Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-14 GTBL042-Callister-v2 August 29, 2007 8:59

604 • Chapter 14 / Synthesis, Fabrication, and Processing of Materials

Propagation involves the linear growth of the polymer chain by the sequen-

tial addition of monomer units to this active growing chain molecule. This may be

represented, again for polyethylene, as follows:

R C  (14.2)

H

H

C·

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C·

H

H

R

Chain growth is relatively rapid; the period required to grow a molecule consisting

of, say, 1000 repeat units is on the order of 10−^2 to 10−^3 s.

Propagation may end or terminate in different ways. First, the active ends of

two propagating chains may link together to form one molecule according to the

following reaction:^4

C (14.3)

HH

HH

( C C·

HH

HH

C )m ·C R

HH

HH

R C ( C

HH

HH

 C )n C

HH

HH

( C

HH

HH

C )m C R

HH

HH

R C C ( C

HH

HH

C )n

The other termination possibility involves two growing molecules that react to form

two “dead chains” as^5

(14.4)

C

HH

HH

( C C·

HH

HH

C )m ·C R

HH

HH

R C ( C

HH

HH

 C )n C

H

H

( C H

HH

HH

C )m C R

HH

H

R C ( C

HH

HH

C  C )n

H

H

thus terminating the growth of each chain.

Molecular weight is governed by the relative rates of initiation, propagation, and

termination. Ordinarily, they are controlled to ensure the production of a polymer

having the desired degree of polymerization.

Addition polymerization is used in the synthesis of polyethylene, polypropylene,

poly(vinyl chloride), and polystyrene, as well as many of the copolymers.

Concept Check 14.7

Cite whether the molecular weight of a polymer that is synthesized by addition poly-

merization is relatively high, medium, or relatively low for the following situations:

(a)Rapid initiation, slow propagation, and rapid termination.

(b)Slow initiation, rapid propagation, and slow termination.

(c)Rapid initiation, rapid propagation, and slow termination.

(d)Slow initiation, slow propagation, and rapid termination.

[The answer may be found at http://www.wiley.com/college/callister (Student Companion Site).]

(^4) This type of termination reaction is referred to ascombination.
(^5) This type of termination reaction is calleddisproportionation.