Regioselective Carbyne Transfer to Ring-Opening Alkyne Metathesis Initiators Gives Access to Telechelic Polymers

Abstract

Regioselective carbyne-transfer reagents derived from (3,3,3-trifluoroprop-1-yn-1-yl)benzene give access to functionalized ring-opening alkyne metathesis polymerization (ROAMP) initiators [R-CHC≡Mo(OC(CH)(CF))] featuring electron-donating or -withdrawing substituents on the benzylidyne. Kinetic studies and linear free-energy relationships reveal that the initiation step of the ring-opening alkyne metathesis polymerization of 5,6,11,12-tetradehydrobenzo[a,e][8]annulene exhibits a moderate positive Hammett reaction constant (ρ = +0.36). ROAMP catalysts featuring electron-withdrawing benzylidynes not only selectively increase the rate of initiation (k) over the rate of propagation (k) but also prevent undesired intra- and intermolecular chain-transfer processes, giving access to linear poly-(o-phenylene ethynylene) with narrow molecular weight distribution. The regioselective carbyne transfer methodology and the detailed mechanistic insight enabled the design of a bifunctional ROAMP-reversible addition-fragmentation chain-transfer (RAFT) initiator complex. ROAMP followed by RAFT polymerization yields hybrid poly-(o-phenylene ethynylene)-block-poly-(methyl acrylate) block copolymers.

Well‐Defined Alkyne Metathesis Catalysts: Developments and Recent Applications

“…[171] Recently,t he influence of the catalyst's alkylidyneu nit on ring-opening alkyne metathesis polymerization was investigated more in-depth. For this purpose, anovel methodwas developed, which allowed the systematic transfer of various functionalized alkylidyne moietiest op ropylidyne complex 60 to form molybdenumc omplexes of the type [p-XC 6 [172] Conventional cross-metathesis of 60 with 1,2-diarylethynes or 1-aryl-1-alkynes in order to transfer the aromatic alkylidyne moietyo nto the molybdenum complex usually suffered from multiple drawbacks like formation of product mixtures, inseparable compounds or generallyl ow selectivity.…”

“…Fischera nd his group established ac onvenient cross-metathesis protocol, which met the requirements for as uccessful reaction such as volatile side products, high regioselectivity and mild reaction conditions. [172] This was accomplished with (3,3,3trifluoroprop-1-yn-1-yl)benzenes as coupling substrates. The electron-deficient trifluoromethyl groupsd irected at ransfer of the aromatic group to the metal centera nd the resulting volatile trifluoropentyne was less likely to undergo furtherr eaction (Scheme 23).…”

“…Furthermore, this strategy produced bifunctional ROAMP-RAFT initiators which were used to form block copolymers. [172] Am ethodf or controlled chain terminationo fl ivingR OAMP through ynamines and ynamides such as 66 was introduced also by the group of Fischer,w hiche nabled selective functional group transfer to the end of polymer chains (Scheme 24). [173] The functionalized end-groups were used to initiate ring-opening polymerization of e-caprolactone to access amphiphilic block copolymers.…”

“…Conventional cross‐metathesis of 60 with 1,2‐diarylethynes or 1‐aryl‐1‐alkynes in order to transfer the aromatic alkylidyne moiety onto the molybdenum complex usually suffered from multiple drawbacks like formation of product mixtures, inseparable compounds or generally low selectivity. Fischer and his group established a convenient cross‐metathesis protocol, which met the requirements for a successful reaction such as volatile side products, high regioselectivity and mild reaction conditions . This was accomplished with (3,3,3‐trifluoroprop‐1‐yn‐1‐yl)benzenes as coupling substrates.…”

Regioselective Termination Reagents for Ring-Opening Alkyne Metathesis Polymerization

“…We investigated the cross-metathesis of 1 with 2 equiv of the terminating reagent 2b featuring a toluene sulfonamide group. 19 F NMR reveals a complete consumption of 1 within 20 min at 24 °C. 1 H NMR shows the formation of 1-methyl-4-(phenylethynyl)benzene (3b) as the only cross-metathesis product, next to a series of paramagnetically shifted resonances that were assigned to the metallacyclobutadiene complex 4b.…”