Polyolefin elastomers are widely employed commodity polymers. There remains a desire to expand the structural diversity of polyolefin elastomers to facilitate their use in additional applications. Polyolefin elastomers may be graft copolymers that comprise: a first polymer chain comprising at least a first olefinic monomer comprising isobutylene and a second olefinic monomer bearing a benzylic carbon atom or an allylic carbon atom, and a second polymer chain bonded to the benzylic carbon atom or the allylic carbon atom of the first polymer chain. The second polymer chain comprises at least one monomer that is not present in the first polymer chain. The second polymer chain may be bonded to the first polymer chain by generating a carbanion upon the benzylic carbon atom or the allylic carbon atom and growing the second polymer chain by anionic polymerization.