Control of the rapid rate of anionic polymerization of styrene monomer has
presented problems. Past attempts to overcome these problems have resulted
in poor economics.
This review deals with a new approach, which turns the previous disadvantage
of high speed into a major economic advantage. This new approach reduces total
fixed capital from $45.5 million for a conventional 250 million lb/yr polystyrene
plant to $32.5 for an anionic plant. The profit would be increased by 1.94
cents/lb at the same selling price (or $4.5 million/yr extra profit).
The process examined uses continuous feed preparation and finishing with batch
polymerization. The polymerization is carried out in a single reactor operated
in two stages. The first stage is at a controlled low temperature where boiling
removes the heat of polymerization. The second adiabatic stage starts when
50% of the styrene has polymerized and polymerization is complete in minutes
as temperature rises to 240°C. Three reactors are used alternately with
one always emptying.
Thermal termination is an order of magnitude slower than propagation at 240°C
but still takes place rapidly throughout the reactor. This termination is
necessary to avoid formation of extremely long molecules that would ultimately
become gels in the product.
