Reversible Deactivation Radical Polymerization

Synthesis and Applications of Functional Polymers

Nikhil K. Singha, Jimmy Mays, Nikhil K. Singha, Jimmy Mays

• Publisher: De Gruyter
• Year: 2020
• Print ISBN:
  9783110639995
• eBook ISBN:
  9783110640175
• Topic: Technologie et ingénierie
• Subtopic: Chimie organique

This book describes strategies and mechanism of reversible deactivation radical polymerization (RDRP) to synthesize functional polymers. Several approaches such as atom transfer radical polymerization and the combination of click chemistry and RDRP are summarized. Contributors from interdisciplinary fields highlight applications in nanotechnology, self-healing materials, oil and water resistant coatings and controlled drug delivery systems.

1 Introduction to reversible deactivation radical polymerization

Bert Klumperman

Department of Chemistry and Polymer Science, Stellenbosch University, Matieland, South Africa

1.1 Introduction

Radical polymerization is among the oldest techniques used for polymer synthesis. It is a typical example of a chain growth reaction in which the active center at a growing chain end is an unpaired electron or radical. A wide variety of monomers that carry a C = C unsaturated functionality can be polymerized via radical polymerization. These monomers include styrene derivatives, acrylates, methacrylates, vinyl esters, N-vinyllactams, and many others. Despite its long history in polymer synthesis, what is now called conventional radical polymerization has always suffered from inherent shortcomings. The two most important of those shortcomings are the lack of stereoselectivity and the inability to control chain topology (broad molar mass distributions, no block copolymers, or other advanced architectures). Since the 1980s, the development of radical polymerization techniques with characteristics that resemble living chain growth polymerization methods (e.g., anionic polymerization) has received considerable attention. Through the 1990s, three main techniques were published that collectively are now known under the IUPAC recommended name reversible deactivation radical polymerization (RDRP). In earlier studies, controlled radical polymerization (CRP) and living radical polymerization (LRP) have extensively been used as names to describe the general concept of RDRP. The three main techniques within the field of RDRP are, in chronological order: nitroxide-mediated polymerization (NMP) [1], atom transfer radical polymerization (ATRP) [2, 3], and reversible addition-fragmentation chain transfer (RAFT) mediated polymerization [4]. In this introductory chapter, these three techniques will be described and discussed in relation to the synthesis of functional polymers.

1.2 Nitroxide-mediated polymerization (NMP)

The lack of control over the molar mass distribution in conventional radical polymerization stems from the continuous initiation and termination of polymer chains throughout the course of the reaction. The statistical nature of these processes leads to a dispersity (Đ) of 1.5 or 2, depending on the mode of termination (combination or disproportionation respectively). To confer a living character to radical polymerization, two main criteria need to be fulfilled. First, initiation should take place early in the polymerization to ensure that all chains have the opportunity to grow throughout the course of the reaction. However, in radical polymerization this would normally lead to an enormous radical concentration with an inevitably high rate of termination. Therefore, a second criterion is needed, which entails the reversible deactivation of radical chain ends to lower the instantaneous radical concentration and consequently minimize the occurrence of bimolecular termination. In NMP, the most common way to initiate chains is via the homolytic dissociation of an alkoxyamine into a transient carbon-centered radical and a persistent nitroxide radical as is shown in Scheme 1.1 for the case of styrene polymerization with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as the mediating nitroxide.

Scheme 1.1: General scheme of nitroxide-mediated polymerization (NMP), initiated by an alkoxyamine.

