or more repeating units. There are
five basic categories of copolymers that have two different repeating units Q
and S. They are
1. ~4lternating: -QS-Q-S-QS-Q-S-QS-
2. Block -Q-Q-Q-Q-QS-SS-SS-
Categories 3. Random: -Q-Q-S-QS-S-Q-S-S-S-
of Copolymers
4. Graft: -Q-Q-Q-Q-Q-Q-Q--Q-Q-Q-
L s-s-s-s-s-s-
5. Statistical (follow certain addition laws)
Examples of each can be found in Young and Lovell.6

R. J. Young and P. A. Lovell, Introduction to Polymers, 2nd ed., Chapman & Hall,
New York, 1991.