Hyperbranched polymers: growing richer in flavours with time

Hyperbranched polymers (HBPs) have been studied for over three decades now; yet several interesting aspects continue to draw the attention of researchers worldwide. This is because of the simplicity of synthesis, their unique globular structure, and the numerous peripherally located functional groups that can be utilised to impart a variety of attributes, such as core-shell amphiphilicity, Janus amphiphilicity, clickable polymeric scaffolds, multifunctional crosslinkers, etc. Several reviews have been written on HBPs with a focus on synthetic strategies, structural diversity, and their potential applications; in this short feature article, we have taken an alternate approach to highlight some of the unique structural features of HBPs and their influence on the properties of HBPs. We also discuss their versatility and adaptability for the generation of several interesting functional polymeric systems. In the latter half, we focus on the utilisation of HBPs as multifunctional scaffolds, that rely on the numerous peripheral terminal groups. We conclude by drawing a structuro-functional analogy between the range of peripherally functionalised HBPs and other analogous, but more complex, polymeric systems. We believe that this review will serve as a visual sounding board that would encourage the development of several other applications for this class of unique polymers.

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.

Degradable thiomethacrylate core-crosslinked star-shaped polymers

Degradable polymers are considered to present a promising solution to combat plastic pollution. However, many polymers are based on ester and amide bonds, which often require high temperatures and acidic/basic...

Triple Functional AB2 Unit-Modulated Facile Preparation of Bioreducible Hyperbranched Copolymers

Facile preparation of hyperbranched polymers (HPs) has been advanced tremendously by the use of either various multifunctional agent-mediated controlled living radical polymerizations or a highly reactive AB_x unit-modulated self-stepwise polymerizations. However, it remains, to our knowledge, a significant challenge to prepare HPs with simultaneously precisely controlled degree of branching (DB) and biorelevant signal-triggered degradation property for controlled release applications due to the respective limitations of the aforementioned two strategies. For this purpose, a triple functional AB₂ unit, A-SS-B₂ chain transfer agent (AB₂ CTA), that integrates the merits of both multifunctional agents and highly reactive AB_x units was designed and synthesized successfully to include a disulfide bond for reduction-triggered polymer degradation toward promoted intracellular release of encapsulated cargoes, a trithiocarbonate group for a universal reversible addition-fragmentation chain transfer (RAFT) polymerization of any vinyl-based monomer, and three terminal groups consisting of one azide and two alkyne functions for the generation of hyperbranched topology via a self-click coupling-based polymerization. A subsequent self-click polymerization of the resulting AB₂ CTA by click coupling in the presence of CuSO₄·5H₂O and sodium ascorbate (NaVc) generated a hyperbranched polymer template (HPT) with precisely modulated DB and a plurality of CTA units for a universal reversible addition-fragmentation chain transfer (RAFT) polymerization of any vinyl-containing monomer. The HPT was next used as a multimacro-CTA for RAFT polymerization of a typical hydrophilic monomer, oligo(ethylene glycol) monomethyl ether methacrylate (OEGMA), to demonstrate the potential of this HPT for a robust and facile production of bioreducible hyperbranched polymers for controlled release applications. The synthesized HPT-4-POEGMA can form unimolecular micelles with enhanced stability due to the hyperbranched structure, and the size of micelles varied in the range from 82.4 to 140.3 nm by a modulation of the molar feed ratio of monomer to HPT and polymerization time. More importantly, HPT-POEGMA micelles incubated with 10 mM glutathione (GSH) showed reduction-triggered cleavage of the disulfide links and polymer degradation for promoted intracellular doxorubicin (DOX) release and enhanced therapeutic efficiency. Taken together, this triple functional AB₂ CTA provided a powerful means for the facile preparation of bioreducible hyperbranched polymers with precisely controlled DB for controlled release applications.

Branched macromonomers from catalytic chain transfer polymerisation (CCTP) as precursors for emulsion-templated porous polymers

Catalytic chain transfer polymerisation (CCTP) is combined for the first time with emulsion-templating to generate polyHIPE materials where functionality and rigidity can be tightly tailored, broadening their scope of application.

