Controlled Cyclopolymerization of 1,5-Hexadiynes to Give Narrow Band Gap Conjugated Polyacetylenes Containing Highly Strained Cyclobutenes

Abstract

For decades, cyclopolymerization of α,ω-diyne derivatives has been an effective method to synthesize various soluble polyacetylenes containing fiveto seven-membered rings in the backbone. However, cyclopolymerization to form fourmembered carbocycles was considered impossible due to their exceptionally high ring strain (∼30 kcal/mol). Herein, we demonstrate the successful cyclopolymerization of rationally designed 1,5-hexadiyne derivatives to afford various polyacetylenes containing highly strained cyclobutenes in each repeat unit. After screening, Ru catalysts containing bulky diisopropylphenyl groups promoted challenging four-membered ring cyclization efficiently from various monomers, enabling the synthesis of high molecular weight (up to 40 kDa) polyacetylenes in a controlled manner. Furthermore, living polymerization allowed for block copolymer synthesis by combining with ring-opening metathesis polymerization as well as block copolymerization of two different 1,5hexadiyne monomers to give a fully conjugated polyacetylene. These new polymers unexpectedly showed much narrower band gaps than conventional substituted polyacetylenes by >0.2 eV. Interestingly, computational studies showed much smaller bond length alternation in the conjugated backbone containing cyclobutenes, resulting in highly delocalized π electrons along the polymer chain and lower band gaps.

Selective Ring Opening Allene Metathesis: Polymerization or Ruthenium Vinylidene Formation

“…Although elevated temperatures are generally deleterious to metathesis, serving to shorten catalyst lifetimes while increasing entropic penalties associated with the polymerization 29 , they have been successfully employed in systems where increased catalytic activity are needed or slow propagation is observed. 30 At elevated temperatures, increased conversions compared to room temperature trials were obtained (44% vs. 81% at ([M] 0 /[I] 0 = 50). Molecular weights remained unchanged, likely due to increased chain transfer.…”

Selective Ring-Opening Allene Metathesis: Polymerization or Ruthenium Vinylidene Formation

“…Although elevated temperatures are generally deleterious to metathesis, serving to shorten catalyst lifetimes, while increasing entropic penalties associated with the polymerization, 33 they have been successfully employed in systems where increased catalytic activity is needed or slow propagation is observed. 34 At elevated temperatures, increased conversions compared to room temperature trials were obtained (44% vs 81% at ([M] 0 /[I] 0 = 50). Molecular weights remained unchanged, likely due to increased chain transfer.…”

“…We hypothesize that elevated temperatures allow the system to combat the slow propagation alongside the relatively unstable propagating species. Although elevated temperatures are generally deleterious to metathesis, serving to shorten catalyst lifetimes, while increasing entropic penalties associated with the polymerization, they have been successfully employed in systems where increased catalytic activity is needed or slow propagation is observed . At elevated temperatures, increased conversions compared to room temperature trials were obtained (44% vs 81% at ([M] 0 /[I] 0 = 50).…”

Controlled Alternating Metathesis Copolymerization of Terminal Alkynes

“…Strategies have also been developed for sequence-controlled ROMP polymerization of low-strained monomers. − Among these, tandem ring-opening–ring-closing enyne metathesis has been studied most. − Even though, the alkyne undergoes spontaneous metathesis polymerization via an intramolecular tandem/cascade process, the intermolecular homo- or copolymerization of such alkyne derivatives with Ru-based metathesis catalysts is still out of reach.…”