Controlled/living radical polymerization in aqueous media: homogeneous and heterogeneous systems

Abstract

Controlled/living radical polymerizations carried out in the presence of water have been examined. These aqueous systems include both the homogeneous solutions and the various heterogeneous media, namely dispersion, suspension, emulsion and miniemulsion. Among them, the most common methods allowing control of the radical polymerization, such as nitroxide-mediated polymerization, atom transfer radical polymerization and reversible transfer, are presented in detail.

Introduction

Since Staudinger [1] proposed the concept of a chain polymerization and the basic structure of a polymer molecule eight decades ago, polymer science and technology has experienced an immense development that has revolutionized the look of the world and the life of human beings. Numerous polymeric materials have been created thanks to continuous progress in understanding the fundamentals of polymerization. One of the greatest contributions to this field from synthetic polymer chemists is the development of living polymerization methodology, which allows the preparation of macromolecules with the maximum degree of structural and compositional homogeneity. As a consequence, well-defined polymers with precise molar masses, compositions, topologies and functionalities can be tailor-made. This is a significant step toward the ultimate goal of polymer synthesis, when the design of novel materials is only limited by the imagination of human beings.

The terms of ‘living polymerization’ and ‘living polymers’ were introduced by Szwarc [2] in 1956, although prior to his classical work, Ziegler [3] and Flory [4] also described similar concepts. By definition, a living polymerization is a chain polymerization that proceeds without the occurrence of irreversible chain breaking processes, i.e. chain transfer and termination. For nearly 30 years after Szwarc reported his insightful work, living polymerization of vinyl monomers had been restricted to anionic polymerization systems. But in 1960s and 1970s, several cationic ring-opening polymerizations of heterocyclic monomers were found to proceed with most undesirable side reactions virtually absent [5], [6], [7]. It was discovered that if a dynamic equilibrium between an active and dormant species was formed, it allowed for fine tuning of the polymerization of tetrahydrofuran [8], [9]. This concept of dynamic equilibrium was eventually extended to vinyl monomers in early 1980s, triggering the breakthrough discovery of cationic vinyl polymerizations that proceeded in a controlled fashion under certain restrictive conditions [10], [11], [12]. Since then, extensive investigations of various CLP mechanisms has been conducted [13], [14]. The late 1980s and the entire 1990s witnessed a rapid expansion of the scope of CLP. To date, the major classes of chain polymerization, i.e. anionic [15], [16], [17], cationic [18], [19], ring-opening metathesis [20], [21], coordination [22] and radical polymerization [23], [24], can become living or controlled under appropriate conditions. However, most of these polymerization techniques are not exempt from chain transfer or termination reactions. To differentiate these imperfect polymerizations from the ideal living polymerization, terms such as controlled, ‘living’, pseudo-living, quasi-living and many others have been used in literature, which initiated an on going debate over nomenclature [25], [26]. Until a uniform terminology is accepted, we will use the term ‘controlled/living polymerization’ to describe all the polymerization processes from which polymers with predetermined molar masses, low polydispersities and high chain-end functionalities can be obtained. The significance of controlled/living polymerization as a synthetic tool is widely recognized. With polymers having uniform and predictable chain length readily available, it provides the best opportunity to control the bulk properties by variations at a molecular level. Furthermore, a variety of novel polymer materials with predetermined composition, topology and functionality, as illustrated in Table 1, can be created.

Among the numerous polymerization techniques, free-radical polymerization is a widely used process from the viewpoint of industrial production and applications [27]. This technique is relatively easy to perform since it does not require stringent purification of the reagents, except the elimination of the dissolved oxygen. It generally leads to high molar mass polymers under relatively mild conditions. Polymerization temperatures can be usually set between room temperature and about 200°C. Many different processes can be applied such as bulk, solution, suspension or emulsion polymerizations. Moreover, a wide range of functional monomers can be polymerized by a radical mechanism and copolymerizations have provided a great variety of random copolymers with many structures and properties. The main limitations of radical polymerization are the lack of control over the molar mass, the molar mass distribution, the end-functionalities and the macromolecular architecture. This is caused by the unavoidable fast radical–radical termination reactions. Mainly for that reason, the recent emergence of many so-called ‘living’ or controlled radical polymerization (CRP) processes has opened a new area in this old polymerization method that had witnessed relatively small progress in the previous years.

Until now, CRP has been predominantly studied in homogeneous organic systems, i.e. bulk or solution polymerizations. With the rapid development of understanding of those controlled systems, one of the biggest present concerns is: can these methods be extended to the aqueous media? Indeed, the polymerizations carried out in aqueous media are receiving more and more attention and, in contrast to the numerous controlled polymerization methods employed in organic media, aqueous polymerization is mainly focused on radical polymerization. Among the most important factors contributing to this trend are increased environmental concern and a sharp growth of pharmaceutical and medical applications for hydrophilic polymers. Another important reason is that in industry radical polymerization is widely performed in aqueous dispersed systems, and particularly, in emulsion polymerization. This technique offers many invaluable practical advantages over homogeneous polymerizations, such as the absence of volatile organic compounds, better control of heat transfer, and the possibility to reach high molar mass polymers with high conversion and a faster rate of polymerization than in bulk or solution systems. The final water suspension of stable polymer particles (also called a latex) is easy to handle owing to a generally low viscosity, even at high solid content, and can be used directly for coating applications or as a dried polymer after removal of water.

Thus, it is obvious that a substantial progress in controlled free-radical polymerization would be achieved if this technique can be applied to either homogeneous or heterogeneous aqueous systems. The first, and main goal is to control the characteristics of the polymer in terms of molar mass, molar mass distribution, architecture, and function. The rapid and very recent development of this domain will be illustrated in this review article. Polymerizations in aqueous solution will be considered first, including the scope and limitations of the existing CRP systems. Then, the attention of the article will be focused on the application of CRP in aqueous dispersed media using the suspension, dispersion, emulsion and miniemulsion polymerization processes. The kinetics of polymerization and control over the polymer characteristics will be examined for each CRP technique used and a comparison with homogeneous polymerization will be made. The repercussions of CRP chemistry on the very mechanism of the heterogeneous polymerization systems will also be addressed with special attention.