CHEM Polymers

Polymers

Introduction

  • Acrylic is a strong, transparent plastic used in various applications.
  • Plastic is ubiquitous but its disposal poses environmental challenges.
  • Chapter overview:
    • Definition of polymers.
    • Formation and uses of addition polymers.
    • Environmental impact of plastic disposal.
    • Plastic recycling methods and related issues.

What Are Polymers?

  • Polymers are large molecules composed of repeating small units called monomers.
  • Examples:
    • Chairs and bedding.
  • Types of Polymers:
    • Natural: wool, natural rubber, silk, starch.
    • Synthetic: poly(ethene), nylon, Terylene, polyvinyl chloride (PVC).
  • Uses of Polymers:
    • Wool, silk, nylon, and Terylene for clothing.
    • Natural rubber for tires.
    • PVC for pipes.
  • Polymers are large organic molecules with thousands/millions of atoms.
  • Monomers are joined by covalent bonds during polymerization.

Key Terms

  • Monomer: A small molecule that combines with many others to form a polymer.
  • Polymer: A large molecule made of many monomers.
  • Polymerization: The process of joining monomers to form a polymer.

Historical Context

  • Early sutures used natural polymers like silk.
  • Modern sutures use synthetic polymers like nylon that can be absorbed by the body.

How Are Addition Polymers Formed and Used?

  • Poly(ethene) is a common plastic used in bags.
  • Poly(ethene) has a large molecular mass and contains thousands of carbon atoms.

Addition Polymerization

  • Monomers join without losing molecules/atoms.
  • Unsaturated monomers (with carbon-carbon double or triple bonds) are used.
  • Alkenes undergo addition polymerization under high pressure and temperature.

Poly(ethene)

  • Simplest addition polymer, also known as polyethylene (PE).
  • Formed from ethene monomers.
  • Process: At high temperature, pressure and with a catalyst, the carbon-carbon double bonds break and monomers join to form a chain.
  • Equation for polymerization of ethene:
    • n (CH2=CH2) \xrightarrow{\text{high temp. and pressure, catalyst}} (-CH2-CH2-)_n
    • nC = C \xrightarrow{\text{high temp. and pressure, catalyst}} -C-C-
    • \text{monomer} \rightarrow \text{repeating unit}
  • Repeat unit of poly(ethene) is \text{-CH2-CH2-}.
  • Polymerization conditions affect poly(ethene) density (LDPE or HDPE).

Deducing Polymer Structure

  • Represent alkene monomers as \text{C=C}, where R represents side groups.
  • Example: Propene (\text{CH3-CH=CH2}) becomes poly(propene).

Worked Example 18A

  • Polymer X is made from 1,1-dichloroethene.
    • Chemical name: poly(1,1-dichloroethene).
    • Three repeat units: \text{-CH2-CCl2-CH2-CCl2-CH2-CCl2-}.

Deducing Monomer Structure from Polymer

  • Example: Polyvinyl acetate (PVA).
  • Steps:
    • Identify the repeat unit.
    • Draw the structure of the repeat unit.
    • Convert the carbon-carbon single bond into a double bond to obtain the monomer.

Uses of Poly(ethene) and Addition Polymers

  • Poly(ethene): Plastic bags, toys, buckets, cling wraps.
  • Polytetrafluoroethylene (PTFE): Non-stick coating for frying pans.
  • Polyvinyl chloride (PVC): Gloves, pipes, raincoats, flooring mats.
  • Polystyrene: Disposable containers; hard, light, and brittle.

How Does the Disposal of Plastics Affect Our Environment?

  • Plastics replace natural materials due to being cheap, easily molded, light, tough, and waterproof.
  • Most plastics are non-biodegradable, posing a disposal problem.

Environmental Problems

  • Land pollution: Plastics accumulate in landfills because they do not decompose.
  • Water pollution: Plastics endanger marine animals and clog waterways.
  • Air pollution: Burning plastics releases poisonous gases like hydrogen chloride from PVC.

How Are Plastics Recycled and What Are the Issues Related to Recycling Plastics?

  • Recycling involves physical and chemical methods.
  • Plastic waste requires pre-treatment: sorting, washing, shredding.

Physical Method

  • Also known as mechanical recycling; the chemical composition remains unchanged.
  • Process: Melting, cooling, and cutting into pellets for new products.

Chemical Method

  • Converts plastic waste into raw materials via chemical reactions like cracking.
  • Cracking produces short-chain alkanes (fuel) and alkenes (chemical feedstock).
  • Requires high temperature and a catalyst.

Tech Connect

  • Mealworms can break down polystyrene, offering a potential recycling solution.

Issues Related to Recycling Plastics

  • Importance of reducing, reusing, and recycling to minimize plastic waste.
  • Low recycling rates: In 2019, only 4% of plastic waste from Singapore households was recycled.
Environmental Issues of Recycling Plastics
  • Wastewater from recycling processes can cause water pollution if untreated.
Economic Issues of Recycling Plastics
  1. High recycling costs compared to landfill disposal or incineration.
    • Transportation, sorting, cleaning, and recycling processes are expensive.
    • Requires manpower, machines, and energy.
  2. Economic viability: Recycled plastics often have lower market value than virgin plastics.
    • High recycling costs can make it difficult for recycling businesses to survive.
Social Issues of Recycling Plastics
  1. Convenience: People find it easier to throw waste away instead of recycling.
  2. Lack of proper recycling habits:
    • Contaminated recyclables (food waste) are incinerated.
    • Non-recyclable items are placed in recycling bins.
  3. Behavioral change: Adopting recycling as a lifestyle takes time and effort.

Addressing Recycling Issues

  • Implementing strict measures and laws to minimize environmental issues.
  • Educating the public through recycling programs.
  • Encouraging recycling through creative activities like Reverse Vending Machines offering rewards.

Cool Career: Materials Scientist

  • Improves material properties and develops new polymers.
  • Example: Development of electrically conductive polymers (Nobel Prize in Chemistry 2000).