Über die Autorin bzw. den Autor

Ian Manners is Professor of Chemistry and Canada Research Chair at the University of Toronto. He received his Ph.D. in the area of transition metal chemistry in 1985 from the University of Bristol, England. After working as a Royal Society Postdoctoral Fellow in Germany in the field of main group chemistry, and then the USA in the area of polymer science, he moved to Toronto in July 1990. He was promoted to Full Professor in 1995.
Professor Manners’ research interests focus on the synthesis, properties, and applications of inorganic molecules, polymers, and materials. He is an author of over 300 publications and holds or coholds 11 patents. His awards includes an Alfred P. Sloan Fellowship from the USA (1994-98), a Corday-Morgan Medal (1997) from the UK, and the Alcan Award (1999) and the Macromolecular Science and Engineering Award (2003) from Canada. He was also awarded the Steacie Prize in 2000 – the top award in all areas of science and engineering in Canada for a person aged 40 or older – and he was elected to the Canadian National Academy of Science in 2001. He currently serves on the Editorial Advisory Board of 8 scientific journals.

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The development of the field of synthetic metal-containing polymers – where metal atoms form an integral part of the main chain or side group structure of a polymer – aims to create new materials which combine the processability or organic polymers with the physical or chemical characteristics associated with the metallic element or complex. This book covers the major developments in the synthesis, properties, and applications of synthetic metal-containing macro-molecules, and includes chapters on the preparation and characterization of metal-containing polymers, metallocene-based polymers, rigid-rod organometallic polymers, coordination polymers, polymers containing main group metals, and also covers dendritic and supramolecular systems. The books describes both polymeric materials with metals in the main chain or side group structure and covers the literature up to the end of 2002.

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Synthetic Metal-Containing Polymers

John Wiley & Sons

Chapter One

1.1 Metal-Containing Polymers

Carbon is not a particularly abundant terrestrial element, ranking 14th among those in the Earth's crust, oceans, and atmosphere. Nevertheless, carbon-based or organic macromolecules form the basis of life on our planet, and both natural and synthetic macromolecules based on carbon chains are ubiquitous in the world around us. Organic polymers are used as plastics, elastomers, films, and fibers in areas as diverse as clothing, food utensils, car tires, compact discs, packaging materials, and prostheses. Moreover, with the additional impetus provided by the Nobel prize winning discovery of electrical conductivity in doped polyacetylene in the mid-1970s, exciting new applications in electroluminescent and integrated optical devices and sensors are also now under development. The remarkable growth in the applications of organic polymeric materials in the latter half of the 20th century can mainly be attributed to their ease of preparation, and the useful mechanical properties and unique propensity for fabrication that are characteristic of long-chain macromolecules. Their ease of preparation is a consequence of the highly developed nature of organic synthesis, which, with its logical functional group chemistry and ready arsenal of metal-catalyzed reactions, allows a diverse range of carbon-based polymers to be prepared from what are currently plentifully available and cheap petroleum-derived monomers. In the late 20th century, organic polymer science has been further advanced by the creation of remarkable polymer architectures such as block copolymers, star polymers, and tree-like molecules or dendrimers, which are attracting intense attention.

In contrast to the situation in organic chemistry, the ability to chemically manipulate atoms of inorganic elements is generally at a much more primitive stage of development. Even seemingly simple small inorganic molecules can still be surprisingly elusive, and the formation of bonds between inorganic elements is still often limited to salt metathesis processes. Inorganic analogues of readily available multiply-bonded organic monomers such as olefins and acetylenes, for example, are generally rather difficult to prepare. The development of routes to polymer chains of substantial length constructed mainly or entirely from inorganic elements has therefore been a challenge. Indeed, apart from the cases of polysiloxanes (1.1), polyphosphazenes (1.2), and polysilanes (1.3), this area has only been significantly expanded since the 1980s and 1990s.

In the case of polymers based on non-metallic main group elements, the development of novel thermal, Lewis acid or base promoted, or transition metal-catalyzed polycondensation strategies that proceed with the elimination of small molecules such as [Me.sub.3]SiOC[H.sub.2]C[F.sub.3], [Me.sub.3]SiC1, [H.sub.2], [H.sub.2]O, and C[H.sub.4], as well as the discovery of ring-opening polymerization (ROP) and related processes, has permitted improved approaches to existing polymer systems (e.g. 1.2 and 1.3) and access to new materials. Examples of the latter include polyoxothiazenes (1.4), polythionylphosphazenes (1.5 and 1.6), polyphosphinoboranes (1.7), polyborazylenes (1.8), and other systems that contain boron-nitrogen rings such as polycyclodiborazanes (1.9).

