Lesson Objective :

At the end of this lesson , you will be able to:

• categorize a given polymer based on different criteria
• identify natural and synthetic polymers
• tell the monomer of natural rubber
• list the properties and applications of natural and synthetic polymers;
• draw the structures of disaccharides
• describe poly saccharides
• draw the structure of starch and cellulose
• draw the structure of polyethylene, polypropylene,polyvinyl chloride, poly tetrafluoroethylene ethylene and poly methyl methyl methocrylate
• give examples polypropylene, polyvinyl chloride and polyethylene products .

Brainstorming question

write the following statements :

• The similarity between starch and cellulose
• The properties of natural rubber
• Similarity between wood and cotton

Key terms/ Concepts

Classification of Polymers

There are a number of methods of classifying polymers. A few of the methods of polymers classification are listed below

A polymer might be made from identical or different types of monomers. This could be used to classify polymers as homo polymer s or copolymers
Homopolymers ; are those made from only one type of monomer. For example, polyethylene is synthesized by the polymerization of one type monomer, ethene(ethylene )

Copolymers; are those prepared by polymerizing more than one kind of monomer unit.
For instance, ethene (H2C=CH2) and propene (H2C=CH–CH3) can be copolymerized to produce a polymer that has two kinds of repeating units

Copolymers are classified as random or regular, based on the way the monomers are arranged along the polymer chain.
 Random polymers contain repeating units arranged in a random fashion.
 Regular polymers contain a sequence of monomers in regular alternating repeating units.

Random copolymer; A-B—–B-A —-A
Regular copolymer ; A-B —-A-B—–A-B…..
Random and Regular copolymers (Where, A and B represent monomer units).
 Another classification system of polymers is based on the nature of the chemical reactions employed in the polymerisation. Here the two major groups are the condensation and the addition polymers.
 Condensation polymers are those prepared from monomers where reaction is accompanied by the loss of a small molecule, usually of water, for example polyesters which are formed by the condensation shown in the reaction .by contrast, addition polymers are those formed by the addition reaction of an unsaturated monomer, such as takes place in the polymerization of vinyl chloride:

 Furthermore, polymers can be classified based on whether they are naturally occurring or made by man (synthetic).
Natural polymers
Natural polymers occur in nature and can be extracted, and often referred to as biopolymers.
Common natural polymers include:

1. Carbohydrates
   a. monosaccharides e.g. glucose, fructose, galactose, and ribose.
   b. diasaccharides e.g. sucrose, lactose and maltose
   Figure (a) and (b) shows the structure of glucose and sucrose respectively

Macromolecules such as polysaccharides (e.g., starches, cellulose, gums, etc.),
proteins (e.g., enzymes), fibbers (e.g., wool, silk, cotton), polyisoprenes (e.g., natural rubber), and nucleic acids (e.g., RNA, DNA) (Figure 4.2).

Synthetic Polymer

Synthetic polymers are those which are human-made polymers.
Synthetic polymers are sometimes referred as ―plastics‖, of which the well-known ones is polyethylene (Figure 4.3).
A Synthetic polymer has no taste or odour and is lightweight, nontoxic and relatively cheap.

Some of its primarily uses are in making squeeze bottles, plastic wrapping, garment bags, trash bags and electrical insulation (Figure below, Fig 4.4).

polyethylene
The monomer of polypropylene is propylene (propene) (CH2=CHCH3).
polyethylene is produced by the addition polymerization of propylene.

Polypropylene is stronger than polyethylene. It is used for making food containers that can safely be washed in a dishwasher. It can also be used for making ropes; fishing nets, carpets, and bottles (below, Figure 4.6).

Polyvinyl Chloride (PVC)
Polyvinyl chloride is the third most widely produced plastic, after polyethylene and polypropylene.
Polyvinyl chloride is prepared by the polymerization of vinyl chloride.

It is commonly used for making pipes, leather-like materials, shoes, raincoats, aprons, wallpaper, floor tile, and phonograph records

Figure 4.8

Polymethyl methacrylate (Perspex)

Polymethyl methacrylate (PMMA) is sold under the trade name Lucite or plexiglass.
Polymethyl methacrylate( PMMA) is prepared by the polymerization of methyl methacrylate (CH₂=C (CH₃ ) COOCH₃.

PMMA is a lightweight glass-like polymer used as a glass substitute for example, in airplane windows and streetlights.

Poly tetra fluoro ethylene (PTFE) Teflon
 Teflon is prepared by the addition polymerization of tetra fluoro ethylene (CF₂=CF₂).
 Teflon has good resistance to chemical attack, and it can be used at any temperature between 73 °C and 260 °C with no effect on its properties

It also has a very low coefficient of friction, which makes it waxy or slippery to touch. As a result, it is particularly suited to applications in food preparation. For example, bread dough does not stick to a Teflon-coated surface.

Teflon is used for coating cooking utensils and for making electric elctrical insulation (Figure 4.11).

Figure 4.11: Some Teflon materials
One way of classification of polymers is to adopt the approach of using their response to thermal treatment and to divide them into thermoplastics and thermo sets.
Thermoplastic polymers
Thermoplastic polymers are those which melt when heated and re-solidify when cooled. They are comprised of essentially linear or lightly branched polymer molecules. They can be remelted or reprocessed (recycled)

Examples of thermoplastics include: polyethylene, polypropylene, polyvinyl chloride, teflon, polymethyl methacrylate, nylon and polyester (Dacron), polystyrene (Rubber), and polyamide.
Thermoset polymers
Thermoset polymers are those which do not melt when heated, but at sufficiently high temperatures, decompose irreversibly.
They are substantially cross-linked materials, consisting of an extensive three-dimensional network of covalent chemical bonding.
They exhibit resistance to heat, corrosion, and mechanical stress.
Thermoset materials are no-recyclable. Examples thermoset polymers include: Bakelite, cyanate esters, epoxy resin, fibber glass.