Polymers

Polymer: High molecular mass substance formed by joining many small repeating units (monomers).

$n \cdot \text{Monomer} \xrightarrow{\text{Polymerization}} \text{Polymer}$

Example: $n \cdot CH_2=CH_2 \to -(CH_2-CH_2)_n-$ (polyethylene).

Terminology:

• Monomer: Repeating unit (small molecule).
• Polymer: Macromolecule (chain of monomers).
• Degree of polymerization (n): Number of repeating units.
• Polymer chain: $-(M)_n-$ where $M$ is the repeating unit.

Classification of Polymers:

A. Based on Source:

B. Based on Structure:

C. Based on Mode of Polymerization:

D. Based on Properties:

E. Based on Molecular Forces:

Types of Polymerization Mechanisms:

1. Addition Polymerization (chain growth):

• Free radical: peroxides initiate (most common)
• Cationic: with $H^+$ acids; gives high MW polymer
• Anionic: with organometallic bases (e.g., $RLi$)
• Coordination (Ziegler-Natta): $TiCl_4 + Al(C_2H_5)_3$ — gives stereoregular (isotactic) polymers

2. Condensation Polymerization (step growth):

• Two functional groups react and lose small molecule ($H_2O$, $HCl$, etc.).
• Examples: ester formation (polyester), amide formation (polyamide).

Homo- vs Co-polymers:

• Homopolymer: Only one monomer type. e.g., polyethylene ($CH_2=CH_2$).
• Copolymer: Two or more monomers. e.g., buna-S (butadiene + styrene), nylon (different diamine + diacid).

Addition (Chain Growth) Polymerization:

Mechanism has three steps:

1. Initiation: Active species (radical, cation, anion) generated.
2. Propagation: Active species adds successive monomers.
3. Termination: Two active chain ends combine or disproportionate.

1. Free Radical Polymerization:

Initiator: Benzoyl peroxide, $H_2O_2$, AIBN (azobisisobutyronitrile), $K_2S_2O_8$.

Mechanism for ethylene → polyethylene:

Initiation: $(C_6H_5COO)_2 \xrightarrow{\Delta} 2C_6H_5COO^\bullet \to 2C_6H_5^\bullet + 2CO_2$ $C_6H_5^\bullet + CH_2=CH_2 \to C_6H_5CH_2CH_2^\bullet$

Propagation: $C_6H_5(CH_2CH_2)_n^\bullet + CH_2=CH_2 \to C_6H_5(CH_2CH_2)_{n+1}^\bullet$ (continues...)

Termination:

• Coupling: $R^\bullet + R'^\bullet \to R-R'$
• Disproportionation: $R^\bullet + R'^\bullet \to R(-H) + R'(+H)$ (gives alkene + alkane)

Important Addition Polymers:

Vinyl Family = monomers with $CH_2=CH-X$ structure (X = H for PE; Cl for PVC; etc.).

2. Cationic Polymerization:

• Initiator: strong acid or Lewis acid ($H^+, BF_3, AlCl_3$).
• Favors electron-rich monomers (isobutylene, vinyl ethers).
• Example: isobutylene + $H^+$ → polyisobutylene (butyl rubber when copolymerized with isoprene).

3. Anionic Polymerization:

• Initiator: strong base or organometallic (RLi, $NaNH_2$).
• Favors electron-poor monomers (styrene, acrylonitrile).
• Living polymerization: chain ends remain active; can add more monomer later.

4. Coordination Polymerization (Ziegler-Natta):

• Catalyst: $TiCl_4 + Al(C_2H_5)_3$ (organometallic).
• Produces stereoregular polymers (isotactic, syndiotactic).
• Used for HDPE, isotactic polypropylene.

Stereoregularity in PP:

• Isotactic: All -CH₃ on same side; crystalline, high MP, hard.
• Syndiotactic: Alternating sides; semi-crystalline.
• Atactic: Random; amorphous, low MP, soft.
• Ziegler-Natta gives isotactic (commercially useful).

Co-polymers:

• Buna-S (SBR — Styrene-Butadiene Rubber): Styrene + 1,3-butadiene; used in tires, conveyor belts.
• Buna-N (NBR — Nitrile rubber): Acrylonitrile + 1,3-butadiene; oil-resistant; used in fuel hoses.
• ABS: Acrylonitrile + butadiene + styrene; tough plastic for housings (Lego pieces, helmets).

Condensation (Step Growth) Polymerization:

Monomers with two functional groups combine, losing a small molecule (water, methanol, HCl, etc.) per linkage.

Common reactions:

• Diamine + dicarboxylic acid → polyamide + water
• Diol + dicarboxylic acid → polyester + water
• Diol + diisocyanate → polyurethane (no loss)

1. Nylon-66:

Monomers: hexamethylenediamine ($H_2N(CH_2)_6NH_2$) + adipic acid ($HOOC(CH_2)_4COOH$).

