Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Light as a Catalytic Switch for Block Copolymer Architectures: Metal−Organic Insertion/Light Initiated Radical (MILRad) Polymerization Anthony Keyes, Hatice E. Basbug Alhan, Uyen Ha, Yu-Sheng Liu, Scott K. Smith, Thomas S. Teets, Dain B. Beezer,* and Eva Harth*
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Department of Chemistry, Center of Excellence in Polymer Chemistry (CEPC), University of Houston, 3585 Cullen Boulevard, Houston, Texas 77030, United States S Supporting Information *
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 (Mn 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 (Mn 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.
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INTRODUCTION Both academia and industry have found great uses for transition-metal-catalyzed olefin polymerizations1 due to their broad molecular weight range, low polydispersity, and highly branched polymers, which all give a tunable set of physical properties based on the preparation. For nonpolar olefins, such as ethylene and propylene, Ziegler−Natta catalysts paved the way for polymerization of these monomers through an insertion-type mechanism.2,3 Brookhart and co-workers made a breakthrough discovery of the cationic α-diimine Pd(II) and Ni(II) catalysts that offered more control over molecular weight and branching.4−6 Since then, a plethora of research has investigated the effects that ligands, monomers, solvents, temperature, time, and light have on organometallic polymerizations.7−23 For example, bichromophoric Ir−Pd complexes have been shown to be activated via visible light irradiation as an external stimulus for the coordination−insertion copolymerization of styrene and trifluoroethyl vinyl ether.24,25 Although a wide variety of conditions could tune the polymer © XXXX American Chemical Society
to a great extent, the copolymerization of polar functionalized monomers such as acrylates with olefins still remains a challenge. Using typical conditions in olefin polymerization for functional α-olefins such as acrylates is not a viable option because copolymers made with ethylene and α-olefins produce polymers with a low degree of acrylate incorporated. Methyl acrylate is inserted, and rearrangement leads to the formation of a stable six-membered chelate hindering further acrylate coordination. Coordination of ethylene, however, is possible which opens up the chelate, leading to MA incorporation at the end of the polymer chain.26,27 This mechanism can be used as an advantageous technique to introduce selective functionalities to the ends of polymer chains, but as mentioned above, the rearrangement that is observed following insertion Received: August 9, 2018 Revised: August 31, 2018
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DOI: 10.1021/acs.macromol.8b01719 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. (A) In the presence of light and the cationic palladium catalyst seen above, MA will undergo polymerization. In the dark, there is no formation of the poly(methyl acrylate). If radical inhibitors, such as galvinoxyl, are introduced into the reaction, no polymerization of MA is observed even in the presence of light. (B) Hexene polymerizes in the dark and light using the cationic palladium catalyst. Polymerization in the light shows lower conversion and higher PDI as compared to the polymerization in the dark. (C) When hexene and MA are polymerized in a onepot method with both monomers present, only polyhexene is observed when performed in dark conditions. When in the light, only poly(methyl acrylate) is observed in the reaction.
chemical bond homolysis of a metal−carbon bond, as has been confirmed in stoichiometric photolysis reactions of the group 10 organometallic complexes.32,33 Recent studies indicate that diimine nickel aryl complexes, ubiquitous in metal-catalyzed cross-coupling reactions and structurally related to olefin polymerization catalysts, exhibit metal-to-ligand charge transfer (MLCT) excited states that can undergo photoinduced electron transfer reactions or result in metal−carbon bond homolysis.34 This suggests the ability of a metal−ligand complex to function as a photocatalytic system. From this understanding, we hypothesized the dormant homolytic nature of the metal−carbon bond could be accessed photochemically to facilitate a radical pathway. On the basis of these studies, we selected a palladium diimine catalyst suitable for performing highly controlled insertion polymerization with the potential to act as a photocatalyst via irradiation of the metal−carbon bond. In this paper, we describe the development of an “ON/OFF” process for the homopolymerization of MA exploiting the selective photoactivation of a palladium diimine catalyst35 via blue light. The behavior of both MA and hexene in the presence and absence of light was investigated (Figure 1). MA was found to not polymerize in the dark, but only by irradiation of the catalyst with blue light. In control experiments, a radical inhibitor was added to the MA polymerization under blue light irradiation, and MA did not polymerize, suggesting the mechanism followed a radical process. When MA was polymerized in the presence of hexene, only poly(methyl acrylate) was observed in a one-pot reaction when irradiated with blue light. For the same one-pot reaction, in the dark, only polyhexene polymers were observed. This key selectivity which depends upon irradiation by light permits one monomer to polymerize over another and allows us to probe deeper into the synthesis of block copolymers. We detail how this orthogonal process was employed to produce well-defined diblock copolymers in both sequential addition and one-pot processes. The novelty of this “ON/OFF” mechanism circumvents the challenges previously documented for the
