5 - Styrenic Block Copolymers

Abstract

Styrenic block copolymers (SBCs) are based on simple molecules of the type A–B–A, where A is polystyrene and B is an elastomeric segment. The most common structure of SBCs is that where the elastomeric segment is a polydiene, such as polybutadiene or polyisoprene. Block copolymers of the S–B–S and S–I–S type are made by anionic polymerization, which is applicable only to three common monomers—namely, (1) styrene (including substituted styrenes), (2) butadiene, and (3) isoprene. The solvents usually used are inert hydrocarbons, such as cyclohexane or toluene. Oxygen, water, or any other impurity must be completely eliminated to prevent undesirable reaction of the highly reactive propagating species. There are essentially three synthetic methods for the preparation of this type of SBCs: sequential, coupling, and multifunctional initiation. In sequential method, polymerization starts at one end of the molecule and continues to the other end. In coupling polymerization starts at each end of the molecule and then the reactive chains are joined together by a coupling or linking agent. In multifunctional initiation, the polymerization starts at the center of the molecule and continues to the ends, using initiators that have more than one active group. A recent development in the synthesis of SBCs is the living carbocationic polymerization, producing well-defined poly (styrene-b-isobutylene-b-styrene) (S–IB–S).