Light as a Catalytic Switch for Block Copolymer Architectures: Metal–Organic Insertion/Light Initiated Radical (MILRad) Polymerization

Abstract

We detail a polymer synthetic methodology that merges the techniques of insertion and radical polymerization methods into a single organometallic catalyst. This metal–organic insertion/light initiated radical (MILRad) polymerization technique proves successful at polymerizing methyl acrylate (MA) and hexene, using light as a critical stimulus to activate the dormant photoresponsive nature of the insertion catalyst. In this study, we describe a novel approach that uses visible light (460 nm) to switch the catalytic activity of a cationic palladium catalyst from an insertion route to a radical process when desired. We discovered that in a mixture of MA and hexene one monomer can be selectively polymerized using light and dark cycles, respectively. As a result, this polymerization process enables the copolymerization of MA and hexene to create homo- and block copolymer architectures facilitated solely by visible light. In this work, we show the synthesis of MA homopolymers in molecular weight ranges (M n 50–400 kDa) with dispersities of ∼1.7. Synthesis of MA (A) and hexene (B) block copolymers were accomplished with a single catalyst in both a sequential and novel one-pot approach, relying solely upon visible light irradiation. A series of BA block copolymers were prepared with tunable monomer compositions, molecular weight ranges of (M n 11–36 kDa), and well-controlled polydispersities (∼1.3–1.6) in a robust rapid synthesis. MILRad polymerization circumvents the need for quantitative conversions during block formation afforded by the orthogonal monomer reactivity dependent upon a light stimulus to acquire distinct polymer architectures with variable block compositions. The use of a photocontrollable “switch” affecting a single organometallic catalyst allows access to block polymers from nonpolar and polar olefins in a novel and facile approach.

A continuing legend: the Brookhart-type α-diimine nickel and palladium catalysts

“…In the process of exploring various Brookhart catalysts, various olefinic monomers have been polymerized and copolymerized. 27,31,[125][126][127] The applicable monomers have also been rapidly growing to include linear 128,129 and branched 130,131 α-olefins, cycloalkyl substituted α-olefins, 132-136 cycloolefins, 137 dienes, 138 trienes, 139,140 internal olefins, [141][142][143][144] norbonene type monomers, [101][102][103][104] and different polar comonomers. 3,27,28 It should be noted that the continuous interest in Brookhart-type catalysts largely originates from their capabilities in copolymerizations of olefins with polar functionalized comonomers.…”

Olefins and Vinyl Polar Monomers: Bridging the Gap for Next Generation Materials

“…More recently,H arth and co-workers have developed anew polymerization technique that combines coordinationinsertion and radical polymerization using asingle catalyst to create well defined block copolymers (Scheme 18). [107] This metal-organic insertion/ light-initiated radical (MILRad) polymerization technique was developed using ap alladium diimine catalyst for the polymerization of hexene and methyl acrylate (MA). It was discovered that upon blue light irradiation, the Pd diimine metal-carbon bond could homolytically cleave and polymerize methyl acrylate via ar adical pathway.T his light initiated radical polymerization proceeds over multiple "ON" and "OFF" cycles,w here no polymerization takes place for MA in the dark.…”

“…More recently, Harth and co‐workers have developed a new polymerization technique that combines coordination–insertion and radical polymerization using a single catalyst to create well defined block copolymers (Scheme ) . This metal‐organic insertion/ light‐initiated radical (MILRad) polymerization technique was developed using a palladium diimine catalyst for the polymerization of hexene and methyl acrylate (MA).…”

Gesteuerte Reaktionsorthogonalität in der Polymer‐ und Materialwissenschaft

“…In einem eleganten Beispiel der Katalysator‐Modulierung für die Dualpolymerisation zeigten Harth et al., dass die katalytische Aktivität eines einzelnen Katalysators gegenüber verschiedenen Polymerisationsprozessen durch externe Stimuli geschaltet werden kann [82] . Im Dunkeln initiierte der eingesetzte kationische Palladiumdiamin‐Katalysator nur die organometallische Insertionspolymerisation von Olefinen, während die Aktivität so geschaltet wurde, dass eine radikalische Polymerisation bei Bestrahlung mit blauem Licht (470 nm) möglich war (Abbildung 28).…”

“… Änderung der Aktivität eines Katalysators für selektive Alken‐ und Acrylatpolymerisationen durch Ein/Aus‐ Schalten der Lichtquelle [82] …”