Types of Polymerization

Polymers are substances of high molecular mass that restrain a large number of reiterating structural units admitted from simple molecules. They implant macro-sized molecules and have a grandly high molecular mass of the order of 10 – 10u. They’re also called macromolecules. The repeating structural units are concluded from an umpteenth simple reactive molecule and are fascinated with each disparate by covalent bonds.

Simple molecules which associate to crystallize polymers are called monomers. The procedure by which simple molecules (i.e., monomerization is modified into polymers is called polymerization). For exemplification, polyethene or generally known as polythene is a polymer deduced from the progression of polymerization of its monomers ethylene polymerization. 

Types of Polymers

Classification based on Origin of Polymers

They are classified as:

1. Natural polymers-The polymers cultivated from nature are called natural polymers. They’re fabricated in plants and animals and are veritably involuntary for life. For exemplification protein, cellulose, starch, nucleic acid, natural rubber, etc.
2. Semi-synthetic polymers- These polymers are frequently deduced from naturally coming about polymers by chemical differences. For exemplification, cellulose is a natural being polymer. On acetylation with acetic anhydride in the presence of sulfuric acid, cellulose forms a cellulose diacetate polymer. It’s used in fabricating threads and materials like films, glasses, etc. Other exemplifications of semisynthetic polymers are vulcanized rubber ( exercised in forming tires etc.), gun cotton which is cellulose nitrate ( used in making explosives), etc.
3. Synthetic polymers- The polymers groomed in the laboratory are called synthetic polymers. For representative-Polythene, Buna-S, PVC, Nylon, etc.

Classification based upon Structures of Polymers

They are classified as:

• Linear polymers: These are polymers in which monomeric units are correlated concurrently to crystallize long and linear chains. These direct polymers are well packed and thus, hold high densities, high tensile (pulling) strength, and high melting points. Some common exemplifications of linear polymers are high-density polythene, polyvinyl chloride, nylons, polyesters, etc.

Linear polymers

• Branched-chain polymers: These are polymers in which monomers are combined to form long chains that have side chains or branches of varying lengths. These branched-chain polymers are packed irregularly and, therefore, have lower tensile strength and melting points than linear polymers. Some common examples are low-density polythene, glycogen, starch, etc.

Branched-chain polymers

• Cross-linked or network polymers: These are polymers in which the monomer units are cross-linked together in a three-dimensional network. These are also called three-dimensional work modes. These polymers are rigid, rigid, and brittle due to the network structure. For example, bakelite, melamine-formaldehyde resins, etc.

Cross-linked or network polymers

Classification based on Mode of Polymerization

They are classified as:

• Addition polymers: A polymer formed by the lump sum addition of repeated monomers to end product molecules is called an addition polymer. These are formed by the repeated addition of monomer molecules possessing double or triple bonds, e.g. polythene is formed from ethene. They are again classified as:
  • Homopolymers: These polymers are formed by the polymerization of single monomeric species, e.g. formation of polythene from ethene as: nCH₂ = CH₂ →  (CH₂-CH₂)_n
  • Copolymers: These polymers are formed by the polymerization of two different monomers, e.g. formation of buna-S from styrene and buta-1,3-diene. Some common addition polymers are polythene, polystyrene, Teflon, buna-S, PVC, etc. Here is the structure of Teflon:

nCF₂ = CF₂ →  –(CF₂–CF₂)_n–

• Condensation polymers: A polymer formed by the condensation of two or more monomers with the removal of simple molecules such as water, alcohol, ammonia, etc. is called a condensation polymer. These are formed by repeated condensation reactions between two different bi-functional or tri-functional monomeric units with the elimination of small molecules such as alcohol, water, HCl, etc. Nylon-6,6 is formed from hexamethylene diamine and adipic acid. Some of the condensation polymers are Terylene or Dacron, Nylon-6, Bakelite, etc.

Classification based on Molecular Forces

Depending on the intermolecular forces, the polymers are classified as:

• Elastomers: Those polymers in which the intermolecular forces of attraction between the polymer chains are weakest and have an elastic character, like that of rubber, are called elastomers. Because of the impending weak forces, polymers can be easily strained by applying small stresses and regain their dominant shape when the stress is deflected. The most important example of an elastomer is natural rubber. These polymers consist of randomly coiled molecular chains. They have the weakest intermolecular forces, e.g. Buna-S, Buna-N, Neoprene, etc.

