Polymers and Aromatic Substitution Reactions Flashcards

Comprehensive flashcards covering biodegradable polymers, high-performance polymers, synthetic and natural fibers, polymer synthesis mechanisms, structure-activity relationships, and electrophilic/nucleophilic aromatic substitution reactions.

What defines a biodegradable polymer?

A polymer that decomposes under aerobic or anaerobic conditions through the action of microorganisms or enzymes into natural, biologically acceptable by-products such as gases ($CO_2$, $N_2$), water, biomass, and inorganic salts.

How do bulk erosion and surface erosion differ in biodegradable polymers?

In bulk erosion, degradation occurs throughout the whole sample because water ingress is faster than the rate of degradation (e.g., PLA, PGA). In surface erosion, mass loss from the surface is faster than water ingress into the bulk (e.g., polyanhydrides, polyorthoesters).

Which catalyst is most commonly used as an initiator for the ring-opening polymerization of glycolide and is approved by the FDA?

Stannous octoate (approved by the FDA as a food stabilizer).

Which monomers undergo copolymerization to form Nylon 2–Nylon 6?

Glycine ($H_2N-CH_2-COOH$) and aminocaproic acid ($H_2N-(CH_2)_5-COOH$).

What are the chemical constituents and key properties of Poly $\beta$-hydroxybutyrate-co-$\beta$-hydroxy valerate (PHBV)?

It is derived by combining 3-hydroxybutanoic acid and 3-hydroxypentanoic acid linked by ester bonds; it is brittle in nature and decomposes into carbon dioxide and water.

How do High Performance Polymers (HPPs) differ structurally and commercially from conventional commodity polymers?

HPPs feature rigid backbones containing aromatic rings, sulfone, amide, imide, or fluorine ($-CF_2-$) groups. Commercially, HPPs are technically driven, application-oriented, and produced in small batches, whereas commodity polymers are volume-driven, price-oriented, and produced on a large scale.

What monomers are used to synthesize Kevlar, and by what mechanism?

1,4-phenylene-diamine and terephthaloyl chloride via condensation polymerization.

What is Poly methyl methacrylate (PMMA) commonly known as, and what are its key applications?

Also known as orlon, lucite, or plexiglass, it is an optically clear, lightweight thermoplastic used in aquariums, hard contact lenses, paints, emulsions, and medical biocompatible devices.

Who synthesized the first synthetic polymer fiber, nylon, and in what year?

Wallace Carothers in $1938$.

What steps occur during electrospinning to produce ultrafine polymer fibers?

A high voltage is applied to a polymer solution in a syringe, deforming a pendant drop into a conical droplet (Taylor cone) and ejecting a polymer jet. The electric field stretches the jet into a whipping motion, evaporating the solvent and depositing ultrafine fibers onto a collector.

What is meant by the functionality of a monomer?

The number of reactive or bonding sites present on a monomer. A functionality of 2 forms linear polymers, while a functionality of 3 forms a 3D network polymer (e.g., Bakelite).

What are the key differences between addition polymerization and condensation polymerization?

Addition polymerization requires double bonds, forms no by-products, yields homo-chain thermoplastics, and chain growth occurs at one active center. Condensation polymerization requires two reactive functional groups per monomer, eliminates small by-product molecules (like $H_2O$, $NH_3$, $HCl$), and yields hetero-chain thermoplastics or thermosets.

How does tacticity affect polymer chain packing and physical strength?

Isotactic (side groups on same side) and syndiotactic (side groups on alternating sides) chains are symmetric and pack closely into strong crystalline regions. Atactic chains (randomly oriented side groups) are asymmetric and cannot pack closely, forming weaker, highly flexible amorphous polymers.

How is glass transition temperature ($T_g$) defined?

The temperature below which an amorphous polymer is hard, brittle, and glassy, and above which it becomes soft, flexible, and rubbery.

What components make up Ziegler-Natta catalysts used in coordination polymerization?

