Recent Advances in Anionic Synthesis of Functionalized Polymers

There has been growing interest and research on new synthetic methods for the preparation of well-defined polymers with in-chain and chain-end functional groups. These functional groups in polymers can participate in (a) reversible ionic association; (b) chain extension, branching or crosslinking reactions with polyfunctional reagents; (c) coupling and linking with reactive groups on other oligomer or polymer chains; and (d) initiation of polymerization of other monomers. It is noteworthy that the use of end-functionalized polybutadienes formed by reaction of poly(butadienyl)lithium with 4,4′-bis(diethylamino)-benzophenone has been reported to provide marked improvements in the wear and traction properties of tires. In order to exploit the unique potential of functionalized polymers, it is important to consider the scope and limitations of current functionalization methodology using anionic polymerization. Anionic polymerization approaches the goal of synthesizing polymers with predictable, well-defined structures in certain systems such as diene, styrene, methacrylate, and heterocyclic monomers, which proceed in the absence of chain termination and chain transfer reactions. These living polymerizations generate stable, anionic polymer chain ends when all of the monomer has been consumed. In principle, these anionic chain ends can react with a variety of electrophilic species to generate a diverse array of functional groups. Unfortunately, many of the reported functionalization reactions have not been well characterized. Another limitation of the use of specific electrophilic functionalization reactions is the necessity of developing, optimizing, and characterizing new procedures for each different functional group. Variables such as chain-end structure, solvent, temperature, concentration, stoichiometry, mode of addition of reagents, and polar additives can have dramatic effects on yield and product distributions. This review will first provide a critical overview of some recent developments in the use of specific functionalization reactions to prepare polymers labeled with carboxyl, hydroxyl, amino, and sulfonate end groups via alkyllithium-initiated polymerization methods. In addition, a recently developed methodology will be described which utilizes the addition reactions of organolithium compounds to substituted 1,1-diphenylethylenes as a general, quantitative functionalization reaction, independent of the specific functional group.