Produktinformation Seite 1 von 1 Polymer Chemistry Allison Calhoun, Andrew James Peacock Properties and Application ISBN 3-446-22283-9 Leseprobe Weitere Informationen oder Bestellungen unter http://www.hanser.de/3-446-22283-9 sowie im Buchhandel http://www.hanser.de/deckblatt/deckblatt1.asp?isbn=3-446-22283-9&style=Leseprobe 05.05.2006 2 Polymer Chemistry 2.1 Introduction In this chapter, we will see how polymers are manufactured from monomers. We will explore the chemical mechanisms that create polymers as well as how polymerization methods affect the final molecular structure of the polymer. We will look at the effect of the chemical structure of monomers, catalysts, radicals, and solvents on polymeric materials. Finally, we will apply our molecular understanding to the real world problem of producing polymers on a commercial scale. 2.2 Thermoplastics and Thermosets Thermoplastics consist of linear or lightly branched chains that can slide past one another under the influence of temperature and pressure. These polymers flow at high temperatures, allowing us to mold them into useful products. When we heat and/or shear thermoplastic polymers, we can change their shape. For example, polyethylene milk containers can be reprocessed back into the melt stage and then formed into a park bench.We fi nd thermoplastics in a wide variety of commonly used items, such as pantyhose (polyamide), compact disks (polycarbonate), grocery bags (polyethylene), house siding (polyvinyl chloride), gas and water pipes (polyethylene, polyvinyl chloride, and polypropylene), and medical intravenous fluid bags (polyvinyl chloride). Thermosets consist of a network of interconnected chains whose positions are fi xed relative to their neighbors. Such polymers do not flow when heated. Instead, when exposed to high temperatures, thermosets degrade into char. Examples of thermosets include some polyurethanes and epoxy resins. To understand the difference between thermoplastic and thermoset polymers, we must look towards the molecular structure of the polymers for insight. In thermoplastics, the individual polymer chains are chemically separate from one another while being physically entangled. The chains can slide over one another when heated and sheared, allowing the polymer to flow or become rubbery. This, in turn, allows the polymer to take on new shapes. The polymer chains in thermosets differ from thermoplastics because their chains are linked to one another through chemical crosslinks. The crosslinks create an extended network in which every chain is attached to every other chain. Therefore, the molecular weight of a fully crosslinked thermoset article is equal to its weight in grams. Thermoset polymers cannot flow because the crosslinks prevent large scale reorganization of their polymer chains. 22 2 Polymer Chemistry a) b) c) Figure 2.1 Effect of crosslinking on molecular architecture: a) no crosslinking – linear molecules, b) light crosslinking – long chain branching and c) complete crosslinking – interconnected network of chains As with most classification schemes, there exist grey areas between the thermoset and thermoplastic categories. The traditional usage of the term thermoset focused on chemicals that polymerized to create the interconnected network, such as urea-formaldehyde resins (the Formica® used in counter tops). This definition becomes blurred by our understanding that traditional thermoplastic materials are often the starting point for the crosslinking process. For example, gamma irradiation of standard polyethylene creates a crosslinked network which is, in essence, a thermoset. Crosslinked polyethylene is used in bulk liquid storage tanks, high voltage electrical insulation, and kayaks. Since the degree of crosslinking can vary, depending on process parameters, such as temperature and radiation dose, there is a continuum between true thermoplastics and true thermosets ranging from no crosslinks (thermoplastics), lightly crosslinked (long chain branched polymers), to mostly crosslinked (rubbers), to fully crosslinked (true thermosets) as shown in Fig. 2.1. 2.3 Chain Growth Polymerization of Thermoplastics Chain growth polymers, which are often referred to as addition polymers, form via chain addition reactions. Figure 2.2 presents a generic chain addition mechanism. Chain addition occurs when the active site of a monomer or polymer chain reacts with an adjacent monomer molecule, which is added to the end of the chain and generates a new active site. The active site is the reactive end of a monomer or polymer that participates in the polymerization reaction. 