Polymers

1.0What is a Polymer?

A polymer consists of large molecules with high molecular weight and numerous identical or diverse smaller molecular components. A monomer denotes the individual unit(s) that constitute the polymer. The term "polymer" originates from two Greek roots: "poly," meaning many, and "mer," signifying unit or part. Polymer refers to large molecules with high molecular mass (ranging from 10³ to 10 ⁷u).

2.0Classification of Polymers

Based on Source

• Natural Polymers: These polymers occur naturally in plants and animals. Examples include proteins, cellulose, starch, certain resins, and rubber.
• Semi-synthetic Polymers: This category includes polymers derived from natural sources but modified through chemical processes. Examples include cellulose derivatives such as cellulose acetate (used in rayon) and cellulose nitrate.
• Synthetic Polymers: Man-made polymers are extensively utilized in everyday life and industry. Examples include plastics like polyethene, synthetic fibres like nylon 6,6, and synthetic rubbers like Buna-S.

 Based on Structure

• Linear polymers are characterized by long, straight-chain repeating units. Examples include polyethene (HDPE), PVC, nylon, and polyester.
• Branched Polymers: These polymers consist of linear chains with additional branches. Examples include amylopectin and glycogen.

3.0Types of Polymerisation Reactions 

Addition Polymerisation or Chain Growth Polymerisation

It involves the formation of reactive intermediates such as free radicals, a carbocation or a carbanion.

For this polymerisation, monomers used are unsaturated compounds like alkenes, alkadienes, and their derivatives. Depending upon the nature of the reactive species involved. Chain growth polymerisation occurs by the following mechanisms:

•  Free radical addition polymerisation
•  Cationic polymerisation
•  Anionic polymerisation

4.0Some Important Addition Polymers

Polyethylene:

• Free radical initiators produce low-density polyethene in wire insulation, squeeze bottle production, toy manufacturing, and flexible pipe fabrication. 

• While ionic catalysts yield high-density polyethylene and are white, translucent, rigid, and linear, high-density polyethene is commonly used to make tubes, pipes, coated wires, insulator parts, buckets, dustbins, bottles, and pipes.

Polypropylene:

• Polypropylene possesses excellent hardness, stiffness, and resistance, making it suitable for manufacturing ropes, carpets, washing machine parts, and various other applications. It also manufactures ropes, toys, pipes, fibres, etc.

Polyvinyl Chloride (PVC) :

• It exhibits resistance to light and atmospheric oxygen, and it is chemically inert. This material is commonly employed for electrical insulators, floor coverings, safety helmets, and other applications. PVC is also used to manufacture raincoats, handbags, vinyl flooring, water pipes, imitation leather, floor covering and gramophone records.

Polystyrene:

• Polystyrene is a transparent, lightweight, and resistant-to-moisture material used as an insulator and wrapping material and in manufacturing toys, radios, and television cabinets. It is also utilized for manufacturing battery cases, refrigerator parts, electric insulators, combs, buttons, and TV cabinets. Polystyrene is commonly known by its trade names, Styrofoam or Styron.

Polyacrylonitrile:

Polyacrylonitrile (PAN) is made from acrylonitrile monomers. It is known for its high tensile strength, chemical resistance, and thermal stability.

PAN fibres are commonly used in textiles, filtration membranes, and carbon fibres as a precursor for producing activated carbon and carbon nanotubes.

Polyvinyl Acetate:

Polyvinyl acetate (PVA) is composed of repeating vinyl acetate units. It is commonly used in adhesives, paints, and coatings and as a component in paper coatings and textile finishes. PVA exhibits excellent adhesive properties, flexibility, and water resistance, making it suitable for various industry applications.

It is extremely tough and resistant to heat and chemicals. It is used for making gaskets, pump parts, coating utensils, high-frequency insulators

Teflon:

Teflon, a trademark for polytetrafluoroethylene (PTFE), is a synthetic fluoropolymer famed for its nonstick characteristics, exceptional heat resistance, and chemical inertness. Its versatile applications include nonstick cookware, electrical insulation, gaskets, seals, and lubricants.

Polymethylmethacrylate:

Polymethylmethacrylate (PMMA), commonly known as acrylic or acrylic glass, is a transparent thermoplastic polymer synthesized from methyl methacrylate monomers. It is a lightweight and durable substitute for glass in various applications, including signage, displays, windows, lenses, and medical implants.     

5.0Condensation Polymerisation or Step Growth Polymerisation

This polymerization involves repetitive condensation reactions between bi-functional or trifunctional monomeric units, forming high molecular mass condensation polymers. 

• Simple molecules like water, alcohol, or hydrogen chloride may be lost. Each step produces a functionalized species, and the reactions proceed independently, hence the term step-growth polymerization. 
• An example is the formation of terylene or dacron from ethylene glycol and terephthalic acid.

