Metallocene-Mediated Olefin Polymerization with B−H Chain Transfer Agents:  Synthesis of Chain-End Functionalized Polyolefins and Diblock Copolymers

A systematic study of the scope and limitations of B−H chain transfer agents during metallocene-mediated olefin polymerization is discussed in this contribution. The polymerization procedures provide a convenient route to prepare chain-end functionalized polyolefins and polyolefin diblock copolymers containing both polyolefin and functional polymer blocks. With the proper choice of borane chain transfer agents, metallocene catalyst systems, and reaction conditions, the chemistry can be applied to a broad range of polyolefin homo- and copolymers, such as polyethylene, polypropylene, syndiotactic polystyrene, poly(ethylene-co-propylene), poly(ethylene-co-1-octene), and poly(ethylene-co-styrene). The molecular weight of the borane-terminated polyolefin is basically inversely proportional to the molar ratio of [borane]/[olefin]. In turn, the terminal borane group is very reactive, which can be quantitatively converted to various functional groups and also can be selectively oxidized to form a stable polymeric radical for living free radical polymerization of functional monomers. This process resembles a transformation reaction from metallocene polymerization to living free radical polymerization via the borane terminal group to produce functional polyolefin diblock copolymers, which are difficult to prepare using conventional initiaors.