Polymer Chemistry

The study of macromolecules composed of repeating structural units (monomers) connected by covalent chemical bonds.

Definitions

  • Monomer: The large molecule from which a polymer is synthesized
  • Polymer: The large molecule which is made up of many repeating units of monomers
  • Repeating Unit: The basic structure of a polymer; by repetition it produces a long polymer chain
  • Degree of Polymerization (DP): Number of monomer units in polymer chain
  • Molecular Weight: DP × Monomer molecular weight
  • Amino Acid Residue: An amino acid unit within a peptide or protein chain
  • Peptide Bond: An amide linkage (–CO–NH–) between the carboxyl group of one amino acid and the amino group of another

Classification

By Origin

Type Examples
Natural Proteins, cellulose, rubber, starch, DNA
Synthetic Nylon, polyethylene, PVC, polystyrene
Semi-synthetic Rayon, cellophone

By Monomer Composition

Homopolymer

Polymers made up from only one type of monomer.

  • General structure: [-A-A-A-A-A-A-]n
  • Examples: Polyethylene, PVC, polystyrene, Teflon

Copolymer

Polymers made up from two or more different monomers.

Type Pattern Description
Random -A-B-B-A-B-A-A-B- Repeating units in purely random fashion
Regular (alternating) -A-B-A-B-A-B-A-B- Regularly alternating units
Block -A-A-A-A-B-B-B-B- Occurs in blocks of different length
Graft A backbone with B branches Chain of one repeating unit grafted onto backbone of another
  • Examples: Saran®, SBR

By Thermal Behavior

Type Properties Examples
Thermoplastics Soften on heating (reversible) PE, PP, PVC, PS
Thermosetting Harden permanently (irreversible) Bakelite, epoxy

By Structure

Type Description Effect
Linear Straight and long continuous chain without lateral linkage or branching Can fold back upon themselves randomly
Branched Branches at regular intervals along chain Difficult to pack in regular array → less crystalline
Cross-linked Linear or branched chains joined by covalent bonds More elastic; more cross-links → more rigid
Network Extensive cross-linking throughout Thermosetting behavior

Polymerization Mechanisms

1. Addition (Chain-Growth) Polymerization

  • Addition reaction in which unsaturated monomers (with C=C double bonds) are joined by covalent bonds
  • No elimination of small molecule
  • Empirical formula of polymer = empirical formula of monomer
  • Peroxide (CH₃OOCH₃) used as initiator
  • Three stages:
    1. Initiation: Radical formation
    2. Propagation: Chain growth
    3. Termination: Chain stops growing

Examples: Polyethylene, PVC, polystyrene, PTFE, polypropylene, neoprene, SBR

2. Condensation (Step-Growth) Polymerization

  • Monomers must have at least two identical or different functional groups
  • Small molecule eliminated (H₂O, methanol, HCl, ammonia)
  • Molecular weight builds slowly

Two major classes:

  1. Polyamide — carboxylic acid with two -COOH reacts with amine with two -NH₂
  2. Polyester — carboxylic acid with two -COOH reacts with alcohol with two -OH

Examples: Nylon 6,6, Nylon 6, Kevlar, Dacron, Terylene, Bakelite

Important Synthetic Polymers

Polyethylene (PE)

C=C                          ; ethene (monomer)
  • LDPE (Recycling code 4): Discovered 1933 by ICI. Conditions: 1200 atm, 200°C, O₂ — free radical mechanism. Highly branched, mp 150°C, density 0.92 g cm⁻³. Uses: plastic bags, wrapping sheet, bottles, electrical insulation.
  • HDPE (Recycling code 2): 1953 by Ziegler & Natta. Conditions: 1 atm, 60°C, TiCl₄ + (C₂H₅)₃Al — Ziegler-Natta mechanism. Linear, density 0.96 g cm⁻³, mp 130–140°C. Stronger and harder. Uses: rigid articles, pipes.

