Discrete and Stereospecific Oligomers Prepared by Sequential and Alternating Single Unit Monomer Insertion

Natural
biopolymers, such as DNA and proteins, have uniform microstructures
with defined molecular weight, precise monomer sequence, and stereoregularity
along the polymer main chain that affords them unique biological functions.
To reproduce such structurally perfect polymers and understand the
mechanism of specific functions through chemical approaches, researchers
have proposed using synthetic polymers as an alternative due to their
broad chemical diversity and relatively simple manipulation. Herein,
we report a new methodology to prepare sequence-controlled and stereospecific
oligomers using alternating radical chain growth and sequential photoinduced
RAFT single unit monomer insertion (photo-RAFT SUMI). Two families
of cyclic monomers, the indenes and the N-substituted maleimides,
can be alternatively inserted into RAFT agents, one unit at a time,
allowing the monomer sequence to be controlled through sequential
and alternating monomer addition. Importantly, the stereochemistry
of cyclic monomer insertion into the RAFT agents is found to be trans-selective along the main chains due to steric hindrance
from the repeating monomer units. All investigated cyclic monomers
provide such trans-selectivity, but analogous acyclic
monomers give a mixed cis- and trans-insertion.