Chemical modification of polymers via catalysis is of great importance as it provides an efficient synthetic route for the production of novel polymers with desirable physical properties. It also allows the introduction of functional groups that are often inaccessible by conventional polymerization techniques. One of the most important chemical modifications is the hydrogenation of unsaturated carbon-carbon double bonds in polymers. Basically, there are two technical routes for the hydrogenation of polymers: homogeneous and heterogeneous. This chapter reviews research and process development with respect to homogeneous catalytic hydrogenation of diene-based polymers.
19.1.1 Diene-Based Polymers
Diene polymers refer to polymers synthesized from monomers that contain two carbon-carbon double bonds (i.e., diene monomers). Butadiene and isoprene are typical diene monomers (see Scheme 19.1). Butadiene monomers can link to each other in three ways to produce cis-1,4-polybutadiene, trans-1,4-polybutadiene and 1,2-polybutadiene, while isoprene monomers can link to each other in four ways. These dienes are the fundamental monomers which are used to synthesize most synthetic rubbers. Typical diene polymers include polyisoprene, polybutadiene and polychloroprene. Diene-based polymers usually refer to diene polymers as well as to those copolymers of which at least one monomer is a diene. They include various copolymers of diene monomers with other monomers, such as poly(butadiene-styrene) and nitrile butadiene rubbers. Except for natural polyisoprene, which is derived from the sap of the rubber tree, Hevea brasiliensis, all other diene-based polymers are prepared synthetically by polymerization methods.