Chemistry Reference
In-Depth Information
ΔH
polym
is 56.5 kJ mol
2
1
for methyl methacrylate and
95.0 kJ mol
2
1
for ethylene polymerization.) This is relatively high compared to
the heats of polymerization in the more common step-growth polymerizations.
The activation energy for free-radical polymerizations is about the same as for
step-growth reactions (Section 8.16.1) but small increases in temperature give rise
to larger increases in heat generation in free-radical reactions because of their
greater exothermic heats of polymerization.
Chain-growth polymerizations that are carried out in the absence of a diluent
are characterized by high viscosity and poor heat transfer. High-molecular-weight
polymer coexists with low-molecular-weight species and monomer, and molecular
weight does not increase with conversion. Mixing becomes difficult in such
chain-growth polymerizations at intermediate levels of conversion, and the heat
transfer efficiency of the reactor decreases as a consequence. The low heat trans-
fer coefficients become more serious as the conversion increases, because the rate
of polymerization and the rate of heat generation often accelerate in these stages
of the reaction (Section 8.13.2). Since the decomposition rate of thermal initiators
is highly dependent on temperature (Section 8.5.1), faster initiation can further
accelerate an increased rate of free-radical polymerization. Runaway reactions
can thus occur unless special precautions are taken.
mol.
(For example,
12.4
Homogeneous and Heterogeneous Polymerization
Processes
Polymerization processes are usefully classified as follows:
1. Homogeneous systems
comprising (a) bulk reactions and (b) solution systems.
2. Heterogeneous systems
comprising (a) heterogeneous bulk polymerizations,
(b) heterogeneous solution polymerizations, (c) suspension systems, (d)
emulsion systems, (e) dispersion polymerization, (f) gas phase polymerization,
and (g) interfacial polymerizations.
12.4.1
Homogeneous Systems
12.4.1.1
Homogeneous Bulk Reactions
In bulk polymerizations, the initial reaction mixture consists essentially of mono-
mer. If the process is a chain-growth reaction, the mixture will also contain initia-
tor and chain transfer agent, if needed. If the polymer and monomer are miscible,
the system remains homogeneous during the polymerization reaction.
Bulk reactions are attractive for step-growth polymerizations. Heat removal is
not a serious problem, because such polymerizations are not highly exothermic.
Mixing and stirring are also not difficult until the last stages of the reaction, since
the product molecular weight and the mixture viscosity remain relatively low until
high conversions are reached.
