Macroporous polymers have become materials of high interest in recent years, because of their potential applications in diverse areas from membranes to medical devices. Star and block polymers are known to template around water droplets to form isoporous arrays. [2ยฑ5] However, the utilization of this discovery has been severely restricted because the synthetic route used, anionic polymerization, is difficult and limited to a small number of (generally nonfunctional) monomers. A number of other synthetic routes to star polymers have become available in recent years, and the development of living radical polymerization provides a great opportunity to significantly broaden the range of functional materials. We have adopted both metal-mediated radical polymerization and reversible addition ยฑ fragmentation transfer (RAFT) polymerization to synthesize star polymers for application in isoporous film and honeycomb-structure production. The porous structures are created from casting solutions of star polymers in an organic solvent onto a glass substrate under a humid atmosphere as described by Francois and co-workers. 8] The star polymers were synthesized either by metalmediated polymerization using a copper (atom-transfer radical polymerization, ATRP) or an iron catalyst system or by RAFT polymerization. Both types of polymerization require a multifunctional initiator, from which arm growth occurs. The preparation of 6-arm polystyrene stars by the RAFT process utilized hexakis(thiobenzoylthiomethyl) benzene (1) as an initiator. [9] The polymerization process is shown in Scheme 1. The pseudo first-order reaction kinetics and molecular-weight development were both found to be consistent with a living radical polymerization process. This RAFT method yielded narrow polydispersity polystyrene stars with molecular weights of up to 400 000, and six arms, each with a thioester end group. One complication in this process is the parallel synthesis of a linear chain along with the star structure (Scheme 1).
ATRP/metal-mediated polymerization was also utilized to synthesize stars. Two different approaches were taken, both based on the application of sugar initiators. The first approach we took was based on the work of Haddleton and co-workers