Chapter 15: Introduction to Polymer Chemistry
Exhaustive Revision Guide - 50 Subjective Questions with Detailed Solutions
Click on any question to reveal its answer.
1. Define polymer and monomer.
Polymer: A polymer is a large macromolecule of high molecular mass formed by the repeated linking together of a large number of small, simple molecules.
Monomer: The small, simple molecules which repeatedly link together to form a polymer are called monomers.
2. What is polymerization? Give the equation for the formation of polythene.
Polymerization: The chemical process by which monomers are linked together to form a large polymer molecule is called polymerization.
Formation of Polythene:
$$n CH_2=CH_2 \xrightarrow{\text{Polymerization}} \ -[-CH_2-CH_2-]_n-$$
(Ethene monomer $\rightarrow$ Polythene polymer)
3. Classify polymers on the basis of their source with one example each.
- Natural Polymers: Obtained from nature (plants and animals). Example: Starch, Cellulose, Natural rubber, Proteins.
- Synthetic Polymers: Man-made polymers synthesized in the laboratory. Example: Polythene, Nylon, Bakelite.
- Semisynthetic Polymers: Derived from natural polymers by chemical modification. Example: Cellulose acetate (Rayon), Vulcanized rubber.
4. Classify polymers on the basis of their structure.
- Linear Polymers: Monomer units are linked together to form long, straight chains. They pack well and have high density and high melting points. Example: High-Density Polythene (HDPE), PVC.
- Branched Chain Polymers: Linear chains have side branches of different lengths. Due to branching, they do not pack closely, resulting in lower density. Example: Low-Density Polythene (LDPE).
- Cross-linked (Network) Polymers: Bifunctional and trifunctional monomers form a 3D network with strong covalent bonds (cross-links) between linear polymer chains. They are hard and rigid. Example: Bakelite, Melamine.
5. Classify polymers on the basis of intermolecular forces.
Based on intermolecular forces (which dictate their mechanical properties), polymers are classified into four types:
- Elastomers
- Fibers
- Thermoplastic Polymers
- Thermosetting Polymers
6. What are Elastomers? Why do they exhibit elasticity?
Elastomers: Polymers that have elastic character; they can be stretched by applying force and return to their original shape when the force is removed. Example: Natural rubber, Buna-S, Neoprene.
Reason for elasticity: The polymer chains are held together by the weakest intermolecular forces, allowing them to stretch. A few covalent 'cross-links' are introduced between the chains to help the polymer retract to its original position once the stretching force is released.
7. What are Fibers? State their characteristics.
Fibers: Polymers that form long, thin, thread-like structures which can be woven into fabrics. Example: Nylon 6,6, Terylene.
Characteristics: They possess high tensile strength and a high modulus. This is because their chains are held together by strong intermolecular forces like hydrogen bonding or strong dipole-dipole interactions. These strong forces lead to close packing of chains, giving them a crystalline nature.
8. Distinguish between Thermoplastic and Thermosetting polymers.
| Thermoplastic Polymers | Thermosetting Polymers |
|---|---|
| They soften on heating and become hard on cooling repeatedly. | They undergo a permanent change upon heating and become hard and infusible. |
| They can be reshaped, remolded, and recycled. | They cannot be reshaped, remolded, or recycled. |
| They consist of linear or slightly branched chains. | They possess extensive 3D cross-linked structures. |
| Example: Polythene, PVC, Polystyrene. | Example: Bakelite, Melamine-formaldehyde resin. |
9. What is Addition Polymerization (Chain Growth Polymerization)? Give an example.
Addition Polymerization: It is a process in which multiple molecules of the same or different monomers simply add together on a large scale to form a polymer, without the elimination of any byproduct molecules.
The monomers are usually unsaturated compounds (alkenes, alkadienes). The empirical formula of the polymer is the same as that of the monomer.
Example: Formation of PVC from vinyl chloride ($CH_2=CH-Cl$).
10. Briefly explain the three steps of the Free Radical Mechanism in addition polymerization.
Addition polymerization often proceeds via a free radical mechanism initiated by an initiator (like benzoyl peroxide). The steps are:
- Chain Initiation: The initiator decomposes to form free radicals ($R^\bullet$). The radical adds to the double bond of the monomer, creating a new, larger free radical.
