Unit 4 - Organic Chemistry: Chapter 29: Synthetic Polymers

Synthetic Polymers

Polymers are large molecules made of repeating units, found both naturally (proteins, sugars) and synthetically (plastic bags, clothing).

Polymerization

Polymerization is the process where small molecules (monomers) join to form a large molecule (polymer).

• Addition Polymerization: Monomers add directly to each other. Example: ethene to poly(ethene).

Addition Polymerization

Involves molecules with carbon-carbon double bonds joining to form long chains. Part of the double bond breaks to form a single bond, allowing bonding to neighboring molecules.

• Initiator: Starts the polymerization process. It is consumed in the reaction, so it is not a catalyst.

• Repeat Unit: The repeating structural unit within the polymer chain.

• n: Represents the large, variable number of repeat units in the polymer.

Example: Polymerization of ethene to form poly(ethene) (polythene).

nH2C=CH2 \rightarrow [-CH2-CH2-]_n

Types of Poly(ethene)

• Low-Density Poly(ethene) (LDPE): Flexible, not very strong, used for thin films like plastic bags.

• High-Density Poly(ethene) (HDPE): Stronger and more rigid, used for plastic bottles.

Polymerization of Alkenes

To deduce the polymerization reaction for any alkene, draw the alkene with the C=C in the middle.

• Open out the double bond and draw continuation bonds on each side to represent the polymer chain.

Polymerization of Propene

Propene ( CH3CH=CH2 ) polymerizes to form poly(propene).

nCH3CH=CH2 \rightarrow [-CH(CH3)-CH2-]_n

Poly(propene) is stronger than poly(ethene) and is used for ropes and crates. Recycling mark: PP

Polymerization of Chloroethene

Chloroethene ( CH_2=CHCl ), also known as vinyl chloride, polymerizes to form poly(chloroethene), commonly known as PVC (polyvinylchloride).

nCH2=CHCl \rightarrow [-CH2-CHCl-]_n

PVC is strong and rigid, used for water pipes and windows. Adding plasticizers makes it flexible for floor coverings and clothing. It is also used for electrical insulation.

Polymerization of Tetrafluoroethene

Tetrafluoroethene ( CF2=CF2 ) polymerizes to form poly(tetrafluoroethene) or PTFE, known as Teflon®.

nCF2=CF2 \rightarrow [-CF2-CF2-]_n

PTFE is unreactive due to strong carbon-fluorine bonds, used as a non-stick coating for pots and pans and for lining containers for corrosive chemicals.

Determining Monomers from Polymers

To deduce the monomer from a polymer structure:

1. Identify the repeat unit by selecting two adjacent carbon atoms in the main chain.

2. Replace the single bond between these carbon atoms with a double bond.

3. Remove the continuation bonds.

Example: Poly(styrene)

Disposal of Addition Polymers

Addition polymers are inert and non-biodegradable due to strong covalent bonds.

Disposal Methods

• Landfill: Polymers remain unchanged for thousands of years.

• Incineration: Releases heat energy for electricity generation but produces carbon dioxide and toxic gases (e.g., carbon monoxide, hydrogen chloride).

Advantages and Disadvantages of Disposal Methods