Storable, Dual-Component Systems for Frontal Ring-Opening Metathesis Polymerization

Frontal
ring-opening metathesis polymerization (FROMP) catalyzed
by Grubbs-type Ru complexes enables new, rapid, and energy-efficient
syntheses of high-performance, structural plastics. Ideal catalysts
survive the extended time periods associated with resin preparation,
storage, and transportation. Current catalysts, however, induce premature
polymerization within hours to days under ambient conditions. In this
work, a thermally latent bis-N-heterocyclic carbene
complex provides exceedingly robust resins, which are viable for 8
weeks. When mixed with CuI coreagents, precatalyst activation
primes the system for rapid reactivity after thermal initiation. In
this study, more than 40 dual-component formulations successfully
catalyzed FROMP of dicyclopentadiene. The polymerization process parameters
(front temperatures and velocities), resin storability, and resultant
polymer properties (e.g., Tg) were determined for each composition. Intriguingly,
the Cu to Ru ratio dramatically impacts the observed frontal velocity
and temperature, as well as the polymer glass-transition temperature;
slower, colder reaction fronts result from formulations with large
Cu to Ru ratios. The resultant polymers display lower Tg values. Mechanistic analysis of a related model system
demonstrated that an excess Cu reagent decreases the activation and
polymerization rates.