Industrial Aspects - Production of Living Polymers

In general, the requirements for purity of the reagents, solvents, and monomers are much higher in anionic polymerization than in free-radical polymerization. Necessary distillations and/or adsorption towers, often with activated aluminum, increase the costs for feed preparation compared to free-radical processes. 

Anionic polymerization of styrene has attracted much attention during recent years and several publications and patents have been published on this issue. The main reason for this interest is the high rate of polymerization which can be achieved compared to free-radical polymerization, and the low content of residual styrene and oligomers in the final polymer [145-147]. 

Polybutadiene rubbers are produced either with transition metal catalysts to give high-cis-butadiene rubber or with hthium alkyls to give medium-ds-butadiene rubber (see Table 7.7). The latter is mainly used as the mbber component in the production of HIPS (high impact polystyrene). 

These rubbers are generally produced in aliphatic solvents [6, 157-159] in a CSTR or a series of CSTRs. The vinyl content, which is usually about 10% in aliphatic solvents, can be varied by the addition of polar compounds (see Table 7.3) over a wide range. 

Copolymers from styrene and butadiene (SBRs) with a more or less random comonomer distribution are produced either in an emulsion polymerization process (cold or hot E-SBR), a free-radical polymerization, which however gives crosslinked polymers, or in a solution process using lithium initiators (S-SBR). There are continuous processes [157,158,160-166] as well as (semi-) batch processes (167,168) with a controlled feed of butadiene [133, 169] to maintain a constant monomer ratio. Randomizers are often used [131, 170] to control the comonomer distribution. 

---