Polypropylene (PP)

C=CC                         ; propene (monomer)
  • Methyl groups increase van der Waals forces but make chains difficult to pack → lower density but higher mp
  • Uses: Ropes, moulds, bottles, kitchenware, carpets, battery containers

Polyvinyl Chloride (PVC)

C=CCl                        ; chloroethene / vinyl chloride (monomer)
  • Rigid (pipes) or plasticized (flooring)

Polystyrene (PS)

c1ccccc1C=C                  ; phenylethene / styrene (monomer)
  • Expanded PS: Foam packaging, insulation
  • Uses: Food packaging containers

Polytetrafluoroethylene (PTFE, Teflon)

F/C(F)=C(F)/F                ; tetrafluoroethylene (monomer)
  • Exceptional chemical resistance, low friction
  • Uses: Non-stick coatings, thread seal tape

Neoprene (Synthetic Rubber)

C=C(Cl)C=C                   ; 2-chloro-1,3-butadiene (chloroprene)
  • First synthetic rubber; polymerization of 2-chloro-1,3-butadiene
  • Resistant to most chemicals
  • Uses: Hoses for petrol, containers for corrosive liquids

Styrene-Butadiene Rubber (SBR)

  • Copolymer of styrene (phenylethene) and 1,3-butadiene in ratio 1:3
  • Can be vulcanized like natural rubber
  • Uses: Car tyres, footware, carpetbacking

Nylon (Polyamides)

Nylon 6,6

NCCCCCCN                     ; hexane-1,6-diamine
O=C(O)CCCCC(=O)O             ; hexane-1,6-dioic acid (adipic acid)
  • Monomers: Hexane-1,6-diamine + hexane-1,6-dioic acid
  • By-product: nH₂O

Nylon 6

NCCCCCC(=O)O                 ; 6-aminohexanoic acid (caprolactam derivative)
  • Monomer: 6-aminohexanoic acid
  • By-product: nH₂O

Kevlar

Nc1ccc(N)cc1                 ; 1,4-diaminobenzene
O=C(O)c1ccc(C(=O)O)cc1       ; terephthalic acid
  • Monomers: 1,4-diaminobenzene + terephthalic acid
  • Properties: Very strong and flexible
  • Uses: Bulletproof vests

Polyesters (PET, Dacron, Terylene)

Dacron (PET)

COC(=O)c1ccc(C(=O)OC)cc1     ; dimethyl terephthalate
OCCO                         ; ethylene glycol (1,2-ethanediol)
  • Monomers: Dimethyl terephthalate + ethylene glycol
  • By-product: Methanol
  • Uses: Clothing, tyre cords, carpets

Terylene

O=C(O)c1ccc(C(=O)O)cc1       ; terephthalic acid
OCCO                         ; ethane-1,2-diol (ethylene glycol)
  • Monomers: Terephthalic acid + ethane-1,2-diol
  • By-product: Water

PET general: Bottles, fibers, films

Bakelite (Phenol-formaldehyde)

  • First synthetic plastic
  • Thermosetting
  • Electrical insulators

Natural Polymers

Proteins

  • Monomers: 20 standard amino acids
  • Polymer type: Polyamide (polypeptide)
  • Functions: Enzymes, structural proteins, transport, hormones

Amino Acid Monomers

Every standard amino acid contains an α-amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a unique side chain (R group) attached to the α-carbon.

N[C@@H](R)C(=O)O

Glycine (R = H) is the only achiral standard amino acid.

In aqueous solution, amino acids exist as zwitterions — dipolar ions with both a positive and a negative charge:

[NH3+]CC(=O)[O-]

Charge and pH

  • Isoelectric point (pI): pH at which the amino acid has no net charge.
  • At pH < pI: net positive charge (migrates to cathode in electrophoresis).
  • At pH > pI: net negative charge (migrates to anode).

Peptide Bond Formation

Amino acids are linked by peptide bonds via condensation (elimination of H₂O):

NCC(=O)O.NCC(=O)O>>NCC(=O)NCC(=O)O
  • N-terminus: Free amino group at one end.
  • C-terminus: Free carboxyl group at the other end.
  • Sequences are written N → C.

Example tripeptide Ala-Gly-Ala:

CC(N)C(=O)NCC(=O)NC(C)C(=O)O

Protein Classification

Type Characteristics Examples
Simple Hydrolyze to amino acids only Albumins, globulins
Fibrous Long, insoluble filaments; structural support Collagen, elastin, keratin
Globular Spherical, soluble; enzymes, hormones, transport Hemoglobin, myoglobin, lysozyme
Conjugated Contain a non-protein prosthetic group Nucleoproteins, glycoproteins, mucoproteins

Levels of Protein Structure

Level Description Stabilizing Forces
Primary Linear amino acid sequence; location of disulfide bridges Peptide bonds (covalent)
Secondary Local folding: α-helices and β-pleated sheets Hydrogen bonds between backbone C=O and N–H
Tertiary Overall 3-D conformation of one polypeptide chain Ionic bonds, H-bonds, disulfide bridges, van der Waals forces
Quaternary Assembly of multiple polypeptide subunits Same as tertiary (inter-subunit)

Collagen (fibrous) forms a triple helix (quaternary).
Hemoglobin (globular) has four subunits (quaternary).
Keratin (fibrous) is rich in cysteine disulfide bridges.

