Overcoming a Tight Coil
To Give a Random “Co”
Polymer Derived from a Mixed Sandwich Cobaltocene
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Abstract
Reversible addition–fragmentation
transfer (RAFT) polymerization
of a η<sup>5</sup>-cyclopentadienylcobalt-η<sup>4</sup>-cyclobutadiene (CpCoCb) containing monomer under a wide variety
of experimental conditions (e.g., different solvents, temperatures,
RAFT agents, concentrations, and [RAFT agent]/[initiator]) was examined.
In all cases the results revealed that although the monomer was being
consumed over the course of the reaction, there was no significant
increase in the molecular weight of the resulting polymer. It was
determined that as the polymer chain grows (DP ≈ 10), a tight
coil morphology was adopted, which hinders the approach of an additional,
sterically demanding CpCoCb-containing monomer. This resulted in premature
termination/chain transfer reactions rather than an increase in the
polymer chain length. To address this problem, methyl acrylate (MA)
with its lower steric demand was copolymerized with the bulky CpCoCb-containing
monomer to act as a spacer. This provided the necessary steric relief
and an opportunity for the metallopolymer to grow. This copolymerization
resulted in dramatic improvements in the polydispersity and molecular
weight of the end material. In subsequent experiments, the random
copolymer was used as a macro-RAFT agent to prepare diblock copolymers,
with good control over the molecular weight, allowing for an examination
of the self-assembly behavior of the block copolymer in the solid
state