Vasiliki Nikolaou

Cu(0)-Mediated Living Radical Polymerization: A Versatile Tool for Materials Synthesis

Materials Synthesis Athina Anastasaki,†,‡ Vasiliki Nikolaou,† Gabit Nurumbetov,† Paul Wilson,†,‡ Kristian Kempe,†,‡ John F. Quinn,‡ Thomas P. Davis,†,‡ Michael R. Whittaker,†,‡ and David M. Haddleton*,†,‡ †Chemistry Department, University of Warwick, Library Road, CV4 7AL, Coventry, United Kingdom ‡ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, Monash Institute of Pharmaceutical Sciences, Monash University (Parkville Campus), 399 Royal Parade, Parkville, Victoria 3152, Australia

Photoinduced sequence-control via one pot living radical polymerization of acrylates

The ability to regulate the activation and deactivation steps via an external stimulus has always been a challenge in polymer chemistry. In an ideal photo-mediated system, whereby high monomer conversion and excellent end group fidelity can be maintained, precise control over the polymer composition and microstructure would be a significant breakthrough. Herein, we report, a versatile, simple and inexpensive method that allows for the synthesis of sequence-controlled multiblock copolymers in a one pot polymerization reaction at ambient temperature. In the absence of a conventional photoredox catalyst and dye-sensitisers, low concentrations of CuBr2 in synergy with Me6-Tren mediate acrylic block copolymerization under UV irradiation (λmax ≈ 360 nm). Four different acrylate monomers were alternated in various combinations within the polymer composition illustrating the potential of the technique. Narrow disperse undecablock copolymers were obtained (Đ < 1.2) with quantitative conversion achieved between the iterative monomer additions. The effect of the chain length was investigated allowing for higher molecular weight multiblock copolymers to be obtained. This approach offers a versatile and inexpensive platform for the preparation of high-order multiblock functional materials with additional applications arising from the precise spatiotemporal “on/off” control and resolution when desired.

Sequence-controlled methacrylic multiblock copolymers via sulfur-free RAFT emulsion polymerization

Translating the precise monomer sequence control achieved in nature over macromolecular structure (for example, DNA) to whole synthetic systems has been limited due to the lack of efficient synthetic methodologies. So far, chemists have only been able to synthesize monomer sequence-controlled macromolecules by means of complex, time-consuming and iterative chemical strategies such as solid-state Merrifield-type approaches or molecularly dissolved solution-phase systems. Here, we report a rapid and quantitative synthesis of sequence-controlled multiblock polymers in discrete stable nanoscale compartments via an emulsion polymerization approach in which a vinyl-terminated macromolecule is used as an efficient chain-transfer agent. This approach is environmentally friendly, fully translatable to industry and thus represents a significant advance in the development of complex macromolecule synthesis, where a high level of molecular precision or monomer sequence control confers potential for molecular targeting, recognition and biocatalysis, as well as molecular information storage. Achieving sequence control in a synthetic polymer is more challenging and time consuming than it is for biopolymers. Now, it has been shown that the synthesis of sequence-controlled multiblock copolymers can be carried out via emulsion polymerization. This approach is environmentally friendly and yields complex multiblock materials with low dispersity and high yields.

Cu(0)-mediated living radical polymerization: recent highlights and applications; a perspective

Cu(0)-mediated living radical polymerization or single electron transfer living radical polymerization (Cu(0)-mediated LRP or SET-LRP) is a versatile polymerization technique that has attracted considerable interest during the past few years for the facile preparation of advanced materials. Importantly, the scope of Cu(0)-mediated LRP has been significantly expanded to include the polymerization of a large variety of functional monomers (e.g. acrylates, methacrylates, acrylamides, methacrylamides, styrene etc.) in several solvents e.g. with the resulting polymers possessing narrow molecular weight distributions (MWDs), fast polymerization rates and very high end-group fidelity (even at quantitative conversions) as exemplified by sequential chain extensions and block copolymerizations. These characteristics render Cu(0)-mediated LRP an ideal candidate for the facile synthesis of complex architectures that have found use in a large diversity of applications including glycopolymers, gene delivery, foldamers, polymer–protein conjugates and many others.

Absolut “copper catalyzation perfected”; robust living polymerization of NIPAM: Guinness is good for SET-LRP

The controlled polymerization of N-isopropyl acrylamide (NIPAM) is reported in a range of international beers, wine, ciders and spirits utilizing Cu(0)-mediated living radical polymerization (SET-LRP). Highly active Cu(0) is first formed in situ by the rapid disproportionation of [Cu(I)(Me6-Tren)Br] in the commercial water–alcohol mixtures. Rapid, yet highly controlled, radical polymerization follows (Đ values as low as 1.05) despite the numerous chemicals of diverse functionality present in these solvents e.g. alpha acids, sugars, phenols, terpenoids, flavonoids, tannins, metallo-complexes, anethole etc. The results herein demonstrate the robust nature of the aqueous SET-LRP protocol, underlining its ability to operate efficiently in a wide range of complex chemical environments.