Many similar synthetic challenges exist in the area of polymers based on metallic elements. At the molecular level, metal chemistry is well developed. For example, the preparation of carefully designed, single-site transition metal catalysts has already had a dramatic impact on polymer science, particularly for the polymerization of alkenes. Inorganic solid-state materials chemistry has also now been developed to the extent that scientists are able to exploit the vast range of possibilities arising from the chemical diversity made available throughout the Periodic Table. The creation of high-temperature ceramic superconductors, state-of-the-art magnetic, electrochromic, or electrooptical materials, and unprecedented catalysts with controlled porosity, are all consequences of chemists' now highly impressive ability to organize atoms of inorganic elements in two and three dimensions. In contrast, the elaboration of efficient synthetic routes to metal-containing polymers has been the real roadblock to the development of 1-D analogues of the well-established 2-D layered and 3-D metal-containing solid-state materials. This is particularly the case if the metal atoms are located directly in the main chain, where they are most likely to exhibit the most profound influence on the properties of the macromolecular material. Over the last decade of the 20th century, there have been clear indications that this synthetic problem is being productively tackled and a wide variety of intriguing new polymer systems have emerged. These developments are the subject of this book, which is written both to review the state-of-the-art and also to further help stimulate both fundamental and applied research in this exciting area that is ripe for exploitation and full of future potential.

1.2 Fundamental Characteristics of Polymeric Materials

Polymers exhibit a range of architectures and unique properties, the study of which represents a major core area of polymer science. Although this book assumes that the reader is familiar with some of the basic concepts of polymer science, such as the structures of common macromolecular materials (polystyrene, polyisoprene, etc.), additional knowledge is certainly desirable for an appreciation of much of the research described and the challenges for the future. In this section, we briefly cover some key points for the benefit of readers unfamiliar with the areas that are relevant to the discussions in subsequent chapters. For detailed background material the reader is referred to the many excellent introductory and advanced books on polymer science and the recent literature cited in this section.

1.2.1 Polymer Molecular Weights

Samples of synthetic polymers are generally formed by reactions where both the start and end of the growth of the macromolecular chain are uncontrolled and are relatively random events. Even chain-transfer reactions, where, for example, one polymer chain stops growing and in the process induces another to begin, are prevalent in many systems. Synthetic polymer samples, therefore, contain molecules with a variety of different chain lengths and are termed polydisperse. For this reason, the resulting molecular weight distribution is characterized by an average molecular weight. The two most common are the weight-average molecular weight, [M.sub.w], and the number-average molecular weight, [M.sub.n]. The quantity [M.sub.w]/[M.sub.n] is termed the polydispersity index (PDI), which measures the breadth of the molecular weight distribution and is [greater than or equal to]1. In the case where the polymer chains are of the same length [M.sub.w] = [M.sub.n] (i.e. PDI=1), the sample is termed monodisperse. Such situations are rare, except in the case of biological macromolecules, but essentially monodisperse systems also occur with synthetic polymers where the polymerization by which they are prepared is termed living. In such cases, initiation is rapid and no termination or chain-transfer reactions occur, under such conditions, the polymer chains initiate at the same instant and grow until the monomer is completely consumed, resulting in macromolecular chains of the same length. In practice, living systems are not perfect; for example, very slow termination reactions generally occur. This leads to polymer samples which are of narrow polydispersity (1.0< PDI < 1.2) rather than perfectly monodisperse (PDI = 1.0). Living systems are of particular interest because they allow the formation of controlled polymer architectures. For example, unterminated chains can be subsequently reacted with a different monomer to form block copolymers.

A variety of different experimental techniques exist for the measurement of [M.sub.w] and [M.sub.n]. Some afford absolute values, while others give estimates that are relative to standard polymers, such as polystyrene, which are used as references. One of the simplest techniques for obtaining a measurement of the molecular weight of a polymer is Gel Permeation Chromatography (GPC) (also known as Size Exclusion Chromatography, SEC). This method affords information on the complete molecular weight distribution as well as values of [M.sub.w] and [M.sub.n] (and hence the PDI). Unfortunately, the molecular weights obtained are relative to that of the polymer standard used to calibrate the instrument unless special adaptations of the experiment are made or standard monodisperse samples of the polymer under study are also available as references. Light-scattering measurements are generally time consuming but permit absolute values of [M.sub.w] to be obtained and also yield a wealth of other information concerning the effective radii of polymer coils in the solvent used, polymer-solvent interactions, and polymer diffusion coefficients. The introduction of light-scattering detectors for GPC instruments has now made it possible for both absolute molecular weights and molecular weight distributions to be determined routinely. It should also be noted that mass spectrometry techniques such as Matrix-Assisted Laser Desorption Ionization - Time of Flight (MALDI-TOF) have now been developed to the stage where they are extremely useful for analysis of the molecular weights of polymers and can give molecular ions for macromolecules with molecular weights substantially greater than 100,000.

