Page 798 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Appendix M

Polymerization Reactors

Polymerization can occur within glass ampules, large-scale batch reactors, within laboratory

beakers, flow-through systems, and so forth. The processes used for small preparation in the
research laboratory can be similar or dissimilar to that employed for the industrial-scale prep-
aration of pound and larger quantities. While the kind or polymerization infl uences molecular
weight and molecular weight distribution, polymer structure, and composition as well as some

of the physical characteristics, the kind of reactor also influences these factors. The reactor must
allow adequate temperature control, mix of reactants and, if needed, catalysts (and at times a
number of additives), reactant homogeneity, blending/mixing, and so on. It must also allow for
the economical “mass production” of the material. While there exists a wide variety of commer-

cial reactors we will look at only three of the most used styles—batch, plug flow, and continuous
stirred tank reactors (CSTR).

M.1 BATCH

In batch reactions, the reactants are added (charged) to the reactor, mixed for a specific time and
temperature, and then removed (discharged). Batch reactors are generally simple and can vary from
being relatively small (such as gallon size) to large (several hundred gallon size) with the reaction
occurring under varying conditions throughout the reaction vessel with time giving products that
vary with time and secondarily, location within the vessel. This second condition is referred to as
the polymerization occurring under nonsteady state or unsteady state conditions.
The general material balance can be described as follows:
Rate of monomer = Rate of monomer + Rate of monomer loss + Rate of polymer
flow into reactor flow from reactor through reaction accumulation
in reactor

In a batch system, the first two terms are zero since monomer is only added once and leaves only
once, after the reaction is completed.
Thus,

0 = Rate of monomer loss through reaction + Rate of polymer accumulation
0 = d[M]/dt + R p
or −d[M]/dt = R p
For free radical polymerization we have
1/2
R = k′[M][I] = k′′[M] or
p
dt = d[M]/k′′[M]
Integration gives
log([M]/[M ]) = −k′′t and
0
–k
[M] = [M ]e ′′t and
o
–k t
% Conversion = 100 ([M ] − [M]/[M ]) = 100(1 − e ′′)
0
0
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