Chapter 28: Polymerisation

Learning Intentions

  • Describe the characteristics of condensation polymerisation in polyesters and polyamides.

  • Understand how polyesters are formed from:

    • A diol and a dicarboxylic acid or a dioyl chloride.

    • A hydroxycarboxylic acid.

  • Explain how polyamides are formed from:

    • A diamine and a dicarboxylic acid or a dioyl chloride.

    • Amino acids.

  • Deduce repeat units, identify monomer(s), and predict the type of polymerisation reaction that produces a given section of a polymer molecule.

  • Recognise that:

    • Polyalkenes are chemically inert and nonbiodegradable.

    • Polyesters and polyamides are biodegradable through:

    • Acidic hydrolysis.

    • Alkaline hydrolysis.

    • Action of light.

28.1 Condensation Polymerisation

  • Definition of Polymerisation: The process of joining many small molecules (monomers) to form long-chain molecules (polymers).

    • Types:

    • Addition Polymerisation

    • Condensation Polymerisation

Condensation Reactions

  • What is a condensation reaction?

    • A reaction where:

    • Two molecules join together.

    • A small molecule, usually H₂O or HCl, is released.

    • It can be conceptualised as:

      • An addition step (molecules bond together).

      • An elimination step (small molecule removed).

Biological Examples of Condensation Reactions

  • Amino acids undergo condensation reactions which form:

    • Peptides (short chains).

    • Proteins (long chains).

Example: Formation of a Tripeptide

  • Three amino acid molecules react to form a tripeptide:

    • Each time two amino acids join, an amide bond (peptide bond) forms and one molecule of water (H₂O) is released.

    • When three amino acids join, two peptide bonds and two molecules of H₂O are released.

Proteins as Condensation Polymers

  • Protein Structure:

    • Proteins can consist of hundreds or thousands of amino acid monomers linked by peptide bonds, releasing H₂O as each joins the chain.

  • The key bond linking amino acids is:

    • Amide bond (peptide bond) formed between the –NH₂ group of one amino acid and the –COOH group of another amino acid.

Recognising Condensation Polymerisation

  • Identified by its monomers:

    • Must have two different functional groups capable of reacting with each other.

  • Mechanisms of Condensation Polymerisation:

    1. Both functional groups in the same molecule (e.g., amino acids).

    2. Functional groups in two different molecules (e.g., nylon 6,6).

Condensation Polymerisation Mechanism

  • During condensation polymerisation, small molecules like H₂O or HCl are produced, indicating elongation of the polymer chain.

  • Exam Tip: If a polymer forms and a small molecule is released → condensation polymerisation. If no small molecule is released → addition polymerisation.

28.2 Synthetic Polyamides

Overview of Polyamides

  • Definition: Polyamides are polymers containing amide links (–CONH–) in their backbone.

  • Have been synthetised using condensation polymerisation since the 1930s.

Formation and Reactants of Polyamides

  • Polyamides form through the reaction of:

    • An amine group (–NH₂)

    • And either:

    • A carboxylic acid group (–COOH)

    • Or an acyl chloride group (–COCl).

Nylons

  • Nylon: A type of synthetic polyamide formed from:

    • –NH₂ groups.

    • –COOH or –COCl groups.

  • Example: Formation of Nylon 6,6

    • Made from 1,6-diaminohexane and hexanedioic acid, both containing 6 carbon atoms.

    • Reaction releases H₂O as amide links form.

Using Acyl Chlorides in Polyamide Synthesis

  • Hexanedioyl dichloride can be used instead of hexanedioic acid, resulting in HCl as a byproduct.

Properties and Uses of Nylon

  • Characteristics:

    • Low density

    • High tensile strength

    • Elasticity

  • Applications include:

    • Clothing and textiles

    • Climbing ropes

    • Reduces fall force.

Cold Drawing

  • Process where molten nylon is extruded to form fibres, enhancing tensile strength through aligned polymer chains and hydrogen bonding.

Kevlar®

  • Definition: A synthetic polyamide featuring benzene rings, highly strong, flexible, and resistant to fire/abrasion.

  • Made from:

    • An aryl diamine (e.g., 1,4-diaminobenzene)

    • An aryl dicarboxylic acid or aryl diacyl chloride.

  • Structure: Long, straight, linear polymer chains allow extensive hydrogen bonding, resulting in exceptional strength.

Properties and Applications of Kevlar®

  • Key Properties:

    • High strength-to-weight ratio

    • Low density

  • Applications include:

    • Bullet-proof vests

    • Fire-proof clothing

  • Uses: Ropes, protective clothing, reinforcement for materials, components in various vehicles/props including racecars and aircraft.

28.3 Biochemical Polyamides

  • Proteins are condensation polymers with amino acids as monomers, joined by amide (peptide) bonds.

  • α-Amino Acids:

    • Contain –NH₂ and –COOH groups attached to the same carbon (α-carbon).

  • Characteristics and classifications of amino acids include:

    • Non-polar, polar, electrically charged.

Protein Structure and Bonding

  • Formed primarily through peptide bonds with added hydrogen bonds, ionic interactions, and disulfide bridges.

  • Example: Insulin, showcasing defined polypeptide chains.

  • Polypeptide Chains: Can have 50 to 2000 amino acids, where each unit is an amino acid residue with terminus designations N-terminal and C-terminal.

Primary Structure of Proteins

  • Defined by the order of amino acids, determining folding and ultimately function.

28.4 Degradable Polymers

Issues with Conventional Polymers

  • Non-biodegradable plastics (e.g., poly(alkenes)) persist in the environment due to chemical inertness.

Biodegradable and Photodegradable Polymers

  • Biodegradable Polymers: Break down over time from microorganisms and hydrolysis, e.g., starch-containing polymers enhance degradation rate.

Hydrolysis Mechanisms

  • Polyamides and polyesters degrade through hydrolysis yielding the respective monomers under acidic or alkaline conditions.

  • Key Degradation Products:

    • Polyamides hydrolyse to amines and carboxylic acids.

    • Polyesters break down to alcohols and carboxylic acids.

Summary of Biodegradability

  • Polyesters and polyamides have the potential for biodegradation, contrasting with the stability of poly(alkenes) due to inertness.

28.5 Polymer Deductions

Expected Tasks in Polymer Questions

  • Predict polymerisation type, deduce repeat units, identify monomers, and assess the structure of given polymer chains.

Predicting Polymerisation Types

  • Addition Polymerisation: Involves C=C double bonds where the bond opens for monomer linkage.

  • Condensation Polymerisation: Requires two functional groups, producing H₂O or HCl.

  • Common Functional Group Pairs:

    • Amine + Carboxylic Acid → polyamide + H₂O

    • Alcohol + Carboxylic Acid → polyester + H₂O.

Deducing Repeat Units

  • For addition polymers, restore C=C to form a repeat unit; for condensation, consider molecule loss during formation to reverse-engineer the structure.

Identifying Monomers from a Polymer Chain

  • For condensation polymers, focus on functional links and reintroduce lost atoms to identify original components.

Example Problem Statement

  • Illustrate the steps in identifying the polymer type and its monomer components through chemical equations and structural diagrams.