Daily Olympiad: Chemistry - Polymers [20260606]

Solution

Correct: D

The polymer formed by the condensation reaction of terephthalic acid and ethylene glycol is Poly(ethylene terephthalate), commonly known as PET, Dacron, or Terylene. It is indeed a polyester, characterized by the ester linkages formed between the dicarboxylic acid and the diol. Polyesters are known for their high tensile strength and are widely used in fibers (textiles, tire cords) and bottles. However, condensation polymerization is defined by the formation of a polymer with the simultaneous loss of small molecules like water, methanol, or HCl. Therefore, it is a condensation polymerization, not an addition polymerization. Addition polymerization involves the direct addition of monomers without the loss of any small molecules.

Solution

Correct: B

The question describes a polymerization method that provides high stereochemical control (isotactic or syndiotactic) through coordinate anionic catalysts. This is a hallmark of Ziegler-Natta polymerization. While free radical polymerization often leads to atactic polymers due to lack of stereochemical control, Ziegler-Natta catalysts are crucial for producing stereoregular polymers like isotactic polypropylene from propene. Isotactic polypropylene has superior mechanical properties compared to its atactic counterpart, making it highly valuable.

Solution

Correct: C

Elastomers, like vulcanized natural rubber, are characterized by their ability to stretch and return to their original shape. This property arises from long, coiled polymer chains connected by weak intermolecular forces, allowing them to be stretched easily. The 'occasional cross-links' (introduced during vulcanization with sulfur) are crucial because they prevent the chains from slipping past each other permanently when stretched, ensuring that the material returns to its original shape once the stress is removed. Without cross-links, raw rubber would be plastic and deform permanently. PVC and Nylon 6,6 are strong, rigid plastics or fibers due to significant intermolecular forces (dipole-dipole, H-bonding) and/or high crystallinity. HDPE is a high-density, crystalline thermoplastic with limited elasticity compared to rubber.

Solution

Correct: B

The monomer H₂C=C(CH₃)-CH=CH₂ is 2-methyl-1,3-butadiene, commonly known as isoprene. The polymer formed from isoprene is polyisoprene. While natural rubber (cis-polyisoprene) is a polymer of isoprene, the question asks for the common name of the polymer formed FROM THIS MONOMER, implying a synthetic process leading to a rubber. Synthetic polyisoprene can be produced to mimic natural rubber. The options need careful evaluation. Neoprene is poly(chloroprene). Buna-N is a copolymer of 1,3-butadiene and acrylonitrile. Butyl Rubber is a copolymer of isobutylene and isoprene. Natural Rubber itself is a biopolymer with specific stereochemistry. Among the given choices, 'Polyisoprene (Natural Rubber)' is the polymer formed from isoprene. However, the question asks for the COMMON NAME OF THE POLYMER. The 'natural rubber' part implies the cis-1,4 configuration. If it's just 'polyisoprene', it's a general term. The polymerization of isoprene to form synthetic polyisoprene, particularly the cis-1,4 form that resembles natural rubber, typically utilizes Ziegler-Natta catalysts (coordination polymerization) for stereochemical control. If we strictly consider 'common name', 'Natural Rubber' is a common name for cis-polyisoprene. So, option B is the most accurate in terms of monomer and mechanism for achieving a specific, useful form of polyisoprene.

Solution

Correct: B

Bakelite is a thermosetting polymer formed by the condensation polymerization of phenol and formaldehyde. It is highly cross-linked and rigid. Melamine-formaldehyde resin is also a thermosetting polymer formed by the condensation polymerization of melamine and formaldehyde. Both are indeed thermosets and form cross-linked structures, meaning they cannot be remelted once formed. Option A is incorrect as both are thermosetting polymers, not thermoplastics. Option C is incorrect as they are condensation polymers, not addition polymers. Option D is incorrect; while both are heat-resistant, Bakelite generally has very high heat and chemical resistance and is known for its electrical insulating properties. Melamine resins are particularly known for being scratch-resistant and used in dinnerware. There isn't a general rule that Melamine has a 'higher degree of cross-linking' or is 'more resistant' overall, as properties depend on processing. Option B accurately states their respective monomers, which is the fundamental difference.

Solution

Correct: B

PHBV is a biodegradable copolymer. Its full name, Poly-β-hydroxybutyrate-co-β-hydroxyvalerate, indicates it's a copolymer of β-hydroxybutyric acid (or 3-hydroxybutanoic acid) and β-hydroxyvaleric acid (or 3-hydroxypentanoic acid). These monomers are typically produced by the fermentation of sugars by certain bacteria (e.g., Alcaligenes eutrophus), which then polymerize them. Thus, PHBV is a biopolymer synthesized by biological systems. Its biodegradability makes it useful for medical implants and packaging. Option A is incorrect as it's a copolymer. Option C describes PGA/PLA (polyglycolic acid/polylactic acid) copolymers. Option D is incorrect as it's biologically produced and degrades relatively quickly.

Solution

Correct: B

In free radical polymerization, termination typically occurs by two main mechanisms: combination (coupling) or disproportionation. For vinyl chloride (and many other vinyl monomers), termination by combination is often predominant, especially at lower temperatures. In combination, two growing polymer radicals react to form a single, longer polymer chain. Disproportionation involves one radical abstracting a hydrogen atom from another, resulting in one saturated and one unsaturated polymer chain. While both can occur, and the ratio depends on factors like monomer type and temperature, for many industrial processes forming PVC, combination is a significant pathway. Option C describes chain transfer to initiator, which is a type of chain transfer, not the primary termination step between two growing chains. Option D is incorrect as chain transfer agents are used to control molecular weight, not always for termination.

