THE ROLE OF THE

42 The Role of the Chemist in Automotive Design
dominant step and therefore the only effective step that consumes the monomer. The
rate of polymerization (R p) is
R
dM ⎡ ⎤
=− ⎣⎢ ⎦⎥
= k ⎡M•⎤M
dt ⎣⎢ ⎦⎥ ⎡ ⎤
⎣⎢ ⎦⎥
p p
3.8 steP growtH PolymerIzatIon
(3.5)
Nylon 6,6 and nylon 6 are hugely important in the automotive industry. Nylon’s
strength and versatility allow for use under the hood as well as in the interior. When
designing, a chemist or design engineer’s first choice of materials begins with nylon.
Nylon is produced by step growth polymerization. Step growth can be categorized by
reactions of molecules with functional groups. This process can be stopped and low
molecular weight oligomers can be obtained [5]. Monomer concentration will not
decrease at the rate that it will in chain growth polymerization, but it will decrease
at a fast rate early in the reaction due to the formation of oligomers [13]. After the
reaction begins, there is a distribution of oligomers that are reacting at a slower
rate. As we can see from the production of nylon 6,6 (Figure 3.10), a molecule of
water is produced when the polymer reacts. For this reason, the term condensation
polymerization is often used. When nylon 6 is produced by step growth, water is not
produced (Figure 3.11).
3.9 IonIc PolymerIzatIon
The electron-withdrawing nature of the constituent on a vinyl monomer (CH 2=CHX)
will affect the polymerization of that monomer. If the constituent is electron donating,
then a cationic initiation mechanism is favored. If the constituent is electron
withdrawing, then an anionic mechanism is favored.
O
O
HO C (CH2 ) 4 C OH +
H
N
H
(CH 2 ) 6
H
N
H
–H 2 O
(CH 2 ) 4
Adipic acid Hexamethylenediamine Nylon 6,6
FIgure 3.10 Nylon 6,6 production.
O
NH
∆
NH
O
C
(CH 2 ) 5
Caprolactam Nylon 6
FIgure 3.11 Nylon 6 production by ring opening caprolactam.
O
C
O
C
NH
n
(CH 2 ) 6
NH
n