Selectively Depolymerizable Polyurethanes from Unsaturated Polyols Cleavable by Olefin Metathesis

Abstract

This communication describes a novel series of linear and crosslinked polyurethanes (PUs) and their selective depolymerization under mild conditions. Two unique polyols are synthesized bearing unsaturated units in a configuration designed to favor ring‐closing metathesis (RCM) to five‐ and six‐membered cycloalkenes. These polyols are co‐polymerized with toluene diisocyanate to generate linear PUs and trifunctional hexamethylene‐ and diphenylmethane‐based isocyanates to generate crosslinked PUs. The polyol design is such that the RCM reaction cleaves the backbone of the polymer chain. Upon exposure to dilute solutions of Grubbs’ catalyst under ambient conditions, the PUs are rapidly depolymerized to low molecular weight, soluble products bearing vinyl and cycloalkene functionalities. These functionalities enable further re‐polymerization by traditional strategies for polymerization of double bonds. It is anticipated that this general approach can be expanded to develop a range of chemically recyclable condensation polymers that are readily depolymerized by orthogonal metathesis chemistry.

Chemical recycling of polyolefins via ring-closing metathesis depolymerization

“…Capitalizing on the low RSEs of cyclohexene and cyclopentene, the Jones lab developed a clever synthetic approach to depolymerizable polyurethanes. 47 They began the study by designing two diol monomers which contain unsaturated alkenes in a configuration favoring ring-closing metathesis to cyclohexene and cyclopentene derivatives. These diol monomers were incorporated into the polymer backbone by their polycondensation with diisocyanates and triisocyanates, generating linear and crosslinked polyurethanes, respectively.…”

Deconstruction of Polymers through Olefin Metathesis

“…In a strategy different from what has been discussed so far, Jones and co-workers synthesized polyurethanes which incorporated diols possessing internal and pendant olefins such that they could be cleaved via ring-closing metathesis between the pendant and internal olefins to form low-strain 5or 6-membered cyclic olefins. 88 In the presence of ∼2 mol % HG2 catalyst, these polymers could be degraded almost quantitatively to yield dicarbamate small molecules with different combinations of either allyl and/ or cycloalkene groups (Scheme 15b). The authors further prepared polymer networks based on this chemistry.…”

“…In a strategy different from what has been discussed so far, Jones and co-workers synthesized polyurethanes which incorporated diols possessing internal and pendant olefins such that they could be cleaved via ring-closing metathesis between the pendant and internal olefins to form low-strain 5- or 6-membered cyclic olefins . Initially, two different polyurethanes were prepared by a step-growth reaction between 2,4-toluene diisocyanate ( M61-NCO ) and either E -nona-3,8-diene-1,5-diol or E -deca-3,9-diene-1,5-diol ( M61a – b ) in the presence of triphenylbismuth (Scheme a, P61a – b ).…”

“…Alkenolysis has not only been utilized to depolymerize rubber polymers for recycling, microstructure analysis, , and synthesis of telechelics, ,,, which have been discussed in detail in several reviews, ,− but has also been recently employed together with dehydrogenation and isomerization to turn polyethylene into value-added propylene monomer. , The mechanism is discussed in Section (Schemes –).…”

Tandem metathesis depolymerization and cyclopolymerization toward flexible–rigid block copolymers with unique damping properties

“…2 It is well-known that the olefin metathesis reaction is a powerful synthetic methodology with numerous applications in organic and polymer syntheses. 3 Consequently, the interest in exploring NR-based high performance or functional materials by the olefin metathesis technique has grown enormously. For instance, NR could be transformed into oligo-1,4- cis -isoprene 1 or telechelic polyisoprene (PIp) oligomers 4 by controlled metathetic depolymerization (MDP).…”