$nH_2N(CH_2)_6NH_2 + nHOOC(CH_2)_4COOH \xrightarrow{\Delta, -H_2O} -[NH(CH_2)_6NHCO(CH_2)_4CO]_n-$

Properties: strong fibre; H-bonding between amide groups; high MP; tensile strength. Uses: fabrics, ropes, bristles, carpets, fishing nets.

2. Nylon-6:

Monomer: caprolactam (cyclic amide, $-CO-NH-(CH_2)_5-$).

$\text{Caprolactam} \xrightarrow{\Delta, \text{ring opening}} -[NH(CH_2)_5CO]_n-$

Properties similar to nylon-66; uses similar (tire cords).

3. Nylon-2-Nylon-6:

Alternating glycine ($H_2NCH_2COOH$) and aminocaproic acid units. Biodegradable because peptide-like bonds are hydrolyzed by enzymes.

4. Dacron (Terylene, PET — Polyethylene terephthalate):

Monomers: ethylene glycol ($HOCH_2CH_2OH$) + terephthalic acid ($HOOC-C_6H_4-COOH$).

$nHOOC-C_6H_4-COOH + nHOCH_2CH_2OH \to -[OC-C_6H_4-COOCH_2CH_2O]_n- + 2nH_2O$

Properties: strong, durable polyester. Uses: clothing (synthetic fibers), bottles (PET water bottles), films, magnetic tape.

5. Bakelite (Phenol-Formaldehyde):

Monomers: phenol ($C_6H_5OH$) + formaldehyde (HCHO).

Two stages: Stage 1 (Novolac): Acid-catalyzed; linear or slightly branched polymer. Stage 2 (Resol/Bakelite): Heated with more formaldehyde; cross-linking via methylene bridges; forms 3D network.

$-CH_2-C_6H_4-OH-CH_2-C_6H_4-OH-...$ with cross-linking.

Properties: hard, infusible, insoluble; thermosetting. Uses: electrical switches, telephone handsets, casings, billiard balls.

6. Melamine-Formaldehyde:

Monomers: melamine ($C_3N_3(NH_2)_3$) + formaldehyde. Forms hard, scratch-resistant; cross-linked. Uses: melamine dinnerware (unbreakable plates), Formica.

7. Urea-Formaldehyde:

Monomers: urea + formaldehyde. Uses: adhesives, particle board, plywood.

8. Glyptal (Polyester from Phthalic acid + Glycerol):

Cross-linked polymer; used in paints, coatings.

9. Polyurethanes:

Monomers: diisocyanate (-NCO-NCO) + diol. $R(NCO)_2 + HOCH_2CH_2OH \to -O-C(=O)NHR-NHC(=O)O-CH_2CH_2-$

No water loss (urethane group is -OCONH-). Used in foam (mattresses, car seats), spandex (elastic fabric), coatings.

Natural Polymers:

Natural Rubber:

• Monomer: Isoprene (2-methyl-1,3-butadiene), $CH_2=C(CH_3)-CH=CH_2$
• Polymer: cis-1,4-polyisoprene
• Structure: long, irregular chains; cis double bonds give the random coiled structure → elastic.

Source: latex from rubber tree (Hevea brasiliensis); coagulated with acid.

Properties of natural rubber:

• Soft, tacky in summer; hard in winter
• Low tensile strength
• Soluble in petroleum solvents
• Reacts with $O_2$, becomes brittle

Vulcanization:

Heating natural rubber with sulfur (~$5\%$ S) at $373$ K introduces S-S cross-links between polyisoprene chains.

Result:

• Increased tensile strength, hardness
• Reduces stickiness
• Resistant to wear
• Used in tires (more S → harder rubber for tires; less S → softer)

Synthetic Rubbers:

Biodegradable Polymers:

Conventional plastics persist in environment for centuries (causing pollution). Biodegradable polymers decompose via:

• Microbial action (enzymes break ester/amide bonds)
• UV light
• Hydrolysis

Examples:

Environmental Importance:

• Replace non-biodegradable plastics (PE, PP)
• Compost back to $CO_2 + H_2O$
• Sustainable materials

Molecular Mass of Polymers:

Two types:

• Number-average molecular mass ($\bar{M}_n$): $\bar{M}_n = \frac{\sum n_i M_i}{\sum n_i}$ where $n_i$ = number of molecules of mass $M_i$.
• Weight-average molecular mass ($\bar{M}_w$): $\bar{M}_w = \frac{\sum n_i M_i^2}{\sum n_i M_i}$.

$\bar{M}_w \geq \bar{M}_n$. Polydispersity Index (PDI) = $\bar{M}_w / \bar{M}_n$. PDI = 1 means uniform (monodisperse, all chains same length); > 1 means range of lengths.