Chapter 15: Introduction to Polymer Chemistry
Highly Detailed Mark-wise Solutions for Board Exam Preparation
1 Mark Questions (Very Short Answer)
Answer: The monomer of natural rubber is Isoprene, and its IUPAC name is 2-methyl-1,3-butadiene.
Answer: Teflon (Polytetrafluoroethylene or PTFE).
Answer: PHBV (Poly $\beta$-hydroxybutyrate - co - $\beta$-hydroxyvalerate) or Nylon-2-nylon-6.
Answer: Elastomers are polymers that exhibit rubber-like elastic properties due to very weak intermolecular forces of attraction. Example: Buna-S, Neoprene.
2 Mark Questions (Short Answer-I)
Answer:
| Thermoplastic Polymers | Thermosetting Polymers |
|---|---|
| 1. These polymers soften on heating and harden on cooling reversibly. | 1. These polymers undergo permanent chemical change (cross-linking) on heating and become hard and infusible. |
| 2. They can be remolded, reshaped, and recycled multiple times. | 2. They cannot be remolded or reused once set in a mould. |
| 3. Structurally, they are mostly linear or slightly branched chain molecules. | 3. Structurally, they are heavily branched and have a strong 3D cross-linked network. |
| 4. Examples: Polythene, PVC, Polystyrene. | 4. Examples: Bakelite, Melamine. |
Answer:
Vulcanization: The process of heating natural rubber with sulfur (about 3-5%) and an appropriate additive at a temperature range of 373 K to 415 K is called vulcanization. During this process, sulfur forms rigid 3D cross-links at the reactive sites of double bonds in the polyisoprene chains.
Purpose: Natural rubber is very soft, sticky at high temperatures, brittle at low temperatures, and has low tensile strength. Vulcanization restricts the movement of polymer chains past each other, which improves its physical properties—making it harder, non-sticky, highly elastic, and resistant to wear and tear.
Answer:
Nylon-6,6 is a copolymer formed from two different bi-functional monomers, both containing 6 carbon atoms.
- Hexamethylenediamine:
$H_2N-CH_2-CH_2-CH_2-CH_2-CH_2-CH_2-NH_2$ or $H_2N-(CH_2)_6-NH_2$ - Adipic acid (Hexanedioic acid):
$HOOC-CH_2-CH_2-CH_2-CH_2-COOH$ or $HOOC-(CH_2)_4-COOH$
3 Mark Questions (Short Answer-II / Preparation)
Answer:
| Addition Polymers | Condensation Polymers |
|---|---|
| 1. Formation: Formed by the repeated addition of monomer molecules possessing double or triple bonds. | 1. Formation: Formed by repeated condensation reaction between two bi-functional or tri-functional monomers. |
| 2. Byproducts: No elimination of simple molecules (byproducts) occurs during polymerization. | 2. Byproducts: Elimination of small molecules like $H_2O, NH_3$, or $HCl$ occurs during the reaction. |
| 3. Monomers: Usually involves a single type of unsaturated monomer (homopolymerization). | 3. Monomers: Involves two different types of monomers with distinct functional groups (copolymerization). |
| 4. Molecular Mass: The molecular mass of the polymer is an exact integral multiple of the monomer. | 4. Molecular Mass: The molecular mass of the polymer is not an exact integral multiple of the monomers due to loss of byproducts. |
| 5. Examples: Polythene, PVC, Teflon. | 5. Examples: Nylon-6,6, Terylene, Bakelite. |
Answer:
Preparation:
Terylene is a polyester formed by the condensation polymerization of Ethylene glycol (ethane-1,2-diol) and Terephthalic acid (benzene-1,4-dicarboxylic acid) at 420-460 K in the presence of a catalyst (zinc acetate-antimony trioxide). Water molecules are eliminated during the process.
$n \text{ } HO-CH_2-CH_2-OH + n \text{ } HOOC-C_6H_4-COOH \xrightarrow{420-460 \text{ K}}$
$[-O-CH_2-CH_2-O-CO-C_6H_4-CO-]_n + 2n \text{ } H_2O$
(Terylene / Dacron)
Uses:
- Used for making crease-resistant fabrics by blending with cotton (terycot) and wool (terywool).
- Used in the manufacture of magnetic recording tapes and safety helmets.
4 Mark Questions (Long Answer / Combined)
Note: In the HSC Board Exam, 4-mark questions in Chemistry are frequently split into sub-questions (e.g., a 2-mark preparation combined with a 2-mark classification question).
(b) What are biodegradable polymers? Give one example with the names of its monomers. [2 Marks] March 2016, March 2020, March 2023
Answer (a): HDPE and Ziegler-Natta Catalyst
Ziegler-Natta Catalyst: It is a mixture of Titanium tetrachloride ($TiCl_4$) and Triethylaluminium ($(C_2H_5)_3Al$). It is highly effective for coordination polymerization.
Preparation of HDPE: High-Density Polythene is obtained by heating ethene at a relatively low temperature (333-343 K) and under a low pressure (6-7 atm) in the presence of the Ziegler-Natta catalyst in a hydrocarbon solvent.
$n \text{ } CH_2=CH_2 \xrightarrow{TiCl_4 + (C_2H_5)_3Al, \text{ 6-7 atm}} [-CH_2-CH_2-]_n$
This forms a highly linear polymer with very close packing, resulting in high density and high tensile strength.
Answer (b): Biodegradable Polymers
Definition: Polymers that can be decomposed or degraded into simpler, environmentally harmless substances by the action of microorganisms (bacteria/fungi) present in the environment are called biodegradable polymers.
Example: PHBV (Poly $\beta$-hydroxybutyrate - co - $\beta$-hydroxyvalerate).
Monomers of PHBV:
- 3-Hydroxybutanoic acid
- 3-Hydroxypentanoic acid
(b) What are Fibres? Give two examples. [2 Marks] Oct 2013, March 2019
Answer (a): Preparation of Bakelite
Bakelite is a thermosetting, cross-linked polymer. It is prepared from Phenol and Formaldehyde ($HCHO$).
- Phenol and formaldehyde condense in the presence of an acid or base catalyst to initially form a linear, soluble polymer called Novolac (used in paints).
- When Novolac is further heated with hexamethylenetetramine (which supplies $-\text{CH}_2-$ bridges), it undergoes extensive cross-linking between the linear chains.
- This creates a highly cross-linked, rigid, and infusible 3D solid network known as Bakelite.
Answer (b): Fibres
Definition: Fibres are a class of polymers which consist of thread-forming solids possessing high tensile strength and a high modulus. In these polymers, the long linear chains are held together by the strongest intermolecular forces, such as hydrogen bonding or strong dipole-dipole interactions. These strong forces lead to close packing of chains, giving them crystalline nature.
Examples: Nylon-6,6; Terylene (Dacron); Silk.
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