Overcoming a Tight CoilTo Give a Random “Co”Polymer Derived from a Mixed Sandwich Cobaltocene.

Reversible addition–fragmentationtransfer (RAFT) polymerizationof a η5-cyclopentadienylcobalt-η4-cyclobutadiene (CpCoCb) containing monomer under a wide varietyof 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 beingconsumed over the course of the reaction, there was no significantincrease in the molecular weight of the resulting polymer. It wasdetermined that as the polymer chain grows (DP ≈ 10), a tightcoil morphology was adopted, which hinders the approach of an additional,sterically demanding CpCoCb-containing monomer. This resulted in prematuretermination/chain transfer reactions rather than an increase in thepolymer chain length. To address this problem, methyl acrylate (MA)with its lower steric demand was copolymerized with the bulky CpCoCb-containingmonomer to act as a spacer. This provided the necessary steric reliefand an opportunity for the metallopolymer to grow. This copolymerizationresulted in dramatic improvements in the polydispersity and molecularweight of the end material. In subsequent experiments, the randomcopolymer was used as a macro-RAFT agent to prepare diblock copolymers,with good control over the molecular weight, allowing for an examinationof the self-assembly behavior of the block copolymer in the solidstate. [ABSTRACT FROM AUTHOR]