Craig A. Bell

A rapid electrochemical method for determining rate coefficients for copper-catalyzed polymerizations

Copper(I) polyamine complexes have emerged as excellent atom-transfer radical polymerization catalysts. The rate of their reaction with organic halide initiators (the so-called activation step) varies across a broad range, depending on both the structure of the copper complex and the initiator. Herein, we report a new technique for determining the rate of copper-catalyzed activation (k(act)) using cyclic voltammetry coupled with electrochemical simulation. This method is applied to measuring k(act) for one of the most active catalysts, [Cu(I)(Me(6)tren)](+) (Me(6)tren = N,N,N-tris-(2-(dimethylamino)ethyl)amine), in reaction with ethyl bromoisobutyrate.

Ultrafast and reversible multiblock formation by the SET-nitroxide radical coupling reaction

The single electron transfer-nitroxide radical coupling (SET-NRC) reaction has been used to produce multiblock polymers with high molecular weights in under 3 min at 50°C by coupling a difunctional telechelic polystyrene (Br-PSTY-Br) with a dinitroxide. The well known combination of dimethyl sulfoxide as solvent and Me6TREN as ligand facilitated the in situ disproportionation of CuIBr to the highly active nascent Cu0 species. This SET reaction allowed polymeric radicals to be rapidly formed from their corresponding halide end-groups. Trapping of these carbon-centred radicals at close to diffusion controlled rates by dinitroxides resulted in high-molecular-weight multiblock polymers. Our results showed that the disproportionation of CuI was critical in obtaining these ultrafast reactions, and confirmed that activation was primarily through Cu0. We took advantage of the reversibility of the NRC reaction at elevated temperatures to decouple the multiblock back to the original PSTY building block through capping the chain-ends with mono-functional nitroxides. These alkoxyamine end-groups were further exchanged with an alkyne mono-functional nitroxide (TEMPO–≡) and ‘clicked’ by a CuI-catalyzed azide/alkyne cycloaddition (CuAAC) reaction with N3–PSTY–N3 to reform the multiblocks. This final ‘click’ reaction, even after the consecutive decoupling and nitroxide-exchange reactions, still produced high-molecular-weight multiblocks efficiently. These SET-NRC reactions would have ideal applications in re-usable plastics and possibly as self-healing materials.

Copper(II) Complexes of a Hexadentate Mixed‐Donor N3S3 Macrobicyclic Cage: Facile Rearrangements and Interconversions

The potentially hexadentate mixed-donor cage ligand 1-methyl-8-amino-3,13,16-trithia-6,10,19-triazabicyclo[6.6.6]eicosane (AMME-N(3)S(3)sar; sar=sarcophagine) displays variable coordination modes in a complex with copper(II). In the absence of coordinating anions, the ligand adopts a conventional hexadentate N(3)S(3) binding mode in the complex [Cu(AMME-N(3)S(3)sar)](ClO(4))(2) that is typical of cage ligands. This structure was determined by X-ray crystallography and solution spectroscopy (EPR and NIR UV/Vis). However, in the presence of bromide ions in DMSO, clean conversion to a five-coordinate bromido complex [Cu(AMME-N(3)S(3)sar)Br](+) is observed that features a novel tetradentate (N(2)S(2))-coordinated form of the cage ligand. This copper(II) complex has also been characterized by X-ray crystallography and solution spectroscopy. The mechanism of the reversible interconversion between the six- and five-coordinated copper(II) complexes has been studied and the reaction has been resolved into two steps; the rate of the first is linearly dependent on bromide ion concentration and the second is bromide independent. Electrochemistry of both [Cu(AMME-N(3)S(3)sar)](2+) and [Cu(AMME-N(3)S(3)sar)Br](+) in DMSO shows that upon reduction to the monovalent state, they share a common five-coordinated form in which the ligand is bound to copper in a tetradentate form exclusively, regardless of whether a six- or five-coordinated copper(II) complex is the precursor.

Functional Degradable Polymers by Radical Ring-Opening Copolymerization of MDO and Vinyl Bromobutanoate: Synthesis, Degradability and Post-Polymerization Modification.

