Cis,syndiotacticA-alt-B copolymers, where A and B are two enantiomerically pure trans-2,3-disubstituted-5,6-norbornenes
with “opposite”
chiralities, can be prepared with stereogenic-at-metal initiators
of the type M(NR)(CHR′)(OR”)(pyrrolide). Formation of
a high percentage of alternating AB copolymer linkages
relies on an inversion of chirality at the metal with each propagating
step and a relatively fast formation of an AB sequence
as a consequence of a preferred diastereomeric relationship between
the chirality at the metal and the chirality of the monomer. This
approach to formation of an alternating AB copolymer
contrasts dramatically with the principle of forming AB copolymers from achiral monomers and catalysts.