Chem 6 Unit 6

UNIT 6: POLYMERS AND POLYMERIZATION

Key Unit Competence

  • To relate the types of polymers to their structural properties and uses.

Learning Objectives

  • At the end of this unit, students will be able to:

    • Define the terms monomer, polymer, and polymerization.

    • Describe the formation of polymers.

    • Describe addition and condensation polymerization.

    • Explain the terms thermosetting and thermosoftening of plastics.

    • Discuss the advantages and disadvantages of both natural and synthetic polymers.

    • Explain the biodegradability property of polymers based on their chemical structure.

    • Use equations to distinguish between condensation and addition polymerization.

    • Prepare phenol-methanol polymer (Bakelite).

    • Relate the structure and properties of polymers to their uses in the plastic and textile industries.

    • Reduce polymer wastes by reusing, recycling, and appropriate disposal.

Introductory Activity

  • Materials commonly used in daily life, such as fibers, plastics, and rubber, are collectively called polymers.

  • Polymers are widely used in various products, including household utensils, automobiles, clothing, furniture, aerospace, biomedical and surgical components.

  • Notable properties of polymeric materials include:

    • Light weight

    • Excellent mechanical properties

    • Easier processing through various methods.

6.1. Definitions of Monomer, Polymer, and Polymerization

6.1.1. Monomer

  • Comes from Greek words: mono (one) and meros (part).

  • Defined as a single unit or small molecular subunit that can chemically bind to other identical or different molecules to form a larger molecule (polymer).

  • Monomers repeat in the polymer chain and serve as the basic structural unit.

  • Example of a monomer: In the reaction of ethylene (nCH2 = CH2), where A is a monomer, the polymer can be represented as (-A-A-A-A….).

  • Other examples of biological macromolecules include carbohydrates, lipids, nucleic acids, and proteins.

6.1.2. Polymer

  • A polymer is a large molecule (macromolecule or giant molecule) made up of smaller repeat units (monomers) connected by intermolecular covalent bonds.

  • Characteristics:

    • High molecular weight range: from 10^3 to 10^7 g/mol.

    • Example: Polyethylene ([ - CH2 — CH2 — ]n) where n indicates the number of repetitions of the monomer ethylene (CH2=CH_2).

6.1.3. Polymerization

  • Defined as the process of linking monomer units through a chemical reaction to form polymer chains.

  • Example: Butadiene with a molecular weight of 54 g/mol polymerizes to form polybutadiene:

    • ext{Butadiene} + ext{Butadiene} + ext{Butadiene} + … → ext{Polybutadiene}.

  • The degree of polymerization (n) represents the number of monomer units in a macromolecule.

  • Types of polymers:

    • Homopolymer: Formed from identical monomers.

    • Copolymer: Formed by two or more different types of monomers.

6.2. Types of Polymerization

6.2.1. Addition Polymerization

  • Addition polymerization connects monomers without losing any atoms from the molecules.

  • The resulting polymer has the same empirical formula as the monomer but with a higher molecular weight.

  • Typically involves unsaturated compounds.

  • Examples: Polyethylene, polypropylene, polystyrene, PVC, rubber, etc.

  • Example of formation:

    • Polyethylene:

    • nCH2 = CH2 o [-CH2 - CH2 -]_n.

    • PVC is formed from vinyl chloride:

    • nCH2=CHCl o [-CH2 - CHCl -]_n (n can vary between 700-1500).

    • Rubber, natural or synthetic, derived from isoprene through 1,4-addition polymer.

Steps in Addition Polymerization:

  1. Chain Initiation: Activation of monomers via free radicals created from peroxides, heat, or light.

  2. Chain Propagation: Successive addition of monomers to form a polymer free radical chain.

  3. Chain Termination: Coupling of two free radicals to produce the final polymer.

6.2.2. Condensation Polymerization

  • Occurs when two or more monomers chemically combine with the elimination of small molecules like water, ammonia, etc.

  • Each monomer contains two functional groups.

  • Types of condensation polymers include:

    • Polyamides: Formed using diols and dicarboxylic acids.

