Synthesis of block, graft, segmented and ion-containing copolymers
Multiphase polymeric materials may be prepared as block, graft, segmented or ion-containing copolymers. This review provides a discussion of the important synthetic techniques that may be utilized to prepare these materials. The review includes critical comments on synthetic processes reported in the literature. Block copolymers have been prepared principally by living anionic mechanisms into various architectures, including di-, tnand star-shaped structures. Multiple blocks, or as they are more commonly known, segmented systems, have produced important materials via polyurethane, polyester-ether, and siloxane chemistry. The general features and architectural microstructures possible are reviewed. Graft copolymers have been utilized traditionally to prepare rubber toughened thermoplastics such as impact polystyrene and ABS. Such materials though valuable, tend to be relatively ill-defined due to the free radical nature of the grafting reactions. More recently, the approach of preparing so-called macromonomers has been introduced by Milkovich and coworkers. Here, one attempts to provide a polymerizable unit at the terminal of a chain. Incorporation of two or more ordered grafts in a subsequent free radical copolymerization has greatly expanded the utility and definition of graft copolymerization. Indeed, many reports have appeared around the world that have utilized this technique in the last two or three years. Lastly, techniques to produce ion-containing copolymers are reviewed. These systems are usually prepared either by direct copolymerization in the case of carboxyl containing systems or via post-reactions for sulfonated ion-containing systems. Quite recently, attempts have been made to provide much better defined location of the ion-containing groups through cationic, step-growth (the "halato" materials) and by well-defined anionic polymerization followed by subseauent hydrolysis of bulky alkyl methacrylate groups. Such ion-containing polymers show promise in allowing for the first time a model system that is suitable for detailed characterization studies. In summary, the multi-phase syntheses described in this review encompass a variety of important materials which can only be expected to expand significantly in the future.