- Chain Propagation: The new free radical attacks another monomer molecule, adding it to the chain and creating a reactive radical at the new end. This process repeats rapidly, growing the chain.
- Chain Termination: The growing chains stop when two free radicals combine with each other to form a stable, dead polymer molecule.
11. What is Condensation Polymerization (Step Growth Polymerization)? Give an example.
Condensation Polymerization: It is a process in which bi- or polyfunctional monomers react together with the elimination of small byproduct molecules like water ($H_2O$), alcohol ($ROH$), or ammonia ($NH_3$) to form a polymer.
The reaction proceeds in a step-wise manner, with each step forming a distinct functional group (like amide or ester linkages).
Example: Formation of Nylon 6,6 from adipic acid and hexamethylenediamine.
12. What are Homopolymers and Copolymers? Give one example of each.
- Homopolymer: A polymer formed from the polymerization of only a single type of monomer.
Example: Polythene (only ethene monomers). - Copolymer: A polymer formed from the polymerization of two or more different types of monomers.
Example: Buna-S (from 1,3-butadiene and styrene monomers).
13. What is Ring Opening Polymerization? Give an example.
Ring Opening Polymerization: Cyclic monomers (like cyclic amides or cyclic ethers) undergo ring-opening upon heating in the presence of a catalyst. The broken rings link together to form a linear polymer. It proceeds via chain growth but resembles condensation polymers chemically.
Example: Polymerization of $\epsilon$-caprolactam to form Nylon 6.
14. How is Low-Density Polythene (LDPE) prepared? Mention two properties.
Preparation: LDPE is prepared by the polymerization of ethene under extremely high pressure (1000 to 2000 atm) and a temperature of 350-570 K, in the presence of traces of oxygen or peroxide initiator. It follows a free-radical mechanism resulting in a highly branched structure.
Properties:
- It is highly flexible, tough, but chemically inert.
- It is a poor conductor of electricity.
Use: Packaging films, squeeze bottles, insulation for wires.
15. How is High-Density Polythene (HDPE) prepared? What catalyst is used?
Preparation: HDPE is prepared by the polymerization of ethene under low pressure (6 to 7 atm) and low temperature (333-343 K) in the presence of a specific catalyst. This forms linear chains that pack closely, giving high density.
Catalyst used: Ziegler-Natta catalyst (a mixture of titanium tetrachloride, $TiCl_4$, and triethyl aluminum, $(C_2H_5)_3Al$).
16. Distinguish between LDPE and HDPE.
| Property | LDPE (Low-Density Polythene) | HDPE (High-Density Polythene) |
|---|---|---|
| Structure | Highly branched chains. | Linear, unbranched chains. |
| Pressure used | High (1000-2000 atm). | Low (6-7 atm). |
| Catalyst | Traces of Oxygen/Peroxide. | Ziegler-Natta catalyst. |
| Physical nature | Soft, flexible, less tough. | Hard, stiff, high tensile strength. |
17. Describe the preparation of Teflon (Polytetrafluoroethylene).
Preparation: Teflon is prepared by heating tetrafluoroethene ($CF_2=CF_2$) gas under high pressure in the presence of a free radical catalyst like ammonium persulfate.
$$n CF_2=CF_2 \xrightarrow{\text{Polymerization, High Pressure}} \ -[-CF_2-CF_2-]_n-$$
Uses: It is chemically inert and resistant to corrosive reagents. It is used in making oil seals, gaskets, and famously for non-stick coating on cooking utensils.
18. Describe the preparation of Polyacrylonitrile (PAN). State its commercial name and use.
Preparation: PAN is an addition polymer obtained by the polymerization of acrylonitrile (vinyl cyanide, $CH_2=CH-CN$) in the presence of a peroxide catalyst.
$$n CH_2=CH(CN) \xrightarrow{\text{Peroxide}} \ -[-CH_2-CH(CN)-]_n-$$
Commercial names: Orlon or Acrilan.
Use: It is a synthetic fiber used as a substitute for wool in making blankets, sweaters, and artificial hair.
