Solution, as the name implies, takes place from monomers which are dissolved in a solvent. The resultant polymer can either be soluble or insoluble in this solvent. The monomer can be presented in gaseous form as well.
The range of process complexity in this type of polymerization is quite striking. In some simple systems, monomer, catalyst, and solvent form a solution which reacts without having to be either heated or cooled. At the other extreme, some continuously manufactured synthetic rubbers require elaborate techniques and equipment for success.
One of the demanding and most expensive problems associated with solution polymerization is the separation of the polymer and the recovery of the unreacted monomer and solvent. Solution processes are widely used in both batch and continuous polymerizations with all processes extensively using agitation equipment such as in the polymerization reactors and product recovery operations.
Solution Reactors: Both batch and continuous reactors use agitated vessels equipped with jackets and even some internal heat transfer coils. If conditions permit, reflux cooling can also be employed. Continuous reaction schemes frequently involve a number of series reactors. Systems can also involve a number of batch reactors with remaining processing steps accomplished continuously.
In a batch reactor, the degree of completion of the reaction is often monitored by the increase of viscosity and its effect upon turbine agitator power draw. The finished viscosity again determines the agitator used satisfactorily to 75,000 cps.
In either the batch or continuous system, the polymerization reaction is terminated in various ways. More frequently, the polymerization is stopped chemically by addition of a “shortstop or catalyst deactivator;” sometimes the method requires no chemical, but depends upon changing the polymerization conditions such as massive solvent addition or temperature lowering. With chemical short stopping, the batch reaction termination occurs in the reactor while continuous reactors are shortstopped in downstream inline agitators.
Recovery of the Product: Separation of the polymer varies with the specific system. Since the polymers made by solution polymerization are so diverse, the method of isolating the polymer after reaction from unreacted monomer and solvent varies considerably.
Advantages of the solution polymerization method over others are the catalyst is not coated by the polymer thereby increasing the efficiency and simplifying the removal of the catalyst from the product, control of temperature, heat transfer, and reaction rates are also simplified.
Disadvantages include limited solubility of the polymer especially at high molecular weights which can increase vessel size, increase viscosity levels, which are more difficult to agitate and handle, and difficulties associated with separating the polymer from the solvent.
Products which use Solution polymerization include synthetic rubbers, butyl rubber, polyethylene, and polyester resins.