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MARTIN E. ROGERS, PhD, is Senior Research Scientist at Luna Innovations in Blacksburg, Virginia.

TIMOTHY E. LONG, PhD, is Professor of Chemistry at Virginia Polytechnic Institute and State University in Blacksburg, Virginia.

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An applications-oriented resource on step-growth polymerization

Step-growth polymers-polymer chains of any length that combine to form a longer polymer chain-comprise a large portion of the commodity plastics industry today, including polyesters, polyamides, and polyurethanes. Synthetic Methods in Step-Growth Polymers provides a concise source of information on synthetic techniques, purification, and characterization methods for step-growth polymers and also addresses future synthetic trends.

This applications-oriented handbook is a one-stop, at-your-fingertips source of information for researchers, technologists, and industrial managers. Encompassing a single reference of the classical and state-of-the-art synthetic techniques for preparing polymers via step-growth polymerization, Martin Rogers and Timothy Long's text provides a historical background of step-growth polymerization, basic information regarding major classes of step-growth polymers, and experimental techniques such as purification, synthesis, and characterization. Chapters include:
* Polyurethanes and Polyureas
* Polyimides and Other High-Temperature Polymers
* Non-Traditional Step-Growth Polymerization-ADMET
* Non-Traditional Step-Growth Polymerization-Transition Metal Coupling
* Depolymerization and Recycling

All chapters are contributed by leading experts in their respective fields. Chemists, chemical engineers, and materials scientists, as well as industrial, academic, and government libraries, will find Synthetic Methods in Step-Growth Polymers to be an unparalleled resource for this category of polymerization.

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Synthetic Methods in Step-Growth Polymers

John Wiley & Sons

Chapter One

Martin E. Rogers Luna Innovations, Blacksburg, Virginia 24060

Timothy E. Long Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061

S. Richard Turner Eastman Chemical Company, Kingsport, Tennessee 37662

1.1 INTRODUCTION

1.1.1 Historical Perspective

Some of the earliest useful polymeric materials, the Bakelite resins formed from the condensation of phenol and formaldehyde, are examples of step-growth processes. However, it was not until the pioneering work of Carothers and his group at DuPont that the fundamental principles of condensation (step-growth) processes were elucidated and specific step-growth structures were intentionally synthesized. Although it is generally thought that Carothers' work was limited to aliphatic polyesters, which did not possess high melting points and other properties for commercial application, this original paper does describe amorphous polyesters using the aromatic diacid, phthalic acid, and ethylene glycol as the diol. As fundamental as this pioneering research by Carothers was, the major thrust of the work was to obtain practical commercial materials for DuPont. Thus, Carothers and DuPont turned to polyamides with high melting points and robust mechanical properties. The first polymer commercialized by DuPont, initiating the "polymer age," was based on the step-growth polymer of adipic acid and hexamethylene diamine-nylon 6,6. It was not until the later work of Whinfield and Dickson in which terephthalic acid was used as the diacid moiety and the benefits of using a para-substituted aromatic diacid were discovered that polyesters became commercially viable.

In these early days of polymer science, the correlation of structure and property in the newly synthesized structures was a daunting challenge. As Carothers said, "problem of the more precise expression of the relationships between the structures and properties of high polymers is complicated by the fact that some of the properties of this class of substances which are of the greatest practical importance and which distinguish them most sharply from simple compounds can not be accurately measured and indeed are not precisely defined. Examples of such properties are toughness and elasticity" (ref. 6, p. 317).

Today, step-growth polymers are a multi-billion-dollar industry. The basic fundamentals of our current understanding of step-growth polymers from monomer functionality to molecular weight distribution to the origins of structure-property relationship all had their beginnings in the pioneering work of Carothers and others at DuPont. A collection of these original papers offers an interesting and informative insight into the development of polymer science and the industry that it spawned.

1.1.2 Applications

In general, step-growth polymers such as polyesters and polyamides possess more robust mechanical properties, including toughness, stiffness, and higher temperature resistance, than polymers from addition polymerization processes such as polyolefins and other vinyl-derived polymers. Even though many commercial step-growth polymerization processes are done on enormous scale using melt-phase processes, most step-growth-based polymers are more expensive than various vinyl-based structures. This is, at least in part, due to the cost of the monomers used in step-growth polymerizations, which require several steps from the bulk commodity petrochemical intermediates to the polymerizable monomer, for example, terephthalic acid from the xylene stream, which requires oxidation and difficult purification technology. These cost and performance factors are key to the commercial applications of the polymers.

Most of the original application successes for step-growth polymers were as substitutes for natural fibers. Nylon-6,6 became an initial enormous success for DuPont as a new fiber. Poly(ethylene terephthalate) (PET) also found its initial success as a textile fiber. An examination of the polymer literature in the 1950s and 1960s shows a tremendous amount of work done on the properties and structures for new fibers. Eventually, as this market began to mature, the research and development community recognized other commercially important properties for step-growth polymers. For example, new life for PET resulted from the recognition of the stretch-blow molding and barrier properties of this resin. This led to the huge container plastics business for PET, which, although maturing, is still fast growing today.