The synthesis of vinyl bromobutanoate (VBr), a new vinyl acetate monomer derivative obtained by the palladium-catalyzed vinyl exchange reaction between vinyl acetate (VAc) and 4-bromobutyric acid is reported. The homopolymerization of this new monomer using the RAFT/MADIX polymerization technique leads to the formation of novel well-defined and controlled polymers containing pendent bromine functional groups able to be modified via postpolymerization modification. Furthermore, the copolymerization of vinyl bromobutanoate with 2-methylene-1,3-dioxepane (MDO) was also performed to deliver a range of novel functional degradable copolymers, poly(MDO-co-VBr). The copolymer composition was shown to be able to be tuned to vary the amount of ester repeat units in the polymer backbone, and hence determine the degradability, while maintaining a control of the final copolymers’ molar masses. The addition of functionalities via simple postpolymerization modifications such as azidation and the 1,3-dipolar cycloaddition of a PEG alkyne to an azide is also reported and proven by 1H NMR spectroscopy, FTIR spectroscopy, and SEC analyses. These studies enable the formation of a novel class of hydrophilic functional degradable copolymers using versatile radical polymerization methods.

Controlling the synthesis of degradable vinyl polymers by xanthate-mediated polymerization

The copolymerization of vinyl acetate (VAc) and 2-methylene-1,3-dioxepane (MDO), as well as the homopolymerization of MDO in the presence of a p-methoxyphenyl xanthate chain transfer agent (CTA) is reported and comparison of the homopolymerization of MDO with other known xanthates was also investigated. In depth investigation showed loss of the xanthate functionality was a result of Z-group fragmentation leading to the formation of carbonodithioate groups, as confirmed by 13C NMR spectroscopy. The use of the xanthate with a substituted phenyl Z-group drastically reduces fragmentation through the Z-group and hence significantly increases chain-end retention during the polymerization using the RAFT/MADIX technique. Post-polymerization modification of the chain-end of poly(MDO) was achieved by in situ aminolysis and base-catalyzed Michael addition of propargyl methacrylate onto the terminal thiol to form alkyne functional poly(MDO).

Methyl acrylate polymerizations in the presence of a copper/N3S3 macrobicyclic cage in DMSO at 25 °C

Macrobicyclic hexa-amine cage ligands are known to completely encapsulate transition metals inside the ligand cavity. Recent work has shown that a five-coordinate bromido complex [Cu(AMME-N3S3sar)Br]+ was observed, featuring a novel tetradentate (N2S2) coordinated form of the cage ligand in DMSO. Reduction to its monovalent state results in no change in the geometry of the complex. The electrochemistry of this complex in the presence of an alkyl halide showed that this complex has a low activation to convert these alkyl halides to their corresponding incipient radicals. Further polymerization kinetics with methyl acrylate showed that the molecular weight was uncontrolled, implying that deactivation was negligible. Therefore, this copper/ligand complex with an alkyl halide initiator only acts as an initiation source. This led us to use this Cu/AMME-N3S3sar complex to initiate MA polymerizations at room temperature in the presence of a RAFT agent. The results showed that the molecular weight distribution was controlled and followed ideal ‘living’ radical behavior, in which the molecular weight polydispersity for a range of different molecular weight targets was less than 1.1.

Synthesis of degradable poly(ε-caprolactone)-based graft copolymers via a “grafting-from” approach

The controlled ring-opening polymerisation (ROP) of α-bromo-e-caprolactone (αBrCL), a derivative of e-caprolactone (eCL), and its copolymerisation with eCL is reported. Functional and monodisperse poly(α-bromo-e-caprolactone) homopolymers and P(αBrCL-co-eCL) copolymers that contain pendant bromine initiators were obtained with targeted molecular weights using diphenyl phosphate (DPP) as the catalyst at room temperature. Using Cu(0)-mediated Controlled Radical Polymerisation (CRP), methyl acrylate (MA) was successfully “grafted from” P(αBrCL-co-eCL) macroinitiators under mild conditions, resulting in polymers with low dispersity, produced with fast reaction times and minimal termination side reactions.

Independent control of elastomer properties through stereocontrolled synthesis

Abstract In most synthetic elastomers, changing the physical properties by monomer choice also results in a change to the crystallinity of the material, which manifests through alteration of its mechanical performance. Using organocatalyzed stereospecific additions of thiols to activated alkynes, high‐molar‐mass elastomers were isolated via step‐growth polymerization. The resulting controllable double‐bond stereochemistry defines the crystallinity and the concomitant mechanical properties as well as enabling the synthesis of materials that retain their excellent mechanical properties through changing monomer composition. Using this approach to elastomer synthesis, further end group modification and toughening through vulcanization strategies are also possible. The organocatalytic control of stereochemistry opens the realm to a new and easily scalable class of elastomers that will have unique chemical handles for functionalization and post synthetic processing.