Aquasorbent guargum grafted hyperbranched poly (acrylic acid): A potential culture medium for microbes and plant tissues

This study reports the synthesis of an unprecedented bio-based aquasorbent guargum-g-hyperbranched poly (acrylic acid); bGG-g-HBPAA by employing graft-copolymerization and "Strathclyde methodology" simultaneously in emulsion and its possible use as a sustainable nutrient bed for the effective growth of Anabaena cylindrica and Vigna radiata seedlings. The formation of bGG-g-HBPAA and the presence of hyperbranched architectures was confirmed from XRD, FTIR, ¹³C NMR, solubility, intrinsic viscosity, BET surface area/ pore size, SEM and rheology analyses. The synthesized grade with a branching percent of 65.4% and a swelling percentage of 13,300% facilitated maximum growth of the cultured species as compared to guargum and its linear graft. Semi synthetic bGG-g-HBPAA culture medium was optically transparent, dried at a controlled rate, held a huge amount of water for growth, provided sufficient space for unhindered growth and featured dimensional stability.

Branched polystyrenes from suspension “Strathclyde” polymerization using a vulcanization accelerator as a chain transfer agent

Branched polymers exhibit a unique three-dimensional (3D) molecular architecture and distinctive physical/chemical properties, and thus have been applied in a wide variety of fields.

Expanding the monomer scope of linear and branched vinyl polymerisations via copper-catalysed reversible-deactivation radical polymerisation of hydrophobic methacrylates using anhydrous alcohol solvents

The use of anhydrous alcohols for Cu-catalysed reversible-deactivation radical polymerisation of a wide range of hydrophobic methacrylates has been explored in detail.

Radiolabeling of Nucleic Acid Aptamers for Highly Sensitive Disease-Specific Molecular Imaging

Aptamers are short single-stranded DNA or RNA oligonucleotide ligand molecules with a unique three-dimensional shape, capable of binding to a defined molecular target with high affinity and specificity. Since their discovery, aptamers have been developed for various applications, including molecular imaging, particularly nuclear imaging that holds the highest potential for the clinical translation of aptamer-based molecular imaging probes. Their easy laboratory production without any batch-to-batch variations, their high stability, their small size with no immunogenicity and toxicity, and their flexibility to incorporate various functionalities without compromising the target binding affinity and specificity make aptamers an attractive class of targeted-imaging agents. Aptamer technology has been utilized in nuclear medicine imaging techniques, such as single photon emission computed tomography (SPECT) and positron emission tomography (PET), as highly sensitive and accurate biomedical imaging modalities towards clinical diagnostic applications. However, for aptamer-targeted PET and SPECT imaging, conjugation of appropriate radionuclides to aptamers is crucial. This review summarizes various strategies to link the radionuclides to chemically modified aptamers to accomplish aptamer-targeted PET and SPECT imaging.

Temperature-dependent structure and dynamics of highly-branched poly(N-isopropylacrylamide) in aqueous solution

Small-angle neutron scattering (SANS) and neutron spin-echo (NSE) have been used to investigate the temperature-dependent solution behaviour of highly-branched poly(N-isopropylacrylamide) (HB-PNIPAM). SANS experiments have shown that water is a good solvent for both HB-PNIPAM and a linear PNIPAM control at low temperatures where the small angle scattering is described by a single correlation length model. Increasing the temperature leads to a gradual collapse of HB-PNIPAM until above the lower critical solution temperature (LCST), at which point aggregation occurs, forming disperse spherical particles of up to 60 nm in diameter, independent of the degree of branching. However, SANS from linear PNIPAM above the LCST is described by a model that combines particulate structure and a contribution from solvated chains. NSE was used to study the internal and translational solution dynamics of HB-PNIPAM chains below the LCST. Internal HB-PNIPAM dynamics is described well by the Rouse model for non-entangled chains.

Main-chain degradable star polymers comprised of pH-responsive hyperbranched cores and thermoresponsive polyethylene glycol-based coronas

Dual stimuliresponsive main-chain degradable star hyperbranched polymers have been synthesized via cyclic ketene acetal radical ring-opening and RAFT-based methacrylate copolymerization.

Highly-ordered onion micelles made from amphiphilic highly-branched copolymers

Uniform onion micelles formed from up to ten nano-structured polymer layers were produced by the aqueous self-assembly of highly-branched copolymers.

