Ring-Expansion Metathesis Polymerization: Catalyst-Dependent Polymerization Profiles

Abstract

Ring-expansion metathesis polymerization (REMP) mediated by recently developed cyclic Ru catalysts has been studied in detail with a focus on the polymer products obtained under varied reaction conditions and catalyst architectures. Depending upon the nature of the catalyst structure, two distinct molecular weight evolutions were observed. Polymerization conducted with catalysts bearing 6-carbon tethers displayed rapid polymer molecular weight growth which reached a maximum value at ca 70% monomer conversion, resembling chain-growth polymerization mechanism. In contrast, 5-carbon tethered catalysts lead to molecular weight growth that resembled a step-growth mechanism with a steep increase occurring only after 95% monomer conversion. The underlying reason for these mechanistic differences appeared to be ready release of 5-carbon tethered catalysts from growing polymer rings, which competed significantly with propagation. Owing to reversible chain transfer and the lack of end groups in REMP, the final molecular weights of cyclic polymers was controlled by thermodynamic equilibria. Large ring sizes in the range of 60 -120 kDa were observed at equilibrium for polycyclooctene and polycyclododecatriene, which were found to be independent of catalyst structure and initial monomer/catalyst ratio. While 6-carbon tethered catalysts slowly incorporated into the formed cyclic polymer, the incorporation of 5-carbon tethered catalysts was minimal, as revealed by ICP-MS. Further polymer analysis was conducted using meltstate magic-angle spinning 13 C NMR spectroscopy of both linear and cyclic polymers, which revealed little or no chain ends for the latter topology.

Recent progress on the synthesis of cyclic polymers via ring‐expansion strategies

“…This seminal work demonstrated one of the first routes toward ultra‐high molecular weight cyclic polymers (>100 kDa). Further investigations showed that extremely high molecular weights of >1000 kDa could be achieved through ligand tuning, although these values equilibrated slowly to ∼500 kDa in the presence of a residual catalyst . More information on the scope and applications of REMP can be found in other recent reviews …”

Cis-Selective Ring-Opening Metathesis Polymerization with Ruthenium Catalysts

“…3 Furthermore, ROMP can also be used to prepare polymers with specific microstructures comprising various tacticities (e.g. atactic, isotactic, syndiotactic), double bond arrangements ( cis/ trans ), or different relative monomer configurations (e.g.…”

Synthesis and Direct Imaging of Ultrahigh Molecular Weight Cyclic Brush Polymers

“…Our group has developed a series of Ru-based catalysts (Scheme 1) that mediate ringexpansion metathesis polymerization (REMP) to produce high MW cyclic polymers under simple experimental conditions without necessitating high dilution conditions by an end-linking approach. [7] With the FrechØt group, we recently showed that cyclic dendronized polymers synthesized using these catalysts indeed adopted toroidal shapes with internal diameters of 5-7 nm. [8] During publication of this manuscript, the Tew group also reported the use of REMP for the synthesis of cyclic polymer templates that were converted to cyclic brush polymers via a "graft-to" approach.…”