Tailoring Polymerization Controllability and Dispersity Through a Photoswitchable Catalyst Strategy

Modulating on-demand polymerization is a challenge in synthetic macromolecules. Herein, tailoring polymerization controllability and dispersity during single-electron transfer mediated living radical polymerization (SET-LRP) of methyl methacrylate (MMA) is achieved. Hexaarylbiimidazole (HABI) is employed as a photoswitchable catalyst, allowing reversible control of catalytic activity between an active and inactive state. In the presence of HABI and with the light on (active state), control SET-LRP of MMA follows first-order kinetics, resulting in polymers with a narrow molecular weight distribution. In contrast, polymerization responds to light and reverts to their original uncontrolled state with light off (inactive state). Therefore, repeatable resetting polymerization can be easily performed. The key to photomodulating dispersity is to use an efficient molecular switch to tailor the breadths of dispersity. Besides, the mechanism of HABI-mediated SET-LRP with switchable ability is proposed.

Using temperature to modify the reaction conditions and outcomes of polymers formed using transfer-dominated branching radical telomerisation (TBRT)

Transfer-dominated Branching Radical Telomerisation (TBRT) enables the production of branched polymers with step-growth backbones using radical telomerisation chemistry. By conducting identical TBRTs over a broad temperature range, the role of temperature in telomer formation and branching has been evaluated. Elevated temperature limits telomer length, thereby allowing a >10% reduction in the amount of telogen required to produce near identical high molecular weight branched polymers.

Accessing new and scalable high molecular weight branched copolymer structures using transfer-dominated branching radical telomerisation (TBRT)

Three new synthesis strategies for branched statistical copolymers containing analogues of step-growth backbones are shown using free radical chemistries and transfer-dominated branching radical polymerisation (TBRT) conditions.

Polymerizable Gd(iii) building blocks for the synthesis of high relaxivity macromolecular MRI contrast agents

A new synthetic strategy for the preparation of macromolecular MRI contrast agents (CAs) is reported. Four gadolinium(iii) complexes bearing either one or two polymerizable methacrylamide groups were synthesized, serving as monomers or crosslinkers for the preparation of water-soluble, polymeric CAs using Reversible Addition–Fragmentation Chain Transfer (RAFT) polymerization. Using this approach, macromolecular CAs were synthesized with different architectures, including linear, hyperbranched polymers and gels. The relaxivities of the polymeric CAs were determined by NMR relaxometry, revealing an up to 5-fold increase in relaxivity (60 MHz, 310 K) for the linear polymers compared with the clinically used CA, Gd-DOTA. Moreover, hyperbranched polymers obtained from Gd(iii) crosslinkers, displayed even higher relaxivities up to 22.8 mM⁻¹ s⁻¹, approximately 8 times higher than that of Gd-DOTA (60 MHz, 310 K). A detailed NMRD study revealed that the enhanced relaxivities of the hyperbranched polymers were obtained by limiting the local motion of the crosslinked Gd(iii) chelate. The versatility of RAFT polymerization of Gd(iii) monomers and crosslinkers opens the doors to more advanced polymeric CAs capable of multimodal, bioresponsive or targeting properties.

A new synthetic strategy for the preparation of efficient macromolecular MRI contrast agents is reported.

Using pyrene to probe the effects of poloxamer stabilisers on internal lipid microenvironments in solid lipid nanoparticles

Solid lipid nanoparticles (SLNs) have proved to be effective nanocarriers with many advantages over other non-lipid-based systems. The development of new SLN formulations is often hindered through poor drug loading capacity and time-consuming optimisation of lipid/stabiliser combinations. One challenge in the development of new SLN formulations is understanding the complex interactions between amphiphilic stabilisers and hydrophobic lipids; the nature of these interactions can significantly impact SLN properties, including the internal polarity within the nanoparticle core. Herein, we report the use of pyrene to probe the internal lipid microenvironment inside SLNs. We investigate the effect of using different poloxamer stabilisers on the internal polarity of SLNs formed using the common solid lipid, Compritol 888 ATO. We show that the polarity of the internal lipid environment is modified by the length of the poly(propylene oxide) (PPO) block of the poloxamer stabiliser, with longer PPO blocks producing SLNs with less polar lipid cores. Blending of stabilisers could also be used to tune the polarity of the core lipid environment, which may allow for adjusting the polarity of the lipid to assist the loading of different therapeutics.

Complex polymer architectures through free-radical polymerization of multivinyl monomers

The construction of complex polymer architectures with well-defined topology, composition and functionality has been extensively explored as the molecular basis for the development of modern polymer materials. The unique reaction kinetics of free-radical polymerization leads to the concurrent formation of crosslinks between polymer chains and rings within an individual chain and, thus, free-radical (co)polymerization of multivinyl monomers provides a facile method to manipulate chain topology and functionality. Regulating the relative contribution of these intermolecular and intramolecular chain-propagation reactions is the key to the construction of architecturally complex polymers. This can be achieved through the design of new monomers or by spatially or kinetically controlling crosslinking reactions. These mechanisms enable the synthesis of various polymer architectures, including linear, cyclized, branched and star polymer chains, as well as crosslinked networks. In this Review, we highlight some of the contemporary experimental strategies to prepare complex polymer architectures using radical polymerization of multivinyl monomers. We also examine the recent development of characterization techniques for sub-chain connections in such complex macromolecules. Finally, we discuss how these crosslinking reactions have been engineered to generate advanced polymer materials for use in a variety of biomedical applications.

Mucus-responsive functionalized emulsions: design, synthesis and study of novel branched polymers as functional emulsifiers

Mucus lines the moist cavities throughout the body, acting as barrier by protecting the underlying cells against the external environment, but it also hinders the permeation of drugs and drug delivery systems. As the rate of diffusion is low, the development of a system which could increase retention time at the mucosal surface would prove beneficial. Here, we have designed a range of branched copolymers to act as functional mucus-responsive oil-in-water emulsifiers comprising the hydrophilic monomer oligo(ethylene glycol) methacrylate and a hydrophobic dodecyl initiator. The study aimed to investigate the importance of chain end functionality on successful emulsion formation, by systematically replacing a fraction of the hydrophobic chain ends with a secondary poly(ethylene glycol) based hydrophilic initiator in a mixed-initiation strategy; a decrease of up to 75 mole percent of hydrophobic chain ends within the branched polymer emulsifiers was shown to maintain comparative emulsion stability. These redundant chain ends allowed for functionality to be incorporated into the polymers via a xanthate based initiator containing a masked thiol group; thiol groups are known to have mucoadhesive character, due to their ability to form disulfide bonds with the cysteine rich areas of mucus. The mucoadhesive nature of emulsions stabilised by thiol-containing branched copolymers was compared to non-functional emulsions in the presence of a biosimilar mucosal substrate and enhanced adherence to the mucosal surface was observed. Importantly, droplet rupture and mucus triggered release of dye-containing oil was seen from previously highly-stable thiol-functional emulsions; this observation was not mirrored by non-functional emulsions where droplet integrity was maintained even in the presence of mucus.

Mucoadhesion and mucus-sensitive materials have many applications. Redundant chain-ends within branched polymer emulsifiers have been functionalized with thiols, without compromising emulsion stability, to create mucus-interacting emulsions.

Preparation of hyperbranched polymers by oxa-Michael addition polymerization

Hyperbranched polymers with high molecular weights were synthesized by t-BuP₂-catalyzed oxa-Michael addition polymerization of trifunctional hydroxyl and diacrylate monomers.