Page 36 - Engineering Plastics Handbook

10 Introduction

Pulsed laser photolysis synthesis and gel permeation chromatography
(GPC) are used to determine propagation rate constants for free-radical
polymerizations. Free-radical polymerization used for industrial pro-
duction of polymers shows better tolerance to trace amounts of oxygen
and impurities compared with other processes [11].
Free-radical polymerization is used to produce graft copolymers. Free-
radical sites on the macromolecule provide sites for an unsaturated
copolymer to graft, a method commonly used for polyolefins [9, 14].
Ionic polymerization uses anionic or cationic catalysts. Monomers with
electron-accepting groups at the double bonds such as vinyls are cat-
alyzed by anionic catalysts such as organometallic compounds [16, 17].
Monomers with electron-donating groups at the double bonds are cat-
alyzed by cationic catalysts such as Lewis acids, Ziegler catalysts, and
Friedel Crafts catalysts [16, 17]. Polyacetals are linear polymers that
can be produced by anionic or cationic chain-growth polymerization of
formaldehyde [2, 14].
Two strategic parameters for semicrystalline polymers are the poly-
merization reaction rate and degree of crystallization [10].
The specific reaction rate (isothermal kinetic data) is obtained from
differential scanning calorimetry (DSC) data. When isothermal ther-
mogravimetry is used, the specific reaction rate is directly related to
sample weight at a given time.

 dC  ( dW dt W − W )
/ )(
k =   (1 − C) n = 0 f
 dt  ( WW )( W − W ) n
−
f
0
f
where k = specific reaction rate
C = fraction of conversion
t = reaction time, s
n = reaction order
W = sample weight at a given time (dW/dt), g
W = initial sample weight, g
0
W = final sample weight, g
f
n
When this equation is valid, a straight-line plot of (1 − C) versus dC/dt
is obtained for the value of n, and k is calculated from the slope. Rate
constants determined for more than one temperature can be used to cal-
culate the Arrhenius activation energy constant. The Arrhenius equa-
tion predicts the rate of a chemical reaction at a given temperature
(Kelvin temperature). The equation is a function of a frequency factor
or preexponential factor A, mathematical quantity e, gas constant R,
temperature T in kelvins, and activation energy E.