Responsive polymers, particles, and assemblies, part A

Understanding the “stuff” of life, such as DNA and proteins – both polymers – was just a fantasy (at best) in the early parts of the 20th century. In this time period, the mere existence of large “macromolecules” was not even fathomable, so the possibility of polymers dictating who we are was simply science fiction. The brave work of Hermann Staudinger (1953 Nobel Prize in Chemistry), Wallace Carothers, and Herman Mark led to the gradual, and eventual global belief that such large molecular weight macromolecules exist, and their work has paved the way for the work of trailblazers such as: Paul J. Flory (1974 Nobel Prize in Chemistry), Jean-Marie Lehn (1987 Nobel Prize in Chemistry), and Pierre-Gilles de Gennes (1991 Nobel Prize in Physics). Through their combined efforts, over many years, polymer science is what it is today, and its development has led to polymers being present in nearly everything people around the world encounter on a daily basis. The ubiquity of polymers is a result of our deep understanding of polymer behavior/physics, and our vast synthetic capabilities, such that polymer chemistry, structure, and ultimately function can be tailored almost at will to have an impact on nearly any application under the sun. This pair of special issues focuses on a very special class of polymers, typically referred to as “smart,” “intelligent,” “stimuli responsive,” and/or “environmentally responsive” polymers. These polymers, like their “nonresponsive” counterparts are indeed high molecular weight structures, composed of monomers as building blocks, and can be synthesized to have a variety of chemistries and morphologies. Unlike traditional polymers, responsive polymers are able to respond to their environment or a stimulus by undergoing some change, whether physical and/or chemical in nature. To date, a number of different responsivities have been engineered/synthesized into polymers, including responsivity to: light, temperature, pH, magnetic and electric fields, and analyte concentration. In fact, a number of years ago Dr. Yoseph Bar-Cohen issued a challenge to synthesize a polymer-based device that is capable of beating a human in arm wrestling. Through the use of electroactive polymers, progress is being made in this area, and soon polymer-based devices will be strong enough to compete with humans (see figure). Illustration depicting the concept of a polymer-based arm with the strength of a human arm. Courtesy of Dr. Yoseph Bar-Cohen, JPL/Caltech/NASA. These two special issues are composed of articles from some of the top researchers operating in the area of responsive polymers. The topics in the special issues can be broken up into three main themes: 1) Synthesis; 2) Fundamental Properties; and 3) Applications. This special issue of the Journal of Polymer Science, Part A: Polymer Chemistry highlights aspects of the topics above to achieve various outcomes. Kuckling and Wycisk describe the use of novel controlled radical polymerization techniques to synthesize stimuli-responsive star polymers, block copolymers, and comb polymers. They go on to detail their use in various applications. Relatedly, Oh and coworkers show that polylactide (PLA)-based block copolymers having disulfide linkages at block junctions could be synthesized using a combination of ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). Furthermore, they show that these polymers are degradable by cleaving the disulfide linkage. In the paper by Hoare and Smeets, the physical, chemical, and biological parameters of microgels that must be tuned to achieve delivery of a specific drug at a specific rate under specific physiological conditions are discussed. They then outline the design parameters that must be taken into consideration when generating responsive polymer-based systems for drug delivery applications. With appropriate synthetic approaches and a fundamental understanding of responsive polymers in hand (described in the associated special issue in the Journal of Polymer Science, Part B: Polymer Physics), a number of applications become possible. Barrett and Goulet-Hanssens detail the use of the azobenzenes to synthesize light responsive polymers. It is well known that azobenzene undergoes a trans to cis conformational change in response to the application of UV light, which can be reconverted to the trans form upon exposure to visible light. In their contribution, they detail the use of this phenomenon to generate photoreversible systems for drug delivery and sensing applications, among other things. Independent contributions by Ngai et al. and Pich et al. detail the use of responsive polymer-based particles for health-related applications, e.g., cell culture and gene/drug delivery. Finally, Suzuki and Umeda detail the synthesis of Janus microgels and self-oscillating microgels. Janus microgels can be used to control assembly of microgels into higher order structures for photonic applications. Relatedly, Serpe and coworkers detail the use of pNIPAm microgel-based assemblies for environmental remediation and sensing applications. In this pair of special issues, we hope it is apparent that while much is known about polymers in general, the research space carved out for responsive polymers and responsive polymer-based systems is, and will continue to be vibrant. Through the evolution of our understanding of the fundamental properties of this interesting class of polymers, and the synthetic tools and techniques available for our use, the future applications of these polymers is vast and seemingly limitless. We hope you enjoy the special issues. Michael J. Serpe University of Alberta

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