6.0Some Important Condensation Polymers 

Polyamides:

Polyamides, also known as nylons, are synthetic fibers characterized by amide linkages. They are commonly prepared by condensation polymerizing diamines with dicarboxylic acids or the condensation of amino acids or their lactams.

Nylon 6 is a type of polyamide synthesized by the ring-opening polymerization of caprolactam, yielding a solid and versatile synthetic fiber.

Nylon 6,6:

Nylon 6,6 is a polyamide synthesized by the condensation polymerization of hexamethylenediamine with adipic acid under high pressure and temperature conditions. This results in a durable and widely used synthetic fiber.

Polyesters:

The polycondensation of dicarboxylic acids and diols forms polyesters. Dacron or terylene, a well-known example, is produced by heating ethylene glycol and terephthalic acid with a zinc acetate-antimony trioxide catalyst.

Dacron fiber, known for its crease resistance, is used as a reinforcing material in blends with cotton and wool and safety helmets.

Phenol-formaldehyde:

Phenol-formaldehyde polymers like Bakelite are some of the earliest synthetic polymers. They are created by condensing phenol with formaldehyde using acid or base catalysts. This reaction produces o- and p-hydroxymethyl phenol derivatives, which combine with phenol to form compounds with interconnected rings via -CH₂ groups. The initial product, Novolac, is utilized in paints.

Novolac, on heating with formaldehyde, undergoes cross-linking to form an infusible solid mass called Bakelite.

Melamine-formaldehyde:

Melamine-formaldehyde is a thermosetting plastic produced through the condensation polymerization of melamine and formaldehyde. Due to its durability and heat resistance, it's widely used in laminates, coatings, adhesives, and moulded products.

7.0Copolymerization

Copolymerization combines two or more monomers to form a copolymer, possibly through chain or step growth mechanisms. Copolymers have repeating units of each monomer within one chain. For example, butadiene-styrene copolymer is a durable alternative to natural rubber used in products like tyres, floor tiles, and cable insulation.

Rubber

Natural rubber is a polymer classified as an elastomer due to its elasticity and ability to undergo long-range reversible extension. Its monomer is isoprene, also known as 2-methyl-1,3-butadiene and natural rubber is often called polyisoprene. The polymerization reaction for natural rubber can be represented as follows:

              𝑛 units of isoprene→Natural rubber (polyisoprene)   

Vulcanisation of Rubber

• Vulcanization involves heating rubber with sulfur, creating sulfur bridges between molecules, and resulting in cross-linking. This cross-linking enhances the rubber's elasticity, durability, and strength. Vulcanized rubber is indeed more elastic than natural rubber.
• Additionally, vulcanization can be induced by free radical generators such as peroxides or azo compounds, as well as metal oxides like zinc oxide (ZnO) or magnesium oxide (MgO). These agents facilitate the formation of cross-links, improving the rubber's properties.

Synthetic Rubber

Synthetic rubber denotes any polymer capable of being vulcanized, stretching to twice its length, and reverting to its original shape when external force is removed. These polymers are usually homopolymers derived from 1,3-butadiene or copolymers of 1,3-butadiene or its derivatives with another unsaturated monomer.

Preparation of Synthetic Rubbers:

• Neoprene: Neoprene, or polychloroprene, is produced through the free radical polymerization of chloroprene.

• Buna-N is a copolymer made from butadiene and acrylonitrile. It exhibits excellent resistance to oil, fuel, and other chemicals, making it commonly used in applications such as O-rings, gaskets, seals, hoses, and automotive parts.

8.0Molecular Mass of Polymers 

Polymer properties are intricately tied to their molecular mass, size, and structure. The growth of polymer chains during synthesis depends on the availability of monomers in the reaction mixture. As a result, polymer samples contain chains of varying lengths, necessitating the expression of molecular mass as an average. Determining polymer molecular mass requires a combination of chemical and physical methods.

It is of two types

• Number average molecular mass (M_n^–) is given by 

Where N₁ is the number of molecules having molecular mass M₁ and so on. It is measured by osmotic pressure measurement.

• Mass-average molecular mass is given by

It is determined by ultracentrifugation or sedimentation.

9.0Biodegradable Polymers

Numerous polymers are resistant to environmental degradation, resulting in the accumulation of non-degradable waste. In response, biodegradable synthetic polymers with functional groups resembling biopolymers have been developed.

Aliphatic polyesters, such as Poly β-hydroxybutyrate-co-β-hydroxyvalerate (PHBV), exemplify this approach. PHBV, synthesized from 3-hydroxybutyric acid and 3-hydroxy pentanoic acid, is utilized in specialized packaging, orthopaedic devices, and drug delivery systems. Its degradation occurs through bacterial processes in the environment.

10.0Polymers of Commercial Importance