High Performance Polymers, 2nd Edition

260 HIGH PERFORMANCE POLYMERS

Transesterification reactions can be promoted by

electron beam irradiation [25]. The transesterification

reaction can be written formally as

−−NEN−−+−−TET ←→ −−NET−−+−−TEN−−

(11.2)

Here E is an ethylene glycol unit, T is a terephthalate

unit, and N is a naphthalate unit.

If the initial mole fractions of NEN and TET units

are a and b, with a + b = 1 and the mole fraction x

of the copolymer NET, then, assuming a second-order

reversible reaction the kinetic law:

dx

dt

= k

1

(a − x)(b − x) − k

2

x

2

(11.3)

emerges. In equilibrium dx/dt = 0andx = x

e

.Set-

ting k = k

1

= k

2

, and inserting in Eq. (11.3)asimpler

form can be obtained:

dx

dt

= k(x

e

−