19. How is Polyvinyl Chloride (PVC) prepared? Mention its uses.
Preparation: PVC is prepared by the free-radical addition polymerization of vinyl chloride ($CH_2=CH-Cl$) in the presence of a peroxide catalyst.
$$n CH_2=CH(Cl) \xrightarrow{\text{Polymerization}} \ -[-CH_2-CH(Cl)-]_n-$$
Uses: It is widely used for making plumbing pipes, raincoats, hand-bags, water pipes, and as electrical insulation for wires.
20. What are Polyamides? Give two examples.
Polyamides: These are condensation polymers possessing repeated amide linkages ($-CO-NH-$) in their chain. They are commonly known as nylons and form very strong synthetic fibers.
Examples: Nylon 6,6 and Nylon 6.
21. Describe the preparation of Nylon 6,6. Write the chemical equation.
Preparation: It is a copolymer formed by the condensation polymerization of hexamethylenediamine (a 6-carbon diamine) and adipic acid (a 6-carbon dicarboxylic acid) under high pressure and temperature (553 K), with the elimination of water molecules.
$$n HOOC-(CH_2)_4-COOH + n H_2N-(CH_2)_6-NH_2 \xrightarrow{553K, \ -nH_2O} \ -[-OC-(CH_2)_4-CO-NH-(CH_2)_6-NH-]_n-$$
Nomenclature: The "6,6" indicates that both monomers contain 6 carbon atoms.
22. Describe the preparation of Nylon 6. Write the monomer used.
Monomer: $\epsilon$-Caprolactam (a 7-membered cyclic amide containing 6 carbon atoms).
Preparation: When $\epsilon$-caprolactam is heated with water at a high temperature (533-543 K), it undergoes ring-opening followed by polymerization to form the polyamide Nylon 6.
$$\text{Ring Opening} \rightarrow \ -[-NH-(CH_2)_5-CO-]_n-$$
It is called "6" because the monomer contains 6 carbon atoms.
23. What are Polyesters? Give an example.
Polyesters: These are condensation polymers that contain repeated ester linkages ($-COO-$) in their main chain. They are formed by the polycondensation of dicarboxylic acids and diols.
Example: Terylene (also known as Dacron).
24. Describe the preparation of Terylene (Dacron). Name the monomers used.
Monomers: Ethylene glycol (Ethane-1,2-diol) and Terephthalic acid (Benzene-1,4-dicarboxylic acid).
Preparation: It is manufactured by heating a mixture of the two monomers at 420-460 K in the presence of a catalyst (zinc acetate-antimony trioxide) through condensation polymerization, releasing water.
$$n HO-CH_2-CH_2-OH + n HOOC-C_6H_4-COOH \rightarrow \ -[-O-CH_2-CH_2-O-OC-C_6H_4-CO-]_n- + nH_2O$$
25. Mention the properties and uses of Terylene (Dacron).
Properties: It is a strong, highly crease-resistant fiber. It absorbs very little water and dries quickly. It is resistant to chemicals and biological degradation.
Uses: Used heavily in the textile industry for making crease-resistant fabrics (often blended with cotton as terrycot or wool as terrywool). Also used for making safety helmets, magnetic recording tapes, and sails for boats.
26. What are Phenol-Formaldehyde polymers? Name the most important one.
These are the oldest synthetic polymers. They are formed by the condensation reaction of phenol with formaldehyde in the presence of either an acidic or basic catalyst. The initial products are linear polymers (Novolac), which upon further heating with formaldehyde form highly cross-linked thermosetting plastics.
The most important and well-known heavily cross-linked phenol-formaldehyde polymer is Bakelite.
27. Describe the preparation of Bakelite.
Step 1: Phenol and Formaldehyde ($HCHO$) react to form o-hydroxymethylphenol and p-hydroxymethylphenol.
Step 2: These monomers undergo condensation to form a linear polymer called Novolac (used in paints).
Step 3: When Novolac is heated with more formaldehyde (using hexamethylenetetramine as a hardening agent), it undergoes extensive cross-linking between the linear chains to form an infusible, hard, solid 3D network called Bakelite.
28. Mention the properties and uses of Bakelite.
Properties: It is a thermosetting polymer. It is hard, infusible, scratch-resistant, and an excellent electrical insulator. It cannot be remolded.
Uses: Used for making electrical switches, plugs, handles of various pressure cookers and frying pans (due to heat resistance), and phonograph records.
