WILLIAM B. SMITH
Polymer Studies by
The Kinetic Parameters for Styrene Polymerizations1
by James A. May, Jr., and William B. Smith
Chemistry, Texas Christian University, Forth worth, Texas
Styrene monomer was polymerized to low conversion using a range of benzoyl peroxide
The resultant polymer samples were characterized by gel permeation
Calculations made from the molecular weight distributions
allowed the determination of the following kinetic parameters: chain transfer to initiator
chain transfer to monomer
and the ratio
are the rate
constants for termination and propagation, respectively.
The molecular weight distribution of a given polymer
is well known to be a function of its mechanism of forma-
tion, and Flory2 has given distribution functions which
can be related to the kinetic parameters for vinyl
polymerization. Until recently, it has usually been
easier to determine the appropriate rate terms and to
calculate molecular weight distributions rather than to
suffer the difficulties and uncertainties of an experi-
mental measurement of a weight di~tribution.~
The advent of
gel permeation chromatography4
offered a rapid and potentially accurate method for
ascertaining molecular weight distributions.
we have demonstrated that gpc results on samples of
polystyrene and polymethyl methacrylate, prepared
under known kinetic conditions, were consistent with
These findings clearly sug-
gest the utility of the gpc technique as a mode of
studying the details of vinyl polymerization.
sequently, we have initiated a more detailed study of
the styrene system with the results reported below.
Styrene monomer was purified by washing three times
10% potassium hydroxide, followed by three
washes with water.
After drying over calcium chloride,
the styrene was carefully distilled under reduced pres-
sure, and only the middle cut (43" (16 mm)) was re-
tained. The benzoyl peroxide, which was used, assayed
100% by iodometric titrations in 2-propanol.
All reactions were carried out using the sealed-am-
poule technique in an oil bath controlled at 60
In every case, a known weight of benzoyl peroxide was
added to a known weight of monomer, with allowance
being made for the subsequent expansion of the mono-
mer upon heating in the bath. Duplicate portions
(30 ml) of the monomer-initiator mixture were placed in
glass ampoules and sealed under vacuum
mm) after a series of three freeze-thaw degassing cycles.
Ampoules were withdrawn from the bath at appropriate
times and plunged into an ice bath. The polystyrene
was precipitated by washing the contents of the am-
poules with a small amount of toluene into 3 1. of cold
The polystyrene was recovered by filtra-
tion, and then it was dried to constant weight at 40"
Evaporation of the filtrate at room temper-