Polymers Notes

Introduction to Polymers

Learning Goals

• Structure 2.4.4 - Polymers: Large molecules (macromolecules) made of repeating subunits called monomers.
• Describe common plastic properties in terms of structure.
• Discuss natural and synthetic polymer examples.
• Structure 3.2: Identify structural features making plastics biodegradable.
• Structure 2.4.5 - Addition polymers: Formed by breaking a double bond in each monomer.
• Represent the repeating unit of an addition polymer from monomer structures.
• Examples: Polymerization reactions of alkenes.
• Monomer structures will be provided or deduced from the polymer.
• Structure 3.2: Identify functional groups enabling molecules to act as monomers in addition reactions.
• Reactivity 2.1: Atom economy is 100% for addition polymerization reactions.

Polymers

• Monomers: Small molecules reacting to form linked chains (polymers).
• Polymers (Macromolecules): Chains held by covalent bonds, composed of thousands of atoms, relatively large molecules.
• Polymers vary in monomer nature, chain length, and branching, leading to diverse properties.

Natural vs. Synthetic Polymers

• Natural Polymers: Widespread in living things (e.g., proteins, starch, DNA).
• Synthetic Polymers: Human-made, emerged in the 1930s with available petroleum industry raw materials.
• Rapid development of plastics followed.
• Plastics: Synthetic polymers with widespread use due to:
  • Light weight
  • Low reactivity
  • Water resistance
  • Strength (in some cases)
• Common plastics: Polythene, PVC, nylon, polystyrene, Kevlar.

Major Differences: Natural vs. Synthetic Polymers

Addition Polymers

• Addition Polymerization: Monomers with double bonds join through multiple addition reactions to form a polymer.

Addition Polymers

• Monomers (A + A + A + A) form a polymer (-A-A-A-A-).
• Ethylene (ethene) polymerizes into polyethylene (linear).
• Propylene (propene) polymerizes into polypropylene (branched).
• $n$ monomers (nA) form a polymer $(A)_n$.

Polymerization of Alkenes: Addition Polymerization

• Process: Alkene adds to itself.
• All original alkene atoms are used to form the polymer.
• Long hydrocarbon chains are formed.
• Equation shows the original monomer and the repeating unit in the polymer.
• $n$ represents a large number.
• Ethene $\rightarrow$ poly(ethene).

Monomer vs. Polymer

Polymerization of Alkenes: Examples of Addition Polymerization

• Ethene $\rightarrow$ Poly(ethene)
• Propene $\rightarrow$ Poly(propene)
• Chloroethene (Vinyl Chloride) $\rightarrow$ Poly(chloroethene) (Polyvinylchloride PVC)
• Tetrafluoroethene $\rightarrow$ Poly(tetrafluoroethene) PTFE "Teflon"

Practice

1. Draw three repeating units of the addition polymer formed from chlorotrifluoroethene.
2. Identify the monomer(s) used to synthesize a given polymer.

Extra Material

• Memorizing specific polymers is not required.
• Skills:
  • Identify the monomer from a polymer chain (repeating unit).
  • Construct/draw a polymer chain with a specific number of repeating units from given monomers (usually 3).

"Styrofoam"

• Styrene polymerizes to form polystyrene using a catalyst (e.g., a proton from sulfuric acid).
• Cationic catalyst ($H<em>2SO</em>4$ or $H^+$).
• Resonance stabilized.

Polystyrene

• Repeating unit.
• $n$ = 20,000 repeating units.

Thermoplastics

• Soften or melt with heat and can be reformed (molded).
• Most addition polymers are thermoplastics.
• Polymer chains held by weak interactions (noncovalent bonds):
  • van der Waal's forces
  • London dispersion forces
  • Dipole-dipole attractions
• Heating disrupts these interactions, allowing chains to become independent.
• Heating and reforming can be repeated indefinitely (if degradation doesn't occur), enabling recycling.

Thermoset Plastics

• Melt initially, but become permanently hardened with further heating.
• Cannot be remolded or recycled once formed.
• Heating causes cross-linking (covalent bonds form between chains), creating a rigid network.

High-Density Polymers

• Linear polymers with chains that can pack closely together.
• Often quite rigid.

Low-Density Polymers

• Branched-chain polymers that cannot pack together closely.
• Often have a degree of cross-linking.
• Often more flexible than high-density polymers.

Common Industrial Monomers and Polymers

Extra - Polyurethane Chemistry

• Diisocyanate + Polyol $\rightarrow$ Polyurethane
  • $n OCN - R - NCO + n HOR' OH \rightarrow [-O-C-N-R-N-C-O-R'-O-]_n$
• Diisocyanate + Amine $\rightarrow$ Polyurea
  • $n OCN - R - NCO + n HN-R' - NH<em>2 \rightarrow [-N-C-N-R-N-C-N-R'-N-]</em>n$
    $\ \ \ \ \ \ \ H \ \ \ \ O \ \ \ \ \ \ H \ \ O$

Polyurethane Foam

$OCN - R - NCO + 2 H<em>2O \rightarrow [HO-C-N-R-N-C-OH] \rightarrow H</em>2N-R-NH<em>2 + CO</em>2$

• Diisocyanate + Water $\rightarrow$ Carbamic acid $\rightarrow$ Diamine + Carbon dioxide

$CO_2 + 2 (OCN-R-NCO) \rightarrow OCN-R-N-C-N-R-N-C-N-R-NCO$
$\ \ \ \ \ \ \ \ \ \ \ \ \ H \ \ \ O \ \ \ \ \ \ \ \ \ H \ \ \ O \ \ \ \ \ \ \ \ \ H$
*A diisocyanatopolyurea

Isocyanate Types

• Aromatic: TDI, MDI
• Aliphatic: HDI, IPDI

Isocyanates - Chemical Structures

• 2,4-Toluene diisocyanate (TDI): $C9H6N2O2$

• 1,6-Hexamethylene diisocyanate (HDI): $OCN(CH2)6 NCO$

• 4,4-Diphenylmethane diisocyanate (MDI)

• Isophorone diisocyanate (IPDI)

Practice - Answers

1. Repeating unit of chlorotrifluoroethene polymer.
2. Monomer(s) used to synthesize a given polymer (with methyl and chlorine groups).