Authors: Francesco De Bon
Source: Padua@research (University of Padova) (article, 2018-11-28)
Field: Chemistry — Organic Chemistry

Electrochemical approaches to Atom Transfer Radical Polymerization Controlled radical polymerizations (CRPs) are among the most powerful methods to obtain polymers with well-defined properties and high commercial value. Atom transfer radical polymerization (ATRP) is probably the most widely used CRP, in academia and industry, thanks to its versatility and simplicity. In ATRP, a metal complex in a low oxidation state, MtzLm (typically a copper-amine system, [CuIL]+) reacts with a dormant polymeric chain Pn–X (where X = Cl, Br) to produce radicals Pn• that can propagate, in the bulk of the solution, by addition to monomer units. In this reaction, the copper complex is oxidized and binds to X–, generating the deactivating species [X-CuIIL]+, which traps the propagating species. ATRP equilibrium is shifted towards the dormant species Pn-X, so that Pn• concentration is very low, and the probability of radical-radical termination events is minimized. Growth of all chains begins virtually at the same time, thanks to the use of alkyl halide (RX) initiators that are more reactive than the dormant species, Pn-X. In such conditions, chain growth is homogenous, and it is possible to obtain polymers with predetermined molecular weight, narrow molecular-weight distribution and high chain-end fidelity. ATRP allows to tailor macromolecules with specific compositions, architectures and position of functional groups.&#13;\nThe aim of this thesis is to contribute to both the understanding and development of ATRP catalyzed by copper complexes, using electrochemical methods as equally analytical tools and efficient means of triggering and controlling the polymerization. The work focused on spreading the use of such systems to efficiently control the polymerization of a series of important monomers. Moreover, the investigated ATRP systems can be considered also “green” for several reasons: (i) most of the work regards the study and development of the reaction in green solvents in which copper complexes generally have high catalytic activity; (ii) electrochemical methods for catalyst regeneration (electrochemically mediated ATRP, eATRP) allowed triggering the polymerization with low loadings of copper complexes; (iii) ionic liquids, a class of non-flammable and easily recyclable solvents, were explored as potential media for eATRP; (iv) the mechanism of catalytic halogen exchange was investigated and defined, abridging the synthesis of block copolymers.&#13;\nATRP catalysts were investigated in the ionic liquid 1-butyl-3-methylimidazolium triflate ([BMIm][OTf]). Both Cu/L speciation and reactivity were found to be suitable for a well-controlled polymerization process. Polymerizations were conducted with electrochemical (re)generation of the active [CuIL]+ complex (eATRP). eATRP of methyl acrylate was investigated in detail by varying a series of parameters such as applied potential, temperature, degree of polymerization and catalyst load ([CuIITPMA]2+, TPMA = tris(2-pyridylmethyl)amine). Application of an electrochemical switch and chain extension with acrylonitrile via catalytic halogen exchange (cHE) proved the livingness of the polymerizations. Experiments triggered in recycled ionic liquid proved that eATRP tolerates well the recycled solvent; polymerizations exhibited good control and high conversion.&#13;\nBlock copolymers (BCP) have relevance in a vast range of applications in everyday life. BCP of acrylonitrile (AN) and butyl acrylate (BA) were investigated as precursors for mesoporous carbons. Thus, eATRP of acrylonitrile was studied considering several aspects, including the effect of applied potential, degree of polymerization, C-X nature and initiator structure. A macroinitiator of PAN was then extended with BA to form PAN-b-PBA copolymer as a precursor for mesoporous carbons. BCP can be obtained also by extension of a PBA chain with AN via cHE, thus avoiding purification procedures and reactivity mismatch when crossing from a less reactive monomer to a more reactive one.&#13;\ncHE was proved to be an efficient tool of polymerization by both SARA and eATRP, in a range of solvents, including water. Methyl methacrylate (MMA) was polymerized thanks to cHE in [BMIm][OTf] and ethanol, to solve the issue of penultimate effect. Fine tuning of the electrolysis conditions afforded PMMA with low dispersion. Further improvements were obtained by using [CuIIPMDETA]2+ as an inexpensive and efficient catalyst in alternative to [CuIITPMA]2+. Tacticity analysis of PMMA obtained in [BMIm][OTf] and ethanol confirmed the poor ability of the ionic solvent to induce stereocontrol to the polymerization.&#13;\nPyridinic complexes such as [CuIITPMA]2+ are stable in very acid conditions (pH = 1). This allowed unprecedented control over conditions of macromolecular growth in water. In addition, it opened a new avenue for the polymerization of ionic liquid monomers (ILMs), a class of building blocks that can give a plethora of new materials. The main reason preventing ATRP of ILM is a cyclization reaction involving the chain-end with the terminal halogen as a leaving group, as in the case of methacrylic acid. Application of three strategies previously developed for ATRP of methacrylic acid allowed to dramatically improve conversion and control over ILMs polymerization. (i) Using C-Cl chain end functionality, which is much more stable than C-Br, (ii) lowering further the pH to completely convert free carboxylate ions to carboxylic acid, which is a much weaker nucleophile, and (iii) enhancing the polymerization rate to avoid the negative contribution of the cyclization side reaction, allowed synthesis of well-controlled high molecular weight poly(ionic liquids), PILs, with degree of polymerization &gt; 500. A simple (poly)halogenated organic initiator such as 2,2-dichloropropionic acid was used to produce linear homotelechelic PILs.&#13;\nElectrochemically mediated ATRP allowed exceptional control over CuI (re)generation. For this reason, it was decided to study the eATRP of vinyl chloride, which was considered impossible until now. The polymerization, triggered in a pressure-resistant electrochemical reactor, was controlled, fast and afforded an acceptable conversion. In addition to linear PVC, a star PVC was also synthesized, highlighting the flexibility of eATRP. In the star architecture, the electrochemical polymerization was by far superior to the chemical one (SARA ATRP). The success of this polymerization has categorically denied the SET-LRP mechanism and its assumptions.&#13;\nOne of the crucial properties of electrochemical eATRP is the inert role played by the cathode material used for the regeneration of [CuIL]+. Any electrode material with good stability in the reaction medium can be used as a cathode. It was therefore decided to study the polymerization of an acrylate using the surface of a stainless steel (SS304) reactor exposed to the polymerization mixture as a cathode. In this way, the reactor has the dual function of electrode and place where the reaction takes place. The results showed that polymerization is fast, controlled and reaches high conversions. Moreover, the absence of release of metal ions during the reaction (Fe, Ni, Cr) confirmed that the polymerization takes place via electrochemical reduction of CuII to CuI, while SS304 acts only as an electron reservoir, not chemically involved in ATRP activation. Such simple and cheap electrochemical setup can make the scale-up of the eATRP a reality in the short term and open new economic prospects. Electrochemistry ; Atom-transfer radical-polymerization ; Chemistry ; Polymerization ; Photochemistry ; Electrode ; Polymer ; Physical chemistry ; Organic chemistry Advanced Polymer Synthesis and Characterization ; Chemistry and Chemical Engineering