29. How is Melamine-formaldehyde polymer prepared? State its use.
Preparation: It is a condensation polymer formed by the reaction of melamine (a heterocyclic triamine) with formaldehyde.
They first form a resin intermediate, which on further condensation heating forms a hard, heavily cross-linked thermosetting polymer.
Use: It is highly durable and used primarily in the manufacture of unbreakable crockery (plates, cups) and decorative laminates.
30. What is Natural Rubber chemically? Write its monomer.
Chemically, natural rubber is a linear polymer called cis-1,4-polyisoprene.
The monomer used is Isoprene (2-methyl-1,3-butadiene).
Due to the cis configuration at all double bonds, the chains do not pack closely. They coil like springs, which gives natural rubber its elastic property.
31. What are the drawbacks of raw natural rubber?
- It becomes soft and sticky at high temperatures (>335 K) and brittle at low temperatures (< 283 K).
- It has low tensile strength and high water absorption capacity.
- It is soluble in non-polar solvents and easily attacked by oxidizing agents.
32. What is vulcanization of rubber? Why is it done?
Vulcanization: It is the process of heating raw natural rubber with Sulfur (3-5%) and an appropriate additive at a temperature range between 373 K to 415 K.
Reason: During heating, sulfur forms cross-links (disulfide bonds) at the reactive sites of the double bonds. These cross-links stiffen the polymer, removing its stickiness, increasing its tensile strength, elasticity, and resistance to temperature and solvents.
33. How is the hardness of vulcanized rubber controlled?
The physical properties of the rubber depend entirely on the amount of sulfur added during vulcanization. More sulfur leads to more cross-linking, making the rubber harder.
- ~5% sulfur: Used for making standard vehicle tires.
- ~30% sulfur: Forms a very hard, inflexible material called ebonite (used in battery cases).
34. What is Synthetic Rubber? Name two synthetic rubbers.
Synthetic rubbers are any vulcanizable artificial polymers capable of getting stretched to twice their length and returning rapidly to their original shape when released.
They are usually homopolymers of 1,3-butadiene derivatives or copolymers of 1,3-butadiene with another unsaturated monomer.
Examples: Neoprene, Buna-S (SBR), Buna-N.
35. Describe the preparation and monomers of Buna-S (SBR).
Buna-S (Styrene-Butadiene Rubber) is a copolymer.
Monomers: 1,3-Butadiene (approx 75%) and Styrene (approx 25%).
Preparation: It is prepared by the free-radical copolymerization of the two monomers in the presence of sodium (Na) as a catalyst (hence the name 'Bu' for butadiene, 'na' for sodium, 'S' for styrene). It is widely used for automobile tires.
36. Describe the preparation and monomers of Buna-N. Where is it used?
Buna-N (Nitrile rubber) is a copolymer.
Monomers: 1,3-Butadiene and Acrylonitrile ($CH_2=CH-CN$).
Preparation: It is obtained by the copolymerization of these monomers in the presence of a peroxide catalyst.
Use: Because of the polar $-CN$ groups, it is highly resistant to the action of petrol, lubricating oils, and organic solvents. It is used in making oil seals and tank linings.
37. Describe the preparation of Neoprene. Write its monomer.
Monomer: Chloroprene (2-chloro-1,3-butadiene).
Preparation: Neoprene is a synthetic homopolymer formed by the free-radical addition polymerization of chloroprene.
It is exceptionally resistant to vegetable/mineral oils and highly resistant to oxidation. Used for manufacturing conveyor belts, gaskets, and hoses.
38. What are Biodegradable Polymers? Why is there a need to develop them?
Biodegradable Polymers: These are polymers that can be broken down or degraded into simple, harmless molecules (like $H_2O, CO_2$) by the action of microorganisms (bacteria, fungi) over a period of time.
Need: Most synthetic plastics are non-biodegradable, meaning they persist in the environment for hundreds of years, causing severe soil and water pollution. Biodegradable polymers are developed as an eco-friendly alternative to mitigate this global environmental crisis.
39. Name a biodegradable aliphatic polyester. Name the monomers used to prepare it.
Polymer: PHBV (Poly-$\beta$-hydroxybutyrate-co-$\beta$-hydroxyvalerate).