2.3 Chain Growth Polymerization of Thermoplastics 23 R* + AB RAB* + AB RABAB* + AB RABABAB* Figure 2.2 Schematic representation of generic chain addition mechanism Chain growth polymers comprise most of the commodity polymers found in consumer products. Common examples include the polyethylene used in trash can liners, the polyvinyl chloride used as wire insulation, and the polypropylene used in food storage containers. Chain growth polymerization begins when a reactive species and a monomer react to form an active site. There are four principal mechanisms of chain growth polymerization: free radical, anionic, cationic, and coordination polymerization. The names of the first three refer to the chemical nature of the active group at the growing end of the monomer. The last type, coordination polymerization, encompasses reactions in which polymers are manufactured in the presence of a catalyst. Coordination polymerization may occur via a free radical, anionic, or cationic reaction. The catalyst acts to increase the speed of the reaction and to provide improved control of the process. Free radical, anionic, and cationic addition polymerization processes occur in four distinct steps: initiation, propagation, chain transfer, and termination. Figure 2.3 illustrates these steps as exemplified by the free radical polymerization of ethylene. In the first step, the initiator, R•, creates an active site on the monomer, as indicated by the unpaired electron. During propagation, the active site reacts with another monomer, thereby adding the monomer residue to the end of the chain and generating a new active site, causing the chain to grow. Chain growth is terminated when the active site becomes deactivated. Chain transfer is an alternate reaction of the active site. In this process, an active site transfers to another molecule creating one terminated species and a new activated species. The choice of one polymerization method over another is defined by the type of monomer and the desired properties of the polymer. Table 2.1 lists advantages and disadvantages of the different chain growth mechanisms. Table 2.2 summarizes some well known addition polymers and the methods by which they can be polymerized. 24 2 Polymer Chemistry Table 2.1 Advantages and Disadvantages of Chain Growth Polymerization Mechanisms Polymerization Advantages Disadvantages mechanism Free radical • Relatively insensitive to trace • Structural irregularities are polymerization impurities introduced during initiation and • Reactions can occur in aqueous termination steps media • Chain transfer reactions lead to • Can use chain transfer to solvent reduced molecular weight and to modify polymerization process branching • Limited control of tacticity • High pressures often required Anionic • Narrow molecular weight • Solvent-sensitive due to the polymerization distribution possibility of chain transfer to the • Limited chain transfer reactions solvent • Predictable molecular weight • Can be slow average • Sensitive to trace impurities • Possibility of forming living • Narrow molecular weight polymers distribution • End groups can be tailored for further reactivity Cationic • Large number of reactive • High reactivity leads to polymerization monomers undesirable side reactions • High reactivity of active site, • Requires very high purity reaction therefore quick reaction times medium • Reactions proceed rapidly even at • Chain transfer leading to low low temperatures molecular weight and high branching • Kinetic mechanisms are poorly understood • Reaction often does not go to completion Coordination • Can engineer polymers with • Mainly applicable to olefi nic polymerization specific tacticities based on the monomers catalyst system • Can limit branching reactions • Polymerization can occur at low pressures and modest temperatures • Otherwise non-polymerizable monomers (e.g., propylene) can be polymerized 2.3 Chain Growth Polymerization of Thermoplastics 25 Table 2.2 Selected Homopolymers and the Potential Methods of Polymerization for Each Polymer Monomer Methods of polymerization available Polyethylene Ethylene Free radical Coordination* Polypropylene Propylene Coordination* Polyvinyl chloride Vinyl chloride Free radical* Polystyrene Styrene Free radical* Cationic Anionic Coordination Polymethyl methacrylate Methyl methacrylate Free radical* Anionic Polyisobutylene Isobutylene Cationic* * Indicates the most frequently used method of polymerization a) CH R CH R + 2 2 H2C C H2 H2 b) CH2 CH C CH + 2 2 C H2C C C H2 H2 H2 c) CH2 CH CH2 CH + 2 + 2 C H2C C H3C H2 H or CH CH 3 + C H2C H2 H2 H2 H2 d) CH2 C C + C C C C H2C H2 H2 H2 or H2 CH2 + C C H H3C Figure 2.3 Chain growth polymerization exemplified by free radical polymerization of polyethylene: a) initiation, b) propagation, c) chain transfer, and d) termination 26 2 Polymer Chemistry 2.3.1 Free Radical Polymerization A monomer that can undergo free radical polymerization must be able to accept