Polymerization mechanisms — free-radical, cationic, anionic, coordination (Ziegler-Natta), step-growth

4.6.4 · Chemistry › Polymers

1. The Big Picture (Pehle forecast!)

Feature	Chain-growth	Step-growth
Reactive species	radical / ion / metal-C bond	functional groups (–OH, –COOH, –NH₂)
Monomer	zyaadatar C=C hota hai	zyaadatar do functional groups hote hain
Growth	active chain mein ek monomer add hota hai	koi bhi do oligomers combine karte hain
High MW kab milti hai	early	sirf bahut high conversion par
Byproduct	koi nahi (addition)	often chhota molecule (H₂O, HCl) — condensation

2. Chain-Growth: teen stages

TEEN STAGES KYUN? Kyunki ek active center ko janam lena hota hai (initiation), khilaya jaana hota hai (propagation), aur maara jaana hota hai (termination). Har ek alag chemical step hai apni khud ki rate ke saath.

2a. Free-radical polymerization

YEH KAISE HOTA HAI (e.g. ethene/styrene ke saath benzoyl peroxide ya AIBN):

Initiation: initiator homolytically split hota hai. $I-I Δ/ h ν 2 I^{∙} I^{∙} + CH_{2} = CHX \to I-CH_{2} - C ∙ HX$ Yeh step kyun? Unpaired electron π-bond ka ek electron pakad leta hai, naya radical agli carbon par ban jaata hai.

Propagation: chain radical baar baar monomer add karta hai. $\sim CH_{2} - C ∙ HX + CH_{2} = CHX \to\sim CH_{2} - CHX - CH_{2} - C ∙ HX$

Termination: do radicals milte hain → combination (join) ya disproportionation (ek H pakad leta hai, ek saturated + ek C=C end banta hai).

2b. Cationic polymerization

Initiator: Lewis/Brønsted acid (e.g. $BF_{3} + H_{2} O$ ). $BF_{3} + H_{2} O \to H^{+} [BF_{3} OH]^{-} + M H-CH_{2} - C^{+} (CH_{3})_{2}$ ELECTRON-DONATING GROUPS KYUN? Yeh electron density ko cation ki taraf push karte hain, isko stabilise karte hain (Markovnikov-jaise). Electron-withdrawing groups ke saath cation bahut unstable ho jaata.

2c. Anionic polymerization — "living" polymers

Initiator: strong base / nucleophile (e.g. $n$ -BuLi, $Na/naphthalene$ ). $Bu^{-} + CH_{2} = CHX \to Bu-CH_{2} - C^{-} HX$

2d. Coordination — Ziegler-Natta

KAISE HOTA HAI (insertion mechanism):

Monomer ka C=C vacant Ti site par coordinate karta hai.
Ti par chain monomer par migrate karti hai (1,2-insertion).
Vacant site dobara khulti hai → agla monomer insert hota hai.

YEH KHAAS KYUN HAI: metal ki geometry har monomer ko same tarike se add karti hai → stereoregular (isotactic) polymer → HDPE (linear, high-density, strong) branched LDPE ki jagah. Yeh low pressure/temperature par kaam karta hai.

3. Step-Growth Polymerization

Example: nylon-6,6 hexamethylenediamine + adipic acid se: $H_{2} N(CH_{2})_{6} NH_{2} + HOOC(CH_{2})_{4} COOH \to [- NH(CH_{2})_{6} NHCO(CH_{2})_{4} CO -]_{n} + H_{2} O$

Formula Carothers equation (derive karo!)

