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There is little doubt the advent of controlled/living radical polymerization (CRP) has revolutionized modern polymer science. Perhaps more than any synthetic development of the last several decades, CRP has unlocked access to a plethora of new functional and well-defined polymers to address a variety of complex applications. Previously considered useful for primarily the facile and rapid preparation of high molecular weight polymers for mostly commodity applications, radical polymerization can now be readily employed to prepare specialty materials based on (co)polymers that are well-defined, have controlled molecular weights and end group functionality, complex topologies, and predefined segment sequences (e.g., block copolymers).

More than any other characteristic, it is the molecular weight of a polymer that controls its properties and potential utility. While conventional radical polymerization had been employed for many years to achieve high molecular weight polymers, precise control of chain length remained difficult. External chain transfer agents proved useful to limit molecular weights, but the obtained polymers were characterized with broad molecular weight distributions. The diverse family of CRP techniques developed over the last 2–3 decades now allows access to polymers of a specific chain length that can be derived from virtually any vinyl monomer polymerizable by a radical method.

Conventional radical polymerization had limited success in the preparation of polymers with specific chain-end functional and/or reactive groups. Although certain functionalized radical initiators showed some potential, the range of functional groups that could be attached efficiently to either end of the macromolecules was relatively narrow. Further, due to the ever-present termination and transfer reactions, which rapidly “kill” the propagating chains, it is virtually impossible to prepare samples in which nearly all macromolecules are chain-end-labeled. In contrast, CRP methods have proved very powerful in the synthesis of polymers containing either identical or different α- and ω-chain-end functional groups. The synthesis of composite materials by CRP is also straightforward.

One of the key benefits of classical radical polymerization is the facility with which many monomers can be incorporated into a single chain by copolymerization. Depending on the relative reactivity of the comonomers, random, alternating and blocky sequences can be achieved. However, block copolymers with well-defined junction points linking segments of differing repeat units are nearly impossible to prepare directly by conventional radical polymerization. While it was possible to prepare block copolymers by coupling two end-functional homopolymers or by polymerization from homopolymers terminated with a moiety that allowed a subsequent polymerization step, facile access to block copolymers was not achieved by a radical mechanism prior to the development of controlled radical methods. Since then, an enormous number of new block copolymers have been prepared by CRP, and this has provided access to a variety of new materials with potential applications as drug delivery agents, cosmetics, coatings, adhesives, thermoplastic elastomers, etc. Additionally, the copolymerization of monomers with different reactivities under CRP conditions yields gradient copolymers, in which the composition changes along the polymer backbone, instead of from one chain to another (as is the case in conventional radical polymerization). Gradient copolymers were inaccessible through traditional radical polymerization and are very promising materials in various fields, such as surfactants, degradable polymers, etc.

Another aspect of chain structure that has become more readily manipulated is chain architecture. By employing new initiators or reversible chain transfer agents with multiple sites capable of initiating chain growth, a wide variety of complex architectures previously considered inaccessible can now be readily prepared by CRP. Stars, combs, and brushes represent only a fraction of branched chain topologies that have been synthesized. This aspect of CRP has enabled both new applications and fundamental insight into polymer structure-property relationships.

Many of the aforementioned characteristics of CRP are in common with those of previously developed (pseudo)living ionic polymerization techniques. However, it is the relative ease with which CRP can be employed to achieve these characteristics that has led to the continued success and growth of the field. Indeed, because most of the methods described in the chapters that follow can be conducted under relatively non-stringent conditions in a variety of solvents and at a wide range of temperatures, the barrier to entry to the field is minimal. Precision polymer synthesis is now possible in laboratories without access to expensive experimental equipment or extensive expertise.

It is important to remember that the CRP techniques are based on the success achieved over many years in the areas of living ionic polymerizations. The success of Szwarc and others in recognizing the far-reaching utility of living anionic polymerization led many to pursue the goal of polymerization control during other chain growth methods. Molecular weight and end group control in cationic polymerization was subsequently achieved by introduction of the concept of dormant-active equilibration (i.e., “reversible deactivation”) that kinetically limited the extent of chain breaking reactions by rendering a significant fraction of chains dormant at any given time. Unfortunately, ionic polymerizations are applicable only to a relatively narrow range of monomers and required stringent reaction conditions (e.g., absence of moisture, carbon dioxide, and many other impurities often encountered in commercially available monomers and solvents). Owing to significant differences in the reactivity ratios of the ionically polymerizable monomers, copolymerization reactions are often challenging, which limits the number of accessible materials. For a relatively long time, it was considered that radical polymerizations that were living or at least resembled living ionic process were impossible to design due to fast termination reactions between the propagating radicals. It was thought that living radical polymerization could only be achieved in system where the mobility of the growing radicals was drastically diminished by, for instance, carrying out the polymerizations in very viscous media and preferably at low temperatures (which not only increases the viscosity but also minimizes transfer). Of course, such approaches were not particularly practical. The principle of reversible deactivation of propagating radicals was extended to radical polymerization in the 1980s and proved to be of tremendous utility in the design of living-like radical polymerizations. It is the specifics of the chemistry employed to achieve reversible deactivation that differentiates the various CRP processes. The fundamental aspects such as kinetics, thermodynamics, structure-reactivity correlations, solvent effects, etc., of the most important CRP methods are the subject of chapters in this book. It should be noted that radical polymerizations are never truly living (termination reactions always take place), but using the reversible deactivation strategy, these reaction show many of the characteristics of living polymerizations, such as control over molecular weight (determined by the ratio of monomer to initiator), narrow molecular weight distribution (if initiation is fast), high degree of chain-end functionalization, which is responsible for the ability to synthesize block copolymers, etc. Owing to the ability to control numerous molecular parameters and due to the fact that radical polymerizations with reversible deactivation resemble closely the classical living ionic polymerizations, we have adopted the term controlled/living radical polymerization in this book.

We hope this book provides insight into the current state of the art in CRP while also giving historical perspective to the field. While many other books have focused on controlled/living polymerizations, including CRP, and have provided extensive insight into the materials that can be prepared therefrom, our intent here is to elucidate the fundamentals governing the most common types of CRP. In many cases, particular attention is dedicated to the kinetic and thermodynamic factors that effect polymerization control. We hope this collection will be useful to both experts and newcomers to this rapidly expanding field.

Nicolay V. Tsarevsky

Southern Methodist University, Dallas

Brent S. Sumerlin

University of Florida

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