Polymers PDF

POLYMERS Outcome At the end of the topic, the students should be able to: understand polymerization identify types of polymerization classify plastics understand rubber as polymer identify miscellaneous polymers Quizlet What does poly mean? Quizlet poly means...

POLYMERS Outcome At the end of the topic, the students should be able to: understand polymerization identify types of polymerization classify plastics understand rubber as polymer identify miscellaneous polymers Quizlet What does poly mean? Quizlet poly means many Quizlet What does mer mean? Quizlet mer means units Polymerization Polymers are giants among molecules, constructed by the sequential stringing together of smaller molecules called monomers. Polymerization Monomer, a molecule that can react with other molecules to form polymers. Polymerization For example, polyethylene is a polymer made from the monomers ethylene. Polymerization For example, polyethylene is a polymer made from the monomers ethylene. Polymerization Hence, polymerization is any process in which relatively small molecules, called monomers, combine chemically to produce a very large chainlike or network molecule, called a polymer.  THE UTILITY OF POLYMERS AND PLASTICS IN ENGINEERING APPLICATIONS ARISES FROM THE FACT THAT MATERIALS CHEMISTS AND ENGINEERS CAN CONTROL THEIR PHYSICAL PROPERTIES The main factors that can be empirically adjusted to modify polymers are the: monomers used, the type of reactions needed to generate the polymers, and the catalysts that are employed to speed the reactions. A careful choice of these factors can ultimately control the physical properties of the resulting polymer. Types of Polymerization  Three Types of Polymerization Addition Condensation Co- polymerization polymerization polymerization Addition polymerization Monomers having double or triple bonds undergo addition polymerization without the liberation of small molecules. The product polymer is exact multiple of the original monomeric molecule e.g., polyethylene from ethylene.  Three Mechanism of Addition Polymerization Free radical Ionic Ziegler-Natta mechanism mechanism polymerization Free radical polymerization INITIATION The spontaneous decomposition of an initiator into free radicals. The next part of initiation involves the addition of this radical to the monomer molecule (M) to initiate the chain. Free radical polymerization PROPAGATION The mechanism of propagation is the reaction of the radical M* with its own monomer M. Continuous addition of new monomer in this manner will finally produce a polymer chain in which the substituents are located on alternate atoms. Free radical polymerization TERMINATIONS The most common terminations are the effect of: i) Collision between two growing chains (ii) Collision of a growing chain with an initiator radical when the latter is proportionately in excess. (iii) Collision between a growing chain with impurities. Ionic polymerization It is an important class of addition polymerization but here, instead of free radicals, the unstable intermediates are either cations or anions. Ionic polymerization Ionic polymerization Coordination or Ziegler-Natta polymerization The ability to control the specific configuration of a polymer was first achieved by Karl Ziegler and Giulio Natta in the 1950s. These scientists discovered new catalysts for the addition polymerization reaction that increased the rate of reaction and also controlled the structure. Coordination or Ziegler-Natta polymerization The catalysts came to be known as Ziegler-Natta catalysts. Their discovery invigorated the study of polymers, and they were awarded the Nobel prize in chemistry in 1963 for their efforts. Coordination or Ziegler-Natta polymerization It was observed by Ziegler and Natta that in presence of a combination of transition metal halides (TiCl4, ZrBr3, and halides of V, Zr, Cr, Mo etc.) along with organometallic compounds (triethyl/trimethyl aluminum) polymerization of olefins leads to stereospecific polymerization. Stereospecific polymerization An organic polymerization in which the spacial arrangements of groups on asymmetric carbon atoms occur in a regular sequence. Stereochemistry of Polymers (i) Isotactic polymers have all the groups in one side of the polymeric backbone and the monomers are joined in a regular head to tail arrangement. Stereochemistry of Polymers (i) Isotactic polymers have all the groups in one side of the polymeric backbone and the monomers are joined in a regular head to tail arrangement. Stereochemistry of Polymers (ii) Syndiotactic polymers have similar head to tail arrangements but here Y groups appear on opposite sides of polymer backbone alternately. Stereochemistry of Polymers (iii) Atactic polymers have Y groups arranged randomly along the polymeric backbone and the material is soft, elastic, rubbery. Stereochemistry of Polymers Here we see polypropylene, The large purple balls here represent those methyl groups. In (a), all of the methyl groups are arranged on the same side of the polymer chain, giving isotactic polypropylene. In (b), the methyl groups are on alternate sides of the chain, making syndiotactic polypropylene. Finally, the random arrangement of the methyl groups in (c) is atactic polypropylene. Condensation Polymerization Combination through different functional groups of monomers with elimination of small molecules like H2O. Condensation Polymerization a.k.a. Nylon or polyamide Condensation Polymerization a.k.a. polyester Nylon or polyamide Products Dacron or polyester Products  Copolymerization Two or more monomers undergoing joint polymerization is called copolymerization reaction such as the production of SBR (Styrene butadiene rubber). Copolymerization Degree of Polymerization The number of repeating units in a polymer. PLASTICS (RESINS) PLASTICS (RESINS) Plastics are a class of high polymers which can be molded into any desired form by heat and pressure. Resins are actually the binders used for plastics and these two terms are used synonymously.  PLASTICS (RESINS) There are two classes of plastics or resins: 1.Thermoplastic resins These plastics soften on heating and harden on cooling and this change is not chemical but physical in nature, hence repeated heating and cooling also does not alter its nature.  PLASTICS (RESINS) 1.Thermoplastic resins The deformation upon heating may seem like a weakness because it means that they are not suitable for high temperature applications.  PLASTICS (RESINS) 1.Thermoplastic resins But most plastic objects, including children’s toys and plastic bottles, are generally used at ambient temperatures. Hence, the melting when heated appreciably is not a major drawback.  PLASTICS (RESINS) 2. Thermosetting resins These are those which are heated during molding and heating is continued until set and hardened. This hardened material cannot be softened again, hence the setting is permanent and irreversible.  PLASTICS (RESINS) 2. Thermosetting resins These polymers can maintain their shape and strength when heated. The name “thermosetting” comes from the fact that these polymers must be heated to set or “lock in” their structures. But once this has been done, the materials offer increased strength and do not lose their shape upon further heating. The different properties of thermoplastic and thermosetting polymers result from the ways in which the polymer chains interact with one another.  Crosslinks Chemically, these cross-links are additional covalent bonds that join the polymer chains to one another. Like most covalent bonds, they are strong enough that they do not readily fail upon heating. So the cross-linked polymer keeps its shape.  Crosslinks An important example of the engineering importance of cross- linking in American industrial history is the discovery of vulcanization. In vulcanization, natural rubber is heated in the presence of sulfur. This produces cross-linking and leads to a harder material that is markedly more resistant to heat.  Natural Rubber Until vulcanization was discovered, natural rubber was difficult to use in applications such as automobile tires because it would become sticky when heated. Natural rubber is no longer widely used, having been replaced by synthetic forms.