Chemistry Reference
In-Depth Information
for different applications, and a complete characterization of the molecular struc-
ture of the polymer would require careful SEC and TREF analyses.
Poly(vinyl chloride) (PVC) is produced by mass, suspension, and emulsion
processes. Mass polymerization is an example of a heterogeneous bulk system.
PVC is virtually insoluble in vinyl chloride because the polymer is about 35%
more dense than the monomer under normal polymerization conditions. Vinyl
chloride, however, is quite soluble in polymer. The two phases in PVC polymeri-
zations are pure monomer and monomer-swollen polymer. Polymerization pro-
ceeds in both phases, but it is very much faster in the polymer-rich phase because
the mobility of macro radicals and mutual
termination reactions are severely
restricted (cf. Section 8.13.2).
Cessation of the growth of PVC radicals is caused almost completely by chain
transfer to monomer (Section 8.8.2) rather than by termination by disproportion-
ation or combination. In other words, the relative magnitudes of the various terms
in Eq. (8-75) are such that the controlling factor is the
C
M
(
K
tr,M
/
k
p
) term. Since
the ratio of these rate constants depends on temperature, the number average
molecular weight of the product polymer is controlled simply by the reaction tem-
perature and shows little dependence on initiator concentration or
5
rate of
polymerization.
A comparison of the basic elements of the bulk polymerizations of styrene
and vinyl chloride is instructive. Styrene and polystyrene are miscible, and the
rate of polymerization and the consequent rate of heat generation will decay
slowly with conversion in the absence of autoacceleration, which is a relatively
weak effect in this system. For this reason polystyrene bulk reactions are operated
with rates of heat generation close to the limits of the heat exchange systems in
the reactor walls. It is not possible to produce PVC with the same reactors, how-
ever. The maximum rate at which heat can be removed from a bulk polymeriza-
tion always decreases with conversion because of the progressive increase in
viscosity of the reaction mixture. The decay in heat transfer efficiency is much
greater in bulk PVC production than in polystyrene manufacture, because the sus-
pension of PVC in its monomer becomes very viscous at low conversions and is
converted to a poorly conducting monomer-wetted powder at about 20% reaction.
In addition, the rate of polymerization and the rate of heat generation rise steadily
in PVC syntheses because the physical state of the system suppresses normal radi-
cal termination reactions. The PVC reaction would run out of control unless pro-
visions were made to increase the rate of heat removal beyond that needed in
homogeneous polymerizations like that of styrene. This is accomplished by using
suspension, emulsion, or special bulk reaction systems.
PVC bulk polymerizations are two-stage processes in which a very porous
PVC seed particle is produced in a vessel provided with very high-speed agita-
tion. The wet polymer powder from this stage is fed to horizontal autoclaves in
which the polymerization is finished. Reaction heat is removed by cooling the
helical ribbon blender-type agitators as well as the vessel jacket.
