Annhelen Lu

Aldol reactions catalyzed by L-proline functionalized polymeric nanoreactors in water

The use of functional core-shell micelles as asymmetric catalytic nanoreactors for organic reactions in water is presented. An unprecedented increase in rate of reaction was achieved, which is proposed to be associated with the ability of the nanostructures to effectively concentrate the reagents in the catalytically active micelle core.

One-pot synthesis of responsive sulfobetaine nanoparticles by RAFT polymerisation: the effect of branching on the UCST cloud point

We describe the one-pot synthesis of temperature-responsive branched polymer nanoparticles. Reversible addition–fragmentation chain transfer (RAFT) polymerisation has been utilised to synthesise ultra-high molecular weight sulfobetaine polymers (up to ca. 500 kDa) with good control over molecular weight (Mn) and dispersity (Mw/Mn). The UCST cloud points of these linear polymers were found to increase with both Mn and concentration, and represent one of the few recent descriptions of polymers exhibiting UCST behaviour in aqueous solution. The incorporation of difunctional monomers results in branched polymers which display vastly reduced transition temperatures compared to their linear counterparts. Furthermore, the incorporation of a permanently hydrophilic monomer results in the formation of stable core–shell particles which no longer exhibit a cloud point in water, even at very high concentrations (ca. 50 mg mL−1). The branched polymers are shown to form discrete well-defined nanoparticles in aqueous solution, and these have been characterised by DLS, SLS, TEM and DOSY. Their reversible swelling behaviour in response to temperature is also demonstrated.

Tuning the catalytic activity of L-proline functionalized hydrophobic nanogel particles in water

L-Proline functionalized PMMA nanogels with a range of catalyst functionalization (0.5–15 wt%) and cross-linking densities (0–50 wt%) were prepared via emulsion polymerization. The catalyst efficiency in water was investigated using a model asymmetric aldol reaction and an unprecedented reduction in catalyst loading, whilst maintaining high catalytic activity, is reported. In these reactions, a marked effect on selectivity was observed and determined to be dependent on the hydrophobicity of the nanogel particles. Furthermore, the effect of increasing cross-linking density on the catalytic efficiency of these particles (and their core–shell analogues) was explored and a significant reduction in activity was observed.

Catalytic Y-tailed amphiphilic homopolymers – aqueous nanoreactors for high activity, low loading SCS pincer catalysts

A new amphiphilic homopolymer bearing an SCS pincer palladium complex has been synthesized by reversible addition fragmentation chain transfer polymerization. The amphiphile has been shown to form spherical and worm-like micelles in water by cryogenic transmission electron microscopy and small angle neutron scattering. Segregation of reactive components within the palladium containing core results in increased catalytic activity of the pincer compound compared to small molecule analogues. This allows carbon-carbon bond forming reactions to be performed in water with reduced catalyst loadings and enhanced activity.

Immobilization of MacMillan catalyst via controlled radical polymerization: catalytic activity and reuse

The MacMillan catalyst is an established organocatalyst capable of catalyzing a variety of organic reactions. Through the synthesis of a novel monomer containing the MacMillan catalytic functionality, a variety of copolymers have been synthesized with the comonomer, diethylene glycol methyl ether methacrylate (DEGMA). Reversible addition–fragmentation chain transfer (RAFT) polymerization was used for the synthesis of these functional polymers with good control over molecular weight, catalyst incorporation and polydispersity. These polymers showed lower critical solution temperature (LCST) behaviour where the cloud point was found to be dependent upon the degree of catalyst incorporation and catalyst loading was also found to have an effect on the Tg of the copolymers. The catalytic activity of the functional copolymers is demonstrated by the Diels–Alder reaction between cyclopentadiene and trans-hexen-1-al and shows enantioselectivity close to those previously reported by MacMillan. The polymers can be reused in multiple Diels–Alder reactions via a pseudo continuous process, maintaining high conversion and enantioselectivity throughout the cycles.

Studying the activity of the MacMillan catalyst embedded within hydrophobic cross-linked polymeric nanostructures

The immobilization of the MacMillan catalyst within a unique hydrophobic environment created by a lightly cross-linked nanogel structure and its resulting catalytic activity is reported. The catalytic activity and selectivity of the catalyst were evaluated using the Diels–Alder (DA) reaction between cyclopentadiene and cinnamaldehyde. The relatively easy synthetic route applied allowed for the synthesis of a collection of nanogels with catalyst incorporations ranging from 0.5 to 25 wt%. In addition, core–shell type nanogels were synthesized to evaluate potential recovery and reuse of the catalytic system. The influence of the concentrator effect and possible partition coefficient on the catalyst activity was investigated. The results indicate catalyst loading/concentration can be more significantly reduced when the catalyst is embedded within the polymeric nanostructures compared to the small molecule equivalent.