Thermoresponsive and Reducible Hyperbranched Polymers Synthesized by RAFT Polymerisation

Here, we report the synthesis of new thermoresponsive hyperbranched polymers (HBPs) via one-pot reversible addition-fragmentation chain transfer (RAFT) copolymerisation of poly(ethylene glycol)methyl ether methacrylate (PEGMEMA, Mn = 475 g/mol), poly(propylene glycol)methacrylate (PPGMA, Mn = 375 g/mol), and disulfide diacrylate (DSDA) using 2-cyanoprop-2-yl dithiobenzoate as a RAFT agent. DSDA was used as the branching agent and to afford the HBPs with reducible disulfide groups. The resulting HBPs were characterised by Nuclear Magnetic Resonance Spectroscopy (NMR) and Gel Permeation Chromatography (GPC). Differential Scanning Calorimetry (DSC) was used to determine lower critical solution temperatures (LCSTs) of these copolymers, which are in the range of 17⁻57 °C. Moreover, the studies on the reducibility of HBPs and swelling behaviours of hydrogels synthesized from these HBPs were conducted. The results demonstrated that we have successfully synthesized hyperbranched polymers with desired dual responsive (thermal and reducible) and crosslinkable (via thiol-ene click chemistry) properties. In addition, these new HBPs carry the multiplicity of reactive functionalities, such as RAFT agent moieties and multivinyl functional groups, which can afford them with the capacity for further bioconjugation and structure modifications.

Preparation and Properties of Branched Polystyrene through Radical Suspension Polymerization

Radical solvent-free suspension polymerization of styrene with 3-mercapto hexyl-methacrylate (MHM) as the branching monomer has been carried out using 2,2'-azobisisobutyronitrile (AIBN) as the initiator to prepare branched polymer beads of high purity. The molecular weight and branching structure of the polymers have been characterized by triple detection size exclusion chromatography (TD-SEC), proton nuclear magnetic resonance spectroscopy (¹H-NMR), and Fourier transform infrared spectroscopy (FTIR). The glass transition temperature and rheological properties have been measured by using differential scanning calorimetry (DSC) and rotational rheometry. At mole ratios of MHM to AIBN less than 1.0, gelation was successfully avoided and branched polystyrene beads were prepared in the absence of any solvent. Branched polystyrene has a relatively higher molecular weight and narrower polydispersity (Mw.MALLS = 1,036,000 g·mol-1, Mw/Mn = 7.76) than those obtained in solution polymerization. Compared with their linear analogues, lower glass transition temperature and decreased chain entanglement were observed in the presently obtained branched polystyrene because of the effects of branching.

Exploration of carboxymethyl guargum grafted hyperbranched poly(acrylic acid) as a scaffold for silver nanoparticles for ultrafast and selective sensing of Hg(ii)

A sensor for Hg²⁺ was developed from a system of silver nanoparticles stabilized by polysaccharide grafted hyperbranched poly(acrylic acid).

Synthesis of highly branched polymers by reversible complexation-mediated copolymerization of vinyl and divinyl monomers

Here, we report the reversible complexation-mediated copolymerization (RCMcP) of vinyl and divinyl monomers for the synthesis of highly branched polymers.

Switchable 19F MRI polymer theranostics: towards in situ quantifiable drug release

A switchable polymeric ¹⁹F magnetic resonance imaging (MRI) contrast agent was synthesised whereby the transverse (T₂) relaxation times increased as a therapeutic was released from a hyperbranched polymer (HBP) scaffold.

From seleno-mediated radical polymerization to seleno-containing branched polymers and dynamic hydrogel

A protocol of introducing redox responsive Se–Se bond into well-defined hydrogel backbone.

pH-Responsive polymers

This review summarizes pH-responsive monomers, polymers and their derivative nano- and micro-structures including micelles, cross-linked micelles, microgels and hydrogels.

Radical polymerization in the presence of a peroxide monomer: an approach to branched vinyl polymers

In this paper, we report radical polymerization in the presence of a peroxide monomer for the preparation of branched vinyl polymers.