The remainder of this introductory chapter covers a few general but important parameters of step-growth polymerization. References are provided throughout the chapter if further information is desired. Further details of specific polymers made by step-growth polymerization are provided in subsequent chapters within this book.

1.2 STRUCTURE-PROPERTY RELATIONSHIPS IN STEP-GROWTH POLYMERS

1.2.1 Molecular Weight

Polymers produced by step-growth polymerization are composed of macromolecules with varying molecular weights. Molecular weights are most often reported as number averages, [bar.Mn], and weight averages, [bar.Mw]. Rudin, in The Elements of Polymer Science and Engineering, provides numerical descriptions of molecular weight averages and the derivation of the molecular weight averages. Other references also define molecular weight in polymers as well as methods for measuring molecular weights. Measurement techniques important to step-growth polymers include endgroup analysis, size exclusion chromatography, light scattering, and solution viscometry.

The physical properties of polymers are primarily determined by the molecular weight and chemical composition. Achieving high molecular weight during polymerization is critical if the polymer is to have sufficient thermal and mechanical properties to be useful. However, molecular weight also influences the polymer melt viscosity and solubility. Ease of polymer processing is dependent on the viscosity of the polymer and polymer solubility. High polymer melt viscosity and poor solubility tend to increase the difficulty and expense of polymer processing.

The relationship between viscosity and molecular weight is well documented. Below a critical molecular weight, the melt viscosity increases in proportion to an increase in molecular weight. At this point, the viscosity is relatively low allowing the material to be easily processed. When the molecular weight goes above a critical value, the melt viscosity increases exponentially with increasing molecular weight. At higher molecular weights, the material becomes so viscous that melt processing becomes more difficult and expensive.

Several references discuss the relation between molecular weight and physical properties such as the glass transition temperature and tensile strength. The nature of thermal transitions, such as the glass transition temperature and crystallization temperature, and mechanical properties are discussed in many polymer texts. Below a critical molecular weight, properties such as tensile strength and the glass transition temperature are low but increase rapidly with increasing molecular weight. As the molecular weight rises beyond the critical molecular weight, changes in mechanical properties are not as significant. When developing polymerization methods, knowledge of the application is necessary to determine the target molecular weight. For example, polymers used as rigid packaging or fibers require high strength and, consequently, high molecular weights.

Thermoplastic commercial step-growth polymers such as polyesters, polycarbonates, and polyamides are generally made with number-average molecular weights in the range of 10,000-50,000 g/mol. Polymers within this molecular weight range are generally strong enough for use as structural materials yet low enough in melt viscosity to be processable at a reasonable cost.

Thermosetting resins are combined with fibers and other fillers to form composites. Thermosetting resins with low viscosities are necessary to wet fibers or other fillers and to allow efficient processing and application prior to curing. When preparing thermosetting resins, such as unsaturated polyesters, phenolics, and epoxides, it is necessary to minimize viscosity by severely limiting molecular weight.

For example, the molecular weight of unsaturated polyesters is controlled to less than 5000 g/mol. The low molecular weight of the unsaturated polyester allows solvation in vinyl monomers such as styrene to produce a low-viscosity resin. Unsaturated polyesters are made with monomers containing carbon-carbon double bonds able to undergo free-radical crosslinking reactions with styrene and other vinyl monomers. Crosslinking the resin by free-radical polymerization produces the mechanical properties needed in various applications.

Step-growth polymerizations can produce polymers with a wide range of physical properties. Polysiloxanes made from the step-growth polymerization of silanols have among the lowest glass transition temperatures. Polydimethyl siloxanes have a glass transition temperature near -125C. On the other hand, step-growth polymerization produces polyimides and polybenzoxazoles with glass transition temperatures of 300C to over 400C.

Even within a particular class of polymers made by step-growth polymerization, monomer composition can be varied to produce a wide range of polymer properties. For example, polyesters and polyamides can be low-[T.sub.g], amorphous materials or high-[T.sub.g], liquid crystalline materials depending on the monomer composition.

The dependence of polymer properties on chemical compositions is reviewed in basic polymer texts. The backbone structure of a polymer defines to a large extent the flexibility and stability of a polymer molecule. Consequently, a great range of polymer properties can be achieved within each class of step-growth polymers by varying the backbone structure using different monomers.

The most common backbone structure found in commercial polymers is the saturated carbon-carbon structure. Polymers with saturated carbon-carbon backbones, such as polyethylene, polypropylene, polystyrene, polyvinyl chloride, and polyacrylates, are produced using chain-growth polymerizations. The saturated carbon-carbon backbone of polyethylene with no side groups is a relatively flexible polymer chain. The glass transition temperature is low at -20C for high-density polyethylene. Side groups on the carbon-carbon backbone influence thermal transitions, solubility, and other polymer properties.