Carbohydrates

  • Cellulose: β-1,4-linked glucose; plant structural polymer
  • Starch: α-1,4-linked glucose; plant energy storage
  • Glycogen: α-1,4-linked glucose; animal energy storage

Natural Rubber

CC(=C)C=C                    ; isoprene / 2-methyl-1,3-butadiene (monomer)
  • Polymer of 2-methyl-1,3-butadiene (isoprene)
  • Exists in two forms: cis and trans depending on spatial arrangement of -CH₂ groups
  • Natural rubber is soft, sticky, not strong or elastic

Vulcanization

  • Process of heating rubber with sulphur atoms
  • Long chains of polyisoprene cross-linked by sulphur atoms
  • Transforms soft rubber to harder cross-linked polymer
  • Too much vulcanization makes rubber hard and brittle
  • Uses: Tyres, footware, gloves, elastic bands, tubings, toys

Advanced Polymer Terminology

Crystallites

  • Regions of the polymer in which the chains are highly ordered with respect to one another.
  • The more crystalline the polymer, the denser, harder, and more resistant to heat it is.
  • Large polymer size → greater van der Waals forces → strongest when chains pack closely in ordered arrays.

Elastomers

  • Polymers that stretch and then revert to their original shape.
  • Randomly oriented amorphous polymers with some cross-linking to prevent chains from slipping over one another.
  • When stretched, random chains align; van der Waals forces are too weak to maintain the stretched arrangement, so the chains return to random shapes when the force is removed.
  • Example: Rubber (natural and synthetic).

Fibers

  • Thin threads produced by passing a molten polymer through small holes in a die.
  • When cooled and drawn out, crystalline regions orientate along the fiber axis, adding considerable tensile strength.
  • Examples: Nylon, Dacron, polyethylene.

Plasticizers

  • Organic compounds added to polymers to increase flexibility.
  • Dissolve in the polymer and lower attractions between chains, allowing them to slide past one another.
  • Example: Dibutyl phthalate (added to PVC to make flexible flooring and tubing).

Modification of Polymer Properties

Polymer properties can be deliberately modified by:

  1. Changing the length of the polymer chain
  2. Varying the chemical composition of the monomer units
  3. Changing the branching of the polymer chains
  4. Cross-linking the polymer chains
  5. Varying the arrangement of the chains in the solid
  6. Modifying the orientation of the monomer units within the chains

Polymer Research Fields

Fiber Reinforced Composites

  • Used in building materials, vehicle parts, etc.
  • Natural fiber vs synthetic fiber: natural fibers (e.g., kenaf) are sustainable and reduce manufacturing cost, but have many hydrophilic –OH groups.
  • Challenge: HDPE is hydrophobic, so natural fiber composites suffer from weak adhesion, poor mechanical properties, and poor moisture resistance.
  • Solution: Use a compatibilizer (e.g., maleic anhydride grafted HDPE) to improve interface adhesion.

Sustainable & Smart Polymers

  • Sustainable GunPla: models made from eggshell-derived biocomposites via injection moulding.
  • Crab shell to solar cell: gel polymer electrolytes based on N-phthaloylchitosan for dye-sensitized solar cells.
  • Self-healing coatings: UV-curable alkyd coatings that repair micro-cracks automatically, protecting buildings and industrial structures.
  • Sustained drug delivery: nanofiber capsules that slowly release drugs into the body, reducing strain on kidneys compared to immediate-release formulations.

Polymer Properties

Property Description
Crystallinity Ordered regions; affects density and strength
Tg (Glass transition) Temperature where polymer becomes rubbery
Tm (Melting point) Temperature where crystalline regions melt
Tensile strength Resistance to breaking under tension
Elasticity Ability to return to original shape

Recycling Codes

Code Polymer
1 PET (polyethylene terephthalate)
2 HDPE (high-density polyethylene)
3 PVC (polyvinyl chloride)
4 LDPE (low-density polyethylene)
5 PP (polypropylene)
6 PS (polystyrene)
7 Other

Related Topics

Sources