Debashish Roy

New directions in thermoresponsive polymers

Interest in thermoresponsive polymers has steadily grown over many decades, and a great deal of work has been dedicated to developing temperature sensitive macromolecules that can be crafted into new smart materials. However, the overwhelming majority of previously reported temperature-responsive polymers are based on poly(N-isopropylacrylamide) (PNIPAM), despite the fact that a wide range of other thermoresponsive polymers have demonstrated similar promise for the preparation of adaptive materials. Herein, we aim to highlight recent results that involve thermoresponsive systems that have not yet been as fully considered. Many of these (co)polymers represent clear opportunities for advancements in emerging biomedical and materials fields due to their increased biocompatibility and tuneable response. By highlighting recent examples of newly developed thermoresponsive polymer systems, we hope to promote the development of new generations of smart materials.

Triply-responsive boronic acid block copolymers: solution self-assembly induced by changes in temperature, pH, or sugar concentration.

Boronic acid-containing block copolymers capable of solution self-assembly into micelles and reverse micelles in response to changes in temperature, pH, and sugar concentration were prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization.

Dynamic-Covalent Macromolecular Stars with Boronic Ester Linkages

Macromolecular stars containing reversible boronic ester linkages were prepared by an arm-first approach by reacting well-defined boronic acid-containing block copolymers with multifunctional 1,2/1,3-diols. Homopolymers of 3-acrylamidophenylboronic acid (APBA) formed macroscopic dynamic-covalent networks when cross-linked with multifunctional diols. On the other hand, adding the diol cross-linkers to block copolymers of poly(N,N-dimethylacrylamide (PDMA))-b-poly(APBA) led to nanosized multiarm stars with boronic ester cores and PDMA coronas. The assembly of the stars under a variety of conditions was considered. The dynamic-covalent nature of the boronic ester cross-links allowed the stars to reconfigure their covalent structure in the presence of monofunctional diols that competed for bonding with the boronic acid component. Therefore, the stars could be induced to dissociate via competitive exchange reactions. The star formation-dissociation process was shown to be repeatable over multiple cycles.

Synthesis of natural–synthetic hybrid materials from cellulose via the RAFT process

The synthesis and characterization of a novel natural-synthetic hybrid material based on cellulose is reported. The reversible addition-fragmentation chain-transfer (RAFT) process was used to graft poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) onto a cellulosic substrate. The weight ratio was increased with an increase in monomer concentration, polymerization time and degree of polymerization (DP). We found that the addition of free chain-transfer agent has a pronounced effect on the weight ratio, chain length of grafted polymer, monomer conversion and homopolymer formation in solution. The cellulose-graft-poly(2-(dimethylamino)ethyl methacrylate) copolymers were characterized by gravimetry, elemental analysis, attenuated total reflectance Fourier transform infrared spectroscopy, scanning electron microscopy, thermal analysis and atomic force microscopy. The dithioester end-group present at the chain end of PDMAEMA was removed via aminolysis. The livingness of the process was utilized to block-copolymerize styrene from the grafted PDMAEMA chains. The hydrophilic/hydrophobic properties of the novel cellulose-g-(PDMAEMA-b-polystyrene) material were illustrated by contact-angle measurements.

RAFT Graft Polymerization of 2-(Dimethylaminoethyl) Methacrylate onto Cellulose Fibre

Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) was grafted from cellulose by reversible addition–fragmentation chain transfer (RAFT) polymerization. The use of a free chain transfer agent in solution allowed for a better control over graft ratio, chain length of grafted polymer, monomer conversion, and homopolymer formation in solution. An increase in polymerization time or degree of polymerization led to an increase in graft ratio, as expected from a living system.

Let There Be Light: Photo-Cross-Linked Block Copolymer Nanoparticles

Polymeric nanoparticles are prepared by selectively cross-linking a photo-sensitive dimethylmaleimide-containing block of a diblock copolymer via UV irradiation. A well-defined photo-cross-linkable block copolymer is prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization of a dimethylmaleimide-functional acrylamido monomer containing photoreactive pendant groups with a poly(N,N-dimethylacrylamide) (PDMA) macro-chain transfer agent. The resulting amphiphilic block copolymers form micelles in water with a hydrophilic PDMA shell and a hydrophobic photo-cross-linkable dimethylmaleimide-containing core. UV irradiation results in photodimerization of the dimethylmaleimide groups within the micelle cores to yield core-cross-linked aggregates. Alternatively, UV irradiation of homogeneous solutions of the block copolymer in a non-selective solvent leads to in situ nanoparticle formation.