Nearly all of the polymers produced by step-growth polymerization contain heteroatoms and/or aromatic rings in the backbone. One exception is polymers produced from acyclic diene metathesis (ADMET) polymerization. Hydrocarbon polymers with carbon-carbon double bonds are readily produced using ADMET polymerization techniques. Polyesters, polycarbonates, polyamides, and polyurethanes can be produced from aliphatic monomers with appropriate functional groups (Fig. 1.1). In these aliphatic polymers, the concentration of the linking groups (ester, carbonate, amide, or urethane) in the backbone greatly influences the physical properties.

Increasing the methylene content increases the melting point, eventually tending toward the [T.sub.m] of polyethylene at low linking group concentrations. The linear aliphatic polyesters and polycarbonates have relatively low [T.sub.g]'s (-70 to -30C) and melting points below 100C. The linear aliphatic polyesters and polycarbonates are not used as structural materials due to the low melting temperatures and limiting hydrolytic stability. Aliphatic polyesters are used as soft-segment polyols in polyurethane production.

In contrast to the polyesters and polycarbonates, the linear aliphatic polyamides and polyurethanes have high melting points and higher glass transition temperatures as the amide and urethane linking groups participate in intermolecular hydrogen bonding. In Chapter 3 of Polymer Chemistry, Stevens discusses the influence of hydrogen bonding in polyamides compared with polyesters. Stevens notes that poly(hexamethylene adipamide) melts at 265C compared to 60C for poly(hexamethylene adipate).

Aromatic groups in the polymer backbone bring rigidity and thermal stability to the polymer molecule (Fig. 1.2). Consequently, the demands of high-strength and high-temperature applications are met by polymers with a high aromatic content in the backbone. Polymers with a particularly high aromatic content can show main-chain liquid crystallinity.

Aromatic polymers are often more difficult to process than aliphatic polymers. Aromatic polyamides have to be processed from very aggressive solvents such as sulfuric acid. The higher melting temperatures and viscosity also make melt processing more difficult. Thermal stability and processing of aromatic polymers can be balanced by the use of flexible spacing groups in between aromatic rings on a polymer backbone. Hexafluoroisopropylidene, isopropylidene, oxygen, carbonyl, and sulfonyl bridging groups between rings increase opportunities for bond rotation, which decreases [T.sub.g]'s and increases solubility. Also, incorporating nonsymmetrical monomers with meta and ortho linkages causes structural disorder in the polymer chain, improving processability. Flexible groups pendant to an aromatic backbone will also increase solubility and processability.

The following chapters will provide detailed discussions of the structure-property relations with various classes of step-growth polymers.

1.2.2 Polymer Architecture

Block copolymers are composed of two different polymer segments that are chemically bonded. The sequential arrangement of block copolymers can vary from diblock or triblock copolymers, with two or three segments respectively, to multiblock copolymers containing many segments. Figure 1.3 is a schematic representation of various block copolymer architectures. The figure also includes graft and radial block copolymers. Step-growth polymerization can be used effectively to produce segmented or multiblock copolymers and graft copolymers. Well-defined diblock and triblock copolymers are generally only accessible by chain-growth polymerization routes.

A variety of morphologies and properties can be achieved with microphase-separated block copolymers. Copolymers of hard and soft polymer segments have a variety of properties depending on their composition. Copolymers with small amounts of a soft segment will behave as a toughened glassy polymer while copolymers made predominately of the soft segment will act as a thermoplastic elastomer.

The thermal properties of block copolymers are similar to physical blends of the same polymer segments. Each distinct phase of the copolymer displays unique thermal transitions, such as a glass transition and/or a crystalline melting point. The thermal transitions of the different phases are affected by the degree of intermixing between the phases.

Segmented or multiblock copolymers can be made by combining a functionally terminated oligomer or prepolymer with at least two monomers. To form a segmented copolymer, the backbone oligomer must not be able to participate in interchange reactions with the monomers. For example, combining a polyester oligomer with a diacid and diamine in a melt polymerization might result in interchange reactions between the monomers and the ester linking groups in the oligomer backbone. In this case a random polyesteramide copolymer would be produced instead of a segmented copolymer. Commercial examples of segmented copolymers produced by step-growth polymerization include polyester-polyether, polyurethane-polyether, and polyurethane-polyester copolymers.

Multifunctional monomers with functionality greater than 2 can be used to form three-dimensional polymer structures during step-growth polymerization. Incorporating multifunctional monomers, [A.sub.x], with AA and BB monomers results in crosslinking between polymer chains and eventual gelation. The point at which gelation occurs depends on the average functionality of the monomer mixture and the conversion of functional groups.

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