Chemistry Reference
In-Depth Information
Heat removal and mixing are problems in bulk chain-growth polymerizations
for the reasons outlined in the previous section. Thus, homogeneous bulk step-
growth reactions are driven to high conversions to achieve high molecular
weights but the corresponding chain-growth polymerizations are often limited to
lower conversions because of problems in keeping the reaction temperature under
control.
Poly(ethylene terephthalate) and nylon-6,6 manufacture are homogeneous bulk
step-growth reactions. The molecular weight of the polymer produced is limited
by the high viscosity of the reaction mixture at very high conversions. Post poly-
merization techniques such as that described in connection with reaction (5-39)
can be used to increase the polymer molecular weight for some applications.
Polystyrene and poly(methyl methacrylate) polymerizations are typical of
homogeneous bulk chain-growth reactions. The molecular weight distributions of
the products made in these reactions are broader than predicted from consider-
ation of classical, homogeneous phase free-radical polymerization kinetics
because of autoacceleration (Section 8.13.2) and temperature rises at higher
conversions.
Crystal polystyrene is produced by thermally initiated (Section 8.5.4) bulk
polymerization of styrene at temperatures of 120
C or more. (The term
crystal
refers to the optical clarity of products made from this polymer, which is not crys-
talline.) The rate of polymerization would decrease with increasing conversion
and decreasing monomer concentration if the reaction were carried out at constant
temperature. For this reason, the polymerization is performed at progressively
increasing temperatures as the reaction mixture moves through a series of reac-
tors. The exothermic heat of polymerization is useful here in raising the reaction
temperature to about 250
C as the process nears completion.
12.4.1.2
Homogeneous Solution Reactions
Both the monomer and polymer are soluble in the solvent in these reactions.
Fairly high polymer concentrations can be obtained by judicious choice of sol-
vent. Solution processes are used in the production of
cis
-polybutadiene with
butyl lithium catalyst in hexane solvent (Section 11.2.7). The cationic polymeriza-
tion of isobutene in methyl chloride (Section 11.4.7) is initiated as a homoge-
neous reaction, but the polymer precipitates as it is formed. Diluents are
necessary in these reactions to control the ionic polymerizations. Their use is
avoided where possible in free-radical chain growth or in step-growth polymeriza-
tions because of the added costs involved in handling and recovering the solvents.
Advantages of solution reactions include better thermal control and mixing
than in bulk polymerizations. Initiator efficiency is also usually better because of
the lower viscosity and better agitation. Disadvantages include the costs of solvent
removal and recovery. Thermoplastic polymers are recovered from solution poly-
merizations as fluffy powders or slabs which will not flow readily enough in the
hoppers of downstream processing machinery. The powders must be compacted
in a separate melting and granulation process. Rubbers like polybutadiene or butyl
