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

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102 • Chapter 4 / Polymer Structures

H

C

H

H

C

H

H

C

H

H

C

Repeat unit

(a)

(b)

H

H

C

H

H

C

H

H

C

H

H

C

H

C H

Figure 4.1 For

polyethylene, (a)a

schematic

representation of

repeat unit and chain

structures, and (b)a

perspective of the

molecule, indicating

the zigzag backbone

structure.

active center is formed by the reaction between an initiator or catalyst species (R·)

and the ethylene monomer, as follows:

C

HH

HH

C R C

HH

HH

R·C·(4.1)

The polymer chain then forms by the sequential addition of monomer units to this

active growing chain molecule. The active site, or unpaired electron (denoted by·),

is transferred to each successive end monomer as it is linked to the chain. This may

be represented schematically as follows:

C

HH

HH

C C

HH

HH

R C

HH

HH

R C C

HH

HH

C· C· (4.2)

The final result, after the addition of many ethylene monomer units, is the polyethy-

lene molecule;^2 a portion of one such molecule and the polyethylene repeat unit are

shown in Figure 4.1a. This polyethylene chain structure can also be represented as

VMSE

Repeat Unit

Structures

PE

HH

HH

—( CC C —)n

or alternatively as

(CH 2 −CH 2 )n

(^2) A more detailed discussion of polymerization reactions including both addition and
condensation mechanisms is given in Section 14.11.