The transient radical has two main fates. The first one is to add to monomer to start the growth of a polymer chain, whereas the second one is recombination with the persistent radical to reform the alkoxyamine. However, the transient radical is a normal carbon-centered radical that can undergo all the usual reactions in radical polymerization including propagation, chain transfer, and termination. In a conventional radical polymerization, the time it takes for a complete chain to grow from initiation to termination is in the order of a second. The reversible deactivation process in NMP cuts this one-second into short intervals with a duration in the order of a millisecond. In between two consecutive intervals of chain growth, a dormant period is inserted that may last in the order of 10 milliseconds. During each activation–deactivation cycle, only one or very few monomer units are added to the polymer chain. As a consequence, all chains grow with a very similar rate, resulting in a narrow molar mass distribution. In addition, the reversible deactivation leads to a situation where only a very small fraction of all chains is in its active form at any point in time. This leads to a strong reduction in irreversible bimolecular termination, which also contributes to a reduction in the width of the molar mass distribution. To some extent, NMP has a self-regulating mechanism that reduces the transient radical concentration, namely the persistent radical effect (PRE) [5]. As soon as irreversible bimolecular termination takes place, an excess of persistent radicals is created. As in any equilibrium reaction, the build-up of concentration of a species leads to a shift in equilibrium. In this specific case, the increased concentration of persistent radicals leads to a shift of the activation–deactivation equilibrium toward the dormant side. This effectively means that the transient radical concentration is reduced, which leads to a reduction in rate of polymerization and a strong reduction in rate of termination. Ultimately, this leads to an enhanced living character of the polymerization, which is shown by a narrow molecular weight distribution (MWD) and the majority of polymer chains carrying well-defined α- and ω-chain end functionalities. Georges and coworkers contributed a lot to the optimization of NMP in the 1990s. They found that the addition of additives like camphor sulfonic acid has a rate-enhancing effect on NMP [6, 7]. The mechanism of rate enhancement is not yet fully elucidated [8].

In the case of NMP, the α-chain end is defined by the choice of the initiating primary radical that forms part of the initial alkoxyamine. Typical examples are esters that resemble acrylate- or methacrylate-propagating radicals as depicted in Figure 1.1. The ω-chain end is an alkoxyamine and there are two ways to make use of its functionality in, for example, conjugation reactions. The first option is to use an alkoxyamine that carries a suitable functionality on the nitroxide fragment, such as, for example, 4-hydroxy-TEMPO. The second option is to end cap the ω-chain end in a bimolecular termination reaction. An example of this strategy is to decompose a radical initiator in the presence of the alkoxyamine terminal polymer chains. When a large excess of radicals is generated of an initiator like 4,4ʹ-azobis(4-cyanopentanoic acid)(ACPA) in the presence of the suitable macro-alkoxyamine, hetero-bimolecular termination will lead to the introduction of an ω-chain end carboxylic acid functionality as shown in Scheme 1.2.

Figure 1.1: Examples of alkoxyamines, (1) 2-hydroxyethyl 2-((2,2,6,6-tetramethylpiperidin-1-yl)oxy)propanoate and (2) 2-((tert-butyl(1-diethoxyphosphoryl)-2,2-dimethylpropyl)amino)oxy)-2-methylpropanoic acid, also known as BlocBuilder or MAMA-SG1.

Scheme 1.2: End-group modification of SG1-terminal PSTY by radical addition with ACPA.

1.3 Transition metal-mediated radical polymerization

In the mid-1990s, Matyjaszewski and Sawamoto first published atom transfer radical polymerization (ATRP) independently [2, 3]. A suitably stabilized alkyl halide is typically used as an initiator where a transition metal-based catalyst in its lower oxidation state (Cu[I], Ru[II], etc) acts as the activator. Cu[I] complexes are most frequently used as activator/catalyst and in a one-electron redox process, the halide gets abstracted from the initiator to create a carbon-centered primary radical and oxidize the copper complex to its Cu[II] state. The carbon-centered transient radical can add to monomer to initiate the growth of a polymer chain, and any carbon-centered radical (primary radical or growing chain) can reversibly be deactivated by the Cu[II] complex. Similar to NMP, a dynamic equilibrium between carbon-centered transient radical and dormant species, in this case halides, is established. The large difference between NMP and ATRP is that in the case of NMP, the dynamic equilibrium is thermally induced, whereas in the case of ATRP the transition metal catalyst is required to mediate the equilibrium. In th...

Table of contents

• Title Page
• Copyright
• Contents
• Preface
• 1 Introduction to reversible deactivation radical polymerization
• 2 Tailor-made polymer–nanohybrid materials via reversible deactivation radical polymerization (RDRP)
• 3 Synthesis of functionally terminated polymers by atom transfer radical polymerization (ATRP) and their applications
• 4 Functional (co)polymers via a combination of reversible deactivation radical polymerization techniques and thiol-based “click”/conjugation chemistries
• 5 Designing macromolecular architecture via reversible deactivation radical polymerization (RDRP) and Diels–Alder reaction
• 6 Recent advances in the reversible deactivation radical (co)polymerization of fluorinated alkenes/acrylates/methacrylates/styrenes
• 7 Polymers prepared via reversible-deactivation radical polymerization (RDRP) for biomedical applications
• Abbreviations
• Index