Synthesis of an ABC triblock copolymer in aqueous solution via RAFT

Significance 

Among the available reversible-deactivation radical polymerization techniques, reversible addition-fragmentation chain transfer (RAFT) stands out. Generally, it makes use of a chain transfer agent in the form of a thiocarbonylthio compound to afford control over the generated molecular weight and polydispersity during a free-radical polymerization. Consequently, it is considered to be one of the most versatile radical polymerization techniques and is rapidly gaining popularity among researchers by day. Several polymer architectures can be achieved via RAFT polymerization, among them, amphiphilic block copolymers are most interesting.

As such, novel self-assembly techniques have been developed in quest to address various needs; such as, the polymerization induced self-assembly (PISA), that mainly employ thermal activation of radical polymerization. With recent technological advances, the light-mediated RAFT-based PISA (photo-PISA) has been developed and lately become the epicenter of much research, as it offers a high degree of control over the polymerization process. Unfortunately, little has been published regarding the PISA generated triblock copolymer self-assembly.

To this effect, Harvard University scientists: Dr. Sudhina Guragain and Professor Juan Perez-Mercader from the Department of Earth and Planetary Science, Origin of Life Initiative reported facile one-pot synthesis of an ABC triblock copolymer of E-D-H in an aqueous solvent using light mediated RAFT. Specifically, they purposed to extend a di-block copolymer that showed pH-dependent water solubility chain with a hydrophobic third block so as to obtain an ABC type amphiphilic triblock copolymer of E-D-H. Their work is currently published in the research journal, Polymer Chemistry.

In brief, the research method employed commenced with the synthesis of the E17-Dx di-block copolymer and the E17-D45-Hx triblock copolymer, respectively. This synthesis of block copolymers was carried out by utilizing visible light-mediated RAFT aqueous dispersion polymerization. Next, the synthesized copolymers were characterized using nuclear magnetic resonance spectroscopy, transmission electron microscopy, dynamic light scattering, fluorescence spectroscopy and gel permeation chromatography.

The authors observed that once the hydrophobic third block was sufficiently chain extended, the block copolymer self-assembled to nano-objects. They also noted that the morphology of the nano-objects depended on the concentration and the achieved degree of polymerization of the hydrophobic block. It was also seen that since the triblock copolymer had a pH sensitive middle block, the morphology could be tuned by altering the pH. Moreover, they noted that an increase in pH resulted in the deprotonation of the PDMAEMA, which in turn, decreased the charge density, hence making the formation of higher order morphologies thermodynamically more favorable and therefore allowing the control of the morphology with pH.

In summary, Sudhina Guragain and Juan Perez-Mercader study presented novel synthesis of a triblock copolymer in one-pot two-steps via photo-induced electron transfer -RAFT aqueous dispersion polymerization. In general, the change in pH not only triggered the change in the morphology, but also caused the change in the surface roughness of the self-assembled structures. Altogether, their work presents the first ever report on visible light-mediated ABC triblock copolymer synthesis and its PISA-based self-assembly in aqueous solution in a single pot.

E-D-Hpoly(ethylene oxide-b-(2-dimethylamino)ethyl methacrylate-b-(2-hydroxypropyl methacrylate))

Reference

Sudhina Guragain, Juan Perez-Mercader. Light-mediated one-pot synthesis of an ABC triblock copolymer in aqueous solution via RAFT and the effect of pH on copolymer self-assembly. Polymer Chemistry, 2018, volume 9, 4000.

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