Aqueous SET-LRP catalyzed with "in situ" generated Cu(0) demonstrates surface mediated activation and bimolecular termination

The aqueous SET-LRP catalyzed with “in situ” generated Cu(0) of the two amphiphilic monomers 2-hydroxyethyl acrylate (HEA) and oligo(ethylene oxide) methyl ether acrylate (OEOMEA) was investigated at temperatures from −22 to +25 °C. The kappp values of both monomers are higher at 0 °C (4.61 min−1 for OEOMEA and 2.60 min−1 for HEA) than at 25 °C (1.60 min−1 for OEOMEA and 1.12 min−1 for HEA). These unexpected and unprecedented results are explained by the lower Cu(0) particle size obtained by the disproportionation of CuBr at 0 °C in H2O. Poly(OEOMEA) obtained by aqueous SET-LRP at 0 °C with the unexpectedly high kappp = 4.61 min−1 exhibits 88% chain-end functionality at 100% monomer conversion, while the theoretical value would have to be ∼0%. This high experimental chain-end functionality was explained by the slow desorption of the hydrophobic backbone containing the propagating radicals of these amphiphilic polymers from the surface of the catalyst due to their strong hydrophobic effect. Polymer radicals adsorbed on the surface of Cu(0) undergo monomer addition and reversible deactivation but do not undergo the bimolecular termination that requires desorption. This amplified adsorption–desorption process that mediates both the activation and the bimolecular termination explains the unexpectedly high chain-end functionality of the polymers synthesized by SET-LRP.

Synthesis of well-defined α,ω-telechelic multiblock copolymers in aqueous medium: in situ generation of α,ω-diols

The synthesis of well-defined α,ω-dihydroxyl telechelic multiblock copolymers by sequential in situ chain extensions via aqueous Cu(0) mediated living radical polymerization (SET-LRP) is reported. The rapid disproportionation of Cu(I)Br in the presence of Me6-TREN in water has been exploited to generate Cu(0) and [Cu(II)Br2/Me6-TREN] in situ, resulting in rapid reaction rate and narrow molecular weight distributions. Under optimized conditions, a telechelic heptablock copolymer was obtained within 2 hours with a final dispersity of ∼1.1 while the monomer conversion was >99% for each block. A range of acrylamides and acrylates have been successfully incorporated within the same polymer backbone, including N-isopropylacrylamide (NIPAAm), N,N-diethylacrylamide (DEA) and N,N-dimethylacrylamide (DMA) and poly(ethylene glycol) methyl ether acrylate (PEGA480). The thermo-responsive nature of these materials was subsequently demonstrated via cloud point measurements as both a function of molecular weight and backbone functionality. In addition, the typically unwanted hydrolysis of the α- and ω-end groups in aqueous media was further exploited via isocyanate post-polymerization modifications to alter the end group functionality.

Photo-induced living radical polymerization of acrylates utilizing a discrete copper(II)–formate complex

A photo-polymerization protocol, utilizing a pre-formed and well-characterized Cu(II) formate complex, [Cu(Me6-Tren)(O2CH)](ClO4), mediated by UV light is described. In the absence of additional reducing agents and/or photosensitizers, ppm concentrations of the oxidatively stable [Cu(Me6-Tren)(O2CH)](ClO4), furnish near-quantitative conversions within 2 h, yielding poly(acrylates) with low dispersities (∼1.10) and exceptional end-group fidelity, capable of undergoing in situ chain extension and block copolymerization.

Polymerization of long chain [meth]acrylates by Cu(0)-mediated and catalytic chain transfer polymerisation (CCTP): high fidelity end group incorporation and modification

The Cu(0)-mediated polymerization of lauryl (C12) and stearyl (C18) acrylate is reported under a range of reaction conditions. Good first order kinetics, a linear evolution of number average molecular weight (Mn) with conversion and dispersity (∼1.10) are observed. The polymerization of lauryl acrylate proceeds either homogenously or in a self-generated biphasic system, depending on the solvent employed, with little deviation in overall polymerization control. The near quantitative retention of ω-bromo end groups is exploited via nucleophilic thio-bromine substitution with thioglycerol to yield highly hydrophobic polymers with polar head groups. Modification is spectroscopically confirmed by both NMR and MALDI-TOF-MS. Highly ω-chain end functional methacrylic analogues have also been prepared by Co(II) mediated catalytic chain transfer polymerisation (CCTP). Reactive vinyl groups at the ω-chain end of poly(lauryl methacrylate) have been targeted with thioglycerol for nucleophilic thio-Michael addition with quantitative modification possible.

Copper(II) gluconate (a non-toxic food supplement/dietary aid) as a precursor catalyst for effective photo-induced living radical polymerisation of acrylates

Copper gluconate, is employed as a precursor catalyst for the photo-induced living radical polymerisation of acrylates. Optimised reaction conditions for efficient ligand transfer leads to well-defined polymers within 2 h with near quantitative conversions (>95%), low dispersities (Đ ∼ 1.16) and high end-group fidelity, as demonstrated by MALDI-ToF-MS. Additionally, in the presence of ppm concentrations of NaBr, similar degree of control could also be attained by facilitating ligand exchange, furnishing narrow dispersed polymers (Đ < 1.12).