Cyclic Graft Copolymer Unimolecular Micelles: Effects of Cyclization on Particle Morphology and Thermoresponsive Behavior

The synthesis of cyclic amphiphilic graft copolymers with a hydrophobic polycarbonate backbone and hydrophilic poly(N-acryloylmorpholine) (PNAM) side arms via a combination of ring-opening polymerization (ROP), cyclization via copper-catalyzed azide-alkyne cycloaddition (CuAAC), and reversible addition-fragmentation chain transfer (RAFT) polymerization is reported. The ability of these cyclic graft copolymers to form unimolecular micelles in water is explored using a combination of light scattering, small-angle X-ray scattering (SAXS), and cryogenic transmission electron microscopy (cryoTEM) analyses, where particle size was found to increase with increasing PNAM arm length. Further analysis revealed differences in the solution conformations, loading capabilities, and morphologies of the cyclic graft copolymers in comparison to equivalent linear graft copolymer unimolecular micelle analogues. Furthermore, the cyclic and linear graft copolymers were found to exhibit significantly different cloud point temperatures. This study highlights how subtle changes in polymer architecture (linear graft copolymer versus cyclic graft copolymer) can dramatically influence a polymer's nanostructure and its properties.

Chiral dendrigraft polymer for asymmetric synthesis of isoquinuclidines

A copper complex of chiral modified dendrigraft amidoamine polymer with a pentaerythritol initiated polyepichlorohydrin core, PEN-G₂, on a solid resin support is employed in the synthesis of isoquinuclidines via aza Diels–Alder reaction between cyclohexenone and imines.

Modular segmented hyperbranched copolymers

Modular segmented hyperbranched polymers, amenable to facile post-polymerization functionalization, were created via two distinct approaches.

Ligand-Unsymmetrical Phenoxyiminato Dinickel Catalyst for High Molecular Weight Long-Chain Branched Polyethylenes

An unsymmetrical bimetallic catalyst, (CF₃/SO₂)FI²-Ni₂, having a CF₃-functionalized phenoxyiminato Ni(II) center, which produces linear high-M_w polyolefin (CF₃/Ni) joined to an adjacent SO₂-functionalized phenoxyiminato Ni(II) center, which produces highly branched low-M_w polyolefin (SO₂/Ni), was synthesized and fully characterized. In ethylene homopolymerizations, (CF₃/SO₂)FI²-Ni₂ affords monomodal (Đ = 1.7), highly, long-chain branched (69 branches/1000 C) polyethylenes with M_w = 24 kg/mol. In contrast, bimetallic (CF₃)FI²-Ni₂ and (SO₂)FI²-Ni₂ produce high-M_w (25 kg/mol) exclusively methyl-branched (40 branches/1000 C) and low-M_w (4.5 kg/mol) highly branched (105 branches/1000 C) polyethylenes, respectively, while tandem monometallic (CF₃)FI-Ni + (SO₂)FI-Ni catalyst mixtures yield a bimodal polyolefin mixture (Đ = 6.4).

Doubly-responsive hyperbranched polymers and core-crosslinked star polymers with tunable reversibility

Thermo- and redox-responsive hyperbranched copolymers were prepared by statistical copolymerization of N-isopropylacrylamide (NIPAM) and N,N′-bis(acryloyl)cystamine (BAC) by reversible addition–fragmentation chain transfer (RAFT) polymerization.

A versatile strategy for synthesis of hyperbranched polymers with commercially available methacrylate inimer

This work reported a facile strategy to synthesize hyperbranched polymers by simply using a commercially available hydroxyl-substituted methacrylate, which can be applied to not only the SCVP of vinyl monomers, but also to the SCROP of cyclic esters.

The electrophilic effect of thiol groups on thiol–yne thermal click polymerization for hyperbranched polythioether

The significant effect of the electrophilicity of the thiol groups on thiol–yne click polymerization was found and well demonstrated.

Hyperbranched polymers: advances from synthesis to applications

This review summarizes the advances in hyperbranched polymers from the viewpoint of structure, click synthesis and functionalization towards their applications in the last decade.

Preferential chiral solvation induced supramolecular chirality in optically inactive star Azo polymers: photocontrollability, chiral amplification and topological effects

The chiral amplification phenomena were observed for the first time in 4- and 6-armed Azo star side-chain polymer systems, but absent in linear and 3-armed counterparts.

Self-assembly of cyclic polymers

This review describes the self-assembly of polymers with a cyclic topology and highlights how cyclization affects the resulting assemblies.