Overcoming a Tight Coil To Give a Random “Co” Polymer Derived from a Mixed Sandwich Cobaltocene

Reversible addition–fragmentation transfer (RAFT) polymerization of a η⁵-cyclopentadienylcobalt-η⁴-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

Multifunctional Block Copolymer: Where Polymetallic and Polyelectrolyte Blocks Meet

Sequential reversible addition–fragmentation transfer (RAFT) polymerization of a mixed sandwich cobaltocene monomer (η⁵-cyclopentadienyl-cobalt-η⁴-cyclobutadiene (CpCoCb)) and a phosphonium salt functionalized styrene monomer resulted in the first example of a unique multifunctional block copolymer consisting of a metallopolymer block and a polyelectrolyte block. The polyelectrolyte block was decorated with a gold anion (AuCl₄^–) via salt metathesis, resulting in a heterobimetallic block copolymer with distinct gold and cobalt sections. Solution self-assembly behavior of this unique metallopolymer-<i>b</i>-polyelectrolyte copolymer before and after salt metathesis was studied. Heterobimetallic micelles with a gold containing core and a cobalt-containing corona were obtained, and then the core was reduced to form gold nanoparticles (AuNPs). Studies on the solid-state self-assembly of this unique block copolymer showed that it phase separated into hexagonally packed cylinders. Salt metathesis of the phase-separated block copolymers was utilized as the first example of a nonstandard selective staining method to exclusively stain the polyelectrolyte domains with the AuCl₄^– anion. Staining the metallopolymer domain by RuO₄ provided the complementary pattern. Pyrolysis of the self-assembled block copolymers resulted in magnetic cobalt-phosphate nanoparticles with 17% char yield