BUNA-N

• Fibres: These are polymers that have strong intermolecular forces between the chains. These forces are either hydrogen bonds or dipole-dipole interactions. These polymers are long, thin, and thread-like and can be woven into fabric. Hence they are used to make fibre. Common examples are nylon-66, terylene, silk, etc. These are thread-forming solids that have high tensile strength and high modulus. They have strong intermolecular forces such as H-bonding, e.g. Polyamides (nylon-6,6), polyesters (terylene), etc.
• Thermoplastic polymers: These are linear or barely radiated long-chain molecules competent of constantly softening on heating and hardening on cooling. These hold intermolecular forces of attraction, intermediate between elastomers and fibres. Example polyethene, polystyrene, polyvinyl, etc.
• Thermosetting polymers: These are polymers that can be fluently softened constantly on heating and hardened on cooling with low-lying alteration in their properties. In these polymers, the intermolecular forces are intermediate between those of the elastomers and the filaments. There’s no cross-linking between chains. Softening occurs when polymer chains move more and furthermore freely due to the lack of cross-links. Common exemplifications of thermoplastics are polythene, polystyrene, polyvinyl chloride, Teflon, etc.

Teflon

Types of Polymerization

Polymerization reactions are of two types:

1. Addition polymerization or chain-growth polymerization.
2. Condensation polymerization or step-growth polymerization.

Addition or Chain Growth Polymerization

When the same or different monomer molecules (unsaturated compounds) are successively added together on a large scale to form a polymer, this type of polymerization is known as addition or chain-growth polymerization.

1. Polyaddition: It is a polymerization reaction that forms polymers through respective independent addition reactions. Polyaddition occurs as a reaction between functional groups on molecules to crystallize a category of altitudinous molar mass, with low degrees of polymerization, resemblant as dimers, trimers, and oligomers.
2. Polycondensation: It is a chemical abridgement for delivering a polymer by associating single or multiple varieties of monomers to crystallize long chains releasing water or a correspondent simple substance.

This involves free radical or ionic mechanisms of which free radical one is the most common mode.

Preparation of Polythene by Addition Polymerization:

In the disposition of polythene from ethene, benzoyl peroxide acts as an initiator or free radical generator. Polythene is of two types:

• Low-Density Polythene (LDP): It is chemically inert and tough but flexible and a poor conductor of electricity.
• High-Density Polythene (HDP): It is chemically inert and tough and is tough due to close packing.

Condensation or Step Growth Polymerization

It is associated with a repetitive condensation reaction between two bi-functional monomers. In these polycondensation reactions, some unpretentious molecules like water, alcohol, etc. are lost and high molecular mass condensation polymers are formed. Such as terylene, nylon-6,6, nylon-6, phenol-formaldehyde polymer, melamine-formaldehyde polymer, etc.

1. Radical Polymerization: A variety of unsaturated compounds such as alkanes or dienes or their derivatives are polymerized by free radical addition polymerization in the presence of a free radical generating initiator (catalyst) such as benzoyl peroxide, acetyl peroxide, tert-butyl peroxide, etc. This type of polymerization is performed by heating the monomer with a small amount of the initiator (usually peroxide) or by exposing the monomer to light.
2. Coordinative Polymerization: Coordination polymerization is a form of polymerization catalyzed by transition metal salts and complexes.
3. Cationic Polymerization: When the initiator is a cation, it binds to the double bond to form an ionic intermediate to which p is added in a process called cationic addition polymerization. It starts with acid. A commonly used acid for the catamite polymer sulfuric acid HSO, HF, BF is in the presence of a small amount of water.
4. Anionic Polymerization: This type of polymerization is initiated by ions, which can be bases or other nucleophiles. The main chain carries the nucleophile addition of ions to an unsaturated monomer. This proceeds through the formation of the carbanion. Butyl lithium or potassium amide K⁺NH₂^–.

Sample Questions

Question 1: What is the primary structural feature required to make a molecule useful in a condensed polymerization reaction?

Answer:

Monomers should be bi-functional that have two functional groups.

Question 2: What are cross-linked polymers? Give one such example.

Answer:

Polymers in which the linear polymer chains are cross-linked to form a three-dimensional network structure are called cross-linked polymers. A common example of this type of polymer is bakelite.

Question 3: What is the distinctness between two notations: nylon-6 and nylon-68?

Answer:

Nylon-6 is obtained from caprolactam which is obtained from cyclohexane. It has only one compared with 6-carbon atoms.

Nylon-66 refers to a polymer obtained from 6-carbon atoms of dicarboxylic acid (adipic acid) and 6 diamine (hexamethylene diamine).

Question 4: Name a polymer formed by step-growth polymerization. name its monomer.

Answer:

Nylon-66. Its monomers are:

• Adipic acid: HOOC (CH₂)₄ COOH
• Hexamethylenediamine: NH₂(CH₂)₆NH₂

Question 5: How do thermoplastic differ from thermosetting polymers?

Answer:

Thermoplastics soften on heating. These can be modulated to any desired size. Thermoplastics must be processed repeatedly. On the other hand, thermosetting plastics become hard and insoluble mass on heating. These cannot be modulated to the desired size and cannot be reprocessed.