Although most polymer samples possess a single molecular weight distribution by GPC and are termed monomodal, for some the molecular weight distribution actually consists of several individual, resolvable distributions. In such cases, the molecular weight distribution is referred to as multimodal. For example, if a high and a low molecular weight fraction can be distinguished then the distribution is termed bimodal (Fig. 1.1 a). Examples of broad and narrow monomodal molecular weight distributions are shown in Fig. 1.1 b and 1.1 c, respectively.

1.2.2 Amorphous, Crystalline, and Liquid-Crystalline Polymers: Thermal Transitions

As polymer chains are usually long and flexible, they would be expected to pack randomly in the solid state to give an amorphous material. This is true for many polymers, particularly those with an irregular chemical structure. Examples are the stereoirregular materials atactic polystyrene (1.10) and atactic polypropylene (1.11), in which the Ph and the Me substituents, respectively, are randomly oriented.

However, polymer chains that have regular structures can pack together in an ordered manner to give crystallites. In general, perfect single crystals are not formed by long polymer chains for entropic reasons, and such materials are therefore often more correctly referred to as semicrystalline, as amorphous regions are also present. At the edges of the crystallites, the macromolecular chains fold and re-enter the crystal. The manner in which this occurs has been a subject of much debate in the polymer science community, but a reasonable picture of the amorphous and crystalline regions of a semicrystalline polymer is shown in Fig. 1.2. Information on the morphology of polymers is revealed by techniques such as powder X-ray diffraction (PXRD), which is often called wide-angle X-ray scattering (WAXS) by polymer scientists, and small-angle X-ray scattering (SAXS). The crystallites exist in a polymer sample below the melting temperature ([T.sub.m]), an order-disorder transition, above which a viscous melt is formed.

The presence of crystallites can lead to profound changes in the properties of a polymeric material. For example, crystallites are often of the appropriate size to scatter visible light and thereby cause the material to appear opaque. They often lead to an increase in mechanical strength, but also to brittleness. Gas permeability generally decreases, as does solubility in organic solvents as an additional lattice energy term must be overcome for dissolution to occur. Examples of crystalline polymers are the stereoregular materials syndiotactic polystyrene (1.12), in which the orientation of the Ph groups alternates in a regular manner, and isotactic polypropylene (1.13), in which the Me groups have the same orientation. This structural regularity allows the polymer chains to pack together in a regular manner as crystallites.

In addition to the melting temperature ([T.sub.m]), which arises from the order-disorder transition for crystallites in a polymer sample, amorphous regions of a polymer show a glass transition ([T.sub.g]). This second-order thermodynamic transition is not characterized by an exotherm or endotherm, but rather by a change in heat capacity, and is related to the onset of large-scale conformational motions of the polymer main chain. Generally, stiff polymer chains and large, rigid side groups generate high [T.sub.g] values. Below the [T.sub.g] an amorphous polymer is a glassy material, whereas above the [T.sub.g] it behaves like a viscous gum, because the polymer chains can move past one another. By linking the polymer chains together through cross-linking reactions, rubbery elastomers can be generated from low [T.sub.g] polymers. Purely amorphous polymers such as atactic polystyrene show only a glass transition ([T.sub.g] [approximately equal to] 100C), whereas semicrystalline polymers show both a [T.sub.m], and a [T.sub.g]. Semicrystalline polymeric materials are rigid plastics below the [T.sub.g] and become more flexible above the [T.sub.g]. Above the [T.sub.m], a viscous melt is formed.

It is noteworthy that the rate of polymer crystallization can be extremely slow and polymers that can potentially crystallize are often isolated in a kinetically stable, amorphous state. The polyester poly(ethylene terephthalate) (1.14) provides a good example. This material has a [T.sub.g] of 69C and a [T.sub.m], of 270C, but crystallization only becomes rapid well above the [T.sub.g]. Rapid cooling from the melt yields an amorphous material, whereas slow cooling or annealing above the [T.sub.g] can yield percentage crystallinities up to 55%. A potentially crystallizable polymer that is in an amorphous state can show an exothermic crystallization transition ([T.sub.c]) at elevated temperatures. The thermal transitions of a polymer are commonly investigated by the technique of differential scanning calorimetry (DSC). A typical DSC trace showing a [T.sub.g], a [T.sub.c], and a [T.sub.m], is shown in Fig. 1.3.

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Excerpted from Synthetic Metal-Containing Polymersby Ian Manners Copyright © 2004 by Wiley-VCH Verlag GmbH & Co. KGaA. Excerpted by permission.
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