Chemistry Reference
In-Depth Information
reasonable period at 100
C without generating significant concentrations of
monomer.
8.17
Free-Radical Polymerization Processes
The kinetic schemes described in this chapter apply to free-radical polymeriza-
tions in bulk monomer, solution, or in suspension. Suspension polymerizations
involve the reactions of monomers that are dispersed in droplets in water. These
monomer droplets contain the initiator, and polymerization is a water-cooled bulk
reaction in effect. Emulsion systems also contain water, monomer, and initiator,
but the kinetics of emulsion polymerizations are different from those of the pro-
cesses listed above. Chapter 10 describes emulsion polymerizations.
PROBLEMS
8-1
Rate constants for termination
k
t
may be of the order of 10
8
liter/mol sec
in free-radical polymerizations. Consider the polymerization of styrene ini-
tiated by di-
t
-butyl peroxide at 60
C. For a solution of 0.01
M
peroxide
and 1.0
M
styrene in ben
zen
e,
the initial
rate of polymerization is
10
2
7
mol/liter sec and
M
n
of the polymer produced is 138,000.
1.5
3
From the above information, estimate
k
p
for styrene at 60
C.
(a)
(b)
What is the average lifetime of a macroradical during the initial stages
of polymerization in this system?
8-2
For a particular application, the molecular weight of the polymer made in
Problem 8-1 is too high. What concentration of
t
-butyl mercaptan should
be used to lower the number molecular weight of the polymer to 85,000?
For this, transfer agent
C
is 3.7 in the polymerization of styrene.
8-3
When bulk styrene is heated to 120
C, polymerization occurs because of
thermal initiation
in
the absence of an added initiator. It is observed that
polystyrene with
M
n
5
200,000 is produced under these conditions at a
rate of 0.011 g polymer/liter/min. Using this information, calculate the
total initial rate of polymerization expected if an initiator with concentra-
tion 0.1
M
,
k
d
5
10
2
4
sec
2
1
, and
f
0.8 is added to this system at 120
C.
5
10
4
;
k
t
5
(Molar mass of styrene
k
tc
;
k
td
5
0.)
5
8-4
Free-radical polymerization can be initiated by a redox system involving
Ce
4
1
and an alcohol:
k
Ce
4
1
!
Ce
3
1
1
RCH
2
OH
H
1
1
R
CHOH
1
2
The propagation reaction can be summarized as usual as