Monomers: It is a copolymer prepared from two hydroxy acids:
- 3-Hydroxybutanoic acid
- 3-Hydroxypentanoic acid
40. Name a biodegradable polyamide. Write the monomers used to prepare it.
Polymer: Nylon-2-nylon-6.
Monomers: It is an alternating copolymer of an amino acid and a synthetic amino acid:
- Glycine ($H_2N-CH_2-COOH$, containing 2 carbons)
- Aminocaproic acid ($H_2N-(CH_2)_5-COOH$, containing 6 carbons)
41. How is the molecular mass of a polymer expressed? Define Number Average Molecular Mass ($\bar{M_n}$).
Polymers consist of chains of varying lengths, so their molecular mass is always expressed as an average rather than a single exact number.
Number Average Molecular Mass ($\bar{M_n}$): It is the total mass of the polymer sample divided by the total number of polymer molecules present in the sample. It is determined by methods based on colligative properties (like osmotic pressure).
$$\bar{M_n} = \frac{\sum N_i M_i}{\sum N_i}$$
42. Define Weight Average Molecular Mass ($\bar{M_w}$).
Weight Average Molecular Mass ($\bar{M_w}$): It is the average mass calculated by taking into account the weight fraction of molecules of each mass. It is sensitive to the presence of heavier molecules in the mixture. It is determined by light scattering or ultracentrifugation techniques.
$$\bar{M_w} = \frac{\sum N_i M_i^2}{\sum N_i M_i}$$
43. What is Poly Dispersity Index (PDI)? What is its value for natural and synthetic polymers?
PDI is the ratio of weight average molecular mass to number average molecular mass.
$$\text{PDI} = \frac{\bar{M_w}}{\bar{M_n}}$$
- For natural polymers (like proteins): Chains are of identical length, so $\bar{M_w} = \bar{M_n}$. Hence, PDI = 1 (Monodisperse).
- For synthetic polymers: Chains are of variable length. $\bar{M_w}$ is always greater than $\bar{M_n}$. Hence, PDI > 1 (Polydisperse).
44. Name the polymer used for making non-breakable plastic crockery.
Melamine-formaldehyde polymer (a cross-linked thermosetting resin) is widely used for making non-breakable plastic crockery.
45. What is the monomer of Polystyrene? Give one use of Polystyrene.
Monomer: Styrene (Vinylbenzene, $C_6H_5-CH=CH_2$).
Use: It is a thermoplastic polymer used extensively as an insulator, wrapping material, and for manufacturing toys, radio and television cabinets.
46. Distinguish between Natural rubber and Vulcanized rubber.
| Natural Rubber | Vulcanized Rubber |
|---|---|
| Soft and sticky. | Hard and non-sticky. |
| Low tensile strength. | High tensile strength. |
| High water absorption. | Low water absorption. |
| No cross-links between chains. | Sulfur cross-links present between chains. |
47. What are plasticizers? Why are they added to polymers?
Plasticizers are chemical substances (usually high boiling liquids) added to certain rigid plastics (like PVC) to decrease their strong intermolecular forces.
Reason: Adding them makes the polymer softer, more flexible, and easier to mold or extrude at lower temperatures without cracking.
48. What is Rayon? How is it made?
Rayon (Viscose Rayon) is a semisynthetic polymer. It is a regenerated cellulose fiber.
It is made by chemically treating natural wood pulp (cellulose) with sodium hydroxide and carbon disulfide to form a thick, viscous liquid called viscose. This liquid is forced through a spinneret into an acid bath, which regenerates the cellulose in the form of fine, continuous, shiny threads (artificial silk).
49. Give two important uses of biodegradable polymers.
- Medical field: Used for surgical sutures (stitches) that dissolve in the body over time, eliminating the need for a second surgery to remove them. Also used for orthopedic devices.
- Drug delivery: Used in controlled-release drug packaging, where a drug capsule slowly degrades in the body, releasing the drug steadily over a required period.
50. Identify the monomer(s) of Natural Rubber and Bakelite.
- Natural Rubber: The monomer is Isoprene (2-methyl-1,3-butadiene).
- Bakelite: The monomers are Phenol and Formaldehyde (Methanal).
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