Maano $N_{0}$ = initial number of monomer molecules, $N$ = kuch bonds banne ke baad bache hue molecules ki count. Har bond banana molecule count ko 1 se reduce karta hai, aur 2 functional groups consume karta hai. Extent of reaction $p$ define karo = reacted functional groups ka fraction. Reacted groups $= p N_{0}$ ; har bond 2 groups use karta hai, isliye bonds formed $= p N_{0} /2$ . Toh $N = N_{0} - p N_{0} /2$ . Number-average degree of polymerization: $\overset{ˉ}{X}_{n} = \frac{N _{0}}{N} = \frac{N _{0}}{N _{0} - p N _{0} /2} = \frac{1}{1 - p /2}$ Standard stoichiometric, two-different-monomer counting ke liye (per repeat-relevant convention) yeh likha jaata hai: $\overset{ˉ}{X}_{n} = \frac{1}{1 - p}$ Do forms kyun? $1/ (1 - p)$ form structural units count karta hai jab dono functional groups network of bonds mein equally belong karte hain; yeh standard textbook result hai. Key insight same hai: $p$ ko large $\overset{ˉ}{X}_{n}$ ke liye 1 ke bahut paas chahiye.

Figure — Polymerization mechanisms — free-radical, cationic, anionic, coordination (Ziegler-Natta), step-growth

4. Kaunsa mechanism hai — quick logic

5. Flashcards

Chain-growth polymerization ke teen stages

Initiation, Propagation, Termination

Free-radical termination ke modes

Combination aur disproportionation

Cationic polymerization mein kaunse substituents wale monomers prefer hote hain

Electron-donating groups (carbocation ko stabilise karte hain)

Anionic polymerization mein kaunse substituents wale monomers prefer hote hain

Electron-withdrawing groups (carbanion ko stabilise karte hain)

Anionic polymerization ko "living" kyun kehte hain?

Do carbanion ends repel karte hain, isliye koi natural termination nahi; chain active rehti hai aur narrow MW distribution deti hai

Ziegler-Natta catalyst ki composition

Transition-metal halide (TiCl₄) + organoaluminium (Al(C₂H₅)₃)

Ziegler-Natta kya deta hai jo free-radical PE nahi de sakta?

Linear, stereoregular (isotactic) HDPE

Carothers equation

X̄ₙ = 1/(1−p), p = extent of reaction

p = 0.99 par step-growth ka degree of polymerization

100

Step-growth ko high MW ke liye bahut high conversion kyun chahiye?

Kyunki X̄ₙ = 1/(1−p) sirf p → 1 par diverge karta hai

Free-radical mein kinetic chain length ν

ν = k_p[M]/√(2k_t R_i)

Zyaada initiator ka free-radical chain length par effect

Chhoti chains (zyaada chains monomer share karti hain)

Woh mechanism jisme koi small-molecule byproduct nahi hota

Chain-growth (addition)

Nylon-6,6 ka byproduct

Water (condensation)

Recall Feynman: ek 12-saal ke bachche ko explain karo

Socho ek lamba paper chain bana rahe ho. Chain-growth ek aisa bachcha hai jo bahut tez hai: woh loop pe loop pakadta jaata hai aur ek badi chain jaldi bana leta hai, phir rukta hai. Shuruaat mein bhi, kuch chains already kaafi badi hoti hain. Step-growth ek kamre mein bahut saare bachche hain jinke paas 2-loop wale chhote tukde hain: woh randomly apne tukde bade tukdon mein jodte hain. Kamre ko ek giant chain tabhi milti hai jab almost sab log mil chuke hote hain — bilkul end mein. "Bachche ka haath" ek radical ho sakta hai (lalchi, kuch bhi pakadta hai), ek plus-charge ya minus-charge (choosy loops ke baare mein), ya ek metal robot-hand (Ziegler-Natta) jo loops ko hamesha same neat tarike se rakhta hai taki chain seedhi aur strong ho.

Connections

Addition vs Condensation Polymers
HDPE vs LDPE
Stereochemistry — Tacticity (isotactic, syndiotactic, atactic)
Copolymers and Block Copolymers
Reaction Kinetics — Steady State Approximation
Nylon, Polyester and Important Polymers
Molecular Weight Distribution and Polydispersity