Depolymerization of Cross-Linked Polybutadiene Networks in Situ Using Latent Alkene Metathesis

Abstract

We report a novel approach whereby cross-linked polybutadiene (PB) networks can be depolymerized in situ based on thermally activated alkene metathesis. A commercially available latent Ru catalyst, HeatMet, was compared to the common second-generation Hoveyda–Grubbs catalyst, HG2, in the metathetic depolymerization of PB. HeatMet was found to possess exceptional stability and negligible activity toward PB under ambient conditions, in solution and in bulk. This enabled cross-linked networks to be prepared containing homogeneously distributed Ru catalyst. The dynamic mechanical properties of networks containing HeatMet and cross-linked using alcohol–isocyanate or thiol–ene chemistry were evaluated during cross-linking and post-cross-linking under isothermal and nonisothermal heating. In both cases, above minimum catalyst loadings ranging from 0.004 to 0.024 mol %, the networks exhibited rapid degelation into a soluble oil upon heating to 100 °C. At these temperatures, extensive depolymerization of the PB segments through ring-closing metathesis of 1,4/1,2 diads by the activated HeatMet introduced network defects in significantly greater proportion than the original number of cross-links. The in situ depolymerization of cross-linked PB networks through latent catalysis, as described here, may enable facile disposal and recycling of PB encapsulants and adhesives, among other applications.

Selectively Depolymerizable Polyurethanes from Unsaturated Polyols Cleavable by Olefin Metathesis

“…Therefore, considerable optimization is needed from an economic standpoint. Along these lines, we have previously shown [ 64 ] that embedding latent metathesis catalysts in polybutadiene elastomers enables their solvent‐free depolymerization to oils at extremely low catalyst loadings–two orders of magnitude lower (0.02 mol%) than used at present (2 mol%). Moreover, metathesis catalysts can potentially be recovered and recycled in order to recover a fraction of their added cost.…”

“…Olefin metathesis is an attractive route to depolymerization of unsaturated rubbers, critically enabled by the nuanced chain microstructure (1,4‐ and 1,2‐addition products) characteristic of polydienes. [ 45–64 ] Adjacent double bonds in side‐ and main‐chain configurations are ring‐closed by Ru carbenes (Grubbs’ catalysts) to cyclopentene and cyclohexene species with concomitant cleavage of the polymer chain. [ 48,57,64 ] The further realization that cyclopentene, functionalized cyclopentenes, and other five‐membered alkene rings can be ring‐opened through judicious selection of reaction conditions has spawned recent development of polypentenamer elastomers and related materials, including networks with continuous recyclability through sequential ring‐opening and ring‐closing metathesis.…”

“…[ 45–64 ] Adjacent double bonds in side‐ and main‐chain configurations are ring‐closed by Ru carbenes (Grubbs’ catalysts) to cyclopentene and cyclohexene species with concomitant cleavage of the polymer chain. [ 48,57,64 ] The further realization that cyclopentene, functionalized cyclopentenes, and other five‐membered alkene rings can be ring‐opened through judicious selection of reaction conditions has spawned recent development of polypentenamer elastomers and related materials, including networks with continuous recyclability through sequential ring‐opening and ring‐closing metathesis. [ 65–72 ] Inspired by this general motif, we report here a series of novel PUs derived from carefully designed, unsaturated polyols, enabling facile depolymerization of the PUs by olefin metathesis.…”

Harnessing autocatalytic reactions in polymerization and depolymerization

“…For example, using programmed protocols, the molecular weight distribution (MWD) profile can be biased towards monotonic, bimodal, square, triangle, and complex MWD profiles in depolymerization reactions. In such configurations, feedback obtained about specific reaction mechanisms in depolymerization of vinyl polymers, [56] hydrolysis of polyesters, [57] metathesis reactions, [58] etc. will be unique, especially when optimized in the context of a continuous flow chemistry set-up.…”

Reactivity and Selectivity in Ruthenium Sulfur‐Chelated Diiodo Catalysts

“…As shown in Table 2, Ru-S-CF 3 -I efficiency promoted RCM of benchmark di-and tri-substituted substrates (entries 1-5) and self-CM of terminal olefins (entries 6,7), although in these latter cases light activation at room temperature proved to be somewhat less efficient. Moreover, and although many ruthenium complexes have been shown to depolymerize cis-polybutadiene (PBD), [18] a stable mixture of Ru-S-CF 3 -I and PBD in THF could be secured in the dark for more than 24 h. Happily, depolymerization readily ensued when the mixture was irradiated with green light. Moreover, when Ru-S-CF 3 -I was mixed with neat DCPD, a cross-linked polymer was obtained after 30 min of irradiation at 510 nm ( Figure 5).…”