of one acrylate monomer prevents the further addition of acrylate monomers to allow polyacrylate formation. This mechanism not only excludes the formation of polyacrylate homopolymers but also decreases the rate of insertion of olefins when acrylates are present in the reaction. Novak and co-workers reported on a polymerization using neutral palladium catalysts which proceeded via a free radical process, resulting in MA homopolymers and copolymers with olefins reaching high molecular weights and dispersities from 3.34 to 5.48.28 This reaction is not metal catalyzed, and the Pd(II) species generates free radicals effectively initiating a radical polymerization. Alternative approaches to polymerize acrylates with ethylene explored neutral nickel complexes through the addition of triphenylphosphine ligands and elevation of temperature and pressure. Although enriched ethylene or acrylate polymers were produced using this method, the ability to switch between the insertion and radical mechanisms was not confirmed.29,30 Moreover, organocobalt species have been reported to copolymerize nonpolar olefins such as ethylene with polar olefins such as vinyl acetate by using temperature to generate a chain-end radical capable of polymerizing ethylene and vinyl comonomers producing blocklike structures.31 Our goal was to develop a selective process that would enable a metal-catalyzed radical polymerization, while at the same time being capable of performing an insertion polymerization, exclusively governed by an outer stimulus. The key to this approach was dependent upon an outer stimulus that influences a single catalyst’s reactivity toward two monomer classes, allowing the controlled copolymerization of polar and nonpolar olefins. One of the limitations in the development of novel thermoplastic elastomers is the absence of a synthetic platform that can polymerize these two monomer classes in a single process with access to controlled block copolymer architectures. The development of our synthetic platform required access to a photocatalytic pathway as an orthogonal method capable of generating a metal-catalyzed radical. In organometallic radical processes, a radical can be created through photoB
DOI: 10.1021/acs.macromol.8b01719 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 2. (A) aExperimental conditions: solvent, dichloromethane (DCM); light source, blue LED light (λmax = 460 nm). bMonomer conversion was determined by 1H NMR spectroscopy. cTheoretical molecular weight was calculated based on 100% of monomer from the equation Mn,th = [M]0/[Pd] × MWM, where [M]0, [Pd], and MWM correspond to initial monomer concentration, initial Pd catalyst concentration, and molar mass of the monomer, respectively. dMolecular weight and polydispersity index (Mw/Mn) were determined by GPC analysis with samples run in THF at 40 °C calibrated to poly(methyl methacrylate) standards. eDegree of polymerization was calculated from the equation DP = [M]0/[Pd] × α, where [M]0, [Pd], and α correspond to initial monomer concentration, initial Pd catalyst concentration, and conversion determined by 1H NMR, respectively. (B) GPC curves were obtained from samples (precipitated) measured in THF at 40 °C using poly(methyl methacrylate) standards. (C) Polymerization of MA in the presence (ON) and absence (OFF) of light.
after a 20 min induction period, reaching high conversions of 67% in 320 min (Figure S8). To test our platform’s ability to target different molecular weights, we targeted lower (20000 g/mol) and higher (500000 g/mol) molecular weights for MA polymers (DPn = 235, 588, 1175, 2350, and 5880 ([MA]/[Pd] = 235, 588, 1175, 2350, and 5880:1)) (Figure 2A,B). Each reaction was run for 12 h to monitor the effect monomer-tocatalyst ratio had on molecular weight targeting, conversion, and dispersity. As expected, decreasing the monomer-tocatalyst ratio led to an increase in conversion. From these studies, we realized that access to a variety of polymer molecular weights was made possible by varying the monomerto-catalyst ratio and could serve as a complementary process that had the potential to be used in tandem with insertion polymerization to develop tailored block copolymers. To illustrate the selectivity for MA polymerization in the light, control reactions were performed in the dark, in the presence of a radical inhibitor (galvinoxyl),28 and in the absence of catalyst. All controls showed no polymerization of MA. These findings detail the photoactivation via blue light which enables the radical polymerization process. “ON/OFF” Polymerization of MA Using Cationic Pd(II). Activation by visible light opened up the possibility for an “ON/OFF” polymerization that further elucidates the activation and deactivation of the radical polymerization with light as a stimulus. We chose to perform two dark (OFF) and three light cycles (ON) to demonstrate temporal control and reversible activation and deactivation of the palladium catalyst. As depicted in Figure 2C, there is linear growth for conversion of the MA during all periods of irradiation (ON). The first
copolymerization of MA and olefins. Moreover, an exciting facile polymerization method is achieved for block copolymer preparation, in which the need to completely consume one monomer before addition of another monomer is eliminated. In this contribution, we report a strategy that lays the foundation for a novel synthetic approach that combines transition-metal-catalyzed olefin polymerizations with an “ON/ OFF” light initiated radical polymerization. Implementing these two selective processes, we were able to develop diblock copolymers of hexene and MA in a sequential and one-pot process which grants access to block copolymers with controlled molecular weights in a facile synthetic platform.
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RESULTS AND DISCUSSION MA Polymerization with Cationic Pd(II) in Blue Light and in the Dark. Our initial target was to investigate the performance of a cationic palladium(II) diimine catalyst35 in tandem with light and functional α-olefins (Figure 1). We chose MA for our preliminary studies because of the electrondeficient nature of the alkene, making it a likely candidate for radical polymerization. UV−Vis measurements were performed for the Pd(II) diimine catalyst, and an absorption maximum was found within the visible light region at 460 nm. This wavelength was applied to irradiate the catalyst for the homopolymerization of MA. The model polymerization of MA under blue light (λmax = 460 nm) was investigated in the presence of [MA]/[Pd] = 1175:1 in DCM (2.79 M), resulting in dispersities (