    • Polyesters: Formed using dicarboxylic acids and diamines.

Examples of Condensation Polymer formation:

  • Nylon (e.g., Nylon-6 and Nylon-6,6)

    • Nylon-6: Synthesized from ext{ε-caprolactam}, polymerizing to form Nylon-6.

    • Nylon-6,6: Formed from adipic acid and 1,6-hexanediamine.

  • Kevlar is formed from benzene-1,4-diamine and benzene-1,4-dioic acid, summarized as:

    • Benzene-1,4-dioic acid + Benzene-1,4-diamine → Kevlar + water.

6.3. Classes and Types of Polymers

6.3.1. Classes of Polymers

6.3.1.1. Natural Polymers

  • Obtained from natural sources, commonly found in plants and animals.

  • Examples include:

    • Silk: Protein fiber produced by silkworms; used in textiles.

    • Cotton: Soft fiber growing in a boll around cotton plant seeds.

    • Natural Rubber: Elastic material obtained from latex of trees, mainly as polyisoprene.

    • Cellulose: Insoluble polysaccharide that forms plant cell walls and fibers.

    • Proteins: Complex molecules made from amino acids that perform various biological functions.

6.3.1.2. Synthetic Polymers

  • Man-made in laboratories, typically from lower mass molecular compounds.

  • Examples: Polyethylene, PVC, nylon, polystyrene, etc.

6.3.2. Types of Polymers

a. Plastics

  • Generally lightweight, strong, and corrosion-resistant materials.

  • Commonly used in a variety of products; they are thermoplastics, meaning they can be melted and reshaped.

b. Rubbers

  • Categories include natural and synthetic; both exhibit elastic properties and regenerate shape after deformation. Examples include isoprene rubber and styrene-butadiene rubber (SBR).

c. Fibers

  • Long materials used in the manufacture of textiles and other materials. Two types exist:

    • Natural Fibers such as cotton, wool; produced by living organisms.

    • Synthetic Fibers made from chemical processes, e.g., nylon, polyester.

6.4. Properties of Polymers

Differences in Degradation

  • Biodegradable polymers: Decompose into natural substances through microbial action under aerobic or anaerobic conditions.

  • Non-biodegradable polymers: Resist degradation. Accumulate in landfills causing environmental issues.

6.4.1. Thermosoftening and Thermosetting Properties

  • Thermosoftening: Plastics that soften upon heating and can be reshaped multiple times.

    • Examples: PVC, polyethylene.

  • Thermosetting: Polymers that harden upon heating and cannot be reshaped; they form a strong covalent bond structure.

    • Examples: Bakelite, epoxy resins.

6.4.2. Biodegradable vs Non-biodegradable Properties

  • Biodegradable: Composed of materials that can decompose naturally (e.g., polylactic acid). They break down into non-toxic components.

  • Non-biodegradable: Usually synthetic, resistant to environmental breakdown. Accumulates as waste, causing environmental hazards.

6.5. Importance of Vulcanization in Rubber Processing

  • Vulcanization: A chemical process that improves rubber's resilience, elasticity, and durability by introducing sulfur cross-links.

  • Discovered by Charles Goodyear in 1839; enhances the properties of natural rubber to improve tensile strength and thermal resistance.

6.6. Uses of Polymers and Their Effects on the Environment

6.6.1. Uses of Polymers

  • Varied Applications: Used in everyday products due to qualities such as strength, durability, and resistance.

  • Commonly used polymers:

    • Polyethylene (PE): Used in film, bags, and containers.

    • Polystyrene (PS): Used in insulations and disposable products.

    • Polyvinyl Chloride (PVC): Used in piping and electrical insulation.

    • Polymethyl Methacrylate (PMMA): Used as an alternative to glass and in optical applications.

6.6.2. Effects of Polymers on the Environment

  • Environmental challenges include:

    • Accumulation in landfills and natural habitats;

    • Biodegradation issues causing pollution;

    • Toxic chemical leaching into wildlife and humans; and,

    • Health risks associated with burning plastic leading to emissions of harmful substances.