A combination of a transition metal halide (such as $TiCl_2$, $TiCl_3$, or $ZrBr_3$) and an organometallic compound (such as triethylaluminum or trimethylaluminum).

Why do conventional polymers act as electrical insulators?

Electrons are tightly bound in covalent bonds along the long chain and side groups, creating a high energy band gap ($E_g$) between localized valence electronic states and the conduction band.

How do polar and non-polar plastics differ in their response to an electric field and alternating current frequency?

Polar plastics (e.g., PMMA, PVC) contain permanent dipoles that rotate to align with the field (dipole polarization), causing their dielectric constant to depend on frequency, temperature, and moisture. Non-polar plastics (e.g., PTFE, PE, PP) undergo only instantaneous electronic polarization, giving them low dielectric constants independent of frequency.

What effect does absorbed water have on the electrical properties of polar polymers?

It raises the overall dielectric constant, lowers electrical resistivity due to higher conductivity, and increases chain mobility through plasticization.

What formula defines Young's Modulus ($E$) on a stress-strain curve?

$E = \frac{\sigma}{\varepsilon}$, where $\sigma$ is stress ($\text{force per unit area}$) and $\varepsilon$ is strain ($\text{elongation per initial length}$).

Why do elastomers exhibit reversible elastic recovery upon stretching?

When unstressed, the randomly coiled polymer chains between loose cross-links disentangle and straighten under load. When the force is removed, cross-links prevent permanent molecular slippage and pull the chains back into their original coiled conformation.

What is the rate-determining step in an electrophilic aromatic substitution (EAS) reaction?

The slow reaction of the electrophile ($E^+$) with a pair of pi electrons from the aromatic ring to form a non-aromatic carbocation intermediate called an arenium ion (sigma complex).

Why are halogen substituents weakly deactivating yet ortho/para directing in EAS?

Halogens withdraw electrons inductively (which dominates the overall rate, making them deactivating), but their lone pairs donate electron density by resonance, which specifically stabilizes the carbocation intermediate when attack occurs at the ortho and para positions.

Why is an amino group ($-NH_2$) on an aromatic ring protected as an acetanilide ($-NHCOCH_3$) before nitration or halogenation?

In strongly acidic reaction mixtures, $-NH_2$ protonates to form $-NH_3^+$, a strongly deactivating meta-director, or reacts uncontrollably with Lewis acids. Protecting it as an acetanilide reduces nitrogen basicity while retaining its activating and ortho/para-directing nature.

What are three major limitations of Friedel-Crafts alkylation reactions?

1) Polyalkylation occurs because the alkylated product is more reactive than the starting material; 2) Carbocation rearrangements can take place; 3) The reaction fails on moderately or strongly deactivated aromatic rings.

Why does Friedel-Crafts acylation avoid the polyacylation and rearrangement drawbacks of Friedel-Crafts alkylation?

The introduced acyl group is electron-withdrawing and deactivates the product ring against further substitution, while the acylium ion ($R-C \equiv O^+$) electrophile is resonance-stabilized and does not rearrange.

Why does electrophilic substitution on naphthalene occur preferentially at position 1 over position 2?

Attack at position 1 forms an arenium ion with 7 resonance structures (including two that preserve an intact aromatic benzene ring), whereas attack at position 2 yields an arenium ion with only 6 resonance structures.

What is the leaving group reactivity order in nucleophilic aromatic substitution via addition-elimination, and why?

$F > Cl > Br > I$; because forming the carbanion (Meisenheimer complex) in the addition step is rate-determining, the most electronegative atom (fluorine) stabilizes the developing negative charge most effectively via induction.

What reactive intermediate is generated during nucleophilic aromatic substitution via elimination-addition using strong bases like $NaNH_2$ in liquid $NH_3$?

Benzyne, a highly strained six-membered ring intermediate containing a formal triple bond formed by in-plane overlap of two $sp^2$ hybrid orbitals.