Solution

Correct: C

The polydispersity index (PDI) is defined as the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn), i.e., PDI = Mw / Mn. In this case, PDI = 150,000 g/mol / 50,000 g/mol = 3.0. A PDI value greater than 1.0 indicates that the polymer sample consists of chains of varying lengths, meaning it has a broad molecular weight distribution. A PDI of 1.0 would indicate a perfectly monodisperse polymer (all chains having the same length), which is rarely achieved in synthetic polymerization. Therefore, a PDI of 3.0 indicates a broad molecular weight distribution. The degree of polymerization is also variable, so option D is incorrect.

Solution

Correct: D

Thermoplastics (like polyethylene, polypropylene) are characterized by linear or branched polymer chains held together by relatively weak intermolecular forces, allowing them to soften upon heating and harden upon cooling, making them recyclable. Thermosetting plastics (like Bakelite, melamine) form extensive three-dimensional cross-linked networks during curing, which makes them rigid and prevents them from being remelted or reformed once hardened. Option D is incorrect. Thermosetting plastics are generally much stronger, more rigid, and more brittle than thermoplastics because of their extensive cross-linking, which restricts chain movement significantly. Thermoplastics vary in strength and rigidity; some are quite strong, but generally, thermosets excel in these properties, especially at high temperatures. The 'higher degree of crystallinity' refers more to specific thermoplastics and doesn't make them inherently stronger or more rigid than a highly cross-linked thermoset.

Solution

Correct: B

Nylon 6,6 is a polyamide, and its polymer chains contain repeating amide (-CO-NH-) linkages. These amide groups are capable of forming strong intermolecular hydrogen bonds between adjacent polymer chains. These extensive and strong hydrogen bonds lead to high cohesive forces, resulting in a highly ordered, crystalline structure and giving Nylon 6,6 its characteristic high tensile strength, stiffness, and high melting point. Polyethylene, on the other hand, is a hydrocarbon polymer with only nonpolar C-C and C-H bonds, primarily relying on weaker van der Waals forces for interchain attraction. While molecular weight and degree of polymerization (options A and D) play a role, the fundamental difference in intermolecular forces (option B) is the primary reason for the vast difference in mechanical properties between Nylon 6,6 and polyethylene. The type of polymerization (option C) doesn't directly explain the mechanical properties in this comparative context.

Solution

Correct: C

Vulcanization is the process of heating natural rubber (polyisoprene) with sulfur. During this process, sulfur atoms react with the double bonds in the polyisoprene chains, forming covalent sulfur bridges (-S-S-) or polysulfidic bridges (-Sₓ-) between different polymer chains. These cross-links restrict the movement of individual polymer chains, preventing them from slipping past each other permanently when stretched, which enhances the rubber's elasticity, tensile strength, and resistance to abrasion, temperature changes, and solvents. Options A, B, and D describe other types of forces or mechanisms not responsible for vulcanization's primary effect.

Solution

Correct: B

Buna-S is a synthetic rubber, also known as SBR (Styrene Butadiene Rubber). It is a copolymer formed by the free radical addition polymerization of 1,3-butadiene and styrene. Buna-N (Nitrile Rubber) is formed from 1,3-butadiene and acrylonitrile. Neoprene is poly(chloroprene). Ethylene and propylene form EPDM rubber or polyethylene/polypropylene. Thus, option B correctly identifies the monomers for Buna-S.

Solution

Correct: D

Copolymers are classified based on the arrangement of their monomer units. A random copolymer is one in which the monomer units are arranged in a completely arbitrary or statistical sequence along the polymer chain. For example, -A-A-B-A-B-B-B-A-. In a block copolymer, different homopolymer blocks are linked together (e.g., -A-A-A-A-B-B-B-B-). An alternating copolymer has a regular, alternating sequence (e.g., -A-B-A-B-A-B-). A graft copolymer has side chains of one type of monomer grafted onto a main chain of another type. The description 'randomly arranged monomer units A and B, where the sequence of A and B along the chain is statistical' precisely defines a random copolymer.

Solution

Correct: D

The distinct properties of HDPE and LDPE stem from their molecular structures and resulting degrees of crystallinity. LDPE is produced under high pressure and high temperature via free-radical polymerization, leading to a highly branched polymer structure. These branches prevent efficient packing of polymer chains, resulting in lower density, lower crystallinity, and thus greater flexibility and softness. HDPE, on the other hand, is produced using Ziegler-Natta catalysts or Phillips catalysts under low pressure and low temperature, which yields a predominantly linear polymer structure with very few branches. This linear structure allows for close packing of chains, leading to high density, high crystallinity, and consequently, greater rigidity, strength, and a higher melting point. Therefore, option D correctly describes the structural differences leading to property variations.

Solution

Correct: C

Polytetrafluoroethylene (PTFE), commonly known as Teflon, is uniquely suited for non-stick coatings. Its structure consists of a carbon backbone fully shielded by fluorine atoms, making the molecule extremely nonpolar and giving it exceptionally low surface energy. This results in an extremely low coefficient of friction (making it 'slippery') and outstanding chemical inertness (resistant to almost all chemicals), as well as high thermal stability. These properties are ideal for non-stick cookware. PVC is used in pipes and insulation but is not non-stick. PAN is used for acrylic fibers. PP is a common plastic for packaging and containers, but not for non-stick coatings.